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RFC 4067
Seamoby WG J. Loughney (editor)
Internet Draft M. Nakhjiri
Category: Experimental C. Perkins
<draft-ietf-seamoby-ctp-03.txt> R. Koodli
Expires: December 2003 June 2003
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].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
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Distribution of this memo is unlimited.
Copyright (C) The Internet Society 2003. All Rights Reserved.
Abstract
This document presents a context transfer protocol that enables
mobile nodes to authorize context transfers between access routers.
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 Conventions Used in This Document
1.2 Abbreviations Used in the Document
2. Protocol Overview
2.1 Context Transfer Packet Formats
2.2 Context Types
2.3 Context Data Block
2.4 Messages
3. Transport, Reliability and Retransmission of Feature Data
4. Open Issues
4.1 Failure Handling ti -5 5. Examples and Signaling Flows
5.1 Network controlled, Initiated by pAR
5.2 Network controlled, Initiated by nAR
5.3 Mobile controlled, Predictive New L2 up/old L2 down
6. Security Considerations
7. IANA Considerations
8. Acknowledgements
9. References
9.1 Normative References
9.2 Non-Normative References
Appendix A. Timing and Trigger Considerations
Appendix B. Congestion Control
Author's Addresses
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1. Introduction
"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 Context Relocation for Seamless Header Compression in IP
Networks [CTHC].
2) An additional motivation is to provide an interoperable solution
that supports various Layer 2 radio access technologies.
This document describes a generic context transfer protocol, 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.
1.1 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].
1.2 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
FPT Feature Profile Types
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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 sends the CT Activate Request to old AR whenever
possible to initiate predictive context transfer. In any case, the
MN always sends the CTAR message to new AR. If the contexts are
already present, nAR would verify the authorization token present
in CTAR with its own computation (using the parameters supplied by
pAR), and subsequently activate 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 B).
The Context Transfer protocol typical lly 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 Previous Access Router (pAR) and
the target node is MN's New Access Router (nAR). Context Transfer
takes place when an event, such as a handover, takes place. We call
such an event as 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 with which the contexts are
associated, the IP addresses of the access routers, and authorization
to transfer context.
Context transfer protocol messages use Feature Profile Types that
identify the way that data is organized for the particular feature
contexts. The Feature Profile Types (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 transfer of
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context data also contain an appropriate token to authorize the
context transfer.
The Previous Access Router transfers feature contexts under two
general scenarios. First, it may receive 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 2.4.1, 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. It also includes
the MN's new IP address (valid on nAR) whenever it is known. 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 2.4.2,
contains the MN's previous IP address and its new IP address (if
known). 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
reliably took place. By always sending the CTAR message to nAR, the
Context Transfer Request (see below) can be sent to pAR whenever
necessary.
In the second scenario, pAR receives a Context Transfer Request (CT
Request) described in Section 2.4.5, message from nAR. The nAR
itself generates the CT Request message either as a result of
receiving the CTAR message or as a response to an internal trigger
(that indicates the MN's attachment). In the CT-Req message, nAR
supplies the MN's previous IP address, the feature contexts to be
transferred, and a token (generated by the MN) authorizing context
transfer. In response to CT Request 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 message (See Section 2.4.3) to
report the status of processing the received contexts.
[1].* contexts, pAR verifies authorization token before transmitting
the [2].* in the CTD message.
When context transfer takes place without the nAR requesting it
(scenario one above), 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. When context transfer happens with an
explicit request from nAR (scenario two above), pAR performs such
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verification since the contexts are still present at pAR. In either
case, token verification takes place at the router possessing the
contexts.
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. 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.
The messages (CTD and CTDR) that perform the context transfer between
the access routers need to be authenticated, so that the access
routers can be certain that the data has not been tampered with
during delivery.
2.1 Context Transfer Packet Format
The packet consists of a common header, message specific header and
one or more data packets. Data packets may be bundled together in
order ensure a more efficient transfer. The total packet size,
including transport protocol and IP protocol headers SHOULD be less
than the path MTU, in order to avoid packet fragmentation.
+----------------------+
| CTP Header |
+----------------------+
| Message Header |
+----------------------+
| CTP Data 1 |
+----------------------+
| CTP Data 2 |
+----------------------+
| ... |
2.2 Context Types
Contexts are identified by context type, which is a 32-bit number.
