Network Working Group
Request for Comments: 2121
Category: Informational
G. Armitage
March 1997

Issues affecting MARS Cluster Size

Status 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.


IP multicast over ATM currently uses the MARS model [1] to manage the use of ATM pt-mpt SVCs for IP multicast packet forwarding. The scope of any given MARS services is the MARS Cluster - typically the same as an IPv4 Logical IP Subnet (LIS). Current IP/ATM networks are usually architected with unicast routing and forwarding issues dictating the sizes of individual LISes. However, as IP multicast is deployed as a service, the size of a LIS will only be as big as a MARS Cluster can be. This document provides a qualitative look at the issues constraining a MARS Cluster's size, including the impact of VC limits in switches and NICs, geographical distribution of cluster members, and the use of VC Mesh or MCS modes to support multicast groups.

1. Introduction

   A MARS Cluster is the set of IP/ATM interfaces that are willing to
   engage in direct, ATM level pt-mpt SVCs to perform IP multicast
   packet forwarding [1]. Each IP/ATM interface (a MARS Client) must
   keep state information regarding the ATM addresses of each leaf node
   (recipient) of each  pt-mpt SVC it has open. In  addition, each MARS
   Client receives MARS_JOIN and MARS_LEAVE messages from the MARS
   whenever there is a requirement that Clients around the Cluster need
   to update their pt-mpt SVCs for a given IP multicast group.
   The definition of Cluster 'size' can mean two things - the number of
   MARS Clients using a given MARS, and the geographic distribution of
   MARS Clients. The number of MARS Clients in a Cluster impacts on the
   amount of state information any given client may need to store while
   managing  outgoing  pt- mpt SVCs. It also impacts on the average rate
   of JOIN/LEAVE traffic that is propagated by the MARS on
   ClusterControlVC, and the number of pt-mpt VCs that may need
   modification each time a MARS_JOIN or MARS_LEAVE appears on

The geographic distribution of clients affects the latency between a client issuing a MARS_JOIN, and it finally being added onto the pt- mpt VCs of the other MARS Clients transmitting to the specified multicast group. (This latency is made up of both the time to propagate the MARS_JOIN, and the delay in the underlying ATM cloud's reaction to the subsequent ADD_PARTY messages.)

When architecting an IP/ATM network it is important to understand the worst case scaling limits applicable to your Clusters. This document provides a primarily qualitative look at the design choices that impose the most dramatic constraints on Cluster size. Since the focus is on worst-case scenarios, most of the analysis will assume multicast groups that are VC Mesh based and have all cluster members as sources and receivers. Engineering using the worst-case boundary conditions, then applying optimisations such as Multicast Servers (MCS), provides the Cluster with a margin of safety. It is hoped that more detailed quantitative analysis of Cluster sizing limits will be prompted by this document.

Section 2 comments on the VC state requirements of the MARS model, while Sections 3 and 4 identify the group change processing load and latency characteristics of a cluster as a function of its size. Section 5 looks at how Multicast Routers (both conventional and combination router/switch architectures) increase the scale of a multicast capable IP/ATM network. Finally, Section 6 discusses how the use of Multicast Servers (MCS) might impact on the worst case Cluster size limits.

2. VC state limitations.

Two characteristics of ATM NICs and switches will limit the number of members a Cluster may contain. They are:

The maximum number of VCs that can be originated from, or terminate on, a port (VCmax).

The maximum number of leaf nodes supportable by a root node (LEAFmax).

We'll assume that the MARS node has similar VCmax and LEAFmax values as Cluster members. VCmax affects the Cluster size because of the following:

The MARS terminates a pt-pt control VC from each cluster member, and originates a VC for ClusterControlVC and ServerControlVC.

When a multicast group is VC Mesh based, a group member terminates a VC from every sender to the group, per group.

When a multicast group is MCS based, the MCS terminates a VC from every sender to the group.

LEAFmax affects the Cluster size because of the following:

ClusterControlVC from the MARS. It has a leaf node per cluster member (MARS Client).

