Subject: [Tsvwg] HS-TCP/FAST-TCP: Kind of Packet Loss?

From: Reiner Ludwig <[email-masked]>
Date: Tue, 22 Jul 2003 14:10:26 +0200

As I understand, the motivation given for HS-TCP and FAST-TCP is that since 
the required cwnd to fill the pipe is so high, it would require 
unrealistically low rates of non-congestion-related packet losses.

Now, I wonder whether anybody has studied what kind of 
non-congestion-related packet losses we see in the target regime of 10 
Gbit/s and beyond ...

Do such optical fiber links still do CRC, i.e., are the 
non-congestion-related packet losses mostly due to packets being discarded 
at the link layer? Or are maybe many of such non-congestion-related packet 
losses due to TCP checksum errors, and thus the TCP receiver silently 
discarding the packet?

If it was the latter, one solution could be the definition of an explicit 
TCP NACK that the TCP sender could use to suppress the reduction of cwnd. 
Clearly, this would have some security considerations, but I guess those 
could be fixed, e.g., with a nonce.

Maybe this would be a good reason to disbale CRC on such optical fiber links.

Did I miss something?

///Reiner

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From: John Heffner <[email-masked]>
Date: Tue, 22 Jul 2003 10:30:33 -0400 (EDT)

> Maybe this would be a good reason to disbale CRC on such optical fiber links.

I think packet corruption is not currently the greatest problem.  There
are a number of other causes of packet loss, including:
  - intermittent flows starting up (punching holes with slow-start)
  - misbehaving switches
  - burstiness, and a small router buffer

When these things cause cwnd to be halved on the order of once every few
minutes, they can seriously degrade the performance of high-BDP flows.  A
faster response time can really help, and I think is ultimately necessary
if we expect end-to-end congestion control to work in messy networks at
large window sizes.

  -John