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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, 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. Each such
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.
2.3 Context Data Block
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V| Context Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Presence Vector (if V = 1) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ context type-dependent data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 'V' bit specifies whether or not the "presence vector" is used.
When the presence vector is in use, the next 32 bits are interpreted
to indicate whether particular data fields are present (and, thus,
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. 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 bytes if
the 'V' bit is set, minus the accumulated size of all the context
data that is implicitly given as a default value.
2.4 Messages
In this section, a list of the available context transfer message
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types is given, along with a brief description of their functions.
Generally, messages use the following generic message header 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |reserve| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Node's Previous IP address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ message data /
\ \
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Mobile Node, for which context transfer protocol operations are
undertaken, is always identified by its previous IP access address.
At any one 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 mobile node's
identifying previous IP address.
2.4.1 Context Transfer Activate Request (CTAR) Message
This message is always sent by MN to nAR to request context transfer
activation. It is for further to study to see if when the CTAR
message is sent by the MN to the nAR. If an acknowledgement is
needed, the MN sets the A flag to 1, other wise the MN does not
expect an acknowledgement. This message may include a list of FPT
(feature profile types) that require transfer. It may include flags
to request secure and/or reliable 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 and its new
IP address (if known).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |A| rsv | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Node's Previous IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Previous Router IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Type=Auth-Token| Type Len | Replay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN Authorization Token |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested Context Type (if present) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Requested Context Type (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The message data for CTAR is the Mobile Node's Previous IP Address,
Previous Router's IP address, MN Authorization Token, followed by a
list of context types. If no context types are specified, then all
contexts for the mobile node are requested.
2.4.2 Context Transfer Activate Acknowledge (CTAA) Message
This is an informative message sent nAR to the MN to acknowledge a
CTAR message. Acknowledgement is optional, since the MN may have
already moved and may not receive the reply. This message may include
a list of FPT (feature profile types) that were not transferred
successfully.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |reserve| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Node's Previous IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Previous Router IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Type=Auth-Token| Type Len | Replay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN Authorization Token |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Failed Context Type (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Failed Context Type (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The message data for CTAR is the Mobile Node's Previous IP Address,
Previous Router's IP address, MN Authorization Token, followed by a
list of context types that were not successfully transferred. If no
context types are specified, then all contexts for the mobile node
are considered successfully transferred.
2.4.3 Context Transfer Data (CTD) Message
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Sent by pAR to nAR, and includes feature data (CTP data). This
message should handle predictive as well as normal CT. A reliability
flag, R, included in this message indicates whether a reply is
required by nAR.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |C|R|rsv| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Elapsed Time (in milliseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Node's Previous Care-of Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Node's New Care-of Address, if C=1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=Auth | Type Length | Algorithm | Key Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| First Context Block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Context Block |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Authorization token type field is present in the predictive
scenario only. The supplied parameters, algorithm, key length and the
key itself, allow nAR to compute a token locally depending on the
contents of the CTAR message.
The algorithm for carrying out the computation of the MN
Authorization Token is HMAC_SHA1. The input data for computing the
token are: the MN's Previous IP address, the FPT objects and the
Replay field. When support for transferring all contexts is
requested, a default FPT is used. The Authorization Token is obtained
by truncating the results of the HMAC_SHA1 computation to retain only
the leading 32 bits.
2.4.4 Context Transfer Data Reply (CTDR) Message
This message is sent by nAR to pAR depending on the value of the
reliability 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
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |C| rsv | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Node's Previous Care-of Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OverallStatus | Ctx-1 Status | Ctx-2 Status | ...... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The OverallStatus is used for reporting overall success or failure,
which could be based on verification of the MN authorization token
for instance. For certain values of the overall status, it may be
that some contexts were successfully transferred and some failed to
be transferred. In this case, then for each context another status
code MUST be provided to indicate to pAR those contexts that have
failed and those that have succeeded, along with the reason.
2.4.5 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type | rsv | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Node's Previous Care-of Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.4.6 Context Transfer Request (CT Request) Message
Sent by nAR to pAR request start of context transfer. This message is
sent as a response to CTAR message by the MN.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message Type |reserve| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mobile Node's Previous Care-of Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Type=Auth-Token| Type Len | Replay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MN Authorization Token |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested Context Type (if present) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Requested Context Type (if present) |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ........ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The message data for CT Request is the Mobile Node's Previous Care-of
Address, MN Authorization Token, followed by a list of context types.
