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Network Working Group J. Loughney, Ed. Request for Comments: 4067 M. Nakhjiri Category: Experimental C. Perkins

                                                             R. Koodli
                                                             July 2005
                  Context Transfer Protocol (CXTP)

Status of This Memo

 This memo defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2005).


 This document presents the Context Transfer Protocol (CXTP) that
 enables authorized context transfers.  Context transfers allow better
 support for node based mobility so that the applications running on
 mobile nodes can operate with minimal disruption.  Key objectives are
 to reduce latency and packet losses, and to avoid the re-initiation
 of signaling to and from the mobile node.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  The Problem. . . . . . . . . . . . . . . . . . . . . . .  2
     1.2.  Conventions Used in This Document. . . . . . . . . . . .  3
     1.3.  Abbreviations Used in the Document . . . . . . . . . . .  3
 2.  Protocol Overview. . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Context Transfer Scenarios . . . . . . . . . . . . . . .  4
     2.2.  Context Transfer Message Format. . . . . . . . . . . . .  5
     2.3.  Context Types. . . . . . . . . . . . . . . . . . . . . .  6
     2.4.  Context Data Block (CDB) . . . . . . . . . . . . . . . .  7
     2.5.  Messages . . . . . . . . . . . . . . . . . . . . . . . .  8
 3.  Transport. . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     3.1.  Inter-Router Transport . . . . . . . . . . . . . . . . . 16
     3.2.  MN-AR Transport. . . . . . . . . . . . . . . . . . . . . 19
 4.  Error Codes and Constants. . . . . . . . . . . . . . . . . . . 20
 5.  Examples and Signaling Flows . . . . . . . . . . . . . . . . . 21
     5.1.  Network controlled, Initiated by pAR, Predictive . . . . 21
     5.2.  Network controlled, Initiated by nAR, Reactive . . . . . 21

Loughney, et al. Experimental [Page 1] RFC 4067 Context Transfer Protocol (CXTP) July 2005

     5.3.  Mobile controlled, Predictive New L2 up/Old L2 down. . . 22
 6.  Security Considerations. . . . . . . . . . . . . . . . . . . . 22
     6.1.  Threats. . . . . . . . . . . . . . . . . . . . . . . . . 22
     6.2.  Access Router Considerations . . . . . . . . . . . . . . 23
     6.3.  Mobile Node Considerations . . . . . . . . . . . . . . . 24
 7.  Acknowledgements & Contributors. . . . . . . . . . . . . . . . 25
 8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     8.1.  Normative References . . . . . . . . . . . . . . . . . . 25
     8.2.  Informative References . . . . . . . . . . . . . . . . . 26
 Appendix A.  Timing and Trigger Considerations . . . . . . . . . . 28
 Appendix B.  Multicast Listener Context Transfer . . . . . . . . . 28

1. Introduction

 This document describes the Context Transfer Protocol, which
  • Representation for feature contexts.
  • Messages to initiate and authorize context transfer, and notify

a mobile node of the status of the transfer.

  • Messages for transferring contexts prior to, during and after


 The proposed protocol is designed to work in conjunction with other
 protocols in order to provide seamless mobility.  The protocol
 supports both IPv4 and IPv6, though support for IPv4 private
 addresses is for future study.

1.1. The Problem

 "Problem Description: Reasons For Performing Context Transfers
 between Nodes in an IP Access Network" [RFC3374] defines the
 following main reasons why Context Transfer procedures may be useful
 in IP networks.
 1) As mentioned in the introduction, the primary motivation is to
    quickly re-establish context transfer-candidate services without
    requiring the mobile host to explicitly perform all protocol flows
    for those services from scratch.  An example of such a service is
    included in Appendix B of this document.
 2) An additional motivation is to provide an interoperable solution
    that supports various Layer 2 radio access technologies.

Loughney, et al. Experimental [Page 2] RFC 4067 Context Transfer Protocol (CXTP) July 2005

1.2. Conventions Used in This Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in RFC 2119 [RFC2119].

1.3. Abbreviations Used in the Document

 Mobility Related Terminology [TERM] defines basic mobility
 terminology.  In addition to the material in that document, we use
 the following terms and abbreviations in this document.
    CXTP            Context Transfer Protocol
    DoS             Denial-of-Service
    FPT             Feature Profile Types
    PCTD            Predictive Context Transfer Data

2. Protocol Overview

 This section provides a protocol overview.  A context transfer can be
 either started by a request from the mobile node ("mobile
 controlled") or at the initiative of the new or the previous access
 router ("network controlled").
  • The mobile node (MN) sends the CT Activate Request (CTAR) to

its current access router (AR) immediately prior to handover

       when it is possible to initiate a predictive context transfer.
       In any case, the MN always sends the CTAR message to the new AR
       (nAR).  If the contexts are already present, nAR verifies the
       authorization token present in CTAR with its own computation
       using the parameters supplied by the previous access router
       (pAR), and subsequently activates those contexts.  If the
       contexts are not present, nAR requests pAR to supply them using
       the Context Transfer Request message, in which it supplies the
       authorization token present in CTAR.
  • Either nAR or pAR may request or start (respectively) context

transfer based on internal or network triggers (see Appendix

 The Context Transfer protocol typically operates between a source
 node and a target node.  In the future, there may be multiple target
 nodes involved; the protocol described here would work with multiple
 target nodes.  For simplicity, we describe the protocol assuming a
 single receiver or target node.

Loughney, et al. Experimental [Page 3] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 Typically, the source node is an MN's pAR and the target node is an
 MN's nAR.  Context Transfer takes place when an event, such as a
 handover, takes place.  We call such an event a Context Transfer
 Trigger.  In response to such a trigger, the pAR may transfer the
 contexts; the nAR may request contexts; and the MN may send a message
 to the routers to transfer contexts.  Such a trigger must be capable
 of providing the necessary information (such as the MN's IP address)
 by which the contexts are identified.  In addition, the trigger must
 be able to provide the IP addresses of the access routers, and the
 authorization to transfer context.
 Context transfer protocol messages use Feature Profile Types (FPTs)
 that identify the way that data is organized for the particular
 feature contexts.  The FPTs are registered in a number space (with
 IANA Type Numbers) that allows a node to unambiguously determine the
 type of context and the context parameters present in the protocol
 messages.  Contexts are transferred by laying out the appropriate
 feature data within Context Data Blocks according to the format in
 Section 2.3, as well as any IP addresses necessary to associate the
 contexts to a particular MN.  The context transfer initiation
 messages contain parameters that identify the source and target
 nodes, the desired list of feature contexts, and IP addresses to
 identify the contexts.  The messages that request the transfer of
 context data also contain an appropriate token to authorize the
 context transfer.
 Performing a context transfer in advance of the MN attaching to nAR
 can increase handover performance.  For this to take place, certain
 conditions must be met.  For example, pAR must have sufficient time
 and knowledge of the impending handover.  This is feasible, for
 instance, in Mobile IP fast handovers [LLMIP][FMIPv6].  Additionally,
 many cellular networks have mechanisms to detect handovers in
 advance.  However, when the advance knowledge of impending handover
 is not available, or if a mechanism such as fast handover fails,
 retrieving feature contexts after the MN attaches to nAR is the only
 available means for context transfer.  Performing context transfer
 after handover might still be better than having to re-establish all
 the contexts from scratch, as shown in [FHCT] and [TEXT].  Finally,
 some contexts may simply need to be transferred during handover
 signaling.  For instance, any context that gets updated on a per-
 packet basis must clearly be transferred only after packet forwarding
 to the MN on its previous link has been terminated.

