DatagramReader
ptolemy.actor.lib.net.DatagramReader

This actor reads datagram packets via a separate thread. The thread responds to datagrams whenever they arrive, giving the actor the ability to read the datagrams asynchronously. Datagrams are connectionless, open-loop internet communications. Each datagram packet contains data plus a return address. The return address consists of an IP address and a socket number. Datagrams use the UDP protocol under which no reply or confirmation is expected. This is in contrast to TCP which expects confirmations and attempts to deliver packets in order to the layer above TCP. This can result in long delays in the delivery of information across the network. Because UDP makes no such attempts, it never hangs and does not need to be timed out. <p>NOTE: This actor has been developed to work in the Discrete Event (DE) and Synchronous Data Flow (SDF) domains. Use elsewhere with caution. <p> <p>The simplest scenario has the thread constantly stalled awaiting a packet. When a packet arrives, the thread quickly queues it in one of the buffers of the actor, calls the getDirector().fireAtCurrentTime(), and then stalls again awaiting the next packet. By stalling again and again, the thread keeps the actor aware at all times of incoming packets. This is particularly important if packets come in more quickly than the model can process them. Depending on the domain (e.g. DE) in which this actor is used, the director may respond to the fireAtCurrentTime() call of the thread by calling the fire() method of the actor. In this case, fire() then broadcasts the data received, along with the return address and return socket number from which the datagram originated. <p>The data portion of the packet is broadcast at the <i>output</i> port. The type of the output is always an array of bytes. <p>The return address and socket number are broadcast as String and int respectively. These tell where the received datagram originated from. <p>The behavior of the actor under less simple scenarios is governed by parameters of this actor. Additional packet(s) can arrive while the director is getting around to calling fire(). Conversely, the director may make extra calls to fire(), even before any datagrams have come in. I call these the eager packet and eager director scenarios respectively. <p>Background: There are two packet buffers. The thread and the fire() method share these buffers and maintain consistency via synchronization on the object <i>_syncFireAndThread</i>. This synchronization prevents conflicts when accessing the shared buffers and when accessing the count of queued packets. <p>The <i>overwrite</i> parameter applies to the eager packet scenario. Setting this parameter to true is useful in cases where it is possible for data to come in too fast for the model to process. This setting alleviates data gluts without undue loss of data when the model is able to keep up. When <i>overwrite</i> is set to true (the default), the actor discards the packet already received in favor of the new packet. If false, the new packet is queued behind the existing one. In the latter case, both buffers are now full. The thread then waits for fire() to consume a queued packet before it stalls again awaiting the next. In all other cases (<i>overwrite</i> true or no queued packets) the thread immediately stalls to await the next packet. <p>The <i>blockAwaitingDatagram</i> parameter applies to the eager director case. This case comes up most often in SDF, where an actor is expected to block in fire until an output can be produced. If true, a call to fire() will block unless or until a datagram has arrived. If false, then fire() returns without waiting, using the <i>defaultOutput</i> parameter in place of real data. The <i>returnAddress</i> and <i>returnSocketNumber</i> ports have default outputs as well, but they are not parameter-programmable. <p> <p>NOTE: This actor has a parameter <i>localSocketNumber</i> for the port number assigned to its local datagram socket. Initially, the local socket number is set to 4004. There is no particular reason for choosing this number, except that is noticeable in the code and in Vergil, thus encouraging you to change it to any desired value in the range 0..65535. Note that socket numbers 0..1023 are generally reserved and numbers 1024 and above are generally available. <p>Some commonly used port numbers (a.k.a. socket numbers) are shown below: <pre> Well-known Ports (Commonly Used Ports) 7 (Echo) 21 (FTP) 23 (TELNET) 25 (SMTP) 53 (DNS) 79 (finger) 80 (HTTP) 110 (POP3) 119 (NNTP) 161 (SNMP) 162 (SNMP Trap) </pre> Reference: http://192.168.1.1/Forward.htm (A webpage hosted from within the Linksys BEFSR41 Cable/DSL Router) <p>NOTE: This actor can also be configured to handle multicase datagram socket. A MulticastSocket is a DatagramSocket with additional capabilities to join groups of other multicast hosts on the internet. A multicast group is specified by a class D IP address and a standard UDP port number. When one member sends a packet to a multicast group, all recipients subscribing to that host and port receive the packet. Currently, The parameter <i>defaultReturnAddress</i> is overloaded to specify a multicast datagram IP address. When the return address is a multicast IP address, The parameter <i>localSocketNumber</i> is used to specify the UDP port number for the multicast group. A multicast IP address ranges from 224.0.0.0 to 239.255.255.255, inclusive. To send a packet to the group, the sender can be either a DatagramSocket or a MulticastSocket. The only difference is that MulticastSocket allows you to control the time-to-live of the datagram. Don't use 224.0.0.1 ~ 224.255.255.255 when the live time of is specified larger than 1. <p>FIXME: we might not want to overload the <i>defaultReturnAddress</i> and the <i>localSocketNumber</i> parameter... <p>Another useful tidbit is the command 'netstat'. This works in a DOS prompt and also in the UNIX-like Bash shell. In either shell, enter 'netstat -an'. This command shows current port allocations! Ports allocated to Ptolemy models are shown along with other port allocations. Other useful network commands include 'ping' and 'tracert'. Both TCP and UDP (datagram) ports are shown by netstat. FIXME: Find out whether a TCP port using a specific number blocks a UDP port from using that same number.

