Seamoby WG J. Loughney (editor)
Internet Draft M. Nakhjiri
Category: Experimental C. Perkins
<draft-ietf-seamoby-ctp-10.txt> R. Koodli
Expires: December 2004 June 2004
Context Transfer Protocol
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [RFC2026].
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Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society 2004. All Rights Reserved.
Abstract
This document presents a context transfer protocol 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, packet losses and avoiding re-initiation of signaling
to and from the mobile node.
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Table of Contents
1. Introduction
1.1 The Problem
1.2 Conventions Used in This Document
1.3 Abbreviations Used in the Document
2. Protocol Overview
2.1 Context Transfer Scenarios
2.2 Context Transfer Message Format
2.3 Context Types
2.4 Context Data Block (CTB)
2.5 Messages
3. Transport
3.1 Inter-Router Transport
3.2 MN-AR Transport
4. Error Codes and Constants
5. Examples and Signaling Flows
5.1 Network controlled, Initiated by pAR, Predictive
5.2 Network controlled, Initiated by nAR, Reactive
5.3 Mobile controlled, Predictive New L2 up/old L2 down
6. Security Considerations
6.1 Threats
6.2 Access Router Considerations
6.3 Mobile Node Considerations
7. IANA Considerations
8. Acknowledgements & Contributors
9. References
9.1 Normative References
9.2 Non-Normative References
Appendix A. Timing and Trigger Considerations
Appendix B. Multicast Listener Context Transfer
Authors' Addresses
Full Copyright Statement
Intellectual Property
Acknowledgement
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1. Introduction
This document describes the Context Transfer Protocol overview, which
provides:
* 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
handovers.
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) The primary motivation, as mentioned in the introduction, is the
need 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 a
service is included in Appendix B.
2) An additional motivation is to provide an interoperable solution
that supports various Layer 2 radio access technologies.
1.2 Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in 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 abbreviation in this document.
CTP Context Transfer Protocol
DoS Denial-of-Service
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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 either 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 whenever
possible to initiate 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 A).
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.
Typically, the source node is a MN's pAR and the target node is 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, the IP addresses of the access
routers, and 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
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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 transfer of context data also
contain an appropriate token to authorize the context transfer.
Performing 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 about 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. 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 is
terminated.
2.1 Context Transfer Scenarios
The Previous Access Router transfers feature contexts under two
general scenarios.
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 present in CTAR message.
Recall that the MN always sends CTAR message to nAR regardless of
whether it sent the CTAR message to pAR. The reason for this is that
there is no means for the MN to ascertain that context transfer
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reliably took 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 highly likely to be present already.
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), described in Section 2.5, message from nAR. 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 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 CTP message consists of a message-specific header and one or more
data blocks. Data blocks may be bundled together in order to ensure
a more efficient transfer. On the inter-AR interface, SCTP is used
so 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, in order to avoid packet
fragmentation. Each message contains a three 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 number MUST be incremented.
2.3 Context Types
Contexts are identified by FPT code, which is a 16-bit unsigned
integer. The meaning of each context type is determined by a
specification document and 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
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this document along with the specification associated with the
particular context type. The following diagram illustrates the
general format for CTP messages:
+----------------------+
| Message Header |
+----------------------+
| CTP Data 1 |
+----------------------+
| CTP Data 2 |
+----------------------+
| ... |
Each context type specification contains the following details:
- Number, size (in bits), and ordering of data fields in the
state variable vector which embodies the context.
- Default values (if any) for each individual datum of the
context state vector.
- 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 date field sizes presented in the specification.
- 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.
2.4 Context Data Block (CTB)
The Context Data Block (CTB) is used both for request and response
operation. 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
sometimes the presence vector is 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 (FTP) | Length |P| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Presence Vector (if P = 1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Feature Profile Type
16 bit integer, assigned by IANA,
indicating the type of data
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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 might be an
error indication. The 'P' bit specifies whether or not the "presence
vector" is used. When the presence vector is in use, the Presence
Vector is interpreted to indicate whether particular data fields are
present (and containing 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 CTB in 8 octet words including the first 4
octets starting from FPT.
