Packet mumcord image

Part of the Plugin data protocol. Flows: Flow-Full-Image, Flow-Direct-Connection.

Packet mumcord.image

Purpose. A chat message can only carry an image that fits the server's message limit, so Mumcord sends a thumbnail in the chat message and keeps the original locally. Other Mumcord users fetch the full image from the original sender. A direct connection is tried first; this packet is the fallback.

dataID	mumcord.image
Encoding	binary, big-endian integers
Version	none; versioned by message kinds 1 to 5
Sent to	one person: requester to the original sender, and the sender's answers back to the requester
Feature gate	full-image (see Packet-mumcord-hello)
Defined in	crates/engine/src/full_image/wire.rs

The reference in the chat message

The thumbnail <img> carries data-mumcord-full="<sha256 as 64 lowercase hex digits>:<size in bytes>". Official clients ignore the attribute and show the thumbnail.

<img src="data:image/jpeg;base64,..." data-mumcord-full="9f86d081884c7d659a2feaa0c55ad015a3bf4f1b2b0b822cd15d6c15b0f00a08:204800">

Message layout

Every message starts with a one-byte kind and a 4-byte transfer id that the requester picks.

Kind	Message	Rest of the message
1	Get	SHA-256 of the wanted image (32 bytes)
2	Queued	- (the request waits behind other transfers)
3	Start	image size in bytes (u32)
4	Chunk	chunk index (u32), then 1 to 960 image bytes (CHUNK_LEN)
5	Unavailable	- (the image is not, or no longer, available, or the sender is busy)

Example

Hex, transfer id 0x0000002a:

Get:    01 0000002a 9f86d081...b0f00a08        (1 + 4 + 32 bytes)
Start:  03 0000002a 00032000                   (size 204800)
Chunk:  04 0000002a 00000000 <960 bytes>       (first chunk: 1 + 4 + 4 + 960 = 969 bytes)

Limits

Limit	Value
Image bytes per chunk	960
Image size	512 KiB, at most 3840 pixels per side
Requests waiting at the sender	4 (more get Unavailable)
Requests per person per minute	6 (more are ignored)
Downloads at once (receiver)	3
Chunk pace	one every 350 ms (about 2.9 per second)

Chunks must arrive in order. The receiver accepts the bytes only if size and SHA-256 match the reference, the content is a real PNG, JPEG, GIF or WebP (by its bytes, not a claimed type) and it decodes completely. Unknown kinds are dropped.

Privacy

Carries an image the user already posted in a channel or private chat. The request reveals that someone opened it.

Used in

Flow-Full-Image; the same image over a direct connection (stream kind 1) is in Flow-Direct-Connection.