Subject: [Iptel] Loop detection and prevention - 2nd try

From: "Stastny Richard" <[email-masked]>
Date: Thu, 10 Mar 2005 23:24:24 +0100

Folks,
 
here the 2nd text proposal:
 
4.2  Adding the "enumdi" parameter to URIs
 
   When a VoIP network element accesses ENUM in e164.arpa for a given
   E.164 number and the result of the query is NXDOMAIN, and the network
   element chooses to pass the call to the next network element by using
   a "tel" URI, the "enumdi" parameter MUST be set.
 
   When a VoIP network element accesses ENUM in e164.arpa for a given
   E.164 number and either:
   o  the result of the query includes a NAPTR RR containing a "tel" URI
      that has the same E.164 number, or
   o  the result of the query includes a NAPTR RR containing a "tel" URI
      with the "enumdi" parameter set,
   then if that retrieved "tel" URI is chosen to be passed to the next
   network element, the sending VoIP network element MUST pass on the
   retrieved URI with the "enumdi" parameter set.
 
4.3 Loop detection and prevention
 
   If the result of the query contains another tel: URI with a different
   E.164 number, and the network element chooses to make another ENUM 
   query with the "tel" URI retrieved in the previous query, an endless
   loop may occur. 
 
   The VoIP network element MUST be able to detect such loops. The implementation
   of a proper algorithm is left to the VoIP network element. This could be
   a simple algorithm such as upper limit of the number queries or 
   sophisticated ones as described e.g. in [KNUTH].
 
   If a loop is detected, the VoIP network element may either drop the call
   and signal back to the previous network element with the proper signaling
   available (e.g. loop detected or service not available) or it may take the
   original received tel URI and act in the same way as described above if the 
   result of the query is NXDOMAIN.
 
[KNUTHl Algoritmos Fundamentales - Vol. I by D. E. Knuth
(out of print)

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