Firewalls will often drop idle ssh connections. You (on your client) and/or your server administrator (on your server), can take steps to ensure connections stay alive and thus are not dropped.

IT Security Office's info on this, as of March 2016:

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Traffic Filtering Issues

Problem : ssh connections silently disconnect when left idle, yielding a "broken pipe" or similar message

Root Cause: an idle ssh connection, by default, sends no traffic, which allows the connection to time out in the Managed Firewall state table. The state table cannot have a timeout value longer than the default 300 seconds

Solution/Workaround: ssh can be configured, either on the ssh client or ssh server side, to send periodic keepalive packets.

on the client, in the file "ssh_config" in either /etc/ or /etc/ssh, use the lines:

Host *

ServerAliveInterval 300

ServerAliveCountMax 2

on the server, in the file "sshd_config" in either /etc/ or /etc/ssh, use the line:

ClientAliveInterval 300

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Managed Firewall Service#TrafficFilteringIssues

Information from Chemistry IT

On networks with firewalls, such as at Cornell, an ssh connection will be dropped if it remains idle.
- A drop can be prevented by configuring the client and/or server to send a “keep alive” packet so the connection is not strictly idle.
- The idle cut-off time on Cornell's networks is about 5 minutes. Thus, a keep-alive of 4 minutes should suffice.
In Chemistry IT, we have set all our servers to 4 minutes and that has seemed to work well for our researchers (as of April 2016).
MacOS clients: No good solution, alas.
- Chemistry IT is trying to craft a better solution at the network level. Contact us for an updated status report.