Installing and Administering LAN/9000 Software

22
• Initial TCP Congestion Window
RFC 2414 defines a formula for calculating the sender's initial
congestion window that usually results in a larger window than in
previous releases.
The default initial congestion window is now calculated using the
following formula:
min((4 * MSS), max(2 * MSS, 4380))
where MSS is the maximum segment size for the underlying link.
With the new congestion window formula, it is possible for TCP to
send a large, initial block of data without waiting for
acknowledgments. This is useful in networks with large bandwidth
and low error rates and particularly useful for short-lived connections
that only need to send ~4Kbytes of data or less.
To modify the initial congestion window, configure the ndd TCP
parameter tcp_cwnd_init. TCP will calculate the initial congestion
window using the following formula:
min((tcp_cwnd_init * MSS), max(2 * MSS, 4380)),
Default 4: (TCP implements RFC 2414)
Range: 1 -4
In prior releases, the initial TCP congestion window was (2 * MSS).
• TCP Congestion Control (RFC 2581) and Fast Recovery (RFC
2582)
RFC 2581 and 2582 may improve TCP performance, particularly in
congested networks or networks with errors. These RFCs specify
refinements to the TCP protocol for setting transmission windows in
congested networks and recovering from lost packets.
RFC 2581 specifies new TCP congestion control algorithms for Slow
Start, Congestion Avoidance, Fast Retransmit and Fast Recovery,
including provisions for TCP SACK (Selective Acknowledgments).
RFC 2582 specifies refinements to the Fast Recovery algorithm to
acknowledge blocks of data and increase the send window when
SACK is not used.
No configuration is needed to enable the implementation of RFC 2581
or RFC 2582.