Specify LINELEN ISUPPORT

_includes/c2s_structure.md

@@ -29,7 +29,7 @@ The specific parts of an IRC message are:
 
 These message parts, and parameters themselves, are separated by one or more ASCII SPACE characters `(' ', 0x20)`.
 
-Most IRC servers limit messages to 512 bytes in length, including the trailing `CR-LF` characters. Implementations which include [message tags](https://ircv3.net/specs/extensions/message-tags.html) need to allow additional bytes for the **tags** section of a message; clients must allow 8191 additional bytes and servers must allow 4096 additional bytes.
+Most IRC servers limit messages to 512 bytes in length, including the trailing `CR-LF` characters. Servers may indicate support for greater message sizes via the {% isupport LINELEN %} `RPL_ISUPPORT` paremeter. Implementations which include [message tags](https://ircv3.net/specs/extensions/message-tags.html) need to allow additional bytes for the **tags** section of a message; clients must allow 8191 additional bytes and servers must allow 4096 additional bytes.
 
 ---
 

_includes/modern-appendix.md

@@ -1373,6 +1373,16 @@ Examples:
 
       KICKLEN=307
 
+{% isupportheader LINELEN %}
+
+      Format: LINELEN=<length>
+
+The `LINELEN` parameter indicates the maximum size in bytes of IRC messages, excluding tags. In other words, `LINELEN` indicates the maximum size of this portion of the ABNF:
+
+      [':' <source> SPACE] <command> <parameters> <crlf>
+
+The value MUST be specified and MUST be a positive integer. If `LINELEN` is not sent by the server, clients SHOULD assume that the server will truncate the messages larger than 512 bytes.
+
 {% isupportheader MAXLIST %}
 
       Format: MAXLIST=<modes>:<limit>{,<modes>:<limit>}