Subject: Re: Number of DP bits?

From: [email-masked] (Konstantinos Dovrolis)
Date: Sat, 1 Nov 1997 20:08:50 -0600

Jon,

> on the issue of "how many bits?" we can easily seperate out IPv4 and
> IPv6 - we don;t have a lot of bits to play with and they are to some
> extent already constrained in IPv4.....so i think one could have a 2
> stage plan....1 bit, then 2 bits
> 

I do not agree with your position above. There are at least 8 different
bits with which one can play: the 3 priority bits, the 3 TOS bits, and 
the 2 bits that were "reserved for future use" (i.e., for now). In other
words 256 different service level selections.

If we need even more bits, one can imagine different protocol numbers for
different "service levels". For example, we can have a protocol number
for the video-related UDP-based applications, and another protocol
number for the audio-related UDP-based applications (don't ask me 
why would this be useful). In any way, there are 8 bits for the 
Protocol field while only a few of them are used. And speaking even more
futuristically, if more bits are needed, the routers can start looking at
the TCP/UDP port numbers for different service levels of the same
application...

My point is not to suggest the use of the Protocol or the UDP/TCP port
numbers. I am just saying that even in IPv4 there are enough bits
to describe very sophisticated service level combinations. The real
question is not if we have the bits, but how complicated this 
"service menu" should be, how much complexity can a gigabit-
per-second IP router can afford in its forwarding path, and of course
the semantics of all these "service menu dishes". Also, if we don't want
to make the same mistakes that were done before (see ATM), we have
to illustrate that this service menu (I like this term!) CAN be
implemented. Saying that "the mechanisms to support this service
is an implementation issue that is left open for vendor differentiation"
is a rather OSI-zed illusion. But this is a another story.

Constantinos

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From: Jon Crowcroft <[email-masked]>
Date: Mon, 03 Nov 1997 08:39:22 +0000

>words 256 different service level selections.

the key word here is IMPLEEMNTATION

yo uare not free to alter the expected behaivour of todays routers by
altering the meaning of some bit.....
 
sure there are lots of bits in the IP header - thep roblem is that a
lot of them are already acted on in some ways.....there's whats
specified (see the relevant RFCs) but there is also what is OUT
there!!!

 >is a rather OSI-zed illusion. But this is a another story.
 
you are right academically.....its a question of what is practically
do-able for IPv4 without breaking either the exisitng routers, or the
proposal itself by making it non-deplyoable.

regards

 jon

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From: [email-masked] (Hyogon Kim)
Date: Mon, 3 Nov 1997 19:43:15 -0500 (EST)

>specified (see the relevant RFCs) but there is also what is OUT

I go with the incremental approach, starting from 1 bit. Reasons being

	1. it doesn't hurt to save the bits for later (in fact, 
		that we are now tied up by the old definitions of
		TOS/precedence fields clearly shows why not to
		hastily use up the bits - so let's do this demand-driven)
	2. it's not clear how and how much the added bits will 
		do for us - in any case they won't be so much of the 
		quantum leap as the first bit 
	2.1 it's likely that if our ideas fail with 1 bit, 
		adding a couple more won't reverse it
	3. same as what Jon says above

Regarding (3), I would like to poll who uses which bits among the
8 bits whether or not it's as specified in the RFC? I only heard Cisco 
using 3 precedence bits to identify classes in their WRED implentation. 
Who uses TOS field? Are there any that use the reserved bit for some 
strange reason?

-- Hyogon
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Hyogon Kim ([email-masked])	       		       |        (201) 829-4728
Research Scientist				       |           MCC 1B-225B
Internet Architecture Research Laboratory	       |      445 South Street 
Bell Communications Research			       |  Morristown, NJ 07960
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From: JJ Krawczyk <[email-masked]>
Date: Thu, 06 Nov 1997 00:00:51 -0500

>Who uses TOS field? Are there any that use the reserved bit for some 

Several routing protocol specs (including BGP and OSPF) say that you SHOULD
send routing packets with the precedence bits set to "internetwork control"
(110).  I know of implementations that do this.

-jj

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From: Paul Ferguson <[email-masked]>
Date: Thu, 06 Nov 1997 08:53:41 -0500

>(110).  I know of implementations that do this.

Yes, so do I.  ;-)

I don't see any reason why this poses a problem, however, since
it is a widely held belief that this is 'correct' behavior. This
is based on the conventional thinking that it is better to
maintain routing system stability in the face of congestion than
to toss a few data packets.

- paul