TRAM                                                        P. Martinsen
Internet-Draft                                                   D. Wing
Intended status: Standards Track                                   Cisco
Expires: December 3, 2015                                   June 1, 2015

                            STUN Traceroute


   After a UDP protocol such as RTP determines a network path is
   experiencing problems, a traceroute is often useful to determine
   which router or which link is contributing to the problem.  However,
   operating system traceroute commands follow a different path than the
   actual UDP flow which complicates troubleshooting.  A superior method
   is shown which is absolutely path-congruent with the UDP protocol
   itself, works on IPv4 and IPv6, and does not require administrative
   privileges on most operating systems.

Status of This Memo

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   This Internet-Draft will expire on December 3, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Notational Conventions  . . . . . . . . . . . . . . . . . . .   3
   3.  Overview of Operation . . . . . . . . . . . . . . . . . . . .   3
   4.  New STUN Attributes . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  PATH-NODE-PROBE . . . . . . . . . . . . . . . . . . . . .   5
   5.  Base Protocol Procedures  . . . . . . . . . . . . . . . . . .   5
     5.1.  Forming STUN Packet Probes  . . . . . . . . . . . . . . .   5
     5.2.  Receiving a STUN Packet Probe . . . . . . . . . . . . . .   6
     5.3.  Receiving ICMP Messages . . . . . . . . . . . . . . . . .   6
   6.  IPv4 and IPv6 Differences . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  Platform Implementation Details  . . . . . . . . . .   9
     A.1.  Setting TTL or HOP_LIMIT on Probes  . . . . . . . . . . .   9
     A.2.  Receiving ICMP Messages . . . . . . . . . . . . . . . . .   9
       A.2.1.  OS-X and iOS  . . . . . . . . . . . . . . . . . . . .   9
       A.2.2.  Linux and Android . . . . . . . . . . . . . . . . . .  10
       A.2.3.  Windows . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Traceroute [RFC1393] is a simple tool available on most operating
   systems and is popular to debug the network by simply getting round-
   trip time along each hop to a remote IP address.  More advanced
   tools, such as MTR, provide more metrics such as packet loss and
   round trip time to each hop over several seconds or minutes.

   To simplify network debugging when dealing with bi-directional real
   time media it is often useful to get as much information as possible
   regarding the network path.  In this specification probe packets are
   sent using the same 5-tuple where (S)RTP media is flowing.  This will
   provide the most accurate results, as probe packets sent on a
   different 5-tuple may take another path due to Equal-Cost Multipath
   (ECMP, [RFC2992]), policy-based routing, and similar techniques.

   To avoid those problems, the probe packets need to be sent from the
   same socket and with the same DiffServ code point the normal (S)RTP

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   media packets.  As shown in Appendix A, most operating systems can
   pass the ICMP "Time to Live Exceeded" error to the application, so
   the application can perform the diagnostics over that network path.

   This specifications uses STUN [RFC5389] packets as probes.  STUN
   packets are designed to be multiplexed together with RTP [RFC3550]
   (and SRTP [RFC3711]) and are unlikely to cause any "problems" for the
   (S)RTP receiver.  To differentiate each hop count, classic traceroute
   uses different UDP port numbers (e.g., TTL=1 uses UDP port 55001,
   TTL=2 uses UDP port 55002, etc.).  The mechanism described here uses
   the same UDP port number (so that the trace is path-congruent with
   the (S)RTP packets), and uses different length UDP packets to
   differentiate each hop count (e.g., TTL=1 uses length 501, TTL=2 uses
   length 502, etc.).

   Using a technique based on ICMP replies avoids a forklift upgrade of
   the network to provide host applications with useful information.
   ICMP is already supported in most network and application stacks.

   Additional network characteristics like MTU and bandwidth
   availability can be discovered by using
   [I-D.petithuguenin-behave-stun-pmtud] and

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Overview of Operation

   An application using (S)RTP to send and receive media like audio and
   video following the guidelines in [RFC4961] uses symmetric send and
   receive ports.  The application opens one socket that it uses to both
   send and receive media on.

