The MPLS WG Archive

["Dutta, Pranjal" <pdutta@riverstonenet.com>]

Hi,
             I think most of the vendors implement min 1 seconds as
hello-intervals as lesser than that might lead to scalability issues (I
am not sure if that is the reason why vendors implement at least 1 sec).
Hardware level detection will be possible only when physical layer has
end-to-end signaling mechanism for failure. In the case where end-to-end
signaling mechanism is not possible, the PLR will still see that link is
up and keep forwarding traffic to downstream LSR, but actually the link
is failed. I am basically looking for some mechanism that can detect
"Node Failure" as early as possible and PLR will initiate node
protection accordingly. I think some kind of OAM mechanism might help.

Thanks,
Pranjal



Date: Wed, 10 Aug 2005 10:02:20 -0400
From: David Charlap <David.Charlap@marconi.com>
Subject: Re: [mpls] Reg: Fast Reroute Timings in RFC 4090
To: mpls@ietf.org
Message-ID: <42FA08EC.8050502@marconi.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Dutta, Pranjal wrote:
> 
>                As per RFC 4090- Fast Reroute Extensions to RSVP-TE for

> LSP Tunnels, it takes 10 ms to reroute the LSP at PLR either by
facility 
> back up or one-to-one back up method. The re-route timing is
definitely 
> lesser when compared to time taken for global rerouting (at Ingress 
> LSR). My concern is PLR will reroute the LSP locally only when it has 
> the fault notification (after missing hellos from the downstream 
> neighbor). Assuming that min 1 second of RSVP hello interval is being 
> used, it will take at least 1 second in total to re-route the traffic 
> locally which will lead to loss of voice/video packets and affects 
> traffic performance. I would like to know if there is some effort
going 
> on in this group to address this issue. Any pointers will be helpful.

First of all, the Hello interval may be faster than 1s, depending on the

implementation.  The RFC actually recommends 5ms intervals, but I don't 
know if anyone actually implements that rate.

Second, a switch may detect a link failure by means other than loss of 
RSVP-Hello.  For instance, the hardware may detect a loss of carrier on 
the interface.  (As a matter of fact, I would expect hardware detection 
of link failure to be the only way that can reroute traffic fast enough 
for real-time applications.)

If a switch loses RSVP Hellos, but does not lose carrier, then the link 
is still physically active.  It is probably still forwarding data-plane 
traffic as well (even though the control plane has probably failed.)  So

actual losses may not be nearly as severe as you are assuming.

-- David



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