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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 argon.cs.virginia.edu.1023 > neon.cs.virginia.edu.login: P 0:1(1) ack 1

44.063317 neon.cs.virginia.edu.login > argon.cs.virginia.edu.1023: P 1:2(1) ack 1 win 8760

44.182705 argon.cs.virginia.edu.1023 > neon.cs.virginia.edu.login: . ack 2 win 17520

48.946471 argon.cs.virginia.edu.1023 > neon.cs.virginia.edu.login: P 1:2(1) ack 2 win 17520

48.947326 neon.cs.virginia.edu.login > argon.cs.virginia.edu.1023: P 2:3(1) ack 2 win 8760

48.982786 argon.cs.virginia.edu.1023 > neon.cs.virginia.edu.login: . ack 3 win 17520

55:00.116581 argon.cs.virginia.edu.1023 > neon.cs.virginia.edu.login: P 2:3(1) ack 3 win 17520

55:00.117497 neon.cs.virginia.edu.login > argon.cs.virginia.edu.1023: P 3:4(1) ack 3 win 8760

55:00.183694 argon.cs.virginia.edu.1023 > neon.cs.virginia.edu.login: . ack 4 win 17520


Rlogin

Rlogin

  • “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


Rlogin1

Rlogin

  • 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 argon.cs.virginia.edu.1023 > tenet.CS.Berkeley.EDU.login: P 1:2(1) ack 2 win 16384

54:16.481929 tenet.CS.Berkeley.EDU.login > argon.cs.virginia.edu.1023: P 2:3(1) ack 2 win 16384

54:16.482154 argon.cs.virginia.edu.1023 > tenet.CS.Berkeley.EDU.login: P 2:3(1) ack 3 win 16383

54:16.559447 tenet.CS.Berkeley.EDU.login > argon.cs.virginia.edu.1023: P 3:4(1) ack 3 win 16384

54:16.559684 argon.cs.virginia.edu.1023 > tenet.CS.Berkeley.EDU.login: P 3:4(1) ack 4 win 16383

54:16.640508 tenet.CS.Berkeley.EDU.login > argon.cs.virginia.edu.1023: P 4:5(1) ack 4 win 16384

54:16.640761 argon.cs.virginia.edu.1023 > tenet.CS.Berkeley.EDU.login: P 4:8(4) ack 5 win 16383

54:16.728402 tenet.CS.Berkeley.EDU.login > argon.cs.virginia.edu.1023: 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.


Flow control congestion control error control

TCP:

Flow Control Congestion ControlError Control


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 control

TCP Flow Control


Tcp flow control1

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


  • Sliding window example

    Sliding Window: Example


    Tcp congestion control

    TCP Congestion Control


    Tcp congestion control1

    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:

      where

    • 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 control2

    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

      endif


    Example of slow start congestion avoidance

    Example of Slow Start/Congestion Avoidance

    Assume that ssthresh = 8

    ssthresh

    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 avoidance1

    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 control1

    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 control2

    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 arq1

    Packet 1

    Packet 0

    Packet 1

    Packet 1

    ACK 0

    ACK 1

    ACK 0

    Background: Stop-and-Wait ARQ

    • Lost Packet

    Timeout

    A

    B


    Background go back n arq

    Background: Go-Back-N ARQ

    Operations:

    • 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 arq1

    Packet 0

    Packet 6

    Packet 5

    Packet 2

    Packet 1

    Packet 3

    Packet 4

    Packet 5

    Packet 6

    Packet 4

    ACK 3

    ACK 6

    Background: Go-Back-N ARQ

    • Lost Packet

    Timeout

    for Packet 2

    Packets 4,5,6are retransmitted

    A

    B

    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 arq1

    Packet 0

    Packet 2

    Packet 1

    Packet 3

    Packet 5

    Packet 6

    Packet 0

    Packet 4

    Packet 7

    Packet 4

    ACK 6

    ACK 2

    ACK 3

    ACK 4

    ACK 1

    ACK 1

    ACK 7

    ACK 5

    ACK 0

    Background: Selective-Repeat ARQ

    • Lost Packet

    Timeout for Packet 4:only Packet 4 is retransmitted

    A

    B

    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

    But:

    TCP does not ACK each segment

    Each connection has only one timer


    Round trip time measurements1

    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 ) srttnrttvarn+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.


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