After an attacker is successful in getting a payload onto a system and getting it to run they still have to worry about whether there will be a successful connection out to a command and control server. There are a number of different transport mechanisms that can be utilized including direct TCP connections, pivoting through a proxy, DNS, or even ICMP to name a few. In this episode of Tradecraft Security Weekly Beau Bullock (@dafthack) details some of these transports that can be used to establish remote command and control over a system.
Dnscat - https://github.com/iagox86/dnscat2
icmpsh - https://github.com/inquisb/icmpsh
Week of PowerShell Shells - http://www.labofapenetrationtester.com/2015/05/week-of-powershell-shells-day-1.html