Windows 7 / Networking

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Selecting DSCP Values

When sending packets, computers add a DSCP value that your network infrastructure can examine to determine how the packet should be prioritized. Although DSCP values can be arbitrary depending on how your network infrastructure is configured, many organizations use a typical DSCP strategy with the following five queues:

• Control traffic Communications transmitted between routers. Typically, these communications require minimal bandwidth, but they should be assigned a high priority because quick transmission can reduce downtime in the event of a hardware failure. You should also use this priority for VoIP control traffic. Use DSCP values of 25 for control traffic.
• Latency-sensitive traffic Traffic, such as VoIP, that must be delivered as quickly as possible. Typically, you should assign this a DSCP value of 46, known as Expedited Forwarding (EF).
• Business critical traffic, also known as Better than Best Effort (BBE) Communications that should receive priority treatment, such as customer service database queries from a line-of-business (LOB) application or streaming video, but that are not highly sensitive to latency. Use a DSCP value of 34.
• Best-effort (BE) traffic Standard traffic, including any traffic not marked with a DSCP number, that should be handled after either of the preceding two queues. This traffic should have a DSCP value of 0, which is the default if no DSCP value is specified.
• Scavenger traffic Low-priority traffic, such as backups, downloading of updates, noncritical file synchronization, and non-work-related traffic that employees might generate. Use a DSCP value of 10 or 8.

Note If you mark traffic from too many applications as high priority, the high-priority queue on routers can grow long enough to add significant latency. This defeats the purpose of QoS. Therefore, you should reserve the highest-priority DSCP marking for real-time communications, such as VoIP.

Table below summarizes these values.

DSCP Interoperability Values

Purpose	Common Use	DSCP Value
VoIP	VoIP traffic, including signaling and control traffic	46
Interactive video	Two-way video conferencing	34
Mission-critical data	Database queries, LOB communications, video streaming	25
Best effort	All other traffic, including e-mail and Web browsing	0
Bulk data	Backups, nonbusiness applications, file transfers	10

Many networks use an even simpler structure with only two priorities: one for latencysensitive traffic and another for BE traffic. However, if you have third-party tools that can use DSCP values to report on network performance for different types of traffic, it is advantageous to define a larger number of DSCP values even if your network infrastructure isn't configured to handle each DSCP value uniquely.

DSCP values might be lost when packets leave your network because most organizations do not trust priorities provided by computers outside the organization. Because sending traffic labeled as high priority can create a denial-of-service (DoS) attack, DSCP values from untrusted computers might be malicious.

Wireless Multimedia (WMM) includes four access categories for prioritizing traffic on 802.11 wireless networks. WMM uses DSCP values to set the priority, so you can take advantage of WMM automatically by specifying DSCP values. Table below shows how DSCP values correspond to WMM access categories.

DSCP Values and WMM Access Categories

DSCP Value	WMM Access Category
48-63	Voice (VO)
32-47	Video (VI)
24-31, 0-7	Best effort (BE)
8-23	Background (BK)

To support fully prioritizing traffic based on DSCP values, your network infrastructure must support the use of multiple queues as defined in RFC 2474.