ILN = Initial onLine Notification

MSNメッセンジャーでのメンバのオンライン通知。
これを受け取らないとメッセージを送ることができない。

http://hypothetic.org/docs/msn/notification/presence.php

Clients shouldn't take special measures to notify the principal when receiving an initial presence notification. For example, the official client doesn't pop up a message when it receives an initial online notification.

The ILN command has a TrID (the same TrID as your CHG or ADD command), and four parameters:

The first parameter is the principal's three letter status code. 
The second parameter is the principal's account name. 
The third parameter is the principal's URL-encoded display name. 
The fourth parameter is the principal's client ID number. 
ILN commands are one of the least well implemented parts of the MSN Messenger 

protocol in that there's no way of knowing when the list of ILNs in response to a CHG is complete, or whether you'll receive an ILN in response to an ADD. If this is a problem for your client, you could ping the server when you receive the CHG reply, and assume the list is complete when you receive the ping reply. Alternatively, if you don't receive any ILNs for several seconds, you could assume no more will come. Even so, you should be prepared to handle ILNs as if they were asynchronous commands.

曖昧にしか読み取れていないけど、久々に「ここ、テストに出るから！」っていう気分を味わった。

対話型のボットとか、常駐型のボットなら簡単だけど通知型のボットが作りにくいなぁ。
ユーザーがオフラインだったときに面倒くさい、timeout 式になりそうな悪寒。
もうちょっと。