Notice that the length of the context data block is defined by the
sum of 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 CTP messages are defined. The MN for which
context transfer protocol operations are undertaken is always
identified by its previous IP access address. At any time, only one
context transfer operation per MN may be in progress 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 IPv6.
2.5.1 Context Transfer Activate Request (CTAR) Message
This message is always sent by MN to nAR to request context transfer.
Even when the MN does not know if contexts need to be transferred,
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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 such a 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 for the message sent on
both pAR and nAR so pAR can determine whether to use a cached
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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 CTP 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
receiver.
Length Message length in units of octets.
MN's Previous IP Address Field contains either:
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IPv4 Address as defined in [RFC 791],
4 octets.
IPv6 Address as defined in [RFC 2373],
16 octets.
nAR / pAR IP Address Field contains either:
IPv4 Address as defined in [RFC 791],
4 octets.
IPv6 Address as defined in [RFC 2373],
16 octets.
Sequence Number A value used to identify requests and
acknowledgements (see Section 3.2).
Authorization Token
An unforgeable value calculated as
discussed below. This authorizes the
receiver of CTAR to perform context
transfer.
Context Block Variable length field defined in
Section 2.4.
If no context types are specified, all contexts for the MN are
requested.
The Authorization Token is calculated as:
First (32, HMAC_SHA1
(Key, (Previous IP address | Sequence Number | CTBs)))
where Key is a shared secret between the MN and pAR, and CTBs is a
concatenation of all the Context Data Blocks specifying the contexts
to be transfered which 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 transferred successfully.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Vers.| Type |V| Reserved | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Mobile Node's Previous IP address ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FPT (if present) | Status code | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Vers. Version number of CTP 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.
Length Message length in units of octets.
MN's Previous IP Address Field contains either:
IPv4 Address as defined in [RFC 791],
4 octets.
IPv6 Address as defined in [RFC 2373],
16 octets.
FPT 16 bit unsigned integer, listing the FTP
that was not successfully transferred.
Status Code An octet, containing failure reason.
2.5.3 Context Transfer Data (CTD) Message
Sent by pAR to nAR, and includes feature data (CTP data). This
message handles predictive as well as normal CT. An acknowledgement
flag, 'A', included in this message indicates whether a reply is
required by pAR.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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 CTP protocol = 0x1
Type CTD = 0x3 (Context Transfer Data)
PCTD = 0x4 (Predictive Context Transfer
Data)
'V' flag When set to '0', IPv6 addresses.
When set to '1', IPv4 addresses.
'A' bit When set, the pAR requests an
acknowledgement.
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 Address as defined in [RFC 791],
4 octets.
IPv6 Address as defined in [RFC 2373],
16 octets.
Algorithm Algorithm for carrying out the computation
of the MN Authorization Token. Currently
only 1 algorithm is defined, HMAC_SHA1 = 1.
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Key Length Length of key, in octets.
Key Shared key between MN and AR for CTP.
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, allowing nAR to compute a
token locally and verify 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 has been 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 receiving either a
CTDR message for the MN's previous IP address 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 that case, the nAR
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. Indicates 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 CTP protocol = 0x1
Type CTDR = 0x5 (Context Transfer Data)
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'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 Address as defined in [RFC 791],
4 octets.
IPv6 Address as defined in [RFC 2373],
16 octets.
Status Code A context-specific return value,
zero for success, nonzero when 'S' is
not set to one.
FPT 16 bit unsigned integer, listing the FTP
that is being acknowledged.
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 CTP 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
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the receiver.
MN's Previous IP Address Field contains either:
IPv4 Address as defined in [RFC 791],
4 octets.
IPv6 Address as defined in [RFC 2373],
16 octets.
2.5.6 Context Transfer Request (CT-Req) Message
Sent by nAR to pAR to request start of context transfer. This message
is sent as a response to CTAR message from the MN. The fields
following the Previous IP address of the MN are included verbatim
from the CTAR 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|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 CTP 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 Address as defined in [RFC 791],
4 octets.
