rfc1254.txt

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   approximately two standard deviations without the timer going off in
   a false alarm.  The same method of estimation may be applicable to
   TP4.  The procedure is:

           Error     = Measured - Estimated
           Estimated = Estimated + Gain_1 * Error
           Deviation = Deviation + Gain_2 * (|Error| - Deviation)
           Timeout   = Estimated + 2 * Deviation

           Where:  Gain_1, Gain_2 are gain factors.

4.1  Response to No Policy in Gateway

   Since packets must be dropped during congestion because of the finite
   buffer space, feedback of congestion to the users exists even when
   there is no gateway congestion policy.  Dropping the packets is an
   attempt to recover from congestion, though it needs to be noted that
   congestion collapse is not prevented by packet drops if end-systems
   accelerate their sending rate in these conditions.  The accurate
   detection of congestive loss by all retransmission timers in the
   end-systems is extremely important for gateway congestion recovery.

4.2  Response to Source Quench

   Given that a Source Quench message has ambiguous meaning, but usually
   is issued for congestion recovery, the response of Slow-start to a
   Source Quench is to return to the beginning of the cycle of increase.
   This is an early response, since the Source Quench on overflow will
   also lead to a retransmission timeout later.

4.3 Response to Random Drop

   The end-system's view of Random Drop is the same as its view of loss
   caused by overflow at the gateway. This is a drawback of the use of
   packet drops as congestion feedback for congestion avoidance: the
   decrease policy on congestion feedback cannot be made more drastic
   for overflows than for the drops the gateway does for congestion
   avoidance.  Slow-start responds rapidly to gateway feedback.  In a
   case where the users are cooperating (all Slow-start), a desired
   outcome would be that this responsiveness would lead quickly to a
   decreased probability of drop.  There would be, as an ideal, a self-
   adjusting overall amount of control.







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4.4  Response to Congestion Indication

   Since the Congestion Indication mechanism attempts to keep gateways
   uncongested, cooperating end-system congestion control policies need
   not reduce demand as much as with other gateway policies.  The
   difference between the Slow-start response to packet drops and the
   End-System Congestion Indication response to Congestion Experienced
   Bits is primarily in the decrease policy.  Slow-start decreases the
   window to one packet on a retransmission timeout.  End-System
   Congestion Indication decreases the window by a fraction (for
   instance, to 7/8 of the original value), when the Congestion
   Experienced Bit is set in half of the packets in a sample spanning
   two round-trip times (two windows full).

4.5  Response to Fair Queuing

   A Fair Queueing policy may issue control indications, as in the
   simulated Bit-Round Fair Queueing with DEC Bit, or it may depend only
   on the passive effects of the queueing.  When the passive control is
   the main effect (perhaps because most users are not responsive to
   control indications), the behavior of retransmission timers will be
   very important.  If retransmitting users send more packets in
   response to increases in their delay and drops, Fair Queueing will be
   prone to degraded performance, though collapse (zero throughput for
   all users) may be prevented for a longer period of time.

5.  Conclusions

   The impact of users with excessive demand is a driving force as
   proposed gateway policies come closer to implementation.  Random Drop
   and Congestion Indication can be fair only if the transport entities
   sharing the gateway are all cooperative and reduce demand as needed.
   Within some portions of the Internet, good behavior of end-systems
   eventually may be enforced through administration.  But for various
   reasons, we can expect non-cooperating transports to be a persistent
   population in the Internet.  Congestion avoidance mechanisms will not
   be deployed all at once, even if they are adopted as standards.
   Without enforcement, or even with penalties for excessive demand,
   some end-systems will never implement congestion avoidance
   mechanisms.

   Since it is outside the context of any of the gateway policies, we
   will mention here a suggestion for a relatively small-scale response
   to users which implement especially aggressive policies. This has
   been made experimentally by [Jac89].  It would implement a low
   priority queue, to which the majority of traffic is not routed.  The
   candidate traffic to be queued there would be identified by a cache
   of recent recipients of whatever control indications the gateway



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   policy makes.  Remaining in the cache over multiple control intervals
   is the criterion for being routed to the low priority queue.  In
   approaching or established congestion, the bandwidth given to the
   low-service queue is decreased.

   The goal of end-system cooperation itself has been questioned.  As
   [She89] points out, it is difficult to define.  A TCP implementation
   that retransmits before it determines that has been loss indicated
   and in a Go-Back-N manner is clearly non-cooperating.  On the other
   hand, a transport entity with selective acknowledgement may demand
   more from the gateways than TCP, even while responding to congestion
   in a cooperative way.

   Fair Queueing maintains its control of allocations regardless of the
   end-system congestion avoidance policies.  [Nag85] and [DKS89] argue
   that the extra delays and congestion drops that result from
   misbehavior could work to enforce good end-system policies.  Are the
   rewards and penalties less sharply defined when one or more
   misbehaving systems cause the whole gateway's performance to be less?
   While the tax on all users imposed by the "over-users" is much less
   than in gateways with other policies, how can it be made sufficiently
   low?

