# AMFPacket Structure for AVC Keyframe
**URL:** https://community.wowza.com/t/amfpacket-structure-for-avc-keyframe/43131
**Category:** Wowza Developer Dojo
**Tags:** wowza-streaming-server-java-api
**Created:** [June 5, 2014, 2:10am UTC](https://community.wowza.com/t/amfpacket-structure-for-avc-keyframe/43131 "2014-06-05T02:10:49Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@Konni\_Hartmann](https://community.wowza.com/u/Konni_Hartmann)
#### Post date: [June 5, 2014, 2:10am UTC](https://community.wowza.com/t/amfpacket-structure-for-avc-keyframe/43131/1 "2014-06-05T02:10:49Z")
Hi,
I am currently trying to understand the Datastructure of AMFPacket for H264 Keyframes (i-Frame). The Stream is a livestream using the baseline profile. I used .getLastKeyFrame() to get a KeyFrame AMFPacket.
When I extract bytes using .getDataBuffer() I receive the following Bytes (in Hex):
```auto
17 01 00 00 00 00 00 00 02 09
10 00 00 00 0F 06 00 01 C0 01
07 09 08 05 84 00 00 03 00 80
00 00 19 5F 65 88 80 0C 00 15
B0 A6 02 63 C0 04 0C 07 FA 2B
FC 0E DE 41 17 82 AF F6 4C 3B
F7 87 5E 31 04 C6 2F 7A 46 EC
```
Based on [http://www.adobe.com/devnet/f4v.html](http://www.adobe.com/devnet/f4v.html) (FLV and F4V File Format Specification) the first 5 Bytes tell me:
17 01 00 00 00
This is a Videodata packet with Frame Type 1 (Keyframe) and CodecID 7 (AVC).
The AVCPacketType is 1 (AVC NALU)
The CompositionTime is 0 (no offset)
According to spec these information are then followed by one or more AVC NALUs (with full frames). So they are not encoded as stated by ANNEX B (no 00 00 00 01 starting sequence) but following avcc. I know that I need the extradata for decoding an avc nalu (as it states the number of bytes used for encoding nalu length (I will extract this information and add it lateron)
By looking at the data, the length field has to be 4 (if smaller than the next NALU would be of length 0). So I would assume the first nalu is of length 2 Byte and contains:
09 10 which would be an AUD (Access Unit Delimiter) but I cant explain the 0x10.
Is this so far correct? I am still not sure where to find the keyframe itself.
Also the first 23 Bytes do not seem to change between keyframes which implies some kind of additional data.
```auto
17 01 00 00 00 00 00 00 02 09
10 00 00 00 0F 06 00 01 C0 01
07 09 08
```
It would be very helpful if someone with a deeper understanding of the used container formats could shed some light on this topis.
Best Regards
Konni
---
### Author:  [@Konni\_Hartmann](https://community.wowza.com/u/Konni_Hartmann)
#### Post date: [June 5, 2014, 5:30am UTC](https://community.wowza.com/t/amfpacket-structure-for-avc-keyframe/43131/2 "2014-06-05T05:30:12Z")
NaluSizeLength = 4 Byte was correct (extracted from VideoCodecConfigPacket)
I think I got it now. There were 3 different NALUs inside the Keyframe Packet:
1st NALU:
Access unit delimiter
2nd NALU:
Supplemental enhancement information (SEI)
3rd NALU:
Coded slice of an IDR picture
Will try to decode the IDR picture now =)
Konni