IPv6 Address as defined in [RFC 2373],
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16 octets.
Sequence Number Copied from the CTAR message, allows the
pAR to distinguish requests for 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
CTAR.
Context Data Request Block
A request block for context data, see
Section 2.4.
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 CTD. If
the CTD fails, the nAR will send CT-Req with the same sequence
number, allowing the pAR to distinguish which context to resend. pAR
drops the context after CTP_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, with no
CT-Req message necessary. 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 the types of access networks in which CTP 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 CTP. Part of the research task in preparing CTP for
consideration as a candidate for possible standardization is to
quantify this issue. However, in order to avoid potential
interference with production applications should a prototype CTP
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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
protocol.
These considerations argue that implementations of CTP MUST support
and prototype deployments of CTP SHOULD use Stream Control Transport
Protocol (SCTP) [SCTP] for 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 features, such as
multiple streams and multiple IP addresses for failover, that are not
necessary for experimental implementation and prototype deployment of
CTP. In this specification, 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. Use
of multiple streams to prevent head-of-line blocking is for future
study. Having 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 'CTP'. Inter-router
CTP 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 in order to accommodate the maximum acceptable
handover delay time, and 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 in order to reduce any extra delay in
sending messages. Within CTP, the nAR SHOULD estimate the retransmit
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timer from the receipt of the first fragment of a CTP 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.
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.
Stream Sequence Number n
Since the 'U' bit is set to one, the
receiver ignores this number. See [SCTP].
Payload Protocol Identifier
Set to 'CTP' (see [IANA]).
User Data Contains the context transfer protocol messages.
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If a CTP 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. An example is piggybacking CTP messages on top of
handover signaling for routing, such as provided by FMIPv6 in ICMP
[FMIPv6]. Implementations of CTP MAY support ICMP for such purposes.
If such piggybacking is used, an experimental message extension for
the protocol on which CTP is piggybacking MUST be designed. Direct
deployment on top of a transport protocol for experimental purposes
is also possible, in that case, the researcher MUST be careful to
accommodate good Internet transport protocol engineering practices,
including using retransmits with exponential backoff.
3.2 MN-AR Transport
The MN-AR interface MUST implement and SHOULD use ICMP for transport
of the CTAR and CTAA messages. Because ICMP contains no provisions
for retransmitting packets if signaling is lost, the CTP 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, since ICMP has no
provision for fragmentation above the IP level. CTP uses the
Experimental Mobility ICMP type [IANA]. The ICMP message format for
CTP 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
interface.
Destination Address
An IP address assigned to the receiving
interface.
Hop Limit 255
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ICMP Fields:
Type Experimental Mobility Type (To be assigned by IANA,
for IPv4 and IPv6, see [IANA])
Code 0
Checksum The ICMP checksum.
Sub-type The Experimental Mobility ICMP subtype for CTP, 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 which fail to
elicit a response are retransmitted. The initial retransmission
occurs after a CTP_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 CTP_RETRY_MAX seconds after the initial transmission.
MNs SHOULD generate the sequence number in the CTAR message
randomly, and, for predictive transfer, MUST use the same sequence
number in a CTAR to the nAR as for the pAR. An AR MUST ignore the
CTAR 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.
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Constant Section Default Value Meaning
--------------------------------------------------------------------
CT_REQUEST_RATE 6.3 10 requests/ Maximum number of
sec. CTAR messages before
AR institutes rate
limiting.
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) ----->|
| | |
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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 | | |
| | |
It is for future study whether the nAR sends the MN a CTAR reject if
CT is not supported.
6. Security Considerations
At this time, the threats to IP handover in general and context
transfer in particular are incompletely understood, particularly on
the MN to AR link, and mechanisms for countering them are not well
defined. Part of the experimental task in preparing CTP 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 masqurading 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
disappearing. Finally, a rogue access router could flood mobile
nodes with packets, attempting DoS attacks, and issue bogus context
transfer requests to surrounding routers.
