[lwip-devel] [bug #21107] LwIP doesn't reset the TTL in ICMP Echo Replies

[Tom Evans]

URL:
  <http://savannah.nongnu.org/bugs/?21107>

                 Summary: LwIP doesn't reset the TTL in ICMP Echo Replies
                 Project: lwIP - A Lightweight TCP/IP stack
            Submitted by: tom_evans
            Submitted on: Tuesday 09/18/2007 at 04:46
                Category: None
                Severity: 3 - Normal
              Item Group: None
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: 

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Details:

LwIP doesn't reset the TTL in ICMP Echo Replies.

This means that when using Windows Tracert, the route is
measured as "twice as long as it should be". I'm currently
over 20 hops away from an LwIP implementation. It is less
than 0.5m away from me physically, but packets are taking
a double trip over the Internet to another country and
through an RF link to get to it. The edited (...)
Traceroute (IP addresses obscured with "xxx") is:

C:>tracert -d -h 60 61.xxx.xxx.xxx

Tracing route to 61.xxx.xxx.xxxover a maximum of 60 hops

  1     1 ms     1 ms     1 ms  10.xxx
  2     1 ms     2 ms     1 ms  210.xxx
  3    14 ms    13 ms    13 ms  210.xxx
...
 10    28 ms    29 ms    31 ms  202.xxx
...
 19   271 ms   271 ms   271 ms  210.xxx
 20     *        *        *     Request timed out.
 21     *        *        *     Request timed out.
...
 48     *        *        *     Request timed out.
 49  1253 ms  1876 ms  2370 ms  61.xxx.xxx.xxx

RFC792 has been slavishly followed. It says "swap the
addresses and send it back". This omission wasn't
picked up on in RFC1122 and 1123 (like I thought it was).
There's discussion in comp.protocols.tcp-ip in 1987 and
1999 on this:

http://groups.google.com/group/comp.protocols.tcp-ip/browse_thread/thread/662fabcdcfb753a6/

http://groups.google.com/group/comp.protocols.tcp-ip/msg/ac1deb7ea4c6b1ea?dmode=source&hl=en

The "modern concensus" is that ICMP should reset the TTL
in the ICMP Echo Reply to whatever is configured on that
host.

The LwIP code should set the TTL and then recalculate
(rather than just "adjust") the TTL.





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