Transportation Layer (1)

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Transportation Layer (1)
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Transportation Layer

• Very similar to the data link layer.
• two hosts connected by a link or two hosts
  connected by a network
• differences
• When two hosts are connected by a link, packets
  will not reorder or duplicate (if the sender
  sends only once). In addition, packets will
  either get to the receiver or get lost.
• When two hosts are connected by a network,
  packets can be duplicated, delayed, lost,
  reordered.
• Implication The problems to be addressed in the
  transport layer are very similar to those in the
  data link layer. However, the solutions may be
  more complex.

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• The transport layer issues service interface,
  addressing, connection management, error control,
  flow control
• Service interface
• connection-oriented and connectionless.
• similar to the network layer. Why one more layer?
• network layer -- part of the communication
  subnet, run by carrier. can't be changed.
• transport layer -- put one more layer on the
  hosts to get the services needed
• potential problem may do the same thing two
  times, which can decrease the communication
  performance.

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• Addressing
• Link want to transfer data to Ethernet card
  08.00.2b.2a.83.62
• Network want to transfer data to IP host
  128.2.222.85
• Transport which entity you will try to address?
• want to talk to one process on host 128.2.222.85.
• what to do use? process ID? how many bits? What
  would be the problem when using the pid as
  transport layer identifier?
• Abstraction port number

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Connection management

• The problem is that network can delay, reorder,
  lose packets
• Time-out/retransmission introduces duplicates of
  data, acknowledgement, connect, close packets
• Worst case scenario consider this bank
  transaction example
• (a) setup connection
• (b) transfer 100
• (c) close connection
• all messages are delayed and replayed.

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Solution 1

• assign a connection-id to each connection, the
  receiver keeps track what connections have been
  terminated.
• How many connection-ids to keep?
• What if receiver crashs and comes back again? It
  forgets what connections have been established?

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Choosing Unique Identifier

• On packet arrival is it real or old?
• New connection request/release or an old one?
  Remember that there is no restriction on reusing
  the port number.
• Unique ID for each TPDU
• How to choose an identifier for each TPDU such
  that it is impossible for other TPDU currently in
  the network associated with this host to have the
  same identifier.

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Choosing Unique Identifier

• When a connection is set up, choose a initial
  sequence number by taking the lower bits of a
  local clock.
• Each TPDU is given a seq. number. With each TPDU
  sent, the seq. number increases.

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Choosing Unique Identifier
These TPDU May still be around

• The clock has finite bits and will wrap around

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Choosing Unique Identifier

• There is a maximum lifetime (T) for a TPDU.
• Forbidden region. When the clock is x, dont send
  TPDUs with seq. number from x to x2T.

Forbidden region
2T
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Connection setup -- Three-Way Handshake

• Connection initiator (the client)
• Chooses unique seqno x and sends req-conn(x)
• Connection respondent (the server)
• Upon receiving req-conn(x)
• Chooses its own unique identifier, y
• Sends ack-conn(y,x)
• Upon receiving ack-conn(y,x), client responds
• With ack-conn(x1,y)

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Three-way handshake
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Three-way handshake
Old message
server
client
req-conn(x)
ack-conn(y,x)
Ignore or reject
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Threeway handshake
Old message
server
client
req-conn(x)
ack-conn(y,x)
Ignore or reject
ack(x1, z)
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Close a connection

• Reaching agreement two approaches
• Abort send close msg to peer, delete state info
• What if close() message lost?
• Graceful send close msg, but before deleting
  state
• Wait for peer to acknowledge close()
• Problem solved?
• Can I decide to close, knowing that
• Other entity also agreed to close and knows that
  I will close
• Can two armies coordinate their attacks
• If communication is unreliable?

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Two army problem
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