Module 15 TCP Flow Control and TCP Congestion Control

1
Module 15TCP Flow Control and TCP Congestion
Control
2

• Textbook sections
• BF Section 12.6 Flow Control
• LG Section 7.8.1 Open-loop Control
• LG Section 7.8.2 Closed-loop Control
• Topics
• TCP Flow Control
• Overview
• The small-packet problem
• TCP Congestion Control
• Open-loop Control
• Closed-loop Control
• Congestion Avoidance

3
1. TCP Flow Control - Overview

• TCP provides a means for the receiver to govern
  the amount of data sent by the sender.
• This is achieved by returning a window with
  every ACK indicating a range of acceptable
  sequence numbers beyond the last segment
  successfully received. The window indicates an
  allowed number of bytes that the sender may
  transmit before receiving further permission.

4
1. TCP Flow Control - The small-packet problem

• The small-packet problem
• There is a special problem associated with small
  packets. For example, when TCP is used for the
  transmission of single-character messages
  originating at a keyboard, the typical result is
  that 41 byte packets (one byte of data, 40 bytes
  of header) are transmitted for each byte of
  useful data.
• This 4000 overhead is annoying but tolerable on
  lightly loaded networks.
• On heavily loaded networks, however, the
  congestion resulting from this overhead can
  result in lost datagrams and retransmissions, as
  well as excessive propagation time caused by
  congestion in switching nodes and gateways. In
  practice, throughput may drop so low that TCP
  connection are aborted.

5
1. TCP Flow Control - The small-packet problem

• The solution to the small-packet problem
• To impose additional restrictions on the sender
• The solution is to inhibit the sending of new TCP
  segments when new outgoing data arrives from the
  user if any previously transmitted data on the
  connection remains unacknowledged. This
  inhibition is to be unconditional no timers,
  tests for size of data received, or other
  conditions are required.. Implementation
  typically requires one or two lines inside a TCP
  program.

6
1. TCP Flow Control - The small-packet problem

• Silly window syndrome/remedies
• Syndrome created by the sender
• Situation Application program on the sender side
  is too slow
• Problem Sender may create many small segments
• Remedy
• Nagles algorithm
• The sending TCP sends the first piece of data it
  receives from the sending application program
  even if it is only one byte
• After sending the first segment, the sending TCP
  accumulates data in the output buffer and waits
  until either the receiving TCP sends an
  acknowledgement or until enough data has
  accumulated to fill a maximum-size segment. At
  this time, the sending TCP can send the segment
• The above step is repeated for the rest of the
  transmission.

7
1. TCP Flow Control - The small-packet problem

• The advantages of Nagles algorithm are its
  simplicity and the fact that it takes both the
  speed of the application program that creates the
  data and the speed of the network that transport
  the data into consideration.
• If the network is slower than the application
  program, then segments are larger.
• If the network is faster than the application
  program, then segments are smaller.
• If the network is much faster than the
  application program, then segments are very
  small. (lightly loaded networks)

8
1. TCP Flow Control - The small-packet problem

• Syndrome created by the receiver
• Situation Application program on the receiver
  side is too slow (receive window becomes very
  small)
• Problem Sender may create many small segments
• Remedy
• Clarks solution
• Acknowledge as soon as the data arrives, but to
  announce a window size of zero until either there
  is enough space to accommodate a segment of
  maximum size or until half of the buffer is
  empty.
• Delayed acknowledgement
• When a segment arrives, it is not acknowledged
  immediately. The receiver waits until there is a
  decent amount of space in its incoming buffer
  before acknowledging the arrived segments
• Advantage reduced traffic
• Disadvantage retransmit the unacknowledged
  segments.

