[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: (draft-vasseur-ccamp-inter-domain-pd-path-comp) 00 01 02 03 04 05 06 RFC 5152

CCAMP Working Group                                 JP Vasseur (Editor)
IETF Internet draft                                 Cisco Systems, Inc.
Proposed Status: Standard                       Arthi Ayyangar (Editor)
                                                Juniper Networks
                                                  Raymond Zhang
                                            Infonet Service Corporation

Expires: October 2005
                                                             April 2005


         draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt


A Per-domain path computation method for computing Inter-domain Traffic
            Engineering (TE) Label Switched Path (LSP)



Status of this Memo

By submitting this Internet-Draft, I certify that any applicable patent
or IPR claims of which I am aware have been disclosed, and any of which
I become aware will be disclosed, in accordance with RFC 3668.

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026. Internet-Drafts are
Working documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups.  Note that other groups may also
distribute working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.

Abstract

This document specifies a per-domain path computation method for
computing inter-domain Traffic Engineering (TE) Multiprotocol Label
Switching (MPLS) and Generalized MPLS (GMPLS) Label Switched (LSP)
paths. In this document a domain is referred to as a collection of
network elements within a common sphere of address management or path
computational responsibility such as IGP areas and Autonomous Systems.


 Vasseur, Ayyangar and Zhang                                   1



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


The principle of per-domain path computation specified in this document
comprises of a GMPLS or MPLS node along an inter-domain LSP path,
computing a path up to the last reachable hop within its domain. It
covers cases where this last hop (domain exit point) may already be
specified in the signaling message as well the case where this last hop
may need to be determined by the GMPLS node.

Conventions used in this document

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 RFC 2119 [RFC2119].

Table of content

1. Terminology ............................................. 3
2. Introduction ............................................ 4
3. General assumptions ..................................... 5
4. Per-Domain path computation algorithm ................... 8
4.1 Example with an inter-area TE LSP ...................... 9
4.1.1 Case 1: T1 is a contiguous TE LSP .................... 9
4.1.2 Case 2: T1 is a stitched or nested TE LSP ............ 10
4.2 Example with an inter-AS TE LSP ........................ 11
4.2.1  Case 1: T1 is a contiguous TE LSP ................... 12
4.2.2  Case 2: T1 is a stitched or nested TE LSP ........... 13
5 Path optimality/diversity ................................ 13
6  MPLS Traffic Engineering Fast Reroute for inter-domain
   TE LSPs ................................................. 13
6.1 Failure of an internal network element ................. 14
6.2 Failure of an inter-ASBR links (inter-AS TE) ........... 14
6.3 Failure of an ABR or an ASBR node ...................... 14
7. Reoptimization of an inter-domain TE LSP ................ 14
7.1 Contiguous TE LSPs ..................................... 14
7.2 Stitched or nested (non-contiguous) TE LSPs ............ 15
8. Security Considerations ................................. 16
9. Intellectual Property Considerations .................... 17
10 References .............................................. 17
10.1 Normative references .................................. 17
10.2 Informative references ................................ 18












 Vasseur, Ayyangar and Zhang                                   2



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005



1.     Terminology


ABR Routers: routers used to connect two IGP areas (areas in OSPF or
L1/L2 in IS-IS)

Boundary LSR: a boundary LSR is either an ABR in the context of inter-
area MPLS TE or an ASBR in the context of inter-AS MPLS TE.

Bypass Tunnel: an LSP that is used to protect a set of LSPs passing
over a common facility.

CSPF: Constraint-based Shortest Path First.

Fast Reroutable LSP: any LSP for which the "Local protection desired"
bit is set in the Flag field of the SESSION_ATTRIBUTE object of its
Path messages or signaled with a FAST-REROUTE object.

Interconnect routers or ASBR routers: routers used to connect together
ASes of a different or the same Service Provider via one or more Inter-
AS links.

Inter-AS MPLS TE LSP: A TE LSP whose head-end LSR and tail-end LSR do
not reside within the same Autonomous System (AS), or whose head-end
LSR and tail-end LSR are both in the same AS but the TE  LSPÆs path
may be across different ASes. Note that this definition also applies to
TE LSP whose Head-end and Tail-end LSRs reside in different sub-ASes
(BGP confederations).

Inter-area MPLS TE LSP: A TE LSP where the head-end LSR and tail-end
LSR do not reside in the same area or both the head-end and tail end
LSR reside in the same area but the TE LSP transits one or more
different areas along the path.

LSR: Label Switch Router

LSP: MPLS Label Switched Path

Local Repair: local protection techniques used to repair TE LSPs
quickly when a node or link along the LSPs path fails.

MP: Merge Point. The LSR where bypass tunnels meet the protected LSP.

NHOP Bypass Tunnel: Next-Hop Bypass Tunnel. A backup tunnel which
bypasses a single link of the protected LSP.

NNHOP Bypass Tunnel: Next-Next-Hop Bypass Tunnel. A backup tunnel which
bypasses a single node of the protected LSP.


 Vasseur, Ayyangar and Zhang                                   3



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


PCE: Path Computation Element. An LSR in charge of computing TE LSP
path for which it is not the Head-end. For instance, an ABR (inter-
area) or an ASBR (Inter-AS) can play the role of PCE.

PCC: Path Computation Client (any head-end LSR) requesting a path
computation from the Path Computation Element.

Protected LSP: an LSP is said to be protected at a given hop if it has
one or multiple associated backup tunnels originating at that hop.

PLR: Point of Local Repair. The head-end of a bypass tunnel.

