Similar to other high-speed serial interfaces operating at gigabits-per-second, USB SuperSpeed (SS) defines port-to-port protocols to ensure the reliable and efficient transfer of protocol packets across each link.

Key characteristics of SS port-to-port operation:

• Packet error checks are performed at every link.
• A packet must be successfully transferred on the current link before it is forwarded to the next link.
• These protocols are fundamentally different from the USB 2.0 Token/Data/Handshake model and are not directly comparable.

Core Features of Port-to-Port Protocols

The SS port-to-port protocol is built around three primary features:

• Link Flow Control
• Link Error Detection and Recovery
• Link Power Management

Link Flow Control

• Uses a credit-based flow control mechanism.
• Tracks buffer availability at the link partner.
• Operates bidirectionally, with each link partner advertising available credits to the other.
• Prevents buffer overflow and ensures smooth packet delivery.

Link Error Detection and Recovery

• Verifies that each header packet is transferred successfully across the link.
• Ensures in-order delivery of header packets.
• Error handling behaviour depends on the error type:
  • Automatic retries until successful transfer, or
  • Link retraining after a specified timeout period.