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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: March 31, 2006                               September 27, 2005


                   Level 3 multihoming shim protocol
                     draft-ietf-shim6-proto-00.txt

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   This Internet-Draft will expire on March 31, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The SHIM6 working group is exploring a layer 3 shim approach for
   providing locator agility below the transport protocols, so that
   multihoming can be provided for IPv6 with failover and load spreading
   properties, 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,



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   should the original one stop working.

   This document picks a particular approach to such a protocol and
   tries to flush out a bunch of details, with the hope that the WG can
   better understand the details in this proposal as well as discovering
   and understanding alternative designs that might be better.  Thus
   this proposal is my no means cast in stone as the direction; quite to
   the contrary it is a depth first exploration of the design space.

Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1  Placement of the shim  . . . . . . . . . . . . . . . . . .   4
   2.   Terminology  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1  Definitions  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2  Notational Conventions . . . . . . . . . . . . . . . . . .   6
   3.   Assumptions  . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.   Protocol Overview  . . . . . . . . . . . . . . . . . . . . .   8
     4.1  Context Tags and Use of Flow Label . . . . . . . . . . . .   9
     4.2  Protocol type overloading  . . . . . . . . . . . . . . . .  10
     4.3  Securing shim6 . . . . . . . . . . . . . . . . . . . . . .  11
     4.4  Overview of Shim Control Messages  . . . . . . . . . . . .  11
   5.   Message Formats  . . . . . . . . . . . . . . . . . . . . . .  14
     5.1  Common Shim6 header  . . . . . . . . . . . . . . . . . . .  14
     5.2  I1 Message Format  . . . . . . . . . . . . . . . . . . . .  15
     5.3  R1 Message Format  . . . . . . . . . . . . . . . . . . . .  16
     5.4  I2 Message Format  . . . . . . . . . . . . . . . . . . . .  17
     5.5  R2 Message Format  . . . . . . . . . . . . . . . . . . . .  19
     5.6  No Context Error Message Format  . . . . . . . . . . . . .  20
     5.7  Locator List Update Message Format . . . . . . . . . . . .  21
     5.8  Locator List Update Acknowledgement Message Format . . . .  22
     5.9  Rehome Request Message Format  . . . . . . . . . . . . . .  23
     5.10   Rehome Acknowledgement Message Format  . . . . . . . . .  23
     5.11   Reachability Probe Message Format  . . . . . . . . . . .  23
     5.12   Reachability Probe Reply Message Format  . . . . . . . .  24
     5.13   Payload Message Format . . . . . . . . . . . . . . . . .  25
     5.14   Keepalive Message Format . . . . . . . . . . . . . . . .  26
     5.15   Locator Pair Test Message Format . . . . . . . . . . . .  27
     5.16   Locator Pair Test Reply Message Format . . . . . . . . .  28
     5.17   Context Locator Pair Explore Message Format  . . . . . .  29
   6.   Option Formats . . . . . . . . . . . . . . . . . . . . . . .  31
     6.1  Validator Option Format  . . . . . . . . . . . . . . . . .  32
     6.2  Locator List Option Format . . . . . . . . . . . . . . . .  32
     6.3  Locator Preferences Option Format  . . . . . . . . . . . .  33
     6.4  CGA Parameter Data Structure Option Format . . . . . . . .  34
     6.5  CGA Signature Option Format  . . . . . . . . . . . . . . .  35
     6.6  ULID Pair Option Format  . . . . . . . . . . . . . . . . .  35
     6.7  Packet In Error Option Format  . . . . . . . . . . . . . .  36



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     6.8  Explorer Results Option Format . . . . . . . . . . . . . .  36
   7.   Conceptual Model of a Host . . . . . . . . . . . . . . . . .  38
     7.1  Conceptual Data Structures . . . . . . . . . . . . . . . .  38
   8.   Establishing Host Pair Contexts  . . . . . . . . . . . . . .  40
     8.1  Sending I1 messages  . . . . . . . . . . . . . . . . . . .  40
     8.2  Receiving I1 messages  . . . . . . . . . . . . . . . . . .  40
     8.3  Receiving R1 messages  . . . . . . . . . . . . . . . . . .  40
     8.4  Retransmitting I1 messages . . . . . . . . . . . . . . . .  40
     8.5  Receiving I2 messages  . . . . . . . . . . . . . . . . . .  40
     8.6  Retransmitting I2 messages . . . . . . . . . . . . . . . .  40
     8.7  Concurrent context establishment . . . . . . . . . . . . .  40
   9.   No Such Content Errors . . . . . . . . . . . . . . . . . . .  41
   10.  Handling ICMP Error Messages . . . . . . . . . . . . . . . .  42
   11.  Taredown of the Host Pair Context  . . . . . . . . . . . . .  43
   12.  Updating the Locator Pairs . . . . . . . . . . . . . . . . .  44
   13.  Various Probe Mechanisms . . . . . . . . . . . . . . . . . .  45
   14.  Rehoming to a Different Locator Pair . . . . . . . . . . . .  46
   15.  Payload Packets before a Switch  . . . . . . . . . . . . . .  47
   16.  Payload Packets after a Switch . . . . . . . . . . . . . . .  48
   17.  Open Issues  . . . . . . . . . . . . . . . . . . . . . . . .  50
   18.  Design Alternatives  . . . . . . . . . . . . . . . . . . . .  52
     18.1   State Cleanup  . . . . . . . . . . . . . . . . . . . . .  52
     18.2   Not Overloading the Flow Label . . . . . . . . . . . . .  52
     18.3   Detecting Context Loss . . . . . . . . . . . . . . . . .  53
   19.  Implications Elsewhere . . . . . . . . . . . . . . . . . . .  55
   20.  Security Considerations  . . . . . . . . . . . . . . . . . .  57
   21.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . .  58
   22.  References . . . . . . . . . . . . . . . . . . . . . . . . .  59
     22.1   Normative References . . . . . . . . . . . . . . . . . .  59
     22.2   Informative References . . . . . . . . . . . . . . . . .  59
        Author's Address . . . . . . . . . . . . . . . . . . . . . .  60
        Intellectual Property and Copyright Statements . . . . . . .  61



















