1. Chi-Cheng Lin, Winona State University CS 313 Introduction to Computer Networking & Telecommunication Chapter 5 Network Layer

2. 2 Topics l Design Issues l Routing Algorithms l Congestion Control l Internetworking

3. 3 Congestion Control l Congestion  Too many packets present in the subnet l Effects  Performance degraded  Packet lost

4. Congestion Control Algorithms (2) When too much traffic is offered, congestion sets in and performance degrades sharply. (Could be achieved by congestion control) (Without congestion control)

5. 5 Causes of Congestion l Causes  Too many packets need an output line  queuing  Problem: not enough memory  packets dropped  Solution(?): adding more memory  New problem: timeout and retransmit  worse  Slow processors  Low bandwidth lines l Congestion tends to feed upon itself and become worse

6. 6 Congestion Control l Congestion = (Load > Resources) l Solutions  Increase resources  Decrease load

7. Approaches to Congestion Control Timescales of approaches to congestion control

8. Traffic-Aware Routing A network in which the East and West parts are connected by two links.

9. Admission Control (a) A congested network. (b) The portion of the network that is not congested. A virtual circuit from A to B is also shown.

10. Traffic Throttling Explicit congestion notification

11. 11 Choke Packets l Approach  Each router monitors output line utilization  Threshold for "warning state"  A receiving router  Checks packet to see if output line in warning state  If yes then send a "choke packet" back to source host original packet tagged and forwarded

12. 12 Choke Packet l Source, upon receiving a choke packet  Reduces traffic by a percentage after receiving choke packet  Choke packet referred to same destination is ignored for a fixed time interval  After time interval expired, listens  If choke packet received then goto the step of reducing traffic else increase traffic

13. 13 Choke Packet l Typically  First choke packet causes data rate reduced to 50%, then 25%, …  Traffic is increased in smaller increments l Why?

14. 14 Hop-by-Hop Choke Packets l Problem in high speed and long distance  slow reaction l Solution  Hop-by-hop choke packets l Buffers needed in routers l Effects:  Quick relief at the price of more buffers

15. 15 Load Shedding l Discard whatever cannot be handled l Which packets to drop?  Application-dependent  Priorities

16. 16 Load Shedding l Strategies  Wine or milk  Priority  Priority classes  Coupled with traffic shaping token bucket  Packet without token sent with lowest priority  Allowing VC set up with exceeding specification  Contingent on low priority  Header field needed  Example: ATM CLP field (1-bit, 0 means high priority) l Rule of thumb  Discard as early as possible!

17. Choke Packets A choke packet that affects only the source.

18. Choke Packets A choke packet that affects each hop it passes through.

19. 19 Internetworking l A collection of interconnected networks.

20. How Networks Differ Some of the many ways networks can differ

21. How Networks Can Be Connected (a) A packet crossing different networks. (b) Network and link layer protocol processing.

22. 22 Tunneling l Encapsulating packets of a protocol in the payload of packets of another protocol l Useful in  Internetworking  VPN  IPv4 to IPv6 transition  …

23. Tunneling Tunneling a packet from Paris to London.

24. Tunneling Tunneling a car from France to England