	CS 580 Client-Server Programming Fall Semester, 2002 Socket Options
	Previous Lecture Notes Index Next © 2002, All Rights Reserved, SDSU & Roger Whitney San Diego State University -- This page last updated 15-Oct-02

Contents of Doc 15, Socket Options

References
Java On-line API http://java.sun.com/j2se/1.4.1/docs/api/

VisualWorks Internet Client Developer's Guide, Socket Programming Chapter 2, docs/NetClientDevGuide.pdf in VW 7 distribution

Unix Network Programming, Stevens, 1990, Berkeley Sockets chapter 6.

TCP/IP Illustrated Vol 1, Stevens, 1994, chapter 20.


Doc 15, Socket Options Slide # 2

Socket Options

Timeouts
Buffer Size
Multi-Homing
No Delay for small data
Linger on close
Keep-Alive
Urgent-Data


Doc 15, Socket Options Slide # 3

Setting Socket Options - VisualWorks
One method to set standard TCP socket options

Uses Berkley sockets naming conventions

SocketAccessor>> setOptionsLevel: SocketAccessor SOL_SOCKET
      name: optionToSet
      value: optionValue.

setOptionsLevel: is always set to SocketAccessor SOL_SOCKET

Values for name: are found in SocketAccessor class under constants-socket options protocol

Example

| serverSocket |
serverSocket := SocketAccessor newTCPserverAtPort: 4444.
serverSocket 
   setOptionsLevel: SocketAccessor SOL_SOCKET
   name: SocketAccessor SO_RCVBUF
   value: 56 * 1024.


Doc 15, Socket Options Slide # 4

Reading Current Values

SocketAccessor>> getOptionsLevel: SocketAccessor SOL_SOCKET
      name: option

Returns result in a byte array

To interpret the bytes

Change the result’s class to UninterpretedBytes
Convert to the type expected

   | childSocket rawBytes bufferSize |
   childSocket := SocketAccessor 
            newTCPclientToHost: 'rugby.sdsu.edu'
            port: 8008.
   rawBytes := childSocket 
            getOptionsLevel: SocketAccessor SOL_SOCKET
            name: SocketAccessor SO_RCVBUF.
   rawBytes changeClassTo: UninterpretedBytes.
   bufferSize := rawBytes longAt: 1.
   ^bufferSize


Doc 15, Socket Options Slide # 5

Timeouts

Socket will time out after specified time of inactivity

Java JDK 1.4 and later
Both Socket and ServerSocket class support:

void setSoTimeout(int timeoutInMilliseconds) throws SocketException
void getSoTimeout() throws SocketException

Must be sent before performing a read

Read throws SocketTimeoutException when socket times out

Not normally used on ServerSockets

VisualWorks
Does not support SO_RCVTIMEO or SO_SNDTIMEO

Need to use non-stream access for timeouts


Doc 15, Socket Options Slide # 6

Buffer Size

Each TCP socket has

Receive buffer
Send Buffer

Buffers are in the TCP stack space (not the VM)

Buffer size should:

Be at least 16KB on Ethernet
Applications that send lots of data use 48KB or 64KB

TCP does not allow the sender to overflow the receiver’s buffer

So the receiver’s receive buffer as large as the sender’s send buffer

Buffers larger than 64KB require special set up


Doc 15, Socket Options Slide # 7

Default Buffer Size
Depends on platform

Has changed over time

OS	Receive buffer Size
Solaris	32KB
Mac OS 10	32KB+ (33304 bytes)


Doc 15, Socket Options Slide # 8

Setting the Buffer Size – VisualWorks
Setting the Receive buffer

aSocketAccesor setOptionsLevel: SocketAccessor SOL_SOCKET
   name: SocketAccessor SO_RCVBUF
   value: newBufferSize.

Setting the Send buffer

aSocketAccesor setOptionsLevel: SocketAccessor SOL_SOCKET
   name: SocketAccessor SO_SNDBUF
   value: newBufferSize.


