Mosh is a remote terminal application like Secure Shell (SSH) with ...
SSH connections require at least one round-trip as the keystroke ha...
SSH operating in a character-at-a-time mode, having each echo and l...
Mosh's approach to the problem is simple: > Mosh performs predic...
Arguably the most important feature of Mosh, perhaps even more impo...
Mosh uses existing infrastructure for authentication and login, foc...
The Mosh client decides how confident to be in a prediction based o...
In these trials, Mosh was able to confidently predict 70% of the ke...
Mosh: An Interactive Remote Shell for Mobile Clients
Keith Winstein and Hari Balakrishnan
M.I.T. Computer Science and Artificial Intelligence Laboratory, Cambridge, Mass.
Mosh (mobile shell) is a remote terminal application
that supports intermittent connectivity, allows roaming,
and speculatively and safely echoes user keystrokes for
better interactive response over high-latency paths. Mosh
is built on the State Synchronization Protocol (SSP),
a new UDP-based protocol that securely synchronizes
client and server state, even across changes of the client’s
IP address. Mosh uses SSP to synchronize a character-
cell terminal emulator, maintaining terminal state at both
client and server to predictively echo keystrokes. Our
evaluation analyzed keystroke traces from six different
users covering a period of 40 hours of real-world us-
age. Mosh was able to immediately display the ef-
fects of 70% of the user keystrokes. Over a commer-
cial EV-DO (3G) network, median keystroke response
latency with Mosh was less than 5 ms, compared with
503 ms for SSH. Mosh is free software, available from It was downloaded more than
15,000 times in the first week of its release.
1 Introduction
Remote terminal applications are almost as old as packet-
switched data networks. The most popular such applica-
tion today is the Secure Shell (SSH) [9], which runs in-
side a terminal emulator. Unfortunately, SSH has two
major weaknesses that make it unsuitable for mobile
use. First, because it runs over TCP, SSH does not sup-
port roaming among IP addresses, or cope with intermit-
tent connectivity while data is pending, and is almost
unusable over marginal paths with non-trivial packet
loss. Second, SSH operates strictly in character-at-a-
time mode, with all echoes and line editing performed
by the remote host. On today’s commercial EV-DO and
UMTS (3G) mobile networks, round-trip latency is typi-
cally in the hundreds of milliseconds when unloaded, and
on both 3G and LTE networks, delays reach several sec-
onds when buffers are filled by a concurrent bulk transfer.
Such delays often make SSH painful for interactive use
on mobile devices.
This paper describes a solution to both problems.
We have built Mosh, the mobile shell, a remote ter-
minal application that supports IP roaming, intermittent
connectivity, and marginal network connections. Mosh
performs predictive client-side echoing and line editing
without any change to server software, and without re-
gard to which application is running. Mosh makes re-
Figure 1: Mosh in use.
mote servers feel more like the local computer, because
most keystrokes are reflected immediately on the user’s
display—even in full-screen programs like a text editor
or mail reader.
These features are possible because Mosh operates at
a different layer from SSH. While SSH securely con-
veys an octet-stream over the network and then hands it
off to a separate client-side terminal emulator to be inter-
preted and rendered in cells on the screen, Mosh contains
a server-side terminal emulator and uses a new protocol
to synchronize terminal screen states over the network,
using the principle of application-layer framing [3].
Because both the server and client maintain an image
of the screen state, Mosh can support intermittent con-
nectivity and local editing, and can adjust its network
traffic to avoid filling network buffers on slow links. As a
result, unlike in SSH, in Mosh “Control-C” always works
to cease output from a runaway process within an RTT.
Mosh’s design makes two principal contributions:
1. State Synchronization Protocol: A new secure
object synchronization protocol on top of UDP to
synchronize abstract state objects in the presence
of roaming, intermittent connectivity, and marginal
networks (§2).
2. Speculation: Mosh maintains the screen state at
both the server and client and uses the above pro-
tocol to synchronize them (§3). The client makes
guesses about the effect each new keystroke will
have on the screen, and when confident renders the
effects immediately. The client verifies its predic-
tions and can repair the screen state if necessary.
We have implemented Mosh in C++ and have exper-
imented across various networks and across disconnec-
tions (§4). Mosh is free software, distributed with a vari-
ety of operating systems and at
Mosh was downloaded more than 15,000 times in its first
week of release in April 2012. An example of Mosh’s in-
terface is shown in Figure 1.
