23.4. Sockets APIs¶

The sockets API is a long-standing API used by user-space applications to access network services in the kernel. A socket is an abstraction, like a Unix file handle, that allows applications to connect to other Internet hosts and exchange reliable byte streams and unreliable datagrams, among other services.

ns-3 provides two types of sockets APIs, and it is important to understand the differences between them. The first is a native ns-3 API, while the second uses the services of the native API to provide a POSIX-like API as part of an overall application process. Both APIs strive to be close to the typical sockets API that application writers on Unix systems are accustomed to, but the POSIX variant is much closer to a real system’s sockets API.

23.4.1. ns-3 sockets API¶

The native sockets API for ns-3 provides an interface to various types of transport protocols (TCP, UDP) as well as to packet sockets and, in the future, Netlink-like sockets. However, users are cautioned to understand that the semantics are not the exact same as one finds in a real system (for an API which is very much aligned to real systems, see the next section).

ns3::Socket is defined in src/network/model/socket.h. Readers will note that many public member functions are aligned with real sockets function calls, and all other things being equal, we have tried to align with a Posix sockets API. However, note that:

ns-3 applications handle a smart pointer to a Socket object, not a file descriptor;
there is no notion of synchronous API or a blocking API; in fact, the model for interaction between application and socket is one of asynchronous I/O, which is not typically found in real systems (more on this below);
the C-style socket address structures are not used;
the API is not a complete sockets API, such as supporting all socket options or all function variants;
many calls use ns3::Packet class to transfer data between application and socket. This may seem peculiar to pass Packets across a stream socket API, but think of these packets as just fancy byte buffers at this level (more on this also below).

23.4.1.1. Basic operation and calls¶

Implementation overview of native sockets API¶

23.4.1.2. Creating sockets¶

An application that wants to use sockets must first create one. On real systems using a C-based API, this is accomplished by calling socket()

int socket(int domain, int type, int protocol);

which creates a socket in the system and returns an integer descriptor.

In ns-3, we have no equivalent of a system call at the lower layers, so we adopt the following model. There are certain factory objects that can create sockets. Each factory is capable of creating one type of socket, and if sockets of a particular type are able to be created on a given node, then a factory that can create such sockets must be aggregated to the Node:

static Ptr<Socket> CreateSocket(Ptr<Node> node, TypeId tid);

Examples of TypeIds to pass to this method are ns3::TcpSocketFactory, ns3::PacketSocketFactory, and ns3::UdpSocketFactory.

This method returns a smart pointer to a Socket object. Here is an example:

Ptr<Node> n0;
// Do some stuff to build up the Node's internet stack
Ptr<Socket> localSocket =
   Socket::CreateSocket(n0, TcpSocketFactory::GetTypeId());

In some ns-3 code, sockets will not be explicitly created by user’s main programs, if an ns-3 application does it. For instance, for ns3::OnOffApplication, the function ns3::OnOffApplication::StartApplication() performs the socket creation, and the application holds the socket pointer.

23.4.1.3. Using sockets¶

Below is a typical sequence of socket calls for a TCP client in a real implementation:

sock = socket(PF_INET, SOCK_STREAM, IPPROTO_TCP);
bind(sock, ...);
connect(sock, ...);
send(sock, ...);
recv(sock, ...);
close(sock);

There are analogs to all of these calls in ns-3, but we will focus on two aspects here. First, most usage of sockets in real systems requires a way to manage I/O between the application and kernel. These models include blocking sockets, signal-based I/O, and non-blocking sockets with polling. In ns-3, we make use of the callback mechanisms to support a fourth mode, which is analogous to POSIX asynchronous I/O.

In this model, on the sending side, if the send() call were to fail because of insufficient buffers, the application suspends the sending of more data until a function registered at the ns3::Socket::SetSendCallback() callback is invoked. An application can also ask the socket how much space is available by calling ns3::Socket::GetTxAvailable(). A typical sequence of events for sending data (ignoring connection setup) might be:

SetSendCallback(MakeCallback(&HandleSendCallback));
Send();
Send();
...
// Send fails because buffer is full
// Wait until HandleSendCallback is called
// HandleSendCallback is called by socket, since space now available
Send(); // Start sending again

Similarly, on the receive side, the socket user does not block on a call to recv(). Instead, the application sets a callback with ns3::Socket::SetRecvCallback() in which the socket will notify the application when (and how much) there is data to be read, and the application then calls ns3::Socket::Recv() to read the data until no more can be read.

23.4.2. Packet vs. buffer variants¶

There are two basic variants of Send() and Recv() supported:

virtual int Send(Ptr<Packet> p) = 0;
int Send(const uint8_t* buf, uint32_t size);

Ptr<Packet> Recv();
int Recv(uint8_t* buf, uint32_t size);

The non-Packet variants are provided for legacy API reasons. When calling the raw buffer variant of ns3::Socket::Send(), the buffer is immediately written into a Packet and the packet variant is invoked.

