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Tools to Visualize the Internet Multicast Backbone

Bradley HUFFAKER <bhuffake@caida.org>
Evi NEMETH <evi@caida.org>
K. CLAFFY <kc@caida.org>
Cooperative Association for Internet Data Analysis


For the past several years, the Internet multicast backbone (MBONE) has been a growing part of the Internet infrastructure, and of strategic interest to the network research community experimenting with multicast technologies. In particular, the MBONE has been a deployment test bed for scalable distribution of group audio and video streams. Rapid growth in topology and traffic volume of the multicast infrastructure has brought inevitable scaling problems, not the least of which is the increased negative impact incurred by accidental misconfiguration of MBONE nodes.

In pursuit of insight into the MBONE topology, its growth characteristics, and the extent of the transition from the tunnel-based architecture to the deployment of native multicast, we developed and applied visualization tools to the database of connectivity information collected with the mwatch [MWAT] utility from University College London. We have used two separate visualization tools for this task: MantaRay [MANTA], for geographic-map based depiction of MBONE data collected by mrinfo queries; and Otter [OTTER], for topological visualizations of the same data.

MantaRay and Otter are part of the fleet of tools from the Cooperative Association for Internet Data Analysis for visualizing various sets of Internet data. Both tools are Java-based and allow users to select parts of the data, focus or zoom in on particular areas, color by various parameters, and generally explore multicast topologies.



The original multicast backbone (or MBONE) is a virtual network of Internet hosts and routers that can send and receive traffic destined for multicast Internet Protocol (IP) addresses. Production Internet routers have supported native multicast in recent years, and the original MBONE provided intermediate support using a virtual layer of tunnels configured over the physical Internet. Note that the term "MBONE" could now be considered to comprise all existing multicast routing infrastructure: tunnels as well as native multicast. For this paper we will use the phrase "tunneled MBONE" to refer to the original Distance Vector Multicast Routing Protocol (DVMRP)-based [DVMRP] implementation, and simply "MBONE" to refer to the global multicast routing infrastructure (which includes PIM [Protocol-Independent Multicast; PIM] infrastructure).

The tunneled MBONE works by connecting individual multicast-capable subnetworks with these logical tunnels whose endpoints encapsulate multicast packets into normal IP packets and send them across the unicast path between the tunnel endpoints. To non-multicast routers this traffic appears as normal IP traffic, while multicast routers treat it as IP encapsulated in IP, rather than the typical TCP (Transmission Control Protocol) or UDP (User Datagram Protocol) encapsulated in IP. These multicast routers recognize such packets as special and process them accordingly: stripping the extra IP header to retrieve the multicast group destination, and consulting the multicast routing table to determine where to forward the packet, whether across native multicast links or across a tunnel to another multicast-capable router. Tunnel endpoints are either routers that support DVMRP or Unix workstations or workstation-based routers that have IP multicast support and run the multicast routing daemon mrouted. Currently the widest use of the MBONE is for real-time video and audio streams.

The next section offers a brief introduction to the multicast routing protocols that support the MBONE. We then describe the multicast data sets we analyzed, and aspects of the MBONE for which visualization could provide insight. Finally, we present an overview of Manta and Otter, the tools we used to visualize the multicast data, and provide example snapshots of various components of the multicast infrastructure.

Multicast routing protocols

A multicast IP address is in the range to and refers to a specific group of Internet hosts who have registered interest in a particular group conversation. These addresses are different from unicast IP addresses, which suggest transmission to only a single host interface (Internet destination). In fact, any host, even those not in the group corresponding to a given multicast address, can send data to that multicast address (i.e., to every host in the group). This functionality adds complexity to the task of maintaining the distribution tree information in a precise and memory efficient way.

