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Network Working Group                                             J. MoyRequest for Comments: 1585                                 Proteon, Inc.Category: Informational                                       March 1994MOSPF: Analysis and ExperienceStatus of this Memo   This memo provides information for the Internet community.  This memo   does not specify an Internet standard of any kind.  Distribution of   this memo is unlimited.Abstract   This memo documents how the MOSPF protocol satisfies the requirements   imposed on Internet routing protocols by "Internet Engineering Task   Force internet routing protocol standardization criteria" ([RFC   1264]).   Please send comments to mospf@gated.cornell.edu.1.  Summary of MOSPF features and algorithms   MOSPF is an enhancement of OSPF V2, enabling the routing of IP   multicast datagrams.  OSPF is a link-state (unicast) routing   protocol, providing a database describing the Autonomous System's   topology.  IP multicast is an extension of LAN multicasting to a   TCP/IP Internet.  IP Multicast permits an IP host to send a single   datagram (called an IP multicast datagram) that will be delivered to   multiple destinations.  IP multicast datagrams are identified as   those packets whose destinations are class D IP addresses (i.e.,   addresses whose first byte lies in the range 224-239 inclusive).   Each class D address defines a multicast group.   The extensions required of an IP host to participate in IP   multicasting are specified in "Host extensions for IP multicasting"   ([RFC 1112]).  That document defines a protocol, the Internet Group   Management Protocol (IGMP), that enables hosts to dynamically join   and leave multicast groups.   MOSPF routers use the IGMP protocol to monitor multicast group   membership on local LANs through the sending of IGMP Host Membership   Queries and the reception of IGMP Host Membership Reports.  A MOSPF   router then distributes this group location information throughout   the routing domain by flooding a new type of OSPF link state   advertisement, the group-membership-LSA (type 6). This in turn   enables the MOSPF routers to most efficiently forward a multicastMoy                                                             [Page 1]

RFC 1585             MOSPF: Analysis and Experience           March 1994   datagram to its multiple destinations: each router calculates the   path of the multicast datagram as a shortest-path tree whose root is   the datagram source, and whose terminal branches are LANs containing   group members.   A separate tree is built for each [source network, multicast   destination] combination.  To ease the computational demand on the   routers, these trees are built "on demand", i.e., the first time a   datagram having a particular combination of source network and   multicast destination is received. The results of these "on demand"   tree calculations are then cached for later use by subsequent   matching datagrams.   MOSPF is meant to be used internal to a single Autonomous System.   When supporting IP multicast over the entire Internet, MOSPF would   have to be used in concert with an inter-AS multicast routing   protocol (something like DVMRP would work).   The MOSPF protocol is based on the work of Steve Deering in   [Deering].  The MOSPF specification is documented in [MOSPF].1.1.  Characteristics of the multicast datagram's path   As a multicast datagram is forwarded along its shortest-path tree,   the datagram is delivered to each member of the destination multicast   group. In MOSPF, the forwarding of the multicast datagram has the   following properties:      o The path taken by a multicast datagram depends both on the        datagram's source and its multicast destination. Called        source/destination routing, this is in contrast to most unicast        datagram forwarding algorithms (like OSPF) that route        based solely on destination.      o The path taken between the datagram's source and any particular        destination group member is the least cost path available. Cost        is expressed in terms of the OSPF link-state metric.      o MOSPF takes advantage of any commonality of least cost paths        to destination group members. However, when members of the        multicast group are spread out over multiple networks, the        multicast datagram must at times be replicated. This replication        is performed as few times as possible (at the tree branches),        taking maximum advantage of common path segments.      o For a given multicast datagram, all routers calculate an        identical shortest-path tree.  This is possible since the        shortest-path tree is rooted at the datagram source, insteadMoy                                                             [Page 2]

