RFC813 - Window and Acknowledgement Strategy in TCP

RFC: 813

WINDOW AND ACKNOWLEDGEMENT STRATEGY IN TCP

David D. Clark

MIT Laboratory for Computer Science

Computer Systems and Communications Group

July, 1982

1. IntrodUCtion

This document describes implementation strategies to deal with two

mechanisms in TCP, the window and the acknowledgement. These mechanisms

are described in the specification document, but it is possible, while

complying with the specification, to produce implementations which yield

very bad performance. Happily, the pitfalls possible in window and

acknowledgement strategies are very easy to avoid.

It is a much more difficult exercise to verify the performance of a

specification than the correctness. Certainly, we have less eXPerience

in this area, and we certainly lack any useful formal technique.

Nonetheless, it is important to attempt a specification in this area,

because different implementors might otherwise choose superficially

reasonable algorithms which interact poorly with each other. This

document presents a particular set of algorithms which have received

testing in the field, and which appear to work properly with each other.

With more experience, these algorithms may become part of the formal

specification: until such time their use is recommended.

2. The Mechanisms

The acknowledgement mechanism is at the heart of TCP. Very simply,

when data arrives at the recipient, the protocol requires that it send

back an acknowledgement of this data. The protocol specifies that the

bytes of data are sequentially numbered, so that the recipient can

acknowledge data by naming the highest numbered byte of data it has

received, which also acknowledges the previous bytes (actually, it

identifies the first byte of data which it has not yet received, but

this is a small detail). The protocol contains only a general assertion

that data should be acknowledged promptly, but gives no more specific

indication as to how quickly an acknowledgement must be sent, or how

much data should be acknowledged in each separate acknowledgement.

The window mechanism is a flow control tool. Whenever appropriate,

the recipient of data returns to the sender a number, which is (more or

less) the size of the buffer which the receiver currently has available

for additional data. This number of bytes, called the window, is the

maximum which the sender is permitted to transmit until the receiver

returns some additional window. Sometimes, the receiver will have no

buffer space available, and will return a window value of zero. Under

these circumstances,the protocol requires the sender to send a small

segment to the receiver now and then, to see if more data is accepted.

If the window remains closed at zero for some substantial period, and

the sender can oBTain no response from the receiver, the protocol

requires the sender to conclude that the receiver has failed, and to

close the connection. Again, there is very little performance

information in the specification, describing under what circumstances

the window should be increased, and how the sender should respond to

such revised information.

A bad implementation of the window algorithm can lead to extremely

poor performance overall. The degradations which occur in throughput

and CPU utilizations can easily be several factors of ten, not just a

fractional increase. This particular phenomenon is specific enough that

it has been given the name of Silly Window Syndrome, or SWS. Happily

SWS is easy to avoid if a few simple rules are observed. The most

important function of this memo is to describe SWS, so that implementors

will understand the general nature of the problem, and to describe

algorithms which will prevent its occurrence. This document also

describes performance enhancing algorithms which relate to

acknowledgement, and discusses the way acknowledgement and window

algorithms interact as part of SWS.

3. SILLY WINDOW SYNDROME

In order to understand SWS, we must first define two new terms.

Superficially, the window mechanism is very simple: there is a number,

called "the window", which is returned from the receiver to the sender.

However, we must have a more detailed way of talking about the meaning

of this number. The receiver of data computes a value which we will

call the "offered window". In a simple case, the offered window

corresponds to the amount of buffer space available in the receiver.

This correspondence is not necessarily exact, but is a suitable model

for the discussion to follow. It is the offered window which is

actually transmitted back from the receiver to the sender. The sender

uses the offered window to compute a different value, the "usable

window", which is the offered window minus the amount of outstanding

unacknowledged data. The usable window is less than or equal to the

offered window, and can be much smaller.

Consider the following simple example. The receiver initially

provides an offered window of 1,000. The sender uses up this window by

sending five segments of 200 bytes each. The receiver, on processing

the first of these segments, returns an acknowledgement which also

contains an updated window value. Let us assume that the receiver of

the data has removed the first 200 bytes from the buffer, so that the

receiver once again has 1,000 bytes of available buffer. Therefore, the

receiver would return, as before, an offered window of 1,000 bytes. The

sender, on receipt of this first acknowledgement, now computes the

additional number of bytes which may be sent. In fact, of the 1,000

bytes which the recipient is prepared to receive at this time, 800 are

already in transit, having been sent in response to the previous offered

window. In this case, the usable window is only 200 bytes.

