Subject: RE: RE: Question about Labels

From: "Morgan, Richard" <[email-masked]>
Date: Mon, 22 Jan 2001 15:49:43 -0000

> OK.. Thats clear now.
> Now, what about multiple services (FECs) that may follow a 
> same path ??
> How the protocol manage the same route, just with another labels ??

Yep one LSP per FEC, they may well take entirely different routes around the
network. For instance one might be low latency and use different physical
links. So it is irrelavant that they use the same path, as each stream is
separated into different LSP's and these are dealt with individually.

> So as I understand the RVSP must be checking for the 
> integrity of the LSP,
> is that correct ??

Yes a static LSP which is hand configured has no fail over, should a link
fail it will become a black hole for packets. RSVP sends periodic Hello
messages up stream, this is done node to node rather than end to end. Should
the hello message not be acknowledged RSVP will give the link a little more
time to recover and on failing this it will send a tear down message down
stream to the originating router. The timers on these checks can be set on
the RSVP path setup. Interestingly this is one of the scalabity problems
with RSVP tunnels - how many RSVP tunnels can a router be expected to
mantain and check? I don't know much about how LDP copes with link failure
but it also has to check.

Rich

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From: David Charlap <[email-masked]>
Date: Mon, 22 Jan 2001 11:30:31 -0500

"Morgan, Richard" wrote:
>> 
>> OK.. Thats clear now.  Now, what about multiple services (FECs) that
>> may follow a same path ??
> Yep one LSP per FEC, they may well take entirely different routes
> around the network. For instance one might be low latency and use
> different physical links. So it is irrelavant that they use the same
> path, as each stream is separated into different LSP's and these are
> dealt with individually.

Note that per FEC does not necessarily mean per IP prefix.  Multiple
prefixes that all leave the MPLS cloud through a common router interface
and have identical traffic characteristics can be considered a single
FEC and can use a single LSP.

> Yes a static LSP which is hand configured has no fail over, should a
> link fail it will become a black hole for packets. RSVP sends periodic
> Hello messages up stream, this is done node to node rather than end to
> end. Should the hello message not be acknowledged RSVP will give the
> link a little more time to recover and on failing this it will send a
> tear down message down stream to the originating router. The timers on
> these checks can be set on the RSVP path setup. Interestingly this is
> one of the scalabity problems with RSVP tunnels - how many RSVP
> tunnels can a router be expected to mantain and check? I don't know
> much about how LDP copes with link failure but it also has to check.

RSVP is a soft-state protocol.  Every setup message (Path and Resv) that
is sent is periodically resent.  If these refreshes stop happening, then
the neighbor switch assumes that it missed a teardown message and does
its own teardown of the corresponding state.

RSVP Hello is an extension, provided by the -TE extensions.  RSVP Hello
can detect node and link failure.  It does not, however, provide for
reliable message delivery, and so does not eliminate the need for
refreshing state.  Note that Hello messages are sent on a per-interface
basis, while refreshes are on a per-LSP basis.

The RSVP working group has a set of optional refresh-reduction
extensions which (among other things) does provide a mechanism for
reliable message delivery.  When used in conjunction with hello
messages, it can eliminate the need for periodic refreshes.  This is one
of the key ways of getting RSVP to scale up to huge numbers of sessions.

WRT LDP, this isn't a problem.  LDP messages are not periodically
refreshed.  Since TCP is used for message delivery, reliability is
assumed.  State remains until neighbor failure is detected or the state
is explicitly torn.  Neighbor failure is detected under one of three
conditions:
	- The neighbor stops broadcasting Hello discovery messages
	- The neighbor stops sending Keepalive messages on the TCP
	  connection.
	- The TCP connection to the neighbor breaks.

-- David

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From: Arturo Nunez <[email-masked]>
Date: Mon, 22 Jan 2001 11:48:05 -0500 (EST)

> FEC and can use a single LSP.
In the worst case.
If  we have different FECs[Services]  that have the same [origin] and the same [desteny]
Will they have different LSPs , created with different labels ??
I guess yes ..

Regards

Arturo Nunez

---

From: Arturo Nunez <[email-masked]>
Date: Mon, 22 Jan 2001 12:20:54 -0500 (EST)

> Will they have different LSPs , created with different labels ??
As I have readed a Label has the following fields

Label|CoS|Stack|TTL

So if we have almost the same path from the origin to the destiny and the
only difference is the FEC , in what field will be the difference:
-	Label or Stack ??

 Regards
 
 Arturo Nunez