Socket Programming: Blocking vs Non-Blocking Sockets and QOS Parameters, Study notes of Computers and Information technologies

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11

Systems Programming

Network Applications Programming

Part 1

Transport Layer - Quality Of Service (QOS) 1 The design of the communication aspects of a distributed application will have a significant impact on the quality of Service (QoS) achieved. For example:

• (^) The choice between TCP or UDP affects the reliability of the data transport.
• (^) The choice of blocking or non-blocking sockets will affect the responsiveness of applications, as well as the efficiency (for example consider a server which receives on many different sockets).
• (^) Correct handling of error codes allows shielding the user form the complexities of communication, and also enables some problems to be automatically dealt with inside the application code. Some aspects of QoS are automatically supported by TCP, others are determined by the design of your application code.

Socket Options 1 Sockets are the endpoints of transport-layer communication. As such these need to be quite versatile. Options that can be selected for use with a particular socket include:

• (^) SO_DEBUG Debugging info
• (^) SO_KEEPALIVE Keep connection alive (reset timer on timeout)
• (^) SO_DONTROUTE Routing bypass
• (^) SO_BROADCAST Allow broadcasts
• (^) SO_USELOOPBACK Bypass H/W if possible
• (^) SO_LINGER Linger on CLOSE if data present
• (^) SO_TYPE (get only) socket type
• (^) SO_SNDBUF (set only) output buffer size
• (^) SO_RCVBUF (set only) input buffer size To get or set socket options, calls to getsockopt() or setsockopt() are made.

Socket Options 2 RECALL - setsockopt() Prototype: int setsockopt( SOCKET s , int level , int optname , const char* optval , int optlen ); If no error occurs, setsockopt returns zero. Otherwise, it returns an error code level is the level at which the option applies (usually SOL_SOCKET ) optname is the name of the option to set optval is the value to set the option to optlen is the length of the value data Example: char cOpt[2]; cOpt[0] = 1; // true cOpt[1] = 0; // null terminate the option array int iError = setsockopt(m_SendSOCKET, SOL_SOCKET, SO_BROADCAST, cOpt, sizeof(cOpt)); if(SOCKET_ERROR == iError) { MessageBox("setsockopt() Failed!","Simple Send Dialog"); }

Socket Address Structure struct sockaddr { unsigned short int sa_family; // address family char sa_data[14]; // up to 14 bytes of address }; This structure stores the socket address family (AF-INET for IP addresses) and the name that has been 'bound' to the socket. sa_family has fixed size of 2 bytes (u_short means unsigned short integer). sa_data has a variable size. A special version of the Socket Address Structure is used with IP addresses: struct sockaddr_in { short int sin_family; // Address family unsigned short int sin_port; // Port number struct in_addr sin_addr; // Internet address }; where: struct in_addr { unsigned long int s_addr; // Internet address ( 32 bits ) };

Buffering (rules for the Transport-layer protocols) The operating system only buffers messages for ports known to be in use (i.e. such that a process is likely to retrieve the message from the buffer). This leads to some important behaviour that must be considered when designing an application’s communication logic:

• (^) If a packet arrives for an unbound port it is dropped by the receiver.
• (^) If a packet arrives for a bound port, but there is not a receive operation outstanding, the packet is retained in a buffer.
• (^) When a recv or recvfrom operation is subsequently issued, the packet is delivered to the process immediately.
• (^) If a packet arrives for a bound port, and there is a recv or recvfrom operation outstanding, the packet is delivered to the process immediately.

Blocking vs Non Blocking behaviour 2 Operations on a Non-Blocking socket will return an error if they cannot complete immediately i.e. they remain responsive. The error code is WOULD-BLOCK and is not an error in the true sense. It signals to the process that the operation could not be completed at that time. For example, a non blocking recv will take a packet from the buffer if one is already there, or it will return with the error code WOULD-BLOCK. The usual way to deal with this situation is to use a timer to wait a short time and then try again. The process can get on with other work in-between the retries. Sockets can be switched between blocking and non-blocking modes by using setsockopt at any time between the socket’s creation and its closure.