Users may find it semantically odd to pass a Packet to a stream socket such as TCP. However, do not let the name bother you; think of ns3::Packet to be a fancy byte buffer. There are a few reasons why the Packet variants are more likely to be preferred in ns-3:

Users can use the Tags facility of packets to, for example, encode a flow ID or other helper data at the application layer.
Users can exploit the copy-on-write implementation to avoid memory copies (on the receive side, the conversion back to a uint8_t* buf may sometimes incur an additional copy).
Use of Packet is more aligned with the rest of the ns-3 API

23.4.3. Sending dummy data¶

Sometimes, users want the simulator to just pretend that there is an actual data payload in the packet (e.g. to calculate transmission delay) but do not want to actually produce or consume the data. This is straightforward to support in ns-3; have applications call Create<Packet> (size); instead of Create<Packet> (buffer, size);. Similarly, passing in a zero to the pointer argument in the raw buffer variants has the same effect. Note that, if some subsequent code tries to read the Packet data buffer, the fake buffer will be converted to a real (zeroed) buffer on the spot, and the efficiency will be lost there.

23.4.4. Use of Send() vs. SendTo()¶

There are two variants of methods used to send data to the socket:

virtual int Send(Ptr<Packet> p, uint32_t flags) = 0;

virtual int SendTo(Ptr<Packet> p, uint32_t flags,
                    const Address &toAddress) = 0;

The first method is used if the socket has already been connected (Socket::Connect()) to a peer address. In the case of stream-based sockets like TCP, the connect call is required to bind the socket to a peer address, and thereafter, Send() is typically used. In the case of datagram-based sockets like UDP, the socket is not required to be connected to a peer address before sending, and the socket may be used to send data to different destination addresses; in this case, the SendTo() method is used to specify the destination address for the datagram.

23.4.5. Socket options¶

23.4.5.1. ToS (Type of Service)¶

The native sockets API for ns-3 provides two public methods (of the Socket base class):

void SetIpTos(uint8_t ipTos);
uint8_t GetIpTos() const;

to set and get, respectively, the type of service associated with the socket. These methods are equivalent to using the IP_TOS option of BSD sockets. Clearly, setting the type of service only applies to sockets using the IPv4 protocol. However, users typically do not set the type of service associated with a socket through ns3::Socket::SetIpTos() because sockets are normally created by application helpers and users cannot get a pointer to the sockets.

Applications have a Tos attribute to simplify the ToS setup:

InetSocketAddress destAddress(ipv4Address, udpPort);
OnOffHelper onoff("ns3::UdpSocketFactory", destAddress);
onoff.SetAttribute("Tos", UintegerValue(tos));

For this to work, the application must eventually call the ns3::Socket::Connect() method to connect to the provided destAddress and the Connect method of the particular socket type must support setting the type of service associated with a socket (by using the ns3::Socket::SetIpTos() method). Currently, the socket types that support setting the type of service in such a way are ns3::UdpSocketImpl and ns3::TcpSocketBase.

The type of service associated with a socket is then used to determine the value of the Type of Service field (renamed as Differentiated Services field by RFC 2474) of the IPv4 header of the packets sent through that socket, as detailed in the next sections.

23.4.5.1.1. Setting the ToS with UDP sockets¶

For IPv4 packets, the ToS field is set according to the following rules:

If the socket is connected, the ToS field is set to the ToS value associated with the socket.
If the socket is not connected, the ToS field is set to the value specified in the destination address (of type ns3::InetSocketAddress) passed to ns3::Socket::SendTo(), and the ToS value associated with the socket is ignored.

23.4.5.1.2. Setting the ToS with TCP sockets¶

For IPv4 packets, the ToS field is set to the ToS value associated with the socket.

23.4.5.2. Priority¶

The native sockets API for ns-3 provides two public methods (of the Socket base class):

void SetPriority(uint8_t priority);
uint8_t GetPriority() const;

to set and get, respectively, the priority associated with the socket. These methods are equivalent to using the SO_PRIORITY option of BSD sockets. Only values in the range 0..6 can be set through the above method.

Note that setting the type of service associated with a socket (by calling ns3::Socket::SetIpTos()) also sets the priority for the socket to the value that the ns3::Socket::IpTos2Priority() function returns when it is passed the type of service value. This function is implemented after the Linux rt_tos2priority function, which takes an 8-bit value as input and returns a value which is a function of bits 3-6 (where bit 0 is the most significant bit) of the input value:

Bits 3-6	Priority
0 to 3	0 (Best Effort)
4 to 7	2 (Bulk)
8 to 11	6 (Interactive)
12 to 15	4 (Interactive Bulk)

The rationale is that bits 3-6 of the Type of Service field were interpreted as the TOS subfield by (the obsolete) RFC 1349. Readers can refer to the doxygen documentation of ns3::Socket::IpTos2Priority() for more information, including how DSCP values map onto priority values.

The priority set for a socket (as described above) is then used to determine the priority of the packets sent through that socket, as detailed in the next sections. Currently, the socket types that support setting the packet priority are ns3::UdpSocketImpl, ns3::TcpSocketBase and ns3::PacketSocket. The packet priority is used, e.g., by queuing disciplines such as the default PfifoFastQueueDisc to classify packets into distinct queues.

23.4.5.2.1. Setting the priority with UDP sockets¶

If the packet is an IPv4 packet and the value to be inserted in the ToS field is not null, then the packet is assigned a priority based on such ToS value (according to the ns3::Socket::IpTos2Priority() function). Otherwise, the priority associated with the socket is assigned to the packet.

23.4.5.2.2. Setting the priority with TCP sockets¶

Every packet is assigned a priority equal to the priority associated with the socket.

23.4.5.2.3. Setting the priority with packet sockets¶