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From: almquist@JESSICA.STANFORD.EDU (Philip Almquist)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Router/Gateway laziness question
Message-ID: <8909121150.AA10548@ucbvax.Berkeley.EDU>
Date: 12 Sep 89 10:48:00 GMT
References: <1989Sep8.190943.9450@brutus.cs.uiuc.edu>
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Johnny,
> ...how about if the router just checked the checksum, looked at the
> destination IP-address, and forwarded the packet out the appropriate
> interface without checking that the options are all acceptable, etc...

	If you're going to play fast and loose with the rules, I
don't understand why you're bothering to verify the checksum...

	But seriously: although your scheme may save some programming
effort, I doubt it it would save very many instructions.  A router has
to do route lookup.  It may have to do an additional lookup to obtain a
link layer address to send the packet to.  It has to strip off the link
layer headers when the packet arrives and add new ones before sending it
back out.  It has to decrement the TTL if you don't like to have packets
live forever in your network (or want to be compliant with RFC1009).
Routers must process certain IP options (RFC1009 requires source route,
record route, and timestamp), and must therefore parse all IP options
sufficiently to determine their type and length.  Last but not least,
routers have to take interrupts and massage their network interface
cards.

	There may be a few instructions you can save here and there by
skipping validity checks, but the the real determinants of router
performance are the network interface cards, the bus bandwidth, and the
choices of data structures and algorithms used in performing the tasks
listed in the previous paragraph.
						Philip