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Network Working Group J. Moy Internet Draft Proteon, Inc. Expiration date: January 1994 July 1993 MOSPF: Analysis and Experience 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. Internet Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a ``work- ing draft'' or ``work in progress.'' Please check the 1id- abstracts.txt listing contained in the internet-drafts Shadow Direc- tories on nic.ddn.mil, nnsc.nsf.net, nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the current status of any Internet Draft. Abstract This memo documents how the MOSPF protocol satisfies the require- ments imposed on Internet routing protocols by "Internet Engineering Task Force internet routing protocol standardization criteria" ([RFC 1264]). This memo provides information for the Internet community. It does not specify any Internet standard. Distribution of this memo is unlimited. Please send comments to mospf@gated.cornell.edu. [Moy] [Page 1] Internet Draft MOSPF: Analysis and Experience July 1993 1. Summary of MOSPF features and algorithms MOSPF is an enhancement of OSPF V2, enabling the routing of IP mul- ticast 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 Inter- net. 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 pack- ets 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 multi- casting are specified in "Host extensions for IP multicasting" ([RFC 1112]). That document defines a protocol, the Internet Group Manage- ment 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 adver- tisement, the group-membership-LSA (type 6). This in turn enables the MOSPF routers to most efficiently forward a multicast 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 desti- nation] 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 mul- ticast destination is received. The results of these "on demand" tree calculations are then cached for later use by subsequent match- ing 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 proto- col (something like DVMRP would work). The MOSPF protocol is based on the work of Steve Deering in [Deer- ing]. The MOSPF specification is documented in [MOSPF]. [Moy] [Page 2] Internet Draft MOSPF: Analysis and Experience July 1993 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 destina- tion 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 uni- cast datagram forwarding algorithms (like OSPF) that route based solely on destination. o The path taken between the datagram's source and any partic- ular destination group member is the least cost path avail- able. 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 repli- cation 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, instead 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 desti- nation group member. This means that, unlike OSPF's treat- ment of regular (unicast) IP data traffic, there is no pro- vision 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]. [Moy] [Page 3] Internet Draft MOSPF: Analysis and Experience July 1993 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 multicast- ing" ([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 ether- net 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 miscel- laneous 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 mul- ticast 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 capa- bility has proved valuable when used by the DVMRP protocol in the 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 con- nectivity without resorting to tunneling. The more central- ized control that exists in most Autonomous Systems, when compared to the Internet as a whole, should make this possi- ble. 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 net- works and non-broadcast multi-access (NBMA) networks. Note however that IGMP is not defined on NBMA networks, so while [Moy] [Page 4] Internet Draft MOSPF: Analysis and Experience July 1993 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 algo- rithm 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 multi- cast datagram having an external source can still be for- warded throughout the Autonomous System. Facilities also exist for forwarding locally generated datagrams to Auto- nomous 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 Auto- nomous System at the same place that unicast datagrams des- tined for that source will exit. o To deal with the fact that the external unicast and multi- cast 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 exam- ple 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. 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 destina- tions. 