Packet forwarding SVCs out of each MARS Client for each IP multicast group being sent to. It has a leaf node for each group member when a group is VC Mesh based.

Packet forwarding SVCs out of each MCS for each IP multicast group being sent to. It has a leaf node for each group member when a group is MCS based.

If we have N cluster members, and M multicast groups active (using VC Mesh mode, and densely populated - all receivers are senders), the following observations may be made:

ClusterControlVC has N leaf nodes, so

N <= LEAFmax.

The MARS terminates a pt-pt VC from each cluster member, and originates ClusterControlVC and ServerControlVC, so

(N+2) <= VCmax.

Each Cluster Member sources 1 VC per group, terminates (N-1) VC per group, originates a pt-pt VC to the MARS, and terminates 1 VC as a leaf on ClusterControlVC, so

(M*N) + 2 <= VCmax.

The VC sourced by each Cluster member per group goes to all other cluster members, so

(N-1) <= LEAFmax.

Since all the above conditions must be simultaneously true, we can see that the most constraining requirement is either:

(M*N) + 2 <= VCmax.


N <= LEAFmax.

The limit involving VCmax is fundamentally controlled by the VC consumption of group members using a VC Mesh for data forwarding, rather than the termination of pt-pt control VCs on the MARS. (It is in practice going to be very dependent on the multicast group membership distributions within the cluster.)

The LEAFmax limit comes from ClusterControlVC, and is independent of the density of group members (or the ratios of senders to receivers) for active multicast groups within the cluster.

Under UNI 3.0/3.1 the most obvious limit on LEAFmax is 2^15 (the leaf node ID is 15 bits wide). However, the signaling driver software for most ATM NICs may impose a limit much lower than this - a function of how much per-leaf node state information they need to store (and are capable of storing) for pt-mpt SVCs.

VCmax is constrained by the ATM NIC hardware (for available segmentation or reassembly instances), or by the VC capacity of the switch port that the NIC is attached to. VCmax will be the smaller of the two.

A MARS Client may impose its own state storage limitations, such that the combined memory consumption of a MARS Client and the ATM NIC's driver in a given host limits both LEAFmax and VCmax to values lower than the ATM NIC alone might have been able to support.

It may be possible to work around LEAFmax limits by distributing the leaf nodes across multiple pt-mpt SVCs operating in parallel. However, such an approach requires further study, and doesn't solve the VCmax limitation associated with a node terminating too many VCs.

A related observation can also be made that the number of MARS Clients in a Cluster may be limited by the memory constraints of the MARS itself. It is required to keep state on all the groups that every one of its MARS Clients have joined. For a given memory limit, the maximum number of MARS Clients must drop if the average number of groups joined per Client rises. Depending on the level of group memberships, this limitation may be more severe than LEAFmax.

3. Signaling load.

In any given cluster there will be an 'ambient' level of MARS_JOIN/LEAVE activity. The dynamic characteristics of this activity will depend on the types of multicast applications running within the cluster. For a constant relative distribution of multicast applications we can assume that, as the number of MARS Clients in a given cluster rises, so does the ambient level of MARS_JOIN/LEAVE activity. This increases the average frequency with which the MARS processes and propagates MARS_JOIN/LEAVE messages.

The existence of MARS_JOIN/LEAVE traffic also has a consequential impact on signaling activity at the ATM level (across the UNI and {P}NNI boundaries). For groups that are VC Mesh supported, each MARS_JOIN or MARS_LEAVE propagated on ClusterControlVC will result in an ADD_PARTY or DROP_PARTY message sent across the UNIs of all MARS Clients that are transmitting to a given group. As a cluster's membership increases, so does the average number of MARS Clients that trigger ATM signaling activity in response to MARS_JOIN/LEAVEs.

The size of a cluster needs to be chosen to provide some level of containment to this ambient level of MARS and UNI/NNI signaling.

Some refinements to the MARS Client behaviour may also be explored to smooth out UNI signaling transients. MARS Clients are currently required to initiate revalidation of group memberships only when the Client next sends a packet to an invalidated group SVC. A Client could apply a similar algorithm to decide when it should issue ADD_PARTYs. For example, after seeing a MARS_JOIN, wait until it actually has a packet to send, send the packet, then initiate the ADD_PARTY. As a result actively transmitting Clients would update their SVCs sooner than intermittently transmitting Clients.

