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rfc:rfc6316

Internet Engineering Task Force (IETF) M. Komu Request for Comments: 6316 Aalto University Category: Informational M. Bagnulo ISSN: 2070-1721 UC3M

                                                             K. Slavov
                                                      S. Sugimoto, Ed.
                                                              Ericsson
                                                             July 2011
  Sockets Application Program Interface (API) for Multihoming Shim

Abstract

 This document specifies sockets API extensions for the multihoming
 shim layer.  The API aims to enable interactions between applications
 and the multihoming shim layer for advanced locator management, and
 access to information about failure detection and path exploration.
 This document is based on an assumption that a multihomed host is
 equipped with a conceptual sub-layer (hereafter called "shim sub-
 layer") inside the IP layer that maintains mappings between
 identifiers and locators.  Examples of the shim are Shim6 and the
 Host Identity Protocol (HIP).

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6316.

Komu, et al. Informational [Page 1] RFC 6316 Multihoming Shim API July 2011

Copyright Notice

 Copyright (c) 2011 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................3
 2. Requirements Language ...........................................4
 3. Terminology and Background ......................................4
 4. System Overview .................................................7
 5. Requirements ....................................................8
 6. Socket Options for Multihoming Shim Sub-Layer ..................10
    6.1. SHIM_ASSOCIATED ...........................................14
    6.2. SHIM_DONTSHIM .............................................15
    6.3. SHIM_HOT_STANDBY ..........................................16
    6.4. SHIM_LOC_LOCAL_PREF .......................................17
    6.5. SHIM_LOC_PEER_PREF ........................................18
    6.6. SHIM_LOC_LOCAL_RECV .......................................19
    6.7. SHIM_LOC_PEER_RECV ........................................20
    6.8. SHIM_LOC_LOCAL_SEND .......................................20
    6.9. SHIM_LOC_PEER_SEND ........................................22
    6.10. SHIM_LOCLIST_LOCAL .......................................23
    6.11. SHIM_LOCLIST_PEER ........................................25
    6.12. SHIM_APP_TIMEOUT .........................................26
    6.13. SHIM_PATHEXPLORE .........................................27

Komu, et al. Informational [Page 2] RFC 6316 Multihoming Shim API July 2011

    6.14. SHIM_DEFERRED_CONTEXT_SETUP ..............................28
    6.15. Applicability ............................................28
    6.16. Error Handling ...........................................29
 7. Ancillary Data for Multihoming Shim Sub-Layer ..................29
    7.1. Get Locator from Incoming Packet ..........................30
    7.2. Set Locator for Outgoing Packet ...........................30
    7.3. Notification from Application to Multihoming Shim
         Sub-Layer .................................................31
    7.4. Applicability .............................................31
 8. Data Structures ................................................32
    8.1. Data Structure for Locator Information ....................32
         8.1.1. Handling Locator behind NAT ........................33
    8.2. Path Exploration Parameter ................................34
    8.3. Feedback Information ......................................35
 9. System Requirements ............................................36
 10. Relation to Existing Sockets API Extensions ...................36
 11. Operational Considerations ....................................37
    11.1. Conflict Resolution ......................................37
    11.2. Incompatibility between IPv4 and IPv6 ....................38
 12. IANA Considerations ...........................................38
 13. Protocol Constant .............................................38
 14. Security Considerations .......................................38
    14.1. Treatment of Unknown Locator .............................39
         14.1.1. Treatment of Unknown Source Locator ...............39
         14.1.2. Treatment of Unknown Destination Locator ..........39
 15. Acknowledgments ...............................................40
 16. References ....................................................40
    16.1. Normative References .....................................40
    16.2. Informative References ...................................41
 Appendix A. Context Forking .......................................42

1. Introduction

 This document defines sockets API extensions by which upper-layer
 protocols may be informed about and control the way in which a
 multihoming shim sub-layer in the IP layer manages the dynamic choice
 of locators.  Initially, the multihoming shim sub-layer refers to
 Shim6 and/or HIP, but it is defined generically.
 The role of the multihoming shim sub-layer (hereafter called "shim
 sub-layer" in this document) is to avoid impacts to upper-layer
 protocols that may be caused when the endhost changes its attachment
 point to the Internet -- for instance, in the case of a rehoming
 event under the multihomed environment.  There is, however, a need
 for an API in the cases where 1) the upper-layer protocol is
 particularly sensitive to impacts, or 2) the upper-layer protocol
 wants to benefit from better knowledge of what is going on
 underneath.

Komu, et al. Informational [Page 3] RFC 6316 Multihoming Shim API July 2011

 There are various kinds of technologies that aim to solve the same
 issue (the multihoming issue).  Note that there will be conflict when
 more than one shim sub-layer is active at the same time.  The
 assumption made in this document is that there is only a single shim
 sub-layer (HIP or Shim6) activated on the system.
 The target readers of this document are application programmers who
 develop application software that may benefit greatly from multihomed
 environments.  In addition, this document aims to provide necessary
 information for developers of shim protocols to implement APIs for
 enabling advanced locator management.

2. Requirements Language

 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 [RFC2119].

3. Terminology and Background

 This section provides terminology used in this document.  Basically,
 most of the terms used in this document are taken from the following
 documents:
 o  Shim6 Protocol Specification [RFC5533]
 o  HIP Architecture [RFC4423]
 o  Reachability Protocol (REAP) [RFC5534]
 In this document, the term "IP" refers to both IPv4 and IPv6, unless
 the protocol version is specifically mentioned.  The following are
 definitions of terms frequently used in this document:
 o  Endpoint Identifier (EID) -- The identifier used by the
    application to specify the endpoint of a given communication.
    Applications may handle EIDs in various ways, such as long-lived
    connections, callbacks, and referrals [SHIM6-APP-REFER].
  • In the case of Shim6, an identifier called a ULID (Upper-Layer

Identifier) serves as an EID. A ULID is chosen from locators

       available on the host.
  • In the case of HIP, an identifier called a Host Identifier

serves as an EID. A Host Identifier is derived from the public

       key of a given host.  For the sake of backward compatibility
       with the sockets API, the Host Identifier is represented in the
       form of a hash of a public key.

Komu, et al. Informational [Page 4] RFC 6316 Multihoming Shim API July 2011

  • Note that the EID appears in the standard sockets API as an

address, and does not appear in the extensions defined in this

       document, which only concern locators.
 o  Locator - The IP address actually used to deliver IP packets.
    Locators are present in the source and destination fields of the
    IP header of a packet on the wire.  A locator as discussed in this
    document could be either an IPv4 address or an IPv6 address.  Note
    that HIP can handle both IPv4 and IPv6 locators, whereas Shim6 can
    handle only IPv6 locators.  For the HIP case, a locator can be a
    private IPv4 address when the host is behind a NAT.  Section 8.1.1
    gives a detailed description about the handling of a locator
    behind a NAT.
  • List of locators - A list of locators associated with an EID.

There are two lists of locators stored in a given context. One

       is associated with the local EID, and the other is associated
       with the remote EID.  As defined in [RFC5533], the list of
       locators associated with an EID 'A' is denoted as Ls(A).
  • Preferred locator - The (source/destination) locator currently

used to send packets within a given context.

  • Unknown locator - Any locator that does not appear in the

locator list of the shim context associated with the socket.

       When there is no shim context associated with the socket, any
       source and/or destination locator requested by the application
       is considered to be an unknown locator.
  • Valid locator - A valid locator means that the locator is

considered to be valid in the security sense. More

       specifically, the validity indicates whether the locator is
       part of a Hash-Based Address (HBA) set [RFC5535].
  • Verified locator - A verified locator means that the locator is

considered to be reachable according to the result of a REAP

       return routability check.  Note that the verification applies
       only to the peer's locator.
 o  Shim - The conceptual sub-layer inside the IP layer.  This sub-
    layer maintains mappings between EIDs and locators.  An EID can be
    associated with more than one locator at a time when the host is
    multihomed.  The term "shim" does not refer to a specific protocol
    but refers to the conceptual sub-layer inside the IP layer.

