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Versions: 00                                                            
INTERNET DRAFT                                           Manolo Sola (1)
draft-sola-pim-static-multicast-00.txt                 Masataka Ohta (2)

                                                   (1) Waseda University
                                       (2) Tokyo Institute of Technology

                                                            August  1998

              Modifications to PIM-SM for Static Multicast

Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).


   The Protocol Independent Multicast - Sparse Mode (PIM-SM) is
   currently defined as an intra-domain multicast protocol. Although in
   PIM-SM more than one Candidate Rendezvous Point (C-RP) may exist,
   only one can be active at a given time, and this will be the one to
   which receivers will send Join messages or sources will send Register
   messages.  The method used in PIM-SM to make public the set of C-RPs
   for a multicast group is to flood all over the domain packets with
   the list of C-RPs using the so-called Bootstrap method.  This
   approach may scale in domains with few routers but does not scale if
   the protocol would have to be applied to provide multicast throughout
   the whole internet.

   In this draft we specify the modifications needed in PIM-SM in order
   to support more than one active RP simultaneously, and also show how
   Static Multicast can be used to make public in an scalable way the
   information regarding RPs as well as the IP multicast address for the
   multicast group.  In the presence of more than one source, having
   more than one RP and placing them according to sources and receivers'

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   distribution on the internet will reduce the average number of hops
   from sources to receivers, and will also improve the multicast
   protocol's fault tolerance.

1.  Introduction

   PIM-SM [PIM-SMv2] delivers multicast traffic to receivers of a
   multicast group by means of a tree rooted at the RP for that
   multicast group.

   As PIM-SM is an intra-domain multicast protocol, it runs over a tree
   which can not cross domain boundaries.

   To connect trees in different domains for the same multicast group,
   an inter-domain multicast protocol is needed and PIM-SM has to be
   extended to interoperate with such a protocol.

   One motivation behind this multicast architecture was to be able to
   improve multicast scalability, in what respect to the size of the
   multicast routing table, by the aggregation of multicast addresses in
   a similar way unicast addresses were aggregated.

   However, it has been shown that it is not possible to aggregate
   multicast routing table entries, even if multicast addresses are
   assigned aggregated [Sola98]. That is, aggregation of multicast
   address assignment is meaningless.

   Other argument which is usually given to justify this multicast
   architecture, is that administrators of different domains may desire
   to run different multicast protocols. However, an intra/inter-domain
   multicast architecture does not allow administrators of different
   domains to run the same multicast protocol (at least 2 different
   protocols, intra-domain and inter-domain, would have to be used).

   This way of delivering multicast lacks also of location independence
   (the protocol is bounded to a domain) and fault tolerance (only one
   C-RP can be active at a given moment).

   In PIM-SM the way to discover the RP for a multicast group is based
   on flooding the RP list to all routers in the domain. However, it is
   necessary to find a method with better scalability properties to
   distribute that information.

   In [static-multicast] there is no need for inter/intra domain
   splitting, multiple independent RPs for the same multicast group are
   allowed, and the tree rooted at each RP is no longer limited to a
   single domain. Furthermore, a scalable solution is presented to the
   problems of how to delegate and allocate multicast addresses, and how

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   to discover RPs for a multicast group.

   In order to complete the Static Multicast architecture, a protocol to
   intercommunicate independent multicast trees for the same multicast
   group has to be defined.

   In this draft we describe the modifications needed in PIM-SM in order
   to support that architecture.

   When an administrator desires to run a multicast group, first of all,
   an IP multicast address must be obtained from an IP multicast
   address' delegation authority.

   The proposed delegation mechanism for multicast address in [static-
   multicast] is summarized in the following paragraph:

      An administrator who has been delegated the set of 2^8 addresses
      {X.Y.Z.0 to X.Y.Z.255} is also delegated the set of 2^4 multicast
      addresses {M.X.Y.Z, with M=224,...,239} as:

          224.X.Y.Z     CNAME     mcast0.Z.Y.X.in-addr.arpa.
              .           .                  .
              .           .                  .
              .           .                  .
          239.X.Y.Z     CNAME     mcast15.Z.Y.X.in-addr.arpa.