Let us now consider how SWS arises. To continue the previous

example, assume that at some point, when the sender computes a useable

window of 200 bytes, it has only 50 bytes to send until it reaches a

"push" point. It thus sends 50 bytes in one segment, and 150 bytes in

the next segment. Sometime later, this 50-byte segment will arrive at

the recipient, which will process and remove the 50 bytes and once again

return an offered window of 1,000 bytes. However, the sender will now

compute that there are 950 bytes in transit in the network, so that the

useable window is now only 50 bytes. Thus, the sender will once again

send a 50 byte segment, even though there is no longer a natural

boundary to force it.

In fact, whenever the acknowledgement of a small segment comes

back, the useable window associated with that acknowledgement will cause

another segment of the same small size to be sent, until some

abnormality breaks the pattern. It is easy to see how small segments

arise, because natural boundaries in the data occasionally cause the

sender to take a computed useable window and divide it up between two

segments. Once that division has occurred, there is no natural way for

those useable window allocations to be recombined; thus the breaking up

of the useable window into small pieces will persist.

Thus, SWS is a degeneration in the throughput which develops over

time, during a long data transfer. If the sender ever stops, as for

example when it runs out of data to send, the receiver will eventually

acknowledge all the outstanding data, so that the useable window

computed by the sender will equal the full offered window of the

receiver. At this point the situation will have healed, and further

data transmission over the link will occur efficiently. However, in

large file transfers, which occur without interruption, SWS can cause

appalling performance. The network between the sender and the receiver

becomes clogged with many small segments, and an equal number of

acknowledgements, which in turn causes lost segments, which triggers

massive retransmission. Bad cases of SWS have been seen in which the

average segment size was one-tenth of the size the sender and receiver

were prepared to deal with, and the average number of retransmission per

successful segments sent was five.

Happily, SWS is trivial to avoid. The following sections describe

two algorithms, one executed by the sender, and one by the receiver,

which appear to eliminate SWS completely. Actually, either algorithm by

itself is sufficient to prevent SWS, and thus protect a host from a

foreign implementation which has failed to deal properly with this

problem. The two algorithms taken together produce an additional

reduction in CPU consumption, observed in practice to be as high as a

factor of four.

4. Improved Window Algorithms

The receiver of data can take a very simple step to eliminate SWS.

When it disposes of a small amount of data, it can artificially reduce

the offered window in subsequent acknowledgements, so that the useable

window computed by the sender does not permit the sending of any further

data. At some later time, when the receiver has processed a

substantially larger amount of incoming data, the artificial limitation

on the offered window can be removed all at once, so that the sender

computes a sudden large jump rather than a sequence of small jumps in

the useable window.

At this level, the algorithm is quite simple, but in order to

determine exactly when the window should be opened up again, it is

necessary to look at some of the other details of the implementation.

Depending on whether the window is held artificially closed for a short

or long time, two problems will develop. The one we have already

discussed -- never closing the window artificially -- will lead to SWS.

On the other hand, if the window is only opened infrequently, the

pipeline of data in the network between the sender and the receiver may

have emptied out while the sender was being held off, so that a delay is

introduced before additional data arrives from the sender. This delay

does reduce throughput, but it does not consume network resources or CPU

resources in the process, as does SWS. Thus, it is in this direction

that one ought to overcompensate. For a simple implementation, a rule

of thumb that seems to work in practice is to artificially reduce the

offered window until the reduction constitutes one half of the available

space, at which point increase the window to advertise the entire space

again. In any event, one ought to make the chunk by which the window is

opened at least permit one reasonably large segment. (If the receiver

is so short of buffers that it can never advertise a large enough buffer

to permit at least one large segment, it is hopeless to expect any sort

of high throughput.)

There is an algorithm that the sender can use to achieve the same

effect described above: a very simple and elegant rule first described

by Michael Greenwald at MIT. The sender of the data uses the offered

window to compute a useable window, and then compares the useable window

to the offered window, and refrains from sending anything if the ratio

of useable to offered is less than a certain fraction. Clearly, if the

computed useable window is small compared to the offered window, this

means that a substantial amount of previously sent information is still

in the pipeline from the sender to the receiver, which in turn means

that the sender can count on being granted a larger useable window in

the future. Until the useable window reaches a certain amount, the

sender should simply refuse to send anything.

