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rfc:rfc5533

Network Working Group E. Nordmark Request for Comments: 5533 Sun Microsystems Category: Standards Track M. Bagnulo

                                                                  UC3M
                                                             June 2009
         Shim6: Level 3 Multihoming Shim Protocol for IPv6

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (c) 2009 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents in effect on the date of
 publication of this document (http://trustee.ietf.org/license-info).
 Please review these documents carefully, as they describe your rights
 and restrictions with respect to this document.

Abstract

 This document defines the Shim6 protocol, a layer 3 shim for
 providing locator agility below the transport protocols, so that
 multihoming can be provided for IPv6 with failover and load-sharing
 properties, without assuming that a multihomed site will have a
 provider-independent IPv6 address prefix announced in the global IPv6
 routing table.  The hosts in a site that has multiple provider-
 allocated IPv6 address prefixes will use the Shim6 protocol specified
 in this document to set up state with peer hosts so that the state
 can later be used to failover to a different locator pair, should the
 original one stop working.

Nordmark & Bagnulo Standards Track [Page 1] RFC 5533 Shim6 Protocol June 2009

Table of Contents

 1. Introduction ....................................................4
    1.1. Goals ......................................................5
    1.2. Non-Goals ..................................................5
    1.3. Locators as Upper-Layer Identifiers (ULID) .................6
    1.4. IP Multicast ...............................................7
    1.5. Renumbering Implications ...................................8
    1.6. Placement of the Shim ......................................9
    1.7. Traffic Engineering .......................................11
 2. Terminology ....................................................12
    2.1. Definitions ...............................................12
    2.2. Notational Conventions ....................................15
    2.3. Conceptual ................................................15
 3. Assumptions ....................................................15
 4. Protocol Overview ..............................................17
    4.1. Context Tags ..............................................19
    4.2. Context Forking ...........................................19
    4.3. API Extensions ............................................20
    4.4. Securing Shim6 ............................................20
    4.5. Overview of Shim Control Messages .........................21
    4.6. Extension Header Order ....................................22
 5. Message Formats ................................................23
    5.1. Common Shim6 Message Format ...............................23
    5.2. Shim6 Payload Extension Header Format .....................24
    5.3. Common Shim6 Control Header ...............................25
    5.4. I1 Message Format .........................................26
    5.5. R1 Message Format .........................................28
    5.6. I2 Message Format .........................................29
    5.7. R2 Message Format .........................................31
    5.8. R1bis Message Format ......................................33
    5.9. I2bis Message Format ......................................34
    5.10. Update Request Message Format ............................37
    5.11. Update Acknowledgement Message Format ....................38
    5.12. Keepalive Message Format .................................40
    5.13. Probe Message Format .....................................40
    5.14. Error Message Format .....................................40
    5.15. Option Formats ...........................................42
         5.15.1. Responder Validator Option Format .................44
         5.15.2. Locator List Option Format ........................44
         5.15.3. Locator Preferences Option Format .................46
         5.15.4. CGA Parameter Data Structure Option Format ........48
         5.15.5. CGA Signature Option Format .......................49
         5.15.6. ULID Pair Option Format ...........................49
         5.15.7. Forked Instance Identifier Option Format ..........50
         5.15.8. Keepalive Timeout Option Format ...................50
 6. Conceptual Model of a Host .....................................51
    6.1. Conceptual Data Structures ................................51

Nordmark & Bagnulo Standards Track [Page 2] RFC 5533 Shim6 Protocol June 2009

    6.2. Context STATES ............................................52
 7. Establishing ULID-Pair Contexts ................................54
    7.1. Uniqueness of Context Tags ................................54
    7.2. Locator Verification ......................................55
    7.3. Normal Context Establishment ..............................56
    7.4. Concurrent Context Establishment ..........................56
    7.5. Context Recovery ..........................................58
    7.6. Context Confusion .........................................60
    7.7. Sending I1 Messages .......................................61
    7.8. Retransmitting I1 Messages ................................62
    7.9. Receiving I1 Messages .....................................62
    7.10. Sending R1 Messages ......................................63
         7.10.1. Generating the R1 Validator .......................64
    7.11. Receiving R1 Messages and Sending I2 Messages ............64
    7.12. Retransmitting I2 Messages ...............................65
    7.13. Receiving I2 Messages ....................................66
    7.14. Sending R2 Messages ......................................67
    7.15. Match for Context Confusion ..............................68
    7.16. Receiving R2 Messages ....................................69
    7.17. Sending R1bis Messages ...................................69
         7.17.1. Generating the R1bis Validator ....................70
    7.18. Receiving R1bis Messages and Sending I2bis Messages ......71
    7.19. Retransmitting I2bis Messages ............................72
    7.20. Receiving I2bis Messages and Sending R2 Messages .........72
 8. Handling ICMP Error Messages ...................................74
 9. Teardown of the ULID-Pair Context ..............................76
 10. Updating the Peer .............................................77
    10.1. Sending Update Request Messages ..........................77
    10.2. Retransmitting Update Request Messages ...................78
    10.3. Newer Information while Retransmitting ...................78
    10.4. Receiving Update Request Messages ........................79
    10.5. Receiving Update Acknowledgement Messages ................81
 11. Sending ULP Payloads ..........................................81
    11.1. Sending ULP Payload after a Switch .......................82
 12. Receiving Packets .............................................83
    12.1. Receiving Payload without Extension Headers ..............83
    12.2. Receiving Shim6 Payload Extension Headers ................83
    12.3. Receiving Shim Control Messages ..........................84
    12.4. Context Lookup ...........................................84
 13. Initial Contact ...............................................86
 14. Protocol Constants ............................................87
 15. Implications Elsewhere ........................................88
    15.1. Congestion Control Considerations ........................88
    15.2. Middle-Boxes Considerations ..............................88
    15.3. Operation and Management Considerations ..................89
    15.4. Other Considerations .....................................90
 16. Security Considerations .......................................91
    16.1. Interaction with IPSec ...................................93

Nordmark & Bagnulo Standards Track [Page 3] RFC 5533 Shim6 Protocol June 2009

    16.2. Residual Threats .........................................94
 17. IANA Considerations ...........................................95
 18. Acknowledgements ..............................................97
 19. References ....................................................97
    19.1. Normative References .....................................97
    19.2. Informative References ...................................97
 Appendix A.  Possible Protocol Extensions ........................100
 Appendix B.  Simplified STATE Machine ............................101
    B.1.  Simplified STATE Machine Diagram ........................108
 Appendix C.  Context Tag Reuse ...................................109
    C.1.  Context Recovery ........................................109
    C.2.  Context Confusion .......................................109
    C.3.  Three-Party Context Confusion .........................110
    C.4.  Summary .................................................110
 Appendix D.  Design Alternatives .................................111
    D.1.  Context Granularity .....................................111
    D.2.  Demultiplexing of Data Packets in Shim6 Communications ..111
      D.2.1.   Flow Label .........................................112
      D.2.2.   Extension Header ...................................115
    D.3.  Context-Loss Detection ................................115
    D.4.  Securing Locator Sets ...................................117
    D.5.  ULID-Pair Context-Establishment Exchange ............120
    D.6.  Updating Locator Sets ...................................121
    D.7.  State Cleanup ...........................................122

1. Introduction

 This document describes a layer 3 shim approach and protocol for
 providing locator agility below the transport protocols, so that
 multihoming can be provided for IPv6 with failover and load-sharing
 properties [11], without assuming that a multihomed site will have a
 provider-independent IPv6 address announced in the global IPv6
 routing table.  The hosts in a site that has multiple provider-
 allocated IPv6 address prefixes will use the Shim6 protocol specified
 in this document to set up state with peer hosts so that the state
 can later be used to failover to a different locator pair, should the
 original one stop working (the term locator is defined in Section 2).
 The Shim6 protocol is a site-multihoming solution in the sense that
 it allows existing communication to continue when a site that has
 multiple connections to the Internet experiences an outage on a
 subset of these connections or further upstream.  However, Shim6
 processing is performed in individual hosts rather than through site-
 wide mechanisms.
 We assume that redirection attacks are prevented using Hash-Based
 Addresses (HBA) as defined in [3].

Nordmark & Bagnulo Standards Track [Page 4] RFC 5533 Shim6 Protocol June 2009

 The reachability and failure-detection mechanisms, including how a
 new working locator pair is discovered after a failure, are specified
 in RFC 5534 [4].  This document allocates message types and option
 types for that sub-protocol, and leaves the specification of the
 message and option formats, as well as the protocol behavior, to RFC
 5534.

1.1. Goals

 The goals for this approach are to:
 o  Preserve established communications in the presence of certain
    classes of failures, for example, TCP connections and UDP streams.
 o  Have minimal impact on upper-layer protocols in general and on
    transport protocols and applications in particular.
 o  Address the security threats in [15] through a combination of the
    HBA/CGA approach specified in RFC 5535 [3] and the techniques
    described in this document.
 o  Not require an extra roundtrip up front to set up shim-specific
    state.  Instead, allow the upper-layer traffic (e.g., TCP) to flow
    as normal and defer the set up of the shim state until some number
    of packets have been exchanged.
 o  Take advantage of multiple locators/addresses for load spreading
    so that different sets of communication to a host (e.g., different
    connections) might use different locators of the host.  Note that
    this might cause load to be spread unevenly; thus, we use the term
    "load spreading" instead of "load balancing".  This capability
    might enable some forms of traffic engineering, but the details
    for traffic engineering, including what requirements can be
    satisfied, are not specified in this document, and form part of
    potential extensions to this protocol.

1.2. Non-Goals

 The problem we are trying to solve is site multihoming, with the
 ability to have the set of site prefixes change over time due to site
 renumbering.  Further, we assume that such changes to the set of
 locator prefixes can be relatively slow and managed: slow enough to
 allow updates to the DNS to propagate (since the protocol defined in
 this document depends on the DNS to find the appropriate locator
 sets).  However, note that it is an explicit non-goal to make
 communication survive a renumbering event (which causes all the
 locators of a host to change to a new set of locators).  This
 proposal does not attempt to solve the related problem of host

Nordmark & Bagnulo Standards Track [Page 5] RFC 5533 Shim6 Protocol June 2009

 mobility.  However, it might turn out that the Shim6 protocol can be
 a useful component for future host mobility solutions, e.g., for
 route optimization.
 Finally, this proposal also does not try to provide a new network-
 level or transport-level identifier name space distinct from the
 current IP address name space.  Even though such a concept would be
 useful to upper-layer protocols (ULPs) and applications, especially
 if the management burden for such a name space was negligible and
 there was an efficient yet secure mechanism to map from identifiers
 to locators, such a name space isn't necessary (and furthermore
 doesn't seem to help) to solve the multihoming problem.
 The Shim6 proposal doesn't fully separate the identifier and locator
 functions that have traditionally been overloaded in the IP address.
 However, throughout this document the term "identifier" or, more
 specifically, upper-layer identifier (ULID), refers to the
 identifying function of an IPv6 address.  "Locator" refers to the
 network-layer routing and forwarding properties of an IPv6 address.

1.3. Locators as Upper-Layer Identifiers (ULID)

 The approach described in this document does not introduce a new
 identifier name space but instead uses the locator that is selected
 in the initial contact with the remote peer as the preserved upper-
 layer identifier (ULID).  While there may be subsequent changes in
 the selected network-level locators over time (in response to
 failures in using the original locator), the upper-level protocol
 stack elements will continue to use this upper-level identifier
 without change.
 This implies that the ULID selection is performed as today's default
 address selection as specified in RFC 3484 [7].  Some extensions are
 needed to RFC 3484 to try different source addresses, whether or not
 the Shim6 protocol is used, as outlined in [9].  Underneath, and
 transparently, the multihoming shim selects working locator pairs
 with the initial locator pair being the ULID pair.  If communication
 subsequently fails, the shim can test and select alternate locators.
 A subsequent section discusses the issues that arise when the
 selected ULID is not initially working, which creates the need to
 switch locators up front.
 Using one of the locators as the ULID has certain benefits for
 applications that have long-lived session state or that perform
 callbacks or referrals, because both the Fully Qualified Domain Name
 (FQDN) and the 128-bit ULID work as handles for the applications.

Nordmark & Bagnulo Standards Track [Page 6] RFC 5533 Shim6 Protocol June 2009

 However, using a single 128-bit ULID doesn't provide seamless
 communication when that locator is unreachable.  See [18] for further
 discussion of the application implications.
 There has been some discussion of using non-routable addresses, such
 as Unique-Local Addresses (ULAs) [14], as ULIDs in a multihoming
 solution.  While this document doesn't specify all aspects of this,
 it is believed that the approach can be extended to handle the non-
 routable address case.  For example, the protocol already needs to
 handle ULIDs that are not initially reachable.  Thus, the same
 mechanism can handle ULIDs that are permanently unreachable from
 outside their site.  The issue becomes how to make the protocol
 perform well when the ULID is known a priori to be unreachable (e.g.,
 the ULID is a ULA), for instance, avoiding any timeout and retries in
 this case.  In addition, one would need to understand how the ULAs
 would be entered in the DNS to avoid a performance impact on
 existing, non-Shim6-aware IPv6 hosts potentially trying to
 communicate to the (unreachable) ULA.

1.4. IP Multicast

 IP multicast requires that the IP Source Address field contain a
 topologically correct locator for the interface that is used to send
 the packet, since IP multicast routing uses both the source address
 and the destination group to determine where to forward the packet.
 In particular, IP multicast routing needs to be able to do the
 Reverse Path Forwarding (RPF) check.  (This isn't much different than
 the situation with widely implemented ingress filtering [6] for
 unicast.)
 While in theory it would be possible to apply the shim re-mapping of
 the IP address fields between ULIDs and locators, the fact that all
 the multicast receivers would need to know the mapping to perform
 makes such an approach difficult in practice.  Thus, it makes sense
 to have multicast ULPs operate directly on locators and not use the
 shim.  This is quite a natural fit for protocols that use RTP [10],
 since RTP already has an explicit identifier in the form of the
 synchronization source (SSRC) field in the RTP headers.  Thus, the
 actual IP address fields are not important to the application.
 In summary, IP multicast will not need the shim to remap the IP
 addresses.
 This doesn't prevent the receiver of multicast to change its
 locators, since the receiver is not explicitly identified; the
 destination address is a multicast address and not the unicast
 locator of the receiver.

Nordmark & Bagnulo Standards Track [Page 7] RFC 5533 Shim6 Protocol June 2009

1.5. Renumbering Implications

 As stated above, this approach does not try to make communication
 survive renumbering in the general case.
 When a host is renumbered, the effect is that one or more locators
 become invalid, and zero or more locators are added to the host's
 network interface.  This means that the set of locators that is used
 in the shim will change, which the shim can handle as long as not all
 the original locators become invalid at the same time; the shim's
 ability to handle this also depends on the time that is required to
 update the DNS and for those updates to propagate.
 But IP addresses are also used as ULIDs, and making the communication
 survive locators becoming invalid can potentially cause some
 confusion at the upper layers.  The fact that a ULID might be used
 with a different locator over time opens up the possibility that
 communication between two ULIDs might continue to work after one or
 both of those ULIDs are no longer reachable as locators, for example,
 due to a renumbering event.  This opens up the possibility that the
 ULID (or at least the prefix on which it is based) may be reassigned
 to another site while it is still being used (with another locator)
 for existing communication.
 In the worst case, we could end up with two separate hosts using the
 same ULID while both of them are communicating with the same host.
 This potential source for confusion is avoided by requiring that any
 communication using a ULID MUST be terminated when the ULID becomes
 invalid (due to the underlying prefix becoming invalid).  This
 behavior can be accomplished by explicitly discarding the shim state
 when the ULID becomes invalid.  The context-recovery mechanism will
 then make the peer aware that the context is gone and that the ULID
 is no longer present at the same locator(s).

Nordmark & Bagnulo Standards Track [Page 8] RFC 5533 Shim6 Protocol June 2009

1.6. Placement of the Shim

  1. ———————-

| Transport Protocols |

  1. ———————-
  1. ————- ————- IP endpoint

| Frag/reass | | Dest opts | sub-layer

  1. ————- ————-
  1. ——————–

| Shim6 shim layer |

  1. ——————–
  1. —– IP routing

| IP | sub-layer

  1. —–
                       Figure 1: Protocol Stack
 The proposal uses a multihoming shim layer within the IP layer, i.e.,
 below the ULPs, as shown in Figure 1, in order to provide ULP
 independence.  The multihoming shim layer behaves as if it is
 associated with an extension header, which would be placed after any
 routing-related headers in the packet (such as any hop-by-hop
 options).  However, when the locator pair is the ULID pair, there is
 no data that needs to be carried in an extension header; thus, none
 is needed in that case.
 Layering the Fragmentation header above the multihoming shim makes
 reassembly robust in the case that there is broken multi-path routing
 that results in using different paths, hence potentially different
 source locators, for different fragments.  Thus, the multihoming shim
 layer is placed between the IP endpoint sublayer (which handles
 fragmentation and reassembly) and the IP routing sublayer (which
 selects the next hop and interface to use for sending out packets).
 Applications and upper-layer protocols use ULIDs that the Shim6 layer
 maps to/from different locators.  The Shim6 layer maintains state,
 called ULID-pair context, per ULID pair (that is, such state applies
 to all ULP connections between the ULID pair) in order to perform
 this mapping.  The mapping is performed consistently at the sender
 and the receiver so that ULPs see packets that appear to be sent
 using ULIDs from end to end.  This property is maintained even though
 the packets travel through the network containing locators in the IP
 address fields, and even though those locators may be changed by the
 transmitting Shim6 layer.

Nordmark & Bagnulo Standards Track [Page 9] RFC 5533 Shim6 Protocol June 2009

 The context state is maintained per remote ULID, i.e., approximately
 per peer host, and not at any finer granularity.  In particular, the
 context state is independent of the ULPs and any ULP connections.
 However, the forking capability enables Shim6-aware ULPs to use more
 than one locator pair at a time for a single ULID pair.
  1. ————————— —————————-

| Sender A | | Receiver B |

  |                          |          |                          |
  |     ULP                  |          |     ULP                  |
  |      | src ULID(A)=L1(A) |          |      ^                   |
  |      | dst ULID(B)=L1(B) |          |      | src ULID(A)=L1(A) |
  |      v                   |          |      | dst ULID(B)=L1(B) |
  |   multihoming shim       |          |   multihoming shim       |
  |      | src L2(A)         |          |      ^                   |
  |      | dst L3(B)         |          |      | src L2(A)         |
  |      v                   |          |      | dst L3(B)         |
  |      IP                  |          |      IP                  |
  ----------------------------          ----------------------------
         |                                     ^
         ------- cloud with some routers -------
                Figure 2: Mapping with Changed Locators
 The result of this consistent mapping is that there is no impact on
 the ULPs.  In particular, there is no impact on pseudo-header
 checksums and connection identification.
 Conceptually, one could view this approach as if both ULIDs and
 locators are present in every packet, and as if a header-compression
 mechanism is applied that removes the need for the ULIDs to be
 carried in the packets once the compression state has been
 established.  In order for the receiver to re-create a packet with
 the correct ULIDs, there is a need to include some "compression tag"
 in the data packets.  This serves to indicate the correct context to
 use for decompression when the locator pair in the packet is
 insufficient to uniquely identify the context.
 There are different types of interactions between the Shim6 layer and
 other protocols.  Those interactions are influenced by the usage of
 the addresses in these other protocols and the impact of the Shim6
 mapping on these usages.  A detailed analysis of the interactions of
 different protocols, including the Stream Control Transmission
 Protocol (SCTP), mobile IP (MIP), and Host Identity Protocol (HIP),
 can be found in [19].  Moreover, some applications may need to have a
 richer interaction with the Shim6 sublayer.  In order to enable that,
 an API [23] has been defined to enable greater control and
 information exchange for those applications that need it.

Nordmark & Bagnulo Standards Track [Page 10] RFC 5533 Shim6 Protocol June 2009

1.7. Traffic Engineering

 At the time of this writing, it is not clear what requirements for
 traffic engineering make sense for the Shim6 protocol, since the
 requirements must both result in some useful behavior as well as be
 implementable using a host-to-host locator agility mechanism like
 Shim6.
 Inherent in a scalable multihoming mechanism that separates the
 locator function of the IP address from identifying function of the
 IP address is that each host ends up with multiple locators.  This
 means that, at least for initial contact, it is the remote peer
 application (or layer working on its behalf) that needs to select an
 initial ULID, which automatically becomes the initial locator.  In
 the case of Shim6, this is performed by applying RFC 3484 address
 selection.
 This is quite different than the common case of IPv4 multihoming
 where the site has a single IP address prefix, since in that case the
 peer performs no destination address selection.
 Thus, in "single prefix multihoming", the site (and in many cases its
 upstream ISPs) can use BGP to exert some control of the ingress path
 used to reach the site.  This capability does not by itself exist in
 "multiple prefix multihoming" approaches such as Shim6.  It is
 conceivable that extensions allowing site or provider guidance of
 host-based mechanisms could be developed.  But it should be noted
 that traffic engineering via BGP, MPLS, or other similar techniques
 can still be applied for traffic on each individual prefix; Shim6
 does not remove the capability for this.  It does provide some
 additional capabilities for hosts to choose between prefixes.
 These capabilities also carry some risk for non-optimal behaviour
 when more than one mechanism attempts to correct problems at the same
 time.  However, it should be noted that this is not necessarily a
 situation brought about by Shim6.  A more constrained form of this
 capability already exists in IPv6, itself, via its support of
 multiple prefixes and address-selection rules for starting new
 communications.  Even IPv4 hosts with multiple interfaces may have
 limited capabilities to choose interfaces on which they communicate.
 Similarly, upper layers may choose different addresses.
 In general, it is expected that Shim6 is applicable in relatively
 small sites and individual hosts where BGP-style traffic engineering
 operations are unavailable, unlikely, or if run with provider-
 independent addressing, possibly even harmful, considering the growth
 rates in the global routing table.

Nordmark & Bagnulo Standards Track [Page 11] RFC 5533 Shim6 Protocol June 2009

 The protocol provides a placeholder, in the form of the Locator
 Preferences option, that can be used by hosts to express priority and
 weight values for each locator.  This option is merely a placeholder
 when it comes to providing traffic engineering; in order to use this
 in a large site, there would have to be a mechanism by which the host
 can find out what preference values to use, either statically (e.g.,
 some new DHCPv6 option) or dynamically.
 Thus, traffic engineering is listed as a possible extension in
 Appendix A.

2. Terminology

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [1].

2.1. Definitions

 This document introduces the following terms:
 upper-layer protocol (ULP)
                     A protocol layer immediately above IP.  Examples
                     are transport protocols such as TCP and UDP;
                     control protocols such as ICMP; routing protocols
                     such as OSPF; and Internet or lower-layer
                     protocols being "tunneled" over (i.e.,
                     encapsulated in) IP, such as the Internet Packet
                     Exchange (IPX), AppleTalk, or IP itself.
 interface           A node's attachment to a link.
 address             An IP-layer name that both contains topological
                     significance and acts as a unique identifier for
                     an interface. 128 bits.  This document only uses
                     the "address" term in the case where it isn't
                     specific whether it is a locator or an
                     identifier.
 locator             An IP-layer topological name for an interface or
                     a set of interfaces. 128 bits.  The locators are
                     carried in the IP address fields as the packets
                     traverse the network.
 identifier          An IP-layer name for an IP-layer endpoint.  The
                     transport endpoint name is a function of the
                     transport protocol and would typically include
                     the IP identifier plus a port number.

Nordmark & Bagnulo Standards Track [Page 12] RFC 5533 Shim6 Protocol June 2009

                     NOTE: This proposal does not specify any new form
                     of IP-layer identifier, but still separates the
                     identifying and locating properties of the IP
                     addresses.
 upper-layer identifier (ULID)
                     An IP address that has been selected for
                     communication with a peer to be used by the
                     upper-layer protocol. 128 bits.  This is used for
                     pseudo-header checksum computation and connection
                     identification in the ULP.  Different sets of
                     communication to a host (e.g., different
                     connections) might use different ULIDs in order
                     to enable load spreading.
                     Since the ULID is just one of the IP locators/
                     addresses of the node, there is no need for a
                     separate name space and allocation mechanisms.
 address field       The Source and Destination Address fields in the
                     IPv6 header.  As IPv6 is currently specified,
                     these fields carry "addresses".  If identifiers
                     and locators are separated, these fields will
                     contain locators for packets on the wire.
 FQDN                Fully Qualified Domain Name
 ULID-pair context   The state that the multihoming shim maintains
                     between a pair of upper-layer identifiers.  The
                     context is identified by a Context Tag for each
                     direction of the communication and also by a
                     ULID-pair and a Forked Instance Identifier (see
                     below).
 Context Tag         Each end of the context allocates a Context Tag
                     for the context.  This is used to uniquely
                     associate both received control packets and Shim6
                     Payload Extension headers as belonging to the
                     context.
 current locator pair
                     Each end of the context has a current locator
                     pair that is used to send packets to the peer.
                     However, the two ends might use different current
                     locator pairs.

Nordmark & Bagnulo Standards Track [Page 13] RFC 5533 Shim6 Protocol June 2009

 default context     At the sending end, the shim uses the ULID pair
                     (passed down from the ULP) to find the context
                     for that pair.  Thus, normally, a host can have
                     at most one context for a ULID pair.  We call
                     this the "default context".
 context forking     A mechanism that allows ULPs that are aware of
                     multiple locators to use separate contexts for
                     the same ULID pair, in order to be able use
                     different locator pairs for different
                     communication to the same ULID.  Context forking
                     causes more than just the default context to be
                     created for a ULID pair.
 Forked Instance Identifier (FII)
                     In order to handle context forking, a context is
                     identified by a ULID pair and a Forked Context
                     Identifier.  The default context has an FII of
                     zero.
 initial contact     We use this term to refer to the pre-shim
                     communication when a ULP decides to start
                     communicating with a peer by sending and
                     receiving ULP packets.  Typically, this would not
                     invoke any operations in the shim, since the shim
                     can defer the context establishment until some
                     arbitrary, later point in time.
 Hash-Based Addresses (HBA)
                     A form of IPv6 address where the interface ID is
                     derived from a cryptographic hash of all the
                     prefixes assigned to the host.  See [3].
 Cryptographically Generated Addresses (CGA)
                     A form of IPv6 address where the interface ID is
                     derived from a cryptographic hash of the public
                     key.  See [2].
 CGA Parameter Data Structure (PDS)
                     The information that CGA and HBA exchange in
                     order to inform the peer of how the interface ID
                     was computed.  See [2] and [3].

