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

Network 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: April 2006
                                                           October 2005


           draft-ietf-ccamp-inter-domain-pd-path-comp-01.txt


   A Per-domain path computation method for establishing Inter-domain
          Traffic Engineering (TE) Label Switched Paths (LSPs)


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Abstract

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

Per-domain computation applies where the full path of an inter-domain
TE LSP cannot be or is not determined at the ingress node of the TE

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LSP, and is not signaled across domain boundaries. This is most likely
to arise owing to TE visibility limitations. The signaling message
indicates the destination and nodes up to the next domain boundary. It
may also indicate further domain boundaries or domain identifiers. The
path through each domain, possibly including the choice of exit point
from the domain, must be determined within the domain.

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.................................................3
3. General assumptions..........................................4
3.1 Common assumptions..........................................4
3.2 Example of topology for the inter-area TE case .............5
3.3 Example of topology for the inter-AS TE case ...............6
4. Per-domain path computation procedures.......................7
4.1 Example with an inter-area TE LSP...........................10
4.1.1 Case 1: T1 is a contiguous TE LSP.........................10
4.1.2 Case 2: T1 is a stitched or nested TE LSP.................11
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 a nested TE LSP...............12
5. Path optimality/diversity....................................13
6. Reoptimization of an inter-domain TE LSP.....................13
6.1 Contiguous TE LSPs..........................................13
6.2. Stitched or nested (non-contiguous) TE LSPs................13
6.3 Path characteristics after reoptimization...................15
7. Security Considerations .....................................15
8. Intellectual Property Considerations ........................15
9. Acknowledgments..............................................15
10. References..................................................15
10.1 Normatives References .....................................16
10.2 Informative References ....................................17














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1. Terminology

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

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

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

Inter-AS TE LSP: A TE LSP that crosses an AS boundary.

Inter-area TE LSP: A TE LSP that crosses an IGP area.

LSR: Label Switching Router

LSP: Label Switched Path

TE LSP: Traffic Engineering Label Switched Path

PCE: Path Computation Element: an entity (component, application or
network node) that is capable of computing a network path or route
based on a network graph and applying computational constraints.

TED: Traffic Engineering Database

The notion of contiguous, stitched and nested TE LSPs is defined in
[INT-DOMAIN-FRWK] and will not be repeated here.

2. Introduction

The requirements for inter-domain Traffic Engineering (inter-area and
inter-AS TE) have been developed by the Traffic Engineering Working
Group and have been stated in [INT-AREA-REQS] and [INT-AS-REQS]. The
framework for inter-domain MPLS Traffic Engineering has been provided
in [INT-DOMAIN-FRWK].

Some of the mechanisms used 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 establishing inter-domain TE LSP where each node
in charge of computing a section of an inter-domain TE LSP path is
always along the path of such TE LSP.

When the visibility of an end to end complete path spanning multiple
domains is not available at the Head-end LSR, one approach described in
this document consists of using a per-domain path computation technique


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during LSP setup to determine the inter-domain TE LSP as it traverses
multiple domains.

The mechanisms proposed in this document are also applicable to MPLS TE
domains other than IGP areas and ASes.

The solution described in this document does not attempt to address all
the requirements specified in [INT-AREA-REQS] and [INT-AS-REQS]. This
is acceptable according to [INT-AS-REQS] which indicates that a
solution may be developed to address a particular deployment scenario
and might, therefore, not meet all requirements for other deployment
scenarios.

It must be pointed out that the inter-domain path computation technique
proposed in this document is one among many others and the choice of
the appropriate technique must be driven by the set of requirements for
the paths attributes and the applicability to a particular technique
with respect to the deployment scenario. 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 (see [PCE-ARCH]). Other offline mechanisms for path computation
are not precluded either. Note also that a Service Provider may elect
to use different inter-domain path computation techniques for different
TE LSP types.