2.1. Context Transfer Scenarios

 The Previous Access Router transfers feature contexts under two
 general scenarios.

Loughney, et al. Experimental [Page 4] RFC 4067 Context Transfer Protocol (CXTP) July 2005

2.1.1. Scenario 1

 The pAR receives a Context Transfer Activate Request (CTAR) message
 from the MN whose feature contexts are to be transferred, or it
 receives an internally generated trigger (e.g., a link-layer trigger
 on the interface to which the MN is connected).  The CTAR message,
 described in Section 2.5, provides the IP address of nAR, the IP
 address of MN on pAR, the list of feature contexts to be transferred
 (by default requesting all contexts to be transferred), and a token
 authorizing the transfer.  In response to a CT-Activate Request
 message or to the CT trigger, pAR predictively transmits a Context
 Transfer Data (CTD) message that contains feature contexts.  This
 message, described in Section 2.5, contains the MN's previous IP
 address.  It also contains parameters for nAR to compute an
 authorization token to verify the MN's token that is present in the
 CTAR message.  Recall that the MN always sends a CTAR message to nAR
 regardless of whether it sent the CTAR message to pAR because there
 is no means for the MN to ascertain that context transfer has
 reliably taken place.  By always sending the CTAR message to nAR, the
 Context Transfer Request (see below) can be sent to pAR if necessary.
 When context transfer takes place without the nAR requesting it, nAR
 requires MN to present its authorization token.  Doing this locally
 at nAR when the MN attaches to it improves performance and increases
 security, since the contexts are likely to already be present.  Token
 verification takes place at the router possessing the contexts.

2.1.2. Scenario 2

 In the second scenario, pAR receives a Context Transfer Request (CT-
 Req) message from nAR, as described in Section 2.5.  The nAR itself
 generates the CT-Req message as a result of receiving the CTAR
 message, or alternatively, from receiving a context transfer trigger.
 In the CT-Req message, nAR supplies the MN's previous IP address, the
 FPTs for the feature contexts to be transferred, the sequence number
 from the CTAR, and the authorization token from the CTAR.  In
 response to a CT-Req message, pAR transmits a Context Transfer Data
 (CTD) message that includes the MN's previous IP address and feature
 contexts.  When it receives a corresponding CTD message, nAR may
 generate a CTD Reply (CTDR) message to report the status of
 processing the received contexts.  The nAR installs the contexts once
 it has received them from the pAR.

2.2. Context Transfer Message Format

 A CXTP message consists of a message-specific header and one or more
 data blocks.  Data blocks may be bundled together to ensure a more
 efficient transfer.  On the inter-AR interface, SCTP is used so

Loughney, et al. Experimental [Page 5] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 fragmentation should not be a problem.  On the MN-AR interface, the
 total packet size, including transport protocol and IP protocol
 headers, SHOULD be less than the path MTU to avoid packet
 fragmentation.  Each message contains a 3 bit version number field in
 the low order octet, along with the 5 bit message type code.  This
 specification only applies to Version 1 of the protocol, and the
 therefore version number field MUST be set to 0x1.  If future
 revisions of the protocol make binary incompatible changes, the
 version number MUST be incremented.

2.3. Context Types

 Contexts are identified by the FPT code, which is a 16 bit unsigned
 integer.  The meaning of each context type is determined by a
 specification document.  The context type numbers are to be tabulated
 in a registry maintained by IANA [IANA] and handled according to the
 message specifications in this document.  The instantiation of each
 context by nAR is determined by the messages in this document along
 with the specification associated with the particular context type.
 The following diagram illustrates the general format for CXTP
             |    Message Header    |
             |     CXTP Data 1      |
             |     CXTP Data 2      |
             |         ...          |
 Each context type specification contains the following details:
  1. Number, size (in bits), and ordering of data fields in the

state variable vector that embodies the context.

  1. Default values (if any) for each individual datum of the

context state vector.

  1. Procedures and requirements for creating a context at a new

access router, given the data transferred from a previous

       access router and formatted according to the ordering rules and
       data field sizes presented in the specification.
  1. If possible, status codes for success or failure related to the

context transfer. For instance, a QoS context transfer might

       have different status codes depending on which elements of the
       context data failed to be instantiated at nAR.

Loughney, et al. Experimental [Page 6] RFC 4067 Context Transfer Protocol (CXTP) July 2005

2.4. Context Data Block (CDB)

 The Context Data Block (CDB) is used both for request and response
 operations.  When a request is constructed, only the first 4 octets
 are typically necessary (See CTAR below).  When used for transferring
 the actual feature context itself, the context data is present, and
 the presence vector is sometimes present.
  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
 |   Feature Profile Type (FPT)  |  Length       |P|  Reserved   |
 |                   Presence Vector (if P = 1)                  |
 ~                              Data                             ~
    Feature Profile Type
                         16 bit integer, assigned by IANA,
                         indicating the type of data
                         included in the Data field.
    Length               Message length in units of 8 octet words.
    'P' bit              0 = No presence vector.
                         1 = Presence vector present.
    Reserved             Reserved for future use.  Set to
                         zero by the sender.
    Data                 Context type-dependent data, whose
                         length is defined by the Length
                         Field.  If the data is not 64 bit
                         aligned, the data field is
                         padded with zeros.
 The Feature Profile Type (FPT) code indicates the type of data in the
 data field.  Typically, this will be context data, but it could be an
 error indication.  The 'P' bit specifies whether the "presence
 vector" is used.  When the presence vector is in use, it is
 interpreted to indicate whether particular data fields are present
 (and contain non-default values).  The ordering of the bits in the
 presence vector is the same as the ordering of the data fields
 according to the context type specification, one bit per data field
 regardless of the size of the data field.  The Length field indicates
 the size of the CDB in 8 octet words, including the first 4 octets
 starting from FPT.

Loughney, et al. Experimental [Page 7] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 Notice that the length of the context data block is defined by the
 sum of the lengths of each data field specified by the context type
 specification, plus 4 octets if the 'P' bit is set, minus the
 accumulated size of all the context data that is implicitly given as
 a default value.