Komu, et al. Informational [Page 5] RFC 6316 Multihoming Shim API July 2011

 o  Identifier/locator adaptation - The adaptation performed at the
    shim sub-layer.  This adaptation may end up re-writing the source
    and/or destination addresses of an IP packet.  In the outbound
    packet processing, the EID pair is converted to the associated
    locator pair.  In the inbound packet processing, the locator pair
    is converted to the EID pair.
 o  Context - The state information shared by a given pair of peers.
    Context stores a binding between the EID and associated locators.
    Contexts are maintained by the shim sub-layer.  Deferred context
    setup is a scenario where a context is established after the
    communication starts.  Deferred context setup is possible if the
    ULID is routable, such as in the case of Shim6.
 o  Reachability detection - The procedure to check reachability
    between a given locator pair.
 o  Path - The sequence of routers that an IP packet goes through to
    reach the destination.
 o  Path exploration - The procedure to explore available paths for a
    given set of locator pairs.
 o  Outage - The incident that prevents IP packets flowing from the
    source locator to the destination locator.  When there is an
    outage, it means that there is no reachability between a given
    locator pair.  The outage may be caused by various reasons, such
    as a shortage of network resources, congestion, and human error
    (faulty operation).
 o  Working address pair - Considered to be "working" if the packet
    can safely travel from the source to the destination, where the
    packet contains the first address from the pair as the source
    address and the second address from the pair as the destination
    address.  If reachability is confirmed in both directions, the
    address pair is considered to be working bi-directionally.
 o  Reachability Protocol (REAP) - The protocol for detecting failure
    and exploring reachability in a multihomed environment.  REAP is
    defined in [RFC5534].

Komu, et al. Informational [Page 6] RFC 6316 Multihoming Shim API July 2011

 In this document, syntax and semantics of the API are given in the
 same way as in the Portable Operating System Interface (POSIX)
 standard [POSIX].  The API specifies how to use ancillary data (aka
 cmsg) to access the locator information with recvmsg() and/or
 sendmsg() I/O calls.  The API is described in C language, and data
 types are defined in the POSIX format; intN_t means a signed integer
 of exactly N bits (e.g., int16_t), and uintN_t means an unsigned
 integer of exactly N bits (e.g., uint32_t).
 The distinction between "connected" sockets and "unconnected" sockets
 is important when discussing the applicability of the sockets API
 defined in this document.  A connected socket is bound to a given
 peer, whereas an unconnected socket is not bound to any specific
 peers.  A TCP socket becomes a connected socket when the TCP
 connection establishment is completed.  UDP sockets are unconnected,
 unless the application uses the connect() system call.

4. System Overview

 Figure 1 illustrates the system overview.  The shim sub-layer and
 REAP component exist inside the IP layer.  Applications use the
 sockets API defined in this document to interface with the shim
 sub-layer and the transport layer for locator management, failure
 detection, and path exploration.
 It is also possible that the shim sub-layer interacts with the
 transport layer; however, such an interaction is outside the scope of
 this document.

Komu, et al. Informational [Page 7] RFC 6316 Multihoming Shim API July 2011

                      +------------------------+
                      |       Application      |
                      +------------------------+
                         ^                 ^
            ~~~~~~~~~~~~~|~Socket Interface|~~~~~~~~~~~~~~
                         |                 v
             +-----------|------------------------------+
             |           |  Transport Layer             |
             +-----------|------------------------------+
                   ^     |
     +-------------|-----|-------------------------------------+
     |             v     v                                     |
     |   +-----------------------------+       +----------+    |  IP
     |   |            Shim             |<----->|   REAP   |    | Layer
     |   +-----------------------------+       +----------+    |
     |                       ^                      ^          |
     +-----------------------|----------------------|----------+
                             v                      v
             +------------------------------------------+
             |                Link Layer                |
             +------------------------------------------+
                       Figure 1: System Overview

5. Requirements

 The following is a list of requirements from applications:
 o  Turn on/off shim.  An application should be able to request to
    turn on or turn off the multihoming support by the shim layer:
  • Apply shim. The application should be able to explicitly

request that the shim sub-layer apply multihoming support.

  • Don't apply shim. The application should be able to request

that the shim sub-layer not apply the multihoming support but

       apply normal IP processing at the IP layer.
  • Note that this function is also required by other types of

multihoming mechanisms, such as the Stream Control Transmission

       Protocol (SCTP) and multipath TCP, to avoid potential conflict
       with the shim sub-layer.

Komu, et al. Informational [Page 8] RFC 6316 Multihoming Shim API July 2011

 o  Locator management.
  • It should be possible to set a preferred source and/or

destination locator within a given context.

  • It should be possible to get a preferred source and/or

destination locator within a given context.

  • It should be possible to set a list of source and/or

destination locators within a given context: Ls(local) and

       Ls(remote).
  • It should be possible to get a list of source and/or

destination locators within a given context: Ls(local) and

       Ls(remote).
 o  Notification from applications and upper-layer protocols to the
    shim sub-layer about the status of the communication.  The
    notification occurs in an event-based manner.  Applications and/or
    upper-layer protocols may provide positive feedback or negative
    feedback to the shim sub-layer.  Note that these types of feedback
    are mentioned in [RFC5534]:
  • Applications and/or upper-layer protocols (e.g., TCP) may

provide positive feedback to the shim sub-layer informing that

       the communication is going well.
  • Applications and/or upper-layer protocols (e.g., TCP) may

provide negative feedback to the shim sub-layer informing that

       the communication status is not satisfactory.  TCP may detect a
       problem when it does not receive any expected ACK message from
       the peer.  The REAP module may be triggered by the negative
       feedback and invoke the path exploration procedure.
 o  Feedback from applications to the shim sub-layer.  Applications
    should be able to inform the shim sub-layer of the timeout values
    for detecting failures, sending keepalives, and starting the
    exploration procedure.  In particular, applications should be able
    to suppress keepalives.
 o  Hot-standby.  Applications may request the shim sub-layer for a
    hot-standby capability.  This means that alternative paths are
    known to be working in advance of a failure detection.  In such a
    case, it is possible for the shim sub-layer to immediately replace
    the current locator pair with an alternative locator pair.

Komu, et al. Informational [Page 9] RFC 6316 Multihoming Shim API July 2011

 o  Eagerness for locator exploration.  An application should be able
    to inform the shim sub-layer of how aggressively it wants the REAP
    mechanism to perform a path exploration (e.g., by specifying the
    number of concurrent attempts of discovery of working locator
    pairs) when an outage occurs on the path between the locator pair
    in use.
 o  Providing locator information to applications.  An application
    should be able to obtain information about the locator pair that
    was actually used to send or receive packets.
  • For inbound traffic, the application may be interested in the

locator pair that was actually used to receive the packet.

  • For outbound traffic, the application may be interested in the

locator pair that was actually used to transmit the packet.

    In this way, applications may have additional control of the
    locator management.  For example, an application becomes capable
    of verifying if its preference for a locator is actually applied
    to the flow or not.
 o  Applications should be able to know if the shim sub-layer supports
    deferred context setup or not.
 o  An application should be able to know if the communication is now
    being served by the shim sub-layer or not.
 o  An application should be able to use a common interface to access
    an IPv4 locator and an IPv6 locator.

6. Socket Options for Multihoming Shim Sub-Layer

 In this section, socket options that are specific to the shim
 sub-layer are defined.
 Table 1 shows a list of the socket options that are specific to the
 shim sub-layer.  All of these socket options are defined at the level
 SOL_SHIM.  When an application uses one of the socket options by
 getsockopt() or setsockopt(), the second argument MUST be set to
 SOL_SHIM.

Komu, et al. Informational [Page 10] RFC 6316 Multihoming Shim API July 2011

 The first column of Table 1 gives the name of the option.  The second
 column indicates whether the value for the socket option can be read
 by getsockopt(), and the third column indicates whether the value for
 the socket option can be written by setsockopt().  The fourth column
 provides a brief description of the socket option.  The fifth column
 shows the type of data structure specified along with the socket
 option.  By default, the data structure type is an integer.
 +-----------------------------+-----+-----+-----------------+-------+
 | optname                     | get | set | description     | dtype |
 +-----------------------------+-----+-----+-----------------+-------+
 | SHIM_ASSOCIATED             | o   |     | Get the         | int   |
 |                             |     |     | parameter that  |       |
 |                             |     |     | indicates       |       |
 |                             |     |     | whether the     |       |
 |                             |     |     | socket is       |       |
 |                             |     |     | associated (1)  |       |
 |                             |     |     | with any shim   |       |
 |                             |     |     | context or not  |       |
 |                             |     |     | (0).            |       |
 | SHIM_DONTSHIM               | o   | o   | Get or set the  | int   |
 |                             |     |     | parameter that  |       |
 |                             |     |     | indicates       |       |
 |                             |     |     | whether or not  |       |
 |                             |     |     | to employ       |       |
 |                             |     |     | multihoming     |       |
 |                             |     |     | support by the  |       |
 |                             |     |     | shim sub-layer. |       |
 | SHIM_HOT_STANDBY            | o   | o   | Get or set the  | int   |
 |                             |     |     | parameter to    |       |
 |                             |     |     | request the     |       |
 |                             |     |     | shim sub-layer  |       |
 |                             |     |     | to prepare a    |       |
 |                             |     |     | hot-standby     |       |
 |                             |     |     | connection.     |       |
 | SHIM_LOC_LOCAL_PREF         | o   | o   | Set the         | Note  |
 |                             |     |     | preference      | 1     |
 |                             |     |     | value for a     |       |
 |                             |     |     | source locator  |       |
 |                             |     |     | for outbound    |       |
 |                             |     |     | traffic.  Get   |       |
 |                             |     |     | the preferred   |       |
 |                             |     |     | locator for the |       |
 |                             |     |     | source locator  |       |
 |                             |     |     | for outbound    |       |
 |                             |     |     | traffic.        |       |

