Re: PPP over UDP

From: Carl Banks (idot@vt.edu)
Date: 03/24/02 

From: idot@vt.edu (Carl Banks)
Date: 23 Mar 2002 16:08:22 -0800

Francis Litterio <franl_removethis@world.std.com> wrote in message news:<uvgbowd0b.fsf@world.std.com>...
> So TCP-over-PPP-over-TCP ought to to work too, right? What is the
> "retransmit problem" you mention above?

I don't know the details, but the basic idea is this:

Suppose a packet gets lost on the lower level TCP. Either TCP could
notice that a packet has been lost and send a retransmit request. A
problem can occur when both TCPs send a retransmit request at the same
time; the high-level TCP's request has to be transmitted over the
low-level TCP, which is sending out its own request. The high-level
request is stalled until the low-level request is filled. Then the
high level request is sent, through the low-level tunnel. In the
meantime, other transmit problems can occur, which multiplies the
confusion.

This, of course plays havoc with the buffers, which can't flush until
all the proper acknowledgments have been sent. It's possible for the
problem to cascade until the buffers are filled and the connection
stalls.

This, however, is not a problem for TCP-over-PPP-over-UDP, where only
the TCP can request retransmits. Also, the irksome little problem of
losing connections due to network shakiness goes away, since in UDP
there are no connections. 

-- 
CARL BANKS

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