Chapter 5 Transport Layer: UDP, TCP

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Chapter 5Transport Layer UDP, TCP

• Professor Rick Han
• University of Colorado at Boulder
• rhan_at_cs.colorado.edu

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Announcements

• Netstat portion of Homework 3 on Web, due March
  12 (two weeks)
• Programming Assignment 2 due Friday March 22 by
  midnight
• Midterm March 14
• Only through Network Layer, Chapter 4
• OH changed today 445-545 due to BACTAC talk
• Next, transport layer

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Recap of Previous Lecture

• IP Multicast
• 1-gtN
• Link-State Multicast MOSPF
• Piggyback off of Dijkstras shortest path tree
• Distance-Vector Multicast DVMRP
• Reverse-path flooding to all nodes
• Reverse-path pruning
• Protocol Independent Multicast PIM
• For sparse wide-area trees
• Unicast to Rendezvous Point, RP then multicasts
• Tree built by explicit joins
• Internet Group Management Protocol (IGMP)
• How leaf nodes join a multicast group

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The Layered Network Stack
Internet Stack
Application Layer
Transport Layer (TCP/UDP)
Network Layer (IP)
Data Link Layer
Physical Layer
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UDP Unreliable Transport Protocols

• IP Provides Best-Effort unreliable datagram
  delivery
• User Datagram Protocol (UDP) provides best-effort
  unreliable datagram delivery
• Packets may be lost
• Due to congestion
• In routers
• At receiver and/or sender
• Due to fading
• Packets may be reordered
• Packets may be duplicated

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UDP (2)

• UDP Features
• Transport protocol above IP
• simply passes IP through to the upper
  (application) layers
• UDP Features
• Gives apps the option to send packets unreliably
• Useful for multicast apps
• Low delay transmission for interactive
  text/audio/video
• Minimal connection setup time
• Minimal connection state connectionless
• No congestion control blast away
• Build own application-level protocol on top of
  UDP

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UDP (3)
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32
Source Port
Dest. Port
UDP length
Checksum
UDP payload
UDP payload data
IP hdr
UDP hdr

• UDP Header
• Only 8 bytes! (vs. IP 20 bytes header)
• Source port for UDP sending process
• Dest port for UDP receiving process
• Length in bytes of UDP header payload
• Checksum over UDP header payload

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UDP (4)

• UDP Checksum calculation
• UDP creates a pseudo-header containing IP
  header info
• Doublechecks that the packet has the correct IP
  address and UDP port

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32-bit IP source address
32-bit IP destination address
Pseudo-Header
Protocol
UDP length
0
Source Port
Dest. Port
UDP Header
UDP length
Checksum

• Discarded if checksum fails

UDP payload data
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UDP Multiplexing/Demultiplexing
Host 1
Host 2
UDP Port 5100
UDP Port 5200
UDP Port 6001
UDP Port 433

• IP layer inspects Protocol field UDP
• Sends to buffer of waiting UDP process with
  matching port

IP
IP
Eth. MAC
Eth MAC
Phys.
Phys.
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TCP Transmission Control Protocol Reliable
Delivery

• TCP provides
• Reliable delivery of packets, AND
• Stream or in-order delivery
• Also, full-duplex service
• Why TCP, when we can build reliable protocols on
  top of UDP?
• Many applications require reliable in-order
  delivery of packets
• Web pages, email, file transfer,
• Why should each application reinvent the wheel?
• Can concentrate expert knowledge into building
  one reliable protocol provided to every
  application

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TCP (2)

• TCP is a true transport protocol, above IP
• TCP Features
• Flow control
• Congestion control
• Connection setup
• Connection state machine
• Reliability, at the cost of some delay
• Achieves reliability by
• Retransmission of a segment after a timeout or
  duplicate ACK
• Sliding window with sequence s

TCP payload
IP hdr
TCP hdr
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TCP (3)

• TCP Header
• 20 bytes
• Source port for TCP sending process
• Dest port for TCP receiving process
• 32-bit Sequence (host is sender)
• 32-bit ACK (host is receiver ACKing data sent
  by other endpoint)

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Source Port
Dest. Port
32-bit Sequence
32-bit Acknowledgement
Window Size
Flags
Checksum
Urgent Pointer
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TCP (3)

• TCP Header
• Flags
• SYN synchronize sequence s to initiate
  connection
• FIN sender is finished sending data
• RST reset the connection,
• More

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Source Port
Dest. Port
32-bit Sequence
32-bit Acknowledgement
Window Size
Flags
Checksum
Urgent Pointer
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TCP (4)

• TCP Header
• Flow control window size Receiver advertises how
  many bytes it is willing to accept into its
  buffer
• Checksum computed over TCP header payload
• Similar to UDP, prepends an IP Pseudo-Header
• Urgent Pointer points to where there is emergency
  data in payload

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32
Source Port
Dest. Port
32-bit Sequence
32-bit Acknowledgement
Window Size
Flags
Checksum
Urgent Pointer
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TCP (5)

• TCP Header
• Variable-sized (not just 20 bytes), can have
  Options after the Urgent Pointer field
• Signalled in Flags field
• Most common option Maximum Segment Size (MSS)
  exchanged during setup

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Source Port
Dest. Port
32-bit Sequence
32-bit Acknowledgement
Window Size
Flags
Checksum
Urgent Pointer
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High-Level TCP Characteristics

• Protocol implemented entirely at the ends
• Fate sharing
• Protocol has evolved over time and will continue
  to do so
• Nearly impossible to change the header
• Uses options to add information to the header
• Change processing at endpoints
• Backward compatibility is what makes it TCP

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Connection Setup

• A and B must agree on initial sequence number
  selection
• Use 3-way handshake

A
B
SYN Seq A
SYNACK-A Seq B
ACK-B
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Connection Setup (2)

• Why a three-way handshake?
• Three-way handshake is necessary and sufficient
  for unambiguous setup/teardown even under
  conditions of
• Loss
• Duplication
• Delay

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Two-Way Handshake
Handshake pictures courtesy of UMass Amherst
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Two-Way Handshake Old Accepts
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Two-Way Handshake Duplicate Requests
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Two-Way Handshake Failure
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Two-Way Handshake W/ Timers
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3-way Handshake Unique IDs
Server (passive)
Client (active)

• Both sender and receiver choose unique IDs to
  label their (x,y) connection
• x chosen by Sender, y by receiver
• Prevents Failure scenario in two-way handshakes
  w/o timers