Nordmark & Bagnulo Standards Track [Page 14] RFC 5533 Shim6 Protocol June 2009

2.2. Notational Conventions

 A, B, and C are hosts.  X is a potentially malicious host.
 FQDN(A) is the Fully Qualified Domain Name for A.
 Ls(A) is the locator set for A, which consists of the locators L1(A),
 L2(A), ...  Ln(A).  The locator set is not ordered in any particular
 way other than maybe what is returned by the DNS.  A host might form
 different locator sets containing different subnets of the host's IP
 addresses.  This is necessary in some cases for security reasons.
 See Section 16.1.
 ULID(A) is an upper-layer identifier for A.  In this proposal,
 ULID(A) is always one member of A's locator set.
 CT(A) is a Context Tag assigned by A.
 STATE (in uppercase) refers to the specific state of the state
 machine described in Section 6.2

2.3. Conceptual

 This document also makes use of internal conceptual variables to
 describe protocol behavior and external variables that an
 implementation must allow system administrators to change.  The
 specific variable names, how their values change, and how their
 settings influence protocol behavior are provided to demonstrate
 protocol behavior.  An implementation is not required to have them in
 the exact form described here, so long as its external behavior is
 consistent with that described in this document.  See Section 6 for a
 description of the conceptual data structures.

3. Assumptions

 The design intent is to ensure that the Shim6 protocol is capable of
 handling path failures independently of the number of IP addresses
 (locators) available to the two communicating hosts, and
 independently of which host detects the failure condition.
 Consider, for example, the case in which both A and B have active
 Shim6 state and where A has only one locator while B has multiple
 locators.  In this case, it might be that B is trying to send a
 packet to A, and has detected a failure condition with the current
 locator pair.  Since B has multiple locators, it presumably has
 multiple ISPs, and (consequently) likely has alternate egress paths

Nordmark & Bagnulo Standards Track [Page 15] RFC 5533 Shim6 Protocol June 2009

 toward A.  B cannot vary the destination address (i.e., A's locator),
 since A has only one locator.  However, B may need to vary the source
 address in order to ensure packet delivery.
 In many cases, normal operation of IP routing may cause the packets
 to follow a path towards the correct (currently operational) egress.
 In some cases, it is possible that a path may be selected based on
 the source address, implying that B will need to select a source
 address corresponding to the currently operating egress.  The details
 of how routing can be accomplished is beyond the scope of this
 document.
 Also, when the site's ISPs perform ingress filtering based on packet
 source addresses, Shim6 assumes that packets sent with different
 source and destination combinations have a reasonable chance of
 making it through the relevant ISP's ingress filters.  This can be
 accomplished in several ways (all outside the scope of this
 document), such as having the ISPs relax their ingress filters or
 selecting the egress such that it matches the IP source address
 prefix.  In the case that one egress path has failed but another is
 operating correctly, it may be necessary for the packet's source
 (node B in the previous paragraph) to select a source address that
 corresponds to the operational egress, in order to pass the ISP's
 ingress filters.
 The Shim6 approach assumes that there are no IPv6-to-IPv6 NATs on the
 paths, i.e., that the two ends can exchange their own notion of their
 IPv6 addresses and that those addresses will also make sense to their
 peer.
 The security of the Shim6 protocol relies on the usage of Hash-Based
 Addresses (HBA) [3] and/or Cryptographically Generated Addresses
 (CGA) [2].  In the case that HBAs are used, all the addresses
 assigned to the host that are included in the Shim6 protocol (either
 as a locator or as a ULID) must be part of the same HBA set.  In the
 case that CGAs are used, the address used as ULID must be a CGA, but
 the other addresses that are used as locators do not need to be
 either CGAs or HBAs.  It should be noted that it is perfectly
 acceptable to run the Shim6 protocol between a host that has multiple
 locators and another host that has a single IP address.  In this
 case, the address of the host with a single address does not need to
 be an HBA or a CGA.

Nordmark & Bagnulo Standards Track [Page 16] RFC 5533 Shim6 Protocol June 2009

4. Protocol Overview

 The Shim6 protocol operates in several phases over time.  The
 following sequence illustrates the concepts:
 o  An application on host A decides to contact an application on host
    B using some upper-layer protocol.  This results in the ULP on
    host A sending packets to host B.  We call this the initial
    contact.  Assuming the IP addresses selected by default address
    selection [7] and its extensions [9] work, then there is no action
    by the shim at this point in time.  Any shim context establishment
    can be deferred until later.
 o  Some heuristic on A or B (or both) determine that it is
    appropriate to pay the Shim6 overhead to make this host-to-host
    communication robust against locator failures.  For instance, this
    heuristic might be that more than 50 packets have been sent or
    received, or that there was a timer expiration while active packet
    exchange was in place.  This makes the shim initiate the 4-way,
    context-establishment exchange.  The purpose of this heuristic is
    to avoid setting up a shim context when only a small number of
    packets is exchanged between two hosts.
    As a result of this exchange, both A and B will know a list of
    locators for each other.
    If the context-establishment exchange fails, the initiator will
    then know that the other end does not support Shim6, and will
    continue with standard (non-Shim6) behavior for the session.
 o  Communication continues without any change for the ULP packets.
    In particular, there are no Shim6 Extension headers added to the
    ULP packets, since the ULID pair is the same as the locator pair.
    In addition, there might be some messages exchanged between the
    shim sublayers for (un)reachability detection.
 o  At some point in time, something fails.  Depending on the approach
    to reachability detection, there might be some advice from the
    ULP, or the shim (un)reachability detection might discover that
    there is a problem.
    At this point in time, one or both ends of the communication need
    to probe the different alternate locator pairs until a working
    pair is found, and then switch to using that locator pair.
 o  Once a working alternative locator pair has been found, the shim
    will rewrite the packets on transmit and tag the packets with the
    Shim6 Payload Extension header, which contains the receiver's

Nordmark & Bagnulo Standards Track [Page 17] RFC 5533 Shim6 Protocol June 2009

    Context Tag.  The receiver will use the Context Tag to find the
    context state, which will indicate which addresses to place in the
    IPv6 header before passing the packet up to the ULP.  The result
    is that, from the perspective of the ULP, the packet passes
    unmodified end-to-end, even though the IP routing infrastructure
    sends the packet to a different locator.
 o  The shim (un)reachability detection will monitor the new locator
    pair as it monitored the original locator pair, so that subsequent
    failures can be detected.
 o  In addition to failures detected based on end-to-end observations,
    one endpoint might know for certain that one or more of its
    locators is not working.  For instance, the network interface
    might have failed or gone down (at layer 2), or an IPv6 address
    might have become deprecated or invalid.  In such cases, the host
    can signal its peer that trying this address is no longer
    recommended.  This triggers something similar to a failure
    handling, and a new working locator pair must be found.
    The protocol also has the ability to express other forms of
    locator preferences.  A change in any preference can be signaled
    to the peer, which will have made the peer record the new
    preferences.  A change in the preferences might optionally make
    the peer want to use a different locator pair.  In this case, the
    peer follows the same locator switching procedure as after a
    failure (by verifying that its peer is indeed present at the
    alternate locator, etc).
 o  When the shim thinks that the context state is no longer used, it
    can garbage collect the state; there is no coordination necessary
    with the peer host before the state is removed.  There is a
    recovery message defined to be able to signal when there is no
    context state, which can be used to detect and recover from both
    premature garbage collection as well as from complete state loss
    (crash and reboot) of a peer.
    The exact mechanism to determine when the context state is no
    longer used is implementation dependent.  For example, an
    implementation might use the existence of ULP state (where known
    to the implementation) as an indication that the state is still
    used, combined with a timer (to handle ULP state that might not be
    known to the shim sublayer) to determine when the state is likely
    to no longer be used.
 NOTE 1: The ULP packets in Shim6 can be carried completely unmodified
 as long as the ULID pair is used as the locator pair.  After a switch
 to a different locator pair, the packets are "tagged" with a Shim6

Nordmark & Bagnulo Standards Track [Page 18] RFC 5533 Shim6 Protocol June 2009

 Extension header so that the receiver can always determine the
 context to which they belong.  This is accomplished by including an
 8-octet Shim6 Payload Extension header before the (extension) headers
 that are processed by the IP endpoint sublayer and ULPs.  If,
 subsequently, the original ULIDs are selected as the active locator
 pair, then the tagging of packets with the Shim6 Extension header is
 no longer necessary.

4.1. Context Tags

 A context between two hosts is actually a context between two ULIDs.
 The context is identified by a pair of Context Tags.  Each end gets
 to allocate a Context Tag, and once the context is established, most
 Shim6 control messages contain the Context Tag that the receiver of
 the message allocated.  Thus, at a minimum, the combination of <peer
 ULID, local ULID, local Context Tag> have to uniquely identify one
 context.  But, since the Shim6 Payload Extension headers are
 demultiplexed without looking at the locators in the packet, the
 receiver will need to allocate Context Tags that are unique for all
 its contexts.  The Context Tag is a 47-bit number (the largest that
 can fit in an 8-octet extension header), while preserving one bit to
 differentiate the Shim6 signaling messages from the Shim6 header
 included in data packets, allowing both to use the same protocol
 number.
 The mechanism for detecting a loss of context state at the peer
 assumes that the receiver can tell the packets that need locator
 rewriting, even after it has lost all state (e.g., due to a crash
 followed by a reboot).  This is achieved because, after a rehoming
 event, the packets that need receive-side rewriting carry the Shim6
 Payload Extension header.

4.2. Context Forking

 It has been asserted that it will be important for future ULPs -- in
 particular, future transport protocols -- to be able to control which
 locator pairs are used for different communication.  For instance,
 host A and host B might communicate using both Voice over IP (VoIP)
 traffic and ftp traffic, and those communications might benefit from
 using different locator pairs.  However, the basic Shim6 mechanism
 uses a single current locator pair for each context; thus, a single
 context cannot accomplish this.
 For this reason, the Shim6 protocol supports the notion of context
 forking.  This is a mechanism by which a ULP can specify (using some
 API not yet defined) that a context, e.g., the ULID pair <A1, B2>,

Nordmark & Bagnulo Standards Track [Page 19] RFC 5533 Shim6 Protocol June 2009

 should be forked into two contexts.  In this case, the forked-off
 context will be assigned a non-zero Forked Instance Identifier, while
 the default context has FII zero.
 The Forked Instance Identifier (FII) is a 32-bit identifier that has
 no semantics in the protocol other than being part of the tuple that
 identifies the context.  For example, a host might allocate FIIs as
 sequential numbers for any given ULID pair.
 No other special considerations are needed in the Shim6 protocol to
 handle forked contexts.
 Note that forking as specified does NOT allow A to be able to tell B
 that certain traffic (a 5-tuple?) should be forked for the reverse
 direction.  The Shim6 forking mechanism as specified applies only to
 the sending of ULP packets.  If some ULP wants to fork for both
 directions, it is up to the ULP to set this up and then instruct the
 shim at each end to transmit using the forked context.

4.3. API Extensions

 Several API extensions have been discussed for Shim6, but their
 actual specification is out of scope for this document.  The simplest
 one would be to add a socket option to be able to have traffic bypass
 the shim (not create any state and not use any state created by other
 traffic).  This could be an IPV6_DONTSHIM socket option.  Such an
 option would be useful for protocols, such as DNS, where the
 application has its own failover mechanism (multiple NS records in
 the case of DNS) and using the shim could potentially add extra
 latency with no added benefits.
 Some other API extensions are discussed in Appendix A.  The actual
 API extensions are defined in [23].

4.4. Securing Shim6

 The mechanisms are secured using a combination of techniques:
 o  The HBA technique [3] for verifying the locators to prevent an
    attacker from redirecting the packet stream to somewhere else.
 o  Requiring a Reachability Probe+Reply (defined in [4]) before a new
    locator is used as the destination, in order to prevent 3rd party
    flooding attacks.

Nordmark & Bagnulo Standards Track [Page 20] RFC 5533 Shim6 Protocol June 2009

 o  The first message does not create any state on the responder.
    Essentially, a 3-way exchange is required before the responder
    creates any state.  This means that a state-based DoS attack
    (trying to use up all memory on the responder) at least provides
    an IPv6 address that the attacker was using.
 o  The context-establishment messages use nonces to prevent replay
    attacks and to prevent off-path attackers from interfering with
    the establishment.
 o  Every control message of the Shim6 protocol, past the context
    establishment, carries the Context Tag assigned to the particular
    context.  This implies that an attacker needs to discover that
    Context Tag before being able to spoof any Shim6 control message.
    Such discovery probably requires any potential attacker to be
    along the path in order to sniff the Context Tag value.  The
    result is that through this technique, the Shim6 protocol is
    protected against off-path attackers.

4.5. Overview of Shim Control Messages

 The Shim6 context establishment is accomplished using four messages;
 I1, R1, I2, R2.  Normally, they are sent in that order from initiator
 and responder, respectively.  Should both ends attempt to set up
 context state at the same time (for the same ULID pair), then their
 I1 messages might cross in flight, and result in an immediate R2
 message.  (The names of these messages are borrowed from HIP [20].)
 R1bis and I2bis messages are defined; they are used to recover a
 context after it has been lost.  An R1bis message is sent when a
 Shim6 control or Shim6 Payload Extension header arrives and there is
 no matching context state at the receiver.  When such a message is
 received, it will result in the re-creation of the Shim6 context
 using the I2bis and R2 messages.
 The peers' lists of locators are normally exchanged as part of the
 context-establishment exchange.  But the set of locators might be
 dynamic.  For this reason, there are Update Request and Update
 Acknowledgement messages as well as a Locator List option.
 Even when the list of locators is fixed, a host might determine that
 some preferences might have changed.  For instance, it might
 determine that there is a locally visible failure that implies that
 some locator(s) are no longer usable.  This uses a Locator
 Preferences option in the Update Request message.

Nordmark & Bagnulo Standards Track [Page 21] RFC 5533 Shim6 Protocol June 2009

 The mechanism for (un)reachability detection is called Forced
 Bidirectional Communication (FBD).  FBD uses a Keepalive message
 which is sent when a host has received packets from its peer but has
 not yet sent any packets from its ULP to the peer.  The message type
 is reserved in this document, but the message format and processing
 rules are specified in [4].
 In addition, when the context is established and there is a
 subsequent failure, there needs to be a way to probe the set of
 locator pairs to efficiently find a working pair.  This document
 reserves a Probe message type, with the packet format and processing
 rules specified in [4].
 The above Probe and Keepalive messages assume we have an established
 ULID-pair context.  However, communication might fail during the
 initial contact (that is, when the application or transport protocol
 is trying to set up some communication).  This is handled using the
 mechanisms in the ULP to try different address pairs as specified in
 [7] and [9].  In future versions of the protocol, and with a richer
 API between the ULP and the shim, the shim might be able to help
 optimize discovering a working locator pair during initial contact.
 This is for further study.

4.6. Extension Header Order

 Since the shim is placed between the IP endpoint sublayer and the IP
 routing sublayer, the Shim header will be placed before any Endpoint
 Extension headers (Fragmentation headers, Destination Options header,
 AH, ESP) but after any routing-related headers (Hop-by-Hop Extensions
 header, Routing header, and a Destinations Options header, which
 precedes a Routing header).  When tunneling is used, whether IP-in-IP
 tunneling or the special form of tunneling that Mobile IPv6 uses
 (with Home Address options and Routing header type 2), there is a
 choice whether the shim applies inside the tunnel or outside the
 tunnel, which affects the location of the Shim6 header.
 In most cases, IP-in-IP tunnels are used as a routing technique;
 thus, it makes sense to apply them on the locators, which means that
 the sender would insert the Shim6 header after any IP-in-IP
 encapsulation.  This is what occurs naturally when routers apply IP-
 in-IP encapsulation.  Thus, the packets would have:
 o  Outer IP header
 o  Inner IP header

Nordmark & Bagnulo Standards Track [Page 22] RFC 5533 Shim6 Protocol June 2009

 o  Shim6 Extension header (if needed)
 o  ULP
 But the shim can also be used to create "shimmed tunnels", i.e.,
 where an IP-in-IP tunnel uses the shim to be able to switch the
 tunnel endpoint addresses between different locators.  In such a
 case, the packets would have:
 o  Outer IP header
 o  Shim6 Extension header (if needed)
 o  Inner IP header
 o  ULP
 In any case, the receiver behavior is well-defined; a receiver
 processes the Extension headers in order.  However, the precise
 interaction between Mobile IPv6 and Shim6 is for further study; it
 might make sense to have Mobile IPv6 operate on locators as well,
 meaning that the shim would be layered on top of the MIPv6 mechanism.

5. Message Formats

 The Shim6 messages are all carried using a new IP protocol number
 (140).  The Shim6 messages have a common header (defined below) with
 some fixed fields, followed by type-specific fields.
 The Shim6 messages are structured as an IPv6 Extension header since
 the Shim6 Payload Extension header is used to carry the ULP packets
 after a locator switch.  The Shim6 control messages use the same
 extension header formats so that a single "protocol number" needs to
 be allowed through firewalls in order for Shim6 to function across
 the firewall.

5.1. Common Shim6 Message Format

 The first 17 bits of the Shim6 header is common for the Shim6 Payload
 Extension header and for the control messages.  It looks as follows:
   0                   1
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Next Header  |  Hdr Ext Len  |P|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Nordmark & Bagnulo Standards Track [Page 23] RFC 5533 Shim6 Protocol June 2009

 Fields:
 Next Header:   The payload that follows this header.
 Hdr Ext Len:   8-bit unsigned integer.  Length of the Shim6 header in
                8-octet units, not including the first 8 octets.
 P:             A single bit to distinguish Shim6 Payload Extension
                headers from control messages.
 Shim6 signaling packets may not be larger than 1280 bytes, including
 the IPv6 header and any intermediate headers between the IPv6 header
 and the Shim6 header.  One way to meet this requirement is to omit
 part of the locator address information if, with this information
 included, the packet would become larger than 1280 bytes.  Another
 option is to perform option engineering, dividing into different
 Shim6 messages the information to be transmitted.  An implementation
 may impose administrative restrictions to avoid excessively large
 Shim6 packets, such as a limitation on the number of locators to be
 used.

5.2. Shim6 Payload Extension Header Format

 The Shim6 Payload Extension header is used to carry ULP packets where
 the receiver must replace the content of the Source and/or
 Destination fields in the IPv6 header before passing the packet to
 the ULP.  Thus, this extension header is required when the locator
 pair that is used is not the same as the ULID pair.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Next Header  |       0       |1|                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
  |                      Receiver Context Tag                     |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Next Header:   The payload that follows this header.
 Hdr Ext Len:   0 (since the header is 8 octets).
 P:             Set to one.  A single bit to distinguish this from the
                Shim6 control messages.

Nordmark & Bagnulo Standards Track [Page 24] RFC 5533 Shim6 Protocol June 2009

 Receiver Context Tag:
                47-bit unsigned integer.  Allocated by the receiver to
                identify the context.

5.3. Common Shim6 Control Header

 The common part of the header has a Next Header field and a Header
 Extension Length field that are consistent with the other IPv6
 Extension headers, even if the Next Header value is always "NO NEXT
 HEADER" for the control messages.
 The Shim6 headers must be a multiple of 8 octets; hence, the minimum
 size is 8 octets.
 The common Shim6 Control message header is as follows:
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Next Header  |  Hdr Ext Len  |P|     Type    |Type-specific|S|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
  |                    Type-specific format                       |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Next Header:   8-bit selector.  Normally set to NO_NXT_HDR (59).
 Hdr Ext Len:   8-bit unsigned integer.  Length of the Shim6 header in
                8-octet units, not including the first 8 octets.
 P:             Set to zero.  A single bit to distinguish this from
                the Shim6 Payload Extension header.
 Type:          7-bit unsigned integer.  Identifies the actual message
                from the table below.  Type codes 0-63 will not
                trigger R1bis messages on a missing context, while
                codes 64-127 will trigger R1bis.
 S:             A single bit set to zero that allows Shim6 and HIP to
                have a common header format yet still distinguishes
                between Shim6 and HIP messages.
 Checksum:      16-bit unsigned integer.  The checksum is the 16-bit
                one's complement of the one's complement sum of the
                entire Shim6 header message, starting with the Shim6

Nordmark & Bagnulo Standards Track [Page 25] RFC 5533 Shim6 Protocol June 2009

                Next Header field and ending as indicated by the Hdr
                Ext Len.  Thus, when there is a payload following the
                Shim6 header, the payload is NOT included in the Shim6
                checksum.  Note that, unlike protocols like ICMPv6,
                there is no pseudo-header checksum part of the
                checksum; this provides locator agility without having
                to change the checksum.
 Type-specific: Part of the message that is different for different
                message types.
  +------------+----------------------------------------------------+
  | Type Value |                       Message                      |
  +------------+----------------------------------------------------+
  |      1     |  I1 (1st establishment message from the initiator) |
  |      2     |  R1 (1st establishment message from the responder) |
  |      3     |  I2 (2nd establishment message from the initiator) |
  |      4     |  R2 (2nd establishment message from the responder) |
  |      5     | R1bis (Reply to reference to non-existent context) |
  |      6     |          I2bis (Reply to an R1bis message)         |
  |     64     |                   Update Request                   |
  |     65     |               Update Acknowledgement               |
  |     66     |                      Keepalive                     |
  |     67     |                    Probe Message                   |
  |     68     |                    Error Message                   |
  +------------+----------------------------------------------------+
                                Table 1

5.4. I1 Message Format

 The I1 message is the first message in the context-establishment
 exchange.

Nordmark & Bagnulo Standards Track [Page 26] RFC 5533 Shim6 Protocol June 2009

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       59      |  Hdr Ext Len  |0|  Type = 1   |   Reserved1 |0|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |R|                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
  |                  Initiator Context Tag                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Initiator Nonce                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                         Options                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Next Header:   NO_NXT_HDR (59).
 Hdr Ext Len:   At least 1, since the header is 16 octets when there
                are no options.
 Type:          1
 Reserved1:     7-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 R:             1-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 Initiator Context Tag:
                47-bit field.  The Context Tag that the initiator has
                allocated for the context.
 Initiator Nonce:
                32-bit unsigned integer.  A random number picked by
                the initiator, which the responder will return in the
                R1 message.
 The following options are defined for this message:
 ULID pair:     When the IPv6 source and destination addresses in the
                IPv6 header does not match the ULID pair, this option
                MUST be included.  An example of this is when
                recovering from a lost context.

Nordmark & Bagnulo Standards Track [Page 27] RFC 5533 Shim6 Protocol June 2009

 Forked Instance Identifier:
                When another instance of an existent context with the
                same ULID pair is being created, a Forked Instance
                Identifier option MUST be included to distinguish this
                new instance from the existent one.
 Future protocol extensions might define additional options for this
 message.  The C-bit in the option format defines how such a new
 option will be handled by an implementation.  See Section 5.15.

5.5. R1 Message Format

 The R1 message is the second message in the context-establishment
 exchange.  The responder sends this in response to an I1 message,
 without creating any state specific to the initiator.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       59      |  Hdr Ext Len  |0|  Type = 2   |   Reserved1 |0|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |           Reserved2           |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Initiator Nonce                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Responder Nonce                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                         Options                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Next Header:   NO_NXT_HDR (59).
 Hdr Ext Len:   At least 1, since the header is 16 octets when there
                are no options.
 Type:          2
 Reserved1:     7-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 Reserved2:     16-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.

Nordmark & Bagnulo Standards Track [Page 28] RFC 5533 Shim6 Protocol June 2009

 Initiator Nonce:
                32-bit unsigned integer.  Copied from the I1 message.
 Responder Nonce:
                32-bit unsigned integer.  A number picked by the
                responder, which the initiator will return in the I2
                message.
 The following options are defined for this message:
 Responder Validator:
                Variable length option.  This option MUST be included
                in the R1 message.  Typically, it contains a hash
                generated by the responder, which the responder uses
                together with the Responder Nonce value to verify that
                an I2 message is indeed sent in response to an R1
                message, and that the parameters in the I2 message are
                the same as those in the I1 message.
 Future protocol extensions might define additional options for this
 message.  The C-bit in the option format defines how such a new
 option will be handled by an implementation.  See Section 5.15.

5.6. I2 Message Format

 The I2 message is the third message in the context-establishment
 exchange.  The initiator sends this in response to an R1 message,
 after checking the Initiator Nonce, etc.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       59      |  Hdr Ext Len  |0|  Type = 3   |   Reserved1 |0|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |R|                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
  |                  Initiator Context Tag                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Initiator Nonce                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Responder Nonce                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       Reserved2                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                         Options                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Nordmark & Bagnulo Standards Track [Page 29] RFC 5533 Shim6 Protocol June 2009

 Fields:
 Next Header:   NO_NXT_HDR (59).
 Hdr Ext Len:   At least 2, since the header is 24 octets when there
                are no options.
 Type:          3
 Reserved1:     7-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 R:             1-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 Initiator Context Tag:
                47-bit field.  The Context Tag that the initiator has
                allocated for the context.
 Initiator Nonce:
                32-bit unsigned integer.  A random number picked by
                the initiator, which the responder will return in the
                R2 message.
 Responder Nonce:
                32-bit unsigned integer.  Copied from the R1 message.
 Reserved2:     32-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.  (Needed to
                make the options start on a multiple of 8 octet
                boundary.)
 The following options are defined for this message:
 Responder Validator:
                Variable length option.  This option MUST be included
                in the I2 message and MUST be generated by copying the
                Responder Validator option received in the R1 message.
 ULID pair:     When the IPv6 source and destination addresses in the
                IPv6 header do not match the ULID pair, this option
                MUST be included.  An example of this is when
                recovering from a lost context.

Nordmark & Bagnulo Standards Track [Page 30] RFC 5533 Shim6 Protocol June 2009

 Forked Instance Identifier:
                When another instance of an existent context with the
                same ULID pair is being created, a Forked Instance
                Identifier option MUST be included to distinguish this
                new instance from the existent one.
 Locator List:  Optionally sent when the initiator immediately wants
                to tell the responder its list of locators.  When it
                is sent, the necessary HBA/CGA information for
                verifying the locator list MUST also be included.
 Locator Preferences:
                Optionally sent when the locators don't all have equal
                preference.
 CGA Parameter Data Structure:
                This option MUST be included in the I2 message when
                the locator list is included so the receiver can
                verify the locator list.
 CGA Signature: This option MUST be included in the I2 message when
                some of the locators in the list use CGA (and not HBA)
                for verification.
 Future protocol extensions might define additional options for this
 message.  The C-bit in the option format defines how such a new
 option will be handled by an implementation.  See Section 5.15.

