Mesh-NOW routes messages through intermediate nodes with a hop-count limit mechanism.

How Routing Works

When a node receives a broadcast data frame, it relays it while the hop budget remains:

static void maybe_relay(const mesh_message_t *mesh_msg)
{
    if (mesh_now_can_relay(mesh_msg)) {
        mesh_now_relay_broadcast(mesh_msg, true);
    }
}

mesh_now_can_relay() checks that hop_count is still below hop_limit. mesh_now_relay_broadcast() increments hop_count and re-broadcasts. Direct frames (DM, ACK, typing) take a different path: they forward one hop toward their target, along a route when one is known, otherwise as a bounded flood.

Hop Limit

The default hop limit is 3, pulled from Kconfig:

#ifndef CONFIG_MESH_NOW_DEFAULT_ROUTE_TTL
#define CONFIG_MESH_NOW_DEFAULT_ROUTE_TTL 3
#endif
#define DEFAULT_ROUTE_TTL CONFIG_MESH_NOW_DEFAULT_ROUTE_TTL

A frame originates with hop_count = 0 and is relayed while hop_count stays below hop_limit. A limit of 3 therefore allows up to 2 intermediate relays before the frame is dropped.

flowchart LR S[Sender] -->|hop_count=0| R1[Relay 1] R1 -->|hop_count=1| R2[Relay 2] R2 -->|hop_count=2| D[Destination] R2 -.->|hop_count=3 reaches limit| X[Drop]

Duplicate Detection

Each node keeps a fixed buffer of the most recent seen message IDs (128 default):

#define MAX_SEEN_MESSAGE_IDS CONFIG_MESH_NOW_MAX_SEEN_MESSAGE_IDS

static uint32_t seen_message_ids[MAX_SEEN_MESSAGE_IDS];
static int seen_message_count = 0;

When a message arrives:

  1. If it is a control frame (beacon, ACK, RREQ, RREP, RERR), skip the duplicate check. Control frames are short-lived and self-deduplicating.
  2. Check if message_id is in the seen buffer.
  3. If seen, drop the message.
  4. If new, add to the buffer and process.
if (!mesh_now_is_control_type(mesh_msg.type)) {
    if (mesh_now_is_message_seen(mesh_msg.message_id)) {
        return;  // duplicate, drop
    }
    mesh_now_mark_message_seen(mesh_msg.message_id);
}
Eviction

When the buffer is full, the oldest entry shifts out and the new ID appends. FIFO eviction keeps the buffer bounded.

Which Messages Relay

Message Type Relayed? Condition
MSG_TYPE_BEACON No Discovery only, handled locally
MSG_TYPE_CHAT Yes Broadcast while hop budget remains
MSG_TYPE_DIRECT Yes Forwarded toward target via route, or flooded if no route
MSG_TYPE_ACK Yes Forwarded toward target along the return path
MSG_TYPE_GROUP Yes Relayed regardless of group membership
MSG_TYPE_PRESENCE Yes Broadcast while hop budget remains
MSG_TYPE_TYPING Yes Forwarded toward target like a direct frame
RREQ / RREP / RERR Yes Relayed by the discovery layer (flood, or toward originator)

Retransmission vs. Relay

These are two different mechanisms:

  • Retransmission is the original sender retrying an unacknowledged direct message (up to 3 retries on a 2-second timeout).
  • Relay is an intermediate node forwarding a message through the mesh (increment hop count, forward toward target or flood).
Reliability
Message Format
Wire Format
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