EE 122: Lecture 89

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EE 122 Lecture 8/9

• Ion Stoica
• September 20/25, 2001

2
Overview

• TCP/IP architecture
• TCP
• IP and TCP headers
• Connection establishment
• Flow control
• RTT estimation

3
Before Internet

• Different packet-switching networks (e.g.,
  ARPANET, ARPA packet radio)
• only nodes on the same network could communicate

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Goal and Challenges

• Share resources of different packet switching
  networks ? interconnect existing networks
• but, packet switching networks differ widely
• Different services
• E.g., degree of reliability
• Different interfaces
• E.g., length of the packet that can be
  transmitted, address format
• Different protocols
• E.g., routing protocols

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Possible solutions

• Reengineer and develop one global packet
  switching network standard
• Not economically feasible
• Have every host implement the protocols of any
  network it wants to communicate with
• Too complex, very high engineering cost

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Solution

• Add an extra layer internetworking layer, also
  called Internet Protocol (IP)
• Hosts implement one higher-level protocol
• Networks interconnected by nodes that run the
  same protocol
• Provide the interface between the new protocol
  and every network

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Solution
Gateways (routers)
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Challenge 1 Different Address Formats

• Provide one common format ? map lower level
  addresses to a common format
• IPv4 address format 32 bit length, hierarchical
  organization

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7
Class A
Network
Host
0
16
14
Network
Class B
Host
1
0
8
21
Network
Host
1
1
0
Class C
28
1
1
1
Class D
0
Multicast address
27
1
1
1
Class E
1
Unused
0
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Challenge 2 Different Packet Sizes

• Define a maximum packet size over all networks.
  Why not?
• Implement fragmentation/re-assembly
• who is doing fragmentation?
• who is doing re-assembly?

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Other Challenges

• Errors ? require end-to-end reliability
• Add a transport layer on top of IP
• Different (routing) protocols ? coordinate these
  protocols

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Transport Layer TCP and UDP

• TCP (Transmission Control Protocol)
• Unbounded but finite length messages
• Byte streaming
• Reliable and in-sequence delivery
• Full duplex
• Application example file transfer, telnet
• UDP (User Datagram Protocol)
• Unreliable datagram delivery
• Adds multiplexing/demultiplexing to IP
• Application example video/audio streaming
• Note The need for UDP was the main reason behind
  splitting TCP and IP (at the beginning there were
  only one TCP/IP layer)

TCP/UDP
Transport Layer
Networking Layer
IP
Link Layer
Physical Layer
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Overview

• TCP/IP architecture
• TCP
• IP and TCP headers
• Connection establishment
• Flow control
• RTT estimation

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Headers

• IP header ? used for IP routing, fragmentation,
  error detection
• UDP header ? used for multiplexing/demultiplexing,
  data error correction
• TCP header ? used for multiplexing/demultiplexing,
  flow and congestion control

Receiver
Sender
Application
Application
data
data
TCP UDP
TCP UDP
data
TCP/UDP
data
TCP/UDP
IP
IP
data
TCP/UDP
IP
data
TCP/UDP
IP
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IP Header
0
4
8
16
19
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Version
HLen
TOS
Length
Identification
Flags
Fragment offset
20 bytes
TTL
Protocol
Header checksum
Source address
Destination address
Options (variable)

• Comments
• HLen header length only in 32-bit words (5 lt
  HLen lt 15)
• TOS (Type of Service) now split in
• Differentiated Service Field (6 bits)
• remaining two bits used by ECN (Early Congestion
  Notification)
• Length the length of the entire
  datagram/segment header data
• Flags Dont Fragment (DF) and More Fragments
  (MF)
• Fragment offset all fragments excepting last
  one contain multiples of 8 bytes
• Header checksum - uses 1s complement

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TCP Header
0
4
10
16
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Destination port
Source port
Sequence number
Acknowledgement
Advertised window
Flags
HdrLen
Checksum
Urgent pointer
Options (variable)

• Sequence number, acknowledgement, and advertised
  window used by sliding-window based flow
  control
• Flags
• SYN, FIN establishing/terminating a TCP
  connection
• ACK set when Acknowledgement field is valid
• URG urgent data Urgent Pointer says where
  non-urgent data starts
• PUSH dont wait to fill segment
• RESET abort connection

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TCP Header (Cont)

• Checksum 1s complement and is computed over
• TCP header
• TCP data
• Pseudo-header (from IP header)
• Note breaks the layering!

Source address
Destination address
TCP Segment length
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Protocol (TCP)
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Overview

• TCP/IP architecture
• TCP
• IP and TCP headers
• Connection establishment
• Flow control
• RTT estimation

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TCP Connection Establishment

• Three-way handshake
• Goal agree on a set of parameters the start
  sequence number for each side
• Staring sequence numbers are random. Why?

Server
Client (initiator)
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Overview

• TCP/IP architecture
• TCP
• IP and TCP headers
• Connection establishment
• Flow control
• RTT estimation

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Basic Problem

• How much traffic do you send?
• Two components
• Flow control
• Make sure that the receiver can receive as fast
  as you send
• Congestion control (see Lecture 10)
• Make sure that the network delivers the packets
  to the receiver

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TCP Flow Control vs. Link Layer Flow Control

• Flow control runs end-to-end, instead of on the
  same link
• RTT can vary widely
• Receivers maximum window size can vary
• Because resources allocated to a connection
  varies
• Senders maximum window size varies
• Because it is used to throttle back the sender
  where there is congestion (see Lecture 10)
• Others
• Sequence numbers counts bytes not packets
• Packets usually called segments

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Flow control Window Size and Throughput
wnd 3

• Sliding-window based flow control
• Higher window ? higher throughput
• Throughput wnd/RTT
• Need to worry about sequence number wrapping
• Remember window size control throughput

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TCP Flow Control

• Receiver window (MaxRcvBuf maximum buffer size
  at receiver)
• Sender window (MaxSendBuf maximum buffer size
  at sender)

AdvertisedWindow MaxRcvBuffer (LastByteRcvd
LastByteRead)
EffectiveWindow AdvertisedWindow
(LastByteSent LastByteAcked)
MaxSendBuffer gt LastByteWritten - LastByteAcked
Sending Application
Receiving Application
LastByteWritten
LastByteRead
LastByteAcked
LastByteSent
NextByteExpected
LastByteRcvd
sequence number increases
sequence number increases
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Overview

• TCP/IP architecture
• TCP
• IP and TCP headers
• Connection establishment
• Flow control
• RTT estimation

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RTT Estimation

• Use exponential averaging

SampleRTT
EstimatedRTT
Time
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Problem

• How to differentiate between the real ACK, and
  ACK of the retransmitted packet

Sender
Receiver
Sender
Receiver
Original Transmission
Original Transmission
SampleRTT
ACK
SampleRTT
Retransmission
Retransmission
ACK
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Karn/Partridge Algorithm

• Measure SampleRTT only for original transmissions
• Exponential backoff ? for each retransmission,
  double EstimatedRTT

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Jacobson/Karels Algorithm

• Problem take into account the variance

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Summary

• TCP/IP architecture
• Add a layer (i.e., the IP layer) to interconnect
  existing networks
• Add a layer (i.e., transport layer) to provide
  multiplexing/demultiplexing, reliability, and
  in-order delivery
• TCP
• Connection establishment ? three way handshake
• Flow control ? based on sliding window protocol
• Congestion control ? next lecture