5.7. R2 Message Format

 The R2 message is the fourth message in the context-establishment
 exchange.  The responder sends this in response to an I2 message.
 The R2 message is also used when both hosts send I1 messages at the
 same time and the I1 messages cross in flight.

Nordmark & Bagnulo Standards Track [Page 31] RFC 5533 Shim6 Protocol June 2009

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       59      |  Hdr Ext Len  |0|  Type = 4   |   Reserved1 |0|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |R|                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
  |                  Responder Context Tag                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Initiator Nonce                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                         Options                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Next Header:   NO_NXT_HDR (59).
 Hdr Ext Len:   At least 1, since the header is 16 octets when there
                are no options.
 Type:          4
 Reserved1:     7-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 R:             1-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 Responder Context Tag:
                47-bit field.  The Context Tag that the responder has
                allocated for the context.
 Initiator Nonce:
                32-bit unsigned integer.  Copied from the I2 message.
 The following options are defined for this message:
 Locator List:  Optionally sent when the responder immediately wants
                to tell the initiator its list of locators.  When it
                is sent, the necessary HBA/CGA information for
                verifying the locator list MUST also be included.
 Locator Preferences:
                Optionally sent when the locators don't all have equal
                preference.

Nordmark & Bagnulo Standards Track [Page 32] RFC 5533 Shim6 Protocol June 2009

 CGA Parameter Data Structure:
                Included when the locator list is included so the
                receiver can verify the locator list.
 CGA Signature: Included when some of the locators in the list use CGA
                (and not HBA) for verification.
 Future protocol extensions might define additional options for this
 message.  The C-bit in the option format defines how such a new
 option will be handled by an implementation.  See Section 5.15.

5.8. R1bis Message Format

 Should a host receive a packet with a Shim6 Payload Extension header
 or Shim6 control message with type code 64-127 (such as an Update or
 Probe message), and the host does not have any context state for the
 received Context Tag, then it will generate a R1bis message.
 This message allows the sender of the packet referring to the non-
 existent context to re-establish the context with a reduced context-
 establishment exchange.  Upon the reception of the R1bis message, the
 receiver can proceed with re-establishing the lost context by
 directly sending an I2bis message.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       59      |  Hdr Ext Len  |0|  Type = 5   |   Reserved1 |0|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |R|                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
  |                     Packet Context Tag                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Responder Nonce                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                         Options                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Next Header:   NO_NXT_HDR (59).
 Hdr Ext Len:   At least 1, since the header is 16 octets when there
                are no options.
 Type:          5

Nordmark & Bagnulo Standards Track [Page 33] RFC 5533 Shim6 Protocol June 2009

 Reserved1:     7-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 R:             1-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 Packet Context Tag:
                47-bit unsigned integer.  The Context Tag contained in
                the received packet that triggered the generation of
                the R1bis message.
 Responder Nonce:
                32-bit unsigned integer.  A number picked by the
                responder which the initiator will return in the I2bis
                message.
 The following options are defined for this message:
 Responder Validator:
                Variable length option.  Typically, a hash generated
                by the responder, which the responder uses together
                with the Responder Nonce value to verify that an I2bis
                message is indeed sent in response to an R1bis
                message.
 Future protocol extensions might define additional options for this
 message.  The C-bit in the option format defines how such a new
 option will be handled by an implementation.  See Section 5.15.

5.9. I2bis Message Format

 The I2bis message is the third message in the context-recovery
 exchange.  This is sent in response to an R1bis message, after
 checking that the R1bis message refers to an existing context, etc.

Nordmark & Bagnulo Standards Track [Page 34] RFC 5533 Shim6 Protocol June 2009

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       59      |  Hdr Ext Len  |0|  Type = 6  |   Reserved1 |0|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |R|                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
  |                  Initiator Context Tag                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Initiator Nonce                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Responder Nonce                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       Reserved2                               |
  |                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                 |                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
  |                     Packet Context Tag                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                         Options                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Next Header:   NO_NXT_HDR (59).
 Hdr Ext Len:   At least 3, since the header is 32 octets when there
                are no options.
 Type:          6
 Reserved1:     7-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 R:             1-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 Initiator Context Tag:
                47-bit field.  The Context Tag that the initiator has
                allocated for the context.
 Initiator Nonce:
                32-bit unsigned integer.  A random number picked by
                the initiator, which the responder will return in the
                R2 message.

Nordmark & Bagnulo Standards Track [Page 35] RFC 5533 Shim6 Protocol June 2009

 Responder Nonce:
                32-bit unsigned integer.  Copied from the R1bis
                message.
 Reserved2:     49-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.  (Note that 17
                bits are not sufficient since the options need to
                start on a multiple-of-8-octet boundary.)
 Packet Context Tag:
                47-bit unsigned integer.  Copied from the Packet
                Context Tag field contained in the received R1bis.
 The following options are defined for this message:
 Responder Validator:
                Variable length option.  Just a copy of the Responder
                Validator option in the R1bis message.
 ULID pair:     When the IPv6 source and destination addresses in the
                IPv6 header do not match the ULID pair, this option
                MUST be included.
 Forked Instance Identifier:
                When another instance of an existent context with the
                same ULID pair is being created, a Forked Instance
                Identifier option is included to distinguish this new
                instance from the existent one.
 Locator List:  Optionally sent when the initiator immediately wants
                to tell the responder its list of locators.  When it
                is sent, the necessary HBA/CGA information for
                verifying the locator list MUST also be included.
 Locator Preferences:
                Optionally sent when the locators don't all have equal
                preference.
 CGA Parameter Data Structure:
                Included when the locator list is included so the
                receiver can verify the locator list.
 CGA Signature: Included when some of the locators in the list use CGA
                (and not HBA) for verification.
 Future protocol extensions might define additional options for this
 message.  The C-bit in the option format defines how such a new
 option will be handled by an implementation.  See Section 5.15.

Nordmark & Bagnulo Standards Track [Page 36] RFC 5533 Shim6 Protocol June 2009

5.10. Update Request Message Format

 The Update Request message is used to update either the list of
 locators, the locator preferences, or both.  When the list of
 locators is updated, the message also contains the option(s)
 necessary for HBA/CGA to secure this.  The basic sanity check that
 prevents off-path attackers from generating bogus updates is the
 Context Tag in the message.
 The Update Request message contains options (the Locator List and the
 Locator Preferences) that, when included, completely replace the
 previous locator list and locator preferences, respectively.  Thus,
 there is no mechanism to just send deltas to the locator list.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       59      |  Hdr Ext Len  |0|  Type = 64  |   Reserved1 |0|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |R|                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
  |                   Receiver Context Tag                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                    Request Nonce                              |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                         Options                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Next Header:   NO_NXT_HDR (59).
 Hdr Ext Len:   At least 1, since the header is 16 octets when there
                are no options.
 Type:          64
 Reserved1:     7-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 R:             1-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 Receiver Context Tag:
                47-bit field.  The Context Tag that the receiver has
                allocated for the context.

Nordmark & Bagnulo Standards Track [Page 37] RFC 5533 Shim6 Protocol June 2009

 Request Nonce:
                32-bit unsigned integer.  A random number picked by
                the initiator, which the peer will return in the
                Update Acknowledgement message.
 The following options are defined for this message:
 Locator List:  The list of the sender's (new) locators.  The locators
                might be unchanged and only the preferences have
                changed.
 Locator Preferences:
                Optionally sent when the locators don't all have equal
                preference.
 CGA Parameter Data Structure (PDS):
                Included when the locator list is included and the PDS
                was not included in the I2/ I2bis/R2 messages, so the
                receiver can verify the locator list.
 CGA Signature: Included when some of the locators in the list use CGA
                (and not HBA) for verification.
 Future protocol extensions might define additional options for this
 message.  The C-bit in the option format defines how such a new
 option will be handled by an implementation.  See Section 5.15.

5.11. Update Acknowledgement Message Format

 This message is sent in response to an Update Request message.  It
 implies that the Update Request has been received and that any new
 locators in the Update Request can now be used as the source locators
 of packets.  But it does not imply that the (new) locators have been
 verified to be used as a destination, since the host might defer the
 verification of a locator until it sees a need to use a locator as
 the destination.

Nordmark & Bagnulo Standards Track [Page 38] RFC 5533 Shim6 Protocol June 2009

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       59      |  Hdr Ext Len  |0|  Type = 65  |   Reserved1 |0|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |R|                             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
  |                   Receiver Context Tag                        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                      Request Nonce                            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                         Options                               +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Next Header:   NO_NXT_HDR (59).
 Hdr Ext Len:   At least 1, since the header is 16 octets when there
                are no options.
 Type:          65
 Reserved1:     7-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 R:             1-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.
 Receiver Context Tag:
                47-bit field.  The Context Tag the receiver has
                allocated for the context.
 Request Nonce: 32-bit unsigned integer.  Copied from the Update
                Request message.
 No options are currently defined for this message.
 Future protocol extensions might define additional options for this
 message.  The C-bit in the option format defines how such a new
 option will be handled by an implementation.  See Section 5.15.

Nordmark & Bagnulo Standards Track [Page 39] RFC 5533 Shim6 Protocol June 2009

5.12. Keepalive Message Format

 This message format is defined in [4].
 The message is used to ensure that when a peer is sending ULP packets
 on a context, it always receives some packets in the reverse
 direction.  When the ULP is sending bidirectional traffic, no extra
 packets need to be inserted.  But for a unidirectional ULP traffic
 pattern, the shim will send back some Keepalive messages when it is
 receiving ULP packets.

5.13. Probe Message Format

 This message and its semantics are defined in [4].
 The goal of this mechanism is to test whether or not locator pairs
 work in the general case.  In particular, this mechanism is to be
 able to handle the case when one locator pair works from A to B and
 another locator pair works from B to A, but there is no locator pair
 that works in both directions.  The protocol mechanism is that, as A
 is sending Probe messages to B, B will observe which locator pairs it
 has received and report that back in Probe messages it sends to A.

5.14. Error Message Format

 The Error message is generated by a Shim6 receiver upon the reception
 of a Shim6 message containing critical information that cannot be
 processed properly.
 In the case that a Shim6 node receives a Shim6 packet that contains
 information that is critical for the Shim6 protocol and that is not
 supported by the receiver, it sends an Error Message back to the
 originator of the Shim6 message.  The Error message is
 unacknowledged.
 In addition, Shim6 Error messages defined in this section can be used
 to identify problems with Shim6 implementations.  In order to do so,
 a range of Error Code types is reserved for that purpose.  In
 particular, implementations may generate Shim6 Error messages with
 Code types in that range, instead of silently discarding Shim6
 packets during the debugging process.

Nordmark & Bagnulo Standards Track [Page 40] RFC 5533 Shim6 Protocol June 2009

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |       59      |  Hdr Ext Len  |0|  Type = 68  |  Error Code |0|
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |            Checksum           |            Pointer            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                         Packet in error                       +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Next Header:   NO_NXT_HDR (59).
 Hdr Ext Len:   At least 1, since the header is 16 octets.  Depends on
                the specific Error Data.
 Type:          68
 Error Code:    7-bit field describing the error that generated the
                Error message.  See Error Code list below.
 Pointer:       16-bit field.  Identifies the octet offset within the
                invoking packet where the error was detected.
 Packet in error:
                As much of invoking packet as possible without the
                Error message packet exceeding the minimum IPv6 MTU.
 The following Error Codes are defined:
 +---------+---------------------------------------------------------+
 |   Code  |                       Description                       |
 |  Value  |                                                         |
 +---------+---------------------------------------------------------+
 |    0    |                Unknown Shim6 message type               |
 |    1    |              Critical option not recognized             |
 |    2    |    Locator verification method failed (Pointer to the   |
 |         |         inconsistent verification method octet)         |
 |    3    |       Locator List Generation number out of sync.       |
 |    4    | Error in the number of locators in a Locator Preference |
 |         |                          option                         |
 | 120-127 |             Reserved for debugging purposes             |
 +---------+---------------------------------------------------------+
                                Table 2

Nordmark & Bagnulo Standards Track [Page 41] RFC 5533 Shim6 Protocol June 2009

5.15. Option Formats

 The format of the options is a snapshot of the current HIP option
 format [20].  However, there is no intention to track any changes to
 the HIP option format, nor is there an intent to use the same name
 space for the option type values.  But using the same format will
 hopefully make it easier to import HIP capabilities into Shim6 as
 extensions to Shim6, should this turn out to be useful.
 All of the TLV parameters have a length (including Type and Length
 fields) that is a multiple of 8 bytes.  When needed, padding MUST be
 added to the end of the parameter so that the total length becomes a
 multiple of 8 bytes.  This rule ensures proper alignment of data.  If
 padding is added, the Length field MUST NOT include the padding.  Any
 added padding bytes MUST be zeroed by the sender, and their values
 SHOULD NOT be checked by the receiver.
 Consequently, the Length field indicates the length of the Contents
 field (in bytes).  The total length of the TLV parameter (including
 Type, Length, Contents, and Padding) is related to the Length field
 according to the following formula:
 Total Length = 11 + Length - (Length + 3) mod 8;
 The total length of the option is the smallest multiple of 8 bytes
 that allows for the 4 bytes of the Option header and option, itself.
 The amount of padding required can be calculated as follows:
 padding = 7 - ((Length + 3) mod 8)
 And:
 Total Length = 4 + Length + padding
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |             Type            |C|             Length            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ~                                                               ~
  ~                          Contents                             ~
  ~                                               +-+-+-+-+-+-+-+-+
  ~                                               |    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Nordmark & Bagnulo Standards Track [Page 42] RFC 5533 Shim6 Protocol June 2009

 Fields:
 Type:          15-bit identifier of the type of option.  The options
                defined in this document are below.
 C:             Critical.  One, if this parameter is critical and MUST
                be recognized by the recipient; zero otherwise.  An
                implementation might view the C-bit as part of the
                Type field by multiplying the type values in this
                specification by two.
 Length:        Length of the Contents, in bytes.
 Contents:      Parameter-specific, defined by Type.
 Padding:       Padding, 0-7 bytes, added if needed.
                +------+------------------------------+
                | Type |          Option Name         |
                +------+------------------------------+
                |   1  |      Responder Validator     |
                |   2  |         Locator List         |
                |   3  |      Locator Preferences     |
                |   4  | CGA Parameter Data Structure |
                |   5  |         CGA Signature        |
                |   6  |           ULID Pair          |
                |   7  |  Forked Instance Identifier  |
                |  10  |   Keepalive Timeout Option   |
                +------+------------------------------+
                                Table 3
 Future protocol extensions might define additional options for the
 Shim6 messages.  The C-bit in the option format defines how such a
 new option will be handled by an implementation.
 If a host receives an option that it does not understand (an option
 that was defined in some future extension to this protocol) or that
 is not listed as a valid option for the different message types
 above, then the Critical bit in the option determines the outcome.
 o  If C=0, then the option is silently ignored, and the rest of the
    message is processed.
 o  If C=1, then the host SHOULD send back a Shim6 Error message with
    Error Code=1, with the Pointer field referencing the first octet
    in the Option Type field.  When C=1, the rest of the message MUST
    NOT be processed.

Nordmark & Bagnulo Standards Track [Page 43] RFC 5533 Shim6 Protocol June 2009

5.15.1. Responder Validator Option Format

 The responder can choose exactly what input is used to compute the
 validator and what one-way function (such as MD5 or SHA1) it uses, as
 long as the responder can check that the validator it receives back
 in the I2 or I2bis message is indeed one that:
 1) computed,
 2) computed for the particular context, and
 3) isn't a replayed I2/I2bis message.
 Some suggestions on how to generate the validators are captured in
 Sections 7.10.1 and 7.17.1.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Type = 1          |0|            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ~                           Validator                           ~
  ~                                               +-+-+-+-+-+-+-+-+
  ~                                               |    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Validator:     Variable length content whose interpretation is local
                to the responder.
 Padding:       Padding, 0-7 bytes, added if needed.  See
                Section 5.15.

5.15.2. Locator List Option Format

 The Locator List option is used to carry all the locators of the
 sender.  Note that the order of the locators is important, since the
 Locator Preferences option refers to the locators by using the index
 in the list.
 Note that we carry all the locators in this option even though some
 of them can be created automatically from the CGA Parameter Data
 Structure.

Nordmark & Bagnulo Standards Track [Page 44] RFC 5533 Shim6 Protocol June 2009

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Type = 2          |0|            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Locator List Generation                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Num Locators |            N Octets of Verification Method    |
  +-+-+-+-+-+-+-+-+                                               |
  ~                                                               ~
  ~                                               +-+-+-+-+-+-+-+-+
  ~                                               |    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ~                     Locators 1 through N                      ~
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Locator List Generation:
                32-bit unsigned integer.  Indicates a generation
                number that is increased by one for each new locator
                list.  This is used to ensure that the index in the
                Locator Preferences refers to the right version of the
                locator list.
 Num Locators:  8-bit unsigned integer.  The number of locators that
                are included in the option.  We call this number "N"
                below.
 Verification Method:
                N octets.  The ith octet specifies the verification
                method for the ith locator.
 Padding:       Padding, 0-7 bytes, added if needed so that the
                Locators start on a multiple-of-8-octet boundary.
                Note that for this option, there is never a need to
                pad at the end since the Locators are a multiple-of-8-
                octets in length.  This internal padding is included
                in the Length field.
 Locators:      N 128-bit locators.
 The defined verification methods are:

Nordmark & Bagnulo Standards Track [Page 45] RFC 5533 Shim6 Protocol June 2009

            +---------+----------------------------------+
            |  Value  |              Method              |
            +---------+----------------------------------+
            |    0    |             Reserved             |
            |    1    |                HBA               |
            |    2    |                CGA               |
            |  3-200  | Allocated using Standards action |
            | 201-254 |         Experimental use         |
            |   255   |             Reserved             |
            +---------+----------------------------------+
                                Table 4

5.15.3. Locator Preferences Option Format

 The Locator Preferences option can have some flags to indicate
 whether or not a locator is known to work.  In addition, the sender
 can include a notion of preferences.  It might make sense to define
 "preferences" as a combination of priority and weight, the same way
 that DNS SRV records have such information.  The priority would
 provide a way to rank the locators, and, within a given priority, the
 weight would provide a way to do some load sharing.  See [5] for how
 SRV defines the interaction of priority and weight.
 The minimum notion of preferences we need is to be able to indicate
 that a locator is "dead".  We can handle this using a single octet
 flag for each locator.
 We can extend that by carrying a larger "element" for each locator.
 This document presently also defines 2-octet and 3-octet elements,
 and we can add more information by having even larger elements if
 need be.
 The locators are not included in the preference list.  Instead, the
 first element refers to the locator that was in the first element in
 the Locator List option.  The generation number carried in this
 option and the Locator List option is used to verify that they refer
 to the same version of the locator list.

Nordmark & Bagnulo Standards Track [Page 46] RFC 5533 Shim6 Protocol June 2009

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Type = 3          |0|            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                     Locator List Generation                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |  Element Len  |  Element[1]   |  Element[2]   |  Element[3]   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ~                              ...                              ~
  ~                                               +-+-+-+-+-+-+-+-+
  ~                                               |    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Case of Element Len = 1 is depicted.
 Fields:
 Locator List Generation:
                32-bit unsigned integer.  Indicates a generation
                number for the locator list to which the elements
                should apply.
 Element Len:   8-bit unsigned integer.  The length in octets of each
                element.  This specification defines the cases when
                the length is 1, 2, or 3.
 Element[i]:    A field with a number of octets defined by the Element
                Len field.  Provides preferences for the ith locator
                in the Locator List option that is in use.
 Padding:       Padding, 0-7 bytes, added if needed.  See
                Section 5.15.
 When the Element length equals one, then the element consists of only
 a one-octet Flags field.  The currently defined set of flags are:
    BROKEN: 0x01
    TRANSIENT: 0x02
 The intent of the BROKEN flag is to inform the peer that a given
 locator is known to be not working.  The intent of TRANSIENT is to
 allow the distinction between more stable addresses and less stable
 addresses when Shim6 is combined with IP mobility, and when we might
 have more stable home locators and less stable care-of-locators.

Nordmark & Bagnulo Standards Track [Page 47] RFC 5533 Shim6 Protocol June 2009

 When the Element length equals two, then the element consists of a
 one-octet Flags field followed by a one-octet Priority field.  This
 Priority field has the same semantics as the Priority field in DNS
 SRV records.
 When the Element length equals three, then the element consists of a
 one-octet Flags field followed by a one-octet Priority field and a
 one-octet Weight field.  This Weight field has the same semantics as
 the Weight field in DNS SRV records.
 This document doesn't specify the format when the Element length is
 more than three, except that any such formats MUST be defined so that
 the first three octets are the same as in the above case, that is, a
 one-octet Flags field followed by a one-octet Priority field, and a
 one-octet Weight field.

5.15.4. CGA Parameter Data Structure Option Format

 This option contains the CGA Parameter Data Structure (PDS).  When
 HBA is used to verify the locators, the PDS contains the HBA
 multiprefix extension in addition to the PDS mandatory fields and
 other extensions unrelated to Shim6 that the PDS might have.  When
 CGA is used to verify the locators, in addition to the PDS option,
 the host also needs to include the signature in the form of a CGA
 Signature option.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Type = 4          |0|            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ~                   CGA Parameter Data Structure                ~
  ~                                               +-+-+-+-+-+-+-+-+
  ~                                               |    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 CGA Parameter Data Structure:
                Variable length content.  Content defined in [2] and
                [3].
 Padding:       Padding, 0-7 bytes, added if needed.  See
                Section 5.15.

Nordmark & Bagnulo Standards Track [Page 48] RFC 5533 Shim6 Protocol June 2009

5.15.5. CGA Signature Option Format

 When CGA is used for verification of one or more of the locators in
 the Locator List option, then the message in question will need to
 contain this option.
   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Type = 5          |0|            Length             |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  ~                        CGA Signature                          ~
  ~                                               +-+-+-+-+-+-+-+-+
  ~                                               |    Padding    |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 CGA Signature: A variable-length field containing a PKCS#1 v1.5
                signature, constructed by using the sender's private
                key over the following sequence of octets:
                1.  The 128-bit CGA Message Type tag [CGA] value for
                    Shim6: 0x4A 30 5662 4858 574B 3655 416F 506A 6D48.
                    (The tag value has been generated randomly by the
                    editor of this specification.).
                2.  The Locator List Generation number of the
                    correspondent Locator List option.
                3.  The subset of locators included in the
                    correspondent Locator List option whose
                    verification method is set to CGA.  The locators
                    MUST be included in the order in which they are
                    listed in the Locator List Option.
 Padding:       Padding, 0-7 bytes, added if needed.  See
                Section 5.15.

5.15.6. ULID Pair Option Format

 I1, I2, and I2bis messages MUST contain the ULID pair; normally, this
 is in the IPv6 Source and Destination fields.  In case the ULID for
 the context differs from the address pair included in the Source and
 Destination Address fields of the IPv6 packet used to carry the I1/
 I2/I2bis message, the ULID Pair option MUST be included in the I1/I2/
 I2bis message.

Nordmark & Bagnulo Standards Track [Page 49] RFC 5533 Shim6 Protocol June 2009

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Type = 6          |0|        Length = 36            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                       Reserved2                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                         Sender ULID                           +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                                                               |
  +                        Receiver ULID                          +
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Reserved2:     32-bit field.  Reserved for future use.  Zero on
                transmit.  MUST be ignored on receipt.  (Needed to
                make the ULIDs start on a multiple-of-8-octet
                boundary.)
 Sender ULID:   A 128-bit IPv6 address.
 Receiver ULID: A 128-bit IPv6 address.

5.15.7. Forked Instance Identifier Option Format

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |           Type = 7          |0|         Length = 4            |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  |                  Forked Instance Identifier                   |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Fields:
 Forked Instance Identifier:
                32-bit field containing the identifier of the
                particular forked instance.

5.15.8. Keepalive Timeout Option Format

 This option is defined in [4].

Nordmark & Bagnulo Standards Track [Page 50] RFC 5533 Shim6 Protocol June 2009

6. Conceptual Model of a Host

 This section describes a conceptual model of one possible data
 structure organization that hosts will maintain for the purposes of
 Shim6.  The described organization is provided to facilitate the
 explanation of how the Shim6 protocol should behave.  This document
 does not mandate that implementations adhere to this model as long as
 their external behavior is consistent with that described in this
 document.

6.1. Conceptual Data Structures

 The key conceptual data structure for the Shim6 protocol is the ULID-
 pair context.  This is a data structure that contains the following
 information:
 o  The state of the context.  See Section 6.2.
 o  The peer ULID: ULID(peer).
 o  The local ULID: ULID(local).
 o  The Forked Instance Identifier: FII.  This is zero for the default
    context, i.e., when there is no forking.
 o  The list of peer locators with their preferences: Ls(peer).
 o  The generation number for the most recently received, verified
    peer locator list.
 o  For each peer locator, the verification method to use (from the
    Locator List option).
 o  For each peer locator, a flag specifying whether it has been
    verified using HBA or CGA, and a bit specifying whether the
    locator has been probed to verify that the ULID is present at that
    location.
 o  The current peer locator is the locator used as the destination
    address when sending packets: Lp(peer).
 o  The set of local locators and the preferences: Ls(local).
 o  The generation number for the most recently sent Locator List
    option.
 o  The current local locator is the locator used as the source
    address when sending packets: Lp(local).

Nordmark & Bagnulo Standards Track [Page 51] RFC 5533 Shim6 Protocol June 2009

 o  The Context Tag used to transmit control messages and Shim6
    Payload Extension headers; this is allocated by the peer:
    CT(peer).
 o  The context to expect in received control messages and Shim6
    Payload Extension headers; this is allocated by the local host:
    CT(local).
 o  Timers for retransmission of the messages during context-
    establishment and update messages.
 o  Depending how an implementation determines whether a context is
    still in use, there might be a need to track the last time a
    packet was sent/received using the context.
 o  Reachability state for the locator pairs as specified in [4].
 o  During pair exploration, information about the Probe messages that
    have been sent and received as specified in [4].
 o  During context-establishment phase, the Initiator Nonce, Responder
    Nonce, Responder Validator, and timers related to the different
    packets sent (I1,I2, R2), as described in Section 7.