9
LG Figure 8.19 TCP end-to-end flow control
Transmitter
Receiver
Send Window
Receive Window
SlastWa-1
RlastWR1
Rlast
...
...
...
Rnext
Rnew
Octets transmitted and ACKed
Slast
Srecent
SlastWs-1
Slast oldest unacknowledged octet Srecent
highest-numbered transmitted octet SlastWa-1
highest-numbered octet that can be
transmitted SlastWs-1 highest-numbered octet
that can be accepted from the application
Rlast highest-numbered octet not yet read by the
application Rnext next expected octet Rnew
highest numbered octet received correctly RlastWR
-1 highest-numbered octet that can be
accommodated in receive buffer
10
2. TCP Congestion Control

• Open-loop control
• Admission control
• Policing
• Traffic policing When the traffic violates the
  agreed-upon contract, the network may choose to
  discard or tag the nonconforming traffic. The
  tagged traffic will be carried by the network but
  given lower priority. If there is any congestion
  down-stream, the tagged traffic is the first one
  to be lost.
• Leaky bucket algorithm
• Traffic shaping
• Leaky bucket traffic shaper
• Token bucket traffic shaper
• Closed-loop control
• TCP congestion control

11
LG Figure 7.53 A leaky bucket
Water poured
irregularly
Note - The bucket depth is used to absorb the
irregularities in the flow. Deep bucket for
bursty flow. Shallow bucket for smooth flow. -
Once the bucket is full, any additional water
entering it spills over the sides and is lost
Leaky bucket
Water drains at
a constant rate
Leaky bucket algorithm A key part of bandwidth
allocation is the mechanism used to specify the
needed bandwidth and limit users to their
allocations. A counter associated with each user
transmitting on a connection is incremented
whenever the user sends a packet and is
decremented periodically. If the counter exceeds
a threshold upon being incremented, the network
discards the packet. The user specifies the rate
at which the counter is decremented (this
determines the average bandwidth) and the value
of the threshold ( a measure of burstiness).
12
LG Figure 7.54 Leaky bucket algorithm used for
policing
Arrival of a packet at time ta
X X - (ta - LCT)
Yes
X lt 0?
No
X 0
Yes
Nonconforming
X gt L?
packet
No
X X I
X value of the leaky bucket counter
LCT ta
X auxiliary variable
conforming packet
LCT last conformance time
13
2. TCP Congestion Control Open-loop Control
Assumptions 1.Packets are assumed to be of fixed
length (I) 2. The leaky bucket will drain at a
continuous rate of 1 unit per packet time
14
2. TCP Congestion Control Open-loop Control
LG Figure 7.55 Behavior of leaky bucket
Nonconforming
Packet
arrival
Time
LI
Bucket
content
I
Time









15
2. TCP Congestion Control Open-loop Control
Given I 4, L 6, and the arrival times of
packets, values in the following table were
generated using the leaky bucket algorithm.
16
2. TCP Congestion Control Open-loop Control
LG Figure 7.58 Possible traffic patterns at the
average rate of 10 kbps
10 Kbps
(a)
Time
0
1
2
3
50 Kbps
(b)
Time
0
1
2
3
100 Kbps
(c)
Time
0
1
2
3
17
2. TCP Congestion Control Open-loop Control
LG Figure 7.59 A leaky bucket traffic shaper
Shaped
Incoming
Size N
traffic
traffic
Server
Packet
18
2. TCP Congestion Control Open-loop Control
LG Figure 7.60 Token bucket traffic shaper
Tokens arrive
periodically
Size K
Token
Shaped
Incoming
Size N
traffic
traffic
Server
Packet
19
2. TCP Congestion Control - Closed-loop Control

• TCP congestion control
• TCP would start a connection with the sender
  injecting multiple segments into the network, up
  to the window size advertised by the receiver.
• While this is OK when the two hosts are on the
  same LAN, if there are routers and slower links
  between the sender and the receiver, problem can
  arise.
• Some intermediate router must queue the packets,
  and it is possible for that router to run out of
  space.

