C: Content-Type: application/beep+xml
       C: Content-ID: <1@example.com>
       C:
       C: <data content='cid:2@example.com'>
       C:     <originator identity='fred@example.com' />
       C:     <recipient identity='apex=presence@example.com' />
       C: </data>
       C: --boundary
       C: Content-Type: application/beep+xml
       C: Content-ID: <2@example.com>
       C:
       C: <reply code='250' transID='1' />
       C: --boundary--
       C: END
</PRE>
  <P>or this: 
  <P><PRE>       C: MSG 1 1 . 42 1234
       C: Content-Type: application/beep+xml
       C:
       C: &lt;data content='#Content'&gt;
       C:     &lt;originator identity='fred@example.com' /&gt;
       C:     &lt;recipient identity='apex=presence@example.com' /&gt;
       C:     &lt;data-content Name='Content'&gt;
       C:         &lt;reply code='250' transID='1' /&gt;
       C:     &lt;/data-content&gt;
       C: &lt;/data&gt;
       C: END
</PRE>
  <P></P>
  <DT>
  <HR>
  <EM><A name=page-13>Page 13</A></EM>
  <DD>
  <P></P>
  <DT><STRONG><A name=sec-4.2>4.2</A> The Subscribe Operation</STRONG>
  <DD>
  <P>When an application wants to (periodically) receive the presence 
  information associated with an endpoint, it sends a "subscribe" element to the 
  service. 
  <P>The "subscribe" element has a "publisher" attribute, a "duration" 
  attribute, a "transID" attribute, and no content: 
  <P>
  <UL>
    <LI>the "publisher" attribute specifies the endpoint associated with the 
    presence entry; 
    <P></P>
    <LI>the "transID" attribute specifies the transaction-identifier associated 
    with this operation; and, 
    <P></P>
    <LI>the "duration" attribute specifies the maximum number of seconds for 
    which the originator is interested in receiving updated presence 
    information. </LI></UL>
  <P>When the service receives a "subscribe" element, we refer to the 
  "publisher" attribute of that element as the "subject", and the service 
  performs these steps: 
  <P></P>
  <DT><STRONG><A name=sec-1>1</A> If the subject is outside of this 
  administrative domain, a "reply"</STRONG>
  <DD>element having code 553 is sent to the originator. 
  <P></P>
  <DT><STRONG><A name=sec-2>2</A> If the subject does not refer to a valid 
  endpoint, a "reply"</STRONG>
  <DD>element having code 550 is sent to the originator. 
  <P></P>
  <DT><STRONG><A name=sec-3>3</A> If the subject's access entry does not contain 
  a</STRONG>
  <DD>"presence:subscribe" token for the originator, a "reply" element having 
  code 537 is sent to the originator. 
  <P></P>
  <DT><STRONG><A name=sec-4>4</A> If the originator already has an in-progress 
  subscribe operation</STRONG>
  <DD>for the subject, then the previous subscribe operation is silently 
  terminated, and processing continues. 
  <P></P>
  <DT><STRONG><A name=sec-5>5</A> If the "transID" attribute refers to an 
  in-progress subscribe or</STRONG>
  <DD>watch operation for the originator, a "reply" element having code 555 is 
  sent to the originator. 
  <P></P>
  <DT><STRONG><A name=sec-6>6</A> Otherwise:</STRONG>
  <DD>
  <P></P>
  <DT><STRONG><A name=sec-1>1</A> A "publish" element, corresponding to the 
  subject's presence</STRONG>
  <DD>entry, is immediately sent to the originator. 
  <P></P>
  <DT>
  <HR>
  <EM><A name=page-14>Page 14</A></EM>
  <DD>
  <P></P>
  <DT><STRONG><A name=sec-2>2</A> For each endpoint currently watching 
  subscribers to the</STRONG>
  <DD>subject's presence information, a "notify" element is immediately as sent 
  (c.f., Step 6.3 of <A 
  href="http://www.apps.ietf.org/rfc/rfc3343.html#sec-4.6">Section 4.6</A>). 