There are two multicast routing protocols in common use: DVMRP (Distance Vector Multicast Routing Protocol) and PIM (Protocol-Independent Multicast). DVMRP originated with Steve Deering [DEER] and its canonical Internet implementation, mrouted, is currently maintained by Bill Fenner of Xerox PARC. PIM was defined by RFC 2117 [PIM] and implemented in Cisco routers by Dino Farinacci. A new multicast paradigm under recent development maps a multicast group to a single fixed source and a group of receivers [EXPR]. This model may be able to accommodate the requirements of many multicast applications, but is not yet implemented or deployed anywhere.

DVMRP infrastructure comprised the vast majority of the MBONE until routers began to support native multicast via PIM, which is what new participants tend to deploy as they add multicast support to their network. In addition, administrators of pieces of the DVMRP infrastructure are independently and gradually transitioning to PIM. We are now even seeing PIM clouds interconnect, similar to unicast exchange points, using the MBGP [MBGP] and MSDP [MSDP] protocols for inter-domain routing support (MBGP to discover topology and MSDP to help construct the inter-domain multicast distribution tree). One important visualization possibility is to depict the relative prominence of various multicast routing protocols in the current Internet infrastructure (including which versions of each, since significant functionality changes are not rare between versions).

Multicast data collection

The most common tool to gather data about MBONE nodes and tunnels is mrinfo [MRINF]. An mrinfo query to an MBONE node returns information on the multicast software running, connectivity to other nodes, and known attributes of any tunnels. Another tool, mwatch, recursively calls the mrinfo program to find all (hopefully) the multicast routers on the MBONE. The mwatch tool was written by Atanu Ghosh of University College London as part of the MICE (Multimedia Integrated Conferencing for European Researchers) project. Piete Brooks (University of Cambridge) enhanced and currently maintains it. (Unfortunately, they have had to deactivate their MBONE topology monitoring effort due to a new policy of their ISP [Internet service provider] to charge for traffic on a per-byte basis.)

The mwatch database was too large to gracefully use for testing while developing these visualization tools, so we built the crawl tool to produce smaller data sets. Crawl starts with a set of root nodes (hostnames), a depth level, and then calls mrinfo on those root nodes. It then collects a list of all the hostnames they are connected to and repeats the process until it has reached the specified depth from these root nodes. Basically, crawl is a modified version of mwatch that can limit the depth of the mrinfo recursion to produce smaller, focused data sets.

As of August 1998, the size of the dataset generated by mwatch is about 4.5 MB; the execution time to traverse the entire MBONE topology increased from about 12 hours when mwatch was first run regularly at the University of Cambridge to over 24 hours as of August 1998. Our utility, crawl, generates a file of approximately 3.5 MB in 13 hours when run across the same scope in topology. The difference appears to derive from mwatch's inclusion of information from previous passes, while crawl retains only information collected in the current pass. The designers of mwatch might have included this memory feature to account for machines that temporarily disappear due to inclement network/host/configuration weather.

A sample of the mrinfo output follows: (mbone.sdsc.edu) [version 3.255,prune,genid,mtrace]: -> (local) [1/1/disabled] -> (pinot.sdsc.edu) [1/1] -> (cs-f-vbns.sdsc.edu) [1/1] -> (medusa.sdsc.edu) [1/1] -> (bigmama.sdsc.edu) [1/1] -> (nccosc-mbone.dren.net) [1/32/tunnel] -> (mbone.cerf.net) [5/32/tunnel] -> (dasher.doct.sdsc.edu) [1/32/tunnel/down/leaf] -> (itlhp1.doct.sdsc.edu) [1/32/tunnel/down/leaf] -> (avatar.sdsc.edu) [1/1/tunnel/down/leaf] -> (mithrandir.sdsc.edu) [1/1/tunnel/down/leaf]

The first line of mrinfo output shows that the host queried is mbone.sdsc.edu and it is running version 3.255 of mrouted with mtrace support. Each additional line represents a connection to another mrouter with characteristics listed in brackets. The word "tunnel" in the attributes means that a multicast path exists between mbone.sdsc.edu and the named host via encapsulated IP. Lines with no "tunnel" attribute are native multicast links, in this case to other mrouters on the local network.

