The TCP Selective Acknowledgement option (specified in RFC2018)
provides a mechanism for the receiver to indicate packets that have
been received out of order (e.g. due to earlier dropped packets).

iPXE often operates in environments in which there is a high
probability of packet loss.  For example, the legacy USB keyboard
emulation in some BIOSes involves polling the USB bus from within a
system management interrupt: this introduces an invisible delay of
around 500us which is long enough for around 40 full-length packets to
be dropped.  Similarly, almost all 1Gbps USB2 devices will eventually
end up dropping packets because the USB2 bus does not provide enough
bandwidth to sustain a 1Gbps stream, and most devices will not provide
enough internal buffering to hold a full TCP window's worth of
received packets.

Add support for sending TCP Selective Acknowledgements.  This provides
the sender with more detailed information about which packets have
been lost, and so allows for a more efficient retransmission strategy.

We include a SACK-permitted option in our SYN packet, since
experimentation shows that at least Linux peers will not include a
SACK-permitted option in the SYN-ACK packet if one was not present in
the initial SYN.  (RFC2018 does not seem to mandate this behaviour,
but it is consistent with the approach taken in RFC1323.)  We ignore
any received SACK options; this is safe to do since SACK is only ever
advisory and we never have to send non-trivial amounts of data.

Since our TCP receive queue is a candidate for cache discarding under
low memory conditions, we may end up discarding data that has been
reported as received via a SACK option.  This is permitted by RFC2018.
We follow the stricture that SACK blocks must not report data which is
no longer held by the receiver: previously-reported blocks are
validated against the current receive queue before being included
within the current SACK block list.

Experiments in a qemu VM using forced packet drops (by setting
NETDEV_DISCARD_RATE to 32) show that implementing SACK improves
throughput by around 400%.

Experiments with a USB2 NIC (an SMSC7500) show that implementing SACK
improves throughput by around 700%, increasing the download rate from
35Mbps up to 250Mbps (which is approximately the usable bandwidth
limit for USB2).

Signed-off-by: Michael Brown <mcb30@ipxe.org>