2.5. Messages

 In this section, the CXTP messages are defined.  The MN for which
 context transfer protocol operations are undertaken is always
 identified by its previous IP access address.  Only one context
 transfer operation per MN may be in progress at a time so that the
 CTDR message unambiguously identifies which CTD message is
 acknowledged simply by including the MN's identifying previous IP
 address.  The 'V' flag indicates whether the IP addresses are IPv4 or

2.5.1. Context Transfer Activate Request (CTAR) Message

 This message is always sent by the MN to the nAR to request a context
 transfer.  Even when the MN does not know if contexts need to be
 transferred, the MN sends the CTAR message.  If an acknowledgement
 for this message is needed, the MN sets the 'A' flag to 1; otherwise
 the MN does not expect an acknowledgement.  This message may include
 a list of FPTs that require transfer.
 The MN may also send this message to pAR while still connected to
 pAR.  In this case, the MN includes the nAR's IP address; otherwise,
 if the message is sent to nAR, the pAR address is sent.  The MN MUST
 set the sequence number to the same value as was set for the message
 sent on both pAR and nAR so pAR can determine whether to use a cached

Loughney, et al. Experimental [Page 8] RFC 4067 Context Transfer Protocol (CXTP) July 2005

  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
 |Vers.|   Type  |V|A| Reserved  |            Length             |
 ~                   MN's Previous IP Address                    ~
 ~                  Previous (New) AR IP Address                 ~
 |                        Sequence Number                        |
 |                     MN Authorization Token                    |
 |            Requested Context Data Block (if present)          |
 |          Next Requested Context Data Block (if present)       |
 |                           ........                            |
    Vers.                Version number of CXTP protocol = 0x1
    Type                 CTAR = 0x1
    'V' flag             When set to '0', IPv6 addresses.
                         When set to '1', IPv4 addresses.
    'A' bit              If set, the MN requests an acknowledgement.
    Reserved             Set to zero by the sender, ignored by the
    Length               Message length in units of octets.
    MN's Previous IP Address Field contains either:
                         IPv4 [RFC791] Address, 4 octets, or
                         IPv6 [RFC3513] Address, 16 octets.
    nAR / pAR IP Address Field contains either:
                         IPv4 [RFC791] Address, 4 octets, or
                         IPv6 [RFC3513] Address, 16 octets.
    Sequence Number      A value used to identify requests and
                         acknowledgements (see Section 3.2).

Loughney, et al. Experimental [Page 9] RFC 4067 Context Transfer Protocol (CXTP) July 2005

    Authorization Token  An unforgeable value calculated as
                         discussed below.  This authorizes the
                         receiver of CTAR to perform context
    Context Block        Variable length field defined in
                         Section 2.4.
 If no context types are specified, all contexts for the MN are
 The Authorization Token is calculated as:
    First (32, HMAC_SHA1
            (Key, (Previous IP address | Sequence Number | CDBs)))
 where Key is a shared secret between the MN and pAR, and CDB is a
 concatenation of all the Context Data Blocks specifying the contexts
 to be transferred that are included in the CTAR message.

2.5.2. Context Transfer Activate Acknowledge (CTAA) Message

 This is an informative message sent by the receiver of CTAR to the MN
 to acknowledge a CTAR message.  Acknowledgement is optional,
 depending on whether the MN requested it.  This message may include a
 list of FPTs that were not successfully transferred.
  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
 |Vers.|  Type   |V|  Reserved   |            Length             |
 ~              Mobile Node's Previous IP address                ~
 |       FPT (if present)        |  Status code  |   Reserved    |
 |                           ........                            |
    Vers.                Version number of CXTP protocol = 0x1
    Type                 CTAA = 0x2
    'V' flag             When set to '0', IPv6 addresses.
                         When set to '1', IPv4 addresses.
    Reserved             Set to zero by the sender and ignored by
                         the receiver.

Loughney, et al. Experimental [Page 10] RFC 4067 Context Transfer Protocol (CXTP) July 2005

    Length               Message length in units of octets.
    MN's Previous IP Address Field contains either:
                         IPv4 [RFC791] Address, 4 octets, or
                         IPv6 [RFC3513] Address, 16 octets.
    FPT                  16 bit unsigned integer, listing the Feature
                         Profile Type that was not successfully
    Status Code          An octet, containing failure reason.
    ........             more FPTs and status codes as necessary

2.5.3. Context Transfer Data (CTD) Message

 Sent by pAR to nAR, and includes feature data (CXTP data).  This
 message handles both predictive and normal CT.  An acknowledgement
 flag, 'A', included in this message indicates whether a reply is
 required by pAR.
 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
|Vers.|   Type  |V|A| Reserved  |          Length               |
|               Elapsed Time (in milliseconds)                  |
~            Mobile Node's Previous Care-of Address             ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^
|            Algorithm          |            Key Length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ PCTD
|                              Key                              | only
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ V
~                   First Context Data Block                    ~
~                    Next Context Data Block                    ~
~                           ........                            ~
    Vers.                Version number of CXTP protocol = 0x1
    Type                 CTD =  0x3 (Context Transfer Data)
                         PCTD = 0x4 (Predictive Context Transfer

Loughney, et al. Experimental [Page 11] RFC 4067 Context Transfer Protocol (CXTP) July 2005

    'V' flag             When set to '0', IPv6 addresses.
                         When set to '1', IPv4 addresses.
    'A' bit              When set, the pAR requests an
    Length               Message length in units of octets.
    Elapsed Time         The number of milliseconds since the
                         transmission of the first CTD message for
                         this MN.
    MN's Previous IP Address Field contains either:
                         IPv4 [RFC791] Address, 4 octets, or
                         IPv6 [RFC3513] Address, 16 octets.
    Algorithm            Algorithm for carrying out the computation
                         of the MN Authorization Token.  Currently
                         only 1 algorithm is defined, HMAC_SHA1 = 1.
    Key Length           Length of key, in octets.
    Key                  Shared key between MN and AR for CXTP.
    Context Data Block   The Context Data Block (see Section 2.4).
 When CTD is sent predictively, the supplied parameters (including the
 algorithm, key length, and the key itself) allow the nAR to compute a
 token locally and verify it against the token present in the CTAR
 message.  This material is also sent if the pAR receives a CTD
 message with a null Authorization Token, indicating that the CT-Req
 message was sent before the nAR received the CTAR message.  CTD MUST
 be protected by IPsec; see Section 6.
 As described previously, the algorithm for carrying out the
 computation of the MN Authorization Token is HMAC_SHA1.  The token
 authentication calculation algorithm is described in Section 2.5.1.
 For predictive handover, the pAR SHOULD keep track of the CTAR
 sequence number and cache the CTD message until a CTDR message for
 the MN's previous IP address has been received from the pAR,
 indicating that the context transfer was successful, or until
 CT_MAX_HANDOVER_TIME expires.  The nAR MAY send a CT-Req message
 containing the same sequence number if the predictive CTD message
 failed to arrive or the context was corrupted.  In this case, the nAR

Loughney, et al. Experimental [Page 12] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 sends a CT-Req message with a matching sequence number and pAR can
 resend the context.