TED: MPLS Traffic Engineering Database

The notion of contiguous, stitched and nested TE LSPs is defined in
[INTER-DOMAIN-SIG] and [LSP-STITCHING] and will not be repeated here.

2.     Introduction

The requirements for inter-area and inter-AS MPLS Traffic Engineering
have been developed by the Traffic Engineering Working Group and have
been stated in [INTER-AREA-REQS] and [INTER-AS-REQS] respectively. The
framework for inter-domain MPLS Traffic Engineering has been provided
in [INTER-DOMAIN-FRAMEWORK].

The set of mechanisms to establish and maintain inter-domain TE LSPs
are specified in [INTER-DOMAIN-SIG] and [LSP-STITCHING].

This document exclusively focuses on the path computation aspects and
defines a method for computing inter-domain TE LSP paths where each
node in charge of computing a section of an inter-domain TE LSP path is
always along the path of such LSP.

When the visibility of an end to end complete path spanning multiple
domains is not available at the head end node, one approach described
in the document consists of using a per-domain path computation scheme
used during LSP setup to determine the inter-domain LSP path as it
traverses multiple domains.

Note that the mechanisms proposed in this document could also be
applicable to MPLS TE domains other than areas and ASes.

According to the wide set of requirements defined in [INTER-AS-TE-REQS]
and [INTER-AREA-TE-REQS], coming up with a single solution covering all
the requirements is certainly possible but may not be desired: indeed,
as described in [INTER-AS-TE-REQS] the spectrum of deployment scenarios
is quite large and designing a solution addressing the super-set of all
the requirements would lead to providing a rich set of mechanisms not
required in several cases. Depending on the deployment scenarios of a
SP, certain requirements stated above may be strict while certain other
requirements may be relaxed.

 Vasseur, Ayyangar and Zhang                                   4



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005



There are different ways in which inter-domain TE LSP path computation
may be performed. For example, if the requirement is to get an end-to-
end constraint-based shortest path across multiple domains, then a
mechanism using one or more distributed PCEs could be used to compute
the shortest path across different domains. Alternatively, one could
also use some static or discovery mechanisms to determine the next
boundary LSR per domain as the inter-domain TE LSP is being signaled.
Other offline mechanisms for path computation are not precluded either.
Depending on the Service ProviderÆs requirements, one may adopt either
of these techniques for inter-domain path computation.

Note that the adequate path computation method may be chosen based upon
the TE LSP characteristics and requirements. This document specifies an
inter-domain path computation technique based on per-domain path
computation and could be used in place or in conjunction with other
techniques.

3.     General assumptions

In the rest of this document, we make the following set of assumptions:

1)  Common assumptions

- Each domain in all the examples below is assumed to be capable of
doing Traffic Engineering (i.e. running OSPF-TE or ISIS-TE and RSVP-
TE). A domain may itself comprise multiple other domains. E.g. An AS
may itself be composed of several other sub-AS(es) (BGP confederations)
or areas/levels.

- The inter-domain LSPs are signaled using RSVP-TE ([RSVP-TE]).

- The path (ERO) for the inter-domain TE LSP traversing multiple
areas/ASes may be signaled as a set of (loose and/or strict) hops. The
hops may identify:
      - The complete strict path end to end across different
      areas/ASes
      - The complete strict path in the source area/AS followed by
      boundary LSRs (and domain identifiers, e.g. AS numbers)
      - The complete list of boundary LSRs along the path
      - The current boundary LSR and the LSP destination

In this case, the set of (loose or strict) hops can either be
statically configured on the Head-end LSR or dynamically computed. In
the former case, the resulting path is statically configured on the
Head-end LSR. In the latter case (dynamic computation), a per-domain
path computation method is defined in this document with some Auto-
discovery mechanism based on BGP and/or IGP information yielding the
next-hop boundary node (domain exit point, say ABR/ASBR) along the path
as the LSP is being signaled, along with crankback mechanisms. Note

 Vasseur, Ayyangar and Zhang                                   5



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


that alternatively next-hop may be statically configured on the Head-
end LSR in which case next-hop auto-discovery would not be needed.

- Furthermore, the boundary LSRs are assumed to be capable of
performing local path computation for expansion of a loose next-hop in
the signaled ERO if the path is not signaled by the head-end LSR as a
set of strict hops or if the strict hop is an abstract node (e.g. an
AS). This can be done by performing a CSPF computation up to that next
loose hop as opposed to the TE LSP destination or by making use of some
PCEs. In any case, no topology or resource information needs to be
distributed between areas/ASes (as mandated per [INTER-AREA-REQS] and
[INTER-AS-REQS]), which is critical to preserve IGP/BGP scalability and
confidentiality in the case of TE LSPs spanning multiple routing
domains.

Note that PCE-based path computation may be mentioned in this document
for the sake of reference but such techniques are outside the scope of
this document.

- The paths for the intra-domain  FA-LSPs or LSP segments or for a
contiguous TE LSP within the area/AS, may be pre-configured or computed
dynamically based on the arriving inter-domain LSP setup request;
depending on the requirements of the transit area/AS. Note that this
capability is explicitly specified as a requirement in [INTER-AS-TE-
REQS]. When the paths for the FA-LSPs/LSP segments are pre-configured,
the constraints as well as other parameters like local protection
scheme for the intra-area/AS FA-LSP/LSP segment are also pre-
configured.

