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rfc:rfc1940

Network Working Group D. Estrin Request for Comments: 1940 USC Category: Informational T. Li

                                                            Y. Rekhter
                                                         cisco Systems
                                                           K. Varadhan
                                                            D. Zappala
                                                                   USC
                                                              May 1996
                       Source Demand Routing:
      Packet Format and Forwarding Specification (Version 1).

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.

1. Overview

 The purpose of SDRP is to support source-initiated selection of
 routes to complement the route selection provided by existing routing
 protocols for both inter-domain and intra-domain routes. This
 document refers to such source-initiated routes as "SDRP routes".
 This document describes the packet format and forwarding procedure
 for SDRP.  It also describes procedures for ascertaining feasibility
 of SDRP routes.  Other components not described here are routing
 information distribution and route computation.  This portion of the
 protocol may initially be used with manually configured routes. The
 same packet format and processing will be usable with dynamic route
 information distribution and computation methods under development.
 The packet forwarding protocol specified here makes minimal
 assumptions about the distribution and acquisition of routing
 information needed to construct the SDRP routes.  These minimal
 assumptions are believed to be sufficient for the existing Internet.
 Future components of the SDRP protocol will extend capabilities in
 this area and others in a largely backward-compatible manner.
 This version of the packet forwarding protocol sends all packets with
 the complete SDRP route in the SDRP header. Future versions will
 address route setup and other enhancements and optimizations.

Estrin, et al Informational [Page 1] RFC 1940 SDRv1 May 1996

2. Model of operations

 An Internet can be viewed as a collection of routing domains
 interconnected by means of common subnetworks, and Border Routers
 (BRs) attached to these subnetworks.  A routing domain itself may be
 composed of further subnetworks, routers interconnecting these
 subnetworks, and hosts.  This document assumes that there is some
 type of routing present within the routing domain, but it does not
 assume that this intra-domain routing is coordinated or even
 consistent.
 For the purposes of this discussion, a BR belongs to only one domain.
 A pair of BRs, each belonging to a different domain, but attached to
 a common subnetwork, form an inter-domain connection. By definition,
 packets that traverse multiple domains must traverse BRs of these
 domains.  Note that a single physical router may act as multiple BRs
 for the purposes of this model.
 A pair of domains is said to be adjacent if there is at least one
 pair of BRs, one in each domain, that form an inter-domain
 connection.
 Each domain has a globally unique identifier, called a Domain
 Identifier (DI). All the BRs within a domain need to know the DI
 assigned to the domain.  Management of the DI space is outside the
 scope of this document.  This document assumes that Autonomous System
 (AS) numbers are used as DIs.  A domain path (or simply path) refers
 to a list of DIs such as might be taken from a BGP AS path [1, 2, 3]
 or an IDRP RD path [4].  We refer to a route as the combination of a
 network address and domain paths. The network addresses are
 represented by NLRI (Network Layer Reachability Information) as
 described in [3].
 This document assumes that the routing domains are congruent to the
 autonomous systems. Thus, within the content of this document, the
 terms autonomous system and routing domain can be used
 interchangeably.
 An application residing at a source host inside a domain,
 communicates with a destination host at another domain.  An
 intermediate router in the path from the source host to the
 destination host may decide to forward the packet using SDRP.  It can
 do this by encapsulating the entire IP packet from the source host in
 an SDRP packet.  The router that does this encapsulation is called
 the "encapsulating router."

Estrin, et al Informational [Page 2] RFC 1940 SDRv1 May 1996

 2.1 SDRP routes
    A component in an SDRP route is either a DI (AS number) or an IP
    address.  Thus, an SDRP route is defined as a sequence of domains
    and routers, syntactically expressed as a sequence of DIs and IP
    addresses.  Thus an SDRP route is a collection of source routed
    hops.
    Each component of the SDRP route is called a "hop."  The packet
    traverses each component of the SDRP route exactly once.  When a
    router corresponding to one of the components of the SDRP route
    receives the packet from a router corresponding to the previous
    component of the SDRP route, the router will process the packet
    according to the SDRP forwarding rules in this packet.  The next
    component of the SDRP route that this router will forward the
    packet to, is called the "next hop," with respect to this router
    and component of the SDRP route.
    An SDRP hop can either be a "strict" source routed hop, or a
    "loose" source routed hop.  A strict source route hop is one in
    which, if the next hop specified is a DI, refers to an immediately
    adjacent domain, and the packet will be forwarded directly to a
    route within the domain; if the next hop specified is an IP
    address, refers to an immediately adjacent router on a common
    subnetwork.  Any other kind of a source route hop is a loose
    source route hop.
    A route is a "strict source route" if the current hop being
    executed is processed as a strict source route hop.  Likewise, a
    route is a "loose source route" if the current hop being executed
    is processed as a loose source route hop.
    It is assumed that each BR participates in the intra-domain
    routing protocol(s) (IGPs) of the domain to which the BR belongs.
    Thus, a BR may forward a packet to any other BR in its own domain
    using intra-domain routing procedures.  Forwarding a packet
    between two BRs that form an inter-domain connection requires
    neither intra-domain nor the inter-domain routing procedures (an
    inter-domain connection is a common Layer 2 subnetwork).
    It is also assumed that all routers participate in the intra-
    domain routing protocol(s) (IGPs) of the domain to which they
    belong.
    While SDRP does not require that all domains have a common network
    layer protocol, all the BRs in the domains along a given SDRP
    route are required to support a common network layer.  This
    document specifies SDRP operations when that common network layer