Komu, et al. Informational [Page 11] RFC 6316 Multihoming Shim API July 2011

 | SHIM_LOC_PEER_PREF          | o   | o   | Set the         | Note  |
 |                             |     |     | preference      | 1     |
 |                             |     |     | value for a     |       |
 |                             |     |     | destination     |       |
 |                             |     |     | locator for     |       |
 |                             |     |     | outbound        |       |
 |                             |     |     | traffic.  Get   |       |
 |                             |     |     | the preferred   |       |
 |                             |     |     | locator for the |       |
 |                             |     |     | destination     |       |
 |                             |     |     | locator for     |       |
 |                             |     |     | outbound        |       |
 |                             |     |     | traffic.        |       |
 | SHIM_LOC_LOCAL_RECV         | o   | o   | Request the     | int   |
 |                             |     |     | shim sub-layer  |       |
 |                             |     |     | to store the    |       |
 |                             |     |     | destination     |       |
 |                             |     |     | locator of the  |       |
 |                             |     |     | received IP     |       |
 |                             |     |     | packet in an    |       |
 |                             |     |     | ancillary data  |       |
 |                             |     |     | object.         |       |
 | SHIM_LOC_PEER_RECV          | o   | o   | Request the     | int   |
 |                             |     |     | shim sub-layer  |       |
 |                             |     |     | to store the    |       |
 |                             |     |     | source locator  |       |
 |                             |     |     | of the received |       |
 |                             |     |     | IP packet in an |       |
 |                             |     |     | ancillary data  |       |
 |                             |     |     | object.         |       |
 | SHIM_LOC_LOCAL_SEND         | o   | o   | Get or set the  | Note  |
 |                             |     |     | source locator  | 1     |
 |                             |     |     | of outgoing IP  |       |
 |                             |     |     | packets.        |       |
 | SHIM_LOC_PEER_SEND          | o   | o   | Get or set the  | Note  |
 |                             |     |     | destination     | 1     |
 |                             |     |     | locator of      |       |
 |                             |     |     | outgoing IP     |       |
 |                             |     |     | packets.        |       |
 | SHIM_LOCLIST_LOCAL          | o   | o   | Get or set the  | Note  |
 |                             |     |     | list of         | 2     |
 |                             |     |     | locators        |       |
 |                             |     |     | associated with |       |
 |                             |     |     | the local EID.  |       |

Komu, et al. Informational [Page 12] RFC 6316 Multihoming Shim API July 2011

 | SHIM_LOCLIST_PEER           | o   | o   | Get or set the  | Note  |
 |                             |     |     | list of         | 2     |
 |                             |     |     | locators        |       |
 |                             |     |     | associated with |       |
 |                             |     |     | the peer's EID. |       |
 | SHIM_APP_TIMEOUT            | o   | o   | Get or set the  | int   |
 |                             |     |     | Send Timeout    |       |
 |                             |     |     | value of REAP.  |       |
 | SHIM_PATHEXPLORE            | o   | o   | Get or set      | Note  |
 |                             |     |     | parameters for  | 3     |
 |                             |     |     | path            |       |
 |                             |     |     | exploration and |       |
 |                             |     |     | failure         |       |
 |                             |     |     | detection.      |       |
 | SHIM_CONTEXT_DEFERRED_SETUP | o   |     | Get the         | int   |
 |                             |     |     | parameter that  |       |
 |                             |     |     | indicates       |       |
 |                             |     |     | whether         |       |
 |                             |     |     | deferred        |       |
 |                             |     |     | context setup   |       |
 |                             |     |     | is supported or |       |
 |                             |     |     | not.            |       |
 +-----------------------------+-----+-----+-----------------+-------+
        Table 1: Socket Options for Multihoming Shim Sub-Layer
 Note 1: Pointer to a shim_locator as defined in Section 8.
 Note 2: Pointer to an array of shim_locator data.
 Note 3: Pointer to a shim_pathexplore as defined in Section 8.
 Figure 2 illustrates how the shim-specific socket options fit into
 the system model of sockets API.  The figure shows that the shim sub-
 layer and the additional protocol components (IPv4 and IPv6) below
 the shim sub-layer are new to the system model.  As previously
 mentioned, all the shim-specific socket options are defined at the
 SOL_SHIM level.  This design choice brings the following advantages:
 1.  The existing sockets APIs continue to work at the layer above the
     shim sub-layer.  That is, those legacy APIs handle IP addresses
     as identifiers.
 2.  With newly defined socket options for the shim sub-layer, the
     application obtains additional control of locator management.

Komu, et al. Informational [Page 13] RFC 6316 Multihoming Shim API July 2011

 3.  The shim-specific socket options can be kept independent from
     address family (IPPROTO_IP or IPPROTO_IPV6) and transport
     protocol (IPPROTO_TCP or IPPROTO_UDP) settings.
                          s1 s2      s3 s4
                           |  |       |  |
          +----------------|--|-------|--|----------------+
          |             +-------+   +-------+             |
          | IPPROTO_TCP |  TCP  |   |  UDP  |             |
          |             +-------+   +-------+             |
          |                |   \     /   |                |
          |                |    -----    |                |
          |                |   /     \   |                |
          |              +------+   +------+              |
          |   IPPROTO_IP | IPv4 |   | IPv6 | IPPROTO_IPV6 |
          |              +------+   +------+              |
          |                  \         /             SOL_SOCKET
          |          +--------\-------/--------+          |
          | SOL_SHIM |          shim           |          |
          |          +--------/-------\--------+          |
          |                  /         \                  |
          |              +------+   +------+              |
          |              | IPv4 |   | IPv6 |              |
          |              +------+   +------+              |
          |                  |          |                 |
          +------------------|----------|-----------------+
                             |          |
                           IPv4       IPv6
                         Datagram   Datagram
       Figure 2: System Model of Sockets API with Shim Sub-Layer

6.1. SHIM_ASSOCIATED

 The SHIM_ASSOCIATED option is used to check whether or not the socket
 is associated with any shim context.
 This option is meaningful when the locator information of the
 received IP packet does not tell whether or not the identifier/
 locator adaptation is performed.  Note that the EID pair and the
 locator pair may be identical in some cases.
 Note that the socket option is read-only, and the option value can be
 read by getsockopt().  The result (0/1/2) is set in the option value
 (the fourth argument of getsockopt()).
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.

Komu, et al. Informational [Page 14] RFC 6316 Multihoming Shim API July 2011

 The data type of the option value is an integer.  The option value
 indicates the presence of shim context.  A return value of 1 means
 that the socket is associated with a shim context at the shim
 sub-layer.  A return value of 0 indicates that there is no shim
 context associated with the socket.  A return value of 2 means that
 it is not known whether or not the socket is associated with a shim
 context, and this MUST be returned only when the socket is
 unconnected.  In other words, the returned value MUST be 0 or 1 when
 the socket is connected.
 For example, the option can be used by the application as follows:
     int optval;
     int optlen = sizeof(optval);
     getsockopt(fd, SOL_SHIM, SHIM_ASSOCIATED, &optval, &optlen);

6.2. SHIM_DONTSHIM

 The SHIM_DONTSHIM option is used to request that the shim layer not
 provide the multihoming support for the communication established
 over the socket.
 The data type of the option value is an integer, and it takes 0 or 1.
 An option value of 0 means that the shim sub-layer is employed if
 available.  An option value of 1 means that the application does not
 want the shim sub-layer to provide the multihoming support for the
 communication established over the socket.
 The default value is set to 0, which means that the shim sub-layer
 performs identifier/locator adaptation if available.
 Any attempt to disable the multihoming shim support MUST be made by
 the application before the socket is connected.  If an application
 makes such an attempt for a connected socket, error code EOPNOTSUPP
 MUST be returned.
 For example, an application can request that the system not apply the
 multihoming support as follows:
     int optval;
     optval = 1;
     setsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, sizeof(optval));

Komu, et al. Informational [Page 15] RFC 6316 Multihoming Shim API July 2011

 For example, the application can check the option value as follows:
     int optval;
     int len;
     len = sizeof(optval);
     getsockopt(fd, SOL_SHIM, SHIM_DONTSHIM, &optval, &len);

