Transport Layer PART II

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Transport LayerPART II
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How Multiplexing/Demultiplexing Work

• Transport layer multiplexing requires-
• The sockets have unique identifiers
• Special fields in the segment that indicate the
  dedicated socket.
• What are these special fields ?
• Source port number field
• Destination port number field
• How many port numbers a host can have ?
• 0 -65535
• 0- 1023 are called-well known port numbers

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How Multiplexing/Demultiplexing Work

• When a segment arrives, the transport layer
  examines the destination port number and directs
  the segment to the corresponding socket

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How Multiplexing/Demultiplexing In UDP

• UDP socket is identified by two-tuple
• Destination IP address
• Destination Port Number
• When a UDP socket is created, the transport layer
  assigns a port number to the socket

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How Multiplexing/Demultiplexing In UDP

• The segment contains source port number (12222)
  and destination port number (13333)
• When the segment arrives at the destination, the
  host directs the segment to socket identified by
  13333

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How Multiplexing/Demultiplexing In UDP

• What happen if two UDP segments have different
  source IP address /Port number, but have the
  same destination IP address and destination port
  number?
• The segments will be directed to the same
  destination socket
• Why?

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How Multiplexing/Demultiplexing In TCP

• TCP segment is identified by 4-tuple
• Source IP address
• Source Port number
• Destination IP address
• Destination Port number

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How Multiplexing/Demultiplexing In TCP

• What happen if two TCP segments have different
  source IP address /Port number, but have the
  same destination IP address and destination port
  number?
• They will be directed two different sockets
• Why?

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How Multiplexing/Demultiplexing In TCP
struct sockaddr_in server, client server.sin_fam
ily AF_INET server.sin_addr.s_addr
INADDR_ANY server.sin_addr.sin_port
htons(13333)
Port number 13333 is a welcoming socket waits
for connection-establishment requests from TCP
client
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How Multiplexing/Demultiplexing In TCP

• When the receiving host receives the incoming
  connection-request segment with destination
  13333, it locates the server socket (sd).
• The server process then creates a new socket
  (nsd) for a connection
• This socket (nsd) inherits the original socket
  (sd)

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How Multiplexing/Demultiplexing In TCP
nsd accept (sd, (struct sockaddr ) client,
client_len)
The newly created socket (nsd) is identified by
Source IP address Source Port
number Destination IP address Destination Port
number
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How Multiplexing/Demultiplexing In TCP
Subsequent arriving segments whose source port,
source IP address, destination port and
destination IP address matches to the 4-tuples
will be demultiplxed to the newly created socket
int read (int nsd, char buf, int buflen)
How about web servers and TCP?
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Connectionless Transport UDP

• no frills, bare bones Internet transport
  protocol
• best effort service, UDP segments may be
• lost
• delivered out of order to application
• connectionless
• no handshaking between UDP sender and UDP
  receiver

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Connectionless Transport UDP

• Why is there a UDP?
• no connection establishment (which can add delay)
• simple no connection state at sender, receiver
  can support many more active clients

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Connectionless Transport UDP

• Why is there a UDP?
• small segment header
• TCP 20 bytes
• UDP 8 bytes
• no congestion control UDP can blast away as fast
  as desired