Network Working Group L. Jin(ed.)
Internet-Draft ZTE
Updates: 4447 6073(if approved) R. Key(ed.)
Intended status: Standards Track Huawei
Expires: December 17, 2012 S. Delord
Alcatel-Lucent
T. Nadeau
Juniper
S. Boutros
Cisco Systems, Inc.
June 15, 2012
Pseudowire Control Word Negotiation Mechanism Update
draft-ietf-pwe3-cbit-negotiation-04.txt
Abstract
The control word negotiation mechanism specified in RFC4447 has a
problem when PE changes the preference for the use of control word
from PREFERRED to NOT-PREFERRED. This draft updates RFC4447 by
introducing a message exchanging mechanism to resolve this control
word negotiation issue.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . . 3
3. Control word re-negotiation by label request message . . . . . 4
3.1. Control word re-negotiation use case . . . . . . . . . . . 5
4. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
8. Contributing Authors . . . . . . . . . . . . . . . . . . . . . 6
9. Normative references . . . . . . . . . . . . . . . . . . . . . 7
Appendix A. Updated C-bit Handling Procedures Diagram . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
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 [RFC2119].
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1. Introduction
The control word negotiation mechanism specified in [RFC4447] section
6.2 has a problem when PE changes the preference for the use of
control word from PREFERRED to NOT-PREFERRED. There would be a wrong
negotiated result of the control word as "not used" while both ends
of pseudowire PE have the use of control word as PREFERRED. This
draft updates [RFC4447] by introducing a message exchanging mechanism
to resolve this negotiation issue.
2. Problem Statement
[RFC4447] section 6 describes the control word negotiation mechanism.
Each PW endpoint has the capability of being configurable with a
parameter that specifies whether the use of the control word is
PREFERRED or NOT PREFERRED. While in some case of control word
negotiation, PE will advertise C-bit=0 in label mapping message with
its locally configured use of control word as PREFERRED. This kind
of behavior will cause some problem when peer PE changes the use of
control word from NOT PREFERRED to PREFERRED.
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 used,
and PE1 sends label mapping with C-bit=0 finally according to
[RFC4447] section 6.2.
3. PE2 then changes its use of control word configuration to
PREFERRED.
4. PE2 will then send label withdraw message to PE1.
5. According to the control word negotiation mechanism, the received
label mapping on PE2 from PE1 indicates C-bit=0, therefore PE2
will still send label mapping with C-bit=0.
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The negotiation result for the control word is still not used, even
though the use of control word configuration on both PE1 and PE2 is
PREFERRED.
3. Control word re-negotiation by label request message
In order to resolve above problem, the control word re-negotiation
mechanism as described in [RFC4447] section 6 is updated by adding
label request message.
When Local PE changes its use of control word from NOT PREFERRED to
PREFERRED and only if it already received the remote label mapping
message with C-bit=0, additional procedure will be added as follow:
-i. Local PE MUST send a label withdraw message to remote PE if
it has previously sent a label mapping, and label release message
to remote PE, and wait until receiving a label release from the
remote PE.
-ii. Local PE MUST send a label request message to remote PE, and
wait until receiving a label mapping message containing the remote
PE locally configured preference for use of control word.
-iii. After receiving remote PE label mapping with C-bit, Local
PE MUST follow procedures defined in [RFC4447] section 6 when
sending its label mapping message.
When the remote PE successfully processed the label withdraw and
label release, and removed the remote label binding, it MUST reset
its use of control word with the locally configured preference, and
send label mapping as a response of label request with locally
configured preference for use of control word.
Note: the FEC element in label request message should be the PE's
local FEC element. Only if FEC element in label request message
could bind together with peer PE's local FEC element, the peer PE
sends label mapping with its bound local FEC element and label as a
response. The label request message format and procedure is
described in [RFC5036].
The multi-segment PW case for T-PE is same. S-PE SHOULD assume an
initial passive role as defined in [RFC6073]. When an S-PE receives
a label request message from one of its adjacent PEs (be it 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 S-PE does not have full information of interface parameter
field in the FEC advertisement. When S-PE receives a label release
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message from remote PE, it MUST send a corresponding label release to
the other remote PE where it has received label mapping.
As local T-PE will send label withdraw before sending label request
to remote peer, the S-PE MUST send the label withdraw upstream before
it advertises the label request. When S-PE receives the label
withdraw, it should process this message to send a label release as a
response and a label withdraw to upstream S-PE/T-PE, then process the
next LDP message, e.g. the label request message.
