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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 5533

SHIM6 WG                                                     E. Nordmark
Internet-Draft                                          Sun Microsystems
Expires: April 26, 2007                                       M. Bagnulo
                                                                    UC3M
                                                        October 23, 2006


                   Level 3 multihoming shim protocol
                     draft-ietf-shim6-proto-06.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   This Internet-Draft will expire on April 26, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The SHIM6 protocol is a layer 3 shim for providing locator agility
   below the transport protocols, so that multihoming can be provided
   for IPv6 with failover and load sharing properties, without assuming
   that a multihomed site will have a provider independent IPv6 address
   prefix which is announced in the global IPv6 routing table.  The
   hosts in a site which has multiple provider allocated IPv6 address
   prefixes, will use the shim6 protocol specified in this document to



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   setup state with peer hosts, so that the state can later be used to
   failover to a different locator pair, should the original one stop
   working.


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.2.  Non-Goals . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.3.  Locators as Upper-layer Identifiers . . . . . . . . . . .   6
     1.4.  IP Multicast  . . . . . . . . . . . . . . . . . . . . . .   7
     1.5.  Renumbering Implications  . . . . . . . . . . . . . . . .   8
     1.6.  Placement of the shim . . . . . . . . . . . . . . . . . .   9
     1.7.  Traffic Engineering . . . . . . . . . . . . . . . . . . .  10
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .  12
     2.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .  12
     2.2.  Notational Conventions  . . . . . . . . . . . . . . . . .  15
   3.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . . .  16
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .  17
     4.1.  Context Tags  . . . . . . . . . . . . . . . . . . . . . .  19
     4.2.  Context Forking . . . . . . . . . . . . . . . . . . . . .  19
     4.3.  API Extensions  . . . . . . . . . . . . . . . . . . . . .  20
     4.4.  Securing shim6  . . . . . . . . . . . . . . . . . . . . .  20
     4.5.  Overview of Shim Control Messages . . . . . . . . . . . .  21
     4.6.  Extension Header Order  . . . . . . . . . . . . . . . . .  22
   5.  Message Formats . . . . . . . . . . . . . . . . . . . . . . .  24
     5.1.  Common shim6 Message Format . . . . . . . . . . . . . . .  24
     5.2.  Payload Extension Header Format . . . . . . . . . . . . .  24
     5.3.  Common Shim6 Control header . . . . . . . . . . . . . . .  25
     5.4.  I1 Message Format . . . . . . . . . . . . . . . . . . . .  27
     5.5.  R1 Message Format . . . . . . . . . . . . . . . . . . . .  28
     5.6.  I2 Message Format . . . . . . . . . . . . . . . . . . . .  29
     5.7.  R2 Message Format . . . . . . . . . . . . . . . . . . . .  31
     5.8.  R1bis Message Format  . . . . . . . . . . . . . . . . . .  33
     5.9.  I2bis Message Format  . . . . . . . . . . . . . . . . . .  34
     5.10. Update Request Message Format . . . . . . . . . . . . . .  36
     5.11. Update Acknowledgement Message Format . . . . . . . . . .  38
     5.12. Keepalive Message Format  . . . . . . . . . . . . . . . .  39
     5.13. Probe Message Format  . . . . . . . . . . . . . . . . . .  39
     5.14. Option Formats  . . . . . . . . . . . . . . . . . . . . .  40
       5.14.1.  Responder Validator Option Format  . . . . . . . . .  42
       5.14.2.  Locator List Option Format . . . . . . . . . . . . .  42
       5.14.3.  Locator Preferences Option Format  . . . . . . . . .  44
       5.14.4.  CGA Parameter Data Structure Option Format . . . . .  46
       5.14.5.  CGA Signature Option Format  . . . . . . . . . . . .  46
       5.14.6.  ULID Pair Option Format  . . . . . . . . . . . . . .  47
       5.14.7.  Forked Instance Identifier Option Format . . . . . .  48



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       5.14.8.  Probe Option Format  . . . . . . . . . . . . . . . .  48
       5.14.9.  Reachability Option Format . . . . . . . . . . . . .  49
       5.14.10. Payload Reception Report Option Format . . . . . . .  49
   6.  Conceptual Model of a Host  . . . . . . . . . . . . . . . . .  50
     6.1.  Conceptual Data Structures  . . . . . . . . . . . . . . .  50
     6.2.  Context States  . . . . . . . . . . . . . . . . . . . . .  51
   7.  Establishing ULID-Pair Contexts . . . . . . . . . . . . . . .  53
     7.1.  Uniqueness of Context Tags  . . . . . . . . . . . . . . .  53
     7.2.  Locator Verification  . . . . . . . . . . . . . . . . . .  53
     7.3.  Normal context establishment  . . . . . . . . . . . . . .  54
     7.4.  Concurrent context establishment  . . . . . . . . . . . .  54
     7.5.  Context recovery  . . . . . . . . . . . . . . . . . . . .  56
     7.6.  Context confusion . . . . . . . . . . . . . . . . . . . .  58
     7.7.  Sending I1 messages . . . . . . . . . . . . . . . . . . .  59
     7.8.  Retransmitting I1 messages  . . . . . . . . . . . . . . .  59
     7.9.  Receiving I1 messages . . . . . . . . . . . . . . . . . .  60
     7.10. Sending R1 messages . . . . . . . . . . . . . . . . . . .  61
       7.10.1.  Generating the R1 Validator  . . . . . . . . . . . .  61
     7.11. Receiving R1 messages and sending I2 messages . . . . . .  62
     7.12. Retransmitting I2 messages  . . . . . . . . . . . . . . .  63
     7.13. Receiving I2 messages . . . . . . . . . . . . . . . . . .  63
     7.14. Sending R2 messages . . . . . . . . . . . . . . . . . . .  65
     7.15. Match for Context Confusion . . . . . . . . . . . . . . .  65
     7.16. Receiving R2 messages . . . . . . . . . . . . . . . . . .  66
     7.17. Sending R1bis messages  . . . . . . . . . . . . . . . . .  67
       7.17.1.  Generating the R1bis Validator . . . . . . . . . . .  67
     7.18. Receiving R1bis messages and sending I2bis messages . . .  68
     7.19. Retransmitting I2bis messages . . . . . . . . . . . . . .  69
     7.20. Receiving I2bis messages and sending R2 messages  . . . .  69
   8.  Handling ICMP Error Messages  . . . . . . . . . . . . . . . .  71
   9.  Teardown of the ULID-Pair Context . . . . . . . . . . . . . .  73
   10. Updating the Peer . . . . . . . . . . . . . . . . . . . . . .  74
     10.1. Sending Update Request messages . . . . . . . . . . . . .  74
     10.2. Retransmitting Update Request messages  . . . . . . . . .  74
     10.3. Newer Information While Retransmitting  . . . . . . . . .  75
     10.4. Receiving Update Request messages . . . . . . . . . . . .  75
     10.5. Receiving Update Acknowledgement messages . . . . . . . .  77
   11. Sending ULP Payloads  . . . . . . . . . . . . . . . . . . . .  78
     11.1. Sending ULP Payload after a Switch  . . . . . . . . . . .  78
   12. Receiving Packets . . . . . . . . . . . . . . . . . . . . . .  80
     12.1. Receiving Payload Extension Headers . . . . . . . . . . .  80
     12.2. Receiving Shim Control messages . . . . . . . . . . . . .  80
     12.3. Context Lookup  . . . . . . . . . . . . . . . . . . . . .  81
   13. Initial Contact . . . . . . . . . . . . . . . . . . . . . . .  83
   14. Protocol constants  . . . . . . . . . . . . . . . . . . . . .  84
   15. Implications Elsewhere  . . . . . . . . . . . . . . . . . . .  85
   16. Security Considerations . . . . . . . . . . . . . . . . . . .  87
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  89



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   18. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  91
   Appendix A.     Possible Protocol Extensions  . . . . . . . . . .  92
   Appendix B.     Simplified State Machine  . . . . . . . . . . . .  94
   Appendix B.1.   Simplified State Machine diagram  . . . . . . . . 100
   Appendix C.     Context Tag Reuse . . . . . . . . . . . . . . . . 101
   Appendix C.1.   Context Recovery  . . . . . . . . . . . . . . . . 101
   Appendix C.2.   Context Confusion . . . . . . . . . . . . . . . . 101
   Appendix C.3.   Three Party Context Confusion . . . . . . . . . . 102
   Appendix D.     Design Alternatives . . . . . . . . . . . . . . . 103
   Appendix D.1.   Context granularity . . . . . . . . . . . . . . . 103
   Appendix D.2.   Demultiplexing of data packets in shim6
                   communications  . . . . . . . . . . . . . . . . . 103
   Appendix D.2.1. Flow-label  . . . . . . . . . . . . . . . . . . . 104
   Appendix D.2.2. Extension Header  . . . . . . . . . . . . . . . . 106
   Appendix D.3.   Context Loss Detection  . . . . . . . . . . . . . 107
   Appendix D.4.   Securing locator sets . . . . . . . . . . . . . . 109
   Appendix D.5.   ULID-pair context establishment exchange  . . . . 112
   Appendix D.6.   Updating locator sets . . . . . . . . . . . . . . 113
   Appendix D.7.   State Cleanup . . . . . . . . . . . . . . . . . . 113
   Appendix E.     Change Log  . . . . . . . . . . . . . . . . . . . 116
   19. References  . . . . . . . . . . . . . . . . . . . . . . . . . 120
     19.1. Normative References  . . . . . . . . . . . . . . . . . . 120
     19.2. Informative References  . . . . . . . . . . . . . . . . . 120
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 122
   Intellectual Property and Copyright Statements  . . . . . . . . . 123


























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1.  Introduction

   This document describes a layer 3 shim approach and protocol for
   providing locator agility below the transport protocols, so that
   multihoming can be provided for IPv6 with failover and load sharing
   properties [15], without assuming that a multihomed site will have a
   provider independent IPv6 address which is announced in the global
   IPv6 routing table.  The hosts in a site which has multiple provider
   allocated IPv6 address prefixes, will use the shim6 protocol
   specified in this document to setup state with peer hosts, so that
   the state can later be used to failover to a different locator pair,
   should the original one stop working.

   We assume that redirection attacks are prevented using the mechanism
   specified in HBA [7].

   The reachability and failure detection mechanisms, including how a
   new working locator pair is discovered after a failure, is specified
   in a separate document [8] This document allocates message types and
   option types for that sub-protocol, and leaves the specification of
   the message and option formats as well as the protocol behavior to
   that document.

1.1.  Goals

   The goals for this approach is to:

   o  Preserve established communications in the presence of certain
      classes of failures, for example, TCP connections and UDP streams.

   o  Have minimal impact on upper layer protocols in general and on
      transport protocols in particular.

   o  Address the security threats in [19] through the combination of
      the HBA/CGA approach specified in a separate document [7] and
      techniques described in this document.

   o  Not require extra roundtrip up front to setup shim specific state.
      Instead allow the upper layer traffic (e.g., TCP) to flow as
      normal and defer the setup of the shim state until some number of
      packets have been exchanged.

   o  Take advantage of multiple locators/addresses for load spreading
      so that different sets of communication to a host (e.g., different
      connections) might use different locators of the host.  Note that
      this might cause load to be spread unevenly, thus we use the term
      "load spreading" instead of "load balancing".  This capability
      might enable some forms of traffic engineering, but the details



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      for traffic engineering, including what requirements can be
      satisfied, are not specified in this document, and form part of a
      potential extensions to this protocol.

1.2.  Non-Goals

   The assumption is that the problem we are trying to solve is site
   multihoming, with the ability to have the set of site prefixes change
   over time due to site renumbering.  Further, we assume that such
   changes to the set of locator prefixes can be relatively slow and
   managed; slow enough to allow updates to the DNS to propagate (since
   the protocol defined in this document depends on the DNS to find the
   appropriate locator sets).  Note, however that it is an explicit non-
   goal to make communication survive a renumbering event (which causes
   all the locators of a host to change to a new set of locators).  This
   proposal does not attempt to solve the related problem of host
   mobility.  However, it might turn out that the shim6 protocol can be
   a useful component for future host mobility solutions, e.g., for
   route optimization.

   Finally, this proposal also does not try to provide a new network
   level or transport level identifier name space distinct from the
   current IP address name space.  Even though such a concept would be
   useful to Upper Layer Protocols (ULPs) and applications, especially
   if the management burden for such a name space was negligible and
   there was an efficient yet secure mechanism to map from identifiers
   to locators, such a name space isn't necessary (and furthermore
   doesn't seem to help) to solve the multihoming problem.

1.3.  Locators as Upper-layer Identifiers

   The approach described in this document does not introduce a new
   identifier name space but instead uses the locator that is selected
   in the initial contact with the remote peer as the preserved Upper-
   Layer Identifier (ULID).  While there may be subsequent changes in
   the selected network level locators over time in response to failures
   in using the original locator, the upper level protocol stack
   elements will continue to use this upper level identifier without
   change.

   This implies that the ULID selection is performed as today's default
   address selection as specified in RFC 3484 [12].  Some extensions are
   needed to RFC 3484 to try different source addresses, whether or not
   the shim6 protocol is used, as outlined in [13].  Underneath, and
   transparently, the multihoming shim selects working locator pairs
   with the initial locator pair being the ULID pair.  If communication
   subsequently fails the shim can test and select alternate locators.
   A subsequent section discusses the issues when the selected ULID is



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   not initially working hence there is a need to switch locators up
   front.

   Using one of the locators as the ULID has certain benefits for
   applications which have long-lived session state or performs
   callbacks or referrals, because both the FQDN and the 128-bit ULID
   work as handles for the applications.  However, using a single 128-
   bit ULID doesn't provide seamless communication when that locator is
   unreachable.  See [22] for further discussion of the application
   implications.

   There has been some discussion of using non-routable addresses, such
   as Unique-Local Addresses (ULAs) [18], as ULIDs in a multihoming
   solution.  While this document doesn't specify all aspects of this,
   it is believed that the approach can be extended to handle the non-
   routable address case..  For example, the protocol already needs to
   handle ULIDs that are not initially reachable.  Thus the same
   mechanism can handle ULIDs that are permanently unreachable from
   outside their site.  The issue becomes how to make the protocol
   perform well when the ULID is known a priori to be not reachable
   (e.g., the ULID is a ULA), for instance, avoiding any timeout and
   retries in this case.  In addition one would need to understand how
   the ULAs would be entered in the DNS to avoid a performance impact on
   existing, non-shim6 aware, IPv6 hosts potentially trying to
   communicate to the (unreachable) ULA.

1.4.  IP Multicast

   IP Multicast requires that the IP source address field contain a
   topologically correct locator for interface that is used to send the
   packet, since IP multicast routing uses both the source address and
   the destination group to determine where to forward the packet.  In
   particular, it need to be able to do the RPF check.  (This isn't much
   different than the situation with widely implemented ingress
   filtering [10] for unicast.)

   While in theory it would be possible to apply the shim re-mapping of
   the IP address fields between ULIDs and locators, the fact that all
   the multicast receivers would need to know the mapping to perform,
   makes such an approach difficult in practice.  Thus it makes sense to
   have multicast ULPs operate directly on locators and not use the
   shim.  This is quite a natural fit for protocols which use RTP [14],
   since RTP already has an explicit identifier in the form of the SSRC
   field in the RTP headers.  Thus the actual IP address fields are not
   important to the application.

   In summary, IP multicast will not need the shim to remap the IP
   addresses.



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   This doesn't prevent the receiver of multicast to change its
   locators, since the receiver is not explicitly identified; the
   destination address is a multicast address and not the unicast
   locator of the receiver.

1.5.  Renumbering Implications

   As stated above, this approach does not try to make communication
   survive renumbering in the general case.

   When a host is renumbered, the effect is that one or more locators
   become invalid, and zero or more locators are added to the host's
   network interface.  This means that the set of locators that is used
   in the shim will change, which the shim can handle as long as not all
   the original locators become invalid at the same time and depending
   on the time that is required to update the DNS and for those updates
   to propagate.

   But IP addresses are also used as ULID, and making the communication
   survive locators becoming invalid can potentially cause some
   confusion at the upper layers.  The fact that a ULID might be used
   with a different locator over time open up the possibility that
   communication between two ULIDs might continue to work after one or
   both of those ULIDs are no longer reachable as locators, for example
   due to a renumbering event.  This opens up the possibility that the
   ULID (or at least the prefix on which it is based) is reassigned to
   another site while it is still being used (with another locator) for
   existing communication.

   In the worst case we could end up with two separate hosts using the
   same ULID while both of them are communicating with the same host.

   This potential source for confusion can be avoided if we require that
   any communication using a ULID must be terminated when the ULID
   becomes invalid (due to the underlying prefix becoming invalid).  If
   that behavior is desired, it can be accomplished by explicitly
   discarding the shim state when the ULID becomes invalid.  The context
   recovery mechanism will then make the peer aware that the context is
   gone, and that the ULID is no longer present at the same locator(s).












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1.6.  Placement of the shim

                            -----------------------
                            | Transport Protocols |
                            -----------------------

             ------ ------- -------------- -------------     IP endpoint
             | AH | | ESP | | Frag/reass | | Dest opts |     sub-layer
             ------ ------- -------------- -------------

                         ---------------------
                         | shim6 shim layer |
                         ---------------------

                                ------                      IP routing
                                | IP |                      sub-layer
                                ------

   Figure 1: Protocol stack

   The proposal uses a multihoming shim layer within the IP layer, i.e.,
   below the ULPs, as shown in Figure 1, in order to provide ULP
   independence.  The multihoming shim layer behaves as if it is
   associated with an extension header, which would be placed after any
   routing-related headers in the packet (such as any hop-by-hop
   options, or routing header).  However, when the locator pair is the
   ULID pair there is no data that needs to be carried in an extension
   header, thus none is needed in that case.

   Layering AH and ESP above the multihoming shim means that IPsec can
   be made to be unaware of locator changes the same way that transport
   protocols can be unaware.  Thus the IPsec security associations
   remain stable even though the locators are changing.  This means that
   the IP addresses specified in the selectors should be the ULIDs.

   Layering the fragmentation header above the multihoming shim makes
   reassembly robust in the case that there is broken multi-path routing
   which results in using different paths, hence potentially different
   source locators, for different fragments.  Thus, effectively the
   multihoming shim layer is placed between the IP endpoint sublayer,
   which handles fragmentation, reassembly, and IPsec, and the IP
   routing sublayer, which selects which next hop and interface to use
   for sending out packets.

   Applications and upper layer protocols use ULIDs which the shim6
   layer map to/from different locators.  The shim6 layer maintains
   state, called ULID-pair context, per ULID pairs (that is, applies to
   all ULP connections between the ULID pair) in order to perform this



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   mapping.  The mapping is performed consistently at the sender and the
   receiver so that ULPs see packets that appear to be sent using ULIDs
   from end to end.  This property is maintained even though the packets
   travel through the network containing locators in the IP address
   fields, and even though those locators may be changed by the
   transmitting shim6 layer. .

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

   ----------------------------          ----------------------------
   | Sender A                 |          | Receiver B               |
   |                          |          |                          |
   |     ULP                  |          |     ULP                  |
   |      | src ULID(A)=L1(A) |          |      ^                   |
   |      | dst ULID(B)=L1(B) |          |      | src ULID(A)=L1(A) |
   |      v                   |          |      | dst ULID(B)=L1(B) |
   |   multihoming shim       |          |   multihoming shim       |
   |      | src L2(A)         |          |      ^                   |
   |      | dst L3(B)         |          |      | src L2(A)         |
   |      v                   |          |      | dst L3(B)         |
   |      IP                  |          |      IP                  |
   ----------------------------          ----------------------------
          |                                     ^
          ------- cloud with some routers -------

   Figure 2: Mapping with changed locators

   The result of this consistent mapping is that there is no impact on
   the ULPs.  In particular, there is no impact on pseudo-header
   checksums and connection identification.

   Conceptually, one could view this approach as if both ULIDs and
   locators are being present in every packet, and with a header
   compression mechanism applied that removes the need for the ULIDs to
   be carried in the packets once the compression state has been
   established.  In order for the receiver to recreate a packet with the
   correct ULIDs there is a need to include some "compression tag" in
   the data packets.  This serves to indicate the correct context to use
   for decompression when the locator pair in the packet is insufficient
   to uniquely identify the context.

1.7.  Traffic Engineering

   At the time of this writing it is not clear what requirements for



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   traffic engineering make sense for the shim6 protocol, since the
   requirements must both result in some useful behavior as well as be
   implementable using a host-to-host locator agility mechanism like
   shim6.

   Inherent in a scalable multihoming mechanism that separates locators
   from identifiers is that each host ends up with multiple locators.
   This means that at least for initial contact, it is the remote peer
   that needs to select which peer locator to try first.  In the case of
   shim6 this is performed by applying RFC 3484 address selection.

   This is quite different than the common case of IPv4 multihoming
   where the site has a single IP address prefix, since in that case the
   peer performs no destination address selection.

   Thus in "single prefix multihoming" the site, and in many cases its
   upstream ISPs, can use BGP to exert some control of the ingress path
   used to reach the site.  This capability can't easily be recreated in
   "multiple prefix multihoming" such as shim6.

   The protocol provides a placeholder, in the form of the Locator
   Preferences option, which can be used by hosts to express priority
   and weight values for each locator.  This is intentionally made
   identical to the DNS SRV [9] specification of priority and weight, so
   that DNS SRV records can be used for initial contact and the shim for
   failover, and they can use the same way to describe the preferences.
   But the Locator Preference option is merely a place holder when it
   comes to providing traffic engineering; in order to use this in a
   large site there would have to be a mechanism by which the host can
   find out what preference values to use, either statically (e.g., some
   new DHCPv6 option) or dynamically.

