INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 Network Working Group S. Bryant Internet Draft G. Swallow Expiration Date: April 2006 L. Martini Cisco Systems D. McPherson Arbor Networks October 2005 PWE3 Control Word for use over an MPLS PSN draft-ietf-pwe3-cw-06.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 design of a PWE3 Control Word to be use over an MPLS packet switched network, and the Pseudo Wire Associated Channel Header. The design of these fields is chosen so that an MPLS Label Switching Router performing MPLS payload inspection will not confuse a PWE3 payload with an IP payload. Bryant et al Expires April 2006 [Page 1] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 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. 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 a label switching router (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 PW 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 is used to distinguish a PW payload from an IP payload carried over an MPLS PSN. It then describes the preferred design of a PW Control Word to be use over an MPLS PSN, and the Pseudo Wire Associated Channel Header. 2. Avoiding ECMP A PW that is carried over an MPLS PSN that uses the contents of the MPLS payload to select the ECMP path may be subjected to packet misordering [BCP]. In cases where the application using the PW is sensitive to packet misordering, or where packet misordering will disrupt the operation of the PW, it is necessary to prevent the PW being subjected to ECMP. 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) which is used for data passing across the PW, and a PW Associated Channel Header (PWACH) that can be used for functions such as OAM. If the first nibble of a PW packet carried over an MPLS PSN has a value of 0, it starts with a PWMCW. If the first nibble of a packet Bryant et al Expires April 2006 [Page 2] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 carried over an MPLS PSN has a value of 1, it starts with a PWACH. The use of any other first nibble value for a PW packet carried over an MPLS PSN is deprecated. If a PW is sensitive to packet misordering and is being carried over an MPLS PSN that uses the contents of the MPLS payload to select the ECMP path, it MUST employ a mechanism which prevents packet misordering. A suitable mechanism is the PWMCW described in Section 3 for data, and the PWACH described in Section 4 for channel associated traffic. The PWMCW or the PWACH MUST immediately follow the bottom of the MPLS label stack. 3. Generic PW MPLS Control Word The Generic PW MPLS Control Word (PWMCW) 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 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 format illustrated in Figure 1 above. To provide consistency between the designs of different types of PW, it SHOULD also use the following preferred 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: Preferred PW MPLS Control Word The meaning of the fields of the Preferred PW MPLS Control Word (Figure 2) is as follows: Flags (bits 4 to 7): Bryant et al Expires April 2006 [Page 3] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 These bits MAY be used by for per-payload signaling. Their semantics MUST be defined in the PW specification. 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 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 MPLS payload is less than 64 bytes, the length field MUST be set to the length of the PW payload plus the length of the PWMCW. Otherwise it MUST be set to zero. Sequence number (Bit 16 to 31): The sequence number implements the sequencing function [RFC3985]. The use of this field is described in Section 4. 4. Sequencing The sequence number mechanism is PW specific. The PW encapsulation specification MAY define a sequence number mechanism to be used, or it may indicate that the mechanism described here is to be used. A pseudo-code description of this mechanism is given in non-normative Appendix 1. The sequence number mechanism described here uses a circular unsigned 16 bit number space that excludes the value zero. 4.1 Setting the Sequence Number For a given PW, and a pair of routers PE1 and PE2, if PE1 supports frame sequencing and frame sequencing is enabled for the PW, then the following procedures MUST be used: o The initial frame transmitted on the PW MUST be sent with sequence number one. Bryant et al Expires April 2006 [Page 4] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 o Subsequent frames MUST increment the sequence number by one for each frame. o The sequence number that follows 65535 (maximum unsigned 16 bit number) is one. If the transmitting router PE1 does not support sequence number processing, or frame sequencing is disabled, then the sequence number field in the control word MUST be set to zero for all frames transmitted on the PW. 4.2 Processing the sequence number If a router PE2 supports receive sequence number processing, and frame sequencing is enabled for this PW, then the following procedure is used: When a PW is initially set up, the "expected sequence number" associated with it MUST be initialized to one. When a frame is received on that PW, the sequence number SHOULD be processed as follows: o If the sequence number on the frame is zero, the sequence integrity of the packets cannot be determined. In this case, the received frame is considered to be in order. o Otherwise if the frame sequence number equals the expected sequence number, the frame is in order. o Otherwise if the frame sequence number is greater than the expected sequence number, and the frame sequence number minus the expected sequence number is less than 32768, the frame is within the allowed receive sequence number window. The implementation MAY treat the packet as is in order. o Otherwise if the frame sequence number is less than the expected sequence number and the expected sequence number minus the frame