1. Transmission Control Protocol (TCP) Retransmission and Time-Out
Lesson 15
Transmission Control Protocol (TCP) Retransmission and Time-Out

2. Transmission Control Protocol (TCP) Retransmission and Time-Out
Retransmission time-out and round-trip time
Retransmission behavior
Calculating the RTO
Using the TCP timestamps option
Karn’s algorithm
Fast retransmit
Fast recovery

3. Retransmission Time-Out and Round-Trip Time
RTO must allow enough time for:
1. The sent TCP segment to traverse the internetwork
2. The TCP segment to be received and processed by the destination node
3. The generation of an ACK for the segment
4. The generated ACK to traverse the internetwork
5. The generated ACK to be received and processed by the sending node
RTO too large
Lowered throughput
RTO too small
Unnecessary retransmissions

4. Congestion Collapse
Begins with a steady increase in the load on the internetwork
The transit time increases
The actual RTT increases than the currently known RTT of sending hosts
When actual RTT becomes larger than the current RTO of sending hosts, segments are retransmitted
Buffers on the internetwork routers fill and packets are discarded

5. Retransmission Behavior
Exponential backoff behavior
1. When the TCP segment is initially sent, the RTO for the segment is set to the currently known RTO for the connection
2. After RTO number of seconds, when the RTO expires, the segment RTO is set to twice the RTO for the segment’s previous transmission and retransmitted
Repeat step 2 for maximum number of retransmissions

6. Dead Gateway Detection
When an individual TCP connection retransmits a segment multiple times, its next-hop IP address is changed to the next default gateway
When 25 percent of all TCP connections using the failed default gateway have been moved to the next default gateway, the default route in the IP routing table is updated with the next default gateway as the next-hop IP address

7. Calculating the RTO Recommended formulas:
SRTT = RTT + 8*(New_RTT - RTT)
Dev = Dev + (|New_RTT - RTT| - Dev)/4
RTO = SRTT + Dev/4
Self-tunes for different environments (LAN vs. WAN) and is sensitive to sudden changes in the RTT
Environments such as the Internet

8. Using the TCP Timestamps Option
TCP timestamps with pauses in data
TCP timestamps for delayed acknowledgments
TCP timestamps and out-of-order segments
TCP timestamps and retransmitted segments

9. TCP Timestamps with Pauses in Data
Block 1, TS Value = 100, TS Echo Reply = 9000
ACK on Block 1, TS Value = 9020, TS Echo Reply = 100
Block 2, TS Value = 158, TS Echo Reply = 9020
ACK on Block 2, TS Value = 9053, TS Echo Reply = 158 1 2 3 4 5 6
TCP Peer A
TCP Peer B .
(pause)
Block 3, TS Value = 2057, TS Echo Reply = 9053
ACK on Block 3, TS Value = 10951, TS Echo Reply = 2057
TS = 9053
TS = 10951

10. TCP Timestamps for Delayed Acknowledgments
Segment 1, TS Value = 100, TS Echo Reply = 9000
(1000 bytes of data)
Segment 2, TS Value = 150, TS Echo Reply = 9000
ACK on Segments 1-3, TS Value = 9250, TS Echo Reply = 100 1 2 3 4
TCP Peer A
TCP Peer B
lastack = 1000
tsrecent = 10
Segment 3, TS Value = 200, TS Echo Reply = 9000
tsrecent = 100
lastack = 4000

11. TCP Timestamps and Out-of-Order Segments
Segment 1, TS Value = 100, TS Echo Reply = 9000
(1000 bytes of data)
Segment 3, TS Value = 250, TS Echo Reply = 9150
ACK on Segment 1, TS Value = 9150, TS Echo Reply = 100 1 2 3 4
TCP Peer A
TCP Peer B
lastack = 1000
tsrecent = 10
Segment 2, TS Value = 200, TS Echo Reply = 9150
tsrecent = 100
lastack = 2000
tsrecent = 200

12. TCP Timestamps and Retransmitted Segments
Segment 1, TS Value = 100, TS Echo Reply = 9000
(1000 bytes of data)
Segment 3, TS Value = 200, TS Echo Reply = 9150
ACK on Segment 1, TS Value = 9150, TS Echo Reply = 100
TCP Peer A
TCP Peer B
lastack = 1000
tsrecent = 10
Segment 2, TS Value = 500, TS Echo Reply = 9150
(1000 bytes of data-retransmitted)
tsrecent = 100
lastack = 2000
tsrecent = 500
Segment 2, TS Value = 150, TS Echo Reply = 9150
(1000 bytes of data-dropped)

13. Karn’s Algorithm
When an ACK for a retransmitted segment arrives, it should not be used to update the RTO.
The RTO of the retransmitted segment (that has been exponentially backed off) should be used as a temporary RTO for subsequent TCP segments.
When an ACK for a nonretransmitted TCP segment arrives, use its RTT to update the RTO.
Then, use the updated RTO for subsequent TCP segments.
Karn’s algorithm is not used with TCP timestamps

14. Fast Retransmit TCP Peer A TCP Peer B Segment 1, Seq #=1000
ACK, Ack #=1000

15. Fast Recovery
Fast retransmit causes the sender to retransmit the missing TCP segment before its RTO expires
If the RTO expires, slow start and congestion avoidance algorithms are used to gradually increase the actual send window up to the advertised receive window
Because the RTO did not expire, congestion avoidance is performed, but not slow start

16. Review Retransmission time-out and round-trip time
Retransmission behavior
Calculating the RTO
Using the TCP timestamps option
Karn’s algorithm
Fast retransmit
Fast recovery