DiffServ

DiffServ, short for Differentiated Services, is a quality-of-service (QoS) architecture for IP networks that aims to provide scalable, per-hop treatment of packets by classifying and marking traffic rather than reserving resources end-to-end. It defines a Differentiated Services Field in the IP header and a set of Per-Hop Behaviors (PHBs) that routers should implement.

In DiffServ, edge devices classify packets and assign a DSCP (Differentiated Services Code Point) value in the IP header. Core routers then use that value to enforce a chosen PHB. The design emphasizes scalable, intra-domain QoS without signaling reservations, and it can work with or without Explicit Congestion Notification (ECN) bits in the same field.

The main PHBs commonly used are Expedited Forwarding (EF), intended for low-loss, low-latency traffic such as voice; Assured Forwarding (AF) with multiple classes and drop precedence levels; and Best Effort (BE), which corresponds to standard, non-marked traffic. AF provides finer granularity through code points such as AF11, AF21, AF31 and their higher-priority variants; EF maps to a high-priority, pre-defined DSCP value.

DiffServ was specified in standards such as RFC 2474 (Differentiated Services Field) and RFC 2475 (Architectural Framework), with ECN specified in RFC 3168. It offers a scalable alternative to resource reservation protocols like RSVP by providing a policy-based mechanism for traffic treatment across routers up to the edge of a domain.

Deployment considerations include the need for consistent policy and cooperative device support across network boundaries, risks from mis-marked traffic and middleboxes that do not honor DSCP values, and limitations in guaranteeing bandwidth without end-to-end coordination.