Networking Working Group JP. Vasseur, Ed.
Internet-Draft Cisco Systems, Inc
Proposed Status: Standard A. Ayyangar, Ed.
Expires: August 11, 2006 Juniper Networks
R. Zhang
BT Infonet
February 7, 2006
A Per-domain path computation method for establishing Inter-domain
Traffic Engineering (TE) Label Switched Paths (LSPs)
draft-ietf-ccamp-inter-domain-pd-path-comp-02.txt
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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
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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 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.
Requirements Language
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].
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. General assumptions . . . . . . . . . . . . . . . . . . . . . 5
3.1. Common assumptions . . . . . . . . . . . . . . . . . . . . 5
3.2. Example of topology for the inter-area TE case . . . . . . 7
3.3. Example of topology for the inter-AS TE case . . . . . . . 7
4. Per-domain path computation procedures . . . . . . . . . . . . 9
4.1. Example with an inter-area TE LSP . . . . . . . . . . . . 12
4.1.1. Case 1: T0 is a contiguous TE LSP . . . . . . . . . . 12
4.1.2. Case 2: T0 is a stitched or nested TE LSP . . . . . . 13
4.2. Example with an inter-AS TE LSP . . . . . . . . . . . . . 13
4.2.1. Case 1: T1 is a contiguous TE LSP . . . . . . . . . . 14
4.2.2. Case 2: T1 is a stitched or nested TE LSP . . . . . . 14
5. Path optimality/diversity . . . . . . . . . . . . . . . . . . 15
6. Reoptimization of an inter-domain TE LSP . . . . . . . . . . . 15
6.1. Contiguous TE LSPs . . . . . . . . . . . . . . . . . . . . 15
6.2. Stitched or nested (non-contiguous) TE LSPs . . . . . . . 16
6.3. Path characteristics after reoptimization . . . . . . . . 17
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. Normative References . . . . . . . . . . . . . . . . . . . 17
10.2. Informative References . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property and Copyright Statements . . . . . . . . . . 21
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1. Terminology
Terminology used in this document
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
[I-D.ietf-ccamp-inter-domain-framework] 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 [RFC4105] and [RFC4216]. The framework
for inter-domain MPLS Traffic Engineering has been provided in
[I-D.ietf-ccamp-inter-domain-framework].
Some of the mechanisms used to establish and maintain inter-domain TE
LSPs are specified in [I-D.ietf-ccamp-inter-domain-rsvp-te] and
[I-D.ietf-ccamp-lsp-stitching].
This document exclusively focuses on the path computation aspects and
defines a method for establishing inter-domain TE LSP where each node
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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 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 ASs.
The solution described in this document does not attempt to address
all the requirements specified in [RFC4105] and [RFC4216]. This is
acceptable according to [RFC4216] 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 [I-D.ietf-pce-architecture]). 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
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 ([RFC3209]).
- 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:
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* 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.
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 [RFC4105] and [RFC4216]), 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 [RFC4216]. 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 [RFC4216].
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- The procedures defined in this document are applicable to any node
(not just boundary node) that receives a Path message with an ERO
that constains 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).
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 =======>
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 [RFC4216]:
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<-- 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 [RFC4216].
Description of Figure 2:
- Three interconnected ASs, respectively AS1, AS2, and AS3. Note
that in some scenarios described in [RFC4216] AS1=AS3.
- The ASBRs in different ASs 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 [RFC3630], [RFC3784], [RFC4203] and [RFC4205]). In
other words, the ASs 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 ASs 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 ASs where
some of those ASs 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
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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 an LSR (e.g. a boundary node such as an 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 [I-D.ietf-ccamp-inter-domain-rsvp-te]. 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
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 PErr message upstream with error code=24
("Routing Problem") and error subcode as described in section 4.3.4
of [RFC3209]. 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.
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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) Once the next-hop boundary LSR has been determined (according to
the procedure described in 1)) or if the next-hop boundary 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 PErr message SHOULD be sent
for the inter-domain TE LSP based on section 4.3.4 of [RFC3209].
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 [I-D.ietf-ccamp-
inter-domain-rsvp-te]. 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 computation and signaling failure and a PErr 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 PErr SHOULD be sent upstream for the inter-
domain TE LSP. In both these cases procedures described in section
4.3.4 of [RFC3209] 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.
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The ERO of an inter-domain TE LSP may comprise abstract nodes such as
ASs. 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.
Note that no summarized TE information is leaked between ASs which is
compliant with the requirements listed in [RFC4105] and [RFC4216].
