INTERNET DRAFT                                         Kamil Sarac, UTD
draft-sarac-mping-00.txt                           Kevin Almeroth, UCSB
                                                     Expires: June 2004

                MPing: A Ping Utility for IP Multicast


Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of RFC2026. Internet Drafts are working documents of
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Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.


   This document describes a mechanism for discovering multicast
   reachability between end systems within/between multicast enabled
   networks. It uses request/response messages to verify multicast
   reachability between the local site and a remote site. With this
   utility, multicast users can test if they can successfully join a
   multicast group of a remote source and receiver its data.

Sarac,Almeroth                                                  [Page 1]

 draft-sarac-mping-00.txt January 2004

1. Introduction

   Verifying reachability between multicast enabled networks is an
   important management task. Multicast data propagates on forwarding
   paths toward the receivers. As the receivers join and leave
   multicast groups, these paths are dynamically created and maintained
   by the routers . In addition, the join mechanism does not use any
   feedback. As a result, problems due to lack of multicast
   reachability become difficult to detect.

   Current approaches to verify multicast reachability consist of a
   number of monitoring efforts that depend on voluntary participation
   from multicast users in collecting the reachability information
   among a number of multicast enabled sites. These efforts include
   sdr-monitor of UC Santa Barbara, Multicast Beacon of NLANR, and
   Multicast Tester of The collected information in
   these systems give a general reachability picture for the overall
   multicast infrastructure. Even though these systems are useful for
   the multicast community in understanding the reachability status of
   the multicast infrastructure, they do not really help a network
   administrator to detect and debug a particular reachability problem
   between his/her network and a remote one. As a result, mechanisms
   are needed to help network administrators to verify the reachability
   status between their networks and the remote ones.

   In unicast, ICMP Echo Request/Reply based PING tool provides a
   convenient approach to discover unicast reachability between two
   systems in the network. For multicast, this mechanism has not been
   considered very useful. In multicast, PING requests are sent to
   multicast group address and these requests trigger group receivers
   to send PING responses to the PINGing host via unicast. This
   operation informs an end system about the fact that there are a
   number of receivers that received the PING request on the group
   address. Contrary to the utility of unicast PING, this mechanism
   does not help us to verify multicast reachability between two given
   end systems in the network. In addition, the mechanism is vulnerable
   to feedback implosion problem. Because of these reasons, this
   approach is not put into practical use by network operators.

   In this draft, we propose a multicast ping utility that can be used
   to discover multicast reachability between a local site and a remote
   site. We call this utility MPING. MPING can be used to verify
   multicast reachability between the local site (as potential
   receiver) and a remote site (as potential multicast source). This
   utility can be very helpful for network operators to debug
   potential multicast reachability problems between their networks and
   the remote ones.

Sarac,Almeroth                                                 [Page 2]

draft-sarac-mping-00.txt                                   January 2004

2. MPING Protocol Operation

   The main idea in MPING is to use a well known multicast group address
   (MPING.MCAST.NET) to exchange an MPING request and response messages
   between the local host and a remote host. More specifically, MPING
   involves the below message exchanges between the local host R and a
   remote host S:

   1) MPING request: This can be implemented as a source specific join
      request to (S,MPING.MCAST.NET) multicast group address. Using the
      source filtering support provided by IGMPv3, R sends a Membership
      Report that includes a join request to above group address. The
      Designated Router (DR) at R's site creates a PIM-Join message for
      (S,MPING.MCAST.NET) and forwards it toward S.

      When the join request reaches the DR at S's site, this router
      needs to inform S about the MPING request. This can be achieved
      by introducing a new message type to IGMP protocol. When a DR
      receives a join request specifically for the MPING.MCAST.NET
      group of a local host, it will use this new message to inform the
      host about the incoming MPING request.

   2) MPING response: On receiving the MPING request, the host S
      creates and sends a MPING response back to the MPINGing host R.
      The MPING response is a special control message that is sent to
      (S,MPING.MCAST.NET) multicast group address by S. This control
      message propagates on the established multicast forwarding path
      toward R.

   3) LEAVE message: On receiving the MPING response or on timeout, the
      MPINGing host R sends a IGMP Membership Report to leave the
      (S,MPING.MCAST.NET) multicast group. Consequently, the DR at R's
      site sends a PIM-Leave message toward the S's site to resolve the
      multicast forwarding path in the network.

   The above mechanism helps an end host R to verify multicast
   reachability between itself as a potential receiver and a remote
   host S as a potential multicast sender. The MPING response returned
   by S may include some useful information as follows:

   1) S may include the list of multicast group addresses that it is
      currently sourcing data for.

   2) MPING request can be answered by the DR router at S's site and
      the response can still contain the above information. This can
      remove the dependency on the host machines to implement the
      required MPING functionality in their operating systems. This
      approach assumes that the router based deployment of MPING is
      easier than end host based deployment.