   Thus traffic engineering is listed as a possible extension in
   Appendix A.

















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2.  Terminology

   This document uses the terms MUST, SHOULD, RECOMMENDED, MAY, SHOULD
   NOT and MUST NOT defined in RFC 2119 [1].  The terms defined in RFC
   2460 [2] are also used.

2.1.  Definitions

   This document introduces the following terms:

   upper layer protocol (ULP)
                       A protocol layer immediately above IP.  Examples
                       are transport protocols such as TCP and UDP,
                       control protocols such as ICMP, routing protocols
                       such as OSPF, and internet or lower-layer
                       protocols being "tunneled" over (i.e.,
                       encapsulated in) IP such as IPX, AppleTalk, or IP
                       itself.


   interface           A node's attachment to a link.


   address             An IP layer name that contains both topological
                       significance and acts as a unique identifier for
                       an interface. 128 bits.  This document only uses
                       the "address" term in the case where it isn't
                       specific whether it is a locator or an
                       identifier.


   locator             An IP layer topological name for an interface or
                       a set of interfaces. 128 bits.  The locators are
                       carried in the IP address fields as the packets
                       traverse the network.


   identifier          An IP layer name for an IP layer endpoint.  The
                       transport endpoint name is a function of the
                       transport protocol and would typically include
                       the IP identifier plus a port number.
                       NOTE: This proposal does not specify any new form
                       of IP layer identifier, but still separates the
                       identifying and locating properties of the IP
                       addresses.






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   upper-layer identifier (ULID)
                       An IP address which has been selected for
                       communication with a peer to be used by the upper
                       layer protocol. 128 bits.  This is used for
                       pseudo-header checksum computation and connection
                       identification in the ULP.  Different sets of
                       communication to a host (e.g., different
                       connections) might use different ULIDs in order
                       to enable load spreading.

                       Since the ULID is just one of the IP locators/
                       addresses of the node, there is no need for a
                       separate name space and allocation mechanisms.


   address field       The source and destination address fields in the
                       IPv6 header.  As IPv6 is currently specified this
                       fields carry "addresses".  If identifiers and
                       locators are separated these fields will contain
                       locators for packets on the wire.


   FQDN                Fully Qualified Domain Name


   ULID-pair context   The state that the multihoming shim maintains
                       between a pair of Upper-layer identifiers.  The
                       context is identified by a context tag for each
                       direction of the communication, and also
                       identified by the pair of ULID and a Forked
                       Instance Identifier (see below).


   Context tag         Each end of the context allocates a context tag
                       for the context.  This is used to uniquely
                       associate both received control packets and
                       payload extension headers as belonging to the
                       context.


   Current locator pair
                       Each end of the context has a current locator
                       pair which is used to send packets to the peer.
                       The two ends might use different current locator
                       pairs though.






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   Default context     At the sending end, the shim uses the ULID pair
                       (passed down from the ULP) to find the context
                       for that pair.  Thus, normally, a host can have
                       at most one context for a ULID pair.  We call
                       this the "default context".


   Context forking     A mechanism which allows ULPs that are aware of
                       multiple locators to use separate contexts for
                       the same ULID pair, in order to be able use
                       different locator pairs for different
                       communication to the same ULID.  Context forking
                       causes more than just the default context to be
                       created for a ULID pair.


   Forked Instance Identifier (FII)
                       In order to handle context forking, a context is
                       identified by a ULID-pair and a forked context
                       identifier.  The default context has a FII of
                       zero.


   Initial contact     We use this term to refer to the pre-shim
                       communication when some ULP decides to start
                       communicating with a peer by sending and
                       receiving ULP packets.  Typically this would not
                       invoke any operations in the shim, since the shim
                       can defer the context establishment until some
                       arbitrary later point in time.


   Hash Based Addresses (HBA)
                       A form of IPv6 address where the interface ID is
                       derived from a cryptographic hash of all the
                       prefixes assigned to the host.  See [7].


   Cryptographically Generated Addresses (CGA)
                       A form of IPv6 address where the interface ID is
                       derived from a cryptographic hash of the public
                       key.  See [6].


   CGA Parameter Data Structure (PDS)
                       The information that CGA and HBA exchanges in
                       order to inform the peer of how the interface ID
                       was computed.  See [6]., [7].



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2.2.  Notational Conventions

   A, B, and C are hosts.  X is a potentially malicious host.

   FQDN(A) is the Fully qualified Domain Name for A.

   Ls(A) is the locator set for A, which consists of the locators L1(A),
   L2(A), ...  Ln(A).  The locator set in not ordered in any particular
   way other than maybe what is returned by the DNS.

   ULID(A) is an upper-layer ID for A. In this proposal, ULID(A) is
   always one member of A's locator set.

   CT(X) is a context tag assigned by X.

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



























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3.  Assumptions

   The design intent is to ensure that the shim6 protocol is capable of
   handling path failures independently of the number of IP addresses
   (locators) available to the two communicating hosts, and
   independently of which host detects the failure condition.

   Consider, for example, the case in which both A and B have active
   shim6 state and where A has only one locator while B has multiple
   locators.  In this case, it might be that B is trying to send a
   packet to A, and has detected a failure condition with the current
   locator pair.  Since B has multiple locators it presumably has
   multiple ISPs, and consequently likely has alternate egress paths
   toward A. However, B cannot vary the destination address (i.e., A's
   locator), since A has only one locator.

   The above scenario leads to the assumption that a host should be able
   to cause different egress paths from its site to be used.  The most
   reasonable approach to accomplish this is to have the host use
   different source addresses and have the source address affect the
   selection of the site egress.  The details of how this can be
   accomplished is beyond the scope of this document, but without this
   capability the ability of the shim to try different "paths" by trying
   different locator pairs will have limited utility.

   The above assumption applies whether or not the ISPs perform ingress
   filtering.

   In addition, when the site's ISPs perform ingress filtering based on
   packet source addresses, shim6 assumes that packets sent with
   different source and destination combinations have a reasonable
   chance of making it through the relevant ISP's ingress filters.  This
   can be accomplished in several ways (all outside the scope of this
   document), such as having the ISPs relax there ingress filters, or
   selecting the egress such that it matches the IP source address
   prefix.

   Further discussion of this issue is captured in [20].

   The shim6 approach assumes that there are no IPv6-to-IPv6 NATs on the
   paths, i.e., that the two ends can exchange their own notion of their
   IPv6 addresses and that those addresses will also make sense to their
   peer.








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4.  Protocol Overview

   The shim6 protocol operates in several phases over time.  The
   following sequence illustrates the concepts:


   o  An application on host A decides to contact an application on host
      B using some upper-layer protocol.  This results in the ULP on
      host A sending packets to host B. We call this the initial
      contact.  Assuming the IP addresses selected by Default Address
      Selection [12] and its extensions [13] work, then there is no
      action by the shim at this point in time.  Any shim context
      establishment can be deferred until later.


   o  Some heuristic on A or B (or both) determine that it is
      appropriate to pay the shim6 overhead to make this host-to-host
      communication robust against locator failures.  For instance, this
      heuristic might be that more than 50 packets have been sent or
      received, or a timer expiration while active packet exchange is in
      place.  This makes the shim initiate the 4-way context
      establishment exchange.

      As a result of this exchange, both A and B will know a list of
      locators for each other.

      If the context establishment exchange fails, the initiator will
      then know that the other end does not support shim6, and will
      continue with standard unicast behavior for the session.


   o  Communication continues without any change for the ULP packets.
      In particular, there are no shim extension headers added to the
      ULP packets, since the ULID pair is the same as the locator pair.
      In addition, there might be some messages exchanged between the
      shim sub-layers for (un)reachability detection.


   o  At some point in time something fails.  Depending on the approach
      to reachability detection, there might be some advice from the
      ULP, or the shim (un)reachability detection might discover that
      there is a problem.

      At this point in time one or both ends of the communication need
      to probe the different alternate locator pairs until a working
      pair is found, and switch to using that locator pair.





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   o  Once a working alternative locator pair has been found, the shim
      will rewrite the packets on transmit, and tag the packets with
      shim6 Payload extension header, which contains the receiver's
      context tag.  The receiver will use the context tag to find the
      context state which will indicate which addresses to place in the
      IPv6 header before passing the packet up to the ULP.  The result
      is that from the perspective of the ULP the packet passes
      unmodified end-to-end, even though the IP routing infrastructure
      sends the packet to a different locator.


   o  The shim (un)reachability detection will monitor the new locator
      pair as it monitored the original locator pair, so that subsequent
      failures can be detected.


   o  In addition to failures detected based on end-to-end observations,
      one endpoint might know for certain that one or more of its
      locators is not working.  For instance, the network interface
      might have failed or gone down (at layer 2), or an IPv6 address
      might have become deprecated or invalid.  In such cases the host
      can signal its peer that this address is no longer recommended to
      try.  This triggers something similar to a failure handling and a
      new working locator pair must be found.

      The protocol also has the ability to express other forms of
      locator preferences.  A change in any preferences can be signaled
      to the peer, which will have made the peer record the new
      preferences.  A change in the preferences might optionally make
      the peer want to use a different locator pair.  In this case, the
      peer follows the same locator switching procedure as after a
      failure (by verifying that its peer is indeed present at the
      alternate locator, etc).


   o  When the shim thinks that the context state is no longer used, it
      can garbage collect the state; there is no coordination necessary
      with the peer host before the state is removed.  There is a
      recovery message defined to be able to signal when there is no
      context state, which can be used to detect and recover from both
      premature garbage collection, as well as complete state loss
      (crash and reboot) of a peer.

      The exact mechanism to determine when the context state is no
      longer used is implementation dependent.  For example, an
      implementation might use the existence of ULP state (where known
      to the implementation) as an indication that the state is still
      used, combined with a timer (to handle ULP state that might not be



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      known to the shim sub-layer) to determine when the state is likely
      to no longer be used.


   NOTE: The ULP packets in shim6 can be carried completely unmodified
   as long as the ULID pair is used as the locator pair.  After a switch
   to a different locator pair the packets are "tagged" with a shim6
   extension header, so that the receiver can always determine the
   context to which they belong.  This is accomplished by including an
   8-octet shim6 Payload Extension header before the (extension) headers
   that are processed by the IP endpoint sublayer and ULPs.  If
   subsequently the original ULIDs are selected as the active locator
   pair then the tagging of packets with the shim6 extension header is
   no longer necesary.

4.1.  Context Tags

   A context between two hosts is actually a context between two ULIDs.
   The context is identified by a pair of context tags.  Each end gets
   to allocate a context tag, and once the context is established, most
   shim6 control messages contain the context tag that the receiver of
   the message allocated.  Thus at a minimum the combination of <peer
   ULID, local ULID, local context tag> have to uniquely identify one
   context.  But since the Payload extension headers are demultiplexed
   without looking at the locators in the packet, the receiver will need
   to allocate context tags that are unique for all its contexts.  The
   context tag is a 47-bit number (the largest which can fit in an
   8-octet extension header).

   The mechanism for detecting a loss of context state at the peer
   assumes that the receiver can tell the packets that need locator
   rewriting, even after it has lost all state (e.g., due to a crash
   followed by a reboot).  This is achieved because after a rehoming
   event the packets that need receive-side rewriting, carry the Payload
   extension header.

4.2.  Context Forking

   It has been asserted that it will be important for future ULPs, in
   particular, future transport protocols, to be able to control which
   locator pairs are used for different communication.  For instance,
   host A and host B might communicate using both VoIP traffic and ftp
   traffic, and those communications might benefit from using different
   locator pairs.  However, the basic shim6 mechanism uses a single
   current locator pair for each context, thus a single context cannot
   accomplish this.

   For this reason, the shim6 protocol supports the notion of context



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   forking.  This is a mechanism by which a ULP can specify (using some
   API not yet defined) that a context for e.g., the ULID pair <A1, B2>
   should be forked into two contexts.  In this case the forked-off
   context will be assigned a non-zero Forked Instance Identifier, while
   the default context has FII zero.

   The Forked Instance Identifier (FII) is a 32-bit identifier which has
   no semantics in the protocol other then being part of the tuple which
   identifies the context.  For example, a host migth allocate FIIs as
   sequential numbers for any given ULID pair.

   No other special considerations are needed in the shim6 protocol to
   handle forked contexts.

   Note that forking as specified does NOT allow A to be able to tell B
   that certain traffic (a 5-tuple?) should be forked for the reverse
   direction.  The shim6 forking mechanism as specified applies only to
   the sending of ULP packets.  If some ULP wants to fork for both
   directions, it is up to the ULP to set this up, and then instruct the
   shim at each end to transmit using the forked context.

4.3.  API Extensions

   Several API extensions have been discussed for shim6, but their
   actual specification is out of scope for this document.  The simplest
   one would be to add a socket option to be able to have traffic bypass
   the shim (not create any state, and not use any state created by
   other traffic).  This could be an IPV6_DONTSHIM socket option.  Such
   an option would be useful for protocols, such as DNS, where the
   application has its own failover mechanism (multiple NS records in
   the case of DNS) and using the shim could potentially add extra
   latency with no added benefits.

   Some other API extensions are discussed in Appendix A

4.4.  Securing shim6

   The mechanisms are secured using a combination of techniques:

   o  The HBA technique [7] for verifying the locators to prevent an
      attacker from redirecting the packet stream to somewhere else.

   o  Requiring a Reachability Probe+Reply before a new locator is used
      as the destination, in order to prevent 3rd party flooding
      attacks.

   o  The first message does not create any state on the responder.
      Essentially a 3-way exchange is required before the responder



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      creates any state.  This means that a state-based DoS attack
      (trying to use up all of memory on the responder) at least
      provides an IPv6 address that the attacker was using.

   o  The context establishment messages use nonces to prevent replay
      attacks, and to prevent off-path attackers from interfering with
      the establishment.

   o  Every control message of the shim6 protocol, past the context
      establishment, carry the context tag assigned to the particular
      context.  This implies that an attacker needs to discover that
      context tag before being able to spoof any shim6 control message.
      Such discovery probably requires to be along the path in order to
      be sniff the context tag value.  The result is that through this
      technique, the shim6 protocol is protected against off-path
      attackers.

4.5.  Overview of Shim Control Messages

   The shim6 context establishment is accomplished using four messages;
   I1, R1, I2, R2.  Normally they are sent in that order from initiator
   and responder, respectively.  Should both ends attempt to set up
   context state at the same time (for the same ULID pair), then their
   I1 messages might cross in flight, and result in an immediate R2
   message.  [The names of these messages are borrowed from HIP [25].]

   R1bis and I2bis messages are defined, which are used to recover a
   context after it has been lost.  A R1bis message is sent when a shim6
   control or Payload extension header arrives and there is no matching
   context state at the receiver.  When such a message is received, it
   will result in the re-creation of the shim6 context using the I2bis
   and R2 messages.

   The peers' lists of locators are normally exchanged as part of the
   context establishment exchange.  But the set of locators might be
   dynamic.  For this reason there is a Update Request and Update
   Acknowledgement messages, and a Locator List option.

   Even when the list of locators is fixed, a host might determine that
   some preferences might have changed.  For instance, it might
   determine that there is a locally visible failure that implies that
   some locator(s) are no longer usable.  This uses a Locator
   Preferences option in the Update Request message.

   The mechanism for (un)reachability detection is called Forced
   Bidirectional Communication (FBD).  FBD uses a Keepalive message
   which is sent when a host has received packets from its peer but has
   not yet sent any packets from its ULP to the peer.  The message type



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   is reserved in this document, but the message format and processing
   rules are specified in [8].

   In addition, when the context is established and there is a
   subsequent failure there needs to be a way to probe the set of
   locator pairs to efficiently find a working pair.  This document
   reserves an Probe message type, with the packet format and processing
   rules specified in [8].

   The above probe and keepalive messages assume we have an established
   ULID-pair context.  However, communication might fail during the
   initial contact (that is, when the application or transport protocol
   is trying to setup some communication).  This is handled using the
   mechanisms in the ULP to try different address pairs as specified in
   [12] [13].  In the future versions of the protocol, and with a richer
   API between the ULP and the shim, the shim might be help optimize
   discovering a working locator pair during initial contact.  This is
   for further study.

4.6.  Extension Header Order

   Since the shim is placed between the IP endpoint sub-layer and the IP
   routing sub-layer, the shim header will be placed before any endpoint
   extension headers (fragmentation headers, destination options header,
   AH, ESP), but after any routing related headers (hop-by-hop
   extensions header, routing header, a destinations options header
   which precedes a routing header).  When tunneling is used, whether
   IP-in-IP tunneling or the special form of tunneling that Mobile IPv6
   uses (with Home Address Options and Routing header type 2), there is
   a choice whether the shim applies inside the tunnel or outside the
   tunnel, which affects the location of the shim6 header.

   In most cases IP-in-IP tunnels are used as a routing technique, thus
   it makes sense to apply them on the locators which means that the
   sender would insert the shim6 header after any IP-in-IP
   encapsulation; this is what occurs naturally when routers apply IP-
   in-IP encapsulation.  Thus the packets would have:

   o  Outer IP header

   o  Inner IP header

   o  Shim6 extension header (if needed)

   o  ULP

   But the shim can also be used to create "shimmed tunnels" i.e., where
   an IP-in-IP tunnel uses the shim to be able to switch the tunnel



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   endpoint addresses between different locators.  In such a case the
   packets would have:

   o  Outer IP header

   o  Shim6 extension header (if needed)

   o  Inner IP header

   o  ULP

   In any case, the receiver behavior is well-defined; a receiver
   processes the extension headers in order.  However, the precise
   interaction between Mobile IPv6 and shim6 is for further study, but
   it might make sense to have Mobile IPv6 operate on locators as well,
   meaning that the shim would be layered on top of the MIPv6 mechanism.



































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5.  Message Formats

   The shim6 messages are all carried using a new IP protocol number [to
   be assigned by IANA].  The shim6 messages have a common header,
   defined below, with some fixed fields, followed by type specific
   fields.

   The shim6 messages are structured as an IPv6 extension header since
   the Payload extension header is used to carry the ULP packets after a
   locator switch.  The shim6 control messages use the same extension
   header formats so that a single "protocol number" needs to be allowed
   through firewalls in order for shim6 to function across the firewall.

5.1.  Common shim6 Message Format

   The first 17 bits of the shim6 header is common for the Payload
   extension header and the control messages and looks as follows:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |  Hdr Ext Len  |P|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   The payload which follows this header.

   Hdr Ext Len:   8-bit unsigned integer.  Length of the shim6 header in
                  8-octet units, not including the first 8 octets.

   P:             A single bit to distinguish Payload extension headers
                  from control messages.

5.2.  Payload Extension Header Format

   The payload extension headers is used to carry ULP packets where the
   receiver must replace the content of the source and/or destination
   fields in the IPv6 header before passing the packet to the ULP.  Thus
   this extension header is required when the locators pair that is used
   is not the same as the ULID pair.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |       0       |1|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                      Receiver Context Tag                     |



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   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   The payload which follows this header.

   Hdr Ext Len:   0 (since the header is 8 octets).

   P:             Set to one.  A single bit to distinguish this from the
                  shim6 control messages.

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

5.3.  Common Shim6 Control header

   The common part of the header has a next header and header extension
   length field which is consistent with the other IPv6 extension
   headers, even if the next header value is always "NO NEXT HEADER" for
   the control messages; only the payload extension header use the Next
   Header field.

   The shim6 headers must be a multiple of 8 octets, hence the minimum
   size is 8 octets.

   The common shim control message header is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Next Header  |  Hdr Ext Len  |0|     Type    |Type-specific|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                    Type-specific format                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   8-bit selector.  Normally set to NO_NXT_HDR (59).

   Hdr Ext Len:   8-bit unsigned integer.  Length of the shim6 header in
                  8-octet units, not including the first 8 octets.

   P:             Set to zero.  A single bit to distinguish this from
                  the shim6 payload extension header.





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   Type:          7-bit unsigned integer.  Identifies the actual message
                  from the table below.  Type codes 0-63 will not
                  trigger R1bis messages on a missing context, while 64-
                  127 will trigger R1bis.

   0:             A single bit (set to zero) which allows shim6 and HIP
                  to have a common header format yet telling shim6 and
                  HIP messages apart.

   Checksum:      16-bit unsigned integer.  The checksum is the 16-bit
                  one's complement of the one's complement sum of the
                  entire shim6 header message starting with the shim6
                  next header field, and ending as indicated by the Hdr
                  Ext Len. Thus when there is a payload following the
                  shim6 header, the payload is NOT included in the shim6
                  checksum.  Note that unlike protocol like ICMPv6,
                  there is no pseudo-header checksum part of the
                  checksum, in order to provide locator agility without
                  having to change the checksum.

   Type-specific: Part of message that is different for different
                  message types.

   +------------+-----------------------------------------------------+
   | Type Value |                       Message                       |
   +------------+-----------------------------------------------------+
   |      1     | I1 (first establishment message from the initiator) |
   |            |                                                     |
   |      2     | R1 (first establishment message from the responder) |
   |            |                                                     |
   |      3     |  I2 (2nd establishment message from the initiator)  |
   |            |                                                     |
   |      4     |  R2 (2nd establishment message from the responder)  |
   |            |                                                     |
   |      5     |  R1bis (Reply to reference to non-existent context) |
   |            |                                                     |
   |      6     |           I2bis (Reply to a R1bis message)          |
   |            |                                                     |
   |     64     |                    Update Request                   |
   |            |                                                     |
   |     65     |                Update Acknowledgement               |
   |            |                                                     |
   |     66     |                      Keepalive                      |
   |            |                                                     |
   |     67     |                    Probe Message                    |
   +------------+-----------------------------------------------------+

                                  Table 1



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5.4.  I1 Message Format

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       59      |  Hdr Ext Len  |0|  Type = 1   |   Reserved1 |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |R|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                  Initiator Context Tag                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Initiator Nonce                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Hdr Ext Len:   At least 1, since the header is 16 octets when there
                  are no options.

