Re: Protocols at the Point of a Gun

[Scott Brickner <sjb@universe.digex.net>]

"Perry E. Metzger" writes:
>Scott Brickner writes:
>> Anyway, you computer creates the IP packet, but then sends it to your
>> ISP's router.  That router *always* makes changes to the packet header
>> because it must decrement the time-to-live field and recompute the
>> header checksum.
>
>There is a trivial trick for making the decrement TTL/change checksum
>operation very fast, based on noting how a decrement would change the
>checksum. Most very high speed routers attempt to avoid doing ANY
>processing of the packets at all beyond this, and IPv6 has no header
>checksum partially in order to reduce this overhead further. Forcing
>routers to do more work is a Very Very Bad Idea.

As I pointed out in a private note to Perry, it's not the high-speed
routers that have to change the packets.  They typically are between
the sort of ISPs that would get "network common carrier" status, and
could rely on the options added (or not) by the other side.  It's only
when the packet crosses the border from outside the "common carrier"
net to inside that the header needs changed, and that's usually at a
terminal server, not a "very high speed router".