Telnet reader will split input after 1448 characters

I am writing a java applet that will print what a telnet client sends to the connection. Unfortunately, the client splits at 1448 characters.

The code that is proving to be a problem:

char[] l = new char[5000];
Reader r = new BufferedReader(new InputStreamReader(s.getInputStream(), "US-ASCII"));
int i = r.read(line);

I cannot change the source of what the telnet client reads from, so I am hoping it is an issue with the above three lines.

Answers


You're expecting to get telnet protocol data units from the TCP layer. It just doesn't work that way. You can only extract telnet protocol data units from the code that implements the telnet protocol. The segmentation of bytes of data at the TCP layer is arbitrary and it's the responsibility of higher layers to reconstruct the protocol data units.

The behavior you are seeing is normal, and unless you're diagnosing a performance issue, you should completely ignore the way the data is split at the TCP level.


The reason you're only getting 1448 bytes at a time is that the underlying protocols divide the transmission into packets. Frequently, this size is around 1500, and there are some bytes used for bookkeeping, so you're left with a chunk of 1448 bytes. The protocols don't guarantee that if you send X bytes in a 'single shot', that the client will receive X bytes in a single shot (e.g. a single call to the receive method).

As has been noted already in the comments above, its up to the receiving program to re-assemble these packets in a way that is meaningful to the client. In generally, you perform receives and append the data you receive to some buffer until you find an agreed-upon 'end of the block of data' marker (such as an end-of-line, new-line, carriage return, some symbol that won't appear in the data, etc.).

If the server is genuinely a telnet server--its output might be line-based (e.g. a single block of data is terminated with a 'end of line': carriage return and linefeed characters). RFC 854 may be helpful--it details the Telnet protocol as originally specified.


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