QOS for videoconferencing

Notes on QOS for videoconferencing

Quality of Service headers are added to network packet headers by many video applications and appliances to allow time-critical video and voice data to receive priority treatment by network switches and routers.
Whether they actually do receive such priority is another matter.

To make matters more complicated, in 1998 the Quality of Service field in the Internet Protocol header was redefined to be the "Differentiated Services Field" instead of the older "Type of Service" field, i.e. redefining the same header bits to have a different meaning. So, older equipment and software refers to TOS, while newer equipment refers to DiffServ, or offers a choice of both. Additionally, Ethernet frames may have a Class of Service field set in VLAN tagged frames.

Example: LifeSize Express

This offers default DiffServ and IntServ settings

Service	DiffServ	IntServ
Video	34	4
Audio	46	5
Data	46	5

Example: Polycom Viewstation FX

This offers a choice of DiffServ and IP Precedence. I'm not sure if there are any defaults.

Service	Value
Video	-
Audio	-
Far End camera control	-

Mapping

Avaya (was Nortel) define mappings between DSCP codepoints and IEEE 802.1p levels, as follows:

Name	DSCP	PCP	priority	802.1p Acronym	Traffic
Standard	0 0x0	1	0 (low)	BK	Background
Bronze	10 0xa	2	2	EE	Excellent Effort
Silver	18 0x12	3	3	CA	Critical
Gold	26 0x1a	4	4	VI	Video
Platinum	34 0x22	5	5	VO	Voice
Premium	46 0x2e	6	6	IC	Control
Network	48 0x30	7 (high)	7	NC	Control

Lifesize, Avaya (9,10) suggest the use of DSCP codepoint 46 (0b101110), expedited forwarding PHB for VoIP. I have used this for the audio channel, and either 26 or 34 for video. One reference suggests 26 for streaming video, and 34 for interactive.
Juniper (16) use codepoint 46 for voice and 34 for video in the "EZQoS" template.

Switches

Once DSCP fields have been set in a video appliance, network packets sent from it should be tagged. This can be seen in monitor tools such as tcpdump (with -v, as "tos"), or Wireshark (as Differentiated Services under Internet Protocol). However, these headers are often stripped off by network switches, and almost certainly on the Internet. On a Avaya (Nortel) switch, ports may be marked as "trusted" or "untrusted". If a port is untrusted, the headers are stripped off. For a packet to traverse the LAN with DSCP intact, the ports on each switch and uplink along the path must be "trusted". In Avaya CLI, the following exmple sets ports to trusted, with a new class called "video":
qos if-group name video class trusted qos if-assign port 4/2,4/6,4/10,5/16,5/20 name video

Path Testing

In the absence of a suitable video appliance, the Linux tool "netcat" may be used to send packets with a particular TOS value. E.g.
nc -u -T 0x8 192.168.3.4 31224
may be used to send tagged UDP packets to a local node port 31224. The command nc -l -u 31224 may be used to listen, or the packets merely observed with a monitor tool.

Software Applications

In principle, it is the responsibility of a software client to set DSCP headers on audio/video packets using e.g. setsockopt(). However, many clients do not, and of those that do, some are still using TOS not DSCP (e.g. Linux 2.6 ip.h defines IPTOS_TOS). It may be possible to add or rewrite DSCP headers in the operating system. E.g. the Linux command
iptables -t mangle -A OUTPUT -p udp --sport 46014:46015 -j DSCP --set-dscp 0x2e
tags outgoing UDP packets on ports 46014,46015 with codepoint 46, for a particular application (EVO) which sends audio/video data on those ports.

ADAC

Newer Avaya/Nortel switches implement auto-discovery for compatible VoIP phones. With this feature enabled, DSCP and PoE may automatically be set when a phone is connected.

Links:

[1]Type of Service (TOS) (wikipedia)
[2]RFC 1349 Type of Service (TOS)
[3]Differentiated services (DSCP) (wikipedia)
[4]RFC 2474 Differentiated Services Field
[5]RFC 3168 Explicit Congestion Notification (ECN)
[6]RFC 2598 Expedited Forwarding PHB
[7]IEEE_P802.1p - class of service in Ethernet (wikipedia)
[8]Nortel Automatic QoS Technical Configuration Guide (PDF)
[9]LifeSize 230 - Configuring QOS
[10]QoS Recommendations for Avaya VoIP Phones (Cisco blog)
[11]Implementing Quality of Service Policies with DSCP (Cisco)
[12]Dan Bricklin's take on QOS
[13]Internet2 Qbone report - Why Premium IP Service Has Not Deployed
[14]Quality of Service Design Overview (Cisco)
[15]RFC 4594 Guidelines for DiffServ Service Classes
[16]Configuring JUNOS EZQoS for CoS

Glossary

PHB	Per-Hop Forwarding Behaviour
DSCP	Differentiated services Code Point
VoIP	Voice over Internet Protocol
IP	Internet Protocol
QOS	Quality of Service
TOS	Type of Service
CoS	Class of Service
DiffServ	Differentiated service
ECN	Explicit Congestion Notification
RFC	Request for Comments (IETF published standard)
IETF	Internet Engineering Task Force
ADAC	Auto-Discovery/Auto-Configuration (of IP phone)
LLDP	Link Layer Detection Protocol
LLDP-MED	Link Layer Detection Protocol - Media Endpoint Discovery
PoE	Power over Ethernet

March 2010 A.Daviel