4. Group change latencies

The group change latency can be defined as the time it takes for all the senders to a group to have correctly updated their forwarding SVCs after a MARS_JOIN or MARS_LEAVE is received from the MARS. This is affected by both the number of Cluster members and the geographical distribution of Cluster members. (Groups that are MCS based create the lowest impact when new members join or leave, since only the MCS needs to update its forwarding SVC.) Under some circumstances, especially modelling or simulation environments, group change latencies within a cluster may be an important characteristic to control.

As noted in the previous section, the ADD_PARTY/DROP_PARTY signaling load created by membership changes in VC Mesh based groups goes up as the number of cluster members rises (assuming worst case scenario of each cluster member being a sender to the group). As the UNI load rises, the ATM network itself may start delivering slower processing of the requested events.

Wide geographic distribution of Cluster members also delays the propagation of MARS_JOIN/LEAVE and ATM UNI/NNI messages. The further apart various members are, the longer it takes for them to receive MARS_JOIN/LEAVE traffic on ClusterControlVC, and the longer it takes for the ATM network to react to ADD_PARTY and DROP_PARTY requests. If the long distance paths are populated by many ATM switches, propagation delays due to per-switch processing will add substantially to delays due to the speed of light.

(Unfortunately, mechanisms for smoothing out the transient ATM signaling load described in section 3 have a consequence of increasing the group change latency, since the goal is for some of the senders to deliberately delay updating their forwarding SVCs. This is an area where the system architect needs to make a situation-specific trade-off.)

It is not clear what affect the internal processing of the MARS itself has on group change latency, and how this might be impacted by cluster size. A component of the MARS processing latency will depend on the specific database implementation and search algorithms as much as on the number of group members for the group being modified at any instant. Since the maximum number of group members for a given group is equal to the number of cluster members, there will be an indirect (even if small) relationship between worst case MARS processing latencies and cluster size.

5. Large IP/ATM networks using Mrouters

Building a large scale, multicast capable IP over ATM network is a tradeoff between Cluster sizes and numbers of Mrouters. For a given number of hosts, the number of clusters goes up as individual clusters shrink. Since Mrouters are the topological intersections between clusters, the number of Mrouters rises as the size of individual clusters shrinks. (The actual number of Mrouters depends largely on the logical IP topology you choose to implement, since a single physical Mrouter may interconnect more than two Clusters at once.) It is a local deployment question as to what the optimal mix of Clusters and Mrouters will be.

Currently two broad classes of Mrouters may be identified:

Those that originate unique VCs into target Clusters, and forward/interleave data at the IP packet level (the Conventional Mrouter).

Those that originate unique VCs into target Clusters, but create internal, cell level 'cut through' paths between VCs from different Clusters (e.g. the Cell Switch Router).

How these Mrouters establish and manage the associations of VCs to IP traffic flows is beyond the scope of this document. However, it is worth looking briefly at their impact on VC consumption and ATM signaling load.

5.1 Impact of the Conventional Mrouter

A conventional Mrouter acts as an aggregation point for both signaling and data plane loads. It hides host specific group membership changes in one cluster from senders within other clusters, and protects group members (receivers) in one cluster from having to be leaf nodes on SVCs from senders in other Clusters.

When acting as an ingress point into a cluster, a conventional Mrouter establishes a single forwarding SVC for IP packets. This single SVC carries data from other clusters interleaved at the IP packet level. Only this single SVC needs to be modified in response to group memberships changes within the target cluster. As a consequence, there is no need for sources in other clusters to be aware of, or react to, MARS_JOIN/LEAVE traffic in the target cluster. (The consequential UNI signaling load identified in section 3 is also localized within the target Cluster.)

MARS Clients within the target cluster also benefit from this data path aggregation because they terminate only one SVC from the Mrouter (per group), rather than multiple SVCs originating from actual senders in other Clusters.

