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DSCPs and Forwarding Service Classes

DiffServ specifications establish a 6-bit field in the IP packet header to indicate the forwarding service class to apply to the packet. The bit values in the DiffServ field form DiffServ code points (DSCPs) that can be set by the application or by a Services Router on the edge of a DiffServ-enabled network.

Each DiffServ forwarding service class has a well-known name and alias. Although not part of the specifications, the aliases are well known through usage. For example, the alias for DSCP 101110 is widely accepted as ef (expedited forwarding).

The 21 well-known DSCPs establish five DiffServ service classes. Table 132 identifies the forwarding service classes and aliases that correspond to the 21 DSCPs.

Table 132: Default Forwarding Service Class-to-DSCP Mapping

DiffServ Service Class Alias	IP DSCP	Forwarding Service Class and Use
ef	101110	Expedited forwarding—The Services Router delivers assured bandwidth, low loss, low delay, and low delay variation (jitter) end-to-end for packets in this service class. Routers accept excess traffic in this class, but in contrast to assured forwarding, out-of-profile expedited-forwarding packets can be forwarded out of sequence or dropped.
af11	001010	Assured forwarding—The Services Router offers a high level of assurance that the packets are delivered as long as the packet flow from the customer stays within a certain service profile that you define. The router accepts excess traffic, but applies a random early discard (RED) drop profile to decide if the excess packets are dropped and not forwarded. Three drop probabilities (low, medium, and high) are defined for this service class.
af12	001100
af13	001110
af21	010010
af22	010100
af23	010110
af31	011010
af32	011100
af33	011110
af41	100010
af42	100100
af43	100110
be	000000	Best-effort—The Services Router does not apply any special CoS handling to packets with 000000 in the DiffServ field, a backward compatibility feature. These packets are usually dropped under congested network conditions.
cs1	001000	Conversational services—The Services Router delivers assured (usually low) bandwidth with low delay and jitter for packets in this service class. Packets can be dropped, but are never delivered out of sequence. Packetized voice is a good example of a conversational service.
cs2	010000
cs3	011000
cs4	100000
cs5	101000
nc1/cs6	110000	Network control—The Services Router delivers packets in this service class with a low priority. (These packets are not delay sensitive.) Typically, these packets represent routing protocol hello or keepalive messages. Because loss of these packets jeopardizes proper network operation, delay is preferable to discard. (See also the conversational services description in this table.)
nc2/cs7	111000

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