      , and that administrator becomes an IP multicast address
      delegation authority, so he or she can further subdelegate any
      subset of the 2^4 multicast addresses in the same way he or she
      can further subdelegate any subset of the 2^8 unicast addresses.

   As some of the multicast address space has already been assigned,
   some addresses can not be used. To ensure that these addresses will
   never be used for static multicast, or to reserve some multicast
   address space for other purposes, the set of valid values for M may
   be reduced.

2.  Summary of the new feautures introduced in this draft

   We call Multi-RP PIM-SM (MRP PIM-SM) to the protocol resulting from
   the modifications to PIM-SM in order to accomplish with the Static
   Multicast architecture. The current version of MRP PIM-SM is based on
   PIM-SM version 2 [PIM-SMv2]. For brevity, we will refer to PIM-SM
   version 2 as PIM-SM.

   In this draft Static Multicast is used for distributing RP and
   multicast group address information.

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   Also, this draft proposes that RP placement and migration strategy
   must be decided by the administrator of the multicast group, and that
   a router must be configured by its administrator in order to behave
   as an RP.

   As a Designated Router (DR) may not have enough routing information
   to decide which is its nearest RP, the format of the Join/Prune
   message is modified and a list of RPs is included in the message
   instead of a unique RP.  This message travels hop by hop, and a
   router forwards it towards the nearest RP in a way which will be
   commented later.  As a Join/Prune is sent with a list of RPs instead
   of only one RP, another new message (Join Ack message) will inform
   the router about the RP that has been finally joined.

   Other new message, the State message, is defined to inform RPs about
   other operative RPs for the same multicast group.  The use of these
   messages will be explained later in this document.

   For the case in which the DR doesn't need to join a RP but to send
   Registered messages to him, first, the nearest RP is discovered and
   then the traffic is forwarded to that RP.

   Four new kind of control messages are proposed:

 2.1.  Join/Prune message

   In order to make possible for non-member sources to send packets to
   their nearest RP, an additional field in the current Join/Prune
   message is defined.  That field, the Control-Only-Join field, has no
   arguments.  With this flag set, a Join/Prune message indicates that
   it is a join to receive only Join Ack messages with no intention to
   receive any data stream.  With this flag unset, a Join/Prune message
   indicates that it is a join to receive Join Ack messages and any data

   An RP is said to be an RP with members for a multicast group if it
   has state instantiated due to the reception of Join/Prune messages
   for that multicast group having the Control-Only-Join field unset.

 2.2.  Join Ack message

   For each multicast group, each RP must multicast periodically this
   message.  It must include the IP address of the RP so that routers in
   that RP's tree for a multicast group will be able to know which RP
   they have joined.  This message must also include a list containing
   the complete list of RPs obtained through DNS, and a RP-unreachable
   flag associated to each RP.

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 2.3.  State message

   It is sent periodically from a RP to each one of the RPs of the
   multicast group.  A RP knows about other reachable RPs through this

 2.4  Join Probe

   It is used to detect RPs marked as unreachable that becomes

3.  RP behaviour

 3.1  Number of RPs and distribution of RP's information

   The administrator of a multicast group G must decide how many RPs
   will exist for G and where they will be located and, following the
   rules in [static-multicast], DNS must be used to make worldwide
   available the information about RPs and the IP multicast address for
   the multicast group.

   For PIM-SM, a new RR should be defined in DNS. We illustrate it with
   an example:

      bbc.com         RVP      england.bbc.com
                      RVP      scotland.bbc.com
                      RVP      wales.bbc.com
                      RVP      north-ireland.bbc.com

   The DNS packet format is identical to that of PTR [RFC1035], and the
   QTYPE value for a RVP query is <to be assigned by IANA>.  The
   administrator of the multicast group must contact the administrator
   of a router that will behave as an RP so that the router can be
   configured properly.  A router must know whether it is an RP by means
   of a configuration paramenter at the router which must be set by the
   administrator of that router.

   When a router configured as an RP for multicast group G starts to
   run, it must query DNS to get the list of RPs for G.