2.5.4. Context Transfer Data Reply (CTDR) Message

 This message is sent by nAR to pAR depending on the value of the 'A'
 flag in CTD, indicating success or failure.
  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
 |Vers.|  Type   |V|S| Reserved  |          Length               |
 ~             Mobile Node's Previous IP Address                 ~
 |        FPT (if present)       |  Status code  |   Reserved    |
 ~                           ........                            ~
    Vers.                Version number of CXTP protocol = 0x1
    Type                 CTDR = 0x5 (Context Transfer Data)
    'V' flag             When set to '0', IPv6 addresses.
                         When set to '1', IPv4 addresses.
    'S' bit              When set to one, this bit indicates
                         that all feature contexts sent in CTD
                         or PCTD were received successfully.
    Reserved             Set to zero by the sender and ignored by
                         the receiver.
    Length               Message length in units of octets.
    MN's Previous IP Address Field contains either:
                         IPv4 [RFC791] Address, 4 octets, or
                         IPv6 [RFC3513] Address, 16 octets.
    FPT                  16 bit unsigned integer, listing the Feature
                         Profile Type that is being acknowledged.
    Status Code          A context-specific return value,
                         zero for success, nonzero when 'S' is
                         not set to one.

Loughney, et al. Experimental [Page 13] RFC 4067 Context Transfer Protocol (CXTP) July 2005

2.5.5. Context Transfer Cancel (CTC) Message

 If transferring a context cannot be completed in a timely fashion,
 then nAR may send CTC to pAR to cancel an ongoing CT process.
  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
 |Vers.|  Type   |V|   Reserved  |            Length             |
 ~               Mobile Node's Previous IP Address               ~
    Vers.                Version number of CXTP protocol = 0x1
    Type                 CTC = 0x6 (Context Transfer Cancel)
    Length               Message length in units of octets.
    'V' flag             When set to '0', IPv6 addresses.
                         When set to '1', IPv4 addresses.
    Reserved             Set to zero by the sender and ignored by
                         the receiver.
    MN's Previous IP Address Field contains either:
                         IPv4 [RFC791] Address, 4 octets, or
                         IPv6 [RFC3513] Address, 16 octets.

2.5.6. Context Transfer Request (CT-Req) Message

 Sent by nAR to pAR to request the start of context transfer.  This
 message is sent as a response to a CTAR message.  The fields
 following the Previous IP address of the MN are included verbatim
 from the CTAR message.

Loughney, et al. Experimental [Page 14] RFC 4067 Context Transfer Protocol (CXTP) July 2005

  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
 |Vers.|  Type   |V|  Reserved   |            Length             |
 ~               Mobile Node's Previous IP Address               ~
 |                        Sequence Number                        |
 |                     MN Authorization Token                    |
 ~        Next Requested Context Data Block (if present)         ~
 ~                           ........                            ~
    Vers.                Version number of CXTP protocol = 0x1
    Type                 CTREQ = 0x7 (Context Transfer Request)
    'V' flag             When set to '0', IPv6 addresses.
                         When set to '1', IPv4 addresses.
    Reserved             Set to zero by the sender and ignored
                         by the receiver.
    Length               Message length in units of octets.
    MN's Previous IP Address Field contains either:
                         IPv4 [RFC791] Address, 4 octets, or
                         IPv6 [RFC3513] Address, 16 octets.
    Sequence Number      Copied from the CTAR message, allows the
                         pAR to distinguish requests from previously
                         sent context.
    MN's Authorization Token
                         An unforgeable value calculated as
                         discussed in Section 2.5.1.  This
                         authorizes the receiver of CTAR to
                         perform context transfer.  Copied from
    Context Data Request Block
                         A request block for context data; see
                         Section 2.4.

Loughney, et al. Experimental [Page 15] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 The sequence number is used by pAR to correlate a request for
 previously transmitted context.  In predictive transfer, if the MN
 sends CTAR prior to handover, pAR pushes context to nAR using PCTD.
 If the CTD fails, the nAR will send a CT-Req with the same sequence
 number, enabling the pAR to determine which context to resend.  The
 pAR deletes the context after CXTP_MAX_TRANSFER_TIME.  The sequence
 number is not used in reactive transfer.
 For predictive transfer, the pAR sends the keying material and other
 information necessary to calculate the Authorization Token without
 having processed a CT-Req message.  For reactive transfer, if the nAR
 receives a context transfer trigger but has not yet received the CTAR
 message with the authorization token, the Authorization Token field
 in CT-Req is set to zero.  The pAR interprets this as an indication
 to include the keying material and other information necessary to
 calculate the Authorization Token, and includes this material into
 the CTD message as if the message were being sent due to predictive
 transfer.  This provides nAR with the information it needs to
 calculate the authorization token when the MN sends CTAR.

3. Transport

3.1. Inter-Router Transport

 Since most types of access networks in which CXTP might be useful are
 not today deployed or, if they have been deployed, have not been
 extensively measured, it is difficult to know whether congestion will
 be a problem for CXTP.  Part of the research task in preparing CXTP
 for consideration as a possible candidate for standardization is to
 quantify this issue.  However, to avoid potential interference with
 production applications should a prototype CXTP deployment involve
 running over the public Internet, it seems prudent to recommend a
 default transport protocol that accommodates congestion.  In
 addition, since the feature context information has a definite
 lifetime, the transport protocol must accommodate flexible
 retransmission, so stale contexts that are held up by congestion are
 dropped.  Finally, because the amount of context data can be
 arbitrarily large, the transport protocol should not be limited to a
 single packet or require implementing a custom fragmentation
 These considerations argue that implementations of CXTP MUST support,
 and prototype deployments of CXTP SHOULD use, the Stream Control
 Transport Protocol (SCTP) [SCTP] as the transport protocol on the
 inter-router interface, especially if deployment over the public
 Internet is contemplated.  SCTP supports congestion control,
 fragmentation, and partial retransmission based on a programmable
 retransmission timer.  SCTP also supports many advanced and complex

