Digipeater Logic

This page describes the digipeater behavior from an operator and user perspective, including all processing rules, path handling, compatibility with legacy formats, and the TEMP PATH feature.

The digipeater settings referenced here are configured in the Digipeater Settings section of the web interface - see Configuration.

Modes

The digipeater operates in one of five modes:

All modes above 0 also handle own callsign and alias in path (TEMP PATH) at all times.

How Path Processing Works

The “first unused address” rule

An APRS path is a comma-separated list of addresses. An address marked with * has already been processed by a digipeater (“used”). An address without * has not been processed yet (“unused”).

Finding the first unused address:

1. Find the position of the last address that has * in the path.
2. The first unused address is the one immediately after it.
3. If no address has *, the entire path is unused - start from the beginning.
4. If there are no more addresses after the last *, there is nothing to relay → ignore.

Processing the first unused address:

• Check whether the digi is capable of handling that address type (see mode table).
• If not → ignore the packet entirely, even if there are processable addresses further right.
• If yes → relay the packet with the updated path:
  • The first unused address is replaced by <digi_callsign>*.
  • If the address was WIDEn-N with N > 1, insert WIDEn-(N-1) after the digi callsign.
  • All addresses after the first unused address are kept unchanged.
  • All addresses before the first unused address: depends on the Path Marker Mode setting (see section below). By default, their * markers are preserved (keep mode). In standard mode they are removed - only the current digi’s callsign carries * in the relayed path.

Mode 2 - WIDE1 Fill-In Digipeater

Handles WIDE1-N addresses only. Ignores any path where the first unused address is not WIDE1.

With “Keep path markers” enabled: F1*,WIDE1-1 → F1*,<call>*

A packet with WIDE2-1 as first unused is ignored even if WIDE1-1 appears later.

Mode 4 - WIDE2 Wide-Area Digipeater

Handles WIDE2-N addresses and WIDE1-1 (N=1 only, per New N-N paradigm). Ignores WIDE1-2 and higher.

With “Keep path markers” enabled: F1*,WIDE2-1 → F1*,<call>* and F1*,WIDE2-2 → F1*,<call>*,WIDE2-1

Sequential example - WIDE2-2

1. Source sends: WIDE2-2
2. First wide-area digi (H1) relays: H1*,WIDE2-1
3. Second wide-area digi (H2) relays: H1,H2*

With “Keep path markers” enabled: step 3 produces H1*,H2* instead.

Mode 3 - Combined WIDE1+WIDE2 Digipeater (Greedy)

Mode 3 processes all WIDE1-N and WIDE2-N addresses in the unused portion of the path in a single pass.

• All WIDE1-N are consumed completely.
• WIDE2-N with N=1 is consumed; with N>1 it is decremented to WIDE2-(N-1).
• The digi inserts its callsign once, at the position of the first consumed WIDE address.
• Addresses before the unused portion follow the Path Marker Mode setting.

With “Keep path markers” enabled: F1*,WIDE2-1 → F1*,<call>* and F1*,WIDE2-2 → F1*,<call>*,WIDE2-1

Path Order Matters

Processing always goes left to right. If the first unused address is a type the digi cannot handle, the packet is ignored - even if a handleable address appears further right.

Example: path WIDE2-1,WIDE1-1 arriving at a mode 2 (WIDE1) digi.

• First unused = WIDE2-1 → mode 2 does not handle WIDE2 → ignored.
• WIDE1-1 at position 2 is never reached.

This is correct APRS behavior. Paths like WIDE2-1,WIDE1-1 are malformed and will not be efficiently relayed.

Compatibility with Legacy Path Formats

Older digipeater software (pre-New N-N) inserts its callsign without * directly before the WIDE element it processed. For example:

• A legacy combined digi receives WIDE1-1,WIDE2-2 and produces: WIDE1*,SQ2-1,WIDE2-1

Here SQ2-1 already relayed the packet but has no *. The WIDE1* is the position marker.