Querying the medusa.sdsc.edu mrouter, mrinfo returns: (medusa.sdsc.edu) [version 11.2,prune,mtrace]: -> (local) [1/6/pim/querier/disabled/down/leaf] -> (cs-f-vbns.sdsc.edu) [1/1/pim] -> (bigmama.sdsc.edu) [1/1/pim] -> (pinot.sdsc.edu) [1/1/pim] -> (mithrandir.sdsc.edu) [1/6/pim] -> (curunir.sdsc.edu) [1/6/pim] -> (tigerfish.sdsc.edu) [1/6/pim]

showing that medusa is a Cisco router running IOS 11.2 with six neighbors. Note that medusa does not show its DVMRP neighbors on its local network, that is, the DVMRP Unix host mbone.sdsc.edu sees and reports its neighbor medusa, but medusa does not mention mbone.sdsc.edu. Cisco's running PIM know that they have DVMRP neighbors, but do not know who they are (i.e., don't retain neighbor state).

An mwatch or crawl traversal of the whole MBONE would never find mbone.sdsc.edu from medusa. Such a traversal is potentially incomplete since nodes can be down (skipped) and DVMRP tunnels can be hidden behind Cisco routers, both of which can create MBONE partitioning, black holes, and failed pruning situations. The last of these, failed pruning, can result in considerable extraneous copies of multicast traffic, imposing unnecessary load on the infrastructure.

Another data collection problem we have experienced is that mrinfo cannot access information on nodes behind firewalls that filter IGMP (Internet Group Multicast Protocol) packets. Fortunately not every site's firewalls are so conservative, but it does pose an obstacle to obtaining a comprehensive picture.

Security for some sites also dictates that global routes are not advertised outside of their internal infrastructure. In such cases the mrinfo query may reach its destination mrouter, but may not be able to get a reply back for lack of a return path. Furthermore, misconfigured routers or incompatible software can also provide potentially inaccurate information, e.g., some routers run native multicast software that does not respond to mrinfo queries.

Aspects of the data worth visualizing

We enumerate the concepts by which we would like to be able to structure visualizations; the keywords listed in bold serve as menu items in the user interface for both our visualization tools.

Both of our visualization tools, Manta and Otter, label the above semantic attributes of the database as generated by mwatch and crawl. The keywords above appear in the menus and color legends associated with various displays as seen in the next section.

Visualization tools

The visualization work we describe in this paper was inspired by limitations inherent in two existing visualization tools, Mapnet and Planet Multicast. Mapnet [MAPN] is a Java applet to visualize the topologies of backbones of major U.S. Internet Service Providers. Planet Multicast [PMUL] uses Virtual Reality Modeling Language (VRML) to produce a three-dimensional view of the Multicast backbone (MBONE).

We have extended Mapnet to visualize the Internet multicast topology. The resulting tool, called MantaRay, displays the geographical placement of MBONE infrastructure, using data from the mwatch utility.

A second tool, Otter, displays topological views of the (same) multicast infrastructure. Otter is derived from Manta and from Plankton [PLANK], the latter tool originally developed to visualize the NLANR Squid Cache Hierarchy.

In both visualization tools we will describe, MantaRay and Otter, nodes represent multicast capable hosts and routers. As mentioned earlier, links represent either native multicast peering, or a tunnel between two multicast nodes through non-multicast speaking nodes. The main difference is that MantaRay is limited to geographic depiction while Otter can support topological layout as well.

MantaRay: geographic depiction

Figure 1: Geographic visualization of a small component of the multicast backbone topology as queried by mwatch/mrinfo (yellow = tunnel, green = native, red = down)

Figure 1 above shows a Manta display of a subset of a particular geographic region of the MBONE (the mid-Atlantic coast of the United States). Not all nodes and links are drawn because we do not have access to geographic coordinate (latitude/longitude) information for all nodes. Unfortunately, accurate latitude/longitude information for Internet hosts and routers is largely unavailable and not easily determined.

