Normally, reservations are serviced when RSVP classifies packets and a queueing mechanism polices the packet. To enable establishment of an SVC to service each new RSVP reservation on the interface, use the following command in interface configuration mode:
Command |
Purpose |
Router(config-if)# ip rsvp svc-required |
Enables creation of an SVC for each new reservation made on the interface or subinterface. |
To ensure defined QoS, SVCs created in response to RSVP reservation requests are established having QoS profiles consistent with the mapped RSVP flow specifications.
The sustainable cell rate (SCR) of an ATM SVC is equal to the RSVP reservation rate; the maximum burst size (MBS) of an ATM SVC is equal to the RSVP burst size. RSVP attempts to compensate for the cell tax when establishing the reservation so that the requested bandwidth is actually available for IP data traffic.
The sustained cell rate formula is given as follows:
r
atm =
r
rsvp * (53/48) * (MPS + DLE + (MPS + DLE) % 48)/MPS
The formula terms used in the equation (and subsequent equations) are described in the table below, followed by an explanation of how the formula was derived.
Table 1 |
SCR Formula Terms |
Term |
Definition |
ratm |
ATM rate (SCR). |
rrsvp |
RSVP rate. |
MPS |
Minimum IP packet size, including the IP headers (300 bytes minimum). |
DLE |
Data-link encapsulation overhead. For RSVP ATM SVCs, ATM adaptation layer 5 (AAL5), Subnetwork Access Protocol (SNAP) encapsulation is used, which imposes a 5-byte encapsulation header on each protocol data unit (PDU). |
% |
Modulus operator. It yields the integer remainder from an integer division operation. For example, 57 % 53 results in 4. |
CPS |
Cell payload size. The total number of bytes in all the payloads of all the cells required to send a single packet with encapsulation. |
UCO |
Unused cell overhead (0 to 47). |
COMP |
Compensation factor. CPS divided by MPS. |
There are two reasons for converting from RSVP rate to the ATM cell rate, as follows:
-
To account for the ATM encapsulation header overhead and cell header overhead
-
To account for the fact that ATM cell sizes are fixed
Because a portion of the last cell is unused, it is possible that a certain IP packet size requires more ATM cell layer bytes.
MPS + DLE is the length of the data packet that needs to be segmented into a number of fixed-length (48-byte payload) pieces that would then be put into a cell and sent.
Because the CPS needs to be greater than or equal to MPS + DLE, CPS must be larger than MPS.
CPS can be calculated as follows:
CPS = ceil((MPS + DLE)/48) * 48
where ceil(x ) is the ceiling operator that returns the smallest integer greater than or equal to the real number x . Upon expanding the implementation of the ceil(x ) operator, the expression can be arithmetically transformed into the following equation:
CPS = MPS + DLE + (MPS + DLE) % 48
where (MPS + DLE) % 48 yields the integer remainder when MPS + DLE is divided by 48. Because (MPS + DLE) % 48 is equal to the UCO, the equation for CPS can be rewritten as follows:
CPS = MPS + DLE + UCO
Because the IP bit rate was calculated by considering only the IP data and header (that is, packets of length MPS or larger), the IP bit rate ( r rsvp) needs to be multiplied by COMP. According to the table above, COMP = CPS/MPS. Thus:
ATM cell payload bit rate =
r
rsvp * COMP =
r
rsvp * CPS/MPS
When expanded, the ATM cell payload bit rate is as follows:
ATM cell payload bit rate =
r
rsvp * (MPS + DLE + UCO)/MPS
Each ATM cell has a 5-byte header and a 48-byte payload, resulting in a 53-byte cell. Because the entire cell needs to be accounted for (not just the payload), we need to multiply the equation by a compensation factor of 53/48, which yields the desired equation:
r
atm =
r
rsvp * (53/48) * (MPS + DLE + UCO)/MPS
Thus, the SCR of the SVC created to carry the RSVP flow is calculated by the following formula:
r
atm =
r
rsvp * (53/48) * (MPS + DLE + (MPS + DLE) % 48)/MPS
The ATM peak cell rate (PCR) is derived using the same formula as the cell rate formula. It is either based on the maximum line rate of the ATM interface or on a configured maximum.
The maximum burst size of the SVC is derived by the following formula:
r
atm =
r
rsvp * (MPS + DLE + UCO)/(MPS * 48)
Note that the actual PCR, SCR, and MBS will be slightly larger than these formulas indicate.
See the task "Limiting the Peak Rate Applied to the PCR for SVCs" for information on setting the PCR of the ATM SVC.
Each new RSVP reservation causes establishment of a new SVC. If an existing reservation is refreshed, no new signalling is needed. If the reservation is not refreshed and it times out, the SVC is torn down. If the reservation is refreshed but the RSVP flowspec has changed, the existing SVC is torn down and a new one with the correct QoS parameters is established.