Estrin, et al Informational [Page 3] RFC 1940 SDRv1 May 1996

    protocol is IP ([5]).
    While this document requires all the BRs to support IP, the
    document does not preclude a BR from additionally supporting other
    network layer protocols as well (e.g., CLNP, IPX, AppleTalk).  If
    a BR supports multiple network layers, then for the purposes of
    this model, the BR must maintain multiple Forwarding Information
    Bases (FIBs), one per network layer.
 2.2 SDRP encapsulation
    Forwarding an IP packet along an SDRP route is accomplished by
    encapsulating the entire packet in an SDRP packet.  An SDRP packet
    consists of the SDRP header followed by the SDRP data.  The SDRP
    header carries the SDRP route constructed by the domain that
    originated the SDRP packet.  The SDRP data carries the original
    packet that the source domain decided to forward via SDRP.
    An SDRP packet is carried across domains as the data portion of an
    IP packet with protocol number 42.
    This document refers to the IP header of a packet that carries an
    SDRP packet as the delivery IP header (or just the delivery
    header).  This document refers to the packet carried as SDRP data
    s the payload packet, and the IP header of the payload packet is
    the payload header.
    Thus, an SDRP Packet can be represented as follows:
              +-------------------+--------------+-------------------
              | Delivery header   |  SDRP header |  SDRP data
              |    (IP header)    |              | (Payload packet)
              +-------------------+--------------+--------------------
    Each SDRP route may have an MTU associated with it. An MTU of an
    SDRP route is defined as the maximum length of the payload packet
    that can be carried without fragmentation of an SDRP packet.  This
    means that the SDRP MTU as seen by the transport layer and
    applications above the transport layer is the actual link MTU less
    the length of the Delivery and SDRP headers.  Procedures for MTU
    discovery are specified in Section 9.
 2.3 D-FIB
    It is assumed that a BR participates in either BGP or IDRP.  A BR
    participating in SDRP augments its FIBs with a D-FIB that contains
    routes to domains.  A route to a domain is a triplet <DI, Next-
    Hop, NLRI>, where DI depicts a destination domain, Next-Hop

Estrin, et al Informational [Page 4] RFC 1940 SDRv1 May 1996

    depicts the IP address of the next-hop BR, and NLRI depicts the
    set of reachable destinations within the destination domain.  D-
    FIBs are constructed based on the information obtained from either
    BGP, IDRP, or configuration information.
    An SDRP packet is forwarded across multiple domains by utilizing
    the forwarding databases (both FIBs and D-FIBs) maintained by the
    BRs.
    The operational status of SDRP routes is monitored via passive
    (Error Reporting) and active (Route Probing) mechanisms. The Error
    Reporting mechanism provides the originator of the SDRP route with
    a failure notification.  The Probing mechanism provides the
    originator of the SDRP route with confirmation of a route's
    feasibility.

3. SDRP Packet format

 The total length of an SDRP packet (header plus data) can be
 determined from the information carried in the delivery IP header.
 The length of the payload packet can be determined from the total
 length of an SDRP packet and the length of its SDRP Header.
 The following describes the format of an SDRP packet.
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Ver |D|S|P|   |   Hop Count   |SourceProtoType|  Payload Type |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                   Source Route Identifier                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Target Router                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                            Prefix                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  PrefixLength |  Notification |SrcRouteLength |   NextHopPtr  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Source Route ...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                Payload ....
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Version and Flags  (1 octet)
    The SDRP version number and control flags are coded in the first
    octet.  Bit 0 is the most significant bit, bit 7 is the least
    significant bit.

Estrin, et al Informational [Page 5] RFC 1940 SDRv1 May 1996

       Version (bits 0 through 2)
          The first three bits  contain the Version field indicating
          the version number of the protocol.  The value of this field
          is set to 1.
       Flags (bits 3 through 7)
          Data packet/Control packet (bit 3)
             If the bit is set to 1, then the packet carries data.
             Otherwise, the packet carries control information.
          Loose/Strict Source Route (bit 4)
             The Loose/Strict Source Route indicator is used when
             making a forwarding decision (see Section 5.2).  If this
             bit is set to 1, it indicates that the next hop is a
             Strict Source Route Hop.  If this bit is set to 0, it
             indicates that the next hop is a Loose Source Route.
          Probe Indicator (bit 5)
             The Probe Indicator is used by the originator of the
             route to request verification of the route's feasibility
             (see Sections 4 and 7.1).  If this bit is set to 1, it
             indicates that the originator is probing the route.  This
             bit should always be set to 0 for control packets.
    Hop Count (1 octet)
       The Hop Count field carries the maximum number of routers an
       SDRP data packet may traverse. It is decremented by 1 as an
       SDRP data packet traverses a router which forwards the packet
       using SDRP forwarding. Once the Hop Count field reaches the
       value of 0, the router should discard the data packet and
       generate a control packet (see Section 5.2.6).  A router that
       receives a packet with a Hop Count value of 0 should discard
       the data packet, and generate a control packet (see Section
       5.2.6).
    Source Route Protocol Type (1 octet)
       The Source Route Protocol Type fields indicates the type of
       information that appears in the source route.  The value 1 in
       this field indicates that the contents of the source route are
       as described in this document and indicates an Explicit Source

Estrin, et al Informational [Page 6] RFC 1940 SDRv1 May 1996

       Route.  The value 2 in this field indicates a Route Setup.  The
       syntax of the source route for this value is identical to a
       value of 1, but also has additional semantics which are defined
       in other documents.
    Payload Protocol Type (1 octet)
       The Payload Protocol Type field indicates the protocol type of
       the payload.  If the payload is an IP datagram, then this field
       should contain the value 1.
       Note that this Payload Protocol Type is not the same as the IP
       protocol type[5,7].
    Source Route Identifier (4 octets)
       The BR  that originates the SDRP packet should insert a 32 bit
       value in this field which will serve as an identifier for the
       source route.  This value needs to be  unique  only in the
       context of the originating BR.
    Target Router (4 octets)
       This field is meaningful only in control packets.
       The Target Router field contains one of the IP addresses of the
       router that originated the SDRP packet that triggered the
       control packet to be returned.
    Prefix (4 octets)
       The Prefix field contains an IP address prefix.  Only the
       number of bits specified in the Prefix Length are significant.
       The Prefix field is used to prevent routing loops when using
       BGP or IDRP to route to the next AS in a loose source route
       (see Section 4).
    Prefix Length (1 octet)
       The Prefix Length field indicates the length in bits of the IP
       address prefix.  A length of zero indicates a prefix that
       matches all IP addresses.
          Notification Code (1 octet)
             This field is only meaningful in control packets.  In
             data packets, this field is transmitted as zero, and
             should be ignored on receipt.

