SCTP in Go

11/20/2017 ∙ by Olivier Van Acker, et al. ∙ Birkbeck, University of London 0

This paper describes a successful attempt to combine two relatively new technologies: Stream Control Transmission Protocol (SCTP) and the programming language Go, achieved by extending the existing Go network library with SCTP. SCTP is a reliable, message-oriented transport layer protocol, similar to TCP and UDP. It offers sequenced delivery of messages over multiple streams, network fault tolerance via multihoming support, resistance against flooding and masquerade attacks and congestion avoidance procedures. It has improvements over wider-established network technologies and is gradually gaining traction in the telecom and Internet industries. Go is an open source, concurrent, statically typed, compiled and garbage-collected language, developed by Google Inc. Go's main design goals are simplicity and ease of use and it has a syntax broadly similar to C. Go has good support for networked and multicore computing and as a system language is often used for networked applications, however it doesn't yet support SCTP. By combining SCTP and Go, software engineers can exploit the advantages of both technologies. The implementation of SCTP extending the Go network library was done on FreeBSD and Mac OS X, the two operating systems that contain the most up to date implementation of the SCTP specification.



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I Introduction

This paper will look into combining two relatively new technologies: a network protocol called SCTP and the programming language Go. Both technologies claim to offer improvements over existing technologies: SCTP does away with the limitations of the widely used TCP protocol [8]; and Go was designed with simplicity and minimized programmer effort in mind, thereby preventing a forest of features getting in the way of program design: Less is exponentially more [6]. The current version of the Go network library does not support the SCTP protocol and this paper examines how easy it is to extend the Go network library with this protocol. The work in this paper is based on the dissertation submitted for an MSc in Computer Science at Birkbeck University in London and is available as an open source project.

I-a Relevance

After ten years SCTP as a technology is becoming more and more relevant. One notable implementation is the use of SCTP as a data channel in the Web Real-Time Communication (WebRTC) standard [11], a HTML 5 extension to enable real time video and audio communication in browsers. Google inc. and the Mozilla Foundation are each planning to release a browser this year implementing the WebRTC standard. Go as a language is very new and it is too early to say what impact it will have. It does however receive a lot of media attention since it is developed by Google. It is also growing in popularity because of its ease of use as a concurrent system language [1].

I-B Outline

Section II presents an overview of Go and SCTP, followed by (section III) a description of how the TCP socket API is integrated in the Go networking library. This is a starting point for the design of an SCTP extension to the network library in Go, described in section IV. Section V explains the implementation. Section VI analysis the results and VII concludes.

Ii Technology overview

In this section I will give an overview of the main features of SCTP and Go.

Ii-a Sctp

SCTP was initially developed in response to the demands of the telecoms industry, which was not satisfied with the reliability and performance of TCP [10, p. 15]. During the design phase the decision was made to make SCTP a less telephony-centric IP protocol [10, p. 16] so that it could also be used for more generic data transport purposes.

Ii-A1 Comparison with TCP

It is fruitful to compare TCP and SCTP, as TCP is the most widely-used protocol [5] and SCTP is very similar to it:

Ii-A2 Multihoming

A host is said to be multihomed if it has multiple IP addresses which can be associated with one or more physical interfaces connected to the same or different networks[2]. TCP can only bind to one network address at each end of the connection. In the event of a network failure there is no way to stay connected and send the data over another physical path without setting up a new connection. SCTP natively supports multihoming at the transport layer. This makes it possible to connect and transfer data to and from multihomed hosts, so that when one network path fails, the connection seamlessly fails over using the remaining paths. And together with concurrent multipath transfer (CMT) [4] it is possible to increase data throughput by transferring data simultaneously over multiple links.

Ii-A3 In-built message boundaries

In TCP there are no markers to indicate the start or end of a specific piece of data (a user message). All data sent over the socket is converted into a byte stream and an application must add its own mechanism to be able to reconstruct the messages. In SCTP the message boundaries of the data sent are preserved in the protocol. In the event of the message being larger than the maximum package size a notofication is sent to the application layer more is on its way.