3. General assumptions

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

3.1.  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. In this case, the path computation technique described
for inter-area and inter-AS MPLS Traffic Engineering just recursively
applies.

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

- The path (ERO) for an inter-domain TE LSP 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 domains
        * The complete strict path in the source domain followed by
        boundary LSRs (or domain identifiers, e.g. AS numbers)
        * The complete list of boundary LSRs along the path
        * The current boundary LSR and the LSP destination



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The set of (loose or strict) hops can either be statically configured
on the Head-end LSR or dynamically computed. A per-domain path
computation method is defined in this document with an optional Auto-
discovery mechanism based on IGP and/or BGP information yielding the
next-hop boundary node (domain exit point, such as ABR/ASBR) along the
path as the TE LSP is being signaled, along with potential crankback
mechanisms. Alternatively the domain exit points may be statically
configured on the Head-end LSR in which case next-hop boundary node
auto-discovery would not be required.

- 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). In any case, no
topology or resource information needs to be distributed between
domains (as mandated per [INT-AREA-REQS] and [INT-AS-REQS]), which is
critical to preserve IGP/BGP scalability and confidentiality in the
case of TE LSPs spanning multiple routing domains.

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

- While certain constraints like bandwidth can be used across different
domains, certain other TE constraints like resource affinity, color,
metric, etc. as listed in [RFC2702] may need to be translated at domain
boundaries. If required, it is assumed that, at the domain boundary
LSRs, there will exist some sort of local mapping based on policy
agreement in order to translate such constraints across domain
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 [INT-AS-REQS].

   3.2. Example of topology for the inter-area TE case

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

<-area 1-><-- area 0 --><--- area 2 --->
 ------ABR1------------ABR3-------
 |    /   |              |  \     |
R0--X1    |              |   X2---X3--R1
 |        |              |  /     |
 ------ABR2-----------ABR4--------
<=========== Inter-area TE LSP =======>

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   Figure 1 - Example of topology for the inter-area TE case

Description of Figure 1:

- ABR1, ABR2, ABR3 and ABR4 are ABRs,
- X1: an LSR in area 1,
- X2, X3: LSRs in area 2,
- An inter-area TE LSP T0 originated at R0 in area 1 and terminating at
R1 in area 2.

Notes:
- The terminology used in the example above corresponds to OSPF but the
path computation technique proposed in this document equally applies to
the case of an IS-IS multi-level network.
- Just a few routers in each area are depicted in the diagram above for
the sake of simplicity.
- The example depicted in Figure 1 shows the case where the Head-end
and Tail-end areas are connected by means of area 0. The case of an
inter-area TE LSP between two IGP areas that does not transit through
area 0 is not precluded.

   3.3. Example of topology for the inter-AS TE case

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

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

               <---BGP--->            <---BGP-->
CE1---R0---X1-ASBR1-----ASBR4--R3---ASBR7----ASBR9----R6
      |\     \ |       / |   / |   / |          |     |
      | \     ASBR2---/ ASBR5  | --  |          |     |
      |  \     |         |     |/    |          |     |
      R1-R2---ASBR3----ASBR6--R4---ASBR8----ASBR10---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)===============>

       Figure 2 - Example of topology for the inter-AS TE case

The diagram depicted in Figure 2 covers all the inter-AS TE deployment
cases described in [INT-AS-REQS].

Description of Figure 2:

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- Three interconnected ASes, respectively AS1, AS2, and AS3. Note that
in some scenarios described in [INT-AS-REQS] AS1=AS3.

- The ASBRs in different ASes are BGP peers. There is usually no IGP
running on the single hop links interconnecting the ASBRs and also
referred to as inter-ASBR links.

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

- Each AS can be made of several IGP areas. The path computation
technique 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. The
case of an Inter-AS TE LSP spanning multiple ASes where some of those
ASes are themselves made of multiple IGP areas can be easily derived
from the examples above: the per-domain path computation technique
described in this document is applied recursively in this case.

