VC & Traffic Class Arbitration

1. Traffic Classes (TC)

What are Traffic Classes?

Traffic Classes (TC) are 3-bit labels (TC0-TC7) in TLP headers that indicate the priority or type of traffic. They enable differentiated QoS handling throughout the PCIe fabric.

Traffic Class Values

TC	Description	Typical Use
TC0	Best Effort (Default)	General traffic
TC1	Best Effort (Low Priority)	Background traffic
TC2-TC3	Best Effort (Higher Priority)	Elevated priority
TC4-TC5	Video/Streaming	Low latency traffic
TC6	Controlled Load	Guaranteed bandwidth
TC7	Network Control	Highest priority

TC in TLP Header

3-bit field in TLP header (bits 6:4 of byte 1 in non-Flit)
Set by Requester based on software configuration
Preserved end-to-end through the fabric

2. Virtual Channels (VC)

What are Virtual Channels?

Virtual Channels (VC) are independent logical transmission pathways within a PCIe link. Each VC has separate flow control and buffering, enabling traffic isolation and differentiated QoS.

VC Characteristics

VC0: Default VC, always present
VC1-VC7: Additional VCs (optional)
Each VC has independent flow control credits
VCs provide traffic isolation

TC to VC Mapping

    Traffic Classes          Virtual Channels
    
    TC0 ─────────────────────► VC0 (always)
    TC1 ─────────────────┬───► VC0 (default)
    TC2 ────────────────┬┤
    TC3 ───────────────┬┤├───► VC1 (optional)
    TC4 ──────────────┬┤├┤
    TC5 ─────────────┬┤├┤├───► VC2 (optional)
    TC6 ────────────┬┤├┤├┤
    TC7 ───────────┬┤├┤├┤├───► VC7 (optional)
                   │││││││
                   └┴┴┴┴┴┴───► TC/VC Mapping Table
    
    Configuration determines which TCs map to which VCs
    TC0 MUST always map to VC0
    Other TCs can map to any enabled VC

3. Arbitration Levels

Two-Level Arbitration

    ┌────────────────────────────────────────────────────────┐
    │                      PORT ARBITER                      │
    │                                                        │
    │    VC0 ──┐                                            │
    │          ├──► Port Arbitration ──► Link Egress        │
    │    VC1 ──┤                                            │
    │          │                                            │
    │    ...   │                                            │
    │          │                                            │
    │    VC7 ──┘                                            │
    │                                                        │
    └────────────────────────────────────────────────────────┘
                            ▲
                            │
    ┌────────────────────────────────────────────────────────┐
    │                  VC ARBITER (per VC)                   │
    │                                                        │
    │    Port A (TC x) ──┐                                  │
    │                    ├──► VC Arbitration ──► VC Queue   │
    │    Port B (TC y) ──┤                                  │
    │                    │                                  │
    │    Port C (TC z) ──┘                                  │
    │                                                        │
    └────────────────────────────────────────────────────────┘
    
    Level 1: VC Arbitration - selects TLP within each VC
    Level 2: Port Arbitration - selects which VC to transmit

VC Arbitration

Arbitrates among TLPs within a single VC from multiple ingress ports:

Round Robin (RR)
Weighted Round Robin (WRR)
Implementation-specific

Port Arbitration

Arbitrates among VCs for access to the link egress:

Strict Priority
Round Robin
Weighted Round Robin
Time-based Weighted Round Robin

4. Arbitration Algorithms

Strict Priority

Higher priority VC always wins
Lower priority can starve under load
Simple implementation

Round Robin (RR)

Each VC gets equal opportunity
Fair but no differentiation
No starvation

Weighted Round Robin (WRR)

Each VC assigned a weight
Weight determines proportion of bandwidth
Higher weight = more bandwidth

    WRR Example (Weights: VC0=4, VC1=2, VC2=1, VC3=1)
    
    Round 1: VC0, VC0, VC0, VC0
    Round 2: VC1, VC1
    Round 3: VC2
    Round 4: VC3
    (repeat)
    
    Bandwidth: VC0=50%, VC1=25%, VC2=12.5%, VC3=12.5%

Time-Based WRR (TBWRR)

Weights expressed in time units
Better for isochronous traffic
More precise bandwidth allocation

5. VC Capability Structure

VC Extended Capability

Offset	Register
00h	Extended Capability Header (ID = 0002h)
04h	Port VC Capability Register 1
08h	Port VC Capability Register 2
0Ch	Port VC Control Register
0Eh	Port VC Status Register
10h+	VC Resource Capability/Control/Status (per VC)

Key Capability Fields

Extended VC Count: Number of VCs beyond VC0
Low Priority Extended VC Count: Low priority VCs
Reference Clock: For time-based arbitration
Port Arbitration Table Entry Size: Arbitration table size

6. Isochronous Traffic

What is Isochronous Traffic?

Time-sensitive traffic requiring guaranteed bandwidth and bounded latency:

Audio/video streaming
Real-time applications
Industrial control

Isochronous VC Requirements

Dedicated VC for isochronous traffic
Time-based arbitration
Reserved bandwidth allocation
Strict latency bounds

7. System Configuration

Configuring TC/VC Mapping

Enumerate VC capabilities of all devices
Determine required VCs and TC mappings
Configure TC/VC Map field in each device
Configure arbitration tables if WRR used
Enable VCs

Arbitration Table Programming

Table defines arbitration schedule
Each entry specifies which VC wins that round
Table loaded into Port Arbitration Table

Important: TC/VC Mapping Consistency

TC/VC mapping must be consistent across the entire path from Requester to Completer. Inconsistent mapping can cause traffic to use different VCs at different points, breaking QoS guarantees.