 3.2  How to discover the nearest RP

   Lets suppose that a router with no state for multicast group G
   receives a Join message.  This message must include:

      a. A complete list of RPs for G as seen by the DR or RP that
      originated this Join message.  The DR or RP used DNS to obtain
      that list.  In a particular case which will be explained later, an

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      intermediate router can also modify this list.

      b. A Control-Only-Join flag.  A DR with no members that need to
      forward multicast data traffic from non-member senders must set
      this flag when sending a Join message.  This will allow the DR to
      receive Join Ack messages to discover the nearest RP without
      receiving multicast data traffic which is not needed because there
      is no members.

   The steps to discover the nearest RP are as follows:

  Step 1.

   In order to determine the next hop for the Join message:

      (1) The entries in the routing table allowing to arrive to the RPs
      in the list of RPs which are not marked as unreachable are
      selected, excluding those associated to the incoming interface by
      which the Join message arrived;

      (2) From this set, all entries with the same best metric are

      (3) From these entries, those with the same more specific route
      are selected;

      (4) From this set of routing entries, one is selected randomly and
      becomes the next hop.

               +----+       +----+          +----+
               | RP0|       | RP1|          | RP2|
               +----+       +----+          +----+
                  |           |                |
            N1 --------  --------- N2        --------
                     3|  |2                   |
                     +----+     +----+     +----+
                     | R1 |-----| R0 |-----| R2 |
                     +----+1   3+----+2    +----+

           Fig.1: Example to ilustrate how Join
                      messages are forwarded.

   Lets illustrate these points with an example.

   In Fig.1 there are three RPs (RP0, RP1, RP2).  RP0 belongs to subnet
   N1=, and RP1 to subnet N2=  The IP
   addresses of RP0, RP1, and RP2 are,, and

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   Lets suppose that there is no state at any router for multicast group
   G, that a Join message for G arrives to R0 through interface 1, and
   that the list of RPs in the message is {RP0, RP1, RP2}.

   Lets also suppose that the routing entries at R0 allowing to arrive
   to {RP0, RP1, RP2} are:

            destination   interface   cost   next hop
            -----------   ---------   ----   --------
           131.112.0/24       3        1       R1
                default       2        1       R2

   In order to determine the next hop to forward the Join message,
   previous points (1) to (4) has to be followed.  In point (1) these
   two entries are selected.  In point (2) again these (2) entries are
   selected because they have the same cost. In point (3) the entry
   associated to interface 3 is selected because the prefix 131.112.0/24
   is more specific than the default entry.

   R0 uses this entry to forward the Join message to R1.

   Lets also suppose that at R1 the routing entries allowing to arrive
   to {RP0, RP1, RP2} according to (1) are

            destination   interface   cost   next hop
            -----------   ---------   ----   --------       3        0       RP0       2        0       RP1

   In this case, both entries are selected in points (2) and (3). In
   point (4), and to solve the tie, one is selected randomly, and the
   Join message is forwarded to the corresponding RP.

  Step 2.

   The router must forward the Join message, and wait for a Join Ack or
   a time-out. The time-out for the forwarded Join message should be set
   to T/R, where T is the time-out of the incoming Join message at the
   router sending the message, and R is the number of entries in the set
   of routing entries.

   If the forwarded Join message times-out, the RPs associated to entry
   in the routing table are marked as unreachable and the process must
   be restarted at (2).

   If the join process succeeds, the router will start to receive Join

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   Ack for G from the RP which has been joined.  This message includes
   the RP which has been joined.

 3.3.  DNS look-up

   As a general rule for DRs or RPs involved in DNS look-ups, DNS has
   only to be queried if the TTL of the records in the last look-up is
   about to expire.

 3.4.  Forwarding tree

   When a router configured as an RP for multicast group G starts to
   run, it must query DNS to get a list of RPs for G. Also an RP with
   receivers for G must send a (S,G) Join message towards each of the
   RPs in the list placing in S the address of the RP to join.  RPs are
   the only routers that can initiate source specific joins.

