Congestion avoidance in TCP

Congestion Avoidance Mechanism
• Congestion control - once congestion happens, TCP will
control the congestion.
– TCP repeatedly increases the load it imposes on the network in
an effort to find the point at which congestion occurs, and then it
backs off from this point.
• Congestion Avoidance - to predict when congestion is about
to happen and then to reduce the rate at which hosts send data
just before packets start being discarded. But it is not yet
widely used.
• Three Methods,
– DEC bit
– Random Early Detection (RED)
– Source Based Congestion Control

DEC Bit
– The first mechanism was developed for use on the Digital
Network Architecture (DNA), a connectionless network with a
connection-oriented transport protocol.
– This mechanism could, therefore, also be applied to TCP and
IP.
– The idea here is to more evenly split the responsibility for
congestion control between the routers and the end nodes.
– Each router monitors the load it is experiencing and explicitly
notifies the end nodes when congestion is about to occur.
– This notification is implemented by setting a binary congestion
bit in the packets that flow through the router; hence the
name DECbit.

– The destination host then copies this congestion bit into
the ACK and it sends back to the source.
– Finally, the source adjusts its sending rate so as to avoid
congestion.
– A single congestion bit is added to the packet header.
• A router sets this bit in a packet if its average queue length is
greater than or equal to 1 at the time the packet arrives.
– This average queue length is measured over a time interval
that spans the last busy + idle cycle, plus the current busy
cycle.
– Using a queue length of 1 as the trigger for setting the
congestion bit is a trade-off between significant queuing
(and hence higher throughput) and increased idle time
(and hence lower delay).

– In other words, a queue length of 1 seems to optimize the
power function.
– The source records how many of its packets resulted in
some router setting the congestion bit.
– In particular, the source maintains a congestion window,
just as in TCP, and watches to see what fraction of the last
window’s worth of packets resulted in the bit being set.
– If less than 50% of the packets had the bit set, then the
source increases its congestion window by one packet.
– If 50% or more of the last window’s worth of packets had
the congestion bit set, then the source decreases its
congestion window to 0.875 times the previous value.

– The value 50% was chosen as the threshold based on
analysis that showed it to correspond to the peak of the
power curve. The “increase by 1, decrease by 0.875” rule
was selected because additive increase/multiplicative
decrease makes the mechanism stable.
Computing average queue length at a
router

Random Early Detection (RED)
– It is similar to the DECbit scheme in that each router is
programmed to monitor its own queue length, and when it
detects that congestion is imminent, to notify the source to
adjust its congestion window.
– RED, invented by Sally Floyd and Van Jacobson in the early
1990s, differs from the DECbit scheme in two major ways:
• The first is that rather than explicitly sending a congestion
notification message to the source, RED is most commonly
implemented such that it implicitly notifies the source of
congestion by dropping one of its packets.
– The source is, therefore, effectively notified by the subsequent
timeout or duplicate ACK.
– RED is designed to be used in conjunction with TCP, which
currently detects congestion by means of timeouts (or some other
means of detecting packet loss such as duplicate ACKs).

– As the “early” part of the RED acronym suggests, the gateway
drops the packet earlier than it would have to, so as to notify the
source that it should decrease its congestion window sooner than
it would normally have.
– In other words, the router drops a few packets before it has
exhausted its buffer space completely, so as to cause the source to
slow down, with the hope that this will mean it does not have to
drop lots of packets later on.

• The second difference between RED and DECbit is, how RED
decides when to drop a packet and what packet it decides to
drop.
– To understand the basic idea, consider a simple FIFO queue.
Rather than wait for the queue to become completely full and
then be forced to drop each arriving packet, we could decide
to drop each arriving packet with some drop probability
whenever the queue length exceeds some drop level.
– This idea is called early random drop. The RED algorithm
defines the details of how to monitor the queue length and
when to drop a packet.
Average Queue Length Calculation:
– Average queue length is calculated using weighted running
average similar to the one used in the original TCP timeout
computation.

• AvgLen = (1 − Weight) × AvgLen + Weight × SampleLen
• where 0 < Weight < 1 and SampleLen is the length of the
queue when a sample measurement is made.
– In most software implementations, the queue length is
measured every time a new packet arrives at the gateway.
– In hardware, it might be calculated at some fixed sampling
interval.
– RED has two queue length thresholds that trigger certain
activity: MinThreshold and MaxThreshold.
– When a packet arrives at the gateway, RED compares the
current AvgLen with these two thresholds, according to
the following rules:

if AvgLen  MinThreshold
 queue the packet
if MinThreshold < AvgLen < MaxThreshold
 calculate probability P
 drop the arriving packet with probability P
if MaxThreshold  AvgLen
drop the arriving packet
– P is a function of both AvgLen and how long it has been
since the last packet was dropped.
– Specifically, it is computed as follows:
• TempP = MaxP × (AvgLen − MinThreshold)/(MaxThreshold −
MinThreshold)
• P = TempP/(1 − count × TempP)

• Random Early Detection (RED)
RED thresholds on a FIFO queue

Source-based Congestion Avoidance
– The general idea of these techniques is to watch for some
sign from the network that some router’s queue is building
up and that congestion will happen soon if nothing is done
about it.
– For example, the source might notice that as packet queues
build up in the network’s routers, there is a measurable
increase in the RTT for each successive packet it sends.
– One particular algorithm exploits this observation as follows:
• The congestion window normally increases as in TCP, but every
two round-trip delays the algorithm checks to see if the
current RTT is greater than the average of the minimum and
maximum RTTs seen so far.
• If it is, then the algorithm decreases the congestion window
by one-eighth.

• Source-based Congestion Avoidance
– A second algorithm does something similar. The decision as to
whether or not to change the current window size is based on
changes to both the RTT and the window size.
– The window is adjusted once every two round-trip delays
based on the product
• (CurrentWindow − OldWindow)×(CurrentRTT − OldRTT)
• If the result is positive, the source decreases the window size by
one-eighth;
• if the result is negative or 0, the source increases the window by
one maximum packet size.

Congestion avoidance in TCP

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