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When I create a QoS policy in Windows 7 through the Group Policy editor and set a DSCP value, does Windows 7 add the 802.1p QoS field to the Ethernet frame? If so, how does it map the different values from DSCP (0-63) to the 802.1p QoS values (0-7)?

So to be clear, my questions are:

  • Does Windows 7 set an 802.1p value for configured QoS policies?
  • If so, how does Windows 7 map the QoS-policy's DSCP value to the 802.1p value?

This question is specifically for outgoing packets.

Note: 802.1p tagging is supported in Windows 7 (and has been since Windows 2000, XP SP2, Windows Server 2003). Microsoft seems to allude to a mapping of DSCP to 802.1p in this document.

syplex
  • 266

3 Answers3

0

In 802.1P, the PRI value of a packet determines its priority. In this mode, the switch requires packets with a VLAN tag, regardless of their IP header. The ToS field in each packet's IP header is used by DSCP priority to determine their priority.

user1802490
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0

There appears to be a crapload of information here (new link).

DSCP and 802.1p are two different methods of QoS, and they do not necessarily correspond, although managed switches can certainly map between the two in the event that one or the other is unsupported on a particular device (with DSCP being the older, more widely-supported one). Windows maps traffic to both independently, with the following values used for 802.1q:

Non-conforming packets  0

Best-effort             0

Controlled load         4

Guaranteed service      5

Network control         7

Qualitative             0

The general trend is higher values = higher priority traffic, but most network devices tend to coalesce these values into a handful of queues/buffers internally (very low, low, medium, high, for example). Also, if your application doesn't label its traffic type, Windows will have nothing to map to and you won't see any change in performance.

Bigbio2002
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0

Support for 802.1p and DSCP has been claimed by Microsoft since many years.
Yet I have found a troubling Microsoft article saying that 802.1p only started with Server 2012 (same code-base as Windows 8).

The Microsoft article Quality of Service (QoS) Overview says:

Previous versions of Windows support priority tagging in the IP header of a network packet. The priority bits are known as Differentiation Service Code Point (DSCP). Windows Server 2012 adds the support of 802.1p tagging. 802.1p is a 3-bit priority value in the layer 2 Ethernet frame. Therefore it applies to non-IP packets such as RDMA over Ethernet.

Both DSCP and 802.1p are said by Microsoft in the article Differentiated Services Code Point (DSCP) overview to be equivalent in functionality. However, as these two standards apply to different layers of the communication protocol, it is unlikely that one can be mapped to the other.

harrymc
  • 498,455