Subject: Re: BER and TCP/IP performance

From: [email-masked]
Date: Wed, 31 Mar 1999 10:18:05 -0800

Jamshid-

Thank you for raising an important point. Not all bit errors are equivalent.
Most (all?) satellite links utilize FEC. Block coding results in lost blocks
when uncorrectable errors occur. The packet loss rate due to corrupted blocks is
much different that that due to a uniform distribution of bit errors. So, the
translation of channel BER (after FEC) to packet loss rate (which is necessary
to interpret the results of the reference below will depend heavily on the
adaptation of IP to link layer.

Having said all that, my understanding is that to get 'decent' performance out
of TCP (ie., a window > 1) packet losses should be limited to less than 1 per
RTT.

There was a thread recently on the pilc list (see http://pilc.grc.nasa.gov/pilc)
on channel BER that might be of interest to tcpsat participants. I am
crossposting to the pilc list and would like to recommend continuing this
discussion there.

--aaron

From: [email-masked] AT mime on 03/31/99 10:23 AM

To:   [email-masked] AT mime@PMDF-IMP@HUB-H4@CCMAIL
cc:   [email-masked] AT mime@PMDF-IMP@HUB-H4@CCMAIL (bcc: Aaron D
      Falk/ES1/ES/HSC)

Subject:  Re: BER and TCP/IP performance

The currently accepted best formula for TCP performance as a function of packet
loss is given in the following paper:

Padhye, J., Firoiu, V., Towsley, D., and Kurose, J., Modeling TCP Throughput: a
     Simple Model and its Empirical Validation, UMASS CMPSCI Tech Report
     TR98-008, Feb. 1998.

The URL is:
     ftp://gaia.cs.umass.edu/pub/Padhye-Firoiu98:TCP-throughput.ps.Z

You can find a compilation of other papers on the subject on the TCP
Friendly website:
     http://www.psc.edu/networking/tcp_friendly.html

You will need to do the same calculation Fred showed to convert BER to packet
loss rates.  I think you will find that BER at 1E-8 is the highest acceptable,
and for fast links you might find 1E-12 to be desirable.

--Jamshid

---

From: [email-masked] (Robert C. Durst)
Date: Wed, 31 Mar 1999 14:13:46 -0500

Aaron,

> of TCP (ie., a window > 1) packet losses should be limited to less than 1 per

It had really ought to be darn better than that, since you're going to be
cutting your rate in half for every loss and building rate back at about 1
additional pkt per 2 RTTs.  For any kind of reasonable (desired) bandwidth over
a long delay link, losses need to be pretty infrequent.

The one-loss-per-RTT that you cite was the magic number (pre SACK) that allows
you not to time out (arguably important, but not sufficient for "decent"
performance).

Regards, 
Bob Durst
[email-masked]

---

From: Eric Travis <[email-masked]>
Date: Wed, 31 Mar 1999 15:19:28 -0500 (EST)

>Having said all that, my understanding is that to get 'decent'
>performance out of TCP (ie., a window > 1) packet losses should 
>be limited to less than 1 per RTT.

Only if the definition of 'decent' translates to an effective 
cwnd of ~1 or 2 segments :o(

You really want your (non-congestion based) losses bunched within 
single RTTs that are fairly widely spaced [how wide will depend 
on what your optimal cwnd for the path would be]. Otherwise TCP 
will be severely under driving your link.

>There was a thread recently on the pilc list (see 
>http://pilc.grc.nasa.gov/pilc) on channel BER that might be 
>of interest to tcpsat participants. I am crossposting to the 
>pilc list and would like to recommend continuing this
>discussion there.
 
Is it me, or haven't we already seen this thread replay 
itself a half dozen times already on this *very* mailing 
list? :o)

This seems like a good time to plug the excellent tcpsat 
mailing list archive that Mark makes available off of:

http://tcpsat.grc.nasa.gov

I'm confident that if one pulls the archive and does a 
grep on BER, errors or loss - you will be able to extract 
virtually all the collected wisdom/opinions from the cyclic 
(and often passionate) "BER Wars" thread that appear every 
2-3 months during 1998. I'm pretty sure it is all there 
already (several times).

The stuff its there for those who want to dig it out; 
Archives are there available, so let's not replay the
old stuff again on either tcpsat or pilc? New stuff 
would be fantastic.

Eric

---

From: Vijay G Bharadwaj <[email-masked]>
Date: Wed, 31 Mar 1999 17:29:47 -0500 (EST)

> will be severely under driving your link.

Do you have data to support this? I have lots of measurements over a
channel simulator that say I can get >90% link utilization on a T1 using
TCP+standard_gadgets at 1E-8 BER, around 85% at 1E-7, and just over 50% at
1E-6. So even at 1E-6 I'm getting way better than your 1-2 segment cwnd
would give me. These are random errors, delay is GEO (250ms). If you have
data to the contrary I'd be very interested. 

> 2-3 months during 1998. I'm pretty sure it is all there 

I just did this. A search on BER gave maybe 6-7 hits, mostly incidental
stuff, and two of them were actually because someone had ROBERT in their
email address. Maybe this list needs a FAQ? ;) I'll look harder, but just
FYI... 

-Vijay