6.2. Access Router Considerations
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The CTP inter-router interface relies on IETF standardized security
mechanisms for protecting traffic between access routers, as opposed
to creating application security mechanisms. IPsec MUST be supported
between access routers.
In order to avoid the introduction of additional latency due to the
need for establishment of 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 mechanisms such as IKE [RFC2409], establish IPSec SAs,
defining the keys, algorithms and IPSec protocols (such as ESP) in
anticipation for 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.
6.3 Mobile Node Considerations
The CTAR message requires the MN and AR to possess a shared secret
key in order 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 the current specification. If both the MN and AR know
certified public keys of the other party, Diffie-Helman can be used
to generate a shared secret [RFC2631]. If an AAA protocol of some
sort is run for network entry, the shared key can be established
using that protocol [PerkCal04].
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 CTP. The more entities
know the key, the more likely a compromise may occur. In order to
mitigate this risk, nAR MUST discard the key immediately after using
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it to validate the authorization token. The MN MUST establish a new
key with the AR for future CTP transactions. The MN and AR SHOULD
exercise care in using a key established for other purposes for also
authorizing context transfer. It is RECOMMENDED that a separate key
be established for context transfer authorization.
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, 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, including the sequence number and authorization
token from the CTAR which 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 fails to validate. The AR MUST NOT
reuse the key for any MN, including the same MN as 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 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 AR
to match the maximum number of handovers that the access network is
expected to support.
7. IANA Considerations
Instructions for IANA allocations are included in [IANA].
8. Acknowledgements & Contributors
This document is the result of a design team formed by the Working
Group chairs of the SeaMoby working group. The team included John
Loughney, Madjid Nakhjiri, Rajeev Koodli and Charles Perkins.
Contributors to the Context Transfer Protocol review are Basavaraj
Patil, Pekka Savola, and Antti Tuominen.
The working group chairs are Pat Calhoun and James Kempf, whose
comments have been very helpful during the creation of this
specification.
Loughney et al. expires December 2004 [Page 24]
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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.
9. References
9.1 Normative References
[RFC2026] S. Bradner, "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA Con-
siderations Section in RFCs", BCP 26, RFC 2434, October 1998.
[RFC2409] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)", RFC
2409, November 1998.
[ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[SCTP] Stewert, R., et. al., "Stream Control Transmission Protocol",
RFC 2960, October, 2000.
[PR-SCTP] Stewert, R., et. al., "SCTP Partial Reliability Extension",
Internet Engineering Task Force. Work in Progress.
[CARD] Liebisch, M., and Singh, A., editors, et. al., "Candidate
Access Router Discovery", Internet Engineering Task Force.
Work in Progress.
[IANA] Kempf, J., "Instructions for Seamoby Experimental Protocol
IANA Allocations", Internet Engineering Task Force. Work in
Progress.
Loughney et al. expires December 2004 [Page 25]
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9.2 Non-Normative References
[CTHC] R. Koodli et al., "Context Relocation for Seamless Header Com-
pression in IP Networks", Internet Draft, Internet Engineering
Task Force, Work in Progress.
[FMIPv6] R. Koodli (Ed), "Fast Handovers for Mobile IPv6", Internet
Engineering Task Force. Work in Progress.
[LLMIP] K. El Malki et. al, "Low Latency Handoffs in Mobile IPv4",
Internet Engineering Task Force. Work in Progress.
[RFC3374] J. Kempf et al., "Problem Description: Reasons For Performing
Context Transfers Between Nodes in an IP Access Network", RFC
3374, May 2001.
[RFC2401] S. Kent, R. Atkinson, "Security Architecture for the Internet
Protocol", RFC 2401, November 1998.
[TERM] J. Manner, M. Kojo, "Mobility Related Terminology", Internet
Engineering Task Force, Work in Progress.
[RFC2631] E. Rescorla, "Diffie-Hellman Key Agreement Method", RFC 2631,
June, 1999.
[PerkCal04]
C. Perkins and P. Calhoun, "AAA Registration Keys for Mobile
IPv4", Internet Engineering Task Force, Work in Progress.