Simple experiments suggest that the exact value of the ratio is not

very important, but that a value of about 25 percent is sufficient to

avoid SWS and achieve reasonable throughput, even for machines with a

small offered window. An additional enhancement which might help

throughput would be to attempt to hold off sending until one can send a

maximum size segment. Another enhancement would be to send anyway, even

if the ratio is small, if the useable window is sufficient to hold the

data available up to the next "push point".

This algorithm at the sender end is very simple. Notice that it is

not necessary to set a timer to protect against protocol lockup when

postponing the send operation. Further acknowledgements, as they

arrive, will inevitably change the ratio of offered to useable window.

(To see this, note that when all the data in the catanet pipeline has

arrived at the receiver, the resulting acknowledgement must yield an

offered window and useable window that equal each other.) If the

expected acknowledgements do not arrive, the retransmission mechanism

will come into play to assure that something finally happens. Thus, to

add this algorithm to an existing TCP implementation usually requires

one line of code. As part of the send algorithm it is already necessary

to compute the useable window from the offered window. It is a simple

matter to add a line of code which, if the ratio is less than a certain

percent, sets the useable window to zero. The results of SWS are so

devastating that no sender should be without this simple piece of

insurance.

5. Improved Acknowledgement Algorithms

In the beginning of this paper, an overly simplistic implementation

of TCP was described, which led to SWS. One of the characteristics of

this implementation was that the recipient of data sent a separate

acknowledgement for every segment that it received. This compulsive

acknowledgement was one of the causes of SWS, because each

acknowledgement provided some new useable window, but even if one of the

algorithms described above is used to eliminate SWS, overly frequent

acknowledgement still has a substantial problem, which is that it

greatly increases the processing time at the sender's end. Measurement

of TCP implementations, especially on large operating systems, indicate

that most of the overhead of dealing with a segment is not in the

processing at the TCP or IP level, but simply in the scheduling of the

handler which is required to deal with the segment. A steady dribble of

acknowledgements causes a high overhead in scheduling, with very little

to show for it. This waste is to be avoided if possible.

There are two reasons for prompt acknowledgement. One is to

prevent retransmission. We will discuss later how to determine whether

unnecessary retransmission is occurring. The other reason one

acknowledges promptly is to permit further data to be sent. However,

the previous section makes quite clear that it is not always desirable

to send a little bit of data, even though the receiver may have room for

it. Therefore, one can state a general rule that under normal

operation, the receiver of data need not, and for efficiency reasons

should not, acknowledge the data unless either the acknowledgement is

intended to produce an increased useable window, is necessary in order

to prevent retransmission or is being sent as part of a reverse

direction segment being sent for some other reason. We will consider an

algorithm to achieve these goals.

Only the recipient of the data can control the generation of

acknowledgements. Once an acknowledgement has been sent from the

receiver back to the sender, the sender must process it. Although the

extra overhead is incurred at the sender's end, it is entirely under the

receiver's control. Therefore, we must now describe an algorithm which

occurs at the receiver's end. Obviously, the algorithm must have the

following general form; sometimes the receiver of data, upon processing

a segment, decides not to send an acknowledgement now, but to postpone

the acknowledgement until some time in the future, perhaps by setting a

timer. The peril of this approach is that on many large operating

systems it is extremely costly to respond to a timer event, almost as

costly as to respond to an incoming segment. Clearly, if the receiver

of the data, in order to avoid extra overhead at the sender end, spends

a great deal of time responding to timer interrupts, no overall benefit

has been achieved, for efficiency at the sender end is achieved by great

thrashing at the receiver end. We must find an algorithm that avoids

both of these perils.

The following scheme seems a good compromise. The receiver of data

will refrain from sending an acknowledgement under certain

circumstances, in which case it must set a timer which will cause the

acknowledgement to be sent later. However, the receiver should do this

only where it is a reasonable guess that some other event will intervene

and prevent the necessity of the timer interrupt. The most obvious

event on which to depend is the arrival of another segment. So, if a

segment arrives, postpone sending an acknowledgement if both of the

following conditions hold. First, the push bit is not set in the

segment, since it is a reasonable assumption that there is more data

coming in a subsequent segment. Second, there is no revised window

information to be sent back.

This algorithm will insure that the timer, although set, is seldom

used. The interval of the timer is related to the expected inter-

segment delay, which is in turn a function of the particular network

through which the data is flowing. For the Arpanet, a reasonable

interval seems to be 200 to 300 milliseconds. Appendix A describes an

adaptive algorithm for measuring this delay.