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1.  Introduction

   The SHIM6 working group, and the MULTI6 WG that preceded it, is
   exploring a layer 3 shim approach for providing locator agility below
   the transport protocols, so that multihoming can be provided for IPv6
   with failover and load spreading properties, 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.

   This document takes the outlines contained in [16] and [15] and
   expands to an actual proposed protocol.

   We assume that redirection attacks are prevented using the mechanism
   specified in HBA [4].

   The WG mailing list is discussing the scheme used for reachability
   detection [5].  The schemes that are being discussed are Context
   Unreachability Detection (CUD) or Force Bidirectional communication
   Detection (FBD).  This document doesn't discuss the tradeoffs between
   the two, but it does suggest a set of keepalive and probe messages
   that are sufficient to handle both.  Once the WG has decided which
   approach to take, we can remove the unneeded messages.

   There is a related but slightly separate issue of how the hosts can
   find which of the locator pairs is working after a failure.  This is
   discussed in [6].  We don't yet know how these details will be done,
   but this draft specifies a separate "Explore" message as a
   placeholder for such a protocol mechanism.

1.1  Placement of the shim

   TBD: Copy material from [16].















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2.  Terminology

   This document uses the terms MUST, SHOULD, RECOMMENDED, MAY, SHOULD
   NOT and MUST NOT defined in RFC 2119 [7].  The terms defined in RFC
   2460 [1] are also used.

2.1  Definitions

   This document introduces the following terms (taken from [16]):

   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 a 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 (stack
                       name in [18]).  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.

   upper-layer identifier (ULID)
                       An IP locator 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



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                       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

   Host-pair context   The state that the multihoming shim maintains for
                       a particular peer.  The peer is identified by one
                       or more ULIDs.


2.2  Notational Conventions

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

   FQDN(A) is the domain name for A.

   Ls(A) is the locator set for A, which consists of the locators L1(A),
   L2(A), ...  Ln(A).

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

   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 7 for a
   description of the conceptual data structures.









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3.  Assumptions

   The general approach of a level3 shim as well as this specific
   proposal makes the following assumptions:

   o  When there is ingress filtering in the ISPs, that the use of all
      {source, destination} locator pairs will cause the packets to exit
      using different ISPs so that all exit ISPs can be tried.  Since
      there might be only one destination locator, when the peer
      supports shim6 but is not multihomed, this implies that the
      selection of the exit ISP should be related to the source address
      in the packets.

   o  Even without ingress filtering, there is the assumption that if
      the host tries all {source, destination} locator pairs, that it
      has done a good enough job of trying to find a working path to the
      peer.  Since we want the protocol to provide benefits even if the
      peer has a single locator, this seems to imply that the choice of
      source locator needs to somehow affect the exit path from the
      site.































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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 B using some upper-
      layer protocol.  This results in the ULP on A sending packets to
      B. We call this the initial contact.  Assuming the IP addresses
      selected by Default Address Selection [9] work, then there is no
      action by the shim at this point in time.  Any shim context
      establishment can be deferred until later.

   o  Some heuristic on A or B (or both) determine that it might make
      sense to make this communication robust against locator failures.
      For instance, this heuristic might be that more than 50 packets
      have been sent or received.  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.

   o  Communication continues without any change for the ULP packets.
      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 advise 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 explore the different alternate locator pairs until a working
      pair is found, and rehome to using that pair.

   o  Once a working alternative locator pair has been found, the shim
      will rewrite the packets on transmit, and tag the packets with a
      shim context tag, and send them on the wire.  The receiver will
      use the <Source Locator, Destination Locator, 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.




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   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 an error
      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 data packets in shim6 are 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" so that the receiver
   can always determine the context to which they belong.  For commonly
   used IP protocols this is done by using a different value in the Flow
   Label field, that is, there is no additional header added to the
   packets.  But for other IP protocol types there is an extra 8 byte
   header inserted, which carries the next header value.

4.1  Context Tags and Use of Flow Label

   A context between to 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, the
   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 MUST uniquely identify one
   context.

   In addition, the non-shim6 messages, which we call payload packets,
   will not contain the ULIDs after a failure.  This introduces the
   requirement that the <peer locator, local locator, local context tag>
   MUST uniquely identify the context.  Since the peer's set of locators
   might be dynamic the simplest form of unique allocation of the local
   context tag is to pick a number that is unique on the host.  Hosts
   which serve multiple ULIDs using disjoint sets of locators can
   maintain the context tag allocation per such disjoint set.

