An Investigation into Transport Protocols and Data Transport ...

5.2. TCP Tuning & Performance Improvements 93
5.2.5 Dynamic Right Sizing & Receiver Window
When the TCP sender’s window is not constrained by the system socket
buffer size nor by the congestion window, it will be throttled only by the receiver’s
advertised window (as per TCP flow control). The size of the receiver
window is a standard control parameter of TCP [Pos81b]. By advertising a
smaller window the receiver can control the number of segments that the
sender is allowed to transmit (similar to imposing a small socket buffer at
the sender).
[FF01] describe modifications to the Linux kernel that allow the kernel to
tune the buffer size advertised by the TCP receiver. Called Dynamic Right
Sizing (DRS), the receiver’s TCP kernel estimates the bandwidth from the
amount of data received in each round-trip time and uses that estimation to
derive the receiver’s window to advertise back to the sender. Assuming that
the sender is limited only by the network condition (i.e. by its cwnd value),
then the amount of data transferred in approximately one RTT is equal to
one windows worth of data. Therefore, by the TCP receiver measuring the
time taken to receive one windows worth of data, it can determine the RTT of
the connection. This value of RTT is used as an upper bound on the latency
of the link and therefore the appropriate receive window size to advertise in
following TCP acks.
Therefore, the effect of DRS on TCP senders is that their cwnd value
is never allowed to grow beyond what is actually achievable through that
connection and is clamped to the value as calculated by the TCP receiver. As
such, the TCP flow never actually imposes the additive increase of congestion
control and hence does not impose the necessary loss that is typical of the
oscillatory behaviour of TCP.