Mosh patch series
This branch changes how Mosh compresses terminal state, schedules competing traffic, and moves bulk data. It is aimed at satellite, radio, metered, and other links where bandwidth is scarce and interactive traffic still needs to arrive first.
01 / Artifacts
Each entry starts with the implementation artifact, then states the reason for the change and the practical effect it is intended to have.
Once both peers advertise zstd, every serialized state update uses zstd level 22. Initial packets stay with zlib for negotiation.
Screen updates are small but frequent, and satellite bandwidth is expensive. State compression is a core part of the link strategy.
There is no size threshold, zlib comparison, or codec-level tuning. After negotiation, the state codec is always zstd-22.
State updates can be sampled, compiled into a zstd dictionary, and supplied during session setup.
Terminal applications repeat structures, prompts, status lines, and escape sequences that a trained dictionary can represent compactly.
Dictionary compression activates only when both peers advertise the same dictionary ID, keeping the choice explicit and verifiable.
Forwarded bytes are coalesced and rate-limited in separate medium- and low-priority token buckets.
A tunnel, agent, or X11 client can generate enough queued traffic to compete with terminal state on a narrow uplink.
Terminal traffic stays ahead. Agent and X11 use medium priority; TCP and SOCKS use low priority.
adam-moshcp moves files through an active session using paced datagrams and forward-error-correction symbols.
Loss recovery and bulk transfer need bounds of their own; an unpaced copy can consume the link needed by the shell.
Bulk packets are sent only when reliable terminal or stream traffic is not queued. Reed–Solomon is the default; RaptorQ is optional.
The branch adds -L, -R, -D, -A, and -X, plus reliable out-of-band OSC 52 clipboard events.
A remote workflow often needs forwarding and clipboard exchange in addition to terminal text.
Those facilities share the Mosh session and its scheduler instead of requiring each workflow to ignore the link budget.
Static Kitty RGB, RGBA, and PNG uploads are decoded once and stored as lossless WebP with image and placement deltas.
Raw pixel uploads are large, while terminal placement changes usually need only a few fields and no new pixels.
Unchanged images are not resent. The client converts WebP back to a standard Kitty RGBA upload only when the local terminal needs it.
The authenticated bootstrap carries the client's exact TERM value into the remote login session.
Mosh previously replaced terminal identity with xterm or xterm-256color, hiding a Kitty-capable Ghostty client.
Remote applications can see xterm-ghostty, select Kitty graphics, and confirm support with the protocol query. The matching terminfo entry is still required remotely.
Programs and manual pages use an adam- prefix, with optional zstd and RaptorQ build detection.
A protocol patch branch needs to coexist with the stock client and server during evaluation.
The patched and upstream installations can remain on the same machine without replacing one another.
02 / Behavior
Compression reduces the amount of data. Scheduling determines which data gets the scarce send opportunity when several features are active at once.
| Traffic | Class | Send rule |
|---|---|---|
| Terminal state + input | primary | Scheduled before forwarded streams and bulk transfer. |
| SSH agent + X11 | medium | Shares a medium-priority token bucket. |
| TCP + SOCKS | low | Uses a separate low-priority token bucket. |
| adam-moshcp | idle | Sends only when reliable terminal or stream traffic is not queued. |
03 / Inspect
The prefixed binaries are intentional: they let this patch set live beside an ordinary Mosh installation.
$ git clone --branch low-bandwidth-mosh --single-branch \ https://github.com/adamdeprince/mosh.git $ cd mosh $ ./autogen.sh $ ./configure --with-zstd $ make $ make check # Run the prefixed client. $ adam-mosh user@host
04 / Limits
The branch is a working patch series, not an upstream Mosh release.
Enhanced protocol features require this branch's client and server. Initial compression remains compatible while peers negotiate support.
State sample logs contain uncompressed terminal updates and may include commands, output, or other sensitive terminal contents.
Appropriate forwarding rates and transfer redundancy depend on the capacity, loss pattern, and cost of the actual path.