6.2. Context STATES

 The STATES that are used to describe the Shim6 protocol are as
 follows:

Nordmark & Bagnulo Standards Track [Page 52] RFC 5533 Shim6 Protocol June 2009

 +---------------------+---------------------------------------------+
 | STATE               | Explanation                                 |
 +---------------------+---------------------------------------------+
 | IDLE                | State machine start                         |
 |                     |                                             |
 | I1-SENT             | Initiating context-establishment exchange   |
 |                     |                                             |
 | I2-SENT             | Waiting to complete context-establishment   |
 |                     | exchange                                    |
 |                     |                                             |
 | I2BIS-SENT          | Potential context loss detected             |
 |                     |                                             |
 | ESTABLISHED         | SHIM context established                    |
 |                     |                                             |
 | E-FAILED            | Context-establishment exchange failed       |
 |                     |                                             |
 | NO-SUPPORT          | ICMP Unrecognized Next Header type          |
 |                     | (type 4, code 1) received, indicating       |
 |                     | that Shim6 is not supported                 |
 +---------------------+---------------------------------------------+
 In addition, in each of the aforementioned STATES, the following
 state information is stored:

Nordmark & Bagnulo Standards Track [Page 53] RFC 5533 Shim6 Protocol June 2009

 +---------------------+---------------------------------------------+
 | STATE               | Information                                 |
 +---------------------+---------------------------------------------+
 | IDLE                | None                                        |
 |                     |                                             |
 | I1-SENT             | ULID(peer), ULID(local), [FII], CT(local),  |
 |                     | INIT Nonce, Lp(local), Lp(peer), Ls(local)  |
 |                     |                                             |
 | I2-SENT             | ULID(peer), ULID(local), [FII], CT(local),  |
 |                     | INIT Nonce, RESP Nonce, Lp(local), Lp(peer),|
 |                     | Ls(local), Responder Validator              |
 |                     |                                             |
 | ESTABLISHED         | ULID(peer), ULID(local), [FII], CT(local),  |
 |                     | CT(peer), Lp(local), Lp(peer), Ls(local),   |
 |                     | Ls(peer), INIT Nonce?(to receive late R2)   |
 |                     |                                             |
 | I2BIS-SENT          | ULID(peer), ULID(local), [FII], CT(local),  |
 |                     | CT(peer), Lp(local), Lp(peer), Ls(local),   |
 |                     | Ls(peer), CT(R1bis), RESP Nonce,            |
 |                     | INIT Nonce, Responder Validator             |
 |                     |                                             |
 | E-FAILED            | ULID(peer), ULID(local)                     |
 |                     |                                             |
 | NO-SUPPORT          | ULID(peer), ULID(local)                     |
 +---------------------+---------------------------------------------+

7. Establishing ULID-Pair Contexts

 ULID-pair contexts are established using a 4-way exchange, which
 allows the responder to avoid creating state on the first packet.  As
 part of this exchange, each end allocates a Context Tag and shares
 this Context Tag and its set of locators with the peer.
 In some cases, the 4-way exchange is not necessary -- for instance,
 when both ends try to set up the context at the same time, or when
 recovering from a context that has been garbage collected or lost at
 one of the hosts.

7.1. Uniqueness of Context Tags

 As part of establishing a new context, each host has to assign a
 unique Context Tag.  Since the Shim6 Payload Extension headers are
 demultiplexed based solely on the Context Tag value (without using
 the locators), the Context Tag MUST be unique for each context.

Nordmark & Bagnulo Standards Track [Page 54] RFC 5533 Shim6 Protocol June 2009

 It is important that Context Tags are hard to guess for off-path
 attackers.  Therefore, if an implementation uses structure in the
 Context Tag to facilitate efficient lookups, at least 30 bits of the
 Context Tag MUST be unstructured and populated by random or pseudo-
 random bits.
 In addition, in order to minimize the reuse of Context Tags, the host
 SHOULD randomly cycle through the unstructured tag name space that is
 reserved for randomly assigned Context Tag values (e.g., following
 the guidelines described in [13]).

7.2. Locator Verification

 The peer's locators might need to be verified during context
 establishment as well as when handling locator updates in Section 10.
 There are two separate aspects of locator verification.  One is to
 verify that the locator is tied to the ULID, i.e., that the host that
 "owns" the ULID is also the one that is claiming the locator
 "ownership".  The Shim6 protocol uses the HBA or CGA techniques for
 doing this verification.  The other aspect is to verify that the host
 is indeed reachable at the claimed locator.  Such verification is
 needed not only to make sure communication can proceed but also to
 prevent 3rd party flooding attacks [15].  These different aspects of
 locator verification happen at different times since the first might
 need to be performed before packets can be received by the peer with
 the source locator in question, but the latter verification is only
 needed before packets are sent to the locator.
 Before a host can use a locator (different than the ULID) as the
 source locator, it must know that the peer will accept packets with
 that source locator as part of this context.  Thus, the HBA/CGA
 verification SHOULD be performed by the host before the host
 acknowledges the new locator by sending either an Update
 Acknowledgement message or an R2 message.
 Before a host can use a locator (different than the ULID) as the
 destination locator, it MUST perform the HBA/CGA verification if this
 was not performed upon reception of the locator set.  In addition, it
 MUST verify that the ULID is indeed present at that locator.  This
 verification is performed by doing a return-routability test as part
 of the Probe sub-protocol [4].
 If the verification method in the Locator List option is not
 supported by the host, or if the verification method is not
 consistent with the CGA Parameter Data Structure (e.g., the Parameter
 Data Structure doesn't contain the multiprefix extension and the
 verification method says to use HBA), then the host MUST ignore the

Nordmark & Bagnulo Standards Track [Page 55] RFC 5533 Shim6 Protocol June 2009

 Locator List and the message in which it is contained.  The host
 SHOULD generate a Shim6 Error message with Error Code=2 and with the
 Pointer referencing the octet in the verification method that was
 found inconsistent.

7.3. Normal Context Establishment

 The normal context establishment consists of a 4-message exchange in
 the order of I1, R1, I2, R2, as can be seen in Figure 3.
       Initiator                          Responder
        IDLE                               IDLE
             ------------- I1 -------------->
        I1-SENT
             <------------ R1 ---------------
                                           IDLE
             ------------- I2 -------------->
        I2-SENT
             <------------ R2 ---------------
        ESTABLISHED                        ESTABLISHED
                Figure 3: Normal Context Establishment

7.4. Concurrent Context Establishment

 When both ends try to initiate a context for the same ULID pair, then
 we might end up with crossing I1 messages.  Alternatively, since no
 state is created when receiving the I1, a host might send an I1 after
 having sent an R1 message.
 Since a host remembers that it has sent an I1, it can respond to an
 I1 from the peer (for the same ULID pair) with an R2, resulting in
 the message exchange shown in Figure 4.  Such behavior is needed for
 reasons such as correctly responding to retransmitted I1 messages,
 which occur when the R2 message has been lost.

Nordmark & Bagnulo Standards Track [Page 56] RFC 5533 Shim6 Protocol June 2009

       Host A                             Host B
        IDLE                               IDLE
             -\
        I1-SENT---\
                   ---\                  /---
                       --- I1 ---\   /---  I1-SENT
                                  ---\
                      /--- I1 ---/    ---\
                 /---                     -->
             <---
  1. \

I1-SENT—\

  1. –\ /—
    1. – R2 —\ /— I1-SENT
      1. –\

/— R2 —/ —\

                 /---                     -->
             <---                          ESTABLISHED
        ESTABLISHED
                    Figure 4: Crossing I1 Messages
 If a host has received an I1 and sent an R1, it has no state to
 remember this.  Thus, if the ULP on the host sends down packets, this
 might trigger the host to send an I1 message itself.  Thus, while one
 end is sending an I1, the other is sending an I2, as can be seen in
 Figure 5.

Nordmark & Bagnulo Standards Track [Page 57] RFC 5533 Shim6 Protocol June 2009

       Host A                             Host B
        IDLE                               IDLE
             -\
               ---\
        I1-SENT    ---\
                       --- I1 ---\
                                  ---\
                                      ---\
                                          -->
                                         /---
                                     /---  IDLE
                                  ---
                      /--- R1--/
                 /---
             <---
  1. \

I2-SENT—\

  1. –\ /—
    1. – I2—\ /— I1-SENT
      1. –\

/— I1 —/ —\

                 /---                     -->
             <---                          ESTABLISHED
  1. \

I2-SENT—\

  1. –\ /—
    1. – R2 —\ /—
      1. –\

/— R2 —/ —\

                 /---                     -->
             <---                          ESTABLISHED
        ESTABLISHED
                     Figure 5: Crossing I2 and I1

7.5. Context Recovery

 Due to garbage collection, we can end up with one end having and
 using the context state, and the other end not having any state.  We
 need to be able to recover this state at the end that has lost it
 before we can use it.

Nordmark & Bagnulo Standards Track [Page 58] RFC 5533 Shim6 Protocol June 2009

 This need can arise in the following cases:
 o  The communication is working using the ULID pair as the locator
    pair but a problem arises, and the end that has retained the
    context state decides to probe alternate locator pairs.
 o  The communication is working using a locator pair that is not the
    ULID pair; hence, the ULP packets sent from a peer that has
    retained the context state use the Shim6 Payload Extension header.
 o  The host that retained the state sends a control message (e.g., an
    Update Request message).
 In all cases, the result is that the peer without state receives a
 shim message for which it has no context for the Context Tag.
 We can recover the context by having the node that doesn't have a
 context state send back an R1bis message, and then complete the
 recovery with an I2bis and R2 message, as can be seen in Figure 6.
         Host A                             Host B
       Context for
       CT(peer)=X                         Discards context for
                                          CT(local)=X
        ESTABLISHED                        IDLE
  1. — payload, probe, etc. —–> No context state

for CT(local)=X

             <------------ R1bis ------------
                                           IDLE
  1. ———— I2bis ———–>

I2BIS_SENT

             <------------ R2 ---------------
        ESTABLISHED                        ESTABLISHED
                  Figure 6: Context Loss at Receiver
 If one end has garbage collected or lost the context state, it might
 try to create a new context state (for the same ULID pair), by
 sending an I1 message.  In this case, the peer (that still has the
 context state) will reply with an R1 message, and the full 4-way
 exchange will be performed again, as can be seen in Figure 7.

Nordmark & Bagnulo Standards Track [Page 59] RFC 5533 Shim6 Protocol June 2009

         Host A                             Host B
       Context for
       CT(peer)=X                         Discards context for
       ULIDs A1, B1                       CT(local)=X
        ESTABLISHED                        IDLE
      Finds  <------------ I1 --------------- Tries to set up
      existing                                for ULIDs A1, B1
      context,
      but CT(peer)                         I1-SENT
      doesn't match
             ------------- R1 --------------->
      Left old context
      in ESTABLISHED
             <------------ I2 ---------------
      Re-create context
      with new CT(peer)                    I2-SENT
      and Ls(peer).
        ESTABLISHED
             ------------- R2 -------------->
        ESTABLISHED                        ESTABLISHED
                   Figure 7: Context Loss at Sender

7.6. Context Confusion

 Since each end might garbage collect the context state, we can have
 the case where one end has retained the context state and tries to
 use it, while the other end has lost the state.  We discussed this in
 the previous section on recovery.  But, for the same reasons, when
 one host retains Context Tag X as CT(peer) for ULID pair <A1, B1>,
 the other end might end up allocating that Context Tag as CT(local)
 for another ULID pair (e.g., <A3, B1>) between the same hosts.  In
 this case, we cannot use the recovery mechanisms since there needs to
 be separate Context Tags for the two ULID pairs.
 This type of "confusion" can be observed in two cases (assuming it is
 A that has retained the state and B that has dropped it):
 o  B decides to create a context for ULID pair <A3, B1>, allocates X
    as its Context Tag for this, and sends an I1 to A.

Nordmark & Bagnulo Standards Track [Page 60] RFC 5533 Shim6 Protocol June 2009

 o  A decides to create a context for ULID pair <A3, B1> and starts
    the exchange by sending I1 to B.  When B receives the I2 message,
    it allocates X as the Context Tag for this context.
 In both cases, A can detect that B has allocated X for ULID pair <A3,
 B1> even though A still has X as CT(peer) for ULID pair <A1, B1>.
 Thus, A can detect that B must have lost the context for <A1, B1>.
 The confusion can be detected when I2/I2bis/R2 is received, since we
 require that those messages MUST include a sufficiently large set of
 locators in a Locator List option that the peer can determine whether
 or not two contexts have the same host as the peer by comparing if
 there is any common locators in Ls(peer).
 The old context that used the Context Tag MUST be removed; it can no
 longer be used to send packets.  Thus, A would forcibly remove the
 context state for <A1, B1, X> so that it can accept the new context
 for <A3, B1, X>.  An implementation MAY re-create a context to
 replace the one that was removed -- in this case, for <A1, B1>.  The
 normal I1, R1, I2, R2 establishment exchange would then pick unique
 Context Tags for that replacement context.  This re-creation is
 OPTIONAL, but might be useful when there is ULP communication that is
 using the ULID pair whose context was removed.
 Note that an I1 message with a duplicate Context Tag should not cause
 the removal of the old context state; this operation needs to be
 deferred until the reception of the I2 message.

7.7. Sending I1 Messages

 When the shim layer decides to set up a context for a ULID pair, it
 starts by allocating and initializing the context state for its end.
 As part of this, it assigns a random Context Tag to the context that
 is not being used as CT(local) by any other context .  In the case
 that a new API is used and the ULP requests a forked context, the
 Forked Instance Identifier value will be set to a non-zero value.
 Otherwise, the FII value is zero.  Then the initiator can send an I1
 message and set the context STATE to I1-SENT.  The I1 message MUST
 include the ULID pair -- normally, in the IPv6 Source and Destination
 fields.  But if the ULID pair for the context is not used as a
 locator pair for the I1 message, then a ULID option MUST be included
 in the I1 message.  In addition, if a Forked Instance Identifier
 value is non-zero, the I1 message MUST include a Context Instance
 Identifier option containing the correspondent value.

Nordmark & Bagnulo Standards Track [Page 61] RFC 5533 Shim6 Protocol June 2009

7.8. Retransmitting I1 Messages

 If the host does not receive an R1 or R2 message in response to the
 I1 message after I1_TIMEOUT time, then it needs to retransmit the I1
 message.  The retransmissions should use a retransmission timer with
 binary exponential backoff to avoid creating congestion issues for
 the network when lots of hosts perform I1 retransmissions.  Also, the
 actual timeout value should be randomized between 0.5 and 1.5 of the
 nominal value to avoid self-synchronization.
 If, after I1_RETRIES_MAX retransmissions, there is no response, then
 most likely the peer does not implement the Shim6 protocol (or there
 could be a firewall that blocks the protocol).  In this case, it
 makes sense for the host to remember not to try again to establish a
 context with that ULID.  However, any such negative caching should be
 retained for at most NO_R1_HOLDDOWN_TIME, in order to be able to
 later set up a context should the problem have been that the host was
 not reachable at all when the shim tried to establish the context.
 If the host receives an ICMP error with "Unrecognized Next Header"
 type (type 4, code 1) and the included packet is the I1 message it
 just sent, then this is a more reliable indication that the peer ULID
 does not implement Shim6.  Again, in this case, the host should
 remember not to try again to establish a context with that ULID.
 Such negative caching should be retained for at most
 ICMP_HOLDDOWN_TIME, which should be significantly longer than the
 previous case.

7.9. Receiving I1 Messages

 A host MUST silently discard any received I1 messages that do not
 satisfy all of the following validity checks in addition to those
 specified in Section 12.3:
 o  The Hdr Ext Len field is at least 1, i.e., the length is at least
    16 octets.
 Upon the reception of an I1 message, the host extracts the ULID pair
 and the Forked Instance Identifier from the message.  If there is no
 ULID-pair option, then the ULID pair is taken from the Source and
 Destination fields in the IPv6 header.  If there is no FII option in
 the message, then the FII value is taken to be zero.
 Next, the host looks for an existing context that matches the ULID
 pair and the FII.
 If no state is found (i.e., the STATE is IDLE), then the host replies
 with an R1 message as specified below.

Nordmark & Bagnulo Standards Track [Page 62] RFC 5533 Shim6 Protocol June 2009

 If such a context exists in ESTABLISHED STATE, the host verifies that
 the locator of the initiator is included in Ls(peer).  (This check is
 unnecessary if there is no ULID-pair option in the I1 message.)
 If the state exists in ESTABLISHED STATE and the locators do not fall
 in the locator sets, then the host replies with an R1 message as
 specified below.  This completes the I1 processing, with the context
 STATE being unchanged.
 If the state exists in ESTABLISHED STATE and the locators do fall in
 the sets, then the host compares CT(peer) for the context with the CT
 contained in the I1 message.
 o  If the Context Tags match, then this probably means that the R2
    message was lost and this I1 is a retransmission.  In this case,
    the host replies with an R2 message containing the information
    available for the existent context.
 o  If the Context Tags do not match, then it probably means that the
    initiator has lost the context information for this context and is
    trying to establish a new one for the same ULID pair.  In this
    case, the host replies with an R1 message as specified below.
    This completes the I1 processing, with the context STATE being
    unchanged.
 If the state exists in other STATE (I1-SENT, I2-SENT, I2BIS-SENT), we
 are in the situation of concurrent context establishment, described
 in Section 7.4.  In this case, the host leaves CT(peer) unchanged and
 replies with an R2 message.  This completes the I1 processing, with
 the context STATE being unchanged.

7.10. Sending R1 Messages

 When the host needs to send an R1 message in response to the I1
 message, it copies the Initiator Nonce from the I1 message to the R1
 message, generates a Responder Nonce, and calculates a Responder
 Validator option as suggested in the following section.  No state is
 created on the host in this case.  (Note that the information used to
 generate the R1 reply message is either contained in the received I1
 message or is global information that is not associated with the
 particular requested context (the S and the Responder Nonce values.))
 When the host needs to send an R2 message in response to the I1
 message, it copies the Initiator Nonce from the I1 message to the R2
 message, and otherwise follows the normal rules for forming an R2
 message (see Section 7.14).

Nordmark & Bagnulo Standards Track [Page 63] RFC 5533 Shim6 Protocol June 2009

7.10.1. Generating the R1 Validator

 As it is stated in Section 5.15.1, the validator-generation mechanism
 is a local choice since the validator is generated and verified by
 the same node, i.e., the responder.  However, in order to provide the
 required protection, the validator needs to be generated by
 fulfilling the conditions described in Section 5.15.1.  One way for
 the responder to properly generate validators is to maintain a single
 secret (S) and a running counter (C) for the Responder Nonce that is
 incremented in fixed periods of time (this allows the responder to
 verify the age of a Responder Nonce, independently of the context in
 which it is used).
 When the validator is generated to be included in an R1 message sent
 in response to a specific I1 message, the responder can perform the
 following procedure to generate the validator value:
 First, the responder uses the current counter C value as the
 Responder Nonce.
 Second, it uses the following information (concatenated) as input to
 the one-way function:
 o  The secret S
 o  That Responder Nonce
 o  The Initiator Context Tag from the I1 message
 o  The ULIDs from the I1 message
 o  The locators from the I1 message (strictly only needed if they are
    different from the ULIDs)
 o  The Forked Instance Identifier, if such option was included in the
    I1 message
 Third, it uses the output of the hash function as the validator value
 included in the R1 message.

7.11. Receiving R1 Messages and Sending I2 Messages

 A host MUST silently discard any received R1 messages that do not
 satisfy all of the following validity checks in addition to those
 specified in Section 12.3:
 o  The Hdr Ext Len field is at least 1, i.e., the length is at least
    16 octets.

Nordmark & Bagnulo Standards Track [Page 64] RFC 5533 Shim6 Protocol June 2009

 Upon the reception of an R1 message, the host extracts the Initiator
 Nonce and the Locator Pair from the message (the latter from the
 Source and Destination fields in the IPv6 header).  Next, the host
 looks for an existing context that matches the Initiator Nonce and
 where the locators are contained in Ls(peer) and Ls(local),
 respectively.  If no such context is found, then the R1 message is
 silently discarded.
 If such a context is found, then the host looks at the STATE:
 o  If the STATE is I1-SENT, then it sends an I2 message as specified
    below.
 o  In any other STATE (I2-SENT, I2BIS-SENT, ESTABLISHED), then the
    host has already sent an I2 message and this is probably a reply
    to a retransmitted I1 message, so this R1 message MUST be silently
    discarded.
 When the host sends an I2 message, it includes the Responder
 Validator option that was in the R1 message.  The I2 message MUST
 include the ULID pair -- normally, in the IPv6 Source and Destination
 fields.  If a ULID-pair option was included in the I1 message, then
 it MUST be included in the I2 message as well.  In addition, if the
 Forked Instance Identifier value for this context is non-zero, the I2
 message MUST contain a Forked Instance Identifier option carrying the
 Forked Instance Identifier value.  Besides, the I2 message contains
 an Initiator Nonce.  This is not required to be the same as the one
 included in the previous I1 message.
 The I2 message may also include the initiator's locator list.  If
 this is the case, then it must also include the CGA Parameter Data
 Structure.  If CGA (and not HBA) is used to verify one or more of the
 locators included in the locator list, then the initiator must also
 include a CGA Signature option containing the signature.
 When the I2 message has been sent, the STATE is set to I2-SENT.

7.12. Retransmitting I2 Messages

 If the initiator does not receive an R2 message after I2_TIMEOUT time
 after sending an I2 message, it MAY retransmit the I2 message, using
 binary exponential backoff and randomized timers.  The Responder
 Validator option might have a limited lifetime -- that is, the peer
 might reject Responder Validator options that are older than
 VALIDATOR_MIN_LIFETIME to avoid replay attacks.  In the case that the
 initiator decides not to retransmit I2 messages, or in the case that

Nordmark & Bagnulo Standards Track [Page 65] RFC 5533 Shim6 Protocol June 2009

 the initiator still does not receive an R2 message after
 retransmitting I2 messages I2_RETRIES_MAX times, the initiator SHOULD
 fall back to retransmitting the I1 message.

7.13. Receiving I2 Messages

 A host MUST silently discard any received I2 messages that do not
 satisfy all of the following validity checks in addition to those
 specified in Section 12.3:
 o  The Hdr Ext Len field is at least 2, i.e., the length is at least
    24 octets.
 Upon the reception of an I2 message, the host extracts the ULID pair
 and the Forked Instance Identifier from the message.  If there is no
 ULID-pair option, then the ULID pair is taken from the Source and
 Destination fields in the IPv6 header.  If there is no FII option in
 the message, then the FII value is taken to be zero.
 Next, the host verifies that the Responder Nonce is a recent one
 (nonces that are no older than VALIDATOR_MIN_LIFETIME SHOULD be
 considered recent) and that the Responder Validator option matches
 the validator the host would have computed for the ULID, locators,
 Responder Nonce, Initiator Nonce, and FII.
 If a CGA Parameter Data Structure (PDS) is included in the message,
 then the host MUST verify if the actual PDS contained in the message
 corresponds to the ULID(peer).
 If any of the above verifications fail, then the host silently
 discards the message; it has completed the I2 processing.
 If all the above verifications are successful, then the host proceeds
 to look for a context state for the initiator.  The host looks for a
 context with the extracted ULID pair and FII.  If none exist, then
 STATE of the (non-existing) context is viewed as being IDLE; thus,
 the actions depend on the STATE as follows:
 o  If the STATE is IDLE (i.e., the context does not exist), the host
    allocates a Context Tag (CT(local)), creates the context state for
    the context, and sets its STATE to ESTABLISHED.  It records
    CT(peer) and the peer's locator set as well as its own locator set
    in the context.  It SHOULD perform the HBA/CGA verification of the
    peer's locator set at this point in time, as specified in
    Section 7.2.  Then, the host sends an R2 message back as specified
    below.

Nordmark & Bagnulo Standards Track [Page 66] RFC 5533 Shim6 Protocol June 2009

 o  If the STATE is I1-SENT, then the host verifies if the source
    locator is included in Ls(peer) or in the Locator List contained
    in the I2 message and that the HBA/CGA verification for this
    specific locator is successful.
  • If this is not the case, then the message is silently discarded

and the context STATE remains unchanged.

  • If this is the case, then the host updates the context

information (CT(peer), Ls(peer)) with the data contained in the

       I2 message, and the host MUST send an R2 message back as
       specified below.  Note that before updating Ls(peer)
       information, the host SHOULD perform the HBA/CGA validation of
       the peer's locator set at this point in time, as specified in
       Section 7.2.  The host moves to ESTABLISHED STATE.
 o  If the STATE is ESTABLISHED, I2-SENT, or I2BIS-SENT, then the host
    verifies if the source locator is included in Ls(peer) or in the
    Locator List contained in the I2 message and that the HBA/CGA
    verification for this specific locator is successful.
  • If this is not the case, then the message is silently discarded

and the context STATE remains unchanged.

  • If this is the case, then the host updates the context

information (CT(peer), Ls(peer)) with the data contained in the

       I2 message, and the host MUST send an R2 message back as
       specified in Section 7.14.  Note that before updating Ls(peer)
       information, the host SHOULD perform the HBA/CGA validation of
       the peer's locator set at this point in time, as specified in
       Section 7.2.  The context STATE remains unchanged.

7.14. Sending R2 Messages

 Before the host sends the R2 message, it MUST look for a possible
 context confusion, i.e., where it would end up with multiple contexts
 using the same CT(peer) for the same peer host.  See Section 7.15.
 When the host needs to send an R2 message, the host forms the message
 and its Context Tag, and copies the Initiator Nonce from the
 triggering message (I2, I2bis, or I1).  In addition, it may include
 alternative locators and necessary options so that the peer can
 verify them.  In particular, the R2 message may include the
 responder's locator list and the PDS option.  If CGA (and not HBA) is
 used to verify the locator list, then the responder also signs the
 key parts of the message and includes a CGA Signature option
 containing the signature.

Nordmark & Bagnulo Standards Track [Page 67] RFC 5533 Shim6 Protocol June 2009

 R2 messages are never retransmitted.  If the R2 message is lost, then
 the initiator will retransmit either the I2/I2bis or I1 message.
 Either retransmission will cause the responder to find the context
 state and respond with an R2 message.