The section on improved window algorithms described both a receiver

algorithm and a sender algorithm, and suggested that both should be

used. The reason for this is now clear. While the sender algorithm is

extremely simple, and useful as insurance, the receiver algorithm is

required in order that this improved acknowledgement strategy work. If

the receipt of every segment causes a new window value to be returned,

then of necessity an acknowledgement will be sent for every data

segment. When, according to the strategy of the previous section, the

receiver determines to artificially reduce the offered window, that is

precisely the circumstance under which an acknowledgement need not be

sent. When the receiver window algorithm and the receiver

acknowledgement algorithm are used together, it will be seen that

sending an acknowledgement will be triggered by one of the following

events. First, a push bit has been received. Second, a temporary pause

in the data stream is detected. Third, the offered window has been

artificially reduced to one-half its actual value.

In the beginning of this section, it was pointed out that there are

two reasons why one must acknowledge data. Our consideration at this

point has been concerned only with the first, that an acknowledgement

must be returned as part of triggering the sending of new data. It is

also necessary to acknowledge whenever the failure to do so would

trigger retransmission by the sender. Since the retransmission interval

is selected by the sender, the receiver of the data cannot make a

precise determination of when the acknowledgement must be sent.

However, there is a rough rule the sender can use to avoid

retransmission, provided that the receiver is reasonably well behaved.

We will assume that sender of the data uses the optional algorithm

described in the TCP specification, in which the roundtrip delay is

measured using an exponential decay smoothing algorithm. Retransmission

of a segment occurs if the measured delay for that segment exceeds the

smoothed average by some factor. To see how retransmission might be

triggered, one must consider the pattern of segment arrivals at the

receiver. The goal of our strategy was that the sender should send off

a number of segments in close sequence, and receive one acknowledgement

for the whole burst. The acknowledgement will be generated by the

receiver at the time that the last segment in the burst arrives at the

receiver. (To ensure the prompt return of the acknowledgement, the

sender could turn on the "push" bit in the last segment of the burst.)

The delay observed at the sender between the initial transmission of a

segment and the receipt of the acknowledgement will include both the

network transit time, plus the holding time at the receiver. The

holding time will be greatest for the first segments in the burst, and

smallest for the last segments in the burst. Thus, the smoothing

algorithm will measure a delay which is roughly proportional to the

average roundtrip delay for all the segments in the burst. Problems

will arise if the average delay is substantially smaller than the

maximum delay and the smoothing algorithm used has a very small

threshold for triggering retransmission. The widest variation between

average and maximum delay will occur when network transit time is

negligible, and all delay is processing time. In this case, the maximum

will be twice the average (by simple algebra) so the threshold that

controls retransmission should be somewhat more than a factor of two.

In practice, retransmission of the first segments of a burst has

not been a problem because the delay measured consists of the network

roundtrip delay, as well as the delay due to withholding the

acknowledgement, and the roundtrip tends to dominate except in very low

roundtrip time situations (such as when sending to one's self for test

purposes). This low roundtrip situation can be covered very simply by

including a minimum value below which the roundtrip estimate is not

permitted to drop.

In our experiments with this algorithm, retransmission due to

faulty calculation of the roundtrip delay occurred only once, when the

parameters of the exponential smoothing algorithm had been misadjusted

so that they were only taking into account the last two or three

segments sent. Clearly, this will cause trouble since the last two or

three segments of any burst are the ones whose holding time at the

receiver is minimal, so the resulting total estimate was much lower than

appropriate. Once the parameters of the algorithm had been adjusted so

that the number of segments taken into account was approximately twice

the number of segments in a burst of average size, with a threshold

factor of 1.5, no further retransmission has ever been identified due to

this problem, including when sending to ourself and when sending over

high delay nets.

6. Conservative Vs. Optimistic Windows

According to the TCP specification, the offered window is presumed

to have some relationship to the amount of data which the receiver is

actually prepared to receive. However, it is not necessarily an exact

correspondence. We will use the term "conservative window" to describe

the case where the offered window is precisely no larger than the actual

buffering available. The drawback to conservative window algorithms is

that they can produce very low throughput in long delay situations. It

is easy to see that the maximum input of a conservative window algorithm

is one bufferfull every roundtrip delay in the net, since the next

bufferfull cannot be launched until the updated window/acknowledgement

information from the previous transmission has made the roundtrip.

In certain cases, it may be possible to increase the overall

throughput of the transmission by increasing the offered window over the

actual buffer available at the receiver. Such a strategy we will call

an "optimistic window" strategy. The optimistic strategy works if the

network delivers the data to the recipient sufficiently slowly that it

can process the data fast enough to keep the buffer from overflowing.

