XKCD said it best:

Sometimes standards are created that are superior in some scenarios, and just don’t get enough love.

To me Stream Control Transmission Protocol (SCTP) is one of those, and it’s really under-utilised in Voice.

Defined by the SIGTRAN working group in 2000 while working to transport SS7 over IP, SCTP takes all the benefits of TCP, mixes in some of the benefits of UDP (No head of line blocking) and mutihoming support, and you’ve got yourself a humdinger of a Transmission Protocol.

Advantages

Reliable Transmission

Like TCP, SCTP includes a reliable transmission mechanism that ensures packets are delivered and retries if they’re not.

Multi Homing

SCTP’s multi homing allows a single connection to be split across multiple paths. This means if you had two paths between Melbourne and Sydney, you could be sending data down both simultaneously.

This means a loss of one transmission path results in the data being sent down another available transmission path.

If you’re doing this using TCP you’d have to wait for the TCP session to expire, BGP to update and then try again. Not so with SCTP.

No Head of Line Blocking

An error / discard with a packet in a TCP stream requires a re-transmission, blocking anything else in that stream from getting through until the error/discarded packet is sorted out. This is referred to as “head of line blocking” and is generally avoided by switching to UDP but that looses the reliability.

4 Way Handshake

Compared to TCP’s 3-way handshake which is susceptible to SYN flooding.

Deployment

If you’ve got a private network, chances are it can support SCTP.

There’s built in SCTP support in almost all Linux kernels since 2002, Cisco iOS and VxWorks all have support, and there’s 3rd party drivers for OSX and Windows.

SCTP is deployed in 3GPP’s LTE / EPC protocol stack for communication over S1-AP and X2 interfaces, meaning if you’ve got a LTE enabled mobile you’re currently using it, not that you’d see the packets.

You’ll find SCTP in SIGTRAN implementations and some TDM-IP gateways, Media Gateways, protocol converters etc, but it’s not widely deployed outside of this.