Subject: RE: RSVP Hello

From: "Kota, Ravikumar" <[email-masked]>
Date: Thu, 23 Aug 2001 11:06:17 -0400

Hi ,

By mistake i have given explanation for discovery message exchange (with
hello message s) which are helpful
for session initialization/establishment in LDP protocol.(udp based exchange
for learning about neighbours and their label spaces )
But the actual question is related to  "hello" message exchange in RSVP.

From RFC 3036 :LDP Specification
1.2. LDP Message Exchange

   There are four categories of LDP messages:

      1. Discovery messages, used to announce and maintain the presence
         of an LSR in a network.

      2. Session messages, used to establish, maintain, and terminate
         sessions between LDP peers.

      3. Advertisement messages, used to create, change, and delete
         label mappings for FECs.

      4. Notification messages, used to provide advisory information and
         to signal error information.

   Discovery messages provide a mechanism whereby LSRs indicate their
   presence in a network by sending a Hello message periodically.  This
   is transmitted as a UDP packet to the LDP port at the `all routers on
   this subnet' group multicast address.  When an LSR chooses to
   establish a session with another LSR learned via the Hello message,
   it uses the LDP initialization procedure over TCP transport.  Upon
   successful completion of the initialization procedure, the two LSRs
   are LDP peers, and may exchange advertisement messages.

   When to request a label or advertise a label mapping to a peer is
   largely a local decision made by an LSR.  In general, the LSR
   requests a label mapping from a neighboring LSR when it needs one,
   and advertises a label mapping to a neighboring LSR when it wishes
   the neighbor to use a label.
-------------------------------------------------------------------------

2.5. Establishing and Maintaining LDP Sessions

2.5.1. LDP Session Establishment

   The exchange of LDP Discovery Hellos between two LSRs triggers LDP
   session establishment.  Session establishment is a two step process:

            -  Transport connection establishment.
            -  Session initialization

   The following describes establishment of an LDP session between LSRs
   LSR1 and LSR2 from LSR1's point of view.  It assumes the exchange of
   Hellos specifying label space LSR1:a for LSR1 and label space LSR2:b
   for LSR2.

Regards
Ravikumar Kota

-----Original Message-----
From: Prabakaran, Thirumali [mailto:[email-masked]]
Sent: Thursday, August 23, 2001 10:33 AM
To: Kota, Ravikumar; 'manoj juneja'; [email-masked]
Subject: RE: RSVP Hello 

Hello Ravikumar Kota,

I added my views and comments,
Please see my inlined comments with [Prabakaran].
Further clarifications/corrections and comments, I am pleased to recv it.

Thanks and regards,
Prabakaran TS.
Software Engineer,
Cratos Networks, Inc.,
100 Nagog Park
Acton, MA  01720
978-263-0094 - Extn:8159

-----Original Message-----
From: Kota, Ravikumar [mailto:[email-masked]]
Sent: Thursday, August 23, 2001 10:00 AM
To: 'manoj juneja'; [email-masked]
Subject: RE: RSVP Hello 

Hi Manoj ,

RSVP hellos will be useful to discover/make adjancies in MPLS cloud.

[Prabakaran] As per draft-ietf-mpls-rsvp-lsp-tunnel-08.txt, In section 5,
the Hello message in RSVP-TE is a optional message, and I think
it is not used for discover/make adjacencies in MPLS cloud, it 
is a mechanism which provides node to node failure detection.  
When such a failure is detected it is handled much the same 
as a link layer communication failure.  

As these messages will be useful to session initialization/establishment,

[Prabakaran]The RSVP Hello message is not used for session
initialization/establishment
in RSVP TE extension, it's purpose is to detect node failures.

hello messages should be exchanged irrespective of session .

[Prabakaran]If two nodes are supporting Hello message and which is enabled
in both
node for node failure detection, then the Hellos can be exchanged between
the nodes.

Also please refer the following paragraph from the Section 5 of the draft
for
more understanding.

"  The Hello extension is specifically designed so that one side can use
   the mechanism while the other side does not.  Neighbor failure
   detection may be initiated at any time.  This includes when neighbors
   first learn about each other, or just when neighbors are sharing Resv
   or Path state "

Regards
Ravikumar Kota
978 869 3314 (cell)
978 263 0094 (x 8157)(work)

-----Original Message-----
From: manoj juneja [mailto:[email-masked]]
Sent: Thursday, August 23, 2001 2:10 AM
To: [email-masked]
Subject: RSVP Hello 

Hi All,
         The RSVP-TE draft states that the RSVP Hellos should be transmitted

when there is a RSVP state between the neighbors i.e. when there is 
Path/Resv state. In other words, if there is a RSVP session between two 
nodes then the RSVP hellos need to be exchanged. If the RSVP session is to 
be released (i.e. graceful termination), does this mean that RSVP hellos 
should not be exchanged between the RSVP neighbors as long as there is no 
RSVP session between them ?

[Prabakaran ] In my understanding, once Hello message exchange is started
until, the protocol is disabled 
on an interface, the messages should be keep on exchanging for node failure
detection, since
in your case after the graceful termination of the the sessions(I think all
the sessions associated
with respect to that link), then there will not be any PSB/RSB associated
with that interface, in that
case what is the use of having hello messages exchanged in between the
nodes, even if the node fails, 
which is detected by Hello message which is of no use to the PSB/RSB, I
think it is a protocol implementation
specific and how the node failure notification is handled when there is no
PSB/RSB associated with it..??

Regards,
manoj.

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