The MPLS WG Archive

[Jean Philippe Vasseur <jvasseur@cisco.com>]

Hi,

At 19:38 25/11/2002 +0200, Chopin He wrote:
>Hi! folks!
>
>I hope this is the right place to ask this stupid MPLS question.
>
>I read couple of I-Ds mentioning "Path Computation Server", But I am a bit 
>unclear about its definition and functionality.
>
>So far, PCS is mentioned in many documents [1][2][3][4][5], one example of 
>its definition is found as:[1]
>
>   "PCS: Path Computation Server (may be any kind of LSR (ABR, ASBR, ...)
>    or a centralized path computation server "
>
>This is quite a detailed explaination, but I understand the LSR or ABR or 
>ASBR are control plane components; while the centralised path computaton 
>server is a management plane component. In some case, (e.g. network 
>management), their roles are lardgely different. Is that possible to 
>define them seperately?
>
>
>Additionaly, it seems to me that PCS's functionality is to response the 
>PCC's Path Computation Request, thus compute the path, and send the result 
>back to PCC. This sounds perfect. While I am still a bit unclear.  What 
>kind of information the PCS need to know? How can the PCS get the relevant 
>information? And so on.
>
>E.g. Brunner said in [3]:
>    "Definitely the path computation server needs topology information in
>    order to perform its task. But how to get that information is out of
>    scope of this document. "
>
>I wonder if there are some specification which defines the PCS's 
>functionality explicitly?
>
>If not, can somebody take the initiative?
>

The PCS's role is to compute TE LSP path for which it is not the HE.
Basically, the PCS needs:
- to get information about the topology, the TE resources, .... This can be 
done in several ways: downloading those informations from a router using a 
Telnet session, having a routing adjacency with a router, ...
- some algorithm(s) to perform TE LSP path computation,
- to provide the computed TE LSP path to the PCC (routers) using Telnet, a 
signalling protocol (Ex: [1])
Example 1: PCS= a UNIX station being able to receive request from routers 
to compute TE LSP path for primary TE LSP placement, inter-area TE LSPs, 
non packet TE LSP paths, bypass tunnel path computation, ...
Example 2: PCS= an ABR (scenario 2 or 4 of draft 
http://www.ietf.org/internet-drafts/draft-kompella-mpls-multiarea-te-03.txt)) 
for inter-area TE LSP path computation,
Example 3: any router is a PCS to compute the bypass tunnel(s) path for 
each of its neighbors to their respective (N)NHOP(s) LSRs. See the 
distributed bypass tunnel path computation scenario of 
http://www.ietf.org/internet-drafts/draft-vasseur-mpls-backup-computation-01.txt

JP.


>Thanks for your time to read this mail.
>
>
>
>Best wishes!
>
>
>--
>Chopin
>
>
>
>[1] Vasseur JP, et al., "RSVP Path computation request and reply 
>messages", <draft-vasseur-mpls-computation-rsvp-03.txt>, IETF work in 
>progress, Jun 2002.
>
>[2] JP Vasseur, "IS-IS Path Computation Server discovery TLV", 
><draft-vasseur-mpls-isis-pcsd-discovery-01.txt>, IETF work in progress, 
>June, 2002
>
>[3] M. Brunner, " COPS usage for Path Computation Servers (COPS-PCS)", 
><draft-brunner-mpls-cops-pcs-00.txt>, IETF work in progress, September, 2002
>
>[4] JP Vasseur, Peter Psenak, "OSPF Path Computation Server discovery", 
><draft-vasseur-mpls-ospf-pcsd-discovery-00.txt>, IETF work in progress, 
>June, 2002
>
>[5] JP Vasseur, Peter Psenak, "OSPF Traffic Engineering capability TLVs ", 
><draft-vasseur-mpls-ospf-te-cap-00.txt>, IETF work in progress, October, 2002
>
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