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

4. Per-domain path computation procedures

The mechanisms for inter-domain TE LSP computation as described in this
document can be used regardless of the nature of the inter-domain TE
LSP (contiguous, stitched or nested).

Note that 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).

When a boundary LSR (e.g. ABR/ASBR) receives a Path message with an ERO
that contains a loose hop or an abstract node that is not a simple
abstract node (that is, an abstract node that identifies more than one
LSR), then it MUST follow the procedures as described in [INTER-DOMAIN-
SIG]. In addition, the following procedures describe the path
computation procedures that SHOULD be carried out on the LSR:

1) If the next hop boundary LSR is not present in the TED.

If the loose next-hop is not present in the TED, the following
conditions MUST be checked:
    - If the IP address of the next hop boundary LSR is outside of the
      current domain,
    - If the domain is PSC (Packet Switch Capable) and uses in-band
      control channel

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If the two conditions above are satisfied then the boundary LSR SHOULD
check if the next-hop has IP reachability (via IGP or BGP). If the
next-hop is not reachable, then a signaling failure occurs and the LSR
SHOULD send back a PathErr message upstream with error code=24
("Routing Problem") and error subcode as described in section 4.3.4 of
[RSVP-TE]. If the next-hop is reachable, then it SHOULD find a domain
boundary LSR (domain boundary point) to get to the next-hop. The
determination of domain boundary point based on routing information is
what we term as "auto-discovery" in this document. In the absence of
such an auto-discovery mechanism, a) 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 signaled loose next-hop in the ERO and hence should be accessible
via the TED or b) there needs to be an alternate scheme that provides
the domain exit points. Otherwise the path computation for the inter-
domain TE LSP will fail.

An implementation MAY support the ability to disable such IP
reachability fall-back option should the next hop boundary LSR not be
present in the TED. In other words, an implementation MAY support the
possibility to trigger a signaling failure whenever the next-hop is not
present in the TED.

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

        a) Case of a contiguous TE LSP. The boundary LSR that processes
        the ERO SHOULD perform 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, then this
        SHOULD be treated as a path computation and signaling failure
        and a PathErr message SHOULD be sent for the inter-domain TE
        LSP based on section 4.3.4 of [RSVP-TE].

        b) Case of stitched or nested LSP

                i) If the boundary LSR is a candidate LSR for intra-
                area H-LSP/S-LSP setup (the LSR has local policy for
                nesting or stitching), and if there is no H-LSP/S-LSP
                from this LSR to the next-hop boundary LSR that
                satisfies the constraints, it SHOULD signal a H-LSP/S-
                LSP to the next-hop boundary LSR. If pre-configured H-
                LSP(s) or S-LSP(s) already exist, then it will try to
                select from among those intra-domain LSPs. Depending on
                local policy, it MAY signal a new H-LSP/S-LSP if this
                selection fails. If the H-LSP/S-LSP is successfully
                signaled or selected, it propagates the inter-domain
                Path message to the next-hop following the procedures
                described in [INTER-DOMAIN-SIG]. If, for some reason
                the dynamic H-LSP/S-LSP setup to the next-hop boundary
                LSR fails, then this SHOULD be treated as a path

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                computation and signaling failure and a PathErr message
                SHOULD be sent upstream for the inter-domain LSP.
                Similarly, if selection of a preconfigured H-LSP/S-LSP
                fails and local policy prevents dynamic H-LSP/S this
                SHOULD be treated as a path computation and signaling
                failure and a PathErr SHOULD be sent upstream for the
                inter-domain TE LSP. In both these cases procedures
                described in section 4.3.4 of [RSVP-TE] SHOULD be
                followed to handle the failure.

                ii) If, however, the boundary LSR is not a candidate
                for intra-domain H-LSP/S-LSP (the LSR does not have
                local policy for nesting or stitching), then it SHOULD
                apply the same procedure as for the contiguous case.

