trunk/protocol-dns/src/main/java/org/apache/directory/server/dns/io/encoder/ServerSelectionRecordEncoder.java

/*
 *  Licensed to the Apache Software Foundation (ASF) under one
 *  or more contributor license agreements.  See the NOTICE file
 *  distributed with this work for additional information
 *  regarding copyright ownership.  The ASF licenses this file
 *  to you under the Apache License, Version 2.0 (the
 *  "License"); you may not use this file except in compliance
 *  with the License.  You may obtain a copy of the License at
 *  
 *    http://www.apache.org/licenses/LICENSE-2.0
 *  
 *  Unless required by applicable law or agreed to in writing,
 *  software distributed under the License is distributed on an
 *  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 *  KIND, either express or implied.  See the License for the
 *  specific language governing permissions and limitations
 *  under the License. 
 *  
 */

package org.apache.directory.server.dns.io.encoder;


import org.apache.directory.server.dns.messages.ResourceRecord;
import org.apache.directory.server.dns.store.DnsAttribute;
import org.apache.mina.core.buffer.IoBuffer;


/**
 * The format of the SRV RR
 * 
 *    Here is the format of the SRV RR, whose DNS type code is 33:
 * 
 *         _Service._Proto.Name TTL Class SRV Priority Weight Port Target
 * 
 *         (There is an example near the end of this document.)
 * 
 *    Service
 *         The symbolic name of the desired service, as defined in Assigned
 *         Numbers [STD 2] or locally.  An underscore (_) is prepended to
 *         the service identifier to avoid collisions with DNS labels that
 *         occur in nature.
 * 
 *         Some widely used services, notably POP, don't have a single
 *         universal name.  If Assigned Numbers names the service
 *         indicated, that name is the only name which is legal for SRV
 *         lookups.  The Service is case insensitive.
 * 
 *    Proto
 *         The symbolic name of the desired protocol, with an underscore
 *         (_) prepended to prevent collisions with DNS labels that occur
 *         in nature.  _TCP and _UDP are at present the most useful values
 *         for this field, though any name defined by Assigned Numbers or
 *         locally may be used (as for Service).  The Proto is case
 *         insensitive.
 * 
 *    Name
 *         The domain this RR refers to.  The SRV RR is unique in that the
 *         name one searches for is not this name; the example near the end
 *         shows this clearly.
 * 
 *    TTL
 *         Standard DNS meaning [RFC 1035].
 * 
 *    Class
 *         Standard DNS meaning [RFC 1035].   SRV records occur in the IN
 *         Class.
 * 
 *    Priority
 *         The priority of this target host.  A client MUST attempt to
 *         contact the target host with the lowest-numbered priority it can
 *         reach; target hosts with the same priority SHOULD be tried in an
 *         order defined by the weight field.  The range is 0-65535.  This
 *         is a 16 bit unsigned integer in network byte order.
 * 
 *    Weight
 *         A server selection mechanism.  The weight field specifies a
 *         relative weight for entries with the same priority. Larger
 *         weights SHOULD be given a proportionately higher probability of
 *         being selected. The range of this number is 0-65535.  This is a
 *         16 bit unsigned integer in network byte order.  Domain
 *         administrators SHOULD use Weight 0 when there isn't any server
 *         selection to do, to make the RR easier to read for humans (less
 *         noisy).  In the presence of records containing weights greater
 *         than 0, records with weight 0 should have a very small chance of
 *         being selected.
 * 
 *         In the absence of a protocol whose specification calls for the
 *         use of other weighting information, a client arranges the SRV
 *         RRs of the same Priority in the order in which target hosts,
 *         specified by the SRV RRs, will be contacted. The following
 *         algorithm SHOULD be used to order the SRV RRs of the same
 *         priority:
 * 
 *         To select a target to be contacted next, arrange all SRV RRs
 *         (that have not been ordered yet) in any order, except that all
 *         those with weight 0 are placed at the beginning of the list.
 * 
 *         Compute the sum of the weights of those RRs, and with each RR
 *         associate the running sum in the selected order. Then choose a
 *         uniform random number between 0 and the sum computed
 *         (inclusive), and select the RR whose running sum value is the
 *         first in the selected order which is greater than or equal to
 *         the random number selected. The target host specified in the
 *         selected SRV RR is the next one to be contacted by the client.
 *         Remove this SRV RR from the set of the unordered SRV RRs and
 *         apply the described algorithm to the unordered SRV RRs to select
 *         the next target host.  Continue the ordering process until there
 *         are no unordered SRV RRs.  This process is repeated for each
 *         Priority.
 * 
 *    Port
 *         The port on this target host of this service.  The range is 0-
 *         65535.  This is a 16 bit unsigned integer in network byte order.
 *         This is often as specified in Assigned Numbers but need not be.
 * 
 *    Target
 *         The domain name of the target host.  There MUST be one or more
 *         address records for this name, the name MUST NOT be an alias (in
 *         the sense of RFC 1034 or RFC 2181).  Implementors are urged, but
 *         not required, to return the address record(s) in the Additional
 *         Data section.  Unless and until permitted by future standards
 *         action, name compression is not to be used for this field.
 * 
 *         A Target of "." means that the service is decidedly not
 *         available at this domain.
 * 
 * @author <a href="mailto:dev@directory.apache.org">Apache Directory Project</a>
 */
public class ServerSelectionRecordEncoder extends ResourceRecordEncoder
{
    protected void putResourceRecordData( IoBuffer byteBuffer, ResourceRecord record )
    {
        byteBuffer.putShort( Short.parseShort( record.get( DnsAttribute.SERVICE_PRIORITY ) ) );
        byteBuffer.putShort( Short.parseShort( record.get( DnsAttribute.SERVICE_WEIGHT ) ) );
        byteBuffer.putShort( Short.parseShort( record.get( DnsAttribute.SERVICE_PORT ) ) );
        putDomainName( byteBuffer, record.get( DnsAttribute.DOMAIN_NAME ) );
    }
}