Quarterly Technical Report, October 2002
This quarter we focused on developing new more advanced protocols to provide replication and
dynamic network routing. We also enhanced the existing software systems by
increasing their modularity and adding new capabilities. The new overlay network
implementation provides a fully application independent platform upon which routing and
reliability protocols can be developed.
Our approach to providing fault-tolerant networks was extended, to
include more dynamic network environments such as ad-hoc wireless
networks. In particular, we addressed the routing survivability problem under
an adversarial model where any intermediate node or group of nodes can
perform byzantine attacks such as creating routing loops, misrouting
packets on non-optimal paths, or selectively dropping packets. Our
on-demand routing algorithm addresses both fault-tolerant and security
concerns and uses an adaptive probing technique that identifies a
faulty link after log n faults have occurred, where n is the length of
the path. A multiplicative increase scheme is used to penalize
``faulty''links which are then rehabilitated over time. This list is
used by the route discovery phase to avoid faulty paths. More details
about this work can be found in a report below.
- New replication protocol:
We continued to work on optimizing and evaluating the replication architecture.
In order to comprehensively analyze the tradeoffs involved when replicating
database systems on wide area networks, we implemented the modular version of the
replication algorithm (Maintaining Database Consistency in P2P networks).
We are practically evaluating the performance of this approach and comparing it
with the performance of our replication architecture.
We are investigating algorithmic and practical solutions for optimizing the cost
of reconfigurations in the presence of faults in a wide-area network environment.
We are also investigating algorithms that minimize the replication costs based on
data partitioning and access patterns.
- Overlay network infrastructure:
We focused our work on the client-daemon communication
in Spines. We develpoped a socket-like API for unreliable, best-effort
UDP communication, and also for the session-based TCP reliable
communication. This brings us the level of transparency necessary for
making Spines easy to use in the socket programming paradigm, in a first
step towoards complete transparency. We also analyzed the possibilities
of poviding IP multicast service using Spines while using only simple
unicast communication at the network level.
We have adapted Wackamole in order to provide n-way fail-over for routers.
An example layout for this application of Wackamole consists of multiple physical
routers that act as a single virtual router. An indivisible set of virtual addresses
on different interfaces is allocated to the physical router currently acting as the
fail over router. If the interface through which the machine is connected to Spread fails,
or the machine itself crashes, the set of virtual addresses will be reallocated to a different
machine. The set of physical routers running Wackamole, each of which is
potentially "the" router, can be conceptualized as a single virtual router.
facilities and enhancing the multiplatform support.
An On-Demand Secure Routing Protocol Resilient to Byzantine Failures
In ACM Workshop on Wireless Security (WiSe) , Atlanta, Georgia, September
and Herbert Rubens.
An ad hoc wireless network is an autonomous self-organizing system of
mobile nodes connected by wireless links where nodes not in direct
range can communicate via intermediate nodes. A common technique used
in routing protocols for ad hoc wireless networks is to establish the
routing paths on-demand, as opposed to continually maintaining a
complete routing table. A significant concern in routing is the
ability to function in the presence of byzantine failures which
include nodes that drop, modify, or mis-route packets in an attempt to
disrupt the routing service.
We propose an on-demand routing protocol for ad hoc wireless networks
that provides resilience to byzantine failures caused by individual or
colluding nodes. Our adaptive probing technique detects a malicious
link after log faults have occurred, where n is the length of
the path. These links are then avoided by multiplicatively increasing
their weights and by using an on-demand route discovery protocol that
finds a least weight path to the destination.
On the Performance of Synchronous Wide-Area Database Replication |
Technical Report CNDS-2002-4, September 2002.
Yair Amir, Claudiu Danilov, Michal
Stanton and Ciprian
A fundamental challenge in database replication is maintaining
a low cost of updates while assuring global system consistency.
The problem is magnified for wide-area replication due to the high latency
and the increased likelihood of network partitions. As a consequence,
most database replication research moved away from strictly consistent
models to update models with weaker semantics, relying on application
knowledge to resolve conflicts.
This paper explores a synchronous replication architecture for local
and wide-area networks that provides strong consistency and performs
considerably better than previous consistent approaches. As a proof of concept,
we implemented transparanet replication for the Postgres database system.
Our results show that sophisticated algorithms and careful distributed systems
design can make symmetric, synchronous, peer database replication a reality over
both local and wide-area networsk.
Plans for Next Quarter:
- Data level resiliency:
We will continue to develop algorithms that will enable smart decision making with
respect to data placement, which will further reduce the costs for providing
We plan to release a new version of Wackamole with enhanced algorithm that better
support survivable routers.
- Overlay network infrastructure:
We plan to release the first version of Spines
as an overlay network research tool and make it available open source.
We also plan to provide multipath routing using the Cost Benefit
framework and implement the IP multicast mechanisms in Spines.
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Johns Hopkins University
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