TCP - Part II

1
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.
2
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.
3
tcpdump of an rlogin session
This is the output of typing 3 (three) characters
44.062449 argon.cs.virginia.edu.1023 gt
neon.cs.virginia.edu.login P 01(1) ack 1
44.063317 neon.cs.virginia.edu.login gt
argon.cs.virginia.edu.1023 P 12(1) ack 1 win
8760 44.182705 argon.cs.virginia.edu.1023 gt
neon.cs.virginia.edu.login . ack 2 win
17520 48.946471 argon.cs.virginia.edu.1023 gt
neon.cs.virginia.edu.login P 12(1) ack 2 win
17520 48.947326 neon.cs.virginia.edu.login gt
argon.cs.virginia.edu.1023 P 23(1) ack 2 win
8760 48.982786 argon.cs.virginia.edu.1023 gt
neon.cs.virginia.edu.login . ack 3 win 17520
5500.116581 argon.cs.virginia.edu.1023 gt
neon.cs.virginia.edu.login P 23(1) ack 3 win
17520 5500.117497 neon.cs.virginia.edu.login gt
argon.cs.virginia.edu.1023 P 34(1) ack 3 win
8760 5500.183694 argon.cs.virginia.edu.1023 gt
neon.cs.virginia.edu.login . ack 4 win 17520
4
Rlogin

• Rlogin is a remote terminal applications
• Originally build 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
• Q What is the total number of bytes
  transmitted for a single keystroke?

5
Rlogin

• We would expect that tcpdump shows this pattern
• However, tcpdump shows this pattern
• So, TCP has delayed the transmission of an ACK

6
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.
• What are the delays of the ACKs in the tcpdump
  example?

7
tcpdump of a wide-area rlogin session
This is the output of typing 9 characters
5416.401963 argon.cs.virginia.edu.1023 gt
tenet.CS.Berkeley.EDU.login P 12(1) ack 2 win
16384 5416.481929 tenet.CS.Berkeley.EDU.login gt
argon.cs.virginia.edu.1023 P 23(1) ack 2 win
16384 5416.482154 argon.cs.virginia.edu.1023 gt
tenet.CS.Berkeley.EDU.login P 23(1) ack 3 win
16383 5416.559447 tenet.CS.Berkeley.EDU.login gt
argon.cs.virginia.edu.1023 P 34(1) ack 3 win
16384 5416.559684 argon.cs.virginia.edu.1023 gt
tenet.CS.Berkeley.EDU.login P 34(1) ack 4 win
16383 5416.640508 tenet.CS.Berkeley.EDU.login gt
argon.cs.virginia.edu.1023 P 45(1) ack 4 win
16384 5416.640761 argon.cs.virginia.edu.1023 gt
tenet.CS.Berkeley.EDU.login P 48(4) ack 5 win
16383 5416.728402 tenet.CS.Berkeley.EDU.login gt
argon.cs.virginia.edu.1023 P 59(4) ack 8 win
16384
8
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.

9
Wide-area Rlogin Observation 2

• There are fewer transmissions than there are
  characters.
• Aida never has multiple segments outstanding.
• This is due to Nagles 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 byte is arriving from
  application one byte at a time)
• Nagles rule reduces the amount of small
  segments.The algorithm can be disabled.

10
Flow Control Congestion ControlError Control
TCP
11
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 are
  different.
• ? But the implementation is combined

12
TCP Flow Control
13
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

14
Sliding Window Flow Control

• Sliding Window Protocol is performed at the byte
  level

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

15
Sliding Window Window Closes

• Transmission of a single byte (with SeqNo 6)
  and acknowledgement is received (AckNo 5,
  Win4)

16
Sliding Window Window Opens

• Acknowledgement is received that enlarges the
  window to the right (AckNo 5, Win6)

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

17
Sliding Window Window Shrinks

• Acknowledgement is received that reduces the
  window from the right (AckNo 5, Win3)

• Shrinking a window should not be used

18
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, AckNo1, ., AckNoWin-1
• Receiver can acknowledge data without opening the
  window
• Receiver can change the window size without
  acknowledging data

19
Sliding Window Example
20
TCP Congestion Control
21
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)

22
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 threshhold Value (ssthresh)
• Initial value is the advertised window size)
• Congestion control works in two modes
• slow start (cwnd lt ssthresh)
• congestion avoidance (cwnd gt ssthresh)

23
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 1
• 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
  1.
• Does Slow Start increment slowly? Not really. In
  fact, the increase of cwnd can be exponential

24
Slow Start Example

• The congestion window size grows very rapidly
• For every ACK, we increase cwnd by 1 irrespective
  of the number of segments ACKed
• TCP slows down the increase of cwnd when cwnd gt
  ssthresh

25
Congestion Avoidance

• Congestion avoidance phase is started if cwnd has
  reached the slow-start threshold value
• If cwnd gt ssthresh then each time an ACK is
  received, increment cwnd as follows
• cwnd cwnd 1/ cwnd
• Where cwnd is the largest integer smaller than
  cwnd
• So cwnd is increased by one segment (MSS bytes)
  only if all segments have been acknowledged.

26
Slow Start / Congestion Avoidance

• Here we give a more accurate version than in our
  earlier discussion of Slow Start
• If cwnd lt ssthresh then Each time an Ack is
  received cwnd cwnd 1
• else / cwnd gt ssthresh /
• Each time an Ack is received cwnd cwnd 1
  / cwnd
• endif

27
Example of Slow Start/Congestion Avoidance

• Assume that ssthresh 8

ssthresh
Cwnd (in segments)
Roundtrip times
28
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 and reduces the size of the sending
  window

29
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 1
• ssthresh is set to half the current size of the
  congestion window
• ssthressh cwnd / 2
• and slow-start is entered

30
Slow Start / Congestion Avoidance

• A typical plot of cwnd for a TCP connection (MSS
  1500 bytes) with TCP Tahoe

31
TCP Error Control
Background on Error Control TCP Error Control
32
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.

33
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
34
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

35
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

36
Background Stop-and-Wait ARQ

• Lost Packet

Timeout
A
B
37
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 i it will retransmit
  packet i and all packets i1, i2,... which have
  been sent, but not been acknowledged

38
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

39
Background Go-Back-N ARQ

• Lost Packet

Timeout for Packet 2
Packets 4,5,6are retransmitted
A
B
Packets 5 and 6 are discarded
40
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

41
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
42
Background Selective-Repeat ARQ

• Lost Packet

Timeout for Packet 4only Packet 4 is
retransmitted
A
B
Packets 5 and 6 are buffered
43
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)

44
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

45
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
46
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
• srttn1 a RTT (1- a ) srttn rttvarn1
  b ( RTT - srttn1 ) (1- b ) rttvarn
• RTOn1 srttn1 4 rttvarn1
• The gains are set to a 1/4 and b 1/8
• srtt0 0 sec, rttvar0 3 sec, Also RTO1
  srtt1 2 rttvar1

47
Karns Algorithm

• If an ACK for a retransmitted segment is
  received, the sender cannot tell if the ACK
  belongs to the original or the retransmission.

Karns Algorithm Dont update srtt on any
segments that have been retransmitted. Each time
when TCP retransmits, it setsRTOn1 max ( 2
RTOn, 64) (exponential backoff)
48
Measuring TCP Retransmission Timers

• Transfer file from Argon to neonn
• Unplug Ethernet of Argon cable in the middle of
  file transfer

49
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.