  <P></P>
  <DT><STRONG><A name=sec-3>3</A> For up to the amount of time indicated by the 
  "duration"</STRONG>
  <DD>attribute of the "subscribe" element, if the subject's presence entry 
  changes, an updated "presence" element is sent to the originator using the 
  publish operation (<A 
  href="http://www.apps.ietf.org/rfc/rfc3343.html#sec-4.4">Section 4.4</A>). 
  Finally, when the amount of time indicated by the "duration" attribute 
  expires, a terminate operation (<A 
  href="http://www.apps.ietf.org/rfc/rfc3343.html#sec-4.5">Section 4.5</A>) is 
  sent to the originator. 
  <P>Note that if the duration is zero-valued, then the subscribe operation is 
  making a one-time poll of the presence information. Accordingly, Step 6.3 
  above does not occur. 
  <P>Regardless of whether a "publish" or "reply" element is sent to the 
  originator, the "transID" attribute is identical to the value found in the 
  "subscribe" element sent by the originator. 
  <P></P>
  <DT><STRONG><A name=sec-4.3>4.3</A> The Watch Operation</STRONG>
  <DD>
  <P>When an application wants to (periodically) receive notices about endpoints 
  that are subscribed to receive presence entry, it sends a "watch" element to 
  the service. 
  <P>The "watch" element has a "publisher" attribute, a "duration" attribute, a 
  "transID" attribute, and no content: 
  <P>
  <UL>
    <LI>the "publisher" attribute specifies the endpoint associated with the 
    presence entry; 
    <P></P>
    <LI>the "transID" attribute specifies the transaction-identifier associated 
    with this operation; and, 
    <P></P>
    <LI>the "duration" attribute specifies the maximum number of seconds for 
    which the originator is interested in watching subscribers. </LI></UL>
  <P>When the service receives a "watch" element, we refer to the "publisher" 
  attribute of that element as the "subject", and the service performs these 
  steps: 
  <P></P>
  <DT><STRONG><A name=sec-1>1</A> If the subject is outside of this 
  administrative domain, a "reply"</STRONG>
  <DD>element having code 553 is sent to the originator. 
  <P></P>
  <DT><STRONG><A name=sec-2>2</A> If the subject does not refer to a valid 
  endpoint, a "reply"</STRONG>
  <DD>element having code 550 is sent to the originator. 
  <P></P>
  <DT>
  <HR>
  <EM><A name=page-15>Page 15</A></EM>
  <DD>
  <P></P>
  <DT><STRONG><A name=sec-3>3</A> If the subject's access entry does not contain 
  a "presence:watch"</STRONG>
  <DD>token for the originator, a "reply" element having code 537 is sent to the 
  originator. 
  <P></P>
  <DT><STRONG><A name=sec-4>4</A> If the originator already has an in-progress 
  watch operation for</STRONG>
  <DD>the subject, then the previous watch operation is silently terminated, and 
  processing continues. 
  <P></P>
  <DT><STRONG><A name=sec-5>5</A> If the "transID" attribute refers to an 
  in-progress subscribe or</STRONG>
  <DD>watch operation for the originator, a "reply" element having code 555 is 
  sent to the originator. 
  <P></P>
  <DT><STRONG><A name=sec-6>6</A> Otherwise:</STRONG>
  <DD>
  <P></P>
  <DT><STRONG><A name=sec-1>1</A> A "reply" element having code 250 is sent to 
  the originator.</STRONG>
  <DD>
  <P></P>
  <DT><STRONG><A name=sec-2>2</A> For each endpoint currently subscribing to the 
  subject's</STRONG>
  <DD>presence information, a "notify" element is immediately sent to the 
  originator (c.f., <A 
  href="http://www.apps.ietf.org/rfc/rfc3343.html#sec-4.6">Section 4.6</A>). 
  <P></P>
  <DT><STRONG><A name=sec-3>3</A> For up to the amount of time indicated by the 
  "duration"</STRONG>
  <DD>attribute of the "watch" element, whenever a subscribe operation succeeds 
  or a subscription is terminated, a "notify" element is sent to the originator. 