7.15. Match for Context Confusion

 When the host receives an I2, I2bis, or R2, it MUST look for a
 possible context confusion, i.e., where it would end up with multiple
 contexts using the same CT(peer) for the same peer host.  This can
 happen when the host has received the above messages, since they
 create a new context with a new CT(peer).  The same issue applies
 when CT(peer) is updated for an existing context.
 The host takes CT(peer) for the newly created or updated context, and
 looks for other contexts which:
 o  Are in STATE ESTABLISHED or I2BIS-SENT
 o  Have the same CT(peer)
 o  Have an Ls(peer) that has at least one locator in common with the
    newly created or updated context
 If such a context is found, then the host checks if the ULID pair or
 the Forked Instance Identifier are different than the ones in the
 newly created or updated context:
 o  If either or both are different, then the peer is reusing the
    Context Tag for the creation of a context with different ULID pair
    or FII, which is an indication that the peer has lost the original
    context.  In this case, we are in a context confusion situation,
    and the host MUST NOT use the old context to send any packets.  It
    MAY just discard the old context (after all, the peer has
    discarded it), or it MAY attempt to re-establish the old context
    by sending a new I1 message and moving its STATE to I1-SENT.  In
    any case, once that this situation is detected, the host MUST NOT
    keep two contexts with overlapping Ls(peer) locator sets and the
    same Context Tag in ESTABLISHED STATE, since this would result in
    demultiplexing problems on the peer.
 o  If both are the same, then this context is actually the context
    that is created or updated; hence, there is no confusion.

Nordmark & Bagnulo Standards Track [Page 68] RFC 5533 Shim6 Protocol June 2009

7.16. Receiving R2 Messages

 A host MUST silently discard any received R2 messages that do not
 satisfy all of the following validity checks in addition to those
 specified in Section 12.3:
 o  The Hdr Ext Len field is at least 1, i.e., the length is at least
    16 octets.
 Upon the reception of an R2 message, the host extracts the Initiator
 Nonce and the Locator Pair from the message (the latter from the
 Source and Destination fields in the IPv6 header).  Next, the host
 looks for an existing context that matches the Initiator Nonce and
 where the locators are Lp(peer) and Lp(local), respectively.  Based
 on the STATE:
 o  If no such context is found, i.e., the STATE is IDLE, then the
    message is silently dropped.
 o  If STATE is I1-SENT, I2-SENT, or I2BIS-SENT, then the host
    performs the following actions.  If a CGA Parameter Data Structure
    (PDS) is included in the message, then the host MUST verify that
    the actual PDS contained in the message corresponds to the
    ULID(peer) as specified in Section 7.2.  If the verification
    fails, then the message is silently dropped.  If the verification
    succeeds, then the host records the information from the R2
    message in the context state; it records the peer's locator set
    and CT(peer).  The host SHOULD perform the HBA/CGA verification of
    the peer's locator set at this point in time, as specified in
    Section 7.2.  The host sets its STATE to ESTABLISHED.
 o  If the STATE is ESTABLISHED, the R2 message is silently ignored,
    (since this is likely to be a reply to a retransmitted I2
    message).
 Before the host completes the R2 processing, it MUST look for a
 possible context confusion, i.e., where it would end up with multiple
 contexts using the same CT(peer) for the same peer host.  See
 Section 7.15.

7.17. Sending R1bis Messages

 Upon the receipt of a Shim6 Payload Extension header where there is
 no current Shim6 context at the receiver, the receiver is to respond
 with an R1bis message in order to enable a fast re-establishment of
 the lost Shim6 context.

Nordmark & Bagnulo Standards Track [Page 69] RFC 5533 Shim6 Protocol June 2009

 Also, a host is to respond with an R1bis upon receipt of any control
 messages that have a message type in the range 64-127 (i.e.,
 excluding the context-setup messages such as I1, R1, R1bis, I2,
 I2bis, R2, and future extensions), where the control message refers
 to a non-existent context.
 We assume that all the incoming packets that trigger the generation
 of an R1bis message contain a locator pair (in the address fields of
 the IPv6 header) and a Context Tag.
 Upon reception of any of the packets described above, the host will
 reply with an R1bis including the following information:
 o  The Responder Nonce is a number picked by the responder that the
    initiator will return in the I2bis message.
 o  Packet Context Tag is the Context Tag contained in the received
    packet that triggered the generation of the R1bis message.
 o  The Responder Validator option is included, with a validator that
    is computed as suggested in the next section.

7.17.1. Generating the R1bis Validator

 One way for the responder to properly generate validators is to
 maintain a single secret (S) and a running counter C for the
 Responder Nonce that is incremented in fixed periods of time (this
 allows the responder to verify the age of a Responder Nonce,
 independently of the context in which it is used).
 When the validator is generated to be included in an R1bis message --
 that is, sent in response to a specific control packet or a packet
 containing the Shim6 Payload Extension header message -- the
 responder can perform the following procedure to generate the
 validator value:
 First, the responder uses the counter C value as the Responder Nonce.
 Second, it uses the following information (concatenated) as input to
 the one-way function:
 o  The secret S
 o  That Responder Nonce
 o  The Receiver Context Tag included in the received packet
 o  The locators from the received packet

Nordmark & Bagnulo Standards Track [Page 70] RFC 5533 Shim6 Protocol June 2009

 Third, it uses the output of the hash function as the validator
 string.

7.18. Receiving R1bis Messages and Sending I2bis Messages

 A host MUST silently discard any received R1bis messages that do not
 satisfy all of the following validity checks in addition to those
 specified in Section 12.3:
 o  The Hdr Ext Len field is at least 1, i.e., the length is at least
    16 octets.
 Upon the reception of an R1bis message, the host extracts the Packet
 Context Tag and the Locator Pair from the message (the latter from
 the Source and Destination fields in the IPv6 header).  Next, the
 host looks for an existing context where the Packet Context Tag
 matches CT(peer) and where the locators match Lp(peer) and Lp(local),
 respectively.
 o  If no such context is found, i.e., the STATE is IDLE, then the
    R1bis message is silently discarded.
 o  If the STATE is I1-SENT, I2-SENT, or I2BIS-SENT, then the R1bis
    message is silently discarded.
 o  If the STATE is ESTABLISHED, then we are in the case where the
    peer has lost the context, and the goal is to try to re-establish
    it.  For that, the host leaves CT(peer) unchanged in the context
    state, transitions to I2BIS-SENT STATE, and sends an I2bis
    message, including the computed Responder Validator option, the
    Packet Context Tag, and the Responder Nonce that were received in
    the R1bis message.  This I2bis message is sent using the locator
    pair included in the R1bis message.  In the case that this locator
    pair differs from the ULID pair defined for this context, then a
    ULID option MUST be included in the I2bis message.  In addition,
    if the Forked Instance Identifier for this context is non-zero,
    then a Forked Instance Identifier option carrying the instance
    identifier value for this context MUST be included in the I2bis
    message.  The I2bis message may also include a locator list.  If
    this is the case, then it must also include the CGA Parameter Data
    Structure.  If CGA (and not HBA) is used to verify one or more of
    the locators included in the locator list, then the initiator must
    also include a CGA Signature option containing the signature.

Nordmark & Bagnulo Standards Track [Page 71] RFC 5533 Shim6 Protocol June 2009

7.19. Retransmitting I2bis Messages

 If the initiator does not receive an R2 message after I2bis_TIMEOUT
 time after sending an I2bis message, it MAY retransmit the I2bis
 message, using binary exponential backoff and randomized timers.  The
 Responder Validator option might have a limited lifetime -- that is,
 the peer might reject Responder Validator options that are older than
 VALIDATOR_MIN_LIFETIME to avoid replay attacks.  In the case that the
 initiator decides not to retransmit I2bis messages, or in the case
 that the initiator still does not receive an R2 message after
 retransmitting I2bis messages I2bis_RETRIES_MAX times, the initiator
 SHOULD fall back to retransmitting the I1 message.

7.20. Receiving I2bis Messages and Sending R2 Messages

 A host MUST silently discard any received I2bis messages that do not
 satisfy all of the following validity checks in addition to those
 specified in Section 12.3:
 o  The Hdr Ext Len field is at least 3, i.e., the length is at least
    32 octets.
 Upon the reception of an I2bis message, the host extracts the ULID
 pair and the Forked Instance Identifier from the message.  If there
 is no ULID-pair option, then the ULID pair is taken from the Source
 and Destination fields in the IPv6 header.  If there is no FII option
 in the message, then the FII value is taken to be zero.
 Next, the host verifies that the Responder Nonce is a recent one
 (nonces that are no older than VALIDATOR_MIN_LIFETIME SHOULD be
 considered recent) and that the Responder Validator option matches
 the validator the host would have computed for the locators,
 Responder Nonce, and Receiver Context Tag as part of sending an R1bis
 message.
 If a CGA Parameter Data Structure (PDS) is included in the message,
 then the host MUST verify if the actual PDS contained in the message
 corresponds to the ULID(peer).
 If any of the above verifications fail, then the host silently
 discards the message; it has completed the I2bis processing.
 If both verifications are successful, then the host proceeds to look
 for a context state for the initiator.  The host looks for a context
 with the extracted ULID pair and FII.  If none exist, then STATE of
 the (non-existing) context is viewed as being IDLE; thus, the actions
 depend on the STATE as follows:

Nordmark & Bagnulo Standards Track [Page 72] RFC 5533 Shim6 Protocol June 2009

 o  If the STATE is IDLE (i.e., the context does not exist), the host
    allocates a Context Tag (CT(local)), creates the context state for
    the context, and sets its STATE to ESTABLISHED.  The host SHOULD
    NOT use the Packet Context Tag in the I2bis message for CT(local);
    instead, it should pick a new random Context Tag just as when it
    processes an I2 message.  It records CT(peer) and the peer's
    locator set as well as its own locator set in the context.  It
    SHOULD perform the HBA/CGA verification of the peer's locator set
    at this point in time, as specified in Section 7.2.  Then the host
    sends an R2 message back as specified in Section 7.14.
 o  If the STATE is I1-SENT, then the host verifies if the source
    locator is included in Ls(peer) or in the Locator List contained
    in the I2bis message and if the HBA/CGA verification for this
    specific locator is successful.
  • If this is not the case, then the message is silently

discarded. The context STATE remains unchanged.

  • If this is the case, then the host updates the context

information (CT(peer), Ls(peer)) with the data contained in the

       I2bis message, and the host MUST send an R2 message back as
       specified below.  Note that before updating Ls(peer)
       information, the host SHOULD perform the HBA/CGA validation of
       the peer's locator set at this point in time, as specified in
       Section 7.2.  The host moves to ESTABLISHED STATE.
 o  If the STATE is ESTABLISHED, I2-SENT, or I2BIS-SENT, then the host
    determines whether at least one of the two following conditions
    hold: i) if the source locator is included in Ls(peer) or, ii) if
    the source locator is included in the Locator List contained in
    the I2bis message and if the HBA/CGA verification for this
    specific locator is successful.
  • If none of the two aforementioned conditions hold, then the

message is silently discarded. The context STATE remains

       unchanged.
  • If at least one of the two aforementioned conditions hold, then

the host updates the context information (CT(peer), Ls(peer))

       with the data contained in the I2bis message, and the host MUST
       send an R2 message back, as specified in Section 7.14.  Note
       that before updating Ls(peer) information, the host SHOULD
       perform the HBA/CGA validation of the peer's locator set at
       this point in time, as specified in Section 7.2.  The context
       STATE remains unchanged.

Nordmark & Bagnulo Standards Track [Page 73] RFC 5533 Shim6 Protocol June 2009

8. Handling ICMP Error Messages

 The routers in the path as well as the destination might generate
 ICMP error messages.  In some cases, the Shim6 can take action and
 solve the problem that resulted in the error.  In other cases, the
 Shim6 layer cannot solve the problem, and it is critical that these
 packets make it back up to the ULPs so that they can take appropriate
 action.
 This is an implementation issue in the sense that the mechanism is
 completely local to the host itself.  But the issue of how ICMP
 errors are correctly dispatched to the ULP on the host are important;
 hence, this section specifies the issue.
 All ICMP messages MUST be delivered to the ULP in all cases, except
 when Shim6 successfully acts on the message (e.g., selects a new
 path).  There SHOULD be a configuration option to unconditionally
 deliver all ICMP messages (including ones acted on by shim6) to the
 ULP.
 According to that recommendation, the following ICMP error messages
 should be processed by the Shim6 layer and not passed to the ULP:
    ICMP error Destination Unreachable, with codes:
       0 (No route to destination)
       1 (Communication with destination administratively prohibited)
       2 (Beyond scope of source address)
       3 (Address unreachable)
       5 (Source address failed ingress/egress policy)
       6 (Reject route to destination)
    ICMP Time exceeded error.
    ICMP Parameter problem error, with the parameter that caused the
    error being a Shim6 parameter.
 The following ICMP error messages report problems that cannot be
 addressed by the Shim6 layer and that should be passed to the ULP (as
 described below):
    ICMP Packet too big error.
    ICMP Destination Unreachable with Code 4 (Port unreachable).
    ICMP Parameter problem (if the parameter that caused the problem
    is not a Shim6 parameter).

Nordmark & Bagnulo Standards Track [Page 74] RFC 5533 Shim6 Protocol June 2009

              +--------------+
              | IPv6 Header  |
              |              |
              +--------------+
              |    ICMPv6    |
              |    Header    |
       - -    +--------------+   - -
              | IPv6 Header  |
              | src, dst as  |   Can be dispatched
      IPv6    | sent by ULP  |   unmodified to ULP
              | on host      |   ICMP error handler
      Packet  +--------------+
              |     ULP      |
      in      |    Header    |
              +--------------+
      Error   |              |
              ~     Data     ~
              |              |
       - -    +--------------+   - -
              Figure 8: ICMP Error Handling without the
                    Shim6 Payload Extension Header
 When the ULP packets are sent without the Shim6 Payload Extension
 header -- that is, while the initial locators=ULIDs are working --
 this introduces no new concerns; an implementation's existing
 mechanism for delivering these errors to the ULP will work.  See
 Figure 8.
 But when the shim on the transmitting side inserts the Shim6 Payload
 Extension header and replaces the ULIDs in the IP address fields with
 some other locators, then an ICMP error coming back will have a
 "packet in error", which is not a packet that the ULP sent.  Thus,
 the implementation will have to apply reverse mapping to the "packet
 in error" before passing the ICMP error up to the ULP, including the
 ICMP extensions defined in [25].  See Figure 9.

Nordmark & Bagnulo Standards Track [Page 75] RFC 5533 Shim6 Protocol June 2009

              +--------------+
              | IPv6 Header  |
              |              |
              +--------------+
              |    ICMPv6    |
              |    Header    |
       - -    +--------------+   - -
              | IPv6 Header  |
              | src, dst as  |   Needs to be
      IPv6    | modified by  |   transformed to
              | shim on host |   have ULIDs
              +--------------+   in src, dst fields,
      Packet  |  Shim6 ext.  |   and Shim6 Ext.
              |    Header    |   header removed
       in     +--------------+   before it can be
              |  Transport   |   dispatched to the ULP
      Error   |    Header    |   ICMP error handler.
              +--------------+
              |              |
              ~     Data     ~
              |              |
       - -    +--------------+   - -
 Figure 9: ICMP Error Handling with the Shim6 Payload Extension Header
 Note that this mapping is different than when receiving packets from
 the peer with Shim6 Payload Extension headers because, in that case,
 the packets contain CT(local).  But the ICMP errors have a "packet in
 error" with a Shim6 Payload Extension header containing CT(peer).
 This is because they were intended to be received by the peer.  In
 any case, since the <Source Locator, Destination Locator, CT(peer)>
 has to be unique when received by the peer, the local host should
 also only be able to find one context that matches this tuple.
 If the ICMP error is a "packet too big", the reported MTU must be
 adjusted to be 8 octets less, since the shim will add 8 octets when
 sending packets.
 After the "packet in error" has had the original ULIDs inserted, then
 this Shim6 Payload Extension header can be removed.  The result is a
 "packet in error" that is passed to the ULP which looks as if the
 shim did not exist.

9. Teardown of the ULID-Pair Context

 Each host can unilaterally decide when to tear down a ULID-pair
 context.  It is RECOMMENDED that hosts do not tear down the context
 when they know that there is some upper-layer protocol that might use

Nordmark & Bagnulo Standards Track [Page 76] RFC 5533 Shim6 Protocol June 2009

 the context.  For example, an implementation might know this if there
 is an open socket that is connected to the ULID(peer).  However,
 there might be cases when the knowledge is not readily available to
 the shim layer, for instance, for UDP applications that do not
 connect their sockets or for any application that retains some
 higher-level state across (TCP) connections and UDP packets.
 Thus, it is RECOMMENDED that implementations minimize premature
 teardown by observing the amount of traffic that is sent and received
 using the context, and tear down the state only after it appears
 quiescent.  A reasonable approach would be to not tear down a context
 until at least 5 minutes have passed since the last message was sent
 or received using the context.  (Note that packets that use the ULID
 pair as a locator pair and that do not require address rewriting by
 the Shim6 layer are also considered as packets using the associated
 Shim6 context.)
 Since there is no explicit, coordinated removal of the context state,
 there are potential issues around Context Tag reuse.  One end might
 remove the state and potentially reuse that Context Tag for some
 other communication, and the peer might later try to use the old
 context (which it didn't remove).  The protocol has mechanisms to
 recover from this, which work whether the state removal was total and
 accidental (e.g., crash and reboot of the host) or just a garbage
 collection of shim state that didn't seem to be used.  However, the
 host should try to minimize the reuse of Context Tags by trying to
 randomly cycle through the 2^47 Context Tag values.  (See Appendix C
 for a summary of how the recovery works in the different cases.)

10. Updating the Peer

 The Update Request and Acknowledgement are used both to update the
 list of locators (only possible when CGA is used to verify the
 locator(s)) and to update the preferences associated with each
 locator.

10.1. Sending Update Request Messages

 When a host has a change in the locator set, it can communicate this
 to the peer by sending an Update Request.  When a host has a change
 in the preferences for its locator set, it can also communicate this
 to the peer.  The Update Request message can include just a Locator
 List option (to convey the new set of locators), just a Locator
 Preferences option, or both a new Locator List and new Locator
 Preferences.

Nordmark & Bagnulo Standards Track [Page 77] RFC 5533 Shim6 Protocol June 2009

 Should the host send a new Locator List, the host picks a new random,
 local generation number, records this in the context, and puts it in
 the Locator List option.  Any Locator Preference option, whether sent
 in the same Update Request or in some future Update Request, will use
 that generation number to make sure the preferences get applied to
 the correct version of the locator list.
 The host picks a random Request Nonce for each update and keeps the
 same nonce for any retransmissions of the Update Request.  The nonce
 is used to match the acknowledgement with the request.
 The Update Request message can also include a CGA Parameter Data
 Structure (this is needed if the CGA PDS was not previously
 exchanged).  If CGA (and not HBA) is used to verify one or more of
 the locators included in the locator list, then a CGA Signature
 option containing the signature must also be included in the Update
 Request message.

10.2. Retransmitting Update Request Messages

 If the host does not receive an Update Acknowledgement R2 message in
 response to the Update Request message after UPDATE_TIMEOUT time,
 then it needs to retransmit the Update Request message.  The
 retransmissions should use a retransmission timer with binary
 exponential backoff to avoid creating congestion issues for the
 network when lots of hosts perform Update Request retransmissions.
 Also, the actual timeout value should be randomized between 0.5 and
 1.5 of the nominal value to avoid self-synchronization.
 Should there be no response, the retransmissions continue forever.
 The binary exponential backoff stops at MAX_UPDATE_TIMEOUT.  But the
 only way the retransmissions would stop when there is no
 acknowledgement is when Shim6, through the REAP protocol or some
 other mechanism, decides to discard the context state due to lack of
 ULP usage in combination with no responses to the REAP protocol.

10.3. Newer Information while Retransmitting

 There can be at most one outstanding Update Request message at any
 time.  Thus until, for example, an update with a new Locator List has
 been acknowledged, any newer Locator List or new Locator Preferences
 cannot just be sent.  However, when there is newer information and
 the older information has not yet been acknowledged, the host can,
 instead of waiting for an acknowledgement, abandon the previous
 update and construct a new Update Request (with a new Request Nonce)
 that includes the new information as well as the information that
 hasn't yet been acknowledged.

Nordmark & Bagnulo Standards Track [Page 78] RFC 5533 Shim6 Protocol June 2009

 For example, if the original locator list was just (A1, A2), and if
 an Update Request with the Locator List (A1, A3) is outstanding, and
 the host determines that it should both add A4 to the locator list
 and mark A1 as BROKEN, then it would need to:
 o  Pick a new random Request Nonce for the new Update Request.
 o  Pick a new random generation number for the new locator list.
 o  Form the new locator list: (A1, A3, A4).
 o  Form a Locator Preference option that uses the new generation
    number and has the BROKEN flag for the first locator.
 o  Send the Update Request and start a retransmission timer.
 Any Update Acknowledgement that doesn't match the current Request
 Nonce (for instance, an acknowledgement for the abandoned Update
 Request) will be silently ignored.

10.4. Receiving Update Request Messages

 A host MUST silently discard any received Update Request messages
 that do not satisfy all of the following validity checks in addition
 to those specified in Section 12.3:
 o  The Hdr Ext Len field is at least 1, i.e., the length is at least
    16 octets.
 Upon the reception of an Update Request message, the host extracts
 the Context Tag from the message.  It then looks for a context that
 has a CT(local) that matches the Context Tag.  If no such context is
 found, it sends an R1bis message as specified in Section 7.17.
 Since Context Tags can be reused, the host MUST verify that the IPv6
 Source Address field is part of Ls(peer) and that the IPv6
 Destination Address field is part of Ls(local).  If this is not the
 case, the sender of the Update Request has a stale context that
 happens to match the CT(local) for this context.  In this case, the
 host MUST send an R1bis message and otherwise ignore the Update
 Request message.
 If a CGA Parameter Data Structure (PDS) is included in the message,
 then the host MUST verify if the actual PDS contained in the packet
 corresponds to the ULID(peer).  If this verification fails, the
 message is silently discarded.

Nordmark & Bagnulo Standards Track [Page 79] RFC 5533 Shim6 Protocol June 2009

 Then, depending on the STATE of the context:
 o  If ESTABLISHED, proceed to process message.
 o  If I1-SENT, discard the message and stay in I1-SENT.
 o  If I2-SENT, send I2 and proceed to process the message.
 o  If I2BIS-SENT, send I2bis and proceed to process the message.
 The verification issues for the locators carried in the Update
 Request message are specified in Section 7.2.  If the locator list
 cannot be verified, this procedure should send a Shim6 Error message
 with Error Code=2.  In any case, if it cannot be verified, there is
 no further processing of the Update Request.
 Once any Locator List option in the Update Request has been verified,
 the peer generation number in the context is updated to be the one in
 the Locator List option.
 If the Update Request message contains a Locator Preference option,
 then the generation number in the preference option is compared with
 the peer generation number in the context.  If they do not match,
 then the host generates a Shim6 Error message with Error Code=3 and
 with the Pointer field referring to the first octet in the Locator
 List Generation number in the Locator Preference option.  In
 addition, if the number of elements in the Locator Preference option
 does not match the number of locators in Ls(peer), then a Shim6 Error
 message with Error Code=4 is sent with the Pointer field referring to
 the first octet of the Length field in the Locator Preference option.
 In both cases of failure, no further processing is performed for the
 Update Request message.
 If the generation numbers match, the locator preferences are recorded
 in the context.
 Once the Locator List option (if present) has been verified and any
 new locator list or locator preferences have been recorded, the host
 sends an Update Acknowledgement message, copying the nonce from the
 request and using the CT(peer) as the Receiver Context Tag.
 Any new locators (or, more likely, new locator preferences) might
 result in the host wanting to select a different locator pair for the
 context -- for instance, if the Locator Preferences option lists the
 current Lp(peer) as BROKEN.  The host uses the reachability
 exploration procedure described in [4] to verify that the new locator
 is reachable before changing Lp(peer).

Nordmark & Bagnulo Standards Track [Page 80] RFC 5533 Shim6 Protocol June 2009

10.5. Receiving Update Acknowledgement Messages

 A host MUST silently discard any received Update Acknowledgement
 messages that do not satisfy all of the following validity checks in
 addition to those specified in Section 12.3:
 o  The Hdr Ext Len field is at least 1, i.e., the length is at least
    16 octets.
 Upon the reception of an Update Acknowledgement message, the host
 extracts the Context Tag and the Request Nonce from the message.  It
 then looks for a context that has a CT(local) that matches the
 Context Tag.  If no such context is found, it sends an R1bis message
 as specified in Section 7.17.
 Since Context Tags can be reused, the host MUST verify that the IPv6
 Source Address field is part of Ls(peer) and that the IPv6
 Destination Address field is part of Ls(local).  If this is not the
 case, the sender of the Update Acknowledgement has a stale context
 that happens to match the CT(local) for this context.  In this case,
 the host MUST send an R1bis message and otherwise ignore the Update
 Acknowledgement message.
 Then, depending on the STATE of the context:
 o  If ESTABLISHED, proceed to process message.
 o  If I1-SENT, discard the message and stay in I1-SENT.
 o  If I2-SENT, send R2 and proceed to process the message.
 o  If I2BIS-SENT, send R2 and proceed to process the message.
 If the Request Nonce doesn't match the nonce for the last sent Update
 Request for the context, then the Update Acknowledgement is silently
 ignored.  If the nonce matches, then the update has been completed
 and the Update retransmit timer can be reset.

11. Sending ULP Payloads

 When there is no context state for the ULID pair on the sender, there
 is no effect on how ULP packets are sent.  If the host is using some
 heuristic for determining when to perform a deferred context
 establishment, then the host might need to do some accounting (count
 the number of packets sent and received) even before there is a ULID-
 pair context.

Nordmark & Bagnulo Standards Track [Page 81] RFC 5533 Shim6 Protocol June 2009

 If the context is not in ESTABLISHED or I2BIS-SENT STATE, then there
 is also no effect on how the ULP packets are sent.  Only in the
 ESTABLISHED and I2BIS-SENT STATEs does the host have CT(peer) and
 Ls(peer) set.
 If there is a ULID-pair context for the ULID pair, then the sender
 needs to verify whether the context uses the ULIDs as locators --
 that is, whether Lp(peer) == ULID(peer) and Lp(local) == ULID(local).
 If this is the case, then packets can be sent unmodified by the shim.
 If it is not the case, then the logic in Section 11.1 will need to be
 used.
 There will also be some maintenance activity relating to
 (un)reachability detection, whether or not packets are sent with the
 original locators.  The details of this are out of scope for this
 document and are specified in [4].