2 State Synchronization Protocol
Mosh works to convey the most recent state of the screen
from server to client at a “frame rate” chosen based on
network conditions. This allows the server to avoid fill-
ing up network buffers, because it does not need to send
every octet generated by the application. (The reverse
direction has less flexibility because the client must send
every keystroke to the server.)
Supporting this is SSP, a lightweight secure datagram
protocol to synchronize the state of abstract objects be-
tween a local node, which controls the object, and a re-
mote host that may be only intermittently connected.
A state-synchronization approach is appropriate for
tasks like editing a document or using an e-mail or chat
application, which control the entire screen and provide
their own means of navigation through a document or
chat session. But it causes trouble for a task like “cat”-
ing a large file to the screen, where the user might rely
on having accurate history on the scrollback buffer.
When these semantics are a problem, the user can use
a pager such as less or more, or can use the screen or
tmux utilities, which are essentially pagers for the entire
terminal. Future versions of Mosh will allow the user to
browse the scrollback history.
The Mosh system runs SSP in each direction, instan-
tiated on two different kinds of objects. From client to
server, the objects represent the history of the user’s in-
put. From server to client, the objects represent the con-
tents of the terminal window.
2.1 Protocol design goals
SSP’s design goals were to:
1. Leverage existing infrastructure for authentication
and login, e.g., SSH.
2. Not require any privileged code.
3. At any time, take the action best calculated to fast-
forward the remote host to the sender’s current state.
4. Accommodate a roaming client whose IP address
changes, without the client’s having to know that a
change has happened.
5. Recover from dropped or reordered packets.
6. Ensure confidentiality and authenticity.
Because SSP doesn’t use any privileged code or au-
thenticate users, and key exchange happens out-of-band,
its security concerns are simplified. To bootstrap the ses-
sion, the user runs a script that logs in to the remote host
using conventional means (e.g., SSH) and runs the un-
privileged server. This program listens on a high UDP
port and prints out a random shared encryption key. The
system then shuts down the SSH connection and talks
directly to the server over UDP.
SSP is organized into two layers. A datagram layer
sends UDP packets over the network, and a transport
layer is responsible for conveying the current object state
to the remote host.
2.2 Datagram Layer
The datagram layer maintains the “roaming” connec-
tion. It accepts opaque payloads from the transport layer,
prepends an incrementing sequence number, encrypts the
packet, and sends the resulting ciphertext in a UDP data-
gram. It is responsible for estimating the timing char-
acteristics of the link and keeping track of the client’s
current public IP address.
The security of the system is built on AES-128 in the
Offset Codebook (OCB) mode [5], which provides con-
fidentiality and authenticity with a single secret key.
To handle reordered and repeated packets, SSP relies
on the principle of idempotency. Each datagram sent to
the remote site represents an idempotent operation at the
recipient—a “diff between a numbered source and tar-
get state. As a result, unlike Datagram TLS and Ker-
beros, SSP does not need to maintain a replay cache or
other message history state, simplifying the design and
Client roaming. Every time the server receives an au-
thentic datagram from the client with a sequence number
greater than any before, it sets the packet’s source IP ad-
dress and UDP port number as its new “target. As a
result, client roaming happens automatically, without the
client’s timing out or even knowing that it has changed
public IP addresses.
Estimating round-trip time and RTT variation. The
datagram layer is also responsible for estimating the
smoothed round-trip time (SRTT) and RTT variation
(RTTVAR) of the connection. Every outgoing datagram
contains a millisecond timestamp and an optional “times-
tamp reply, containing the most recently-received times-
tamp from the remote host.
We use the algorithm of TCP [7] with three changes:
1. Because every datagram has a unique sequence
number, there is no ambiguity between the times-
tamps of retransmissions of the same payload.
2. SSP adjusts the “timestamp reply” by the amount of
time since it received the corresponding timestamp.
Therefore, policies like delayed ACKs do not affect
the accuracy of the RTT estimates.
3. We reduce the lower limit on the retransmission
timeout to be 50 ms instead of one second. SSH
runs over TCP and rarely benefits from fast re-
transmissions, meaning it generally cannot detect a
dropped keystroke in less than a second.