When Local PE changes the use of control word from PREFERRED to NOT
PREFERRED, Local PE would be able to re-negotiate the Control Word to
be not used following the procedures defined in [RFC4447], and no
label request message to peer PE will be needed. In that case, Local
PE will always send new label mapping with C-bit reflecting the
locally configured preference for use of Control Word.
The diagram in Appendix A in this document updates the control word
negotiation diagram in [RFC4447] Appendix A.
3.1. Control word re-negotiation use case
The procedure of PE1 and PE2 for the use case in figure 1 should be
as follows:
1. PE2 changes locally configured preference for use of control word
to PREFERRED.
2. PE2 will then send label withdraw and release messages to PE1,
and wait until receiving label release from PE1.
3. PE1 will send label release in reply to label withdraw message
from PE2. After that and processing the label release from PE2,
it would reset the use of control word to the locally configured
preference as PREFERRED.
4. Upon receipt of Label release message from PE1, PE2 would reset
use of control word to the locally configured preference as
PREFERRED, and MUST send label request message to PE1, and wait
until receiving a label mapping message.
5. PE1 must send label mapping message with C-bit=1 again to PE2 as
response of label request.
6. PE2 receives the label mapping from PE1 and gets the remote label
binding information. PE2 should wait for PE1 label binding
before sending its label binding with C-bit set.
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7. PE2 will send label mapping to PE1 with C-bit=1, and follow
procedures defined in [RFC4447] section 6.
It is to be noted that the above assume that PE1 is configured to
prefer the use of control word, however in step 5 if PE1 doesn't
prefer or support control word, PE1 would send the label mapping
message with C-bit=0, this would result in PE2 in step 7 sending a
label mapping with C-bit=0 as per [RFC4447] section 6.
By sending 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 label mapping message
received from peer PE1 and locally configured preference for use of
control word, PE2 should determine the use of PW control word
according to [RFC4447] section 6.
4. Backward Compatibility
Since control word negotiation mechanism is updated by adding label
request message, and still follows the basic procedure described in
[RFC4447] section 6, it is fully compatible with existing
implementations. The remote PE (PE1 in figure 1) which already
implement label request message could be compatible with the PE (PE2
in figure 1) following the mechanism of this document.
5. Security Considerations
This document does not introduce any additional security constraints.
6. IANA Considerations
This document does not require IANA assignment.
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. Contributing Authors
Vishwas Manral
Hewlett-Packard Co.
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19111 Pruneridge Ave, Bldg 44,
Cupertino, CA 95014-0691
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
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4447] 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.
[RFC5036] Andersson, L., Minei, I., and B. Thomas, "LDP
Specification", RFC 5036, October 2007.
[RFC6073] Martini, L., Metz, C., Nadeau, T., Bocci, M., and M.
Aissaoui, "Segmented Pseudowire", RFC 6073, January 2011.
Appendix A. Updated C-bit Handling Procedures Diagram
-----------------------------------
| |
| ------------------
| Y | Received Label | N
| -------| Mapping Msg? |--------------
| | ------------------ |
| -------------- |
| | | |
| ------- ------- |
| | C=0 | | C=1 | |
| ------- ------- |
| | | |
| | ---------------- |
| | | Control Word | N |
| | | Capable? |----------- |
| | ---------------- | |
| | Y | | |
| | | | |
| | ---------------- | |
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| | | Control Word | N | |
| | | Preferred? |---- | |
| | ---------------- | | |
| | Y | | | |
| --------------------- | | | |
| | Control Word | | | | ----------------
| | change Preferred | | | | | Control Word |
| | to not-Preferred? | | | | | Preferred? |
| --------------------- | | | ----------------
| Y | | N | | | N | Y |
| | | | | | | |
| | Send Send Send Send Send Send
| | C=0 C=1 C=0 C=0 C=0 C=1
| | | | | |
| ------------------- ----------------------------------
| | Send withdraw | | If receive the same as sent, |
| | if already sent | | PW setup is complete. If not: |
| | label mapping, | ----------------------------------
| | and release msg | | | | |
| ------------------- ------------------- -----------
| | | Receive | | Receive |
| ------------------- | C=1 | | C=0 |
| | Receiving label | ------------------- -----------
| | release message | | |
| ------------------- Wait for the Send
| | next message Wrong C-bit
| ------------------- |
| | Send label | Send Label
| | request message | Mapping Message
| -------------------
| |
-------------
Authors' Addresses
Lizhong Jin (editor)
ZTE Corporation
889, Bibo Road
Shanghai, 201203, China
Email: lizhong.jin@zte.com.cn
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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
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