- While certain constraints like bandwidth can be used across different
areas/ASes, certain other TE constraints like resource affinity, color,
metric, etc. as listed in [RFC2702] could be translated at areas/ASes
boundaries. If required, it is assumed that, at the area/AS boundary
LSRs, there will exist some sort of local mapping based on offline
policy agreement, in order to translate such constraints across area/AS
boundaries. It is expected that such an assumption particularly applies
to inter-AS TE: for example, the local mapping would be similar to the
Inter-AS TE Agreement Enforcement Polices stated in [INTER-AS-TE-REQS].

2) Example of topology for the inter-area TE case

The following example will be used for the inter-area TE case in this
document.

<--area1--><---area0---><----area2----->
 ------ABR1------------ABRÆ1-------
 |    /   |              |  \     |
R0--X1    |              |   X2---X3--R1
 |        |              |  /     |
 -------ABR2-----------ABRÆ2------
<=========== Inter-area TE LSP =======>

 Vasseur, Ayyangar and Zhang                                   6



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005



Assumptions

- ABR1, ABR2, ABRÆ1 and ABRÆ2 are ABRs,
- X1: an LSR in area 1,
- X2, X3: LSRs in area 2,
- An inter-area TE LSP T0 originated at R0 in area1 and terminating at
R1 in area2,

Notes:
- The terminology used in the example above corresponds to OSPF but the
path computation methods proposed in this document equally applies to
the case of an IS-IS multi-levels network.
- Just a few routers in each area are depicted in the diagram above for
the sake of simplicity.

3) Example of topology for the inter-AS TE case:

We will consider the following general case, built on a superset of the
various scenarios defined in [INTER-AS-TE-REQS]:


     <-- AS 1 ---> <------- AS 2 -----><--- AS 3 ---->

               <---BGP--->            <---BGP-->
CE1---R0---X1-ASBR1-----ASBR4ù-R3---ASBR7-ù--ASBR9----R6
      |\     \ |       / |   / |   / |          |      |
      | \     ASBR2---/ ASBR5  | --  |          |      |
      |  \     |         |     |/    |          |      |
      R1-R2ù--ASBR3ù----ASBR6ù-R4---ASBR8ù---ASBR10ù--R7---CE2

      <======= Inter-AS TE LSP(LSR to LSR)===========>
or

<======== Inter-AS TE LSP (CE to ASBR =>

or

<================= Inter-AS TE LSP (CE to CE)===============>             Formatted:

The diagram above covers all the inter-AS TE deployment cases described
in [INTER-AS-TE-REQS].

Assumptions:

- Three interconnected ASes, respectively AS1, AS2, and AS3. Note that
AS3 might be AS1 in some scenarios described in [INTER-AS-TE-REQS],

- The various ASBRs are BGP peers, without any IGP running on the
single hop links interconnecting the ASBRs and also referred to as
inter-ASBR links,

 Vasseur, Ayyangar and Zhang                                   7



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005



- Each AS runs an IGP (IS-IS or OSPF) with the required IGP TE
extensions (see [OSPF-TE] and [IS-IS-TE]). In other words, the ASes are
TE enabled,

- Each AS can be made of several IGP areas. The path computation
techniques described in this document applies to the case of a single
AS made of multiple IGP areas, multiples ASes made of a single IGP
areas or any combination of the above. For the sake of simplicity, each
routing domain will be considered as single area in this document.

- An inter-AS TE LSP T1 originated at R0 in AS1 and terminating at R6
in AS3.

4.     Per-domain path computation algorithm

Regardless of the nature of the inter-domain TE LSP (contiguous,
stitched or nested), a similar set of mechanisms for local TE LSP path
computation (next hop resolution) can be used.

When an ABR/ASBR receives a Path message with a loose next-hop or an
abstract node in the ERO, then it carries out the following actions:

1) It checks if the loose next-hop is accessible via the TED. If the
loose next-hop is not present in the TED, then it checks if the next-
hop at least has IP reachability (via IGP or BGP). If the next-hop is
not reachable, then the path computation stops and the LSR sends back a
PathErr upstream. If the next-hop is reachable, then it finds an
ABR/ASBR to get to the next-hop. In the absence of an auto-discovery
mechanism, the ABR in the case of inter-area TE or the ASBR in the
next-hop AS in the case of inter-AS TE should be the loose next-hop in
the ERO and hence should be accessible via the TED, otherwise the path
computation for the inter-domain TE LSP will fail.

2) If the next-hop boundary LSR is present in the TED.

      a) Case of a contiguous TE LSP. The ABR/ASBR just performs an
      ERO expansion (unless not allowed by policy) after having
      computed the path to the next loose hop (ABR/ASBR) that obeys
      the set of required constraints. If no path satisfying the set
      of constraints can be found, the path computation stops and a
      Path Error MUST be sent for the inter-domain TE LSP.

      b) Case of stitched or nested LSP

             i) if the ABR/ASBR (receiving the LSP setup request) is
             a candidate LSR for intra-area FA-LSP/LSP segment
             setup, and if there is no FA-LSP/LSP segment from this
             LSR to the next-hop boundary LSR (satisfying the
             constraints) it SHOULD signal a FA-LSP/LSP segment to
             the next-hop boundary LSR. If pre-configured FA-LSP(s)