2.3 Transport Layer
The transport layer synchronizes the contents of the local
state to the remote host, and is agnostic to the type of
objects sent and received.
Transport sender behavior: The transport sender up-
dates the receiver to the current state of the object by
sending an Instruction: a self-contained message listing
the source and target states and the binary “diff between
them. This “diff is a logical one, calculated by the ob-
ject implementation. The ultimate semantics of the pro-
tocol depend on the type of object, and are not dictated
by SSP. For example, for user inputs, the diff contains
every intervening keystroke, whereas for screen states, it
is only the minimal message that transforms the client’s
frame to the current one.
Transport sender timing: Because SSP can construct
a diff between any two object states, it is not required
to send every octet it receives from the host and can
modulate the “frame rate” based on network conditions.
The minimum interval between frames is set at half the
smoothed RTT estimate, so there is about one Instruction
in flight to the receiver at any time.
As a result, when a process goes haywire and floods
the terminal, network buffers do not fill up and increase
latency, so unlike in prior work, Control-C and other in-
terrupt sequences continue to work.
The transport sender uses delayed acks, similar to
TCP, to cut down on excess packets. In more than 99.9%
of cases in our experiments, a delay of 100 ms was suffi-
cient to let the delayed ACK piggyback on host data.
The server also pauses from the first time its object has
changed before sending off an Instruction, because up-
dates to the screen tend to clump together, and it would
be wasteful to send off a new frame with a partial update
and then have to wait the full “frame rate” interval be-
fore sending another. A collection interval of 8 ms was
chosen as optimal after analyzing application traces (§ 4).
SSP sends an occasional heartbeat to allow the server
to learn when the client has roamed to a new IP address,
and to allow the client to warn the user when it hasn’t
recently heard from the server. The heartbeat also keeps
the connection open when the client is behind a network
We cap the maximum frame rate at 50 Hz, roughly the limit of
human perception, to save unnecessary traffic on low-latency paths.
address translator. We chose an interval of 3 seconds
to compromise between responsiveness and the desire to
reduce unnecessary chatter.
3 A Remote Terminal with Speculative
Local Echo
To support the Mosh application, we implemented a ter-
minal emulator that obeys the SSP object interface. The
client sends all keystrokes to the server, which applies
them and maintains the authoritative state of the termi-
nal, which it in turn synchronizes back to the client.
The client intelligently guesses the effect that
keystrokes will have on the terminal, and in most cases
can speculatively apply such effects immediately. The
client observes the success of its predictions to decide
how confident to be and whether to display the predic-
tions to the user.
On high-delay connections, we underline unconfirmed
predictions so the user doesn’t become misled. This
underline trails behind the user’s cursor and disappears
gradually as responses arrive from the server. Occasional
mistakes can be removed within an RTT and do not cause
lasting effect.
3.1 Implementing the terminal emulator
Mosh’s terminal emulator implements the subset of
the ISO/IEC 6429/ECMA-48 language [1] used by
typical terminal emulators, including the xterm,
gnome-terminal,, and PuTTY pro-
grams for X11, OS X, and Windows. This protocol was
popularized by Digital Equipment Corp. in the 1970s and
80s and specifies a series of escape sequences to move
the cursor, render characters in bold and colors, erase ar-
eas of the screen, etc. The protocol is bidirectional, as
the host can query the terminal for its current character
position and ask it to identify itself.
3.2 Speculative local echo
Because Mosh operates at the terminal emulation layer
and maintains the screen state at both the server and
client, it is possible for the client to make predictions
about the effect of user keystrokes and later verify its
predictions against the authoritative screen state coming
from the server.
Most Unix applications operate similarly in response
to user keystrokes. They either echo the key at the current
cursor location or not. As a result, it is possible to ap-
proximate a local user interface for arbitrary remote ap-
plications. We use this technique to boost the perceived
interactivity of a Mosh session over a high-latency net-
work or one with packet loss.
Our general strategy is for the Mosh client to make
an echo prediction each time the user hits a key, but not
necessarily to display this prediction immediately.
The predictions are made in groups known as
“epochs, with the intention that either all of the pre-
dictions in an epoch will be correct, or none will. An
epoch begins tentatively, making predictions only in the
background. If any prediction from a certain epoch is
confirmed by the server, the rest of the predictions in that
epoch are immediately displayed to the user, along with
any future predictions in the same epoch.
