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draft-vasseur-mpls-loose-path-reopt-01.txt February 2003
Jean-Philippe Vasseur(Editor)
Cisco Systems, Inc.
Yuichi Ikejiri
NTT Communications Corporation
IETF Internet Draft
Expires: August, 2003
February, 2003
draft-vasseur-mpls-loose-path-reopt-01.txt
Reoptimization of an explicit loosely routed MPLS TE paths
Status of this Memo
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026. Internet-Drafts are
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Abstract
The aim of this document is to propose a mechanism for the
reoptimization of loosely routed explicit paths. A loosely routed
explicit path is as a path specified as a combination of strict and
loose hop(s) that contains at least one loose hop and zero or more
strict hop(s). The path calculation (ERO expansion) to reach a loose
hop is made on the previous hop defined in the TE LSP path. This draft
proposes a mechanism that allows:
- the TE LSP Head-end LSR to trigger a reoptimization on every loose
hops along the path,
- an LSR to signal to the TE LSP head-end that a better path exists
to reach a loose than the path in use. A better path is defined as a
path with a lower cost, where the cost is defined by the metric used
to compute the path.
This primarily applies to inter-area TE LSPs and inter-AS TE LSPs when
the path is defined as a list of loose hops (generally the loose hops
are the ABRs) but the following mechanism is also applicable to any
loosely routed explicit paths within a single routing domain.
1. Establishment of an explicit loosely routed TE LSP path
A loosely routed explicit path is as a path specified as a combination
of strict and loose hop(s) that contains at least one loose hop and
zero or more strict hop(s). Loose hops are specified in the ERO object
of the Path message with the L flag of the Ipv4 prefix sub-object set,
as defined in RFC3209. In this case, each LSR along path can perform a
partial route computation to reach the next loose hop and then performs
an ERO expand, before forwarding the RSVP Path message downstream.
Note that the examples in the rest of this draft will be provided in
the context of MPLS inter-area TE but the proposed mechanism also
applies to loosely routed path within a single routing domain.
Furthermore, this mechanism could also be used in the context of
loosely routed paths in the context of TE LSPs spanning several
autonomous systems.
Also examples will be provided with OSPF as the IGP but the mechanisms
similarly apply to IS-IS.
An example of an explicit loosely routed TE LSP signalling (see also
[MULTI-AREA-TE scenario 1]
<---area 1--><-area 0--><-area 2->
R1---R2----R3---R6 R8-----R10
| | | / |\ |
| | | -- | --\ |
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| | |/ | \|
|---R4----R5---R7----R9-----R11
Assumptions
- R3, R5, R8 and R9 are ABRs
- A TE LSP1 from R1 (Head-End LSR) to R11 (Tail-end LSR) is defined
with the following loosely routed path: R1-R3-R8-R11. R3, R8 and R11
are defined as loose hops.
Step 1: the TE LSP 1 Head-end (R1) builds the following ERO object:
R1(S)-R2(S)-R3(S)-R8(L)-R11(L)
where:
S: Strict hop (L=0)
L: Loose hop (L=1)
The R1-R2-R3 path obeys the TE LSP constraint
Step 2: the RSVP Path message is then forwarded by R1 following the ERO
path and reaches R3 with the following content: R8(L)-R11(L)
Step 3: R3 determines that the next hop (R8) is a loose hop (not
directly connected to R3) and then performs an ERO expand operation to
reach the next loose hops R8. The new ERO becomes: R6(S)-R7(S)-R8(S)-
R11(L).
Step 4: the same procedure applies at R8.
...
2. Reoptimization of an explicit loosely routed TE LSP
Once the TE LSP is set up, the TE LSP is maintained through normal RSVP
procedures. Then a more optimal path might appear between an LSR and
its next loose hop (suppose in the example above that a link between R6
and R8 is added that provides a shorter path between R3 and R8 (R3-R6-
R8) than the existing R3-R6-R7-R8 path). Currently if the better path
is not visible from the Head-end LSR, it cannot make use of this better
path and perform a make before break when appropriate. This is for
instance the case in the example above as the better path appears in an
area, which is not the Head-end area.
This draft proposes a mechanism that allows:
- The TE LSP Head-end LSR to trigger on every LSR whose next hop is a
loose hop the re evaluation of the current path in order to detect a
potential more optimal path,
- An LSR whose next hop is a loose-hop to signal (using a new ERROR-
SPEC sub code carried in a Path Error Notify message) to the TE LSP
head-end that a better path exists (a path with a lower cost, where the
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draft-vasseur-mpls-loose-path-reopt-01.txt February 2003
cost is defined by the metric used to compute the path - - see [SEC-
METRIC], [METRIC]).
