An Investigation into Transport Protocols and Data Transport ...

5.1. TCP Hardware Requirements 81
1000
800
12000
10000
1500B
9000B
Goodput (mbit/sec)
600
400
cwnd (packets)
8000
6000
4000
200
1500B
9000B
0
0 1000 2000 3000 4000 5000 6000 7000
Time (seconds)
2000
0
0 1000 2000 3000 4000 5000 6000 7000
Time (seconds)
(a) Goodput
(b) CWND
Figure 5.2: Effect of TCP performance with different MTU sizes.
is used to impose an upper limit on MSS.
These values do not prevent senders from placing larger packet sizes into
the network. However, large segments packets will be ‘fragmented’ [Cla82a,
ZRT95] into smaller packets to overcome the size difference. This imposes
the extra overhead of increased packet header sizes and a higher chance
of packets lost to BER. At the receiver, the individual fragments are then
reassembled to form the original packet. To over come the problems of packet
fragmentation, Path MTU Discovery is often conducted to determine the
appropriate segment sizes on the network [MD90, Lah00].
The effect of using large MTU’s is shown in Figure 5.2 based on tests run
on the DataTAG testbed (See Appendix C.3). TCP counts acknowledgments
by segments rather than by bytes. As such, in order to achieve the same
throughput, larger MSS flows only require a smaller value of cwnd as more
bytes are transferred per unit value of cwnd.
On the other hand, the transport of each large MSS segments requires an
increase in latency in order for routers and switches to transport the segment.
This could lead to a larger RT O which may have a negative influence on TCP
timeouts and hence lead to a deterioration in average throughput. However,