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IETF RFC 6723
Last modified on Wednesday, September 5th, 2012
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Internet Engineering Task Force (IETF) L. Jin, Ed.
Request for Comments: 6723 ZTE
Updates: 4447, 6073 R. Key, Ed.
Category: Standards Track Huawei
ISSN: 2070-1721 S. Delord
Alcatel-Lucent
T. Nadeau
Juniper
S. Boutros
Cisco Systems, Inc.
September 2012
Update of the Pseudowire Control-Word Negotiation Mechanism
 Abstract
The control-word negotiation mechanism specified in RFC 4447 has a
problem when a PE (Provider Edge) changes the preference for the use
of the control word from NOT PREFERRED to PREFERRED. This document
updates RFC 4447 and RFC 6073 by adding the Label Request message to
resolve this control-word negotiation issue for single-segment and
multi-segment pseudowires.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6723.
Jin, et al. Standards Track PAGE 1 
RFC 6723 Update of PW C-Bit Negotiation September 2012
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
4. Control-Word Renegotiation by Label Request Message . . . . . . 4
4.1. Control-Word Renegotiation for Multi-Segment PW . . . . . . 5
4.2. Control-Word Renegotiation Use Case . . . . . . . . . . . . 5
5. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Updated Diagram of C-Bit Handling Procedures . . . . . 8
1. Introduction
The control-word negotiation mechanism specified in [RFC 4447],
Section 6.2, encounters a problem when a PE changes the preference
for the use of the control word from NOT PREFERRED to PREFERRED.
[RFC 4447] specifies that if both endpoints prefer the use of the
control word, then the pseudowire control word should be used.
However, in the case where a PE changes its preference from NOT
PREFERRED to PREFERRED and both ends of the PW (pseudowire) PE have
the use of control word set as PREFERRED, an incorrect negotiated
result of the control word as "not used" occurs. This document
updates the control-word negotiation mechanism in [RFC 4447] by adding
a Label Request message to resolve this negotiation issue for single-
segment PW. Multi-segment PW in [RFC 6073] inherits the control-word
negotiation mechanism in [RFC 4447], and this document updates
[RFC 6073] by adding the processing of Label Request message on the
Jin, et al. Standards Track PAGE 2
RFC 6723 Update of PW C-Bit Negotiation September 2012
S-PE (Switching Provider Edge). When the PE changes the preference
for the use of control word from PREFERRED to NOT PREFERRED, it
should follow [RFC 4447], and there is no problem.
2. 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].
3. Problem Statement
[RFC 4447], Section 6, describes the control-word negotiation
mechanism. Each PW endpoint has a configurable parameter that
specifies whether the use of the control word is PREFERRED or NOT
PREFERRED. During control-word negotiation, if one PE advertises a C
bit set to 0 in the Label Mapping message with its locally configured
use of control word as PREFERRED, and a corresponding peer PE changes
its use of control word from NOT PREFERRED to PREFERRED, this causes
an incorrect negotiated control-word result of "not used".
The following case will describe the negotiation problem in detail:
+-------+ +-------+
| | PW | |
| PE1 |====================| PE2 |
| | | |
+-------+ +-------+
Figure 1
1. Initially, the use of control word on PE1 is configured as
PREFERRED, and on PE2 as NOT PREFERRED.
2. The negotiation result for the control word of this PW is not
used, and ultimately PE1 sends the Label Mapping message with C
bit set to 0 according to [RFC 4447], Section 6.2.
3. PE2 then changes its use of control-word configuration from NOT
PREFERRED to PREFERRED, by deleting PW configuration with NOT
PREFERRED use of control word, and configuring the PW again with
PREFERRED use of control word.
4. PE2 will then send the Label Withdraw message to PE1, and
correspondingly will receive the Label Release message from PE1.
Jin, et al. Standards Track PAGE 3
RFC 6723 Update of PW C-Bit Negotiation September 2012
5. According to the control-word negotiation mechanism, the
previously received Label Mapping message on PE2 from PE1 carries
the C bit set to 0; therefore, PE2 will still send the Label
Mapping message with the C bit set to 0.
The negotiation result for the control word is still not used, even
though the use of control-word configuration on both PE1 and PE2 are
PREFERRED.
4. Control-Word Renegotiation by Label Request Message
The control-word negotiation mechanism in [RFC 4447], Section 6, is
updated to add the Label Request message described in this section.
The renegotiation process begins when the local PE has received the
remote Label Mapping message with the C bit set to 0, and at the
point its use of control word is changed from NOT PREFERRED to
PREFERRED. The following additional procedure will be carried out:
i. The local PE MUST send a Label Release message to remote PE.
