QoS (Quality of Service)

QoS (Quality of Service) regroupe les mécanismes réseau permettant de garantir performance différenciée selon les types de trafic — priorité, bande passante minimum, latence maximum, jitter contrôlé. Essentiel pour voice (VoIP), video (streaming, conferencing), gaming, applications critiques business.

Composants QoS :
(1) **Classification** — identifier le type de trafic (ACL, NBAR Cisco, application identification). Marquer avec DSCP (Differentiated Services Code Point, 6 bits dans IPv4 ToS / IPv6 Traffic Class), CoS (Class of Service, 802.1p en Ethernet), MPLS EXP.
(2) **Marking** — assigner valeur DSCP (EF=46 voice, AF41=34 video, CS6=48 control, DF=0 best-effort).
(3) **Queuing** — multiple queues par priorité (Priority Queuing, Weighted Fair Queuing, CBWFQ, LLQ).
(4) **Congestion management** — WRED (Weighted Random Early Detection) drop preventif.
(5) **Shaping** — buffer excès trafic, smooth bursts.
(6) **Policing** — drop excès si dépassement contracted rate.

DiffServ (Differentiated Services) — model dominant : per-hop behavior based on DSCP. PHBs : EF (Expedited Forwarding pour voice), AF (Assured Forwarding 4 classes × 3 drop precedences), DF (Default).

IntServ (Integrated Services) — model RSVP-based per-flow reservation, rarely deployed (scalability).

Use cases : enterprise WAN avec voice (Cisco UC, Microsoft Teams), datacenter east-west traffic prioritization, SD-WAN application-aware steering, ISPs differentiated services. Compétences CCNA, CCNP.