4 ms·

Yes, TCP header had 16 bits allocated to "Urgent Pointer". It's (unfortunately) still there for compatibility, but RFC 9293, which updated RFC 793 after more th

Yes, TCP header had 16 bits allocated to "Urgent Pointer". It's (unfortunately) still there for compatibility, but RFC 9293, which updated RFC 793 after more than 40 years, has the following to say about it: > As a result of implementation differences and middlebox interactions, new applications SHOULD NOT employ the TCP urgent mechanism. However, TCP implementations MUST still include support for the urgent mechanism.

LegionMammal978 3y ago
As it happens, that feature is used in the FTP protocol (RFC 959) to abort the current file transfer (or to immediately print a status message), since some old implementations would have to stop listening on the control connection while a transfer was in progress and could only be woken up with an interrupt. At least vsftpd supports it, I haven't looked at any other implementations. The main difficulty is that the libc API for reading urgent data is somewhat painful, and most higher-level TCP libraries haven't bothered with it for obvious reasons. Of course, basic ftp:// clients (the only kind that are really used today) would probably rather just close the control connection, which has the same effect.