How the new logic handles this

When the last * in the path belongs to a WIDE-type address (e.g. WIDE1*), the digi applies legacy normalization: it skips over any callsigns without * that appear between the WIDE marker and the next processable address (own callsign, alias, or another WIDE address).

With “Keep path markers” enabled: WIDE1* and other * markers in the pre-relay portion are preserved - e.g. WIDE1*,SQ2-1,WIDE2-1 → WIDE1*,SQ2-1,<call>*

Exhausted paths (last * on a WIDE element with no remaining addresses) are always ignored correctly.

Path Optimization

When the digi’s own callsign or configured alias is the first unused address and the path contains legacy WIDE * markers before it, the digi performs path optimization: it converts the legacy format to the new clean format in a single relay.

Optimization rules applied to all addresses before own callsign:

• WIDE-type addresses with * (e.g. WIDE1*) → always removed (they were position markers, not real digis)
• Other callsign addresses → follow the Path Marker Mode setting

Then own callsign is appended with *. Everything after remains unchanged.

Examples

sourceCallsign = SQ2CPA-1

Note: F1* in the last row - F1 is a callsign address (not WIDE-type), so path optimization is not triggered. * on F1 is removed or kept based on the Path Marker Mode setting, not by optimization logic.

Why this matters: In both modes, the relayed path contains no unused addresses after SQ2CPA-1* - so any digi receiving the packet will find no further work to do and correctly ignores it. Loop prevention relies on the 30-second deduplication cache, not on the presence of * on previous addresses.

TEMP PATH

TEMP PATH allows a tracker or portable station to act as a relay digi for another station that cannot reach the main APRS network directly. Active in all modes above 0.

Configuration

One optional alias callsign can be configured (dgi_temp_call, max 9 characters; the TEMP PATH Alias field in the web interface). When a packet arrives with this alias or the digi’s own callsign as the first unused address, the digi relays it - replacing the alias or callsign with its own callsign marked *.

Use case

A handheld radio (HH) is operated in an area with no direct WIDE1 digipeater coverage, but a car tracker (SQ2CPA-10) is nearby with good coverage.

Option A - HH puts the car’s callsign in the path explicitly:

HH sends: SQ2CPA-10,WIDE1-1,WIDE2-1

Car tracker receives, sees SQ2CPA-10 as first unused (matches own callsign) → relays: SQ2CPA-10*,WIDE1-1,WIDE2-1

A WIDE1 digi then relays: SQ2CPA-10,<W1>*,WIDE2-1 A WIDE2 digi then relays: SQ2CPA-10,<W1>,<W2>*

With “Keep path markers”: SQ2CPA-10*,<W1>*,WIDE2-1 and SQ2CPA-10*,<W1>*,<W2>*

Option B - Car configured with dgi_temp_call = MOBILE:

HH sends: MOBILE,WIDE1-1,WIDE2-1

Car receives, sees MOBILE as first unused (matches alias) → relays using own callsign: SQ2CPA-10*,WIDE1-1,WIDE2-1

The path continues normally from there.

Option C - Car configured with dgi_temp_call = SQ2CPA-9 (HH’s callsign):

HH sends: SQ2CPA-9,WIDE1-1,WIDE2-1

Car receives, sees SQ2CPA-9 (matches alias) → relays as: SQ2CPA-10*,WIDE1-1,WIDE2-1

HH does not need to know the car’s callsign - just puts its own callsign in the path.

Loop prevention

If the car receives its own relay back (SQ2CPA-10*,WIDE1-1,WIDE2-1):

• Last * = SQ2CPA-10* (a callsign, not WIDE) → no legacy normalization
• First unused = WIDE1-1
• Mode 1: WIDE1-1 is not handled → ignored ✓
• Mode 2+: same payload hash was already seen within 30 seconds → ignored ✓

Path Validation - Duplicate Address Detection

Before processing, the path is validated. A packet is rejected if any address appears more than once in the path (case-insensitive, ignoring *).

Frame Deduplication

The digi maintains a cache of recently seen frames (identified by a hash of the source and payload, excluding the path). If the same frame is received again within 30 seconds, it is not retransmitted.