Ii-A4 Protection against Denial of Service (DOS) attacks

Another disadvantage of TCP is that it is open to ’SYN flood attack’, a specific type of attack which can drain the server of resources, resulting in a denial of service (nothing else can connect). To prevent this, SCTP uses a four-way handshake to establish the connection, whereas TCP only uses a three-way handshake. With a four-way handshake the originator has to double-acknowledge itself (”is it really you?”) by resending a cookie it previously received from the destination server before that server will assign resources to the connection. This prevents timeouts on the server side and thus makes this type of denial of service impossible. To reduce start up delay, actual data can also be sent during the second part of the handshake.

(a) TCP handshake
(b) SCTP handshake
Figure 1: Initiating a network connection

Ii-A5 SCTP Multistreaming

SCTP user messages in a single SCTP socket connection can be sent and delivered to the application layer over independent streams. In case of two sets of user messages A and B, each set delivered sequentially, the messages of set A can be sent over a different stream than B. And in case a messages in set A gets missing and part of the sequence needs to be resent this will not effect the data of set B if it is sent over a different stream. This tackels the ’head of line blocking’ problem (figure 2) where messages already delivered need to be redelivered because they have to arrive in order and becuase one message did not arrive.

Figure 2: Head of line blocking

Ii-A6 Associations

A connection between a SCTP server and client is called an association. An association gets initiated by a request from a SCTP client. So a listening server can accept incoming requests from multiple clients. Messages sent over the assocation have an association id attached to them. to make it possible to know where they come from. Within a single association you can have multiple streams for data transmission (See figure 3)

Figure 3: Associations and streams

Ii-A7 Two programming models

SCTP has two interfaces for implementation in a networked application: one-to-one and one-to-many. A single socket in a one-to-many model can have multiple incoming associations, meaning that multiple clients can connect to a single server listening on a single socket. The one-to-one model can only have a single association per socket. The one-to-one model is for easy migration of existing TCP applications, it maps one-to-one to the system calls TCP makes to establish a connection. But it can only have one connection per association, and thus only a single client can connect to a server. The one-to-one interface makes migrating an existing application a relatively painless exercise. If you want to upgrade your existing TCP application to a one-to-many SCTP application, significant retooling is needed. [7, p. 267]

Figure 4: Socket API

Ii-A8 Socket API

Figure 4 is an overview of two applications communicating over an IP network using a transport protocol (TCP, UDP or SCTP). The software engineer writing the client and server in the application layer only has to know about the function calls exposing the functionality in the transport layer. This collection of functions is called the socket API. A socket is an endpoint for networked communication; multiple processes can open sockets via the API and data written into one socket can be read from another. The socket API consist of functions which can open and close the socket, send data over it and set the behavior of a socket using ’socket options’. An example of such behavior is the enabling or disabling of data buffering before sending to reduce the number of packets to sent over the network and therefore improve efficiency (Nagle’s algorithm).

Figure 5: TCP and SCTP socket API

Ii-A9 SCTP Socket API

Figure 5 is an overview of both the TCP and SCTP socket APIs and gives the order in which the system calls would be used in a simple client server application sending and receiving messages. The server first creates a socket, this returns a file descriptor which can be used by the bind function to to make a request to assign a address to it. After this the server can start listening to incoming connections with the listen() function. After this point TCP is different from SCTP. The TCP client actively connects to the peer and the server accepts the connection with the accept() and connect() functions. With SCTP the connection set up hand handshake happens implicitly when sending and receiving a message. At this point the server and client can exchange messages and finally the connection terminates with the close() function.

Figure 6: Ancillary data embedded in sendmsg() data structure

Ii-A10 Socket I/O functions and ancillary data

The sendmsg() and recvmsg() functions (see appendix B-A for definition) are the most general of all socket I/O functions [7, p. 390] and can be used in combination with all protocols in the transport layer. Both functions have room for message data and ancillary data (see appendix B-B). SCTP adds extra meta data which contains information about the message being sent and its connection. Table I gives an overview of the ancillary data mapped to the return variables.

sid stream identifier
ssn stream sequence number
ppid identifier set by peer
aid association id
Table I: Ancillary data mappings

Ii-B Go language overview

In this section I will give a short overview of the main features and characteristics of the Go language and provide more detail where relevant to the project.

