Yahoo! Messenger Protocol - Technical Overview

Technical Overview

The YMSG protocol communicates between the client application, and a server, using a TCP/IP connection on port 5050 by default. Other ports may be used if this port is blocked. Alternatively, an HTTP route is available for clients behind a well-secured firewall, with HTTP requests being used to upload messages from the client, while downloading all messages which have accumulated on the server since the last request.

The client remains logged in for as long as the TCP/IP connection is kept open. Or, in the case of a client connected via HTTP, until the client fails to send a request for some time ('ping' messages are sent every thirty seconds or so).

YMSG packets consist of a twenty byte header, that consists of four bytes for "YMSG", two bytes for the version (current is version 19),two bytes for the vendor id (typically 0), two bytes for the packet length(content only, does not include the 20 byte header), two bytes for the Service(the type of ymsg packet), 4 bytes for the Status(not related to buddy status, though is used to login as invisible), and four bytes for the Session id. The rest of the packet is a variable length table of key/value pairs, where the key is an ASCII representation of a numeric code representing the field type, and the value is its associated data. A two byte separator, the hexadecimal values C0 and 80, are used to delimit each entry in this table.

Some parts of YMSG rely on other protocols. For example, file transfer is initially negotiated using YMSG, but the actual transfer of the file is done via HTTP. Webcams also use YMSG to discover and request permission to view, but HTTP to actually feed JPEG 2000 images from one client to another. Chatroom categories, rooms and lobbies are retrieved using HTTP as XML documents. Regular webcam connections use H.323. Yahoo! with Voice uses SIP. For calls, VoIP is handled indirectly by Yahoo! servers so the chat client does not have direct access to it. Yahoo! Chat and Conference Voice however use an older from of audio compression called TrueSpeech, developed by the DSP group.

The chatroom categories can be retrieved from here.