RSVP is a topology aware Call Admission Control (CAC) mechanism. As with any CAC mechanism the main purpose is to determine whether a call has enough bandwidth to proceed. RSVP supports any network topology and can pass through non-rsvp aware routers. Non-rsvp aware router will ignore and forward all rsvp messages.
RSVP signalling uses the same IP route as the media stream would take. If bandwidth on any link throughout the network is not sufficient, the reservation fails. If there is sufficient bandwidth throughout the network the reservation succeeds.
RSVP calculation
Lets work with G729 codec. Now before I get into how RSVP calculates bandwidth requirements, let do a quick refresher on payload sizes, and rtp stream size requirements for G729.
Below is a quick calculation on the three sampling sizes available for configuration in CUCM and CME.
10ms sampling size
IP Header = 20bytes
UDP Header = 8bytes
RTP Header = 12bytes
Payload = 10bytes
Total 50bytes per packet
1000ms / 10ms (sampling size) = 100packets per seconds (pps)
100pps x 50bytes = 5000bytes per second
Convert to kbps = 40kbps. This is needed to for a single call with G729 with 10ms sampling size.
20ms sampling (Default for both CUCM and CME)
IP Header 20bytes
UDP Header 8bytes
RTP Header 12bytes
Payload 20bytes
Total 60bytes per packet.
1000ms / 20ms = 50pps
50pps x 60bytes = 3000bytes per second
Convert to kbps = 24kbps. This is needed to for a single call with G729 with 20ms sampling size.
30ms Sampling size
IP Header 20bytes
UDP Header 8bytes
RTP Header 12bytes
Payload 30bytes
Total 70bytes per packet
1000ms / 30ms = 33pps
33pps x 70bytes = 2310bytes per second
Convert to kbps = 18.4kbps. This is needed to for a single call with G729 with 30ms sampling size.
As we can see the 10ms sampling size requires the highest amount of bandwidth being 40kbps. Keep that in mind for a later discussion.
RSVP bandwidth pool is configured on each router’s interface with the ip rsvp bandwidth bandwidth command.
The bandwidth entered should equal the number of calls that is allowed to stream across the interface simultaneously. Working from the default (20ms sampling size), if we want to restrict calls to a maximum of 5 calls, we would enter 136kbps for the bandwidth. You are probably saying, the bandwidth should be 120kbps as 5 x 24kbps equals 120kbps. BUT RSVP doesn’t know the sampling size when the its setting up the reservation. All RSVP knows is the codec type, in this case its G729 which uses a maximum of 40kbps, remember going back to the previous discussion, a 10ms smapling size equals 40kbps. RSVP allows for the maximum size for a G729 call initially, until the rsvp process can look inside the voice stream and determine what sampling size the codec is using. Generally default is left, there the size is 24kbps, RSVP then throttles down the bandwidth needed to 24kbps. Therefore we always add 16kbps (the difference between a 20mx sampling size packet to a 10ms sampling size packet) to the rsvp bandwidth. This allows the last call to be initiated at 40kbps, before being throttled down. If we didn’t add the 16kbps onto the bandwidth needed, the last call would never be allowed to be setup as the RSVP process would have 24kbps left in the bandwidth pool, but 40kbps is needed to initially setup the reservation.
RSVP for CAC purposes is a DiffServ Model, not InetServ model. Yes RSVP traditionally speaking is an InetServ model as the RSVP process sits on the Control and Data Plane of the router. However RSVP used by CUCM sits only on the Control Plane, hence controls the Call Admission Component and not the actual QoS which runs on the Data Plane level. The QoS is left up to the LLQ mechanism.
Configuring RSVP
Commands
Interface G0/0
ip rsvp bandwidth kbps
Dspfarm profile 2 mtp
Codec g729
Codec pass-through
Rsvp
Maximum sessions software 4 (Ensure this number is greater than or equal to the number of calls desired.
Sccp local Loopback 0
Sccp ccm 10.0.0.1 identifier 1 version 7+
Sccp ccm 10.0.0.2 identifier 2 version 7+
Sccp
Sccp ccm group 1
Bind interface loopback 0
Associate ccm 1 priority 1
Associate ccm 2 priority 2
Associate profile 2 register HQ-RSVP
Notes:
*Can only use one codec assigned to an MTP resource, although you can have variations of the codec in same MTP resource. Eg G729a G729ab etc
*The MTP resource can also be used for other purposes, not just rsvp. So its always a good idea to allow maximum sessions higher to what is allowed by RSVP CAC as other process maybe using ghe MTP resource and consuming a session.