RFC 1018 (rfc1018) - Page 2 of 3
Some comments on SQuID
Alternative Format: Original Text Document
RFC 1018 Some Comments on SQuID August 1987
behavior proposed in RFC- 1016 may be appropriate for both cases.
Since we assume that Funneling congestion is the result of short-
lived phenomena, it is appropriate for gateways which are the sites
of this congestion to attempt to smooth it without excessive control
actions. This is the basis for the "hint" in the ICMP specification
that maybe an SQ should be sent only when a datagram is dropped. It
is the basis for the idea in RFC- 1016 that a gateway should be slow
to cry "congestion" (SQK = 70% of queue space filed), even if
persistent in attempting to eliminate it (SQLW = 50% of queue space
filled). Since Funneling congestion is the result of the actions of
multiple senders, the growth of internal queues is the only
reasonable place to watch for its existence or measure its effects.
Mismatch congestion, on the other hand, is the result of incorrect or
inadequate information about available transmission bandwidth on the
part of a single sender. The sending host has available to it
information about destination host capacity (TCP window size and ACK
rate) and about local link capacity (from the hardware/software
interface to the directly-connected network), but no real information
about the capacity of the Internet path. As noted in RFC-1016, hosts
can obtain the best throughput if their datagrams are never dropped,
and the probability of dropped datagrams is minimized when hosts send
at the appropriate steady-state rate (no "bunching"). Therefore, it
is a disservice to a host which is the source of Mismatch congestion
to wait a "long" time before taking control action. It would be
preferable to provide immediate feedback, via SQ's, to the host as
soon as datagrams with too short an inter-arrival time begin to
arrive. The sending host could then immediately (begin to) adjust
its behavior for the indicated destination.
There are, of course, many implementation issues which would need to
be addressed in order to implement the type of SQ-sending behavior
suggested here. Perhaps, though, they are not as severe as they
might appear. Two specific issues and possible solutions, are:
1. How should a gateway differentiate between Funneling and
mismatch congestion? Perhaps whenever there are more than q"
items on an output queue to a slower subnet which have been
received from a faster subnet, then look to see if any h" of them
have the same source. It so assume Mismatch and send an SQ to
that source. The "q" test might be implemented by a small set of
counters which are incremented when a packet is placed on an
output queue and decremented when a packet is sent. The search
for a common source might require more cycles but be performed
less often. The value of "q" would have to be small enough to
give an early warning, but bigger than a small multiple of "h".
The value of "h" would have to be big enough to avoid triggering
McKenzie