If local PE has previously sent a Label Mapping message, it
MUST send a Label Withdraw message to remote PE and wait until
it has received a Label Release message from the remote PE.
Note: the above-mentioned sending of the Label Release message
and Label Withdraw message does not require a specific
sequence.
ii. The local PE MUST send a Label Request message to the peer PE,
and then MUST wait until it receives a Label Mapping message
containing the peer's current configured preference for use of
control word.
iii. After receiving the remote peer PE Label Mapping message with
the C bit, the local PE MUST follow the procedures defined in
[RFC 4447], Section 6, when sending its Label Mapping message.
The remote PE will follow [RFC 4447], and once the remote PE has
successfully processed the Label Withdraw message and Label Release
message, it will reset its use of control word with the locally
configured preference. Then, the remote PE will send a Label Mapping
message with locally configured preference for use of control word as
a response to Label Request message as specified in [RFC 5036].
Note: for the local PE, before processing new request to change the
configuration, the above message-exchanging process should be
finished. The FEC (Forwarding Equivalence Class) element in the
Label Request message should be the PE's local PW FEC element. As a
response to the Label Request message, the peer PE should send a
Jin, et al. Standards Track PAGE 4
RFC 6723 Update of PW C-Bit Negotiation September 2012
Label Mapping message with its own local PW FEC element. The Label
Request message format and procedure is described in [RFC 5036].
4.1. Control-Word Renegotiation for Multi-Segment PW
The multi-segment PW case for a T-PE (Terminating Provider Edge)
operates similarly as the PE in single-segment PW described in the
above section. An initial passive role is defined in [RFC 6073] for
the S-PE when processing of the Label Mapping message. [RFC 6073] is
updated by applying this passive role to the processing of Label
Request message. When an S-PE receives a Label Request message from
one of its adjacent PEs (which may be an S-PE or another T-PE), it
MUST send a matching Label Request message to other adjacent PE
(again, it may be an S-PE or a T-PE). This is necessary since an
S-PE does not have complete information of the interface parameter
field in the FEC advertisement. When the S-PE receives a Label
Release message from remote PE, it MUST send a corresponding Label
Release message to the other remote PE when it holds a label for the
PW from the remote PE.
Note: because the local T-PE will send a Label Withdraw message
before sending a Label Request message to the remote peer, the S-PE
MUST process the Label Withdraw message before the Label Request
message. When the S-PE receives the Label Withdraw message, it
should process this message to send a Label Release message as a
response and a Label Withdraw message to an upstream S-PE/T-PE. The
S-PE will then process the next LDP message, e.g. the Label Request
message.
When the local PE changes the use of control word from PREFERRED to
NOT PREFERRED, the local PE would then renegotiate the control word
so that it is not used by deleting the PW configuration with
PREFERRED use of control word, and configuring the PW again with NOT
PREFERRED use of control word. All of these procedures have been
defined in [RFC 4447], Section 5.4.1.
The diagram in Appendix A of this document updates the control-word
negotiation diagram in [RFC 4447] Appendix A.
4.2. Control-Word Renegotiation Use Case
The procedure of PE1 and PE2 for the use case in Figure 1 will become
as follows:
1. PE2 changes locally configured preference for use of control word
to PREFERRED.
Jin, et al. Standards Track PAGE 5
RFC 6723 Update of PW C-Bit Negotiation September 2012
2. PE2 will then send the Release messages to PE1. PE2 will also
send the Label Withdraw message, and wait until it has received
the Label Release message from PE1.
3. PE1 will send the Label Release message in response to the Label
Withdraw message from PE2. After processing the Label Release
from PE2, PE1 will then reset the use of control word to the
locally configured preference as PREFERRED.
4. Upon receipt of the Label Release message from PE1, PE2 will send
the Label Request message to PE1, and proceed to wait until a
Label Mapping message is received.
5. PE1 will send a Label Mapping message with the C bit set to 1
again to PE2 in response to the Label Request message.
6. PE2 receives the Label Mapping message from PE1 and gets the
remote label binding information. PE2 will wait for the PE1
Label Mapping message before sending its Label Mapping message
with the C bit set.
7. PE2 will send the Label Mapping to PE1 with C bit set to 1, and
follow procedures defined in [RFC 4447], Section 6.
While it is assumed that PE1 is configured to prefer the use of the
control word, in step 5, if PE1 doesn't prefer or support the control
word, PE1 would then send the Label Mapping message with the C bit
set to 0. As a result, PE2 in step 7 would send a Label Mapping
message with the C bit set 0 as per [RFC 4447], Section 6.