If no context types are specified, then all contexts for the mobile
node are requested. The fields including and following the
Authorization Token Type are inserted from the CTAR message.
The algorithm for carrying out the computation is HMAC_SHA1. The
Authorization token is obtained by truncating the results of the
HMAC_SHA1 computation to retain only the leading 32 bits. The input
data for computing the token are: the MN's Previous IP address, the
FPT objects and the Replay field. When support for transferring all
contexts is requested, a default FPT is used.
3. Transport, Reliability and Retransmission of Feature Data
CTP runs over UDP using port number <TBD>. Some feature contexts may
specify a reliability factor, MAX_RETRY_INTERVAL, which is the length
of time that the pAR is allowed to perform retransmissions before
giving up on the context transfer for that feature context. The
longer the allowed retry interval, the more retransmissions will be
possible for that feature context.
For feature contexts that specify MAX_RETRY_INTERVAL, pAR SHOULD
retransmit an unacknowledged CTD message after waiting for
RETRANSMISSION_DELAY milliseconds. This time value is configurable
based on the type of network interface; slower network media
naturally will be configured with longer values for the
RETRANSMISSION_DELAY. Except for the value of the elapsed time
field, the payload of each retransmitted CTD packet is identical to
the payload of the initially transmitted CTD packet, in order to
maintain the ability of the mobile device to present a correctly
calculated authentication token. The number of retransmissions, each
delayed by RETRANSMISSION_DELAY, depends on the maximum value for
MAX_RETRY_INTERVAL as specified by any of the contexts. The value
of the Elapsed Time field is the number of milliseconds since the
transmission of the first CTD message
UDP provides an optional checksum, which SHOULD be checked. If the
checksum is incorrect, nAR SHOULD return a CTDR packet to pAR with
the status value BAD_UDP_CHECKSUM, even if the 'R' bit is not set in
the CTD.
4. Error Codes
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This section lists error codes used by UDP
BAD_UDP_CHECKSUM 0x01
5. Examples and Signaling Flows
5.1 Network controlled, Initiated by pAR
MN nAR pAR
| | |
T | | CT trigger
I | | |
M | |<------- CTD ----------|
E |--------CTAR--------->| |
: | | |
| | |-------- CTDR -------->|
V | | |
| | |
5.2 Network controlled, initiated by nAR
in 6
MN nAR pAR
| | |
T | CT trigger |
I | | |
M | |----- CT Request ----->|
E | | |
: | |<------- CTD ----------|
| | | |
V |--------CTAR--------->| |
| |----- CTDR (opt) ----->|
| | |
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 Request ------>|
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M | | |
E | |<-------- CTD ---------|
: | | |
| | | |
V | | |
| | |
In case CT cannot be supported, a CTAR reject (TBD) may be sent to
the MN by nAR.
6. Security Considerations
6.1. Threats.
The Context Transfer Protocol transfers state between access routers.
If the mobile nodes are not authenticated and authorized before
moving on the network, there is a potential for DoS attacks to shift
state between ARs, causing network disruptions.
Additionally, DoS attacks can be launched from mobile nodes towards
the access routers by requesting multiple context transfers and then
disappearing. Additionally, a rogue access router could flood mobile
nodes with packets, attempting DoS attacks.
6.2. Security Details
CTP 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 to context
transfer due to the need for establishment of secure channel between
the context transfer peers (ARs), the two ARs SHOULD establish such
secure channel in advance. If IPSec is used, for example, 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 of mobile node's context(s). 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.
Furthermore, either one or both of the pAR and nAR need to be able
authenticate the mobile and authorize mobile's credential before
authorizing the context transfer and release of context to the
mobile. In case the context transfer is request by the MN, a
mechanism MUST be provided so that requests are authenticated
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(regardless of the security of context transfer itself) to prevent
the possibility of rogue MNs launching DoS attacks by sending large
number of CT requests as well as cause large number of context
transfers between ARs. Another important consideration is that the
mobile node should claim it's own context, and not some other
masquerader. One method to achieve this is to provide an
authentication cookie to be included with the context transfer
message sent from the pAR to the nAR and confirmed by the mobile node
at the nAR.
