Subject: [L2tpext] l2tp ports

From: "Avinash Natarajan" <[email-masked]>
Date: Thu, 25 Sep 2003 14:25:38 +0530

Hi,
    i want to confirm if my understanding of how ports are set in lac/lns is right. i have a nat
which checks for the entire session info before letting a pkt pass thru that causes l2tp to
not operate over it.
    is the following port related data right for l2tp operation?

-----------------------------------------------------------------------------------------------
LAC(initial packet):
Src port = X
Dstn port = 1701

LNS (response to the above pkt):
Src port = Y (some free port,cud also be 1701)
Dstn port = X

LAC (rest of the tunnel's life after establishment)
Src port = X
Dstn port = Y

LNS (rest of the tunnel's life after establishment)
Src port = Y
Dstn port = X
-------------------------------------------------------------------------------------------------

  also, is there any scenario(any know implementation of lns) in which an LNS might decide to choose a 
source ip different from that on which it received the initial packet. there is no mention of such a thing in the
l2tp std but i saw such a scenario being discussed in linux netfilters forum with respect to l2tp operation 
over nat.

-Avinash

"Because i live in a large tin can and i work in an egg carton Flirting is the only joy i have:)"