Internet DRAFT - draft-ietf-pwe3-cbit-negotiation
draft-ietf-pwe3-cbit-negotiation
Network Working Group L. Jin(ed.)
Internet-Draft ZTE
Updates: 4447, 6073 (if approved) R. Key(ed.)
Intended status: Standards Track Huawei
Expires: January 4, 2013 S. Delord
Alcatel-Lucent
T. Nadeau
Juniper
S. Boutros
Cisco Systems, Inc.
July 3, 2012
Pseudowire Control Word Negotiation Mechanism Update
draft-ietf-pwe3-cbit-negotiation-05.txt
Abstract
The control word negotiation mechanism specified in RFC4447 has a
problem when a PE changes the preference for the use of the control
word from NOT PREFERRED to PREFERRED. This document updates RFC4447
and RFC6073 by adding the Label Request message to resolve this
control word negotiation issue for single-segment and multi-segment
pseudowires.
Status of this Memo
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
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 re-negotiation use case . . . . . . . . . . . 6
5. Backward Compatibility . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9. Contributing Authors . . . . . . . . . . . . . . . . . . . . . 7
10. Normative references . . . . . . . . . . . . . . . . . . . . . 8
Appendix A. Updated C-bit Handling Procedures Diagram . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
The control word negotiation mechanism specified in [RFC4447] section
6.2 encounters a problem when a PE (Provider Edge) changes the
preference for the use of the control word from NOT PREFERRED to
PREFERRED. [RFC4447] specifies that if both endpoints prefer the use
of the control word, then the pseudowire control word should be used.
However, in the case whereby 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 [RFC4447] by adding
Label Request message to resolve this negotiation issue for single-
segment PW. Multi-segment PW in [RFC6073] inherits the control word
negotiation mechanism in [RFC4447], and this document updates
[RFC6073] by adding the processing of Label Request message on S-PE.
When PE changes the preference for the use of control word from
PREFERRED to NOT PREFERRED, it should follow [RFC4447], 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 [RFC2119].
3. Problem Statement
[RFC4447] 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 whereby one PE advertises a C bit set 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 causes an incorrect
negotiated control word result of "not used".
The following case will describe the negotiation problem in detail:
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+-------+ +-------+
| | 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 [RFC4447] 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.
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 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 [RFC4447] 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
a change occurs of its use of control word 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 Label Release message and Label Withdraw message sending
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does not require specific sequence.
ii. The local PE MUST send a Label Request message to 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 C bit, local PE MUST follow the procedures defined in
[RFC4447] section 6 when sending its Label Mapping message.
The remote PE will follow [RFC4447], 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 Label Mapping
message with locally configured preference for use of control word as
a response of Label Request message as specified in [RFC5036].
Note: for the local PE, before processing new configuration changing
request, 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 of
the Label Request message, the peer PE should send Label Mapping
message with its own local PW FEC element. The Label Request message
format and procedure is described in [RFC5036].
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 [RFC6073] for
S-PE (Switching Provider Edge) for the processing of Label Mapping
message. [RFC6073] 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 (may be 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 Label Withdraw message before
sending 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
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Label Withdraw message to 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 PW control
word to be 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
[RFC4447] section 5.4.1.
The diagram in Appendix A in this document updates the control word
negotiation diagram in [RFC4447] Appendix A.
4.2. Control word re-negotiation 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.
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 C bit set to 1 again
to PE2 as response of 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 C bit set.
7. PE2 will send the Label Mapping to PE1 with C bit set to 1, and
follow procedures defined in [RFC4447] 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 C bit set to
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0. As a result, PE2 in step 7 would send a Label Mapping message
with C bit set 0 as per [RFC4447] 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 [RFC4447] section 6.
5. 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, this document is fully compatible with existing
implementations. For single-segment pseudowire, the remote PE (PE1
in figure 1) which already implements [RFC4447] and Label Request
message as defined in [RFC5036] could be compatible with the PE (PE2
in figure 1) following the mechanism of this document. For the
multi-segment pseudowire, the T-PE is 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 [RFC4447] and [RFC6073] also
apply to this document, and this document does not introduce any
additional security constraints.
7. IANA Considerations
This document does not require IANA assignment.
8. 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.
9. 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
10. 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.
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Appendix A. Updated C-bit Handling Procedures Diagram
-----------------------------------
| |
| ------------------
| 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 |
| | Receiving Label | | C=1 | | C=0 |
| | Release message | ------------------- -----------
| ------------------- | |
| | Wait for the Send
| ------------------- next message Wrong C-bit
| | Send Label | |
| | Request message | Send Label
| ------------------- Mapping Message
| |
-------------
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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
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