[trel-dnssd] trel platform selects first peer address instead of wider scope one

[abtink]

The trel platform implementation seem to select the first address (at index [0]) when invoking callback to reprot a discovered TREL peer info:

ot-br-posix/src/trel_dnssd/trel_dnssd.cpp

Lines 306 to 310 in 4b826a1

	peerInfo.mRemoved = false;
	memcpy(&peerInfo.mSockAddr.mAddress, &aInstanceInfo.mAddresses[0], sizeof(peerInfo.mSockAddr.mAddress));
	peerInfo.mSockAddr.mPort = aInstanceInfo.mPort;
	peerInfo.mTxtData = aInstanceInfo.mTxtData.data();
	peerInfo.mTxtLength = aInstanceInfo.mTxtData.size();

The platform/trel.h documenation mentions:

...
 * events are not guaranteed, however. When a TREL service instance is discovered, a new ongoing DNS-SD query for an
 * AAAA record should be started on the hostname indicated in the SRV record of the discovered instance. If multiple
 * host IPv6 addressees are discovered for a peer, one with highest scope among all addresses MUST be reported (if
 * there are multiple address at same scope, one must be selected randomly).
 *

This behavior seems to be missing in the current implementation.

---

Thanks To @gabekassel for noticing that different peers seems to be using a mixture of ULA/GUA addresses:

Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Input: trel peers
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: | No  | Ext MAC Address  | Ext PAN Id       | IPv6 Socket Address                              |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: +-----+------------------+------------------+--------------------------------------------------+
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |   1 | f2295d25707238d4 | dcbc1020cee5f6af | [2603:7000:c00:a315:8:3e3:857f:e5d4]:59248       |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |   2 | be860a24c6976a2c | dcbc1020cee5f6af | [fdbc:1020:cee5:f6af:829:8a3c:7f40:396a]:57947   |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |   3 | 0efd70f1c6ded375 | dcbc1020cee5f6af | [fdbc:1020:cee5:f6af:4f7:a1ac:6f7f:1ebb]:63834   |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |   4 | 0320674a52350037 | dcbc1020cee5f6af | [fd38:eb5c:f30e:1:6697:14ff:fe5b:ad92]:49635     |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |   5 | 6ab2f87ca6ca8952 | dcbc1020cee5f6af | [fdbc:1020:cee5:f6af:1c0f:dcc3:56f6:d71d]:65458  |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |   6 | 1a00eed7a68c2657 | dcbc1020cee5f6af | [fdbc:1020:cee5:f6af:88a:f289:c402:41d7]:58104   |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |   7 | ca7f47968e01c42c | dcbc1020cee5f6af | [fdbc:1020:cee5:f6af:102b:222e:a28a:c46d]:59180  |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |   8 | 4a26f1158748ddf2 | dcbc1020cee5f6af | [fdbc:1020:cee5:f6af:1c96:2ba9:12f1:186b]:62782  |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |   9 | 8a32076938e722c7 | dcbc1020cee5f6af | [2603:7000:c00:a315:c6a8:16ff:fe21:dad4]:59246   |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |  10 | 5e515b7ebb4cc45e | 30b0cd0eb498ae7c | [fdbc:1020:cee5:f6af:4945:c045:b65b:d6c0]:64705  |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |  11 | b6aedcf9c86863ba | dcbc1020cee5f6af | [fdbc:1020:cee5:f6af:2:fd54:3ef1:be4f]:62431     |
Dec  4 18:21:49.145 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |  12 | 8262022218ce4872 | dcbc1020cee5f6af | [fdbc:1020:cee5:f6af:1caa:1d4f:963f:9b58]:55343  |
Dec  4 18:21:49.146 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |  13 | 32e5835da26a658d | dcbc1020cee5f6af | [2603:7000:c00:a315:c6a8:16ff:fe27:8b14]:50566   |
Dec  4 18:21:49.146 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: |  14 | 6679afc43e7cf38e | dcbc1020cee5f6af | [fdbc:1020:cee5:f6af:1099:e91f:6dd0:c7fa]:62185  |
Dec  4 18:21:49.146 eero local5.debug otbr-agent[666778]: [D] Cli-----------: Output: Done```

[abtink]

One point on this: Using wider scope would be better but at the end, as long as we can reach the other peer using the selected address, it should be fine and things should work okay.

[abtink]

• Submitted [trel-dnssd] prioritize wider-scope IPv6 address of TREL peers #2118 for this.