[cisco-voip] Re: cs3 instead of af31 in CM 3.3.4

[Carter, Bill]

CS3 = AF24
 
IP Precedence 3    = 011
CS3                       = 011000
AF24                      = 011000
AF31                      = 011010
AF32                      = 011100
AF33                      = 011110
 
In the IP Precedence world only the first 3 bits are used (0-2).
In the DSCP world six bits are used (0-5)
DSCP Class of Service uses six bits. First 3 define queuing class, and
the last 3 are 0 (3-5)
DSCP Assured Forward uses six bits.  First 3 define queuing class (0-2),
last 3 define drop Precedence (3-5).
 
The last 3 bits signify drop preference.
XXX010 = Low Drop Probability
XXX100 = Medium Drop Probability
XXX110 = High Drop Probability
 
In the DSCP standard, Assured Forwarding (AF) has two parts, the first
defines queuing placement (bits 0-2), the second defines drop precedence
(bits 3-5).   Class Selector (CS) only defines queuing placement (bits
0-2).  The remaining 3 bits are left at 0 (bits 3-5).
 
Thus DSCP (CS and AF) supports backward compatibility for devices that
only support IP Precedence.  Since the DSCP first 3 bits define class of
service which maps to IP Precedence classes.  When a device not
supporting DSCP receives a packet with DSCP marking, it will read the
first 3 bits and treat the packet as if it was marked with IP
Precedence.
 
I can't find a statement on the drop preference of a packet marked with
a CS value and the last 3 drop bits set to 0 (011000).  Under the AF
logic where "bigger-is worse"  XXX000 should have a lower drop
preference than XXX010, XXX100, or XXX110.  I don't know is this is how
network devices actually handle drop precedence is DCP.
 
I believe the standards bodies were suggesting CS3 for VoIP signaling
and Cisco was using AF31.  Frankly AF31 seems more logical. 
 
 
References: 
RFCs 2474, 2475, 2597.
"IP Telephony Self-Study Cisco DQOS Exam Certification Guide" Cisco
Press. pages 114-129.
 
 


________________________________

	From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Voll, Scott
	Sent: Monday, February 27, 2006 10:42 AM
	To: Matt Slaga (US)
	Cc: Cisco VoIP
	Subject: RE: [cisco-voip] Re: cs3 instead of af31 in CM 3.3.4
	
	

	Negative Ghost Rider* in CM 4.1.3 the signaling is CS3. No
longer AF31.  This is a known change.

	 

	Scott

	 

	*My favorite quote from Top Gun the movie.

	 

	
________________________________


	From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Matt Slaga (US)
	Sent: Monday, February 27, 2006 8:32 AM
	To: Voll, Scott; Wes Sisk
	Cc: Cisco VoIP
	Subject: RE: [cisco-voip] Re: cs3 instead of af31 in CM 3.3.4

	 

	I think this guy was trying to do the opposite, change the
default of DSCP af31 to CS3.

	 

	
________________________________


	From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Voll, Scott
	Sent: Monday, February 27, 2006 11:15 AM
	To: Wes Sisk
	Cc: Cisco VoIP
	Subject: RE: [cisco-voip] Re: cs3 instead of af31 in CM 3.3.4

	Why did Cisco's default change from DSCP to CS?  Are we going to
see these continue to change for all packets?  Video, audio, etc.

	 

	Scott

	 

	
________________________________


	From: cisco-voip-bounces at puck.nether.net
[mailto:cisco-voip-bounces at puck.nether.net] On Behalf Of Wes Sisk
	Sent: Sunday, February 26, 2006 10:09 AM
	To: Waleed Samir
	Cc: Cisco VoIP
	Subject: [cisco-voip] Re: cs3 instead of af31 in CM 3.3.4

	 

	there is a setting in ccmadmin under service->service parameters
to changes the DSCP bits used for various signaling.

	 

	/Wes

	 

	On Feb 26, 2006, at 10:08 AM, Waleed Samir wrote:

	
	
	
	

	Ok Thanks for your guiding

	 

	 

	
________________________________


	From: Wes Sisk [mailto:wsisk at cisco.com] 
	Sent: Sunday, February 26, 2006 5:06 PM
	To: Waleed Samir
	Subject: Re: cs3 instead of af31 in CM 3.3.4

	 

	I'm sorry, i do not answer direct questions.  If this is
regarding a TAC case please provide a number, otherwise, plese post your
question on the cisco-voip alias so everyone can benefit from the
answer.

	 

	Thank You,

	/Wes

	 

	On Feb 25, 2006, at 7:35 PM, Waleed Samir wrote:

	
	
	

	 

	Hi

	 

	I need to change the setting of the Callmanager 3.3.4 to send
the signaling traffic with CS3 instead of the AF31 to follow the new
standard.

	How can I make this change and do you have any documents or
links related to this subject.

	Many Thanks

	Waleed Samir

	
	
	

	 

	 

	 

	 

	
________________________________


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