The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

Vadlamani rao wrote:
> 
> What happens if the Path message using the RSVP-TE mechanism is lost
> in the network?

Then it gets lost.  The last router that successfully received the Path
will refresh it when its refresh interval expires.  The refresh message,
which is identical to the Path message it tried to send out originally,
will finish establishing the Path.

> Currently, if I understand correctly there is no way for the ingress
> LSR (which initiates the LSP setup) to know about it, since the
> refresh timer starts only after the RESV is received.

No.  Every router starts its refresh timer as soon as state is
established.  To do otherwise will guarantee failure if a single message
is lost.

The ingress LSR will not know about the loss, but it doesn't have to. 
It will simply experience a (perhaps substantial) delay between sending
out its Path message and receiving the Resv message.

If it chooses not to wait, and tears down the Path state, it is free to
do so.  It is also free to immediately begin refreshing the Path state
and wait indefinitely for a response.

> If this is true, the HELLO mechanism specified in the draft should
> have been mandatory to be implemented and not optional.

Hello can be used to detect link failure.  It does nothing to prevent or
detect Path/Resv packet loss.  If a link goes down, Hello will detect it
quickly.  (Of course, if carrier is lost on a link, it will be detected
without Hello processing on most hardware.)  If the packet is lost
without link failure (maybe the receiving router is overloaded and has
run out of receive buffer space), Hello may not detect anything unusual.

-- David