DiffServ values for VoIP and Videoconferencing (Q121)

The information in this article applies to:

DiffServ is defined in IETF RFC 2474 and RFC 2475. Other RFCs define the particular "diffserv markings" or DSCP (DiffServ codepoint) field values. Among those are per-hop behavior (PHB) classes "Assured Forwarding" (AF), as defined in RFC 2597, and "Expedited Forwarding" (EF), as defined in RFC 3246.

Within real-time multimedia systems, we need DSCP values that are generally recognized and accepted across the industry as appropriate DSCP values for call signaling, audio, and video transport.

The table below defines these generally agreed values used in the industry today (most documented in RFC 4594).

When a terminal is transmitting both audio and video, arguably both media streams should have equal precedence and, therefore, the packets for both streams should be marked with the value AF41. However, others argue that audio is more important than the video and should still be given higher precedence.