An Investigation into Transport Protocols and Data Transport ...

9.1. Transfer Tests Across Dedicated Private Wide Area Networks 212
1e+07
1e+06
TCP Slow Path
TCP Fast Path
100000
Frequency
10000
1000
100
10
1
10 100 1000 10000 100000 1e+06 1e+07
Processing Time Per Ack (Ticks)
Figure 9.18: Histogram of SACK Processing Time in processor ticks of Standard
TCP under Dummynet with 600Mbit/sec link capacity and 40ms RTT.
as they work in kernel space.
The number of packets in flight is roughly equal to the bandwidth delay
product (See Section 4.1). Therefore, the TCP sender, assuming perfect
delayed acking, will need to process half of the number of packets in flight;
with each ack containing a maximum of 3 SACK Blocks. As the processing
of each SACK Block results in a linear traversal of the unacknowledged
packets in transmission buffers (and subsequent updates to the TCP Control
Block) such that data segments that need to be retransmitted can be identified,
there is a huge processing burden with large bandwidth delay products.
Therefore, under large fluxes of SACK Blocks, the system processor is completely
saturated by the need to process SACK information.
The processing time required to process both normal (TCP Fast Path)
and duplicate (TCP Slow Path) acknowledgments are shown in Figure 9.18.
It clearly demonstrates that even though there is a higher number of normal
acks, duplicate acks can take up-to 10 times longer to processes.