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012// ***************************************************************************************************************************
013package org.apache.juneau.http;
014
015import org.apache.juneau.http.annotation.*;
016
017/**
018 * Represents a parsed <l>Via</l> HTTP response header.
019 *
020 * <p>
021 * Informs the client of proxies through which the response was sent.
022 *
023 * <h5 class='figure'>Example</h5>
024 * <p class='bcode w800'>
025 *    Via: 1.0 fred, 1.1 example.com (Apache/1.1)
026 * </p>
027 *
028 * <p>
029 * Informs the client of proxies through which the response was sent.
030 *
031 * <p>
032 * <h5 class='figure'>Example</h5>
033 * <p class='bcode w800'>
034 *    Via: 1.0 fred, 1.1 example.com (Apache/1.1)
035 * </p>
036 *
037 * <p>
038 * <h5 class='topic'>RFC2616 Specification</h5>
039 *
040 * The Via general-header field MUST be used by gateways and proxies to indicate the intermediate protocols and
041 * recipients between the user agent and the server on requests, and between the origin server and the client on
042 * responses.
043 * It is analogous to the "Received" field of RFC 822 and is intended to be used for tracking message forwards,
044 * avoiding request loops, and identifying the protocol capabilities of all senders along the request/response chain.
045 *
046 * <p class='bcode w800'>
047 *    Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
048 *    received-protocol = [ protocol-name "/" ] protocol-version
049 *    protocol-name     = token
050 *    protocol-version  = token
051 *    received-by       = ( host [ ":" port ] ) | pseudonym
052 *    pseudonym         = token
053 * </p>
054 *
055 * <p>
056 * The received-protocol indicates the protocol version of the message received by the server or client along each
057 * segment of the request/response chain.
058 * The received-protocol version is appended to the Via field value when the message is forwarded so that information
059 * about the protocol capabilities of upstream applications remains visible to all recipients.
060 *
061 * <p>
062 * The protocol-name is optional if and only if it would be "HTTP".
063 * The received-by field is normally the host and optional port number of a recipient server or client that subsequently
064 * forwarded the message.
065 * However, if the real host is considered to be sensitive information, it MAY be replaced by a pseudonym.
066 * If the port is not given, it MAY be assumed to be the default port of the received-protocol.
067 *
068 * <p>
069 * Multiple Via field values represents each proxy or gateway that has forwarded the message.
070 * Each recipient MUST append its information such that the end result is ordered according to the sequence of
071 * forwarding applications.
072 *
073 * <p>
074 * Comments MAY be used in the Via header field to identify the software of the recipient proxy or gateway, analogous
075 * to the User-Agent and Server header fields.
076 * However, all comments in the Via field are optional and MAY be removed by any recipient prior to forwarding the
077 * message.
078 *
079 * <p>
080 * For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named "fred",
081 * which uses HTTP/1.1 to forward the request to a public proxy at nowhere.com, which completes the request by
082 * forwarding it to the origin server at www.ics.uci.edu.
083 * The request received by www.ics.uci.edu would then have the following Via header field:
084 * <p class='bcode w800'>
085 *    Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)
086 * </p>
087 *
088 * <p>
089 * Proxies and gateways used as a portal through a network firewall SHOULD NOT, by default, forward the names and ports
090 * of hosts within the firewall region.
091 * This information SHOULD only be propagated if explicitly enabled.
092 * If not enabled, the received-by host of any host behind the firewall SHOULD be replaced by an appropriate pseudonym
093 * for that host.
094 *
095 * <p>
096 * For organizations that have strong privacy requirements for hiding internal structures, a proxy MAY combine an
097 * ordered subsequence of Via header field entries with identical received-protocol values into a single such entry.
098 * For example...
099 * <p class='bcode w800'>
100 *    Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy
101 * </p>
102 *
103 * <p>
104 * ...could be collapsed to...
105 * <p class='bcode w800'>
106 *    Via: 1.0 ricky, 1.1 mertz, 1.0 lucy
107 * </p>
108 *
109 * <p>
110 * Applications SHOULD NOT combine multiple entries unless they are all under the same organizational control and the
111 * hosts have already been replaced by pseudonyms.
112 * Applications MUST NOT combine entries which have different received-protocol values.
113 *
114 * <ul class='seealso'>
115 *    <li class='extlink'>{@doc RFC2616}
116 * </ul>
117 */
118@Header("Via")
119public final class Via extends HeaderStringArray {
120
121   /**
122    * Constructor.
123    *
124    * @param value The value for this header.
125    */
126   public Via(String[] value) {
127      super(value);
128   }
129
130   /**
131    * Returns a parsed <c>Via</c> header.
132    *
133    * @param value The <c>Via</c> header string.
134    * @return The parsed <c>Via</c> header, or <jk>null</jk> if the string was null.
135    */
136   public static Via forString(String value) {
137      if (value == null)
138         return null;
139      return new Via(value);
140   }
141
142   private Via(String value) {
143      super(value);
144   }
145}