Why is SCTP not much used/known

SCTP requires more design within the application to get the best use of it. There are more options than TCP, the Sockets-like API came later, and it is young. However I think most people that take the time to understand it (and who know the shortcomings of TCP) appreciate it -- it is a well designed protocol that builds on our ~30 years of knowledge of TCP and UDP.

One of the aspects that requires some thought is that of streams. Streams provide (usually, I think you can turn it off) an order guarantee within them (much like a TCP connection) but there can be multiple streams per SCTP connection. If your application's data can be sent over multiple streams then you avoid head-of-line blocking where the receiver starves due to one mislaid packet. Effectively different conversations can be had over the same connection without impacting each other.

Another useful addition is that of multi-homing support -- one connection can be across multiple interfaces on both ends and it copes with failures. You can emulate this in TCP, but at the application layer.

Proper link heartbeating, which is the first thing any application using TCP for non-transient connections implements, is there for free.

My personal summary of SCTP is that it doesn't do anything you couldn't do another way (in TCP or UDP) with substantial application support. The thing it provides is the ability to not have to implement that code (badly) yourself.

FYI, SCTP is mandated as supported for Diameter (cf RADIUS next gen). see RFC 3588

   Diameter clients MUST support either TCP or SCTP, while agents and
   servers MUST support both.  Future versions of this specification MAY
   mandate that clients support SCTP.

We have been deploying SCTP in several applications now, and encountered significant problem with SCTP support in various home routers. They simply don't handle SCTP correctly. I believe this is primarily a performance issue (the SCTP protocol specification require checksums for the whole packets to be recalculated and not just for headers).

Like many other promising protocols SCTP is sadly dead in the water until D-link and Netgear fixes their broken NAT boxes.

SCTP is not very much known and not used/deployed a lot because:

• Widespread: Not widely integrated in TCP/IP stacks (in 2013: still missing natively in latest Mac OSX and Windows. 2020 update: still not in Windows nor Mac OS X)
• Libraries: Few high level bindings in easy to use languages (Disclaimer: i'm maintainer of pysctp, SCTP easy stack support for Python)
• NAT: Doesn't cross NAT very well/at all (less than 1% internet home & enterprise routers do NAT on SCTP).
• Popularity: No general public app use it
• Programming paradigm: it changed a bit: it's still a socket, but you can connect many hosts to many hosts (multihoming), datagram is ordered and reliable, erc...
• Complexity: SCTP stack is complex to implement (due to above)
• Competition: Multipath TCP is coming and should address multihoming needs / capabilities so people refrain from implementing SCTP if possible, waiting for MTCP
• Niche: Needs SCTP fills are very peculiar (ordered reliable datagrams, multistream) and not needed by much applications
• Security: SCTP evades security controls (some firewalls, most IDSes, all DLPs, does not appear on netstat except CentOS/Redhat/Fedora...)
• Audit-ability: Something like 3 companies in the world routinely do audits of SCTP security (Disclaimer: I work in one of them)
• Learning curve: Not much toolchain to play with SCTP (check the excellent withsctp that combines nicely with netcat or use socat, 2020 edit: nmap supports it for a few years now )
• Under the hood: Used mostly in telecom and everytime you send SMS, start surfing the net on your mobile or make phone calls, you're often triggering messages that flow over SCTP (SIGTRAN/SS7 with GSM/UMTS, Diameter with LTE/IMS/RCS, S1AP/X2AP with LTE), so you actually use it a lot but you never know about it ;-) 2020 edit: it's being removed from the core 5G network (no more Diameter, HTTP/2 instead) and will be only used in the 5G radio access network between antennas and core.

Indeed, SCTP is used mostly in the telecom area. Traditionally, telecom switches use SS7 (Signaling System No. 7) to interconnect different entities in the telecom network. For example - the telecom provider's subscriber data base(HLR), with a switch (MSC), the subscriber is connected too (MSC).

The telecom area is moving to higher speeds and more reachable environment. One of these changes is to replace SS7 protocol by some more elegant, fast and flexible IP-based protocol.

The telecom area is very conservative. The SS7 network has been used here for decades. It is very a reliable and closed network. This means a regular user has no access to it.

The IP network, in contrast, is open and not reliable, and telecoms will not convert to it if it won't handle at least the load that SS7 handles. This is why SCTP was developed. It tries:

• to mimic all advantages of the SS7 network accumulated over the decades.
• to create a connection-oriented protocol better than TCP in speed, security, and redundancy

The latest releases of Linux already have SCTP support.