Estrin, et al Informational [Page 7] RFC 1940 SDRv1 May 1996

             This document defines the following values for the
             Notification Code:
             1 - No Route Available
             2 - Strict Source Route Failed
             3 - Transit Policy Violation
             4 - Hop Count Exceeded
             5 - Probe Completed
             6 - Unimplemented SDRP version
             7 - Unimplemented Source Route Protocol Type
             8 - Setup Request Rejected
    Source Route Length (1 octet)
       The Source Route Length field indicates the length in 32 bit
       words of the domain level source route carried in the SDRP
       Header.
    Next Hop Pointer (1 octet)
       The Next Hop Pointer field indicates the offset of the high-
       order byte of the next hop along the route that the packet has
       to be forwarded.  This offset is relative to the start of the
       Source Route field; so if the value of the Next Hop Pointer
       field equals the value of the Source Route Length field, then
       the entire source route has been completely traversed.  All
       other source routes are said to be incompletely traversed.
    Source Route (variable)
       The components of the source route are syntactically IP
       addresses.
       An IP address from network 128.0.0.0 is used to encode a next
       hop that is a domain.  The least significant two octets contain
       the DI, which is an Internet Autonomous System number.

Estrin, et al Informational [Page 8] RFC 1940 SDRv1 May 1996

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      128      .       0       |             D. I.             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       An IP address from the network 127.0.0.0 is used to encode
       characteristics of the source route.  The least significant
       three octets are used as a Source Route Change field.
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      127      |          Source     Route     Change          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       Source Route Change (3 octets)
          Loose/Strict Source Route Change (bit 1)
             The Loose/Strict Source Route Change bit reflects a new
             value of the Loose/Strict Source Route bit in the SDRP
             header.  The value of the Loose/Strict Source Route
             Change bit is copied into the Loose/Strict Source Route
             bit in the SDRP header when a Source Route Change field
             is encountered in processing an SDRP packet.
          The rest of the Source Route Change field is transmitted as
          zero, and should be ignored on receipt.
    Payload (variable)
       The Payload field carries the datagram originated by the end-
       system within the domain that constructed the SDRP packet. The
       Payload field forms the data portion of the SDRP packet.  In a
       control packet this field may be empty or may carry the payload
       header of the packet that triggered the control message (see
       5.2.5).  Note that there is no padding between the Source Route
       and the Payload, and that the Payload may start at any
       arbitrary octet boundary.

Estrin, et al Informational [Page 9] RFC 1940 SDRv1 May 1996

4. Originating SDRP Data packets

 This document assumes that a router that originates SDRP packets is
 preconfigured with a set of SDRP routes.  Procedures for constructing
 these routes are outside the scope of this document.  SDRP packet
 forwarding may be deployed initially without additional routing
 protocol support.
 An application on a source host generates packets that must be
 delivered to a given destination.  The packet traverses the Internet
 by following normal hop-by-hop routing information.  An intermediate
 router in the path between the source host and the destination host
 may decide to forward some of these packets via SDRP.
 When this router receives an IP datagram, the router uses the
 information in the datagram and the local criteria to determine
 whether the datagram should be forwarded along a particular SDRP
 route.  Associated with each set of criteria is a set of one or more
 SDRP routes that should be used to route matching packets.  The exact
 nature of the criteria is a local matter.  The only restrictions this
 document places on the applicability of SDRP routes is that an IP
 datagram that contains a strict source route should not be forwarded
 along an SDRP route, that SDRP encapsulation should never be applied
 to an SDRP packet, and that if SDRP is used with inter-domain routes,
 the destination domain must also run SDRP.
 If the router decides to forward a datagram along a particular SDRP
 route, the router constructs the SDRP packet by placing the original
 datagram into the Payload field of the SDRP packet and constructing
 the SDRP header based on the selected SDRP route.  The Next Hop
 pointer is set to 0 (the first entry in the Source Route field of the
 SDRP packet).  The value of the Time To Live field in the payload
 header should be copied into the Hop Count field of the SDRP header.
 Even if we assume that interior routing is loop free, it is possible,
 either due to the state of inter-domain routing or due to other SDRP
 routers, that a domain level source route that does not terminate
 with the intended destination domain may lead a packet into a routing
 loop.  Originating SDRP routers that wish to insure that this does
 not occur should include a final domain level hop of the
 destination's domain, i.e. specify the SDRP route as <DI1, DI2, DI3>
 instead of <DI1, DI2>, if the destination host is in domain DI3.  The
 means for determining the DI of the destination domain is outside of
 the scope of this document.
 Similarly, when using SDRP for interior routing, it is possible that
 the source route does not coincide with IGP routing.  In this case,
 one means of preventing a loop is to specify the last hop router's IP