[MIPv6] D. Johnson, C. Perkins, and J. Arkko, "Mobility Support in
IPv6", Internet Engineering Task Force, Work in Progress.
[RFC2710] S. Deering, W. Fenner, and B. Haberman, " Multicast Listener
Discovery (MLD) for IPv6," RFC 2710, October, 1999.
[RFC2461] T. Narten, E. Nordmark, and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)," RFC 2461, December, 1998.
Loughney et al. expires December 2004 [Page 26]
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[RFC2462] S. Thompson, and T. Narten, "IPv6 Address Autoconfiguration,"
RFC 2462, December, 1998.
[RFC3095] C. Borman, ed., "RObust Header Compression (ROHC)", RFC 3095,
July, 2001.
[BT] IEEE, "IEEE Standard for information technology - Telecommuni-
cation 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.
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 ser-
vices. Mobile IP latency and packet loss is being 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 conjunc-
tion to 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 pro-
tocols become available.
Furthermore, some of those optimized mobile IP handover mechanisms
may provide more flexibility in choosing the timing and order for
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 speed 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 pro-
file definitions for specific applications, none of the drafts sub-
mitted to Seamoby were accepted as Working Group items. At this time,
work continues to develop a context profile definition for RFC 3095
header compression context [RFC3095] and to characterize the perfor-
mance gains obtainable by using header compression, but the work is
not yet complete. In addition, there are several commercial wireless
products that reportedly use non-standard, non-interoperable context
Loughney et al. expires December 2004 [Page 27]
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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 net-
works. This is one reason why CTP is being proposed as an Experimen-
tal protocol, rather than Standards Track. However, it nevertheless
seems valuable to have a simple example that shows how handover could
benefit from using CTP. 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 application-spe-
cific 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 sig-
nificantly 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 IPv4.
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 com-
pression on the new link, with IPv6 header being 40 octets, and a
required Router Alert Hop-by-Hop option being 8 octets including
padding. The total MLD message size is 72 octets per subscribed mul-
ticast address. RFC 2710 recommends that nodes send 2 to 3 MLD Report
messages per address subscription, since the Report message is unac-
knowledged. 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 optimize 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, but for purposes of this calculation, we
assume that the IPv6 implementation is conformant and that the mes-
sage is sent. The amount of time required for MLD signaling will, of
course, 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 two 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
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substantially more.
This example might seem a bit contrived because MLD isn't used in the
3G cellular protocols and wireless local area network protocols typi-
cally have enough bandwidth, if radio propagation conditions are
optimal, so sending a single MLD message might not be viewed as such
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 and one without PPP.
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.
In addition 802.15.1 handover times 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 CTP 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 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
established.
The pAR sends one MLD context block per subscribed IPv6 multicast
address.
No changes are required in the MLD state machine.
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Upon receipt of a CTP Context Data Block for MLD, the state machine
takes the following actions:
- 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 Con-
text Data Block. This transition is exactly the same as if the
router had received a Report message.
- 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 adver-
tises the Subnet Prefix field from the Context Data Block. Since MLD
routers do not keep track of which nodes are listening to munticast
addresses, only whether a particular multicast address is being lis-
tened to, proxying the subscription should cause no difficulty.
Authors' Addresses
Rajeev Koodli
Nokia Research Center
313 Fairchild Drive
Mountain View, California 94043
USA
Rajeev.koodli@nokia.com
John Loughney
Nokia
Itdmerenkatu 11-13
00180 Espoo
Finland
john.loughney@nokia.com
Madjid F. Nakhjiri
Motorola Labs
1031 East Algonquin Rd., Room 2240
Schaumburg, IL, 60196
USA
madjid.nakhjiri@motorola.com
Loughney et al. expires December 2004 [Page 30]
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Charles E. Perkins
Nokia Research Center
313 Fairchild Drive
Mountain View, California 94043
USA
charliep@iprg.nokia.com
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Loughney et al. expires December 2004 [Page 31]
Internet-Draft June 2004
Internet Society.
Loughney et al. expires December 2004 [Page 32]