6.3. SHIM_HOT_STANDBY

 The SHIM_HOT_STANDBY option is used to control whether or not the
 shim sub-layer employs a hot-standby connection for the socket.  A
 hot-standby connection is an alternative working locator pair to the
 current locator pair.  This option is effective only when there is a
 shim context associated with the socket.
 The data type of the option value is an integer.
 The option value can be set by setsockopt().
 The option value can be read by getsockopt().
 By default, the value is set to 0, meaning that hot-standby
 connection is disabled.
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 For example, an application can request establishment of a hot-
 standby connection by using the socket option as follows:
     int optval;
     optval = 1;
     setsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval,
                sizeof(optval));

Komu, et al. Informational [Page 16] RFC 6316 Multihoming Shim API July 2011

 For example, an application can get the option value by using the
 socket option as follows:
     int optval;
     int len;
     len = sizeof(optval);
     getsockopt(fd, SOL_SHIM, SHIM_HOT_STANDBY, &optval, &len);

6.4. SHIM_LOC_LOCAL_PREF

 The SHIM_LOC_LOCAL_PREF option is used to set the preference value
 for a source locator for outbound traffic, or to get the preference
 value of the source locator for outbound traffic that has the highest
 preference value.
 This option is effective only when there is a shim context associated
 with the socket.
 By default, the option value is set to NULL, meaning that the option
 is disabled.
 The preference of a locator is defined by a combination of priority
 and weight as per DNS SRV [RFC2782].  Note that the Shim6 base
 protocol defines the preference of a locator in the same way.
 The data type of the option value is a pointer to the shim_locator
 information data structure as defined in Section 8.1.
 When an application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 Error code EINVALIDLOCATOR is returned when the validation of the
 specified locator fails.
 An application can set the preference value for a source locator for
 outbound traffic by setsockopt() with the socket option.  Note that
 lc_ifidx and lc_flags (as defined in Section 8.1) have no effect in a
 set operation.  Below is an example of such a set operation.

Komu, et al. Informational [Page 17] RFC 6316 Multihoming Shim API July 2011

     struct shim_locator lc;
     struct in6_addr ip6;
     /* ...set the locator (ip6)... */
     memset(&lc, 0, sizeof(shim_locator));
     lc.lc_family = AF_INET6;  /* IPv6 */
     lc.lc_ifidx = 0;
     lc.lc_flags = 0;
     lc.lc_prio = 1;
     lc.lc_weight = 10;
     memcpy(&lc.lc_addr, &ip6, sizeof(in6_addr));
     setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc,
                sizeof(optval));
 An application can get the source locator for outbound traffic that
 has the highest preference value by using the socket option.  Below
 is an example of such a get operation.
     struct shim_locator lc;
     int len;
     len = sizeof(lc);
     getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_PREF, &lc, &len);

6.5. SHIM_LOC_PEER_PREF

 The SHIM_LOC_PEER_PREF option is used to set the preference value for
 a destination locator for outbound traffic, or to get the preference
 value of the destination locator for outbound traffic that has the
 highest preference value.
 This option is effective only when there is a shim context associated
 with the socket.
 By default, the option value is set to NULL, meaning that the option
 is disabled.
 As defined earlier, the preference of a locator is defined by a
 combination of priority and weight as per DNS SRV [RFC2782].  When
 there is more than one candidate destination locator, the shim
 sub-layer makes a selection based on the priority and weight
 specified for each locator.
 The data type of the option value is a pointer to the shim_locator
 information data structure as defined in Section 8.1.

Komu, et al. Informational [Page 18] RFC 6316 Multihoming Shim API July 2011

 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 Error code EINVALIDLOCATOR is returned when the validation of the
 requested locator fails.
 Error code EUNREACHABLELOCATOR is returned when the requested locator
 is determined to be unreachable according to a reachability check.
 The usage of the option is the same as that of SHIM_LOC_LOCAL_PREF.

6.6. SHIM_LOC_LOCAL_RECV

 The SHIM_LOC_LOCAL_RECV option can be used to request that the shim
 sub-layer store the destination locator of the received IP packet in
 an ancillary data object that can be accessed by recvmsg().  This
 option is effective only when there is a shim context associated with
 the socket.
 The data type of the option value is an integer.  The option value
 MUST be binary (0 or 1).  By default, the option value is set to 0,
 meaning that the option is disabled.
 An application can set the option value by setsockopt().
 An application can get the option value by getsockopt().
 See Section 7 for the procedure to access locator information stored
 in the ancillary data objects.
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 For example, an application can request the shim sub-layer to store a
 destination locator by using the socket option as follows:
     int optval;
     optval = 1;
     setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval,
                sizeof(optval));

Komu, et al. Informational [Page 19] RFC 6316 Multihoming Shim API July 2011

 For example, an application can get the option value as follows:
     int optval;
     int len;
     len = sizeof(optval);
     getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, &optval, &len);

6.7. SHIM_LOC_PEER_RECV

 The SHIM_LOC_PEER_RECV option is used to request that the shim
 sub-layer store the source locator of the received IP packet in an
 ancillary data object that can be accessed by recvmsg().  This option
 is effective only when there is a shim context associated with the
 socket.
 The data type of the option value is an integer.  The option value
 MUST be binary (0 or 1).  By default, the option value is set to 0,
 meaning that the option is disabled.
 The option value can be set by setsockopt().
 The option value can be read by getsockopt().
 See Section 7 for the procedure to access locator information stored
 in the ancillary data objects.
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 The usage of the option is the same as that of the
 SHIM_LOC_LOCAL_RECV option.

6.8. SHIM_LOC_LOCAL_SEND

 The SHIM_LOC_LOCAL_SEND option is used to request that the shim
 sub-layer use a specific locator as the source locator for the IP
 packets to be sent from the socket.  This option is effective only
 when there is a shim context associated with the socket.
 The data type of the option value is a pointer to the shim_locator
 data structure.

Komu, et al. Informational [Page 20] RFC 6316 Multihoming Shim API July 2011

 An application can set the local locator by setsockopt(), providing a
 locator that is stored in a shim_locator data structure.  When a
 zero-filled locator is specified, the pre-existing setting of the
 local locator is inactivated.
 An application can get the local locator by getsockopt().
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 Error code EINVALIDLOCATOR is returned when an invalid locator is
 specified.
 For example, an application can request the shim sub-layer to use a
 specific local locator by using the socket option as follows:
     struct shim_locator locator;
     struct in6_addr ia6;
     /* an IPv6 address preferred for the source locator is copied
        to the parameter ia6 */
     memset(&locator, 0, sizeof(locator));
     /* fill shim_locator data structure */
     locator.lc_family = AF_INET6;
     locator.lc_ifidx = 0;
     locator.lc_flags = 0;
     locator.lc_prio = 0;
     locator.lc_weight = 0;
     memcpy(&locator.lc_addr, &ia6, sizeof(ia6));
     setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
                sizeof(locator));
 For example, an application can get the designated local locator by
 using the socket option as follows:

Komu, et al. Informational [Page 21] RFC 6316 Multihoming Shim API July 2011

     struct shim_locator locator;
     memset(&locator, 0, sizeof(locator));
     getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
                sizeof(locator));
     /* check locator */

6.9. SHIM_LOC_PEER_SEND

 The SHIM_LOC_PEER_SEND option is used to request that the shim
 sub-layer use a specific locator for the destination locator of IP
 packets to be sent from the socket.  This option is effective only
 when there is a shim context associated with the socket.
 The data type of the option value is a pointer to the shim_locator
 data structure.
 An application can set the remote locator by setsockopt(), providing
 a locator that is stored in a shim_locator data structure.  When a
 zero-filled locator is specified, the pre-existing setting of the
 remote locator is inactivated.
 An application can get the specified remote locator by getsockopt().
 The difference between the SHIM_LOC_PEER_SEND option and the
 SHIM_LOC_PEER_PREF option is that the former guarantees the use of a
 requested locator when applicable, whereas the latter does not.
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 Error code EINVALIDLOCATOR is returned when the validation of the
 requested locator fails.
 Error code EUNVERIFIEDLOCATOR is returned when reachability for the
 requested locator has not been verified yet.
 Error code EUNREACHABLELOCATOR is returned when the requested locator
 is determined to be unreachable according to a reachability check.
 The usage of the option is the same as that of the
 SHIM_LOC_LOCAL_SEND option.

