





Network Working Group                                       D. Farinacci
Request for Comments: 2337                                 Cisco Systems
Category: Experimental                                          D. Meyer
                                                           Cisco Systems
                                                              Y. Rekhter
                                                           Cisco Systems
                                                              April 1998


  Intra-LIS IP multicast among routers over ATM using Sparse Mode PIM

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  It does not specify an Internet standard of any kind.
   Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

2. Abstract

   This document describes how intra-LIS IP multicast can be efficiently
   supported among routers over ATM without using the Multicast Address
   Resolution Server (MARS). The method described here is specific to
   Sparse Mode PIM [PIM-SM], and relies on the explicit join mechanism
   inherent in PIM-SM to notify routers when they should create group
   specific point-to-multipoint VCs.

3. Overall model

   This document focuses on forwarding of multicast traffic among PIM-SM
   routers connected to an ATM network. Routers on an ATM network are
   partitioned into Logical IP Subnets, or LISs.  This document deals
   with handling multicast within a single LIS. Handling inter-LIS
   multicast traffic, including handling shortcuts, is outside the scope
   of this document.  In addition, this document does not address
   forwarding of multicast traffic to or from hosts connected to an ATM
   network.










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4. Router behavior

   This document requires that each router within a LIS knows IP and ATM
   addresses of all other routers within the LIS. The mapping between IP
   and ATM addresses may be provided by an ARP server [RFC2225], or by
   any other means (e.g., static configuration).

   Each PIM router within a LIS is required to maintain a single
   (shared) point-to-multipoint distribution VC rooted at the router
   with all other PIM routers in the LIS as the leaf nodes. The VC is
   expected to be used for forwarding of multicast traffic (both data
   and control) among routers within the LIS. For example, this VC would
   be used for distributing PIM [PIM-SM] control messages (Join/Prune
   messages).

   In addition, if a PIM router receives a IGMP report from an non-PIM
   neighbor, then the router may add the reporter to the existing shared
   distribution VC or to the group specific distribution VC (if it
   exists). The PIM router may also create a specific VC for this IGMP
   proxy.

4.1. Establishing Dedicated, Per Group Point-to-Multipoint VCs

   Routers may also maintain group specific, dedicated point-to-
   multipoint VCs. In particular, an upstream router for a group may
   choose to become the root of a group specific point-to-multipoint VC
   whose leaves are the downstream routers that have directly connected
   or downstream receivers for the group. While the criteria for
   establishing a group specific point-to-multipoint VC are local to a
   router, issues such as the volume of traffic associated with the
   group and the fanout factor within the LIS should be considered.
   Finally, note that a router must minimally support a single shared
   point-to-multipoint VC for distribution of control messages and data
   (to all group addresses).

   A router can choose to establish a dedicated point-to-multipoint VC
   (or add another leaf to an already established dedicated point-to-
   multipoint VC) when it receives a PIM Join or IGMP report messages
   from another device in the same LIS. When a router that is the root
   of a point-to-multipoint VC receives PIM Prune message or IGMP leave,
   it removes the originator of the message from its dedicated point-
   to-multipoint VC.









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4.2. Switching to a Source-Rooted Tree

   If at least one of the routers within a LIS decides to switch to a
   source-rooted tree (by sending (S,G) PIM Joins), then all other
   routers within the LIS that have downstream members for G should
   switch to that source-rooted tree as well. Since a router that
   switches to a source-rooted tree sends PIM Join messages for (S,G)
   over its shared point-to-multipoint VC, the other routers within the
   LIS are able to detect this. Once a router that has downstream
   members for G detects this, the router should send (S,G) PIM Join
   message as well (otherwise the router may receive duplicate traffic
   from S).

   Note that it is possible for a non-PIM router in the LIS to fail to
   receive data if the injection point moves to router to which there is
   not an existing VC.

