VoIP DSCP Rewrite Rules

This topic describes the voice over IP Application Layer Gateway (VoIP ALG) mechanism for modifying the Differentiated Services Code Point (DSCP) field of Real-Time Transport Protocol (RTP) packets. The VoIP ALG mechanism is applicable for the RTP session, which is recognized by the ALG.

DSCP is a modification of the type of service byte for class of service (CoS). Six bits of this byte are reallocated for use as the DSCP field, where each DSCP specifies a particular per-hop behavior that is applied to a packet.

A rewrite rule modifies the appropriate CoS bits in an outgoing packet to meet the requirements of the targeted peer. Each rewrite rule reads the current CoS value that is configured at the VoIP ALG level. Every packet that hits the VoIP ALG is marked by this CoS value.

This feature supports ALG DSCP marking for H323, Session Initiation Protocol (SIP), Media Gateway Control Protocol (MGCP), and Skinny Client Control Protocol (SCCP). It provides a 6-bit DSCP value configuration for each of these. When the first RTP packet hits the ALG, this feature receives the 6-bit DSCP value form the configuration and sets it to the RTP session that the packet has created. This first RTP packet and the following RTP packets passing through the RTP session are marked according to the 6-bit DSCP value in the session.

To avoid VoIP quality degradation caused by network congestion, the RTP packets are required to mark the DSCP bit to ensure they get higher routing priority. A downstream router can put those packets in a higher priority queue for faster forwarding. To provide this functionality, there needs to be a per-VoIP mechanism for modifying the DSCP field of RTP packets according to the specific configuration. This will ensure that all RTP packets based on User Datagram Protocol/Transport Control Protocol (UDP/TCP) that encounter the ALG will be assigned a specific DSCP bit.

This example shows how to configure VoIP DSCP.