   As an optimization, to ensure that payload packets, even after the
   locators have been switched from being the original ones, do not
   require an extra shim extension header, the proposal uses the Flow
   Label field in the IPv6 header to carry the context tag for common
   upper layer protocols such as TCP and UDP.  This works as follows:

   o  The allocation and usage of the flow label during the initial
      communication is unchanged.  Thus the TCP, UDP, etc packets are
      sent with a flow label which is allocated according to [11].

   o  When the context is created, each endpoint picks an unused context
      tag based on the constraints above, which is also not used as a



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      flow label for the set of locators.

   o  The context tag is used by the shim to refer to the shim state in
      the control messages (such as probes, and locator updates.

   o  The payload traffic (TCP, UDP, etc.) continue to flow unchanged.

   o  Should there be a need to switch to a different locator pair, then
      the TCP, UDP, etc packets will be sent using an alternate locator
      pair, and with a flow label that is the same as the (20 bit)
      context tag.

   The mechanism for detecting a loss of context state at the peer that
   is currently proposed in this document 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).  The next
   section specifies how this can be done.

   Note that there is no need for a single context to have more than one
   context tag; whether the locator pair is <A1, B2>, <A1, B3> or <A2,
   B3> the same context tag is used in the flow label field.  Only the
   payload packets using the original <A1, B1> locator pair use the flow
   label (which is different than the context tag).

   Whether we overload the flow label field to carry the context tag or
   not, any protocol (such as RSVP or NSIS) which signals information
   about flows from the host stack to devices in the path, need to be
   made aware of the locator agility introduced by a layer 3 shim, so
   that the signaling can be performed for the locator pairs that are
   currently being used.

4.2  Protocol type overloading

   The mechanism for detecting a loss of context state at the peer that
   is currently proposed in this document 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).  There is an
   alternative to detection of lost state outlined in Section 18.

   The idea is to steal a partial bit from the protocol type fields that
   are used in the Next Header values, so that the common upper layer
   protocols can be identified.

   For example:

   o  TCP has protocol 6; TCP using alternate locators has protocol P.





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   o  UDP has protocol 17; TCP using alternate locators has protocol
      P+1.

   o  ICMPv6 has protocol 58; ICMPv6 using alternate locators has
      protocol P+2.

   o  SCTP has protocol 132; SCTP using alternate locators has protocol
      P+3.

   o  DCCP has protocol ??; DCCP using alternate locators has protocol
      P+4.

   o  ESP has protocol 50; ESP using alternate locators has protocol
      P+5.

   o  AH has protocol 51; AH using alternate locators has protocol P+6.

   o  FRAG has protocol 44; FRAG using alternate locators has protocol
      P+7.

   Thus with 7 or so additional protocol field values we can do a
   reasonable job of overloading the flow label field and get detection
   of lost context state.

4.3  Securing shim6

   The mechanisms are secured using a combination of techniques:

   o  The HBA technique [4] for validating 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
      provides an IPv6 address that the attacker was using.

   o  The context establishment messages use nonces to prevent replay
      attacks.


4.4  Overview of Shim Control Messages

   The shim context establishment is accomplished using four messages;



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   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 [17].]

   There is a No Contex error message defined, when a control or payload
   packet arrives and there is no matching context state at the
   receiver.  When such a message is received, it will result in the
   destruction of the shim context and a re-establishment.

   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 Locator List Update message and
   acknowledgement.

   Even though 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.  Currently this mechanism has a
   separate message pair (Rehome Request and acknowledgement), but
   perhaps this can be encoded using the Locator List Update message
   pair with a preference option and no change to the list of locators.

   At least two approaches (CUD and FBD) have been discussed for the
   shim (un)reachability detection [5].  This document attempt to define
   messages for both cases; once the WG has picked an approach we can
   delete any unneeded messages.

   The CUD approach uses a probe message and acknowledgement, which can
   be suppressed e.g. using positive advise from the ULP.  This message
   pair also seems needed to verify that the host is indeed present at a
   new locator before the data stream is redirected to that locator, in
   order to prevent 3rd party DoS attacks.

   The FBD approach uses a keepalive message, which is sent when a host
   has received packets from the peer, but the ULP has not given the
   host an opportunity to send any payload packet to the peer.

   The above probe and keepalive messages assume we have an established
   host-pair context.  However, communication might fail during the
   initial context (that is, when the application or transport protocol
   is trying to setup some communication).  If we want the shim to be
   able to optimize discovering a working locator pair in that case, we
   need a mechanism to test the reachability of locators independent of
   some context.  We define a locator pair test message and
   acknowledgement for this purpose, even though it isn't yet clear
   whether we need such a thing.



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   Finally, when the context is established and there is a failure there
   needs to be a way to explore the set of locator pairs to efficiently
   find a working pair.  We define an explore message as a placeholder
   for some mechanism in this space [6].















































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5.  Message Formats

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

5.1  Common Shim6 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 only used for data payload
   which is carried with a shim6 header on the front.

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

   The common 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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |                    Type specific format                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   8-bit selector.  Normally set to NO_NXT_HDR (59).
                  Indicates the next header value for the shim6 payload
                  messages.

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

   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.






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   Type:          8-bit unsigned integer.  Identifies the actual message
                  from the table below.

  +------------+-----------------------------------------------------+
  | 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     |                   No Context Error                  |
  |            |                                                     |
  |      6     |                 Locator List Update                 |
  |            |                                                     |
  |      7     |         Locator List Update Acknowledgement         |
  |            |                                                     |
  |      8     |                    Rehome Request                   |
  |            |                                                     |
  |      9     |                Rehome Acknowledgement               |
  |            |                                                     |
  |     10     |                  Reachability Probe                 |
  |            |                                                     |
  |     11     |               Reachability Probe Reply              |
  |            |                                                     |
  |     12     |                       Payload                       |
  |            |                                                     |
  |     13     |                      Keepalive                      |
  |            |                                                     |
  |     14     |                  Locator Pair Test                  |
  |            |                                                     |
  |     15     |               Locator Pair Test Reply               |
  |            |                                                     |
  |     16     |             Context Locator pair explore            |
  +------------+-----------------------------------------------------+

                                  Table 1


5.2  I1 Message Format

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





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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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       1       | Res |      Initiator Context Tag              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Initiator Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          1

   Res:           4-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Initiator Context Tag: 20-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 allowed in the message:

   ULID pair:     TBD Do we need to carry the ULIDs, or assume they are
                  the same as the address fields in the IPv6 header?
                  Depends on how we handle failures during initial
                  contact.


