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isLooped checks for any duplicate prefixes #1976

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isLooped checks for any duplicate prefixes #1976

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20 changes: 13 additions & 7 deletions examples/simple_repeater/MyMesh.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -402,15 +402,21 @@ static uint8_t max_loop_strict[] = { 0, /* 1-byte */ 1, /* 2-byte */ 1, /* 3

bool MyMesh::isLooped(const mesh::Packet* packet, const uint8_t max_counters[]) {
uint8_t hash_size = packet->getPathHashSize();
uint8_t hash_count = packet->getPathHashCount();
uint8_t n = 0;
uint8_t path_byte_len = packet->getPathByteLen();
uint8_t limit = max_counters[hash_size];
const uint8_t* path = packet->path;
while (hash_count > 0) { // count how many times this node is already in the path
if (self_id.isHashMatch(path, hash_size)) n++;
hash_count--;
path += hash_size;

for (uint8_t i = 0; i < path_byte_len; i += hash_size) {
uint8_t n = 0;

if (self_id.isHashMatch(&path[i], hash_size)) n++;

for (uint8_t j = i + hash_size; j < path_byte_len; j += hash_size) {
if (memcmp(&path[i], &path[j], hash_size) == 0) n++;
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@robekl robekl Mar 17, 2026

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loop detection will treat any repeated path hash segment as a loop, so equal 1-byte hashes can be considered loops if they collide and will cause healthy packets to be dropped. Is that okay?

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@everett1992 everett1992 Mar 17, 2026

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That's been the case since loop detection was added, tho the check and false positive rate is lower without this change because it only triggered on collisions with the self-id.

There are different thresholds for loop detection,

These two tests are passing with original code and my change. An ID has to appear 4 times, and match your ID to be dropped.

runTest(1, {0xAA, 0xAA, 0xAA}, max_loop_minimal, false, "3 repeats and self: under minimal limit");
runTest(1, {0xAA, 0xAA, 0xAA, 0xAA}, max_loop_minimal, true, "4 repeats and self: over minimal limit");

These tests pass with my change, the id must appear 5 times.

runTest(1, {0x11, 0x11, 0x11, 0x11}, max_loop_minimal, false, "4 repeats: under minimal limit");
runTest(1, {0x11, 0x11, 0x11, 0x11, 0x11}, max_loop_minimal, true, "5 repeats: over minimal limit");

Assuming random distribution of node ids there's a 0.15% chance to drop a full 64 hop packet.

About 0.15% — roughly 1 in 664 chance that when you pick 64 random numbers from 1–254, at least one
number shows up 5 or more times.

}
if (n >= limit) return true;
}
return n >= max_counters[hash_size];
return false;
}

bool MyMesh::allowPacketForward(const mesh::Packet *packet) {
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