   Type:          1

   Reserved1:     7-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   R:             1-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Initiator Context Tag: 47-bit field.  The Context Tag the initiator
                  has allocated for the context.

   Initiator Nonce: 32-bit unsigned integer.  A random number picked by
                  the initiator which the responder will return in the
                  R1 message.

   The following options are defined for this message:







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   ULID pair:     When the IPv6 source and destination addresses in the
                  IPv6 header does not match the ULID pair, this option
                  MUST be included.  An example of this is when
                  recovering from a lost context.

   Forked Instance Identifier: When another instance of an existent
                  context with the same ULID pair is being created, a
                  Forked Instance Identifier option is included to
                  distinguish this new instance from the existent one.

   Future protocol extensions might define additional options for this
   message.  The C-bit in the option format defines how such a new
   option will be handled by an implementation.  See Section 5.14.

5.5.  R1 Message Format

   The R1 message is the second message in the context establishment
   exchange.  The responder sends this in response to an I1 message,
   without creating any state specific to the initiator.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       59      |  Hdr Ext Len  |0|  Type = 2   |   Reserved1 |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |           Reserved2           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Initiator Nonce                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Responder Nonce                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Hdr Ext Len:   At least 1, since the header is 16 octets when there
                  are no options.

   Type:          2







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   Reserved1:     7-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Reserved2:     16-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Initiator Nonce: 32-bit unsigned integer.  Copied from the I1
                  message.

   Responder Nonce: 32-bit unsigned integer.  A number picked by the
                  responder which the initiator will return in the I2
                  message.

   The following options are defined for this message:

   Responder Validator: Variable length option.  Typically a hash
                  generated by the responder, which the responder uses
                  together with the Responder Nonce value to verify that
                  an I2 message is indeed sent in response to a R1
                  message, and that the parameters in the I2 message are
                  the same as those in the I1 message.

   Future protocol extensions might define additional options for this
   message.  The C-bit in the option format defines how such a new
   option will be handled by an implementation.  See Section 5.14.

5.6.  I2 Message Format

   The I2 message is the third message in the context establishment
   exchange.  The initiator sends this in response to a R1 message,
   after checking the Initiator Nonce, etc.




















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       59      |  Hdr Ext Len  |0|  Type = 3   |   Reserved1 |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |R|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                  Initiator Context Tag                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Initiator Nonce                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Responder Nonce                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Reserved2                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Hdr Ext Len:   At least 2, since the header is 24 octets when there
                  are no options.

   Type:          3

   Reserved1:     7-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   R:             1-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Initiator Context Tag: 47-bit field.  The Context Tag the initiator
                  has allocated for the context.

   Initiator Nonce: 32-bit unsigned integer.  A random number picked by
                  the initiator which the responder will return in the
                  R2 message.

   Responder Nonce: 32-bit unsigned integer.  Copied from the R1
                  message.







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   Reserved2:     32-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.  (Needed to
                  make the options start on a multiple of 8 octet
                  boundary.)

   The following options are defined for this message:

   Responder Validator: Variable length option.  Just a copy of the
                  Responder Validator option in the R1 message.

   ULID pair:     When the IPv6 source and destination addresses in the
                  IPv6 header does not match the ULID pair, this option
                  MUST be included.  An example of this is when
                  recovering from a lost context.

   Forked Instance Identifier: When another instance of an existent
                  context with the same ULID pair is being created, a
                  Forked Instance Identifier option is included to
                  distinguish this new instance from the existent one.

   Locator list:  Optionally sent when the initiator immediately wants
                  to tell the responder its list of locators.  When it
                  is sent, the necessary HBA/CGA information for
                  verifying the locator list MUST also be included.

   Locator Preferences: Optionally sent when the locators don't all have
                  equal preference.

   CGA Parameter Data Structure: Included when the locator list is
                  included so the receiver can verify the locator list.

   CGA Signature: Included when the some of the locators in the list use
                  CGA (and not HBA) for verification.

   Future protocol extensions might define additional options for this
   message.  The C-bit in the option format defines how such a new
   option will be handled by an implementation.  See Section 5.14.

5.7.  R2 Message Format

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







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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       59      |  Hdr Ext Len  |0|  Type = 4   |   Reserved1 |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |R|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                  Responder Context Tag                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Initiator Nonce                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Hdr Ext Len:   At least 1, since the header is 16 octets when there
                  are no options.

   Type:          4

   Reserved1:     7-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   R:             1-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Responder Context Tag: 47-bit field.  The Context Tag the responder
                  has allocated for the context.

   Initiator Nonce: 32-bit unsigned integer.  Copied from the I2
                  message.

   The following options are defined for this message:

   Locator List:  Optionally sent when the responder immediately wants
                  to tell the initiator its list of locators.  When it
                  is sent, the necessary HBA/CGA information for
                  verifying the locator list MUST also be included.

   Locator Preferences: Optionally sent when the locators don't all have
                  equal preference.





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   CGA Parameter Data Structure: Included when the locator list is
                  included so the receiver can verify the locator list.

   CGA Signature: Included when the some of the locators in the list use
                  CGA (and not HBA) for verification.

   Future protocol extensions might define additional options for this
   message.  The C-bit in the option format defines how such a new
   option will be handled by an implementation.  See Section 5.14.

5.8.  R1bis Message Format

   Should a host receive a packet with a shim Payload extension header
   or shim6 control message with type code 64-127 (such as an Update or
   Probe message), and the host does not have any context state for the
   received context tag, then it will generate a R1bis message.

   This message allows the sender of the packet referring to the non-
   existent context to re-establish the context with a reduced context
   establishment exchange.  Upon the reception of the R1bis message, the
   receiver can proceed reestablishing the lost context by directly
   sending an I2bis message.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       59      |  Hdr Ext Len  |0|  Type = 5   |   Reserved1 |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |R|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                     Packet Context Tag                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Responder Nonce                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Hdr Ext Len:   At least 1, since the header is 16 octets when there
                  are no options.






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   Type:          5

   Reserved1:     7-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   R:             1-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Packet Context Tag: 47-bit unsigned integer.  The context tag
                  contained in the received packet that triggered the
                  generation of the R1bis message.

   Responder Nonce: 32-bit unsigned integer.  A number picked by the
                  responder which the initiator will return in the I2bis
                  message.

   The following options are defined for this message:

   Responder Validator: Variable length option.  Typically a hash
                  generated by the responder, which the responder uses
                  together with the Responder Nonce value to verify that
                  an I2bis message is indeed sent in response to a R1bis
                  message.

   Future protocol extensions might define additional options for this
   message.  The C-bit in the option format defines how such a new
   option will be handled by an implementation.  See Section 5.14.

5.9.  I2bis Message Format

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


















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       59      |  Hdr Ext Len  |0|  Type = 6  |   Reserved1 |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |R|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                  Initiator Context Tag                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Initiator Nonce                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Responder Nonce                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Reserved2                               |
   |                                 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                 |                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                     Packet Context Tag                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Hdr Ext Len:   At least 3, since the header is 32 octets when there
                  are no options.

   Type:          6

   Reserved1:     7-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   R:             1-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Initiator Context Tag: 47-bit field.  The Context Tag the initiator
                  has allocated for the context.

   Initiator Nonce: 32-bit unsigned integer.  A random number picked by
                  the initiator which the responder will return in the
                  R2 message.






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   Responder Nonce: 32-bit unsigned integer.  Copied from the R1bis
                  message.

   Reserved2:     49-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.  (Note that 17
                  bits are not sufficient since the options need start
                  on a multiple of 8 octet boundary.)

   Packet Context Tag: 47-bit unsigned integer.  Copied from the Packet
                  Context Tag contained in the received R1bis.

   The following options are defined for this message:

   Responder Validator: Variable length option.  Just a copy of the
                  Responder Validator option in the R1bis message.

   ULID pair:     When the IPv6 source and destination addresses in the
                  IPv6 header does not match the ULID pair, this option
                  MUST be included.

   Forked Instance Identifier: When another instance of an existent
                  context with the same ULID pair is being created, a
                  Forked Instance Identifier option is included to
                  distinguish this new instance from the existent one.

   Locator list:  Optionally sent when the initiator immediately wants
                  to tell the responder its list of locators.  When it
                  is sent, the necessary HBA/CGA information for
                  verifying the locator list MUST also be included.

   Locator Preferences: Optionally sent when the locators don't all have
                  equal preference.

   CGA Parameter Data Structure: Included when the locator list is
                  included so the receiver can verify the locator list.

   CGA Signature: Included when the some of the locators in the list use
                  CGA (and not HBA) for verification.

   Future protocol extensions might define additional options for this
   message.  The C-bit in the option format defines how such a new
   option will be handled by an implementation.  See Section 5.14.

5.10.  Update Request Message Format

   The Update Request Message is used to update either the list of
   locators, the locator preferences, and both.  When the list of
   locators is updated, the message also contains the option(s)



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   necessary for HBA/CGA to secure this.  The basic sanity check that
   prevents off-path attackers from generating bogus updates is the
   context tag in the message.

   The update message contains options (the Locator List and the Locator
   Preferences) that, when included, completely replace the previous
   locator list and locator preferences, respectively.  Thus there is no
   mechanism to just send deltas to the locator list.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       59      |  Hdr Ext Len  |0|  Type = 64  |   Reserved1 |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |R|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                   Receiver Context Tag                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Request Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Hdr Ext Len:   At least 1, since the header is 16 octets when there
                  are no options.

   Type:          64

   Reserved1:     7-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   R:             1-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Receiver Context Tag: 47-bit field.  The Context Tag the receiver has
                  allocated for the context.

   Request Nonce: 32-bit unsigned integer.  A random number picked by
                  the initiator which the peer will return in the
                  acknowledgement message.

   The following options are defined for this message:



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   Locator List:  The list of the sender's (new) locators.  The locators
                  might be unchanged and only the preferences have
                  changed.

   Locator Preferences: Optionally sent when the locators don't all have
                  equal preference.

   CGA Parameter Data Structure (PDS): Included when the locator list is
                  included and the PDS was not included in the
                  I2/I2bis/R2 messages, so the receiver can verify the
                  locator list.

   CGA Signature: Included when the some of the locators in the list use
                  CGA (and not HBA) for verification.

   Future protocol extensions might define additional options for this
   message.  The C-bit in the option format defines how such a new
   option will be handled by an implementation.  See Section 5.14.

5.11.  Update Acknowledgement Message Format

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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       59      |  Hdr Ext Len  |0|  Type = 65  |   Reserved1 |0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            Checksum           |R|                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                             |
   |                   Receiver Context Tag                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Request Nonce                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:





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   Next Header:   NO_NXT_HDR (59).

   Hdr Ext Len:   At least 1, since the header is 16 octets when there
                  are no options.

   Type:          65

   Reserved1:     7-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   R:             1-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Receiver Context Tag: 47-bit field.  The Context Tag the receiver has
                  allocated for the context.

   Request Nonce: 32-bit unsigned integer.  Copied from the Update
                  Request message.

   No options are currently defined for this message.

   Future protocol extensions might define additional options for this
   message.  The C-bit in the option format defines how such a new
   option will be handled by an implementation.  See Section 5.14.

5.12.  Keepalive Message Format

   This message format is defined in [8].

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

5.13.  Probe Message Format

   This message and its semantics are defined in [8].

   The idea behind that mechanism is to be able to handle the case when
   one locator pair works in from A to B, and another locator pair works
   from B to A, but there is no locator pair which works in both
   directions.  The protocol mechanism is that as A is sending probe
   messages to B, B will observe which locator pairs it has received
   from and report that back in probe messages it is sending to A.





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5.14.  Option Formats

   The format of the options is a snapshot of the current HIP option
   format [25].  However, there is no intention to track any changes to
   the HIP option format, nor is there an intent to use the same name
   space for the option type values.  But using the same format will
   hopefully make it easier to import HIP capabilities into shim6 as
   extensions to shim6, should this turn out to be useful.

   All of the TLV parameters have a length (including Type and Length
   fields) which is a multiple of 8 bytes.  When needed, padding MUST be
   added to the end of the parameter so that the total length becomes a
   multiple of 8 bytes.  This rule ensures proper alignment of data.  If
   padding is added, the Length field MUST NOT include the padding.  Any
   added padding bytes MUST be zeroed by the sender, and their values
   SHOULD NOT be checked by the receiver.

   Consequently, the Length field indicates the length of the Contents
   field (in bytes).  The total length of the TLV parameter (including
   Type, Length, Contents, and Padding) is related to the Length field
   according to the following formula:

   Total Length = 11 + Length - (Length + 3) % 8;

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |             Type            |C|             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                                                               ~
   ~                          Contents                             ~
   ~                                               +-+-+-+-+-+-+-+-+
   ~                                               |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type:          15-bit identifier of the type of option.  The options
                  defined in this document are below.

   C:             Critical.  One if this parameter is critical, and MUST
                  be recognized by the recipient, zero otherwise.  An
                  implementation might view the C bit as part of the
                  Type field, by multiplying the type values in this
                  specification by two.






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   Length:        Length of the Contents, in bytes.

   Contents:      Parameter specific, defined by Type.

   Padding:       Padding, 0-7 bytes, added if needed.

                +------+---------------------------------+
                | Type |           Option Name           |
                +------+---------------------------------+
                |   1  |       Responder Validator       |
                |      |                                 |
                |   2  |           Locator List          |
                |      |                                 |
                |   3  |       Locator Preferences       |
                |      |                                 |
                |   4  |   CGA Parameter Data Structure  |
                |      |                                 |
                |   5  |          CGA Signature          |
                |      |                                 |
                |   6  |            ULID Pair            |
                |      |                                 |
                |   7  |    Forked Instance Identifier   |
                |      |                                 |
                |  10  |           Probe Option          |
                |      |                                 |
                |  11  |       Reachability Option       |
                |      |                                 |
                |  12  | Payload Reception Report Option |
                +------+---------------------------------+

                                  Table 2

   Future protocol extensions might define additional options for the
   SHIM6 messages.  The C-bit in the option format defines how such a
   new option will be handled by an implementation.

   If a host receives an option that it does not understand (an option
   that was defined in some future extension to this protocol) or is not
   listed as a valid option for the different message types above, then
   the Critical bit in the option determines the outcome.

   o  If C=0 then the option is silently ignored, and the rest of the
      message is processed.

   o  If C=1 then the host SHOULD send back an ICMP parameter problem
      (type 4, code 1), with the Pointer referencing the first octet in
      the option Type field.  When C=1 the message MUST NOT be
      processed.



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5.14.1.  Responder Validator Option Format

   The responder can choose exactly what input is used to compute the
   validator, and what one-way function (MD5, SHA1) it uses, as long as
   the responder can check that the validator it receives back in the I2
   or I2bis message is indeed one that:

   1)- it computed,

   2)- it computed for the particular context, and

   3)- that it isn't a replayed I2/I2bis message.

   Some suggestions on how to generate the validators are captured in
   Section 7.10.1 and Section 7.17.1.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 1          |0|            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                           Validator                           ~
   ~                                               +-+-+-+-+-+-+-+-+
   ~                                               |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Validator:     Variable length content whose interpretation is local
                  to the responder.

   Padding:       Padding, 0-7 bytes, added if needed.  See
                  Section 5.14.

5.14.2.  Locator List Option Format

   The Locator List Option is used to carry all the locators of the
   sender.  Note that the order of the locators is important, since the
   Locator Preferences refers to the locators by using the index in the
   list.

   Note that we carry all the locators in this option even though some
   of them can be created automatically from the CGA Parameter Data
   Structure.







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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 2          |0|            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Locator List Generation                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Num Locators |            N Octets of Verification Method    |
   +-+-+-+-+-+-+-+-+                                               |
   ~                                                               ~
   ~                                               +-+-+-+-+-+-+-+-+
   ~                                               |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                     Locators 1 through N                      ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Locator List Generation: 32-bit unsigned integer.  Indicates a
                  generation number which is increased by one for each
                  new locator list.  This is used to ensure that the
                  index in the Locator Preferences refer to the right
                  version of the locator list.

   Num Locators:  8-bit unsigned integer.  The number of locators that
                  are included in the option.  We call this number "N"
                  below.

   Verification Method: N octets.  The i'th octet specifies the
                  verification method for the i'th locator.

   Padding:       Padding, 0-7 bytes, added if needed so that the
                  Locators start on a multiple of 8 octet boundary.
                  NOTE that for this option there is never a need to pad
                  at the end, since the locators are a multiple of 8
                  octets in length.  This internal padding is included
                  in the length field.

   Locators:      N 128-bit locators.

   The defined verification methods are:










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                           +-------+----------+
                           | Value |  Method  |
                           +-------+----------+
                           |   0   | Reserved |
                           |       |          |
                           |   1   |    HBA   |
                           |       |          |
                           |   2   |    CGA   |
                           |       |          |
                           | 3-255 | Reserved |
                           +-------+----------+

                                  Table 3

5.14.3.  Locator Preferences Option Format

   The Locator Preferences option can have some flags to indicate
   whether or not a locator is known to work.  In addition, the sender
   can include a notion of preferences.  It might make sense to define
   "preferences" as a combination of priority and weight the same way
   that DNS SRV records has such information.  The priority would
   provide a way to rank the locators, and within a given priority, the
   weight would provide a way to do some load sharing.  See [9] for how
   SRV defines the interaction of priority and weight.

   The minimum notion of preferences we need is to be able to indicate
   that a locator is "dead".  We can handle this using a single octet
   flag for each locator.

   We can extend that by carrying a larger "element" for each locator.
   This document presently also defines 2-octet and 3-octet elements,
   and we can add more information by having even larger elements if
   need be.

   The locators are not included in the preference list.  Instead, the
   first element refers to locator that was in the first element in the
   Locator List option.  The generation number carried in this option
   and the Locator List option is used to verify that they refer to the
   same version of the locator list.












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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 3          |0|            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Locator List Generation                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Element Len  |  Element[1]   |  Element[2]   |  Element[3]   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              ...                              ~
   ~                                               +-+-+-+-+-+-+-+-+
   ~                                               |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Case of Element Len = 1 is depicted.

   Fields:

   Locator List Generation: 32-bit unsigned integer.  Indicates a
                  generation number for the locator list to which the
                  elements should apply.

   Element Len:   8-bit unsigned integer.  The length in octets of each
                  element.  This specification defines the cases when
                  the length is 1, 2, or 3.

   Element[i]:    A field with a number of octets defined by the Element
                  Len field.  Provides preferences for the i'th locator
                  in the Locator List option that is in use.

   Padding:       Padding, 0-7 bytes, added if needed.  See
                  Section 5.14.

   When the Element length equals one, then the element consists of only
   a one octet flags field.  The currently defined set of flags are:

      BROKEN: 0x01

      TEMPORARY: 0x02

   The intent of the BROKEN flag is to inform the peer that a given
   locator is known to be not working.  The intent of TEMPORARY is to
   allow the distinction between more stable addresses and less stable
   addresses when shim6 is combined with IP mobility, when we might have
   more stable home locators, and less stable care-of-locators.

   When the Element length equals two, then the element consists of a 1
   octet flags field followed by a 1 octet priority field.  The priority



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   has the same semantics as the priority in DNS SRV records.

   When the Element length equals three, then the element consists of a
   1 octet flags field followed by a 1 octet priority field, and a 1
   octet weight field.  The weight has the same semantics as the weight
   in DNS SRV records.

   This document doesn't specify the format when the Element length is
   more than three, except that any such formats MUST be defined so that
   the first three octets are the same as in the above case, that is, a
   of a 1 octet flags field followed by a 1 octet priority field, and a
   1 octet weight field.

5.14.4.  CGA Parameter Data Structure Option Format

   This option contains the CGA Parameter Data Structure (PDS).  When
   HBA is used to verify the locators, the PDS contains the HBA
   multiprefix extension.  When CGA is used to verify the locators, in
   addition to the PDS option, the host also needs to include the
   signature in the form of a CGA Signature option.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 4          |0|            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                   CGA Parameter Data Structure                ~
   ~                                               +-+-+-+-+-+-+-+-+
   ~                                               |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   CGA Parameter Data Structure: Variable length content.  Content
                  defined in [6] and [7].

   Padding:       Padding, 0-7 bytes, added if needed.  See
                  Section 5.14.

5.14.5.  CGA Signature Option Format

   When CGA is used for verification of one or more of the locators in
   the Locator List option, then the message in question will need to
   contain this option.







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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 5          |0|            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                        CGA Signature                          ~
   ~                                               +-+-+-+-+-+-+-+-+
   ~                                               |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   CGA Signature: A variable-length field containing a PKCS#1 v1.5
                  signature, constructed by using the sender's private
                  key over the following sequence of octets:

                  1.  The 128-bit CGA Message Type tag [CGA] value for
                      SHIM6, 0x4A 30 5662 4858 574B 3655 416F 506A 6D48.
                      (The tag value has been generated randomly by the
                      editor of this specification.).

                  2.  The Locator List Generation value of the
                      correspondent Locator List Option.

                  3.  The subset of locators included in the
                      correspondent Locator List Option which
                      verification method is set to CGA.  The locators
                      MUST be included in the order they are listed in
                      the Locator List Option.

   Padding:       Padding, 0-7 bytes, added if needed.  See
                  Section 5.14.

5.14.6.  ULID Pair Option Format

   I1, I2, and I2bis messages MUST contain the ULID pair; normally this
   is in the IPv6 source and destination fields.  In case that the ULID
   for the context differ from the address pair included in the source
   and destination address fields of the IPv6 packet used to carry the
   I1/I2/I2bis message, the ULID pair option MUST be included in the I1/
   I2/I2bis message.