   MRP-PIM-SM always uses RP-trees and does not allow to switch from RP-
   trees to source's shortest path trees (SP-trees).  The reason for not
   allowing SP-trees is due to the multicast architecture being used.
   The administrator of the multicast group decides the number of RPs
   and where they will be located. As is commented in [Sola98], the
   administrator should place RPs looking for an acceptable average
   number of hops between sources and receivers. Within that scheme, the
   scalability drawbacks of source-specific trees overcome their

   When a Register message from a source arrives to an RP, the RP
   decapsulate the packet, encapsulate the packet again using as the
   source address its own address, and forwards the packet as in PIM-SM.

   When a receiver receives a packet sent to the multicast group G, it
   decapsulates the packet and process it as in PIM-SM.

4.  Designated Router (DR) and intermediate router's behaviour

 4.1.  First member for G

   If a DR has no state for G and a join request for G arrives, the DR
   must query DNS to get a list of RPs for G.  DRs join the nearest RP
   sending, according to the rules given previously, (*,G) messages with
   the Control-Only-Join falg unset and the complete list of RPs.

 4.2.  Sources and DR with no members

   A Designated Router (DR) always sends packets from sources in
   Register messages towards the nearest RP. If the nearest RP is
   unknown, it is discovered sending a Join message with the Control-

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   Only-Join flag set and the complete list of RPs. The Join Ack to this
   message will include that RP.

 4.3.  Join messages with different lists of RPs

   If a router with Join state for G receives a Join message for
   multicast group G, and the list of RPs in the message is not equal to
   the list of RPs at the router, the router must query DNS and build a
   new list of RPs.  If the new list is not equal to the one at the
   router, the router must prune the current up-branch and send a new
   Join message based on the new list.

 4.4.  Unreachable RPs becoming reachable

   A new message is defined, the Join Probe message, to detect when an
   RP marked as down becomes reachable.

   Routers should send Join Probes to next hops (excluding the one
   through which the normal Join is sent) towards RPs nearer than the
   currently joined RP.

   The Join Probe message is similar to the Join message except that it
   does not affect up data path and it includes only the RPs nearer than
   the currently joined RP.

   If Join Ack is returned to Join Probe, the acked RP is removed from
   the list of unreachable RPs, and will be considered again for the
   next Join to be sent.

   In this case, a Join Ack is sent selectively, that is, only to those
   routers which requested a Join Probe for that RP

5.  Eliminate intra/inter-domain procedures

   As the new multicast architecture does not make any intra/inter-
   domain assumptions, the procedures, timers, and data structures
   introduced in PIM-SM to interoperate with an inter-domain protocol
   should be eliminated.

6. Security Considerations

   It should be noted that PIM-SM offers a floor control capability as
   good as that of unicast communication. That is, with a floor control
   at the RP based on source IP addresses, attacking senders, even with
   a forged source address, can not inject disturbing packets on the
   multicast tree unless they were located between an RP and receivers,
   even in which case only receivers downstream of the attacker are

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   [Sola98] M. Sola, M. Ohta, T. Maeno, "Scalability of Internet
   Multicast Protocols," INET'9

   [static-multicast] M. Ohta, J. Crowcroft, "Static Multicast," Work in
   progress, ftp://ftp.ietf.org/internet-drafts/draft-ohta-static-m

   [PIM-SMv2] D. Estrin et al. "Protocol Independent Multicast-Sparse
   Mode (PIM-SM): Protocol Specification," RFC 2362,

   [RFC1035] P. Mokapertis, "Domain Names - Implementation and
   Specification," RFC 1035, ftp://ftp.isi.edu/in-notes/rfc-1035.txt

Author's Address

   Manolo Sola
   Waseda University
   School of Science and Engineering
   Department of Information and Computer Science
   3-4-1  Okubo, Shinjuku
   Tokyo 169-8555, JAPAN

   Phone: +81-3-5734-3299
   Fax: +81-3-5734-3415
   EMail: sola@jet.es

   Masataka Ohta
   Tokyo Institute of Technology
   Computer Center
   12-12-1, O-okayama, Meguro-ku
   Tokyo 152, JAPAN

   Phone: +81-3-5734-3299
   Fax: +81-3-5734-3415
   EMail: mohta@necom830.hpcl.titech.ac.jp

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