Some user keystrokes are likely to alter the host’s echo
state from echoing to not, or are otherwise hard to pre-
dict, including the up- and down-arrow keys and control
characters. These cause Mosh to lose confidence and in-
crement the epoch, so that future predictions are made in
the background again.
In practice, this approach accommodates a wide vari-
ety of application behaviors, including multi-mode edi-
tors like vi (which sometimes echo conventionally and
sometimes don’t), and the possibility that the user might
type a command at the prompt (e.g., passwd) that stops
server-side echoes after the ENTER key is typed.
Because the decision to perform local echo is made en-
tirely based on the application’s observed behavior, ap-
plications need not be rewritten to accommodate local
echo. Unlike prior work, Mosh’s local echo works even
with full-screen programs (like emacs) that put the ter-
minal driver in “raw” mode and do their own echoing.
In typical use, Mosh can display immediately the ef-
fects of almost all “typing,” which constitutes more than
two-thirds of user keystrokes in our captures. The re-
maining keystrokes are principally “navigation” (such as
“n” to move to the next e-mail message in a mail reader),
which cannot be predicted locally.
Server-side assistance for prediction evaluation
For the above algorithm to work properly, the Mosh
client must be able to reliably determine whether its
echo predictions are correct. Early versions of Mosh at-
tempted to do this with the client only, by simply exam-
ining whether a predicted echo was present on the screen
by the time the Mosh server had acknowledged the cor-
responding keystroke.
Unfortunately, in trials, we found that applications
sometimes take tens of milliseconds after input is pre-
sented to them before echoing to the screen. This can
lead the Mosh server to acknowledge an input keystroke
before the echo is present in the screen state, and causes
the client to conclude that its prediction was incorrect,
even though the echo is on the way. This produces an-
noying flicker as the echo is (mistakenly) removed from
the screen, then reinstated when it eventually arrives
from the server.
Our initial solution to this problem was a client-side
timeout, so that a prediction is not considered incor-
rect until the corresponding keystroke has been acknowl-
edged by the server and a certain amount of time has
elapsed. Unfortunately, because of network jitter that can
delay the eventual echo beyond the timeout, this too pro-
duced an annoying number of false-negatives and result-
ing flicker. (By contrast, setting the timeout long enough
to accommodate large amounts of jitter causes mistaken
predictions to linger on the screen for too long.)
Our final solution was to implement a server-side time-
out of 50 ms, chosen to contain the vast majority of le-
gitimate application echoes on loaded servers, while still
fast enough to rapidly detect mistaken predictions. The
terminal object that is synchronized to the client contains
an “echo ack” field, representing the latest keystroke that
has been presented to the application for at least 50 ms
and whose effects ought to be reflected in the current
screen. The client has no timeouts of its own, and con-
sequently network jitter does not adversely affect the
client’s ability to evaluate whether a prediction is correct.
The cost is increased network traffic, because the server
often sends an extra datagram 50 ms after a keystroke to
convey the echo ack.
In practice, this has eliminated the flicker caused by
4 Results
We evaluated Mosh using traces contributed by six users,
covering about 40 hours of real-world usage and includ-
ing 9,986 total keystrokes. These traces included the
timing and contents of all writes from the user to a re-
mote host and vice versa. The users were asked to con-
tribute “typical, real-world sessions. In practice, the
traces include use of popular programs such as the bash
and zsh shells, the alpine and mutt e-mail clients, the
emacs and vim text editors, the irssi and barnowl chat
clients, the links text-mode Web browser, and several
programs unique to each user.
To evaluate typical usage of a “mobile” terminal, we
replayed the traces over an otherwise unloaded Sprint
commercial EV-DO (3G) cellular Internet connection in
Cambridge, Mass. A client-side process played the user
portion of the traces, and a server-side process waited for
the expected user input and then replied (in time) with
the prerecorded server output. We sped up long periods
with no activity. The average round-trip time on the link
was about half a second.