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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 6          |0|        Length = 36            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Reserved2                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Sender ULID                           +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                        Receiver ULID                          +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Reserved2:     32-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.  (Needed to
                  make the ULIDs start on a multiple of 8 octet
                  boundary.)

   Sender ULID:   A 128-bit IPv6 address.

   Receiver ULID: A 128-bit IPv6 address.

5.14.7.  Forked Instance Identifier Option Format

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Type = 7          |0|         Length = 4            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Forked Instance Identifier                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Forked Instance Identifier: 32-bit field containing the identifier of
                  the particular forked instance.

5.14.8.  Probe Option Format

   This option is defined in [8].






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5.14.9.  Reachability Option Format

   This option is defined in [8].

5.14.10.  Payload Reception Report Option Format

   This option is defined in [8].












































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6.  Conceptual Model of a Host

   This section describes a conceptual model of one possible data
   structure organization that hosts will maintain for the purposes of
   shim6.  The described organization is provided to facilitate the
   explanation of how the shim6 protocol should behave.  This document
   does not mandate that implementations adhere to this model as long as
   their external behavior is consistent with that described in this
   document.

6.1.  Conceptual Data Structures

   The key conceptual data structure for the shim6 protocol is the ULID
   pair context.  This is a data structure which contains the following
   information:

   o  The state of the context.  See Section 6.2.

   o  The peer ULID; ULID(peer)

   o  The local ULID; ULID(local)

   o  The Forked Instance Identifier; FII.  This is zero for the default
      context i.e., when there is no forking.

   o  The list of peer locators, with their preferences; Ls(peer)

   o  The generation number for the most recently received, verified
      peer locator list.

   o  For each peer locator, the verification method to use (from the
      Locator List option).

   o  For each peer locator, a bit whether it has been verified using
      HBA or CGA, and a bit whether the locator has been probed to
      verify that the ULID is present at that location.

   o  The preferred peer locator - used as destination; Lp(peer)

   o  The set of local locators and the preferences; Ls(local)

   o  The generation number for the most recently sent Locator List
      option.

   o  The preferred local locator - used as source; Lp(local)

   o  The context tag used to transmit control messages and payload
      extension headers - allocated by the peer; CT(peer)



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   o  The context to expect in received control messages and payload
      extension headers - allocated by the local host; CT(local)

   o  Timers for retransmission of the messages during context
      establishment and update messages.

   o  Depending how an implementation determines whether a context is
      still in use, there might be a need to track the last time a
      packet was sent/received using the context.

   o  Reachability state for the locator pairs as specified in [8].

   o  During pair exploration, information about the probe messages that
      have been sent and received as specified in [8].

6.2.  Context States

   The states that are used to describe the shim6 protocol are as
   follows:

   +---------------------+---------------------------------------------+
   | State               | Explanation                                 |
   +---------------------+---------------------------------------------+
   | IDLE                | State machine start                         |
   |                     |                                             |
   | I1-SENT             | Initiating context establishment exchange   |
   |                     |                                             |
   | I2-SENT             | Waiting to complete context establishment   |
   |                     | exchange                                    |
   |                     |                                             |
   | I2BIS-SENT          | Potential context loss detected             |
   |                     |                                             |
   |                     |                                             |
   | ESTABLISHED         | SHIM context established                    |
   |                     |                                             |
   | E-FAILED            | Context establishment exchange failed       |
   |                     |                                             |
   | NO-SUPPORT          | ICMP Unrecognized Next Header type          |
   |                     | (type 4, code 1) received indicating        |
   |                     | that shim6 is not supported                 |
   +---------------------+---------------------------------------------+










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   In addition, in each of the aforementioned states, the following
   state information is stored:

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
























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7.  Establishing ULID-Pair Contexts

   ULID-pair contexts are established using a 4-way exchange, which
   allows the responder to avoid creating state on the first packet.  As
   part of this exchange each end allocates a context tag, and it shares
   this context tag and its set of locators with the peer.

   In some cases the 4-way exchange is not necessary, for instance when
   both ends try to setup the context at the same time, or when
   recovering from a context that has been garbage collected or lost at
   one of the hosts.

7.1.  Uniqueness of Context Tags

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

   In addition, in order to minimize the reuse of context tags, the host
   SHOULD randomly cycle through the 2^47 context tag values,(e.g.
   following the guidelines described in [17]).

7.2.  Locator Verification

   The peer's locators might need to be verified during context
   establishment as well as when handling locator updates in Section 10.

   There are two separate aspects of locator verification.  One is to
   verify that the locator is tied to the ULID, i.e., that the host
   which "owns" the ULID is also the one that is claiming the locator
   "ownership".  The shim6 protocol uses the HBA or CGA techniques for
   doing this verification.  The other is to verify that the host is
   indeed reachable at the claimed locator.  Such verification is needed
   both to make sure communication can proceed, but also to prevent 3rd
   party flooding attacks [19].  These different verifications happen at
   different times, since the first might need to be performed before
   packets can be received by the peer with the source locator in
   question, but the latter verification is only needed before packets
   are sent to the locator.

   Before a host can use a locator (different than the ULID) as the
   source locator, it must know that the peer will accept packets with
   that source locator as being part of this context.  Thus the HBA/CGA
   verification SHOULD be performed by the host before the host
   acknowledges the new locator, by sending an Update Acknowledgement
   message, or an R2 message.




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   Before a host can use a locator (different than the ULID) as the
   destination locator it MUST perform the HBA/CGA verification if this
   was not performed before upon the reception of the locator set.  In
   addition, it MUST verify that the ULID is indeed present at that
   locator.  This verification is performed by doing a return-
   routability test as part of the Probe sub-protocol [8].

   If the verification method in the Locator List option is not
   supported by the host, or if the verification method is not
   consistent with the CGA Parameter Data Structure (e.g., the Parameter
   Data Structure doesn't contain the multiprefix extension, and the
   verification method says to use HBA), then the host MUST ignore the
   Locator List and the message in which it is contained, and the host
   SHOULD generates an ICMP parameter problem (type 4, code 0), with the
   Pointer referencing the octet in the Verification method that was
   found inconsistent.

7.3.  Normal context establishment

   The normal context establishment consists of a 4 message exchange in
   the order of I1, R1, I2, R2 as can be seen in Figure 24.

        Initiator                          Responder

         IDLE                               IDLE
              ------------- I1 -------------->
         I1-SENT
              <------------ R1 ---------------
                                            IDLE
              ------------- I2 -------------->
         I2-SENT
              <------------ R2 ---------------
         ESTABLISHED                        ESTABLISHED

   Figure 24: Normal context establishment

7.4.  Concurrent context establishment

   When both ends try to initiate a context for the same ULID pair, then
   we might end up with crossing I1 messages.  Alternatively, since no
   state is created when receiving the I1, a host might send a I1 after
   having sent a R1 message.

   Since a host remembers that it has sent an I1, it can respond to an
   I1 from the peer (for the same ULID-pair), with a R2, resulting in
   the message exchange shown in Figure 25.  Such behavior is needed for
   other reasons such as to correctly respond to retransmitted I1
   messages, which occur when the R2 message has been lost.



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        Host A                             Host B

         IDLE                               IDLE
              -\
         I1-SENT---\
                    ---\                  /---
                        --- I1 ---\   /---  I1-SENT
                                   ---\
                       /--- I1 ---/    ---\
                  /---                     -->
              <---

              -\
         I1-SENT---\
                    ---\                  /---
                        --- R2 ---\   /---  I1-SENT
                                   ---\
                       /--- R2 ---/    ---\
                  /---                     -->
              <---                          ESTABLISHED
         ESTABLISHED

   Figure 25: Crossing I1 messages

   If a host has received an I1 and sent an R1, it has no state to
   remember this.  Thus if the ULP on the host sends down packets, this
   might trigger the host to send an I1 message itself.  Thus while one
   end is sending an I1 the other is sending an I2 as can be seen in
   Figure 26.






















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        Host A                             Host B

         IDLE                               IDLE
              -\
                ---\
         I1-SENT    ---\
                        --- I1 ---\
                                   ---\
                                       ---\
                                           -->

                                          /---
                                      /---  IDLE
                                   ---
                       /--- R1--/
                  /---
              <---

              -\
         I2-SENT---\
                    ---\                  /---
                        --- I2---\   /---   I1-SENT
                                   ---\
                       /--- I1 ---/    ---\
                  /---                     -->
              <---                          ESTABLISHED

              -\
         I2-SENT---\
                    ---\                  /---
                        --- R2 ---\   /---
                                   ---\
                       /--- R2 ---/    ---\
                  /---                     -->
              <---                          ESTABLISHED
         ESTABLISHED

   Figure 26: Crossing I2 and I1

7.5.  Context recovery

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

   This need can arise in the following cases:




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   o  The communication is working using the ULID pair as the locator
      pair, but a problem arises, and the end that has retained the
      context state decides to probe alternate locator pairs.

   o  The communication is working using a locator pair that is not the
      ULID pair, hence the ULP packets sent from a peer that has
      retained the context state use the shim6 Payload extension header.

   o  The host that retained the state sends a control message (e.g. an
      Update Request message).

   In all the cases the result is that the peer without state receives a
   shim message for which it has to context for the context tag.

   In all of those cases we can recover the context by having the node
   which doesn't have a context state, send back an R1bis message, and
   have then complete the recovery with a I2bis and R2 message as can be
   seen in Figure 27.

          Host A                             Host B

        Context for
        CT(peer)=X                         Discards context for
                                           CT(local)=X

         ESTABLISHED                        IDLE

              ---- payload, probe, etc. -----> No context state
                                               for CT(local)=X

              <------------ R1bis ------------
                                            IDLE

              ------------- I2bis ----------->
         I2BIS_SENT
              <------------ R2 ---------------
         ESTABLISHED                        ESTABLISHED

   Figure 27: Context loss at receiver

   If one end has garbage collected or lost the context state, it might
   try to create a new context state (for the same ULID pair), by
   sending an I1 message.  The peer (that still has the context state)
   will reply with an R1 message and the full 4-way exchange will be
   performed again in this case as can be seen in Figure 28.






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          Host A                             Host B

        Context for
        CT(peer)=X                         Discards context for
        ULIDs A1, B1                       CT(local)=X

         ESTABLISHED                        IDLE

       Finds  <------------ I1 --------------- Tries to setup
       existing                                for ULIDs A1, B1
       context,
       but CT(peer)                         I1-SENT
       doesn't match
              ------------- R1 --------------->
       Left old context
       in ESTABLISHED

              <------------ I2 ---------------
       Recreate context

       with new CT(peer)                    I2-SENT
       and Ls(peer).

         ESTABLISHED
              ------------- R2 -------------->
         ESTABLISHED                        ESTABLISHED

   Figure 28: Context loss at sender

7.6.  Context confusion

   Since each end might garbage collect the context state we can have
   the case when one end has retained the context state and tries to use
   it, while the other end has lost the state.  We discussed this in the
   previous section on recovery.  But for the same reasons, when one
   host retains context tag X as CT(peer) for ULID pair <A1, B1>, the
   other end might end up allocating that context tag as CT(local) for
   another ULID pair, e.g., <A3, B1> between the same hosts.  In this
   case we can not use the recovery mechanisms since there needs to be
   separate context tags for the two ULID pairs.

   This type of "confusion" can be observed in two cases (assuming it is
   A that has retained the state and B has dropped it):

   o  B decides to create a context for ULID pair <A3, B1>, and
      allocates X as its context tag for this, and sends an I1 to A.





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   o  A decides to create a context for ULID pair <A3, B1>, and starts
      the exchange by sending I1 to B. When B receives the I2 message,
      it allocates X as the context tag for this context.

   In both cases, A can detect that B has allocated X for ULID pair <A3,
   B1> even though that A still X as CT(peer) for ULID pair <A1, B1>.
   Thus A can detect that B must have lost the context for <A1, B1>.

   The confusion can be detected when I2/I2bis/R2 is received since we
   require that those messages MUST include a sufficiently large set of
   locators in a Locator List option that the peer can determine whether
   or not two contexts have the same host as the peer by comparing if
   there is any common locators in Ls(peer).

   The requirement is that the old context which used the context tag
   MUST be removed; it can no longer be used to send packets.  Thus A
   would forcibly remove the context state for <A1, B1, X>, so that it
   can accept the new context for <A3, B1, X>.  An implementation MAY
   re-create a context to replace the one that was removed; in this case
   for <A1, B1>.  The normal I1, R1, I2, R2 establishment exchange would
   then pick unique context tags for that replacement context.  This re-
   creation is OPTIONAL, but might be useful when there is ULP
   communication which is using the ULID pair whose context was removed.

   Note that an I1 message with a duplicate context tag should not cause
   the removal of the old context state; this operation needs to be
   deferred until the reception of the I2 message.

7.7.  Sending I1 messages

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

7.8.  Retransmitting I1 messages

   If the host does not receive an I2 or R2 message in response to the



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   I1 message after I1_TIMEOUT time, then it needs to retransmit the I1
   message.  The retransmissions should use a retransmission timer with
   binary exponential backoff to avoid creating congestion issues for
   the network when lots of hosts perform I1 retransmissions.  Also, the
   actual timeout value should be randomized between 0.5 and 1.5 of the
   nominal value to avoid self-synchronization.

   If, after I1_RETRIES_MAX retransmissions, there is no response, then
   most likely the peer does not implement the shim6 protocol, or there
   could be a firewall that blocks the protocol.  In this case it makes
   sense for the host to remember to not try again to establish a
   context with that ULID.  However, any such negative caching should
   retained for at most NO_R1_HOLDDOWN_TIME, to be able to later setup a
   context should the problem have been that the host was not reachable
   at all when the shim tried to establish the context.

   If the host receives an ICMP error with "Unrecognized Next Header"
   type (type 4, code 1) and the included packet is the I1 message it
   just sent, then this is a more reliable indication that the peer ULID
   does not implement shim6.  Again, in this case, the host should
   remember to not try again to establish a context with that ULID.
   Such negative caching should retained for at most ICMP_HOLDDOWN_TIME,
   which should be significantly longer than the previous case.

7.9.  Receiving I1 messages

   A host MUST silently discard any received I1 messages that do not
   satisfy all of the following validity checks in addition to those
   specified in Section 12.2:

   o  The Hdr Ext Len field is at least 1, i.e., the length is at least
      16 octets.

   Upon the reception of an I1 message, the host extracts the ULID pair
   and the Forked Instance Identifier from the message.  If there is no
   ULID-pair option, then the ULID pair is taken from the source and
   destination fields in the IPv6 header.  If there is no FII option in
   the message, then the FII value is taken to be zero.

   Next the host looks for an existing context which matches the ULID
   pair and the FII.

   If no state is found (i.e., the state is IDLE), then the host replies
   with a R1 message as specified below.

   If such a context exists in ESTABLISHED state, the host verifies that
   the locator of the Initiator is included in Ls(peer) (This check is
   unnecessary if there is no ULID-pair option in the I1 message).



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   If the state exists in ESTABLISHED state and the locators do not fall
   in the locator sets, then the host replies with a R1 message as
   specified below.  This completes the I1 processing, with the context
   state being unchanged.

   If the state exists in ESTABLISHED state and the locators do fall in
   the sets, then the host compares CT(peer) for the context with the CT
   contained in the I1 message.

   o  If the context tags match, then this probably means that the R2
      message was lost and this I1 is a retransmission.  In this case,
      the host replies with a R2 message containing the information
      available for the existent context.

   o  If the context tags do not match, then it probably means that the
      Initiator has lost the context information for this context and it
      is trying to establish a new one for the same ULID-pair.  In this
      case, the host replies with a R1 message as specified below.  This
      completes the I1 processing, with the context state being
      unchanged.

   If the state exists in other state (I1-SENT, I2-SENT, I2BIS-SENT), we
   are in the situation of Concurrent context establishment described in
   Section 7.4.  In this case, the host leaves CT(peer) unchanged, and
   replies with a R2 message.  This completes the I1 processing, with
   the context state being unchanged.

7.10.  Sending R1 messages

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

   When the host needs to send a R2 message in response to the I1
   message, it copies the Initiator Nonce from the I1 message to the R2
   message, and otherwise follows the normal rules for forming an R2
   message (see Section 7.14).

7.10.1.  Generating the R1 Validator

   One way for the responder to properly generate validators is to
   maintain a single secret (S) and a running counter for the Responder
   Nonce.



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   In the case the validator is generated to be included in a R1
   message, for each I1 message.  The responder can increase the
   counter, use the counter value as the responder nonce, and use the
   following information concatenated as input to the one-way function:

   o  The the secret S

   o  That Responder Nonce

   o  The Initiator Context Tag from the I1 message

   o  The ULIDs from the I1 message

   o  The locators from the I1 message (strictly only needed if they are
      different from the ULIDs)

   o  The forked instance identifier if such option was included in the
      I1 message

   and then the output of the hash function is used as the validator
   octet string.

7.11.  Receiving R1 messages and sending I2 messages

   A host MUST silently discard any received R1 messages that do not
   satisfy all of the following validity checks in addition to those
   specified in Section 12.2:

   o  The Hdr Ext Len field is at least 1, i.e., the length is at least
      16 octets.

   Upon the reception of an R1 message, the host extracts the Initiator
   Nonce and the Locator Pair from the message (the latter from the
   source and destination fields in the IPv6 header).  Next the host
   looks for an existing context which matches the Initiator Nonce and
   where the locators are contained in Ls(peer) and Ls(local),
   respectively.  If no such context is found, then the R1 message is
   silently discarded.

   If such a context is found, then the host looks at the state:

   o  If the state is I1-SENT, then it sends an I2 message as specified
      below.

   o  In any other state (I2-SENT, I2BIS-SENT, ESTABLISHED) then the
      host has already sent an I2 message then this is probably a reply
      to a retransmitted I1 message, so this R1 message MUST be silently
      discarded.



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   When the host sends an I2 message, then it includes the Responder
   Validator option that was in the R1 message.  The I2 message MUST
   include the ULID pair; normally in the IPv6 source and destination
   fields.  If a ULID-pair option was included in the I1 message then it
   MUST be included in the I2 message as well.  In addition, if the
   Forked Instance Identifier value for this context is non-zero, the I2
   message MUST contain a Forked Instance Identifier Option carrying
   this value.  Besides, the I2 message contains an Initiator Nonce.
   This is not required to be the same than the one included in the
   previous I1 message.

   The I2 message also includes the Initiator's locator list and the CGA
   parameter data structure.  If CGA (and not HBA) is used to verify the
   locator list, then Initiator also signs the key parts of the message
   and includes a CGA signature option containing the signature.

   When the I2 message has been sent, the state is set to I2-SENT.

7.12.  Retransmitting I2 messages

   If the initiator does not receive an R2 message after I2_TIMEOUT time
   after sending an I2 message it MAY retransmit the I2 message, using
   binary exponential backoff and randomized timers.  The Responder
   Validator option might have a limited lifetime, that is, the peer
   might reject Responder Validator options that are older than
   VALIDATOR_MIN_LIFETIME to avoid replay attacks.  Thus the initiator
   SHOULD fall back to retransmitting the I1 message when there is no R2
   received after retransmitting the I2 message I2_RETRIES_MAX times.

7.13.  Receiving I2 messages

   A host MUST silently discard any received I2 messages that do not
   satisfy all of the following validity checks in addition to those
   specified in Section 12.2:

   o  The Hdr Ext Len field is at least 2, i.e., the length is at least
      24 octets.

   Upon the reception of an I2 message, the host extracts the ULID pair
   and the Forked Instance identifier from the message.  If there is no
   ULID-pair option, then the ULID pair is taken from the source and
   destination fields in the IPv6 header.  If there is no FII option in
   the message, then the FII value is taken to be zero.

   Next the host verifies that the Responder Nonce is a recent one
   (Nonces that are no older than VALIDATOR_MIN_LIFETIME SHOULD be
   considered recent), and that the Responder Validator option matches
   the validator the host would have computed for the ULID, locators,



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   responder nonce, and FII.

   If a CGA Parameter Data Structure (PDS) is included in the message,
   then the host MUST verify if the actual PDS contained in the message
   corresponds to the ULID(peer).

   If any of the above verifications fails, then the host silently
   discards the message and it has completed the I2 processing.

   If all the above verifications are successful, then the host proceeds
   to look for a context state for the Initiator.  The host looks for a
   context with the extracted ULID pair and FII.  If none exist then
   state of the (non-existing) context is viewed as being IDLE, thus the
   actions depend on the state as follows:

   o  If the state is IDLE (i.e., the context does not exist) the host
      allocates a context tag (CT(local)), creates the context state for
      the context, and sets its state to ESTABLISHED.  It records
      CT(peer), and the peer's locator set as well as its own locator
      set in the context.  It SHOULD perform the HBA/CGA verification of
      the peer's locator set at this point in time, as specified in
      Section 7.2.  Then the host sends an R2 message back as specified
      below.

   o  If the state is I1-SENT, then the host verifies if the source
      locator is included in Ls(peer) or, it is included in the Locator
      List contained in the the I2 message and the HBA/CGA verification
      for this specific locator is successful

      *  If this is not the case, then the message is silently discarded
         and the context state remains unchanged.

      *  If this is the case, then the host updates the context
         information (CT(peer), Ls(peer)) with the data contained in the
         I2 message and the host MUST send a R2 message back as
         specified below.  Note that before updating Ls(peer)
         information, the host SHOULD perform the HBA/CGA validation of
         the peer's locator set at this point in time as specified in
         Section 7.2.  The host moves to ESTABLISHED state.

   o  If the state is ESTABLISHED, I2-SENT, or I2BIS-SENT, then the host
      verifies if the source locator is included in Ls(peer) or, it is
      included in the Locator List contained in the the I2 message and
      the HBA/CGA verification for this specific locator is successful

      *  If this is not the case, then the message is silently discarded
         and the context state remains unchanged.




