4.3.1 The DiffServ Philosophy

DiffServ (RFC 2475) moves complexity to the edge of the network. Core routers only look at a 6-bit DSCP (Differentiated Services Code Point) in the IP header.

Study Deep: DSCP Code Points

The 6-bit DSCP field replaced the older 3-bit IP Precedence. It allows for much finer control:

• EF (Expedited Forwarding): Code 46. "Strict Priority." Used for VoIP.
• AF (Assured Forwarding): Codes like 11, 12, 13... These allow for "Gold, Silver, Bronze" tiers where some packets are dropped before others during a crunch.
• CS (Class Selector): Used for backward compatibility with older routers.

The DS Field and Per-Hop Behaviors (PHB)

1. Expedited Forwarding (EF): "Hard" QoS. Low loss, low latency, guaranteed bandwidth. Perfect for VoIP.
2. Assured Forwarding (AF): "Soft" QoS. 4 classes (1-4) with 3 drop priorities each.

• Example: AF11 (Low drop), AF12 (Medium drop), AF13 (High drop).

1. Class Selector (CS): Backward compatibility with IP Precedence.

4.3.3 The Traffic Conditioner Pipeline

Conditioning happens at the ingress (entry) of the DiffServ domain:

1. Classifier: Selects packets based on rules.
2. Meter: Compares the stream to a Token Bucket ($r, b$).
3. Marker: Sets the DSCP bit (e.g., mark as AF11 if compliant, AF13 if non-compliant).
4. Shaper/Dropper: Delays cells or drops them to meet the SLA.

4.3.4 Token Bucket Math for DiffServ:

$B(t) = min(Bucket_Size, B(t-Delta t) + r Delta t)$ If $Packet_Size leq B(t)$, the packet is Green (compliant).