Estrin, et al Informational [Page 10] RFC 1940 SDRv1 May 1996

 address as the last address within the source route.  The
 encapsulating router can do this by specifying the source route to
 reach destination host IP3 as <IP1, IP2, IP3> instead of <IP1, IP2>.
 The source address field in the delivery header should contain an IP
 address of the router. The value of the Don't Fragment flag of the
 delivery header is copied from the Don't Fragment flag of the payload
 header.  The value of the Type Of Service field in the delivery
 header is copied from the Type Of Service field in the payload
 header.  If the payload header contains an IP security option, that
 option is replicated as an option in the delivery header.  All other
 IP options in the payload header must be ignored.
 If the SDRP route that is used is learned from IDRP, then the TOS
 corresponding to this route is copied into the TOS field in the
 delivery header.
 The resulting SDRP packet is then forwarded as described in Section
 5.2.2.
 If the encapsulating router decides to forward a datagram along a
 particular SDRP route that has an MTU smaller than the length of the
 datagram, then if the payload header has the Don't Fragment flag set
 to 1, the router should generate an ICMP Destination Unreachable
 message with a code meaning "fragmentation needed and DF set" in
 accordance with [6].  The ICMP message must be sent to the original
 source host.  The router should then discard the original datagram.
 If a router has learned an MTU for a particular SDRP route, either
 via ICMP messages or via configuration information, and it determines
 that an SDRP packet must be fragmented before transmission, then it
 first calculates the the effective MTU seen by the payload packet.
 If the effective MTU is greater than or equal to 512 bytes, the
 router SHOULD first fragment the payload packet using normal IP
 fragmentation.  SDRP packets are then constructed for each fragment,
 as describe above.  Otherwise, the router should first form the SDRP
 packet, and then fragment it.
 A router may use locally originated  SDRP packets to verify the
 feasibility of its SDRP routes. To do this the router sets the value
 of the Probe Indicator field in the SDRP packet to 1.  Receipt of an
 SDRP control packet by the originating router with the "Probe
 Completed" Notification Code (see Section 7.1) indicates feasibility
 of the SDRP route.  Persistent lack of SDRP control packets with the
 "Probe Completed" Notification Code should be used as an indication
 that the associated SDRP route is not feasible.

Estrin, et al Informational [Page 11] RFC 1940 SDRv1 May 1996

5. Processing SDRP packets

 We say that a router receives an SDRP packet if the destination
 address field in the delivery header of the packet arriving at the
 router contains one of the IP addresses of the router.
 When a router receives an SDRP packet, the router extracts the Source
 Route Protocol field from the SDRP header.

5.1 Supporting Transit Policies

 A router may be able to verify that a packet that it is given to
 forward does not violate any of the transit policies that may exist,
 of the domain to which the router belongs.  Specific verification
 mechanisms are a matter that is local to the router and are outside
 the scope of this document.
 The restriction on the verification mechanisms is that they may take
 into account only the contents of the SDRP header, the payload
 header, and transport protocol header of the payload packet.
 With SDRP a domain may enforce its transit policies by applying
 filters based on the information present in the IP Header. For
 example a router may initially carefully filter all SDRP traffic from
 all possible sources. A filter that allows certain SDRP traffic from
 selected sources to pass through the router could then be installed
 dynamically to pass similar types of traffic.  Thus, by caching
 appropriate filtering information, a transit domain can efficiently
 support transit policies.  Other mechanisms for supporting transit
 policy and implementation techniques are not precluded by this
 document.
 If the router detects that the SDRP packet violates a domain's
 transit policy it sends back an SDRP control packet to the
 encapsulating router and discards the violating packet.
 SDRP control packets are not subject to transit policies.
 If a router does not discard an SDRP packet due to a transit policy
 violation, then the router attempts to forward it as specified in
 Section 5.2.

5.2 Forwarding SDRP packets

 Procedures for forwarding of an SDRP packet depend on
    a) whether the router has the routing information needed to
       forward the packet;

Estrin, et al Informational [Page 12] RFC 1940 SDRv1 May 1996

    b) whether the SDRP route has been completely traversed;
    c) whether the SDRP route is strict or loose, and
    d) whether the packet is a data or control packet.
 When forwarding an SDRP packet (either data or control) a router
 should not modify the following fields in the delivery header:
    a) Source Address
    b) Don't Fragment flag
 If the Source Route Protocol Type of a packet indicates a Route Setup
 and the router does not or cannot support setup, the router MAY send
 the encapsulating router a control packet with a Notification Code of
 Setup Request Rejected.  It MAY then modify the data packet so that
 the Source Route Protocol Type is Explicit Source Route and the Probe
 Indicator bit is 0, then forwards the packet as described below.  The
 router MAY send notification of a failed setup request only
 periodically.  Alternately, a router MAY silently drop the Route
 Setup packet.

5.2.1 Forwarding algorithm pseudo-code

 The following pseudo-code gives an overview of the SDRP forwarding
 algorithm.  Please consult the text below for more details.
 Let LOCAL_DI be the DI of the domain of the local system, let
 NEXT_HOP be the next hop in the source route if the source route has
 not been completely traversed, let NEXT_DI be the DI portion of
 NEXT_HOP if NEXT_HOP is from network 128.0.0.0, and let NEXT_ROUTER
 be the IP address of the next router if the packet is to be forwarded
 using SDRP.  We say that NEXT_DI is adjacent if the local domain is
 adjacent to the domain that has NEXT_DI as its DI, and we say that
 NEXT_ROUTER is adjacent if it represents an IP address of a router
 that shares a link with the current router.  Normal IP forwarding
 refers to forwarding that can be accomplished using FIBs constructed
 via BGP, IDRP or one or more IGPs.
 The pseudo code requires sending control messages in a number of
 places.  All such control messages must be sent to the encapsulating
 router, which is indicated in the source address of the delivery
 header.  Note too that all intermediate SDRP routers that process an
 SDRP packet must ensure that the source address of the delivery
 header is left untouched, since this source address is the address of
 the encapsulating router to which any control messages must be sent.