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      *  If this is the case, then the host updates the context
         information (CT(peer), Ls(peer)) with the data contained in the
         I2 message and the host MUST send a R2 message back as
         specified in Section 7.14.  Note that before updating Ls(peer)
         information, the host SHOULD perform the HBA/CGA validation of
         the peer's locator set at this point in time as specified in
         Section 7.2.  The context state remains unchanged.

7.14.  Sending R2 messages

   Before the host sends the R2 message it MUST look for a possible
   context confusion i.e. where it would end up with multiple contexts
   using the same CT(peer) for the same peer host.  See Section 7.15.

   When the host needs to send an R2 message, the host forms the message
   using its locators and its context tag, copies the Initiator Nonce
   from the triggering message (I2, I2bis, or I1), and includes the
   necessary options so that the peer can verify the locators.  In
   particular, the R2 message includes the Responder's locator list and
   the PDS option.  If CGA (and not HBA) is used to verify the locator
   list, then the Responder also signs the key parts of the message and
   includes a CGA Signature option containing the signature.

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

7.15.  Match for Context Confusion

   When the host receives an I2, I2bis, or R2 it MUST look for a
   possible context confusion i.e. where it would end up with multiple
   contexts using the same CT(peer) for the same peer host.  This can
   happen when it has received the above messages since they create a
   new context with a new CT(peer).  Same issue applies when CT(peer) is
   updated for an existing context.

   The host takes CT(peer) for the newly created or updated context, and
   looks for other contexts which:

   o  Are in state ESTABLISHED or I2BIS-SENT.

   o  Have the same CT(peer).

   o  Where Ls(peer) has at least one locator in common with the newly
      created or updated context.

   If such a context is found, then the host checks if the ULID pair or



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   the Forked Instance Identifier different than the ones in the newly
   created or updated context:

   o  If either or both are different, then the peer is reusing the
      context tag for the creation of a context with different ULID pair
      or FII, which is an indication that the peer has lost the original
      context.  In this case, we are in the Context confusion situation,
      and the host MUST NOT use the old context to send any packets.  It
      MAY just discard the old context (after all, the peer has
      discarded it), or it MAY attempt to re-establish the old context
      by sending a new I1 message and moving its state to I1-SENT.  In
      any case, once that this situation is detected, the host MUST NOT
      keep two contexts with overlapping Ls(peer) locator sets and the
      same context tag in ESTABLISHED state, since this would result in
      demultiplexing problems on the peer.

   o  If both are the same, then this context is actually the context
      that is created or updated, hence there is no confusion.

7.16.  Receiving R2 messages

   A host MUST silently discard any received R2 messages that do not
   satisfy all of the following validity checks in addition to those
   specified in Section 12.2:

   o  The Hdr Ext Len field is at least 1, i.e., the length is at least
      16 octets.

   Upon the reception of an R2 message, the host extracts the Initiator
   Nonce and the Locator Pair from the message (the latter from the
   source and destination fields in the IPv6 header).  Next the host
   looks for an existing context which matches the Initiator Nonce and
   where the locators are Lp(peer) and Lp(local), respectively.  Based
   on the state:

   o  If no such context is found, i.e., the state is IDLE, then the
      message is silently dropped.

   o  If state is I1-SENT, I2-SENT, or I2BIS-SENT then the host performs
      the following actions: If a CGA Parameter Data Structure (PDS) is
      included in the message, then the host MUST verify that the actual
      PDS contained in the message corresponds to the ULID(peer) as
      specified in Section 7.2.  If the verification fails, then the
      message is silently dropped.  If the verification succeeds, then
      the host records the information from the R2 message in the
      context state; it records the peer's locator set and CT(peer).
      The host SHOULD perform the HBA/CGA verification of the peer's
      locator set at this point in time, as specified in Section 7.2.



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      The host sets its state to ESTABLISHED.

   o  If the state is ESTABLISHED, the R2 message is silently ignored,
      (since this is likely to be a reply to a retransmitted I2
      message).

   Before the host completes the R2 processing it MUST look for a
   possible context confusion i.e. where it would end up with multiple
   contexts using the same CT(peer) for the same peer host.  See
   Section 7.15.

7.17.  Sending R1bis messages

   Upon the receipt of a shim6 payload extension header where there is
   no current SHIM6 context at the receiver, the receiver is to respond
   with an R1bis message in order to enable a fast re-establishment of
   the lost SHIM6 context.

   Also a host is to respond with a R1bis upon receipt of any control
   messages that has a message type in the range 64-127 (i.e., excluding
   the context setup messages such as I1, R1, R1bis, I2, I2bis, R2 and
   future extensions), where the control message refers to a non
   existent context.

   We assume that all the incoming packets that trigger the generation
   of an R1bis message contain a locator pair (in the address fields of
   the IPv6 header) and a Context Tag.

   Upon reception of any of the packets described above, the host will
   reply with an R1bis including the following information:

   o  The Responder Nonce is a number picked by the responder which the
      initiator will return in the I2bis message.

   o  Packet Context Tag is the context tag contained in the received
      packet that triggered the generation of the R1bis message.

   o  The Responder Validator option is included, with a validator that
      is computed as suggested in the next section.

7.17.1.  Generating the R1bis Validator

   One way for the responder to properly generate validators is to
   maintain a single secret (S) and a running counter for the Responder
   Nonce.

   In the case the validator is generated to be included in a R1bis
   message, for each received payload extension header or control



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   message, the responder can increase the counter, use the counter
   value as the responder nonce, and use the following information
   concatenated as input to the one-way function:

   o  The the secret S

   o  That Responder Nonce

   o  The Receiver Context tag included in the received packet

   o  The locators from the received packet

   and then the output of the hash function is used as the validator
   octet string.

7.18.  Receiving R1bis messages and sending I2bis messages

   A host MUST silently discard any received R1bis messages that do not
   satisfy all of the following validity checks in addition to those
   specified in Section 12.2:

   o  The Hdr Ext Len field is at least 1, i.e., the length is at least
      16 octets.

   Upon the reception of an R1bis message, the host extracts the Packet
   Context Tag and the Locator Pair from the message (the latter from
   the source and destination fields in the IPv6 header).  Next the host
   looks for an existing context where the Packet Context Tag matches
   CT(peer) and where the locators match Lp(peer) and Lp(local),
   respectively.

   o  If no such context is not found, i.e., the state is IDLE, then the
      R1bis message is silently discarded.

   o  If the state is I1-SENT, I2-SENT, or I2BIS-SENT, then the R1bis
      message is silently discarded.

   o  If the state is ESTABLISHED, then we are in the case where the
      peer has lost the context and the goal is to try to re-establish
      it.  For that, the host leaves CT(peer) unchanged in the context
      state, transitions to I2BIS-SENT state, and sends a I2bis message,
      including the computed Responder Validator option, the Packet
      Context Tag, and the Responder Nonce received in the R1bis
      message.  This I2bis message is sent using the locator pair
      included in the R1bis message.  In the case that this locator pair
      differs from the ULID pair defined for this context, then an ULID
      option MUST be included in the I2bis message.  In addition, if the
      Forked Instance Identifier for this context is non-zero, then a



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      Forked Instance Identifier option carrying the instance identifier
      value for this context MUST be included in the I2bis message.

7.19.  Retransmitting I2bis messages

   If the initiator does not receive an R2 message after I2bis_TIMEOUT
   time after sending an I2bis message it MAY retransmit the I2bis
   message, using binary exponential backoff and randomized timers.  The
   Responder Validator option might have a limited lifetime, that is,
   the peer might reject Responder Validator options that are older than
   VALIDATOR_MIN_LIFETIME to avoid replay attacks.  Thus the initiator
   SHOULD fall back to retransmitting the I1 message when there is no R2
   received after retransmitting the I2bis message I2bis_RETRIES_MAX
   times.

7.20.  Receiving I2bis messages and sending R2 messages

   A host MUST silently discard any received I2bis messages that do not
   satisfy all of the following validity checks in addition to those
   specified in Section 12.2:

   o  The Hdr Ext Len field is at least 3, i.e., the length is at least
      32 octets.

   Upon the reception of an I2bis message, the host extracts the ULID
   pair and the Forked Instance identifier from the message.  If there
   is no ULID-pair option, then the ULID pair is taken from the source
   and destination fields in the IPv6 header.  If there is no FII option
   in the message, then the FII value is taken to be zero.

   Next the host verifies that the Responder Nonce is a recent one
   (Nonces that are no older than VALIDATOR_MIN_LIFETIME SHOULD be
   considered recent), and that the Responder Validator option matches
   the validator the host would have computed for the ULID, locators,
   responder nonce, and FII as part of sending an R1bis message.

   If a CGA Parameter Data Structure (PDS) is included in the message,
   then the host MUST verify if the actual PDS contained in the message
   corresponds to the ULID(peer).

   If any of the above verifications fails, then the host silently
   discard the message and it has completed the I2bis processing.

   If both verifications are successful, then the host proceeds to look
   for a context state for the Initiator.  The host looks for a context
   with the extracted ULID pair and FII.  If none exist then state of
   the (non-existing) context is viewed as being IDLE, thus the actions
   depend on the state as follows:



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   o  If the state is IDLE (i.e., the context does not exist) the host
      allocates a context tag (CT(local)), creates the context state for
      the context, and sets its state to ESTABLISHED.  The host SHOULD
      NOT use the Packet Context Tag in the I2bis message for CT(local);
      instead it should pick a new random context tag just as when it
      processes an I2 message.  It records CT(peer), and the peer's
      locator set as well as its own locator set in the context.  It
      SHOULD perform the HBA/CGA verification of the peer's locator set
      at this point in time as specified in Section 7.2.  Then the host
      sends an R2 message back as specified in Section 7.14.

   o  If the state is I1-SENT, then the host verifies if the source
      locator is included in Ls(peer) or, it is included in the Locator
      List contained in the the I2 message and the HBA/CGA verification
      for this specific locator is successful

      *  If this is not the case, then the message is silently
         discarded.  The the context state remains unchanged.

      *  If this is the case, then the host updates the context
         information (CT(peer), Ls(peer)) with the data contained in the
         I2 message and the host MUST send a R2 message back as
         specified below.  Note that before updating Ls(peer)
         information, the host SHOULD perform the HBA/CGA validation of
         the peer's locator set at this point in time as specified in
         Section 7.2.  The host moves to ESTABLISHED state.

   o  If the state is ESTABLISHED, I2-SENT, or I2BIS-SENT, then the host
      verifies if the source locator is included in Ls(peer) or, it is
      included in the Locator List contained in the the I2 message and
      the HBA/CGA verification for this specific locator is successful

      *  If this is not the case, then the message is silently
         discarded.  The the context state remains unchanged.

      *  If this is the case, then the host updates the context
         information (CT(peer), Ls(peer)) with the data contained in the
         I2 message and the host MUST send a R2 message back as
         specified in Section 7.14.  Note that before updating Ls(peer)
         information, the host SHOULD perform the HBA/CGA validation of
         the peer's locator set at this point in time as specified in
         Section 7.2.  The context state remains unchanged.









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8.  Handling ICMP Error Messages

   The routers in the path as well as the destination might generate
   various ICMP error messages, such as host unreachable, packet too
   big, and Unrecognized Next Header type.  It is critical that these
   packets make it back up to the ULPs so that they can take appropriate
   action.

   This is an implementation issue in the sense that the mechanism is
   completely local to the host itself.  But the issue of how ICMP
   errors are correctly dispatched to the ULP on the host are important,
   hence this section specifies the issue.

                +--------------+
                | IPv6 Header  |
                |              |
                +--------------+
                |    ICMPv6    |
                |    Header    |
         - -    +--------------+   - -
                | IPv6 Header  |
                | src, dst as  |   Can be dispatched
        IPv6    | sent by ULP  |   unmodified to ULP
                | on host      |   ICMP error handler
        Packet  +--------------+
                |     ULP      |
        in      |    Header    |
                +--------------+
        Error   |              |
                ~     Data     ~
                |              |
         - -    +--------------+   - -

   Figure 29: ICMP error handling without payload extension header

   When the ULP packets are sent without the payload extension header,
   that is, while the initial locators=ULIDs are working, this
   introduces no new concerns; an implementation's existing mechanism
   for delivering these errors to the ULP will work.  See Figure 29.

   But when the shim on the transmitting side inserts the payload
   extension header and replaces the ULIDs in the IP address fields with
   some other locators, then an ICMP error coming back will have a
   "packet in error" which is not a packet that the ULP sent.  Thus the
   implementation will have to apply the reverse mapping to the "packet
   in error" before passing the ICMP error up to the ULP.  See
   Figure 30.




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                +--------------+
                | IPv6 Header  |
                |              |
                +--------------+
                |    ICMPv6    |
                |    Header    |
         - -    +--------------+   - -
                | IPv6 Header  |
                | src, dst as  |   Needs to be
        IPv6    | modified by  |   transformed to
                | shim on host |   have ULIDs
                +--------------+   in src, dst fields,
        Packet  |  SHIM6 ext.  |   and SHIM6 ext.
                |    Header    |   header removed
         in     +--------------+   before it can be
                |  Transport   |   dispatched to the ULP
        Error   |    Header    |   ICMP error handler.
                +--------------+
                |              |
                ~     Data     ~
                |              |
         - -    +--------------+   - -

   Figure 30: ICMP error handling with payload  extension header

   Note that this mapping is different than when receiving packets from
   the peer with a payload extension headers, because in that case the
   packets contain CT(local).  But the ICMP errors have a "packet in
   error" with an payload extension header containing CT(peer).  This is
   because they were intended to be received by the peer.  In any case,
   since the <Source Locator, Destination Locator, CT(peer)> has to be
   unique when received by the peer, the local host should also only be
   able to find one context that matches this tuple.

   If the ICMP error is a Packet Too Big, the reported MTU must be
   adjusted to be 8 octets less, since the shim will add 8 octets when
   sending packets.

   After the "packet in error" has had the original ULIDs inserted, then
   this payload extension header can be removed.  The result is a
   "packet in error" that is passed to the ULP which looks as if the
   shim did not exist.









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9.  Teardown of the ULID-Pair Context

   Each host can unilaterally decide when to tear down a ULID-pair
   context.  It is RECOMMENDED that hosts do not tear down the context
   when they know that there is some upper layer protocol that might use
   the context.  For example, an implementation might know this if there
   is an open socket which is connected to the ULID(peer).  However,
   there might be cases when the knowledge is not readily available to
   the shim layer, for instance for UDP applications which do not
   connect their sockets, or any application which retains some higher
   level state across (TCP) connections and UDP packets.

   Thus it is RECOMMENDED that implementations minimize premature
   teardown by observing the amount of traffic that is sent and received
   using the context, and only after it appears quiescent, tear down the
   state.  A reasonable approach would be not to tear down a context
   until at least 5 minutes have passed since the last message was sent
   or received using the context.

   Since there is no explicit, coordinated removal of the context state,
   there are potential issues around context tag reuse.  One end might
   remove the state, and potentially reuse that context tag for some
   other communication, and the peer might later try to use the old
   context (which it didn't remove).  The protocol has mechanisms to
   recover from this, which work whether the state removal was total and
   accidental (e.g., crash and reboot of the host), or just a garbage
   collection of shim state that didn't seem to be used.  However, the
   host should try to minimize the reuse of context tags by trying to
   randomly cycle through the 2^47 context tag values.  (See Appendix C
   for a summary how the recovery works in the different cases.)





















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10.  Updating the Peer

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

10.1.  Sending Update Request messages

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

   Should the host send a new Locator List, the host picks a new random
   local generation number, records this in the context, and puts it in
   the Locator List option.  Any Locator Preference option, whether send
   in the same Update Request or in some future Update Request, will use
   that generation number to make sure the preferences get applied to
   the correct version of the locator list.

   The host picks a random Request Nonce for each update, and keeps the
   same nonce for any retransmissions of the Update Request.  The nonce
   is used to match the acknowledgement with the request.

10.2.  Retransmitting Update Request messages

   If the host does not receive an Update Acknowledgement R2 message in
   response to the Update Request message after UPDATE_TIMEOUT time,
   then it needs to retransmit the Update Request message.  The
   retransmissions should use a retransmission timer with binary
   exponential backoff to avoid creating congestion issues for the
   network when lots of hosts perform Update Request retransmissions.
   Also, the actual timeout value should be randomized between 0.5 and
   1.5 of the nominal value to avoid self-synchronization.

   Should there be no response, the retransmissions continue forever.
   The binary exponential backoff stops at MAX_UPDATE_TIMEOUT.  But the
   only way the retransmissions would stop when there is no
   acknowledgement, is when the shim, through the Probe protocol or some
   other mechanism, decides to discard the context state due to lack of
   ULP usage in combination with no responses to the Probes.






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10.3.  Newer Information While Retransmitting

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

   For example, if the original locator list was just (A1, A2), and if
   an Update Request with the Locator List (A1, A3) is outstanding, and
   the host determines that it should both add A4 to the locator list,
   and mark A1 as BROKEN, then it would need to:

   o  Pick a new random Request Nonce for the new Update Request.

   o  Pick a new random Generation number for the new locator list.

   o  Form the new locator list - (A1, A3, A4)

   o  Form a Locator Preference option which uses the new generation
      number and has the BROKEN flag for the first locator.

   o  Send the Update Request and start a retransmission timer.

   Any Update Acknowledgement which doesn't match the current request
   nonce, for instance an acknowledgement for the abandoned Update
   Request, will be silently ignored.

10.4.  Receiving Update Request messages

   A host MUST silently discard any received Update Request messages
   that do not satisfy all of the following validity checks in addition
   to those specified in Section 12.2:

   o  The Hdr Ext Len field is at least 1, i.e., the length is at least
      16 octets.

   Upon the reception of an Update Request message, the host extracts
   the Context Tag from the message.  It then looks for a context which
   has a CT(local) that matches the context tag.  If no such context is
   found, it sends a R1bis message as specified in Section 7.17.

   Since context tags can be reused, the host MUST verify that the IPv6
   source address field is part of Ls(peer) and that the IPv6



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   destination address field is part of Ls(local).  If this is not the
   case, the sender of the Update Request has a stale context which
   happens to match the CT(local) for this context.  In this case the
   host MUST send a R1bis message, and otherwise ignore the Update
   Request message.

   If a CGA Parameter Data Structure (PDS) is included in the message,
   then the host MUST verify if the actual PDS contained in the packet
   corresponds to the ULID(peer).  If this verification fails, the
   message is silently discarded.

   Then, depending on the state of the context:

   o  If ESTABLISHED: Proceed to process message.

   o  If I1-SENT, discard the message and stay in I1-SENT.

   o  If I2-SENT, then send R2 and proceed to process the message.

   o  If I2BIS-SENT, then send R2 and proceed to process the message.

   The verification issues for the locators carried in the Locator
   Update message are specified in Section 7.2.  If the locator list can
   not be verified, this procedure might send an ICMP Parameter Problem
   error.  In any case, if it can not be verified, there is no further
   processing of the Update Request.

   Once any Locator List option in the Update Request has been verified,
   the peer generation number in the context is updated to be the one in
   the Locator List option.

   If the Update message contains a Locator Preference option, then the
   Generation number in the preference option is compared with the peer
   generation number in the context.  If they do not match, then the
   host generates an ICMP parameter problem (type 4, code 0) with the
   Pointer field referring to the first octet in the Generation number
   in the Locator Preference option.  In addition, if the number of
   elements in the Locator Preference option does not match the number
   of locators in Ls(peer), then an ICMP parameter problem is sent with
   the Pointer referring to the first octet of the Length field in the
   Locator Preference option.  In both cases of failures, no further
   processing is performed for the Locator Update message.

   If the generation number matches, the locator preferences are
   recorded in the context.

   Once the Locator List option (if present) has been verified and any
   new locator list or locator preferences have been recorded, the host



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   sends an Update Acknowledgement message, copying the nonce from the
   request, and using the CT(peer) in as the Receiver Context Tag.

   Any new locators, or more likely new locator preferences, might
   result in the host wanting to select a different locator pair for the
   context.  For instance, if the Locator Preferences lists the current
   Lp(peer) as BROKEN.  The host uses the Probe message in [8] to verify
   that the new locator is reachable before changing Lp(peer).

10.5.  Receiving Update Acknowledgement messages

   A host MUST silently discard any received Update Acknowledgement
   messages that do not satisfy all of the following validity checks in
   addition to those specified in Section 12.2:

   o  The Hdr Ext Len field is at least 1, i.e., the length is at least
      16 octets.

   Upon the reception of an Update Acknowledgement message, the host
   extracts the Context Tag and the Request Nonce from the message.  It
   then looks for a context which has a CT(local) that matches the
   context tag.  If no such context is found, it sends a R1bis message
   as specified in Section 7.17.

   Since context tags can be reused, the host MUST verify that the IPv6
   source address field is part of Ls(peer) and that the IPv6
   destination address field is part of Ls(local).  If this is not the
   case, the sender of the Update Acknowledgement has a stale context
   which happens to match the CT(local) for this context.  In this case
   the host MUST send a R1bis message, and otherwise ignore the Update
   Acknowledgement message.

   Then, depending on the state of the context:

   o  If ESTABLISHED: Proceed to process message.

   o  If I1-SENT, discard the message and stay in I1-SENT.

   o  If I2-SENT, then send R2 and proceed to process the message.

   o  If I2BIS-SENT, then send R2 and proceed to process the message.

   If the Request Nonce doesn't match the Nonce for the last sent Update
   Request for the context, then the Update Acknowledgement is silently
   ignored.  If the nonce matches, then the update has been completed
   and the Update retransmit timer can be reset.





