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Abstract:
This presentation introduces Mosh (Mobile Shell), a modern alternative to SSH designed to address the limitations of traditional remote terminal access in the face of modern network conditions like mobility, intermittent connectivity, and high latency. The speaker argues that SSH's reliance on TCP and raw byte stream transmission is ill-suited for today's internet, leading to broken connections upon roaming (IP address changes) and frustrating delays, especially for user input feedback. Mosh utilizes a novel UDP-based protocol called SSP (State Synchronization Protocol), which synchronizes the abstract state of the terminal screen rather than a continuous byte stream. This architecture allows Mosh to seamlessly handle client roaming, persist connections through sleep/wake cycles and network changes, and significantly improve perceived responsiveness. Mosh further enhances the user experience with predictive local echo, displaying typed characters immediately even on high-latency links. The presentation includes a demonstration comparing Mosh and SSH under simulated roaming and high-latency conditions, highlights Mosh's adoption, and posits that the underlying state synchronization paradigm could benefit other mobile applications that currently struggle with robust connectivity.
Mosh: A Mobile Shell for Robust Remote Access
0:00:06 Introduction to Mosh: Mosh is presented as a new mobile shell, an alternative to SSH, designed to support roaming, intermittent connectivity, and provide a better user interface.
0:00:28 SSH Limitations: SSH uses TCP, sends keyboard input to the server, and waits for the server to send back screen updates, including the echo of typed characters. This leads to problems:
No Roaming: Connections break if your IP address changes (e.g., switching Wi-Fi, laptop sleep/resume).
Poor Performance: Suffers on high-latency or lossy networks (like Amtrak Wi-Fi or cellular) due to TCP's behavior.
Delayed Feedback: Typing feels sluggish as you wait for characters to echo back from the server.
0:01:14 Root Cause Identified: The speaker argues SSH's use of a raw octet stream over TCP is the wrong abstraction. Users primarily need the current screen state, not every intermediate byte. TCP buffers can cause long delays for output and make interrupting processes (Ctrl+C) unresponsive.
0:02:14 Mosh's Core Innovation: SSP: Mosh uses the State Synchronization Protocol (SSP) over UDP.
Object Synchronization: SSP synchronizes abstract objects (in Mosh's case, the terminal screen state) rather than byte streams.
Diff/Patch Mechanism: It works by computing differences (diffs) between states and sending only those diffs to be applied (patched) by the receiver.
Resilience: Designed for low latency, roaming, and lossy network paths.
A terminal emulator runs on the server, generating screen state objects.
SSP synchronizes this screen state object from server to client.
Keystrokes are synchronized from client to server using SSP.
Uses SSH for initial authentication but then switches to its own UDP protocol.
0:04:36 Seamless Roaming: Since Mosh uses UDP and identifies sessions potentially by port (or other means not tied strictly to IP), the server simply sends updates to the last known IP address and port from which it received an authenticated packet from the client. Roaming becomes transparent.
0:05:14 User Experience Boost: Predictive Local Echo: Mosh predicts whether typed characters will be echoed by the remote application.
Instant Feedback: Typed characters are displayed immediately on the local terminal, underlined to indicate they are speculative predictions.
Algorithm: Uses "epochs"; if any character in a predicted group (epoch) is confirmed by the server, the whole group is shown confidently (un-underlined). Mistakes are simply corrected when the actual server state arrives.
Roaming (0:06:35): Mosh handles a switch from Wi-Fi to cellular seamlessly, notifying the user of temporary unreachability and reconnecting. SSH connection breaks silently and requires manual termination and restart.
High Latency (0:07:21): Under simulated high latency (download running), Mosh remains usable with responsive local echo, while SSH typing becomes extremely laggy and frustrating.
0:07:52 Evaluation & Findings: Mosh displays ~70% of user keystrokes instantly. It works well over lossy links. The project also uncovered numerous Unicode handling bugs in various Unix terminals.
0:08:14 Adoption & Availability: Mosh is widely packaged for Linux distributions, macOS (via package managers), and FreeBSD. It has gained significant user interest and positive feedback.
0:08:53 Broader Implications: The presenter argues that the state synchronization paradigm used by Mosh (SSP) should be adopted by other mobile applications (like Gmail, Skype) that often fail silently or lose data when network conditions change or the user roams. An example of a months-old unsent Gmail draft due to a poorly handled network transition is shown (0:09:16).
0:10:30 Q&A - Firewall Ports: Mosh requires UDP ports to be open on the server (typically a range like 60000-61000). Each concurrent connection needs its own port, as the port helps identify the session when the client's IP changes.