Komu, et al. Informational [Page 22] RFC 6316 Multihoming Shim API July 2011

6.10. SHIM_LOCLIST_LOCAL

 The SHIM_LOCLIST_LOCAL option is used to get or set the locator list
 associated with the local EID of the shim context associated with the
 socket.  This option is effective only when there is a shim context
 associated with the socket.
 The data type of the option value is a pointer to the buffer in which
 a locator list is stored.  See Section 8 for the data structure for
 storing the locator information.  By default, the option value is set
 to NULL, meaning that the option is disabled.
 An application can get the locator list by getsockopt().  Note that
 the size of the buffer pointed to by the optval argument SHOULD be
 large enough to store an array of locator information.  The number of
 the locator information is not known beforehand.
 The local locator list can be set by setsockopt().  The buffer
 pointed to by the optval argument MUST contain an array of locator
 structures.
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 Error code EINVALIDLOCATOR is returned when the validation of any of
 the specified locators failed.
 Error code ETOOMANYLOCATORS is returned when the number of locators
 specified exceeds the limit (SHIM_MAX_LOCATORS), or when the size of
 the buffer provided by the application is not large enough to store
 the locator list provided by the shim sub-layer.
 For example, an application can set a list of locators to be
 associated with the local EID by using the socket option as follows.
 Note that an IPv4 locator can be handled by HIP and not by Shim6.

Komu, et al. Informational [Page 23] RFC 6316 Multihoming Shim API July 2011

     struct shim_locator locators[SHIM_MAX_LOCATORS];
     struct sockaddr_in *sin;
     struct sockaddr_in6 *sin6;
     memset(locators, 0, sizeof(locators));
     ...
     /* obtain local IP addresses from local interfaces */
     ...
     /* first locator (an IPv6 address) */
     locators[0].lc_family = AF_INET6;
     locators[0].lc_ifidx = 0;
     locators[0].lc_flags = 0;
     locators[0].lc_prio = 1;
     locators[0].lc_weight = 0;
     memcpy(&locators[0].lc_addr, &sa6->sin6_addr,
            sizeof(sa6->sin6_addr));
     ...
     /* second locator (an IPv4 address) */
     locators[1].lc_family = AF_INET;
     locators[1].lc_ifidx = 0;
     locators[1].lc_flags = 0;
     locators[1].lc_prio = 0;
     locators[1].lc_weight = 0;
     memcpy(&locators[1].lc_addr, &sa->sin_addr,
            sizeof(sa->sin_addr));
     setsockopt(fd, SOL_SHIM, SHIM_LOCLIST_LOCAL, locators,
                sizeof(locators));
 For example, an application can get a list of locators that are
 associated with the local EID by using the socket option as follows:
     struct shim_locator locators[SHIM_MAX_LOCATORS];
     memset(locators, 0, sizeof(locators));
     getsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_RECV, locators,
                sizeof(locators));
     /* parse locators */
     ...

Komu, et al. Informational [Page 24] RFC 6316 Multihoming Shim API July 2011

6.11. SHIM_LOCLIST_PEER

 The SHIM_LOCLIST_PEER option is used to get or set the locator list
 associated with the peer EID of the shim context associated with the
 socket.  This option is effective only when there is a shim context
 associated with the socket.
 The data type of the option value is a pointer to the buffer where a
 locator list is stored.  See Section 8 for the data structure for
 storing the locator information.  By default, the option value is set
 to NULL, meaning that the option is disabled.
 An application can get the locator list by getsockopt().  Note that
 the size of the buffer pointed to by the optval argument SHOULD be
 large enough to store an array of locator information.  The number of
 the locator information is not known beforehand.
 An application can set the locator list by setsockopt().  The buffer
 pointed to by the optval argument MUST contain an array of locator
 list items.
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 Error code EINVALIDLOCATOR is returned when the validation of any of
 the specified locators failed.
 Error code EUNVERIFIEDLOCATOR is returned when reachability for the
 requested locator has not been verified yet.
 Error code EUNREACHABLELOCATOR is returned when the requested locator
 is determined to be unreachable according to a reachability check.
 Error code ETOOMANYLOCATORS is returned when the number of locators
 specified exceeds the limit (SHIM_MAX_LOCATORS), or when the size of
 the buffer provided by the application is not large enough to store
 the locator list provided by the shim sub-layer.
 The usage of the option is the same as that of SHIM_LOCLIST_LOCAL.

Komu, et al. Informational [Page 25] RFC 6316 Multihoming Shim API July 2011

6.12. SHIM_APP_TIMEOUT

 The SHIM_APP_TIMEOUT option is used to get or set the Send Timeout
 value of REAP [RFC5534].  This option is effective only when there is
 a shim context associated with the socket.
 The data type of the option value is an integer.  The value indicates
 the period of timeout in seconds to send a REAP Keepalive message
 since the last outbound traffic.  By default, the option value is set
 to 0, meaning that the option is disabled.  When the option is
 disabled, the REAP mechanism follows its default Send Timeout value
 as specified in [RFC5534].
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 When there is no REAP instance on the system, error code EOPNOTSUPP
 is returned to the application.
 For example, an application can set the timeout value by using the
 socket option as follows:
     int optval;
     optval = 15; /* 15 seconds */
     setsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval,
                sizeof(optval));
 For example, an application can get the timeout value by using the
 socket option as follows:
     int optval;
     int len;
     len = sizeof(optval);
     getsockopt(fd, SOL_SHIM, SHIM_APP_TIMEOUT, &optval, &len);

Komu, et al. Informational [Page 26] RFC 6316 Multihoming Shim API July 2011

6.13. SHIM_PATHEXPLORE

 The application MAY use this socket option to get or set parameters
 concerning path exploration.  Path exploration is a procedure to find
 an alternative locator pair to the current locator pair.  As the REAP
 specification defines, a peer may send Probe messages to find an
 alternative locator pair.
 This option is effective only when there is a shim context associated
 with the socket.
 The data type of the option value is a pointer to the buffer where a
 set of information for path exploration is stored.  The data
 structure is defined in Section 8.
 By default, the option value is set to NULL, meaning that the option
 is disabled.
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 For example, an application can set parameters for path exploration
 by using the socket option as follows:
     struct shim6_pathexplore pe;
     pe.pe_probenum = 4;        /* times */
     pe.pe_keepaliveto = 10;    /* seconds */
     pe.pe_initprobeto = 500;   /* milliseconds */
     pe.pe_reserved = 0;
     setsockopt(fd, SOL_SHIM, SHIM_PATHEXPLORE, &pe, sizeof(pe));
 For example, an application can get parameters for path exploration
 by using the socket option as follows:
     struct shim6_pathexplore pe;
     int len;
     len = sizeof(pe);
     getsockopt(fd, SOL_SHIM, SHIM_PATHEXPLORE, &pe, &len);

Komu, et al. Informational [Page 27] RFC 6316 Multihoming Shim API July 2011

6.14. SHIM_DEFERRED_CONTEXT_SETUP

 The SHIM_DEFERRED_CONTEXT_SETUP option is used to check whether or
 not deferred context setup is possible.  Deferred context setup means
 that the context is established in parallel with the data
 communication.  Note that Shim6 supports deferred context setup and
 HIP does not, because EIDs in HIP (i.e., Host Identifiers) are non-
 routable.
 Note that the socket option is read-only, and the option value can be
 read by getsockopt().
 The data type for the option value is an integer.  The option value
 MUST be binary (0 or 1).  The option value of 1 means that the shim
 sub-layer supports deferred context setup.
 When the application specifies the socket option to an unconnected
 socket, error code EOPNOTSUPP is returned to the application.
 For example, an application can check whether deferred context setup
 is possible or not as follows:
     int optval;
     int len;
     len = sizeof(optval);
     getsockopt(fd, SOL_SHIM, SHIM_DEFERRED_CONTEXT_SETUP,
                &optval, &len);

6.15. Applicability

 All the socket options defined in this section except for the
 SHIM_DONTSHIM option are applicable to applications that use
 connected sockets.
 All the socket options defined in this section except for the
 SHIM_ASSOCIATED, SHIM_DONTSHIM, and SHIM_CONTEXT_DEFERRED_SETUP
 options are effective only when there is a shim context associated
 with the socket.

Komu, et al. Informational [Page 28] RFC 6316 Multihoming Shim API July 2011

6.16. Error Handling

 If successful, getsockopt() and setsockopt() return 0; otherwise, the
 functions return -1 and set errno to indicate an error.
 The following are new error values defined for some shim-specific
 socket options indicating that the getsockopt() or setsockopt()
 finished incompletely:
 EINVALIDLOCATOR
    This indicates that the locator is not part of the HBA set
    [RFC5535] within the shim context associated with the socket.
 EUNVERIFIEDLOCATOR
    This indicates that the reachability of the locator has not been
    confirmed.  This error is applicable to only the peer's locator.
 EUNREACHABLELOCATOR
    This indicates that the locator is not reachable according to the
    result of the reachability check.  This error is applicable to
    only the peer's locator.