Estrin, et al Informational [Page 13] RFC 1940 SDRv1 May 1996

   if the packet is a control packet begin
     if the Target Router equals an address assigned to the
       local router begin
       remove the delivery header
       process information carried in the control packet
       return
     end if
     if the packet can be forwarded using normal IP forwarding begin
       set Next Hop Pointer to Source Route Length
       forward the packet using normal IP forwarding
       return
     end if
   end if
   if the version field is not 1 begin
     if the packet is a data packet begin
       generate a control packet with "Unimplemented SDRP version"
     end if
     discard the packet
     return
   end if
   if the source route protocol type is not 1 begin
     if the packet is a data packet begin
       generate a control packet with "Unimplemented source route
         protocol type"
     end if
     discard the packet
     return
   end if
   if the Hop Count field is greater than 0 begin
     decrement the Hop Count field
   end if
   if the Hop Count field is 0 begin
     if the packet is a data packet begin
       generate a control packet with "Hop Count Exceeded"
    end if
     discard the packet
     return
   end if
   if the packet is a data packet begin
     if the packet violates transit policy begin
       generate a control packet with "Transit Policy Violation"

Estrin, et al Informational [Page 14] RFC 1940 SDRv1 May 1996

       discard the data packet
       return
     end if
   end if
   set mode to NONE
   set advanced to FALSE
   if Next Hop Ptr does not equal Source Route Length begin
     set NEXT_HOP to the next hop in the source route
     while mode equals NONE begin
       if NEXT_HOP is from network 127.0.0.0 begin
         set the Loose/Strict Source Route bit equal to
             the Loose/Strict Source Route Change bit
       else if NEXT_HOP is from network 128.0.0.0 begin
         set NEXT_DI to the least significant two octets of NEXT_HOP
         if NEXT_DI is not equal to LOCAL_DI begin
           set mode to DOMAIN
         end if
       else if NEXT_HOP does not equal an address assigned to the
         local router begin
         set mode to LOCAL
       end if
       if mode equals NONE begin
         set advanced to TRUE
         increment the Next Hop Pointer field
         if Next Hop Pointer equals Source Route Length begin
           set mode to COMPLETE
         else
           set NEXT_HOP to the next hop in the source route
         end if
       end if
     end while
   end if
   if mode equals DOMAIN begin
     set route to NONE
     if the source route is loose begin
       if not advanced begin
         find the route, if any, based on Prefix and Prefix Length
         if the route is an aggregate formed at the local router begin
           set route to NONE
         end if
       end if
       if route equals NONE begin
         select a BGP or IDRP route, if any, with a path that includes
           NEXT_DI and is not an aggregate formed at the local router
         if route equals NONE begin

Estrin, et al Informational [Page 15] RFC 1940 SDRv1 May 1996

           if the packet is a data packet begin
            generate a control packet with "No Route Available"
           end if
           discard the packet
           return
         end if
         copy the NLRI from the route to the Prefix and Prefix Length
       end if
       if the route is an IDRP route begin
         set appropriate TOS in delivery header
       end if
       set NEXT_ROUTER from the route
     else
       set NEXT_ROUTER from the routing information for NEXT_DI
         using the D-FIB
       if route equals NONE begin
         if the packet is a data packet begin
           generate a control packet with "No Route Available"
         end if
         discard the packet
         return
       end if
       if NEXT_DI is not adjacent begin
         if the packet is a data packet begin
           generate a control packet with "Strict Source Route Failed"
         end if
         discard the packet
         return
       end if
     end if
     end if
   end if
   if mode equals LOCAL begin
     set NEXT_ROUTER equal to NEXT_HOP
     if the source route is strict and NEXT_ROUTER is not
       adjacent begin
       if the packet is a data packet begin
         generate a control packet with "Strict Source Route Failed"
       end if
       discard the packet
       return
     end if
   end if
   if mode equals LOCAL or mode equals DOMAIN begin
     set the destination address of the delivery header equal

Estrin, et al Informational [Page 16] RFC 1940 SDRv1 May 1996

       to NEXT_ROUTER
     checksum the delivery header
     route packet to NEXT_ROUTER using normal IP forwarding
     return
   end if
   if the packet is a control packet begin
     discard the packet
   end if
   remove the delivery header and the SDRP Header
   if there is no normal IP route to the payload destination begin
     generate a control packet with "No Route Available"
     discard the data packet
     return
   end if
   forward the payload using normal IP forwarding
   if the probe bit is set begin
     generate a control packet with "Probe Completed"
   end if

5.2.2 Handling an SDRP control packet.

 An SDRP control packet is indicated by 0 in the Data packet/Control
 packet bit in the Flags field in the SDRP Header.
 If the Target Router field of the received SDRP packet contains an IP
 address that is assigned to the router that received this SDRP
 packet, then the router should use the information carried in the
 Notification Code field, the Source Route Identifier field and the
 information carried in the Payload field to update the status of its
 SDRP routes. Details of such procedures are described in Section 7.
 Otherwise, the router checks whether it can forward the packet to the
 router specified in the Target Router field by using the routing
 information present in its local FIB. If forwarding is possible then
 the local system sets the destination address of the delivery header
 to the address specified in the Target Router field, and hands the
 packet off for normal IP forwarding.  If normal IP forwarding is
 impossible then the packet may be forwarded in the same manner as an
 SDRP data packet (described below) but with the following exceptions.
  1. Control packets are not subject to transit policies.
  2. In no case should a control packet be generated in response to

an error caused by a control packet.

  1. If the source route is completely traversed and the packet still

cannot be forwarded via normal IP routing, the packet should be

      silently dropped.

Estrin, et al Informational [Page 17] RFC 1940 SDRv1 May 1996

5.2.3 Handling an SDRP data packet.

 An SDRP data packet is indicated by a one in the Data packet/Control
 packet bit in the Flags field in the SDRP Header.
 An SDRP data packet is forwarded by sending the packet along the
 source route in the SDRP Header.  When the source route is completely
 traversed and the packet has reached the destination domain, the
 payload may be removed from the data packet and forwarded normally.
 Further details are described below.