We pursued a variety of approaches and heuristics for finding geographical coordinates. Often the closest we could get was the InterNIC database records for the domain name that corresponded to the IP address. These records contain one geographical location per each entire domain (the "headquarters"), which may have no relation to the location of the particular IP address we need to locate on the map. If the IP address does not map to a domain name, one can attempt to map it directly to an autonomous system (AS) via a core BGP (Border Gateway Protocol) routing table and then use the InterNIC-registered headquarters location for that autonomous system. This method involves the same problematic assumption, particularly unacceptable for mapping nodes of national backbone service providers. Nonetheless we derived a basic image, which we can then refine as we gain more accurate location data. Note: To help with such visualization studies, we encourage service providers to put geographical information into their DNS (Domain Name System) configuration as outlined in RFC 1876 [DNSL].

MantaRay features

MantaRay's layout of nodes is strictly geographical, which means we cannot draw infrastructure for which we lack geographical information. MantaRay can manipulate the image with focus and zoom capabilities, change node size and line size to fit the complexity of the image, and also color the nodes and links by different attributes. We illustrate some of these features in the images that follow.

Figure 2: The small window displays information for the white link.

Manta identifies nodes and links in the status text window on the lower right of the Java applet window. When the mouse moves over a link or node, this window contains raw data corresponding to that object in the database, as shown in figure 2.

Zoom and focus on a particular region

Manta can zoom and focus on a particular geographic region. Figure 3 below shows the Washington, DC area shown in Figure 2, at three levels of granularity: full size, zoomed in once (2X), and then zoomed again (4X). Unfortunately, zooming in too far magnifies the background world map image so much as to become almost useless for geographical identification.

Washington, DC is a rich area with respect to network connectivity and there are many nodes and links in close proximity. Manta adjusts the positions of nodes if their geographical information would place them on top of each other. For multiple links between the same two cities, Manta draws distinct parallel lines, which although distorting the geographic representation, provides a more compelling sense of the connectivity. The granularity of the geographic information is often at the level of city rather than exact lat/longs, so there are also many placement collisions in larger cities although the ISP points of presence are likely spread across the city.

Full size (100%)

Zoomed once (50%)

Zoomed twice (25%)

Figure 3: Zoom and focus on the Washington, DC area (yellow = tunnel, green = native, red = down)

Color by link status: down, tunnel, or native

Color is the primary attribute we use to distinguish semantic attributes of the database generated by mrinfo. Figure 4 colors the links according to their "status": down (red), tunnels (yellow), and native multicast (green).

Figure 4: MBONE infrastructure of major U.S. Internet service providers (yellow = tunnel, green = native, red = down)

Color by link attribute: degree, metric, and threshold

Manta offers the ability to color links in the graph according to its corresponding metric or threshold, as well as by the degree of a node (i.e., number of links from it to other nodes). The metric represents the cost of the link from a routing perspective. The threshold represents a ttl (time-to-live) limit: the router will not forward packets whose ttl is less than the threshold value.

A link with a node of degree X at one end and of degree Y at the other end is colored with the color corresponding to the larger of X and Y. Similarly for links with differing thresholds and metrics. A threshold value of 0 is used for native PIM links, since PIM does not use the concept of threshold for forwarding.

There are two coloring modes: spectrum mode and binary mode. Spectrum mode sorts the values into buckets and assigns each bucket a color. Spectrum displays include a color legend. Figure 5 is an example of spectrum coloring.

Figure 5: Color by metric (spectrum)

In contrast to spectrum mode, which uses an array of colors, binary mode uses only two colors. Binary mode uses the truth value of a Boolean statement to determine color: blue for false, yellow for true. The user specifies the desired Boolean expression via a pop-up window. Objects with no value for the specified attribute are ignored. For example, we can color all links with metric less than 10 yellow (true) and all others blue (false). In binary mode, Manta can also hide (mask) all links for whom the Boolean value of the given statement is false.