   It is important to note that the functionality described here can be
   done on most OSes without any administrative privileges.

   Figure 1 depicts the various components needed for this to work.  The
   application opens up its media socket as it would in normal cases
   where media is to be sent and received.  It also opens up a ICMP
   socket or installs an error listener on the media socket.

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                     POLL/      Network Node      Network Node
    +-----+            |             |                  |
    |  A  |* ICMP +++ /+\++++++++++++|++++ <ICMP Reply> |
    |  L  |  SOCKET  |   |           |    ++++++++++++  |
    |  I  |          |   |           |                + |
    |  C  |* MEDIA ==|===|===========|=================+|====<(S)RTP>
    |  E  |  SOCKET --\-/------------|------------------X <STUN Probe>
    +-----+            |             |                  |(TTL expired)

                           ====== Media Path
                           ------ STUN Probes (on same 5 tuple as Media)
                           ++++++ ICMP reply

                                 Figure 1

   The application also need to listen on the sockets for any incoming
   ICMP packets or socket error messages.  This is usually done with the
   socket calls select() or poll().  How to actually receive the ICMP
   messages will vary from OS to OS.  See Appendix A for implementation
   details on various OSes.

   Once the application have media running and is listening for ICMP
   replies it can start sending probes to detect networks nodes in the
   media path.  This is done by sending STUN messages and setting the
   TTL/MAX_HOP limit in the IPv4/IPv6 header.  Appendix A.1 explains how
   to set this on various platforms.

   The STUN packet is sent on the same socket as the media packet are
   sent and received on.  Mixing (S)RTP and STUN is well known behavior
   and should not cause any problems.

   Along the path, every layer 3 network node (a.k.a. router) decreases
   the IPv4 TTL or IPv6 HOP_LIMIT field.  If the field becomes 0 the
   network node responds with a ICMP error "Time to Live Exceeded" (TTL
   Exceeded) or "Hop Limit Exceeded in Transit" (Time Exceeded Message).

   The application will receive a ICMP error in response to the
   offending probe packet.  The source IP address of the ICMP packet
   will be the sending network node.  This enables the application to
   trace the path towards the destination.  The ICMP reply contains at
   least 8 bytes of the offending packet.  The IP fragment of the
   offending packet in the ICMP reply can be used to determining if this
   ICMP reply actually was a reply to an offending packet the
   application did send out.

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4.  New STUN Attributes

   This STUN extension defines the following new attribute:

         0xXXX0: PATH-NODE-PROBE


   This attribute have a length of 8.  Padding is needed to hit the
   required STUN 32 bit STUN attribute boundary.

       0 1 2 3 4 5 6 7
      | HOP           |

                    Figure 2: PATH-NODE-PROBE Attribute

   The HOP field indicates what hop in the network path (relative to the
   application) the application is trying to learn the IP address of.
   This field should be set to the same value as the TTL/HOP_LIMIT field
   in the IPv4/IPv6 header of the probe packet leaving the application.
   Note that the TTL/HOP_LIMIT field in the IPv4/IPv6 header will
   decrease as the packet traverses the path.  The HOP field in the
   attribute will remain unchanged.

   This attribute is useful for clients when receiving the whole
   offending IP packet in the ICMP reply.  The attribute will be
   reflected back in a STUN response if the remote application supports
   is.  This makes it easier to correlate sent probe packets and ICMP

5.  Base Protocol Procedures

   The procedures are simple; send a probe packet that may or may not
   trigger a reply from one of the nodes in the network path and then
   listen and parse any incoming replies.  The reply might be an ICMP
   Time To Live Exceeded (from an intermediate hop), a STUN response
   (from the (S)RTP peer), or any other ICMP error message.

5.1.  Forming STUN Packet Probes

   To reduce chances of a STUN traceroute probe being stopped by various
   middle-boxes it is RECOMMENDED to use a STUN binding request as
   described in ICE [RFC5245].