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
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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: T0 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 (ABR1) 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-ABR1-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 PErr is received indicating the TE LSP has
suffered a failure, an implementation might support the possibility
to retry a 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 PErr 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
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is allowed for this inter-area TE LSP (see [I-D.ietf-ccamp-
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 PErr up to the
Head-end LSR (R0) without trying to select another ABR.
4.1.2. Case 2: T0 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 [I-D.ietf-ccamp-inter-domain-
rsvp-te], 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 PErr 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 [I-D.ietf-ccamp-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 PErr
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
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).
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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 PErr 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 [I-D.ietf-ccamp-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
PErr 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 [I-D.ietf-pce-architecture]).
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.
[I-D.ietf-ccamp-loose-path-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.
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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 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
[RFC3209]. 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
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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 [I-D.ietf-ccamp-loose-path-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.
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. IANA Considerations
This document makes no request for any IANA action.
8. Security Considerations
Signaling of inter-domain TE LSPs raises security issues that have
been described in section 7 of [I-D.ietf-ccamp-inter-domain-rsvp-te];
however the path computation aspects specified in this document do
not raise additional security concerns.
9. Acknowledgements
We would like to acknowledge input and helpful comments from Adrian
Farrel, Jean-Louis Le Roux, Dimitri Papadimitriou and Faisal Aslam.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2702] Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
McManus, "Requirements for Traffic Engineering Over MPLS",
RFC 2702, September 1999.
[RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
Tunnels", RFC 3209, December 2001.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630,
September 2003.
[RFC3784] Smit, H. and T. Li, "Intermediate System to Intermediate
System (IS-IS) Extensions for Traffic Engineering (TE)",
RFC 3784, June 2004.
[RFC4203] Kompella, K. and Y. Rekhter, "OSPF Extensions in Support
of Generalized Multi-Protocol Label Switching (GMPLS)",
RFC 4203, October 2005.
[RFC4205] Kompella, K. and Y. Rekhter, "Intermediate System to
Intermediate System (IS-IS) Extensions in Support of
Generalized Multi-Protocol Label Switching (GMPLS)",
RFC 4205, October 2005.
10.2. Informative References
[I-D.ietf-ccamp-crankback]
Farrel, A., "Crankback Signaling Extensions for MPLS and
GMPLS RSVP-TE", draft-ietf-ccamp-crankback-05 (work in
progress), May 2005.
[I-D.ietf-ccamp-inter-domain-framework]
Farrel, A., "A Framework for Inter-Domain MPLS Traffic
Engineering", draft-ietf-ccamp-inter-domain-framework-04
(work in progress), July 2005.
[I-D.ietf-ccamp-inter-domain-pd-path-comp]
Vasseur, J., "A Per-domain path computation method for
establishing Inter-domain Traffic Engineering (TE) Label
Switched Paths (LSPs)",
draft-ietf-ccamp-inter-domain-pd-path-comp-01 (work in
progress), October 2005.
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[I-D.ietf-ccamp-inter-domain-rsvp-te]
Ayyangar, A. and J. Vasseur, "Inter domain GMPLS Traffic
Engineering - RSVP-TE extensions",
draft-ietf-ccamp-inter-domain-rsvp-te-02 (work in
progress), October 2005.
[I-D.ietf-ccamp-loose-path-reopt]
Vasseur, J., "Reoptimization of Multiprotocol Label
Switching (MPLS) Traffic Engineering (TE) loosely routed
Label Switch Path (LSP)",
draft-ietf-ccamp-loose-path-reopt-02 (work in progress),
February 2006.
[I-D.ietf-ccamp-lsp-stitching]
Ayyangar, A. and J. Vasseur, "Label Switched Path
Stitching with Generalized MPLS Traffic Engineering",
draft-ietf-ccamp-lsp-stitching-02 (work in progress),
September 2005.
[I-D.ietf-pce-architecture]
Farrel, A., "A Path Computation Element (PCE) Based
Architecture", draft-ietf-pce-architecture-04 (work in
progress), January 2006.
[RFC4105] Le Roux, J., Vasseur, J., and J. Boyle, "Requirements for
Inter-Area MPLS Traffic Engineering", RFC 4105, June 2005.
[RFC4216] Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous System
(AS) Traffic Engineering (TE) Requirements", RFC 4216,
November 2005.
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Authors' Addresses
JP Vasseur (editor)
Cisco Systems, Inc
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
Email: jpv@cisco.com
Arthi Ayyangar (editor)
Juniper Networks
1194 N.Mathilda Avenue
Sunnyvale, CA 94089
USA
Email: arthi@juniper.net
Raymond Zhang
BT Infonet
2160 E. Grand Ave.
El Segundo, CA 90025
USA
Email: raymond_zhang@bt.infonet.com
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