5.2.4 Checking the SDRP version number

 An SDRP packet that has a version number other than 1 should be
 discarded.  If the SDRP packet was a data packet, then a control
 packet with the Notification Code "Unimplemented SDRP version" should
 be generated as specified in section 6.

5.2.5 Checking the Source Route Protocol Type

 This document describes Source Route Protocol Type 1.  An SDRP router
 may support multiple Source Route Protocol Types; however an SDRP
 router is NOT required to support all defined Source Route Types.
 Any packet that has a Source Route Protocol Type which is not
 supported should be discarded.  If the SDRP packet was a data packet,
 then a control packet with the Notification Code "Unimplemented
 Source Route Protocol Type" should be generated as specified in
 section 6.

5.2.6 Decrementing and checking Hop Count

 If an SDRP packet is to be forwarded and the Hop Count field is non-
 zero, the Hop Count field should be decremented.  If the resulting
 value is zero and the packet was a data packet, then a control packet
 with the Notification Code "Hop Count Exceeded" should be generated
 and sent to the encapsulating router as specified in section 6, and
 the packet should be discarded.  If the resulting value is zero and
 the packet was a control packet, the packet should be discarded.  The
 payload of the control packet should carry the payload header
 followed by 64 bits of the payload data of the data packet.

5.2.7 Upholding transit policies

 It is not a goal of SDRP to create a security routing system.
 Therefore, we need to qualify our use of the term "upholding transit
 policy".  It is assumed that transit policies have the nature of a
 "gentleperson's agreement", and are upheld by all the participants.
 In other words, it is assumed that there will be no malicious

Estrin, et al Informational [Page 18] RFC 1940 SDRv1 May 1996

 attempts to violate transit policies and that parties will rely on
 auditing and post facto detection of violations. When a security
 architecture is developed for IP or other network protocols then it
 may be applied to increase the assurance of transit policy
 enforcement. These issues are beyond the scope of this document.
 A router may examine any data packet to verify if it complies with
 local transit policies, as described in section 5.1.  If the
 verification fails, the router generates a control packet.  If the
 verification referred to only the contents of the SDRP header, then
 the payload field of the control packet should be empty. If the
 verification referred to both the contents of the SDRP header and the
 payload header, then the payload field of the control packet should
 carry the payload header.  If the verification referred to the
 transport protocol header, then the payload field of the control
 packet should carry the payload header and the transport header.
 The Notification Code field of the SDRP header in the control packet
 is set to Transit Policy Violation.  The procedures for constructing
 the rest of the SDRP Header of the control packet are specified in
 Section 6.

5.2.8 Partially traversed source routes

 If a router receives an SDRP packet with a partially traversed source
 route, it extracts the next hop of the source route from the Source
 Route field. The router locates the high-order byte of the
 appropriate hop by using the Next Hop Pointer field as a 32 bit word
 offset relative to the start of the Source Route field.  The next hop
 is always four octets long.  The following procedure is used to
 interpret the next hop.
 Syntactically, each element in the source route appears as an IP
 address.  There are three encodings for the next hop:
 a) The next hop is an address in network 127.0.0.0.  In this case,
 the Loose/Strict Source Route field is set equal to the Loose/Strict
 Source Route Change bit.  Then the Next Hop Pointer is incremented,
 the next hop is read from the Source Route field, and these three
 cases are examined again.
 b) The next hop is an address in network 128.0.0.0.  In this case,
 the DI of the next domain is extracted from the least significant two
 octets of the next hop.  If the extracted DI is the same as the DI of
 the local domain, then the Next Hop Pointer is incremented, the next
 hop is read from the Source Route field, and these three cases are
 examined again.  Otherwise, if the extracted DI is different from the
 DI of the local domain, the next hop is the extracted DI, and the

Estrin, et al Informational [Page 19] RFC 1940 SDRv1 May 1996

 forwarding process may proceed.
 c) The next hop is any other IP address.  If the next hop is equal to
 any IP address assigned to the local router, the Next Hop Pointer is
 incremented, the next hop is read from the Source Route field, and
 these three cases examined again.  Otherwise, the next hop is the IP
 address of the next router in the source route and the forwarding
 process may proceed.
 The above procedure for interpreting the next hop in the source route
 finishes when the next hop is either a router other than the local
 router or an encoded DI that is not the local DI or a completed
 source route.
 If upon termination of this procedure the source route is completely
 traversed, see section 5.2.9.

5.2.8.1 Finding a route to the next hop

 If the next hop is not a DI, then the destination address in the
 delivery header is replaced by the next hop address and the resulting
 packet can then be forwarded using normal IP forwarding.  Otherwise,
 a DI was extracted from the next hop in the source route, and the
 following procedure is used to find a route to the next domain.
 Given the DI of the next domain, the router next consults its D-FIB.
 If no entry exists in the D-FIB for the next domain, then the packet
 should be discarded.  If the packet was a data packet, a control
 message with Notification Code "No Route Available" should be
 generated as specified in Section 6. No other actions are necessary.
 If there is a D-FIB entry, the router next examines the SDRP header
 to determine if the packet specified a strict source route.  If so,
 and the next domain is not adjacent to the local domain, then a
 control packet with the Notification Code "Strict Source Route
 Failed" should be generated, as specified in section 6, and the
 original packet should be discarded.  No other actions are necessary.
 If source route is loose, then BGP or IDRP information must be used
 to insure that there is no loop in reaching the next hop.  If the
 Next Hop Pointer was incremented when determining the next hop, then
 the router must select a BGP or IDRP route with a path that includes
 the extracted DI, and the NLRI for this route is copied into the
 Prefix Length and Prefix fields.
 Otherwise, the Next Hop Pointer was not incremented, and the router
 should use the information carried in the Prefix and Prefix Length as
 an index into its BGP or IDRP routing table.  If it finds a matching

