IP datagram consists of a header and its data.
When the network layer has data to send, it puts the transport segement inside a new IP datagram. The IP's job is to deliver the datagram to its destination.
Then the IP sends the datagram to the link layer and it becomes the link data.
Four properties of the IP service:
- Datagram: individually routed packets. Hop-by-hop routing.
- Unreliable: packets might be dropped
- Best effort: ...but only if necessary
- Connectionless: No per-flow state, packets might be mis-sequenced. It means each packet is individual and has no shared info.
In the IP datagram, there is a IP SA (source address) and IP DA (destination address).
The router has a forwarding table to tell where to send the packet by matching its address.
Why is the IP so simple? <- no strict guarantee on data quality
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It is expected to be simple, faster, and minimal.
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It is expected to implement feature at the end hosts (end-to-end principle).
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It allows reliable or unreliable services built on top.
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It should work at any link layer, no assumption is made.
Five features on the IP service model:
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it tries to prevent packets from looping forever by limiting the footprint of the packet.
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it will fragment packets if they are too long. It can break, send, and reassemble long datagram.
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it uses a header checksum to reduce chances of delivering datagram to wrong destination.
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it allows new versions of IP: ipv4 and ipv6
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it allows new options for its header.