From: Carlos G Mendioroz (tron@huapi.ba.ar)
Date: Thu May 06 2004 - 09:09:38 GMT-3
Howard,
I have a couple of doubts about this, inline:
Howard C. Berkowitz wrote:
> At 9:53 PM -0400 5/4/04, Kenneth Wygand wrote:
>
>> Howard,
>>
>> Thank you for your complete and very comprehensive analysis. My
>> question, however, was theoretical in application and is not intended
>> for a production environment. Technically the bits in the TOS field
>> can be set to anything provided they support the overall
>> implementation of the network policy in effect.
>
>
> Correct. I did want to emphasize the point that network/internetwork
> control protocols MUST have absolute priority, and, in production
> environments, it's often wise to have either out-of-band or prioritized
> telnet for remote consoles.
>
>> The real basis of my question was if a question says "set a specific
>> priority or DSCP for a specific type or subset of traffic and use
>> "best effort" for everything else", do I retag all other traffic with
>> all zero's for the TOS byte or do I trust them the values of the TOS
>> byte for this traffic. What does "best effort" actually imply, all
>> zero's or unchanged?
>>
>
> It's a little more subtle than that. Under the differentiated service
> model, there are two main kinds of traffic: guaranteed service (GS) and
> best effort (BE). Controlled load is a special case and not worth
> considering in a general discussion.
>
> BE is a somewhat misleading term when one looks at the Cisco RSVP
> implementation of GS. Since you can reserve only 75 percent of an
> interface bandwidth for GS, leaving 25 percent for BE, the
> highest-priority BE really has a guarantee that it will get 25 percent
> of the total bandwidth.
I think the 25 percent is for control overhead and layer 2 overhead,
cause bandwidths are set al layer 3. So you don't have that 25%.
(I guess that's the max-reserved-bandwidth we are talking about)
Also, I'm confused by your talking about RSVP in a differentiated
service implementation.
Last, what do you mean by "highest-priority BE" ? If it is BE, then it
has no priority, or am I missing something ?
>
> If this were a question in the CCIE lab, you really have already
> identified the key issue, which, I believe, is a legitimate question to
> ask a proctor: "I need to determine what amount of policy enforcement
> you want. Do I force differentiated service priority, or should I trust
> the data originator? Further, should I follow best practice and not
> reprioritize routing protocols and related network control mechanisms?"
>
> [If the proctor answered "no" to the last question, I might indeed walk
> away muttering that he needed a thorough thrashing with a large clue
> stick. Nevertheles...]
Current QoS courses talk a lot about trust boundaries, and tell not to
believe in markings coming from the outside. Acording to that, you
should remark (to BE) in what your design defines as edge devices, and
nowhere else. So I would do that...
>
> Given that you'll use a router as data source in the lab, AFAIK, it will
> not set non-routine priority on anything other than VoIP.
>
> Is that answer closer to your question? :-)
>
>>
>> -----Original Message-----
>> From: nobody@groupstudy.com on behalf of Howard C. Berkowitz
>> Sent: Tue 5/4/2004 5:48 PM
>> To: ccielab@groupstudy.com
>> Cc:
>> Subject: Re: Best Effort Definition
>>
>>
>>
>> At 4:31 PM -0400 5/4/04, Kenneth Wygand wrote:
>> >Hello Group,
>> >
>> >
>> >
>> >I think I know the answer to this one but I just want to get some
>> more
>> >opinions...
>> >
>> >
>> >
>> >If I am performing QoS (whether it be CoS, IP Precedence, ToS
>> Bits, or
>> >DSCP), supposed I would like to "mark all traffic going from
>> router A to
>> >router B as IP Precedence 6 while other traffic receiving "best
>> effort"
>> >service"... obviously through some kind of classification,
>> marking and
>> >queuing I would make sure all traffic from router A to router B
>> receives
>> >the type of service it requires.
>>
>> Why are you using priority 6? Priority 5 is intended as the highest
>> to be used for application traffic.
>>
>> >However, what about all other traffic?
>>
>> Priorities 6 and 7 are reserved for time-criticall routing protocols,
>> network management, etc. Never interfere with the priorities for
>> these services, or you may create a situation where the routers lock
>> up and you cannot get control. Along the same lines, you might want
>> to create fine-grained rules to have telnet from a control console at
>> priority 5.
>>
>> >When requesting that other traffic gets "best effort", should one
>> leave
>> >the QoS markings as-is, or actually remark them back to all 0's?
>>
>> Leave them as-is. I can't say with certainty that any other
>> applications will set priority, but they rarely do without a good
>> reason. If, for example, you use TFTP to reload NVRAM during
>> production hours, I might give it priority 4.
>>
>> To put it in perspective, the original military purposes of the
>> precedences were having the highest for network and internetwork
>> control. The next was used, among other things, for Emergency Command
>> Precedence, which is an order to launch a nuclear weapon. Considering
>> that the sender of such a message may become part of a mushroom cloud
>> at any time, that message HAS to take priority -- but even then, the
>> network/internetwork control had even higher precedence, because if
>> they weren't working, the network might not be there to carry the
>> Emergency Action Message at ECP precedence.
>
>
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-- Carlos G Mendioroz <tron@huapi.ba.ar> LW7 EQI Argentina
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