Ii-B1 Data

Go has a syntax broadly similar to the C and Java languages. Variables are declared either by variable name followed by data type, or by name only with data type inferred (known as ’type inference’).’ With the initialization operator := variables can be declared and initialized in a single statement. If not initialized explicitly, the variable is set to a default value. Here are some examples:

var number int
var first_name, last_name string
var greeting = hello("olivier")
fractal := make([]uint64, 10)

Go has two ways to create data structures: i) with ’new’, where the data structure is initialized to zero; ii) with ’make’, where the data structure is initialized to a specified value.

var p *[]int = new([]int)
var v  []int = make([]int, 100)

An asterisk indicates that the variable is a pointer.

Ii-B2 Functions

Unlike most commonly-used languages Go functions and methods can return multiple values. The following function expects a string and integer as input parameters and returns a string and an error. If the count integer is zero the function will return a nil and an error, otherwise it returns the greeting and nil as error.

func hello(name string, count int) (greeting string, err error) {
    if count = 0 {
        return nil, errors.New("Cannot say hello zero times")
    greeting = "Hello" + name, nil

This hello function can be be called in the following manner:

greeting, err := hello("paard", 0)
if err != nil {
} else {

Or if we are not interested in one of the return values, a blank identifier _ can be used to discard the value:

greeting, _ := hello("paard", 1)

Ii-B3 Object and methods

In Go data structures (objects) are defined as follows:

type Person struct {
    name string
    age int

Functionality can be added to an object by defining methods associated with it. A difference between Go and other object-oriented languages like Java is that in Go this functionality is defined outside of the object, for example:

func (p Person) SayHello(name String) {
    return "Hello " + name ", my name is " +

In the above example the SayHello method has a string as input and is associated with a Person object p.

Methods can be associated with any type, not just objects. In the following example the Add() method is associated with an Array of strings:

type NameArray []string
func (na NameArray) Add(name string) []string {

It’s worth noting that Go has objects but not inheritance.

Ii-B4 Interfaces

As with other object oriented languages, you can specify behavior of an object by using interfaces and also implement multiple interfaces per object. The interfaces does not have to be explicitly named: as soon as the type implements the methods of the interface the compiler knows the relation between the two. In the following example, in function main, a cat object gets created, it talks and then gets cast (type changed) to an animal object: type animal interface {     Talk() } type Cat func (c Cat) Talk() {     fmt.Println("Meow") } func main() {     var c Cat     c.Talk()     a := animal(c)   // Cast from cat to animal     a.Talk() }

Ii-B5 Pointers

Although Go has pointers (references to memory locations), they are implemented differently than in a language like C. Pointer arithmetic is not possible so Go pointers can only be used to pass values by reference. At a lower level Go pointers can be converted to C pointers.

Ii-B6 Concurrency

Go has a basic primitive for concurrency called a goroutine. The name is not only a play on a programming component called coroutine but is also its implementation of it in the language. Coroutines are methods which call on each other, data comes in, gets processed and gets passed to a the next coroutine. One of the advantages of coroutines is they are generally easier to create and understand [Knuth V1 p193]. But the main advantage is that it lends itself very well for distributed computing where data gets passed around from one processor to another, maybe even on a different computer.

In Go every function can become a goroutine by simple putting go in front of it. A gorouting can than communicate its input and output via channels. This concept is called Communicating sequential processes (CSP) and is surprisingly versatile in its use. [3] Here an example:

1package main
3func receiveChan(ci chan int) {
4  for {
5    i := <-ci
6    println(i)
7  }
10func main() {
11  ci := make(chan int)
12  go receiveChan(ci)
14  for i := 0; i < 10; i++ {
15    ci <- i
16  }

The receiveChan() function has as input a channel of integers. On line 4 an endless for loop starts where line 5 waits for an integer to come in on the channel. The main functions first creates a channel of integers. Line 12 starts the function receiveChan as a Go routine in the background. This is followed by a loop sending 10 integers over the channel to the receiveChan function.

