PuTTY sending extra characters at the beginning of TELNET connection initialisation

Question

When a TELNET session is initialised using PuTTY, I see that PuTTY is sending the following characters to the server at the beginning of the connection:

..... .....'.........

I was able to capture this using WireShark. The hex characters are as follows:

ff fb 1f ff fb 20 ff fb 18 ff fb 27 ff fd 01 ff fb 03 ff fd 03

Can you please help me understand why PuTTY sends these characters? I assumed it is something with the connection, but then, these characters are found in the DATA category of the WireShark capture.

Here is a video: https://www.youtube.com/watch?v=WGZru4KPmsM

Answer 1

Thanks to Some programmer dude and Elmi.

These extra chunk of characters are the option negotiating sequences from PuTTY (client) to the server. Different telnet clients (For example: ZOC Terminal or native macOS terminal's telnet) use different options. Negotiations allow the client to have an enriched experience.

A list of negotiation sequences in telnet protocol and their explanation is given here

If you are using PuTTY and do not need these sequences, go to Connection > Telnet and set the Telnet negotiation mode to Passive

In the words of Some programmer dude:

Perhaps there's settings in the other client to not start with sending these option negotiating sequences? Or perhaps PuTTY is more telnet-compliant than the other clients? There are unfortunately many so-called "telnet clients" which only handles sending plain text and just don't care about the rich telnet protocol.

Here is a link to Elmi's post