Dr. Mark Humphrys

School of Computing. Dublin City University.

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Network Layer

This layer (and higher) normally not embedded in the network hardware
but rather now running as processes on CPU.

Network layer runs on source and destination machines, and also on routers in between.
Transport layer is the first end-to-end layer.

The whole network

Data Link layer: Just move data to end of a single link.

Network layer: Move data across the entire network. May involve many point-to-point links along the way.


  1. Routing
  2. Addressing
  3. Internetworking
  4. Traffic (Congestion control)
  5. Quality of service (Congestion control, delays, jitter)

  6. Hide from higher layers the topology of the network.


5.3.6 Jitter

Jitter - variation in packet arrival times.
This is important for streaming audio/video. Want near-constant transit time.


Streaming audio/video - can buffer at client-side. Read from buffer at constant rate.
Internet phone - delay in buffering not so acceptable.

Here we start playing when we have the first 6 packets in the buffer.
Unfortunately, no.8 is so delayed that when its slot comes up, it has still not arrived, and the play has to pause.

Note high delay is ok, so long as constant transit time - just wait longer before starting to play the stream - then packets will seem to arrive fast.

See RealPlayer.

5.4 Quality of service

Applications and their demands/requirements.
Reliability "Low": No checksums computed or verified.

5.6 IP

Internet protocol stack:
IP - Network Layer for Internet.
Unreliable, connectionless, datagram service.
For reliable, connection-oriented service: acks and re-transmits added in higher level (TCP).

Takes message and breaks it into packets.
Can be up to 64 k bytes.
In practice these are usually no more than 1500 bytes (so can fit in one Ethernet frame).
Re-assembled by network layer at far end.

Packet from host 1 to host 2 has to cross 6 networks.

IP packets

IP packet header.
Meaning of fields.
Header Checksum is just a simple checksum to check for errors at each hop along the route, and discard packet if any. Not a high-overhead system of acks and re-transmits.

Some of the options that can be in the variable-length Options field.
Full list.

ancientbrain.com      w2mind.org      humphrysfamilytree.com

On the Internet since 1987.

Wikipedia: Sometimes I link to Wikipedia. I have written something In defence of Wikipedia. It is often a useful starting point but you cannot trust it. Linking to it is like linking to a Google search. A starting point, not a destination. I automatically highlight in red all links to Wikipedia and Google search and other possibly-unreliable user-generated content.