Figure 6: Threshold greater than 32 (yellow = links with threshold > 32, gray = links with threshold <= 32)

Figure 7: Only nodes/links with degree > 10

Figures 6 and 7 are examples of binary coloring. Note that metrics and thresholds are quantities associated with directional links between nodes. Since we display links unidirectionally, we assign a "true" color to the link if it meets the condition in both directions.

Color by Internet service provider (ISP)

Manta can also draw each Internet service provider's (ISP's) multicast infrastructure in a different color. We currently use an admittedly coarse heuristic to determine who is an ISP: nodes whose domain names end in .net. Links take on a particular ISP label if both ends have the same ISP string in their hostnames, or if one has an ISP hostname and the other is "unknown." If the data contain too many ISPs to comfortably distinguish with a color key, Manta truncates the list and only displays those with the most multicast nodes. On smaller data sets that do "fit" the color spectrum, Manta uses the last color in the spectrum for the category "unknown." Figure 8 below shows the U.S. ISPs with the most multicast nodes and links in the gathered data set, and also the vBNS. [VBNS].

Figure 8: U.S. MBONE infrastructure of selected ISPs

Limitations of Manta

MantaRay's visualizations rely on geographical placement of the nodes, a serious drawback since the latitude/longitude data available are incomplete. In July we ran a complete mwatch traversal of the multicast backbone, with a data file comprised of approximately 10,000 nodes and 25,000 links. Only about 40% of those nodes were in our lat/long database.

To overcome this limitation, we pursued methods for visualizing topology that would not rely on difficult-to-obtain geographic information. Using techniques developed for Plankton [PLANK], CAIDA's tool for visualizing the NLANR Web caching hierarchy, we extended the software platform to support more general topological visualization tasks. We've called the resulting tool Otter. Much of the data in sample Otter graphs are from the output of crawl.

Otter: general-purpose visualization

The feature set of Otter is richer than that of Manta. We will again illustrate with visualizations of the multicast data that allow us to take large ASCII data sets and usefully illustrate multicast topology, broken configurations, tunnel peering relationships, and the extent and rate of native multicast deployment in the commercial Internet.

Otter's graph layout algorithms

Otter's initial layout algorithms derived from those of the Plankton tool for visualizing the NLANR Caching Hierarchy [IRCHE]. Otter places "root" nodes first, where roots and their locations are specified in the data file, and, like any other nodes, subject to user adjustment when running Otter interactively.

There are currently two different methods for arranging root nodes: (1) circular, which places root nodes on the circumference of a circle; and (2) semi-geographical, which places roots based on lat/long information. For semi-geographical placement, Otter finds the maximum and minimum latitude and longitude coordinates of the complete set of root nodes in the data file, and uses these values to determine the four corners of a rectangle, which Otter then scales to occupy half the viewing area. The upper left corner represents the highest latitude and longitude values; the lower right corner the lowest values.

Otter then places the root nodes within the rectangle in their corresponding semi-geographical location. Root nodes that do not have associated lat/long data are placed according to the algorithm used for non-root nodes, described in the next paragraph. Semi-geographical placement can cause clusters of nodes in metropolitan areas to overlap. For viewability, Otter separates these clusters so that nodes are at least the length of their diameter from each other.

After placing the roots, Otter places non-root nodes in semi-circles around their parent node using a breadth-first scan from the roots and guided by the following heuristics:

  1. the more children a node has, the farther away it should be so that the children do not overlap;
  2. children that themselves have lots of children should be even farther away so that the grandchildren do not overlap.