Then once the existence of a better path is notified to the Head-end it
can perform a make before break.
3. Signalling extensions
3.1. ERO expansion signaling request
For EXPLICIT ROUTE object C-Type 1 and 7, we define a new flag in the
SESSION_ATTRIBUTE object:
ERO expansion request: 0x20
This flag indicates to every LSR having a loose hops specified as their
next hop in the ERO that a new ERO expansion is requested.
3.2. New Path Error sub-code
The format of a Path Error is the following:
<PathErr message> ::= <Common Header> [ <INTEGRITY> ]
<SESSION> <ERROR_SPEC>
[ <POLICY_DATA> ...]
[ <sender descriptor> ]
<sender descriptor> ::= (see earlier definition)
IPv4 ERROR_SPEC object: Class = 6, C-Type = 1
+-------------+-------------+-------------+-------------+
| IPv4 Error Node Address (4 bytes) |
+-------------+-------------+-------------+-------------+
| Flags | Error Code | Error Value |
+-------------+-------------+-------------+-------------+
Various Error Codes and Error values have been defined in RFC2205 and
RFC3209.
The ERROR-CODE 25 corresponds to a Path Error - Notify Error. We
propose to add two new sub-codes:
4 Better path exists
5 No better path
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4. Mode of operation
4.1. TE LSP reroute
The TE LSP reroute (make before break) is always considered as being
performed on the Head-end LSR.
4.2. Reoptimization triggers
There are two possible reoptimization triggers:
- timer-based: a reoptimization is triggered (look for a more optimal
path) when a configurable timer expires,
- event-driven: a reoptimization is triggered when a particular event
happens (such as a ''Link-UP'' event).
4.3. Head-end reoptimization request versus mid-point reoptimization
indication
The need for reoptimization (a better path exists) of an explicit
loosely routed TE LSP can be either:
- requested by the Head-end LSR,
- determined by any mid-point LSR whose next hop is a loose hop
having detected that a better path (than the existing path)
exists.
4.3.1. Head-end reoptimization request
In this mode, when a timer-based reoptimization is triggered on the
head-end LSR (reoptimization timer has fired) or the operator manually
requests a reoptimization, the head-end LSR immediately sends a Path
message with the ''ERO expansion request'' bit of the SESSION_ATTRIBUTE
object set.
Upon receiving a Path message with the ''ERO expansion request'' bit of
the SESSION_ATTRIBUTE object set, every LSR, for which the next
abstract node contained in the ERO is defined as a loose hop, must
perform a new ERO expansion (path re evaluation):
- if a better path can be found to reach the next loose hop
(than the path currently in use), the LSR must immediately send
a Path Error to the head-end (Error code 25 (Notify), sub-
code=4 (better path exists)),
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- if no better path can be found, the LSR must send a Path
Error to the head-end (Error code 25 (Notify), sub-code=5 (no
better path))
By better path, we mean a path having a lower cost. By default, an LSR
uses the IGP metric in their CSPF to detect the shortest path that
obeys a set of constraints. Note that the head-end might use the
METRIC-TYPE object (defined in [PATH-COMP]) in its path message to
request the LSR having a next hop defined as a loose hop in the ERO to
use the TE metric to determine the best path.
Let call Ln the list of LSRs defined as loose hops in the ERO sent in
the Path message by the Head-end LSR: Ln=<l1, l2, ..., ln>. Let's now
call Pn=<p1, p2, ..., pn> the list of LSRs pi such that li is a next
(loose) hop of pi for i=1...n
Example 2:
<---area 1--><-area 0--><-area 2->
R1---R2----R3---R6 R8-----R10
| | | / |\ |
| | | -- | --\ |
| | |/ | \|
|---R4----R5---R7----R9-----R11
A TE LSP1 from R1 (Head-End LSR) to R11 (Tail-end LSR) is defined with
the following loosely routed path: R1-R3-R8-R11. R3, R8 and R11 are
defined as loose hops.
Ln=<R3,R8,R11>
Pn=<R1,R3,R8>
As soon as a positive response is received from an LSR pi (sub-code=4,
''Better path exists''), the Head-end LSR must:
- clear the ''ERO expansion request'' bit,
- perform a make before break
If no positive response is received then the Head-end LSR must keep
refreshing the current TE LSP.