 Vasseur, Ayyangar and Zhang                                   8



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


             or LSP segment(s) already exist, then it SHOULD try to
             select from among those intra-area/AS LSPs. Depending
             on local policy, it MAY signal a new FA-LSP/LSP segment
             if this selection fails. If the FA-LSP/LSP segment is
             successfully signaled or selected, it propagates the
             inter-domain Path message to the next-hop following the
             procedures described in [LSP-HIER]. If, for some reason
             the dynamic FA-LSP/LSP segment setup to the next-hop
             boundary LSR fails, the path computation stops and a
             PathErr is sent upstream for the inter-domain LSP.
             Similarly, if selection of a preconfigured FA-LSP/LSP
             segment fails and local policy prevents dynamic FA-
             LSP/LSP segment setup, then the path computation stops
             and a PathErr is sent upstream for the inter-domain TE
             LSP.

             ii) If, however, the boundary LSR is not a FA-LSP/LSP
             segment candidate, then it SHOULD simply compute a CSPF
             path up to the next-hop boundary LSR carry out an ERO
             expansion to the next-hop boundary LSR) and propagate
             the Path message downstream. The outgoing ERO is
             modified after the ERO expansion to the loose next-hop.

             Note that in both cases, path computation may be
             stopped due to some local policy.


4.1.   Example with an inter-area TE LSP

4.1.1.  Case 1: T1 is a contiguous TE LSP

When the path message reaches ABR1, it first determines the egress LSR
from its area 0 along the LSP path (say ABRÆ1), either directly from
the ERO (if for example the next hop ABR is specified as a loose hop in
the ERO) or by using some constraint-aware auto-discovery mechanism. In
the former case, every inter-AS TE LSP path is defined as a set of
loose and strict hops but at least the ABRs traversed by the inter-area
TE LSP MUST be specified as loose hops on the head-End LSR.

- Example 1 (set of strict hops end to end): R0-X1-ABR1-ABRÆ1-X2-X3-R1
- Example 2 (set of loose hops): R0-ABR1(loose)-ABRÆ1(loose)-R1(loose)
- Example 3 (mix of strict and loose hops): R0-X1-ASBR1-ABRÆ1(loose)-
X2-X3-R1

At least, the set of ABRs from the TE LSP head-end to the tail-End MUST
be present in the ERO as a set of loose hops. Optionally, a set of
paths can be configured on the head-end LSR, ordered by priority. Each
priority path can be associated with a different set of constraints.
Typically, it might be desirable to systematically have a last resort
option with no constraint to ensure that the inter-area TE LSP could
always be set up if at least a path exists between the inter-area TE

 Vasseur, Ayyangar and Zhang                                   9



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


LSP source and destination. Note that in case of set up failure or when
an RSVP Path Error is received indicating the TE LSP has suffered a
failure, an implementation might support the possibility to retry a
particular path option a specific amount of time (optionally with
dynamic intervals between each trial) before trying a lower priority
path option. Any path can be defined as a set of loose and strict hops.
In other words, in some cases, it might be desirable to rely on the
dynamic path computation in some area, and exert a strict control on
the path in other areas (defining strict hops).

Once it has computed the path up to the next ABR, ABR1 sends the Path
message for the inter-area TE LSP to ABRÆ1. ABRÆ1 then repeats the a
similar procedure and the Path message for the inter-area TE LSP will
reach the destination R1. If ABRÆ1 cannot find a path obeying the set
of constraints for the inter-area TE LSP, the path computation stops
and ABRÆ1 MUST send a PathErr message to ABR1. Then ABR1 can in turn
triggers a new computation by selecting another egress boundary LSR
(ABRÆ2 in the example above) if crankback is allowed for this inter-
area TE LSP (see [CRANBACK]). If crankback is not allowed for that
inter-area TE LSP or if ABR1 has been configured not to perform
crankback, then ABR1 MUST stop any path computation for the TE LSP and
MUST forward a PathErr up to the head-end LSR (R0) without trying to
select another egress LSR.

4.1.2.  Case 2: T1 is a stitched or nested TE LSP

When the path message reaches ABR1, ABR1 first determines the egress
LSR from its area 0 along the LSP path (say ABRÆ1), either directly
from the ERO or by using some constraint-aware auto-discovery
mechanism.

ABR1 will check if it has a FA-LSP or LSP segment to ABRÆ1 matching the
constraints carried in the inter-area TE LSP Path message. If not, ABR1
will compute the path for a FA-LSP or LSP segment from ABR1 to ABRÆ1
satisfying the constraint and will set it up accordingly. Note that the
FA-LSP or LSP segment could have also been pre-configured.

Once the ABR has selected the FA-LSP/LSP segment for the inter-area
LSP, using the signaling procedures described in [LSP-HIER], ABR1 sends
the Path message for inter-area TE LSP to ABRÆ1. Note that irrespective
of whether ABR1 does nesting or stitching, the Path message for the
inter-area TE LSP is always forwarded to ABRÆ1. ABRÆ1 then repeats the
exact same procedures and the Path message for the inter-area TE LSP
will reach the destination R1. If ABRÆ1 cannot find a path obeying the
set of constraints for the inter-area TE LSP, then ABRÆ1 MUST send a
PathErr message to ABR1. Then ABR1 can in turn either select another
FA-LSP/LSP segment to ABRÆ1 if such an LSP exists or select another
egress boundary LSR (ABRÆ2 in the example above) if crankback is
allowed for this inter-area TE LSP (see [CRANBACK]). If crankback is
not allowed for that inter-area TE LSP or if ABR1 has been configured
not to perform crankback, then ABR1 MUST forward a PathErr up to the

 Vasseur, Ayyangar and Zhang                                  10



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


inter-area head-end LSR (R0) without trying to select another egress
LSR.

