1. TCP
EE122 Discussion
10/18/13

2. TCP Sequence Numbers
Initial sequence number for the connection is not zero
Sequence number refers to the first byte in the TCP packet payload

3. Example (from class) Packet arrives: – Seq: 2323 – Ack: 4001 – W=3000
– [no payload]
Appropriate response?
Seq: 4001, payload:
Seq: 2001, payload:
Seq: 4001, payload:
Seq: 5001, payload:
Seq: 8001, payload:

5. Congestion Control [1]
• Avoid overwhelming the network • cwnd – congestion window • SSTHRESH –threshold until which exponential slow-start happens • TCP is either in slow-start mode or AIMD mode

6. Congestion Control [2] • TCP is either in slow-start mode or AIMD mode
• Slow-start: for every ACK, increase cwnd by MSS
• “AI”: for every window of packets being ACK-ed, increase cwnd by MSS

7. ‘Vanilla’ TCP Exponential slow-start till SSTHRESH
Set timer, which expires when packets are lost
At each new data ACK the timeout is RESET
On time-out, set SSTHRESH to (cwnd/2), and cwnd itself to 1
– Timeout = receiving nothing for an entire RTO
Usually a disaster!