RETRANSMISSION
Following
two schemes are used :
1. Fast Retransmit
2. Fast Recovery
When a
source sends a segment TCP sets a timer. If this value is set too low it will
result in many unnecessary treransmissions. If set too high it results in
wastage of banwidth and hence lower throughput. In Fast Retransmit scheme the
timer value is set fairly higher than the RTT. The sender can therefore detect
segment loss before the timer expires. This scheme presumes that the sender
will get repeated ACK for a lost packet.
Round Trip Time (RTT): In
Internet environment the segments may travel across different intermediate networks and through multiple routers. The
networks and routers may have different delays, which may vary over time. The
RTT therefore is also variable. It makes difficult to set timers. TCP allows
varying timers by using an adaptive retransmission algorithm. It works as
follows.
1. Note the
time (t1) when a segment is sent and the time (t2) when its ACK is received.
2. Compute
RTT(sample) = (t 2 - t 1 )
3. Again
Compute RTT(new) for next segment.
4. Compute
Average RTT by weighted average of old and new values of RTT
5. RTT(est)
= a *RTT(old) + (1-a) * RTT (new) where 0 < a < 1
A high
value of 'a' makes the estimated RTT insensitive to changes that last for a
short time and RTT relies on the history of the network. A low value makes it
sensitive to current state of the network. A typical value of 'a' is 0.75
6. Compute
Time Out = b * RTT(est) where b> 1 A low value of 'b' will ensure quick
detection of a packet loss. Any small delay will however cause unnecessary
retransmission. A typical value of 'b' is kept at .2
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