|Title||GMPLS Signaling Procedure for Egress Control
Network Working Group L. Berger
Request for Comments: 4003 Movaz Networks
Updates: 3473 February 2005
Category: Standards Track
GMPLS Signaling Procedure for Egress Control
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 (C) The Internet Society (2005).
This document clarifies the procedures for the control of the label
used on an output/downstream interface of the egress node of a Label
Switched Path (LSP). This control is also known as "Egress Control".
Support for Egress Control is implicit in Generalized Multi-Protocol
Label Switching (GMPLS) Signaling. This document clarifies the
specification of GMPLS Signaling and does not modify GMPLS signaling
mechanisms and procedures.
The ability to control the label used on the output/downstream
interface of an egress node was one of the early requirements for
GMPLS. In the initial GMPLS documents, this was called "Egress
Control". As the GMPLS documents progressed, the ability to control
a label on an egress interface was generalized to support control of
a label on any interface. This generalization is seen in Section 6
of [RFC3471] and Section 5.1 of [RFC3473]. When this functionality
was generalized, the procedures to support control of a label at the
egress were also generalized. Although the result was intended to
cover egress control, this intention is not clear to all. This note
reiterates the procedures to cover control of a label used on an
egress output/downstream interface.
For context, the following is the text from the GMPLS signalling
document dated June 2000 about how ERO (Explicit Route Object) for
6. Egress Control
The LSR at the head-end of an LSP can control the termination of
the LSP by using the ERO. To terminate an LSP on a particular
outgoing interface of the egress LSR, the head-end may specify the
IP address of that interface as the last element in the ERO,
provided that interface has an associated IP address.
There are cases where the use of IP address doesn't provide enough
information to uniquely identify the egress termination. One case
is when the outgoing interface on the egress LSR is a component
link of a link bundle. Another case is when it is desirable to
"splice" two LSPs together, i.e., where the tail of the first LSP
would be "spliced" into the head of the second LSP. This last
case is more likely to be used in the non-PSC classes of links.
The Egress Label subobject may appear only as the last subobject
in the ERO/ER. Appearance of this subobject anywhere else in the
ERO/ER is treated as a "Bad strict node" error.
During an LSP setup, when a node processing the ERO/RR performs
Next Hop selection finds that the second subobject is an Egress
Label Subobject, the node uses the information carried in this
subobject to determine the handling of the data received over that
LSP. Specifically, if the Link ID field of the subobject is non
zero, then this field identifies a specific (outgoing) link of the
node that should be used for sending all the data received over
the LSP. If the Label field of the subobject is not Implicit NULL
label, this field specifies the label that should be used as an
outgoing label on the data received over the LSP.
Procedures by which an LSR at the head-end of an LSP obtains the
information needed to construct the Egress Label subobject are
outside the scope of this document.
2. Egress Control Procedures
This section is intended to complement Sections 5.1.1 and 5.2.1 of
[RFC3473]. The procedures described in those sections are not
modified. This section clarifies procedures related to the label
used on an egress output/downstream interface.
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].
2.1. ERO Procedures
Egress Control occurs when the node processing an ERO is the egress
and the ERO contains one or more subobjects related to the
output/downstream interface. In this case, the outgoing/downstream
interface is indicated in the ERO as the last listed local interface.
Note that an interface may be numbered or unnumbered.
To support Egress Control, an egress checks to see whether the
received ERO contains an outgoing/downstream interface. If it does,
the type of the subobject or subobjects following the interface is
examined. If the associated LSP is unidirectional, one subobject is
examined. Two subobjects are examined for bidirectional LSPs. If
the U-bit of the subobject being examined is clear (0), then the
value of the label MUST be used for transmitting traffic associated
with the LSP on the indicated outgoing/downstream interface.
If the U-bit of the subobject being examined is set (1), then the
value of the label is used for upstream traffic associated with the
bidirectional LSP. Specifically, the label value will be used for
the traffic associated with the LSP that will be received on the
indicated outgoing/downstream interface.
Per [RFC3473], any errors encountered while processing the ERO,
including if the listed label(s) are not acceptable or cannot be
supported in forwarding, SHOULD result in the generation of a PathErr
message with the error code "Routing Error" and error value of "Bad
Explicit Route Object".
2.2. RRO Procedures
If an ERO is used to specify outgoing interface information at the
egress and label recording is indicated for the LSP, the egress
should include the specified interface information and the specified
label or labels in the corresponding RRO (Route Record Object).
3. Security Considerations
This document clarifies procedures defined in [RFC3473] but does not
define any new procedures. As such, no new security considerations
Valuable comments and input were received from Adrian Farrel, Alan
Kullberg, and Dimitri Papadimitriou.
5. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Functional Description", RFC 3471,
[RFC3473] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Resource ReserVation Protocol-Traffic
Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.
Movaz Networks, Inc.
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