1. Packet communication Johan Lukkien
Computer Networks 2002/2003
Packet communication
Johan Lukkien
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

2. Packet-based communication
The effect of multiple hops
performance
Breaking a message up in packets
transmission strategies
Getting a packet to a destination
methods
anomalies
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

3. Multiple hops: Store-and-forward
Simple model
m links, each latency 0 (for simplicity)
packet size P
address info, size A
channel throughput: w sec/bit
setting up a communication: s (including handling)
Transmission time: m(s+(A+P)w) seconds
Latency: (m-1)(s+(A+P)w)
A/P should be small
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

4. Sending a message: packet switching
Message size L
Store-and-forward (“message switching”)
transmission: m(s+(A+L)w)
bad latency: (m-1)(s+(A+L)w)
excessive buffer requirements
Divide into packets: packet-switched
use multiple links concurrently, in sequence
transmission: (m-1)(s+(A+P)w) + L/P(s+(A+P)w)
Note: L+mP rather than Lm
latency: (m-1)(A+P)w
Packets can even take different routes
can disturb the packet sequence
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

5. Sending a message: cut-through
Cut-through / wormhole:
only the first packet contains address info
the packets take the same route
transmission: m(s+(A+P)w) + (L/P-1)(s+Pw)
Limits: P=L (s&f), P=1
Order is maintained
Used most often in special purpose computers
high-performance multi-computers
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

6. Getting a packet to its destination
Flooding: send it everywhere except where it came from (broadcasting)
Routing: send it along a path
specify the path within the packet or
determine the next step at intermediate nodes
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

7. Flooding: example Note: links may be broadcast networks 14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

8. Flooding: example Adding an extra link
More messages are generated and messages don’t die
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

9. Flooding Use in an acyclic (sub)graph: spanning tree Use a method...
otherwise anomalies occur
Use a method...
to see if a packet has been seen before or
to throw a packet away after some time
In principle, sharing of total capacity!
each link contains each message
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

10. Routing: virtual circuits
First, an end-to-end connection is set up
at each intermediate node
store the direction to forward a packet
the address information on a packet or its origin identifies this information
address info on a packet is adapted on each hop
Communication continues for some time
order of packets is maintained
Then the circuit is destroyed again
Advantages
potentially supports guaranteeing bandwidth and delay
though this needs essentially physical support
address info can be very small, even zero
how?
small packets (“cells”) are possible
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

11. Routing: datagrams
At each intermediate node the next step is determined
based on information in the packet only
just the destination address
or the complete path
decisions for similar packets may change over time
convergence towards destination is a concern
End-to-end order of packet sequence is not preserved
Substantial address information required
A/P should not be too large
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking

12. Comparison: circuit switched
Build an end-to-end physical connection
that’s where the switching goes
Use this physical channel for the duration of a session
no address info needed in message
format of messages etc. only relevant to end systems
bad use of resources
Destroy the connection
Not packet oriented
14-Jul-19
Johan J. Lukkien,
TU/e Computer Science, System Architecture and Networking