Brief description of IIP-Mobile patch
-------------------------------------
by Pasi Sarolahti, 5.4.2002,
modified 19.6.2003: JS, TK.


'patch-2.4.18-frto-20020419' implements the TCP variants tested during the
IWTCP projects in Univerity of Helsinki, Department of Computer
Science. The primary purpose of the patch is to enable performance
tests in an isolated network, thus it is not intended to be used in
general networking. The code is very experimental in nature (read: not
beautiful or easily maintainable), and there may be bugs. Some of the
modifications, e.g. fixes on congestion window undoing on timestamps
(Eifel), are submitted to Alexey Kuznetsov, but many of the
modifications remain unsubmitted due to their incomplete state. They
can be used for tests in closed network but not in a general Linux
system.

The TCP variants are controlled by sysctl options that can be found as
entries in /proc/sys/net/ipv4. The descriptions below are very brief
and assume prior knowledge about the Linux TCP implementation and TCP
enhancements.

The patch adds following TCP options:

iip_rto_behaviour	This option enables F-RTO, which may avoid some unnecessary
			retransmissions after spurious retransmission timeouts.
			Setting 1 means the conventional Linux RTO recovery, value 3 enables
			F-RTO.

iip_srwnd_addr		IIP Mobile project studied the effects of
			sharing the receiver's advertised window space
			between the connections under same
			host in order to avoid slow start overshooting
			in the beginning of the connection. Advertised
			window sharing is applied only for connections
			having peer address of 10.0.0.x, where the
			last octet x is placed in this variable. If
			this variable is 0, advertised window sharing
			is disabled.

iip_srwnd_size		Size of the shared advertised window. Applies
			only for connections from address specified
			above.

iip_srwnd_min		Each connection gets to advertise at least
			this much of window. Applies only for
			connections with shared advertised window.

iip_srwnd_max		An upper limit for advertised window size per
			each connection. Applies only for connections
			with shared advertised window.

iip_cbi			This option controls whether Linux destination
			cache is used for TCP variables. In IETF terms
			this is roughly equal to Control Block
			Interdependence (RFC 2140). (Values: 0 off, 1 on). 
			Please see the notes!

iip_ratehalving		Linux implements Rate-halving algorithm explained in
 			the Internet draft draft-ratehalving.txt by
			default. This option allows disabling it. (Values: 0 off, 1 on).

iip_limitedxmit		Controls whether Limited transmit explained in 
			RFC 3042 is in use or not. (Values: 0 off, 1 on).

iip_delack_mode		Setting 1 uses a static delay of 200 ms for
			each ACK. Setting 2 uses the unmodified Linux
			algorithm for delayed ACKs, dynamically
			adjusting the ACK delay and using quick
			acknowledgements.

iip_iw			This option allows setting the TCP Initial Window size. 
			Please see the notes!


The standard Linux options:
--------------------------
tcp_sack		Enables SACK
tcp_fack		Enables Forward Acknowledgements, if SACK is
			in use. See below.
tcp_dsack		Enables D-SACK. Does not work very well in
			standard Linux, but with this patch it usually
			does.
tcp_timestamps		Enables TCP timestamps and with this patch
			the Eifel algorithm.


Other points of note:
====================
- SACK behaviour is slightly modified for configuration having FACK
  disabled. The patch implements behaviour close to
  RFC 3517. The RFC has some additional precautions
  against reordering not included in the patch.

- Behaviour with timestamps is equal to Eifel algorithm, which is
  slightly different of the original Linux algorithm (which does not
  undo congestion window if there are multiple consequtive RTOs)

- If CBI is used the iip_iw setting is overriden.

- This patch has issues with the PAWS checking if TCP Timestamps option
  is enabled. If first SYN-ACK of a connection is lost, the following 
  retransmissions of the SYN-ACK are discarded, which causes the connection
  to fail.