How does the Announce packet work?

Development
1

Recently I have been trying to understand how syncthing works under the hood. I started with the Local Discovery Protocol v4. While reading, I got stuck trying to understand how the Announcement message conveys its length without explicitly transmitting the length in a protobuf field or a length-prefixed frame.

I looked into the source code but couldn’t figure out much. So I ran wireshark on to study the packets sent. I got the magic number that syncthing uses. After stripping the magic number from the byte sequence, I ran the protobuf decoder (protoc --decode) on the message and it decoded fine. So I want to know how syncthing distinguishes between two announce packets.

I am a newbie in networking and stuff so please bear with me. Thanks

2

Local discovery uses UDP, and UDP has its own length field. The discovery packets are always single UDP packets, so each layer 4 packet is one layer 7 packet as well.

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3

Ahh! Thank you, I didnt knew that in UDP a single recieve call will return a single UDP packet and excess bytes that can’t be held in the storage buffer are discarded. That clarifies a lot. Thanks again for all the points that you shared.

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4

Yup, UDP doesn’t use the “stream” semantics like TCP or other high level protocols.

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