Loughney, et al. Experimental [Page 16] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 features, such as multiple streams and multiple IP addresses for
 failover that are not necessary for experimental implementation and
 prototype deployment of CXTP.  The use of such SCTP features is not
 recommended at this time.
 The SCTP Payload Data Chunk carries the context transfer protocol
 messages.  The User Data part of each SCTP message contains an
 appropriate context transfer protocol message defined in this
 document.  The messages sent using SCTP are CTD (Section 2.5.3), CTDR
 (Section 2.5.4), CTC (Section 2.5.5), and CT-Req (Section 2.5.6).  In
 general, each SCTP message can carry feature contexts belonging to
 any MN.  If the SCTP checksum calculation fails, the nAR returns the
 BAD_CHECKSUM error code in a CTDR message.
 A single stream is used for context transfer without in-sequence
 delivery of SCTP messages.  Each message corresponds to a single MN's
 feature context collection.  A single stream provides simplicity.
 The use of multiple streams to prevent head-of-line blocking is for
 future study.  Unordered delivery allows the receiver to not block
 for in-sequence delivery of messages that belong to different MNs.
 The Payload Protocol Identifier in the SCTP header is 'CXTP'.
 Inter-router CXTP uses the Seamoby SCTP port [IANA].
 Timeliness of the context transfer information SHOULD be accommodated
 by setting the SCTP maximum retransmission value to
 CT_MAX_TRANSFER_TIME to accommodate the maximum acceptable handover
 delay time.  The AR SHOULD be configured with CT_MAX_TRANSFER_TIME to
 accommodate the particular wireless link technology and local
 wireless propagation conditions.  SCTP message bundling SHOULD be
 turned off to reduce an extra delay in sending messages.  Within
 CXTP, the nAR SHOULD estimate the retransmit timer from the receipt
 of the first fragment of a CXTP message and avoid processing any IP
 traffic from the MN until either context transfer is complete or the
 estimated retransmit timer expires.  If both routers support PR-SCTP
 [PR-SCTP], then PR-SCTP SHOULD be used.  PR-SCTP modifies the
 lifetime parameter of the Send() operation (defined in Section 10.1 E
 in [SCTP]) so that it applies to retransmits as well as transmits;
 that is, in PR-SCTP, if the lifetime expires and the data chunk has
 not been acknowledged, the transmitter stops retransmitting, whereas
 in the base protocol the data would be retransmitted until
 acknowledged or the connection timed out.

Loughney, et al. Experimental [Page 17] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 The format of Payload Data Chunk taken from [SCTP] is shown in the
 following diagram.
  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 = 0    | Reserved|U|B|E|    Length                     |
 |                              TSN                              |
 |      Stream Identifier S      |   Stream Sequence Number n    |
 |                 Payload Protocol Identifier                   |
 ~                 User Data (seq n of Stream S)                 ~
    'U' bit              The Unordered bit.  MUST be set to 1 (one).
    'B' bit              The Beginning fragment bit.  See [SCTP].
    'E' bit              The Ending fragment bit.  See [SCTP].
    TSN                  Transmission Sequence Number.  See [SCTP].
    Stream Identifier S
                         Identifies the context transfer protocol
    Stream Sequence Number n
                         Since the 'U' bit is set to one, the
                         receiver ignores this number.  See [SCTP].
    Payload Protocol Identifier
                         Set to 'CXTP' (see [IANA]).
    User Data            Contains the context transfer protocol
 If a CXTP deployment will never run over the public Internet, and it
 is known that congestion is not a problem in the access network,
 alternative transport protocols MAY be appropriate vehicles for
 experimentation.  For example, piggybacking CXTP messages on top of
 handover signaling for routing, such as provided by FMIPv6 in ICMP
 [FMIPv6].  Implementations of CXTP MAY support ICMP for such
 purposes.  If such piggybacking is used, an experimental message
 extension for the protocol on which CXTP is piggybacking MUST be
 designed.  Direct deployment on top of a transport protocol for
 experimental purposes is also possible.  In this case, the researcher

Loughney, et al. Experimental [Page 18] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 MUST be careful to accommodate good Internet transport protocol
 engineering practices, including using retransmits with exponential

3.2. MN-AR Transport

 The MN-AR interface MUST implement and SHOULD use ICMP to transport
 the CTAR and CTAA messages.  Because ICMP contains no provisions for
 retransmitting packets if signaling is lost, the CXTP protocol
 incorporates provisions for improving transport performance on the
 MN-AR interface.  The MN and AR SHOULD limit the number of context
 data block identifiers included in the CTAR and CTAA messages so that
 the message will fit into a single packet, because ICMP has no
 provision for fragmentation above the IP level.  CXTP uses the
 Experimental Mobility ICMP type [IANA].  The ICMP message format for
 CXTP messages 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
 |     Type      |     Code      |           Checksum            |
 |   Subtype     |                   Reserved                    |
 |   Message...
 +-+-+-+-+-+-+-+-+-+-+-+- - - -
 IP Fields:
    Source Address       An IP address assigned to the sending
    Destination Address
                         An IP address assigned to the receiving
    Hop Limit            255
 ICMP Fields:
    Type           Experimental Mobility Type (To be assigned by IANA,
                   for IPv4 and IPv6, see [IANA])
    Code           0
    Checksum       The ICMP checksum.

Loughney, et al. Experimental [Page 19] RFC 4067 Context Transfer Protocol (CXTP) July 2005

    Sub-type       The Experimental Mobility ICMP subtype for CXTP,
                   see [IANA].
    Reserved       Set to zero by the sender and ignored by
                   the receiver.
    Message        The body of the CTAR or CTAA message.
    CTAR messages for which a response is requested but fail to elicit
    a response are retransmitted.  The initial retransmission occurs
    after a CXTP_REQUEST_RETRY wait period.  Retransmissions MUST be
    made with exponentially increasing wait intervals (doubling the
    wait each time).  CTAR messages should be retransmitted until
    either a response (which might be an error) has been obtained, or
    until CXTP_RETRY_MAX seconds after the initial transmission.
    MNs SHOULD generate the sequence number in the CTAR message
    randomly (also ensuring that the same sequence number has not been
    used in the last 7 seconds), and, for predictive transfer, MUST
    use the same sequence number in a CTAR message to the nAR as for
    the pAR.  An AR MUST ignore the CTAR message if it has already
    received one with the same sequence number and MN IP address.
    Implementations MAY, for research purposes, try other transport
    protocols.  Examples are the definition of a Mobile IPv6 Mobility
    Header [MIPv6] for use with the FMIPv6 Fast Binding Update
    [FMIPv6] to allow bundling of both routing change and context
    transfer signaling from the MN to AR, or definition of a UDP
    protocol instead of ICMP.  If such implementations are done, they
    should abide carefully by good Internet transport engineering
    practices and be used for prototype and demonstration purposes
    only.  Deployment on large scale networks should be avoided until
    the transport characteristics are well understood.

4. Error Codes and Constants

 Error Code      Section    Value        Meaning
 BAD_CHECKSUM    3.1        0x01         Error code if the
                                         SCTP checksum fails.