4.2.   Example with an inter-AS TE LSP

The procedures for establishing an inter-AS TE LSP are very similar to
those of an inter-area TE LSP described above. The main difference is
related to the presence of inter-ASBRs link(s).

The links interconnecting ASBRs are usually not TE enabled and no IGP
is running at the AS boundaries. An implementation supporting inter-AS
MPLS TE MUST obviously allow the set up of inter-AS TE LSP over the
region interconnecting multiple ASBRs. In other words, an ASBR
compliant with this document MUST support the set up of TE LSP over
ASBR to ASBR links, performing all the usual operations related to MPLS
Traffic Engineering (call admission control, à) as defined in [RSVP-
TE].

In term of computation of an inter-AS TE LSP path, an interesting
optimization consists of allowing the ASBRs to flood the TE information
related to the inter-ASBR link(s) although no IGP TE is enabled over
those links (and so there is no IGP adjacency over the inter-ASBR
links). This of course implies for the inter-ASBR links to be TE-
enabled although no IGP is running on those links. This allows a head-
end LSR to make a more appropriate route selection up to the first ASBR
in the next hop AS and will significantly reduce the number of
signaling steps in route computation. This also allows the entry ASBR
in an AS to make a more appropriate route selection up to the entry
ASBR in the next hop AS taking into account constraints associated with
the ASBR-ASBR links. Moreover, this reduces the risk of call set up
failure due to inter-ASBR links not satisfying the inter-AS TE LSP set
of constraints. Note that the TE information is only related to the
inter-ASBR links: the TE LSA/LSP flooded by the ASBR includes not only
the TE-enabled links contained in the AS but also the inter-ASBR links.

Note that no summarized TE information is leaked between ASes which is
compliant with the requirements listed in [INTER-AREA-TE-REQS] and
[INTER-AS-TE-REQS].

Example:

               <---BGP--->            <---BGP-->
CE1---R0---X1-ASBR1-----ASBR4ù-R3---ASBR7-ù--ASBR9---R6
      |\     \ |       / |   / |   / |          |     |
      | \     ASBR2---/ ASBR5  | --  |          |     |
      |  \     |         |     |/    |          |     |
      R1-R2ù--ASBR3ù----ASBR6ù-R4---ASBR8ù---ASBR10---R7---CE2

For instance, in the diagram depicted above, when ASBR1 floods its IGP
TE LSA (opaque LSA for OSPF)/LSP (TLV 22 for IS-IS) in its routing

 Vasseur, Ayyangar and Zhang                                  11



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


domain, it reflects the reservation states and TE properties of the
following links: X1-ASBR1, ASBR1-ASBR2 and ASBR1-ASBR4.

Thanks to such an optimization, the inter-ASBRs TE link information
corresponding to the links originated by the ASBR is made available in
the TED of other LSRs in the same area/AS that the ASBR belongs to.
Consequently, the CSPF computation for an inter-AS TE LSP path can also
take into account the inter-ASBR link(s). This will improve the chance
of successful path computation up to the next AS in case of a
bottleneck on some inter-ASBR links and it potentially reduces one
level of crankback. Note that no topology information is flooded and
these links are not used in IGP SPF computations. Only the TE
information for the links originated by the ASBR is advertised.

4.2.1.  Case 1: T1 is a contiguous TE LSP

The inter-AS TE path may be configured on the head-end LSR as a set of
strict hops, loose hops or a combination of both.

- Example 1 (set of strict hops end to end): R0-X1-ASBR1-ASBR4-ASBR5-
R3-ASBR7-ASBR9-R6
- Example 2 (set of loose hops): R0-ASBR4(loose)-ASBR9(loose)-R6(loose)
- Example 3 (mix of strict and loose hops): R0-R2-ASBR3-ASBR2-ASBR1-
ASBR4(loose)-ASBR10(loose)-ASBR9-R6

When a next hop is a loose hop, a dynamic path calculation (also called
ERO expansion) is required taking into account the topology and TE
information of its own AS and the set of TE LSP constraints. In the
example 1 above, the inter-AS TE LSP path is statically configured as a
set of strict hops; thus, in this case, no dynamic computation is
required. Conversely, in the example 2, a per-AS path computation is
performed, respectively on R0 for AS1, ASBR4 for AS2 and ASBR9 for AS3.
Note that when an LSR has to perform an ERO expansion, the next hop
must either belong to the same AS, or must be the ASBR directly
connected to the next hops AS. In this later case, the ASBR
reachability MUST be announced in the IGP TE LSA/LSP originated by its
neighboring ASBR. Indeed, in the example 2 above, the TE LSP path is
defined as: R0-ASBR4(loose)-ASBR9(loose)-R6(loose). This implies that
R0 must compute the path from R0 to ASBR4, hence the need for R0 to get
the TE reservation state related to the ASBR1-ASBR4 link (flooded in
AS1 by ASBR1). In addition, ASBR1 MUST also announce the IP address of
ASBR4 specified in the T1 path configuration.

If an auto-discovery mechanism is available, every LSR receiving an
RSVP Path message, will have to determine automatically the next hop
ASBR, based on the IGP/BGP reachability of the TE LSP destination. With
such a scheme, the head-end LSR and every downstream ASBR loose hop
(except the last loose hop that computes the path to the final
destination) automatically computes the path up to the next ASBR, the
next loose hop based on the IGP/BGP reachability of the TE LSP
destination. If a particular destination is reachable via multiple

 Vasseur, Ayyangar and Zhang                                  12



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


loose hops (ASBRs), local heuristics may be implemented by the head-end
LSR/ASBRs to select the next hop an ASBR among a list of possible
choices (closest exit point, metric advertised for the IP destination
(ex: OSPF LSA External - Type 2), local policy,...). Once the next ASBR
has been determined, an ERO expansion is performed as in the previous
case.

