Lync Quality of Service Behavior

In Lync Server, by default, Quality of Service (QoS) is not enabled for the Windows Lync soft client, yet it is enabled for Lync Phone Edition clients.  Thus it is important to understand that Lync Phone Edition devices will stamp a Differentiated Services Code Point (DSCP) value of 40 on media traffic while the standard Lync client will leave this field null.

In at least one field-engagement there were reported call quality issues between Lync Phone Edition devices in different sites across a WAN, yet anyone on the Windows Lync client was unaffected.  The WAN routers were prioritizing traffic stamped with a DSCP value other than 40, so the 0 value traffic from the computers was being treated normally, but the 40 value traffic from the phones was being deprioritized, thus dropping a lot of packets of that traffic type.  Once the QoS tagging was updated in Lync to the proper value then call quality returned to normal for all endpoints uniformly.

The following steps show how to validate the default configuration in Lync and can be used to change the setting if desired.

Default Configuration

• Using the Lync Server Control Panel browse to the Clients > Device Configuration section and view the properties of the default Global object.

• As shown the Voice quality of service setting is enabled and configured to a value of 40.  This setting can be changed to 0 is disable DSCP tagging.

• Alternatively the same steps can also be performed from the Lync Server Management Shell by issuing the following cmdlet:

PS C:\> Get-CsUCPhoneConfiguration

Identity             : Global
CalendarPollInterval : 00:03:00
EnforcePhoneLock     : True
PhoneLockTimeout     : 00:10:00
MinPhonePinLength    : 6
SIPSecurityMode      : High
VoiceDiffServTag     : 40
Voice8021p           : 0
LoggingLevel         : Off

• Run the Set-CsUCPhoneConfiguration cmdlet to specify the desired value for DSCP stamping or to disable the behavior by setting a value of 0.

Set-CsUCPhoneConfiguration -Identity global -VoiceDiffServTag 0

DiffServ Classifications

Before looking at the actual traffic it would be prudent to quickly review the different DiffServ classifications which are applicable to prioritizing media in Lync.

• Default is what unmarked traffic is classified as (using a null DSCP value) and is delivered on a best-effort bases just the same as all other non-prioritized traffic.

• Class Selector (CS) is a grouping of values that are backwards compatible with the pre-DiffServ method of prioritizing traffic called Type of Service (ToS).  Multiple Precedence Levels are defined as per RFC791 and the following table shows the DSCP decimal values for each level.  Note how DSCP 40 is categorized as this will be discussed shortly.

Precedence Level	DiffServ Marking	ToS Name	Description
7	DSCP 56 (CS7)	Network Control	Reserved for IP routing protocols
6	DSCP 48 (CS6)	Internetwork Control	Reserved for IP routing protocols
5	DSCP 40 (CS5)	CRITIC/ECP	Express Forwarding (EF)
4	DSCP 32 (CS4)	Flash Override	Class 4
3	DSCP 24 (CS3)	Flash	Class 3
2	DSCP 16 (CS2)	Immediate	Class 2
1	DSCP 8 (CS1)	Priority	Class 1
0	DSCP 0 (Default)	Routine	Best Effort

• Assured Forwarding (AF) is a group of 12 different levels of prioritization that assures delivery as long as this traffic does not exceed defined rates, as anything that exceeds this rate has an increased probability of being dropped.  The table below shows the multiple classes available to assign different traffic across the varying levels of prioritization available with AF.  The DSCP decimal values are also identified for each AF classification as defined by RFC2597.

Drop Precedence	Class 1	Class 2	Class 3	Class 4
Low	DSCP 10 (AF11)	DSCP 18 (AF21)	DSCP 26 (AF31)	DSCP 34 (AF41)
Medium	DSCP 12 (AF12)	DSCP 20 (AF22)	DSCP 28 (AF32)	DSCP 36 (AF42)
High	DSCP 14 (AF13)	DSCP 22 (AF23)	DSCP 30 (AF33)	DSCP 38 (AF43)

• Expedited Forwarding (EF) is a single-value classification that is often granted the highest priority (below classification reserved for routing protocol traffic) and should be controlled through bandwidth management and call admission control policies on the network.  This class is ideal for real-time media as it provides a mechanism for reduced loss, delay, and jitter.  Many networks will already be controlling EF traffic as well as limiting the total amount of bandwidth that EF traffic can consume on the network as if too much traffic is stamped with EF then assured delivery can become compromised.  RFC2598 allocates a single DSCP value for all traffic in this classification as decimal 46 (hexadecimal 2E, binary 101110).

One common point of confusion in the field is that Microsoft has always defaulted to using a DSCP value of 40 in practice and their documentation with OCS and Lync, and in some places this is referred to simply as ‘EF”, yet a DSCP value of 46 is also defined as ‘EF’.  The mix-up here is because value a decimal DSCP value of 40 denotes Class Selector 5 (CS5) which falls into Precedence Level 5, also known as Express Forwarding (EF).  So the acronym ‘EF’ could mean either the Express Forwarding Precedence Level (DSCP 40) or the Expedited Forwarding classification (DSCP 46) depending on the context used.

Validating Default Media Traffic

A peak at traffic captured between a CX3000 (192.168.103.104) and the Lync Server during a conference call shows that the media is stamped with a DSCP hexadecimal value of 0×28, which is equal to 40 in decimal.  The binary value of 101000 is also shown in the DiffServ field data.  Additional information shown in the IP header should also look familiar, like “Class Selector 5”.

But the same media traffic captured from a Windows 7 Lync client (192.168.103.10) shows a DSCP value of 0 (don’t even ask if that is in hex or decimal ).

In the event that all traffic should be prioritized equally, be it from the soft-client or the phones then either remove tagging on the device configuration policy, or enable tagging on the Windows clients.  Clearly one approach would treat all media traffic as best-effort (0) while the other would provide a level of Quality of Service delivery, with the assumption that the network equipment is configured to prioritize DSCP 40 traffic (or whichever value is desired).

The Enabling Quality of Service with Microsoft Lync Server 2010 white paper goes into details on how to manage QoS tagging for all Lync components.