Subject: Re: pilc minutes for IETF 48

From: Vernon Schryver <[email-masked]>
Date: Mon, 9 Oct 2000 09:27:17 -0600 (MDT)

> From: Reiner Ludwig <[email-masked]>

> ...
> >   - TCP goes slightly slower, and with sufficient consecutive out-of-order
> >      segments, can send duplicate ACKs which can cause retransmissions.
> >
> >Is that last the concern?
>
> Yes, it is.
>
> As soon as the link's capacity goes beyond 4x MTU, any link layer 
> retransmission would cause a *spurious* fast retransmit / fast recovery in 
> TCP. That assumes that (1) the following 3 packets do not incur a link 
> layer retransmission and (2) out-of-order delivery is permitted even within 
> a flow.
>
> For some wireless networks this could become a real pain ...

Has the pain been simulated or measured on real wireless links?

What does it mean to say "link's capacity goes beyond 4x MTU"?  I would
have thought that a link's capacity is measured in bits/second instead of
bits.  What TCP implementation ACK's based on MTU instead of MSS?  What
does the MTU of one link in the path have to do with the MSS?  Do you mean
something about the size of the queues in the radio modem/routers?

In other words, I see the point, but I also wonder if some of the concerns
are as valid as the concerns that lead the WAP people to conclude that
TCP/IP cannot be used on wireless links because we all know that TCP/IP
cannot run on links as slow as 9600 bits/second.  So they've said in the
main IETF list and elsewhere including the trade rags...and never mind
that some of us used TCP over 9600, 2400, and even 1200 bit/sec modems,
and that some of those very fast (for the day) 9600 bit/sec modems had
ARQ and delays that varied between about 120 ms. and 300 ms. depending
on sizes of their queues.  (Telebit Trailblazers with PEP)

Vernon Schryver    [email-masked]

---

From: Reiner Ludwig <[email-masked]>
Date: Wed, 11 Oct 2000 15:41:02 +0200

>Has the pain been simulated or measured on real wireless links?

I only know of these two studies (in literature):
1. A Comparison of Mechanisms for Improving TCP Performance over Wireless 
Links. Hari Balakrishnan, Venkat Padmanabhan, Srinivasan Seshan, and Randy 
H. Katz.
2. J.C.R. Bennett, C. Partridge and N. Shectman, Packet Reordering is Not 
Pathological Network Behavior, In IEEE/ACM Transactions on Networking, 
Decem-ber 1999.

>something about the size of the queues in the radio modem/routers?

OK, so my wording was a bit loose. With the "link's capacity" I mean the 
number of TCP segments (MSS) that fit into a *link layer RTT*.

///Reiner