INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Jun 2005 Network Working Group S. Bryant Internet Draft G. Swallow Expiration Date: December 2005 Cisco Systems D. McPherson Arbor Networks June 2005 PWE3 Control Word for use over an MPLS PSN draft-ietf-pwe3-cw-04.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract This document describes the preferred designs of the PWE3 MPLS Control Word, and the Pseudo Wire Associated Channel Header. The design of these fields is chosen so that an MPLS LSR performing MPLS payload inspection will not confuse a PWE3 payload with an IP payload. Conventions used in this document Bryant et al Expires December 2005 [Page 1] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Jun 2005 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. Introduction The standard MPLS encapsulations have no explicit protocol identifier. In order for a pseudo wire (PW) [RFC3985] to operate correctly over an MPLS packet switched network (PSN) that performs MPLS payload inspection, a PW packet must not appear to the LSR as if it were an IP packet [BCP]. An example of an LSR that performs MPLS payload inspection is one that is performing equal-cost multiple-path load-balancing (ECMP) [RFC2992]. If ECMP were performed on PWE3 packets, the packets in the PW may not all follow the same path through the PSN. This may result in misordered packet delivery to the egress PE. The inability to ensure that all packets belonging to a PW follow the same path may also prevent the PW OAM [VCCV] mechanism from correctly monitoring the PW. This draft specifies how a PW header distinguishes a PW payload from an IP payload carried over an MPLS PSN. 2. PWE3 Packet Identification All IP packets [RFC791][RFC1883] start with a version number that is checked by LSRs performing MPLS payload inspection. To prevent the incorrect processing of packets carried within a PW, PW packets carried over an MPLS PSN MUST NOT start with the value 4 (IPv4) or the value 6 (IPv6) in the first nibble [BCP], as those are assumed to carry normal IP payloads. This document defines a PW header and two general formats of that header. These two formats are the PW MPLS Control Word (PWMCW) used for data passing across the PW, and a PW Associated Channel Header (PW-ACH) that can be used for functions such as OAM. If the first nibble of a PWE3 packet carried over an MPLS PSN has a value of 0, it starts with a PWMCW. If the first nibble of a packet carried over an MPLS PSN has a value of 1, it starts with a PW-ACH. The use of any other first nibble value for a PWE3 packet carried over an MPLS PSN is deprecated. A PW carried over an MPLS PSN that uses the contents of the MPLS payload to select the ECMP path SHOULD employ the PW MPLS Control Word described in Section 3 for data, or the PW Associated Channel Header described in Section 4 for channel associated traffic. The PWE3 Control Word or the PW Associated Channel Header MUST immediately follow the bottom of the MPLS label stack. Bryant et al Expires December 2005 [Page 2] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Jun 2005 3. Generic PW MPLS Control Word The Generic PW MPLS Control Word is shown in Figure 1. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0| Specified by PW Encapsulation | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1: Generic PW MPLS Control Word The PW set-up protocol or configuration mechanism determines whether a PW uses a PW MPLS Control Word (PWMCW). Bits 0..3 differ from the first four bits of an IP packet [BCP] and hence provide the necessary MPLS payload discrimination. When a PWMCW is used, it MUST adhere to the Generic MPLS Control Word format as illustrated in Figure 1 above. It SHOULD also follow 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 0| Flags |FRG| Length | Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: PW Preferred MPLS Control Word The meaning of the fields of the PW Preferred MPLS Control Word (Figure 2) is as follows: Flags (bits 4 to 7): These bits are available for per-payload signaling. FRG (bits 8 and 9): These bits are used when fragmenting a PW payload. Their use is described in [FRAG] which is currently a work in progress. When the PW is of a type that will never need payload fragmentation, these bits may be used as general purpose flags. Length (bits 10 to 15): When the PSN path between the PEs includes an Ethernet, the PW packet arriving at the CE-bound PE from the PSN may Bryant et al Expires December 2005 [Page 3] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Jun 2005 include padding appended by the Ethernet data link layer. The CE-bound PE uses the length field to determine the size of the padding added by the PSN, and hence extract the PW payload from the PW packet. If the entire packet length is less than 64 bytes, the length field MUST be set to the length of the PW payload plus the length of the control word. Otherwise it MUST be set to zero. Sequence number (Bit 16 to 31): The sequence number implements the sequencing function [RFC3985]. The definition of this field is PW specific. 