This prevents duplicate transmissions when the same packet arrives via multiple paths, and provides a safety net against relay loops.

Path Marker Mode

This setting controls whether previous * markers in the relayed path are preserved or stripped.

Why this matters

In standard mode, the relayed path is clean and shows only who relayed last:

• F1*,WIDE2-1 received by H1 → relayed as F1,H1*

In “keep” mode, the full relay history is visible:

• F1*,WIDE2-1 received by H1 → relayed as F1*,H1*

Most APRS software and iGate displays show only the last * entry regardless, so for end users the practical difference is minimal. Standard mode produces shorter, cleaner paths. Keep mode is useful if you want the full relay chain visible in raw packet logs.

Loop prevention works identically in both modes - it is always based on the 30-second frame deduplication cache, not on the presence of * markers.

Configuration

The setting is available in the web interface under the Digipeater configuration section as “Keep path markers (*)”. It is checked by default (keep mode).

Cross-Digi (RX/TX on Different Frequencies)

When the receive and transmit frequencies are different (cross-digi mode), the digi applies a simpler rule: it strips all WIDE markers from the path and inserts its own callsign with *. The first-unused-address algorithm, legacy normalization, path optimization, and TEMP PATH do not apply in this mode.

Complete Path Examples

Standard WIDE2-2 traversal (new format, two wide-area digis)

1. Source transmits: WIDE2-2
2. H1 (first W2 digi, mode 4) relays: H1*,WIDE2-1
3. H2 (second W2 digi, mode 4) relays: H1,H2*
4. IGate receives H1,H2* - both hops consumed, no further relay.

With “Keep path markers”: step 3 produces H1*,H2*

Mixed WIDE1+WIDE2 traversal (fill-in then wide-area)

1. Source transmits: WIDE1-1,WIDE2-1
2. F1 (fill-in digi, mode 2) relays: F1*,WIDE2-1
3. H1 (wide-area digi, mode 4) relays: F1,H1*

With “Keep path markers”: step 3 produces F1*,H1*

TEMP PATH with path optimization (legacy W1 in network)

Car callsign = SQ2CPA-10, dgi_temp_call = SQ2CPA-9.

1. HH transmits: SQ2CPA-9,WIDE1-1,WIDE2-1
2. Car (SQ2CPA-10) receives, SQ2CPA-9 is first unused → matches alias → relays: SQ2CPA-10*,WIDE1-1,WIDE2-1
3. Fill-in digi (mode 2) receives → relays: SQ2CPA-10,F1*,WIDE2-1
4. Wide-area digi (mode 4) receives → relays: SQ2CPA-10,F1,H1*

With “Keep path markers”: step 3 = SQ2CPA-10*,F1*,WIDE2-1, step 4 = SQ2CPA-10*,F1*,H1*

Old-format path arriving at own callsign digi

1. Legacy combined digi processed WIDE1-1,WIDE2-2 and produced: SQ2CPA-2,WIDE1*,SQ2CPA-1,WIDE2-1
2. SQ2CPA-1 digi (mode 1+) receives this, detects WIDE1* before own callsign → path optimization:
   • WIDE1* → removed (always)
   • SQ2CPA-2 → kept as-is (default) or marked SQ2CPA-2* (keep markers mode)
   • SQ2CPA-1 → own callsign → SQ2CPA-1*
   • WIDE2-1 → unchanged
3. Relayed path (default): SQ2CPA-2,SQ2CPA-1*,WIDE2-1 (with “Keep path markers”: SQ2CPA-2*,SQ2CPA-1*,WIDE2-1)
4. SQ2CPA-2 hears this - last * = SQ2CPA-1*, no addresses after it (for the no-WIDE2-1 case) → ignored ✓. For the WIDE2-1 case, the 30-second deduplication prevents re-relay.
5. Wide-area digi (mode 4) receives → relays: SQ2CPA-2,SQ2CPA-1,H1* (with “Keep path markers”: SQ2CPA-2*,SQ2CPA-1*,H1*)