5.3  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.








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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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       2       |                Reserved                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Initiator Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Responder Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          2

   Reserved:      24-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
                  initiator which the initiator will return in the I2
                  message.

   The following options are allowed in the message:

   Responder Validator: Variable length mandatory 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.


5.4  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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       3       | Res |      Initiator Context Tag              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Initiator Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Responder Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          3

   Res:           4-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Initiator Context Tag: 20-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.

   The following options are allowed in the message:

   Responder Validator: Variable length mandatory option.  Copied from
                  the Validator in the R1 message.

   ULID pair:     TBD Do we need to carry the ULIDs, or assume they are
                  the same as the address fields in the IPv6 header?

   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
                  validating the locator list MUST also be included.




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   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 validation.


5.5  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.

    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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       4       | Res |      Responder Context Tag              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Initiator Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          4

   Res:           4-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Responder Context Tag: 20-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 allowed in the message:




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   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
                  validating 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 validation.


5.6  No Context Error Message Format

   Should a host receive a packet (payload packet or shim6 control
   message such a a locator update) and the host does not have any
   context state for the locators (in the IPv6 source and destination
   fields) and the context tag, then it will generate a No Context
   Error.  The error includes the packet that was received, subject to
   the packet not exceeding 1280 octets.

    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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       5       |                Reserved                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          5

   Reserved:      24-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   The following options are allowed in the message:





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   Packet in Error: Variable length mandatory option containing the IPv6
                  packet that was in error, starting with the IPv6
                  header, and normally containing the full packet.  If
                  the resulting No Context Error message would exceed
                  1280 octets, the Packet In Error option will not
                  include the full packet in error in order to limit the
                  error to 1280 octets.


5.7  Locator List Update Message Format

   The Locator List Update (LLU) Message contains a complete replacement
   of the senders locator list, and the options 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.

    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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       6       | Res |      Initiator Context Tag              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Request Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          6

   Res:           4-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Initiator Context Tag: 20-bit field.  The Context Tag the initiator
                  has allocated for the context.

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

   The following options are allowed in the message:




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   Locator List:  The list of the senders (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: 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 validation.


5.8  Locator List Update Acknowledgement Message Format

   This message is sent in response to a LLU 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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       7       | Res |      Responder Context Tag              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Request Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          7

   Res:           4-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Initiator Context Tag: 20-bit field.  The Context Tag the responder
                  has allocated for the context.

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

   The following options are allowed in the message:




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   TBD any options?:


5.9  Rehome Request Message Format

   TBD Is there any use to have a separate Rehome pair of messages?  The
   sender can indicates its new knowledge of one of its locators (such
   as it no longer working) using the LLU message.  Would it be useful
   to be able to specify just failure or preference changes without
   listing the actual locators?  This would require that the locator
   list is ordered so that a Rehome Request can refer to the locators by
   some short index.

   Perhaps this functionality can be accomplished by sending a Locator
   Update message and only including new Locator Preferences, without
   including any Locator List option?  If so, we don't need a separate
   message.

5.10  Rehome Acknowledgement Message Format

   TBD: See above.

5.11  Reachability Probe Message Format

   The Reachability Probe message is used to prevent 3rd party DoS
   attacks, and can also be used to verify whether a context is
   reachable at a given locator should that be needed for the general
   reachability detection mechanism (e.g., if we pick the CUD mechanism
   where one end sends probes and expects a reply).

   Before a host uses a locator for the peer that is different than the
   ULID, it needs to verify that the peer is indeed present at that
   locator by sending a Context Verify and receiving an acknowledgement.
   This message includes the ULID pair as well as the context tag, so
   that the peer can indeed verify that it has that ULID and that the
   context tag is correct.















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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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      10       | Res |       Receiver Context Tag              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Request Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          10

   Res:           4-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Receiver Context Tag: 20-bit field.  The Context Tag the peer has
                  allocated for the context.

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

   The following options are allowed in the message:

   ULID pair:     The ULID pair that is being probed.


5.12  Reachability Probe Reply Message Format

   This is sent in response to a Reachability Probe message.  Although,
   if the receiver of the RT does not have a matching context it will
   send a No Context Error 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      59       |  Hdr Ext Len  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      11       | Res |       Receiver Context Tag              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Request Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          11

   Res:           4-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Receiver Context Tag: 20-bit field.  The Context Tag the peer has
                  allocated for the context.

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

   The following options are allowed in the message:

   ULID pair:     The ULID pair that is being probed.  Copied from the
                  Probe message.


5.13  Payload Message Format

   The payload message is used for payload which do not have a
   designated "foo-inside-shim6" protocol type, as specified in
   Section 4.2.

   Since the shim is placed between the IP endpoint sub-layer and the IP
   routing sub-layer in the host, 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



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   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 effects the location of the shim6 header.

    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       |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      12       |                Reserved                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   The payload which follows this header.

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

   Checksum:      The checksum of the 8 octets.

   Type:          12

   Reserved:      Reserved for future use.  Zero on transmit.  MUST be
                  ignored on receipt.


