CSE 561 Notes - October 21, 2002


Protocol

• Goals
• end-to-end bandwith
• (minimize lost packets)
• efficiency vs. predictability
• fairness
• host latency
• stability/predictability/Q.O.S.
• robust w.r.t. unforeseen topologies/workloads
• security
• (we'll ignore this for now)
• How do we allocate resources?
• link bandwith
• buffers
• Which brings us to...

TCP (duplex/sliding window)

• What happens when you blast the window?
• lost packets
• if routber buffer < window size
• if link bandwith < access link

• Therefore the technique is to:
• (1) Find out bottleneck bandwith
• slow start: exponential increase until packet loss observed
• (2) slow start again until you get to highest bandwith that works
• (3) then use AIMD (additive increase multiplicative decrease)
• each time there is multiplicative decrease, use slow start again

• Fiarness of TCP?
• One analysis says:

• A's throughput (xput) and B's xput will converge to most fair point.
• How to avoid the timeout persiod (see seond picture): fast retransmit
• if received 3 duplicate acks in a row:
• assume loss and retransmit
• (remember: TCP will awk a recieved packet with last in-order received packet)