Loughney, et al. Experimental [Page 20] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 Constant             Section    Default Value  Meaning
 CT_REQUEST_RATE       6.3       10 requests/   Maximum number of
                                    sec.        CTAR messages before
                                                AR institutes rate
 CT_MAX_TRANSFER_TIME  3.1       200 ms         Maximum amount of time
                                                pAR should wait before
                                                aborting the transfer.
 CT_REQUEST_RETRY      3.2       2 seconds      Wait interval before
                                                initial retransmit
                                                on MN-AR interface.
 CT_RETRY_MAX          3.2     15 seconds       Give up retrying
                                                on MN-AR interface.

5. Examples and Signaling Flows

5.1. Network Controlled, Initiated by pAR, Predictive

               MN                    nAR                     pAR
               |                      |                       |
          T    |                      |                  CT trigger
          I    |                      |                       |
          M    |                      |<------- CTD ----------|
          E    |------- CTAR -------->|                       |
          :    |                      |                       |
          |    |                      |-------- CTDR -------->|
          V    |                      |                       |
               |                      |                       |

5.2. Network Controlled, Initiated by nAR, Reactive

               MN                    nAR                     pAR
               |                      |                       |
          T    |                 CT trigger                   |
          I    |                      |                       |
          M    |                      |--------- CT-Req ----->|
          E    |                      |                       |
          :    |                      |<------- CTD ----------|
          |    |                      |                       |
          V    |------- CTAR -------->|                       |
               |                      |----- CTDR (opt) ----->|
               |                      |                       |

Loughney, et al. Experimental [Page 21] RFC 4067 Context Transfer Protocol (CXTP) July 2005

5.3. Mobile Controlled, Predictive New L2 up/Old L2 down

 CTAR request to nAR
               MN                    nAR                     pAR
               |                      |                       |
         new L2 link up               |                       |
               |                      |                       |
          CT trigger                  |                       |
               |                      |                       |
          T    |------- CTAR -------->|                       |
          I    |                      |-------- CT-Req ------>|
          M    |                      |                       |
          E    |                      |<-------- CTD ---------|
          :    |                      |                       |
          |    |                      |                       |
          V    |                      |                       |
               |                      |                       |
 Whether the nAR sends the MN a CTAR reject message if CT is not
 supported is for future study.

6. Security Considerations

 At this time, the threats to IP handover in general and context
 transfer in particular are not widely understood, particularly on the
 MN to AR link, and mechanisms for countering them are not well
 defined.  Part of the experimental task in preparing CXTP for
 eventual standards track will be to better characterize threats to
 context transfer and design specific mechanisms to counter them.
 This section provides some general guidelines about security based on
 discussions among the Design Team and Working Group members.

6.1. Threats

 The Context Transfer Protocol transfers state between access routers.
 If the MNs are not authenticated and authorized before moving on the
 network, there is a potential for masquerading attacks to shift state
 between ARs, causing network disruptions.
 Additionally, DoS attacks can be launched from MNs towards the access
 routers by requesting multiple context transfers and then by
 disappearing.  Finally, a rogue access router could flood mobile
 nodes with packets, attempt DoS attacks, and issue bogus context
 transfer requests to surrounding routers.

Loughney, et al. Experimental [Page 22] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 Consistency and correctness in context transfer depend on
 interoperable feature context definitions and how CXTP is utilized
 for a particular application.  For some considerations regarding
 consistency and correctness that have general applicability but are
 articulated in the context of AAA context transfer, please see [EAP].

6.2. Access Router Considerations

 The CXTP inter-router interface relies on IETF standardized security
 mechanisms for protecting traffic between access routers, as opposed
 to creating application security mechanisms.  IPsec [RFC2401] MUST be
 supported between access routers.
 To avoid the introduction of additional latency due to the need for
 establishing a secure channel between the context transfer peers
 (ARs), the two ARs SHOULD establish such a secure channel in advance.
 The two access routers need to engage in a key exchange mechanism
 such as IKE [RFC2409], establish IPSec SAs, and define the keys,
 algorithms, and IPSec protocols (such as ESP) in anticipation of any
 upcoming context transfer.  This will save time during handovers that
 require secure transfer.  Such SAs can be maintained and used for all
 upcoming context transfers between the two ARs.  Security should be
 negotiated prior to the sending of context.
 Access Routers MUST implement IPsec ESP [ESP] in transport mode with
 non-null encryption and authentication algorithms to provide per-
 packet authentication, integrity protection and confidentiality, and
 MUST implement the replay protection mechanisms of IPsec.  In those
 scenarios where IP layer protection is needed, ESP in tunnel mode
 SHOULD be used.  Non-null encryption should be used when using IPSec
 ESP.  Strong security on the inter-router interface is required to
 protect against attacks by rogue routers, and to ensure
 confidentiality on the context transfer authorization key in
 predicative transfer.
 The details of IKE key exchange and other details of the IPsec
 security associations between routers are to be determined as part of
 the research phase associated with finalizing the protocol for
 standardization.  These details must be determined prior to
 standardization.  Other working groups are currently working on
 general security for routing protocols.  Ideally, a possible solution
 for CXTP will be based on this work to minimize the operational
 configuration of routers for different protocols.  Requirements for
 CXTP will be brought to the appropriate IETF routing protocol
 security working groups for consideration.

Loughney, et al. Experimental [Page 23] RFC 4067 Context Transfer Protocol (CXTP) July 2005

6.3. Mobile Node Considerations

 The CTAR message requires the MN and AR to possess a shared secret
 key to calculate the authorization token.  Validation of this token
 MUST precede context transfer or installation of context for the MN,
 removing the risk that an attacker could cause an unauthorized
 transfer.  How the shared key is established is out of scope of this
 specification.  If both the MN and AR know certified public keys of
 the other party, Diffie-Hellman can be used to generate a shared
 secret key [RFC2631].  If an AAA protocol of some sort is run for
 network entry, the shared key can be established using that protocol
 If predictive context transfer is used, the shared key for
 calculating the authorization token is transferred between ARs.  A
 transfer of confidential material of this sort poses certain security
 risks, even if the actual transfer itself is confidential and
 authenticated, as is the case for inter-router CXTP.  The more
 entities know the key, the more likely a compromise may occur.  To
 mitigate this risk, nAR MUST discard the key immediately after using
 it to validate the authorization token.  The MN MUST establish a new
 key with the AR for future CXTP transactions.  The MN and AR SHOULD
 exercise care in using a key established for other purposes for also
 authorizing context transfer.  The establishment of a separate key
 for context transfer authorization is RECOMMENDED.
 Replay protection on the MN-AR protocol is provided by limiting the
 time period in which context is maintained.  For predictive transfer,
 the pAR receives a CTAR message with a sequence number, transfers the
 context along with the authorization token key, and then drops the
 context and the authorization token key immediately upon completion
 of the transfer.  For reactive transfer, the nAR receives the CTAR,
 requests the context that includes the sequence number and
 authorization token from the CTAR message that allows the pAR to
 check whether the transfer is authorized.  The pAR drops the context
 and authorization token key after the transfer has been completed.
 The pAR and nAR ignore any requests containing the same MN IP address
 if an outstanding CTAR or CTD message is unacknowledged and has not
 timed out.  After the key has been dropped, any attempt at replay
 will fail because the authorization token will fail to validate.  The
 AR MUST NOT reuse the key for any MN, including the MN that
 originally possessed the key.
 DoS attacks on the MN-AR interface can be limited by having the AR
 rate limit the number of CTAR messages it processes.  The AR SHOULD
 limit the number of CTAR messages to the CT_REQUEST_RATE.  If the
 request exceeds this rate, the AR SHOULD randomly drop messages until
 the rate is established.  The actual rate SHOULD be configured on the

Loughney, et al. Experimental [Page 24] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 AR to match the maximum number of handovers that the access network
 is expected to support.

