Network Working Group V. Jain Internet-Draft R. Penno Expires: January 2002 S. McGeown Nortel Networks Ly Loi TahoeNetworks August, 2001 L2TP Tunnel Switching draft-ietf-l2tpext-tunnel-switching-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of 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 The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. Abstract For some time now several equipment manufactures have been implementing what is called L2TP tunnel switching or L2TP multihop. Although this technology has several applications and is quite widespread, there has been no effort to standardize the nomenclature and methods associated with it. The goal of this document is to achieve a common denominator in what is tunnel switching, its advantages and nomenclature associated with it. Penno, et al. [Page 1] Internet-Draft draft-ietf-l2tpext-switching-00.txt August, 2001 Specification of Requirements The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this document, are to be interpreted as described in RFC 2119 [3]. Table of Contents 1. What is L2TP Tunnel Switching. . . . . . . . . . . . . . . 3 2. Tunnel Switching Nomenclature . . . . . . . . . . . . . . .3 3. Why L2TP Tunnel Switching. . . . . . . . . . . . . . . . . 4 4. Disadvantages of Tunnel Switching. . . . . . . . . . . . . 5 5. Extensions to enhance tunnel switching support. . . . . . .6 6. References . . . . . . . . . . . . . . . . . . . . . . . . 8 7. Acknowledgments. . . . . . . . . . . . . . . . . . . . . . 8 8. Author's Addresses. . . . . . . . . . . . . . . . . . . . 8 Full Copyright Statement. . . . . . . . . . . . . . . . . 9 Penno, et al. [Page 2] Internet-Draft draft-ietf-l2tpext-switching-00.txt August 2001 1. What is L2TP Tunnel Switching L2TP tunneling allows processing of layer2 packets to be divorced from the termination of layer2 circuit. L2TP tunnel switching facilitates moving the termination of a layer2 session further to another LNS potentially unknown to the first LAC. It does so by re-tunneling the layer2 session over another L2TP tunnel to a different LNS. The knowledge of whether to switch a layer2 session to another L2TP tunnel can be static or dynamic (for example when a PPP session is established). _______ _______ _______ _______ _______ | L2 | | | | | | | | | User | | LAC A | | LNS A | LAC B | | LNS B | |_______| |_______| |_______|_______| |_______| |------ L2- ------| |---- L2/L2TP ----| |-- L2 --| |------ L2/L2TP -------| |------- tunnel switching ------| The figure above presents a typical tunnel switching scenario. The user opens a layer2 (for example PPP) session to LAC A, which puts the layer2 session into a L2TP tunnel that terminates on LNS A. If LNS A decides to further tunnel the layer2 session, it puts the layer2 session on another L2TP tunnel originating on LAC B and terminating on LNS B. LNS A and LAC B reside on the same device. The process of getting the layer2 session terminating on LNS A and further tunneling it to another LNS, in the example above LNS B, is called tunnel switching. 2. Tunnel Switching Nomenclature Ingress Tunnel Aggregator (ITA): These devices are represented by the first layer of LACs, represented in the picure by LAC A. This is the node which terminates layer2 circuit and initiates the first L2TP tunnel. Penno, et al. [Page 3] Internet-Draft draft-ietf-l2tpext-switching-00.txt August 2001 Tunnel Switching Aggregator (TSA): These are the devices that act as LNS as well as LAC for a particular layer2 session therefore it typically re-tunnels layer2 session to another LNS. Egress Tunnel Aggregator (ETA): These are devices that terminate the layer2 session. They are represented in the picture by LNS B. 3. Why L2TP Tunnel Switching This section discusses the advantages of L2TP tunnel switching. * Often, the administrative domain of a LAC, an ILEC or CLEC, is not same as that of LNS, typically an ISP terminating subscriber's Layer2 connection. In such situations, a multi tier deployment with tunnel switching helps the LEC (ILEC or CLEC) mask its internal network architecture from the ISPs. In particular, it eases the configuration across different administrative domain. For example, for every new ITA added in the system, ISP doesn't need to reconfigure its LNSs - for them LACs are always same (TSAs). * L2TP tunnel switching divorces the location of "decision-maker" LNS. Certain deployments do not have decision-making capabilities on LAC For example PC based LACs might not have mechanisms to be configured with policies a service provider wants to adopt; On other hand, it might not be desirable to expose such policies to every customer LAC CPE. The decision to choose the right LNS, for load balancing or other administrative purposes, when multiple LNSs are available, might not be done best by the first LAC always. Not all LACs should be expected to exhibit such rich functionality, features and flexibility. * L2TP tunnel switching allows using a common L2TP tunnel on LAC for sessions that are actually destined to different LNSs. This enables wholesaling layer2 sessions destined to any LNS go over a few tunnels. * L2TP tunnel switching might reduce the total number of tunnels in a meshed environment. The advantage fewer tunnels would primarily be a configuration and provisioning ease. The reduction of tunnels can be seen from three different angles, from the entire system, from the ITA and from the last ETA point of view. * Entire System In a traditional deployment, the total number of L2TP sessions between N LACs (ITA) and M LNSs (ETA) is N*M, assuming there is at least one layer2 session from every LAC that needs to be terminated on each LNS. With tunnel switching, this number can be reduced to (#ETA + #ITA) * (#TSA). Penno, et al. [Page 4] Internet-Draft draft-ietf-l2tpext-switching-00.txt August 2001 Of course the advantage on the reduction of tunnels in the system only holds when the (#TSA)is less than the number of LNS1s (see picture below). * ITA From the first layer of LACs point of view, the reduction in the number of tunnels holds whenever ITA*TSA < TSA*ETA, which is true for most deployments. * ETA On the other hand, the advantage on the reduction of tunnels from the