Windows 7 / Networking

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

PurposeCommon UseDSCP Value
VoIPVoIP traffic, including signaling and control traffic46
Interactive videoTwo-way video conferencing34
Mission-critical dataDatabase queries, LOB communications, video streaming25
Best effortAll other traffic, including e-mail and Web browsing0
Bulk dataBackups, nonbusiness applications, file transfers10

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

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In this tutorial:

  1. Configuring Windows Networking
  2. Usability Improvements
  3. Network And Sharing Center
  4. Network Explorer
  5. How Windows Finds Network Resources
  6. How Windows Publishes Network Resources
  7. How Windows Creates the Network Map
  8. Network Map
  9. Set Up A Connection Or Network Wizard
  10. Manageability Improvements
  11. Network Location Types
  12. Policy-Based QoS
  13. Selecting DSCP Values
  14. Planning Traffic Throttling
  15. Configuring QoS Policies
  16. Configuring System-Wide QoS Settings
  17. Configuring Advanced QoS Settings
  18. Testing QoS
  19. Windows Firewall and IPsec
  20. Windows Connect Now in Windows 7
  21. Core Networking Improvements
  22. Networking BranchCache
  23. How Hosted Cache Works
  24. How Distributed Cache Works
  25. Configuring BranchCache
  26. BranchCache Protocols
  27. File Sharing Using SMB
  28. Web Browsing with HTTP (Including HTTPS)
  29. DNSsec
  30. GreenIT
  31. Efficient Networking
  32. What Causes Latency, How to Measure It, and How to Control It
  33. TCP Receive Window Scaling
  34. Scalable Networking
  35. Improved Reliability
  36. IPv6 Support
  37. 802.1X Network Authentication
  38. Server Message Block (SMB) 2.0
  39. Strong Host Model
  40. Wireless Networking
  41. Improved APIs
  42. Network Awareness
  43. Improved Peer Networking
  44. Services Used by Peer-to-Peer Networking
  45. Managing Peer-to-Peer Networking
  46. Peer-to-Peer Name Resolution
  47. EAP Host Architecture
  48. Layered Service Provider (LSP)
  49. Windows Sockets Direct Path for System Area Networks
  50. How to Configure Wireless Settings
  51. Configuring Wireless Settings Manually
  52. Using Group Policy to Configure Wireless Settings
  53. How to Configure TCP/IP
  54. DHCP
  55. Configuring IP Addresses Manually
  56. Command Line and Scripts
  57. How to Connect to AD DS Domains
  58. How to Connect to a Domain When 802.1X Authentication Is Not Enabled
  59. How to Connect to a Domain When 802.1X Authentication Is Enabled