rfc1254.txt

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RFC 1254           Gateway Congestion Control Survey         August 1991


   similar. This may be one reason that the combination of Bit-Round
   Fair Queueing with Congestion Indication had particularly good
   simulated performance in [DKS89].

   The aggregation of users has the expected drawbacks, as well.  A
   TELNET user in a queue with an FTP user does not get delay benefits;
   and host pairs with high volume of connections get treated the same
   as a host pair with small number of connections and as a result gets
   unfair services.

   A problem can be mentioned about Fair Queueing, though it is related
   to implementation, and perhaps not properly part of a policy
   discussion.  This is a concern that the resources (processing) used
   for determining where to queue incoming packets would themselves be
   subject to congestion, but not to the benefits of the Fair Queuing
   discipline.  In a situation where the gateway processor was not
   adequate to the demands on it, the gateway would need an alternative
   policy for congestion control of the queue awaiting processing.
   Clever implementation can probably find an efficient way to route
   packets to the queues they belong in before other input processing is
   done, so that processing resources can be controlled, too.  There is
   in addition, the concern that bit-by-bit round FQ requires non-FCFS
   queueing even within the same source destination pairs to allow for
   fairness to different connections between these end systems.

   Another potential concern about Fair Queueing is whether it can scale
   up to gateways with very large source-destination populations.  For
   example, the state in a Fair Queueing implementation scales with the
   number of active end-to-end paths, which will be high in backbone
   gateways.

3.5.2  Stochastic Fairness Queuing

   Stochastic Fairness Queueing (SFQ) has been suggested as a technique
   [McK90] to address the implementation issues relating to Fair
   Queueing.  The first overhead that is reduced is that of looking up
   the source-destination address pair in an incoming packet and
   determining which queue that packet will have to be placed in.  SFQ
   does not require as many memory accesses as Fair Queueing to place
   the packet in the appropriate queue.  SFQ is thus claimed to be more
   amenable to implementation for high-speed networks [McK90].

   SFQ uses a simple hash function to map from the source-destination
   address pair to a fixed set of queues.  Since the assignment of an
   address pair to a queue is probabilistic, there is the likelihood of
   multiple address pairs colliding and mapping to the same queue.  This
   would potentially degrade the additional fairness that is gained with
   Fairness Queueing.  If two or more address pairs collide, they would



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   continue to do so.  To deal with the situation when such a collision
   occurs, SFQ periodically perturbs the hash function so that these
   address pairs will be unlikely to collide subsequently.

   The price paid for achieving this implementation efficiency is that
   SFQ requires a potentially large number of queues (we must note
   however, that these queues may be organized orthogonally from the
   buffers in which packets are stored. The buffers themselves may be
   drawn from a common pool, and buffers in each queue organized as a
   linked list pointed to from each queue header).  For example, [McK90]
   indicates that to get good, consistent performance, we may need to
   have up to 5 to 10 times the number of active source-destination
   pairs. In a typical gateway, this may require around 1000 to 2000
   queues.

   [McK90] reports simulation results with SFQ. The particular hash
   function that is studied is using the HDLC's cyclic redundancy check
   (CRC).  The hash function is perturbed by multiplying each byte by a
   sequence number in the range 1 to 255 before applying the CRC.  The
   metric considered is the standard deviation of the number of packets
   output per source-destination pair.  A perfectly fair policy would
   have a standard deviation of zero and an unfair policy would have a
   large standard deviation.  In the example studied (which has up to 20
   source-destination (s-d) pairs going through a single overloaded
   gateway), SFQ with 1280 queues (i.e., 64 times the number of source-
   destination pairs), approaches about 3 times the standard deviation
   of Fairness Queueing.  This must be compared to a FCFS queueing
   discipline having a standard deviation which is almost 175 times the
   standard deviation of Fairness Queueing.

   It is conjectured in [McK90] that a good value for the interval in
   between perturbations of the hash function appears to be in the area
   between twice the queue flush time of the stochastic fairness queue
   and one-tenth the average conversation time between a source-
   destination pair.

   SFQ also may alleviate the anticipated scaling problems that may be
   an issue with Fair Queueing by not strictly requiring the number of
   queues equal to the possible source-destination pairs that may be
   presenting a load on a particular gateway. However, SFQ achieves this
   property by trading off some of the fairness for implementation
   efficiency.

   [McK90] examines alternative strategies for implementation of Fair
   Queueing and SFQ and estimates their complexity on common hardware
   platforms (e.g., a Motorola 68020).  It is suggested that mapping an
   IP address pair may require around 24 instructions per packet for
   Fair Queueing in the best case; in contrast SFQ requires 10



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   instructions in the worst case.  The primary source of this gain is
   that SFQ avoids a comparison of the s-d address pair on the packet to
   the identity of the queue header.  The relative benefit of SFQ over
   Fair Queueing is anticipated to be greater when the addresses are
   longer.

