After considering several alternative communication protocols, simple network sockets were chosen for the communication layer, since

• they can be used for local as well as remote sessions, and
• can be tunneled through SSH.

The details of the protocol are described below.

Startup

User requests a new session from Anthias and specifies if it is a local session, or a remote session.

Local session

If local session, then

• Anthias opens a pseudo-tty,
• sets TERM_EXTENDED=AnthiasLocal
• starts user's login shell in it.

Remote session

If remote session,

• Anthias prompts user to enter remote hostname/ip.
• Anthias connects to remote host using SSH.
• Anthias sets env-variables:
  1. TERM_EXTENDED=AnthiasRemote
  2. ANTHIAS_REMOTE_ADDR=#ip_address

(This is required so that remote application can bind to the same ip-address as the SSH session.)

• Anthias opens a SSH channel and executes the user's shell in it.

Shell startup

At this point, the user is presented with an innocuous xterm-like interface. There are no special widgets, no directory browsing, nothing. But now, if the user (or the login process) starts an Anthias-capable shell, the ball starts rolling. We will call this hypothetical shell, the AShell.

AShell Registeration

• AShell checks if TERM_EXTENDED is set.

• AShell creates a socket. If TERM_EXTENDED is AnthiasRemote, then it binds the socket to ANTHIAS_REMOTE_ADDR. Otherwise it binds to localhost.

• AShell emits "ANTHIAS_REGISTER_SEQUENCE #my_ip #my_port_num" onto the terminal.

• When Anthias sees the ANTHIAS_REGISTER_SEQUENCE, it creates a socket and connects it to the AShell's socket. If the session is local, then this is

straight forward. If it is remote, then the SSH service is used to port-forward.

• Anthias emits "ANTHIAS_REGISTER_RESPONSE [success|failure]" onto the terminal.

• From now on, communication between AShell and Anthias takes place over the socket.

Communication basics

Communication between AShell and Anthias will be in binary. The advantages of this approach are,

• It will save extraneous string parsing and comparisons.
• It will save bandwidth, memory, and ultimately, processing time.
• Skipping to next message can be efficiently done by skipping a known number of bytes.

Message Types

Here we list the message types used by Anthias to communicate with AShell.

AShell initiated messages

• FirstTokenCompletions
• ChangePWD

Anthias initiated messages

• Version
• ExecuteCommand