When a web server returns a JPEG image (mime type image/jpeg), how is that encoded?

Question

If you make an HTTP request to a web server, and it returns a response of type image/jpeg, how is the binary data actually encoded? Is it the original byte-level content of

Answer 1

Strangely, it's not "straight through".

Apart from adding the MIME header, the webserver appears to strip out all the jpeg markers (0xFF, 0xNN) but leaves the rest intact. This seems strange as I don't know how the web browser is then recognising the start of the image frame.

I found this out by writing my own simple webserver in an embedded system - I thought I'd only need to add the MIME header and send the rest of the jfif-jpeg file untouched, but the browser says "the image cannot be displayed because it contains errors"!

here's the start of the original jpeg/jfif in hex

ff d8 ff e0 00 10 4a 46 49 46 00

[SOI][APP0][length]J F I F NULL

As per the spec.

The received file contains this, after the header:

0d 0a 0d 0a 00 10 4a 46 49 46 00

The first 4 bytes are cr/lf/cr/lf at the end of the header, then NO markers, but it does contain the data field. The same thing is repeated for the other markers e.g. start of frame.

Strange huh? I don't think it's an mime encoding issue, as the rest of the data looks intact - including FFs in the data etc.

Anyone know what's going on here? PS to look closer, just request a .jpg from any website using putty or similar and save what you get, and compare it to the original, or even the saved-as version.