IP Datagrams

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IP Datagrams
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Connectionless Service

• Application programs unaware of physical networks
• Application may want either connectionless or
  connection-oriented service
• TCP/IP allows for either
• IP (delivery) is basically connectionless each
  datagram is on its own!

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Routing

• Router moves packet from one network to another
• Cant use network frame format as these will be
  very different from each other across different
  hardware networks
• IP defines universal, virtual packet format
• Similar to format of frames

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IP Datagram
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IP Datagram Continued

• Overall size not fixed
• As little as one octet, as many as 64k
• Header has fixed format fields have fixed size
• Routing by network number
• Dont confuse datagram with frames
• Routing use of next-hop tables
• Use of masking and and operation
• Destination address in datagram is ultimate
  destination IP address unchanged as datagram
  passes across network

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Best-effort Delivery

• IP does not guarantee delivery! There is no
• Datagram duplication
• Out of order delivery
• Corruption of data
• Loss
• Higher layer must deal with these

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IP Header
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Fields

• 4-bit protocol number -- 4
• 4-bit header length -- 5
• Total length both header and data
• Identification, flags, fragment offset for
  reassembly
• TTL hop count for loops

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Fields Continued

• Type whats in the datagram
• Header checksum
• Source and destination addresses
• Options

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Questions

• 20.1, 20.8