                Note that in both cases, path computation and signaling
                process may be stopped due to policy violation.

The ERO of an inter-domain TE LSP may comprise abstract nodes such as
ASes. In such a case, upon receiving the ERO whose next hop is an AS,
the boundary LSR has to determine the next-hop boundary LSR which may
be determined based on the "auto-discovery" process mentioned above. If
multiple ASBRs candidates exist the boundary LSR may apply some
policies based on peering contracts that may have been pre-negotiated.
Once the next-hop boundary LSR has been determined a similar procedure
as the one described above is followed.

Note related to the inter-AS TE case

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 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 inter-ASBR links
and MUST be able to perform all the usual operations related to MPLS
Traffic Engineering (call admission control, ...).

In terms of computation of an inter-AS TE LSP path, an interesting
optimization technique 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 an
LSR (could be entry ASBR) in the previous AS to make a more appropriate
route selection up to the entry ASBR in the immediately downstream AS
taking into account the constraints associated with the inter-ASBR
links. 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.

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

For example, consider the diagram depicted in Figure 2: when ASBR1
floods its IGP TE LSA ((opaque LSA for OSPF)/LSP (TLV 22 for IS-IS)) in
its routing 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 domain that the ASBR belongs to.
Consequently, the path 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 signaling along the next AS in case of resource shortage
or unsatisfied constraints on 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 outgoing links directly connected to the ASBR is
advertised.

Note that an Operator may decide to operate a stitched segment or 1-hop
hierarchical LSP for the inter-ASBR link.

   4.1. Example with an inter-area TE LSP

4.1.1 Case 1: T1 is a contiguous TE LSP

The Head-end LSR (R0) first determines the next hop ABR (which could be
manually configured by the user or dynamically determined by using
auto-discovery mechanism). R0 then computes the path to reach the
selected next hop ABR and signals the Path message. When the Path
message reaches ABR1, it first determines the next hop ABR from its
area 0 along the LSP path (say ABR3), either directly from the ERO (if
for example the next hop ABR is specified as a loose hop in the ERO) or
by using the auto-discovery mechanism specified above.

- Example 1 (set of loose hops): R0-ABR1(loose)-ABR3(loose)-R1(loose)
- Example 2 (mix of strict and loose hops): R0-X1-ASR1-ABR3(loose)-X2-
X3-R1

Note that 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. It may 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 TE path exists between the inter-
area TE LSP source and destination. In case of set up failure or when
an RSVP PathErr is received indicating the TE LSP has suffered a
failure, an implementation might support the possibility to retry a

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particular path option configurable amount of times (optionally with
dynamic intervals between each trial) before trying a lower priority
path option.

Once it has computed the path up to the next hop ABR (ABR3), ABR1 sends
the Path message along the computed path. Upon receiving the Path
message, ABR3 then repeats a similar procedure. If ABR3 cannot find a
path obeying the set of constraints for the inter-area TE LSP, the
signaling process stops and ABR3 sends a PathErr message to ABR1. Then
ABR1 can in turn trigger a new path computation by selecting another
egress boundary LSR (ABR4 in the example above) if crankback is allowed
for this inter-area TE LSP (see [CRANKBACK]). 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 the signaling process and
MUST forward a PathErr up to the Head-end LSR (R0) without trying to
select another ABR.

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

The Head-end LSR (R0) first determines the next hop ABR (which could be
manually configured by the user or dynamically determined by using
auto-discovery mechanism). R0 then computes the path to reach the
selected next hop ABR and signals the Path message. When the Path
message reaches ABR1, it first determines the next hop ABR from its
area 0 along the LSP path (say ABR3), either directly from the ERO (if
for example the next hop ABR is specified as a loose hop in the ERO) or
by using an auto-discovery mechanism, specified above.

