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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

McKenzie [Page 1]  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 [Page 2]  RFC 1018 Some Comments on SQuID August 1987

    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.

McKenzie [Page 3]