We replayed the traces over two different remote shell
applications, SSH and Mosh, and recorded the user inter-
face response latency to each simulated user keystroke,
as seen by the user. The Mosh predictive algorithm and
Figure 2: Cumulative distribution of keystroke response
times with Sprint EV-DO (3G) Internet service
0 0.2 0.4 0.6 0.8 1
Keystroke response time (seconds)
median: 5 ms
mean: 173 ms
median: 503 ms
mean: 515 ms
SSP were frozen prior to collecting the traces and were
not adjusted in response to their contents or results.
The cumulative distributions and statistics of
keystroke response time are shown in Figure 2. When
Mosh was confident enough to display its predictions,
the response was nearly instant. This occurred about
70% of the time. But many of the remaining keystrokes
were “navigation, such as moving to the next e-mail
message, and Mosh cannot make a prediction in these
cases. For keystrokes it could not predict, Mosh’s
latency distribution was similar to that of SSH.
Mosh displayed an erroneous prediction, which it
fixed within an RTT, for 0.9% of the keystrokes. These
generally occurred because of word-wrap (characters
that were printed near the end of a line get moved to the
next line at an unpredictable time).
Appropriateness of timing parameters
We also used the user traces to examine our choice of
timing parameters for the SSP sender. Here, we assess
the choice for the “collection interval”: the pause time
after receiving a write from the host, in order to collect
writes that may be following in close succession. We
disregard the possible benefits of speculative local echo
and focus on network performance.
Figure 3 shows the artificial delay introduced by the
Mosh server on the applications’s screen updates in our
traces. Recall that the server obeys two rules: always
We subsequently “unfroze” and modified the Mosh algorithm in
response to the data, including moving the collection interval to 8 ms
and adding the server-side timeout and “echo ack” feature to reduce
false-negative predictions on slow servers (§3.2). These changes im-
proved Mosh in real-world use but would have little effect on this eval-
uation, because it used a long-delay link with an unloaded server.
wait at least the frame-rate interval after a previous
frame, and always wait at least the “collection interval”
after receiving an initial write from the application. This
parameter represents a tradeoff: too short could cause
the server to send a tiny initial datagram and then wait
before sending more data. But too long would hurt the
responsiveness of a typical session.
The ideal value depends on how often, empirically, ap-
plications tend to wait between their writes. We had ini-
tially guessed that a value of 15 ms would be reasonable;
based on the results and user feedback, we adjusted that
to 8 ms, the minimum of the curve.
Predictive echo on other networks
After tuning the algorithm as discussed above, we
evaluated the same user traces replayed over a wireless
Internet service loaded with a concurrent TCP download,
and a trans-oceanic wired link. Again, Mosh displayed
about 70% of the keystrokes instantly, sometimes (but
not always) increasing the variance in latencies seen by
the user. We summarize these results as follows:
Verizon LTE service in Cambridge, Mass., running
one concurrent TCP download:
Median latency Mean σ
SSH 5.36 s 5.03 s 2.14 s
Mosh < 0.005 s 1.70 s 2.60 s
MIT-Singapore Internet path (to Amazon EC2
data center):
Median latency Mean σ
SSH 273 ms 272 ms 9 ms
Mosh < 5 ms 86 ms 132 ms
Resilience to high packet loss
We also tested SSP’s resilience to packet loss without
the benefit of predictive local echo. In general, SSP’s
delay-based rate control and ability to skip intermediate
states allow it to handle links with non-congestive packet
loss, which TCP was not designed to handle.
We set up a test network with a Linux-based router, us-
ing the netem tool to create an artificial RTT of 100 ms
and a 29% probability of i.i.d. packet loss in each di-
rection, resulting in 50% round-trip packet loss. As
expected, TCP
produces huge delays because of loss-
induced exponential backoffs:
Median Mean σ
SSH 0.416 s 16.8 s 52.2 s
Mosh (no predictions) 0.222 s 0.329 s 1.63 s
5 Related Work
GNU screen and OpenBSD tmux are popular “termi-
nal multiplexers” that allow the user to detach from and
Linux 2.6.32 default TCP (cubic)
Figure 3: Average protocol-induced delay from varying
collection interval (with frame interval of 250 ms)
0.1 1 10 100
Average delay (ms)
Collection interval after first write (ms)
Mosh collection interval
later reattach to a terminal session. (Graphical remote-
desktop programs, such as VNC, also allow reconnec-
tion.) screen and tmux provide several other features,
such as multiplexing and scrollback buffers, and are of-
ten used concurrently with Mosh.