  Finally, when the amount of time indicated by the "duration" attribute 
  expires, a terminate operation (<A 
  href="http://www.apps.ietf.org/rfc/rfc3343.html#sec-4.5">Section 4.5</A>) is 
  sent to the originator. 
  <P>Note that if the duration is zero-valued, then the watch operation is 
  making a one-time poll of the presence information. Accordingly, Step 6.3 
  above does not occur. 
  <P>Regardless of whether a "notify" or "reply" element is sent to the 
  originator, the "transID" attribute is identical to the value found in the 
  "presence" element sent by the originator. 
  <P></P>
  <DT><STRONG><A name=sec-4.4>4.4</A> The Publish Operation</STRONG>
  <DD>
  <P>When an application wants to modify the presence entry associated with an 
  endpoint, it sends a "publish" element to the service. In addition, the 
  service sends a "publish" element to endpoints that have subscribed to see 
  presence information (c.f., <A 
  href="http://www.apps.ietf.org/rfc/rfc3343.html#sec-4.2">Section 4.2</A>). 
  <P>The "publish" element has a "publisher" attribute, a "transID" attribute, a 
  "timeStamp" attribute, and contains a "presence" element: 
  <P>
  <UL>
    <LI>the "publisher" attribute specifies the endpoint to be associated with 
    the presence entry; </LI></UL>
  <P></P>
  <DT>
  <HR>
  <EM><A name=page-16>Page 16</A></EM>
  <DD>
  <P>
  <UL>
    <LI>the "transID" attribute specifies the transaction-identifier associated 
    with this operation; 
    <P></P>
    <LI>the "timeStamp" attribute specifies the application's notion of the 
    current date and time; and, 
    <P></P>
    <LI>the "presence" element contains the desired presence entry for the 
    endpoint. </LI></UL>
  <P>When the service sends a "publish" element, the "transID" attribute 
  specifies the transaction-identifier associated with the subscribe operation 
  that caused this "publish" element to be sent, and the "timeStamp" attribute 
  specifies the service's notion of the current date and time. No reply is sent 
  by the receiving endpoint. 
  <P>When the service receives a "publish" element, we refer to the "publisher" 
  attribute of that element as the "subject", and the service performs these 
  steps: 
  <P></P>
  <DT><STRONG><A name=sec-1>1</A> If the "publisher" attribute of the "publish" 
  element doesn't</STRONG>
  <DD>match the "publisher" attribute of the "presence" element contained in the 
  "publish" element, a "reply" element having code 503 is sent to the 
  originator. 
  <P></P>
  <DT><STRONG><A name=sec-2>2</A> If the subject is outside of this 
  administrative domain, a "reply"</STRONG>
  <DD>element having code 553 is sent to the originator. 
  <P></P>
  <DT><STRONG><A name=sec-3>3</A> If the subject does not refer to a valid 
  endpoint, a "reply"</STRONG>
  <DD>element having code 550 is sent to the originator. 
  <P></P>
  <DT><STRONG><A name=sec-4>4</A> If the subject's access entry does not contain 
  a</STRONG>
  <DD>"presence:publish" token for the originator, a "reply" element having code 
  537 is sent to the originator. 
  <P></P>
  <DT><STRONG><A name=sec-5>5</A> If the "lastUpdate" attribute of the "publish" 
  element is not</STRONG>
  <DD>semantically identical to the "lastUpdate" attribute of the subject's 
  presence entry, a "reply" element having code 555 is sent to the originator. 
  (This allows a simple mechanism for atomic updates.) 
  <P></P>
  <DT><STRONG><A name=sec-6>6</A> Otherwise:</STRONG>
  <DD>
  <P></P>
  <DT><STRONG><A name=sec-1>1</A> The subject's presence entry is updated from 
  the "publish"</STRONG>
  <DD>element. 
  <P></P>
  <DT><STRONG><A name=sec-2>2</A> The "lastUpdate" attribute of the presence 
  entry is set to the</STRONG>
  <DD>service's notion of the current date and time. 
  <P></P>
  <DT>
  <HR>