Once it has computed the path up to the next ASBR, ASBR1 sends the Path
message for the inter-area TE LSP to ASBR4 (supposing that ASBR4 is the
selected next hop ASBR). ASBR4 then repeats the exact same procedures
and the Path message for the inter-AS TE LSP will reach the destination
R1. If ASBR4 cannot find a path obeying the set of constraints for the
inter-AS TE LSP, then ASBR4 MUST send a PathErr message to ASBR1. Then
ASBR1 can in turn either select another ASBR (ASBR5 in the example
above) if crankback is allowed for this inter-AS TE LSP (see
[CRANBACK]). If crankback is not allowed for that inter-AS TE LSP or if
ASBR1 has been configured not to perform crankback, then ABR1 MUST stop
the path computation and MUST forward a PathErr up to the head-end LSR
(R0) without trying to select another egress LSR. In this case, the
head-end LSR can in turn select another sequence of loose hops, if
configured. Alternatively, the head-end LSR may decide to retry the
same path; this can be useful in case of set up failure due an outdated
IGP TE database in some downstream AS. An alternative could also be for
the head-end LSR to retry to same sequence of loose hops after having
relaxed some constraint(s).

4.2.2.  Case 2: T1 is a stitched or nested TE LSP

The signaling procedures are very similar to the inter-area LSP setup
case described earlier. In this case, the FA-LSPs or LSP segments will
only be originated by the ASBRs at the entry to the AS.

5.     Path optimality/diversity

Since the inter-domain path is computed on a per domain (area, AS)
basis, one cannot guarantee that the shortest inter-domain path can be
found.

Moreover, computing two diverse paths might not be possible in some
topologies (due to the well-known ôtrappingö problem).

As already pointed out, the required path computation method can be
selected by the Operator on a per LSP basis.

6.     MPLS Traffic Engineering Fast Reroute for inter-domain TE LSPs

The signaling aspects of Fast Reroute and related constraints for each
TE LSP types in the case of inter-domain TE LSP has been covered in
[INTER-DOMAIN-SIG] and will not be repeated in this document.


 Vasseur, Ayyangar and Zhang                                  13



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


There are multiple failure scenarios to consider in the case of Fast
Reroute for inter-domain TE LSPs.

6.1.   Failure of an internal network element

The case of a failure of a network element within an area/AS such as a
link, SRLG or a node does not differ from Fast Reroute for intra-domain
TE LSP.

6.2.   Failure of an inter-ASBR links (inter-AS TE)

In order to protect inter-domain TE LSPs from the failure of an inter-
ASBR link, this requires the computation of a backup tunnel path that
crosses an non IGP TE-enabled region (between two ASes). If the inter-
ASBR TE related information is flooded in the IGPs, each ASBR is
capable of computing the path according to the backup tunnel
constraints. Otherwise, the backup tunnel path MUST be statically
configured.

6.3.   Failure of an ABR or an ASBR node

The constraints to be taken into account during the backup tunnel path
computation significantly differs upon the TE LSP type, network element
to protect (entry/exit boundary node) and the Fast Reroute method in
use (facility backup versus one-to-one). Those constraints have been
explored in detail in [INTER-DOMAIN-SIG] but since the backup tunnel is
itself an inter-domain TE LSP, its path computation can be performed
according to the two path computation methods described in this
document.

7.     Reoptimization of an inter-domain TE LSP

The ability to reoptimize an existing inter-domain TE LSP path is of
course a requirement. The reoptimization process significantly differs
based upon the nature of the TE LSP and the mechanism in use for the TE
LSP path computation.

The following mechanisms can be used for re-optimization, which are
dependent on the nature of the inter-domain TE LSP.

7.1.   Contiguous TE LSPs

After an inter-AS TE LSP has been set up, a more optimal route might
appear in the various traversed ASes. Then in this case, it is
desirable to get the ability to reroute an inter-AS TE LSP in a non-
disruptive fashion (making use of the so called Make Before Break
procedure) to follow this more optimal path. This is a known as a TE
LSP reoptimization procedure.

[LOOSE-REOPT] proposes a mechanisms allowing:


 Vasseur, Ayyangar and Zhang                                  14



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


      - The 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 potentially more optimal path. This is done via
      explicit signaling request: the head-end LSR sets the ôERO
      Expansion requestö bit of the SESSION-ATTRIBUTE object carried
      in the RSVP Path message.

      - An LSR whose next hop is a loose-hop to signal to the head-
      end LSR that a better path exists. This is performed by sending
      an RSVP Path Error Notify message (ERROR-CODE = 25), sub-code 6
      (Better path exists).

      This indication may be sent either:

            - In response to a query sent by the head-end LSR,
             - Spontaneously by any LSR having detected a more
             optimal path

Such a mechanism allows for the reoptimization of a TE LSP if and only
if a better path is some downstream area/AS is detected.

The reoptimization event can either be timer or event-driven based (a
link UP event for instance).

Note that the reoptimization MUST always be performed in a non-
disruptive fashion.

Once the head-end LSR is informed of the existence of a more optimal
path either in its head-end area/AS or in another AS, the inter-AS TE
Path computation is triggered using the same set of mechanisms as when
the TE LSP is first set up. Then the inter-AS TE LSP is set up
following the more optimal path, making use of the make before break
procedure. In case of a contiguous LSP, the reoptimization process is
strictly controlled by the head-end LSR which triggers the make-before-
break procedure, regardless of the location where the more optimal path
is.