Doc 15, Socket Options Slide # 9

Setting the Buffer Size – Java
void setReceiveBufferSize(int size) throws SocketException
int getReceiveBufferSize() throws SocketException

void setSendBufferSize(int size) throws SocketException
int getSendBufferSize() throws SocketException

A Socket object has both a send & receive buffer

A ServerSocket only has a receive buffer


Doc 15, Socket Options Slide # 10

Java Example
In this example the default buffer size will be fine

Setting the buffer size just to show how to do it

import java.net.*;
import java.io.*;
import java.util.Date;
public class ServerWithTimeout extends Thread {
   static final int CLIENT_TIMEOUT = 3 * 1000; // in milliseconds
   static final int BUFFER_SIZE = 16 * 1024;
   ServerSocket acceptor;
   
   public static void main(String[] args) throws IOException {
      int port = Integer.parseInt( args[1]);   
      
      ServerWithTimeout server = new ServerWithTimeout( port );
      server.start();
   }
   
   public ServerWithTimeout(int port ) throws IOException {
      acceptor = new ServerSocket(port);
      acceptor.setReceiveBufferSize( BUFFER_SIZE );
   }


Doc 15, Socket Options Slide # 11

Java Example Continued


   public void run() 
      {
      while (true) 
         {
         try
            {
            Socket client = acceptor.accept();
            processRequest( client );
            }
         catch (IOException acceptError)
            {
            // for a later lecture
            }
         }
      }
   
   void processRequest( Socket  client) throws IOException
      {
      try
         {
         client.setReceiveBufferSize( BUFFER_SIZE);
         client.setSoTimeout( CLIENT_TIMEOUT);
         processRequest( 
            client.getInputStream(),
            client.getOutputStream());
         }
      finally
         {
         client.close();
         }
      }


Doc 15, Socket Options Slide # 12

Java Example Continued


   void processRequest(InputStream in,OutputStream out)
      throws IOException 
      {
      BufferedReader parsedInput = null;
      PrintWriter parsedOutput = null;
      try
         {
         parsedInput = new BufferedReader(new InputStreamReader(in));
         parsedOutput = new PrintWriter(out,true);
   
         String inputLine = parsedInput.readLine();
   
         if (inputLine.startsWith("date")) 
            {
            Date now = new Date();
            parsedOutput.println(now.toString());
            }
         }
      catch (SocketTimeoutException clientTooSlow)
         {
         parsedOutput.println("Connection timed out");
         }
      }
   }
Doc 15, Socket Options Slide # 13

Nagle’s Algorithm

Delays transmission of new TCP packets while any data remains unacknowledged

Allows TCP to merge data into larger packets before sending

Introduced to avoid lots of small packets across a WAN

Delay is on be default


Doc 15, Socket Options Slide # 14

Java

class Socket 
   {
   void setTcpNoDelay(Boolean noDelay) throws SocketException
   void getTcpNoDelay() throws SocketException
   }

Set noDelay to true to turn delay off

VisualWorks

aSocketAccesor setOptionsLevel: SocketAccessor SOL_SOCKET
   name: SocketAccessor TCP_NODELAY
   value: delayValue.

Set delayValue to

1 to turn delay off
0 to turn delay on

or use the shorter method

aSocketAccesor tcpNoDelay: noDelay


Doc 15, Socket Options Slide # 15

Linger on close

Determines what happens when a socket is closed

How long does the socket remain after close to

Acknowledge packets
Retransmit lost packets

Default is to

Allow the application to continue
TCP handles sending unsent data & rejecting new requests


Doc 15, Socket Options Slide # 16

Keep Alive

Send packet on inactive connection to prevent timeouts

At least 2 hour delay between sending keep alive packets

Long delay limits it usefulness


Doc 15, Socket Options Slide # 17

Urgent (Out of Band) Data

Urgent data can be read out of order

Read before data that was sent before it

Java 1.4

Supports sending of urgent data

Does not promote urgent data in the input stream

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