20
2. TCP Congestion Control - Closed-loop Control

• Definitions
• Receiver window (rwnd)
• The most recently advertised receive window
• Receive-side limit
• Congestion window (cwnd)
• A TCP state variable that limits the amount of
  data a TCP can send. At any given time, a TCP
  must not send data with a sequence number higher
  than the sum of the highest acknowledged sequence
  number and the minimum of receive window and
  congestion window
• actual sender window size minimum (
  receive-advertised sender window size, congestion
  window size)
• The congestion window is a sender-side limit on
  the amount of data the sender can transmit into
  the network before receiving an ACK
• Initial window (IW)
• The initial window is the size of the senders
  congestion window after the three-way handshake
  is completed
• Slow start threshold (Congestion threshold)
• The value of the congestion window where slow
  start phase stops and congestion avoidance phase
  starts. The initial value is 65,535 bytes
• Segment
• Any TCP/IP data or acknowledgement packet (or
  both)

21
2. TCP Congestion Control - Closed-loop Control
LG Figure 7.63 Dynamics of TCP congestion window
Congestion occurs
Congestion
20
avoidance
16
15
Congestion
window
Threshold
10
Slow
start
5
0
Round-trip times
22
2. TCP Congestion Control - Closed-loop Control

• Slow start phase
• Algorithm
• The Initialization of the congestion window
• for a given connection sets congestion window to
  one segment and slow start threshold to 65,535
  bytes
• The increment of the congestion window
• The sender starts by transmitting one segment and
  waiting for its ACK. When that ACK is received,
  the congestion window is incremented from one to
  two, and two segments can be sent. When each of
  those two segments is acknowledged, the
  congestion window is increased to four. This
  provides an exponential growth.
• Limitation of the segments sent
• The TCP output routine never sends more than the
  minimum of congestion and the advertised receive
  window.
• Transition from slow start phase to congestion
  avoidance phase
• Slow start phase stops when congestion window
  reaches the slow start threshold. At this point,
  a congestion avoidance phase takes over.

23
2. TCP Congestion Control - Closed-loop Control

• Slow start phase
• Flow control
• The advertised receiver window is flow control
  imposed by the receiver.
• The advertised receiver window is related to the
  amount of available buffer space at the receiver
  for a connection
• The congestion window is flow control imposed by
  the sender
• The congestion window is based on the sender's
  assessment of perceived network congestion

24
2. TCP Congestion Control - Closed-loop Control

• Congestion avoidance phase
• This phase assume that the pipe is running close
  to full utilization
• Algorithm
• Increase the congestion window by one segment for
  each round-trip time.
• The congestion window stops increasing when TCP
  detects that the network is congested
• When congestion is detected, the slow start
  threshold is first set to one-half of the current
  window size (the minimum of the congestion window
  and the advertised window, but at least two
  segments). Next the congestion window is set to
  one maximum-sized segment.
• Restart phase
• With updated slow start threshold and congestion
  window, the system restarts, using the slow stat
  algorithm.

25
2. TCP Congestion Control - Closed-loop Control

• Congestion detection
• Time-out
• TCP assumes that congestion occurs in a network
  when an acknowledgement does not arrive before
  the time-out expires because of segment loss
• Duplicate ACK
• A TCP receiver SHOULD send an immediate duplicate
  ACK when an out-of-order segment arrives. The
  purpose of this ACK is to inform the sender that
  a segment was received out-of-order and which
  sequence number is expected.
• From the senders perspective, duplicate ACKs can
  be caused by a number of network problems
• They can be caused by dropped segments. In this
  case, all segments after the dropped segment will
  trigger duplicate ACKS.
• They can be caused by the re-ordering of data
  segments by the network (not a rare event along
  some network paths)
• They can be caused by replication of ACK or data
  segment by the network.

26
2. TCP Congestion Control - Closed-loop Control

• Discussion
• Weakness of the slow start congestion
  avoidance algorithm
• Periodic packet losses
• During the congestion avoidance phase, the
  window increase linearly by approximately 1 for
  each round trip. Eventually new packet losses
  result in another timeout and this process
  repeats. Oscillation of such high magnitude in
  sending window size leads to drastic round trip
  time and queue length fluctuation. The periodic
  packet losses at each peak of the fluctuation
  have sever impact on the network performance.
  Yet, window oscillation is the very measure used
  in the multiplicative decrease /additive increase
  algorithm to probe the network conditions.

