Wildcard Pseudowire Type
A codepoint that lets one pseudowire endpoint decide the protocol type for both directions.
Martini, L. and G. Swallow, "Wildcard Pseudowire Type", RFC 4863, DOI 10.17487/RFC4863, May 2007, <https://www.rfc-editor.org/info/rfc4863>.
@misc{rfc4863,
series = {Request for Comments},
number = 4863,
howpublished = {RFC 4863},
publisher = {RFC Editor},
doi = {10.17487/RFC4863},
url = {https://www.rfc-editor.org/info/rfc4863},
author = {L. Martini and G. Swallow},
title = {{Wildcard Pseudowire Type}},
pagetotal = 6,
year = 2007,
month = may,
} RFC text
Contents
Network Working Group L. Martini
Request for Comments: 4863 G. Swallow
Category: Standards Track Cisco Systems, Inc.
May 2007
Wildcard Pseudowire Type
Status 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 IETF Trust (2007).
Abstract
Pseudowire signaling requires that the Pseudowire Type (PW Type) be
identical in both directions. For certain applications the
configuration of the PW Type is most easily accomplished by
configuring this information at just one PW endpoint. In any form of
LDP-based signaling, each PW endpoint must initiate the creation of a
unidirectional LSP. In order to allow the initiation of these two
LSPs to remain independent, a means is needed for allowing the PW
endpoint (lacking a priori knowledge of the PW Type) to initiate the
creation of an LSP. This document defines a Wildcard PW Type to
satisfy this need.
Table of Contents
1. Introduction ....................................................2
1.1. Conventions and Terminology ................................2
2. Wildcard PW Type ................................................3
3. Procedures ......................................................3
3.1. Procedures When Sending the Wildcard FEC ...................3
3.2. Procedures When Receiving the Wildcard FEC .................3
4. Security Considerations .........................................4
5. IANA Considerations .............................................4
6. References ......................................................4
6.1. Normative References .......................................4
6.2. Informative References .....................................4
1. Introduction
Pseudowire signaling requires that the Pseudowire Type (PW Type) be
identical in both directions. For certain applications the
configuration of the PW Type is most easily accomplished by
configuring this information at just one PW endpoint. In any form of
LDP-based signaling, each PW endpoint must initiate the creation of a
unidirectional LSP.
By the procedures of [CONTROL], both Label Mapping messages must
carry the PW type, and the two unidirectional mapping messages must
be in agreement. Thus within the current procedures, the PW endpoint
that lacks configuration must wait to receive a Label Mapping message
in order to learn the PW Type, prior to signaling its unidirectional
LSP.
For certain applications this can become particularly onerous. For
example, suppose that an ingress Provider Edge (PE) is serving as
part of a gateway function between a layer 2 network and layer 2
attachment circuits on remote PEs. Suppose further that the initial
setup needs to be initiated from the layer 2 network, but the layer 2
signaling does not contain sufficient information to determine the PW
Type. However, this information is known at the PE supporting the
targeted attachment circuit.
In this situation, it is often desirable to allow the initiation of
the two LSPs that compose a pseudowire to remain independent. A
means is needed for allowing a PW endpoint (lacking a priori
knowledge of the PW Type) to initiate the creation of an LSP. This
document defines a wildcard PW Type to satisfy this need.
1.1. Conventions and Terminology
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 [KEYWORDS].
This document introduces no new terminology. However, it assumes
that the reader is familiar with the terminology contained in
[CONTROL] and RFC 3985, "Pseudo Wire Emulation Edge-to-Edge (PWE3)
Architecture" [ARCH].
2. Wildcard PW Type
In order to allow a PE to initiate the signaling exchange for a
pseudowire without knowing the pseudowire type, a new PW Type is
defined. The codepoint is 0x7FFF. The semantics are the following:
1. To the targeted PE, this value indicates that it is to determine
the PW Type (for both directions) and signal that in a Label
Mapping message back to the initiating PE.
2. For the procedures of [CONTROL], this PW Type is interpreted to
match any PW Type other than itself. That is, the targeted PE
may respond with any valid PW Type other than the wildcard PW
Type.
3. Procedures
3.1. Procedures When Sending the Wildcard FEC
When a PE that is not configured to use a specific PW Type for a
particular pseudowire wishes to signal an LSP for that pseudowire, it
sets the PW Type to "wildcard". This indicates that the target PE
should determine the PW Type for this pseudowire.
When a Label Mapping message is received for the pseudowire, the PE
checks the PW Type.
If the PW Type can be supported, the PE uses this as the PW Type for
both directions.
If the PW Type cannot be supported or is "wildcard", it MUST respond
to this message with a Label Release message with an LDP Status Code
of "Generic Misconfiguration Error". Further actions are beyond the
scope of this document, but could include notifying the associated
application (if any) or notifying network management.
3.2. Procedures When Receiving the Wildcard FEC
When a targeted PE receives a Label Mapping message indicating the
wildcard PW Type, it follows the normal procedures for checking the
Attachment Group Identifier (AGI) and Target Attachment Individual
Identifier (TAII) values. If the targeted PE is not configured to
use a specific, non-wildcard PW Type, it MUST respond to this message
with a Label Release message with an LDP Status Code of "Generic
Misconfiguration Error".
Otherwise, it treats the Label Mapping message as if it had indicated
the PW Type it is configured to use.
4. Security Considerations
This document has little impact on the security aspects of [CONTROL].
The message exchanges remain the same. However, a malicious agent
attempting to connect to an access circuit would require one less
piece of information. To mitigate against this, a pseudowire control
entity receiving a request containing the wildcard FEC type SHOULD
only proceed with setup if explicitly configured to do so for the
particular AI in the TAI. Further, the reader should note the
security considerations of [CONTROL], in general, and those
pertaining to the Generalized PWid FEC Element, in particular.
