where one side can think a connection exists, where the other side thinks
the connection was never established.
The original problem was first reported by Ty Sarna in PR #5909. The
original fix I made to the code didn't cover all cases. The problem this
fix addresses was reported by Christoph Badura via private e-mail.
Many thanks to Bill Sommerfeld for helping me to test this code, and
for finding a subtle bug.
- Don't use tcp_respond(), instead create the tcp/ip header from scratch,
and send it ourself.
- Reuse the mbuf that carried the SYN, or allocate one if that is not
available.
- Cache the route we look up to do the Path MTU Discovery check, and
transfer the reference to that route to the inpcb when the connection
completes.
* Macro'ize a small, but often repeated code fragment.
syn_cache_unreach() should remove the entry, or just continue on.
Algorithm is to only remove the entry if we've had more than one unreach
error and have retransmitted 3 or more times. This prevents the following
scenario, as noted in PR #5909 (PR from Ty Sarna, scenario from
Charles Hannum):
* Host A sends a SYN.
* Host A retransmits the SYN.
* Host B gets the first SYN and sends a SYN-ACK.
* Host B gets the second SYN and sends a SYN-ACK.
* One of the SYN-ACK bounces with an
ICMP unreachable, causing the `SYN cache' entry to be
removed with no notification.
* Host A receives the other SYN-ACK, sends an ACK, and goes to
ESTABLISHED state.
Should fix PR #5909.
- Don't use home-grown queue manipulation. Use <sys/queue.h> instead. The
data structures are a little larger, but we are otherwise wasting the
memory chunk anyway (we're already a 64-byte malloc bucket).
- Fix a bug in the cache-is-full case: if the oldest element removed from
the first non-empty bucket was the only element in the bucket, the
bucket wouldn't be removed from the bucket cache, causing queue corruption
later.
- Optimize the syn cache timers by using PRT timers rather than home-grown
decrement-and-propagate timers.
This code is now a fair bit smaller, and significantly easier to read
and understand.
the protocol dispatch layer for TCP timers. This saves having to
modify a potentially large number of timer values (which were shorts,
and expanded to ... a lot of code on the Alpha).
the burst size allowed, but rather a fixed number of packets, as
described in the Internet Draft. Default allowed burst is 4 packets,
per the Draft.
Make the use of CWM and the allowed burst size tunable via sysctl.
conditional (tcp_compat_42). The kernel config option TCP_COMPAT_42
will still enable this by default, or disable this by default if the
option is not included (i.e. current behavior). This will be made a
sysctl soon.
TCP connections by using the MTU of the interface. Also added
a knob, mss_ifmtu, to force all connections to use the MTU of
the interface to calculate the advertised MSS.
code, as clarified in the TCPIMPL WG meeting at IETF #41: If the SYN
(active open) or SYN,ACK (passive open) was retransmitted, the initial
congestion window for the first slow start of that connection must be
one segment.
in the packet. This fixes a bug that was resulting in extra packets
in retransmissions (the second packet would be 12 bytes long,
reflecting the RFC1323 timestamp option size).
The sysctl'able variable "tcp_init_win", when set to 0, selects an
auto-tuning algorithm for selecting the initial window, based on transmit
segment size, per discussion in the IETF tcpimpl working group.
Default initial window is still 1 segment, but will soon become 2 segments,
per discussion in tcpimpl.
- Don't overload t_maxseg. Previous behavior was to set it to the min
of the peer's advertised MSS, our advertised MSS, and tcp_mssdflt
(for non-local networks). This breaks PMTU discovery running on
either host. Instead, remember the MSS we advertise, and use it
as appropriate (in silly window avoidance).
- Per last bullet, split tcp_mss() into several functions for handling
MSS (ours and peer's), and performing various tasks when a connection
becomes ESTABLISHED.
- Introduce a new function, tcp_segsize(), which computes the max size
for every segment transmitted in tcp_output(). This will eventually
be used to hook in PMTU discovery.
(like the alpha). Biggest problem: IP headers were overlayed with
structure which included pointers, and which therefore didn't overlay
properly on 64-bit machines. Solution: instead of threading pointers
through IP header overlays, add a "queue element" structure to do
the threading, and point it at the ip headers.
* Convert several data structures to use queue.h.
* Split in_pcbnotify() into two parts; one for notifying a specific PCB, and
one for notifying all PCBs for a particular foreign address.
* Don't add the extra 1/8 of the mss when ramping up the congestion window.
* Scale the RTT values slightly to adjust for rounding errors.
* Set the lower bound of the RTO to RTT+2.
