Some comments on SQuID

Network Working Group                                        A. McKenzie
Request for Comments: 1018                                      BBN Labs
                                                             August 1987
                         Some Comments on SQuID

Status of this Memo

   This memo is a discussion of some of the ideas expressed in RFC-1016
   on Source Quench.  This memo introduces the distinction of the cause
   of congestion in a gateway between the effects of "Funneling" and
   "Mismatch".  It is offered in the same spirit as RFC-1016; to
   stimulate discussion.  The opinions offered are personal, not
   corporate, opinions.  Distribution of this memo is unlimited.

Discussion

   It appears to me that there are at least two qualitatively different
   types of congestion which may occur at Internet gateways.  One form
   of congestion is the result of the merger of several independent data
   streams from diverse sources at a common point in their communication
   path.  I'll refer to this as "Funneling".  The architecture of the
   Internet (apparently) assumes that traffic flows are bursty and
   asynchronous; therefore congestion which occurs at the result of
   Funneling will typically be the result of "bad luck" as several
   independent bursts happen to arrive at a common point simultaneously.
   It is expected that Funneling congestion will be short-lived, just as
   individual bursts are.  I don't claim that any such assumptions are
   documented or formalized; nevertheless I got a clear sense of this
   class of assumptions both from reading the protocol documentation and
   from personal recollections of long-past design meetings.

   A second form of Internet congestion may arise during a prolonged
   (non-bursty) data transfer between hosts when the resulting traffic
   must pass through a node connecting two communications subsystems
   with significantly different throughput rates.  I'll refer to this as
   "Mismatching".  By contrast with Funneling, Mismatching can be caused
   by the innocent action of a single host, is highly repeatable
   (definitely not just "bad luck"), and will be long-lived.

   RFC- 1016 discusses two interrelated strategies; one for when to send
   a SQ, and a second for what to do when an SQ is received.  There is
   also a discussion of some experiments, which deal almost exclusively
   with Mismatching congestion. (I understand that the simulation can
   generate multiple flows, but these simply further increase the degree
   of Mismatch; the flow under study is long-lived by design.)  It seems
   to me that the strategy RFC- 1016 proposes for sending SQ's, based on
   queue length, may be appropriate for Funneling Congestion, but
   inappropriate for Mismatch congestion, as discussed below.  The host
   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
      on common cases of source datagram fragmentation by an
      intermediate gateway.

      2. How can a gateway determine which subnets are "slower" and
      faster", as well as appropriate inter-arrival times?  There may be
      lots of clever ways for a gateway to measure the dynamic bandwidth
      of its directly-connected subnets.  However, I'm more in favor of
      starting with configuration parameters related to the known (at
      installation time) general characteristics of subnet types (e.g.
      Ethernet is 10Mbps, ARPANET is 50 Kbps, SATNET is 100 Kbps, etc).
      This sort of approximation is quite adequate for determining which
      subnet is faster, or what inter-arrival time is appropriate for
      packets being routed to a slower subnet.

Summary

   Funneling congestion and Mismatch congestion are qualitatively
   different, and it would not be surprising if different SQ-sending
   strategies were best for dealing with them.  RFC- 1016 suggests a
   specific SQ-sending strategy which may be inappropriate for dealing
   with Mismatch congestion.  This RFC suggests guidelines for an
   additional SQ-sending strategy for dealing with Mismatch.  Hosts
   implementing the SQuID algorithm of RFC-1016 should be expected to
   achieve better performance if they received SQ's sent according to
   either or both of these strategies.  However, all these ideas are
   still only in half-baked form; real engineering is clearly needed.