Default DSCP Mappings

Table 1 shows the mapping of DiffServ service class meanings (aliases) to DSCPs.

None of the aliases are established by DiffServ specifications. The aliases are well-known only through usage. For example, it is widely accepted that the alias for DSCP 101110 is ef (expedited forwarding). The 21 well-known DSCPs establish 5 DiffServ service classes:

• Best-effort (be)—The router does not apply any special CoS handling to packets with 000000 in the DiffServ field, a backward compatibility feature. There is usually a high probability that these packets will be dropped under congested network conditions.

• Assured forwarding (af)—The 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 (the service provider defines the values). The router accepts excess traffic, but applies a random early detection (RED) drop profile to decide if the excess packets should be dropped and not forwarded. Three drop probabilities (low, medium, and high) are defined for this service class.

• Expedited forwarding (ef)—The 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.

• Conversational services (cs)—The router delivers assured (usually low) bandwidth with low delay and jitter for packets in this service class. Packets can be dropped, but never delivered out of sequence. Packetized voice is a good example of a conversational service.

• Network control (nc)—The 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 and loss of these packets jeopardizes proper network operation, so delay is preferable to discard.