Estrin, et al Informational [Page 20] RFC 1940 SDRv1 May 1996

 route then it must select the corresponding D-FIB entry.  If the
 route was formed locally by aggregation, then the router must consult
 its D-FIB and select any route with a path that includes the
 extracted DI.  The NLRI for this route should be copied into the
 Prefix Length and Prefix fields.
 In either case, the D-FIB entry includes the IP address of the next
 SDRP-speaking router to which the SDRP packet should be routed.  The
 destination address in the delivery header is replaced by this
 address.  The resulting packet can then be forwarded using normal IP
 forwarding.

5.2.8.2 Last Hop Optimization

 A small optimization can be performed if there is only a single DI or
 IP address in the source route that has not been traversed.
 In this case, if the next hop in the SDRP route is a DI, that DI is
 adjacent to the router processing this packet, the route has a route
 to the destination address in the payload header in its FIB, and this
 FIB route passes through the adjacent domain, then the source route
 may be considered completely traversed and processing may proceed as
 in section 5.2.9.
 If the next hop in the SDRP route is an IP address, that IP address
 is adjacent to the router processing this packet, the router has a
 route to the destination address in the payload header in its FIB,
 and this FIB route passes through the adjacent IP address, then the
 source route may be considered completely traversed and processing
 may proceed as in section 5.2.9.
 Since the last hop optimization may only be done if the last hop is
 directly adjacent, and reachable, it is irrelevant whether the SDRP
 route specifies that this is a strict source route or a loose source
 route hop.

5.2.9 Completely Traversed source routes

 If the SDRP packet received by a router with a completely-traversed
 source route is a control packet and if the Target Router field
 carries an IP address assigned to the router, then the packet should
 be processed as specified in Section 7.  Otherwise, if the SDRP
 packet is a control packet, and the packet cannot be forwarded via
 either SDRP or normal IP forwarding, the packet should be silently
 dropped.
 The Hop Count field has already been decremented when processing the
 SDRP header.  The Hop Count field should now be copied from the SDRP

Estrin, et al Informational [Page 21] RFC 1940 SDRv1 May 1996

 header into the IP TTL field in the payload header.  The resulting
 payload packet is then forwarded using normal IP forwarding.  If
 there is no FIB entry for the destination, then the packet should be
 discarded and a control message with Notification Code "No Route
 Available" should be generated as specified in Section 6.  If the
 packet can be forwarded and if the Probe Indication bit is set to one
 in the SDRP header, then a control message with Notification Code
 "Probe Completed" should be generated as specified in section 6. If a
 control packet is generated, then it must be sent to the
 encapsulating router.  The payload of the control packet should carry
 the first 64 bits of the SDRP header and the payload header.

6. Originating SDRP control packets

 A router sends a control packet in response to either error
 conditions, or to successful completion of a probe request (indicated
 via Probe Indication in the Flags field).
 The Data Packet/Control Packet field is set to indicate Control
 Packet.  The following fields are copied from the SDRP header of the
 Data packet that caused the generation of the Control packet:
  1. Loose/Strict Source Route
  2. Source Route Protocol Type
  3. Source Route Identifier
  4. Source Route Length field
  5. Payload Protocol Type
 A Control packet should not carry a Probe Indication field.
 A router should never originate a Control packet as the result of an
 error caused by a control packet.
 The Target Router is copied from the source IP address of the
 delivery header of the SDRP Data packet.  This causes the control
 packet to be returned to the encapsulating router.
 The router generating a control packet checks its FIB for a route to
 the destination depicted by the Target Router field.  If such a route
 is present, then the value of the Destination Address field in the
 delivery header is set to the Target Router, the Source Address field
 in the delivery header is set to the IP address of one of the
 interfaces attached to the local system, and the packet is forwarded
 via normal IP forwarding.
 If the FIB does not have a route to the destination depicted by the
 Target Router field, the local system constructs the Source Route
 field of the Control packet by reversing the SDRP route carried in

Estrin, et al Informational [Page 22] RFC 1940 SDRv1 May 1996

 the Source Route field of the Data packet, sets the value of the Next
 Hop Pointer to the value of the Source Route Length field minus the
 value of the Next Hop Pointer field of the SDRP data packet that
 caused generation of the Control Packet.  All Loose/Strict Source
 Route change bits in the new source route should be set to 0 (loose
 source route).
 The contents of the Payload field depends on the reason for
 generating a control packet.
 The resulting packet is then handled via SDRP Forwarding procedures
 described in Section 5.2.

7. Processing control information

 A router participating in SDRP may receive control information in two
 forms, SDRP control packets from other routers and ICMP messages from
 routers that do not participate in SDRP, but are involved in
 forwarding SDRP packets.

7.1 Processing SDRP control packets

 Most control packets carry information about some SDRP routes used by
 the router.  To correlate information carried in the SDRP control
 packet with the SDRP routes used by the router, the router uses
 information carried in the SDRP header of the control packet, and
 optionally in the SDRP payload of the control packet (if present).
 In general, receipt of any SDRP control packet that carries one of
 the following Notification codes
  1. No Route Available
  1. Strict Source Route Failed
  1. Unimplemented SDRP Version
  1. Unimplemented Source Route Probe Type
 indicates that the corresponding SDRP route is presently not
 feasible, and thus should not be used for packet forwarding.  The
 router must mark the affected routes as not feasible, and may use
 alternate routes if available.
 The router may at some later point attempt to use an SDRP route that
 was marked as infeasible.  The criteria used for retrying routes is
 outside the scope of this document and a subject of further study.
 It need not be standardizes and can be a matter of local control.