Ii-B7 Much more

There is more to the language like garbage collection, first class functions, arrays, slices, however the explanation of this falls outside the scope of this paper. More information can be found on the Go website 111

Iii Go networking

The following section contains the findings of my research on how Go combines the system level TCP/IP socket API into an easy-to-use network library and how that works internally. Much of the TCP functionality is very similar to the SCTP stack and this will serve as an example of how to implement SCTP in Go.

Iii-a The network library

Go provides a portable interface for network I/O. Basic interaction is provided by the Dial, Listen, ListenPacket and Accept functions which return implementations of the Conn, PacketConn and Listener interfaces. The Listen function is for byte stream data connections and the ListenPacket is for data transfer which includes messages boundaries, like UDP or Unix domain sockets. These functions simplify the access to the socket API, but if needed the application developer can still access the socket API in more detail.

Here is an example of a typical TCP client and server application. The client sends a single message and the server waits to receive a message, prints it out after receiving and starts listening again. First a simple client:

1package main
2import "net"
4func main() {
5    conn, err := net.Dial("tcp", "localhost:1234")
6    if err != nil {
7        return
8    }
9    defer conn.Close()
10    conn.Write([]byte("Hello world!"))

The function main() is the entry point of the program. The first step the client performs is ’dialing’ a server with the TCP protocol (line 5). The Dial() function returns a connection which is an implementation of the Conn interface. After checking for errors (6-8) the defer keyword before the connection close command indicates the connection can be finished as soon as it is no longer needed. In this case it happens immediately after the write so it does not make much sense to defer it, but in larger programs with multiple exit points you only need a single (deferred) close statement, which makes it easier to understand and maintain the program.

Next the server:

1package main
2import "net"
4func main() {
5    listen, err := net.Listen("tcp", "localhost:1234")
6    if err != nil {
7        return
8    }
9    buffer := make([]byte, 1024)
10    for {
11        conn, err := listen.Accept()
12        if err != nil {
13            continue
14        }
15        conn.Read(buffer)
16        println(string(buffer))
17    }

The server gets created with the Listen() method and returns an object which implements the Listener interface. On line 9 a byte array buffer gets created for the received data. The following for loop (10 - 17) will continuously wait to accept an incoming connection (11), check for errors after connect, read from it (15) and convert the byte array to a string before printing it (16).

Iii-B Under the hood

Go’s network communication library uses the same socket API as any C program. In this section I will examine what happens when a network connection is set up and which socket API calls are made at what point. The example uses TCP. To illustrate how Go accesses the socket API I will take the TCP client described in the previous section as an example and show how it interacts with the kernel by opening a connection.

Socket and Connect

For a TCP client to create a connection and send some data the following system calls need to be made in this order:

  1. resolve IP address

  2. socket()

  3. setsockopt() (optional)

  4. connect()

In Go all this is wrapped in the net.Dial() call.

Figure 8 shows a sequence diagram of method calls after a client calls Dial(). First the hostname:port gets parsed and resolved to an IP address (1.1)222There are more method calls behind this but they are not relevant for this example. Net.dialAddr() (1.3) determines the transport protocol type and calls the relevant method, net.DialTCP() in this case (1.3.1). Next net.internetSocket() gets called which internally calls socket() in the syscall package. Syscall.socket() is an auto-generated method based on C header files which describe the socket API.

Every network protocol in Go has its own connection type. As you can see in figure 8 the generic Dial() method eventually reaches the specific DialTCP() method which returns a TCP- specific connection type. This type gets cast to the more generic Conn type which can be used in the client application. If TCP-specific functionality is needed the Conn type can be recast to a TCPConn which then makes it possible to access TCP-specific functionality.

Iii-C Auto-generation

Native Go functions to access kernel system calls and data structures can be auto-generated via scripts. In FreeBSD, if the description of the system call is present in the syscalls.master file 333System call name/number master file:, the function will be available through the syscall package. Most system calls which are used in Go are wrapped in methods to make them fit better the language. The structures which are passed to the system calls are created in the same way. The source code directory of the syscall package contains a file with a link pointing to a header file which describes the structures.

