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Congested TCP/LDP - How to confirm that the label actually made it to the (targeted) peer!

  • From: Manish Gupta <manish@xebeo.com>
  • Date: Wed, 19 Feb 2003 15:46:28 -0500
  • Resent-Date: Wed, 19 Feb 2003 17:22:23 -0500
  • To: mpls-ops@mplsrc.com
  • User-Agent: Mozilla/5.0 (X11; U; FreeBSD i386; en-US; rv:1.1b) Gecko/20020813

Hello

    I have one doubt which I am not being able to resolve by reading
    LDP specs. Hoping that somebody on the list will be able to help
    me out.

    Here is the scenario:

   [LSR-A] ----- <targeted ldp session> ----- [LSR-B]

   note.1: LSR-A and LSR-B are *NOT* connected directly.

   Time-1: <A> and <B> establish a targeted ldp session.

   Time-2: <A> sends a label map to <B>.

   Time-3: <B> receives label map from <A>

   In the above scenario, <A> has no means to detect when did
   <B> actually received its label. Once <A> puts the packet on
   the wire, it assumes (thanks to reliable TCP) that label will
   reach <B>. The difference between Time-3 and Time-2
   could be significant if network connecting <A> and <B> is
   congested. The packet may end up sitting in TCP queues
   for sometime.

   If I am not mistaken, <A> has no means to detect this time
   difference and may (incorrectly) start sending out labeled
   packets to <B> and if data packets reach <B> before label
   could be installed, they will get dropped (data packet
   does not have to take the same path as the label pdu).

   Can anybody explain how one can handle this? Build some
   sort of end-to-end LDP level reliability.

   thanks
   manish

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