ABR1 then checks if it has a H-LSP or S-LSP to ABR3 matching the
constraints carried in the inter-area TE LSP Path message. If not, ABR1
computes the path for a H-LSP or S-LSP from ABR1 to ABR3 satisfying the
constraint and sets it up accordingly. Note that the H-LSP or S-LSP
could have also been pre-configured.

Once ABR1 has selected the H-LSP/S-LSP for the inter-area LSP, using
the signaling procedures described in [INTER-DOMAIN-SIG], ABR1 sends
the Path message for inter-area TE LSP to ABR3. Note that irrespective
of whether ABR1 does nesting or stitching, the Path message for the
inter-area TE LSP is always forwarded to ABR3. ABR3 then repeats the
exact same procedures. If ABR3 cannot find a path obeying the set of
constraints for the inter-area TE LSP, ABR3 sends a PathErr message to
ABR1. Then ABR1 can in turn either select another H-LSP/S-LSP to ABR3
if such an LSP exists or select another egress boundary LSR (ABR4 in
the example above) if crankback is allowed for this inter-area TE LSP
(see [CRANKBACK]). If crankback is not allowed for that inter-area TE
LSP or if ABR1 has been configured not to perform crankback, then ABR1
forwards the PathErr up to the inter-area Head-end LSR (R0) without
trying to select another egress LSR.

   4.2. Example with an inter-AS TE LSP


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The path computation 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).

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 loose hops): ASBR4(loose)-ASBR9(loose)-R6(loose)
- Example 2 (mix of strict and loose hops): R2-ASBR3-ASBR2-ASBR1-ASBR4-
ASBR10(loose)-ASBR9-R6

In the example 1 above, 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 is announced in
the IGP TE LSA/LSP originated by its neighboring ASBR. In the example 1
above, the TE LSP path is defined as: 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's path
configuration.

Once it has computed the path up to the next hop 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. If ASBR4 cannot find a path obeying the set of constraints
for the inter-AS TE LSP, then ASBR4 sends 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
[CRANKBACK]). If crankback is not allowed for that inter-AS TE LSP or
if ASBR1 has been configured not to perform crankback, ABR1 stops the
signaling process and forwards 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 path computation procedures are very similar to the inter-area LSP
setup case described earlier. In this case, the H-LSPs or S-LSPs are
originated by the ASBRs at the entry to the AS.



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5. Path optimality/diversity

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

Moreover, computing two diverse paths using a per-domain path
computation approach may not be possible in some topologies (due to the
well-known "trapping" problem).

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

If the per-domain path computation technique does no meet the set of
requirements for a particular TE LSP (e.g. path optimality,
requirements for a set of diversely routed TE LSPs, ...) other techniques
such as PCE-based path computation techniques may be used (see [PCE-
ARCH]).

6. Reoptimization of an inter-domain TE LSP

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

The following mechanisms can be used for reoptimization and are
dependent on the nature of the inter-domain TE LSP.

   6.1. Contiguous TE LSPs

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

[LOOSE-REOPT] proposes a mechanism that allows the Head-end LSR to be
notified of the existence of a more optimal path in a downstream
domain. The Head-end LSR may then decide to gracefully reroute the TE
LSP using the so-called 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 of the more optimal path.

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

In the case of a stitched or nested inter-domain TE LSP, the
reoptimization process is treated as a local matter to any domain. The

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main reason is that the inter-domain TE LSP is a different LSP (and
therefore different RSVP session) from the intra-domain S-LSP or H-LSP
in an area or an AS. Therefore, reoptimization in a domain is done by
locally reoptimizing the intra-domain H-LSP or S-LSP.  Since the inter-
domain TE LSPs are transported using S-LSP or H-LSP across each domain,
optimality of the inter-domain TE LSP in a domain is dependent on the
optimality of the corresponding S-LSP or H-LSPs. If, after an inter-
domain LSP is setup, a more optimal path is available within an domain,
the corresponding S-LSP  or H-LSP will be reoptimized using "Make-
Before-Break" techniques discussed in [RSVP-TE]. Reoptimization of the
H-LSP or S-LSP automatically reoptimizes the inter-domain TE LSPs that
the H-LSP or the S-LSP transports. Reoptimization parameters like
frequency of reoptimization, criteria for reoptimization like metric or
bandwidth availability, etc can vary from one domain to another and can
be configured as required, per intra-domain TE S-LSP or H-LSP if it is
preconfigured or based on some global policy within the domain.

