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Egress Control (was Re: spc connections)
At 03:51 PM 11/15/2003, Kireeti Kompella wrote:
Hi Lou,
On Sat, 15 Nov 2003, Lou Berger wrote:
> As I mentioned in that mail, I agree that explicitly documenting
> egress label control seems like the thing to do now. I suggest that this
> be done in a standalone document as it has application outside of
> overlay/UNI. The text below is overly specific.
First, let's make the text (procedures) more general. Then we can
figure out where it goes. My preference is to keep it in this doc.
Kireeti.
-------
See below for the my proposal on the clarification. I'll submit
this as a draft once folks have a chance to comment. Assuming the
draft goes forward, I think it should be on the Informational track.
Also, Having some text in the overlay document would be fine as well.
Lou
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GMPLS Signaling Procedure For Egress Control
Abstract
This note clarifies the procedures for the control of a label used on an
egress output/downstream interface. Such control is also know and
"Egress Control". Support for Egress Control is implicit in Generalized
Multi-Protocol Label Switching (GMPLS) Signaling [RFC3471] and
[RFC3473]. This note does not modify GMPLS signaling mechanisms and
procedures and should be viewed as an informative clarification of
[RFC3473].
1. Background
The ability to control a label used on an egress output/downstream
interface was one of the early requirements for GMPLS. In the
initial GMPLS drafts, this was called "Egress Control". As the GMPLS
drafts 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]. In generalizing this functionality, the
procedures to support control of a label at the egress were also
generalized. While the resulting 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 signaling draft
dated June 2000:
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 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.
and
6.2. Procedures
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 compliment Section 5.1.1 and 5.2.1 of
[RFC3473]. The procedures described in that section are not
modified. This section clarifies procedures related to the label
used on an egress output/downstream interface.
2.1. ERO Procedures
Egress Control occurs when the node processing an ERO is the egress
and the ERO contains one or more label subobjects. 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, and bundled or unbundled.
To support Egress Control, an egress checks to see if the received
ERO contains an outgoing/downstream interface. If it does, the type
of the subobject or subobjects following the interface are examined.
If the associated LSP is unicast, 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 is
copied into a new Label_Set object. This Label_Set object indicates
the label value that 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 message
containing a "Bad EXPLICIT_ROUTE object" error.
2.2. RRO Procedures
In the case where an ERO is used to specify outgoing interface
information at the egress, the egress should include the specified
interface information and label or labels, if present, in the
corresponding RRO.