#208 VOS socket options: QoS/ToS field priority handling needs update

[Bernd Löhr]

The QoS value (0-7) is mapped to MSB bits 7-5 of IP_TOS field, bit 2 is set for local use.

The usage as TOS field is deprecated (since 1998) and its definition has been succeeded by RFC 2474 and RFC 3168. Also, usage depends heavily on IP-routers.
This field is now called the "DS" (Differentiated Services) field and the upper 6 bits contain a value called the "DSCP" (Differentiated Services Code Point). QoS as priority value should now be mapped to DSCP values and the Explicit Congestion Notification field (ECN) should be set to 0.

[Holger Waldmann]

This change seems to contradict IEC61375-3-4 (ECN) which defines the priority 0..7 to be set in the upper 3 bits of the DSCP field.
This should result in dscpMap[] = {0, 8, 16, 24, 32, 40, 48, 56}
Instead the change defines dscpMap[] = {0, 8, 18, 24, 34, 40, 48, 56}
The notable difference is that priority 2 and 4 have an additonal bit set to 1, that according to IEC61375-3-4 shall be 0.
Can someone please comment, why TRDP does not respect IEC61375-3-4?

[Bernd Löhr]

Dear Mr. Waldmann,
thank you for your response – I will reply as this "somenone":
After reviewing the mentioned code and mapping, I can see some contradiction between the current implementation and IEC61375-3-4 Chapter 4.6ff.
What might have been irritating is the numbering of the bits in the standard versus RFC 2474.
The mapping {0, 8, 16, 24, 32, 40, 48, 56} is the correct one - It will be changed to these values in the trunk.
Regards
Bernd Löhr