7. Ancillary Data for Multihoming Shim Sub-Layer

 This section provides definitions of ancillary data to be used for
 locator management and notification from/to the shim sub-layer to/
 from the application.
 When the application performs locator management by sendmsg() or
 recvmsg(), a member of the msghdr structure (given in Figure 3)
 called msg_control holds a pointer to the buffer in which one or more
 shim-specific ancillary data objects may be stored.  An ancillary
 data object can store a single locator.  It should be possible to
 process the shim-specific ancillary data object by the existing
 macros defined in the POSIX standard and [RFC3542].
      struct msghdr {
              caddr_t msg_name;       /* optional address */
              u_int   msg_namelen;    /* size of address */
              struct  iovec *msg_iov; /* scatter/gather array */
              u_int   msg_iovlen;     /* # elements in msg_iov */
              caddr_t msg_control;    /* ancillary data, see below */
              u_int   msg_controllen; /* ancillary data buffer len */
              int     msg_flags;      /* flags on received message */
      };
                      Figure 3: msghdr Structure

Komu, et al. Informational [Page 29] RFC 6316 Multihoming Shim API July 2011

 In the case of an unconnected socket, msg_name stores the socket
 address of the peer.  Note that the address is not a locator of the
 peer but the identifier of the peer.  SHIM_LOC_PEER_RECV can be used
 to get the locator of the peer node.
 Table 2 is a list of the shim-specific ancillary data that can be
 used for locator management by recvmsg() or sendmsg().  In any case,
 the value of cmsg_level MUST be set to SOL_SHIM.
   +---------------------+-----------+-----------+-----------------+
   | cmsg_type           | sendmsg() | recvmsg() |   cmsg_data[]   |
   +---------------------+-----------+-----------+-----------------+
   | SHIM_LOC_LOCAL_RECV |           |     o     |      Note 1     |
   | SHIM_LOC_PEER_RECV  |           |     o     |      Note 1     |
   | SHIM_LOC_LOCAL_SEND |     o     |           |      Note 1     |
   | SHIM_LOC_PEER_SEND  |     o     |           |      Note 1     |
   | SHIM_FEEDBACK       |     o     |           | shim_feedback{} |
   +---------------------+-----------+-----------+-----------------+
                 Table 2: Shim-Specific Ancillary Data
 Note 1: cmsg_data[] within msg_control includes a single
 sockaddr_in{} or sockaddr_in6{} and padding if necessary

7.1. Get Locator from Incoming Packet

 An application can get locator information from the received IP
 packet by specifying the shim-specific socket options for the socket.
 When SHIM_LOC_LOCAL_RECV and/or SHIM_LOC_PEER_RECV socket options are
 set, the application can retrieve a local and/or remote locator from
 the ancillary data.
 When there is no shim context associated with the socket, the shim
 sub-layer MUST return zero-filled locator information to the
 application.

7.2. Set Locator for Outgoing Packet

 An application can specify the locators to be used for transmitting
 an IP packet by sendmsg().  When the ancillary data of cmsg_type
 SHIM_LOC_LOCAL_SEND and/or SHIM_LOC_PEER_SEND are specified, the
 application can explicitly specify the source and/or the destination
 locators to be used for the communication over the socket.  If the
 specified locator pair is verified, the shim sub-layer overrides the
 locator(s) of the outgoing IP packet.  Note that the effect is
 limited to the datagram transmitted by the sendmsg().

Komu, et al. Informational [Page 30] RFC 6316 Multihoming Shim API July 2011

 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 Error code EINVALIDLOCATOR is returned when validation of the
 specified locator fails.
 Error code EUNVERIFIEDLOCATOR is returned when reachability for the
 requested locator has not been verified yet.  The application is
 recommended to use another destination locator until the reachability
 check for the requested locator is done.
 Error code EUNREACHABLELOCATOR is returned when the requested locator
 is determined to be unreachable according to a reachability check.
 The application is recommended to use another destination locator
 when receiving the error.

7.3. Notification from Application to Multihoming Shim Sub-Layer

 An application MAY provide feedback to the shim sub-layer about the
 communication status.  Such feedback is useful for the shim sub-layer
 to monitor the reachability status of the currently used locator pair
 in a given shim context.
 The notification can be made by sendmsg() specifying a new ancillary
 data called SHIM_FEEDBACK.  The ancillary data can be handled by
 specifying the SHIM_FEEDBACK option in cmsg_type.
 When there is no shim context associated with the socket, error code
 ENOENT is returned to the application.
 See Section 8.3 for details of the data structure to be used.
 It is outside the scope of this document to describe how the shim
 sub-layer would react when feedback is provided by an application.

7.4. Applicability

 All the ancillary data for the shim sub-layer is applicable to
 connected sockets.
 Care is needed when the SHIM_LOC_*_RECV socket option is used for
 stream-oriented sockets (e.g., TCP sockets) because there is no one-
 to-one mapping between a single send or receive operation and the
 data (e.g., a TCP segment) being received.  In other words, there is

Komu, et al. Informational [Page 31] RFC 6316 Multihoming Shim API July 2011

 no guarantee that the locator(s) set in the SHIM_LOC_*_RECV ancillary
 data is identical to the locator(s) that appears in the IP packets
 received.  The shim sub-layer SHOULD provide the latest locator
 information to the application in response to the SHIM_LOC_*_RECV
 socket option.

8. Data Structures

 This section gives data structures for the shim sub-layer.  These
 data structures are either used as a parameter for setsockopt() or
 getsockopt() (as mentioned in Section 6), or as a parameter for
 ancillary data to be processed by sendmsg() or recvmsg() (as
 mentioned in Section 7).

8.1. Data Structure for Locator Information

 As defined in Section 6, the SHIM_LOC_*_PREF, SHIM_LOC_*_SEND, and
 SHIM_LOCLIST_* socket options need to handle one or more locator
 information points.  Locator information includes not only the
 locator itself but also additional information about the locator that
 is useful for locator management.  A new data structure is defined to
 serve as a placeholder for the locator information.
 Figure 4 illustrates the data structure called shim_locator, which
 stores locator information.
      struct shim_locator {
              uint8_t    lc_family;       /* address family */
              uint8_t    lc_proto;        /* protocol */
              uint16_t   lc_port;         /* port number */
              uint16_t   lc_prio;         /* preference value */
              uint16_t   lc_weight;       /* weight */
              uint32_t   lc_ifidx;        /* interface index */
              struct in6_addr lc_addr;    /* address */
              uint16_t   lc_flags;        /* flags */
      };
                   Figure 4: Shim Locator Structure
 lc_family
    Address family of the locator (e.g., AF_INET, AF_INET6).  It is
    required that the parameter contains a non-zero value indicating
    the exact address family of the locator.

Komu, et al. Informational [Page 32] RFC 6316 Multihoming Shim API July 2011

 lc_proto
    Internet Protocol number for the protocol that is used to handle a
    locator behind a NAT.  The value MUST be set to zero when there is
    no NAT involved.  When the locator is behind a NAT, the value MUST
    be set to IPPROTO_UDP.
 lc_port
    Port number that is used for handling a locator behind a NAT.
 lc_prio
    Priority of the locator.  The range is 0-65535.  The lowest
    priority value means the highest priority.
 lc_weight
    Weight value indicates a relative weight for locators with the
    same priority value.  The range is 0-65535.  A locator with higher
    weight value is prioritized over the other locators with lower
    weight values.
 lc_ifidx
    Interface index of the network interface to which the locator is
    assigned.  This field is applicable only to local locators, and
    has no effect in set operations.
 lc_addr
    Contains the locator.  In the case of IPv4, the locator MUST be
    formatted in the IPv4-mapped IPv6 address as defined in [RFC4291].
    The locator MUST be stored in network byte order.
 lc_flags
    Each bit of the flags represents a specific characteristic of the
    locator.  The Hash-Based Address (HBA) is defined as 0x01.  The
    Cryptographically Generated Address (CGA) is defined as 0x02.
    This field has no effect in set operations.

8.1.1. Handling Locator behind NAT

 Note that the locator information MAY contain a locator behind a
 Network Address Translator (NAT).  Such a situation may arise when
 the host is behind the NAT and uses a local address as a source
 locator to communicate with the peer.  Note that a NAT traversal
 mechanism for HIP is defined, which allows a HIP host to tunnel
 control and data traffic over UDP [RFC5770].  Note also that the
 locator behind a NAT is not necessarily an IPv4 address and can be an
 IPv6 address.  Below is an example where the application sets a UDP
 encapsulation interface as a source locator when sending IP packets.