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11.  Sending ULP Payloads

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

   If the context is not in ESTABLISHED or I2BIS-SENT state, then it
   there is also no effect on how the ULP packets are sent.  Only in the
   ESTABLISHED and I2BIS-SENT states does the host have CT(peer) and
   Ls(peer) set.

   If there is a ULID-pair context for the ULID pair, then the sender
   needs to verify whether context uses the ULIDs as locators, that is,
   whether Lp(peer) == ULID(peer) and Lp(local) == ULID(local).

   If this is the case, then packets can be sent unmodified by the shim.
   If it is not the case, then the logic in Section 11.1 will need to be
   used.

   There will also be some maintenance activity relating to
   (un)reachability detection, whether packets are sent with the
   original locators or not.  The details of this is out of scope for
   this document and is specified in [8].

11.1.  Sending ULP Payload after a Switch

   When sending packets, if there is a ULID-pair context for the ULID
   pair, and the ULID pair is no longer used as the locator pair, then
   the sender needs to transform the packet.  Apart from replacing the
   IPv6 source and destination fields with a locator pair, an 8-octet
   header is added so that the receiver can find the context and inverse
   the transformation.

   If there has been a failure causing a switch, and later the context
   switches back to sending things using the ULID pair as the locator
   pair, then there is no longer a need to do any packet transformation
   by the sender, hence there is no need to include the 8-octet
   extension header.

   First, the IP address fields are replaced.  The IPv6 source address
   field is set to Lp(local) and the destination address field is set to
   Lp(peer).  NOTE that this MUST NOT cause any recalculation of the ULP
   checksums, since the ULP checksums are carried end-to-end and the ULP
   pseudo-header contains the ULIDs which are preserved end-to-end.




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   The sender skips any "routing sub-layer extension headers" that the
   ULP might have included, thus it skips any hop-by-hop extension
   header, any routing header, and any destination options header that
   is followed by a routing header.  After any such headers the shim6
   extension header will be added.  This might be before a Fragment
   header, a Destination Options header, an ESP or AH header, or a ULP
   header.

   The inserted shim6 Payload extension header includes the peer's
   context tag.  It takes on the next header value from the preceding
   extension header, since that extension header will have a next header
   value of SHIM6.







































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12.  Receiving Packets

   As in normal IPv6 receive side packet processing the receiver parses
   the (extension) headers in order.  Should it find a shim6 extension
   header it will look at the "P" field in that header.  If this bit is
   zero, then the packet must be passed to the shim6 payload handling
   for rewriting.  Otherwise, the packet is passed to the shim6 control
   handling.

12.1.  Receiving Payload Extension Headers

   The receiver extracts the context tag from the payload extension
   header, and uses this to find a ULID-pair context.  If no context is
   found, the receiver SHOULD generate a R1bis message (see
   Section 7.17).

   Then, depending on the state of the context:

   o  If ESTABLISHED: Proceed to process message.

   o  If I1-SENT, discard the message and stay in I1-SENT.

   o  If I2-SENT, then send R2 and proceed to process the message.

   o  If I2BIS-SENT, then send R2 and proceed to process the message.

   With the context in hand, the receiver can now replace the IP address
   fields with the ULIDs kept in the context.  Finally, the Payload
   extension header is removed from the packet (so that the ULP doesn't
   get confused by it), and the next header value in the preceding
   header is set to be the actual protocol number for the payload.  Then
   the packet can be passed to the protocol identified by the next
   header value (which might be some function associated with the IP
   endpoint sublayer, or a ULP).

   If the host is using some heuristic for determining when to perform a
   deferred context establishment, then the host might need to do some
   accounting (count the number of packets sent and received) for
   packets that does not have a shim6 extension header and for which
   there is no context.  But the need for this depends on what
   heuristics the implementation has chosen.

12.2.  Receiving Shim Control messages

   A shim control message has the checksum field verified.  The Shim
   header length field is also verified against the length of the IPv6
   packet to make sure that the shim message doesn't claim to end past
   the end of the IPv6 packet.  Finally, it checks that the neither the



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   IPv6 destination field nor the IPv6 source field is a multicast
   address.  If any of those checks fail, the packet is silently
   dropped.

   The message is then dispatched based on the shim message type.  Each
   message type is then processed as described elsewhere in this
   document.  If the packet contains a shim message type which is
   unknown to the receiver, then an ICMPv6 Parameter Problem error is
   generated and sent back.  The pointer field in the Parameter Problem
   is set to point at the first octet of the shim message type.  The
   error is rate limited just like other ICMP errors [5].

   All the control messages can contain any options with C=0.  If there
   is any option in the message with C=1 that isn't known to the host,
   then the host MUST send an ICMPv6 Parameter Problem, with the Pointer
   field referencing the first octet of the Option Type.

12.3.  Context Lookup

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

   There is one state machine per context identified that is
   conceptually identified by ULID pair and Forked Instance Identifier
   (which is zero by default), or identified by CT(local).  However, the
   detailed lookup rules are more complex, especially during context
   establishment.

   Clearly, if the required context is not established, it will be in
   IDLE state.

   During context establishment, the context is identified as follows:

   o  I1 packets: Deliver to the context associated with the ULID pair
      and the Forked Instance Identifier.


   o  I2 packets: Deliver to the context associated with the ULID pair
      and the Forked Instance Identifier.


   o  R1 packets: Deliver to the context with the locator pair included
      in the packet and the Initiator nonce included in the packet (R1
      does not contain ULID pair nor the CT(local)).  If no context
      exist with this locator pair and Initiator nonce, then silently



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      discard.


   o  R2 packets: Deliver to the context with the locator pair included
      in the packet and the Initiator nonce included in the packet (R2
      does not contain ULID pair nor the CT(local)).  If no context
      exists with this locator pair and INIT nonce, then silently
      discard.


   o  R1bis packet: deliver to the context that has the locator pair and
      the CT(peer) equal to the Packet Context Tag included in the R1bis
      packet.


   o  I2bis packets: Deliver to the context associated with the ULID
      pair and the Forked Instance Identifier.


   o  Payload extension headers: Deliver to the context with CT(local)
      equal to the Receiver Context Tag included in the packet.


   o  Other control messages (Update, Keepalive, Probe): Deliver to the
      context with CT(local) equal to the Receiver Context Tag included
      in the packet.  Verify that the IPv6 source address field is part
      of Ls(peer) and that the IPv6 destination address field is part of
      Ls(local).  If not, send a R1bis message.


   o  ICMP errors which contain a shim6 payload extension header or
      other shim control packet in the "packet in error": Use the
      "packet in error" for dispatching as follows.  Deliver to the
      context with CT(peer) equal to the Receiver Context Tag, Lp(local)
      being the IPv6 source address, and Lp(peer) being the IPv6
      destination address.


   In addition, the shim on the sending side needs to be able to find
   the context state when a ULP packet is passed down from the ULP.  In
   that case the lookup key is the pair of ULIDs and FII=0.  If we have
   a ULP API that allows the ULP to do context forking, then presumably
   the ULP would pass down the Forked Instance Identifier.








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13.  Initial Contact

   The initial contact is some non-shim communication between two ULIDs,
   as described in Section 2.  At that point in time there is no
   activity in the shim.

   Whether the shim ends up being used or not (e.g., the peer might not
   support shim6) it is highly desirable that the initial contact can be
   established even if there is a failure for one or more IP addresses.

   The approach taken is to rely on the applications and the transport
   protocols to retry with different source and destination addresses,
   consistent with what is already specified in Default Address
   Selection [12], and some fixes to that specification [13] to make it
   try different source addresses and not only different destination
   addresses.

   The implementation of such an approach can potentially result in long
   timeouts.  For instance, a naive implementation at the socket API
   which uses getaddrinfo() to retrieve all destination addresses and
   then tries to bind() and connect() to try all source and destination
   address combinations waiting for TCP to time out for each combination
   before trying the next one.

   However, if implementations encapsulate this in some new connect-by-
   name() API, and use non-blocking connect calls, it is possible to
   cycle through the available combinations in a more rapid manner until
   a working source and destination pair is found.  Thus the issues in
   this domain are issues of implementations and the current socket API,
   and not issues of protocol specification.  In all honesty, while
   providing an easy to use connect-by-name() API for TCP and other
   connection-oriented transports is easy; providing a similar
   capability at the API for UDP is hard due to the protocol itself not
   providing any "success" feedback.  But even the UDP issue is one of
   APIs and implementation.
















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14.  Protocol constants

   The protocol uses the following constants:

   I1_RETRIES_MAX = 4

   I1_TIMEOUT = 4 seconds

   NO_R1_HOLDDOWN_TIME = 1 min

   ICMP_HOLDDOWN_TIME = 10 min

   I2_TIMEOUT = 4 seconds

   I2_RETRIES_MAX = 2

   I2bis_TIMEOUT = 4 seconds

   I2bis_RETRIES_MAX = 2

   VALIDATOR_MIN_LIFETIME = 30 seconds

   UPDATE_TIMEOUT = 4 seconds

   The retransmit timers (I1_TIMEOUT, I2_TIMEOUT, UPDATE_TIMEOUT) are
   subject to binary exponential backoff, as well as randomization
   across a range of 0.5 and 1.5 times the nominal (backed off) value.
   This removes any risk of synchronization between lots of hosts
   performing independent shim operations at the same time.

   The randomization is applied after the binary exponential backoff.
   Thus the first retransmission would happen based on a uniformly
   distributed random number in the range [0.5*4, 1.5*4] seconds, the
   second retransmission [0.5*8, 1.5*8] seconds after the first one,
   etc.
















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15.  Implications Elsewhere

   The general shim6 approach, as well as the specifics of this proposed
   solution, has implications elsewhere.  The key implications are:

   o  Applications that perform referrals, or callbacks using IP
      addresses as the 'identifiers' can still function in limited ways,
      as described in [22].  But in order for such applications to be
      able to take advantage of the multiple locators for redundancy,
      the applications need to be modified to either use fully qualified
      domain names as the 'identifiers', or they need to pass all the
      locators as the 'identifiers' i.e., the 'identifier' from the
      applications perspective becomes a set of IP addresses instead of
      a single IP address.

   o  Firewalls that today pass limited traffic, e.g., outbound TCP
      connections, would presumably block the shim6 protocol.  This
      means that even when shim6 capable hosts are communicating, the I1
      messages would be dropped, hence the hosts would not discover that
      their peer is shim6 capable.  This is in fact a feature, since if
      the hosts managed to establish a ULID-pair context, then the
      firewall would probably drop the "different" packets that are sent
      after a failure (those using the shim6 payload extension header
      with a TCP packet inside it).  Thus stateful firewalls that are
      modified to pass shim6 messages should also be modified to pass
      the payload extension header, so that the shim can use the
      alternate locators to recover from failures.  This presumably
      implies that the firewall needs to track the set of locators in
      use by looking at the shim6 control exchanges.  Such firewalls
      might even want to verify the locators using the HBA/CGA
      verification themselves, which they can do without modifying any
      of the shim6 packets they pass through.

   o  Signaling protocols for QoS or other things that involve having
      devices in the network path look at IP addresses and port numbers,
      or IP addresses and Flow Labels, need to be invoked on the hosts
      when the locator pair changes due to a failure.  At that point in
      time those protocols need to inform the devices that a new pair of
      IP addresses will be used for the flow.  Note that this is the
      case even though this protocol, unlike some earlier proposals,
      does not overload the flow label as a context tag; the in-path
      devices need to know about the use of the new locators even though
      the flow label stays the same.

   o  MTU implications.  The path MTU mechanisms we use are robust
      against different packets taking different paths through the
      Internet, by computing a minimum over the recently observed path
      MTUs.  When shim6 fails over from using one locator pair to



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      another pair, this means that packets might travel over a
      different path through the Internet, hence the path MTU might be
      quite different.  Perhaps such a path change would be a good hint
      to the path MTU mechanism to try a larger MTU?

      The fact that the shim will add an 8 octet payload extension
      header to the ULP packets after a locator switch, can also affect
      the usable path MTU for the ULPs.  In this case the MTU change is
      local to the sending host, thus conveying the change to the ULPs
      is an implementation matter.

   o  The precise interaction between Mobile IPv6 and shim6 is for
      further study, but it might make sense to have Mobile IPv6 operate
      on locators, meaning that the shim would be layered on top of the
      MIPv6 mechanism.




































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16.  Security Considerations

   This document satisfies the concerns specified in [19] as follows:

   o  The HBA technique [7] for verifying the locators to prevent an
      attacker from redirecting the packet stream to somewhere else.

   o  Requiring a Reachability Probe+Reply before a new locator is used
      as the destination, in order to prevent 3rd party flooding
      attacks.

   o  The first message does not create any state on the responder.
      Essentially a 3-way exchange is required before the responder
      creates any state.  This means that a state-based DoS attack
      (trying to use up all of memory on the responder) at least
      requires the attacker to create state, cosnuming his own resources
      and also it provides an IPv6 address that the attacker was using.

   o  The context establishment messages use nonces to prevent replay
      attacks, and to prevent off-path attackers from interfering with
      the establishment.

   o  Every control message of the shim6 protocol, past the context
      establishment, carry the context tag assigned to the particular
      context.  This implies that an attacker needs to discover that
      context tag before being able to spoof any shim6 control message.
      Such discovery probably requires to be along the path in order to
      be sniff the context tag value.  The result is that through this
      technique, the shim6 protocol is protected against off-path
      attackers.

   Some of the residual threats in this proposal are:

   o  An attacker which arrives late on the path (after the context has
      been established) can use the R1bis message to cause one peer to
      recreate the context, and at that point in time the attacker can
      observe all of the exchange.  But this doesn't seem to open any
      new doors for the attacker since such an attacker can observe the
      context tags that are being used, and once known it can use those
      to send bogus messages.

   o  An attacker which is present on the path so that it can find out
      the context tags, can generate a R1bis message after it has moved
      off the path.  For this packet to be effective it needs to have a
      source locator which belongs to the context, thus there can not be
      "too much" ingress filtering between the attackers new location
      and the communicating peers.  But this doesn't seem to be that
      severe, because once the R1bis causes the context to be re-



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      established, a new pair of context tags will be used, which will
      not be known to the attacker.  If this is still a concern, we
      could require a 2-way handshake "did you really loose the state?"
      in response to the error message.

   o  It might be possible for an attacker to try random 47-bit context
      tags and see if they can cause disruption for communication
      between two hosts.  In particular, in the case of payload packets,
      the effects of such attack would be similar of those of an
      attacker sending packets with spoofed source address.  In the case
      of control packets, it is not enough to find the correct context
      tag, but additional information is required (e.g. nonces, proper
      source addresses) (see previous bullet for the case of R1bis).  If
      a 47-bit tag, which is the largest that fits in an 8-octet
      extension header, isn't sufficient, one could use an even larger
      tag in the shim6 control messages, and use the low-order 47 bits
      in the payload extension header.

   o  When the payload extension header is used, an attacker that can
      guess the 47-bit random context tag, can inject packets into the
      context with any source locator.  Thus if there is ingress
      filtering between the attacker, this could potentially allow to
      bypass the ingress filtering.  However, in addition to guessing
      the 47-bit context tag, the attacker also needs to find a context
      where, after the receiver's replacement of the locators with the
      ULIDs, the the ULP checksum is correct.  But even this wouldn't be
      sufficient with ULPs like TCP, since the TCP port numbers and
      sequence numbers must match an existing connection.  Thus, even
      though the issues for off-path attackers injecting packets are
      different than today with ingress filtering, it is still very hard
      for an off-path attacker to guess.  If IPsec is applied then the
      issue goes away completely.

   o  The validator included in the R1 and R1bis packets are generated
      as a hash of several input parameters.  However, most of the
      inputs are actually determined by the sender, and only the secret
      value S is unknown to the sender.  However, the resulting
      protection is deemed to be enough since it would be easier for the
      attacker to just obtain a new validator sending a I1 packet than
      performing all the computations required to determine the secret
      S. However, it is recommended that the host changes the secret S
      periodically.









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17.  IANA Considerations

   IANA is directed to allocate a new IP Protocol Number value for the
   SHIM6 Protocol.

   IANA is directed to record a CGA message type for the SHIM6 Protocol
   in the [CGA] namespace registry with the value 0x4A30 5662 4858 574B
   3655 416F 506A 6D48.

   IANA is directed to establish a SHIM6 Parameter Registry with two
   components: SHIM6 Type registrations and SHIM6 Options registrations.

   The initial contents of the SHIM6 Type registry are as follows:

   +------------+-----------------------------------------------------+
   | Type Value |                       Message                       |
   +------------+-----------------------------------------------------+
   |      0     |                       RESERVED                      |
   |            |                                                     |
   |      1     | I1 (first establishment message from the initiator) |
   |            |                                                     |
   |      2     | R1 (first establishment message from the responder) |
   |            |                                                     |
   |      3     |  I2 (2nd establishment message from the initiator)  |
   |            |                                                     |
   |      4     |  R2 (2nd establishment message from the responder)  |
   |            |                                                     |
   |      5     |  R1bis (Reply to reference to non-existent context) |
   |            |                                                     |
   |      6     |           I2bis (Reply to a R1bis message)          |
   |            |                                                     |
   |    7-59    |       Can be allocated using Standards Action       |
   |            |                                                     |
   |    60-63   |                 For Experimental use                |
   |            |                                                     |
   |     64     |                    Update Request                   |
   |            |                                                     |
   |     65     |                Update Acknowledgement               |
   |            |                                                     |
   |     66     |                      Keepalive                      |
   |            |                                                     |
   |     67     |                    Probe Message                    |
   |            |                                                     |
   |   68-123   |       Can be allocated using Standards Action       |
   |            |                                                     |
   |   124-127  |                 For Experimental use                |
   +------------+-----------------------------------------------------+




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   The initial contents of the SHIM6 Options registry are as follows:

            +-------------+----------------------------------+
            |     Type    |            Option Name           |
            +-------------+----------------------------------+
            |      0      |             RESERVED             |
            |             |                                  |
            |      1      |        Responder Validator       |
            |             |                                  |
            |      2      |           Locator List           |
            |             |                                  |
            |      3      |        Locator Preferences       |
            |             |                                  |
            |      4      |   CGA Parameter Data Structure   |
            |             |                                  |
            |      5      |           CGA Signature          |
            |             |                                  |
            |      6      |             ULID Pair            |
            |             |                                  |
            |      7      |    Forked Instance Identifier    |
            |             |                                  |
            |     8-9     | Allocated using Standards action |
            |             |                                  |
            |      10     |           Probe Option           |
            |             |                                  |
            |      11     |        Reachability Option       |
            |             |                                  |
            |      12     |  Payload Reception Report Option |
            |             |                                  |
            |   13-16383  | Allocated using Standards action |
            |             |                                  |
            | 16384-32767 |       For Experimental use       |
            +-------------+----------------------------------+


















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18.  Acknowledgements

   Over the years many people active in the multi6 and shim6 WGs have
   contributed ideas a suggestions that are reflected in this
   specification.  Special thanks to the careful comments from Geoff
   Huston, Shinta Sugimoto, Pekka Savola, Dave Meyer, Deguang Le and Jim
   Bound on earlier versions of this draft.












































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Appendix A.  Possible Protocol Extensions

   During the development of this protocol, several issues have been
   brought up as important one to address, but are ones that do not need
   to be in the base protocol itself but can instead be done as
   extensions to the protocol.  The key ones are:

   o  As stated in the assumptions in Section 3, the in order for the
      shim6 protocol to be able to recover from a wide range of
      failures, for instance when one of the communicating hosts is
      singly-homed, and cope with a site's ISPs that do ingress
      filtering based on the source IPv6 address, there is a need for
      the host to be able to influence the egress selection from its
      site.  Further discussion of this issue is captured in [20].

   o  Is there need for keeping the list of locators private between the
      two communicating endpoints?  We can potentially accomplish that
      when using CGA but not with HBA, but it comes at the cost of doing
      some public key encryption and decryption operations as part of
      the context establishment.  The suggestion is to leave this for a
      future extension to the protocol.

   o  Defining some form of end-to-end "compression" mechanism that
      removes the need for including the Shim6 Payload extension header
      when the locator pair is not the ULID pair.

   o  Supporting the dynamic setting of locator preferences on a site-
      wide basis, and use the Locator Preference option in the shim6
      protocol to convey these preferences to remote communicating
      hosts.  This could mirror the DNS SRV record's notion of priority
      and weight.

   o  Potentially recommend that more application protocols use DNS SRV
      records to allow a site some influence on load spreading for the
      initial contact (before the shim6 context establishment) as well
      as for traffic which does not use the shim.

   o  Specifying APIs for the ULPs to be aware of the locators the shim
      is using, and be able to influence the choice of locators
      (controlling preferences as well as triggering a locator pair
      switch).  This includes providing APIs the ULPs can use to fork a
      shim context.

   o  Whether it is feasible to relax the suggestions for when context
      state is removed, so that one can end up with an asymmetric
      distribution of the context state and still get (most of) the shim
      benefits.  For example, the busy server would go through the
      context setup but would quickly remove the context state after



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      this (in order to save memory) but the not-so-busy client would
      retain the context state.  The context recovery mechanism
      presented in Section 7.5 would then be recreate the state should
      the client send either a shim control message (e.g., probe message
      because it sees a problem), or a ULP packet in an payload
      extension header (because it had earlier failed over to an
      alternative locator pair, but had been silent for a while).  This
      seems to provide the benefits of the shim as long as the client
      can detect the failure.  If the client doesn't send anything, and
      it is the server that tries to send, then it will not be able to
      recover because the shim on the server has no context state, hence
      doesn't know any alternate locator pairs.

   o  Study whether a host explicitly fail communication when a ULID
      becomes invalid (based on RFC 2462 lifetimes or DHCPv6), or should
      we let the communication continue using the invalidated ULID (it
      can certainly work since other locators will be used).

   o  Study what it would take to make the shim6 control protocol not
      rely at all on a stable source locator in the packets.  This can
      probably be accomplished by having all the shim control messages
      include the ULID-pair option.

   o  If each host might have lots of locators, then the currently
      requirement to include essentially all of them in the I2 and R2
      messages might be constraining.  If this is the case we can look
      into using the CGA Parameter Data Structure for the comparison,
      instead of the prefix sets, to be able to detect context
      confusion.  This would place some constraint on a (logical) only
      using e.g., one CGA public key, and would require some carefully
      crafted rules on how two PDSs are compared for "being the same
      host".  But if we don't expect more than a handful locators per
      host, then we don't need this added complexity.

   o  ULP specified timers for the reachability detection mechanism
      (which can be useful particularly when there are forked contexts).

   o  Pre-verify some "backup" locator pair, so that the failover time
      can be shorter.

   o  Study how shim6 and Mobile IPv6 might interact.  There existing an
      initial draft on this topic [21].