4. PW Associated Channel For some features of PWs, such as OAM, an associated channel is required. An associated channel is a channel that is multiplexed over the PW so that it follows exactly the same path through the PSN as the PW. Note that the use of the term "channel" is not a "PW channel type" as used in subsection 5.1.2 of [RFC3985] When MPLS is used as the PSN, the PW Associated Channel is identified by the following header: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |0 0 0 1| FmtID | Reserved | Channel Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: PW Associated Channel Header The meanings of the fields in the PW Associated Channel Header (Figure 3) are: FmtID: This is the Format Identifier for the remaining 3 octets of the header. A Format Identifier value of 0 indicates that the 3 octets are as shown in Figure 3. Reserved: MUST be sent as 0, and ignored on reception. Channel Type: The PW Associated Channel Type is defined in the IANA PW Associated Channel Type registry [IANA]. Bryant et al Expires December 2005 [Page 4] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Jun 2005 Bits 0..3 MUST be 0001. This allows the packet the packet to be distinguished from an IP packet [BCP] and from a PWE3 data packet. Note that L2TPv3 [RFC3931] has its own mechanisms for providing this associated channel, and is therefore out of the scope of this document. 5. IANA considerations IANA needs to set up a registry of "Pseudowire Associated Channel Types". These are 16-bit values. Registry entries are assigned by using the "IETF Consensus" policy defined in [RFC2434]. IANA also needs to set up a registry of "Pseudowire Format Identifiers". These are 4-bit values. Registry entries are assigned by using the "IETF Consensus" policy defined in [RFC2434]. 6. Security Considerations An application using PW Associated Channel to provide an OAM [VCCV] or other message channel MUST be aware that this can potentially be misused. Any application using the Associated Channel must therefore fully consider the resultant security issues, and provide mechanisms to prevent an attacker from using this as a mechanism to disrupt the operation of the PW or the PE, and to stop this channel from being used as a conduit to deliver packets elsewhere. If a PW has been configured to operate without a CW, the PW Associated Channel Type mechanism described in the document MUST NOT be used. This is to prevent user payloads being fabricated in such a way that they mimic the PW Associated Channel Header, and thereby provide a method of attacking the application that is using the Associated Channel. 7. Acknowledgements The authors wish to thank David Allan, Luca Martini, Thomas Nadeau Yaakov Stein, and Mark Townsley for their input to this work. 8. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that Bryant et al Expires December 2005 [Page 5] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Jun 2005 it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. 9. Full copyright statement Copyright (C) The Internet Society (2005). 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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 10. Normative References Internet-drafts are works in progress available from http://www.ietf.org/internet-drafts/ [RFC791] RFC-791: DARPA Internet Program, Protocol Specification, ISI, September 1981. [RFC1883] RFC-1883: Internet Protocol, Version 6 (IPv6), S. Deering, et al, December 1995 Bryant et al Expires December 2005 [Page 6] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Jun 2005 11. Informative References Internet-drafts are works in progress available from [BCP] Swallow, G. et al, "Avoiding Equal Cost Multipath Treatment in MPLS Networks", Internet Draft , September 2004, Work in Progress. [FRAG] Malis, A., Townsley, M., "PWE3 Fragmentation and Reassembly", Internet Draft, , February 2005, Work in Progress. [IANA] Martini, L., Townsley M., "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3) ", Internet Draft, , April 2005, Work in Progress. [RFC2434] RFC-2434: Guidelines for Writing an IANA Considerations Section in RFCs, Narten, T., Alvestrand, H., October 1998 [RFC2992] RFC-2992: Analysis of an Equal-Cost Multi-Path Algorithm, C. Hopps, November 2000 [RFC3985] RFC-3985: PWE3 Architecture, Bryant, S. ed., Pate, P. ed., March 2005 [VCCV] Nadeau, T., Aggarwal, T., "Pseudo Wire (PW) Virtual Circuit Connection Verification (VCCV)", Internet Draft, , Feb. 2005, Work in Progress. 12. Authors' Addresses Stewart Bryant Cisco Systems, 250, Longwater, Green Park, Bryant et al Expires December 2005 [Page 7] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Jun 2005 Reading, RG2 6GB, United Kingdom. Email: stbryant@cisco.com Danny McPherson Arbor Networks, Inc. Email: danny@arbor.net George Swallow Cisco Systems, Inc. 1414 Massachusetts Ave Boxborough, MA 01719 Email: swallow@cisco.com Bryant et al Expires December 2005 [Page 8]