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 [Moy] [Page 5] Internet Draft MOSPF: Analysis and Experience July 1993 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 con- sumed 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 dis- cussion of OSPF's authentication mechanism, see Appendix 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 pro- tocol. The multicast applications included with the Proteon MOSPF implemen- tation 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 a 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 pro- vides a lightweight transport that is resistant to reordering. The Proteon MOSPF implementation is currently running in approxi- mately 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 [Moy] [Page 6] Internet Draft MOSPF: Analysis and Experience July 1993 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 fig- ure, 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. 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. [Moy] [Page 7] Internet Draft MOSPF: Analysis and Experience July 1993 +---+ +-------------------------------|RT1| | +---+ | +---------+ | | | | | +---+ +---+ +---+ | | |RT5|---------|RT2| |NAZ| | | +---+ +----+---+ +---+ | | | | | | | | +------------------------+ | | | + | | | | | | | | +---+ | +------------+ + | |--|RT7| | | | | | +---+ | +---+ | +---+ | | |RT4|--------|-----------|RT3|----| | +---+ | +---+ | | | | | + + | | | +---+ | +---------------|-----------|RT6|------------| | +---+ | + + Figure 1: Initial MOSPF test setup o Interoperability of MOSPF and non-multicast OSPF routers. Due to the commercial tunneling applications developed by Proteon that use IP multicast, MOSPF has been deployed in a number of opera- tional 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). [Moy] [Page 8] Internet Draft MOSPF: Analysis and Experience July 1993 o TOS-based routing in MOSPF. 6. A brief analysis of MOSPF scaling MOSPF uses the Dijkstra algorithm to calculate the path of a multi- cast datagram through any given OSPF area. This calculation encom- passes 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 calcula- tion). This is the cost of a single path calculation; however, MOSPF calculates a separate path for each [source network, multicast des- tination, TOS] tuple. This is potentially a lot of Dijkstra calcula- tions. MOSPF proposes to deal with this calculation burden by calculating datagram paths in an "on demand" fashion. That is, the path is cal- culated 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 calcula- tions 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. 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 participat- ing 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 applica- tion data (the cost of this soon dwarfing the cost of the Dijks- tras). [Moy] [Page 9] Internet Draft MOSPF: Analysis and Experience July 1993 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 gen- erate 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 calcula- tion (the one using 0.0.0.0 as the source). 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). [Moy] [Page 10] Internet Draft MOSPF: Analysis and Experience July 1993 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 transi- tions, 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. 8. 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 for- warding when the router is unable to calculate the detailed MOSPF forwarding cache entries. o Experience needs to be gained with the interactions between mul- tiple multicast routing algorithms (e.g., MOSPF and DVMRP). [Moy] [Page 11] Internet Draft MOSPF: Analysis and Experience July 1993 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 12] Internet Draft MOSPF: Analysis and Experience July 1993 9. References [Bharath-Kumar] Bharath-Kumar, K. and Jaffe, J. M. Routing to multi- ple destinations in Computer Networks. IEEE Transac- tions 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 inter- network. Stanford Technical Report STAN-CS-92-1415, Department of Computer Science, Stanford University, December 1991. [OSPF] Moy, J. OSPF Version 2. Internet Draft. March 1993. [OSPF MIB] Baker F. and Coltun R. OSPF Version 2 Management Information Base. Internet Draft. November 1992. [MOSPF] Moy, J. Multicast Extensions to OSPF. Internet Draft. March 1993. [RFC 1075] Waitzman, D., Partridge, C. and Deering, S. Distance Vector Multicast Routing Protocol. RFC 1175, November 1988. [RFC 1112] Deering, S.E. Host extensions for IP multicasting. RFC 1112, May 1988. [RFC 1209] Piscitello, D.M. Transmission of IP datagrams over the SMDS Service. RFC 1209, March 1991. [RFC 1245] Moy, J.,ed. OSPF protocol analysis. RFC 1245, July 1991. [RFC 1246] Moy, J.,ed. Experience with the OSPF protocol. RFC 1245, July 1991. [RFC 1264] Hinden, R. Internet Routing Protocol Standardization Criteria. RFC 1264, October 1991. [RFC 1390] Katz, D. Transmission of IP and ARP over FDDI Net- works. RFC 1390, January 1993. [RFC 1354] Baker, F. IP forwarding table MIB. RFC 1354, July 1992. [Moy] [Page 13] Internet Draft MOSPF: Analysis and Experience July 1993 Security Considerations Security issues are not discussed in this memo. Author's Address John Moy Proteon, Inc. 9 Technology Drive Westborough, MA 01581 Phone: (508) 898-2800 Email: jmoy@proteon.com This document expires in January 1994. [Moy] [Page 14]