Conventional Mrouters help control the limiting factors described in sections 2, 3, and 4. A hypothetical 10000 node Cluster could be broken into two 5000 node Clusters, or four 2500 node Clusters, etc, to reduce VC consumption. Or you might have 200 nodes of the overall 10000 that are known to join and leave groups rapidly, whilst the other 9800 are fairly steady - so you deploy clusters of 200, 2500, 2500, 2500, 2300 hosts respectively.

5.2. Impact of the Cell Switch Router (CSR).

Another class of Mrouter, the Cell Switch Router (CSR) attempts to utilize IP level flow information to dynamically manage the switching of data through the device below the IP level. Once the CSR has identified a flow of IP traffic, and associated it with an inbound and outbound SVC, it begins to function as an ATM cell level device rather than a packet level device.

Even when operating in this mode the CSR isolates attached Clusters from each other's MARS_JOIN/LEAVE activities, in the same manner as a conventional Mrouter. This occurs because the CSR manages its forwarding SVCs just like a normal MARS Client - responding to MARS_JOIN/LEAVE messages within the target cluster by updating the pt-mpt trees rooted on its own ATM ports.

However, since AAL5 AAL_SDUs cannot be interleaved at the cell level on a single SVC, a CSR cannot simultaneously perform cell level cut- through and aggregate the IP packet flows from multiple senders onto a single SVC into a target Cluster. As a result, the CSR must construct a separate forwarding SVC into a target cluster for each SVC it is a leaf of in a source Cluster (to to ensure that cells from individual sources are not interleaved prior to reaching the re- assembly engines of the group members in the target cluster).

Interestingly, the UNI signaling load offered within the target Cluster by the CSR is potentially greater than that of a conventional Mrouter. If there are N senders in the source Cluster, the CSR will have built N identical pt-mpt SVCs out to the group members within the target Cluster. If a new MARS_JOIN is issued within the target Cluster, the CSR must issue N ADD_PARTYs to update the N SVCs into the target Cluster. (Under similar circumstances a conventional Mrouter would have issued only one ADD_PARTY for its single SVC into the target Cluster.)

Thus, without the ability to provide internal cut-through forwarding with AAL_SDU boundaries intact, the CSR only provides for the isolation of MARS_JOIN/LEAVE traffic within clusters. It cannot provide the data path aggregation of a conventional Mrouter.

6. The impact of Multicast Servers (MCSs)

Since the focus of this document is on worst-case scenarios, most of the analysis has assumed multicast groups that are VC Mesh based and have all cluster members as sources and receivers. The impact of using an MCS to support a multicast group can be dramatic in the context of the group's resource consumption, but less so in the over-all context of cluster size limits.

The intra-cluster, per group impact of an MCS is somewhat analogous to the inter-cluster impact of a conventional Mrouter. The MCS aggregates the data flows (only 1 SVC terminates on each group member, independent of the number of senders), and isolates MARS_JOIN/LEAVE traffic (which is shifted to ServerControlVC rather than ClusterControlVC). The resulting UNI signaling traffic and load is reduced too, as only the forwarding SVC out of the MCS needs to be modified for every membership change in the MCS supported group.

Deploying a mixture of MCS and VC Mesh based groups will certainly improve resource utilization. However, the actual extent of the improvements (and consequently how large the cluster can be made) will depend greatly on the dynamics of your typical applications and which characteristics from sections 2, 3, and 4 are your primary limitations.

For example, if VCmax or LEAFmax (section 2) are primary limitations, one must keep in mind that each MCS itself suffers the same NIC limits as the MARS and MARS Clients. Even though using an MCS dramatically reduces the number of VCs per MARS Client per group, each MCS still needs to terminate 1 SVC per sender - potentially up to 1 SVC from each Cluster member. (This may become 1 SVC per member per group if the MCS supports multiple groups simultaneously.)

Assume we have a Cluster where every group is MCS based, each MCS supports only one group, and both VCmax and LEAFmax apply equally to MCS nodes as MARS and MARS Clients nodes. If we have N cluster members, M groups, and all receivers are senders for a given MCS supported group, the following observations may be made:

Each MCS forwarding SVC has N leaf nodes, so

N <= LEAFmax.