Figure 9 below shows examples of both semi-geographical and topological layout for mrinfo data on BBNplanet's multicast infrastructure in July 1998.

semi-geographical layout

topological layout

Figure 9: Otter placement algorithm (bbnplanet.net nodes as reported by mrinfo, July 1998)

In these Otter visualizations of multicast topology, red nodes are "roots," and represent the starting set for the crawl run that produced the data set. We typically select as root nodes known multicast routers of large ISP domains. White nodes are also multicast-capable routers, but not in the root set. Links represent Internet paths over which multicast traffic may flow, either as native multicast (with both endpoints participating) or as encapsulated tunneled traffic. Intermediate routers that do not understand multicast are not shown.

Otter has the same display focus, zoom, and manipulation capabilities illustrated for Manta earlier. One can often improve the layout of the autoplacement algorithms with manual adjustment. Figure 10 shows a particular layout before and after hand tuning. The root nodes are from the domain sdsc.edu, the San Diego Supercomputing Center. Crawl generated the data using a depth parameter of three; Otter is using topological placement.



Figure 10: SDSC multicast infrastructure, before and after hand-tuned placement (July 1998)

Like Manta, Otter identifies raw database attributes of nodes and links in the status text field on the lower right of the Java applet window, according to which link or node the mouse sits on.

One can also label or color nodes by domain name, as shown in Figures 11a and 11b.

Figure 11a: SDSC labeled with host names (July 1998)

Figure 11b: SDSC colored by second level domain (July 1998)

Figures 12a-d illustrate how Otter uses color to display specific attributes of multicast infrastructure. In each case, the graph displayed is the SDSC multicast network drawn with the circular placement algorithm.

Coloring by tunnel status shows the distribution of DVMRP, PIM, and broken tunnels. DVMRP peers are labeled "tunnel," PIM peers "native," and others either "unidirectional," meaning a DVMRP host hidden by a Cisco router, or "down," meaning that one end of the tunnel is not configured correctly or the mrouter is actually not reachable or not running a multicast routing daemon.

Figure 12a: Color by tunnel status (sdsc.edu, July 1998)

Coloring by multicast version shows which version of either mrouted or IOS (International Organization for Standardization). This coloring also provides some indication of the transition to native multicast: values such as 11.2 are Cisco IOS version numbers (which support native multicast via PIM), while 3.8 refers to mrouted and 3.255 to the DVMRP protocol.

Figure 12b: Color by multicast version (sdsc.edu, July 1998)

A tunnel metric represents the routing "cost" to send packets over that link. It is typically not used unless a host has multiple tunnels via providers and wants to load them differently. Most sites just set the value to 1. Metric can be used to provide redundancy via backup tunnels that take over if the primary tunnel with lower metric is down. The values for metric and threshold are typically negotiated by the system administrators of the tunnel endpoints when they establish the tunnel.

Figure 12c: Color by tunnel metric (sdsc.edu, July 1998)

Coloring by threshold can yield insight into scoping across sites. Packets will not be forwarded over a link with threshold X, unless they have a ttl (time to live) value greater than X. Many sites try to use the threshold parameter to limit, or administratively "scope," the propagation of their multicast sessions (e.g., internal video conferencing) so they do not escape to the global Internet.

Figure 12d: Color by tunnel threshold (sdsc.edu, July 1998)

Otter's utility for ISPs

The ability to display such multicast link parameters and network topology can help network engineers find configuration or design problems. By looking at tunnel status, multicast version, and multicast protocol over time, one can also gauge the rate of change toward native multicast in the infrastructure.

Internet Service Providers multicast infrastructure

Otter has some limitations in visualizing large data sets: Java is still rather slow and suboptimally stable. However Otter does support the manipulation of interactive visual depictions of smaller topologies. Crawl can gather a restricted dataset and use Otter to visualize multicast topology, connectivity, and design.

