RE: Graceful restart - inter-protocol dependencies

[ruiquan.jing@ties.itu.int]

Hi David,

We should specify which version of RSVP we are talking. What I have said is 
about GMPLS RSVP-TE as used in ASON. 

When there is a failed link within the network, the ingress node will know the 
failure by transport plan OAM mechanism such as SDH path layer AIS, or by RSVP-
TE Notify message. And usually these mechanisms are faster than routing 
protocol convergence speed. After the ingress node knows which link or route 
has failed, it will do the rerouting while excluding the failed link or route. 

Hope I say it clearly :-)

Rick 

-----Original Message-----
From: owner-ccamp@ops.ietf.org [mailto:owner-ccamp@ops.ietf.org] On Behalf Of 
David Charlap
Sent: Thursday, September 29, 2005 10:35 PM
To: 'IETF MPLS List'; 'IETF CCAMP List'
Subject: Re: Graceful restart - inter-protocol dependencies

But RSVP doesn't work this way.  It is a soft-state protocol.

If the route table changes such that the next-hop interface changes, RSVP is 
supposed to reroute the connection accordingly.

This probably won't happen if the next-hop is a strict-hop in an ERO, but it 
can easily happen if the next-hop is loose or if the LSP is signaled without 
an ERO (meaning routing is consulted to determine the next-hop to the egress
node.)

If there is a paragraph in an RFC that says RSVP-TE should ignore all route-
table changes once an LSP comes up, I missed it.

-- David

Jing Ruiquan wrote:
> I think only the  originating node should do the rerouting with 
> RSVP-TE .At the same time, all the nodes will converge their IGP 
> routing
data base.
> 
> Rick
> 
> -----Original Message-----
> From: owner-ccamp@ops.ietf.org [mailto:owner-ccamp@ops.ietf.org] On 
> Behalf Of David Charlap
> Sent: Thursday, September 29, 2005 2:37 AM
> To: IETF MPLS List; IETF CCAMP List
> Subject: Graceful restart - inter-protocol dependencies
> 
> Is there any point to implementing RSVP-TE's graceful restart without 
> also implementing graceful restart for routing protocols?
> 
> On the one hand, RSVP doesn't require routing to recover LSPs.  It 
> knows the next-hop interface, because of the preserved data-plane
connection.
>   Whatever other information it may need, the switch can either 
> preserve this information or recover it from a neighbor using the 
> RecoveryPath object (draft-ietf-ccamp-rsvo-restart-ext-03).
> 
> On the other hand, nodes more than one hop upstream of the failure 
> will detect the loss of routing-connectivity to the failed node if IGP 
> graceful restart is not also implemented.  They may reroute the LSP 
> away from the failed node, or tear it down altogether, even though the 
> data plane is still active and RSVP graceful restart is recovering the
control-plane state.
> 
> An originating node may consider the destination unreachable, as a 
> result of losing the routes even though the data-plane for the LSP is 
> still up (which can be confirmed via OAM.)
> 
> A transit node, when processing an loose ERO-hop, may choose to 
> reroute or fail the LSP if its local topology information says that 
> the failed (and
> restarting) node is not available.  It might even choose to do this 
> for an established LSP, as a result of Path refresh processing.
> 
> My questions are:
> 
> 1: Does this mean it's pointless to use RSVP graceful restart without
>     also using IGP graceful restart (for whatever IGP is active).
> 
> 2: Is IGP graceful restart sufficient to prevent this problem?  For
>     instance, OSPF's restart procedure requires all preserved state to
>     be thrown away if a topology change is detected.
> 
> 3: An originating node can use OAM to validate the data plane of an
>     LSP, and choose to ignore what routing tells it about the
>     destination's reachability.  But what about transit nodes?  As far
>     as I know, MPLS doesn't support segment-OAM, and it would be
>     prohibitive for every transit node to run its own OAM streams
>     to detect self-to-end connectivity.
> 
> 4: Are there other solutions to this problem?
> 
> One possible solution might be route-pinning, but RSVP doesn't have a 
> built-in mechanism for this.  The usual workaround (signal the LSP, 
> requesting route-recording, then turn the RRO into an ERO for 
> subsequent
> refreshes) can work, but are there situations where even this would be 
> insufficient to prevent a transit node from rerouting/tearing the 
> connection in this particular situation (where RSVP is doing a 
> graceful restart but the IGP is not)?
> 
> -- David