Network Working Group S. Bryant Internet Draft G. Swallow Expiration Date: APR 2005 Cisco Systems D. McPherson Arbor Networks Oct 2004 PWE3 Control Word for use over an MPLS PSN draft-ietf-pwe3-cw-00.txt Status of this Memo By submitting this Internet-Draft, we certify that any applicable patent or other IPR claims of which we are aware have been disclosed, or will be disclosed, and any of which we become aware will be disclosed, in accordance with RFC 3668. 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 a "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 Control Word, and the PW Associated Channel Header. The design of these fields is chosen so that an MPLS LSR performing deep packet inspection will not confuse a PWE3 payload with an IP payload. Conventions used in this document Bryant et al Expires APR 2005 [Page 1] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN SEPT 2004 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 Packets are carried in MPLS label stacks without any protocol identifier. In order for a pseudo wire (PW) [ARCH] to operate correctly over an MPLS packet switched network (PSN) that performs deep packet 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 deep packet 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 though the PSN. This may result in misordered packet deliver to the egress PE. The inability to ensure that all packets belonging to a PW follow the same path also prevents 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 which is checked by LSRs performing deep packet inspection. To prevent the incorrect inspection of packets, PW packets carried over an MPLS PSN SHOULD NOT start with the value 4 or the value 6 in the first nibble [BCP]. This document defines a PW header and two general formats of that header. These two formats are the PW Control Word (PW-CW) 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 PW-CW. 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 Control Word described in Section 3 for data, and the PW Associated Channel Header described in Section Error! Reference source not found. for channel associated traffic. These fields MUST immediately follow the bottom of the MPLS label stack. Bryant et al Expires APR 2005 [Page 2] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN SEPT 2004 3. Generic PW Control Word The 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: PW Control Word The PW set-up protocol or configuration mechanism determines whether a PW uses a PW Control Word (PW-CW). Bits 0..3 differ from the first four bits of an IP packet [BCP] and hence provide the necessary MPLS payload discrimination. When a PW-CW is used, it SHOULD have the following preferred form: 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 Control Word The meaning of the fields of the PWE3 Preferred Control Word (Figure 2) are as follows: Flags (bits 4 to 7): These bits are available for per payload signalling. Their definition is encapsulation specific. FRG (bits 8 and 9): These bits are used when fragmenting a PW payload. Their use is defined in [FRAG] which is currently 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): The length field is used to determine the size of a PW payload that might have been padded to the minimum Ethernet MAC frame size during its transit across the PSN. If the Bryant et al Expires APR 2005 [Page 3] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN SEPT 2004 MPLS payload (defined as the PW-CW + the PW payload + any additional PW headers) is less than 46 bytes, the length MUST be set to the length of the MPLS payload. If the MPLS payload is between 46 bytes and 63 bytes the implementation MAY either set to the length of the MPLS payload, or it MAY set it to 0. If the length of the MPLS payload is greater than 63 bytes the length MUST be set to 0. Note to the reader: In the definition above, both the MUSTs are needed to make the mechanism work, the MAY provides backwards compatibility with deployed systems. Sequence number (Bit 16 to 31): If the sequence number is not used, it is set to zero by the sender and ignored by the receiver. Otherwise it specifies the sequence number of a packet. A circular list of sequence numbers is used. A sequence number takes a value from 1 to 65535 (2**16-1). The sequence number window size for packet acceptance is dependent on the parameters of the PSN, and SHOULD be configurable. The mechanism used by the decapsulating PE to (re)acquire the correct sequence number is implementation dependent. 4. PW Associated Channel For some features of PWs, such as OAM, an associated channel is required. When MPLS is used as the PSN, the PW Assocated 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 meaning of the fields in the PW Associated Channel Header (Figure 3) are as follows: FmtID: Format ID for the remaining 3 octets of the header. A FmtID of 0 indicates that the 3 octets are as depicted above. Reserved: Must be sent as 0, and ignored on receive. Bryant et al Expires APR 2005 [Page 4] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN SEPT 2004 Channel Type: The PW Associated Channel Type is defined in the IANA PW Associated Channel Type registry [IANA]. Bits 0..3 MUST be 0x01, and hence differ from the first four bits of an IP packet [BCP]. This provides the necessary MPLS payload discrimination. Note that L2TPv3 has its own mechanisms for providing this associated channel. 5. IANA considerations IANA needs to set up a registry of "PW Associated Channel Type". These are 16-bit values. PW Associated Channel Type values 1 through 127 are to be assigned by IANA using the "IETF Consensus" policy defined in RFC2434. PW Associated Channel Type values 128 through 256 are to be assigned by IANA, using the "First Come First Served" policy defined in RFC2434. PW Associated Channel Type values 1 through 65535 are vendor-specific, and values in this range are not to be assigned by IANA. A PW Associated Channel Type of up to 65 characters is required for any assignment from this registry. The value 0 is reserved. Initial PW Associated Channel Type value allocations are specified in "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3)" [IANA], and should be incorporated by IANA into the registry. 6. Security Considerations An application using this mechanism 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 using this as a mechanism to disrupt the operation of the PW or the PE, and to stop this channel 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. Bryant et al Expires APR 2005 [Page 5] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN SEPT 2004 7. 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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. 8. 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 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 9. 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. Bryant et al Expires APR 2005 [Page 6] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN SEPT 2004 [RFC1883] RFC-1883: Internet Protocol, Version 6 (IPv6), S. Deering, et al, December 1995 [RFC2992] RFC-2992: Analysis of an Equal-Cost Multi-Path Algorithm, C. Hopps, November 2000 [RFC2424] RFC-2424: Guidelines for Writing an IANA Considerations Section in RFCs, Alvestrand and Narten, October 1998. 10. Informative References Internet-drafts are works in progress available from [ARCH] Bryant, S., Pate, P., "PWE3 Architecture", Internet Draft, < draft-ietf-pwe3-arch-07.txt>, October 2003, Work in Progress. [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 2004, Work in Progress. [IANA] Martini, L., Townsley M., "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3)", Internet Draft, , June 2004, Work in Progress. [VCCV] Nadeau, T., Aggarwal, T., "Pseudo Wire (PW) Virtual Circuit Connection Verification (VCCV)", Internet Draft, , February 2004, Work in Progress. Bryant et al Expires APR 2005 [Page 7] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN SEPT 2004 11. Authors' Addresses Stewart Bryant Cisco Systems, 250, Longwater, Green Park, Reading, RG2 6GB, United Kingdom. Email: stbryant@cisco.com Danny McPherson Arbor Networks Email: danny@arbor.net George Swallow Cisco Systems, Inc. 1414 Massachusetts Ave Boxborough, MA 01719 Email: swallow@cisco.com Bryant et al Expires APR 2005 [Page 8]