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   Since the STUN packet can traverse the whole media-path and reach the
   remote peer it is RECOMMENDED the agent follows the guidelines for
   sending connectivity checks defined in ICE [RFC5245].  Adding a
   USERNAME attribute and integrity protecting the STUN message enables
   the remote peer to authenticate the STUN message and create an
   appropriate response.  If the remote peer is unable to authenticate
   the STUN request it will not send any response.  Getting a response
   from the remote peer is useful as it is an indication the probe have
   traveled the whole network path.

   When forming the STUN packet probe the agent SHOULD add the PATH-
   NODE-PROBE attribute and MAY add a PADDING attribute as described in
   [RFC5780] Section 7.6.  The PATH-NODE-PROBE attribute is useful for
   STUN servers receiving the STUN probe and it can be used to correlate
   any ICMP replies if the reply contains the complete offending packet.
   Adding the PADDING attribute is useful for clients that needs to have
   several outstanding probe packets on the same 5-tuple.  The length of
   the offending packet reported back in any ICMP reply will make it
   possible to correlate this to the correct probe.

   The agent sending the STUN packet probe MUST store the length of the
   UDP packet (as reported in the IP header) containing the STUN probe.

   Before sending the probe on the wire it is important to set the
   appropriate TTL or HOP_LIMIT field in the IPv4 or IPv6 header before
   the packet is sent.  How to do this on various OSes are described in
   Appendix A.1.

   The probe MUST also be sent with the same DSCP value as the (S)RTP
   packets.  This is normally not a problem as the STUN probes and
   (S)RTP packets are sent on the same socket.

5.2.  Receiving a STUN Packet Probe

   An agent that listens for STUN requests (a.k.a STUN server) that
   receives a STUN request with a PATH-NODE-PROBE attribute, MUST
   include a PATH-NODE-PROBE attribute with the same value in the
   generated response.

   Any PADDING attributes as defined in [RFC5780] SHOULD be ignored by
   the STUN server.

5.3.  Receiving ICMP Messages

   After an agent sends a STUN probe it must be ready to receive a ICMP
   reply or a STUN reply.  Details on how to do this on various OSes are
   described in Appendix A.2.

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   To prevent ICMP spoofing attacks [RFC5927] , the received ICMP packet
   MUST be validated by port number and length in the IP fragment of the
   offending packet contained in the ICMP payload.  Port number
   validation checks that the port number in the offending IP fragment
   of the probe packet contained in the ICMP payload corresponds to the
   (S)RTP media (and STUN probe) 5-tuple.  The length validation checks
   IP packet length field in the IP fragment of the offending packet
   received in the ICMP reply.  This value MUST correspond to any length
   stored when the agent sent the STUN probe.  If the agent uses the
   PADDING (Defined in [RFC5780]) attribute to generate different length
   on the STUN probes it is possible to have several outstanding probes,
   thus speeding up the trace.

6.  IPv4 and IPv6 Differences

   Core functionality is the same.  In IPv6 the IPv4 TTL field is
   renamed to HOP_LIMIT to better reflect what it actually represent.

7.  IANA Considerations

   The code-point for the new STUN attribute defined in this
   specification is described in Section 4.

8.  Security Considerations

   ICMP messages does leak network topology, which is a well-known
   threat to networks and mitigations have long existed in routers and
   firewalls so that networks can be configured to not leak this
   topology information beyond their borders.

   ICMP spoofing and DOS attack prevention exist in routers deployed on
   the Internet today.

   No new threats have been added in this specification.

9.  Acknowledgements

   Trond Andersen for actually implementing this and Wilson Chen for
   helping out with different OS behavior testing.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

   [RFC4961]  Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
              BCP 131, RFC 4961, July 2007.

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245, April

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5780]  MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
              Using Session Traversal Utilities for NAT (STUN)", RFC
              5780, May 2010.

