Core-Based Trees

A historic shared-tree IP-multicast routing architecture in which group receivers join toward a configured core router and traffic follows the resulting bidirectional distribution tree.

Version

v3 · 2026-09-19 · History

Domain-specific #

1564

Domain group

Applied Sciences & Engineering

Origin domain

Computer Science & Software Engineering

Subdomain

Computer Networking → Computer Science & Software Engineering

Aliases

CBT, CBTv2, Core Based Tree multicast

Core Idea

Core-Based Trees (CBT) is a historic IP-multicast routing architecture that builds one shared bidirectional distribution tree per multicast group around a configured core router. A receiver-side router sends a join toward the core; routers along the path create group state and the branch attaches when it reaches the existing tree. Senders can inject traffic at an on-tree router, and packets follow the shared tree to receivers.

The recognition invariant is multicast group + selected core/rendezvous router + receiver-initiated joins + shared bidirectional tree + per-group on-tree state + protocol maintenance. RFC 2189 was Experimental and is now Historic; retention describes an important design identity, not a recommendation for contemporary deployment.

How would you explain it like I'm…

One Shared Message Tree

Imagine sending one message to lots of friends at once. Everyone agrees on one central meeting spot. Friends who want the messages make a path to that spot, and all the paths join into one big shared tree. Messages travel along the tree's branches to everyone who joined.

One Shared Group Tree

Core-Based Trees is an old way of sending internet messages to a whole group at once, called multicast. For each group, one special router is picked as the 'core,' like the trunk of a tree. When someone wants to receive the group's messages, their nearby router sends a 'join' message toward the core, and each router along the way remembers that it's part of the tree. The new branch stops as soon as it reaches a part of the tree that already exists. Anyone sending to the group puts the message onto the tree, and it flows along the branches in both directions to all the receivers. It's no longer recommended for use today but was an important design idea.

Shared Bidirectional Multicast Tree

Core-Based Trees (CBT) is a historic routing architecture for IP multicast, where one packet is delivered to every member of a group. Instead of building a separate tree from each sender, CBT builds one shared, bidirectional tree per group, centered on a configured core router. Receivers' routers send join requests toward the core; each router on the path records state for the group, and the new branch attaches when it hits the existing tree. A sender injects traffic at a router on the tree, and packets travel along the shared tree in both directions to reach receivers. Routers must keep per-group state and run protocol maintenance to keep the tree intact. The specification, RFC 2189, was Experimental and is now classified as Historic, so CBT matters as a design model rather than something to deploy now.

 

Core-Based Trees (CBT) is a historic IP-multicast routing architecture that constructs a single shared, bidirectional distribution tree per multicast group, rooted around a configured core (rendezvous) router. Tree construction is receiver-initiated: a receiver-side router sends a join toward the core, routers along the path instantiate per-group forwarding state, and the branch is grafted once the join reaches a router already on the tree. Senders inject packets at an on-tree router and traffic flows along the shared tree toward all receivers in both directions, rather than along per-source shortest-path trees. Its recognition invariant is the combination of a multicast group, a selected core router, receiver-initiated joins, a shared bidirectional tree, per-group on-tree state, and protocol maintenance of that tree. RFC 2189 specified it as Experimental and it is now Historic; its significance is as a design identity for shared-tree multicast, not a recommendation for contemporary deployment.

Scope of Application

CBT addressed scalable inter-domain or wide-area many-to-many multicast where source-specific state would grow poorly. Its shared tree made router state depend more on groups than on source–group pairs, at the cost of core dependence and paths that could be longer than source-rooted alternatives.

The architecture remains useful in protocol history, comparison of multicast tree designs, rendezvous placement research, routing-state analysis, and failure/security reasoning.

Clarity

The “core” is logically central to tree construction but need not be topologically central or aware that it was configured as a core. Version 2 specifies a single active core per group tree, even though broader descriptions sometimes speak loosely of multiple cores or candidate cores.

Manages Complexity

The shared tree compresses many senders’ delivery state into one group structure. Routers need not maintain a separate shortest-path tree for every source, and receivers can attach by following existing unicast reachability toward a common rendezvous.

The compression transfers complexity to core selection, path stretch, single-tree failure consequences, bootstrap, and group-wide control consistency.

Abstract Reasoning

1. Define group, candidate core, active core, and administrative scope.
2. Trace a receiver’s join toward the core using unicast routing.
3. Mark where the join first intersects the existing tree.
4. Construct parent/child interface state along the new branch.
5. Trace sender traffic entering and traversing the tree.
6. Test duplicate suppression, loop prevention, and reverse-path conditions.
7. Analyze core, link, and router failure and state repair.
8. Compare state scaling and path stretch against source-rooted alternatives.
9. Preserve protocol-version and Historic-status boundaries.

Knowledge Transfer

The portable structure is rendezvous-centered shared traversal: many participants attach toward a common meeting point, and the union of attachment paths becomes the reusable delivery structure. The proposed immediate parent is Network Traversal.

Relationships to Other Abstractions

Current abstraction Core-Based Trees Domain-specific

Parents (1) — outgoing typed relations

• Core-Based Trees is a kind of Network Traversal Prime

  Network Traversal is the broader abstraction this entry instantiates.

Hierarchy path (1) — routes to 1 parentless root

• Core-Based Trees → is a kind of Network Traversal → presupposes Network

Neighborhood in Abstraction Space

Core-Based Trees sits in a sparse region of the domain-specific corpus (99^th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Unclustered & Miscellaneous (2769 abstractions)

Nearest neighbors

• Skew Heap — 0.75
• Graph-Structured Stack — 0.75
• Adversarial Queueing Network — 0.75
• Node graph architecture — 0.74
• Fallacy of Stable Topology — 0.74

Computed from structural-signature embeddings · 2026-10-25