Network Working Group S. Bryant
Internet Draft G. Swallow
Expiration Date: September 2005 Cisco Systems
D. McPherson
Arbor Networks
March 2005
PWE3 Control Word for use over an MPLS PSN
draft-ietf-pwe3-cw-03.txt
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Abstract
This document describes the preferred designs of the PWE3 MPLS
Control Word, and the PW 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
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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 SHOULD 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, and the PW Associated Channel
Header described in Section 4 for channel associated traffic. These
Control Words 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 is however strongly
recommended that it also follows 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) are as follows:
Flags (bits 4 to 7):
These bits are available for per-payload signalling. Their
definition is encapsulation specific as defined in
[pointer?].
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):
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The value of the length field, if non-zero, can be used to
remove any padding added by the PSN. 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.
If a non-zero length field is received, the PW payload MUST
be trimmed if required.
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 both the PW and
the MPLS PSN, and SHOULD be configurable. The mechanism used
by the decapsulating PE to syncronise the expected sequence
number with the received sequence number is implementation
dependent.
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 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 shown in Figure 3.
Reserved:
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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 0x01, and hence differ from the first four bits of
an IP packet [BCP]. This provides the necessary MPLS payload
discrimination.
Note that L2TPv3 [REFERENCE] 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].
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
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
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to pertain to the implementation or use of the technology described
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9. 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
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IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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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-
07.txt>, October 2004, 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. , To be published.
[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,
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Green Park,
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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