This section discusses the Transport Layer

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• This section discusses the Transport Layer
• Overview
• User Datagram Protocol
• Transport Control Protocol
• TCP Service Primitives
• TCP Operation

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• CH 11 Transport Protocols
• TCP/IP transport layer IP are closely coupled
• IP is always present in the network layer
• regardless of underlying network (LAN, WAN,
• inter-network)
• all transport layer PDUs encapsulated in IP
  packets

• OSI network oriented protocols reflect
  underlying network type
• User Datagram Protocol (UDP) connectionless
• Transport Control Protocol (TCP) connection
  oriented
• trade-off cost/rate of lost packets vs delay
  throughput
• required for reliable delivery

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• Network Interface LAN/WAN frame encapsulates IP
  packet
• provides MAC address
• IP address provides net_id host_id
• IP protocol field type of protocol ICMP, TCP,
  IP
• Port Address Transport Protocol Field used to
  address
• application w/i host (ISO TSAP)
• Application Address composite TCP/IP address

• e.g. 128.3.2.3, 53
• net_id 128.3
• host_id 2.3
• port_address 53

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PDU/layer
Address Selector
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• 11.1 UDP
• Characteristics of IP
• provides connectionless service for individually
  addressed
• packets
• minimizes overheads resulting from connections
  error control
• IP checksum only applies to IP header

• UDP connectionless transport protocol
• single PDU associated with UDP protocol
• exploit overhead advantages of IP
• single short request/response exchange
• error control unneeded

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• source port and destination port
• 16 bit integers identify port on a host
• source port is optional, included only if reply
• is expected, otherwise set to 0
• length total bytes in entire UDP packet

• checksum 1s compliment over entire UDP PDU
• checksum is optional, if not used ? set to 0
• if computed checksum 000 ? use 111
• (1s compliment 0 represented by 000or 111)

• knowledge of addressing is an application issue
• source must know port address and IP address
• use name server, etc

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11.2 TCP reliable stream transport service

• TCP Operation - supports reliable, duplex packet
  transmission
• error detection 1s compliment checksum
• error control ? go-back-N
• flow control ? sliding window

• TCP data is submitted for transmission in
  segments (blocks)
• TCP message stream consists of message segments
• all segments (thus all PDU types) have same
  header format
• validity of fields in segment header is
  indicated by code field

• Error Control Flow Control ? data treated as a
  byte stream
• SEQ ACK numbers relate to byte position in
  message
• stream, not to position of message block in the
  sequence

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TCP Packet
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TCP Packet Fields
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• default maximum bytes in data field segment 536
• assumed enroute networks were WANs with higher
  BER
• if host operates with low BER (LAN) ?can use
  larger segments

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• 11.2.1 Reliable Stream Service Operation
• service primitives (OPEN, CLOSE, ) provided to
  application
• goal provide error free, ordered delivery
  without lost or
• duplicate packets

• stream oriented transmission all user data
  associated with a
• connection treated as 2 separate data streams
• each stream forms part of a duplex connection
• each stream comprised of a string of bytes

• segments unit of data transfer for reliable
  service
• may consist of multiple short user messages
• may be part of a single large message (file, )
• max segment length is TCP function

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• segment transmission
• source TCP normally decides when to transmit new
  segment
• user can alter message transmission unit via
  parameters
• remote TCP stores segments in memory associated
  with an
• application, buffer is
  delivered when full

client server model client always initiates
requests server responds to client requests, may
support multiple clients concurrently
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• Phases of TCP Operations
• Service Primitives used to perform
• Connection Set Up
• Data Transfer
• Connection Termination

(1) Connection Set Up- server side

• indicate readiness to accept connection requests
• passive open wait to receive connection
  requests
• - as opposed to active

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(1) Connection Set-up - client side -
initiated connection establishment (active)
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(2) data transfer phase
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(3) clearing connection
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CLIENT
SERVER
TCP
TCP
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Parameters for TCP Service Primitives source
destination address IP address of source_IP
dest_IP source destination ports port
addresses of protocol entities

• timeout max time that source TCP should wait for
  segment ACK
• user specified
• normally greater than 2? time to live (IP)

timeout action action on time-out, normally
close connection
precedence parameters that specify IP Headers
service type field - pass
through parameter ? QoS request
security range server can specify security level
to potential clients
push flag indicates transmit immediately to TCP
urgent flag indicates transmit out of normal
flow control to TCP
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• local connection name allocated by local TCP
  entity at connection
• set-up
• with multiple logical connections - allows TCP
  to relate primitives
• to same connection OSI connection endpoint
  identifier
• port address only allows TCP to relate received
  segment to server
• application
• many parameters associated w/ low level
  operation of TCP/UDP
• - often same value used ? default value for many
• - if value not specifically set ? default assumed

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Parameters for TCP Service Primitives
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TCP User Service Primitives
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TCP User Service Primitives
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TCP User Service Primitives
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• 1. Establishing a connection ? use 3-way
  handshake
• allows for general case when anyone can initiate
  connection
• in client-server model ? client always initiates
  connection
• flow of data in each direction independently
  controlled
• each side informs other of initial sequence
  number
• - avoids ambiguity
• - initial sequence numbers ACKed as part of
  handshake

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Example 1

• Client Initiates Connection
• send a segment with SEQ flag set
• proposed sequence number set Seq_no X
• Server accepts connection
• record incoming connections sequence number X
• send reply segment with both SEQ, ACK flags set
• - reverse sequence number set Seq Y
• - ACK number set ack_no X1

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Example 2

• Simultaneous Connection Initiated
• both sides send segment with SEQ flag set
• proposed sequence numbers set seq_no X
• record incoming connections sequence number X
• send reply segments with ACK flag set Ack
  number X1

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2. Data Transfer flow/error control Go Back N
w/ sliding window
if segment was corrupt ?incorrect SEQ/ACK field
received - return RST segment with SEQ, ACK flags
and fields set
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• 3. graceful connection termination
• Client finished, closes connection
• Server finished, closes connection
• TCP terminates connection on both sides

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• (3) Connection Termination - Abort Sequence
• immediately terminates both sides of connection v
• Client TCP sends segment with RST set to server
• ServerTCP immediately terminates both sides of
  connection
• sends TERMINATE primitive with reason code to
  server

RST