An Investigation into Transport Protocols and Data Transport ...

5.2. TCP Tuning & Performance Improvements 84
5.2 TCP Tuning & Performance Improvements
5.2.1 Standardised Changes to TCP
Inadequate Window
The 16-bit TCP Sequence Number and Acknowledgment fields as proposed
in [Pos81b] imposes a maximum TCP window size of 2 16 = 64KB. Without
the Large Window extensions [JBB92], the maximum throughput of a TCP
connection is limited by the RTT as given by Equation 4.1. On a typical
long distance link with a RTT of 100ms, the maximum throughput of the
TCP connection is therefore limited to 64KB
100ms = 5.12Mbit/sec.
The TCP extension for large windows [JBB92] is implemented using a
3-byte TCP Option that defines an implicit scale factor to multiply the window
size as reported in each TCP packet. It increases the size of the TCP
advertised window to 32-bits and then uses a scale factor to carry this 32-bit
value in the 16-bit window field of the TCP header.
It is sent only in a
SYN segment at the TCP handshake and hence the scaling factor is fixed
in each direction when a new connection is opened. The maximum window
guaranteed is 2 30 -bytes which imposes a limit of 1GB.
The use of scaling the window imposes problems of sequence numbers
wrapping around and therefore the possibility of acking incorrect segments
in the window.
This is dealt with internally to ensure that the sequence
number is within 2 31 -bytes of the left edge of the window. Old acks are also
dealt with using PAWS and through the IP TTL field [Pos81a].