4.2.1. Adding New Members to a Source-Rooted Tree

   As mentioned above, this document requires that once one router in a
   LIS decides to switch to the source tree for some (S,G), all routers
   in the LIS that have downstream members must also switch to the (S,G)
   source tree. Now, when a new router wants to receive traffic from G,
   it starts sending (*,G)-Joins on it's shared point-to-multipoint VC
   toward the RP for G. The root of the (S,G)-source-rooted tree will
   know to add the new router to the point-to-multipoint VC servicing
   the (S,G)-source-rooted tree by observing the (*,G)-joins on it's
   shared point-to-multipoint VC. However, the new router must also
   switch to the (S,G)-source-rooted tree. In order to accomplish this,
   the newly added router must:

      (i).    Notice that it has been added to a new
              point-to-multipoint VC

      (ii).   Notice (S,G) traffic coming down this new
              point-to-multipoint VC

      (iii).  Send (S,G) joins toward S, causing it to switch to the
              source-rooted tree. The router learns that the VC is used
              to distribute (S,G) traffic in the previous steps.











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4.3. Handing the "Packet Reflection" Problem

   When a router receives a multicast packet from another router in its
   own LIS, the router should not send the packet on any of the routers
   distribution point-to-multipoint VCs associate with the LIS. This
   eliminates the problem of "packet reflection". Sending the packet on
   the routers' distribution VCs associated with other LISs is
   controlled by the multicast routing procedures.

5. Brief Comparison with MARS

   The intra-LIS multicast scheme described in this document is intended
   to be a less complex solution to an important subset of the
   functionality provided by the Multicast Address Resolution Server, or
   MARS [MARS]. In particular, it is designed to provide intra-LIS
   multicast between routers using PIM-SM, and does not consider the
   case of host-rooted point-to-multicast multicast distribution VCs.

   Although MARS supports both of the current schemes for mapping the IP
   multicast service model to ATM (multicast server and meshes of
   point-to-multipoint VCs), it does so at at cost and complexity higher
   than of the scheme described in this document. In addition, MARS
   requires new encapsulations, whereas this proposal works with either
   LLC/SNAP or with NLPID encapsulation. Another important difference is
   that MARS allows point-to-multipoint VCs rooted either at a source or
   at a multicast server (MCS). The approach taken here is to constrain
   complexity by focusing on PIM-SM (taking advantage of information
   available in explicit joins), and by allowing point-to-multipoint VCs
   to be rooted only at the routers (which is roughly analogous to the
   complexity and functionality of rooting point-to-multipoint VCs at
   the sources).

   In summary, the method described in this document is designed for the
   router-to-router case, and takes advantage of the explicit-join
   mechanism inherent in PIM-SM to provide a simple mechanism for
   intra-LIS multicast between routers. MARS, on the other hand, accepts
   different tradeoffs in complexity-functionality design space. In
   particular, while the MARS paradigm provides a general neighbor
   discovery mechanism, allows host to participate, and is protocol
   independent, it does so at considerable cost.











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6. Security Considerations

   In general, the security issues relevant to the proposal outlined in
   the memo are subsumed by those faced by PIM-SM. While work in
   proceeding on security for PIM-SM, it is worthwhile noting that
   several issues have been raised in conjunction with multicast routing
   and with PIM-SM in particular. These issues include but are not
   limited to:

      (i).   Unauthorized Senders

      (ii).  Unauthorized Receivers

      (iii). Unauthorized use of the RP

      (iv).  Unauthorized "last hop" switching to shortest path
             tree.

6.1. General Comments on Multicast Routing Protocol Security

   Historically, routing protocols used within the Internet have lacked
   strong authentication mechanisms [RFC1704]. In the late 1980s,
   analysis revealed that there were a number of security problems in
   Internet routing protocols then in use [BELLOVIN89].  During the
   early 1990s it became clear that adversaries were selectively
   attacking various intra-domain and inter-domain routing protocols
   (e.g. via TCP session stealing of BGP sessions) [CERTCA9501,
   RFC1636]. More recently, cryptographic authentication mechanisms have
   been developed for RIPv2, OSPF, and the proprietary EIGRP routing
   protocols.  BGP protection, in the form of a Keyed MD5 option for
   TCP, has also become widely deployed.

