Subject: RE: Kicking TCP

From: Marie-Jose Montpetit <[email-masked]>
Date: Tue, 7 Mar 2000 04:02:22 -0800

I would tend to agree that some simulations maybe needed.

However the kicking strategy is closely related to something I have been
thinking for a while on how to "advertise" that a link is down or degraded.
ICMP extensions were also our first idea or raising the "cost" of a route
when the link is down (when multiple routes are available).

Maybe what is needed in LINK is a section on suggested mechanisms to survive
capacity degradation?

Marie-Jose

-----Original Message-----
From: Zachary Amsden [mailto:[email-masked]]
Sent: Monday, March 06, 2000 11:54 PM
To: [email-masked]
Cc: [email-masked]
Subject: Re: Kicking TCP 

Yes, kicking TCP is useful in limited situations, but you needn't go to much

trouble at all to do it - you can easily add a kernel hack to reset the 
retransmission timer on the transmitting socket.  This will trigger 
retranmissions without losing any TCP state, and avoids any protocol 
modifications.

This serves a useful purpose on what I call "blackout" lines - lines of 
communication that temporarily go dead for several seconds and require 
complicated renegotiation (like some wireless links).  On public connections

with interactive users, having a kick command is a very bad idea from a 
congestion point of view.

A neat solution to this problem would be to cache link characteristics with 
routes, and to tweak TCP strategy routines based on link types - i.e. a 
kick_timeout that limits the maximum retransmission backoff allowed, after 
which the retransmit timer starts it's cycle over again, or stays at the 
maximum.  (Modifying cwnd is also possible, but I wouldn't recommend it 
without analysis).

One would really need to do some serious simulation runs to analyze the 
effects of kicking connections under these circumstances.

If you still can't complete the transmission with this strategy, I can't
think 
of any other strategy that would help - if you don't get any ACKs in return,

kicking is useless.

-- 
Zachary Amsden  [email-masked]  3-6919  31-2-510  Core Protocols

---

From: Phil Karn <[email-masked]>
Date: Tue, 7 Mar 2000 23:22:26 -0800

>thinking for a while on how to "advertise" that a link is down or degraded.

You can already do that with the ICMP Destination Unreachable (down)
or Source Quench (degraded) messages. But some would claim that no explicit
notification is really needed; packet drops will tell TCP all it needs to
know.

Phil