The Internetwork Protocol (IP) [RFC791] defined a best effort network layer service for connecting computers to form a computer network. This original specification defined an IP header field, known as the "Type of Service" (ToS). This is no longer the recommended use for this field, and it has been replaced by a new usage called the DSCP value. (For historical reference the bottom of this page includes a description of this legacy method).
The DSCP is defined in RFC 2474 and 2475. It is a method that classifies the way an IP packet is queued while waiting to be forwarded within a router. Strictly speaking this defines network policies and rules, not the priority of individual packets (this is decided by the router itself). A router or end host assigns one of 64 possible forwarding behaviors known as Per Hop Behaviors (PHBs). A PHB provides a particular service level (bandwidth, queuing, and dropping decisions) in accordance with network policy. Using DSCP, a system (or router) can assign Assured forwarding (AF) codepoints, using one of four AF classes and one of three drop precedence values. It is also allowed to eneter your own non-standard codepoints.
The values assigned to the set of DSCP tags are backwards compatible with the legacy ToS IP precedence bits; so non-DSCP compliant, ToS-enabled devices should not conflict with the DSCP mapping. A default DSCP mapping to "best effort" forwarding will be applied if network rules are not defined or the devices are non-DSCP compliant.
LAN that support priority at Layer 2 (e.g. Ethernet LANs that support the Tag field) can use the DSCP value to select an appropriate priority in the header of frames sent over a specific LAN. This mapping is normally perfomed by a router.
This was an early tagging method that classifies and prioritizes packets based on a hierarchy of different classes of traffic (following common practice in military networks). When these packets are sent out on the network, the higher priority packets are transferred first. Using Priority Packet, Legacy IP Precedence tagging lets you assign the following specific priority levels:
|ToS Value||ToS Description|
|4 (100)||Flash Override|
|6 (110)||Internet Control|
|7 (111)||Network Control|
J. Postel Internet Protocol.(STD0005) RFC791
IP Packet Header
IP Packet Processing
Grossman, D., "New Terminology and Clarifications for Diffserv", RFC 3260, April 2002.
Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.
Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z., and W. Weiss, "An Architecture for Differentiated Service", RFC 2475, December 1998.
Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski, "Assured Forwarding PHB Group", RFC 2597, June 1999.
Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec, J., Courtney, W., Davari, S., Firoiu, V., and D. Stiliadis, "An Expedited Forwarding PHB (Per-Hop Behavior)", RFC 3246, March 2002.