Undocumented ut_holepunch Extension (https://github.com/arvidn/libtorrent/blob/c1ade2b75f8f7771509a19d427954c8c851c4931/src/bt_peer_connection.cpp#L1576)

ut_holepunch.ext

+-------------------------------------------------------------------+
|                                                                   |
|                                                13/27  16/30       |
|                                                ^  ^   ^  ^        |
|                                                +--++  +--+        |
|        0  4  5  6  7  8  9  12   9              26 |  |           |
|       +--------------------------------------------+--++          |
|       |XXXX||X||X||X||X||XXXX OR XXXXXXXXXXXXXXXX||XXXX|          |
|       +^--^--^--^--^--^--^--^----^--------------^--^--^+          |
|        +--+  |  |  |  |  |--|    +--------------+  +--+           |
|        |     |  |  |  |  |-----+ |                   |            |
|        |     |  |  |  +-----+  | |                   |            |
|        |     |  |  +------+ |  +-+-------------+     |            |
|     +--+ +---+  +-+       | |                  |     |            |
|     |    |        |       | +-------+          |     |            |
|     |    |        |       |         |          |     |            |
|    ++----+--------+-------+---------+----------+-----+-------+    |
|    |Len||Ext_Msg||Ext_Id||Msg_Type||Addr_Type||Addr||Err_Code|    |
|    +---------------------------------------------------------+    |
|                                                                   |
+-------------------------------------------------------------------+
|Msg Type    |ByteCode|Usage                                        |
|Rendezvous  |       0|Send connect message to the initiating       |
|            |        |peer with address in this mesage + Send      |
|            |        |connect message to the address in this mesage|
|            |        |with the address of the initiating peer      |
|Connect     |       1|Tell a peer to connect to the address        |
|Failed      |       2|A previous request failed for some reason    |
+-------------------------------------------------------------------+
|Addr Type   |ByteCode|Usage                                        |
|Ipv4        |       0|Address in the message is an ipv4 address    |
|Ipv6        |       1|Address in the message is an ipv6 address    |
+-------------------------------------------------------------------+
|Err Code    |ByteCode|Usage                                        |
|NoSuchPeer  |       1|???                                          |
|NotConnected|       2|We cant rendezvous the connection as we arent|
|            |        |connected to the peer                        |
|NoSupport   |       3|Other peer doesnt support the extension      |
|            |        |(if the peer that send the message doesnt    |
|            |        |support the extension, dont respond)         |
|NoSelf      |       4|Our address was listed in the message        |
+------------+------------------------------------------------------+