From: Harris, Joe F (Joe_Harris@xxxxxxxxxxxx)
Date: Mon Jan 07 2002 - 20:48:17 GMT-3
If traffic shaping is enabled the router will check to see if there is
credit built up so it can transmit the packet. If there is no credit
available, the router queues the packet to whatever queue has been set up
for that virtual circuit, such as First In/First Out (FIFO), Custom Queuing,
Priority Queuing, ect...If there is credit available, the router sends the
packet to the output interface and deducts the packet size from the
available credit. The router will start the "xmit" process on the interface
and at this point the packet is forwarded. Of note here is that Priority
Queuing and Custom Queuing queue sizes can be adjusted via the CLI, however
the FIFO shaping queue on the VC defualts to 40 and cannot be changed. Now
After every time interval (TC), which are measured in milliseconds, the
router begins the a loop which will the following:
1. The router will add the byte increment, which is Bc/8, to the available
credit. The router then checks to see if there is available credit to send
the packet from this virtual circuit. If so, the process dequeues a packet.
2. It then checks the "qcount", which is the number of packets currently in
the interface output queue, and saves the value that it recorded.
3. Then the packet is sent to the output interface and starts the transmit
process, It also charges the virtual circuit for this packet.
4. Then the value of the "qcount" that was saved earlier is checked to see
if it's greater than 0. If so, it breaks out of the loop. This output queue
check is done in order to prevent one virtual circuit from overrunning the
interface output queue and causing packets to be dropped.
5. If the output queue count is zero, the router continues sending more
packets until there is no more data to send, the VC has used all of its
credit or the output queue becomes greater than zero.
Now lets bring BECN responses into the picture. If a router receives any
BECNs during the current time interval, it decreases the transmit rate by
25%. The rate will continue to drop with each BECN (with a limit of one drop
per time interval) until the traffic rate gets to the minimum acceptable
incoming or outgoing committed information rate (MINCIR), once it reaches
MINCIR is stops decreasing the transmit rate.
Once the traffic rate has decreased, it takes 16 time intervals of receiving
no BECNs to start to increase the traffic rate again. Traffic increases
(Be+Bc)/16, or a better way of describing it is the byte limit that shows up
in the "show traffic" and "show frame pvc xxx" divided by 16. So it takes
much longer to get back to commited information rate (CIR) than it does to
drop to MINCIR. This is similar to slow start in TCP/IP. One way you can
make this length of time much shorter is to set "Be" 7 times the value of
"Bc", which would ensure it gets back to CIR immediately after going through
16 time intervals without a BECN. Now this traffic increase only occurs when
traffic shaping is active, so if traffic shaping is not active, the transmit
increment stays the same even if BECNs are not received.
So here are some terms that are related to BECNs and to your original
question:
TC - The Bc/CIR time interval, this is the inernalized version of TC where
the time interval can't exceed 125ms.
CIR - This is the rate you want to send out at. this is not the same as the
CIR you get from your service provider, unless you are not allowed to send
above CIR and CIR is measured in bits per second.
Bc - This is the amount of data to send out per each Tc interval and it is
measured in bits.
Be - This is the amount excess data allowed to be sent during the first time
interval once credit is built up and is measured in bits.
Mincir - Is the amount of data to be sent during periods of congestion and
it defaults to one half of CIR.
Interval - is an equation that equates to Bc/CIR, with the maximum size
being 125ms.
Byte increment - is another equation that equates to Bc/8 and it must be
greater than 125.
limit - equals byte increment + Be/8 and is measured in bytes.
So if you put these together you get this. Suppose you have router with a
physical port speed of 64KB. You purchase a 16KB CIR from your service
provider. The provider tells you that you can data out onto the PVC up to
port speed as long as there is no congestion in their network, but that when
there is congestion they will guarantee only CIR on the PVC. So in this case
your frame relay configuration would look something like:
int serial3/0
ip address 192.168.10.1 255.255.255.252
encap frame
frame-relay traffic-shaping
frame-relay map ip 192.1678.10.2 100 broadcast
frame-relay interface-dlci 100 class shaping
!
!
map-class frame-relay shaping
frame-relay cir 64000
frame-relay bc 8000
frame-relay be 0
frame-relay mincir 16000
With traffic shaping, TC = Bc/CIR and this should not be larger than 1/8
second for effective shaping to occur. So in the example listed above, TC =
8K/64K = 1/8 second. So this is what the router would do for each TC.
8000(Bc+Be), 8000(Bc), 8000, 8000, 8000, 8000, 8000, 8000
And this would equal 64k.
If the port speed was 128KB but you configured the Be = 64000, then this is
what you would see:
72000, 8000, 8000, 8000, 8000, 8000, 8000, 8000
-Joe
-----Original Message-----
From: Jim Brown [mailto:Jim.Brown@CaseLogic.com]
Sent: Monday, January 07, 2002 4:35 PM
To: 'omar guarisco'; ccielab@groupstudy.com
Subject: RE: FRTS ...CIR vs. MinCIR command
The way I understand it is the burstable port speed equates to CIR and the
minCIR is actually the guaranteed CIR limit from the provider. When shaping,
a congestion notification will throttle the bandwidth back to the minCIR
value.
-----Original Message-----
From: omar guarisco [mailto:oguarisco44@hotmail.com]
Sent: Monday, January 07, 2002 3:11 PM
To: ccielab@groupstudy.com
Subject: FRTS ...CIR vs. MinCIR command
With FRTS what is the difference configuring the CIR versus configuring the
minCIR ???
If I buy a Service from Provider which states that the PVC between Main Site
A and Branch office B the CIR is 64 Kbit/s....I would configure on FRTS the
CIR as 64....but what about MinCIR ???
Tnx for the help
Omar
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