User Datagram Protocol UDP Transmission Control Protocol TCP

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User Datagram Protocol (UDP)Transmission Control
Protocol (TCP)
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User Datagram Protocol (UDP)

• Connectionless
• Before sending a UDP packet, we do not need to
  set up a connection first
• No error, flow, congestion control
• No packet resequencing
• Multiplexing feature via protocol ports
• UDP checksum mechanism violates strict protocol
  layering concept

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UDP Header
Fig. 11.2 p.144

• UDP length the length (in bytes) of the UDP
  segment including header. The minimum value is 8
  bytes.
• UDP checksum (optional) checksum of the UDP
  pseudo header, UDP header, and the UDP data

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Fields for UDP checksum computation
Fig. 11.3 p.145

• UDP pseudo header
• The checksum algorithm calculates checksum over
  even number of bytes. A pad byte of 0 is appended
  is necessary.

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Transmission Control Protocol (TCP)

• Connection-oriented
• Before sending a TCP packet, we need to set up a
  TCP connection first
• A TCP connection is a full-duplex logical
  communication channel
• The sending and receiving nodes need to be
  configured
• However, the routers on the connection path need
  not be configured
• Provide a reliable, in-sequence byte-stream
  delivery service
• With error, flow, and congestion control
• Can resequence arrived packets

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TCP Reliability

• Reliability is provided by
• Reliable connection startup Data on old
  connection does not confuse new connections
• Graceful connection shutdown Data sent before
  closing a connection is not lost
• Data segmented for transmission and acknowledged
  by destination Timeout Retransmission provided
  if data unacknowledged
• Checksum provided to catch errors
• Resequencing of out-of-order data discarding of
  duplicate data
• Window flow control gt sender cannot overrun
  receiver buffers

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TCP Header Format
Fig. 17.2 p.225
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TCP Header

• Source Port (16 bits)
• Destination Port (16 bits)
• Sequence Number (32 bits) sequence number of the
  first byte in the segment
• If SYN bit is set, this is the initial
  sequence number (ISN) and the first data byte is
  ISN1
• Ack number (32 bits) sequence number of the next
  byte expected

• Header length (4 bits) in unit of 4 bytes
• (to handle the variable option field)
• Reserved (6 bits)

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TCP Header (continued)

• Control (6 bits)

• URG the urgent field is valid if this is set
• ACK the acknowledgement number filed is valid if
  this is set
• PSH tells the receiver to pass this data to the
  application as soon as possible (Flush)
• RST tells the receiver to reset the connection
  (Reset)
• SYN synchronize sequence number to initiate a
  connection
• FIN tell the receiver there is no more data left
  (Final)

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TCP Header (continued)

• Window size (16 bits) of bytes the receiver is
  willing to accept
• Checksum (16 bits) covers the segment plus a
  pseudo header (similar to UDP)
• Includes the following fields from IP header
  source and destination address, protocol, segment
  length
• Urgent pointer (16 bits) Points to the byte
  following urgent data
• Lets receiver know how much data it should
  deliver right away

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Connection Establishment

• Three way handshake
• Step 1 client host sends TCP SYN segment to
  server
• specifies initial seq
• no data
• Step 2 server host receives SYN, replies with
  SYNACK segment
• server allocates buffers
• specifies server initial seq.
• Step 3 client receives SYNACK, replies with ACK
  segment, which may contain data

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Connection Termination
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Why applications use UDP?

• No connection establishment
• No connection state
• Small packet header overhead
• Unregulated send rate

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Why applications use UDP? (continued)

• No connection establishment
• TCP uses a three-way handshake before it starts
  to transfer data
• UDP just blasts away without any formal
  preliminaries.
• Thus UDP does not introduce any delay to
  establish a connection

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Why applications use UDP? (continued)

• No connection state
• TCP maintains connection state in the end systems
• Connection state includes
• Receive and send buffers
• Congestion-control parameters
• Sequence and acknowledgment number parameters
• This state information is needed to implement
  TCP's reliable data transfer service and to
  provide congestion control
• UDP does not maintain connection state and does
  not track any of these parameters
• A server can typically support many more active
  clients when the application runs over UDP rather
  than TCP

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Why applications use UDP? (continued)

• Small packet header overhead
• The TCP segment has 20 bytes of header overhead
  in every segment
• UDP only has 8 bytes of overhead

UDP
TCP
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Why applications use UDP? (continued)

• Unregulated send rate
• TCP congestion control mechanism
• one or more links between sender and receiver
  become congested gt reduce send rate
• UDP send rate is only constrained by
• the rate at which the application generates data
• the capabilities of the source (CPU, clock rate,
  and so on)
• the access bandwidth to the Internet
• UDP receiver does not necessarily receive all the
  data
• network congested gt packet lost due to router
  buffer overflow
• UDP Receive rate can be limited if network
  congested

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What applications use TCP?

• Applications that need the reliable data transfer
  service of TCP
• Examples e-mail
• remote terminal access
• the Web
• file transfer
• and etc.

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What applications use UDP?

• UDP is commonly used with multimedia
  applications, such as
• Internet phone
• real-time video conferencing
• streaming of stored audio and video
• All of these applications can tolerate a small
  fraction of packet loss
• UDP is used to carry network management (SNMP)
  messages
• UDP is preferred to TCP in this case
• Network management applications must often run
  when the network is in a stressed (congested)
  state when reliable, congestion-controlled
  data transfer is difficult to achieve