sequence number is greater than or equal to 32768, the frame is within the allowed receive sequence number window. The implementation MAY treat the packet as is in order. o Otherwise the frame is out of order. If the frame is in order, it can be delivered immediately. If the frame sequence number was not zero, then the expected sequence number is set to the frame sequence number plus one. The expected sequence number that follows 65535 (maximum unsigned 16 bit number) is one. Bryant et al Expires April 2006 [Page 5] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 Frames which are received out of order MAY either be dropped or reordered. The choice between dropping or re-ordering an out of sequence frame is at the discretion of the receiver. If a PE negotiated not to use receive sequence number processing, and it received a non zero sequence number, then it SHOULD send a PW status message indicating a receive fault, and disable the PW. 5. PW Associated Channel For some PW features, 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 (PWAC) 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|Version| Reserved | Channel Type | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: PW Associated Channel Header The meanings of the fields in the PW Associated Channel Header (PWACH) (Figure 3) are: Version: This is the version number of the PWACH. This specification defines version 0. 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]. Bits 0..3 MUST be 0001. This allows the packet to be distinguished from an IP packet [BCP] and from a PW data packet. Bryant et al Expires April 2006 [Page 6] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 6. 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]. The value 0X21 indicates that the Associated Channel carries an IPv4 packet. 7. Security Considerations An application using a PW Associated Channel must be aware that the channel 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. The selection of a suitable security mechanism for an application using a PW Associated Channel is outside the scope of this document. 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. 8. Acknowledgements The authors wish to thank David Allan, Thomas Nadeau, Yaakov Stein, and Mark Townsley for their input to this work. 9. 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 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. Bryant et al Expires April 2006 [Page 7] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 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. 10. 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. 11. 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 April 2006 [Page 8] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 12. 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 , July 2005, Work in Progress. [FRAG] Malis, A., Townsley, M., "PWE3 Fragmentation and Reassembly", Internet Draft, , September 2005, Work in Progress. [IANA] Martini, L., Townsley M., "IANA Allocations for pseudo Wire Edge to Edge Emulation (PWE3) ", Internet Draft, , September 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 Bryant et al Expires April 2006 [Page 9] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 13. Authors' Addresses Stewart Bryant Cisco Systems, 250, Longwater, Green Park, Reading, RG2 6GB, United Kingdom. Email: stbryant@cisco.com Luca Martini Cisco Systems, Inc. 9155 East Nichols Avenue, Suite 400 Englewood, CO, 80112 Email: lmartini@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 14. Appendix 1 Sequence Number Processing This appendix is non-normative. This appendix provides a pseudo-code description of the sequence number processing mechanism described in Section 4.2. unsigned16 RECEIVED /* frame sequence number unsigned16 EXPECTED = 1 /* expected sequence number /* initialized to one boolean sequencingDisabled boolean dropOutOfOrder /* policy on in-window out of sequence /* frames updateExpected() begin EXPECTED := RECEIVED + 1; /* Because EXPECTED is an unsigned16 it will wrap /* from 65535 to 0 /* zero is skipped if (EXPECTED = 0) EXPECTED := 1; return; end; Bryant et al Expires April 2006 [Page 10] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 On receipt of a PW packet from PSN: begin if (RECEIVED = 0) then begin processFrame(); return; end; if (sequencingDisabled) then begin /* A frame was received with non-zero sequence number, but /* sequencing is disabled indicateReceiveFault(); disablePW(); return; end; /* The received sequence is the expected sequence number if ((RECEIVED = EXPECTED) then begin /* packet is in order processFrame(); updateExpected(); return; end; /* Test for received sequence number is greater than /* the expected sequence number and is within the /* allowed receive sequence number window if ((RECEIVED > EXPECTED) and ((RECEIVED - EXPECTED) < 32768) then begin /* frame is in the window, but there are late/missing /* frames if (dropOutOfOrder) then begin /* policy is to receive immediately, dropping /* out of sequence frames processFrame(); updateExpected(); return; end else begin /* policy is to wait for late packets processMissingFrames(); return; end; end; /* Test for the received sequence is less than the /* expected sequence number and is within the allowed /* receive sequence number window if ((RECEIVED < EXPECTED) and ((EXPECTED - RECEIVED) >= 32768) then begin /* frame is in the window, but there are late/missing /* frames Bryant et al Expires April 2006 [Page 11] INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Oct 2005 if (dropOutOfOrder) then begin /* policy is to receive immediately, dropping /* out of sequence frames processFrame(); updateExpected(); return; end else begin /* policy is to wait for late packets processMissingFrames(); return; end; end; /* Received packet was outside the allowed receive /* sequence number window processOutOfWindow(); end; Bryant et al Expires April 2006 [Page 12]