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Network Working Group                                          S. BryantRequest for Comments: 4385                                    G. SwallowCategory: Standards Track                                     L. Martini                                                           Cisco Systems                                                            D. McPherson                                                          Arbor Networks                                                           February 2006Pseudowire Emulation Edge-to-Edge (PWE3)Control Word for Use over an MPLS PSNStatus of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2006).Abstract   This document describes the preferred design of a Pseudowire   Emulation Edge-to-Edge (PWE3) Control Word to be used over an MPLS   packet switched network, and the Pseudowire 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.1.  Introduction   The standard MPLS encapsulations have no explicit protocol   identifier.  In order for a pseudowire (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 Operations and Management (OAM) [VCCV] mechanism from   correctly monitoring the PW.Bryant, et al.              Standards Track                     [Page 1]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 2006   This document specifies how the PW control word 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 Pseudowire Associated Channel Header.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 inRFC 2119 [RFC2119].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] [RFC2460] 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), which 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, this indicates that the packet 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 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 that prevents packet   misordering.  A suitable mechanism is the PWMCW described inSection3 for data, and the PWACH described inSection 5 for channel-   associated traffic.   The PWMCW or the PWACH MUST immediately follow the bottom of the MPLS   label stack.Bryant, et al.              Standards Track                     [Page 2]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 20063.  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):          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.Bryant, et al.              Standards Track                     [Page 3]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 2006   Length (bits 10 to 15):          When the PSN path between the PEs includes an Ethernet          segment, 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 inSection 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 the non-   normative Appendix.   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   packet sequencing and packet sequencing is enabled for the PW, then   the following procedures MUST be used:     o The initial packet transmitted on the PW MUST be sent with       sequence number one.     o Subsequent packets MUST increment the sequence number by one for       each packet.     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 packet sequencing is disabled, then the sequence   number field in the control word MUST be set to zero for all packets   transmitted on the PW.Bryant, et al.              Standards Track                     [Page 4]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 20064.2.  Processing the Sequence Number   If a router PE2 supports receive sequence number processing, and   packet 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 packet is received on that PW, the sequence number SHOULD be   processed as follows:     o If the sequence number on the packet is zero, the sequence       integrity of the packets cannot be determined.  In this case, the       received packet is considered to be in order.     o Otherwise if the packet sequence number equals the expected       sequence number, the packet is in order.     o Otherwise if the packet sequence number is greater than the       expected sequence number, and the packet sequence number minus       the expected sequence number is less than 32768, the packet is       within the allowed receive sequence number window.  The       implementation MAY treat the packet as in order.     o Otherwise if the packet sequence number is less than the expected       sequence number and the expected sequence number minus the packet       sequence number is greater than or equal to 32768, the packet is       within the allowed receive sequence number window.  The       implementation MAY treat the packet as in order.     o Otherwise the packet is out of order.   If the packet is found to be in order, it MAY be delivered   immediately.   If the packet sequence number was not zero, then the expected   sequence number is set to the packet sequence number plus one.  The   expected sequence number that follows 65535 (maximum unsigned 16-bit   number) is one.   Packets that are received out of order MAY either be dropped or   reordered.  The choice between dropping or reordering an out-of-   sequence packet 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.Bryant, et al.              Standards Track                     [Page 5]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 20065.  PW Associated Channel   For some PW features, an associated channel is required.  An   associated channel is a channel that is multiplexed in the PW with   user traffic, and thus follows the same path through the PSN as user   traffic.  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.6.  IANA Considerations   IANA has 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.  The   value 0x57 indicates that the Associated Channel carries an IPv6   packet.Bryant, et al.              Standards Track                     [Page 6]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 20067.  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.Bryant, et al.              Standards Track                     [Page 7]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 20069.  Normative References   [RFC791]   Postel, J., "Internet Protocol", STD 5,RFC 791, September              1981.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6              (IPv6) Specification",RFC 2460, December 1998.10.  Informative References   [BCP]      Swallow, G., Bryant, S., and L. Andersson, "Avoiding Equal              Cost Multipath Treatment in MPLS Networks", Work in              Progress, September 2005.   [FRAG]     Malis, A. and M. Townsley, "PWE3 Fragmentation and              Reassembly", Work in Progress, November 2005.   [IANA]     Martini, L., "IANA Allocations for Pseudowire Edge to Edge              Emulation (PWE3)", Work in Progress, November 2005.   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an              IANA Considerations Section in RFCs",BCP 26,RFC 2434,              October 1998.   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path              Algorithm",RFC 2992, November 2000.   [RFC3985]  Bryant, S. and P. Pate, "Pseudo Wire Emulation Edge-to-              Edge (PWE3) Architecture",RFC 3985, March 2005.   [VCCV]     Nadeau, T. and R. Aggarwal, "Pseudowire Virtual Circuit              Connectivity Verification (VCCV)", Work in Progress,              August 2005.Bryant, et al.              Standards Track                     [Page 8]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 2006Appendix.  Sequence Number Processing   This appendix is non-normative.   This appendix provides a pseudo-code description of the sequence   number processing mechanism described inSection 4.2.   unsigned16 RECEIVED     /* packet sequence number   unsigned16 EXPECTED = 1 /* expected sequence number                           /* initialized to one   boolean sequencingDisabled   boolean dropOutOfOrder  /* policy on in-window out of sequence                           /* packets   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;   On receipt of a PW packet from PSN:   begin       if (RECEIVED = 0) then begin           processPacket();           return;       end;       if (sequencingDisabled) then begin           /* A packet 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           processPacket();           updateExpected();           return;       end;Bryant, et al.              Standards Track                     [Page 9]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 2006       /* 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           /* packet is in the window, but there are late/missing           /* packets           if (dropOutOfOrder) then begin               /* policy is to receive immediately, dropping               /* out of sequence packets               processPacket();               updateExpected();               return;           end else begin               /* policy is to wait for late packets               processMissingPackets();               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           /* packet is in the window, but there are late/missing           /* packets           if (dropOutOfOrder) then begin               /* policy is to receive immediately, dropping               /* out of sequence packets               processPacket();               updateExpected();               return;           end else begin               /* policy is to wait for late packets               processMissingPackets();               return;           end;       end;       /* Received packet was outside the allowed receive       /* sequence number window       processOutOfWindow();   end;Bryant, et al.              Standards Track                    [Page 10]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 2006Authors' Addresses   Stewart Bryant   Cisco Systems,   250, Longwater,   Green Park,   Reading, RG2 6GB,   United Kingdom.   EMail: stbryant@cisco.com   George Swallow   Cisco Systems, Inc.   1414 Massachusetts Ave   Boxborough, MA 01719   EMail:  swallow@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.netBryant, et al.              Standards Track                    [Page 11]

RFC 4385       PW3 Control Word for Use over an MPLS PSN   February 2006Full Copyright Statement   Copyright (C) The Internet Society (2006).   This document is subject to the rights, licenses and restrictions   contained inBCP 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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is provided by the IETF   Administrative Support Activity (IASA).Bryant, et al.              Standards Track                    [Page 12]