11.1. Sending ULP Payload after a Switch

 When sending packets, if there is a ULID-pair context for the ULID
 pair, and if the ULID pair is no longer used as the locator pair,
 then the sender needs to transform the packet.  Apart from replacing
 the IPv6 Source and Destination fields with a locator pair, an
 8-octet header is added so that the receiver can find the context and
 inverse the transformation.
 If there has been a failure causing a switch, and later the context
 switches back to sending things using the ULID pair as the locator
 pair, then there is no longer a need to do any packet transformation
 by the sender; hence, there is no need to include the 8-octet
 Extension header.
 First, the IP address fields are replaced.  The IPv6 Source Address
 field is set to Lp(local) and the Destination Address field is set to
 Lp(peer).  Note that this MUST NOT cause any recalculation of the ULP
 checksums, since the ULP checksums are carried end-to-end and the ULP
 pseudo-header contains the ULIDs that are preserved end-to-end.
 The sender skips any "Routing Sublayer Extension headers" that the
 ULP might have included; thus, it skips any Hop-by-Hop Extension
 header, any Routing header, and any Destination Options header that
 is followed by a Routing header.  After any such headers, the Shim6
 Extension header will be added.  This might be before a Fragment
 header, a Destination Options header, an ESP or AH header, or a ULP
 header.

Nordmark & Bagnulo Standards Track [Page 82] RFC 5533 Shim6 Protocol June 2009

 The inserted Shim6 Payload Extension header includes the peer's
 Context Tag.  It takes on the Next Header value from the preceding
 Extension header, since that Extension header will have a Next Header
 value of Shim6.

12. Receiving Packets

 The receive side of the communication can receive packets associated
 to a Shim6 context, with or without the Shim6 Extension header.  In
 case the ULID pair is being used as a locator pair, the packets
 received will not have the Shim6 Extension header and will be
 processed by the Shim6 layer as described below.  If the received
 packet does carry the Shim6 Extension header, as in normal IPv6
 receive-side packet processing, the receiver parses the (extension)
 headers in order.  Should it find a Shim6 Extension header, it will
 look at the "P" field in that header.  If this bit is zero, then the
 packet must be passed to the Shim6 payload handling for rewriting.
 Otherwise, the packet is passed to the Shim6 control handling.

12.1. Receiving Payload without Extension Headers

 The receiver extracts the IPv6 Source and Destination fields and uses
 this to find a ULID-pair context, such that the IPv6 address fields
 match the ULID(local) and ULID(peer).  If such a context is found,
 the context appears not to be quiescent; this should be remembered in
 order to avoid tearing down the context and for reachability
 detection purposes as described in [4].  The host continues with the
 normal processing of the IP packet.

12.2. Receiving Shim6 Payload Extension Headers

 The receiver extracts the Context Tag from the Shim6 Payload
 Extension header and uses this to find a ULID-pair context.  If no
 context is found, the receiver SHOULD generate an R1bis message (see
 Section 7.17).
 Then, depending on the STATE of the context:
 o  If ESTABLISHED, proceed to process message.
 o  If I1-SENT, discard the message and stay in I1-SENT.
 o  If I2-SENT, send I2 and proceed to process the message.
 o  If I2BIS-SENT, send I2bis and proceed to process the message.

Nordmark & Bagnulo Standards Track [Page 83] RFC 5533 Shim6 Protocol June 2009

 With the context in hand, the receiver can now replace the IP address
 fields with the ULIDs kept in the context.  Finally, the Shim6
 Payload Extension header is removed from the packet (so that the ULP
 doesn't get confused by it), and the Next Header value in the
 preceding header is set to be the actual protocol number for the
 payload.  Then the packet can be passed to the protocol identified by
 the Next Header value (which might be some function associated with
 the IP endpoint sublayer or a ULP).
 If the host is using some heuristic for determining when to perform a
 deferred context establishment, then the host might need to do some
 accounting (count the number of packets sent and received) for
 packets that do not have a Shim6 Extension header and for which there
 is no context.  But the need for this depends on what heuristics the
 implementation has chosen.

12.3. Receiving Shim Control Messages

 A shim control message has the Checksum field verified.  The Shim
 Header Length field is also verified against the length of the IPv6
 packet to make sure that the shim message doesn't claim to end past
 the end of the IPv6 packet.  Finally, it checks that neither the IPv6
 Destination field nor the IPv6 Source field is a multicast address or
 an unspecified address.  If any of those checks fail, the packet is
 silently dropped.
 The message is then dispatched based on the shim message type.  Each
 message type is then processed as described elsewhere in this
 document.  If the packet contains a shim message type that is unknown
 to the receiver, then a Shim6 Error message with Error Code=0 is
 generated and sent back.  The Pointer field is set to point at the
 first octet of the shim message type.
 All the control messages can contain any options with C=0.  If there
 is any option in the message with C=1 that isn't known to the host,
 then the host MUST send a Shim6 Error message with Error Code=1 with
 the Pointer field referencing the first octet of the Option Type.

12.4. Context Lookup

 We assume that each shim context has its own STATE machine.  We
 assume that a dispatcher delivers incoming packets to the STATE
 machine that it belongs to.  Here, we describe the rules used for the
 dispatcher to deliver packets to the correct shim context STATE
 machine.

Nordmark & Bagnulo Standards Track [Page 84] RFC 5533 Shim6 Protocol June 2009

 There is one STATE machine per identified context that is
 conceptually identified by the ULID pair and Forked Instance
 Identifier (which is zero by default) or identified by CT(local).
 However, the detailed lookup rules are more complex, especially
 during context establishment.
 Clearly, if the required context is not established, it will be in
 IDLE STATE.
 During context establishment, the context is identified as follows:
 o  I1 packets: Deliver to the context associated with the ULID pair
    and the Forked Instance Identifier.
 o  I2 packets: Deliver to the context associated with the ULID pair
    and the Forked Instance Identifier.
 o  R1 packets: Deliver to the context with the locator pair included
    in the packet and the Initiator Nonce included in the packet (R1
    does not contain a ULID pair or the CT(local)).  If no context
    exists with this locator pair and Initiator Nonce, then silently
    discard.
 o  R2 packets: Deliver to the context with the locator pair included
    in the packet and the Initiator Nonce included in the packet (R2
    does not contain a ULID pair or the CT(local)).  If no context
    exists with this locator pair and Initiator Nonce, then silently
    discard.
 o  R1bis packets: Deliver to the context that has the locator pair
    and the CT(peer) equal to the Packet Context Tag included in the
    R1bis packet.
 o  I2bis packets: Deliver to the context associated with the ULID
    pair and the Forked Instance Identifier.
 o  Shim6 Payload Extension headers: Deliver to the context with
    CT(local) equal to the Receiver Context Tag included in the
    packet.
 o  Other control messages (Update, Keepalive, Probe): Deliver to the
    context with CT(local) equal to the Receiver Context Tag included
    in the packet.  Verify that the IPv6 Source Address field is part
    of Ls(peer) and that the IPv6 Destination Address field is part of
    Ls(local).  If not, send an R1bis message.

Nordmark & Bagnulo Standards Track [Page 85] RFC 5533 Shim6 Protocol June 2009

 o  Shim6 Error messages and ICMP errors that contain a Shim6 Payload
    Extension header or other shim control packet in the "packet in
    error": Use the "packet in error" for dispatching as follows.
    Deliver to the context with CT(peer) equal to the Receiver Context
    Tag -- Lp(local) being the IPv6 source address and Lp(peer) being
    the IPv6 destination address.
 In addition, the shim on the sending side needs to be able to find
 the context state when a ULP packet is passed down from the ULP.  In
 that case, the lookup key is the pair of ULIDs and FII=0.  If we have
 a ULP API that allows the ULP to do context forking, then presumably
 the ULP would pass down the Forked Instance Identifier.

13. Initial Contact

 The initial contact is some non-shim communication between two ULIDs,
 as described in Section 2.  At that point in time, there is no
 activity in the shim.
 Whether or not the shim ends up being used (e.g., the peer might not
 support Shim6), it is highly desirable that the initial contact can
 be established even if there is a failure for one or more IP
 addresses.
 The approach taken is to rely on the applications and the transport
 protocols to retry with different source and destination addresses,
 consistent with what is already specified in "Default Address
 Selection for IPv6" [7] as well as with some fixes to that
 specification [9], to make it try different source addresses and not
 only different destination addresses.
 The implementation of such an approach can potentially result in long
 timeouts.  For instance, consider a naive implementation at the
 socket API that uses getaddrinfo() to retrieve all destination
 addresses and then tries to bind() and connect() to try all source
 and destination address combinations and waits for TCP to time out
 for each combination before trying the next one.
 However, if implementations encapsulate this in some new connect-by-
 name() API and use non-blocking connect calls, it is possible to
 cycle through the available combinations in a more rapid manner until
 a working source and destination pair is found.  Thus, the issues in
 this domain are issues of implementations and the current socket API,
 and not issues of protocol specification.  In all honesty, while
 providing an easy to use connect-by-name() API for TCP and other
 connection-oriented transports is easy, providing a similar

Nordmark & Bagnulo Standards Track [Page 86] RFC 5533 Shim6 Protocol June 2009

 capability at the API for UDP is hard due to the protocol itself not
 providing any "success" feedback.  Yet, even the UDP issue is one of
 APIs and implementation.

14. Protocol Constants

 The protocol uses the following constants:
 I1_RETRIES_MAX = 4
 I1_TIMEOUT = 4 seconds
 NO_R1_HOLDDOWN_TIME = 1 min
 ICMP_HOLDDOWN_TIME = 10 min
 I2_TIMEOUT = 4 seconds
 I2_RETRIES_MAX = 2
 I2bis_TIMEOUT = 4 seconds
 I2bis_RETRIES_MAX = 2
 VALIDATOR_MIN_LIFETIME = 30 seconds
 UPDATE_TIMEOUT = 4 seconds
 MAX_UPDATE_TIMEOUT = 120 seconds
 The retransmit timers (I1_TIMEOUT, I2_TIMEOUT, UPDATE_TIMEOUT) are
 subject to binary exponential backoff as well as to randomization
 across a range of 0.5 and 1.5 times the nominal (backed off) value.
 This removes any risk of synchronization between lots of hosts
 performing independent shim operations at the same time.
 The randomization is applied after the binary exponential backoff.
 Thus, the first retransmission would happen based on a uniformly
 distributed random number in the range of [0.5*4, 1.5*4] seconds; the
 second retransmission, [0.5*8, 1.5*8] seconds after the first one,
 etc.

Nordmark & Bagnulo Standards Track [Page 87] RFC 5533 Shim6 Protocol June 2009

15. Implications Elsewhere

15.1. Congestion Control Considerations

 When the locator pair currently used for exchanging packets in a
 Shim6 context becomes unreachable, the Shim6 layer will divert the
 communication through an alternative locator pair, which in most
 cases will result in redirecting the packet flow through an
 alternative network path.  In this case, it is recommended that the
 Shim6 follows the recommendation defined in [21] and informs the
 upper layers about the path change, in order to allow the congestion
 control mechanisms of the upper layers to react accordingly.

15.2. Middle-Boxes Considerations

 Data packets belonging to a Shim6 context carrying the Shim6 Payload
 header contain alternative locators other than the ULIDs in the
 Source and Destination Address fields of the IPv6 header.  On the
 other hand, the upper layers of the peers involved in the
 communication operate on the ULID pair presented to them by the Shim6
 layer, rather than on the locator pair contained in the IPv6 header
 of the actual packets.  It should be noted that the Shim6 layer does
 not modify the data packets but, because a constant ULID pair is
 presented to upper layers irrespective of the locator pair changes,
 the relation between the upper-layer header (such as TCP, UDP, ICMP,
 ESP, etc) and the IPv6 header is modified.  In particular, when the
 Shim6 Extension header is present in the packet, if those data
 packets are TCP, UDP, or ICMP packets, the pseudo-header used for the
 checksum calculation will contain the ULID pair, rather than the
 locator pair contained in the data packet.
 It is possible that some firewalls or other middle-boxes will try to
 verify the validity of upper-layer sanity checks of the packet on the
 fly.  If they do that based on the actual source and destination
 addresses contained in the IPv6 header without considering the Shim6
 context information (in particular, without replacing the locator
 pair by the ULID pair used by the Shim6 context), such verifications
 may fail.  Those middle-boxes need to be updated in order to be able
 to parse the Shim6 Payload header and find the next header.  It is
 recommended that firewalls and other middle-boxes do not drop packets
 that carry the Shim6 Payload header with apparently incorrect upper-
 layer validity checks that involve the addresses in the IPv6 header
 for their computation, unless they are able to determine the ULID
 pair of the Shim6 context associated to the data packet and use the
 ULID pair for the verification of the validity check.

Nordmark & Bagnulo Standards Track [Page 88] RFC 5533 Shim6 Protocol June 2009

 In the particular case of TCP, UDP, and ICMP checksums, it is
 recommended that firewalls and other middle-boxes do not drop TCP,
 UDP, and ICMP packets that carry the Shim6 Payload header with
 apparently incorrect checksums when using the addresses in the IPv6
 header for the pseudo-header computation, unless they are able to
 determine the ULID pair of the Shim6 context associated to the data
 packet and use the ULID pair to determine the checksum that must be
 present in a packet with addresses rewritten by Shim6.
 In addition, firewalls that today pass limited traffic, e.g.,
 outbound TCP connections, would presumably block the Shim6 protocol.
 This means that even when Shim6-capable hosts are communicating, the
 I1 messages would be dropped; hence, the hosts would not discover
 that their peer is Shim6-capable.  This is, in fact, a benefit since,
 if the hosts managed to establish a ULID-pair context, the firewall
 would probably drop the "different" packets that are sent after a
 failure (those using the Shim6 Payload Extension header with a TCP
 packet inside it).  Thus, stateful firewalls that are modified to
 pass Shim6 messages should also be modified to pass the Shim6 Payload
 Extension header so that the shim can use the alternate locators to
 recover from failures.  This presumably implies that the firewall
 needs to track the set of locators in use by looking at the Shim6
 control exchanges.  Such firewalls might even want to verify the
 locators using the HBA/CGA verification themselves, which they can do
 without modifying any of the Shim6 packets through which they pass.

15.3. Operation and Management Considerations

 This section considers some aspects related to the operations and
 management of the Shim6 protocol.
 Deployment of the Shim6 protocol: The Shim6 protocol is a host-based
 solution.  So, in order to be deployed, the stacks of the hosts using
 the Shim6 protocol need to be updated to support it.  This enables an
 incremental deployment of the protocol since it does not require a
 flag day for the deployment -- just single host updates.  If the
 Shim6 solution will be deployed in a site, the host can be gradually
 updated to support the solution.  Moreover, for supporting the Shim6
 protocol, only end hosts need to be updated and no router changes are
 required.  However, it should be noted that, in order to benefit from
 the Shim6 protocol, both ends of a communication should support the
 protocol, meaning that both hosts must be updated to be able to use
 the Shim6 protocol.  Nevertheless, the Shim6 protocol uses a deferred
 context-setup capability that allows end hosts to establish normal
 IPv6 communications and, later on, if both endpoints are Shim6-
 capable, establish the Shim6 context using the Shim6 protocol.  This

Nordmark & Bagnulo Standards Track [Page 89] RFC 5533 Shim6 Protocol June 2009

 has an important deployment benefit, since Shim6-enabled nodes can
 talk perfectly to non-Shim6-capable nodes without introducing any
 problem into the communication.
 Configuration of Shim6-capable nodes: The Shim6 protocol itself does
 not require any specific configuration to provide its basic features.
 The Shim6 protocol is designed to provide a default service to upper
 layers that should satisfy general applications.  The Shim6 layer
 would automatically attempt to protect long-lived communications by
 triggering the establishment of the Shim6 context using some
 predefined heuristics.  Of course, if some special tunning is
 required by some applications, this may require additional
 configuration.  Similar considerations apply to a site attempting to
 perform some forms of traffic engineering by using different
 preferences for different locators.
 Address and prefix configuration: The Shim6 protocol assumes that, in
 a multihomed site, multiple prefixes will be available.  Such
 configuration can increase the operation work in a network.  However,
 it should be noted that the capability of having multiple prefixes in
 a site and multiple addresses assigned to an interface is an IPv6
 capability that goes beyond the Shim6 case, and it is expected to be
 widely used.  So, even though this is the case for Shim6, we consider
 that the implications of such a configuration is beyond the
 particular case of Shim6 and must be addressed for the generic IPv6
 case.  Nevertheless, Shim6 also assumes the usage of CGA/HBA
 addresses by Shim6 hosts.  This implies that Shim6-capable hosts
 should configure addresses using HBA/CGA generation mechanisms.
 Additional consideration about this issue can be found at [19].

15.4. Other Considerations

 The general Shim6 approach as well as the specifics of this proposed
 solution have implications elsewhere, including:
 o  Applications that perform referrals or callbacks using IP
    addresses as the 'identifiers' can still function in limited ways,
    as described in [18].  But, in order for such applications to be
    able to take advantage of the multiple locators for redundancy,
    the applications need to be modified to either use Fully Qualified
    Domain Names as the 'identifiers' or they need to pass all the
    locators as the 'identifiers', i.e., the 'identifier' from the
    application's perspective becomes a set of IP addresses instead of
    a single IP address.
 o  Signaling protocols for QoS or for other things that involve
    having devices in the network path look at IP addresses and port
    numbers (or at IP addresses and Flow Labels) need to be invoked on

Nordmark & Bagnulo Standards Track [Page 90] RFC 5533 Shim6 Protocol June 2009

    the hosts when the locator pair changes due to a failure.  At that
    point in time, those protocols need to inform the devices that a
    new pair of IP addresses will be used for the flow.  Note that
    this is the case even though this protocol, unlike some earlier
    proposals, does not overload the Flow Label as a Context Tag; the
    in-path devices need to know about the use of the new locators
    even though the Flow Label stays the same.
 o  MTU implications.  By computing a minimum over the recently
    observed path MTUs, the path MTU mechanisms we use are robust
    against different packets taking different paths through the
    Internet.  When Shim6 fails over from using one locator pair to
    another, this means that packets might travel over a different
    path through the Internet; hence, the path MTU might be quite
    different.  In order to deal with this change in the MTU, the
    usage of Packetization Layer Path MTU Discovery as defined in [24]
    is recommended.
    The fact that the shim will add an 8-octet Shim6 Payload Extension
    header to the ULP packets after a locator switch can also affect
    the usable path MTU for the ULPs.  In this case, the MTU change is
    local to the sending host; thus, conveying the change to the ULPs
    is an implementation matter.  By conveying the information to the
    transport layer, it can adapt and reduce the Maximum Segment Size
    (MSS) accordingly.

16. Security Considerations

 This document satisfies the concerns specified in [15] as follows:
 o  The HBA [3] and CGA [2] techniques for verifying the locators to
    prevent an attacker from redirecting the packet stream to
    somewhere else, prevent threats described in Sections 4.1.1,
    4.1.2, 4.1.3, and 4.2 of [15].  These two techniques provide a
    similar level of protection but also provide different
    functionality with different computational costs.
    The HBA mechanism relies on the capability of generating all the
    addresses of a multihomed host as an unalterable set of
    intrinsically bound IPv6 addresses, known as an HBA set.  In this
    approach, addresses incorporate a cryptographic one-way hash of
    the prefix set available into the interface identifier part.  The
    result is that the binding between all the available addresses is
    encoded within the addresses themselves, providing hijacking
    protection.  Any peer using the shim protocol node can efficiently
    verify that the alternative addresses proposed for continuing the
    communication are bound to the initial address through a simple
    hash calculation.

Nordmark & Bagnulo Standards Track [Page 91] RFC 5533 Shim6 Protocol June 2009

    In a CGA-based approach, the address used as the ULID is a CGA
    that contains a hash of a public key in its interface identifier.
    The result is a secure binding between the ULID and the associated
    key pair.  This allows each peer to use the corresponding private
    key to sign the shim messages that convey locator set information.
    The trust chain in this case is the following: the ULID used for
    the communication is securely bound to the key pair because it
    contains the hash of the public key, and the locator set is bound
    to the public key through the signature.
    Either of these two mechanisms, HBA and CGA, provides time-shifted
    attack protection (as described in Section 4.1.2 of [15]), since
    the ULID is securely bound to a locator set that can only be
    defined by the owner of the ULID.  The minimum acceptable key
    length for RSA keys used in the generation of CGAs MUST be at
    least 1024 bits.  Any implementation should follow prudent
    cryptographic practice in determining the appropriate key lengths.
 o  3rd party flooding attacks, described in Section 4.3 of [15], are
    prevented by requiring a Shim6 peer to perform a successful
    Reachability probe + reply exchange before accepting a new locator
    for use as a packet destination.
 o  The first message does not create any state on the responder.
    Essentially, a 3-way exchange is required before the responder
    creates any state.  This means that a state-based DoS attack
    (trying to use up all memory on the responder) at least requires
    the attacker to create state, consuming his own resources; it also
    provides an IPv6 address that the attacker was using.
 o  The context-establishment messages use nonces to prevent replay
    attacks, which are described in Section 4.1.4 of [15], and to
    prevent off-path attackers from interfering with the
    establishment.
 o  Every control message of the Shim6 protocol, past the context
    establishment, carry the Context Tag assigned to the particular
    context.  This implies that an attacker needs to discover that
    Context Tag before being able to spoof any Shim6 control message
    as described in Section 4.4 of [15].  Such discovery probably
    requires an attacker to be along the path in order to sniff the
    Context Tag value.  The result is that, through this technique,
    the Shim6 protocol is protected against off-path attackers.

Nordmark & Bagnulo Standards Track [Page 92] RFC 5533 Shim6 Protocol June 2009

16.1. Interaction with IPSec

 Shim6 has two modes of processing data packets.  If the ULID pair is
 also the locator pair being used, then the data packet is not
 modified by Shim6.  In this case, the interaction with IPSec is
 exactly the same as if the Shim6 layer was not present in the host.
 If the ULID pair differs from the current locator pair for that Shim6
 context, then Shim6 will take the data packet, replace the ULIDs
 contained in the IP Source and Destination Address fields with the
 current locator pair, and add the Shim6 extension with the
 corresponding Context Tag.  In this case, as is mentioned in Section
 1.6, Shim6 conceptually works as a tunnel mechanism, where the inner
 header contains the ULID and the outer header contains the locators.
 The main difference is that the inner header is "compressed" and a
 compression tag, namely the Context Tag, is added to decompress the
 inner header at the receiving end.
 In this case, the interaction between IPSec and Shim6 is then similar
 to the interaction between IPSec and a tunnel mechanism.  When the
 packet is generated by the upper-layer protocol, it is passed to the
 IP layer containing the ULIDs in the IP Source and Destination field.
 IPSec is then applied to this packet.  Then the packet is passed to
 the Shim6 sublayer, which "encapsulates" the received packet and
 includes a new IP header containing the locator pair in the IP Source
 and Destination field.  This new IP packet is in turn passed to IPSec
 for processing, just as in the case of a tunnel.  This can be viewed
 as if IPSec is located both above and below the Shim6 sublayer and as
 if IPSec policies apply both to ULIDs and locators.
 When IPSec processed the packet after the Shim6 sublayer has
 processed it (i.e., the packet carrying the locators in the IP Source
 and Destination Address field), the Shim6 sublayer may have added the
 Shim6 Extension header.  In that case, IPSec needs to skip the Shim6
 Extension header to find the selectors for the next layer's protocols
 (e.g., TCP, UDP, Stream Control Transmission Protocol (SCTP)).
 When a packet is received at the other end, it is processed based on
 the order of the extension headers.  Thus, if an ESP or AH header
 precedes a Shim6 header, that determines the order.  Shim6 introduces
 the need to do policy checks, analogous to how they are done for
 tunnels, when Shim6 receives a packet and the ULID pair for that
 packet is not identical to the locator pair in the packet.

Nordmark & Bagnulo Standards Track [Page 93] RFC 5533 Shim6 Protocol June 2009

16.2. Residual Threats

 Some of the residual threats in this proposal are:
 o  An attacker that arrives late on the path (after the context has
    been established) can use the R1bis message to cause one peer to
    re-create the context and, at that point in time, can observe all
    of the exchange.  But this doesn't seem to open any new doors for
    the attacker since such an attacker can observe the Context Tags
    that are being used and, once known, can use those to send bogus
    messages.
 o  An attacker present on the path in order to find out the Context
    Tags can generate an R1bis message after it has moved off the
    path.  For this packet to be effective, it needs to have a source
    locator that belongs to the context; thus, there cannot be "too
    much" ingress filtering between the attacker's new location and
    the communicating peers.  But this doesn't seem to be that severe
    because, once the R1bis causes the context to be re-established, a
    new pair of Context Tags will be used, which will not be known to
    the attacker.  If this is still a concern, we could require a
    2-way handshake, "did you really lose the state?", in response to
    the error message.
 o  It might be possible for an attacker to try random 47-bit Context
    Tags and see if they can cause disruption for communication
    between two hosts.  In particular, in the case of payload packets,
    the effects of such an attack would be similar to those of an
    attacker sending packets with a spoofed source address.  In the
    case of control packets, it is not enough to find the correct
    Context Tag -- additional information is required (e.g., nonces,
    proper source addresses; see previous bullet for the case of
    R1bis).  If a 47-bit tag, which is the largest that fits in an
    8-octet Extension header, isn't sufficient, one could use an even
    larger tag in the Shim6 control messages and use the low-order 47
    bits in the Shim6 Payload Extension header.
 o  When the Shim6 Payload Extension header is used, an attacker that
    can guess the 47-bit random Context Tag can inject packets into
    the context with any source locator.  Thus, if there is ingress
    filtering between the attacker and its target, this could
    potentially allow the attacker to bypass the ingress filtering.
    However, in addition to guessing the 47-bit Context Tag, the
    attacker also needs to find a context where, after the receiver's
    replacement of the locators with the ULIDs, the ULP checksum is
    correct.  But even this wouldn't be sufficient with ULPs like TCP,
    since the TCP port numbers and sequence numbers must match an
    existing connection.  Thus, even though the issues for off-path

Nordmark & Bagnulo Standards Track [Page 94] RFC 5533 Shim6 Protocol June 2009

    attackers injecting packets are different than today with ingress
    filtering, it is still very hard for an off-path attacker to
    guess.  If IPsec is applied, then the issue goes away completely.
 o  The validator included in the R1 and R1bis packets is generated as
    a hash of several input parameters.  While most of the inputs are
    actually determined by the sender, and only the secret value S is
    unknown to the sender, the resulting protection is deemed to be
    enough since it would be easier for the attacker to just obtain a
    new validator by sending an I1 packet than to perform all the
    computations required to determine the secret S.  Nevertheless, it
    is recommended that the host change the secret S periodically.

