CS244a: An Introduction to Computer Networks

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CS244a An Introduction to Computer Networks

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

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

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The Transport Layer

• What is the transport layer for?
• What characteristics might it have?
• Reliable delivery
• Flow control

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Review of the transport layer
Athena.MIT.edu
Leland.Stanford.edu
Nick
Dave
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Layering The OSI Model
layer-to-layer communication
Application
Application
7
7
Presentation
Presentation
6
6
Session
Session
5
5
Peer-layer communication
Transport
Transport
Router
Router
4
4
Network
Network
Network
Network
3
3
Link
Link
Link
Link
2
2
Physical
Physical
Physical
Physical
1
1
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Layering Our FTP Example
Application
Application
Presentation
Transport
Session
Transport
Network
Network
Link
Link
Physical
The 4-layer Internet model
The 7-layer OSI Model
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TCP Characteristics

• TCP is connection-oriented.
• 3-way handshake used for connection setup.
• 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
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
IP Data
IP Hdr
TCP Hdr
TCP Data
0
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Src port
Dst port
Sequence
Src/dst port numbers and IP addresses uniquely
identify socket
Ack Sequence
TCP Header and Data IP Addresses
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 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

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

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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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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
• EstimatedRTTk ? EstimatedRTTk-1 (1 - ?)
  SampleRTT
• RTO 2 EstimatedRTT
• Characteristics of the original algorithm
• Variance is assumed to be fixed.
• But in practice, variance increases as congestion
  increases.

Determined empirically
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TCP Retransmission and Timeouts

• Router queues grow when there is more
  traffic, until they become unstable.
• As load grows, variance of delay grows
  rapidly.

• There will be some (unknown) distribution
  of RTTs.
• We are trying to estimate an RTO to
  minimize the probability of a false timeout.

Probability
variance
RTT
mean
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TCP Retransmission and Timeouts

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

Same as before
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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).
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User Datagram Protocol (UDP) Characteristics

• UDP is a connectionless datagram service.
• There is no connection establishment packets may
  show up at any time.
• UDP packets are self-contained.
• UDP is unreliable
• No acknowledgements to indicate delivery of data.
• Checksums cover the header, and only optionally
  cover the data.
• Contains no mechanism to detect missing or
  mis-sequenced packets.
• No mechanism for automatic retransmission.
• No mechanism for flow control, and so can
  over-run the receiver.

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User-Datagram Protocol (UDP)
A1
A2
B1
B2
App
App
App
App
OS
UDP
Like TCP, UDP uses port number to demultiplex
packets
IP
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User-Datagram Protocol (UDP)Packet format
SRC port
DST port
By default, only covers the header.
checksum
length
DATA

• Why do we have UDP?
• It is used by applications that dont need
  reliable delivery, or
• Applications that have their own special needs,
  such as streaming of real-time audio/video.