The MPLS WG Archive

["Kishore Tiruveedhula" <tiruveedhula@avici.com>]

If the both links uses the same local LDP ID (LSR Id:label space id), then
it should create single LDP session (single TCP connection).

I guess your case might be something like below:

If Router A (assuming passive) LDP ID on link1 is Ra:La1, on link2 is Ra:La2
and
Router B (assuming Active) LDP ID on both links is same Rb:Lb.


Router A link1 (LDP ID is Ra:La1) <--> Router B link1 (LDP ID is Rb:Lb)
Router A link2 (LDP ID is Ra:La2) <--> Router B link2 (LDP ID is Rb:Lb)


In this case, Router B opens two TCP connections and Router B accepts both
connections,
but it may be accepted the TCP connections
      link1 of Router A to link2 of Router B  and
      link2 of Router A to link1 of Router B.

In this case, the session initialization fails when it verifies the
Initialization message with its Hello adjacency ( by using Receiver LDP
Identifier in CommonSession Parameter TLV).


Thanks,
Kishore

-----Original Message-----
From: Tempia Bonda Alberto [mailto:alberto.tempiabonda@telecomitalia.it]
Sent: Wednesday, September 27, 2006 11:21 AM
To: mpls@lists.ietf.org
Cc: Fantolino Luca
Subject: [mpls] Question regarding LDP session setup


Hello,

I have a question about LDP session establishment between two LSRs
connected by multiple links.
A problem has occurred in the network during LDP session set up between
two routers linked by two Ethernet LANs. Below I describe the behavior.

 1. Router A sends a LDP HELLO message on each Ethernet link at the same
time; the two messages are identical, including the router ID and the
source IP address (IP address assigned to an internal interface).

 2. Router B tries to open two TCP connections with router A on port
646: it sends two TCP SYN packets (using two different local ports).

 3. Router A accepts both connections (TCP SYN ACK) and router B
completes the TCP connections set up (TCP ACK): two TCP connections are
now available for LDP message exchange.

 4. Router B (active role) sends two copies of the same LDP
INITIALIZATION message to router A through the two TCP connections.

 5. Router A sends a LDP INITIALIZATION message on the first TCP
connection and clears the second TCP connection (TCP FIN).

 6. Router A sends a LDP NOTIFICATION message with E-bit set and then
resets the second TCP connection.

What happens is that two TCP connections are created and Router B tries
to open two separate LDP sessions. It tries to open a second LDP session
when the first one is not yet established with the result that router A
closes both TCP connections and no LDP session is set up.

It is not clear to me whether this behavior is correct according to RFC
3036. In particular, is it correct that two TCP sessions are created and
a router tries to open two LDP sessions (it sends two LDP INITIALIZATION
messages) with the same peer?

Thanks,
Alberto and Luca
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