If the receiver is faster than the sender, one could, with luck, permit

an infinitely optimistic window, in which the sender is simply permitted

to send full-speed. If the sender is faster than the receiver, however,

and the window is too optimistic, then some segments will cause a buffer

overflow, and will be discarded. Therefore, the correct strategy to

implement an optimistic window is to increase the window size until

segments start to be lost. This only works if it is possible to detect

that the segment has been lost. In some cases, it is easy to do,

because the segment is partially processed inside the receiving host

before it is thrown away. In other cases, overflows may actually cause

the network interface to be clogged, which will cause the segments to be

lost elsewhere in the net. It is inadvisable to attempt an optimistic

window strategy unless one is certain that the algorithm can detect the

resulting lost segments. However, the increase in throughput which is

possible from optimistic windows is quite substantial. Any systems with

small buffer space should seriously consider the merit of optimistic

windows.

The selection of an appropriate window algorithm is actually more

complicated than even the above discussion suggests. The following

considerations are not presented with the intention that they be

incorporated in current implementations of TCP, but as background for

the sophisticated designer who is attempting to understand how his TCP

will respond to a variety of networks, with different speed and delay

characteristics. The particular pattern of windows and acknowledgements

sent from receiver to sender influences two characteristics of the data

being sent. First, they control the average data rate. Clearly, the

average rate of the sender cannot exceed the average rate of the

receiver, or long-term buffer overflow will occur. Second, they

influence the burstiness of the data coming from the sender. Burstiness

has both advantages and disadvantages. The advantage of burstiness is

that it reduces the CPU processing necessary to send the data. This

follows from the observed fact, especially on large machines, that most

of the cost of sending a segment is not the TCP or IP processing, but

the scheduling overhead of getting started.

On the other hand, the disadvantage of burstiness is that it may

cause buffers to overflow, either in the eventual recipient, which was

discussed above, or in an intermediate gateway, a problem ignored in

this paper. The algorithms described above attempts to strike a balance

between excessive burstiness, which in the extreme cases can cause

delays because a burst is not requested soon enough, and excessive

fragmentation of the data stream into small segments, which we

identified as Silly Window Syndrome.

Under conditions of extreme delay in the network, none of the

algorithms described above will achieve adequate throughput.

Conservative window algorithms have a predictable throughput limit,

which is one windowfull per roundtrip delay. Attempts to solve this by

optimistic window strategies may cause buffer overflows due to the

bursty nature of the arriving data. A very sophisticated way to solve

this is for the receiver, having measured by some means the roundtrip

delay and intersegment arrival rate of the actual connection, to open

his window, not in one optimistic increment of gigantic proportion, but

in a number of smaller optimistic increments, which have been carefully

spaced using a timer so that the resulting smaller bursts which arrive

are each sufficiently small to fit into the existing buffers. One could

visualize this as a number of requests flowing backwards through the net

which trigger in return a number of bursts which flow back spaced evenly

from the sender to the receiver. The overall result is that the

receiver uses the window mechanism to control the burstiness of the

arrivals, and the average rate.

To my knowledge, no such strategy has been implemented in any TCP.

First, we do not normally have delays high enough to require this kind

of treatment. Second, the strategy described above is probably not

stable unless it is very carefully balanced. Just as buses on a single

bus route tend to bunch up, bursts which start out equally spaced could

well end up piling into each other, and forming the single large burst

which the receiver was hoping to avoid. It is important to understand

this extreme case, however, in order to understand the limits beyond

which TCP, as normally implemented, with either conservative or simple

optimistic windows can be expected to deliver throughput which is a

reasonable percentage of the actual network capacity.

7. Conclusions

This paper describes three simple algorithms for performance

enhancement in TCP, one at the sender end and two at the receiver. The

sender algorithm is to refrain from sending if the useable window is

smaller than 25 percent of the offered window. The receiver algorithms

are first, to artificially reduce the offered window when processing new

data if the resulting reduction does not represent more than some

fraction, say 50 percent, of the actual space available, and second, to

refrain from sending an acknowledgment at all if two simple conditions

hold.

Either of these algorithms will prevent the worst ASPects of Silly

Window Syndrome, and when these algorithms are used together, they will

produce substantial improvement in CPU utilization, by eliminating the

process of excess acknowledgements.

