Paper Review: IP Multicast Channels: EXPRESS Support for Large-scale
Reviewer: Mark Meras (mm446)
This paper by Holbrook and Cheriton proposes a new extension to the IP
multicast model called EXPRESS. EXPRESS creates a channel model for
delivering large amounts of data from one source to many subscribers.
- IP multicast is currently based on a group model of communication in
which a set of hosts are grouped together into a single address. This
approach has several problems:
- It violates the current ISP billing models
- It provides no indication of group size
- It does not provide a mechanism to restrict membership
- The group model allocates world-wide unique addresses, of which there
are only 256 million
- It does not scale well
- A better approach can be used, the so-called EXPRESS (EXPlicity
REquested Single-Source) model. The EXPRESS model provides for channels
that allow a pair (S,E) to be subscribed to by many subscribers, where S
is the source (host) of the channel. Thus, the EXPRESS model capitalizes
on the one-to-many nature of most IP multicast applications.
- A new protocol, ECMP, is proposed to allow for subscriptions and
counting. Reverse-path forwarding is used to keep the dynamic tree for IP
multicast. ECMP sets up the Forwarding Information Base (FIB) entries at
the routers that are used to look up the (S,E) pairs for multicast
- A session relay approach can be used for almost single source
multicasting, such as distance learning. In this application, one host is
the relay to which everyone is subscribed. To send a message to all
subscribers, a host simply sends it to the relay to be forwarded. Session
relay is very easy to implement using the EXPRESS model.
Critique of Contribution
The new EXPRESS model seems to be a very important contribution to the
field of IP multicasting. It uses an existing infrastructure (IP
multicasting over TCP or UDP), requiring only a small-bandwidth protocol
to keep track of the subscribers. It addresses the world-wide uniqueness
problem of group-based IP multicast, and closely mimics the dynamics of
other broadcast mediums, such as radio and television.
How will the pricing schemes work for the sending host if the number of
subscribing hosts cannot be determined beforehand? Will it be a "possible
viewer" scheme that is currently used for cable TV, or can something more
intelligent be proposed using the Counting feature of EXPRESS?