Seamoby WG Internet Draft J. Loughney (editor) Document: draft-ietf-seamoby-ctp-00.txt Nokia M. Nakhjiri Motorola C. Perkins Nokia Expires: October 2002 October 2002 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 [1]. 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 The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Abstract This document presents a context transfer protocol that enables authorized context transfers. Context transfers allows better support for node based mobility so that the applications running on mobile nodes can operate with minimal disruption. Key objectives are to reducing latency, packet losses and avoid re-initiation of signaling to and from the mobile node. Loughney Expires û April 2003 [Page 1]
Context Transfer Protocol October 2002 Table of Contents 1. Introduction...................................................3 1.1 Conventions Used in This Document..........................4 1.2 Abbreviation Used in the Document..........................4 2. Protocol Overview..............................................4 2.1 Context Transfer Packet Format.............................4 2.2 Messages...................................................5 2.3 Message Types..............................................5 2.4 Data Format................................................6 3. Timing and Trigger Considerations..............................6 3.1 Starting Event.............................................7 4. Transport Considerations.......................................7 4.1 Congestion Control.........................................8 4.2 ICMP.......................................................8 4.3 UDP........................................................9 4.4 TCP........................................................9 4.5 SCTP.......................................................9 5. Open Issues....................................................9 5.1 Reliability................................................9 5.2 Transport.................................................10 5.3 Failure Handling..........................................10 5.4 Zone of Operation.........................................10 6. Examples and Signaling Flows..................................10 7. Security Considerations.......................................10 8. IANA Considerations...........................................11 9. Acknowledgements..............................................12 10. References...................................................12 10.1 Normative References.....................................12 10.2 Non-Normative References.................................12 Author's Addresses...............................................13 Loughney Expires - April 2003 [Page 2]
Context Transfer Protocol October 2002 1. Introduction "Problem Description: Reasons For Performing Context Transfers between Nodes in an IP Access Network" [3374] 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. 2) An additional motivation is to provide an interoperable solution that works for any Layer 2 radio access technology. Access Routers typically establish state in order to effect certain forwarding treatments to packet streams belonging to nodes sharing the access router. For instance, an access router may establish an AAA session state and a QoS state for a node's packet streams. When the link connecting a mobile node and the access router is bandwidth- constrained, the access router may maintain header compression state on behalf of the mobile node. When such a node moves to a different access router, this state or context relocation during handover provides many important benefits, including: . Seamless operation of application streams. The handover node (i.e., the Mobile Node) does not need to re-establish its contexts at the new access router . Performance benefits. When contexts need to be reestablished, performance of transport protocols would suffer until the contexts are in place. For instance, when the required QoS state is not present, a stream's packets would not receive the desired per-hop behavior treatment, subjecting them to higher likelihood of unacceptable delays and packet losses. . Bandwidth savings. Re-establishing multiple contexts over an expensive, low-speed link can be avoided by relocating contexts over a potentially higher-speed wire. . Reduced susceptibility to errors, since much more of the protocol operates over reliable networks, replacing renegotiations over a potentially error-prone link. In [3374], a detailed description of motivation, the need and benefits of context transfer are outlined. In this document, we describe a generic context transfer protocol, which provides: Loughney Expires - April 2003 [Page 3]
Context Transfer Protocol October 2002 . 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 [2119]. 1.2 Abbreviation Used in the Document AR Access Router CT Context Transfer CTP Context Transfer Protocol DoS Denial-of-Service FPT Feature Profile Types MIP Mobile IP MN Mobile Node nAR New Access Router pAR Previous Access Router SA Security Association 2. Protocol Overview This section provides a protocol overview. 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, Loughney Expires - April 2003 [Page 4]
Context Transfer Protocol October 2002 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 Messages 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 | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | flags | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Authorization Token | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / data / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 2.3 Message Types Context Transfer Start Request (CTSR) Message Sent by MN or nAR to pAR request start of context transfer. It is for further to study to see if when the CTSR message is sent by the MN to the nAR, it should also be relayed to pAR). Acknowledgement is optional, since if the CTSR message is sent by the MN, the MN may have already moved and may not receive the reply. 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. Context Transfer Initiate Ack (CTIN-Ack) Message Loughney Expires - April 2003 [Page 5]
Context Transfer Protocol October 2002 Sent by nAR to pAR, showing nAR's readiness to accept MN's context. This message is used when reliability is required. In other words, if the reliability flag is set, this message must be sent. Context Transfer Data (CTD) Message Sent by pAR to nAR, and includes feature data (CTP data). This message should handle predictive as well as normal CT. A reliability flag included in this message indicates whether a reply is required by nAR. 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. Context Transfer Cancel (CTA) Message Sent by nAR to pAR to cancel an ongoing CT process, for example mobile node has moved, or nAR has waited too long for retransmissions. 2.4 Data 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data Type | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ / context feature data / \ \ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 3. 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 many types of services. 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 minimally to the disruption of services in conjunction 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 protocols become more stable. Loughney Expires - April 2003 [Page 6]