RFC 1585             MOSPF: Analysis and Experience           March 1994        of being rooted at the calculating router (as is done in the        unicast case). Tie-breakers have been defined to ensure        that, when several equal-cost paths exist, all routers agree        on which single path to use. As a result, there is a single        path between the datagram's source and any particular        destination group member. This means that, unlike OSPF's        treatment of regular (unicast) IP data traffic, there is no        provision for equal-cost multipath.      o While MOSPF optimizes the path to any given group member, it        does not necessarily optimize the use of the internetwork as        a whole. To do so, instead of calculating source-based        shortest-path trees, something similar to a minimal spanning        tree (containing only the group members) would need to be        calculated.  This type of minimal spanning tree is called a        Steiner tree in the literature.  For a comparison of        shortest-path tree routing to routing using Steiner trees,        see [Deering2] and [Bharath-Kumar].      o When forwarding a multicast datagram, MOSPF conforms to the        link-layer encapsulation standards for IP multicast        datagrams as specified in "Host extensions for IP multicasting"        ([RFC 1112]), "Transmission of IP datagrams over the        SMDS Service" ([RFC 1209]) and "Transmission of IP and ARP        over FDDI Networks" ([RFC 1390]). In particular, for ethernet        and FDDI the explicit mapping between IP multicast        addresses and data-link multicast addresses is used.1.2.  Miscellaneous features   This section lists, in no particular order, the other miscellaneous   features that the MOSPF protocol supports:      o MOSPF routers can be mixed within an Autonomous System (and        even within a single OSPF area) with non-multicast OSPF        routers. When this is done, all routers will interoperate in        the routing of unicasts.  Unicast routing will not be        affected by this mixing; all unicast paths will be the same        as before the introduction of multicast. This mixing of        multicast and non-multicast routers enables phased        introduction of a multicast capability into an internetwork.        However, it should be noted that some configurations of MOSPF        and non-MOSPF routers may produce unexpected failures in        multicast routing (see Section 6.1 of [MOSPF]).      o MOSPF does not include the ability to tunnel multicast        datagrams through non-multicast routers. A tunneling capability        has proved valuable when used by the DVMRP protocol in theMoy                                                             [Page 3]

RFC 1585             MOSPF: Analysis and Experience           March 1994        MBONE.  However, it is assumed that, since MOSPF is an intra-AS        protocol, multicast can be turned on in enough of the Autonomous        System's routers to achieve the required connectivity without        resorting to tunneling. The more centralized control that exists        in most Autonomous Systems, when compared to the Internet as a        whole, should make this possible.      o In addition to calculating a separate datagram path for each        [source network, multicast destination] combination, MOSPF        can also vary the path based on IP Type of Service (TOS). As        with OSPF unicast routing, TOS-based multicast routing is        optional, and routers supporting it can be freely mixed with        those that don't.      o MOSPF supports all network types that are supported by the base        OSPF protocol: broadcast networks, point-to-points networks and        non-broadcast multi-access (NBMA) networks.  Note however that        IGMP is not defined on NBMA networks, so while these networks        can support the forwarding of multicast datagrams, they cannot        support directly connected group members.      o MOSPF supports all Autonomous System configurations that are        supported by the base OSPF protocol. In particular, an algorithm        for forwarding multicast datagrams between OSPF areas        is included.  Also, areas with configured virtual links can        be used for transit multicast traffic.      o A way of forwarding multicast datagrams across Autonomous        System boundaries has been defined. This means that a multicast        datagram having an external source can still be forwarded        throughout the Autonomous System. Facilities also exist for        forwarding locally generated datagrams to Autonomous System exit        points, from which they can be further distributed. The        effectiveness of this support will depend upon the nature of the        inter-AS multicast routing protocol.  The one assumption that        has been made is that the inter-AS multicast routing protocol        will operate in an reverse path forwarding (RPF) fashion:        namely, that multicast datagrams originating from an external        source will enter the Autonomous System at the same place that        unicast datagrams destined for that source will exit.      o To deal with the fact that the external unicast and multicast        topologies will be different for some time to come, a        way to indicate that a route is available for multicast but        not unicast (or vice versa) has been defined. This for example        would allow a MOSPF system to use DVMRP as its inter-AS        multicast routing protocol, while using BGP as its inter-AS        unicast routing protocol.Moy                                                             [Page 4]

RFC 1585             MOSPF: Analysis and Experience           March 1994      o For those physical networks that have been assigned multiple        IP network/subnet numbers, multicast routing can be disabled        on all but one OSPF interface to the physical network.  This        avoids unwanted replication of multicast datagrams.      o For those networks residing on Autonomous System boundaries,        which  may  be  running multiple multicast routing protocols        (or multiple copies of the same multicast routing protocol),        MOSPF  can  be configured to encapsulate multicast datagrams        with unicast (rather than multicast) link-level destinations.        This also avoids unwanted replication of multicast datagrams.      o MOSPF provides an optimization for IP multicast's "expanding        ring search" (sometimes called "TTL scoping") procedure. In        an expanding ring search, an application finds the nearest        server by sending out successive multicasts, each with a        larger TTL. The first responding server will then be the        closest (in terms of hops, but not necessarily in terms of        the OSPF metric). MOSPF minimizes the network bandwidth        consumed by an expanding ring search by refusing to forward        multicast datagrams whose TTL is too small to ever reach a        group member.2.  Security architecture   All MOSPF protocol packet exchanges (excluding IGMP) are specified by   the base OSPF protocol, and as such are authenticated. For a   discussion of OSPF's authentication mechanism, seeAppendix D of   [OSPF].3.  MIB support   Management support for MOSPF has been added to the base OSPF V2 MIB   [OSPF MIB]. These additions consist of the ability to read and write   the configuration parameters specified in Section B of [MOSPF],   together with the ability to dump the new group-membership-LSAs.4.  Implementations   There is currently one MOSPF implementation, written by Proteon, Inc.   It was released for general use in April 1992. It is a full MOSPF   implementation, with the exception of TOS-based multicast routing. It   also does not contain an inter-AS multicast routing protocol.   The multicast applications included with the Proteon MOSPF   implementation include: a multicast pinger, console commands so that   the router itself can join and leave multicast groups (and so respond   to multicast pings), and the ability to send SNMP traps to aMoy                                                             [Page 5]