   In the sections on Selective Feedback Congestion Indication and Bit-
   Round Fair Queueing we have pointed out that more needs to be done on
   two particular fronts:

      How can increased computational overhead be avoided?

      The allocation of resources to source-destination pairs is, in
      many scenarios, unfair to individual users. Bit-Round Fair
      Queueing offers a potential administrative remedy, but even if it
      is applied, how should the unequal allocations be propagated
      through multiple gateways?

   The first question has been taken up by [McK90].

   Since Selective Feedback Congestion Indication (or Congestion
   Indication used with Fair Queueing) uses a network bit, its use in
   the Internet requires protocol support; the transition of some
   portions of the Internet to OSI protocols may make such a change
   attractive in the future.  The interactions between heterogeneous
   congestion control policies in the Internet will need to be explored.

   The goals of Random Drop Congestion Avoidance are presented in this
   survey, but without any claim that the problems of this policy can be
   solved.  These goals themselves, of uniform, dynamic treatment of
   users (streams/flows), of low overhead, and of good scaling



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   characteristics in large and loaded networks, are significant.

Appendix:  Congestion and Connection-oriented Subnets

   This section presents a recommendation for gateway implementation in
   an areas that unavoidably interacts with gateway congestion control,
   the impact of connection-oriented subnets, such as those based on
   X.25.

   The need to manage a connection oriented service (X.25) in order to
   transport datagram traffic (IP) can cause problems for gateway
   congestion control.  Being a pure datagram protocol, IP provides no
   information delimiting when a pair of IP entities need to establish a
   session between themselves.  The solution involves compromise among
   delay, cost, and resources.  Delay is introduced by call
   establishment when a new X.25 SVC (Switched Virtual Circuit) needs to
   be established, and also by queueing delays for the physical line.
   Cost includes any charges by the X.25 network service provider.
   Besides the resources most gateways have (CPU, memory, links), a
   maximum supported or permitted number of virtual circuits may be in
   contest.

   SVCs are established on demand when an IP packet needs to be sent and
   there is no SVC established or pending establishment to the
   destination IP entity.  Optionally, when cost considerations, and
   sufficient numbers of unused virtual circuits allow, redundant SVCs
   may be established between the same pair of IP entities.  Redundant
   SVCs can have the effect of improving performance when coping with
   large end-to-end delay, small maximum packet sizes and small maximum
   packet windows.  It is generally preferred though, to negotiate large
   packet sizes and packet windows on a single SVC.  Redundant SVCs must
   especially be discouraged when virtual circuit resources are small
   compared with the number of destination IP entities among the active
   users of the gateway.

   SVCs are closed after some period of inactivity indicates that
   communication may have been suspended between the IP entities.  This
   timeout is significant in the operation of the interface.  Setting
   the value too low can result in closing of the SVC even though the
   traffic has not stopped.  This results in potentially large delays
   for the packets which reopen the SVC and may also incur charges
   associated with SVC calls.  Also, clearing of SVCs is, by definition,
   nongraceful.  If an SVC is closed before the last packets are
   acknowledged, there is no guarantee of delivery.  Packet losses are
   introduced by this destructive close independent of gateway traffic
   and congestion.

   When a new circuit is needed and all available circuits are currently



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   in use, there is a temptation to pick one to close (possibly using
   some Least Recently Used criterion).  But if connectivity demands are
   larger than available circuit resources, this strategy can lead to a
   type of thrashing where circuits are constantly being closed and
   reopened.  In the worst case, a circuit is opened, a single packet
   sent and the circuit closed (without a guarantee of packet delivery).
   To counteract this, each circuit should be allowed to remain open a
   minimum amount of time.

   One possible SVC strategy is to dynamically change the time a circuit
   will be allowed to remain open based on the number of circuits in
   use.  Three administratively controlled variables are used, a minimum
   time, a maximum time and an adaptation factor in seconds per
   available circuit.  In this scheme, a circuit is closed after it has
   been idle for a time period equal to the minimum plus the adaptation
   factor times the number of available circuits, limited by the maximum
   time.  By administratively adjusting these variables, one has
   considerable flexibility in adjusting the SVC utilization to meet the
   needs of a specific gateway.

Acknowledgements

   This paper is the outcome of discussions in the Performance and
   Congestion Control Working Group between April 1988 and July 1989.
   Both PCC WG and plenary IETF members gave us helpful reviews of
   earlier drafts.  Several of the ideas described here were contributed
   by the members of the PCC WG.  The Appendix was written by Art
   Berggreen.  We are particularly thankful also to Van Jacobson, Scott
   Shenker, Bruce Schofield, Paul McKenney, Matt Mathis, Geof Stone, and
   Lixia Zhang for participation and reviews.

Reference