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The
Secure Shell, or SSH, is a popular program that lets computer users log onto
remote machines. Software developers use it for large collaborative projects,
students use it to work from university servers, customers of commercial cloud computing
services use it access their accounts, and system administrators use it to
manage computers on their networks.
First
released in 1995, SSH was designed for an Internet consisting of stationary
machines, and it hasn’t evolved with the mobile Internet. Among other problems,
it can’t handle roaming: If you close your laptop at the office and reopen it
at home, your SSH session will have died; the same goes for an SSH session on a
tablet computer that switches from a Wi-Fi connection to the cellular network.
Researchers
at Massachusetts Institute of Technology (MIT)’s Computer Science and
Artificial Intelligence Laboratory recently presented a paper describing a new
remote-login program called Mosh, for mobile shell, which solves many of SSH’s
problems. The researchers also believe that the communication scheme underlying
Mosh could improve the performance of a host of other mobile applications.
Even
before they presented the paper, they made Mosh freely available on a number of
different websites; it’s now been downloaded at least 70,000 times. “That’s
from the ones that we’re able to track,” says Keith Winstein, a graduate
student in MIT’s Department of Electrical Engineering and Computer Science and
lead developer of Mosh.
Echo location
Besides roaming, another of the problems that Mosh addresses is the delayed “echoing” of keystrokes in SSH. During a standard SSH session, when a user
strikes a key on the keyboard, nothing appears onscreen until information about
the keystroke travels to the remote machine, which performs a computation and
sends back the result. That’s because, in many applications commonly run
through SSH, keystrokes don’t necessarily correspond directly to displayed
symbols: In an email program, for instance, the “N” key might call up the next
email; similarly, when a user enters a password, it shouldn’t appear onscreen.
Mosh
has an algorithm running in the background that deduces when keystrokes should
be displayed and when they shouldn’t. Until the remote computer confirms Mosh’s
predictions, the characters onscreen are underlined. “I have never seen it
display anything wrong,” says Hari Balakrishnan, a professor in the Department
of Electrical Engineering and Computer Science and Winstein’s coauthor on the
Usenix paper.
The
reason Mosh handles roaming so much better than SSH does is that it abandons
the Transmission Control Protocol, or TCP—the framework that governs almost all
the traffic in today’s Internet.
“TCP
has some wonderful ideas embedded in it—congestion control, ways of doing
reliability, and so forth,” Balakrishnan says. “But it has this one big, big
problem: It provides a reliable, in-order byte-stream abstraction between two
fixed endpoints. If you were to pick the worst possible abstraction for the
mobile world, it would be that.”
With
mobile applications, Balakrishnan explains, it’s not as crucial that every byte
of information be displayed in exactly the order in which it was sent. If
you’ve lost connectivity while using the map application on a smartphone, for
instance, when the network comes back up, you probably want an accurate map of
your immediate surroundings; you don’t want to wait while the phone reloads
data about where you were when the network went down.
State currency
Winstein and Balakrishnan developed their own communications protocol, which
they call SSP, for state synchronization protocol. SSP, Balakrishnan says,
works more like the protocols that govern videoconferencing, where getting
timely data about the most recent state of the application is more important
than getting exhaustive data about previous states.
Mosh
is already proving itself useful: At his computer in his office, Balakrishnan
pulls up the connection log for one of the servers in MIT’s Athena network; a
third of the people logged into it are using Mosh. But in ongoing research,
Winstein and Balakrishnan are investigating how SSP can be improved and
extended so that other applications can use it as well.
“We
have sort of a broader agenda here,” Winstein says. “Mosh is a gracefully
mobile application. But there’s a lot of even more popular network applications
that have the same problems, like Gmail, or Google Chat, or Skype. None of
these programs gracefully handle mobility, even though they’re intended for
mobile devices.”
“Mosh
is a fine bit of engineering, focused on the precise requirements of the
application at hand,” says Jon Howell, a researcher at Microsoft Research who
specializes in web applications. Winstein, Howell says, “spent a lot of effort
on the gritty practical details of correct terminal behavior.”
Howell
points out that there’s a good deal of existing research on state
synchronization, the technique underlying SSP, and he’s not sure that SSP
offers any clear advantages over its predecessors. “If [Winstein] pushes the
SSP idea farther, I suspect he’ll find himself covering some of the [same]
territory,” Howell says. “Perhaps he’ll discover a new strategy or a new class
of applications for which synchronization is particularly useful.”