RFC 1585             MOSPF: Analysis and Experience           March 1994   multicast address. Proteon is also using IP multicast to support the   tunneling of other protocols over IP.  For example, source route   bridging is tunneled over a MOSPF domain, using one IP multicast   address for explorer frames and mapping the segment/bridge found in a   specifically-routed frame's RIF field to other IP multicast   addresses.  This last application has proved popular, since it   provides a lightweight transport that is resistant to reordering.   The Proteon MOSPF implementation is currently running in   approximately a dozen sites, each site consisting of 10-20 routers.   Table 1 gives a comparison between the code size of Proteon's base   OSPF implementation and its MOSPF implementation. Two dimensions of                      lines of C   bytes of 68020 object code          ___________________________________________________          OSPF base   11,693       63,160          MOSPF       15,247       81,956            Table 1: Comparison of OSPF and MOSPF code sizes   size are indicated: lines of C (comments and blanks  included),  and   bytes  of 68020 object code. In both cases, the multicast extensions   to OSPF have engendered a 30% size increase.   Note that in these sizes, the code used to configure and monitor the   implementation has been included. Also, in the MOSPF code size   figure, the IGMP implementation has been included.5.  Testing   Figure 1 shows the topology that was used for the initial debugging   of Proteon's MOSPF implementation.  It consists of seven MOSPF   routers, interconnected by ethernets, token rings, FDDIs and serial   lines. The applications used to test the routing were multicast ping   and the sending of traps to a multicast address (the box labeled   "NAZ" was a network analyzer that was occasionally sending IGMP Host   Membership Reports and then continuously receiving multicast SNMP   traps). The "vat" application was also used on workstations (without   running the DVMRP "mrouted" daemon; see "Distance Vector Multicast   Routing Protocol", [RFC 1075]) which were multicasting packet voice   across the MOSPF domain.Moy                                                             [Page 6]

RFC 1585             MOSPF: Analysis and Experience           March 1994   The MOSPF features tested in this setup were:   o   Re-routing in response to topology changes.   o   Path verification after altering costs.   o   Routing multicast datagrams between areas.   o   Routing multicast datagrams to and from external addresses.   o   The various tiebreakers employed when constructing datagram       shortest-path trees.   o   MOSPF over non-broadcast multi-access networks.   o   Interoperability of MOSPF and non-multicast OSPF routers.                                              +---+              +-------------------------------|RT1|              |                               +---+              |             +---------+         |              |                  |              |              |  +---+         +---+    +---+   |              |  |RT5|---------|RT2|    |NAZ|   |              |  +---+    +----+---+    +---+   |              |           |      |        |     |              |           |   +------------------------+              |           |                         |      +              |           |                         |      |              |           |                         |      |  +---+              |   +------------+      +             |      |--|RT7|              |            |          |             |      |  +---+              |          +---+        |           +---+    |              |          |RT4|--------|-----------|RT3|----|              |          +---+        |           +---+    |              |                       |                    |              |               +       +                    |              |               |           +---+            |              +---------------|-----------|RT6|------------|                              |           +---+            |                              +                            +                  Figure 1: Initial MOSPF test setupMoy                                                             [Page 7]

