Subject: RE: RFC : 3036 : Session establishment for multiple adjacencies.

From: "Sharma, Prem" <[email-masked]>
Date: Wed, 18 Jul 2001 20:31:49 -0700

Hi Vikas,

TCP connections are established between peer sessions and there could be
multiple adjacencies under per session. The very fact the multiple
adjacencies are control of one session implies that hellos sent over the
adjacencies have the same LDP identifier. Irrespective of the no. of
adjacencies, one TCP connection is good enough between peer sessions. If
there is a one to one mapping between the physical interfaces of the machine
and hello adjacencies, the label exchanged are applicable for all the
interfaces under the control of session.

Hope it clarifies.
Prem

-----Original Message-----
From: Vikas D [mailto:[email-masked]]
Sent: Wednesday, July 18, 2001 11:48 AM
To: [email-masked]; [email-masked]
Subject: Re: RFC : 3036 : Session establishment for multiple
adjacencies.

All,

A small doubt in Section 2.5.2, "Transport Connection
Establishment"
of LDP RFC.  It says:

             .............snip..................
 "An LSR MUST advertise the same transport address in all Hellos that
  advertise the same label space.  This requirement ensures that two
  LSRs linked by multiple Hello adjacencies using the same label
spaces
  play the same connection establishment role for each adjacency."
             .............snip....................

Now, under what circumstances would two LSRs, having multiple
hello adjacencies , using same label spaces on all adjacencies
try to establish multiple TCP sessions?

Is this not already precluded by the following rule in the same
section (Sec 2.5.2 of RFC 3036):

             .............snip..................
 "  1. If LSR1 does not already have an LDP session for the exchange
       of label spaces LSR1:a and LSR2:b it attempts to open a TCP
       connection for a new LDP session with LSR2."
             .............snip..................

Thanks
- Vikas

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---

From: Vikas D <[email-masked]>
Date: Wed, 18 Jul 2001 20:54:39 -0700 (PDT)

Prem,

That's what my question is.  We both agree that if the LDP identifier
is same, there needs to be one session.  Then, why does the LDP RFC
talk about trying to establish multiple sessions for two LSRs linked
by multiple Hello adjacencies using the same label spaces.  Read the
following line carefully:

"This requirement ensures that two LSRs linked by multiple Hello
adjacencies using the same label spaces play the same connection
establishment role for each adjacency."

I was thinking about it and the only reason (it is a very good reason
though) I could think of was to take care of the race condition that
may arise when both the transport addresses on the hello adjacencies
are different and each of them acts as master for one of the
adjacencies.

Hope I got it right.

Thanks
- Vikas

---

From: Eric Gray <[email-masked]>
Date: Thu, 19 Jul 2001 10:51:15 -0400

Prem/Chetan,

    I feel certain that the real misunderstanding here is in the fact
that use of "transport address" in a hello message has little to do
with the LDP ID and even less with establishing muliple sessions
between two LSRs that are both using per-platform label space.
As Chetan points out, these addresses are used to determine who
plays what role in a single LDP session.  What may not have been
made clear in this thread is that 'roles' refers to active/passive -
or who it will be that is responsible for initiating the single session.
If you don't require both LSRs to use no more than one transport
address each, then you have to define a determintistic mechanism
for ensuring that they will both come to the same conclusion as to
who plays the active role.

--
Eric Gray

Vikas D wrote:

> > TCP connections are established between peer sessions and there
> > could be multiple adjacencies under per session. The very fact the
> > multiple adjacencies are control of one session implies that hellos
> > sent over the adjacencies have the same LDP identifier. Irrespective
> > of the no. of adjacencies, one TCP connection is good enough between
> > peer sessions. If there is a one to one mapping between the physical
> > interfaces of the machine and hello adjacencies, the label exchanged
> > are applicable for all the interfaces under the control of session.
> >
> > Hope it clarifies.
> > Prem
> >
> >
> >
> >  "An LSR MUST advertise the same transport address in all Hellos
> > that advertise the same label space.  This requirement ensures that
> > two LSRs linked by multiple Hello adjacencies using the same label
> > spaces play the same connection establishment role for each adjacency."

---

From: Vikas D <[email-masked]>
Date: Fri, 20 Jul 2001 09:46:52 -0700 (PDT)

All,

There appears to be an error in 

    draft-ietf-mpls-generalized-cr-ldp-03.txt

Section 5.1 - Procedures (for Explicit Label Control) requires the
generated Label Set TLV and/or Upstream Label TLV to be included in
the outgoing MAPPING message.

Should it not be the "Label Request" message that carries these TLVs?

- Vikas