Each MCS terminates an SVC from N senders, originates 1 SVC forwarding path, originates a pt-pt control SVC to the MARS, and terminates 1 SVC as a leaf on ServerControlVC, so

N + 3 <= VCmax.

MARS ClusterControlVC has N leaf nodes, so

N <= LEAFmax.

MARS ServerControlVC has M leaf nodes, so

M <= LEAFmax.

The MARS terminates a pt-pt VC from each cluster member, a pt-pt VC from each MCS, originates ClusterControlVC, and originates ServerControlVC, so

N + M + 2 <= VCmax.

Each Cluster Member sources 1 VC per group, terminates 1 VC per group, originates a pt-pt VC to the MARS, and terminates 1 VC as a leaf on ClusterControlVC, so

2*M + 2 <= VCmax.

Since all the above conditions must be simultaneously true, we can see that the most constraining requirements are:

      N + M + 2 <= VCmax (if M <= N)
      2*M + 2 <= VCmax (if M >= N)
      N <= LEAFmax.

(Assuming that in general M+2 > 3, so the VCmax constraint at each MCS is not a limiting factor.)

We can get a feel for the relative impacts of VC Mesh groups vs MCS based groups by considering a cluster where M1 represents the number of VC Mesh based groups, and M2 represents the number of MCS based groups. Again we assume worst case group density (all N cluster members are group members, all receivers are also senders).

As noted in section 2, the VCmax constraint in VC Mesh mode comes from each MARS Client, and is:

      N*M1 <= VCmax - 2

For the MCS case we have two scenarios, M2 <= N and M2 >= N.

If M2 <= N we can see the VC consumption by VC Mesh based groups will become the applicable constraint on cluster size N when:

      N + M2 <= N*M1
      M1 >= 1 + (M2/N)

Thus, if there is more than 1 VC Mesh based group, and less MCS based groups than cluster members (M2 < N), the constraint on cluster size is dictated by the VC Mesh characteristics: N*M1 <= VCmax - 2. (If M2 == N, then there may be 2 VC Mesh based groups before the VC Mesh characteristics are the dictating factor.)

Now, if M2 > N (more MCS based groups, and hence MCSes, than cluster members) the calculation is more complex since in this case VCmax at the MARS Client is the limiting parameter for both VC Mesh and MCS cases. The limit becomes:

      N*M1 + 2*M2 <= VCmax - 2

However, on face value this is an odd situation anyway, since it implies more MCS entities than hosts or router interfaces into the cluster (given the assumption of one group per MCS).

The impact of MCS entities that simultaneously support multiple groups is left for future study.

7. Open Issues

There is a wide range of qualitative analysis that can be extracted from typical MARS deployment scenarios. This document does not attempt to develop any numerical models for VC consumptions, end to end latencies, etc.

8. Conclusion

This document has provided a high level, qualitative overview of the parameters affecting the size of MARS Clusters. Limitations on the number of leaf nodes a pt-mpt SVC may support, sizes of the MARS database, propagation delays of MARS and UNI messages, and the frequency of MARS and UNI control messages are all identified as issues that will constrain Clusters. Conventional Mrouters are identified as useful aggregators of IP multicast traffic and signaling information. Cell Switch Routers are noted to offer only some of the aggregation attributes of conventional Mrouters. Large scale IP multicasting over ATM requires a combination of Mrouters and appropriately sized MARS Clusters. Finally, it has been shown that in a simple cluster where there are less MCS based groups than cluster members, two or more VC Mesh based groups are sufficient to render the use of Multicast Servers irrelevant to the worst case cluster size limit.

Security Considerations

Security issues are not discussed in this memo.


Thanks must go to Rajesh Talpade (Georgia Tech) for specific input on aspects of the VC Mesh vs MCS tradeoffs, and Joel Halpern (Newbridge) for general input on the document's focus.

Author's Address

Grenville Armitage
Bellcore, 445 South Street
Morristown, NJ, 07960

   Phone +1 201 829 2635


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