   SFQ offers promising implemenatation benefits.  However, the price to
   be paid in terms of having a significantly larger number of queues
   (and the consequent data structures and their management) than the
   number of s-d pairs placing a load on the gateway is a concern.  SFQ
   is also attractive in that it may be used in concert with the DEC-bit
   scheme for Selective Feedback Congestion Indication to provide
   fairness as well as congestion avoidance.

4.  END-SYSTEM CONGESTION CONTROL POLICIES

   Ideally in gateway congestion control, the end-system transport
   entities adjust (decrease) their demand in response to control
   exerted by the gateway.  Schemes have been put in practice for
   transport entities to adjust their demand dynamically in response to
   congestion feedback.  To accomplish this, in general, they call for
   the user to gradually increase demand until information is received
   that the load on the gateway is too high.  In response to this
   information, the user decreases load, then begins an exploratory
   increases again.  This cycle is repeated continuously, with the goal
   that the total demand will oscillate around the optimal level.

   We have already noted that a Slow-start connection may give up
   considerable bandwidth when sharing a gateway with aggressive
   transport entities.  There is currently no way to enforce that end-
   systems use a congestion avoidance policy, though the Host
   Requirements RFC [HR89] has defined Slow-start as mandatory for TCP.
   This adverse effect on Slow-start connections is mitigated by the
   Fair Queueing policy.  Our conclusions discuss further the
   coexistence of different end-system strategies.

   This section briefly presents two fielded transport congestion
   control and avoidance schemes, Slow-start and End-System Congestion
   Indication, and the responses by means of which they cooperate with
   gateway policies.  They both use the control paradigm described
   above.  Slow-start, as mentioned in Section 1, was developed by
   [Jac88], and widely fielded in the BSD TCP implementation.  End-
   system Congestion Indication was developed by [JRC87].  It is fielded
   in DEC's Digital Network Architecture, and has been specified as well
   for ISO TP4 [NIST88].

   Both Slow-start and End-system Congestion Indication view the
   relationship between users and gateways as a control system. They



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   have feedback and control components, the latter further broken down
   into a procedure bringing a new connection to equilibrium, and a
   procedure to maintain demand at the proper level.  They make use of
   policies for increasing and decreasing the transport sender's window
   size.  These require the sender to follow a set of self-restraining
   rules which dynamically adjust the send window whenever congestion is
   sensed.

   A predecessor of these, CUTE, developed by [Jai86], introduced the
   use of retransmission timeouts as congestion feedback.  The Slow-
   start scheme was also designed to use timeouts in the same way.  The
   End-System policies for Congestion Indication use the Congestion
   Experienced Bit delivered in the network header as the primary
   feedback, but retransmission timeouts also provoke an end-system
   congestion response.

   In reliable transport protocols like TCP and TP4, the retransmission
   timer must do its best to satisfy two conflicting goals. On one hand,
   the timer must rapidly detect lost packets. And, on the other hand,
   the timer must minimize false alarms.  Since the retransmit timer is
   used as a congestion signal in these end-system policies, it is all
   the more important that timeouts reliably correspond to packet drops.
   One important rule for retransmission is to avoid bad sampling in the
   measurements that will be used in estimating the round-trip delay.
   [KP87] describes techniques to ensure accurate sampling.  The Host
   Requirements RFC [HR89] makes these techniques mandatory for TCP.

   The utilization of a resource can be defined as the ratio of its
   average arrival rate to its mean service rate. This metric varies
   between 0 and 1.0. In a state of congestion, one or more resources
   (link, gateway buffer, gateway CPU) has a utilization approaching
   1.0.  The average delay (round trip time) and its variance approach
   infinity. Therefore, as the utilization of a network increases, it
   becomes increasingly important to take into account the variance of
   the round trip time in estimating it for the retransmission timeout.

   The TCP retransmission timer is based on an estimate of the round
   trip time.  [Jac88] calls the round trip time estimator the single
   most important feature of any protocol implementation that expects to
   survive a heavy load. The retransmit timeout procedure in RFC-793
   [Pos81b] includes a fixed parameter, beta, to account for the
   variance in the delay. An estimate of round trip time using the
   suggested values for beta is valid for a network which is at most 30%
   utilized.  Thus, the RFC-793 retransmission timeout estimator will
   fail under heavy congestion, causing unnecessary retransmissions that
   add to the load, and making congestive loss detection impossible.

   Slow-start TCP uses the mean deviation as an estimate of the variance



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   to improve the estimate. As a rough view of what happens with the
   Slow-start retransmission calculation, delays can change by
   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