Blocking vs Non Blocking behaviour 3 Setting a socket to be Blocking or Non-Blocking A socket by default operates in ‘Blocking’ mode. It can be changed between the two modes, to suit application requirements. Class header file (.h) #define BLOCKING 0 #define NON_BLOCKING 1 SOCKET m_RecvSOCKET; Class source file (.cpp) (create socket) unsigned long lArg = NON_BLOCKING; // or BLOCKING int iError = ioctlsocket(m_RecvSOCKET,FIONBIO,&lArg); if(SOCKET_ERROR == iError) { // Error handling, e.g. use WSAGetLastError() to get more information }

Blocking vs Non Blocking behaviour 5 Communication Deadlock If two communicating processes both set their sockets to ‘blocking’ mode, a problem can arise: If the processes work in ‘lock-step’ / synchronised fashion then all is well. i.e. Step 1: A sends, B receives Step 2: B sends, A receives Step 3: A sends, B receives … However, if the situation arises, where synchronisation is lost, and both processes wait for a message from the other one, a problem arises: Solutions: multithreaded applications, or use non-blocking socket at one end recv() blocked (cannot send) Buffer (empty) Process A (^) Process B recv() blocked (cannot send) Buffer (empty)

Timers 1 Distributed applications typically have an event-driven user interface – i.e. the client process is usually activated by the user selecting activities such as initiating a request to be sent to the server. Such event-driven processes ‘sleep’ when there are no events to process (this is the classic ‘windows’ concept, but also occurs in many other technologies). A ‘Timer’ is needed if an event needs to be generated periodically and automatically, such as checking to see if a message has arrived in the buffer when using a non-blocking socket. Check buffer for received message Check buffer for received message Check buffer for received message Method active Event Send message from buffer Time Timer expired Timer expired Timer expired Send button pressed Timer period

Timers 3 Source file (.cpp) contents include: (In the ‘message map’ section) ON_WM_TIMER() // handle timer events (General code) void class_name::InitiateTimer() { // Start timer number 1, Interval 250 milliseconds (Frequency = 4Hz) m_nTimer = SetTimer( 1, 250, 0 ); } void class_name::StopTimer() { KillTimer( m_nTimer ); // Stop the timer } void class_name::OnTimer( unsigned int nIDEvent ) { // Timer event-handler // Code here runs when the timer expires, e.g. non-blocking receive) } // nIDEvent signals which timer has expired (ignore if only using one)

User Interface 1 List control 1 Example use – within the O’s and X’s game, this was used to list the other players available to have a game with. List Control Names are added to the list when a player registers with the server Names are removed from the list when a player becomes part of a game To create - Add a new resource from the toolbox. Right-click and select ‘Properties’ to change its name etc. Example code (header file) // Define the maximum length of strings that can be placed in the listbox #define MAX_ALIAS_LEN 30 // This does not have to be part of the actual class // Declare the variable in the header file (.h) for the class CListBox m_PlayerListBox; // Can use right click on resource, ‘add variable’

User Interface 1 List control 3 Example code continued - source file for the class (.cpp) // To add an entry to the listbox char sPlayerAlias[MAX_ALIAS_LEN + 1]; strcpy( sPlayerAlias , ”Champion” ); m_PlayerListBox.AddString( sPlayerAlias ); // Set default selection to the top item in the list m_PlayerListBox.SetCurSel( 0 ); // Get the index of the actual item selected int iSelection = m_PlayerListBox.GetCurSel(); // Get the actual text for the selected item char sBuffer[MAX_ALIAS_LEN + 1]; int iLen = m_PlayerListBox.GetText( iSelection , sBuffer );

User Interface 2 Slider control 1 Example use – to change the speed of a game, or to select a sound volume, difficulty level etc. To create - Add a new resource from the toolbox. Right-click and select ‘Properties’ to change its name etc. Example code (header file) // Declare the slider control CSliderCtrl m_Speed; // Can use right click on resource, ‘add variable’ Slider control