Seamoby WG J. Loughney (editor) Internet Draft M. Nakhjiri Category: Experimental C. Perkins
<draft-ietf-seamoby-ctp-09.txt><draft-ietf-seamoby-ctp-10.txt> R. Koodli Expires: October 13,December 2004 April 14,June 2004 Context Transfer Protocol Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed,This document is an Internet-Draft and any of which I become aware will be disclosed,is in accordancefull conformance with RFC 3668.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- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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. 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 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 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 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 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 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 bytesoctets 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 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, which is the high orderbit in the Length field,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 bytesoctets 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 bytesoctets 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, 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 bytes.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. 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|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. 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 bytes.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. 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 bytes.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. 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) '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 bytes.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 bytes.octets. 'V' flag When set to '0', IPv6 addresses. When set to '1', IPv4 addresses. Reserved Set to zero by the sender and ignored by the receiver. MN's Previous IP Address Field contains either: IPv4 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 bytes.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. 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 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, unless fragmented,message corresponds to a single MN's feature context collection. A single stream provides simplicity. Use of multiple streams to prevent head- of-linehead-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 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. 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 ICMP Fields: Type SeamobyExperimental Mobility Type (To be assigned by IANA, for IPv4 and IPv6, see [IANA]) Code 10 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. 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 | | | | | | in 05.2 Network controlled, initiated by nAR, Reactive MN nAR pAR | | | T | CT trigger | I | | | M | |--------- CT-Req ----->| 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-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. ThreatsThreats. 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 DoSmasqurading 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 The CTP interrouterinter-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 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,context along with the authorization token key, then drops the context and the authorization token key immediately upon completion of the transfer, along with the authorization token key.transfer. For reactive transfer, the nAR receives the CTAR, requests the context along withcontext, including the sequence number and authorization token, allowingtoken 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 during the transfer process.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 other MNs.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. 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 ConsiderationsCon- 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. 9.2 Non-Normative References [CTHC] R. Koodli et al., "Context Relocation for Seamless Header CompressionCom- 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.[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. [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 services.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 conjunctionconjunc- 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 protocolspro- 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 profilepro- file definitions for specific applications, none of the drafts submittedsub- 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 performanceperfor- 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 transfer protocols, though none is as yet widely deployed. As a consequence, it is difficult at this time to point to a solid example of how context transfer could result in a commercially viable, widely deployable, interoperable benefit for wireless networks.net- works. This is one reason why CTP is being proposed as an ExperimentalExperimen- 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-specificapplication-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 significantlysig- 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 bytes,octets, to which the headers must be added, because there is no header compressioncom- pression on the new link, with IPv6 header being 40 bytes,octets, and a required Router Alert Hop-by-Hop option being 8 bytesoctets including pad- ding.padding. The total MLD message size is 72 bytesoctets per subscribed multicastmul- ticast address. RFC 2710 recommends that nodes send 2 to 3 MLD Report messages per address subscription, since the Report message is unacknowledged.unac- knowledged. Assuming 2 MLD messages sent for a subscribed address, the MN would need to send 144 bytesoctets 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 bytesoctets 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 messagemes- 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 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 typicallytypi- 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. 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 ContextCon- 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 advertisesadver- 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 listenedlis- 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.firstname.lastname@example.org John Loughney Nokia Itdmerenkatu 11-13 00180 Espoo Finland email@example.com Madjid F. Nakhjiri Motorola Labs 1031 East Algonquin Rd., Room 2240 Schaumburg, IL, 60196 USA firstname.lastname@example.org Charles E. Perkins Nokia Research Center 313 Fairchild Drive Mountain View, California 94043 USA email@example.com Full Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFOR- MATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 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