Subject: Re: Correcting LINK (Advice for Internet Subnetwork Designers)

From: Phil Karn <[email-masked]>
Date: Wed, 28 Feb 2001 17:15:05 -0800

Hi all,

I'm confused by this section still in the LINK draft:

Assuming constant, uniform bit-errors
(instead of packet errors), and assuming that the BER is small enough
for the approximation [p=BER*PACKET_SIZE] to apply, a simple
derivation will show that larger packet sizes still result in
increased TCP performance.  For this reason (and others) it is
advisable to support larger packet sizes where possible.

To derive this, simply plug in p = BER*PACKET_SIZE into the simple
formula for performance.  The result is p = O(sqrt(PACKET_SIZE)),
providing larger performance for larger packet sizes.

If the approximation p = BER*PACKET_SIZE breaks down, and in
particular if the BER is high enough that BER*PACKET_SIZE approaches
(or exceeds) 1, the packet loss rate p will tend to 100%,
resulting in zero throughput.

(end of excerpt)

I can't see how this can be true. Yes, if the *packet* loss
probability is independent of packet size, clearly it's better to send
large packets. But if the packet loss probability increases with size
(as it would with independent bit errors), then I don't see how large
packets could still be better.

Any comments on this?

Phil

---

From: Joe Touch <[email-masked]>
Date: Wed, 28 Feb 2001 18:53:46 -0800

> packets could still be better.

Packet loss probability would have to be linear (or worse)
to size; anything less would justify larger packets.

One reason this can occur is because doubling the transport
PDU doesn't quite double the bits on the wire. Network and link
bits aren't doubled, so the resulting packet is somewhat smaller
e.g., for 512 byte payloads:

	original packet (e.g.)		doubled transport
	512 data			1024 data
	20 TCP				20 TCP				
	20 IP				20 IP
	14 ethernet			14 ethernet
	---------------------		---------------------
	566 bytes on the wire		1078 bytes on the wire
	
	2 packets = 1132 bytes, nearly 5% larger (thus 5% more likely
	to be hit by independent bit errors, right??)

Granted, 5% is small, but the smaller the packet, the larger that gets.

(hmmm - almost sounds a paper for next years' Sigcomm ;-)

PS - let me know if I've missed something; this is somewhat outside my
sphere...

Joe