11 ms·

> ...I'd be curious to know how they get around laggy interrupts, e.g. cat'ing a massive file and trying to Ctrl-C out of it. Presumably with the TCP URG flag.

> ...I'd be curious to know how they get around laggy interrupts, e.g. cat'ing a massive file and trying to Ctrl-C out of it. Presumably with the TCP URG flag. That said, I only read years ago that's how it's done, without ever checking myself. I've found references to in in telnet, but not ssh, so perhaps the latter uses some other mechanism.

geofft 9y ago
TCP urgent data only indicates that the application should try really hard to read up to a certain point. It doesn't provide a way to abandon transmission of data that's already been sent with write(). It also generally does not convince routers on the way to prioritize the urgent packet before packets already in their buffers. The way you'd want to use it is not to transmit the Ctrl-C urgently (that direction is clear), but to transmit the interruption of the reply urgently back to the client. And I think there's nothing that will save the server from having to actually send all the bytes that you managed to attempt to send before you received the Ctrl-C. Also, it's completely broken: https://tools.ietf.org/html/rfc6093 https://tools.ietf.org/html/rfc6093 mosh works around this by rendering the terminal window server-side and sending diffs of the terminal window (think VNC or streaming video) to the client. So the in-flight data from server to client is at most a screenful. If the data isn't sent in the first place, you don't have to worry about unsending it. (Of course the tradeoff is loss of scrollback.)
- btym 9y ago
  >Of course the tradeoff is loss of scrollback. That could always be buffered on the server side.
  - joombaga 9y ago
    If you use a terminal multiplexer (screen or tmux) it effectively is.
  - geofft 9y ago
    You won't get native scrollback with server-side buffering. Precisely because it's scrollback that's off the screen, there's no way to send ANSI escape sequences to edit it after the fact; it has to actually be written to the screen and then scrolled off. So you can't really do anything where you transmit the current screen at higher priority and scrollback at lower priority. (Unless you want to do something where mosh will write out scrollback when it receives it and then write the actual current screen below it, but I bet that will cause a ton of flicker.) You can probably do some things to heuristically improve scrollback, e.g., telling the terminal sync thing to sync 1000 lines of scrollback if the connection seems good and only the current screen if it seems poor. You won't get reliable scrollback but maybe it's good enough.
tptacek 9y ago
Virtually nothing uses TCP URG anymore; it is for all intents and purposes obsolete.

geofft 9y ago

TCP urgent data only indicates that the application should try really hard to read up to a certain point. It doesn't provide a way to abandon transmission of data that's already been sent with write(). It also generally does not convince routers on the way to prioritize the urgent packet before packets already in their buffers. The way you'd want to use it is not to transmit the Ctrl-C urgently (that direction is clear), but to transmit the interruption of the reply urgently back to the client. And I think there's nothing that will save the server from having to actually send all the bytes that you managed to attempt to send before you received the Ctrl-C. Also, it's completely broken: https://tools.ietf.org/html/rfc6093 https://tools.ietf.org/html/rfc6093 mosh works around this by rendering the terminal window server-side and sending diffs of the terminal window (think VNC or streaming video) to the client. So the in-flight data from server to client is at most a screenful. If the data isn't sent in the first place, you don't have to worry about unsending it. (Of course the tradeoff is loss of scrollback.)

btym 9y ago
>Of course the tradeoff is loss of scrollback. That could always be buffered on the server side.
- joombaga 9y ago
  If you use a terminal multiplexer (screen or tmux) it effectively is.
- geofft 9y ago
  You won't get native scrollback with server-side buffering. Precisely because it's scrollback that's off the screen, there's no way to send ANSI escape sequences to edit it after the fact; it has to actually be written to the screen and then scrolled off. So you can't really do anything where you transmit the current screen at higher priority and scrollback at lower priority. (Unless you want to do something where mosh will write out scrollback when it receives it and then write the actual current screen below it, but I bet that will cause a ton of flicker.) You can probably do some things to heuristically improve scrollback, e.g., telling the terminal sync thing to sync 1000 lines of scrollback if the connection seems good and only the current screen if it seems poor. You won't get reliable scrollback but maybe it's good enough.

tptacek 9y ago

Virtually nothing uses TCP URG anymore; it is for all intents and purposes obsolete.