RFC 1585             MOSPF: Analysis and Experience           March 1994   Due to the commercial tunneling applications developed by Proteon   that use IP multicast, MOSPF has been deployed in a number of   operational but non-Internet-connected sites.  MOSPF has been also   deployed in some Internet-connected sites (e.g., OARnet) for testing   purposes. The desire of these sites is to use MOSPF to attach to the   "mbone".  However, an implementation of both MOSPF and DVMRP in the   same box is needed; without this one way communication has been   achieved (sort of like lecture mode in vat) by configuring multicast   static routes in the MOSPF implementation. The problem is that there   is no current way to inject the MOSPF source information into DVMRP.   The MOSPF features that have not yet been tested are:   o   The interaction between MOSPF and virtual links.   o   Interaction between MOSPF and other multicast routing protocols       (e.g., DVMRP).   o   TOS-based routing in MOSPF.6.  A brief analysis of MOSPF scaling   MOSPF uses the Dijkstra algorithm to calculate the path of a   multicast datagram through any given OSPF area. This calculation   encompasses all the transit nodes (routers and networks) in the area;   its cost scales as O(N*log(N)) where N is the number of transit nodes   (same as the cost of the OSPF unicast intra-area routing   calculation). This is the cost of a single path calculation; however,   MOSPF calculates a separate path for each [source network, multicast   destination, TOS] tuple. This is potentially a lot of Dijkstra   calculations.   MOSPF proposes to deal with this calculation burden by calculating   datagram paths in an "on demand" fashion. That is, the path is   calculated only when receiving the first datagram in a stream.  After   the calculation, the results are cached for use by later matching   datagrams. This on demand calculation alleviates the cost of the   routing calculations in two ways: 1) It spreads the routing   calculations out over time and 2) the router does fewer calculations,   since it does not even calculate the paths of datagrams whose path   will not even touch the router.   Cache entries need never be timed out, although they are removed on   topological changes.  If an implementation chooses to limit the   amount of memory consumed by the cache, probably by removing selected   entries, care must be taken to ensure that cache thrashing does not   occur.Moy                                                             [Page 8]

RFC 1585             MOSPF: Analysis and Experience           March 1994   The effectiveness of this "on demand" calculation will need to be   proven over time, as multicast usage and traffic patterns become more   evident.   As a simple example, suppose an OSPF area consists of 200 routers.   Suppose each router represents a site, and each site is participating   simultaneously with three other local sites (inside the area) in a   video conference. This gives 200/4 = 50 groups, and 200 separate   datagram trees. Assuming each datagram tree goes through every router   (which probably won't be true), each router will be doing 200   Dijkstras initially (and on internal topology changes). The time to   run a 200 node Dijkstra on a 10 mips processor was estimated to be 15   milliseconds in "OSPF protocol analysis" ([RFC 1245]). So if all 200   Dijkstras need to be done at once, it will take 3 seconds total on a   10 mips processor. In contrast, assuming each video stream is   64Kb/sec, the routers will constantly forward 12Mb/sec of application   data (the cost of this soon dwarfing the cost of the Dijkstras).   In this example there are also 200 group-membership-LSAs in the area;   since each group membership-LSA is around 64 bytes, this adds 64*200   = 12K bytes to the OSPF link state database.   Other things to keep in mind when evaluating the cost of MOSPF's   routing calculation are:   o Assuming that the guidelines of "OSPF protocol analysis" ([RFC     1245]) are followed and areas are limited to 200 nodes, the cost     of the Dijkstra will not grow unbounded, but will instead be     capped at the Dijkstra for 200 nodes.  One need then worry about     the number of Dijkstras, which is determined by the number of     [datagram source, multicast destination] combinations.   o A datagram whose destination has no group members in the domain     can still be forwarded through the MOSPF system. However, the     Dijkstra calculation here depends only on the [datagram source,     TOS], since the datagram will be forwarded along to "wild-card     receivers" only. Since the number of group members in a 200     router area is probably also bounded, the possibility of     unbounded calculation growth lies in the number of possible     datagram sources. (However, it should be noted that some future     multicast applications, such as distributed computing, may generate     a large number of short-lived multicast groups).   o By collapsing routing information before importing it into the     area/AS, the number of sources can be reduced dramatically. In     particular, if the AS relies on a default external route, most     external sources will be covered by a single Dijkstra calculation     (the one using 0.0.0.0 as the source).Moy                                                             [Page 9]

