> The Synchronous Idle (SYN) symbol is a heartbeat, and is especially useful for streaming data, such as to keep a connection alive. > > SYN tells the data

> The Synchronous Idle (SYN) symbol is a heartbeat, and is especially useful for streaming data, such as to keep a connection alive. > > SYN tells the data reader that data streaming is still in progress. > > SYN has no effect on the output content. > > Example of a unit that contains a Synchronous Idle: > > a␖b␞ Why would this go in-band inside a document format? Just why? If you want keep-alives, use a kind of connection that supports out-of-band keepalives. If you download the same document twice, and the second time the server is heavily loaded (or it's waiting on some dependency, or whatever), presumably the server will helpfully generate some SYNs in the middle of the document to keep the connection alive (?), but now you've got the same document "spelled" two different ways, that won't checksum alike. SYN along with the weirdness of > Escape + [non-USV-special] character: the character is ignored means that you have arbitrarily many ways of writing semantically-same documents.