5.14  Keepalive Message Format

   The keepalive message would be used if we decide to do the Force
   Bidirectional communication as a way to get verification that the
   locator pair continues to work.  If we are not going to do FBD we
   probably will not need this 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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      13       | Res |         Receiver Context Tag            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:





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   Next Header:   NO_NXT_HDR (59).

   Type:          13

   Res:           4-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Receiver Context Tag: 20-bit field.  The Context Tag the peer has
                  allocated for the context.

   The following options are allowed in the message:

   TBD any options?:


5.15  Locator Pair Test Message Format

   The above Reachability Probe message probes a context.  This message
   just probes a locator.  If we are going to handle failure during
   initial contact using the shim, then the shim needs to be able to
   find out what locators are working (and that they correspond to a
   desirable ULID) without assuming there is a context setup, and
   without knowing the actual ULID.  The latter is needed so that we can
   handle the case when the AAAA RRset contains any combination of
   multiple hosts and multiple IP addresses for a given host.  Having
   the responder send back the ULID that corresponds to a particular
   locator allows the initiator to take the AAAA RRset and determine
   which IPv6 addresses therein are for different hosts.

   Once we understand how the shim will be involved in locator failures
   during initial contact, then we can determine whether we need this
   mechanism, and whether it can be overloaded on the Probe Message
   (e.g., by making the Receiver Context tag optional in the
   Reachability Probe 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      59       |  Hdr Ext Len  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      14       |                Reserved                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Request Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Target ULID                                +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          14

   Reserved:      24-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Request Nonce: 32-bit unsigned integer.  A random number picked by
                  the sender which the target will return in the reply
                  message.

   Target ULID:   128-bit IPv6 address.

   The following options are allowed in the message:

   TBD any options?:


5.16  Locator Pair Test Reply Message Format

   If a host receives a Locator Pair Test message, and the Target ULID
   is one of its IP addresses, then it will send this reply.

   TBD: If ULID doesn't match, does it just ignore the test message?  Or
   send some error?

   TBD: Should the responder instead return its ULID, so that it is
   easier for the sender to determine which of the IPv6 addresses from



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   the DNS correspond to different hosts vs. different locators for the
   same host?

    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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      15       |                Reserved                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Request Nonce                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Target ULID                                +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          15

   Reserved:      24-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

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

   Target ULID:   128-bit IPv6 address.  Copied from the test message.
                  TBD: Or should the host be able to fill this in to
                  make it easier for the peer to determine which
                  locators refer to the same host?

   The following options are allowed in the message:

   TBD any options?:


5.17  Context Locator Pair Explore Message Format

   This is a placeholder for the protocol mechanism outlined in [6].
   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



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   from B to A, but there is no locator pair which works in both
   directions.  The protocol mechanism is that as A is sending explore
   packets to B, B will observe which locator pairs it has received from
   and report that back in explore packets it is sending to A.

    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  |         Checksum              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      16       | Res |       Receiver Context Tag              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Sequence Number                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Options                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Next Header:   NO_NXT_HDR (59).

   Type:          16

   Res:           4-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Receiver Context Tag: 20-bit field.  The Context Tag the peer has
                  allocated for the context.

   Sequence Number: 32-bit unsigned integer.  Used to determine which
                  packets have been received by the peer.

   The following options are allowed in the message:

   Explorer Results: Indication of what Explorer messages the sender has
                  recently received from the peer.













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6.  Option Formats

   The options follow the same layout as in RFC 2461 [2].  Thus they all
   are a multiple of 8 octets.

    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      |    Length     |              ...              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              ...                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Type:          8-bit identifier of the type of option.  The options
                  defined in this document are below.

   Length:        8-bit unsigned integer.  The length of the option
                  (including the type and length fields) in units of 8
                  octets.  The value 0 is invalid.  Nodes MUST silently
                  discard an ND packet that contains an option with
                  length zero.

                +------------------------------+------+
                |          Option Name         | Type |
                +------------------------------+------+
                |           Validator          |   1  |
                |                              |      |
                |         Locator List         |   2  |
                |                              |      |
                |      Locator Preferences     |   3  |
                |                              |      |
                | CGA Parameter Data Structure |   4  |
                |                              |      |
                |         CGA Signature        |   5  |
                |                              |      |
                |           ULID Pair          |   6  |
                |                              |      |
                |        Packet In Error       |   7  |
                |                              |      |
                |       Explorer Results       |   8  |
                +------------------------------+------+

                                  Table 2






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6.1  Validator Option Format

   The responder can choose exactly what input uses to compute the
   validator, and what one-way function (MD5, SHA1) it uses, as long as
   the reponder can verify that the validator it receives back in the I2
   packet is indeed one that 1) it computed, 2) it computed for the
   particular context, and 3) that it isn't a replayed I2 message.

   One way for the responder to do this is to maintain a single secret
   (S) and a running counter for the Responder Nonce.  For each I1
   message, the responder can then increase the counter, use the counter
   value as the responder nonce, and use the following information 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)


    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    |    Length     |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   ~                           Validator                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

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


6.2  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 and the Explorer packet refers to the locators by
   using the index in the list.

   TBD: Do we need this when all the locators are contained in the PDS?



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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    |    Length     |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                           Reserveds                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                            Locators                           ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Reserved:      48-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Locators:      A variable number of 128-bit locators.  The number of
                  locators present can be determined by the option
                  length field.


6.3  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 [8] for how
   SRV defines the interaction of priority and weight.

