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TCP - Part II. Relates to Lab 5. This is an extended module that covers TCP data transport, and flow control, congestion control, and error control in TCP. Interactive and bulk data. TCP applications can be put into the following categories bulk data transfer - ftp, mail, http

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tcp part ii

TCP - Part II

Relates to Lab 5. This is an extended module that covers TCP data transport, and flow control, congestion control, and error control in TCP.

interactive and bulk data
Interactive and bulk data

TCP applications can be put into the following categories

bulk data transfer - ftp, mail, http

interactive data transfer - telnet, rlogin

TCP has algorithms to deal which each type of applications efficiently.

tcpdump of an rlogin session
tcpdump of an rlogin session

This is the output of typing 3 (three) characters :

44.062449 > P 0:1(1) ack 1

44.063317 > P 1:2(1) ack 1 win 8760

44.182705 > . ack 2 win 17520

48.946471 > P 1:2(1) ack 2 win 17520

48.947326 > P 2:3(1) ack 2 win 8760

48.982786 > . ack 3 win 17520

55:00.116581 > P 2:3(1) ack 3 win 17520

55:00.117497 > P 3:4(1) ack 3 win 8760

55:00.183694 > . ack 4 win 17520

  • “Rlogin” is a remote terminal application
  • Originally built only for Unix systems.
  • Rlogin sends one segment per character (keystroke)
  • Receiver echoes the character back.
  • So, we really expect to have four segments per keystroke
  • We would expect that tcpdump shows this pattern:
  • However, tcpdump shows this pattern:
  • So, TCP has delayed the transmission of an ACK
delayed acknowledgement
Delayed Acknowledgement
  • TCP delays transmission of ACKs for up to 200ms
  • The hope is to have data ready in that time frame. Then, the ACK can be piggybacked with the data segment.
  • Delayed ACKs explain why the ACK and the “echo of character” are sent in the same segment.
tcpdump of a wide area rlogin session
tcpdump of a wide-area rlogin session

This is the output of typing 9 characters :

54:16.401963 > tenet.CS.Berkeley.EDU.login: P 1:2(1) ack 2 win 16384

54:16.481929 tenet.CS.Berkeley.EDU.login > P 2:3(1) ack 2 win 16384

54:16.482154 > tenet.CS.Berkeley.EDU.login: P 2:3(1) ack 3 win 16383

54:16.559447 tenet.CS.Berkeley.EDU.login > P 3:4(1) ack 3 win 16384

54:16.559684 > tenet.CS.Berkeley.EDU.login: P 3:4(1) ack 4 win 16383

54:16.640508 tenet.CS.Berkeley.EDU.login > P 4:5(1) ack 4 win 16384

54:16.640761 > tenet.CS.Berkeley.EDU.login: P 4:8(4) ack 5 win 16383

54:16.728402 tenet.CS.Berkeley.EDU.login > P 5:9(4) ack 8 win 16384

wide area rlogin observation 1
Wide-area Rlogin: Observation 1
  • Transmission of segments follows a different pattern.
  • The delayed acknowled-gment does not kick in
  • Reason is that there is always data at aida when the ACK arrives.
wide area rlogin observation 2
Wide-area Rlogin: Observation 2
  • There are fewer transmissions than there are characters.
  • Aida never has multiple segments outstanding.
  • This is due to Nagle’s Algorithm:

Each TCP connection can have only one small (1-byte) segment outstanding that has not been acknowledged.

  • Implementation:Send one byte and buffer all subsequent bytes until acknowledgement is received.Then send all buffered bytes in a single segment. (Only enforced if data is arriving from application one byte at a time)
  • Nagle’s rule reduces the amount of small segments.The algorithm can be disabled.
what is flow congestion error control
What is Flow/Congestion/Error Control ?
  • Flow Control: Algorithms to prevent that the sender overruns the receiver with information?
  • Congestion Control: Algorithms to prevent that the sender overloads the network
  • Error Control: Algorithms to recover or conceal the effects from packet losses

 The goal of each of the control mechanisms is different.

 But the implementation is combined

tcp flow control13
TCP Flow Control
  • TCP implements sliding window flow control
      • Sending acknowledgements is separated from setting the window size at sender.
      • Acknowledgements do not automatically increase the window size
      • Acknowledgements are cumulative
sliding window flow control
Sliding Window Flow Control

Sliding Window Protocol is performed at the byte level:

Here: Sender can transmit sequence numbers 6,7,8.

sliding window window closes
Sliding Window: “Window Closes”

Transmission of a single byte (with SeqNo = 6) and acknowledgement is received (AckNo = 5, Win=4):

sliding window window opens
Sliding Window: “Window Opens”

Acknowledgement is received that enlarges the window to the right (AckNo = 5, Win=6):

A receiver opens a window when TCP buffer empties (meaning that data is delivered to the application).

sliding window window shrinks
Sliding Window: “Window Shrinks”

Acknowledgement is received that reduces the window from the right (AckNo = 5, Win=3):

