Industry standards for QoS, and how they apply to DSCP in PTP 650/700

The Internet standards for Quality of Service (QoS) have been specified and re-specified many times since the IP header was standardized in 1981. Back at the beginning of Internet history, RFC 791 defined the second byte of the header as the Type of Service (ToS) field.

In the latest standards (RFC 2474, RFC 3168, RFC 3260) the first six bits of the second byte are defined as the Differentiated Services field. The value in this field is called the Differentiated Services Code Point (DSCP). The last two bits of the second byte are now known as the Explicit Congestion Notification (ECN) field.

Other QoS standards have been introduced and superseded in the years since 1981. Here are the highlights:

The six-bit DSCP provides 64 unique values.

RFC 2474 defines some specific uses for DSCP values ending in three zeros (XXX000). These are defined as Default and Class Selector (CS) codepoints CS1 to CS7, and are used to provide backwards compatibility to the original IP Precedence field.

RFC 2597 (1999) defines the Assured Forwarding (AF) codepoints AF11, AF12, AF13, AF21, AF22, AF23, AF31, AF32, AF33, AF41, AF42, and AF43.

RFC 3246 (2002) defines the Expedited Forwarding (EF) codepoint.

RFC 5865 (2010) defines the Voice Admit (VA) codepoint.

PTP 650 and PTP 700 provide a flexible QoS scheme based on the DSCP field. You can select QoS treatment based on the IP header using the Data Priority Scheme attribute. Here is an extract from the QoS Configuration web-page:

The rectangular array of controls map each DSCP value to one of eight queues, where Q7 is the highest priority and Q0 is the lowest priority. The names of the special codepoints are added after the decimal numeric value of the DSCP.

PTP 650 and PTP 700 set default mapping based on standards and on the recognised meanings of the codepoints. If your network is different, you can set up any custom mapping needed.

PTP 650 and PTP 700 completely ignore the ECN field in the ToS byte.

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