   At present, most multicast routing protocols lack strong
   cryptographic protection.  One possible approach to this is to
   incorporate a strong cryptographic protection mechanism (e.g. Keyed
   HMAC MD5 [RFC2104]) within the routing protocol itself.  Alternately,
   the routing protocol could be designed and specified to use the IP
   Authentication Header (AH) [RFC1825, RFC1826, RFC2085] to provide
   cryptographic authentication.

   Because the intent of any routing protocol is to propagate routing
   information to other parties, confidentiality is not generally
   required in routing protocols.  In those few cases where local
   security policy might require confidentiality, the use of the IP
   Encapsulating Security Payload (ESP) [RFC1825, RFC1827] is
   recommended.





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   Scalable dynamic multicast key management is an active research area
   at this time. Candidate technologies for scalable dynamic multicast
   key management include CBT-based key management [RFC1949] and the
   Group Key Management Protocol (GKMP) [RFC2093,RFC2094].  The IETF IP
   Security Working Group is actively working on GKMP extensions to the
   standards-track ISAKMP key management protocol being developed in the
   same working group.

7. References

   [BELLOVIN89] S. Bellovin, "Security Problems in the TCP/IP
                Protocol Suite", ACM Computer Communications Review,
                Volume 19, Number 2, pp. 32-48, April 1989.

   [CERTCA9501] CERT, "IP Spoofing Attacks and Hijacked Terminal
                Connections", ftp://ftp.cert.org/cert_advisories/,
                January 1995.

   [MARS]       Armitage, G., "Support for Multicast over UNI 3.0/3.1
                based ATM Networks.", RFC 2022, November 1996.

   [PIM-SM]     Estrin, D, et. al., "Protocol Independent Multicast
                Sparse Mode (PIM-SM): Protocol Specification", Work in
                Progress.

   [RFC1636]    Braden, R., Clark, D., Crocker, S., and C. Huitema,
                "Report of IAB Workshop on Security in the Internet
                Architecture February 8-10, 1994", RFC 1636, June 1994.

   [RFC1704]    Haller, N., and R. Atkinson, "On Internet
                Authentication", RFC 1704, October 1994.

   [RFC1825]    Atkinson, R., "IP Security Architecture", RFC 1825,
                August 1995.

   [RFC1826]    Atkinson, R., "IP Authentication Header", RFC 1826,
                August 1995.

   [RFC1827]    Atkinson, R., "IP Encapsulating Security Payload",
                RFC 1827, August 1995.

   [RFC1949]    Ballardie, A., "Scalable Multicast Key Distribution",
                RFC1949, June 1996.

   [RFC2085]    Oehler, M., and R. Glenn, "HMAC-MD5 IP Authentication
                with Replay Prevention", RFC 2085, February 1997.





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   [RFC2093]    Harney, H., and C. Muckenhirn, "Group Key Management
                Protocol (GKMP) Specification", RFC 2093, July 1997.

   [RFC2094]    Harney, H., and C. Muckenhirn, "Group Key Management
                Protocol (GKMP) Architecture", RFC 2094, July 1997.

   [RFC2104]    Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed
                Hashing for Message Authentication", RFC 2104, February
                1997.

   [RFC2225]    Laubach, M., and J. Halpern, "Classical IP and ARP over
                ATM", RFC 2225, April 1998.

8. Acknowledgments

   Petri Helenius provided several insightful comments on earlier
   versions of this document.

9. Author Information

   Dino Farinacci
   Cisco Systems
   170 Tasman Dr.
   San Jose, CA 95134

   Phone: (408) 526-4696
   EMail: dino@cisco.com


   David Meyer
   Cisco Systems
   170 Tasman Dr.
   San Jose, CA 95134

   Phone: (541) 687-2581
   EMail: dmm@cisco.com


   Yakov Rekhter
   cisco Systems, Inc.
   170 Tasman Dr.
   San Jose, CA 95134

   Phone: (914) 528-0090
   EMail: yakov@cisco.com






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10.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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