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NAT Behavior Discovery Using Session Traversal Utilities for NAT (STUN)
RFC 5780

Internet Engineering Task Force (IETF)                      D. MacDonald
Request for Comments: 5780                                   B. Lowekamp
Category: Experimental                                             Skype
ISSN: 2070-1721                                                 May 2010

NAT Behavior Discovery Using Session Traversal Utilities for NAT (STUN)

Abstract

   This specification defines an experimental usage of the Session
   Traversal Utilities for NAT (STUN) Protocol that discovers the
   presence and current behavior of NATs and firewalls between the STUN
   client and the STUN server.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5780.

MacDonald & Lowekamp          Experimental                      [Page 1]
RFC 5780                 NAT Behavior Discovery                 May 2010

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

MacDonald & Lowekamp          Experimental                      [Page 2]
RFC 5780                 NAT Behavior Discovery                 May 2010

Table of Contents

   1.  Applicability  . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  5
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Example Diagnostic Use . . . . . . . . . . . . . . . . . .  6
     2.2.  Example Use with P2P Overlays  . . . . . . . . . . . . . .  7
     2.3.  Experimental Goals . . . . . . . . . . . . . . . . . . . .  8
   3.  Overview of Operations . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Determining NAT Mapping  . . . . . . . . . . . . . . . . . 10
     3.2.  Determining NAT Filtering  . . . . . . . . . . . . . . . . 10
     3.3.  Binding Lifetime Discovery . . . . . . . . . . . . . . . . 10
     3.4.  Diagnosing NAT Hairpinning . . . . . . . . . . . . . . . . 11
     3.5.  Determining Fragment Handling  . . . . . . . . . . . . . . 11
     3.6.  Detecting a Generic Application Level Gateway (ALG)  . . . 11
   4.  Discovery Process  . . . . . . . . . . . . . . . . . . . . . . 11
     4.1.  Source Port Selection  . . . . . . . . . . . . . . . . . . 12
     4.2.  Checking for UDP Connectivity with the STUN Server . . . . 13
     4.3.  Determining NAT Mapping Behavior . . . . . . . . . . . . . 14
     4.4.  Determining NAT Filtering Behavior . . . . . . . . . . . . 14
     4.5.  Combining and Ordering Tests . . . . . . . . . . . . . . . 15
     4.6.  Binding Lifetime Discovery . . . . . . . . . . . . . . . . 15
   5.  Client Behavior  . . . . . . . . . . . . . . . . . . . . . . . 17
     5.1.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . . 17
     5.2.  Security . . . . . . . . . . . . . . . . . . . . . . . . . 18

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