Note that in the case of loose hop reoptimization, the TE LSP may
follow a preferable path within one or more domain(s) whereas in the
case of PCE-based path computation techniques, the reoptimization
process may lead to following a completely different inter-domain path
(including a different set of ABRs/ASBRs) since end-to-end shortest
path is computed.

7.2.   Stitched or nested (non-contiguous) TE LSPs

In the case of a stitched or nested inter-domain TE LSP, the re-
optimization process is treated as a local matter to any Area/AS. The
main reason is that the inter-domain TE LSP is a different LSP (and
therefore different RSVP session) from the intra-domain LSP segment or
FA-LSP in an area or an AS. Therefore, reoptimization in an area/AS is

 Vasseur, Ayyangar and Zhang                                  15



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


done by locally reoptimizing the intra-domain FA LSP or LSP segment.
Since the inter-domain TE LSPs are transported using LSP segments or
FA-LSP across each domain, optimality of the inter-domain TE LSP in an
area/AS is dependent on the optimality of the corresponding LSP
segments or FA-LSPs. If, after an inter-domain LSP is setup, a more
optimal path is available within an area/AS, the corresponding LSP
segment(s) or FA-LSP will be re-optimized using "make-before-break"
techniques discussed in [RSVP-TE]. Reoptimization of the FA LSP or LSP
segment automatically reoptimizes the inter-domain TE LSPs that the LSP
segment transports. Reoptimization parameters like frequency of
reoptimization, criteria for reoptimization like metric or bandwidth
availability; etc can vary from one area/AS to another and can be
configured as required, per intra-area/AS TE LSP segment or FA-LSP if
it is preconfigured or based on some global policy within the area/AS.

Hence, in this scheme, since each area/AS takes care of reoptimizing
its own LSP segments or FA-LSPs, and therefore the corresponding inter-
domain TE LSPs, the make-before-break can happen locally and is not
triggered by the head-end LSR for the inter-domain LSP. So, no
additional RSVP signaling is required for LSP re-optimization and
reoptimization is transparent to the HE LSR of the inter-domain TE LSP.

If, however, an operator desires to manually trigger reoptimization at
the head-end LSR for the inter-domain TE LSP, then this solution does
not prevent that. A manual trigger for reoptimization at the head-end
LSR SHOULD force a reoptimization thereby signaling a "new" path for
the same LSP (along the optimal path) making use of the make-before-
break procedure. In response to this new setup request, the boundary
LSR may either initiate new LSP segment setup, in case the inter-domain
TE LSP is being stitched to the intra-area/AS LSP segment or it may
select an existing or new FA-LSP in case of nesting. When the LSP setup
along the current optimal path is complete, the head end should
switchover the traffic onto that path and the old path is eventually
torn down. Note that the head-end LSR does not know a priori whether a
more optimal path exists. Such a manual trigger from the head-end LSR
of the inter-domain TE LSP is, however, not considered to be a frequent
occurrence.

Note that stitching or nesting rely on local optimization: the
reoptimization process allows to locally reoptimize each TE LSP segment
or FA-LSP: hence, the reoptimization is not global and consequently the
end to end path may no longer to optimal, should it be optimal when set
up.

Procedures described in [LOOSE-REOPT] MUST be used if the operator does
not desire local re-optimization of certain inter-domain LSPs. In this
case, any re-optimization event within the domain MUST be reported to
the head-end node. This SHOULD be a configurable policy.

8.     Security Considerations


 Vasseur, Ayyangar and Zhang                                  16



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


When signaling an inter-AS TE, an Operator may make use of the already
defined security features related to RSVP (authentication). This may
require some coordination between Service Providers to share the keys
(see RFC 2747 and RFC 3097).

9.     Intellectual Property Considerations

  The IETF takes no position regarding the validity or scope of any
  Intellectual Property Rights or other rights that might be claimed to
  pertain to the implementation or use of the technology described in
  this document or the extent to which any license under such rights
  might or might not be available; nor does it represent that it has
  made any independent effort to identify any such rights. Information
  on the procedures with respect to rights in RFC documents can be
  found in BCP 78 and BCP 79.

  Copies of IPR disclosures made to the IETF Secretariat and any
  assurances of licenses to be made available, or the result of an
  attempt made to obtain a general license or permission for the use of
  such proprietary rights by implementers or users of this
  specification can be obtained from the IETF on-line IPR repository at
  http://www.ietf.org/ipr.

  The IETF invites any interested party to bring to its attention any
  copyrights, patents or patent applications, or other proprietary
  rights that may cover technology that may be required to implement
  this standard. Please address the information to the IETF at ietf-
  ipr@ietf.org..

  IPR Disclosure Acknowledgement

  By submitting this Internet-Draft, I certify that any applicable
  patent or other IPR claims of which I am aware have been disclosed,
  and any of which I become aware will be disclosed, in accordance with
  RFC 3668.

10.    Acknowledgments

We would like to acknowledge input and helpful comments from Adrian
Farrel.

11 References

10.1.  Normative References

[RFC] Bradner, S., "Key words for use in RFCs to indicate requirements
levels", RFC 2119, March 1997.

[RFC3667] Bradner, S., "IETF Rights in Contributions", BCP 78, RFC
3667, February 2004.


 Vasseur, Ayyangar and Zhang                                  17



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


[RFC3668] Bradner, S., Ed., "Intellectual Property Rights in IETF
Technology", BCP 79, RFC 3668, February 2004.