last LNS (LNS2) point of view in comparison with LNS1 is considerable, since the M*(#TSA) is usually much less than M*N. ____ ______ ______ ______ ______ | PC | | LAC1 |__________| LNS1 | LAC2 |______________| LNS2 | |____| |______|\ ___|______|______| |______| \ / \__/ ___/\ ____ ______ / \ ______ ______ ______ | PC | | LAC1 |_______\__| LNS1 | LAC2 |______________| LNS2 | |____| |______| |______|______| |______| . . . . . . . . / \ . . ____ ______ / \ ______ ______ ______ | PC | | LAC1 |/ \_| LNS1 | LAC2 |______________| LNS2 | |____| |______| |______|______| |______| 4. Disadvantages of L2TP tunnel switching: * Focal point of failure: As TSA aggregates more and more tunnels, it becomes a focal point for failure, as compared to if ITAs had tunnels to ETAs directly. * Multiple Negotiations: Subscriber might be authenticated/LCP multiple times because each TSA might have its own criteria to determine if subscriber should be authenticated or if LCP parameters negotiated (proxy-LCP) are appropriate. Penno, et al. [Page 5] Internet-Draft draft-ietf-l2tpext-switching-00.txt August 2001 * Session limit within an L2TP tunnel: Bundling sessions within a single L2TP tunnel makes one deployment more likely to hit the 65k limit inherent of the L2TP protocol faster than if you have unique tunnels. Care should be taken while deploying L2TP tunnel switching to not exceed this limit due to aggregation of various sessions in limited number of tunnels. 5. Extensions to enhance tunnel switching support In this section we present new AVPs (and motivation behind them) that can enhance tunnel switching support beyond what is usually deployed today. These extensions are only applicable for L2TP tunnel switching. 5.1 Problem Statement When an TSA <-> ETA tunnel collapses for one reason or another (link failure, etc), the initial ITA<->TSA link continues to function normally, even though there is nowhere for the layer2 traffic to go once it gets to this TSA point. This causes a major disruption of service impact, as several subscribers who were knocked off the network will not be able to get back on the network. This creates a "black hole" condition, which directs all new sessions to the TSA, which has no path to the ETA. All new session attempts for this ETA fail. 5.2 Tunnel Set Dependency A new object L2TP Dependency should be defined to maintain a relationship between the ITA to TSA tunnels and the TSA to ETA tunnels. This object can be utilized by ITA to route away L2TP sessions from failures in the TSA to ETA connection. Information about failures between TSA and ETA should be provided to ITA through a new set of AVPs defined below. 5.3 Tunnel Dependency Load AVP (All Control Messages) The Tunnel Dependency Load AVP, Attribute Type XS, indicates the capacity to carry L2TP sessions from TSA to ETA for certain "service profile" (e.g., a ISP or Domain Name) Penno, et al. [Page 6] Internet-Draft draft-ietf-l2tpext-switching-00.txt August 2001 The Attribute Value field for this AVP 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of Services Profiles | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SPN Length | Service Profile Name ... (arbitrary length) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Maximum Load | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Current Load | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The Number of Services Profiles indicates the number of occurrences of the tuple (Service Profile Name, Maximum Load, Current Load). The Service Profile Name is up to 256 bytes long, but MUST be at least 1 octet. This name should be as broadly unique as possible, because the tunnels between ITA to TSA can contain sessions for different service profiles. The Maximum Load indicates the Maximum reference capacity for a service profile. It could be the number of maximum tunnels supported by the system at a point in time, maximum amount of bandwidth, or some other metric that reflects ratio The Current Load indicated the current capacity of the system, it could be the number of active tunnels at a point in time, amount of utilized bandwidth, or some other metric that reflects ratio. This AVP MAY be be hidden (the H-bit can be either 0 or 1). The M-bit for this AVP MUST be set to 0. 5.4. Loop Prevention AVP The Loop Prevention AVP, Attribute Type TBD, can be used to detect the existence of loops in the TSA network. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Number of Nodes | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HN Length | HostName ... (arbitrary length) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | IP address of the Node | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Penno, et al. [Page 7] Internet-Draft draft-ietf-l2tpext-switching-00.txt August 2001 The Number of Nodes indicates the occurrences of the tuple (Host Name, IP Address). Each tuple identifies a node in the tunnel- switched-path. The Hostname is MUST be same as used on the Hostname AVP when the tunnel was established. The IP address field represents the IP address which was used to establish the tunnel. This AVP is updated by LAC when a new tunnel is being established. It adds (Hostname, IP address) tuple to the existing AVP and increments Number of Nodes. 6. References [RFC 2661] W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn, B. Palter, "Layer 2 Tunnel Protocol (L2TP)", RFC2661, August 1999. [RFC 1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 7. Acknowledgments Thanks to W. Mark Townsley for his valuable comments 8. Author's Addresses Vipin Jain Nortel Networks, Inc. 2305 Mission College Boulevard Building SC9 Santa Clara, CA 95054 Email: vipin@nortelnetworks.com Reinaldo Penno Nortel Networks, Inc. 2305 Mission College Boulevard Building SC9 Santa Clara, CA 95054 Email: rpenno@nortelnetworks.com Penno, et al. [Page 8] Internet-Draft draft-ietf-l2tpext-switching-00.txt August 2001 Ly Loi Tahoe Networks, Inc. 3052 Orchard Drive San Jose, CA 95134 Phone: +1 408.944.8630 Email: lll@tahoenetworks.com Full Copyright Statement Copyright (C) The Internet Society (2000). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC editor function is currently provided by the Internet Society.