Sending Window Size
Time
27
2. TCP Congestion Control - Closed-loop Control

• Duplicate ACK
• Every time a data packet arrives at the receiving
  side, the receiver responds with an
  acknowledgement, even if this sequence number has
  already been acknowledged.
• Thus, when a packet arrives out of order, TCP
  cannot yet acknowledge the data the packet
  contains because earlier data has not yet
  arrived. TCP resend the same acknowledgement it
  sent the last time.
• This second transmission of the same
  acknowledgement is called a duplicate ACK.
• When the sending side sees a duplicate ACK, it
  knows that the other side must have received a
  packet out of order, which suggests that an
  earlier packet might have been lost.
• Since it is also possible that the earlier packet
  has only been delayed rather than lost, the
  sender waits until it sees some number of
  duplicate ACKs and then retransmission the
  missing packet.
• In practice, TCP waits until it has seen three
  duplicate ACKs before retransmitting the packet.

28
2. TCP Congestion Control - Closed-loop Control

• Fast Retransmit
• Reason
• Rely only on TCP timeouts to detect congestion
  could lead to long period of time during which
  the connection went dead while waiting for a
  timer to expire.
• Fast retransmit is a heuristic that sometimes
  triggers the retransmission of a dropped segment
  sooner than the regular timeout mechanism. It
  does not replace regular timeouts it just
  enhances that facility.
• Fast retransmit is based on the use of duplicate
  ACK.

29
2. TCP Congestion Control - Closed-loop Control

• Fast Retransmit
• Description
• Since we do not know whether a duplicate ACK is
  caused by a lost segment or just a reordering of
  segments, we wait for a small number of duplicate
  ACKs to be received.
• It is assumed that if there is just a reordering
  of the segments, there will be only one or two
  duplicate ACKS before the reordered segments is
  processed, which will then generate a new ACK.
  If three or more duplicate ACKs are received in a
  row, it is a strong indication that a segment has
  been lost.
• We then perform a retransmission of what appears
  to be the missing segment, without waiting for a
  retransmission timer to expire.

30
2. TCP Congestion Control - Closed-loop Control

• Fast Recovery
• Description
• After fast retransmit, congestion avoidance, but
  not slow start is performed. This the the fast
  recovery algorithm.
• The reason for not performing slow start after
  fast retransmit is that the receipt of the
  duplicate ACKs tells us more than just a packet
  has been lost. Since the receiver can only
  generate the duplicate ACK when another segment
  is received, that segment has left the network,
  and is in the receivers buffer. That is, there
  is still data flowing between the two ends, and
  we do not want to reduce the flow abruptly by
  going into slow start.
• Fast retransmit and fast recovery are usually
  implemented together.

31
2. TCP Congestion Control - Closed-loop Control

• Fast retransmit/fast recovery algorithm
• When the third duplicate ACK is received, set
  ssthresh to one-half of the minimum of the
  current congestion window (cwnd) and the
  receivers advertised window. Retransmit the
  missing segment. Set cwnd to ssthresh plus 3
  times the segment size.
• Each time another duplicate ACK arrives,
  increment cwnd by the segment size and transmit a
  packet (if allowed by the new value of cwnd)
• When the next ACK arrives that acknowledges new
  data, set cwnd to ssthresh (the value set in step
  1). This should be the ACK of the retransmission
  from step 1. one round-trip time after the
  retransmission. Additionally, this ACK should
  acknowledge all the intermediate segments sent
  between the lost packet and the receipt of the
  third duplicate ACK. This step is congestion
  avoidance, since were slowing down to one-half
  the rate we were at when the packet was lost.

32
Sender
Receiver
Packet 1
Packet 2
ACK 1
Packet 3
ACK 2
Packet 4
ACK 2
Packet 5
Packet 6
ACK 2
ACK 2
Retransmit
packet 3
ACK 6
33
(No Transcript)
34
2. TCP Congestion Control - Closed-loop Control

• Discussion
• Weakness of TCP congestion control using slow
  start and congestion avoidance only
• TCP needs to create losses in order to find the
  available bandwidth of the connection
• TCP repeatedly increase 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 points.
• Alternative approaches
• Congestion avoidance
• Add small amount functionality into the routers
  to assist the end node in the anticipation of
  congestion.