7. Acknowledgements & Contributors

 This document is the result of a design team formed by the chairs of
 the SeaMoby working group.  The team included John Loughney, Madjid
 Nakhjiri, Rajeev Koodli and Charles Perkins.
 Basavaraj Patil, Pekka Savola, and Antti Tuominen contributed to the
 Context Transfer Protocol review.
 The working group chairs are Pat Calhoun and James Kempf, whose
 comments have been very helpful in the creation of this
 The authors would also like to thank Julien Bournelle, Vijay
 Devarapalli, Dan Forsberg, Xiaoming Fu, Michael Georgiades, Yusuf
 Motiwala, Phil Neumiller, Hesham Soliman, and Lucian Suciu for their
 help and suggestions with this document.

8. References

8.1. Normative References

 [RFC791]    Postel, J., "Internet Protocol", STD 5, RFC 791,
             September 1981.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2409]   Harkins, D. and D. Carrel, "The Internet Key Exchange
             (IKE)", RFC 2409, November 1998.
 [RFC3513]   Hinden, R. and S. Deering, "Internet Protocol Version 6
             (IPv6) Addressing Architecture", RFC 3513, April 2003.
 [ESP]       Kent, S. and R. Atkinson, "IP Encapsulating Security
             Payload (ESP)", RFC 2406, November 1998.
 [SCTP]      Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
             Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
             Zhang, L., and V. Paxson, "Stream Control Transmission
             Protocol", RFC 2960, October 2000.
 [PR-SCTP]   Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
             Conrad, "Stream Control Transmission Protocol (SCTP)
             Partial Reliability Extension", RFC 3758, May 2004.

Loughney, et al. Experimental [Page 25] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 [IANA]      Kempf, J., "Instructions for Seamoby and Experimental
             Mobility Protocol IANA Allocations", RFC 4065, July 2005.

8.2. Informative References

 [FHCT]      R. Koodli and C. E. Perkins, "Fast Handovers and Context
             Transfers", ACM Computing Communication Review, volume
             31, number 5, October 2001.
 [TEXT]      M. Nakhjiri, "A time efficient context transfer method
             with Selective reliability for seamless IP mobility",
             IEEE VTC-2003-Fall, VTC 2003 Proceedings, Vol.3, Oct.
 [FMIPv6]    Koodli, R., Ed., "Fast Handovers for Mobile IPv6", RFC
             4068, July 2005.
 [LLMIP]     K. El Malki et al., "Low Latency Handoffs in Mobile
             IPv4", Work in Progress.
 [RFC3374]   Kempf, J., "Problem Description: Reasons For Performing
             Context Transfers Between Nodes in an IP Access Network",
             RFC 3374, September 2002.
 [RFC2401]   Kent, S. and R. Atkinson, "Security Architecture for the
             Internet Protocol", RFC 2401, November 1998.
 [TERM]      Manner, J. and M. Kojo, "Mobility Related Terminology",
             RFC 3753, June 2004.
 [RFC2631]   Rescorla, E., "Diffie-Hellman Key Agreement Method", RFC
             2631, June 1999.
 [PerkCal04] Perkins, C. and P. Calhoun, "Authentication,
             Authorization, and Accounting (AAA) Registration Keys for
             Mobile IPv4", RFC 3957, March 2005.
 [MIPv6]     Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
             in IPv6", RFC 3775, June 2004.
 [RFC2710]   Deering, S., Fenner, W., and B. Haberman, "Multicast
             Listener Discovery (MLD) for IPv6", RFC 2710, October
 [RFC2461]   Narten, T., Nordmark, E., and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 2461, December

Loughney, et al. Experimental [Page 26] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 [RFC2462]   Thomson, S. and T. Narten, "IPv6 Stateless Address
             Autoconfiguration", RFC 2462, December 1998.
 [RFC3095]   Bormann, C., Burmeister, C., Degermark, M., Fukushima,
             H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T.,
             Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro,
             K., Wiebke, T., Yoshimura, T., and H. Zheng, "RObust
             Header Compression (ROHC): Framework and four profiles:
             RTP, UDP, ESP, and uncompressed ", RFC 3095, July 2001.
 [BT]        IEEE, "IEEE Standard for information technology -
             Telecommunication and information exchange between
             systems - LAN/MAN - Part 15.1: Wireless Medium Access
             Control (MAC) and Physical Layer (PHY) specifications for
             Wireless Personal Area Networks (WPANs)", IEEE Standard
             802.15.1, 2002.
 [EAP]       Aboba, B., Simon, D., Arkko, J., Eron, P., and H.
             Levokowetz, "Extensible Authentication Protocol (EAP) Key
             Management Framework", Work in Progress.

Loughney, et al. Experimental [Page 27] RFC 4067 Context Transfer Protocol (CXTP) July 2005

Appendix A. Timing and Trigger Considerations

 Basic Mobile IP handover signaling can introduce disruptions to the
 services running on top of Mobile IP, which may introduce unwanted
 latencies that practically prohibit its use for certain types of
 services.  Mobile IP latency and packet loss are optimized through
 several alternative procedures, such as Fast Mobile IP [FMIPv6] and
 Low Latency Mobile IP [LLMIP].
 Feature re-establishment through context transfer should contribute
 zero (optimally) or minimal extra disruption of services in
 conjunction with handovers.  This means that the timing of context
 transfer SHOULD be carefully aligned with basic Mobile IP handover
 events, and with optimized Mobile IP handover signaling mechanisms,
 as those protocols become available.
 Furthermore, some of those optimized mobile IP handover mechanisms
 may provide more flexibility in choosing the timing and ordering for
 the transfer of various context information.