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Appendix B.  Simplified State Machine

   The states are defined in Section 6.2.  The intent is that the
   stylized description below be consistent with the textual description
   in the specification, but should they conflict, the textual
   description is normative.

   The following table describes the possible actions in state IDLE and
   their respective triggers:

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive I1          | Send R1 and stay in IDLE                    |
   |                     |                                             |
   | Heuristics trigger  | Send I1 and move to I1-SENT                 |
   | a new context       |                                             |
   | establishment       |                                             |
   |                     |                                             |
   | Receive I2, verify  | If successful, send R2 and move to          |
   | validator and       | ESTABLISHED                                 |
   | RESP nonce          |                                             |
   |                     | If fail, stay in IDLE                       |
   |                     |                                             |
   | Receive I2bis,      | If successful, send R2 and move to          |
   | verify validator    | ESTABLISHED                                 |
   | and RESP nonce      |                                             |
   |                     | If fail, stay in IDLE                       |
   |                     |                                             |
   | R1, R1bis, R2       | N/A (This context lacks the required info   |
   |                     | for the dispatcher to deliver them)         |
   |                     |                                             |
   | Receive payload     | Send R1bis and stay in IDLE                 |
   | extension header    |                                             |
   | or other control    |                                             |
   | packet              |                                             |
   +---------------------+---------------------------------------------+














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   The following table describes the possible actions in state I1-SENT
   and their respective triggers:

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









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   The following table describes the possible actions in state I2-SENT
   and their respective triggers:

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive R2, verify  | If successful move to ESTABLISHED           |
   | INIT nonce          |                                             |
   |                     | If fail, stay in I2-SENT                    |
   |                     |                                             |
   | Receive I1          | Send R2 and stay in I2-SENT                 |
   |                     |                                             |
   | Receive I2          | Send R2 and stay in I2-SENT                 |
   | verify validator    |                                             |
   | and RESP nonce      |                                             |
   |                     |                                             |
   | Receive I2bis       | Send R2 and stay in I2-SENT                 |
   | verify validator    |                                             |
   | and RESP nonce      |                                             |
   |                     |                                             |
   | Receive R1          | Discard and stay in I2-SENT                 |
   |                     |                                             |
   | Timeout, increment  | If counter =< I2_RETRIES_MAX, send I2 and   |
   | timeout counter     | stay in I2-SENT                             |
   |                     |                                             |
   |                     | If counter > I2_RETRIES_MAX, send I1 and go |
   |                     | to I1-SENT                                  |
   |                     |                                             |
   | R1bis               | N/A (Dispatcher doesn't deliver since       |
   |                     | CT(peer) is not set)                        |
   |                     |                                             |
   | Receive payload or  | Accept and send I2 (probably R2 was sent    |
   | extension header    | by peer and lost)                           |
   | other control       |                                             |
   | packet              |                                             |
   +---------------------+---------------------------------------------+















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   The following table describes the possible actions in state I2BIS-
   SENT and their respective triggers:

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive R2, verify  | If successful move to ESTABLISHED           |
   | INIT nonce          |                                             |
   |                     | If fail, stay in I2BIS-SENT                 |
   |                     |                                             |
   | Receive I1          | Send R2 and stay in I2BIS-SENT              |
   |                     |                                             |
   | Receive I2          | Send R2 and stay in I2BIS-SENT              |
   | verify validator    |                                             |
   | and RESP nonce      |                                             |
   |                     |                                             |
   | Receive I2bis       | Send R2 and stay in I2BIS-SENT              |
   | verify validator    |                                             |
   | and RESP nonce      |                                             |
   |                     |                                             |
   | Receive R1          | Discard and stay in I2BIS-SENT              |
   |                     |                                             |
   | Timeout, increment  | If counter =< I2_RETRIES_MAX, send I2bis    |
   | timeout counter     | and stay in I2BIS-SENT                      |
   |                     |                                             |
   |                     | If counter > I2_RETRIES_MAX, send I1 and    |
   |                     | go to I1-SENT                               |
   |                     |                                             |
   | R1bis               | N/A (Dispatcher doesn't deliver since       |
   |                     | CT(peer) is not set)                        |
   |                     |                                             |
   | Receive payload or  | Accept and send I2bis (probably R2 was      |
   | extension header    | sent by peer and lost)                      |
   | other control       |                                             |
   | packet              |                                             |
   +---------------------+---------------------------------------------+















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   The following table describes the possible actions in state
   ESTABLISHED and their respective triggers:

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive I1, compare | If no match, send R1 and stay in ESTABLISHED|
   | CT(peer) with       |                                             |
   | received CT         | If match, send R2 and stay in ESTABLISHED   |
   |                     |                                             |
   |                     |                                             |
   | Receive I2, verify  | If successful, then send R2 and stay in     |
   | validator and RESP  | ESTABLISHED                                 |
   | nonce               |                                             |
   |                     | Otherwise, discard and stay in ESTABLISHED  |
   |                     |                                             |
   | Receive I2bis,      | If successful, then send R2 and stay in     |
   | verify validator    | ESTABLISHED                                 |
   | and RESP nonce      |                                             |
   |                     | Otherwise, discard and stay in ESTABLISHED  |
   |                     |                                             |
   | Receive R2          | Discard and stay in ESTABLISHED             |
   |                     |                                             |
   | Receive R1          | Discard and stay in ESTABLISHED             |
   |                     |                                             |
   | Receive R1bis       | Send I2bis and move to I2BIS-SENT           |
   |                     |                                             |
   |                     |                                             |
   | Receive payload or  | Process and stay in ESTABLISHED             |
   | extension header    |                                             |
   | other control       |                                             |
   | packet              |                                             |
   |                     |                                             |
   | Implementation      | Discard state and go to IDLE                |
   | specific heuristic  |                                             |
   | (E.g., No open ULP  |                                             |
   | sockets and idle    |                                             |
   | for some time )     |                                             |
   +---------------------+---------------------------------------------+












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   The following table describes the possible actions in state E-FAILED
   and their respective triggers:

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Wait for            | Go to IDLE                                  |
   | NO_R1_HOLDDOWN_TIME |                                             |
   |                     |                                             |
   | Any packet          | Process as in IDLE                          |
   +---------------------+---------------------------------------------+

   The following table describes the possible actions in state NO-
   SUPPORT and their respective triggers:

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Wait for            | Go to IDLE                                  |
   | ICMP_HOLDDOWN_TIME  |                                             |
   |                     |                                             |
   | Any packet          | Process as in IDLE                          |
   +---------------------+---------------------------------------------+




























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Appendix B.1.  Simplified State Machine diagram

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



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Appendix C.  Context Tag Reuse

   The shim6 protocol doesn't have a mechanism for coordinated state
   removal between the peers, because such state removal doesn't seem to
   help given that a host can crash and reboot at any time.  A result of
   this is that the protocol needs to be robust against a context tag
   being reused for some other context.  This section summarizes the
   different cases in which a tag can be reused, and how the recovery
   works.

   The different cases are exemplified by the following case.  Assume
   host A and B were communicating using a context with the ULID pair
   <A1, B2>, and that B had assigned context tag X to this context.  We
   assume that B uses only the context tag to demultiplex the received
   payload extension headers, since this is the more general case.
   Further we assume that B removes this context state, while A retains
   it.  B might then at a later time assign CT(local)=X to some other
   context, and we have several cases:

   o  The context tag is reassigned to a context for the same ULID pair
      <A1, B2>.  We've called this "Context Recovery" in this document.

   o  The context tag is reassigned to a context for a different ULID
      pair between the same to hosts, e.g., <A3, B3>.  We've called this
      "Context Confusion" in this document.

   o  The context tag is reassigned to a context between B and other
      host C, for instance for the ULID pair <C3, B2>.  That is a form
      of three party context confusion.

Appendix C.1.  Context Recovery

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

Appendix C.2.  Context Confusion

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



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

Appendix C.3.  Three Party Context Confusion

   The third case does not have a place where the old state on A can be
   verified, since the new context is established between B and C. Thus
   when B receives payload extension headers with X as the context tag,
   it will find the context for <C3, B2>, hence rewrite the packets to
   have C3 in the source address field and B2 in the destination address
   field before passing them up to the ULP.  This rewriting is correct
   when the packets are in fact sent by host C, but if host A ever
   happens to send a packet using the old context, then the ULP on A
   sends a packet with ULIDs <A1, B2> and the packet arrives at the ULP
   on B with ULIDs <C3, B2>.

   This is clearly an error, and the packet will most likely be rejected
   by the ULP on B due to a bad pseudo-header checksum.  Even if the
   checksum is ok (probability 2^-16), the ULP isn't likely to have a
   connection for those ULIDs and port numbers.  And if the ULP is
   connection-less, processing the packet is most likely harmless; such
   a ULP must be able to copy with random packets being sent by random
   peers in any case.

   This broken state, where packets sent from A to B using the old
   context on host A might persist for some time, but it will not remain
   for very long.  The unreachability detection on host A will kick in,
   because it does not see any return traffic (payload or Keepalive
   messages) for the context.  This will result in host A sending Probe
   messages to host B to find a working locator pair.  The effect of
   this is that host B will notice that it does not have a context for
   the ULID pair <A1, B2> and CT(B) = X, which will make host B send an
   R1bis packet to re-establish that context.  The re-established
   context, just like in the previous section, will get a unique CT(B)
   assigned by host B, thus there will no longer be any confusion.

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






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Appendix D.  Design Alternatives

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

Appendix D.1.  Context granularity

   Over the years various suggestions have been made whether the shim
   should, even if it operates at the IP layer, be aware of ULP
   connections and sessions, and as a result be able to make separate
   shim contexts for separate ULP connections and sessions.  A few
   different options have been discussed:

   o  Each ULP connection maps to its own shim context.

   o  The shim is unaware of the ULP notion of connections and just
      operates on a host-to-host (IP address) granularity.

   o  Hybrids where the shim is aware of some ULPs (such as TCP) and
      handles other ULPs on a host-to-host basis.

   Having shim state for every ULP connection potentially means higher
   overhead since the state setup overhead might become significant;
   there is utility in being able to amortize this over multiple
   connections.

   But being completely unaware of the ULP connections might hamper ULPs
   that want different communication to use different locator pairs, for
   instance for quality or cost reasons.

   The protocol has a shim which operates with host-level granularity
   (strictly speaking, with ULID-pair granularity, to be able to
   amortize the context establishment over multiple ULP connections.
   This is combined with the ability for shim-aware ULPs to request
   context forking so that different ULP traffic can use different
   locator pairs.

Appendix D.2.  Demultiplexing of data packets in shim6 communications

   Once a ULID-pair context is established between two hosts, packets
   may carry locators that differ from the ULIDs presented to the ULPs
   using the established context.  One of main functions of the SHIM6
   layer is to perform the mapping between the locators used to forward
   packets through the network and the ULIDs presented to the ULP.  In
   order to perform that translation for incoming packets, the SHIM6



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   layer needs to first identify which of the incoming packets need to
   be translated and then perform the mapping between locators and ULIDs
   using the associated context.  Such operation is called
   demultiplexing.  It should be noted that because any address can be
   used both as a locator and as a ULID, additional information other
   than the addresses carried in packets, need to be taken into account
   for this operation.

   For example, if a host has address A1 and A2 and starts communicating
   with a peer with addresses B1 and B2, then some communication
   (connections) might use the pair <A1, B1> as ULID and others might
   use e.g., <A2, B2>.  Initially there are no failures so these address
   pairs are used as locators i.e. in the IP address fields in the
   packets on the wire.  But when there is a failure the shim6 layer on
   A might decide to send packets that used <A1, B1> as ULIDs using <A2,
   B2> as the locators.  In this case B needs to be able to rewrite the
   IP address field for some packets and not others, but the packets all
   have the same locator pair.

   In order to accomplish the demultiplexing operation successfully,
   data packets carry a context tag that allows the receiver of the
   packet to determine the shim context to be used to perform the
   operation.

   Two mechanisms for carrying the context tag information have been
   considered in depth during the shim protocol design.  Those carrying
   the context tag in the flow label field of the IPv6 header and the
   usage of a new extension header to carry the context tag.  In this
   appendix we will describe the pros and cons of each approach and
   justify the selected option.

Appendix D.2.1.  Flow-label

   A possible approach is to carry the context tag in the Flow Label
   field of the IPv6 header.  This means that when a shim6 context is
   established, a Flow Label value is associated with this context (and
   perhaps a separate flow label for each direction).

   The simplest approach that does this is to have the triple <Flow
   Label, Source Locator, Destination Locator> identify the context at
   the receiver.

   The problem with this approach is that because the locator sets are
   dynamic, it is not possible at any given moment to be sure that two
   contexts for which the same context tag is allocated will have
   disjoint locator sets during the lifetime of the contexts.

   Suppose that Node A has addresses IPA1, IPA2, IPA3 and IPA4 and that



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   Host B has addresses IPB1 and IPB2.

   Suppose that two different contexts are established between HostA and
   HostB.

   Context #1 is using IPA1 and IPB1 as ULIDs.  The locator set
   associated to IPA1 is IPA1 and IPA2 while the locator set associated
   to IPB1 is just IPB1.

   Context #2 uses IPA3 and IPB2 as ULIDs.  The locator set associated
   to IPA3 is IPA3 and IPA4 and the locator set associated to IPB2 is
   just IPB2.

   Because the locator sets of the Context #1 and Context #2 are
   disjoint, hosts could think that the same context tag value can be
   assigned to both of them.  The problem arrives when later on IPA3 is
   added as a valid locator for IPA1 and IPB2 is added as a valid
   locator for IPB1 in Context #1.  In this case, the triple <Flow
   Label, Source Locator, Destination Locator> would not identify a
   unique context anymore and correct demultiplexing is no longer
   possible.

   A possible approach to overcome this limitation is simply not to
   repeat the Flow Label values for any communication established in a
   host.  This basically means that each time a new communication that
   is using different ULIDs is established, a new Flow Label value is
   assigned to it.  By this mean, each communication that is using
   different ULIDs can be differentiated because it has a different Flow
   Label value.

   The problem with such approach is that it requires that the receiver
   of the communication allocates the Flow Label value used for incoming
   packets, in order to assign them uniquely.  For this, a shim
   negotiation of the Flow Label value to use in the communication is
   needed before exchanging data packets.  This poses problems with non-
   shim capable hosts, since they would not be able to negotiate an
   acceptable value for the Flow Label.  This limitation can be lifted
   by marking the packets that belong to shim sessions from those that
   do not.  These marking would require at least a bit in the IPv6
   header that is not currently available, so more creative options
   would be required, for instance using new Next Header values to
   indicate that the packet belongs to a shim6 enabled communication and
   that the Flow Label carries context information as proposed in the
   now expired NOID draft. .  However, even if this is done, this
   approach is incompatible with the deferred establishment capability
   of the shim protocol, which is a preferred function, since it
   suppresses the delay due to the shim context establishment prior to
   initiation of the communication and it also allows nodes to define at



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   which stage of the communication they decide, based on their own
   policies, that a given communication requires to be protected by the
   shim.

   In order to cope with the identified limitations, an alternative
   approach that does not constraints the flow label values used by
   communications that are using ULIDs equal to the locators (i.e. no
   shim translation) is to only require that different flow label values
   are assigned to different shim contexts.  In such approach
   communications start with unmodified flow label usage (could be zero,
   or as suggested in [16]).  The packets sent after a failure when a
   different locator pair is used would use a completely different flow
   label, and this flow label could be allocated by the receiver as part
   of the shim context establishment.  Since it is allocated during the
   context establishment, the receiver of the "failed over" packets can
   pick a flow label of its choosing (that is unique in the sense that
   no other context is using it as a context tag), without any
   performance impact, and respecting that for each locator pair, the
   flow label value used for a given locator pair doesn't change due to
   the operation of the multihoming shim.

   In this approach, the constraint is that Flow Label values being used
   as context identifiers cannot be used by other communications that
   use non-disjoint locator sets.  This means that once that a given
   Flow Label value has been assigned to a shim context that has a
   certain locator sets associated, the same value cannot be used for
   other communications that use an address pair that is contained in
   the locator sets of the context.  This is a constraint in the
   potential Flow Label allocation strategies.

   A possible workaround to this constraint is to mark shim packets that
   require translation, in order to differentiate them from regular IPv6
   packets, using the artificial Next Header values described above.  In
   this case, the Flow Label values constrained are only those of the
   packets that are being translated by the shim.  This last approach
   would be the preferred approach if the context tag is to be carried
   in the Flow Label field.  This is not only because it imposes the
   minimum constraints to the Flow Label allocation strategies, limiting
   the restrictions only to those packets that need to be translated by
   the shim, but also because Context Loss detection mechanisms greatly
   benefit from the fact that shim data packets are identified as such,
   allowing the receiving end to identify if a shim context associated
   to a received packet is suppose to exist, as it will be discussed in
   the Context Loss detection appendix below.

Appendix D.2.2.  Extension Header

   Another approach, which is the one selected for this protocol, is to



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   carry the context tag in a new Extension Header.  These context tags
   are allocated by the receiving end during the shim6 protocol initial
   negotiation, implying that each context will have two context tags,
   one for each direction.  Data packets will be demultiplexed using the
   context tag carried in the Extension Header.  This seems a clean
   approach since it does not overload existing fields.  However, it
   introduces additional overhead in the packet due to the additional
   header.  The additional overhead introduced is 8 octets.  However, it
   should be noted that the context tag is only required when a locator
   other than the one used as ULID is contained in the packet.  Packets
   where both the source and destination address fields contain the
   ULIDs do not require a context tag, since no rewriting is necessary
   at the receiver.  This approach would reduce the overhead, because
   the additional header is only required after a failure.  On the other
   hand, this approach would cause changes in the available MTU for some
   packets, since packets that include the Extension Header will have an
   MTU 8 octets shorter.  However, path changes through the network can
   result in different MTU in any case, thus having a locator change,
   which implies a path change, affect the MTU doesn't introduce any new
   issues.

Appendix D.3.  Context Loss Detection

   In this appendix we will present different approaches considered to
   detect context loss and potential context recovery strategies.  The
   scenario being considered is the following: Node A and Node B are
   communicating using IPA1 and IPB1.  Sometime later, a shim context is
   established between them, with IPA1 and IPB1 as ULIDs and
   IPA1,...,IPAn and IPB1,...,IPBm as locator set respectively.

   It may happen, that later on, one of the hosts, e.g.  Host A looses
   the shim context.  The reason for this can be that Host A has a more
   aggressive garbage collection policy than HostB or that an error
   occurred in the shim layer at host A resulting in the loss of the
   context state.

   The mechanisms considered in this appendix are aimed to deal with
   this problem.  There are essentially two tasks that need to be
   performed in order to cope with this problem: first, the context loss
   must be detected and second the context needs to be recovered/
   reestablished.

   Mechanisms for detecting context. loss

   These mechanisms basically consist in that each end of the context
   periodically sends a packet containing context-specific information
   to the other end.  Upon reception of such packets, the receiver
   verifies that the required context exists.  In case that the context



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   does not exist, it sends a packet notifying the problem to the
   sender.

   An obvious alternative for this would be to create a specific context
   keepalive exchange, which consists in periodically sending packets
   with this purpose.  This option was considered and discarded because
   it seemed an overkill to define a new packet exchange to deal with
   this issue.

   An alternative is to piggyback the context loss detection function in
   other existent packet exchanges.  In particular, both shim control
   and data packets can be used for this.

   Shim control packets can be trivially used for this, because they
   carry context specific information, so that when a node receives one
   of such packets, it will verify if the context exists.  However, shim
   control frequency may not be adequate for context loss detection
   since control packet exchanges can be very limited for a session in
   certain scenarios.

   Data packets, on the other hand, are expected to be exchanged with a
   higher frequency but they do not necessarily carry context specific
   information.  In particular, packets flowing before a locator change
   (i.e. packet carrying the ULIDs in the address fields) do not need
   context information since they do not need any shim processing.
   Packets that carry locators that differ from the ULIDs carry context
   information.

   However, we need to make a distinction here between the different
   approaches considered to carry the context tag, in particular between
   those approaches where packets are explicitly marked as shim packets
   and those approaches where packets are not marked as such.  For
   instance, in the case where the context tag is carried in the Flow
   Label and packets are not marked as shim packets (i.e. no new Next
   Header values are defined for shim), a receiver that has lost the
   associated context is not able to detect that the packet is
   associated with a missing context.  The result is that the packet
   will be passed unchanged to the upper layer protocol, which in turn
   will probably silently discard it due to a checksum error.  The
   resulting behavior is that the context loss is undetected.  This is
   one additional reason to discard an approach that carries the context
   tag in the Flow Label field and does not explicitly mark the shim
   packets as such.  On the other hand, approaches that mark shim data
   packets (like the Extension Header or the Flow Label with new Next
   Header values approaches) allow the receiver to detect if the context
   associated to the received packet is missing.  In this case, data
   packets also perform the function of a context loss detection
   exchange.  However, it must be noted that only those packets that



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   carry a locator that differs form the ULID are marked.  This
   basically means that context loss will be detected after an outage
   has occurred i.e. alternative locators are being used.

