4.1.1 The IntServ Approach

Integrated Services provides Per-Flow guarantees. It treats the network like a circuit-switched telephone system, where a path is "reserved" before data starts flowing.

Study Deep: The Scalability Bottleneck

Why don't we use IntServ for the whole Internet?

• State Explosion: If a core router has 1,000,000 active flows (typical for a 100G link), it must maintain 1,000,000 RSVP timers. If the refresh timer is 30s, the router must process 33,000 RSVP packets every second just to keep the status quo.
• Alternative: This is why DiffServ was created—to group those 1,000,000 flows into just 4 or 8 "classes" of traffic.

Core Components Overview:

1. Packet Classifier: Directs incoming packets into the correct "bucket" based on their 5-tuple (Src IP, Dst IP, Proto, Src Port, Dst Port).
2. Packet Scheduler: Manages the output queues (e.g., using WFQ) to ensure each flow meets its deadline.
3. Admission Control: The gatekeeper that checks if surplus capacity exists.
4. Policy Control: Checks if the user has the administrative right to make a reservation.

4.1.2 The RSVP Signaling Flow

Unlike routing protocols which are sender-driven, RSVP is Receiver-Oriented.

The Process:

1. PATH Message: Sent by the sender to describe the traffic characteristics using $Tspec$ (Traffic Specification).
2. RESV Message: Sent by the receiver back towards the sender. It contains the $Rspec$ (Reservation Specification) defining the desired QoS.
3. Soft State: Reservations are temporary; they must be refreshed periodically (e.g., every 30s) or they time out.

4.1.3 IntServ Service Classes

• Guaranteed Service (RFC 2212): For applications requiring firm delay bounds. No packet loss due to queue overflow.
• Controlled Load Service (RFC 2211): Provides a service level similar to an "unloaded" network, even if the actual link is loaded.

4.1.4 The Implementation Challenge

While mathematically elegant, IntServ failed to scale to the global Internet.

• CPU Overhead: Routers must run the RSVP state machine for millions of individual flows.
• Memory Overhead: Per-flow state storage ($O(N)$).
• Complexity: Every router in the path must support RSVP.