Re: TCP Checksum Interoperability

[Fred Baker <[e-mail]>]

At 03:13 PM 4/5/2002, Matt Crawford wrote:
>I think that (A) most or all extant IPv4 routers violate 791
>if they happen hold a packet more than a second, and (B) IPv6
>invalidated TCP's correctness by defining the Hop Limit field to be a
>hop limit and have no connection to time.  A TCP riding on IPv6 may
>receive old segments an unbounded time later without any other
>network element breaking a spec.

for the record, while IPv4 TTL remains theoretically a time in seconds, it 
effectively became a hop count a long time ago. Thus sayeth RFC 1812:

5.3.1 Time to Live (TTL)

    The Time-to-Live (TTL) field of the IP header is defined to be a
    timer limiting the lifetime of a datagram.  It is an 8-bit field and
    the units are seconds.  Each router (or other module) that handles a
    packet MUST decrement the TTL by at least one, even if the elapsed
    time was much less than a second.  Since this is very often the case,
    the TTL is effectively a hop count limit on how far a datagram can
    propagate through the Internet.

    When a router forwards a packet, it MUST reduce the TTL by at least
    one.  If it holds a packet for more than one second, it MAY decrement
    the TTL by one for each second.

In that it "MAY" decrement the value, it also may choose not to...