/**
* Disable/Enable local loopback of multicast datagrams
* The option is used by the platform's networking code as a hint
* for setting whether multicast data will be looped back to
* the local socket.
*
* <p>Because this option is a hint, applications that want to
* verify what loopback mode is set to should call
* {@link #getLoopbackMode()}
* @param disable <code>true</code> to disable the LoopbackMode
* @throws SocketException if an error occurs while setting the value
* @since 1.4
* @see #getLoopbackMode
*/
public void setLoopbackMode(boolean disable) throws SocketException {
getImpl().setOption(SocketOptions.IP_MULTICAST_LOOP, Boolean.valueOf(disable));
}
9、public final static int IP_TOS = 0x3;
这个参数是用来控制IP头中的TOS字段的，是用来控制和优化IP包的路径的，在Socket源代码里有一个设置的方法：
/**
* Sets traffic class or type-of-service octet in the IP
* header for packets sent from this Socket.
* As the underlying network implementation may ignore this
* value applications should consider it a hint.
*
* <P> The tc <B>must</B> be in the range <code> 0 <= tc <=
* 255</code> or an IllegalArgumentException will be thrown.
* <p>Notes:
* <p> For Internet Protocol v4 the value consists of an octet
* with precedence and TOS fields as detailed in RFC 1349. The
* TOS field is bitset created by bitwise-or'ing values such
* the following :-
* <p>
* <UL>
* <LI><CODE>IPTOS_LOWCOST (0x02)</CODE></LI>
* <LI><CODE>IPTOS_RELIABILITY (0x04)</CODE></LI>
* <LI><CODE>IPTOS_THROUGHPUT (0x08)</CODE></LI>
* <LI><CODE>IPTOS_LOWDELAY (0x10)</CODE></LI>
* </UL>
* The last low order bit is always ignored as this
* corresponds to the MBZ (must be zero) bit.
* <p>
* Setting bits in the precedence field may result in a
* SocketException indicating that the operation is not
* permitted.
* <p>
* As RFC 1122 section 4.2.4.2 indicates, a compliant TCP
* implementation should, but is not required to, let application
* change the TOS field during the lifetime of a connection.
* So whether the type-of-service field can be changed after the
* TCP connection has been established depends on the implementation
* in the underlying platform. Applications should not assume that
* they can change the TOS field after the connection.
* <p>
* For Internet Protocol v6 <code>tc</code> is the value that
* would be placed into the sin6_flowinfo field of the IP header.
*
* @param tc an <code>int</code> value for the bitset.
* @throws SocketException if there is an error setting the
* traffic class or type-of-service
* @since 1.4
* @see #getTrafficClass
*/
public void setTrafficClass(int tc) throws SocketException {
if (tc < 0 || tc > 255)
throw new IllegalArgumentException("tc is not in range 0 -- 255");
if (isClosed())
throw new SocketException("Socket is closed");
getImpl().setOption(SocketOptions.IP_TOS, new Integer(tc));
}
从源代码的注释看，TOS设置了是否生效，和底层的操作系统的实现有关。应用程序无法保证TOS的变更会对socket连接产生影响。个人认为，TOS在一般情况下用不到。
10、public final static int SO_LINGER = 0x0080;
先看Socket源代码：
/**
* Enable/disable SO_LINGER with the specified linger time in seconds.
* The maximum timeout value is platform specific.
*
* The setting only affects socket close.
*
* @param on whether or not to linger on.
* @param linger how long to linger for, if on is true.
* @exception SocketException if there is an error
* in the underlying protocol, such as a TCP error.
* @exception IllegalArgumentException if the linger value is negative.
* @since JDK1.1
* @see #getSoLinger()
*/
public void setSoLinger(boolean on, int linger) throws SocketException {
if (isClosed())
throw new SocketException("Socket is closed");
if (!on) {
getImpl().setOption(SocketOptions.SO_LINGER, new Boolean(on));
} else {
if (linger < 0) {
throw new IllegalArgumentException("invalid value for SO_LINGER");
}
if (linger > 65535)
linger = 65535;
getImpl().setOption(SocketOptions.SO_LINGER, new Integer(linger));
}
}