Komu, et al. Informational [Page 33] RFC 6316 Multihoming Shim API July 2011

        struct shim_locator locator;
        struct in6_addr ia6;
        /* copy the private IPv4 address to the ia6 as an IPv4-mapped
           IPv6 address */
        memset(&locator, 0, sizeof(locator));
        /* fill shim_locator data structure */
        locator.lc_family = AF_INET;
        locator.lc_proto = IPPROTO_UDP;
        locator.lc_port = 50500;
        locator.lc_ifidx = 0;
        locator.lc_flags = 0;
        locator.lc_prio = 0;
        locator.lc_weight = 0;
        memcpy(&locator.lc_addr, &ia6, sizeof(ia6));
        setsockopt(fd, SOL_SHIM, SHIM_LOC_LOCAL_SEND, &locator,
                   sizeof(locator));
                 Figure 5: Handling Locator behind NAT

8.2. Path Exploration Parameter

 As defined in Section 6, SHIM_PATHEXPLORE allows an application to
 set or read the parameters for path exploration and failure
 detection.  A new data structure called shim_pathexplore is defined
 to store the necessary parameters.  Figure 6 illustrates the data
 structure.  The data structure can be passed to getsockopt() or
 setsockopt() as an argument.
      struct shim_pathexplore {
              uint16_t  pe_probenum;      /* # of initial probes */
              uint16_t  pe_keepaliveto;   /* Keepalive Timeout */
              uint16_t  pe_keepaliveint;  /* Keepalive Interval */
              uint16_t  pe_initprobeto;   /* Initial Probe Timeout */
              uint32_t  pe_reserved;      /* reserved */
      };
                   Figure 6: Path Explore Structure

Komu, et al. Informational [Page 34] RFC 6316 Multihoming Shim API July 2011

 pe_probenum
    Indicates the number of initial Probe messages to be sent.  The
    value MUST be set as per [RFC5534].
 pe_keepaliveto
    Indicates the timeout value in seconds for detecting a failure
    when the host does not receive any packets for a certain period of
    time while there is outbound traffic.  When the timer expires, the
    path exploration procedure will be carried out by sending a REAP
    Probe message.  The value MUST be set as per [RFC5534].
 pe_keepaliveint
    Indicates the interval of REAP Keepalive messages in seconds to be
    sent by the host when there is no outbound traffic to the peer
    host.  The value MUST be set as per [RFC5534].
 pe_initprobeto
    Indicates the retransmission timer of the REAP Probe message in
    milliseconds.  Note that this timer is applied before exponential
    back-off is started.  A REAP Probe message for the same locator
    pair may be retransmitted.  The value MUST be set as per
    [RFC5534].
 pe_reserved
    A reserved field for future extension.  By default, the field MUST
    be initialized to zero.

8.3. Feedback Information

 As mentioned in Section 7.3, applications can inform the shim
 sub-layer about the status of unicast reachability of the locator
 pair currently in use.  The feedback information can be handled by
 using ancillary data called SHIM_FEEDBACK.  A new data structure
 named shim_feedback is illustrated in Figure 7.
      struct shim_feedback {
              uint8_t   fb_direction;    /* direction of traffic */
              uint8_t   fb_indicator;    /* indicator (1-3) */
              uint16_t  fb_reserved;     /* reserved */
      };
               Figure 7: Feedback Information Structure

Komu, et al. Informational [Page 35] RFC 6316 Multihoming Shim API July 2011

 fb_direction
    Indicates the direction of reachability between the locator pair
    in question.  A value of 0 indicates outbound direction, and a
    value of 1 indicates inbound direction.
 fb_indicator
    A value indicating the degree of satisfaction of a unidirectional
    reachability for a given locator pair.
  • 0: Default value. Whenever this value is specified, the

feedback information MUST NOT be processed by the shim

       sub-layer.
  • 1: Unable to connect. There is no unidirectional reachability

between the locator pair in question.

  • 2: Unsatisfactory. The application is not satisfied with the

unidirectional reachability between the locator pair in

       question.
  • 3: Satisfactory. There is satisfactory unidirectional

reachability between the locator pair in question.

 fb_reserved
    Reserved field.  MUST be ignored by the receiver.

9. System Requirements

 As addressed in Section 6, most of the socket options and ancillary
 data defined in this document are applicable to connected sockets.
 It is assumed that the kernel is capable of maintaining the
 association between a connected socket and a shim context.  This
 requirement is considered to be reasonable because a pair of source
 and destination IP addresses is bound to a connected socket.

10. Relation to Existing Sockets API Extensions

 This section explains the relation between the sockets API defined in
 this document and the existing sockets API extensions.
 As mentioned in Section 6, the basic assumption is that the existing
 sockets API continues to work above the shim sub-layer.  This means
 that the existing sockets API deals with identifiers, and the sockets
 API defined in this document deals with locators.

Komu, et al. Informational [Page 36] RFC 6316 Multihoming Shim API July 2011

 SHIM_LOC_LOCAL_SEND and SHIM_LOC_PEER_SEND socket options are
 semantically similar to the IPV6_PKTINFO sockets API in the sense
 that both provide a means for an application to set the source IP
 address of outbound IP packets.
 SHIM_LOC_LOCAL_RECV and SHIM_LOC_PEER_RECV socket options are
 semantically similar to the IP_RECVDSTADDR and IPV6_PKTINFO sockets
 APIs in the sense that both provide a means for an application to get
 the source and/or destination IP address of inbound IP packets.
 getsockname() and getpeername() enable an application to get the
 "name" of the communication endpoints, which is represented by a pair
 of IP addresses and port numbers assigned to the socket.
 getsockname() gives the IP address and port number assigned to the
 socket on the local side, and getpeername() gives the IP address and
 port number of the peer side.

11. Operational Considerations

 This section gives operational considerations of the sockets API
 defined in this document.

11.1. Conflict Resolution

 There can be a conflicting situation when different applications
 specify different preferences for the same shim context.  For
 instance, suppose that applications A and B establish communication
 with the same EID pair while both applications have different
 preferences in their choice of local locator.  The notion of context
 forking in Shim6 can resolve the conflicting situation.
 It is possible that socket options defined in Section 6 cause a
 conflicting situation when the target context is shared by multiple
 applications.  In such a case, the socket handler should inform the
 shim sub-layer that context forking is required.  In Shim6, when a
 context is forked, a unique identifier called the Forked Instance
 Identifier (FII) is assigned to the newly forked context.  The forked
 context is then exclusively associated with the socket through which
 a non-default preference value was specified.  The forked context is
 maintained by the shim sub-layer during the lifetime of the
 associated socket instance.  When the socket is closed, the shim
 sub-layer SHOULD delete the associated context.
 When the application specifies SHIM_LOC_*_SEND specifying a different
 source or destination locator that does not have the highest priority
 and weight specified by the SHIM_LOC_*_PREF, the shim sub-layer
 SHOULD supersede the request made by SHIM_LOC_*_SEND over the
 preference specified by SHIM_LOC_*_PREF.

Komu, et al. Informational [Page 37] RFC 6316 Multihoming Shim API July 2011

 When the peer provides preferences of the locators (e.g., a Shim6
 peer sends a locator with a Locator Preferences Option) that conflict
 with preferences specified by the applications either by
 SHIM_LOC_PEER_SEND or SHIM_LOC_PEER_PREF, the shim sub-layer SHOULD
 supersede the preferences made by the applications over the
 preferences specified by the peer.

11.2. Incompatibility between IPv4 and IPv6

 The shim sub-layer performs identifier/locator adaptation.
 Therefore, in some cases, the whole IP header can be replaced with a
 new IP header of a different address family (e.g., conversion from
 IPv4 to IPv6 or vice versa).  Hence, there is an issue regarding how
 to make the conversion with minimum impact.  Note that this issue is
 common in other protocol conversion techniques [RFC2765] [RFC6145].
 As studied in the previous works on protocol conversion [RFC2765],
 [RFC6145] some of the features (IPv6 routing headers, hop-by-hop
 extension headers, and destination headers) from IPv6 are not
 convertible to IPv4.  In addition, the notion of source routing is
 not exactly the same in IPv4 and IPv6.  This means that an error may
 occur during the conversion of the identifier and locator.  It is
 outside the scope of this document to describe how the shim sub-layer
 should behave in such erroneous cases.

12. IANA Considerations

 There are no IANA considerations for the socket options (SHIM_*), the
 ancillary data, and the socket level (SOL_SHIM) that are defined in
 this document.  All the numbers concerned are not under the control
 of the IETF or IANA, but they are platform-specific.

13. Protocol Constant

 This section defines a protocol constant.
 SHIM_MAX_LOCATORS  The maximum number of locators to be included in a
    locator list.  The value is set to 32.

14. Security Considerations

 This section gives security considerations of the API defined in this
 document.

Komu, et al. Informational [Page 38] RFC 6316 Multihoming Shim API July 2011

14.1. Treatment of Unknown Locator

 When sending IP packets, there is a possibility that an application
 will request the use of an unknown locator for the source and/or
 destination locators.  Note that the treatment of an unknown locator
 can be a subject of security considerations, because the use of an
 invalid source and/or destination locator may cause a redirection
 attack.