Sarac,Almeroth                                                 [Page 3]

draft-sarac-mping-00.txt                                   January 2004

3. Multiple Simultaneous MPING Requests

   One issue with the above mechanism is the interaction of multiple
   simultaneous MPING requests going toward an end host S. Due to the
   possibility of multiple simultaneous requests, a MPING request may
   not reach the source site but instead may graft on the corresponding
   multicast tree in the middle of the network. By the time the MPING
   request reaches a router on the multicast tree, the MPING response
   may have already been sent by the queried host and this response may
   have already crossed this grafting on-tree router. In this
   situation, the PIM-Join message corresponding the second MPING
   request may not return a MPING response. As a result, the second
   host may mistakenly conclude the lack of reachability toward the

   As an example, consider the example scenario in Figure 1. In this
   scenario, an end host K sends a MPING request toward a remote end
   host S. This request creates the multicast forwarding path between K
   and S in the network which includes a router A. On receiving the
   request, the end host S sends a MPING response back to K on the
   (S,MPING.MCAST.NET) multicast group. Assume that just after this
   MPING response passes the router A toward its destination K, the
   router A receives a second MPING request that has originated from
   end host L. Since the router A is already on the multicast tree,
   instead of forwarding this new MPING request (which is nothing but a
   PIM-Join message for the (S,MPING.MCAST.NET) multicast group) it
   will graft this new branch on the tree. However, since the MPING
   response has already passed the router A, this second MPING request
   will not result in a MPING response. And this situation may be
   mistakenly interpreted by the end host L as lack of reachability
   between L and S.

                                | S |
                                | A |
                                 /  \
                                /    \
                               /      \
                             -----   -----
                             | K |   | L |
                             -----   -----

        Figure 1. A sample scenario with multiple MPING  requests.

Sarac,Almeroth                                                  [Page 4]

draft-sarac-mping-00.txt                                    January 2004

   In the above situation, before the end host L decides on the lack of
   reachability, it may need to run several MPING requests toward the
   end host S. In addition to this, we require the end host S to send
   multiple MPING responses with some periodicity (every 5 seconds) up
   to some defined time (up to 10 responses). This will increase the
   chances for L to receive a response to its MPING request. In the
   above example scenario, when the end host L sends its MPING request,
   it will wait for over 5 seconds to receive a response from the end
   host S. By the time the MPING request of L arrives the router A, if
   S has not sent out 10 responses yet, the end host L will receive a
   response for its MPING request. On the other hand, if the end host S
   already sent out 10 responses, we expect the end host K to receive a
   MPING response and then send a Leave message to resolve the
   previously created multicast path between its site toward the S's
   site. The Leave message will cause the multicast states to be removed
   in the routers between K and A. At this point, when the end host L
   waits for 5 seconds without receiving any MPING responses, it will
   also send a Leave message and this message will resolve the multicast
   forwarding path between itself and S. Then, the end host L will
   initiate a second MPING request which will most likely go all the way
   toward S's site and will result in a MPING response. Note that even
   if this second MPING request gets grafted on the multicast tree
   before it reaches S's site (due to potentially another MPING request
   from another MPINGing end host), because of the repeated MPING
   responses coming from the end host S, it will most likely receive a
   MPING response from S.

4. Required Modifications

   MPING requires a few modifications to routers and end hosts to be
   able to realize its function. These modifications are discussed

4.1 Required Modifications to End Systems

   End systems need to support a new IGMP message type for MPING. On
   receiving this IGMP message, they will create and send a number of
   responses to their MPING.MCAST.NET group address. Optionally these
   responses may include information about the multicast groups that
   this host is sourcing data currently.

4.2 Required Modifications to Routers

   Similar to end systems, edge routers need to be modified to recognize
   MPING requests and act on them. That is, on receiving a PIM-Join for
   (S,MPING.MCAST.NET) for a host S in its directly

Sarac,Almeroth                                                  [Page 5]

draft-sarac-mping-00.txt                                    January 2004

   attached subnet, a router will create an IGMP message and forward it
   to the end system to inform it about the MPING request. Optionally,
   routers may also participate in MPING operation by sending MPING
   responses on behalf of their local end hosts as discussed in
   Section 3.

5. Limitations

   The MPING mechanism presented in this draft is useful for verifying
   multicast reachability among remote end hosts. The maximum benefit of
   this mechanism can be realized with wide scale deployment in the
   network. In other words, the lack of MPING deployment at a multicast
   site may cause a MPINGing host to draw wrong conclusions about the
   reachability status to this site. Compared to end host deployment, we
   expect that MPING deployment in DR routers is relatively easier. In
   addition, the service provided by this utility is most useful for
   network administrators for debugging multicast services in their
   network. This fact should give strong incentives to network
   administrators for deploying the service in their routers.

6.  Intellectual Property

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7.  Full Copyright Statement

   Copyright (C) The Internet Society (2003). All Rights Reserved.

   This document and translations of it may be copied and furnished to
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8.  Author's Address

   Kamil Sarac <>
   Department of Computer Science
   University of Texas at Dallas
   Richardson, TX 75083, USA

   Kevin C. Almeroth <>
   Department of Computer Science
   University of California Santa Barbara
   Santa Barbara, CA 93106, USA

Sarac,Almeroth                                             [Page 8]