Links

A transport link is a point-to-point or multicast channel between two Zenoh nodes over which transport frames are exchanged.

Link Properties

is_reliable

Whether the underlying link guarantees delivery and ordering. TCP, TLS, QUIC, and Serial (with a reliable framing layer) are reliable. UDP unicast and UDP multicast are not.

is_streamed

Whether the link is stream-oriented (true) or datagram-oriented (false). Stream links require a 2-byte little-endian length prefix on each batch (see Frame Format). Datagram links do not.

mtu

Maximum transmission unit in bytes. For stream links, the effective MTU is the negotiated batch_size. For datagram links, it is constrained by the UDP payload limit and any IP fragmentation.

Supported Link Protocols

Protocol Reliable Streamed Default Port Notes

TCP

Yes

Yes

7447

Standard reliable transport; stream-framed

TLS

Yes

Yes

7447

Encrypted TCP; stream-framed

QUIC

Yes

Yes

7447

Encrypted, multiplexed; stream-framed

UDP Unicast

No

No

7447

Best-effort unicast; datagram-framed

UDP Multicast

No

No

7446

One-to-many; used for scouting and multicast sessions

Serial

Configurable

Yes

—

Constrained / embedded devices; stream-framed

Default multicast group: 224.0.0.224 (IPv4), ff02::1 (IPv6).

Batch Size

For each link, the effective maximum batch size is:

  • On unicast links: the minimum of the two endpoints' batch_size values negotiated during INIT.

  • On multicast links: the minimum of the link MTU and the local node’s batch_size.

  • Absolute maximum: 65 535 bytes (the range of the 2-byte length prefix on stream links).

Session Establishment per Link Type

Link type Establishment mechanism

Unicast (TCP, TLS, QUIC)

INIT SYN/ACK then OPEN SYN/ACK; see Session Establishment (INIT & OPEN)

Multicast (UDP multicast)

No handshake; periodic JOIN advertisement; see JOIN

Link Failure Detection

On unicast links: lease expiry detected by absence of KEEP_ALIVE or any other message. On multicast links: absence of JOIN messages within the JOIN lease period.

See Keep-Alive for the lease / keep-alive interval specification.