System Administration Guide: IP Services

Using the dscpmk Marker for Forwarding Packets

The marker receives traffic flows after the flows are processed by the classifier or by the metering modules. The marker marks the traffic with a forwarding behavior. This forwarding behavior is the action to be taken on the flows after the flows leaving the IPQoS system. Forwarding behavior to be taken on a traffic class is defined in the per-hop behavior (PHB). The PHB assigns a priority to a traffic class, which indicates the precedence flows of that class in relation to other traffic classes. PHBs only govern forwarding behaviors on the IPQoS system's contiguous network. For more information on PHBs, refer to Per-Hop Behaviors.

Packet forwarding is the process of sending traffic of a particular class to its next destination on a network. For a host such as an IPQoS system, a packet is forwarded from the host to the local network stream. For a Diffserv router, a packet is forwarded from the local network to the router's next hop.

The marker marks the DS field in the packet header with a well-known forwarding behavior that is defined in the IPQoS configuration file. Thereafter, the IPQoS system and subsequent Diffserv-aware systems forward the traffic as indicated in the DS field until the mark changes. To assign a PHB, the IPQoS system marks a value in the DS field of the packet header. This value is called the differentiated services codepoint (DSCP). The Diffserv architecture defines two types of forwarding behaviors, EF and AF, which use different DSCPs. For overview information about DSCPs, refer to DS Codepoint.

The IPQoS system reads the DSCP for the traffic flow and evaluates the flow's precedence in relation to other outgoing traffic flows. The IPQoS system then prioritizes all concurrent traffic flows and releases each flow onto the network by its priority.

The Diffserv router receives the outgoing traffic flows and reads the DS field in the packet headers. The DSCP enables the router to prioritize and schedule the concurrent traffic flows. The router forwards each flow by the priority that is indicated by the PHB. Note that the PHB cannot apply beyond the boundary router of the network unless Diffserv-aware systems on subsequent hops also recognize the same PHB.

Expedited Forwarding (EF) PHB

Expedited forwarding (EF) guarantees that packets with the recommended EF codepoint 46 (101110) receive the best treatment that is available on release to the network. Expedited forwarding is often compared to a leased line. Packets with the 46 (101110) codepoint are guaranteed preferential treatment by all Diffserv routers en route to the packets' destination. For technical information about EF, refer to RFC 2598, An Expedited Forwarding PHB.

Assured Forwarding (AF) PHB

Assured forwarding (AF) provides four different classes of forwarding behaviors that you can specify to the marker. The next table shows the classes, the three drop precedences that are provided with each class, and the recommended DSCPs that are associated with each precedence. Each DSCP is represented by its AF value, its value in decimal, and its value in binary.

Table 37–2 Assured Forwarding Codepoints


Class 1 

Class 2 

Class 3 

Class 4 

Low-Drop Precedence

AF11 = 

10 (001010) 

AF21 = 

18 (010010) 

AF31 = 

26 (011010) 

AF41 = 

34 (100010) 

Medium-Drop Precedence

AF12 = 

12 (001100) 

AF22 = 

20 (010100) 

AF32 = 

28 (011100) 

AF42 =  

36 (100100) 

High-Drop Precedence

AF13 = 

14 (001110) 

AF23 = 

22 (010110) 

AF33 = 

30 (011110) 

AF43 = 

38 (100110) 

Any Diffserv-aware system can use the AF codepoint as a guide for providing differentiated forwarding behaviors to different classes of traffic.

When these packets reach a Diffserv router, the router evaluates the packets' codepoints along with DSCPs of other traffic in the queue. The router then forwards or drops packets, depending on the available bandwidth and the priorities that are assigned by the packets' DSCPs. Note that packets that are marked with the EF PHB are guaranteed bandwidth over packets that are marked with the various AF PHBs.

Coordinate packet marking between any IPQoS systems on your network and the Diffserv router to ensure that packets are forwarded as expected. For example, suppose IPQoS systems on your network mark packets with AF21 (010010), AF13 (001110), AF43 (100110), and EF (101110) codepoints. You then need to add the AF21, AF13, AF43, and EF DSCPs to the appropriate file on the Diffserv router.

For a technical explanation of the AF codepoint table, refer to RFC 2597. Router manufacturers Cisco Systems and Juniper Networks have detailed information about setting the AF PHB on their web sites. You can use this information to define AF PHBs for IPQoS systems as well as routers. Additionally, router manufacturers' documentation contains instructions for setting DS codepoints on their equipment.

Supplying a DSCP to the Marker

The DSCP is 6 bits in length. The DS field is 1 byte long. When you define a DSCP, the marker marks the first 6 significant bits of the packet header with the DS codepoint. The remaining 2 least-significant bits are unused.

To define a DSCP, you use the following parameter within a marker action statement:


The dscp_map parameter is a 64-element array, which you populate with the (DSCP) value. dscp_map is used to map incoming DSCPs to outgoing DSCPs that are applied by the dscpmk marker.

You must specify the DSCP value to dscp_map in decimal notation. For example, you must translate the EF codepoint of 101110 into the decimal value 46, which results in dscp_map{0-63:46}. For AF codepoints, you must translate the various codepoints that are shown in Table 37–2 to decimal notation for use with dscp_map.