One document matched: draft-krishnan-mpls-reroute-rsvpext-01.txt
Differences from draft-krishnan-mpls-reroute-rsvpext-00.txt
Internet Engineering Task Force Ram Krishnan
Internet Draft Dimitry Haskin
Expires: December 1999 Nexabit Networks
June 1999
Extensions to RSVP to Handle Establishment of Alternate
Label-Switched Paths for Fast Re-route
draft-krishnan-mpls-reroute-rsvpext-01.txt
Status
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document describes the RSVP extensions that may facilitate
creation of an alternative label switched path to handle fast data
packet reroute upon failure in a primary label switched path in an
Multi-protocol Label Switching (MPLS) network as described in [3].
As such, this draft is a companion draft to [3]. The proposed
extensions present no backward compatibility issues.
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1. Introduction
A mechanism to establish an alternate label-switched path (LSP) that
is used for quickly re-routing traffic in the event of a network
element failure or congestion along the primary LSP is described in
[3]. Only one alternate LSP needs to be created in this approach as
opposed to an approach that requires multiple alternate LSPs to be
created at each intermediate switch along the LSP. Such an approach
reduces computational complexity and the associated signalling
overhead. It is required that the alternate backup LSP does not
share any network elements (links or label-switched routers (LSR))
with the exception of the source and destination LSRs of the primary
LSP.
This document defines objects necessary for signalling the creation
and establishment of the alternate LSP when the primary and
alternative LSPs are initiated from a single router using RSVP [4].
2. Description of the Approach
The main idea behind the approach in [3] is to redirect traffic at
the point of failure in the primary LSP back to the source end-point
of the primary LSP in the reverse direction after which the traffic
flow is sent along via the alternate disjoint LSP between source and
destination switches of the protected primary LSP.
Referring to Figure 1, there is an MPLS network consisting of 7
interconnected switches.
Figure 1:
+--------+ 24 +--------+ 46 +--------+
+-->| Switch |------->| Switch |------->| Switch |---+
: | 2 |--------| 4 |--------| 6 | :
: | | | | | | :
12 : +--------+ +--------+ +--------+ : 67
: / / / \ :
: / / / \ V
+--------+ 31 +--------+ 53 +--------+ 75 +--------+
| Switch |<-------| Switch |<-------| Switch |<......| Switch |
| 1 |--------| 3 |--------| 5 |-------| 7 |
=>| |=======>| |=======>| |======>| |=>
+--------+ 13 +--------+ 35 +--------+ 57 +--------+
A primary LSP between switches 1 and 7 is shown by a double-dashed
links labeled 13, 35, and 57. Arrows indicate direction of the data
traffic.
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The initial segment of the alternative LSP runs between the
destination LSR and the source LSR in the reverse direction of the
primary path traversing through every switch between the last hop
switch and the source LSR. The dashed line between switches 5 and 1
illustrates such a segment of the alternative path.
The second and final segment of the alternative path is set between
the source switch and the destination switch along a transmission
path disjoint from the primary LSP. The dashed line between Switches
1 and 7 through Switches 2, 4, and 6 illustrates the final segment
of the alternative LSP.
The initial and final segments of the alternative path are linked to
form an entire alternative path from the last hop switch to the
destination switch. In Figure 1 the entire alternative path consists
of the LSP links labeled 53, 31, 12, 24, 46, and 67 if the
alternative path originates at the last hop switch.
As soon as a link failure or congestion along the protected path is
detected an operational switch at ingress of failed link reroutes
incoming traffic around of the failure or congestion by linking
upstream portion of the primary path to the downstream portion of
the alternative path. Thus if the link between Switches 3 and 5
fails, the primary and alternative paths are linked at Switch 3
forming the following label switched path for the traffic flow:
13->31->12->24->46->67.
3. Extensions to RSVP for Alternate LSP Establishment
Clearly, a label-switched path needs to be set up similar to the
establishment of the primary LSP. The presence of LABEL-REQ object
in the PATH message and LABEL object in the RESV message enables the
downstream-on-demand label allocation policy by which the labels are
exchanged among the neighbors. As shown in Figure 1, the alternate
LSP is composed of two components: the disjoint segment between the
source end-point and the destination end-point of the primary LSP
(12->24->46->67 in the example) and the segment in the reverse
direction between the destination end-point and the source end-point
traversing the same network elements as the primary LSP (75->53-
>31).
