Tag Archives: Stream Control Transmission Protocol

SCTP Multihoming

One of the key advantages of SCTP over TCP is the support for Multihoming,

From an application perspective, this enables one “socket”, to be shared across multiple IP Addresses, allowing multiple IP paths and physical NICs.

Through multihoming we can protect against failures in IP Routing and physical links, from a transport layer protocol.

So let’s take a look at how this actually works,

For starters there’s a few ways multihoming can be implemented, ideally we have multiple IPs on both ends (“client” and “server”), but this isn’t always achievable, so SCTP supports partial multi-homing, where for example the client has only one IP but can contact the server on multiple IP Addresses, and visa-versa.

The below image (Courtesy of Wikimedia) shows the ideal scenario, where both the client and the server have multiple IPs they can be reached on.

Arkrishna, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

This would mean a failure of any one of the IP Addresses or the routing between them, would see the other secondary IP Addresses used for Transport, and the application not even necessarily aware of the interruption to the primary IP Path.

The Process

For starters, our SCTP Client/Server will each need to be aware of the IPs that can be used,

This is advertised in the INIT message, sent by the “client” to a “server” when the SCTP session is established.

SCTP INIT sent by the client at, but advertising two IPs

In the above screenshot we can see the two IPs for SCTP to use, the primary IP is the first one ( and also the from IP, and there is just one additional IP ( although there could be more.

In a production environment you’d want to ensure each of your IPs is in a different subnet, with different paths, hardware and routes.

So the INIT is then responded to by the client with an INIT_ACK, and this time the server advertises it’s IP addresses, the primary IP is the From IP address ( and there is just one additional IP of,

SCTP INIT ACK showing Server’s Multi-homed IP Options

It’s worth noting that according to RFC 4960 Multi-homing is Optional and so is the IP Address Header, if it’s not advertised the sender is single-homed.

Next up we have the cookie exchange, which is used to protect against synchronization attacks, and then our SCTP session is up.

So what happens at this point? How do we know if a path is up and working?

Well the answer is heartbeat messages,

Sent from each of the IPs on the client to each of the IPs on the server, to make sure that there’s a path from every IP, to every other IP.

SCTP Heartbeats from each local IP to each remote IP

This means the SCTP stacks knows if a path fails, for example if the route to IP on the server were to fail, the SCTP stack knows it has another option,, which it’s been monitoring.

So that’s multi-homed SCTP in action – While a lot of work has historically been done with LACP for aggregating multiple NICs together, and VRRP for ensuring a host is alive, SCTP handles this in a clean and efficient way.

I’ve attached a PCAP showing multi-homing on a Diameter S6a (HSS) interface between an MME and a HSS.

Kamailio Bytes – SCTP

I’ve talked about how cool SCTP is in the past, so I thought I’d describe how easy it is to start using SCTP as the Transport protocol in Kamailio.

I’m working on a Debian based system, and I’ll need to install libsctp-dev to use the SCTP module.

apt-get install libsctp-dev

Next we’ll edit the Kamailio config to load module sctp in the loadmodules section:

loadmodule "sctp.so"

Now we’ll start listening on SCTP, so where your current listen= entries are we’ll add one:


I’ve loaded Dispatcher for this example, and we’ll add a new entry to Dispatcher so we can ping ourselves.

We’ll use kamctl to add a new dispatcher entry of our loopback IP ( but using SCTP as the transport.

kamctl dispatcher add 1 'sip:;transport=sctp' 0 0 '' 'Myself SCTP'

Now I’ll restart Kamailio and check kamcmd:

kamcmd dispatcher.list

All going well you’ll see the entry as up in Dispatcher:

And firing up tcpdump should show you that sweet SCTP traffic:

tcpdump -i lo -n sctp

Sadly by default TCPdump doesn’t show our SIP packets as they’re in SCTP, you can still view this in Wireshark though:

Here’s a copy of the packet capture I took:

I’ve put a copy of my basic config on GitHub.

Now get out there and put SCTP into the real world!

SCTP (And why you should be using it)

XKCD said it best:

XKCD Standards

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.


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.


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.