Quantum Datagram Control Protocol (QDCP) for IP Optical Environments

Introduction Hybrid quantum-classical networking is emerging as a foundation for distributed quantum information processing. Recent experiments on commercial fiber networks have shown that quantum states can be dynamically routed by classical headers embedded in IP-like packets. To configure downstream optical switches and mitigate errors, a lightweight, extensible protocol is needed. QDCP is intended to be that protocol, running over UDP and supporting modular Type-Length-Value (TLV) extensions. QDCP supports applications aligned with scenarios defined by the IRTF Quantum Internet Research Group (QIRG) . By the no-cloning theorem, quantum information cannot be copied, buffered, or retransmitted without disturbing the underlying state. In the present work, where practical quantum memories and error-corrected storage are not yet available at network scale, quantum information is therefore transmitted as a datagram: loss is terminal, and retransmission is physically meaningless. The accompanying classical control header is sent without guaranteed delivery. If the classical information is lost in transit, the associated quantum state is presumed lost as well. Future implementations may leverage advances in quantum memory, error correction, or entanglement-assisted repeaters to decouple classical and quantum reliability, potentially incorporating reliable classical transports such as QUIC or TCP for control-plane robustness.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Protocol Overview QDCP defines a fixed header followed by TLV-encoded fields. The header carries version and flag information; TLVs encode control- plane parameters such as quantum link layer protocol, polarization state, center frequency, or error-mitigation metadata. UDP provides transport simplicity and compatibility with existing IP infrastructure. Unknown TLVs MUST be ignored to ensure forward compatibility. While UDP imposes a maximum datagram length (65,535 bytes), this limitation has no impact on the amount of quantum information conveyed. The quantum payload is not encapsulated within the UDP packet itself but is passed through at the physical layer, with UDP carrying only the associated classical control header. Thus the UDP size constraint applies solely to the metadata, not to the optical or quantum state being transported.

QDCP Packet Format The QDCP packet consists of a fixed header followed by a sequence of Type-Length-Value (TLV) payloads. Packet Format:

QDCP Packet Header and TLV Payloads

Version (4 bits): Protocol version number (currently 0x1).
Flags (4 bits): Reserved for future use.
Length (16 bits): Specifies length of entire packet.
Reserved (8 bits): Set to zero; ignored on receipt.
TLV Payloads: Sequence of variable-length TLVs.

TLV Structures Each TLV consists of a type, a reserved field, a length (in bytes), and a value. The length specifies the length of the value, not the entire TLV. All fields are in network byte order. TLV Format:

TLV Format Defined TLV Types:

Initial TLV Type Assignments

Example Use Cases This section illustrates how the Quantum Datagram Control Protocol (QDCP) can be applied in practical network environments.

Dynamic ROADM Configuration QDCP packets carrying TLVs for ROADM Output Port ID () allow classical headers to steer entangled photons through commercial reconfigurable optical add-drop multiplexers (ROADMs). This enables dynamic path selection across metro and campus-scale optical networks, as demonstrated in recent hybrid IP packet experiments ().

Real-Time Error Mitigation TLVs containing polarization parameters and error-mitigation vectors (Type 0x08) allow active compensation of SU(2) rotations induced by deployed fiber (). Classical light encodes detection signals in the header, enabling dynamic updates to the error mitigator without disturbing quantum states.

Hybrid IP Packet Orchestration The QDCP framework aligns with the IRTF QIRG goals and use-cases (). By transporting control-plane metadata in TLVs, classical headers and quantum payloads can be synchronized and routed through existing IP infrastructure.

Timestamp Alignment TLVs carrying local and photon arrival timestamps can provide synchronization similar to RTP (). This enables sub-nanosecond correlation of entangled photon arrivals across nodes. The mechanisms to achieve such precision for distributed-clock synchronization (e.g. NTP, PTP, White Rabbit) are out of scope for this document. TLVs carrying "Duration of Quantum Information" specify the period during which the optical bypass must remain active to support quantum information transport. After the indicated duration expires, the bypass is automatically reverted back to its normal state to resume classical control-plane processing.

WDM/TDM Extensions Additional TLVs may specify per-wavelength parameters, enabling wavelength-division multiplexing (WDM) or time-division multiplexing (TDM) of entangled states (). This supports scaling of quantum internet bandwidth across multiple frequency channels while preserving compatibility with ITU-T DWDM grids ().

Example TLV Blocks This section specifies the TLV structure for specific TLV types.

0x08: Error Mitigation Vector Error mitigation can be done by sending different known polarization states with respect to the output of the chip and identifying the SU(2) transformation applied to these states by the fiber once they reach the receiver (). The value of the Error Mitigation TLV will be composed of a sequence of 64 bit structures, where each structure corresponds to a specific polarization state that is transmitted. The structure of each 64 bit block is as follows: Error Mitigation Value Structure:

Error Mitigation Value Structure The first eight bits specify which polarization is being sent. For simplicity, assume transmitted states must be in a horizontal, vertical, diagonal, anti-diagonal, right-circular, or left-circular polarization. The polarization section for each of these 64-bit structures can then be specified using the following mapping table. Polarization to Reserved Bit Mapping:

Polarization to Reserved Bit Mappings The duration in nanoseconds specifies how long the the specific polarization will be transmitted for. The arrival time specifies how long after the reception of the QDCP packet this specific polarization will arrive with nanosecond precision. To accurately identify the SU(2) transformation, at least two non-orthogonal polarizations are required to be sent. Zhang et al. experimentally used Horizontal and Right-Circular polarizations for error mitigation, both other combinations are also valid. For concreteness, consider the example where Horizontal and Right-Circular polarizations are transmitted for error correction. Example Error Mitigation TLV Structure:

Error Mitigation TLV including Right-Circular and Horizontal Polarizations. 0x08 is the TLV type. 0x00 is the reserved field. 0x10 is the length of the value, which is 16 bytes in this case. The next 0x00 represents horizontal polarization according to . The first 0x400 represents the duration of the horizontal polarization in nanoseconds and the second represents the arrival time of the horizontal polarization in nanoseconds. The next 0x04 represents right-circular polarization, with the following 0x400 representing the duration of the right-circular polarization and the 0x800 representing the arrival time of the right-circular polarization.

UDP Port Assignment Implementations SHOULD use a configurable default port. IANA is requested to allocate a well-known port for QDCP.

IANA Considerations - Allocate a UDP port for QDCP. - IANA is also requested to establish a QDCP TLV Types Registry with initial assignments as defined in Section 4.

Security Considerations QDCP inherits the risks of UDP: spoofing, injection, replay. It MUST be run in trusted environments or protected by DTLS/IPsec. TLVs may reveal network state information and MUST be protected if confidentiality is required. Use of DTLS/IPsec and reliable classical transport mechanisms are reserved for future work.

Acknowledgements The authors would like to thank Steve Schwartz and Wes Harding for their constructive feedback and detailed comments. Their suggestions helped broaden the scope of this document beyond the initial implementation and guided refinements to the protocol design and terminology.