An Investigation into Transport Protocols and Data Transport ...

5.1. TCP Hardware Requirements 78
5.1.1 Memory Requirements
The reliable data transport design of TCP requires that a sliding window’s
worth of data be stored in memory in order to maintain state about the TCP
connection. Without such knowledge, it would be impossible for TCP to be
able to replicate data reliably across a network. The amount of physical
memory allocated to this sliding window is called the socket buffer memory.
Should the assigned value of this socket buffer memory be less than the
required value of the sliding window for optimal TCP transfers, then the
performance of TCP suffers as it is starved of physical memory space to keep
history for the number of packets in flight.
This is true of both the sending and the receiving TCP end-points; the
TCP receiver must also have a large enough assigned socket buffer memory, or
else TCP’s flow control at the sender will restrict the throughput to maintain
a rate sustainable by the TCP receiver (See Section 4.2.2).
The amount of memory that needs to be allocated to the socket buffer
memory, on both the sending and receiving hosts, is related to the Bandwidth
Delay Product (See Section 4.2.2) [CJ89].
The effect on a TCP connection of having limited memory is shown in
Figure 5.1. The graph shows the statistically multiplexed result of running
various socket buffer sizes from the production network between CERN and
RAL using the machines and configuration as shown in Section 3.2.2. The
test consisted of 490 individual bulk transfers using iperf [TQD + 03] with
the socket buffer sizes configured as shown. The theoretical prediction of
the estimated throughput is shown. It can be seen clearly that with small
socket buffers, the throughput is proportionally restricted by the amount of
memory available to the TCP socket.