By sending a Label Request message, PE2 will get the locally
configured preference for use of control word of peer PE1 in the
received Label Mapping message. By using the new C bit from the
Label Mapping message received from peer PE1 and the locally
configured preference for use of control word, PE2 should determine
the use of PW control word according to [RFC 4447], Section 6.
5. Backward Compatibility
Since control-word negotiation mechanism is updated by adding the
Label Request message, and still follows the basic procedure
described in [RFC 4447], Section 6, this document is fully compatible
with existing implementations. For single-segment pseudowire, the
remote PE (PE1 in Figure 1) which already implements [RFC 4447], and
the Label Request message as defined in [RFC 5036] could be compatible
with the PE (PE2 in Figure 1) following the mechanism of this
Jin, et al. Standards Track PAGE 6
RFC 6723 Update of PW C-Bit Negotiation September 2012
document. For the multi-segment pseudowire, the T-PE is the same as
PE in single-segment pseudowire; the S-PE should be upgraded with the
mechanism defined in this document.
6. Security Considerations
The security considerations specified in [RFC 4447] and [RFC 6073] also
apply to this document, and this document does not introduce any
additional security constraints.
7. Acknowledgements
The authors would like to thank Stewart Bryant, Andrew Malis, Nick
Del Regno, Luca Martini, Venkatesan Mahalingam, Alexander Vainshtein,
Adrian Farrel, and Spike Curtis for their discussion and comments.
8. Contributors
Vishwas Manral
Hewlett-Packard Co.
19111 Pruneridge Ave., Bldg. 44
Cupertino, CA 95014-0691
US
EMail: vishwas.manral@hp.com
Reshad Rahman
Cisco Systems, Inc.
2000 Innovation Drive
Ottawa, Ontario K2K 3E8
CANADA
EMail: rrahman@cisco.com
9. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 4447] Martini, L., Rosen, E., El-Aawar, N., Smith, T., and G.
Heron, "Pseudowire Setup and Maintenance Using the Label
Distribution Protocol (LDP)", RFC 4447, April 2006.
[RFC 5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC 6073] Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
Aissaoui, "Segmented Pseudowire", RFC 6073, January 2011.
Jin, et al. Standards Track PAGE 7
RFC 6723 Update of PW C-Bit Negotiation September 2012
Appendix A. Updated Diagram of C-Bit Handling Procedures
-----------------------------------
| |
| ------------------
| Y | Received Label | N
| -------| Mapping msg? |--------------
| | ------------------ |
| -------------- |
| | | |
| ------- ------- |
| | C=0 | | C=1 | |
| ------- ------- |
| | | |
| | ---------------- |
| | | Control Word | N |
| | | Capable? |----------- |
| | ---------------- | |
| | Y | | |
| | | | |
| | ---------------- | |
| | | Control Word | N | |
| | | Preferred? |---- | |
| | ---------------- | | |
| | Y | | | |
| --------------------- | | | |
| |Control Word change| | | | ----------------
| |from NOT PREFERRED | | | | | Control Word |
| | to PREFERRED? | | | | | Preferred? |
| --------------------- | | | ----------------
| Y | | N | | | N | Y |
| | Delete, and | | | | |
| | configure Send Send Send Send Send
| | new PW again C=1 C=0 C=0 C=0 C=1
| | | | | |
| ---------------------------- ----------------------------------
| |Send Label Release msg, | | If receive the same as sent, |
| |send Label Withdraw msg if| | PW setup is complete. If not: |
| |has sent Label Mapping msg| ----------------------------------
| ---------------------------- | | | |
| | ------------------- -----------
| ------------------- | Receive | | Receive |
| | Receive Label | | C=1 | | C=0 |
| | Release message | ------------------- -----------
| ------------------- | |
Jin, et al. Standards Track PAGE 8
RFC 6723 Update of PW C-Bit Negotiation September 2012
| | Wait for the Send
| ------------------- next message Wrong C-bit
| | Send Label | |
| | Request message | Send Label
| ------------------- Mapping message
| |
-------------
Authors' Addresses
Lizhong Jin (editor)
ZTE Corporation
889, Bibo Road
Shanghai, 201203, China
EMail: lizhong.jin@zte.com.cn
Raymond Key (editor)
Huawei
EMail: raymond.key@ieee.org
Simon Delord
Alcatel-Lucent
EMail: simon.delord@gmail.com
Thomas Nadeau
Juniper
EMail: tnadeau@juniper.net
Sami Boutros
Cisco Systems, Inc.
3750 Cisco Way
San Jose, California 95134, USA
EMail: sboutros@cisco.com
Jin, et al. Standards Track PAGE 9
RFC TOTAL SIZE: 20307 bytes
PUBLICATION DATE: Wednesday, September 5th, 2012
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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