CS492B Project

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CS492B Project 2 TCP Tutorial 1

• 2003. 9. 15
• Jin Hyun Ju

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CS244a An Introduction to Computer Networks

• Handout 6 The Transport Layer, User Datagram
  Protocol (UDP) , and Transmission Control
  Protocol (TCP)

Nick McKeown Professor of Electrical Engineering
and Computer Science, Stanford
University nickm_at_stanford.edu http//www.stanford.
edu/nickm
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Outline

• The TCP Protocol
• TCP Characteristics
• TCP Connection setup
• TCP Segments
• TCP Sequence Numbers
• TCP Sliding Window
• ACK Policy
• Timeouts and Retransmission
• (Congestion Control and Avoidance)

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TCP Characteristics

• TCP is connection-oriented.
• TCP provides a stream-of-bytes service.
• TCP is reliable
• Acknowledgements indicate delivery of data.
• Checksums are used to detect corrupted data.
• Sequence numbers detect missing, or mis-sequenced
  data.
• Corrupted data is retransmitted after a timeout.
• Mis-sequenced data is re-sequenced.
• (Window-based) Flow control prevents over-run of
  receiver.
• TCP uses congestion control to share network
  capacity among users. Well study this in the
  next lecture.

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TCP is connection-oriented
(Active) Client
(Passive) Server
(Active) Client
(Passive) Server
Syn
Fin
Syn Ack
(Data ) Ack
Ack
Fin
Ack
TIME_WAIT
CLOSED
Connection Setup 3-way handshake
Connection Close/Teardown 2 x 2-way handshake
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TCP supports a stream of bytes service
Host A
Byte 0
Byte 1
Byte 2
Byte 3
Byte 80
Host B
Byte 0
Byte 1
Byte 2
Byte 3
Byte 80
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which is emulated using TCP segments
Host A
Byte 0
Byte 1
Byte 2
Byte 3
Byte 80

• Segment sent when
• Segment full (MSS bytes),
• Not full, but times out, or
• Pushed by application.

TCP Data
TCP Data
Host B
Byte 0
Byte 1
Byte 2
Byte 3
Byte 80
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The TCP Segment Format
TCP Hdr
TCP Data
0
15
31
Src port
Dst port
Sequence
Ack Sequence
Flags
Window Size
HLEN 4
RSVD 6
URG
ACK
PSH
RST
SYN
FIN
Checksum
Urg Pointer
(TCP Options)
TCP Data
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Sequence Numbers
Host A
ISN (initial sequence number)
Sequence number 1st byte
TCP HDR
TCP Data
Ack sequence number next expected byte
TCP HDR
TCP Data
Host B
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Initial Sequence Numbers
(Active) Client
(Passive) Server
Syn ISNA
Syn Ack ISNB
Ack
Connection Setup 3-way handshake
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TCP Window

• Pipelining
• Retransmission
• Flow control
• (Congestion control)

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TCP Sliding Window

• How much data can a TCP sender have outstanding
  in the network?
• How much data should TCP retransmit when an error
  occurs? Just selectively repeat the missing data?
• How does the TCP sender avoid over-running the
  receivers buffers?

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TCP Sliding Window
Window Size
Outstanding Un-ackd data
Data OK to send
Data not OK to send yet
Data ACKd
ACK

• Retransmission policy is Go Back N.
• Current window size is advertised by receiver
• (usually 4k 8k Bytes when connection set-up).

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TCP Sliding Window
Round-trip time
Window Size
Host A
Host B
ACK
ACK
(1) RTT gt Window size
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Delayed ACK

• Data types in TCP segment
• Bulk data (FTP, Email, News, HTTP) 90
• Interactive data (Telnet, Rlogin) 10
• one key-stroke generates one segment, carrying
  only 1 byte.
• 90 of Telnet Rlogin packets carry less than 10
  bytes of user data
• TCP can be silly without any optimization!

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Delayed ACK
Event in-order segment arrival, no
gaps, everything else already ACKed in-order
segment arrival, no gaps, one delayed ACK
pending out-of-order segment arrival higher-than-
expect seq. gap detected arrival of segment
that partially or completely fills gap
TCP Receiver action delayed ACK. Wait up to
500ms for next segment. If no next segment, send
ACK immediately send single cumulative ACK
send duplicate ACK, indicating seq. of next
expected byte immediate ACK if segment
starts at lower end of gap
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TCP Retransmission and Timeouts
Round-trip time (RTT)
Retransmission TimeOut (RTO)
Guard Band
Host A
Estimated RTT
Data1
Data2
ACK
ACK
Host B
TCP uses an adaptive retransmission timeout
value Congestion Changes in Routing
RTT changes frequently
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TCP Retransmission and Timeouts

• Picking the RTO is important
• Pick a values thats too big and it will wait too
  long to retransmit a packet,
• Pick a value too small, and it will unnecessarily
  retransmit packets.
• The original algorithm for picking RTO
• EstimatedRTT ? EstimatedRTT (1 - ?)
  SampleRTT
• RTO 2 EstimatedRTT
• Characteristics of the original algorithm
• Variance is assumed to be fixed.
• But in practice, variance increases as congestion
  increases.

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TCP Retransmission and Timeouts

• Newer Algorithm includes estimate of variance in
  RTT
• Difference SampleRTT - EstimatedRTT
• EstimatedRTT EstimatedRTT (?Difference)
• Deviation Deviation ?( Difference -
  Deviation )
• RTO ? EstimatedRTT ? Deviation
• ? ? 1
• ? ? 4

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TCP Retransmission and TimeoutsKarns Algorithm
Host A
Host B
Host A
Host B
Retransmission
Retransmission
Wrong RTT Sample
Wrong RTT Sample
Problem How can we estimate RTT when packets
are retransmitted? Solution On retransmission,
dont update estimated RTT (and double RTO).