Preliminary experiments with these algorithms suggest that they

work, and work very well. Both the sender and receiver algorithms have

been shown to eliminate SWS, even when talking to fairly silly

algorithms at the other end. The Multics mailer, in particular, had

suffered substantial attacks of SWS while sending large mail to a number

of hosts. We believe that implementation of the sender side algorithm

has eliminated every known case of SWS detected in our mailer.

Implementation of the receiver side algorithm produced substantial

improvements of CPU time when Multics was the sending system. Multics

is a typical large operating system, with scheduling costs which are

large compared to the actual processing time for protocol handlers.

Tests were done sending from Multics to a host which implemented the SWS

suppression algorithm, and which could either refrain or not from

sending acknowledgements on each segment. As predicted, suppressing the

return acknowledgements did not influence the throughput for large data

transfer at all, since the throttling effect was elsewhere. However,

the CPU time required to process the data at the Multics end was cut by

a factor of four (In this experiment, the bursts of data which were

being sent were approximately eight segments. Thus, the number of

acknowledgements in the two experiments differed by a factor of eight.)

An important consideration in evaluating these algorithms is that

they must not cause the protocol implementations to deadlock. All of

the recommendations in this document have the characteristic that they

suggest one refrain from doing something even though the protocol

specification permits one to do it. The possibility exists that if one

refrains from doing something now one may never get to do it later, and

both ends will halt, even though it would appear superficially that the

transaction can continue.

Formally, the idea that things continue to work is referred to as

"liveness". One of the defects of ad hoc solutions to performance

problems is the possibility that two different approaches will interact

to prevent liveness. It is believed that the algorithms described in

this paper are always live, and that is one of the reasons why there is

a strong advantage in uniform use of this particular proposal, except in

cases where it is explicitly demonstrated not to work.

The argument for liveness in these solutions proceeds as follows.

First, the sender algorithm can only be stopped by one thing, a refusal

of the receiver to acknowledge sent data. As long as the receiver

continues to acknowledge data, the ratio of useable window to offered

window will approach one, and eventually the sender must continue to

send. However, notice that the receiver algorithm we have advocated

involves refraining from acknowledging. Therefore, we certainly do have

a situation where improper operation of this algorithm can prevent

liveness.

What we must show is that the receiver of the data, if it chooses

to refrain from acknowledging, will do so only for a short time, and not

forever. The design of the algorithm described above was intended to

achieve precisely this goal: whenever the receiver of data refrained

from sending an acknowledgement it was required to set a timer. The

only event that was permitted to clear that timer was the receipt of

another segment, which essentially reset the timer, and started it going

again. Thus, an acknowledgement will be sent as soon as no data has

been received. This has precisely the effect desired: if the data flow

appears to be disrupted for any reason, the receiver responds by sending

an up-to-date acknowledgement. In fact, the receiver algorithm is

designed to be more robust than this, for transmission of an

acknowledgment is triggered by two events, either a cessation of data or

a reduction in the amount of offered window to 50 percent of the actual

value. This is the condition which will normally trigger the

transmission of this acknowledgement.

APPENDIX A

Dynamic Calculation of Acknowledgement Delay

The text suggested that when setting a timer to postpone the

sending of an acknowledgement, a fixed interval of 200 to 300

milliseconds would work properly in practice. This has not been

verified over a wide variety of network delays, and clearly if there is

a very slow net which stretches out the intersegment arrival time, a

fixed interval will fail. In a sophisticated TCP, which is expected to

adjust dynamically (rather than manually) to changing network

conditions, it would be appropriate to measure this interval and respond

dynamically. The following algorithm, which has been relegated to an

Appendix, because it has not been tested, seems sensible. Whenever a

segment arrives which does not have the push bit on in it, start a

timer, which runs until the next segment arrives. Average these

interarrival intervals, using an exponential decay smoothing function

tuned to take into account perhaps the last ten or twenty segments that

have come in. Occasionally, there will be a long interarrival period,

even for a segment which is does not terminate a piece of data being

pushed, perhaps because a window has gone to zero or some glitch in the

sender or the network has held up the data. Therefore, examine each

interarrival interval, and discard it from the smoothing algorithm if it

exceeds the current estimate by some amount, perhaps a ratio of two or

four times. By rejecting the larger intersegment arrival intervals, one

should obtain a smoothed estimate of the interarrival of segments inside

a burst. The number need not be exact, since the timer which triggers

acknowledgement can add a fairly geNerous fudge factor to this without

causing trouble with the sender's estimate of the retransmission

interval, so long as the fudge factor is constant.