14.1.1. Treatment of Unknown Source Locator

 The shim sub-layer checks to determine if the requested locator is
 available on any local interface.  If not, the shim sub-layer MUST
 reject the request and return an error message with the
 EINVALIDLOCATOR code to the application.  If the locator is confirmed
 to be available, the shim sub-layer SHOULD initiate the procedure to
 update the locator list.
 Use of the following socket options and ancillary data requires
 treatment of an unknown source locator:
 o  SHIM_LOC_LOCAL_SEND
 o  SHIM_LOC_LOCAL_PREF
 o  SHIM_LOCLIST_LOCAL

14.1.2. Treatment of Unknown Destination Locator

 If the shim sub-layer turns out to be Shim6, the Shim6 layer MUST
 reject the request for using an unknown destination locator.
 If the shim sub-layer turns out to be HIP, the HIP layer MUST reject
 the request for using an unknown destination locator.  There is,
 however, an exceptional case where the HIP layer SHOULD accept the
 request, provided that the HIP association is in the UNASSOCIATED
 state.  Details of locator handling in HIP are described in
 Section 4.6 of [RFC6317].
 Use of the following socket options and ancillary data requires
 treatment of an unknown destination locator:
 o  SHIM_LOC_PEER_SEND
 o  SHIM_LOC_PEER_PREF
 o  SHIM_LOCLIST_PEER

Komu, et al. Informational [Page 39] RFC 6316 Multihoming Shim API July 2011

15. Acknowledgments

 The authors would like to thank Jari Arkko, who participated in the
 discussion that led to the first version of this document, and Tatuya
 Jinmei, who thoroughly reviewed the early draft version of this
 document and provided detailed comments on sockets API-related
 issues.  Thomas Henderson provided valuable comments, especially from
 the HIP perspective.
 The authors sincerely thank the following people for their helpful
 comments regarding the document: Samu Varjonen, Dmitriy Kuptsov,
 Brian Carpenter, Michael Scharf, Sebastien Barre, and Roni Even.

16. References

16.1. Normative References

 [POSIX]     "IEEE Std. 1003.1-2008 Standard for Information
             Technology -- Portable Operating System Interface
             (POSIX).  Open group Technical Standard: Base
             Specifications, Issue 7", September 2008,
             <http://www.opengroup.org/austin>.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3542]   Stevens, W., Thomas, M., Nordmark, E., and T.  Jinmei,
             "Advanced Sockets Application Program Interface (API) for
             IPv6", RFC 3542, May 2003.
 [RFC4423]   Moskowitz, R. and P. Nikander, "Host Identity Protocol
             (HIP) Architecture", RFC 4423, May 2006.
 [RFC5533]   Nordmark, E. and M. Bagnulo, "Shim6: Level 3 Multihoming
             Shim Protocol for IPv6", RFC 5533, June 2009.
 [RFC5534]   Arkko, J. and I. van Beijnum, "Failure Detection and
             Locator Pair Exploration Protocol for IPv6 Multihoming",
             RFC 5534, June 2009.

Komu, et al. Informational [Page 40] RFC 6316 Multihoming Shim API July 2011

16.2. Informative References

 [RFC2765]   Nordmark, E., "Stateless IP/ICMP Translation Algorithm
             (SIIT)", RFC 2765, February 2000.
 [RFC2782]   Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
             specifying the location of services (DNS SRV)", RFC 2782,
             February 2000.
 [RFC4291]   Hinden, R. and S. Deering, "IP Version 6 Addressing
             Architecture", RFC 4291, February 2006.
 [RFC5535]   Bagnulo, M., "Hash-Based Addresses (HBA)", RFC 5535,
             June 2009.
 [RFC5770]   Komu, M., Henderson, T., Tschofenig, H., Melen, J., and
             A. Keranen, Ed., "Basic Host Identity Protocol (HIP)
             Extensions for Traversal of Network Address Translators",
             RFC 5770, April 2010.
 [RFC6145]   Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
             Algorithm", RFC 6145, April 2011.
 [RFC6317]   Komu, M. and T. Henderson, "Basic Socket Interface
             Extensions for the Host Identity Protocol (HIP)",
             RFC 6317, July 2011.
 [SHIM6-APP-REFER]
             Nordmark, E., "Shim6 Application Referral Issues", Work
             in Progress, July 2005.

Komu, et al. Informational [Page 41] RFC 6316 Multihoming Shim API July 2011

Appendix A. Context Forking

 In this section, an issue concerning context forking and its relation
 to the multihoming shim API are discussed.
 Shim6 supports the notion of context forking.  A peer may decide to
 fork a context for a certain reason (e.g., an upper-layer protocol
 prefers to use a different locator pair than the one defined in an
 available context).  The procedure of context forking is done
 similarly to the normal context establishment, performing the 4-way
 message exchange.  A peer who has decided to fork a context initiates
 the context establishment.  Hereafter, we call this peer the
 "initiator".  The peer of the initiator is called the "responder".
 Once the forked context is established between the peers, on the
 initiator side, it is possible to apply forked context to the packet
 flow, since the system maintains an association between the forked
 context and the socket owned by the application that has requested
 the context forking.  How this association is maintained is an
 implementation-specific issue.  However, on the responder side, there
 is a question of how the outbound packet can be multiplexed by the
 shim sub-layer, because there is more than one Shim6 context that
 matches with the ULID pair of the packet flow.  There is a need to
 differentiate packet flows not only by the ULID pairs but by some
 other information and associate a given packet flow with a specific
 context.
 Figure 8 gives an example of a scenario where two communicating peers
 fork a context.  Initially, there has been a single transaction
 between the peers, by the application 1 (App1).  Accordingly, another
 transaction is started, by application 2 (App2).  Both of the
 transactions are made based on the same ULID pair.  The first context
 pair (Ctx1) is established for the transaction of App1.  Given the
 requests from App2, the shim sub-layer on Peer 1 decides to fork a
 context.  Accordingly, a forked context (Ctx2) is established between
 the peers, which should be exclusively applied to the transaction of
 App2.  Ideally, multiplexing and demultiplexing of packet flows that
 relate to App1 and App2 should be done as illustrated in Figure 8.
 However, as mentioned earlier, the responder needs to multiplex
 outbound flows of App1 and App2 somehow.  Note that if a context
 forking occurs on the initiator side, a context forking needs to also
 occur on the responder side.

Komu, et al. Informational [Page 42] RFC 6316 Multihoming Shim API July 2011

            Peer 1                                 Peer 2
          (initiator)                            (responder)
     +----+         +----+                  +----+         +----+
     |App1|         |App2|                  |App1|         |App2|
     +----+         +----+                  +----+         +----+
       |^             |^                      ^|             ^|
       v|             v|                      |v             |v
  -----S1-------------S2-----            -----S1-------------S2-----
       ||             ||                      ||             ||
       ||             ||                      ||             ||
      Ctx1           Ctx2                    Ctx1           Ctx2
  ULID:<A1,B1>   ULID:<A1,B1>            ULID:<B1,A1>    ULID:<B1,A1>
  Loc: <A1,B2>   Loc: <A1,B3>            Loc: <B2,A1>    Loc: <B3,A1>
  FII: 0         FII: 100                FII: 0          FII: 100
       |^             |^                      ^|             ^|
       ||             ||                      ||             ||
       ||             ||                      ||             ||
       \..............||....................../|             ||
        \.............||......................./             ||
                      ||                                     ||
                      \|...................................../|
                       \....................................../
                       Figure 8: Context Forking
 How to solve the issue described above is a topic for further study.

Komu, et al. Informational [Page 43] RFC 6316 Multihoming Shim API July 2011

Authors' Addresses

 Miika Komu
 Aalto University
 Espoo
 Finland
 Phone: +358505734395
 Fax:   +358947025014
 EMail: miika@iki.fi
 URI:   http://cse.aalto.fi/research/groups/datacommunications/people/
 Marcelo Bagnulo
 Universidad Carlos III de Madrid
 Av. Universidad 30
 Leganes  28911
 SPAIN
 Phone: +34 91 6248837
 EMail: marcelo@it.uc3m.es
 URI:   http://it.uc3m.es/marcelo
 Kristian Slavov
 Ericsson Research Nomadiclab
 Hirsalantie 11
 Jorvas  FI-02420
 Finland
 Phone: +358 9 299 3286
 EMail: kristian.slavov@ericsson.com
 Shinta Sugimoto (editor)
 Nippon Ericsson K.K.
 Koraku Mori Building
 1-4-14, Koraku, Bunkyo-ku
 Tokyo  112-0004
 Japan
 Phone: +81 3 3830 2241
 EMail: shinta.sugimoto@ericsson.com

Komu, et al. Informational [Page 44]

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