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Versions: 00 01 02 03
Network Working Group A. Vainshtein
Internet-Draft ECI Telecom
Updates: 5586 (if approved) L. Andersson
Intended status: Standards Track Huawei Technologies Co., Ltd
Expires: November 27, 2015 A. Farrel
Juniper Networks
May 26, 2015
Handling the TC and TTL fields in a Label Stack Entry when the Generic
Associated Channel Label is Present
draft-vainshtein-mpls-gal-tc-ttl-handling-00
Abstract
This document clarifies handling of the Traffic Class (TC) and Time-
to-Live (TTL) fields of a Label Stack Entry that contains the Generic
Associated Channel (G-ACh) Label (GAL). These clarifications are
intended to aid interoperability of implementations.
Original handling was defined in RFC 5586, and this document updates
that RFC.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 27, 2015.
Copyright Notice
Copyright (c) 2015 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2.2. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3
3. New Procedures . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. New Procedures for Handling the TC Field in an LSE That
Contains the GAL . . . . . . . . . . . . . . . . . . . . 4
3.2. New Procedures for Handling the TTL Field in an LSE
Containing GAL . . . . . . . . . . . . . . . . . . . . . 4
3.3. Scope of the new Procedures . . . . . . . . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
7.1. Normative References . . . . . . . . . . . . . . . . . . 5
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
[RFC5586] introduced an alert mechanism for the Generic Associated
Channel (G-ACh) that uses a Generic Associated Channel Label (GAL).
In particular, RFC 5586 allocated one of the values from the special
purpose label space to be the GAL, specified that the Label Stack
Entry (LSE) containing GAL must be always at the bottom of the label
stack in the case of MPLS transport profile (MPLS-TP) Label Switched
Paths (LSPs), and that G-ACh packets must not be forwarded based on
the GAL.
Per [RFC3032] each LSE contains, in addition to the label value and
bottom-of-stack (BoS) flag, two additional fields:
o Traffic Class (TC) field - 3 bits (renamed from Experimental (EXP)
field [RFC5462]). [RFC5586] defined that the handling of this
field in an LSE that contains the GAL is as specified and
referenced in RFC 5462.
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o Time-to-Live (TTL) field - 8 bits. [RFC5586] defined that the
handling of this field in an LSE that contains the GAL is in
accordance with [RFC3443].
Implementations of [RFC5586] have encountered interoperability
problems in their interpretation of these two fields when present in
an LSE that contains the GAL. Section 4.2.1.1 of [RFC5586] states:
The TTL field of the GAL LSE MUST be set to at least 1. The exact
value of the TTL is application specific.
When this LSE becomes the top entry in the label stack (because the
previous label has been popped) some receiving implementations have
attempted to interpret the fields and this has resulted in errors,
packet drops, or poor performance. In particular, packets with an
LSE with TTL set to zero have been dropped as "expired" while those
with TTL set to one can be trapped to the generic (slow) MPLS
exception handler with appropriate rate limiting before the GAL is
noticed (which would otherwise result in trapping the packet to a
fast OAM handler). This document clarifies the rules for setting and
processing them in the Label Stack Entry that includes the GAL.
The above-mentioned references are not useful for the implementers
and testers because they don't give enough information about the
correct processing actions. For example, [RFC5462] says only that
the use of TC field for Quality of Service (QoS) and Explicit
Congestion Notification (ECN) "is intended to be flexible'. On the
other hand, while [RFC3443] is very detailed with regard to
processing of the TTL field, it mainly deals with issues that are
irrelevant for an LSE that contains the GAL.
This document defines handling of the TC and TTL fields in an LSE
that contains GAL in an unambiguous way without referring to any
other documents. It updates [RFC5586] in that regard.
2. Terminology
2.1. Requirements Language
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].
2.2. Abbreviations
BoS: Bottom of Stack
G-ACh: Generic Associated Channel
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GAL: Generic Associated Channel Label
LER: Label Edge Router
LSE: Label Stack Entry
LSP: Label Switching Path
LSR: Label Switching Router
PW: Pseudowire
TC: Traffic Class field (formerly named EXP)
TTL: Time-to-Live
3. New Procedures
3.1. New Procedures for Handling the TC Field in an LSE That Contains
the GAL
Setting the value of the TC field in an LSE that contains the GAL is
done by the LER that originates the G-ACh packet and is a matter of
local policy for that LER. It is RECOMMENDED that implementations
set the TC field of an LSE that contains the GAL to all zero (0b000).
The LER that inspects an LSE that contains the GAL MUST ignore the
value of the TC field.
3.2. New Procedures for Handling the TTL Field in an LSE Containing GAL
Setting the value of the TTL in an LSE that contains the GAL is done
by the LER that originates the G-ACh packet and is a matter of local
policy for that LER. The LER that originates the G-ACh packet SHOULD
NOT set this value to 0 or 1: this will avoid possible
misinterpretation by the LER that inspects an LSE that contains the
GAL if that LER does not comply with the this document. It is
RECOMMENDED that implementations set the TTL of an LSE that contains
the GAL to 0xff.
The LER that examines an LSE that contains the GAL MUST ignore the
value of the TTL field.
3.3. Scope of the new Procedures
[RFC5586] disallowed the use of the GAL in PWs, but that limitation
was relaxed in [RFC6423].
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The new procedures defined in this document for handling TC field and
the TTL field in an LSE that contains the GAL apply equally all
possible uses of the GAL including the so-called "Section G-ACh"
where the GAL is the only label in the label stack, and the use of
the GAL in LSPs and PWs.
4. IANA Considerations
This document makes no requests for IANA action.
5. Security Considerations
This document makes a minor update to the processing for MPLS packets
containing the GAL and does not change any of the security
fundamentals of MPLS. For a discussion of security considerations
relating to MPLS, please refer to [RFC5920].
Note that the rules set out in this document specify that a receiver
must ignore the values in the two MPLS LSE fields that are discussed.
As such, this clarification removes a potential (and minor) attack
vector where those fields could be malignly set and might cause
incorrect action by the receiver.
6. Acknowledgments
The authors would like to thank Nadav Baadany whose question
triggered this work, and Jeff Haas, Jie Dong, and Mach Chen for their
comments on this document.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3032] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
Encoding", RFC 3032, January 2001.
[RFC3443] Agarwal, P. and B. Akyol, "Time To Live (TTL) Processing
in Multi-Protocol Label Switching (MPLS) Networks", RFC
3443, January 2003.
[RFC5462] Andersson, L. and R. Asati, "Multiprotocol Label Switching
(MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
Class" Field", RFC 5462, February 2009.
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[RFC5586] Bocci, M., Vigoureux, M., and S. Bryant, "MPLS Generic
Associated Channel", RFC 5586, June 2009.
[RFC6423] Li, H., Martini, L., He, J., and F. Huang, "Using the
Generic Associated Channel Label for Pseudowire in the
MPLS Transport Profile (MPLS-TP)", RFC 6423, November
2011.
7.2. Informative References
[RFC5920] Fang, L., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
Authors' Addresses
Alexander Vainshtein
ECI Telecom
Petah Tikva
Israel
Email: alexander.vainshtein@ecitele.com
Loa Andersson
Huawei Technologies Co., Ltd
Stockholm
Sweden
Email: loa@mail01.huawei.com
Adrian Farrel
Juniper Networks
Email: adrian@olddog.co.uk
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