So I wanted to see what traffic was going to 239.254.2.4. That is a UDP MultiCast IP address. Looking back at the trace you can see that three packets are sent every second pretty continuously. The three packets have three different lengths. 60, 150 and 134. Here is a quick look at three consecutive packets payload content. The 60 length packets look consistent each time. The other two have minor changes. My assumption is that these are GPS data. So it should have some of the same data as these NEMA 2000 sentences: Estimated Error Information (PGRME) $PGRME,<1>,M,<2>,M,<3>,M*hh<CR><LF>
Map Datum (PGRMM) $PGRMM,<1> *hh<CR><LF>
Global Positioning System Fix Data (GGA) $GPGGA,<1>,<2>,<3>,<4>,<5>,<6>,<7>,<8>,<9>,M,<10>,M,<11>,<12>*hh<CR><LF>
So my Lattitude and Longitude when running these tests was N 42.359289, W 82.953903 I don't see the N and the W in the data! So the 60 length packet could be either 1) and explanation of what is coming or 2) the map datum. All of the data below came from: http://cloudshark.org/captures/824f2c3bc420 This is the Boat Three packet capture where I start up the Garmin with everything else already on and ready. 60 Length Packets 9b0000000400000003030100 9b0000000400000003030100 9b0000000400000003030100 150 Length Packets 99 00 00 00 64 00 .......t......d. 00 00 00 00 01 00 00 00 00 00 b9 06 00 00 dd 5a ...............Z 6d 61 6a a8 e7 3f 47 58 a1 81 45 2a f7 bf 00 00 maj..?GX..E*.... 00 00 a4 80 21 41 00 00 00 00 00 00 00 00 00 00 ....!A.......... 00 00 00 00 00 00 00 00 00 00 bd 3f 29 43 00 00 ...........?)C.. 00 00 00 00 00 00 51 59 04 69 51 59 04 69 51 59 ......QY.iQY.iQY 04 69 51 59 04 69 51 59 04 69 51 59 04 69 00 00 .iQY.iQY.iQY.i.. 00 00 00 00 00 00 ...... 99 00 00 00 64 00 .......t......d. 00 00 00 00 04 00 00 00 00 00 b9 06 00 00 99 24 ...............$ 73 3f 6a a8 e7 3f e2 c9 a6 4a 45 2a f7 bf 4c 37 s?j..?...JE*..L7 89 81 60 81 21 41 00 00 00 00 00 00 00 00 00 00 ..`.!A.......... 00 00 00 00 00 00 00 00 00 00 59 5a 07 43 8f c2 ..........YZ.C.. 75 3d 80 45 54 bf ce c1 0e 41 e2 af b0 41 6c 8f u=.ET....A...Al. be 41 b8 1e a5 3f 29 5c ef 3f 85 eb 11 40 00 00 .A...?)\.?...@.. 00 00 00 00 00 00 ...... 99 00 00 00 64 00 .......t......d. 00 00 00 00 04 00 00 00 00 00 b9 06 00 00 99 24 ...............$ 73 3f 6a a8 e7 3f e2 c9 a6 4a 45 2a f7 bf 00 00 s?j..?...JE*.... 00 00 62 81 21 41 00 00 00 00 00 00 00 00 00 00 ..b.!A.......... 00 00 00 00 00 00 00 00 00 00 59 5a 07 43 8f c2 ..........YZ.C.. 75 3d 80 45 54 bf ce c1 0e 41 e2 af b0 41 6c 8f u=.ET....A...Al. be 41 b8 1e a5 3f 29 5c ef 3f 85 eb 11 40 00 00 .A...?)\.?...@.. 00 00 00 00 00 00 ...... 134 Length Packets 9d 00 00 00 54 00 .......d......T. 00 00 05 f0 0a 3b ce 00 05 02 fc 08 47 1f 00 05 .....;......G... 0d b8 0b 0f 2c 00 05 0c 60 09 24 ea 00 05 1d 98 ....,...`.$..... 08 17 36 01 05 04 80 0c 1c 4c 00 05 19 c4 09 22 ..6......L....." 18 01 05 0a fc 08 35 37 00 05 11 9c ff 01 83 00 ......57........ 00 1a 9c ff 04 a1 00 00 ff ff 7f ff ff ff 00 ff ................ ff 7f ff ff ff 00 ...... 9d 00 00 00 54 00 .......d......T. 00 00 05 8c 0a 3b ce 00 05 02 fc 08 47 1f 00 05 .....;......G... 0d b8 0b 0f 2c 00 05 0c 60 09 24 ea 00 05 1d fc ....,...`.$..... 08 17 36 01 05 04 80 0c 1c 4c 00 05 19 c4 09 22 ..6......L....." 18 01 05 0a fc 08 35 37 00 05 11 9c ff 01 83 00 ......57........ 00 1a 9c ff 04 a1 00 00 ff ff 7f ff ff ff 00 ff ................ ff 7f ff ff ff 00 ...... 9d 00 00 00 54 00 .......d,m....T. 00 00 05 8c 0a 3b ce 00 05 02 fc 08 47 1f 00 05 .....;......G... 0d b8 0b 0f 2c 00 05 0c fc 08 24 ea 00 05 1d 98 ....,.....$..... 08 17 36 01 05 04 80 0c 1c 4c 00 05 19 c4 09 22 ..6......L....." 18 01 05 0a fc 08 35 37 00 05 11 9c ff 01 83 00 ......57........ 00 1a 9c ff 04 a1 00 00 ff ff 7f ff ff ff 00 ff ................ ff 7f ff ff ff 00 ...... |