Hence, in this scheme, since each domain takes care of reoptimizing its
own S-LSPs or H-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 reoptimization and reoptimization is
transparent to the Head-end 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 more optimal path) making use of the Make-
Before-Break procedure. In response to this new setup request, the
boundary LSR may either initiate new S-LSP setup, in case the inter-
domain TE LSP is being stitched to the intra-domain S-LSP or it may
select an existing or new H-LSP in case of nesting. When the LSP setup
along the current path is complete, the Head-end LSR 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 S-LSP
or H-LSP: hence, the reoptimization is not global and consequently the
end to end path may no longer be optimal should it be optimal when
being set up.

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


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   6.3. Path characteristics after reoptimization

Note that in the case of loose hop reoptimization of contiguous inter-
domain TE LSP or local reoptimization of stitched/nested S-LSP where
boundary LSRs are specified as loose hops, the TE LSP may follow a
preferable path within one or more domain(s) but would still traverse
the same set of boundary LSRs. In contrast, in the case of PCE-based
path computation techniques, because end to end optimal path is
computed, the reoptimization process may lead to following a completely
different inter-domain path (including a different set of boundary
LSRs).

7. Security Considerations

Signaling of inter-domain TE LSPs raises security issues that have been
described in section 7 of [INTER-DOMAIN-SIG]; however the path
computation aspects specified in this document do not raise additional
security concerns.

8. 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.

9. Acknowledgments

We would like to acknowledge input and helpful comments from Adrian
Farrel, Jean-Louis Le Roux and Dimitri Papadimitriou.

10. References



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   10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to indicate
requirements levels", RFC 2119, March 1997.

[RFC3979] Bradner, S., Ed., "Intellectual Property Rights in IETF
Technology", BCP 79, RFC 3979, March 2005.

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

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

[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.

[G-OSPF]Rekhter Y., Kompella K, et al., "OSPF Extensions in Support of
Generalized Multi-Protocol Label Switching", draft-ietf-ccamp-ospf-
gmpls-extensions, work in progress.

[G-ISIS] Rekhter Y., Kompella K, et al., "IS-IS Extensions in Support
of Generalized Multi-Protocol Label Switching", draft-ietf-isis-gmpls-
extensions, work in progress.

   10.2. Informative references

[CRANKBACK] Farrel A., et al., "Crankback Signaling Extensions for MPLS
and GMPLS RSVP-TE", draft-ietf-ccamp-crankback, work in progress.

[INT-AREA-REQ] Le Roux, J.L., Vasseur, J.P., Boyle, J., "Requirements
for inter-area MPLS Traffic Engineering", RFC 4105, June 2005.

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



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draft-ietf-ccamp-inter-domain-pd-path-comp-01.txt          October 2005


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

[LSP-STITCHING] Ayyangar, A., Vasseur, JP. "Label Switched Path
Stitching with Generalized MPLS Traffic Engineering", draft-ietf-ccamp-
lsp-stitching-00, work under progress.

[LOOSE-PATH-REOPT] Vasseur, Ikejiri and Zhang "Reoptimization of an
explicit loosely routed MPLS TE paths", draft-ietf-ccamp-loose-path-
reopt, work in Progress.

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

[PCE-ARCH] Farrel A., Vasseur J.P, Ash J, "Path Computation Element
(PCE) Architecture", draft-ietf-pce-architecture, work in progress.

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

Raymond Zhang
BT/Infonet
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

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draft-ietf-ccamp-inter-domain-pd-path-comp-01.txt          October 2005


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