Subject: Re: (IPng 5912) Re: Last Call: Internet Protocol, Version 6

From: Christian Huitema <[email-masked]>
Date: Tue, 23 Jun 1998 11:54:05 -0400 (EDT)

Masataka has a point.  If you take the example of a binary file transfer,
even throwing in IPSEC, TCP headers and TCP ACK, an MTU of 1280 bytes
results in approximately 91% efficiency, while a PMTU of 1500 bytes
results in 92% efficiency. (Efficiency being the number of total bytes
sent on the network versus the number of bytes in the file.) So, if you
are in a mostly Ethernet environment, there is not a great incentive to
properly implement PMTU discovery.

The situation changes if the station can send large frames, of say 4K.  In
that case, the efficiency raises to 97%, and, maybe more important, the
number of packets is divided by three, which reduce the number of
interrupts, software cycles, etc.

In short, if the guaranteed MTU is large enough, implementers wont bother
with PMTU discovery.  Large enough depends on the application:

 * The DNS today uses an "MTU" of 512 octets, but that is probably
   not large enough anymore, with the advent of DNSSEC.

 * A typical audio packet is no more than 80 bytes, but a video
application
   could easily 4K, and in some cases 64K.

 * The median web page size, in my cache, is about 2K. The average is
   about 4K, and about 40% would fit in a single 1280 byte packet.

But the real question is, is this important? The working group actually
debated it, and came to the conclusion that, no, raising the limit would
not dramatically harm any of the popular underlying networks.  The key
argument were multicast and DNS.  We don't really know how to do a
multicast PMTU discovery, and we certainly don't know how to embed PMTU
discovery within a single round trip delay.  So, on balance, raising the
minimum MTU to 1280 helps, and only hurts those who are lazy enough to
settle for 91% efficiency when they could get 92%.

-- 
Christian Huitema
----------
See you at INET'98, Geneva 21-24,July 98 http://www.isoc.org/inet98/

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From: Masataka Ohta <[email-masked]>
Date: Wed, 24 Jun 98 16:16:22 JST

Christian;

> with PMTU discovery.  Large enough depends on the application:

For header processing overhead, it rather depends on chip speed.

Network engineers may tend to think that the network speed increases
while chip speed stays the same.

The proper argument is that the application speed stays the same while
the chip speed exponentially increases.

Today, it is already obvious that 1280 bytes of minimum MTU is large
enough for video applications, even if it is unicast ones.

Well, it must have been large enough to make multicast applications work.

						Masataka Ohta