3.1 Establishing the Disjoint Segment of the Alternate LSP.
No new RSVP objects are necessary for establishing the disjoint
segment of the alternate LSP. Procedures similar to the creation of
the primary LSP can be used to establish this disjoint segment. As
mentioned earlier, care should be exercised to make sure that this
segment of the alternate path is completely disjoint from the
primary LSP. For instance, the disjoint segment can be explicitly
specified using the Explicit Route Object (ERO) in the PATH message
[4].
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3.2 Establishing the Reverse Segment of the Alternate LSP.
New RSVP objects are required in the PATH and RESV messages to
establish the reverse segment of the alternate LSP.
A new Flag option is defined in the Flags field of the SESSION-
ATTRIBUTE object that specifies Fast-reroute based on reverse-path
setup.
Flags
0x08 = Fast reroute based on reverse-path alternate LSP. When this
flag is set, all transit LSRs set up an alternate LSP based on the
mechanism specified in this document.
Two new optional objects are required: a REVERSE-LABEL-REQ object in
the RESV message and REVERSE-LABEL object in the PATH message are
used for setting up the reverse segment of the alternate LSP. The
term REVERSE refers to the establishment of an alternate LSP in the
reverse direction of the primary LSP. The function and format of
these objects are similar to the LABEL-REQ and the LABEL object used
to set-up the primary LSP.
When the destination end-point of the primary LSP receives a PATH
message consisting of the SESSION-ATTRIBUTE object, it includes the
optional REVERSE-LABEL-REQ object in the corresponding RESV message
if Fast-reroute is enabled in the SESSION-ATTRIBUTE object. Each LSR
in the path of the primary LSP allocates a label for the reverse
segment of the alternate LSP and stores the label in the PSB for
inclusion in the corresponding PATH message. An LSR that receives a
RESV message with the REVERSE-LABEL-REQ object should allocate and
include the REVERSE-LABEL object in the corresponding PATH message,
unless it is unable to allocate a label in the specified label range
in the REVERSE-LABEL-REQ object. In that case, the LSR should send a
PATHERR message with the appropriate error codes.
REVERSE-LABEL object
REVERSE-LABEL Class = [TBD] C-Type = 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length(bytes) | Class-Num | C-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
// (Object contents) //
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (top label) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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The contents of the REVERSE-LABEL object are a stack of labels, and
the top of the stack is in the right four octets of the contents.
REVERSE-LABEL-REQ object
REVERSE-LABEL-REQ Class = [TBD] C-Type = 1
The format of the REVERSE-LABEL-REQ object is similar to that of the
LABEL-REQ object with the exception of the Class number. Three
possible C-Types are supported: Label request without a label range,
Label request with an ATM label range and a Label request with a
Frame Relay label range.
The source end-point of the LSP allocates a label in the PATH
message for the reverse segment of the alternate LSP, in response to
a label request from its downstream neighbor. This label is used as
the incoming label in its cross-connect table while the outgoing
label used by the source end-point is allocated by its immediate
downstream neighbor in the disjoint segment of the alternate LSP.
The proposed extensions are backward compatible with those LSRs that
do not recognize the optional REVERSE-LABEL_REQ and REVERSE-LABEL
objects.
4. References
[1] Rosen, E. et al., "Multiprotocol Label Switching Architecture",
Internet Draft, draft-ietf-mpls-arch-05.txt, April 1999.
[2] Awduche, D. et al., "Requirements for Traffic Engineering over
MPLS", Internet Draft, draft-ietf-mpls-traffic-eng-00.txt, October
1998.
[3] Haskin, D. et al., "A Method for Setting an Alternate Label-
Switched Paths to Handle Fast Re-route", work in progress, draft-
haskin-fast-reroute-00.txt, June 1999.
[4] Awduche, D. et al., "Extensions to RSVP for LSP tunnels", work
in progress, draft-ietf-mpls-rsvp-lsp-tunnel-01.txt, March 1999.
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5. Authors' Addresses
Ram Krishnan
Nexabit Networks, Inc.
200 Nickerson Road
Marlborough, MA 01752
E-mail: ram@nexabit.com
Dimitry Haskin
Nexabit Networks, Inc.
200 Nickerson Road
Marlborough, MA 01752
E-mail: dhaskin@nexabit.com
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