Subject: Re: TTL value of VC Label?

From: Skip Booth <[email-masked]>
Date: Thu, 1 May 2003 15:07:20 -0400 (Eastern Daylight Time)

On Thu, 1 May 2003, Eric Rosen wrote:

>
> Wei> packets arrived from  one PW will remove its  current PW encapsulation,
> Wei> e.g. tunnel  and vc labels,  and then add  a new PW  encapsulation.  In
> Wei> this way, it  allows the data path to have  a unified packet processing
> Wei> behavior regardless what PW encapsulation is used in each leg.
>
> I guess  I don't agree.  If  you're splicing together  two MPLS pseudowires,
> you should  be able to  use existing MPLS  forwarding hardware, and  just do
> ordinary MPLS label replacement with ordinary MPLS TTL processing.

Not quite true as you would want to go through the control word processing
including any sequence number options.  This isn't straight MPLS forwarding IMO.
There's probably a degenerate case where you could handle this like straight
MPLS forwarding though, but that shouldn't be the base assumption.

>
> In fact,  even in the  case where you  are moving from one  encapsulation to
> another I think you want to make sure that the outgoing TTL is one less than
> the  incoming   TTL,  in  order   to  maintain  the  usual   loop  detection
> functionality of TTL.

I tend to agree here.  I guess the question is whether the TTL's meaning is
really end to end, i.e AC to AC, or per PW hop.  It probably has more value
meaning end to end so that a service provider could limit the number of PW
splices that occur for the end to end service.  However, it's more difficult to
map the VC label TTL into the IP header TTL for AToM to L2TP tunnel splicing.
They certainly aren't treated exactly the same way by the intermediate P
routers.

-Skip

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---

From: Eric Rosen <[email-masked]>
Date: Thu, 01 May 2003 15:14:52 -0400

Eric> If you're splicing  together two MPLS pseudowires, you  should be able
Eric> to use  existing MPLS forwarding  hardware, and just do  ordinary MPLS
Eric> label replacement with ordinary MPLS TTL processing.

Skip> Not  quite true  as you  would  want to  go through  the control  word
Skip> processing including any sequence number options.

I don't  think so, the  control word processing  should only be done  at the
ultimate endpoint, there's no point to doing it multiple times.

---

From: Wei Luo <[email-masked]>
Date: Thu, 01 May 2003 15:23:57 -0700

> They certainly aren't treated exactly the same way by the intermediate P

It's a "surprise" how quickly we switched topic on the thread :-).  At any rate,
having an end-to-end TTL is more valuable than per-PW segement TTL.  Therefore,
a fixed TTL value should not be imposed on the VC label if we expect to have
features like "traceroute" or "loop prevention".  I would agree not to specify a
fixed value for the VC label TTL.

As for mapping TTL from MPLS to L2TP, it's definitely not a good idea to map the
VC labe TTL to the IP header TTL, as the IP header TTL has a semantics of
Layer-3 IP hops.  Instead, it should be mapped to a TTL field in L2TP header,
either the session header or the L2-specific sublayer, none of which has such a
field currently.

---Wei