The MPLS WG Archive

[Kappler Cornelia <Cornelia.Kappler@icn.siemens.de>]

I'm wondering about the definition for bandwidth reservation for LSPs. Sorry if this is obvious. 

My understanding is the following: Bandwidth for LSPs can be reserved using e.g. RSVP-TE. However, what does "reservation" mean? It means that at LSP establishment time, the LSR checks whether it has enough resources available. However, it only does a "book keeping" of its resources. It does not need to control their actual use. Admission control and policing must be performed by the LERs. If they don't do it, bandwidth is not guaranteed.

I derived this understanding from conversations with others. Cisco seems to implement it this way. Checking on RFCs and IDs however, the picture is not entirely straight:

draft-ietf-mpls-diff-ext-08.txt (MPLS Support of Differentiated Services) corroborates the above view:

"Establishing an E-LSP or L-LSP with bandwidth reservation means that bandwidth requirements for the LSP are signaled at LSP establishment time. Such signaled bandwidth requirements may be used by LSRs at establishment time to perform admission control of the signaled LSP over the Diff-Serv resources provisioned (e.g. via configuration, SNMP or policy protocols) for the relevant PSC(s)"

I.e. there is no mention of using the signaled bandwidth for on-line control of resource usage. 

I would expect to find something on the subject in the RSVP-TE draft, draft-ietf-mpls-rsvp-lsp-tunnel-07.txt. However, it only seems to talk about "reserving" bandwidth without defining what that is. It also says:

"an implementation SHOULD support the Controlled-Load service"

Checking, on "Controlled-Load service" one finds:

"Network elements must not assume that data sources or upstream elements have taken action to "police" controlled-load flows by limiting their traffic to conform to the flow's TSpec.  Each network element providing controlled-load service MUST independently ensure that the requirements given above are met in the presence of nonconformant arriving traffic for one or more controlled-load flows."

I.e. each individual LSP is responsible for guaranteeing bandwidth after all. 

Additionally, RFC 2205 on RSVP states

"During reservation setup, an RSVP QoS request is passed to two local decision modules, "admission control" and "policy control". ... If both checks succeed, parameters are set in the packet classifier and in the link layer interface (e.g., in the packet scheduler) to obtain the desired QoS. "

So RSVP seems to require the individual routers make sure all flows receive their due. Since RSVP-TE is just an extension of RSVP I would assume it works with the same router requirements.

I am confused. Which one is it, must LSPs assure bandwidth reservations are met, or not?

Thanks for your help,

-- Cornelia Kappler