RFC 1018 (rfc1018) - Page 2 of 3

Some comments on SQuID

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