1. 1 Spring Semester 2007, Dept. of Computer Science, Technion Internet Networking recitation #11 TCP Eiffel (RFC 3522)

2. 2 Internet Networking Motivation  Allows a TCP sender to detect if it had entered loss recovery unnecessarily.  Detects whether a fast retransmit or a timeout was spurious upon the first acceptable ACK that arrives during loss recovery.  Robust against the loss of ACKs.  Requires TCP Timestamps option be enabled for a connection (RFC 1323).  Eliminates the retransmission ambiguity in TCP  Based on the timestamp of the first acceptable ACK that arrives during loss recovery  Decides whether loss recovery was entered unnecessarily.  Performs a response algorithms to back out of loss recovery by restoring a TCP sender's congestion control state.  Restoring the TCP sender's congestion control state.  Avoiding unnecessary go-back-N retransmits.

3. 3 Internet Networking Retransmission Ambiguity  A TCP sender unable to distinguish whether the first acceptable ACK that arrives after a retransmit was sent in response to the original transmit or the retransmit.  This problem occurs after a timeout-based retransmit and after a fast retransmit.  The Eiffel detection algorithm uses the TCP Timestamps option to eliminate the retransmission ambiguity.  Useful in environments where TCP's loss recovery and congestion control algorithms may often get falsely triggered. This can be caused by packet reordering, packet duplication, or a sudden delay increase in the data or the ACK path that results in a spurious timeout.

4. 4 Internet Networking Falsely Trigger TCP Loss Recovery  Spurious timeout  May be caused by increased delay that suddenly occurs in the data and/or the ACK path.  Then an acceptable ACK to arrive too late, i.e., only after a TCP sender's retransmission timer has expired.  Unnecessarily forces a TCP sender into slow start and causes a go-back-N retransmissions. 1 2 Ack(2) Ack(3) 2 Time out

5. 5 Internet Networking Falsely Trigger TCP Loss Recovery (cont.)  Packet reordering  IP does not guarantee in-order delivery of packets.  May cause a single spurious retransmit (the fast retransmit), and the unnecessary halving of a TCP sender's congestion window as a result of the subsequent fast recovery phase. 1 2 Ack(1) 3 Ack(4) 4 Ack(1)

6. 6 Internet Networking Falsely Trigger TCP Loss Recovery (cont.)  Packet duplication  May cause a single spurious retransmit (the fast retransmit), and the unnecessary halving of a TCP sender's congestion window as a result of the subsequent fast recovery phase.

7. 7 Internet Networking The Idea  Allows a TCP sender to detect a posteriori whether it has entered loss recovery unnecessarily.  TCP sender should be able to make this decision upon the first acceptable ACK that arrives after the timeout-based retransmit or the fast retransmit has been sent.  Requires extra information in ACKs by which the TCP sender can unambiguously distinguish whether that first acceptable ACK was sent in response to the original transmit or the retransmit.  Such extra information is provided by the TCP Timestamps option.

8. 8 Internet Networking TCP Timestamps Option (RFC 1323)  The sender places a timestamp in each data segment.  The receiver reflects these timestamps back in ACK segments.  Can be used to measure the RTT.  Can be used to correlate between data segments and their ACK’s.

9. 9 Internet Networking The Detection Algorithm  A TCP sender always stores the timestamp of the retransmit sent in the beginning of loss recovery.  Retransmit as a result of timeout or dup acks. 1 2 Ack(1) 3 Ack(4) 4 Ack(1) Resend packet 1, record time stamp

10. 10 Internet Networking The Detection Algorithm (Cont.)  If the echo timestamp of the first acceptable ACK, that arrives after the retransmit was sent, is smaller then the stored timestamp of that retransmit  that ACK must have been sent in response to an original transmit.  ‘Acceptable ACK’ - an ACK that acknowledges previously unacknowledged data.  Hence, the TCP sender must have entered loss recovery unnecessarily. 1 2 Ack(1) 3 Ack(4) 4 Ack(1) Smaller time-stamp here Resend packet 1, record time stamp

11. 11 Internet Networking The Detection Algorithm Example 1 2 Ack(2) Ack(4) 2 Time out event 3 Ack(3) 3 Without Eiffel DetectionWith Eiffel Detection 4 1 2 Ack(2) Ack(4) 2 3 Ack(3) 5 6 Time out event Detect a spurious timeout 44

12. 12 Internet Networking Conclusions  The Eiffel algorithm eliminates the retransmission ambiguity.  Allows to recognize whether a retransmit was triggered by congestion in the network.  Detects unnecessary congestion control algorithm activation caused by:  packet reordering,  packet duplication,  spurious timeout.  Alleviates the consequences of a falsely triggered loss recovery.