INTERNET DRAFT PWE3 Control Word for use over an MPLS PSN Jul 2006
Network Working Group S. Bryant
Internet Draft G. Swallow
Expiration Date: January 2006 Cisco Systems
D. McPherson
Arbor Networks
July 2005
PWE3 Control Word for use over an MPLS PSN
draft-ietf-pwe3-cw-05.txt
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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.
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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 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.
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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
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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].
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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.
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
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of such proprietary rights by implementers or users of this
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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
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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
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11. Informative References
Internet-drafts are works in progress available from
<http://www.ietf.org/internet-drafts/>
[BCP] Swallow, G. et al, "Avoiding Equal Cost Multipath
Treatment in MPLS Networks", Internet Draft
<draft-ietf-mpls-ecmp-bcp-00.txt>, September 2004,
Work in Progress.
[FRAG] Malis, A., Townsley, M., "PWE3 Fragmentation and
Reassembly", Internet Draft, <draft-ietf-pwe3-
fragmentation-08.txt>, February 2005, Work in
Progress.
[IANA] Martini, L., Townsley M., "IANA Allocations for
pseudo Wire Edge to Edge Emulation (PWE3) ",
Internet Draft, <draft-ietf-pwe3-iana-allocation-
09.txt>, 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, <draft-ietf-pwe3-vccv-04.txt>, Feb. 2005,
Work in Progress.
12. Authors' Addresses
Stewart Bryant
Cisco Systems,
250, Longwater,
Green Park,
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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
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