1996-04-11 - Re: Protocols at the Point of a Gun

Header Data

From: “Perry E. Metzger” <perry@piermont.com>
To: Scott Brickner <sjb@universe.digex.net>
Message Hash: 4b780c5fa1fc5c0b160b3106787028b5d65ed2b6bcbf5c513e01eaa129cfe95b
Message ID: <199604111740.NAA21264@jekyll.piermont.com>
Reply To: <199604111714.NAA02244@universe.digex.net>
UTC Datetime: 1996-04-11 23:58:35 UTC
Raw Date: Fri, 12 Apr 1996 07:58:35 +0800

Raw message

From: "Perry E. Metzger" <perry@piermont.com>
Date: Fri, 12 Apr 1996 07:58:35 +0800
To: Scott Brickner <sjb@universe.digex.net>
Subject: Re: Protocols at the Point of a Gun
In-Reply-To: <199604111714.NAA02244@universe.digex.net>
Message-ID: <199604111740.NAA21264@jekyll.piermont.com>
MIME-Version: 1.0
Content-Type: text/plain

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.