Iii-D Go non-blocking networking

All networking in Go is non blocking which is not the default in the TCP/IP model. As soon as a application tries to retrieve data from a socket, e.g. via readmsg(), it will not block until some data comes in, instead it will immediately move on in the program. Reading data from the socket is normally done in an endless loop. To make Go actually wait for sending data back to the application Go implements the reactor design pattern. The implementation of this software pattern makes use of a mechanism in the kernel where you can ask the kernel to send a signal back to the application if certain events occur whilst you keep doing other things in the program. For example data being written to the socket descriptor. On BSD the system call kqeueu() and kevent() are used to register and queue events. The functionality of this is handled by Go’s runtime.

Iv Design

In the following section I will describe how the different Go network methods will map to the SCTP socket API.

Figure 7: Go SCTP network API

Iv-a Mapping APIs

The SCTP socket API will follow closely the wrapping of the TCP socket API in the Go network library as described in section III-B. Figure 7 show the mapping of the socket API to the different functions and methods. At the server side the socket(), bind() and listen() functions are bundled into the ListenPacket() function which resides in the network package (net). The ListenPacket() function returns an implementation of the PacketConn interface (See appendix A-B). The client wraps the socket() function into the Dail() function. This functions returns a Conn (connection) interface which can used for writing messages (Conn.WriteTo()) and these user messages can be received vie the ReadFrom() method on the PacketConn interface.

Iv-B SCTP specific

Iv-B1 Receive SCTP specific information

To access SCTP specific functionality, such as which stream the message has been sent on, or the association id, net.ListenSCTP() can be used. This method returns a SCTP specific type (SCTPConn) which has the method ReadFromSCTP() and WriteToSCTP() added to it. These methods return and set the information contained by the SCTP receive information structure, added as ancillary data when the system call recvmsg() returns.

Iv-B2 Send SCTP specific information

To be able to set SCTP specific send information such as stream id or association id via the SCTP Send Information Structure, the several methods on the SCTPConn object can be used (See table II):

Number of output streams (*SCTPConn) InitNumStreams(n int) error
Max number of input streams (*SCTPConn) InitMaxInStream(n int) error
Max number of attempts to connect (*SCTPConn) InitMaxAttempts(n int) error
Timeout (*SCTPConn) InitMaxInitTimeout(n int) error
Table II: Initialization parameters

A typical server which has access to SCTP specific functionality would look like this:

package main import (     "net"     "strconv" ) func main() {    addr, _ := net.ResolveSCTPAddr("sctp", "localhost:4242")    conn, _ := net.ListenSCTP("sctp", addr)    defer conn.Close()    for {       message := make([]byte, 1024)        _, _, stream, _ := conn.ReadFromSCTP(message)        println("stream " + strconv.Itoa(int(stream)) + ": " + string(message))     } }

In this program ListenSCTP returns a SCTP connection type. This type implements Conn and PacketConn interface and has the ReadFromSCTP method added to it. The println() functions prints the stream id and the user message.

V Implementation

In this section I will describe how the SCTP network functionality can fit into the existing Go network framework. The main goal of the SCTP application programming interface (API) design is to combine lower-level system calls in an easy-to-use framework. This framework hides the underlying complexity of the socket API but if needed gives access to all the functionality provided by the protocol. To make sure SCTP fits in the Go design philosophy, less is more, I will make use as much as possible of the existing components and interfaces in the Go network package. In the following section I’ll e

V-a Server

For a server to be able to set up a SCTP association it needs to create a socket, bind to it, optionally set some socket options and start listening to it. A typical server will access the socket API in the following sequence:

  1. socket()

  2. bind()

  3. listen()

  4. recvmsg()

The socket(), bind() and listen() functions will be wrapped into a Listen() method which returns a connection type. There are three variations: net.Listen(), net.ListenPacket() and net.ListenSCTP(). The Go network library provides the net.ListenPacket() method for packet-oriented networking like UDP, and net.Listen() for stream-oriented networking like TCP. SCTP which is packet-oriented, can also make use of the net.ListenPacket() method. ListenPacket() returns an implementation of the PacketConn interface which can be used to read and write messages (recvmsg()). A simple SCTP echo server using the PacketConn interface might look like this:

package main
import "net"
func main() {
   conn, _ := net.ListenPacket("sctp", "localhost:4242")
   defer conn.Close()
   message := make([]byte, 1024)

After the the main entry point a connection object is created via the ListenPacket() method. The parameters of this method indicate that the connection should use the SCTP protocol and listen on localhost port 4242. The next line defers the closing of the connection when it is not needed anymore. After creating a byte array buffer to store incoming data a message is read from the connection. The ReadFrom() method will block until a message is received. Finally the message is printed and the program ends.

