DisplayPort Daisy Chain HDCP Repeater Rules

When broadcasting protected digital media across a DisplayPort daisy chain using Multi-Stream Transport (MST), intermediate displays act as High-bandwidth Digital Content Protection (HDCP) repeaters. Under HDCP compliance standards, these repeaters must enforce strict downstream topology, authentication, and content-stream management rules to prevent unauthorized interception of encrypted video streams.

The Role of an HDCP Repeater

In a DisplayPort daisy chain, only the final monitor functions purely as a standard HDCP receiver (sink). Any intermediate monitor that receives an encrypted DisplayPort stream and passes it along to another display via DisplayPort Out is classified as an HDCP repeater. The primary rule governing an HDCP repeater is that it must decrypt the upstream content, manage the streams, and immediately re-encrypt the data before forwarding it to downstream devices.

Downstream Topology and Device Limits

An HDCP repeater must discover and report the physical structure of all downstream devices back to the primary transmitter (the host PC or media player):

  • Depth Limit: The repeater checks the tree depth of the chain. HDCP standards limit topology depth to a maximum of 7 levels.
  • Device Count: The repeater compiles a list of all downstream receiver IDs (RxIDs in HDCP 2.x) or Key Selection Vectors (KSVs in HDCP 1.x). The total number of connected devices across the entire topology cannot exceed the standard limit (typically up to 31 devices for HDCP 1.x or up to 127 devices for HDCP 2.x).
  • Topology Exceeded Handling: If the downstream chain exceeds either the depth or device count limit, the repeater must report a topology error to the upstream transmitter, which will block or terminate the transmission of protected content.

Downstream Authentication and Stream Propagation

A repeater cannot transmit encrypted media to a downstream branch until that specific downstream link has successfully completed the HDCP handshake:

  1. Downstream Authentication: The repeater initiates an independent HDCP authentication handshake with each attached downstream receiver.
  2. Upstream Reporting: Once downstream devices are authenticated, the repeater transmits their IDs to the upstream transmitter for validation against system revocation lists (SRLs).
  3. Independent Re-Encryption: After upstream authorization, the repeater decrypts incoming MST packets and re-encrypts the designated stream specifically for the target downstream device.

Content Stream Management and Version Compatibility

HDCP repeaters must enforce content-type restrictions (HDCP Type 0 vs. Type 1):

  • Type 0 Content: Can be transmitted to downstream devices supporting either HDCP 2.x or legacy HDCP 1.x, provided the repeater converts the encryption scheme appropriately.
  • Type 1 Content: Strict premium content (such as 4K UHD HDR media) requires end-to-end HDCP 2.2 or higher. An HDCP repeater is strictly prohibited from transmitting Type 1 content across any downstream link that negotiates down to HDCP 1.x.

If any downstream device in the chain fails to authenticate, is revoked, or lacks the necessary HDCP version required for the content stream, the repeater must block the transmission of that stream to the non-compliant link while maintaining authorized streams to other compliant displays in the chain.