Subject: Re: TTL value of VC Label?

From: David Watkinson <[email-masked]>
Date: Mon, 05 May 2003 17:03:09 -0400

Eric Rosen wrote:

> David> Setting the TTL is application dependant.  The originating PE will know
> David> if the PSN tunnel reaches the destination node for the PW.
>
> I don't think it will always know this.

For applications that do not know the number of hops will have to set the TTL to a
larger value (e.g. the same TTL used for originated IP packets).  Many
applications will know that the far-end IP address of the tunnel is the same as
the IP address of the termination point of the PW.  For those cases, it could set
the TTL to 2.

>
>
> David> In other cases,  for example, DTLS, a  TTL of 4 may be  needed due to
> David> the two additional hops taken between the L2PE and PE at each end.
>
> You could have an L2PE/PE hop at  one end but not at the other.  In general,
> you wouldn't know whether the other end has such a hop or not.

Yes, that is true, however setting a TTL=4 is much safer than using a TTL=3.

I still think that a statement such as "The setting of the TTL value in the PW
label is application dependant" is a suitable replacement for the existing text.

---

From: Luca Martini <[email-masked]>
Date: Wed, 07 May 2003 15:49:51 -0600

Eric,

I would like to point out that several of the vendors reading this list have 
experienced bugs in their MPLS routers that cause the MPLS LFIB to 
misprogrammed , and traffic to be sent to the wrong location.
Initially we where attempting to mitigate such problems by making sure that a 
PW packet could not enter the SP network in this fashion.

How about this ?

"The setting of the TTL value in the PW
label is application dependent, however in a strict point to point application 
the TTL should be appropriately set to 2"

Luca

David Watkinson wrote:
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