Receive SCTP-specific information

To access SCTP-specific functionality, such as which stream the message has been sent on, or the association id, net.ListenSCTP() can be used. This method returns a SCTP-specific type (SCTPConn) which has the method ReadFromSCTP() added to it:

(*SCTPConn).ReadFromSCTP(message *string)(sid int, ssn int, ppid int, aid int, addr SCTPAddr, err error)

The ReadFromSCTP() method returns the information contained by the SCTP receive information structure, added as ancillary data when the system call recvmsg() returns. A typical server which has access to SCTP-specific functionality would look like this:

package main
import (
func main() {
    addr, _ := net.ResolveSCTPAddr("sctp", "localhost:4242")
    conn, _ := net.ListenSCTP("sctp", addr)
    defer conn.Close()
    for {
       message := make([]byte, 1024)
       _, _, stream, _ := conn.ReadFromSCTP(message)
       println("stream " + strconv.Itoa(int(stream)) + ": " + string(message))

In this program ListenSCTP() returns a SCTP connection type. This type implements the Conn and PacketConn interfaces and adds the ReadFromSCTP() method.

V-B Client

In Go a client connection sets itself up with a call to the Dial() method. The Dial() method returns the generic Conn interface. Every network protocol in Go has its own Dial() method which returns a protocol-specific connection type. In the case of SCTP this is the PacketConn type which has underneath it a specific SCTP connection type (SCTPConn). A simple SCTP client sending a single message would look like this:

package main
import "net"
func main() {
    addr, _ := net.ResolveSCTPAddr("sctp", "localhost:4242")
    conn, _ := net.DialSCTP("sctp", nil, addr)
    defer conn.Close()
    message := []byte("paard")
    conn.WriteTo(message, addr)

The DialSCTP() method creates the socket and sets default socket options. Sending the message via WriteTo() will implicitly set up the connection.

Send SCTP-specific information

To be able to set SCTP-specific send information such as stream id or association id via the SCTP Send Information Structure, the WriteToSCTP() method can be used:

(*SCTPConn).WriteToSCTP(message *string, addr SCTPAddr, sid int, ssn int, ppid int, aid int, err error)

Creating and binding of the socket

The sequence diagram in figure 9 gives an overview of how a socket is created and bind to. At point 1.1.3 in this figure net.internetSocket() returns a socket descriptor which is used to create the actual SCTP connection type. At this point the SCTP initialization structure is set to its default values together with the NODELAY socket option. The ’no delay’ socket option disables buffering of bytes sent over the connection. This is also the default setting for the TCP implementation in Go.

Vi Analysis

Vi-a Performance

For performance testing a client-server application was designed, with the client sending messages and the server receiving them. Both a C and a Go version of this application were created and compared and run against each other. The data throughput of an SCTP client-server application written in Go is approximately twice as high as the same program written in C. Most of the delay happens in the Go client. Unlike the C implementation, the Go application needs to go through some additional method calls before it reaches the system call which sends the data. Go is also garbage-collected, which causes an extra delay because several data structures are redeclared each time a message is sent. Another factor to consider is that the implementation of SCTP in Go is against version 1.0.2 of Go. This version does not have a compiler which is good at optimizing. Newer versions of Go address this issue.

Vi-B Fitting like a glove

Since Go already provided methods and interfaces for message based data transmission which could be reused and because of it similarity with the TCP socket API making SCTP available in the Go network library was a relatively straightforward task. I could reuse of the ancillary data structure from the Unix socket API and I only had to add the SCTP specific ancillary data to the structure. It was easy to follow the design philosophy ’less is exponentially more’: the SCTP socket API which is very similar to TCP could be wrapped in an easier to use API just like it is done with TCP. This resulted in a Go SCTP API which can be used in the most simple way hiding all complexity of the protocol or if needed it is possible to dive deeper and make use of more specific SCTP functionality.

