New TCP Socket Architecture
In the following, a new architecture for TCP socket implementation is proposed. This is to replace the old TcpSocketImpl class in NS-3.8 so that different favors of TCP can be easily implemented.
As of change set 6273:8d70de29d514 in the Mercurial, TCP simulation is implemented by class TcpSocketImpl, in src/internet-stack/tcp-socket-impl.h and src/internet-stack/tcp-socket-impl.cc. The TcpSocketImpl class is implementing TCP NewReno, despite the Doxygen comment claims that is implementing Tahoe.
The TcpSocketImpl class is derived from TcpSocket class, which in turn, is derived from Socket class. The TcpSocket class is merely an empty class defining the interface for attribute get/set. Examples of the attributes configured by the interface of TcpSocket class are the send and receive buffer sizes, initial congestion window size, etc. The Socket class, however, provides the interface for the L7 application to call.
How TcpSocketImpl works
TCP state machine transitions are defined in tcp-l4-protocol.h and tcp-l4-protocol.cc. The class TcpSocketImpl does not maintain the transition rule but keeps track on the state of the current socket.
When an application needs a TCP connection, it has to get a socket from TcpL4Protocol::CreateSocket(). This call will allocate a TcpSocketImpl object and configure it (e.g. assign it to a particular node). The TcpL4Protocol object is unique on a TCP/IP stack and serve as a mux/demux layer for the real sockets, namely, TcpSocketImpl.
Once the TcpSocketImpl object is created, it is in CLOSED state. The application can instruct it to Bind() to a port number and then Connect() or Listen() as the traditional BSD socket does.
The Bind() call is to register its port in the TcpL4Protocol object as an Ipv4EndPoint, and set up the callback functions (by way of FinishBind()), so that mux/demux can be done.
The Listen() call puts the socket into LISTEN state. The Connect() call, on the other hand, initiates three way handshake.
The new structure, TcpSocketBase class, is having the same relationship to TcpSocket and Socket classes as TcpSocketImpl. However, instead of providing a concrete TCP implementation, it is designed to meet the following goals:
- Provide only the function common to all TCP classes, namely, the implementation of TCP state machine
- Minimize the code footprint and make it modular to make it easier to understand
To be continued