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
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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
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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
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.
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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
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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.
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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.
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7. Intellectual Property Statement
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8. Full copyright statement
Copyright (C) The Internet Society (2004). This document is subject
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This document and the information contained herein are provided on
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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.
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[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
<http://www.ietf.org/internet-drafts/>
[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
<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-05.txt>, February 2004, 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-05.txt>,
June 2004, Work in Progress.
[VCCV] Nadeau, T., Aggarwal, T., "Pseudo Wire (PW) Virtual
Circuit Connection Verification (VCCV)", Internet
Draft, <draft-ietf-pwe3-vccv-02.txt>, February 2004,
Work in Progress.
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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
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