ISPs might also want to use Otter to verify that their multicast configuration as seen by mrinfo is sensible, and does not contain loops or other quirks that might generate duplicate packets. In collecting these data, we noticed that parts of many multicast infrastructures of ISPs do not respond to our queries, and thus appear in Otter as disconnected or partitioned. There are a variety of possible reasons: firewalls blocking our probes, routing, or misconfiguration.

We have run our crawl script on several national ISP's multicast infrastructure, including that of MCI for a case study we present below. Appendix A contains several other visualizations of various ISP's multicast infrastructure.

Partitioning: a case study

When we first examined Internet MCI's (sold to Cable & Wireless in 1998) multicast infrastructure, using data either from the mwatch database or from a crawl run, Otter displayed MCI's MBONE connectivity as six separate disconnected subgraphs. We knew that the nature of multicast requires a connected distribution graph and concluded that our mrinfo data must be incomplete. We suspected that firewalls were blocking our queries to mrouters. However, correspondence with MCI engineers clarified that the MCI mrouters did not have a default unicast route, only a few specific routes within their internal infrastructure.

This security-related configuration aspect of their backbone routers allowed mrinfo queries to reach MCI mrouters but did not allow responses from those mrouters to get back to our probe machines. After MCI added to those mrouters a route to reach our probe host, crawl was able to find, and Otter thus able to draw, the fully connected MCI multicast infrastructure. Figure 12 shows the different partitions that crawl initially found in MCI's multicast network.

Figure 12: Partitions of MCI as seen by mwatch, 12 May 1998

We leveraged an Otter feature intended for something else to determine the actual number of connected components. Specifically, Otter allows the user to specify which nodes to use as "root nodes" by entering a search string and then rendering as root nodes all nodes with that string in their name. If no matches occur, Otter will by default randomly use a single node from each connected component of the graph as the set of root nodes. Typing in a nonsense string causes Otter's search to fail, so it will use the default algorithm instead. This mechanism found the disconnected components in the mci.net multicast infrastructure when visual inspection had failed to discern them. In the next version of Otter, identifying and labeling connected components will be a standard feature.

Figure 13: The MCI multicast network differences, 26 July

Figure 13 shows the correct connected MCI multicast network after the routing information had been changed to allow answers to our queries to reach us, notably larger than that seen in Figure 12 before the routing change.


Manta's geographical depiction of the multicast infrastructure overlaid on the familiar United States or world map allows network engineers to verify connection and link status in an intuitive way. However, the difficulty of obtaining accurate latitude and longitude data for each node severely limits its usefulness and extensibility. Our mechanisms for resolving lat/long information: ferreting it out of sources like the ARIN or InterNIC databases for each domain location, friends who work at the institution in question, knowledge of the host naming scheme, or other ad hoc methods, have suboptimal scaling properties to say the least. And many network topologies are so densely populated that geographic depiction often leaves many nodes so close together as to detract from visability of the graph.

In response to these limitations, we developed Otter to support topological and semi-geographical placement algorithms, allowing the network engineer to view his infrastructure at a more abstract level. Note that this method can render the proximity of nodes in the graph misleading, since there is no relation to physical distance.

A major limitation of both tools is the mwatch/mrinfo data itself. Firewalls and routing policies that block the mrinfo queries and/or answers leave holes in the data that we can collect. Thus any visualization of the multicast infrastructure based on this data collection method is just a snapshot of the mrouters reachable at a fixed point in time from our query machine.

Also, the two major multicast routing protocols still do not always interoperate well together. A PIM-speaking router connected with a tunnel to a DVMRP mrouter with a direct connection on the local network will not acknowledge to mrinfo that the connection exists. The tunnel will thus appear in the database as unidirectional, with the DVMRP side announcing it and the PIM side denying it. Unless there is an alternative path to that DVMRP mrouter, it will not appear in the data set collected.

Despite the limitations, we hope that these tools will allow ISPs and organizations deploying multicast infrastructure to easily visualize its topology and aspects of its configuration. Such tools can facilitate timely detection and correction of misconfiguration or poor design decisions.