Shrinking a window should not be used

window management in tcp
Window Management in TCP
  • The receiver is returning two parameters to the sender
  • The interpretation is:
      • I am ready to receive new data with

SeqNo= AckNo, AckNo+1, …., AckNo+Win-1

  • Receiver can acknowledge data without opening the window
  • Receiver can change the window size without acknowledging data
tcp congestion control21
TCP Congestion Control
  • TCP has a mechanism for congestion control. The mechanism is implemented at the sender
  • The window size at the sender is set as follows:


  • flow control window is advertised by the receiver
  • congestion window is adjusted based on feedback from the network

Send Window = MIN (flow control window, congestion window)

tcp congestion control22
TCP Congestion Control
  • The sender has two additional parameters:
    • Congestion Window (cwnd)Initial value is 1 MSS (=maximum segment size) counted as bytes
    • Slow-start threshold Value (ssthresh)

Initial value is the advertised window size)

  • Congestion control works in two modes:
    • slow start (cwnd < ssthresh)
    • congestion avoidance (cwnd >= ssthresh)
slow start
Slow Start
  • Initial value:
        • cwnd = 1 segment
  • Note: cwnd is actually measured in bytes: 1 segment = MSS bytes
  • Each time an ACK is received, the congestion window is increased by MSS bytes.
        • cwnd = cwnd + MSS
    • If an ACK acknowledges two segments, cwnd is still increased by only 1 segment.
    • Even if ACK acknowledges a segment that is smaller than MSS bytes long, cwnd is increased by MSS.
  • Does Slow Start increment slowly? Not really. In fact, the increase of cwnd can be exponential
slow start example
Slow Start Example
  • The congestion window size grows very rapidly
    • For every ACK, we increase cwnd by 1 irrespective of the number of segments ACK’ed
  • TCP slows down the increase of cwnd when cwnd > ssthresh
congestion avoidance
Congestion Avoidance
  • Congestion avoidance phase is started if cwnd has reached the slow-start threshold value
  • If cwnd >= ssthresh then each time an ACK is received, increment cwnd as follows:
      • cwnd = cwnd + MSS(MSS/ cwnd)
  • So cwnd is increased by one segment (=MSS bytes) only if all segments have been acknowledged.
slow start congestion avoidance
Slow Start / Congestion Avoidance
  • Here we give a more accurate version than in our earlier discussion of Slow Start:

Ifcwnd <= ssthreshthenEach time an Ack is received: cwnd = cwnd +MSS

else /* cwnd > ssthresh */

Each time an Ack is received :cwnd = cwnd + MSS. MSS/ cwnd


example of slow start congestion avoidance
Example of Slow Start/Congestion Avoidance

Assume that ssthresh = 8


Cwnd (in segments)

Roundtrip times

responses to congestion
Responses to Congestion
  • Most often, a packet loss in a network is due to an overflow at a congested router (rather than due to a transmission error)
  • So, TCP assumes there is congestion if it detects a packet loss
  • A TCP sender can detect lost packets via:
      • Timeout of a retransmission timer
      • Receipt of a duplicate ACK
  • When TCP assumes that a packet loss is caused by congestion it reduces the size of the sending window
tcp tahoe
TCP Tahoe
  • Congestion is assumed if sender has timeout or receipt of duplicate ACK
  • Each time when congestion occurs,
    • cwnd is reset to one:

cwnd = MSS

    • ssthresh is set to half the current size of the congestion window:

ssthressh = cwnd / 2

    • and slow-start is entered
slow start congestion avoidance30
Slow Start / Congestion Avoidance
  • A typical plot of cwnd for a TCP connection (MSS = 1500 bytes) with TCP Tahoe:
tcp error control

TCP Error Control

Background on Error Control

TCP Error Control

background arq error control
Background: ARQ Error Control
  • Two types of errors:
    • Lost packets
    • Damaged packets
  • Most Error Control techniques are based on:

1. Error Detection Scheme (Parity checks, CRC).

2. Retransmission Scheme.

  • Error control schemes that involve error detection and retransmission of lost or corrupted packets are referred to as Automatic Repeat Request (ARQ) error control.
background arq error control33
Background: ARQ Error Control

All retransmission schemes use all or a subset of the following procedures:

Positive acknowledgments (ACK)

Negative acknowledgment (NACK)

All retransmission schemes (using ACK, NACK or both) rely on the use of timers

The most common ARQ retransmission schemes are:

Stop-and-Wait ARQ

Go-Back-N ARQ

Selective Repeat ARQ

background arq error control34
Background: ARQ Error Control
  • The most common ARQ retransmission schemes:
    • Stop-and-Wait ARQ
    • Go-Back-N ARQ
    • Selective Repeat ARQ
  • The protocol for sending ACKs in all ARQ protocols are based on the sliding window flow control scheme
background stop and wait arq
Background: Stop-and-Wait ARQ
  • Stop-and-Wait ARQ is an addition to the Stop-and-Wait flow control protocol:
  • Packets have 1-bit sequence numbers (SN = 0 or 1)
  • Receiver sends an ACK (1-SN) if packet SN is correctly received
  • Sender waits for an ACK (1-SN) before transmitting the next packet with sequence number 1-SN
  • If sender does not receive anything before a timeout value expires, it retransmits packet SN
background stop and wait arq36