   As of this draft we define the preferences to include three 8-bit
   fields: a priority, a weight, and 8-bits of flags.  The intent is
   that the TBD flags can  carry information such as "this locator is
   not working", and "this locator is temporary".  The latter allows
   making 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.

   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.  This assumes that the Locator List option is
   stable.  See Section 17.








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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    |    Length     |     Pri[1]    |  Weight[1]    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Flags[1]    |    Pri[2]     |  Weight[2]    |   Flags[2]    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              ...                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Pri[i]:        8-bit unsigned integer.  The Priority associated with
                  the i'th locator in the Locator List option that is in
                  use.

   Weight[i]:     8-bit unsigned integer.  The Weight associated with
                  the i'th locator in the Locator List option that is in
                  use.

   Flags[i]:      8-bit unsigned integer.  The flags associated with the
                  i'th locator in the Locator List option that is in
                  use.

   The set of flags is TBD: Assume there will be two initially: BROKEN
   and TEMPORARY.

6.4  CGA Parameter Data Structure Option Format

   This option contains the CGA parameter data structure (hereafter
   called the PDS).  When HBA is used to validate the locators, the PDS
   contains the HBA multiprefix extension.  When CGA is used to validate
   the locators, in addition to the CGA PDS, the signature will need to
   be included as 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    |    Length     |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   ~                   CGA Parameter Data Structure                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:







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   CGA Parameter Data Structure: Variable length content.  Content
                  defined in [4].


6.5  CGA Signature Option Format

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

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

   Fields:

   CGA Signature: Variable length content.  Content defined in [4].


6.6  ULID Pair Option Format

   It isn't clear whether we need this option.  It depends whether we
   want to be able to setup a context for a ULID pair when that ULID
   pair can't be used to communicate.  Thus the IPv6 addresses in the
   context establishment would not be the ULIDs.

    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    |    Length     |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                           Reserveds                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Sender ULID                           +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                        Receiver ULID                          +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:





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   Reserved:      48-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Sender ULID:   A 128-bit IPv6 address.

   Receiver ULID: A 128-bit IPv6 address.


6.7  Packet In Error 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    |    Length     |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
   |                           Reserveds                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~           IPv6 header, shim6/TCP/UDP header, etc              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:

   Reserved:      48-bit field.  Reserved for future use.  Zero on
                  transmit.  MUST be ignored on receipt.

   Packet:        A variable length field which contains the packet in
                  error starting with the IPv6 header.


6.8  Explorer Results Option Format

   TBD: This needs to indicate which explorer packets (sequence numbers,
   source and destination locators) that have been recently received, in
   order to detect which locator pairs work when there is no locator
   pair which works in both directions.  When indicating locators it
   makes sense to use the offset in the Locator List (that was carries
   in the LLU option), since this takes less space than including the
   locators themselves.
   p
    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 = 8    |    Length     |              TBD              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   ~                              ...                              ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Fields:



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   TBD:


















































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7.  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.

7.1  Conceptual Data Structures

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

   o  The peer ULID; ULID(peer)

   o  The local ULID; ULID(local)

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

   o  For each peer locator, a bit whether it has been validated using
      HBA, 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 preferred local locator - used as source; Lp(local)

   o  The context tag used to transmit packets - allocated by the peer;
      CT(peer)

   o  The context to expect in received packets - allocated by the local
      host; CT(local)

   o  Reachability state for the locator pairs.

   o  During pair exploration, information about the explore packets
      that have been sent and received.

   The receiver finds the context by looking it up using <Source
   Locator, Destination Locator, CT(local)>, where the context tag is in
   the Flow Label field for ULP payload packets, and in the shim headers
   for control messages.  The sender needs to be able to find the
   context state when a packet is passed down from the ULP.  In that



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   case the lookup key is the pair of ULIDs.


















































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8.  Establishing Host Pair Contexts

   TBD

8.1  Sending I1 messages

8.2  Receiving I1 messages

8.3  Receiving R1 messages

8.4  Retransmitting I1 messages

8.5  Receiving I2 messages

8.6  Retransmitting I2 messages

8.7  Concurrent context establishment


































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9.  No Such Content Errors

   TBD
















































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10.  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 payload type unknown.  It is critical that these packets
   make it back up to the ULPs so that they can take appropriate action.

   When the ULP packets are sent unmodified, 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.  But when the shim on the transmitting side
   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.

   This mapping is different than when receiving ULP packets from the
   peer, because in that case the packets contain CT(local).  But the
   ICMP errors have a "packet in error" with CT(peer) since 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.

   Should the ULP packet have been conveyed using the protocol type
   encoding (Section 4.2), then that encoding must be undone for the
   packet in error before it is delivered to the ULP.  If the ULP packet
   had been encapsulated in a shim6 payload message, then this extension
   header must be removed.  The result needs to be that the ULP receives
   an ICMP error where the contained "packet in error" looks as if the
   shim did not exist.



















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11.  Taredown of the Host Pair Context

   Each host can unilaterally decide when to tare down a host-pair
   context.  It is RECOMMENDED that hosts not tare 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 is 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 not 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
   taredown by observing the amount of traffic that is sent and received
   using the context, and only after it appears quiescent, tare down the
   state.



































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12.  Updating the Locator Pairs

   TBD
















































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13.  Various Probe Mechanisms

   TBD
















































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14.  Rehoming to a Different Locator Pair

   TBD
















































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15.  Payload Packets before a Switch

   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 host-
   pair context.  This need to count packets might also appear on the
   receive side, depending on what heuristics the implementation has
   chosen.

   If there is a host-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 will be sent unmodified by the
   shim.  If it is not the case, then the logic in Section 16 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 will be covered is follow-ons to [5].




