17. IANA Considerations

 IANA allocated a new IP Protocol Number value (140) for the Shim6
 Protocol.
 IANA recorded a CGA message type for the Shim6 protocol in the CGA
 Extension Type Tags registry with the value 0x4A30 5662 4858 574B
 3655 416F 506A 6D48.
 IANA established a Shim6 Parameter Registry with four components:
 Shim6 Type registrations, Shim6 Options registrations, Shim6 Error
 Code registrations, and Shim6 Verification Method registrations.
 The initial contents of the Shim6 Type registry are as follows:
 +------------+-----------------------------------------------------+
 | Type Value |                       Message                       |
 +------------+-----------------------------------------------------+
 |      0     |                       RESERVED                      |
 |      1     | I1 (first establishment message from the initiator) |
 |      2     | R1 (first establishment message from the responder) |
 |      3     |  I2 (2nd establishment message from the initiator)  |
 |      4     |  R2 (2nd establishment message from the responder)  |
 |      5     |  R1bis (Reply to reference to non-existent context) |
 |      6     |           I2bis (Reply to a R1bis message)          |
 |    7-59    |           Allocated using Standards action          |
 |    60-63   |                 For Experimental use                |
 |     64     |                    Update Request                   |
 |     65     |                Update Acknowledgement               |
 |     66     |                      Keepalive                      |
 |     67     |                    Probe Message                    |
 |     68     |                    Error Message                    |
 |   69-123   |           Allocated using Standards action          |
 |   124-127  |                 For Experimental use                |
 +------------+-----------------------------------------------------+

Nordmark & Bagnulo Standards Track [Page 95] RFC 5533 Shim6 Protocol June 2009

 The initial contents of the Shim6 Options registry are as follows:
          +-------------+----------------------------------+
          |     Type    |            Option Name           |
          +-------------+----------------------------------+
          |      0      |             RESERVED             |
          |      1      |        Responder Validator       |
          |      2      |           Locator List           |
          |      3      |        Locator Preferences       |
          |      4      |   CGA Parameter Data Structure   |
          |      5      |           CGA Signature          |
          |      6      |             ULID Pair            |
          |      7      |    Forked Instance Identifier    |
          |     8-9     | Allocated using Standards action |
          |      10     |     Keepalive Timeout Option     |
          |   11-16383  | Allocated using Standards action |
          | 16384-32767 |       For Experimental use       |
          +-------------+----------------------------------+
 The initial contents of the Shim6 Error Code registry are as follows:
      +------------+--------------------------------------------+
      | Code Value |                 Description                |
      +------------+--------------------------------------------+
      |      0     |         Unknown Shim6 message type         |
      |      1     |       Critical Option not recognized       |
      |      2     |     Locator verification method failed     |
      |      3     | Locator List Generation number out of sync |
      |      4     |       Error in the number of locators      |
      |    5-19    |      Allocated using Standards action      |
      |   120-127  |       Reserved for debugging purposes      |
      +------------+--------------------------------------------+
 The initial contents of the Shim6 Verification Method registry are as
 follows:
            +---------+----------------------------------+
            |  Value  |        Verification Method       |
            +---------+----------------------------------+
            |    0    |             RESERVED             |
            |    1    |                CGA               |
            |    2    |                HBA               |
            |  3-200  | Allocated using Standards action |
            | 201-254 |       For Experimental use       |
            |   255   |             RESERVED             |
            +---------+----------------------------------+

Nordmark & Bagnulo Standards Track [Page 96] RFC 5533 Shim6 Protocol June 2009

18. Acknowledgements

 Over the years, many people active in the multi6 and shim6 WGs have
 contributed ideas and suggestions that are reflected in this
 specification.  Special thanks to the careful comments from Sam
 Hartman, Cullen Jennings, Magnus Nystrom, Stephen Kent, Geoff Huston,
 Shinta Sugimoto, Pekka Savola, Dave Meyer, Deguang Le, Jari Arkko,
 Iljitsch van Beijnum, Jim Bound, Brian Carpenter, Sebastien Barre,
 Matthijs Mekking, Dave Thaler, Bob Braden, Wesley Eddy, Pasi Eronen,
 and Tom Henderson on earlier versions of this document.

19. References

19.1. Normative References

 [1]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [2]   Aura, T., "Cryptographically Generated Addresses (CGA)",
       RFC 3972, March 2005.
 [3]   Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535, June 2009.
 [4]   Arkko, J. and I. van Beijnum, "Failure Detection and Locator
       Pair Exploration Protocol for IPv6 Multihoming", RFC 5534,
       June 2009.

19.2. Informative References

 [5]   Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
       specifying the location of services (DNS SRV)", RFC 2782,
       February 2000.
 [6]   Ferguson, P. and D. Senie, "Network Ingress Filtering:
       Defeating Denial of Service Attacks which employ IP Source
       Address Spoofing", BCP 38, RFC 2827, May 2000.
 [7]   Draves, R., "Default Address Selection for Internet Protocol
       version 6 (IPv6)", RFC 3484, February 2003.
 [8]   Nordmark, E., "Multihoming without IP Identifiers", Work
       in Progress, July 2004.
 [9]   Bagnulo, M., "Updating RFC 3484 for multihoming support", Work
       in Progress, November 2007.

Nordmark & Bagnulo Standards Track [Page 97] RFC 5533 Shim6 Protocol June 2009

 [10]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
       "RTP: A Transport Protocol for Real-Time Applications", STD 64,
       RFC 3550, July 2003.
 [11]  Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
       Multihoming Architectures", RFC 3582, August 2003.
 [12]  Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, "IPv6
       Flow Label Specification", RFC 3697, March 2004.
 [13]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
       Requirements for Security", BCP 106, RFC 4086, June 2005.
 [14]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
       Addresses", RFC 4193, October 2005.
 [15]  Nordmark, E. and T. Li, "Threats Relating to IPv6 Multihoming
       Solutions", RFC 4218, October 2005.
 [16]  Huitema, C., "Ingress filtering compatibility for IPv6
       multihomed sites", Work in Progress, September 2005.
 [17]  Bagnulo, M. and E. Nordmark, "SHIM - MIPv6 Interaction", Work
       in Progress, July 2005.
 [18]  Nordmark, E., "Shim6-Application Referral Issues", Work
       in Progress, July 2005.
 [19]  Bagnulo, M. and J. Abley, "Applicability Statement for the
       Level 3 Multihoming Shim Protocol (Shim6)", Work in Progress,
       July 2007.
 [20]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
       "Host Identity Protocol", RFC 5201, April 2008.
 [21]  Schuetz, S., Koutsianas, N., Eggert, L., Eddy, W., Swami, Y.,
       and K. Le, "TCP Response to Lower-Layer Connectivity-Change
       Indications", Work in Progress, February 2008.
 [22]  Williams, N. and M. Richardson, "Better-Than-Nothing Security:
       An Unauthenticated Mode of IPsec", RFC 5386, November 2008.
 [23]  Komu, M., Bagnulo, M., Slavov, K., and S. Sugimoto, "Socket
       Application Program Interface (API) for Multihoming Shim", Work
       in Progress, November 2008.
 [24]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
       Discovery", RFC 4821, March 2007.

Nordmark & Bagnulo Standards Track [Page 98] RFC 5533 Shim6 Protocol June 2009

 [25]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "Extended
       ICMP to Support Multi-Part Messages", RFC 4884, April 2007.

Nordmark & Bagnulo Standards Track [Page 99] RFC 5533 Shim6 Protocol June 2009

Appendix A. Possible Protocol Extensions

 During the development of this protocol, several issues have been
 brought up that are important to address but that do not need to be
 in the base protocol itself; instead, these can be done as extensions
 to the protocol.  The key ones are:
 o  As stated in the assumptions in Section 3, in order for the Shim6
    protocol to be able to recover from a wide range of failures (for
    instance, when one of the communicating hosts is single-homed) and
    to cope with a site's ISPs that do ingress filtering based on the
    source IPv6 address, there is a need for the host to be able to
    influence the egress selection from its site.  Further discussion
    of this issue is captured in [16].
 o  Is there need for keeping the list of locators private between the
    two communicating endpoints?  We can potentially accomplish that
    when using CGA (not when using HBA), but only at the cost of doing
    some public key encryption and decryption operations as part of
    the context establishment.  The suggestion is to leave this for a
    future extension to the protocol.
 o  Defining some form of end-to-end "compression" mechanism that
    removes the need to include the Shim6 Payload Extension header
    when the locator pair is not the ULID pair.
 o  Supporting the dynamic setting of locator preferences on a site-
    wide basis and using the Locator Preference option in the Shim6
    protocol to convey these preferences to remote communicating
    hosts.  This could mirror the DNS SRV record's notion of priority
    and weight.
 o  Specifying APIs in order for the ULPs to be aware of the locators
    that the shim is using and to be able to influence the choice of
    locators (controlling preferences as well as triggering a locator-
    pair switch).  This includes providing APIs that the ULPs can use
    to fork a shim context.
 o  Determining whether it is feasible to relax the suggestions for
    when context state is removed so that one can end up with an
    asymmetric distribution of the context state and still get (most
    of) the shim benefits.  For example, the busy server would go
    through the context setup but would quickly remove the context
    state after this (in order to save memory); however, the not-so-
    busy client would retain the context state.  The context-recovery
    mechanism presented in Section 7.5 would then re-create the state
    should the client send either a shim control message (e.g., Probe
    message because it sees a problem) or a ULP packet in a Shim6

Nordmark & Bagnulo Standards Track [Page 100] RFC 5533 Shim6 Protocol June 2009

    Payload Extension header (because it had earlier failed over to an
    alternative locator pair but had been silent for a while).  This
    seems to provide the benefits of the shim as long as the client
    can detect the failure.  If the client doesn't send anything and
    it is the server that tries to send, then it will not be able to
    recover because the shim on the server has no context state and
    hence doesn't know any alternate locator pairs.
 o  Study what it would take to make the Shim6 control protocol not
    rely at all on a stable source locator in the packets.  This can
    probably be accomplished by having all the shim control messages
    include the ULID-pair option.
 o  If each host might have lots of locators, then the current
    requirement to include essentially all of them in the I2 and R2
    messages might be constraining.  If this is the case, we can look
    into using the CGA Parameter Data Structure for the comparison,
    instead of the prefix sets, to be able to detect context
    confusion.  This would place some constraint on a (logical) only
    using, for example, one CGA public key; it would also require some
    carefully crafted rules on how two PDSs are compared for "being
    the same host".  But if we don't expect more than a handful of
    locators per host, then we don't need this added complexity.
 o  ULP-specified timers for the reachability detection mechanism
    (which can be particularly useful when there are forked contexts).
 o  Pre-verify some "backup" locator pair, so that the failover time
    can be shorter.
 o  Study how Shim6 and Mobile IPv6 might interact [17].

Appendix B. Simplified STATE Machine

 The STATEs are defined in Section 6.2.  The intent is for the
 stylized description below to be consistent with the textual
 description in the specification; however, should they conflict, the
 textual description is normative.

Nordmark & Bagnulo Standards Track [Page 101] RFC 5533 Shim6 Protocol June 2009

 The following table describes the possible actions in STATE IDLE and
 their respective triggers:
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Receive I1          | Send R1 and stay in IDLE                    |
 |                     |                                             |
 | Heuristics trigger  | Send I1 and move to I1-SENT                 |
 | a new context       |                                             |
 | establishment       |                                             |
 |                     |                                             |
 | Receive I2, verify  | If successful, send R2 and move to          |
 | validator and       | ESTABLISHED                                 |
 | RESP Nonce          |                                             |
 |                     | If fail, stay in IDLE                       |
 |                     |                                             |
 | Receive I2bis,      | If successful, send R2 and move to          |
 | verify validator    | ESTABLISHED                                 |
 | and RESP Nonce      |                                             |
 |                     | If fail, stay in IDLE                       |
 |                     |                                             |
 | R1, R1bis, R2       | N/A (This context lacks the required info   |
 |                     | for the dispatcher to deliver them)         |
 |                     |                                             |
 | Receive Payload     | Send R1bis and stay in IDLE                 |
 | Extension header    |                                             |
 | or other control    |                                             |
 | packet              |                                             |
 +---------------------+---------------------------------------------+

Nordmark & Bagnulo Standards Track [Page 102] RFC 5533 Shim6 Protocol June 2009

 The following table describes the possible actions in STATE I1-SENT
 and their respective triggers:
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Receive R1, verify  | If successful, send I2 and move to I2-SENT  |
 | INIT Nonce          |                                             |
 |                     | If fail, discard and stay in I1-SENT        |
 |                     |                                             |
 | Receive I1          | Send R2 and stay in I1-SENT                 |
 |                     |                                             |
 | Receive R2, verify  | If successful, move to ESTABLISHED          |
 | INIT Nonce          |                                             |
 |                     | If fail, discard and stay in I1-SENT        |
 |                     |                                             |
 | Receive I2, verify  | If successful, send R2 and move to          |
 | validator and RESP  | ESTABLISHED                                 |
 | Nonce               |                                             |
 |                     | If fail, discard and stay in I1-SENT        |
 |                     |                                             |
 | Receive I2bis,      | If successful, send R2 and move to          |
 | verify validator    | ESTABLISHED                                 |
 | and RESP Nonce      |                                             |
 |                     | If fail, discard and stay in I1-SENT        |
 |                     |                                             |
 | Timeout, increment  | If counter =< I1_RETRIES_MAX, send I1 and   |
 | timeout counter     | stay in I1-SENT                             |
 |                     |                                             |
 |                     | If counter > I1_RETRIES_MAX, go to E-FAILED |
 |                     |                                             |
 | Receive ICMP payload| Move to E-FAILED                            |
 | unknown error       |                                             |
 |                     |                                             |
 | R1bis               | N/A (Dispatcher doesn't deliver since       |
 |                     | CT(peer) is not set)                        |
 |                     |                                             |
 | Receive Payload     | Discard and stay in I1-SENT                 |
 | Extension header    |                                             |
 | or other control    |                                             |
 | packet              |                                             |
 +---------------------+---------------------------------------------+

Nordmark & Bagnulo Standards Track [Page 103] RFC 5533 Shim6 Protocol June 2009

 The following table describes the possible actions in STATE I2-SENT
 and their respective triggers:
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Receive R2, verify  | If successful, move to ESTABLISHED          |
 | INIT Nonce          |                                             |
 |                     | If fail, stay in I2-SENT                    |
 |                     |                                             |
 | Receive I1          | Send R2 and stay in I2-SENT                 |
 |                     |                                             |
 | Receive I2,         | Send R2 and stay in I2-SENT                 |
 | verify validator    |                                             |
 | and RESP Nonce      |                                             |
 |                     |                                             |
 | Receive I2bis,      | Send R2 and stay in I2-SENT                 |
 | verify validator    |                                             |
 | and RESP Nonce      |                                             |
 |                     |                                             |
 | Receive R1          | Discard and stay in I2-SENT                 |
 |                     |                                             |
 | Timeout, increment  | If counter =< I2_RETRIES_MAX, send I2 and   |
 | timeout counter     | stay in I2-SENT                             |
 |                     |                                             |
 |                     | If counter > I2_RETRIES_MAX, send I1 and go |
 |                     | to I1-SENT                                  |
 |                     |                                             |
 | R1bis               | N/A (Dispatcher doesn't deliver since       |
 |                     | CT(peer) is not set)                        |
 |                     |                                             |
 | Receive Payload     | Accept and send I2 (probably R2 was sent    |
 | Extension header    | by peer and lost)                           |
 | or other control    |                                             |
 | packet              |                                             |
 +---------------------+---------------------------------------------+

Nordmark & Bagnulo Standards Track [Page 104] RFC 5533 Shim6 Protocol June 2009

 The following table describes the possible actions in STATE I2BIS-
 SENT and their respective triggers:
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Receive R2, verify  | If successful, move to ESTABLISHED          |
 | INIT Nonce          |                                             |
 |                     | If fail, stay in I2BIS-SENT                 |
 |                     |                                             |
 | Receive I1          | Send R2 and stay in I2BIS-SENT              |
 |                     |                                             |
 | Receive I2,         | Send R2 and stay in I2BIS-SENT              |
 | verify validator    |                                             |
 | and RESP Nonce      |                                             |
 |                     |                                             |
 | Receive I2bis,      | Send R2 and stay in I2BIS-SENT              |
 | verify validator    |                                             |
 | and RESP Nonce      |                                             |
 |                     |                                             |
 | Receive R1          | Discard and stay in I2BIS-SENT              |
 |                     |                                             |
 | Timeout, increment  | If counter =< I2_RETRIES_MAX, send I2bis    |
 | timeout counter     | and stay in I2BIS-SENT                      |
 |                     |                                             |
 |                     | If counter > I2_RETRIES_MAX, send I1 and    |
 |                     | go to I1-SENT                               |
 |                     |                                             |
 | R1bis               | N/A (Dispatcher doesn't deliver since       |
 |                     | CT(peer) is not set)                        |
 |                     |                                             |
 | Receive Payload     | Accept and send I2bis (probably R2 was      |
 | Extension header    | sent by peer and lost)                      |
 | or other control    |                                             |
 | packet              |                                             |
 +---------------------+---------------------------------------------+

Nordmark & Bagnulo Standards Track [Page 105] RFC 5533 Shim6 Protocol June 2009

 The following table describes the possible actions in STATE
 ESTABLISHED and their respective triggers:
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Receive I1, compare | If no match, send R1 and stay in ESTABLISHED|
 | CT(peer) with       |                                             |
 | received CT         | If match, send R2 and stay in ESTABLISHED   |
 |                     |                                             |
 |                     |                                             |
 | Receive I2, verify  | If successful, send R2 and stay in          |
 | validator and RESP  | ESTABLISHED                                 |
 | Nonce               |                                             |
 |                     | Otherwise, discard and stay in ESTABLISHED  |
 |                     |                                             |
 | Receive I2bis,      | If successful, send R2 and stay in          |
 | verify validator    | ESTABLISHED                                 |
 | and RESP Nonce      |                                             |
 |                     | Otherwise, discard and stay in ESTABLISHED  |
 |                     |                                             |
 | Receive R2          | Discard and stay in ESTABLISHED             |
 |                     |                                             |
 | Receive R1          | Discard and stay in ESTABLISHED             |
 |                     |                                             |
 | Receive R1bis       | Send I2bis and move to I2BIS-SENT           |
 |                     |                                             |
 |                     |                                             |
 | Receive Payload     | Process and stay in ESTABLISHED             |
 | Extension header    |                                             |
 | or other control    |                                             |
 | packet              |                                             |
 |                     |                                             |
 | Implementation-     | Discard state and go to IDLE                |
 | specific heuristic  |                                             |
 | (e.g., No open ULP  |                                             |
 | sockets and idle    |                                             |
 | for some time )     |                                             |
 +---------------------+---------------------------------------------+

Nordmark & Bagnulo Standards Track [Page 106] RFC 5533 Shim6 Protocol June 2009

 The following table describes the possible actions in STATE E-FAILED
 and their respective triggers:
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Wait for            | Go to IDLE                                  |
 | NO_R1_HOLDDOWN_TIME |                                             |
 |                     |                                             |
 | Any packet          | Process as in IDLE                          |
 +---------------------+---------------------------------------------+
 The following table describes the possible actions in STATE NO-
 SUPPORT and their respective triggers:
 +---------------------+---------------------------------------------+
 | Trigger             | Action                                      |
 +---------------------+---------------------------------------------+
 | Wait for            | Go to IDLE                                  |
 | ICMP_HOLDDOWN_TIME  |                                             |
 |                     |                                             |
 | Any packet          | Process as in IDLE                          |
 +---------------------+---------------------------------------------+

Nordmark & Bagnulo Standards Track [Page 107] RFC 5533 Shim6 Protocol June 2009

B.1. Simplified STATE Machine Diagram

                                        Timeout/Null    +------------+
                          I1/R1      +------------------| NO SUPPORT |
          Payload or Control/R1bis   |                  +------------+
                      +---------+    |                              ^
                      |         |    |               ICMP Error/Null|
                      |         V    V                              |
                    +-----------------+  Timeout/Null  +----------+ |
                    |                 |<---------------| E-FAILED | |
                  +-|      IDLE       |                +----------+ |
   I2 or I2bis/R2 | |                 |                          ^  |
                  | +-----------------+       (Tiemout#>MAX)/Null|  |
                  |    ^            |                            |  |
                  |    |            +------+                     |  |
 I2 or I2bis/R2   |    |       Heuristic/I1|            I1/R2    |  |
   Payload/Null   |    |                   |       Control/Null  |  |
    I1/R1 or R2   | +--+                   |       Payload/Null  |  |
  R1 or R2/Null   | |Heuristic/Null        |  (Tiemout#<MAX)/I1  |  |
    +----------+  | |                      |         +--------+  |  |
    |          V  V |                      |         |        V  |  |
  +-------------------+   R2/Null          |        +----------------+
  |                   |   I2 or I2bis/R2   +------->|                |
  |   ESTABLISHED     |<----------------------------|    I1-SENT     |
  |                   |                             |                |
  +-------------------+                             +----------------+
     |     ^        ^                                   |   ^       ^
     |     |        |R2/Null              +-------------+   |       |
     |     |        +----------+          |R1/I2            |       |
     |     |                   |          V                 |       |
     |     |               +------------------+             |       |
     |     |               |                  |-------------+       |
     |     |               |     I2-SENT      | (Timeout#>Max)/I1   |
     |     |               |                  |                     |
     |     |               +------------------+                     |
     |     |                 |              ^                       |
     |     |                 +--------------+                       |
     |     |                I1 or I2bis or I2/R2                    |
     |     |           (Timeout#<Max) or Payload/I2                 |
     |     |                 R1 or R1bis/Null                       |
     |     +-------+                              (Timeout#>Max)/I1 |
     |      R2/Null|     +------------------------------------------+
     |             V     |
     |         +-------------------+
     |         |                   |<-+ (Timeout#<Max)/I2bis
     +-------->|   I2bis-SENT      |  | I1 or I2 or I2bis/R2
   R1bis/I2bis |                   |--+ R1 or R1bis/Null
               +-------------------+    Payload/I2bis

Nordmark & Bagnulo Standards Track [Page 108] RFC 5533 Shim6 Protocol June 2009

Appendix C. Context Tag Reuse

 The Shim6 protocol doesn't have a mechanism for coordinated state
 removal between the peers because such state removal doesn't seem to
 help, given that a host can crash and reboot at any time.  A result
 of this is that the protocol needs to be robust against a Context Tag
 being reused for some other context.  This section summarizes the
 different cases in which a Tag can be reused, and how the recovery
 works.
 The different cases are exemplified by the following case.  Assume
 hosts A and B were communicating using a context with the ULID pair
 <A1, B2>, and that B had assigned Context Tag X to this context.  We
 assume that B uses only the Context Tag to demultiplex the received
 Shim6 Payload Extension headers, since this is the more general case.
 Further, we assume that B removes this context state, while A retains
 it.  B might then at a later time assign CT(local)=X to some other
 context, at which time, we have several possible cases:
 o  The Context Tag is reassigned to a context for the same ULID pair
    <A1, B2>.  We've called this "context recovery" in this document.
 o  The Context Tag is reassigned to a context for a different ULID
    pair between the same two hosts, e.g., <A3, B3>.  We've called
    this "context confusion" in this document.
 o  The Context Tag is reassigned to a context between B and another
    host C, for instance, for the ULID pair <C3, B2>.  That is a form
    of three-party context confusion.

C.1. Context Recovery

 This case is relatively simple and is discussed in Section 7.5.  The
 observation is that since the ULID pair is the same, when either A or
 B tries to establish the new context, A can keep the old context
 while B re-creates the context with the same Context Tag CT(B) = X.

C.2. Context Confusion

 This case is a bit more complex and is discussed in Section 7.6.
 When the new context is created, whether A or B initiates it, host A
 can detect when it receives B's locator set (in the I2 or R2 message)
 in that it ends up with two contexts to the same peer host
 (overlapping Ls(peer) locator sets) that have the same Context Tag:
 CT(peer) = X.  At this point in time, host A can clear up any
 possibility of causing confusion by not using the old context to send
 any more packets.  It either just discards the old context (it might
 not be used by any ULP traffic, since B had discarded it) or it re-

Nordmark & Bagnulo Standards Track [Page 109] RFC 5533 Shim6 Protocol June 2009

 creates a different context for the old ULID pair (<A1, B2>), for
 which B will assign a unique CT(B) as part of the normal context-
 establishment mechanism.

C.3. Three-Party Context Confusion

 The third case does not have a place where the old state on A can be
 verified since the new context is established between B and C.  Thus,
 when B receives Shim6 Payload Extension headers with X as the Context
 Tag, it will find the context for <C3, B2> and, hence, will rewrite
 the packets to have C3 in the Source Address field and B2 in the
 Destination Address field before passing them up to the ULP.  This
 rewriting is correct when the packets are in fact sent by host C, but
 if host A ever happens to send a packet using the old context, then
 the ULP on A sends a packet with ULIDs <A1, B2> and the packet
 arrives at the ULP on B with ULIDs <C3, B2>.
 This is clearly an error, and the packet will most likely be rejected
 by the ULP on B due to a bad pseudo-header checksum.  Even if the
 checksum is okay (probability 2^-16), the ULP isn't likely to have a
 connection for those ULIDs and port numbers.  And if the ULP is
 connection-less, processing the packet is most likely harmless; such
 a ULP must be able to copy with random packets being sent by random
 peers in any case.
 This broken state, where packets are sent from A to B using the old
 context on host A, might persist for some time but will not remain
 for very long.  The unreachability detection on host A will kick in
 because it does not see any return traffic (payload or Keepalive
 messages) for the context.  This will result in host A sending Probe
 messages to host B to find a working locator pair.  The effect of
 this is that host B will notice that it does not have a context for
 the ULID pair <A1, B2> and CT(B) = X, which will make host B send an
 R1bis packet to re-establish that context.  The re-established
 context, just like in the previous section, will get a unique CT(B)
 assigned by host B; thus, there will no longer be any confusion.

C.4. Summary

 In summary, there are cases where a Context Tag might be reused while
 some peer retains the state, but the protocol can recover from it.
 The probability of these events is low, given the 47-bit Context Tag
 size.  However, it is important that these recovery mechanisms be
 tested.  Thus, during development and testing, it is recommended that
 implementations not use the full 47-bit space but instead keep, for
 example, the top 40 bits as zero, only leaving the host with 128
 unique Context Tags.  This will help test the recovery mechanisms.

Nordmark & Bagnulo Standards Track [Page 110] RFC 5533 Shim6 Protocol June 2009

Appendix D. Design Alternatives

 This document has picked a certain set of design choices in order to
 try to work out a bunch of the details and to stimulate discussion.
 But, as has been discussed on the mailing list, there are other
 choices that make sense.  This appendix tries to enumerate some
 alternatives.

