[En-Nut-Discussion] [PATCH] Implement TCP sequence number wraparound

[Marti Raudsepp]

Hello list,

It seems that neither Ethernut 4.8 nor 4.9 currently attempt to handle 
TCP sequence number wraparound. This means that each TCP connection is 
limited to transmitting/receiving 4 GB of data in the best case. 
Technically it's in violation of the TCP spec.

Transmitting at the maximum rate of our application (1.10 MB/s both ways 
by our measurements) means that we can hit this limit in just 1 hour.

Further, since the initial TX sequence number is initialized from 
NutGetMillis(), the maximum TCP session lifetime approaches 0 as uptime 
approaches 49.71 days. Nut/OS has no control at all over RX sequence 
numbers which are assigned by the other endpoint.

So I went through every sequence number comparison in the TCP code and 
changed it. My initial approach was to change all sequence number values 
to signed int32_t and change comparisons of style "a > b" into
"a - b > 0", but I figured it might be confusing to people, because 
sequence numbers are usually represented as positive integers and the 
two comparisons would seem equivalent mathematically.

In the end I settled for adding two macros to tcputil.h:

#define SeqIsAfter(x, low) \
   ((int32_t)(x - low) > 0)

#define SeqIsBetween(x, low, high) \
   ((uint32_t)(x - low) <= (uint32_t)(high - low))

SeqIsBetween is my replacement for the IsInLimits function; the macro is 
very simple, so a function seemed like overkill.

The patch applies cleanly to both 4.8.6 and 4.9.8, but I have only 
tested with 4.8.6 on AT91SAM7X. So far I have tested wraparound in the 
common case (no packet loss). I feel confident that I haven't broken 
anything that worked before. But no guarantees, test and scrutinize it 
yourself. :)

However, testing wraparound around packet loss/reordering edge cases is 
pretty complicated, and Nut/OS's short TCP window doesn't help. I'll see 
what I can do with Linux netem network emulator.

----
Hereby I submit 3 patches for review:

01_tcp_seqno_wraparound.patch:
Implements handling of TCP sequence number wraparound. This is the 
actual patch that I described above.

02_remove_bogus_todo.patch:
This just removes a bogus TODO item that has already been implemented.

03_tcp_ntohl_to_htonl.patch:
I found a few places in the TCP code where the "htonl" macro was used 
insteasd of "ntohl". Since these operations are intechangeable, it's a 
coding style fix only.


Regards,
Marti Raudsepp
voicecom.ee