[RSVP] Braden, et al, " Resource ReSerVation Protocol (RSVP) û Version
1, Functional Specificationö, RFC 2205, September 1997.

[RSVP-TE] Awduche, et al, "Extensions to RSVP for LSP Tunnels", RFC
3209, December 2001.

[REFRESH-REDUCTION] Berger et al, ôRSVP Refresh Overhead Reduction
Extensionsö, RFC2961, April 2001.

[FAST-REROUTE] Ping Pan, et al, "Fast Reroute Extensions to RSVP-TE for
LSP Tunnels", draft-ietf-mpls-rsvp-lsp-fastreroute-03.txt, December
2003.

[OSPF-TE] Katz, D., Yeung, D., Kompella, K., "Traffic Engineering
Extensions to OSPF Version 2", RFC 3630, September 2003.

[ISIS-TE] Li, T., Smit, H., "IS-IS extensions for Traffic Engineering",
RFC 3784, June 2004.

10.2.  Informative references

[INT-AREA-REQ] Le Roux, J.L., Vasseur, J.P., Boyle, J., "Requirements
for inter-area MPLS Traffic Engineering", draft-ietf-tewg-interarea-
mpls-te-req-03.txt, work in progress.

[INT-AS-REQ] Zhang, R., Vasseur, J.P., "MPLS Inter-AS Traffic
Engineering Requirements", draft-ietf-tewg-interas-mpls-te-req-09.txt,
work in progress.

[INT-DOMAIN-FRWK] Farrel, A., Vasseur, J.P., Ayyangar, A., "A Framework
for Inter-Domain MPLS Traffic Engineering", draft-ietf-ccamp-inter-
domain-framework-00.txt, work in progress.

[FACILITY-BACKUP] Le Roux, J.L., Vasseur, J.P. et al. "Framework for
PCE based MPLS Facility Backup Path Computation", draft-leroux-pce-
backup-comp-frwk-00.txt, work in progress

[INTER-DOMAIN-SIG] Ayyangar, A., Vasseur, JP. ôInter-domain MPLS
Traffic Engineering û RSVP extensionsö, draft-ietf-ccamp-inter-domain-
rsvp-te, work in progress.

[LSP-STITCHING] Ayyangar, A., Vasseur, JP. ôLabel Switched Path
Stitching with Generalized MPLS Traffic Engineeringö, draft-ietf-ccamp-
lsp-stitching-00, Work under progress.





 Vasseur, Ayyangar and Zhang                                  18



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005



[LSP-ATTRIBUTE] Farrel A. et al, "Encoding of Attributes for
Multiprotocol Label Switching (MPLS) Label Switched Path (LSP)
Establishment Using RSVP-TE", draft-ietf-mpls-rsvpte-attributes-04,(work
in progress).

[GMPLS-OVERLAY] G. Swallow et al, "GMPLS RSVP Support for the Overlay
Model", (work in progress).

[EXCLUDE-ROUTE] Lee et all, Exclude Routes - Extension to RSVP-TE,
draft-ietf-ccamp-rsvp-te-exclude-route-00.txt, work in progress.

[LSPPING] Kompella, K., Pan, P., Sheth, N., Cooper, D.,Swallow, G.,
Wadhwa, S., Bonica, R., " Detecting Data Plane Liveliness in MPLS",
Internet Draft <draft-ietf-mpls-lsp-ping-02.txt>, October 2002. (Work
in Progress)

[MPLS-TTL], Agarwal, et al, "Time to Live (TTL) Processing in MPLS
Networks", RFC 3443 Updates RFC 3032) ", January 2003

[LOOSE-PATH-REOPT] Vasseur, Ikejiri and Zhang ôReoptimization of an
explicit loosely routed MPLS TE pathsö, draft-ietf-ccamp-loose-path-
reopt-01.txt, July 2004, Work in Progress.

[NODE-ID] Vasseur, Ali and Sivabalan, ôDefinition of an RRO node-id
subobjectö,  draft-ietf-mpls-nodeid-subobject-05.txt, work in progress.

[LSP-HIER] Kompella K., Rekhter Y., "LSP Hierarchy with Generalized
MPLS TE", draft-ietf-mpls-lsp-hierarchy-08.txt, March 2002.

[MPLS-TTL], Agarwal, et al, "Time to Live (TTL) Processing in MPLS
Networks", RFC 3443 (Updates RFC 3032) ", January 2003.


Authors' Address:

Jean-Philippe Vasseur (Editor)
Cisco Systems, Inc.
300 Beaver Brook Road
Boxborough , MA - 01719
USA
Email: jpv@cisco.com

Arthi Ayyangar (Editor)
Juniper Networks, Inc
1194 N.Mathilda Ave
Sunnyvale, CA 94089
USA
e-mail: arthi@juniper.net



 Vasseur, Ayyangar and Zhang                                  19



draft-ietf-ccamp-inter-domain-pd-path-comp-00.txt            April 2005


Raymond Zhang
Infonet Services Corporation
2160 E. Grand Ave.
El Segundo, CA 90025
USA
Email: raymond_zhang@infonet.com


Full Copyright Statement

Copyright (C) The Internet Society (2005).  This document is subject to
the rights, licenses and restrictions contained in BCP 78, and except
as set forth therein, the authors retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.



























 Vasseur, Ayyangar and Zhang                                  20


Html markup produced by rfcmarkup 1.129c, available from https://tools.ietf.org/tools/rfcmarkup/