Subject: Re: Initial text on QoS

From: Vernon Schryver <[email-masked]>
Date: Fri, 10 Mar 2000 15:14:32 -0700 (MST)

> From: Phil Karn <[email-masked]>

>            ...  An IP packet presented to this subnetwork with the
> "low delay" bit set in its TOS field should disable hop-by-hop
> retransmission within the subnetwork for that particular
> packet. Another example is link forward error correction, which can
> increase reliability at the expense of throughput without
> significantly affecting delay.

Why turn off retransmission for "low delay"?  The application protocols
I can think of off hand that currently and commonly use "low delay" are
NTP/UDP (as of a fairly recent change to (x?)ntpd), `timed` or TSP/UDP
(another time protocol), telnet, and rlogind.  I know of some other
applications, but they're much less common and some are even proprietary,
but follow those two families.  Turning off retransmission for the UDP
applications is sounds like a good idea, but not good for the TCP
applications.  For them, you want to retransmit to avoid the longer
end-to-end timeout, and perhaps move them higher in your hop-by-hop
retransmission queue.  In other words, turning off hop-by-hop
retransmissions for "low delay" sounds either too specific a remedy or to
general.

Vernon Schryver    [email-masked]