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16.  Payload Packets after a Switch

   When sending packets, if there is a host-pair context for the ULID
   pair, and the ULID pair is no longer used as the locator pair, then
   the sender needs to transfer the packet.  The transformation depends
   on the payload type, since some protocol values can be carried
   without adding a shim6 extension header, and others need an 8-octet
   header.

   Before the payload dependent transformation, 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.

   The sender skips any "routing sub-layer extension headers", thus it
   skips any hop-by-hop extension header, any routing header, and any
   destination options header that is followed by a routing header.  The
   (extension) header that follows after that is viewed as the ULP
   header.

   If the ULP header is of a type listed in Section 4.2, then it is
   replaced by the "foo-in-shim6 value for that protocol type.  And in
   this case, the context tag CT(peer) is placed in the flow label field
   in the IPv6 header.  Then the packet can be passed to the IP routing
   sub-layer.

   If the ULP header type is not listed in that section, then a shim6
   Payload extension header is inserted in the packet before the ULP
   header.  In this case the context tag CT(peer) is also placed in the
   flow label field, and the packet is passed down to the routing sub-
   layer.  TBD: We could use the Reserved field in the payload message
   instead of using flow label in this case.

   The receiver parses the (extension) headers in order.  Should it find
   a shim6 extension header it will look at the type field in that
   header.  If the type is Payload message, then the packet must be
   passed to the shim6 payload handling for rewriting.  If the receiver
   finds one of the eight additional payload type (for "foo-inside-
   shim6"), then it treats this analogous to the case of a shim6 payload
   extension header.

   In both cases the receiver extracts the context tag from the IPv6
   flow label field, and uses this together with the IPv6 source and
   destination address fields to find a host-pair context.  If no
   context is found, the receiver SHOULD generate a No Such Context
   error message (see Section 9).



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   With the context in hand, the receiver can now replace the IP address
   fields with the ULIDs kept in the context.  Finally, the traces of
   the shim are removed from the packet; any payload extension header is
   removed, 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 ULP.













































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17.  Open Issues

   The following open issues are known:

   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.

   o  Forking the context state.  On the mailing list we've discussed
      the need to fork the context state, so that different ULP streams
      can be sent using different locator pairs.  No protocol extensions
      are needed if any forking is done independently by each endpoint.
      But if we want A to be able to tell B that certain traffic (a
      5-tuple?) should be forked, then we need a way to convey this in
      the shim6 protocol.  The hard part would be defining what
      selectors can be specified for the filter which determines which
      traffic uses which of the forks.  So the question is whether we
      really need signaling for forking, or whether it is sufficient to
      allow each endpoint to do its own selection of which locator pair
      it is using for which traffic.

   o  If we allow forking, it seems like the mechanism for reachability
      detection, whether it is CUD or FBD, must be applied separately
      for each locator pair that is in use.  Without forking a single
      locator pair will be in use for each host-pair context, hence
      things would be simpler.

   o  Having the Locator List option contain all the prefixes implies
      extra bytes when the locators are also in the CGA Parameter Data
      Structure option.  To optimize this will still need to provide an
      ordered list, so that the Locator Preferences can refer to the
      locators by "index".  (The Explore Results option might need to
      refer to them by index as well.)

   o  The index to a locator might get out of synch between the two ends
      if messages with a new Locator List option is lost.  It might make
      sense to include a "generation" or "locator list version" number
      in the Locator List option so that the Locator Preference (and
      Explorer Result) options can refer to a particular version of the
      list.

   o  The specified mechanism (of relying on No Such Context errors)
      doesn't always detect the loss of the context on the peer when the
      original ULID=locators are used.  See Section 18 for other
      options.




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   o  Which messages need sequence numbers to prevent parts of the
      protocol to operate on stale information should the shim6
      information get out of date?  Just the Locator List Update
      message?

   o  The CGA PDS might not need to be included in every LLU message.
      If it is associated with the ULID, it is sufficient to exchange it
      once.  Then a HBA-protected LLU would not need anything (it can
      just change the preferences for the locators in any case), and a
      CGA-protected LLU would just need the signature option.

   o  In the LLU do we need to indicate which locators need to be
      validated using HBA vs. CGA?  Or it could tell which locators are
      in the HBA extension in the PDS, and assume any others need CGA
      validation.

   o  What happens when a host runs out of 20 bit context tags?  When is
      it safe for a host to reuse a context tag?  With the unilateral
      taredown one end might discard the context state long before the
      other end.































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18.  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 section tries to enumerate some
   alternatives.

18.1  State Cleanup

   This document uses a timer based cleanup mechanism, as specified in
   Section 11.

   An alternative would be to use an explicit CLOSE mechanism, akin to
   the one specified in HIP [17].  If an explicit CLOSE handshake and
   associated timer is used, then there would no longer be a need for
   the No Context Error message due to a peer having garbage collected
   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.

   If there is no need for the No Context Error message, this also means
   that it might be possible to remove the need to explicitly
   identifying the shim6 payload packets after a locator switch, neither
   using the foo-inside-shim6 protocol number nor using the shim6
   Payload message.  In essence, the receiver could identify the context
   based on the locator pair and the Flow Label in the received packets.

   There might be some debugging and operational issues with removing
   the explicit identification of the shim6 packets after a locator
   switch.  Should the receiver have lost the context state, then there
   will be no indication that something is going wrong.  The shim on the
   receiver would happily pass up the packets unmodified to the ULP, and
   the ULP would most likely see a checksum error.  The checksum error
   is caused by the ULP packet having different IP addresses than the
   packet that the sending ULP passed down to its shim.