D.1. Context Granularity

 Over the years, various suggestions have been made whether the shim
 should, even if it operates at the IP layer, be aware of ULP
 connections and sessions and, as a result, be able to make separate
 shim contexts for separate ULP connections and sessions.  A few
 different options have been discussed:
 o  Each ULP connection maps to its own shim context.
 o  The shim is unaware of the ULP notion of connections and just
    operates on a host-to-host (IP address) granularity.
 o  Hybrids in which the shim is aware of some ULPs (such as TCP) and
    handles other ULPs on a host-to-host basis.
 Having shim state for every ULP connection potentially means higher
 overhead since the state-setup overhead might become significant;
 there is utility in being able to amortize this over multiple
 connections.
 But being completely unaware of the ULP connections might hamper ULPs
 that want different communication to use different locator pairs, for
 instance, for quality or cost reasons.
 The protocol has a shim that operates with host-level granularity
 (strictly speaking, with ULID-pair granularity) to be able to
 amortize the context establishment over multiple ULP connections.
 This is combined with the ability for Shim6-aware ULPs to request
 context forking so that different ULP traffic can use different
 locator pairs.

D.2. Demultiplexing of Data Packets in Shim6 Communications

 Once a ULID-pair context is established between two hosts, packets
 may carry locators that differ from the ULIDs presented to the ULPs
 using the established context.  One of the main functions of the
 Shim6 layer is to perform the mapping between the locators used to
 forward packets through the network and the ULIDs presented to the
 ULP.  In order to perform that translation for incoming packets, the

Nordmark & Bagnulo Standards Track [Page 111] RFC 5533 Shim6 Protocol June 2009

 Shim6 layer needs to first identify which of the incoming packets
 need to be translated and then perform the mapping between locators
 and ULIDs using the associated context.  Such operation is called
 "demultiplexing".  It should be noted that, because any address can
 be used both as a locator and as a ULID, additional information,
 other than the addresses carried in packets, needs to be taken into
 account for this operation.
 For example, if a host has addresses A1 and A2 and starts
 communicating with a peer with addresses B1 and B2, then some
 communication (connections) might use the pair <A1, B1> as ULID and
 others might use, for example, <A2, B2>.  Initially there are no
 failures, so these address pairs are used as locators, i.e., in the
 IP address fields in the packets on the wire.  But when there is a
 failure, the Shim6 layer on A might decide to send packets that used
 <A1, B1> as ULIDs using <A2, B2> as the locators.  In this case, B
 needs to be able to rewrite the IP address field for some packets and
 not others, but the packets all have the same locator pair.
 In order to accomplish the demultiplexing operation successfully,
 data packets carry a Context Tag that allows the receiver of the
 packet to determine the shim context to be used to perform the
 operation.
 Two mechanisms for carrying the Context Tag information have been
 considered in depth during the shim protocol design: those carrying
 the Context Tag in the Flow Label field of the IPv6 header and those
 using a new Extension header to carry the Context Tag.  In this
 appendix, we will describe the pros and cons of each mechanism and
 justify the selected option.

D.2.1. Flow Label

 A possible approach is to carry the Context Tag in the Flow Label
 field of the IPv6 header.  This means that when a Shim6 context is
 established, a Flow Label value is associated with this context (and
 perhaps a separate Flow Label for each direction).
 The simplest way to do this is to have the triple <Flow Label, Source
 Locator, Destination Locator> identify the context at the receiver.
 The problem with this approach is that, because the locator sets are
 dynamic, it is not possible at any given moment to be sure that two
 contexts for which the same Context Tag is allocated will have
 disjoint locator sets during the lifetime of the contexts.
 Suppose that Node A has addresses IPA1, IPA2, IPA3, and IPA4 and that
 Host B has addresses IPB1 and IPB2.

Nordmark & Bagnulo Standards Track [Page 112] RFC 5533 Shim6 Protocol June 2009

 Suppose that two different contexts are established between Host A
 and Host B.
 Context #1 is using IPA1 and IPB1 as ULIDs.  The locator set
 associated to IPA1 is IPA1 and IPA2, while the locator set associated
 to IPB1 is just IPB1.
 Context #2 uses IPA3 and IPB2 as ULIDs.  The locator set associated
 to IPA3 is IPA3 and IPA4, while the locator set associated to IPB2 is
 just IPB2.
 Because the locator sets of Context #1 and Context #2 are disjoint,
 hosts could think that the same Context Tag value can be assigned to
 both of them.  The problem arrives when, later on, IPA3 is added as a
 valid locator for IPA1 in Context #2 and IPB2 is added as a valid
 locator for IPB1 in Context #1.  In this case, the triple <Flow
 Label, Source Locator, Destination Locator> would not identify a
 unique context anymore, and correct demultiplexing is no longer
 possible.
 A possible approach to overcome this limitation is to simply not
 repeat the Flow Label values for any communication established in a
 host.  This basically means that each time a new communication that
 is using different ULIDs is established, a new Flow Label value is
 assigned to it.  By these means, each communication that is using
 different ULIDs can be differentiated because each has a different
 Flow Label value.
 The problem with such an approach is that it requires the receiver of
 the communication to allocate the Flow Label value used for incoming
 packets, in order to assign them uniquely.  For this, a shim
 negotiation of the Flow Label value to use in the communication is
 needed before exchanging data packets.  This poses problems with non-
 Shim6-capable hosts, since they would not be able to negotiate an
 acceptable value for the Flow Label.  This limitation can be lifted
 by marking the packets that belong to shim sessions from those that
 do not.  These markings would require at least a bit in the IPv6
 header that is not currently available, so more creative options
 would be required, for instance, using new Next Header values to
 indicate that the packet belongs to a Shim6-enabled communication and
 that the Flow Label carries context information as proposed in [8].
 However, even if new Next Header values are used in this way, such an
 approach is incompatible with the deferred-establishment capability
 of the shim protocol, which is a preferred function since it
 suppresses delay due to shim context establishment prior to the
 initiation of communication.  Such capability also allows nodes to

Nordmark & Bagnulo Standards Track [Page 113] RFC 5533 Shim6 Protocol June 2009

 define at which stage of the communication they decide, based on
 their own policies, that a given communication requires protection by
 the shim.
 In order to cope with the identified limitations, an alternative
 approach that does not constrain the Flow Label values that are used
 by communications using ULIDs equal to the locators (i.e., no shim
 translation) is to only require that different Flow Label values are
 assigned to different shim contexts.  In such an approach,
 communications start with unmodified Flow Label usage (could be zero
 or as suggested in [12]).  The packets sent after a failure when a
 different locator pair is used would use a completely different Flow
 Label, and this Flow Label could be allocated by the receiver as part
 of the shim context establishment.  Since it is allocated during the
 context establishment, the receiver of the "failed over" packets can
 pick a Flow Label of its choosing (that is unique in the sense that
 no other context is using it as a Context Tag), without any
 performance impact, respecting that, for each locator pair, the Flow
 Label value used for a given locator pair doesn't change due to the
 operation of the multihoming shim.
 In this approach, the constraint is that Flow Label values being used
 as context identifiers cannot be used by other communications that
 use non-disjoint locator sets.  This means that once a given Flow
 Label value has been assigned to a shim context that has a certain
 locator sets associated, the same value cannot be used for other
 communications that use an address pair that is contained in the
 locator sets of the context.  This is a constraint in the potential
 Flow Label allocation strategies.
 A possible workaround to this constraint is to mark shim packets that
 require translation, in order to differentiate them from regular IPv6
 packets, using the artificial Next Header values described above.  In
 this case, the Flow Label values constrained are only those of the
 packets that are being translated by the shim.  This last approach
 would be the preferred approach if the Context Tag is to be carried
 in the Flow Label field.  This is the case not only because it
 imposes the minimum constraints to the Flow Label allocation
 strategies, limiting the restrictions only to those packets that need
 to be translated by the shim, but also because context-loss detection
 mechanisms greatly benefit from the fact that shim data packets are
 identified as such, allowing the receiving end to identify if a shim
 context associated to a received packet is supposed to exist, as will
 be discussed in the context-loss detection appendix below.

Nordmark & Bagnulo Standards Track [Page 114] RFC 5533 Shim6 Protocol June 2009

D.2.2. Extension Header

 Another approach, which is the one selected for this protocol, is to
 carry the Context Tag in a new Extension header.  These Context Tags
 are allocated by the receiving end during the Shim6 protocol initial
 negotiation, implying that each context will have two Context Tags,
 one for each direction.  Data packets will be demultiplexed using the
 Context Tag carried in the Extension header.  This seems a clean
 approach since it does not overload existing fields.  However, it
 introduces additional overhead in the packet due to the additional
 header.  The additional overhead introduced is 8 octets.  However, it
 should be noted that the Context Tag is only required when a locator
 other than the one used as ULID is contained in the packet.  Packets
 where both the Source and Destination Address fields contain the
 ULIDs do not require a Context Tag, since no rewriting is necessary
 at the receiver.  This approach would reduce the overhead because the
 additional header is only required after a failure.  On the other
 hand, this approach would cause changes in the available MTU for some
 packets, since packets that include the Extension header will have an
 MTU that is 8 octets shorter.  However, path changes through the
 network can result in a different MTU in any case; thus, having a
 locator change, which implies a path change, affect the MTU doesn't
 introduce any new issues.

D.3. Context-Loss Detection

 In this appendix, we will present different approaches considered to
 detect context loss and potential context-recovery strategies.  The
 scenario being considered is the following: Node A and Node B are
 communicating using IPA1 and IPB1.  Sometime later, a shim context is
 established between them, with IPA1 and IPB1 as ULIDs and with
 IPA1,...,IPAn and IPB1,...,IPBm as locator sets, respectively.
 It may happen that, later on, one of the hosts (e.g., Host A) loses
 the shim context.  The reason for this can be that Host A has a more
 aggressive garbage collection policy than Host B or that an error
 occurred in the shim layer at Host A and resulted in the loss of the
 context state.
 The mechanisms considered in this appendix are aimed at dealing with
 this problem.  There are essentially two tasks that need to be
 performed in order to cope with this problem: first, the context loss
 must be detected and, second, the context needs to be recovered/
 re-established.
 Mechanisms for detecting context loss.

Nordmark & Bagnulo Standards Track [Page 115] RFC 5533 Shim6 Protocol June 2009

 These mechanisms basically consist in each end of the context that
 periodically sends a packet containing context-specific information
 to the other end.  Upon reception of such packets, the receiver
 verifies that the required context exists.  In the case that the
 context does not exist, it sends a packet notifying the sender of the
 problem.
 An obvious alternative for this would be to create a specific context
 keepalive exchange, which consists in periodically sending packets
 with this purpose.  This option was considered and discarded because
 it seemed an overkill to define a new packet exchange to deal with
 this issue.
 Another alternative is to piggyback the context-loss detection
 function in other existent packet exchanges.  In particular, both
 shim control and data packets can be used for this.
 Shim control packets can be trivially used for this because they
 carry context-specific information.  This way, when a node receives
 one such packet, it will verify if the context exists.  However, shim
 control frequency may not be adequate for context-loss detection
 since control packet exchanges can be very limited for a session in
 certain scenarios.
 Data packets, on the other hand, are expected to be exchanged with a
 higher frequency but do not necessarily carry context-specific
 information.  In particular, packets flowing before a locator change
 (i.e., a packet carrying the ULIDs in the address fields) do not need
 context information since they do not need any shim processing.
 Packets that carry locators that differ from the ULIDs carry context
 information.
 However, we need to make a distinction here between the different
 approaches considered to carry the Context Tag -- in particular,
 between those approaches where packets are explicitly marked as shim
 packets and those approaches where packets are not marked as such.
 For instance, in the case where the Context Tag is carried in the
 Flow Label and packets are not marked as shim packets (i.e., no new
 Next Header values are defined for shim), a receiver that has lost
 the associated context is not able to detect that the packet is
 associated with a missing context.  The result is that the packet
 will be passed unchanged to the upper-layer protocol, which in turn
 will probably silently discard it due to a checksum error.  The
 resulting behavior is that the context loss is undetected.  This is
 one additional reason to discard an approach that carries the Context
 Tag in the Flow Label field and does not explicitly mark the shim
 packets as such.  On the other hand, approaches that mark shim data
 packets (like those that use the Extension header or the Flow Label

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 with new Next Header values) allow the receiver to detect if the
 context associated to the received packet is missing.  In this case,
 data packets also perform the function of a context-loss detection
 exchange.  However, it must be noted that only those packets that
 carry a locator that differs from the ULID are marked.  This
 basically means that context loss will be detected after an outage
 has occurred, i.e., alternative locators are being used.
 Summarizing, the proposed context-loss detection mechanisms use shim
 control packets and Shim6 Payload Extension headers to detect context
 loss.  Shim control packets detect context loss during the whole
 lifetime of the context, but the expected frequency in some cases is
 very low.  On the other hand, Shim6 Payload Extension headers have a
 higher expected frequency in general, but they only detect context
 loss after an outage.  This behavior implies that it will be common
 that context loss is detected after a failure, i.e., once it is
 actually needed.  Because of that, a mechanism for recovering from
 context loss is required if this approach is used.
 Overall, the mechanism for detecting lost context would work as
 follows: the end that still has the context available sends a message
 referring to the context.  Upon the reception of such message, the
 end that has lost the context identifies the situation and notifies
 the other end of the context-loss event by sending a packet
 containing the lost context information extracted from the received
 packet.
 One option is to simply send an error message containing the received
 packets (or at least as much of the received packet that the MTU
 allows to fit).  One of the goals of this notification is to allow
 the other end that still retains context state to re-establish the
 lost context.  The mechanism to re-establish the lost context
 consists in performing the 4-way initial handshake.  This is a time-
 consuming exchange and, at this point, time may be critical since we
 are re-establishing a context that is currently needed (because
 context-loss detection may occur after a failure).  So another
 option, which is the one used in this protocol, is to replace the
 error message with a modified R1 message so that the time required to
 perform the context-establishment exchange can be reduced.  Upon the
 reception of this modified R1 message, the end that still has the
 context state can finish the context-establishment exchange and
 restore the lost context.

D.4. Securing Locator Sets

 The adoption of a protocol like SHIM, which allows the binding of a
 given ULID with a set of locators, opens the door for different types
 of redirection attacks as described in [15].  The goal, in terms of

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 security, for the design of the shim protocol is to not introduce any
 new vulnerability into the Internet architecture.  It is a non-goal
 to provide additional protection other than what is currently
 available in the single-homed IPv6 Internet.
 Multiple security mechanisms were considered to protect the shim
 protocol.  In this appendix we will present some of them.
 The simplest option to protect the shim protocol is to use cookies,
 i.e., a randomly generated bit string that is negotiated during the
 context-establishment phase and then is included in subsequent
 signaling messages.  By these means, it would be possible to verify
 that the party that was involved in the initial handshake is the same
 party that is introducing new locators.  Moreover, before using a new
 locator, an exchange is performed through the new locator, verifying
 that the party located at the new locator knows the cookie, i.e.,
 that it is the same party that performed the initial handshake.
 While this security mechanism does indeed provide a fair amount of
 protection, it leaves the door open for so-called time-shifted
 attacks.  In these attacks, an attacker on the path discovers the
 cookie by sniffing any signaling message.  After that, the attacker
 can leave the path and still perform a redirection attack since, as
 he is in possession of the cookie, he can introduce a new locator
 into the locator set and can also successfully perform the
 reachability exchange if he is able to receive packets at the new
 locator.  The difference with the current single-homed IPv6 situation
 is that in the current situation the attacker needs to be on-path
 during the whole lifetime of the attack, while in this new situation
 (where only cookie protection is provided), an attacker that was once
 on the path can perform attacks after he has left the on-path
 location.
 Moreover, because the cookie is included in signaling messages, the
 attacker can discover the cookie by sniffing any of them, making the
 protocol vulnerable during the whole lifetime of the shim context.  A
 possible approach to increase security is to use a shared secret,
 i.e., a bit string that is negotiated during the initial handshake
 but that is used as a key to protect following messages.  With this
 technique, the attacker must be present on the path and sniffing
 packets during the initial handshake, since this is the only moment
 when the shared secret is exchanged.  Though it imposes that the
 attacker must be on path at a very specific moment (the establishment
 phase), and though it improves security, this approach is still
 vulnerable to time-shifted attacks.  It should be noted that,
 depending on protocol details, an attacker may be able to force the
 re-creation of the initial handshake (for instance, by blocking

Nordmark & Bagnulo Standards Track [Page 118] RFC 5533 Shim6 Protocol June 2009

 messages and making the parties think that the context has been
 lost); thus, the resulting situation may not differ that much from
 the cookie-based approach.
 Another option that was discussed during the design of this protocol
 was the possibility of using IPsec for protecting the shim protocol.
 Now, the problem under consideration in this scenario is how to
 securely bind an address that is being used as ULID with a locator
 set that can be used to exchange packets.  The mechanism provided by
 IPsec to securely bind the address that is used with cryptographic
 keys is the usage of digital certificates.  This implies that an
 IPsec-based solution would require a common and mutually trusted
 third party to generate digital certificates that bind the key and
 the ULID.  Considering that the scope of application of the shim
 protocol is global, this would imply a global public key
 infrastructure (PKI).  The major issues with this approach are the
 deployment difficulties associated with a global PKI.  The other
 possibility would be to use some form of opportunistic IPSec, like
 Better-Than-Nothing-Security (BTNS) [22].  However, this would still
 present some issues.  In particular, this approach requires a leap-
 of-faith in order to bind a given address to the public key that is
 being used, which would actually prevent the most critical security
 feature that a Shim6 security solution needs to achieve from being
 provided: proving identifier ownership.  On top of that, using IPsec
 would require to turn on per-packet AH/ESP just for multihoming to
 occur.
 In general, SHIM6 was expected to work between pairs of hosts that
 have no prior arrangement, security association, or common, trusted
 third party.  It was also seen as undesirable to have to turn on per-
 packet AH/ESP just for the multihoming to occur.  However, Shim6
 should work and have an additional level of security where two hosts
 choose to use IPsec.
 Another design alternative would have employed some form of
 opportunistic or Better-Than-Nothing Security (BTNS) IPsec to perform
 these tasks with IPsec instead.  Essentially, HIP in opportunistic
 mode is very similar to SHIM6, except that HIP uses IPsec, employs
 per-packet ESP, and introduces another set of identifiers.
 Finally, two different technologies were selected to protect the shim
 protocol: HBA [3] and CGA [2].  These two techniques provide a
 similar level of protection but also provide different functionality
 with different computational costs.
 The HBA mechanism relies on the capability of generating all the
 addresses of a multihomed host as an unalterable set of intrinsically
 bound IPv6 addresses, known as an HBA set.  In this approach,

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 addresses incorporate a cryptographic one-way hash of the prefix set
 available into the interface identifier part.  The result is that the
 binding between all the available addresses is encoded within the
 addresses themselves, providing hijacking protection.  Any peer using
 the shim protocol node can efficiently verify that the alternative
 addresses proposed for continuing the communication are bound to the
 initial address through a simple hash calculation.  A limitation of
 the HBA technique is that, once generated, the address set is fixed
 and cannot be changed without also changing all the addresses of the
 HBA set.  In other words, the HBA technique does not support dynamic
 addition of address to a previously generated HBA set.  An advantage
 of this approach is that it requires only hash operations to verify a
 locator set, imposing very low computational cost to the protocol.
 In a CGA-based approach, the address used as ULID is a CGA that
 contains a hash of a public key in its interface identifier.  The
 result is a secure binding between the ULID and the associated key
 pair.  This allows each peer to use the corresponding private key to
 sign the shim messages that convey locator set information.  The
 trust chain in this case is the following: the ULID used for the
 communication is securely bound to the key pair because it contains
 the hash of the public key, and the locator set is bound to the
 public key through the signature.  The CGA approach then supports
 dynamic addition of new locators in the locator set, since in order
 to do that the node only needs to sign the new locator with the
 private key associated with the CGA used as ULID.  A limitation of
 this approach is that it imposes systematic usage of public key
 cryptography with its associate computational cost.
 Either of these two mechanisms, HBA and CGA, provides time-shifted
 attack protection, since the ULID is securely bound to a locator set
 that can only be defined by the owner of the ULID.
 So the design decision adopted was that both mechanisms, HBA and CGA,
 are supported.  This way, when only stable address sets are required,
 the nodes can benefit from the low computational cost offered by HBA,
 while when dynamic locator sets are required, this can be achieved
 through CGAs with an additional cost.  Moreover, because HBAs are
 defined as a CGA extension, the addresses available in a node can
 simultaneously be CGAs and HBAs, allowing the usage of the HBA and
 CGA functionality when needed, without requiring a change in the
 addresses used.

D.5. ULID-Pair Context-Establishment Exchange

 Two options were considered for the ULID-pair context-establishment
 exchange: a 2-way handshake and a 4-way handshake.

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 A key goal for the design of this exchange was protection against DoS
 attacks.  The attack under consideration was basically a situation
 where an attacker launches a great amount of ULID-pair establishment-
 request packets, exhausting the victim's resources similarly to TCP
 SYN flooding attacks.
 A 4-way handshake exchange protects against these attacks because the
 receiver does not create any state associated to a given context
 until the reception of the second packet, which contains prior-
 contact proof in the form of a token.  At this point, the receiver
 can verify that at least the address used by the initiator is valid
 to some extent, since the initiator is able to receive packets at
 this address.  In the worst case, the responder can track down the
 attacker using this address.  The drawback of this approach is that
 it imposes a 4-packet exchange for any context establishment.  This
 would be a great deal if the shim context needed to be established up
 front, before the communication can proceed.  However, thanks to the
 deferred context-establishment capability of the shim protocol, this
 limitation has a reduced impact in the performance of the protocol.
 (However, it may have a greater impact in the situation of context
 recovery, as discussed earlier.  However, in this case, it is
 possible to perform optimizations to reduce the number of packets as
 described above.)
 The other option considered was a 2-way handshake with the
 possibility to fall back to a 4-way handshake in case of attack.  In
 this approach, the ULID-pair establishment exchange normally consists
 of a 2-packet exchange and does not verify that the initiator has
 performed a prior contact before creating context state.  In case a
 DoS attack is detected, the responder falls back to a 4-way handshake
 similar to the one described previously, in order to prevent the
 detected attack from proceeding.  The main difficulty with this
 attack is how to detect that a responder is currently under attack.
 It should be noted that, because this is a 2-way exchange, it is not
 possible to use the number of half-open sessions (as in TCP) to
 detect an ongoing attack; different heuristics need to be considered.
 The design decision taken was that, considering the current impact of
 DoS attacks and the low impact of the 4-way exchange in the shim
 protocol (thanks to the deferred context-establishment capability), a
 4-way exchange would be adopted for the base protocol.

D.6. Updating Locator Sets

 There are two possible approaches to the addition and removal of
 locators: atomic and differential approaches.  The atomic approach
 essentially sends the complete locator set each time a variation in
 the locator set occurs.  The differential approach sends the

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 differences between the existing locator set and the new one.  The
 atomic approach imposes additional overhead since all of the locator
 set has to be exchanged each time, while the differential approach
 requires re-synchronization of both ends through changes (i.e.,
 requires that both ends have the same idea about what the current
 locator set is).
 Because of the difficulties imposed by the synchronization
 requirement, the atomic approach was selected.

D.7. State Cleanup

 There are essentially two approaches for discarding an existing state
 about locators, keys, and identifiers of a correspondent node: a
 coordinated approach and an unilateral approach.
 In the unilateral approach, each node discards information about the
 other node without coordination with the other node, based on some
 local timers and heuristics.  No packet exchange is required for
 this.  In this case, it would be possible that one of the nodes has
 discarded the state while the other node still hasn't.  In this case,
 a No Context Error message may be required to inform the other node
 about the situation; possibly a recovery mechanism is also needed.
 A coordinated approach would use an explicit CLOSE mechanism, akin to
 the one specified in HIP [20].  If an explicit CLOSE handshake and
 associated timer is used, then there would no longer be a need for
 the No Context Error message due to a peer having garbage collected
 at its end of the context.  However, there is still potentially a
 need to have a No Context Error message in the case of a complete
 state loss of the peer (also known as a crash followed by a reboot).
 Only if we assume that the reboot takes at least the time of the
 CLOSE timer, or that it is okay to not provide complete service until
 CLOSE-timer minutes after the crash, can we completely do away with
 the No Context Error message.
 In addition, another aspect that is relevant for this design choice
 is the context confusion issue.  In particular, using a unilateral
 approach to discard context state clearly opens up the possibility of
 context confusion, where one of the ends unilaterally discards the
 context state, while the other does not.  In this case, the end that
 has discarded the state can re-use the Context Tag value used for the
 discarded state for another context, creating potential context
 confusion.  In order to illustrate the cases where problems would
 arise, consider the following scenario:
 o  Hosts A and B establish context 1 using CTA and CTB as Context
    Tags.

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 o  Later on, A discards context 1 and the Context Tag value CTA
    becomes available for reuse.
 o  However, B still keeps context 1.
 This would create context confusion in the following two cases:
 o  A new context 2 is established between A and B with a different
    ULID pair (or Forked Instance Identifier), and A uses CTA as the
    Context Tag.  If the locator sets used for both contexts are not
    disjoint, we have context confusion.
 o  A new context is established between A and C, and A uses CTA as
    the Context Tag value for this new context.  Later on, B sends
    Payload Extension header and/or control messages containing CTA,
    which could be interpreted by A as belonging to context 2 (if no
    proper care is taken).  Again we have context confusion.
 One could think that using a coordinated approach would eliminate
 such context confusion, making the protocol much simpler.  However,
 this is not the case, because even in the case of a coordinated
 approach using a CLOSE/CLOSE ACK exchange, there is still the
 possibility of a host rebooting without having the time to perform
 the CLOSE exchange.  So, it is true that the coordinated approach
 eliminates the possibility of context confusion due to premature
 garbage collection, but it does not prevent the same situations due
 to a crash and reboot of one of the involved hosts.  The result is
 that, even if we went for a coordinated approach, we would still need
 to deal with context confusion and provide the means to detect and
 recover from these situations.

Nordmark & Bagnulo Standards Track [Page 123] RFC 5533 Shim6 Protocol June 2009

Authors' Addresses

 Erik Nordmark
 Sun Microsystems
 17 Network Circle
 Menlo Park, CA 94025
 USA
 Phone: +1 650 786 2921
 EMail: erik.nordmark@sun.com
 Marcelo Bagnulo
 Universidad Carlos III de Madrid
 Av. Universidad 30
 Leganes, Madrid  28911
 SPAIN
 Phone: +34 91 6248814
 EMail: marcelo@it.uc3m.es
 URI:   http://www.it.uc3m.es

Nordmark & Bagnulo Standards Track [Page 124]

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