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Traffic Rate Reporting and Adjusting by ANCP
ANCP monitors the subscriber traffic reported by the DSLAM and passes the traffic information to AAA and CoS. The net data rate is the portion of the total subscriber traffic rate that is used to transmit user information. This is the rate that ANCP reports by default and is also called the unadjusted traffic rate. However, each DSL line type has a certain technology overhead; so the rate for user data is actually less than the net data rate.
You can configure ANCP to adjust the net data rate by a fixed percentage for each line type. This adjusted or calculated rate provides CoS with a more accurate accounting of subscriber traffic and thus enables CoS to correctly shape subscriber traffic.
ANCP reports the traffic rates differently to AAA and CoS.
- ANCP always reports the unadjusted and adjusted rates for both upstream and downstream traffic rates to AAA in response to a AAA request.
- ANCP always reports unadjusted downstream traffic rates to CoS in support of CoS traffic shaping. It never reports upstream traffic rates to CoS because CoS does not shape upstream traffic. You can configure ANCP to report the adjusted downstream traffic rate to CoS.
When you remove a shaping rate configuration that ANCP previously applied, the traffic shaping rate reverts to the CoS session shaping as determined by the CoS traffic control profiles specified in the dynamic profile. If ANCP remains running but loses a connection to a particular neighbor whose subscriber traffic is adjusted as a result of ANCP, the adjusted rate remains in effect. The rate currently in effect changes only when ANCP restores the connection and sends fresh updates to CoS, or when you remove the qos-adjust statement.
Traffic Rate Adjustment
When a DSLAM determines the data rate on the subscriber local loop, it ignores the additional headers on the DSL line that are associated with the overhead of the access mode (ATM or Ethernet) and the technology of the DSL line type. However, when ANCP subsequently reports a net data rate, by default it includes this overhead and therefore reports a slightly higher value than the actual subscriber data rate seen by the DSLAM.
CoS attempts to avoid traffic drops in the access node by adjusting the traffic shaping rate that it applies to downstream traffic for a particular VLAN or set of VLANs. The discrepancy between the actual user data rate and the ANCP-reported net data rate reduces the accuracy of CoS traffic shaping.
ANCP can send CoS more accurate information by dynamically adjusting the net data rate down according to a fixed percentage that accounts for the traffic overhead. You configure the adjustment by including one or more of the qos-adjust-dsl-line-type statements at the [edit protocols ancp] hierarchy level. Each of these statements sets an adjustment factor for a particular DSL line type such as ADSL or VDS2. The adjustment factor is a percentage value that ANCP applies to the traffic rates it receives from the DSLAM and may subsequently report. The percentage accounts for the traffic overhead for that line type. That is, you configure the statements for all relevant line types, and ANCP applies the appropriate adjustment when it identifies the line type for the interface. The adjustment factor applies globally for all subscribers of the particular DSL line type associated with the statement: ADSL, ADSL2, ADSL+, SDS1, VDS1, or VDS2. Although ANCP calculates the rate, it does not report the adjusted downstream traffic rate to CoS unless you also include the qos-adjust statement at the [edit protocols ancp] hierarchy level
The qos-adjust-dsl-line-type statements are enabled by default with an adjustment factor of 100 percent, meaning that, by default ANCP effectively makes no adjustment to the rates. In this case, when ANCP is configured to report rates to CoS, it simply reports the net data rate that includes the overhead.
 | Note: ANCP automatically includes the adjusted rate when it reports to RADIUS. |
Traffic Rate Reporting and Shaping Scenarios
Table 1 lists combinations of ANCP and CoS configurations and their consequences for traffic rate reporting and shaping. In this example, the following configuration applies:
- The DSL line type is ADSL.
- The ADSL adjustment factor is set to 85 percent.
- The value for Actual-Net-Data-Rate-Downstream is 1 Mbps.
- The value for Actual-Net-Data-Rate-Upstream is 5 Mbps.
- A CoS traffic control profile named tcp1 is configured
as follows.traffic-control-profiles {tcp1 {shaping-rate 5Moverhead-accounting bytes 124}}
Table 1: ANCP Traffic Rate Reporting and Shaping Scenarios
Scenario | ANCP configuration includes the qos-adjust statement | CoS configuration includes the tcp1 traffic control profile | Traffic rate adjusted by | Expected Rate |
|---|---|---|---|---|
1 | No | Yes | CoS | <5Mbps |
2 | Yes | Yes | ANCP and CoS | <1Mbps |
3 | Yes | No | ANCP | <1Mbps |
- Scenario 1—ANCP adjusts the rate it receives from the DSLAM. ANCP does not report any rate to CoS. The CoS adjustment is determined by the overhead accounting bytes set in the traffic control profile, tcp1. The expected final traffic rate is the tcp1 shaping rate minus the adjustments performed by CoS based on the overhead accounting bytes in the profile, or 5 Mbps - 124 bps.
- Scenario 2—ANCP adjusts the rate it receives from the DSLAM. ANCP reports the adjusted rate to CoS. CoS then applies an overhead adjustment to the ANCP-adjusted rate. The CoS adjustment is determined by the overhead accounting bytes set in the traffic control profile, tcp1. The expected final traffic rate is the product of the DSLAM-reported rate times the ANCP adjustment factor for the line, minus the adjustments performed by CoS based on the overhead accounting bytes in the profile, or (1 Mbps x 0.85) - 124 bps.
- Scenario 3—ANCP adjusts the rate it receives from the DSLAM. ANCP reports the adjusted rate to CoS. CoS has no traffic control profile, so it does not further adjust the rate received from ANCP. The expected final traffic rate is the product of the DSLAM-reported rate times the ANCP adjustment factor for the line, or 1 Mbps x 0.85.
Recommended Traffic Shaping Rates
To handle a situation where the router does not receive information from the access node about the downstream and upstream calculated traffic rates for an interface, you can specify recommended advisory values for shaping the traffic sent to the interface so that it matches the subscriber local loop speed.
The transmit speed is the recommended traffic value in bits per second used for downstream traffic for an ANCP interface, and is conveyed in the Juniper Networks VSA, Downstream-Calculated-Qos-Rate (IANA 4874, 26–141). The receive speed is the recommended traffic value in bits per second used for upstream traffic for an ANCP interface, and is conveyed in the Juniper Networks VSA, Upstream-Calculated-Qos-Rate VSA (IANA 4874, 26-142).
To set the recommended shaping rate that is used as the default values for these VSAs, include the downstream-rate and upstream-rate statements at the [edit interfaces interface-name unit logical-unit-number advisory-options] hierarchy level.
ANCP Keepalives
ANCP sends a keepalive message to CoS at specific intervals. If CoS does not receive a keepalive in the expected time, it reverts the shaping rate changes it made in response to ANCP. You can adjust how long CoS waits for a keepalive message by including the maximum-helper-restart-time statement at the [edit protocols ancp] hierarchy level. The interval between keepalive messages is automatically set to one-third the value of the maximum helper restart time. For example, if you set the maximum helper restart time to 120 seconds, then ANCP sends keepalive messages every 40 seconds. In this example, if CoS does not receive a keepalive message within 120 seconds, then it reverts any ANCP-derived policy changes.