35
3. Congestion Avoidance - DECbit

• DECbit
• Concept
• K.K. Ramakrishnan and Raj Jains research at
  Digital Equipment Corporation titled A Binary
  feedback Scheme for Congestion Avoidance in
  Computer Networks.
• 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 notes 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 binary
  congestion bit into the ACK it sends back to the
  source
• Finally, the source adjusts its sending rate so
  as to avoid congestion.

36
3. Congestion Avoidance - DECbit

• DECbit
• Algorithm
• Router Policy
• Congestion detection The router sets the
  congestion avoidance bit in the packet when the
  average queue length at the router at the time
  the packet arrives is greater or equal to one.
• Average queue length The average queue length
  is determined based on the number of packets in
  the network router that are queued and in service
  averaged over an interval T. The interval T is
  the last (busy idle) cycle time plus the busy
  period of the current cycle. The router is busy
  when it is transmitting and idle when it is not

37
3. Congestion Avoidance - DECbit
Note The figure below shows the queue length at
a router as a function of time. Essentially, the
router calculate the area under the curve and
divides this value by the time interval to
compute the average queue length.
Queue length
Current time
Time
Previous cycle
Current cycle
Averaging interval
38
3. Congestion Avoidance - DECbit

• Algorithm
• User Policy
• The user updates the window size after receiving
  acknowledgements for a number of packets
  transmitted. This number is the sum of the
  previous window size (Wp) and the current window
  size (Wc) at which the transport connection is
  operating. The bits returned in the
  acknowledgment are stored by the user.
• In practice, the source maintains a congestion
  window, just as in TCP, and watches to see what
  fraction of the last windows worth of packets
  resulted in the binary congestion bit being set.
  the bits corresponding to the last Wc packets for
  which acknowledgments are returned are examined.
• 3. 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 windows worth of
  packets had the congestion bit set, then the
  source decreases its congestion window to 0.875
  time the previous values.

39
3. Congestion Avoidance - DECbit

• Discussion
• Weakness of DECbit congestion avoidance
• DECbit requires modification in the packet header
• DECbit requires modification of router software

40
3. Congestion Avoidance - DECbit
Information flow in the DECbit congestion
avoidance mechanism
congestion avoidance bit
congestion avoidance bit
Data Packet
Destination Node
Router
Source Node
Congested Router
...
Acknowledgement Packet
congestion avoidance bit
41
3. Congestion Avoidance - Random Early Detection
(RED)

• Concept
• Each router is programmed to monitor its own
  queue length.
• When the router detects that congestion is
  imminent, it will notify the source to adjust its
  congestion window.
• Implicit notification
• The router implicitly notified the source of
  imminent 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 detects congestion by means of
  timeouts or duplicated ACKs.
• 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 lot of
  packets later on

42
3. Congestion Avoidance - Random Early Detection
(RED)

• Algorithm
• if CurrentAvgLen ? Minthreshold
• then queue the packet
• if MinThreshold lt CurrentAvgLen lt MaxThreshold
• then calculate probability P and drop the
  arriving packet with probability P
• if MaxThreshold ? CurrentAvtgLen
• then drop the arriving packet
• Notes on the algorithm
• If the average queue length is smaller than the
  lower threshold, no action is taken.
• If the average queue length is larger than the
  upper threshold, then the packet is always
  dropped.
• If the average queue length is between the two
  thresholds, then the newly arriving packet is
  dropped with some probability P.

43
3. Congestion Avoidance - Random Early Detection
(RED)

• Calculation of average queue length
• CurrentAvgLen (1 - Weight) x PreviousAvgLen
  weight x SampleLen
• where 0 lt weight lt 1, and
• SampleLen is the length of the queue when a
  sample measurement is made. In most software
  implementation, the queue length is measured
  every a new packet arrives at the router. In
  hardware, it might be calculated at some fixed
  sampling interval.

44
3. Congestion Avoidance - Random Early Detection
(RED)

• Calculation of Probability (P)
• TempP MaxP x (CurrentAvgLen - MinThreshold) /
  (MaxThreshold - MinThreshold)
• P TempP / (1 - count x TempP)

TempP
1.0
MaxP
CurrentAvgLen
MinThreshold
MaxThreshold