RFC 1585             MOSPF: Analysis and Experience           March 1994   One other factor to be considered in MOSPF scaling is how often cache   entries need to be recalculated, as a result of a network topology   change. The rules for MOSPF cache maintenance are explained in   Section 13 of [MOSPF]. Note that the further away the topology change   happens from the calculating router, the fewer cache entries need to   be recalculated. For example, if an external route changes, many   fewer cache entries need to be purged as compared to a change in a   MOSPF domain's internal link. For scaling purposes, this is exactly   the desired behavior. Note that "OSPF protocol analysis" ([RFC 1245])   bears this out when it shows that changes in external routes (on the   order of once a minute for the networks surveyed) are much more   frequent than internal changes (between 15 and 50 minutes for the   networks surveyed).7.  Known difficulties   The following are known difficulties with the MOSPF protocol:   o When a MOSPF router itself contains multicast applications, the     choice of its application datagrams' source addresses should be     made with care.  Due to OSPF's representation of serial lines,     using a serial line interface address as source can lead to     excess data traffic on the serial line.  In fact, using any     interface address as source can lead to excess traffic, owing to     MOSPF's decision to always multicast the packet onto the source     network as part of the forwarding process (see Section 11.3 of     [MOSPF]). However, optimal behavior can be achieved by assigning     the router an interface-independent address, and using this as     the datagram source.     This concern does not apply to regular IP hosts (i.e., those     that are not MOSPF routers).   o It is necessary to ensure, when mixing MOSPF and non-multicast     routers on a LAN, that a MOSPF router becomes Designated Router.     Otherwise multicast datagrams will not be forwarded on the LAN,     nor will group membership be monitored on the LAN, nor will the     group-membership-LSAs be flooded over the LAN. This can be an     operational nuisance, since OSPF's Designated Router election     algorithm is designed to discourage Designated Router transitions,     rather than to guarantee that certain routers become     Designated Router. However, assigning a DR Priority of 0 to all     non-multicast routers will always guarantee that a MOSPF router     is selected as Designated Router.Moy                                                            [Page 10]

RFC 1585             MOSPF: Analysis and Experience           March 19948.  Future work   In the future, it is expected that the following work will be done on   the MOSPF protocol:   o More analysis of multicast traffic patterns needs to be done, in     order to see whether the MOSPF routing calculations will pose an     undue processing burden on multicast routers.  If necessary,     further ways to ease this burden may need to be defined. One     suggestion that has been made is to revert to reverse path     forwarding when the router is unable to calculate the detailed     MOSPF forwarding cache entries.   o Experience needs to be gained with the interactions between multiple     multicast routing algorithms (e.g., MOSPF and DVMRP).   o Additional MIB support for the retrieval of forwarding cache     entries, along the lines of the "IP forwarding table MIB" ([RFC     1354]), would be useful.Moy                                                            [Page 11]

RFC 1585             MOSPF: Analysis and Experience           March 19949.  References    [Bharath-Kumar] Bharath-Kumar, K., and J. Jaffe, "Routing to                    multiple destinations in Computer Networks", IEEE                    Transactions on Communications, COM-31[3], March                    1983.    [Deering]       Deering, S., "Multicast Routing in Internetworks                    and Extended LANs", SIGCOMM Summer 1988                    Proceedings, August 1988.    [Deering2]      Deering, S., "Multicast Routing in a Datagram                    Internetwork", Stanford Technical Report                    STAN-CS-92-1415, Department of Computer Science,                    Stanford University, December 1991.    [OSPF]          Moy, J., "OSPF Version 2",RFC 1583, Proteon,                    Inc., March 1994.    [OSPF MIB]      Baker F., and R. Coltun, "OSPF Version 2 Management                    Information Base",RFC 1253, ACC, Computer Science                    Center, August 1991.    [MOSPF]         Moy, J., "Multicast Extensions to OSPF",RFC 1584,                    Proteon, Inc., March 1994.    [RFC 1075]      Waitzman, D., Partridge, C. and S. Deering,                    "Distance Vector Multicast Routing Protocol",RFC1075, BBN STC, Stanford University, November 1988.    [RFC 1112]      Deering, S., "Host Extensions for IP Multicasting",                    Stanford University,RFC 1112, May 1988.    [RFC 1209]      Piscitello, D., and J. Lawrence, "Transmission of IP                    Datagrams over the SMDS Service",RFC 1209, Bell                    Communications Research, March 1991.    [RFC 1245]      Moy, J., Editor, "OSPF Protocol Analysis",RFC1245, Proteon, Inc., July 1991.    [RFC 1246]      Moy, J., Editor, "Experience with the OSPF                    Protocol",RFC 1245, Proteon, Inc., July 1991.    [RFC 1264]      Hinden, R., "Internet Routing Protocol                    Standardization Criteria",RFC 1264, BBN, October                    1991.Moy                                                            [Page 12]

RFC 1585             MOSPF: Analysis and Experience           March 1994    [RFC 1390]      Katz, D., "Transmission of IP and ARP over FDDI                    Networks",RFC 1390, cisco Systems, Inc., January                    1993.    [RFC 1354]      Baker, F., "IP Forwarding Table MIB",RFC 1354,                    ACC, July 1992.Security Considerations   Security issues are not discussed in this memo, tho seeSection 2.Author's Address   John Moy   Proteon, Inc.   9 Technology Drive   Westborough, MA 01581   Phone: (508) 898-2800   EMail: jmoy@proteon.comMoy                                                            [Page 13]