Appendix B. Multicast Listener Context Transfer

 In the past, credible proposals have been made in the Seamoby Working
 Group and elsewhere for using context transfer to the speed of
 handover of authentication, authorization, and accounting context,
 distributed firewall context, PPP context, and header compression
 context.  Because the Working Group was not chartered to develop
 context profile definitions for specific applications, none of the
 documents submitted to Seamoby were accepted as Working Group items.
 At this time, work to develop a context profile definition for RFC
 3095 header compression context [RFC3095] and to characterize the
 performance gains obtainable by using header compression continues,
 but is not yet complete.  In addition, there are several commercial
 wireless products that reportedly use non-standard, non-interoperable
 context transfer protocols, though none is as yet widely deployed.
 As a consequence, it is difficult at this time to point to a solid
 example of how context transfer could result in a commercially
 viable, widely deployable, interoperable benefit for wireless
 networks.  This is one reason why CXTP is being proposed as an
 Experimental protocol, rather than Standards Track.  Nevertheless, it
 seems valuable to have a simple example that shows how handover could
 benefit from using CXTP.  The example we consider here is
 transferring IPv6 MLD state [RFC2710].  MLD state is a particularly
 good example because every IPv6 node must perform at least one MLD
 messaging sequence on the wireless link to establish itself as an MLD
 listener prior to performing router discovery [RFC2461] or duplicate
 address detection [RFC2462] or before sending/receiving any

Loughney, et al. Experimental [Page 28] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 application-specific traffic (including Mobile IP handover signaling,
 if any).  The node must subscribe to the Solicited Node Multicast
 Address as soon as it comes up on the link.  Any application-specific
 multicast addresses must be re-established as well.  Context transfer
 can significantly speed up re-establishing multicast state by
 allowing the nAR to initialize MLD for a node that just completed
 handover without any MLD signaling on the new wireless link.  The
 same approach could be used for transferring multicast context in
 An approximate quantitative estimate for the amount of savings in
 handover time can be obtained as follows: MLD messages are 24 octets,
 to which the headers must be added, because there is no header
 compression on the new link, where the IPv6 header is 40 octets, and
 a required Router Alert Hop-by-Hop option is 8 octets including
 padding.  The total MLD message size is 72 octets per subscribed
 multicast address.  RFC 2710 recommends that nodes send 2 to 3 MLD
 Report messages per address subscription, since the Report message is
 unacknowledged.  Assuming 2 MLD messages sent for a subscribed
 address, the MN would need to send 144 octets per address
 subscription.  If MLD messages are sent for both the All Nodes
 Multicast address and the Solicited Node Multicast address for the
 node's link local address, a total of 288 octets are required when
 the node hands over to the new link.  Note that some implementations
 of IPv6 are optimized by not sending an MLD message for the All Nodes
 Multicast Address, since the router can infer that at least one node
 is on the link (itself) when it comes up and always will be.
 However, for purposes of this calculation, we assume that the IPv6
 implementation is conformant and that the message is sent.  The
 amount of time required for MLD signaling will depend on the per node
 available wireless link bandwidth, but some representative numbers
 can be obtained by assuming bandwidths of 20 kbps or 100 kbps.  With
 these 2 bit rates, the savings from not having to perform the pre-
 router discovery messages are 115 msec. and 23 msec., respectively.
 If any application-specific multicast addresses are subscribed, the
 amount of time saved could be more substantial.
 This example might seem a bit contrived as MLD is not used in the 3G
 cellular protocols, and wireless local area network protocols
 typically have enough bandwidth if radio propagation conditions are
 optimal.  Therefore, sending a single MLD message might not be viewed
 as a performance burden.  An example of a wireless protocol where MLD
 context transfer might be useful is IEEE 802.15.1 (Bluetooth)[BT].
 IEEE 802.15.1 has two IP "profiles": one with PPP and one without.
 The profile without PPP would use MLD.  The 802.15.1 protocol has a
 maximum bandwidth of about 800 kbps, shared between all nodes on the
 link, so a host on a moderately loaded 802.15.1 access point could
 experience the kind of bandwidth described in the previous paragraph.

Loughney, et al. Experimental [Page 29] RFC 4067 Context Transfer Protocol (CXTP) July 2005

 In addition, 802.15.1 handover times are typically run upwards of a
 second or more because the host must resynchronize its frequency
 hopping pattern with the access point, so anything the IP layer could
 do to alleviate further delay would be beneficial.
 The context-specific data field for MLD context transfer included in
 the CXTP Context Data Block message for a single IPv6 multicast
 address has the following 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
    |                                                               |
    +             Subnet Prefix on nAR Wireless Interface           +
    |                                                               |
    |                                                               |
    +                                                               +
    |                                                               |
    +               Subscribed IPv6 Multicast Address               +
    |                                                               |
    +                                                               +
    |                                                               |
 The Subnet Prefix on a nAR Wireless Interface field contains a subnet
 prefix that identifies the interface on which multicast routing
 should be established.  The Subscribed IPv6 Multicast Address field
 contains the multicast address for which multicast routing should be
 The pAR sends one MLD context block per subscribed IPv6 multicast
 No changes are required in the MLD state machine.
 Upon receipt of a CXTP Context Data Block for MLD, the state machine
 takes the following actions:
  1. If the router is in the No Listeners present state on the

wireless interface on which the Subnet Prefix field in the

       Context Data Block is advertised, it transitions into the
       Listeners Present state for the Subscribed IPv6 Multicast
       Address field in the Context Data Block.  This transition is
       exactly the same as if the router had received a Report

Loughney, et al. Experimental [Page 30] RFC 4067 Context Transfer Protocol (CXTP) July 2005

  1. If the router is in the Listeners present state on that

interface, it remains in that state but restarts the timer, as

       if it had received a Report message.
 If more than one MLD router is on the link, a router receiving an MLD
 Context Data Block SHOULD send the block to the other routers on the
 link.  If wireless bandwidth is not an issue, the router MAY instead
 send a proxy MLD Report message on the wireless interface that
 advertises the Subnet Prefix field from the Context Data Block.
 Since MLD routers do not keep track of which nodes are listening to
 multicast addresses (only whether a particular multicast address is
 being listened to) proxying the subscription should cause no

Loughney, et al. Experimental [Page 31] RFC 4067 Context Transfer Protocol (CXTP) July 2005

Authors' Addresses

 Rajeev Koodli
 Nokia Research Center
 313 Fairchild Drive
 Mountain View, California 94043
 John Loughney
 Itdmerenkatu 11-13
 00180 Espoo
 Madjid F. Nakhjiri
 Motorola Labs
 1301 East Algonquin Rd., Room 2240
 Schaumburg, IL, 60196
 Charles E. Perkins
 Nokia Research Center
 313 Fairchild Drive
 Mountain View, California 94043

Loughney, et al. Experimental [Page 32] RFC 4067 Context Transfer Protocol (CXTP) July 2005

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Loughney, et al. Experimental [Page 33]

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