The Spread Wide Area Group Communication System
Yair Amir and Jonathan Stanton
Department of Computer Science
The Johns Hopkins University
Building a wide area group communication system is a challenge. This paper presents the design and protocols of
the Spread wide area group communication system. Spread integrates two lowlevel protocols: one for local area
networks called Ring, and one for the wide area network connecting them, called Hop. Spread decouples the
dissemination and local reliability mechanisms from the global ordering and stability protocols. This allows many
optimizations useful for wide area network settings. Spread is operational and publicly available on the Web.
There exist some fundamental difficulties with highperformance group communication over
widearea networks. These difficulties include:
. The characteristics (loss rates, amount of buffering) and performance (latency, bandwidth) vary
widely in different parts of the network.
. The packet loss rates and latencies are significantly higher and more variable then on LANs.
. It is not as easy to implement efficient reliability and ordering on top of the available wide area
multicast mechanisms as it is on top of local area hardware broadcast and multicast. Moreover,