Packet 1

Packet 0

Packet 1

Packet 1




Background: Stop-and-Wait ARQ
  • Lost Packet




background go back n arq
Background: Go-Back-N ARQ


  • A station may send multiple packets as allowed by the window size
  • Receiver sends a NAK i if packet i is in error. After that, the receiver discards all incoming packets until the packet in error was correctly retransmitted
  • If sender receives a NAK iit will retransmit packet i and all packets i+1, i+2,... which have been sent, but not been acknowledged
example of go back n arq
In Go-back-N, if packets are correctly delivered, they are delivered in the correct sequence

Therefore, the receiver does not need to keep track of `holes’ in the sequence of delivered packets

Example of Go-Back-N ARQ
background go back n arq39

Packet 0

Packet 6

Packet 5

Packet 2

Packet 1

Packet 3

Packet 4

Packet 5

Packet 6

Packet 4



Background: Go-Back-N ARQ
  • Lost Packet


for Packet 2

Packets 4,5,6are retransmitted



Packets 5 and 6

are discarded

background selective repeat arq
Background: Selective-Repeat ARQ
  • Similar to Go-Back-N ARQ. However, the sender only retransmits packets for which a time-out occured is received
  • Advantage over Go-Back-N:
    • Fewer Retransmissions.
  • Disadvantages:
    • More complexity at sender and receiver
    • Each packet must be acknowledged individually (no cumulative acknowledgements)
    • Receiver may receive packets out of sequence
example of selective repeat arq
Example of Selective-Repeat ARQ

Receiver must keep track of `holes’ in the sequence of delivered packets

Sender must maintain one timer per outstanding packet

background selective repeat arq42

Packet 0

Packet 2

Packet 1

Packet 3

Packet 5

Packet 6

Packet 0

Packet 4

Packet 7

Packet 4










Background: Selective-Repeat ARQ
  • Lost Packet

Timeout for Packet 4:only Packet 4 is retransmitted



Packets 5 and 6

are buffered

error control in tcp
Error Control in TCP
  • TCP implements a variation of the Go-back-N retransmission scheme
  • TCP maintains a Retransmission Timer for each connection:
    • The timer is started during a transmission. A timeout causes a retransmission
  • TCP couples error control and congestion control (I.e., it assumes that errors are caused by congestion)
  • TCP allows accelerated retransmissions (Fast Retransmit)
tcp retransmission timer
TCP Retransmission Timer
  • Retransmission Timer:
    • The setting of the retransmission timer is crucial for efficiency
    • Timeout value too small results in unnecessary retransmissions
    • Timeout value too large long waiting time before a retransmission can be issued
    • A problem is that the delays in the network are not fixed
    • Therefore, the retransmission timers must be adaptive
round trip time measurements
Round-Trip Time Measurements
  • The retransmission mechanism of TCP is adaptive
  • The retransmission timers are set based on round-trip time (RTT) measurements that TCP performs

The RTT is based on time difference between segment transmission and ACK


TCP does not ACK each segment

Each connection has only one timer

round trip time measurements46
Round-Trip Time Measurements
  • Retransmission timer is set to a Retransmission Timeout (RTO) value.
  • RTO is calculated based on the RTT measurements.
  • The RTT measurements are smoothed by the following estimators srtt and rttvar:

srttn+1 = a RTT + (1- a ) srttn rttvarn+1 = b ( | RTT - srttn+1 | ) + (1- b ) rttvarn

RTOn+1 = srttn+1 + 4 rttvarn+1

  • The gains are set toa =1/4 and b =1/8
  • srtt0 = 0 sec, rttvar0 = 3 sec, Also: RTO1 = srtt1 + 2 rttvar1
karn s algorithm
Karn’s Algorithm
  • If an ACK for a retransmitted segment is received, the sender cannot tell if the ACK belongs to the original or the retransmission.

Karn’s Algorithm:

Don’t update srtt on any segments that have been retransmitted.

Each time when TCP retransmits, it sets:RTOn+1 = max ( 2 RTOn, 64)(exponential backoff)

measuring tcp retransmission timers
Measuring TCP Retransmission Timers

Transfer file from Argon to neon

Unplug Ethernet of Argon cable in the middle of file transfer

interpreting the measurements
Interpreting the Measurements
  • The interval between retransmission attempts in seconds is:

1.03, 3, 6, 12, 24, 48, 64, 64, 64, 64, 64, 64, 64.

  • Time between retrans-missions is doubled each time (Exponential Backoff Algorithm)
  • Timer is not increased beyond 64 seconds
  • TCP gives up after 13th attempt and 9 minutes.