18.2  Not Overloading the Flow Label

   This document overloads the Flow Label field as a context tag for
   packets that are sent after the locators have been switched, that is,
   the packets where the sending shim has replaced the ULIDs with some
   other locator pair.




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   An alternative would be to not do this, and instead always use the
   shim6 Payload message to encapsulate the payloads when the locators
   are different than the ULIDs.  While this doesn't remove the need to
   have any QoS signaling protocol be aware of the shim6 architectural
   implications Section 19, it does offer some other simplifications to
   the protocol, namely that there would no longer be a need to use
   designated protocol number values for the "foo-inside-shim6"; the
   cases when those protocol numbers are used would instead use the
   Payload message.  The downside of always using the Payload message
   after a failure is that the path MTU usable by the ULP would be 8
   octets less.

18.3  Detecting Context Loss

   This document specifies that context loss is detected by receiving a
   No Such Context error message from the peer.  Such messages are
   generated in response to a shim6 message that contain a the peer's
   context tag, including the shim6 Payload messages, when the receiver
   doesn't have matching context.  They are also generated in response
   to data packets after a locator switch (because such payload packets
   are identified as such using the overloaded protocol field specified
   in Section 4.2).

   This approach has the disadvantage of the overloaded protocol type,
   and it also doesn't detect the loss of context state when the
   original ULIDs are used as locators, because there might be no shim6
   messages exchanged if the reachability detection manages to suppress
   any extra packets.

   Discussion: it isn't clear we could remove the protocol type
   overloading with this approach, because without protocol type
   overloading it is undefined in what order the receiver would do
   things.  Normally the receiver follows the next header chain and
   processes things in order.  This also works with an overloaded
   protocol type; that next header value is basically indicating that
   there is a zero length shim payload header.  Thus the sender can
   control whether this happens before the processing of some other
   extension header or after.  Without any such indication in the
   packet, the receiver would find a shim context based on the <Source
   Locator, Destination Locator, Flow Label>.  But would it process this
   before or after some other extension header, such as a MIPv6 Home
   Address Option, or IP-in-IP encapsulation header?

   An alternative would be to remove the protocol field overloading and
   mandate that there be some low-frequency periodic Reachability Probe/
   Reply messages, even when there is bidirectional communication and
   the ULPs report that they are doing fine.  Such an approach would be
   able to detect state loss even before there is a locator switch.



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   Presumably such probes can be suppressed when there are no ULP
   packets being sent to the peer.

















































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19.  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 referals, or callbacks using IP
      addresses as the 'identifiers' can still function in limited ways,
      as described in [12].  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
      packets 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 host-pair context, then the
      firewall would probably drop the "different" packets that are sent
      after a failure (either using a "TCP-inside-shim6" protocol
      number, or using the shim6 payload packet with a TCP packet inside
      it).  Thus stateful firewalls  that are modified to allow shim6
      packets through should also be modified to allow the payload
      packets through after a failure.  This presumably implies that the
      firewall needs to track the set of locators in use by looking at
      the shim6 exchanges.

   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, as well as a new Flow
      Label being used.

   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
      another pair, this means that packets might travel over a
      different path through the Internt, 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, at least for uncommon payload types, will



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      add an 8 octet extension header (the payload message) 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.















































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20.  Security Considerations

   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 No Such Context error 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 No Such Context error 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 error causes the context to be torn
      down and re-established, a new pair of context tags will be used,
      which will not be known to the attacker.  If this is still a
      concern, we could require a 2-way handshake "did you really loose
      the state?" in response to the error message.

   o  It might be possible for an attacker to try random 24-bit context
      tags and see if they can cause disruption for communication
      between two hosts.  We can make this harder by using a larger
      context tag (64-bits?) in the shim6 control messages, and use the
      low-order 24 bits as the flow label.






















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21.  Acknowledgements

   Over the years many people active in the multi6 and shim6 WGs have
   contributed ideas a suggestions that are reflected in this draft.

   Thanks to Marcelo Bagnulo for providing comments on earlier versions
   of this draft.












































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22.  References

22.1  Normative References

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

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

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

   [4]  Bagnulo, M., "Hash Based Addresses (HBA)",
        draft-ietf-shim6-hba-00 (work in progress), July 2005.

   [5]  Beijnum, I., "Shim6 Reachability Detection",
        draft-ietf-shim6-reach-detect-00 (work in progress), July 2005.

   [6]  Arkko, J., "Failure Detection and Locator Selection Design
        Considerations", draft-ietf-shim6-failure-detection-00 (work in
        progress), July 2005.

22.2  Informative References

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

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

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

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

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

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

   [13]  Abley, J., "Shim6 Applicability Statement",
         draft-ietf-shim6-applicability-00 (work in progress),
         July 2005.




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

   [15]  Bagnulo, M. and J. Arkko, "Functional decomposition of the
         multihoming protocol", draft-ietf-shim6-functional-dec-00 (work
         in progress), July 2005.

   [16]  Nordmark, E. and M. Bagnulo, "Multihoming L3 Shim Approach",
         draft-ietf-shim6-l3shim-00 (work in progress), July 2005.

   [17]  Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-03
         (work in progress), June 2005.

   [18]  Lear, E. and R. Droms, "What's In A Name:Thoughts from the
         NSRG", draft-irtf-nsrg-report-10 (work in progress),
         September 2003.


Author's Address

   Erik Nordmark
   Sun Microsystems
   17 Network Circle
   Menlo Park, CA 94043
   USA

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






















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