Vi-C Issues during implementation

During the implementation there where two major changes which caused an unexpected setback. The first as mentioned before was the changing SCTP socket API which made large parts of the implementation I already had in place obsolete. This forced me to rewrite the majority of the implementation. The second issue which proved to be a challenge was the first release (1.0) of Go. Until this release I only sporadicly synced my version of Go with the latest changes of the main development tree of the canonical Go source code repository. Building up to the new version the Go development team did a considerable amount of work. So with the 1.0 release a large amount of changes needed to be incorporated into my own branch. Since there where lots of changes in the internals of Go I had to deal with a large amount of merge conflicts in certain areas, specifically around the implementation of the generic Dial and Listen interfaces. Most of the work in this area had to be redone.

Vi-D Extensions

There are many extensions to SCTP described in multiple RFCs. A complete SCTP implementation should include [9]:

  1. RFC4960 (basic SCTP)

  2. RFC3758 (partial reliability)

  3. RFC4895 (authentication)

  4. RFC5061 (dynamic addresses)

The last three in this list have not be included in this implementation.

Vii Conclusion

Because of its similarity with existing protocols available in the Go networking library SCTP fits easily into the existing Go network library. The biggest challenge implementing this was the ongoing work on the SCTP specification and Go itself which made the implementation of this project a moving target. Only recently (mid way 2012) the APIs of Go and SCTP have been stabilized. It should be noted however that there are many extensions to SCTP described in multiple RFCs. The work in this paper only looked at a bare minimum which is needed to make SCTP work in Go.

Vii-a Future work

User land implementation of SCTP in Go

Not all operating systems support the SCTP protocol natively. It is however possible to have SCTP running on top of the UDP protocol, outside the kernel (user land). To make this work a user land implementation of SCTP on top of UDP needs to be added to Go. Once this is done SCTP could become available on all different platforms supported by the language.


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Appendix A Go network types and interfaces

A-a Conn interface

type Conn interface {
    Read(b []byte) (n int, err error)
    Write(b []byte) (n int, err error)
    Close() error
    LocalAddr() Addr
    RemoteAddr() Addr
    SetDeadline(t time.Time) error
    SetReadDeadline(t time.Time) error
    SetWriteDeadline(t time.Time) error

A-B PacketConn interface

type PacketConn interface {
    ReadFrom(b []byte) (n int, addr Addr, err error)
    WriteTo(b []byte, addr Addr) (n int, err error)
    Close() error
    LocalAddr() Addr
    SetDeadline(t time.Time) error
    SetReadDeadline(t time.Time) error
    SetWriteDeadline(t time.Time) error

A-C Listener interface

type Listener interface {
    Accept() (c Conn, err error)
    Close() error
    Addr() Addr

Figure 8: TCP Client setting up connection

Figure 9: Creating and bind

Appendix B Socket IO functions

B-a Socket IO function definition

ssize_t sendmsg(int s, const struct msghdr *msg, int flags)
ssize_t recvmsg(int s, struct msghdr *msg, int flags)

B-B Message header structure and ancillary data

struct msghdr {   void         *msg_name;      /* optional address */   socklen_t     msg_namelen;   /* size of address */   struct iovec *msg_iov;       /* scatter/gather array */   int           msg_iovlen;    /* # elements in msg_iov */   void         *msg_control;   /* ancillary data */   socklen_t     msg_controllen;/* ancillary data buffer len */   int           msg_flags;     /* flags on message */ };

The msg_control argument, which has length msg_controllen, points to a buffer for other protocol control related messages or other miscellaneous ancillary data. The messages are of the form: struct cmsghdr {   socklen_t  cmsg_len;    /* data byte count, including hdr */   int        cmsg_level;  /* originating protocol */   int        cmsg_type;   /* protocol-specific type */  /* followed by u_char cmsg_data[]; */ };