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
Move data across the entire network.
May involve many point-to-point links
along the way.
- Traffic (Congestion control)
- Quality of service (Congestion control, delays, jitter)
- Hide from higher layers the
topology of the network.
- variation in packet arrival times.
This is important for streaming audio/video.
Want near-constant transit time.
Streaming audio/video - can buffer
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.
5.4 Quality of service
Applications and their demands/requirements.
No checksums computed or verified.
Internet protocol stack
IP - Network Layer for Internet.
For reliable, connection-oriented service:
and re-transmits added in higher level
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
Re-assembled by network layer at far end.
Packet from host 1 to host 2 has to cross 6 networks.
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.