Context Transfer Protocol October 2002 In addition whenever possible, the context transfer procedure MUST take measures to minimize the service disruption (packet loss) caused by the context transfer procedure itself. This will be elaborated further in the future updates of this draft. 3.1 Starting Event CT can be either started by a request from the MN or at the initiative of new AR or pAR. . MN can send the CT start request to old or new AR. The former is preferred since the MN is expected to have more satisfactory trust relationship with pAR than with nAR. Also sending the request to nAR would add extra latency since the new AR, itself, after processing the MN's request will need to request context transmission from pAR. . Either new AR or old AR may request or start (respectively) context transfer based on internal or network triggers. Here we prefer the following approach: The MN will send a context transfer request to the old AR (including proper authentication material). After checking authentication and if the pAR has a trust relationship with nAR, the old AR starts the context transfer procedure. 4. Transport Considerations It is not the intention of this work to define a new general-purpose transport protocol. In this section we outline the issues of running CTP over well-known protocols. A general note, however, on congestion control. ICMP and UDP do not provide congestion control, while TCP/SCTP do provide congestion control. Congestion control may delay the transfer of context, but do protect the network. This trade off needs to be further studied in terms of applicability of context transfer. Some issues affecting this will be the type of signaling - for example, intra-domain vs. inter-domain. Of the existing transport mechanisms, both TCP and SCTP provide checksums. UDP provides only an optional checksum. In order to make efficient use of multiplexing functionality of context transfer protocol, the UDP checksum MUST be de-activated. This will decrease the likelihood that the entire context data packet is dropped due to individual feature data corruptions. Loughney Expires - April 2003 [Page 7]
Context Transfer Protocol October 2002 4.1 Congestion Control Context transfer is essentially an optimization to enable smooth handovers and to prevent the need of re-initializing signaling to and from a mobile node after handover. 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 control 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 approximately 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 will be fine. Additionally, physically adjacent access routers should be with in 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. 4.2 ICMP Pros . No explicit connection setup needed. . Reliability handled at the upper layer. . Co-ordination with mobility solutions easier to implement. Cons . No transport level signaling . No reliability . No ability to fragment data. . Upper Size limits on packets. Loughney Expires - April 2003 [Page 8]
Context Transfer Protocol October 2002 . ICMP messages have low priority in case of congestion 4.3 UDP Pros . No explicit connection setup needed. . Reliability handled at the upper layer. . Co-ordination with mobility solutions easier to implement. Cons . No transport level signaling . No reliability . No ability to fragment data. 4.4 TCP Pros . Supports TLS . Built in reliability Cons . 3-way handshake, mitigated if TCP connections are re-used. Open issues . Single TCP connection or multiple TCP connections? 4.5 SCTP Pros . Supports TLS . Built in reliability . Message-oriented . Multiple streams prevent head-of-line blocking for multiple users. Cons . 4-way handshake (though data can be included on 4th), mitigated if TCP connections are re-used. Open issues . Partial Reliability SCTP may have interesting applicability here. 5. Open Issues This section lists some open issues that need further discussion. 5.1 Reliability Loughney Expires - April 2003 [Page 9]
Context Transfer Protocol October 2002 Should reliability be handled at the transport layer? Should the reliability needs of contexts be handled at the CTP data level or CTP message level? How to handle multiple contexts to be transferred with differing reliability needs. 5.2 Transport Currently, the issue of which transport protocol should be supported is open. 5.3 Failure Handling Failure of Context Transfer should at least cause no harm to the network or to the user session. Failure reporting to the mobile node may be needed. It is for further study how to handle failures of context transfer. 5.4 Zone of Operation Currently, the authors are restricting discussion of CTP to intra- domain signaling. Discussion of inter-domain signaling left for later discussions. Inter-domain signaling is more complicated. For example, the physically adjacent routers may be more than several IP hops away. Additionally, provisioning inter-domain signaling links may be much more complicated. Restricting CTP to intra-domain signaling simplifies security, transport and provision concerns. Additionally, a restriction to intra-domain signaling may help to ensure CT completes in sufficient time to meet time sensitive requirements. 6. Examples and Signaling Flows To be added. 7. Security Considerations The Context Transfer Protocol essentially 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-style attacks by bad guys trying to shift state between ARs, causing network disruptions. In general, CTP MUST rely on IETF standardized security mechanisms for protecting traffic between access routers, as opposed to creating application security mechanisms. Both IPsec and TLS [TLS] may be Loughney Expires - April 2003 [Page 10]
Context Transfer Protocol October 2002 reasonable mechanisms for securing the context transfer 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 AR need to engage in a key exchange mechanisms such as IKE [IKE], 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 MSUT be provided so that requests are authenticated (regardless of the security of context transfer itself) to prevent the possibility of rouge 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. 8. IANA Considerations This section provides guidance to the Internet Assigned Numbers Authority (IANA) regarding registration of values related to the Diameter protocol, in accordance with BCP 26 [IANA]. 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. Loughney Expires - April 2003 [Page 11]
Context Transfer Protocol October 2002 9. Acknowledgements The authors would like to thank Rajeev Koodli, whose prior Context Transfer Framework was used as the basis for part of this work. 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 and Charles Perkins. The Working Group chairs, Pat Calhoun and James Kempf monitored the design team. 10. References 10.1 Normative References [2026] S. Bradner, "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996. [2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [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. [IANA] Narten, Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [IKE] 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. [TLS] T. Dierks, C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999. 10.2 Non-Normative References [3374] J. Kempf et al., "Problem Description: Reasons For Performing Context Transfers Between Nodes in an IP Loughney Expires - April 2003 [Page 12]
Context Transfer Protocol October 2002 Access Network", RFC 3374, Internet Engineering Task Force, RFC 3374, May 2001. [2401] 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. Author's Addresses 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 Loughney Expires - April 2003 [Page 13]