   Summarizing, the proposed context loss detection mechanisms uses shim
   control packets and payload extension headers to detect context loss.
   Shim control packets detect context loss during the whole lifetime of
   the context, but the expected frequency in some cases is very low.
   On the other hand, payload extension headers have a higher expected
   frequency in general, but they only detect context loss after an
   outage.  This behavior implies that it will be common that context
   loss is detected after a failure i.e. once that it is actually
   needed.  Because of that, a mechanism for recovering from context
   loss is required if this approach is used.

   Overall, the mechanism for detecting lost context would work as
   follows: the end that still has the context available sends a message
   referring to the context.  Upon the reception of such message, the
   end that has lost the context identifies the situation and notifies
   the context loss event to the other end by sending a packet
   containing the lost context information extracted from the received
   packet.

   One option is to simply send an error message containing the received
   packets (or at least as much of the received packet that the MTU
   allows to fit in).  One of the goals of this notification is to allow
   the other end that still retains context state, to reestablish the
   lost context.  The mechanism to reestablish the loss context consists
   in performing the 4-way initial handshake.  This is a time consuming
   exchange and at this point time may be critical since we are
   reestablishing a context that is currently needed (because context
   loss detection may occur after a failure).  So, another option, which
   is the one used in this protocol, is to replace the error message by
   a modified R1 message, so that the time required to perform the
   context establishment exchange can be reduced.  Upon the reception of
   this modified R1 message, the end that still has the context state
   can finish the context establishment exchange and restore the lost
   context.

Appendix D.4.  Securing locator sets

   The adoption of a protocol like SHIM that allows the binding of a
   given ULID with a set of locators opens the doors for different types
   of redirection attacks as described in [19].  The goal in terms of
   security for the design of the shim protocol is not to introduce any
   new vulnerability in the Internet architecture.  It is a non-goal to
   provide additional protection than the currently available in the
   single-homed IPv6 Internet.



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   Multiple security mechanisms were considered to protect the shim
   protocol.  In this appendix we will present some of them.

   The simplest option to protect the shim protocol was to use cookies
   i.e. a randomly generated bit string that is negotiated during the
   context establishment phase and then it is included in following
   signaling messages.  By this mean, it would be possible to verify
   that the party that was involved in the initial handshake is the same
   party that is introducing new locators.  Moreover, before using a new
   locator, an exchange is performed through the new locator, verifying
   that the party located at the new locator knows the cookie i.e. that
   it is the same party that performed the initial handshake.

   While this security mechanisms does indeed provide a fair amount of
   protection, it does leave the door open for the so-called time
   shifted attacks.  In these attacks, an attacker that once was on the
   path, it discovers the cookie by sniffing any signaling message.
   After that, the attacker can leave the path and still perform a
   redirection attack, since as he is in possession of the cookie, he
   can introduce a new locator in the locator set and he can also
   successfully perform the reachability exchange if he is able to
   receive packets at the new locator.  The difference with the current
   single-homed IPv6 situation is that in the current situation the
   attacker needs to be on-path during the whole lifetime of the attack,
   while in this new situation where only cookie protection if provided,
   an attacker that once was on the path can perform attacks after he
   has left the on-path location.

   Moreover, because the cookie is included in signaling messages, the
   attacker can discover the cookie by sniffing any of them, making the
   protocol vulnerable during the whole lifetime of the shim context.  A
   possible approach to increase the security was to use a shared secret
   i.e. a bit string that is negotiated during the initial handshake but
   that is used as a key to protect following messages.  With this
   technique, the attacker must be present on the path sniffing packets
   during the initial handshake, since it is the only moment where the
   shared secret is exchanged.  While this improves the security, it is
   still vulnerable to time shifted attacks, even though it imposes that
   the attacker must be on path at a very specific moment (the
   establishment phase) to actually be able to launch the attack.  While
   this seems to substantially improve the situation, it should be noted
   that, depending on protocol details, an attacker may be able to force
   the recreation of the initial handshake (for instance by blocking
   messages and making the parties think that the context has been
   lost), so the resulting situation may not differ that much from the
   cookie based approach.

   Another option that was discussed during the design of the protocol



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   was the possibility of using IPsec for protecting the shim protocol.
   Now, the problem under consideration in this scenario is how to
   securely bind an address that is being used as ULID with a locator
   set that can be used to exchange packets.  The mechanism provided by
   IPsec to securely bind the address used with the cryptographic keys
   is the usage of digital certificates.  This implies that an IPsec
   based solution would require that the generation of digital
   certificates that bind the key and the ULID by a common third trusted
   party for both parties involved in the communication.  Considering
   that the scope of application of the shim protocol is global, this
   would imply a global public key infrastructure.  The major issues
   with this approach are the deployment difficulties associated with a
   global PKI.

   Finally two different technologies were selected to protect the shim
   protocol: HBA [7] and CGA [6].  These two approaches provide a
   similar level of protection but they provide different functionality
   with a different computational cost.

   The HBA mechanism relies on the capability of generating all the
   addresses of a multihomed host as an unalterable set of intrinsically
   bound IPv6 addresses, known as an HBA set.  In this approach,
   addresses incorporate a cryptographic one-way hash of the prefix-set
   available into the interface identifier part.  The result is that the
   binding between all the available addresses is encoded within the
   addresses themselves, providing hijacking protection.  Any peer using
   the shim protocol node can efficiently verify that the alternative
   addresses proposed for continuing the communication are bound to the
   initial address through a simple hash calculation.  A limitation of
   the HBA technique is that once generated the address set is fixed and
   cannot be changed without also changing all the addresses of the HBA
   set.  In other words, the HBA technique does not support dynamic
   addition of address to a previously generated HBA set.  An advantage
   of this approach is that it requires only hash operations to verify a
   locator set, imposing very low computational cost to the protocol.

   In a CGA based approach the address used as ULID is a CGA that
   contains a hash of a public key in its interface identifier.  The
   result is a secure binding between the ULID and the associated key
   pair.  This allows each peer to use the corresponding private key to
   sign the shim messages that convey locator set information.  The
   trust chain in this case is the following: the ULID used for the
   communication is securely bound to the key pair because it contains
   the hash of the public key, and the locator set is bound to the
   public key through the signature.  The CGA approach then supports
   dynamic addition of new locators in the locator set, since in order
   to do that, the node only needs to sign the new locator with the
   private key associated with the CGA used as ULID.  A limitation of



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   this approach is that it imposes systematic usage of public key
   cryptography with its associate computational cost.

   Any of these two mechanisms HBA and CGA provide time-shifted attack
   protection, since the ULID is securely bound to a locator set that
   can only be defined by the owner of the ULID.

   So, the design decision adopted was that both mechanisms HBA and CGA
   are supported, so that when only stable address sets are required,
   the nodes can benefit from the low computational cost offered by HBA
   while when dynamic locator sets are required, this can be achieved
   through CGAs with an additional cost.  Moreover, because HBAs are
   defined as a CGA extension, the addresses available in a node can
   simultaneously be CGAs and HBAs, allowing the usage of the HBA and
   CGA functionality when needed without requiring a change in the
   addresses used.

Appendix D.5.  ULID-pair context establishment exchange

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

   A key goal for the design of this exchange was that protection
   against DoS attacks.  The attack under consideration was basically a
   situation where an attacker launches a great amount of ULID-pair
   establishment request packets, exhausting victim's resources, similar
   to TCP SYN flooding attacks.

   A 4 way-handshake exchange protects against these attacks because the
   receiver does not creates any state associate to a given context
   until the reception of the second packet which contains a prior
   contact proof in the form of a token.  At this point the receiver can
   verify that at least the address used by the initiator is at some
   extent valid, since the initiator is able to receive packets at this
   address.  In the worse case, the responder can track down the
   attacker using this address.  The drawback of this approach is that
   it imposes a 4 packet exchange for any context establishment.  This
   would be a great deal if the shim context needed to be established up
   front, before the communication can proceed.  However, thanks to
   deferred context establishment capability of the shim protocol, this
   limitation has a reduced impact in the performance of the protocol.
   (It may however have a greater impact in the situation of context
   recover as discussed earlier, but in this case, it is possible to
   perform optimizations to reduce the number of packets as described
   above)

   The other option considered was a 2-way handshake with the
   possibility to fall back to a 4-way handshake in case of attack.  In



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   this approach, the ULID-pair establishment exchange normally consists
   in a 2-packet exchange and it does not verify that the initiator has
   performed a prior contact before creating context state.  In case
   that a DoS attack is detected, the responder falls back to a 4-way
   handshake similar to the one described previously in order to prevent
   the detected attack to proceed.  The main difficulty with this attack
   is how to detect that a responder is currently under attack.  It
   should be noted, that because this is 2-way exchange, it is not
   possible to use the number of half open sessions (as in TCP) to
   detect an ongoing attack and different heuristics need to be
   considered.

   The design decision taken was that considering the current impact of
   DoS attacks and the low impact of the 4-way exchange in the shim
   protocol thanks to the deferred context establishment capability, a
   4-way exchange would be adopted for the base protocol.

Appendix D.6.  Updating locator sets

   There are two possible approaches to the addition and removal of
   locators: atomic and differential approaches.  The atomic approach
   essentially send the complete locators set each time that a variation
   in the locator set occurs.  The differential approach send the
   differences between the existing locator set and the new one.  The
   atomic approach imposes additional overhead, since all the locator
   set has to be exchanged each time while the differential approach
   requires re-synchronization of both ends through changes i.e. that
   both ends have the same idea about what the current locator set is.

   Because of the difficulties imposed by the synchronization
   requirement, the atomic approach was selected.

Appendix D.7.  State Cleanup

   There are essentially two approaches for discarding an existing state
   about locators, keys and identifiers of a correspondent node: a
   coordinated approach and an unilateral approach.

   In the unilateral approach, each node discards the information about
   the other node without coordination with the other node based on some
   local timers and heuristics.  No packet exchange is required for
   this.  In this case, it would be possible that one of the nodes has
   discarded the state while the other node still hasn't.  In this case,
   a No-Context error message may be required to inform about the
   situation and possibly a recovery mechanism is also needed.

   A coordinated approach would use an explicit CLOSE mechanism, akin to
   the one specified in HIP [25].  If an explicit CLOSE handshake and



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   associated timer is used, then there would no longer be a need for
   the No Context Error message due to a peer having garbage collected
   its end of the context.  However, there is still potentially a need
   to have a No Context Error message in the case of a complete state
   loss of the peer (also known as a crash followed by a reboot).  Only
   if we assume that the reboot takes at least the CLOSE timer, or that
   it is ok to not provide complete service until CLOSE timer minutes
   after the crash, can we completely do away with the No Context Error
   message.

   In addition, other aspect that is relevant for this design choice is
   the context confusion issue.  In particular, using an unilateral
   approach to discard context state clearly opens the possibility of
   context confusion, where one of the ends unilaterally discards the
   context state, while the peer does not.  In this case, the end that
   has discarded the state can re-use the context tag value used for the
   discarded state for a another context, creating a potential context
   confusion situation.  In order to illustrate the cases where problems
   would arise consider the following scenario:

   o  Hosts A and B establish context 1 using CTA and CTB as context
      tags.

   o  Later on, A discards context 1 and the context tag value CTA
      becomes available for reuse.

   o  However, B still keeps context 1.

   This would become a context confusion situation in the following two
   cases:

   o  A new context 2 is established between A and B with a different
      ULID pair (or Forked Instance Identifier), and A uses CTA as
      context tag, If the locator sets used for both contexts are not
      disjoint, we are in a context confusion situation.

   o  A new context is established between A and C and A uses CTA as
      context tag value for this new context.  Later on, B sends Payload
      extension header and/or control messages containing CTA, which
      could be interpreted by A as belonging to context 2 (if no proper
      care is taken).  Again we are in a context confusion situation.

   One could think that using a coordinated approach would eliminate
   these context confusion situations, making the protocol much simpler.
   However, this is not the case, because even in the case of a
   coordinated approach using a CLOSE/CLOSE ACK exchange, there is still
   the possibility of a host rebooting without having the time to
   perform the CLOSE exchange.  So, it is true that the coordinated



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   approach eliminates the possibility of a context confusion situation
   because premature garbage collection, but it does not prevents the
   same situations due to a crash and reboot of one of the involved
   hosts.  The result is that even if we went for a coordinated
   approach, we would still need to deal with context confusion and
   provide the means to detect and recover from this situations.













































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Appendix E.  Change Log

   [RFC Editor: please remove this section]

   The following changes have been made since draft-ietf-shim6-proto-05:

   o  Removed the possibility to keep on uding the ULID after a
      renumbering event

   o  Editorial corrections resulting from Dave Meyer's and Jim Bound's
      reviews.

   The following changes have been made since draft-ietf-shim6-proto-04:

   o  Defined I1_RETRIES_MAX as 4.

   o  Added text in section 7.9 clarifying the no per context state is
      stored at the receiver in order to reply an I1 message.

   o  Added text in section 5 and in section 5.14 in particular, on
      defining additional options (including critical and non critical
      options).

   o  Added text in the security considerations about threats related to
      secret S for generating the validators and recommendation to
      change S periodically.

   o  Added text in the security considerations about the effects of
      attacks based on guessing the context tag being similar to
      spoofing source addresses in the case of payload packets.

   o  Added clarification on what a recent nonce is in I2 and I2bis.

   o  Removed (empty) open issues section.

   o  Editorial corrections.

   The following changes have been made since draft-ietf-shim6-proto-03:

   o  Editorial clarifications based on comments from Geoff, Shinta,
      Jari.

   o  Added "no IPv6 NATs as an explicit assumption.

   o  Moving some things out of the Introduction and Overview sections
      to remove all SHOULDs and MUSTs from there.





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   o  Added requirement that any Locator Preference options which use an
      element length greater than 3 octets have the already defined
      first 3 octets of flags, priority and weight.

   o  Fixed security hole where a single message (I1) could cause
      CT(peer) to be updated.  Now a three-way handshake is required
      before CT(peer) is updated for an existing context.

   The following changes have been made since draft-ietf-shim6-proto-02:

   o  Replaced the Context Error message with the R1bis message.

   o  Removed the Packet In Error option, since it was only used in the
      Context Error message.

   o  Introduced a I2bis message which is sent in response to an I1bis
      message, since the responders processing is quite in this case
      than in the regular R1 case.

   o  Moved the packet formats for the Keepalive and Probe message types
      and Event option to [8].  Only the message type values and option
      type value are specified for those in this document.

   o  Removed the unused message types.

   o  Added a state machine description as an appendix.

   o  Filled in all the TBDs - except the IANA assignment of the
      protocol number.

   o  Specified how context recovery and forked contexts work together.
      This required the introduction of a Forked Instance option to be
      able to tell which of possibly forked instances is being
      recovered.

   o  Renamed the "host-pair context" to be "ULID-pair context".

   o  Picked some initial retransmit timers for I1 and I2; 4 seconds.

   o  Added timer values as protocol constants.  The retransmit timers
      use binary exponential backoff and randomization (between .5 and
      1.5 of the nominal value).

   o  Require that the R1/R1bis verifiers be usable for some minimum
      time so that the initiator knows for how long time it can safely
      retransmit I2 before it needs to go back to sending I1 again.
      Picked 30 seconds.




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   o  Split the message type codes into 0-63, which will not generate
      R1bis messages, and 64-127 which will generate R1bis messages.
      This allows extensibility of the protocol with new message types
      while being able to control when R1bis is generated.

   o  Expanded the context tag from 32 to 47 bits.

   o  Specified that enough locators need to be included in I2 and R2
      messages.  Specified that the HBA/CGA verification must be
      performed when the locator set is received.

   o  Specified that ICMP parameter problem errors are sent in certain
      error cases, for instance when the verification method is unknown
      to the receiver, or there is an unknown message type or option
      type.

   o  Renamed "payload message" to be "payload extension header".

   o  Many editorial clarifications suggested by Geoff Huston.

   o  Modified the dispatching of payload extension header to only
      compare CT(local) i.e., not compare the source and destination
      IPv6 address fields.

   The following changes have been made since draft-ietf-shim6-proto-00:

   o  Removed the use of the flow label and the overloading of the IP
      protocol numbers.  Instead, when the locator pair is not the ULID
      pair, the ULP payloads will be carried with an 8 octet extension
      header.  The belief is that it is possible to remove these extra
      bytes by defining future shim6 extensions that exchange more
      information between the hosts, without having to overload the flow
      label or the IP protocol numbers.

   o  Grew the context tag from 20 bits to 32 bits, with the possibility
      to grow it to 47 bits.  This implies changes to the message
      formats.

   o  Almost by accident, the new shim6 message format is very close to
      the HIP message format.

   o  Adopted the HIP format for the options, since this makes it easier
      to describe variable length options.  The original, ND-style,
      option format requires internal padding in the options to make
      them 8 octet length in total, while the HIP format handles that
      using the option length field.





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   o  Removed some of the control messages, and renamed the other ones.

   o  Added a "generation" number to the Locator List option, so that
      the peers can ensure that the preferences refer to the right
      "version" of the Locator List.

   o  In order for FBD and exploration to work when there the use of the
      context is forked, that is different ULP messages are sent over
      different locator pairs, things are a lot easier if there is only
      one current locator pair used for each context.  Thus the forking
      of the context is now causing a new context to be established for
      the same ULID; the new context having a new context tag.  The
      original context is referred to as the "default" context for the
      ULID pair.

   o  Added more background material and textual descriptions.



































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19.  References

19.1.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [2]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
        Specification", RFC 2460, December 1998.

   [3]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
        for IP Version 6 (IPv6)", RFC 2461, December 1998.

   [4]  Thomson, S. and T. Narten, "IPv6 Stateless Address
        Autoconfiguration", RFC 2462, December 1998.

   [5]  Conta, A. and S. Deering, "Internet Control Message Protocol
        (ICMPv6) for the Internet Protocol Version 6 (IPv6)
        Specification", RFC 2463, December 1998.

   [6]  Aura, T., "Cryptographically Generated Addresses (CGA)",
        RFC 3972, March 2005.

   [7]  Bagnulo, M., "Hash Based Addresses (HBA)",
        draft-ietf-shim6-hba-01 (work in progress), September 2006.

   [8]  Arkko, J. and I. Beijnum, "Failure Detection and Locator Pair
        Exploration Protocol for IPv6  Multihoming",
        draft-ietf-shim6-failure-detection-06 (work in progress),
        September 2006.

19.2.  Informative References

   [9]   Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
         specifying the location of services (DNS SRV)", RFC 2782,
         February 2000.

   [10]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
         Defeating Denial of Service Attacks which employ IP Source
         Address Spoofing", BCP 38, RFC 2827, May 2000.

   [11]  Narten, T. and R. Draves, "Privacy Extensions for Stateless
         Address Autoconfiguration in IPv6", RFC 3041, January 2001.

   [12]  Draves, R., "Default Address Selection for Internet Protocol
         version 6 (IPv6)", RFC 3484, February 2003.

   [13]  Bagnulo, M., "Updating RFC 3484 for multihoming support",



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         draft-bagnulo-ipv6-rfc3484-update-00 (work in progress),
         December 2005.

   [14]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications", STD 64,
         RFC 3550, July 2003.

   [15]  Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
         Multihoming Architectures", RFC 3582, August 2003.

   [16]  Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, "IPv6
         Flow Label Specification", RFC 3697, March 2004.

   [17]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
         Requirements for Security", BCP 106, RFC 4086, June 2005.

   [18]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
         Addresses", RFC 4193, October 2005.

   [19]  Nordmark, E. and T. Li, "Threats Relating to IPv6 Multihoming
         Solutions", RFC 4218, October 2005.

   [20]  Huitema, C., "Ingress filtering compatibility for IPv6
         multihomed sites", draft-huitema-shim6-ingress-filtering-00
         (work in progress), September 2005.

   [21]  Bagnulo, M. and E. Nordmark, "SHIM - MIPv6 Interaction",
         draft-bagnulo-shim6-mip-00 (work in progress), July 2005.

   [22]  Nordmark, E., "Shim6 Application Referral Issues",
         draft-ietf-shim6-app-refer-00 (work in progress), July 2005.

   [23]  Bagnulo, M. and J. Abley, "Applicability Statement for the
         Level 3 Multihoming Shim Protocol (Shim6)",
         draft-ietf-shim6-applicability-01 (work in progress),
         June 2006.

   [24]  Huston, G., "Architectural Commentary on Site Multi-homing
         using a Level 3 Shim", draft-ietf-shim6-arch-00 (work in
         progress), July 2005.

   [25]  Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-06
         (work in progress), June 2006.

   [26]  Eronen, P., "IKEv2 Mobility and Multihoming Protocol (MOBIKE)",
         draft-ietf-mobike-protocol-08 (work in progress),
         February 2006.




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Authors' Addresses

   Erik Nordmark
   Sun Microsystems
   17 Network Circle
   Menlo Park, CA 94025
   USA

   Phone: +1 650 786 2921
   Email: erik.nordmark@sun.com


   Marcelo Bagnulo
   Universidad Carlos III de Madrid
   Av. Universidad 30
   Leganes, Madrid  28911
   SPAIN

   Phone: +34 91 6248814
   Email: marcelo@it.uc3m.es
   URI:   http://www.it.uc3m.es






























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