Dialogic Voice Cards - About the TOS/DSCP Class of Service field in NaturalAccess RTP
Information
Dialogic NaturalAccess Software is use to create development and runtime environment for creating voice, fxs, conferencing and video application using the Dialogic CG Series Media Boards.
Overview
The Dialogic NaturalAccess™ Media Stream Protocol Processing (MSPP) API provides support for the TOS and DSCP Class of Service field in an IPv4 header.
The TOS field was the original class of service field defined for IPv4 packets in RFC 791. It defines 7 levels of precedence (3 bits) in an 8-bit field in the IP header. The TOS uses the most significant 3 bits of the 8-bit field. For example, the value 0xA0 represents TOS level 5, as the most significant 3 bits (101) are the value 5.
The DSCP field is a newer specification (defined in RFC 2475 and RFC 2476) that uses the same 8-bit field used by TOS in the IP header. The DSCP defines 63 levels of precedence (6 bits). DSCP is backwards compatible with TOS so that the most significant 3 bits are used for precedence, while the next 3 bits are used for finer granularity. An incoming packet using the older TOS standard will still be routed with the correct precedence.
The NaturalAccess MSPP API contains a TypeOfService field used when defining an IPv4 endpoint for use with RTP:
typedef struct tag_RTPRTCP_ENDPOINT_PARMS
{
DWORD size;
BYTE TypeOfService;
DWORD startRtcp;
RTCP_SESSION_PARMS rtcpParms;
DWORD RtpTsFreq;
DWORD Session_bw;
DWORD dtmf_event_control;
DWORD frameQuota;
DWORD linkEvents;
RTP_ PAYLOAD_MAP PayloadMap;
} RTPRTCP_ENDPOINT_PARMS; |
The TypeOfService field must be set to the correct 8-bit value used in the IP header. If a IP precedence level of 5 is desired, then the most significant 3 bits should be set to 5, while the remaining bits are left as zeroes. For example, for IP precedence level 5:
RTPRTCP_ENDPOINTPARMS.TypeOfService = 0xA0 /* sets IP precedence level 5 */ |
By default, the TypeOfService field is set to zero.
If DSCP is desired, then the correct 8-bit TypeOfService setting should be used when defining the IPv4 endpoint.
Glossary of Acronyms / Terms
MSPP - Media Stream Protocol Processing
TOS - Type of Services, defined in RFC 791
DSCP - Differentiated Services Code Point, defined in RFC 2474 and 2475