Author(s): Winthrop Williams, Joern Janneck, Xiaojun Liu, Edward Lee (Based on TiltSensor actor written by Chamberlain Fong, Xiaojun Liu, Edward Lee)
Version:$Id: DatagramReader.doc.html,v 1.1 2006/02/22 18:40:26 mangal Exp $
Pt.Proposed Rating:Yellow (winthrop)
Pt.Accepted Rating:Yellow (winthrop)


returnAddress
This port outputs the IP address portion of the received datagram packet. The type of this output is String. This is the IP address of the remote datagram socket which sent the packet to the socket of this actor. Under IPv4, this string has the familiar form "128.32.1.1". This output defaults (when no datagram has been received and blocking is false) to the IP address of the socket.
returnSocketNumber
This port outputs the socket (a.k.a port) number portion of the received datagram packet. The type of this output is int. This is the socket number of the remote datagram socket which sent the packet to this actor's socket. This is an integer in the range 0 through 65535. This output defaults (when no datagram has been received and blocking is false) to this actor's local socket number.
output
This port outputs the data portion of the received datagram packet. The type of <i>output</i> may depend on the datagram received, which may vary even during a single run of a model. <b>The user is encouraged to play with the configuration of this port</b> to best suit the need at hand.
trigger
The trigger input port reads and discards a token from each channel that has a token. The type of this port has been set to GENERAL, permitting any token type to be accepted. The hasToken() and get(int) methods are called on this input, but their contents are discarded. The presence of a connection to this input serves a purpose by causing the director to schedule the firing of this actor at an appropriate place in the sequence of firings of actors. This is particularly useful in the SDF domain. Without a trigger input, the SDF scheduler would be unable to schedule a firing of this actor unless it can be scheduled as the first actor to be fired. Thus, without this input, configurations in SDF would be limited. (@See ptolemy.actor.lib.Source for an archetypal trigger input.)


localSocketNumber
This actor's local socket (a.k.a. port) number. <b>This is a system resource allocated to this actor.</b> No other actor with the same local socket number may run at the same time. Currently, When the return address is a multicast IP address, this parameter is also used to specify the UDP port number for the multicast group.
actorBufferLength
Length (in bytes) of each of the actor's two packet buffers for receiving a datagram. This length does not include the bytes needed for storing the datagram's return address and other housekeeping information. This buffer need only be big enough to hold the payload (a.k.a. data portion) of the datagram.
platformBufferLength
Length (in bytes) of the buffer within java and/or the platform layers below java. Java documents refers to all this collectively as the platform. The size of this buffer is controlled via the getReceiveBufferSize() and setReceiveBufferSize() methods. @see java.net.DatagramSocket. Caution #1 - The platform treats setReceiveBufferSize() as a suggestion only. It supposedly reports the actual buffer size granted in subsequent calls to getReceiveBufferSize(). However, my experiments with this showed it granting buffers as large as 2 gigabytes, with no apparent limit except the maximum representable integer value. Thus, I suggest taking this with a grain of salt. Caution #2 - the get/setReceiveBufferSize() calls block when called as long as another thread is in a receive() call on that same socket. This is undocumented in Java's documentation. Also note that the setReceiveBufferSize() method is not available in early JDK's, which makes it important to have setPlatformBufferLength set to false when generating code.
setPlatformBufferLength
Determine whether the platformBufferLength parameter will be used to set the platform's receive buffer size. This parameter must contain a boolean token, and has a default of false.
overwrite
Whether to overwrite when inundated with datagrams or let them pile up. Default is true. If false, datagrams will queue up (mostly in the platform, some in the actor). The datagram used at each invocation of fire will be the oldest in the queue. On the other hand, if <i>overwrite</i> is true, then minimal queuing will occur and the most recent data will be used when fire() is called. Older data will be discarded.
blockAwaitingDatagram
Whether to block in fire(). If fire() is called before the datagram has arrived, the actor must either block awaiting the datagram or use its <i>defaultOutput</i>. This blocking parameter controls which choice fire() will make. This parameter is useful for SDF models, where it is generally set to true. It has no effect in DE models unless the trigger input has been connected. Type is Boolean. Default value is true.
defaultReturnAddress
The default for the <i>returnAddress</i> output. This token is broadcast when the actor is fired, but no actual datagram is available to broadcast and <i>blockAwaitingDatagram</i> is false. If blocking were true, the actor would simply stall in fire() until a datagram arrives. Type is string. Default value is "localhost". Currently, this parameter can be overloaded to specify a multicast datagram IP address. A multicast IP address ranges from 224.0.0.0 to 239.255.255.255, inclusive.
defaultReturnSocketNumber
The default the <i>returnSocketNumber</i> output. This token is broadcast when the actor is fired, but no actual datagram is available to broadcast and <i>blockAwaitingDatagram</i> is false. If blocking were true, the actor would simply stall in fire() until a datagram arrives. Type is integer. Default value is 0.
defaultOutput
The default for the <i>output</i> output. This default token is broadcast when the actor is fired, but no actual datagram data is available to broadcast and <i>blockAwaitingDatagram</i> is false. If blocking were true, the actor would simply stall in fire() until a datagram arrives. Type is defined by the expression entered. Default type & value is the integer 0.