The Head-end LSR SHOULD clear the ''ERO expansion request'' bit as soon
as:
- at least a Path Error containing a positive reply (better path
exists) is received,
or
- all LSRs <p1, ..pn> have sent a negative reply (not better path),
or
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- K consecutive Path messages with the ''ERO expansion request'' bit set
have been sent.
A recommended value for K is 4.
4.3.2. Mid-point reoptimization indication
In this mode, an LSR whose next abstract node is a loose hop can
locally trigger an ERO expansion (when a configurable timer expires or
on event-driven basis (link-up event for example)). If a better path is
found compared to the existing one, the LSR sends a Path Error to the
head-end (Error code 25 (Notify), sub-code=4 (better path exists)). The
Head-end LSR must then immediately perform a make before break.
Note that those modes are not exclusive: both the timer and even-driven
reoptimization triggers can be implemented on the Head-end and/or any
mid-point LSR with potentially different timer values for the timer
driven reoptimization case.
4.3.3. ERO caching
Once a mid-point LSR has determined that a better path exists (after a
reoptimization request has been received by the Head-end LSR or the
reoptimization timer on the mid-point has fired), the more optimal path
MAY be cached on the mid-point for a limited amount of time to avoid
having to recompute a route once the Head-LSR performs a make before
break.
5. Interoperability
An LSR non supporting the ''ERO expansion request'' bit of the SESSION-
ATTRIBUTE object should just ignore it.
Any Head-end LSR non supporting this draft receiving a Path Error
Notify message with sub-code = 4 or 5 should just ignore the Path
message.
6. Security Considerations
The practice described in this draft does not raise specific security
issues beyond those of existing TE.
7. Acknowledgment
The authors would like to thank Carol Iturralde, Miya Kohno and
Francois Le Faucheur for their useful and valuable comments.
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8. Intellectual Property
The contributor represents that he has disclosed the existence of any
proprietary or intellectual property rights in the contribution that
are reasonably and personally known to the contributor. The
contributor does not represent that he personally knows of all
potentially pertinent proprietary and intellectual property rights
owned or claimed by the organization he represents (if any) or third
parties.
References
[TE-REQ] Awduche et al, Requirements for Traffic Engineering over MPLS,
RFC2702, September 1999.
[OSPF-TE] Katz, Yeung, Traffic Engineering Extensions to OSPF, draft-
katz-yeung-ospf-traffic-05.txt, June 2001.
[ISIS-TE] Smit, Li, IS-IS extensions for Traffic Engineering, draft-
ietf-isis-traffic-03.txt, June 2001.
[RSVP-TE] Awduche et al, "RSVP-TE: Extensions to RSVP for LSP Tunnels",
RFC3209, December 2001.
[METRICS] Fedyk et al, ''Multiple Metrics for Traffic Engineering with
IS-IS and OSPF'', draft-fedyk-isis-ospf-te-metrics-01.txt, November
2000.
[DS-TE] Le Faucheur et al, ''Requirements for support of Diff-Serv-aware
MPLS Traffic Engineering'', draft-ietf-tewg-diff-te-reqts-01.txt, June
2001.
[MULTI-AREA-TE] Kompella at all, ''Multi-area MPLS Traffic Engineering'',
draft-kompella-mpls-multiarea-te-03.txt, June 2002.
[PATH-COMP] Vasseur et al, ''RSVP Path computation request and reply
messages'', draft-vasseur-mpls-computation-rsvp-03.txt, November 2001.
[SEC-METRIC] Le Faucheur et all,'' Use of Interior Gateway Protocol
(IGP) Metric as a second MPLS Traffic Engineering Metric'', draft-ietf-
tewg-te-metric-igp-02.txt, September, 2002.
[INTER-AS-TE-REQS] Zhang et al, ''MPLS Inter-AS Traffic Engineering
requirements'', draft-tewg-interas-te-req-01.txt (work in progress).
[INTER-AS-TE] Vasseur and Zhang, ''MPLS Inter-AS Traffic Engineering'',
draft-vasseur-inter-as-te-00.txt, February 2003, work in progress.
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Authors' addresses:
Jean Philippe Vasseur
Cisco Systems, Inc.
300 Apollo Drive
Chelmsford, MA 01824
USA
Email: jpv@cisco.com
Yuichi Ikejiri
NTT Communications Corporation
1-1-6, Uchisaiwai-cho, Chiyoda-ku
Tokyo 100-8019
JAPAN
Email: y.ikejiri@ntt.com
Vasseur and Ikejiri 9
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