5. IANA Considerations
IANA has made the following allocation from the IETF consensus range
of the "Pseudowire Type" registry as defined in [IANA].
PW Type Description
0x7FFF Wildcard
6. References
6.1. Normative References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[CONTROL] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T.,
and G. Heron, "Pseudowire Setup and Maintenance Using
the Label Distribution Protocol (LDP)", RFC 4447, April
2006.
[IANA] Martini, L., "IANA Allocations for Pseudowire Edge to
Edge Emulation (PWE3)", BCP 116, RFC 4446, April 2006.
6.2. Informative References
[ARCH] Bryant, S., Ed., and P. Pate, Ed., "Pseudo Wire
Emulation Edge-to-Edge (PWE3) Architecture", RFC 3985,
March 2005.
Authors' Addresses
Luca Martini
Cisco Systems
9155 East Nichols Avenue, Suite 400
Englewood, CO, 80112
EMail: lmartini@cisco.com
George Swallow
Cisco Systems
1414 Massachusetts Ave,
Boxborough, MA 01719
EMail: swallow@cisco.com
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AI ELI5
On this page
§1The Problem: Coordinating Pseudowire Types
In pseudowire technology, two Provider Edge routers (the endpoints) must agree on what type of traffic they are carrying across an MPLS network. Both endpoints traditionally need to be preconfigured with the identical Pseudowire Type (PW Type) value.
This works fine in most cases, but creates challenges in certain setups. For example, a PE router serving as a gateway between a layer 2 network and remote attachment circuits might receive setup requests from the layer 2 side that lack information about which PW Type should be used. The PE knows the type (because its local circuit determines it), but the far-end PE does not and cannot know it without being told.
The traditional approach requires the unconfigured endpoint to wait for a Label Mapping message to learn the type before starting its own signaling. This adds complexity and delay.
When building a bridge between two towns, you need both towns to use the same kind of bridge. Normally both towns have to decide and agree ahead of time. But what if one town already knows what kind they want?
§2LDP-Based Pseudowire Signaling
This RFC builds on Label Distribution Protocol (LDP) based pseudowire signaling, defined in RFC 4447. In LDP signaling, each pseudowire endpoint initiates the creation of a unidirectional LSP (Label Switched Path) independently. Both endpoints send Label Mapping messages that must carry the same PW Type for the pseudowire to function correctly.
The RFC assumes the reader is familiar with the Pseudowire Emulation Edge-to-Edge (PWE3) architecture, which defines how protocols other than IP are carried across MPLS networks. It references RFC 3985 for architectural details and RFC 2119 for the standard keywords MUST, SHOULD, and MAY that define requirements.
LDP is like a messaging system between routers. Each router sends a message saying what type of traffic it wants to carry. The type has to match on both sides.
§3The Wildcard Type Solution
RFC 4863 introduces a new Pseudowire Type codepoint: 0x7FFF, called the wildcard type. When a PE sends a Label Mapping message with this value, it signals the receiving PE to perform two tasks: first, determine the actual PW Type for both directions of the pseudowire, and second, signal that type back in its own Label Mapping message.
The wildcard type has special matching semantics: it matches any valid PW Type other than itself. This allows the far-end PE to respond with whatever type it is configured to use.
Instead of deciding the bridge type before you start, you can say 'you decide' (the wildcard), and the other side tells you what they want.
§4Sending and Receiving the Wildcard FEC
When a PE wishes to initiate pseudowire signaling without knowing the PW Type, it sets the type to wildcard (0x7FFF). The targeted PE receives this and, following normal procedures for checking the Attachment Group Identifier (AGI) and Target Attachment Individual Identifier (TAII) values, responds by treating the message as if the wildcard had indicated its own configured PW Type.
If the targeted PE is not configured with a specific non-wildcard PW Type, it must reject the request with a Label Release message and a 'Generic Misconfiguration Error' status code.
If a PE that sends the wildcard receives a Label Mapping message with a PW Type it cannot support, or if the response is itself wildcard, it must likewise reject with an error message. Once the PE receives a supported PW Type from the far end, it uses that value for both directions of the pseudowire.
The requesting router says 'you pick'. The other router picks its type and sends it back. If either side says 'I cannot do that', they send an error message and the setup fails.
- Label Mapping: Wildcard (0x7FFF)Initiator does not know the PW Type
- Label Mapping: Actual PW TypeTargeted PE responds with its configured type
§5Security and Operational Constraints
The wildcard mechanism itself does not change the fundamental security of LDP pseudowire signaling. Message exchanges remain the same as in RFC 4447. However, an attacker attempting to connect to an access circuit requires one less piece of configuration information.
To mitigate this risk, the RFC recommends that a PE receiving a wildcard FEC should only proceed with setup if explicitly configured to accept wildcards for the particular Attachment Individual (AI) in question. Further, operators should apply the full security considerations of RFC 4447, particularly those pertaining to the Generalized PWid FEC Element.
Letting one side pick is simpler, but routers should only allow it for circuits they trust, so attackers cannot misuse the feature.
§6IANA Registration
RFC 4863 requests one IANA allocation from the Pseudowire Type registry defined in RFC 4446 (IANA Allocations for PWE3): the codepoint 0x7FFF for the wildcard type.
This single registry entry enables the entire mechanism described in the RFC. No other protocol allocations are required.
The Internet Assigned Numbers Authority set aside one special number (0x7FFF) in the Pseudowire Type list to mean 'you decide'.