Maintain state for the advertised window length, and only ever increase
it (instead of calculating it afresh on each transmit).  This avoids
triggering "treason uncloaked" messages on Linux peers.

Respond to zero-length TCP keepalives (i.e. empty data packets
transmitted outside the window).  Even if the peer wouldn't otherwise
expect an ACK (because its packet consumed no sequence space), force an
ACK if it was outside the window.

We don't yet generate TCP keepalives.  It could be done, but it's unclear
what benefit this would have.  (Linux, for example, doesn't start sending
keepalives until the connection has been idle for two hours.)

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>

separate xfer method.

Add missing .alloc_iob entries to several xfer_interface_operations
structures.

to generate the initial transmission; this cuts off around 0.3s per
instantiated connection.

in tcp_rx_data().

Clarify comments on discarding duplicate or out-of-order data.

implemented; should be done at some point.)

obtained via xfer_window().

xfer_window() function, which can return a scalar rather than a
boolean.

implement sendto()/recvfrom() equivalents.

request(), seek() or deliver_xxx() in order to start the data flow.
Autonomous generators must be genuinely autonomous (having their own
process), or otherwise arrange to be called.  TCP does this by
starting the retry timer immediately.

Add some debugging statements.

Achieved via Perl using:

perl -pi -e 's/pk_buff/io_buffer/g; s/Packet buffer/I\/O buffer/ig; ' \
	-e 's/pkbuff\.h/iobuf.h/g; s/pkb_/iob_/g; s/_pkb/_iob/g; ' \
	-e 's/pkb/iobuf/g; s/PKB/IOB/g;'

to avoid drops even on slow NICs.

almost everywhere else.

tcpip_tx().  This avoids the irritating wait when you accidentally type
"kernel pxelinux.0" before bringing up the network interface.

Add ENETUNREACH to strerror()'s list.

transmissions.

skb_tailroom().  Add pkb_headroom().

  "when SYN is ACKed and we have already received SYN", or
  "when SYN is received and we have already had SYN ACKed"

rather than just

  "when SYN is ACKed"

This avoids spuriously calling the connected() method when we receive
a RST,ACK in response to a SYN.

out-of-range ACK.

Simplify checksum generation on the TX datapath.

Truncate TX length to TCP window at time of transmission rather than at
time of adding to TX packet; this is conceptually cleaner and also allows
the application to call tcp_send() multiple times to build up a single
packet.

"flags that are currently being sent".  This allows at least one special
case (checking that we haven't already sent a FIN in tcp_rx_fin()) to be
collapsed.

TCP_CLOSED state (i.e. after the final FIN/ACK exchange), and has been
removed from the list of TCP connections.

already stopped.

Don't call start_timer() when sending a dataless ACK.  This may or may
not be the right thing to do; I can't tell.

Back out broken "send ACK only if required to" logic temporarily.

subsequent runs.