Future directions

In the future we hope to use Manta and Otter to track evolution of the multicast infrastructure over time, in particular the trends as we move from DVMRP to PIM or other multicast routing protocols, or to more recent versions of router software. Actually we hope to evolve Otter to embody all the functionality of Manta so that we would not have to support both tools. The main requirement is to integrate into Otter the strong geographical binding of Manta's implementation, which allows more optimized geographic depiction.

One obstacle with continued MBONE visualization research is that the mwatch caretakers have stopped ongoing operational maintenance and execution of the code. We will need to find or provide support for continuous operation of either it or another scalable crawl-like tool. We would also like to support animation capabilities to allow organizations to track deployment of their own multicast infrastructure. Finally, we would be grateful for the opportunity to run crawl from multiple sites at the same time to see how the location of the querying machine impacts the data collected.


Both MantaRay and Otter are freely available. We encourage network engineers responsible for an ISP's multicast infrastructure to play with them and provide feedback on their usefulness and suggestions for new features that would be helpful in managing a robust multi-provider multicast infrastructure.



Source Code








We would like to thank Bill Fenner of Xerox PARC for his help in interpreting the mrinfo output and for explaining DVMRP/PIM intricacies.

Piete Brooks from the University of Cambridge clarified the history of mwatch for us and gave us access to its database. Thanks, Piete.

Nancy Bachman rescued us once again with her helpful comments and edits.

We extend special thanks to Doug Pasko and Jeff Young, then iMCI engineers who helped us solve the disconnected graph problem and added our measurement probes to their routing tables.

Note: This paper uses a lot of color images for explanation and examples. Color printout strongly recommended. Home URL: http://www.caida.org/Tools/Manta/Paper/


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Collection of Multicast information, http://www.cl.cam.ac.uk/mbone/#Mrouted
Java Application for viewing various Network Data, http://www.caida.org/Tools/Otter
D. Estrin, D. Farinacci, A. Helmy, D. Thaler, S. Deering, M. Handley, V. Jacobson, C. Liu, P. Sharma, and L. Wei, RFC 2117: Protocol Independent Multicast-Sparse Mode (PIM-SM): Protocol Specification. June 1997.
T. Munzner, K. Claffy, B. Fenner, E. Hoffman, "Planet Multicast: visualization of the Mbone," http://oceana.nlanr.net/PlanetMulticast, NLANR, Oct-30-1996.
B. Huffaker, J. Jung, D. Wessels, K. Claffy, "Visualization of the Growth and Topology of the NLANR Caching Hierarchy," http://www.caida.org/Tools/Plankton/Paper/plankton.html, CAIDA, Mar-30-1998.
Very high performance Backbone Network Service, http://www.vbns.net

Appendix A: Otter visualizations of various ISPs' multicast infrastructure

We provide several examples of typical Manta and Otter displays that would be easy for network engineers to customize for their own network. We have also included the domain cisco.com because it has an extensive multicast infrastructure. These graphs illustrate different Otter features and are in no particular order.

Dataset: ans.net, crawl depth 2
Placement: topological with hand tuning
Color: by threshold

Dataset: bbnplanet.net, crawl depth 2
Placement: cluster experiment - nodes are clustered by IP address
Color: by tunnel type

Dataset: cerfnet.net, crawl depth 2
Placement: topological
Color: by threshold (with line size increased)

Dataset: cisco.com, crawl depth 2
Placement: topological
Color: by multicast protocol

Dataset: mci.net, crawl depth 2
Placement: semi-geographical
Color: by threshold symmetry between directions

Dataset: sprintlink.net, crawl depth 2
Placement: semi-geographic
Color: protocol-type-(ip match between nodes)

Dataset: uu.net, crawl depth 2
Placement: semi-geographical
Color: by threshold

Dataset: vbns.net, depth 2
Placement: semi-geographical
Color: by multicast protocol

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