An Investigation into Transport Protocols and Data Transport ...

5.1. TCP Hardware Requirements 82
this effect should be negligible on long latency paths where the extra latency
of processing the larger segment is small compared to the end-to-end latency.
Further information can be found in [Ste94, MDK + 00].
5.1.3 CPU Requirements
A context switch requires that the CPU state needs to be stored and restored.
The transport of segments to and from a PC requires the context switching
of hardware and software control such that separate compute processes of
the operating system can gain access to the CPU resources.
Gigabit speeds require the transport of 1,000,000,000
8×1500
= 83, 333 packets per
second using standard MTU sized packets. This means that a packet needs
to be processed about every 12µsec.
As TCP uses feedback from acknowledgments, the TCP sender must also
process all incoming acks. This means that a TCP sender must send out data
and receive acks in order to enable reliable transport. Also, TCP requires
the extra processing overhead of the contents of acks in order to determine
what data needs to be retransmitted 1 .
At higher speeds, more packets need to be handled by the end systems
which results in higher CPU utilisation.
The overheads required in the context switching of interrupts can be
mitigated via interrupt coalescing (See Section A.2.4) and the use of advanced
queue management techniques such as NAPI (See Section 5.2.3). Using larger
sized packets (See Section 5.1.2) may also help reduce the CPU utilisation
as long as fragmentation does not occur.
Figure 5.3 shows the CPU load during individual TCP transfers between
1 Also SACKs prove to be a serious overhead in computational terms.