Rate-Limiting for VoIP Traffic Overview
This topic describes how use rate limiting to improve the flow of VoIP traffic.
Introduction to Rate Limiting
Because BGF traffic flows involve voice traffic, the flows require quality of service that:
- Provides the bandwidth that the flow requires.
- Ensures that flows do not consume more resources than they need.
- Regulates flows that are nonconforming and present vastly greater rates of traffic.
The BGF provides a two-rate policer that you can apply to the ingress traffic of any gate.
This quality of service is provided through a two-rate three-color policing functionality on the MultiServices PIC or MS-DPC. This policer complies with RFC 2698, A Two Rate Three Color Marker, September, 1999. With the rate limiting capability, the MultiServices PIC or MS-DPC can police flows to conform to:
- Committed information rate (CIR)
- Peak information rate (PIR)
- Committed burst size (CBS)
- Peak burst size (PBS)
How the Rate Limiting Feature Works
You use rate limiting with gates. To enable rate limiting for a gate, you need to provide traffic management package (TMAN) parameters. You can configure these parameters in the Junos OS CLI (Table 1) or they can come from the H.248 signaling commands received from the gateway controller. Traffic management parameters that come from the gateway controller override parameters configured in the CLI.
Table 1: Traffic Parameters Configured in the CLI
| Parameter | Description | Equivalent PGCP Signaling Command |
|---|---|---|
Sustained data rate (SDR) | Provides the CIR | Tman/sdr |
Peak data rate (PDR) | Provides the PIR | Tman/pdr |
Maximum burst size (MBS) | Provides the burst size. Both the CBS and the PBS defined in RFC 2698 map to the maximum burst size. | Tman/mbs |
For each traffic management parameter, you can configure a value that applies to all gate streams and a value that applies only to RTCP gate streams. For RTCP streams, you can specify a fixed value for the parameters or you can specify the value as a percentage of the RTP rate. When RTP and RTCP are represented as a single stream, RTCP is policed whenever RTP is policed. You can also specify that RTCP bandwidth is be included in the SDR for streams other than RTCP.
The gateway controller can send traffic management parameters to the BGF in gate open and gate modify signaling requests. When the PIC or DPC receives these parameters, it marks the packets red, yellow, or green as specified in RFC 2698. A packet is marked:
- Red if it exceeds the PIR.
- Yellow if it exceeds the CIR.
- Green if it does not exceed the CIR.
Packets that are marked red are dropped by the PIC or DPC.
Default Values for Rate-Limiting Parameters
If the policy command H.248 message from the gateway controller is on (tman/pol=on), but the rate-limiting parameters are not specified in the message and the Junos rate-limiting parameters have not been configured, the BGF uses following default values:
- Peak data rate—10,000 bytes per second for all streams and 5 percent of the RTP gates’ PDR for RTCP streams.
- Sustained data rate—10,000 bytes per second for all streams and 5 percent of the RTP gate’s SDR for RTCP streams.
- Maximum burst size—1000 bytes for all streams and the MBS of the RTP gate for RTCP streams.
Rate Limiting and Fast Update Filters
When a VoIP flow configured through the BGF violates the SDR by three times the configured rate, fast update filters are installed on the gate to allow the rate-limiting drop action to occur on the PFE instead of the PIC or DPC.
A fast update filter is similar to a regular filter that is defined in the [edit firewall] hierarchy, except that the system can incrementally add or update terms.
For fast update filters, a term equals a gate definition. You can see gate definitions in the show services pgcp gates gateway command output.
The fast update filter match is performed based on the most specific defined term. For each filter, a default term is installed to allow traffic to pass through (otherwise, all traffic is dropped because it is the default firewall action). For example, two terms are listed when there are two filters.
Filters are in effect until the gate is destroyed. If the client loses its connection for over 30 seconds, the existing filters are deleted, and default fast update filters are installed.
Rate-Limiting Statistics Display
To display statistics for a gate including rate-limiting statistics and the number of packets dropped because of FUF filters, use the show services pgcp gate gateway gateway-name gate-id gate-id statistics command.