Estrin, et al Informational [Page 23] RFC 1940 SDRv1 May 1996

 Receipt of an SDRP control packet that carries "Probe Completed"
 Notification code indicates that the corresponding SDRP route is
 feasible.
 Receipt of an SDRP control packet that carries the "Transit Policy
 Violation" Notification Code shall be interpreted as follows:
  1. If the control packet carries no payload data then the

corresponding SDRP route violates transit policy regardless of

      the content of the payload packet carried along that route.
    - If the control packet carries only the payload header, then
      the corresponding SDRP route violates transit policy due to
      the content of the payload header.
    - If the control packet carries the payload header and the
      transport header, then the corresponding SDRP route violates
      transit policy for the particular combination of payload and
      transport header contents.
 If a router receives an SDRP control packet that carries "Hop Count
 Exceeded" Notification Code, the router should use the information in
 the payload of the Control packet to construct an ICMP Time Exceeded
 Message with code "time to live exceeded in transit" and send the
 message to the host indicated by the source address in the Payload
 Header.

7.2 Processing ICMP messages

 To correlate information carried in the ICMP messages with the SDRP
 routes used by the router, the router uses the portion of the SDRP
 datagram returned by ICMP.  This must contain the Source Route
 Identifier of the SDRP route used by the router.
 ICMP Destination Unreachable messages with a code meaning
 "fragmentation needed and DF set" should be used for SDRP MTU
 discovery as described in Section 9.
 All other ICMP Unreachable messages indicate that the associated
 route is not feasible.

8. Constructing D-FIBs.

 A BR constructs its D-FIB as a result of participating in either BGP
 or IDRP. A BR must advertise a route to destinations within its
 domain to all of its external peers (BRs in adjacent domains), via
 BGP or IDRP.  In BGP and IDRP, a BR must advertise a route to
 destinations within its domain to all of its external peers (BRs in
 adjacent domains).

Estrin, et al Informational [Page 24] RFC 1940 SDRv1 May 1996

 If a BR receives a route to an adjacent domain from a BR in that
 domain and selects that route as part of its BGP or IDRP Decision
 Process, then it must propagate this route (via BGP or IDRP) to all
 other BRs within its domain.  A BR may also propagate such a route if
 it depicts an autonomous system other than the adjacent domain.
 Since AS numbers are encoded as network numbers in network 128.0.0.0,
 it is possible to also advertise a route to a domain in BGP or IDRP.

9. SDRP MTU Discovery

 To participate in Path MTU Discovery ([6]) a router may maintain
 information about the maximum length of the payload packet that can
 be carried without fragmentation along a particular SDRP route.
 SDRP provides two complimentary techniques to support MTU Discovery.
 The first one is passive and is based on the receipt of the ICMP
 Destination Unreachable messages (as described in Section 7.2).  By
 combining information provided in the ICMP message with local
 information about the SDRP route the local system can determine the
 length of a payload packet that would require fragmentation.
 The second one is active and employs the Probe Indicator bit.  If an
 SDRP data packet that carries the Probe Indicator bit in the SDRP
 header and Don't Fragment flag in the delivery header triggers the
 last router on the SDRP route to return an SDRP Control packet (with
 the Notification Code "Probe Completed"), then the information
 carried in the payload header of the control packet can be used to
 determine the length of the payload packet that went through the SDRP
 route without fragmentation.

10. Acknowledgments

 The authors would like to thank Scott Bradner (Harvard University),
 Noel Chiappa (Consultant), Joel Halpern (Newbridge Networks),
 Christian Huitema (INRIA), and Curtis Villamizar (ANS) for their
 comments on various aspects of this document.

Security Considerations

 Security issues are not discussed in this memo.

Estrin, et al Informational [Page 25] RFC 1940 SDRv1 May 1996

Authors' Addresses

 Deborah Estrin
 USC/Information Sciences Institute
 4676 Admiralty Way
 Marina Del Rey, Ca 90292-6695.
 Phone: +1 310 822 1511 x 253
 EMail: estrin@isi.edu
 Tony Li
 cisco Systems, Inc.
 1525 O'Brien Drive
 Menlo Park, CA 94025
 Phone: +1 415 526 8186
 EMail: tli@cisco.com
 Yakov Rekhter
 Cisco systems
 170 West Tasman Drive
 San Jose, CA, USA
 Phone: +1 914 528 0090
 Fax: +1 408 526-4952
 EMail: yakov@cisco.com
 Kannan Varadhan
 USC/Information Sciences Institute
 4676 Admiralty Way
 Marina Del Rey, Ca 90292-6695.
 Phone: +1 310 822 1511 x 402
 EMail: kannan@isi.edu
 Daniel Zappala
 USC/Information Sciences Institute
 4676 Admiralty Way
 Marina Del Rey, Ca 90292-6695.
 Phone: +1 310 822 1511 x 352
 EMail: daniel@isi.edu

Estrin, et al Informational [Page 26] RFC 1940 SDRv1 May 1996

References

 [1] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol 3
     (BGP-3), RFC 1267, October 1991.
 [2] Rekhter, Y., and P. Gross, "Application of the Border Gateway
     Protocol in the Internet", RFC 1268, October 1991.
 [3] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
     RFC 1654, July 1994.
 [4] Hares, S., "IDRP for IP", IDR Working Group, 1994.
     Work in Progress.
 [5] Postel, J., "Internet Protocol - DARPA Internet Program
     Protocol Specification", STD 5, RFC 791, September 1981.
 [6] Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
     November 1990.
 [7] Reynolds, J., and J. Postel, "ASSIGNED NUMBERS", STD 2,
     RFC 1700, October 1994.

Estrin, et al Informational [Page 27]

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