6.3. IPsec Considerations
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.
7. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the
context transfer protocol, in accordance with BCP 26 [RFC2434].
7.1 Context Profile Types Registry
This document authorized IANA to create a registry for Context Profile
Types, introduced in this document. For future Context Profile Types,
it is recommended that allocations be done on the basis of Designated
Expert.
The Context Profile Type introduced in this document requires IANA Type
Numbers for each set of feature contexts that make use of Profile Types.
For registration requests where a Designated Expert should be consulted,
the responsible IESG area director should appoint the Designated Expert.
7.2 UDP Port
CTP requires a UDP port assignment.
8. Acknowledgements
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. The
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working group chairs are Pat Calhoun and James Kempf, whose comments
have been very helpful during the creation of this specification.
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 Require-
ment Levels", BCP 14, RFC 2119, March 1997.
[RFC2402] S. Kent, R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998
[CT-REQ] G. Kenward et al., "General Requirements for Context
Transfer", Internet Draft, Internet Engineering Task Force,
Work in Progress.
[CTF] R. Koodli, C.E. Perkins, "Context Transfer Framework for
Seamless Mobility", 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.
IP [RFC2434] Narten, Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC2409] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[LLMIP] K. El Malki et. al, "Low Latency Handoffs in Mobile IPv4",
Internet Engineering Task Force. Work in Progress.
[ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security Pay-
load (ESP)", RFC 2406, November 1998.
Loughney et al. expires December 2003 [Page 16]
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9.2 Non-Normative References
[CTHC] R. Koodli et al., "Context Relocation for Seamless Header
Compression in IP Networks", Internet Draft, 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, Internet Engineering Task Force, 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.
[RFC2246] T. Dierks, C. Allen, "The TLS Protocol Version 1.0", RFC 2246,
January 1999.
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
(such as BETH) may provide more flexibility is choosing the timing
and order for transfer of various context information.
Appendix B. Congestion Control
Context transfer enables smooth handovers and prevents the need of
re-initializing signaling to and from a mobile node after handover.
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Context transfer takes place between access routers.
The goal of congestion control is to prevent congestion by noting
packet loss at the transport layer and reducing the congestion con-
trol window when packet loss occurs, thus effectively restricting the
available in-flight window for packet sending. Additionally, TCP &
SCTP deploy slow-start mechanisms at start-up, in order to prevent
congestion problems at the start of a new TCP/SCTP session.
As some context is time-critical, delays due to congestion control
may reduce the performance of mobile nodes; additionally, signaling
from the mobile node may be increased if the context transfer of time
critical data fails.
Therefore, some analysis is needed on the role of congestion control
and context transfer. Important considerations should be network-
provisioning, intra-domain vs. inter-domain signaling as well as
other considerations. A quick analysis follows.
It is assumed that intra-domain time-critical context transfer should
take no more than one kilobyte, based on existing implementation of
some context transfer solutions. Contexts that are significantly
larger are assumed not so time critical. For a larger number of
users, say one thousand users requesting a smooth handover all in the
same second, the total bandwidth needed is still a small fraction of
a typical Ethernet or frame relay or ATM link between access routers.
So even bursty traffic is unlikely to introduce local congestion.
Furthermore, physically adjacent access routers should be within one
or two IP hops of each other, so the effects of context transfer
should be localized. If transferring real-time contexts triggers
congestive errors, the access network may be seriously under-
provisioned.
In order to handle multiple contexts to be transferred with differing
reliability needs, each context has to be considered separately by
the sending access router nAR. If a CTD message is retransmitted
because the CTDR message was not received in time, then those con-
texts that are "too late" are included anyway as part of the
retransmitted CTD data, in order to ease the ability to verify the
Mobile Authorization Token.
Authors' Addresses
Rajeev Koodli
Nokia Research Center
313 Fairchild Drive
Mountain View, California 94043
USA
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Rajeev.koodli@nokia.com
John Loughney
Nokia
It„merenkatu 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
Charles E. Perkins
Nokia Research Center
313 Fairchild Drive
Mountain View, California 94043
USA
charliep@iprg.nokia.com
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Loughney et al. expires December 2003 [Page 19]
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