REX [4] is a remote execution protocol built atop the
Self-certifying File System [6]. It uses TCP, but provides
automatic roaming in some cases: when the client finds
that a TCP connection aborts or a connection timeout oc-
curs, it reinitiates the TCP connection and queues pend-
ing data in the mean time. However, it could take several
minutes or longer for a TCP connection timeout to occur,
especially if the client has no pending data of its own.
Mosh differs from terminal multiplexers and REX
in that its roaming is immediate and automatic, using
application-level timers that assess the state of connec-
tivity end-to-end. Mosh is also distinct in that it skips
over intermediate screen states, even while connected, to
accommodate high-latency or loss-prone paths.
Some BSD-style operating systems support the
LINEMODE option [2] for TELNET, in which character
echoing and line editing is performed by the client. Un-
fortunately, LINEMODE does not work with programs
that put the terminal into “raw” mode, including shells
like bash, and full-screen applications like emacs, vi, or
pine. SSH does not have an equivalent of LINEMODE.
SUPDUP [8] included a Local Editing Protocol in
which an entire text editor session could be executed lo-
cally and uploaded to the server in batches. SUPDUP re-
quired the host application to encode its interactive func-
tionality in the SUPDUP language. Mosh does not re-
quire modifications to host applications, but still handles
most typing and cursor movement keystrokes.
6 Conclusion
This paper presented the design, implementation, and
evaluation of Mosh, a mobile shell that performs well
over marginal networks. Mosh handles intermittent con-
nectivity and changes in IP addresses, and provides good
interactive performance over long-delay network paths.
In our empirical evaluation of 40 hours of keystroke ac-
tivity from six users, we found that mean and median
response times were dramatically reduced on several dif-
ferent types of connections. Mosh achieved this im-
provement by accurately predicting the response to 70%
of user keystrokes. Mosh’s wide adoption upon release
suggests that it fulfills a previously unmet need among
mobile network users.
SSP is a relatively rare example of a gracefully-mobile
networking protocol. Today, many programs intended
for mobility, including e-mail and chat programs on pop-
ular smartphones, cannot cope gracefully with roaming
and intermittent connectivity: the very conditions pre-
sented by mobile networks. We believe many of these
applications would benefit from SSP’s design principles.
7 Acknowledgments
We thank Nickolai Zeldovich and Chris Lesniewski-Laas
for helpful comments on this work. We also thank An-
ders Kaseorg, Quentin Smith, Richard Tibbetts, Keegan
McAllister, and the users who provided us with keystroke
traces. This work was supported in part by NSF grants
1040072 and 0721702.
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SSH connections require at least one round-trip as the keystroke has to be sent to the server to correctly render the next buffer. Mosh decouples this, providing client-side prediction and having the client repair incorrect predictions after synchronizing with the server. Arguably the most important feature of Mosh, perhaps even more important than the reduction of keystroke latency. Being able to freely roam between networks while maintaining a connection to the server *feels* like magic. The Mosh client decides how confident to be in a prediction based on the success of its past predictions. A low confidence prediction will not be shown to the user. Mosh's approach to the problem is simple: > Mosh performs predictive client-side echoing and line editing without any change to server software, and without regard to which application is running. Mosh makes remote servers feel more like the local computer, because most keystrokes are reflected immediately on the user’s display — even in full-screen programs like a text editor or mail reader. SSH operating in a character-at-a-time mode, having each echo and line editing operations occurring on the remote host results in a terrible user experience when using an unreliable network connection. Additionally, if the amount of lost packets is excessive the connection may be dropped and the entire terminal state will be lost, forcing the user to start from scratch or use a terminal multiplexer like tmux or screen. Mosh uses existing infrastructure for authentication and login, focusing only on solving the problem of terminal state prediction and synchronization, and roaming connections. Mosh is a remote terminal application like Secure Shell (SSH) with better support for connections over an unreliable network. Unlike SSH, Mosh allows roaming connections and speculative echoing of keystrokes. In these trials, Mosh was able to confidently predict 70% of the keystrokes, resulting in a nearly instant response to the user. Many of the remaining 30% of keystrokes included things like navigation using arrow keys or using shortcuts to change “views” in a program. For cases like this, Mosh's response times were similar to those of SSH.