10.2.  Informative References

              Martinsen, P., Andersen, T., Salgueiro, G., and M. Petit-
              Huguenin, "Traversal Using Relays around NAT (TURN)
              Bandwidth Probe", draft-martinsen-tram-
              turnbandwidthprobe-00 (work in progress), May 2015.

              Petit-Huguenin, M., "Path MTU Discovery Using Session
              Traversal Utilities for NAT (STUN)", draft-petithuguenin-
              behave-stun-pmtud-03 (work in progress), March 2009.

              "ICMP test github repo", <

   [RFC1393]  Malkin, G., "Traceroute Using an IP Option", RFC 1393,
              January 1993.

   [RFC2992]  Hopps, C., "Analysis of an Equal-Cost Multi-Path
              Algorithm", RFC 2992, November 2000.

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.

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Appendix A.  Platform Implementation Details

   This section provides examples and hint on how probe packets can be
   sent and ICMP messages received on various OSes.  For a complete
   example please refer to [ICMPTest].

A.1.  Setting TTL or HOP_LIMIT on Probes

   Setting the appropriate value in the IPv4 or IPv6 header is the same
   for most platforms.  Use

             setsockopt(sockHandle, IPPROTO_IP, IP_TTL, &sock_ttl,

   for IPv4 or

             IPPROTO_IPV6, IPV6_UNICAST_HOPS, &sock_ttl,

   for IPv6.

   Sending the probes on the same socket as media is flowing requires
   the implementations to only set this when sending the probe packet.
   Remember to set it back to initial value when sending media.  Most
   OSes seems to handle the setsockopt call correctly and not set the
   value in the IP header of any buffered packets.

A.2.  Receiving ICMP Messages

A.2.1.  OS-X and iOS

   Creating a socket to listen for incoming ICMP messages can be done

       icmpSocket=socket(config.remoteAddr.ss_family, SOCK_DGRAM,
                         IPPROTO_ICMP); <<<

   This is done in addition to the normal socket used to send media on
   (RTP) and probes.  (Yes, even if the probe are sent on the media
   socket the ICMP reply will be on the ICMP sockets..)

   Code in the while(1) loop of poll would look something like:

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           if (ufds[i].revents & POLLIN) {
               if(i == rtpSock){
                   //Handle "normal" data here.
               if(i == icmpSock){//This is the ICMP socket
                  //Handle ICMP packets here.

A.2.2.  Linux and Android

   For unprivileged recipient of the ICMP messages an error handler must
   be installed.  This can be done like:

      setsockopt (config.sockfd, SOL_IP,
                  IP_RECVERR, &val, sizeof(val)) < 0);

   In the poll() section of the code something like this needs to be

       struct msghdr msg;

       if (ufds[dataSock].revents & POLLERR) {
           if (recvmsg(sockfd, &msg, MSG_ERRQUEUE ) == -1) {
               //Ignore for now. Will get it later..
           //possible ICMP message
           //use cmsg to read the structures in msg

   Failing to call rcvmsg seems to let the msg fall through to the
   kernel.  Looks like it will close down the socket because of the
   received error.  So be careful!

   For application with the right administrative privileges it is
   possible create a separate ICMP listen socket as described in the
   previous section.  The socket() call would then look like:

               icmpSocket=socket(config.remoteAddr.ss_family, SOCK_RAW,

   The poll() loop will be as described for OS-X and iOS.  No need for a
   error handler.

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A.2.3.  Windows

   The following code in select() or poll() will read and detect any
   incoming ICMP messages on the send socket.

               if (FD_ISSET(sendsocket, &read_flags)) {
                   cc = recvfrom(sendsocket, receivepacket,
                       sizeof(receivepacket), 0,
                       (struct sockaddr *)&receiveaddr, (int*)&fromlen);
                   if (cc < 0 && GETERRORCODE == WSAENETRESET) {
                      //ICMP packet handling here
                      //to get the address of the router sending the
                      //ICMP reply

Authors' Addresses

   Paal-Erik Martinsen
   Cisco Systems, Inc.
   Philip Pedersens Vei 22
   Lysaker, Akershus  1325


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134


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