Technical Report, July 2001
Our goal is to develop a Cost-Benefit framework for fault tolerant communication and
information access that addresses extremely powerful adversaries that were never handled in
the past. The project will develop the theory and algorithms required to overcome strong
network attacks, while providing theoretically provable performance bounds. We will build a
system that incorporates these algorithms, and that exhibits good performance in practice.
Our technical approach includes the following innovative topics:
- Analysis of strong adversary models: In order to understand how robust our solutions must
be, we need to first understand what is the model of possible attacks and errors. This project
introduces a collection of adversary models. They range from a simple predictable slow
adversary, to a somewhat limited "stable path" adversary (which allows communication over a
path to be successfully completed), to a totally unpredictable adversary (which can
selectively block traffic based on type, source, destination, etc.). Many of these models have
not been considered in the literature so far.
- New routing and dissemination protocols: We present a suite of novel routing protocols
tailored to the above adversary models and prove that these protocols perform in a
near-optimal manner. Specifically, we present novel solutions that, in case an operational
path exists, will be able to find it. Even when no path exist for more than a very short time
(shorter than network round-trip time) we still are able to pass packets between source
Our goal is to support the performance and
correctness properties of these protocols by rigorous analysis. We aim our analysis not to
assume anything about either the topology or the traffic patterns in the network, and not to
assume a known correlation between past and future behavior of the adversary.
- New replication protocol: When there is no theoretic possibility of communication, say in
the case of a cut in the network, one can still continue the operation by making sure that the
data is replicated in most areas, or at least in the areas where disconnection is likely. We
will develop a suite of replication protocols that can handle a range of adversaries and can
gracefully degrade performance and semantics as the network hostility increases. We aim
at being able to replicate an ACID database as this is the most demanding replication
- A Cost-Benefit decision framework: This framework is used to select the most suitable
protocol as network conditions change, both for network-level protocols such as routing and
flow control, and data-level protocols such as replication. The main idea is to consider the
marginal benefit obtained by the application when consuming a given resource, versus the
"opportunity cost" of using this resource. The latter is the benefit that may potentially be
lost by other applications if this resource is committed. In the network level, the decision
is based on application tolerance to delay, and the reliability of the network. In the data
level, the decision is based on the cost of inaccessibility of data, the cost of updating
replicas, and the synchronization cost of replication.
- An overlay network architecture: We develop an overlay network architecture that will make
these protocols practical since they are too complex to have any hope to be implemented in
general Internet routers anytime soon.
- Analysis of strong adversary models, and routing and dissemination protocols: We have implemented an initial version of gravitational flow technique for routing that can overcome strong adversary model. Our protocols operate without using the concept of a global path, which is standard in the common "weak adversary" models.
Roughly, the operation of the gravitational flow protocol is based on the number of unacknowledged packets that were sent in each direction. Packets that reach the destination are acknowledged. Packets that are stuck create "back pressure" so that this link is not used for further packets. As the network connectivity change, pressure points may be released with links coming back up, and free paths may be clogged as links go down.
We have built a prototype for the gravitational flow protocol that sends Mpeg video stream through an overlay network in our lab with controlled link-down and link-up events. This is still a work in progress.
- Overlay network infrastructure: We have completed a basic congestion control for the Hop link-level protocol - a link level modified
selective-repeat protocol that is TCP-fair, uses less CPU, and allows us full control over the forwarding of messages on
the overlay network. The basic congestion control was implemented both in the ns2 simulator and in our overlay network implementation.
We are still investigating tradeoffs regarding the different mechanisms in terms of our ability to control rate versus window,
and in terms of cpu consumption.
- Cost benefit decision making: We designed two alternative global flow control protocols
for multi-sender multi-group multicast in overlay networks schemes. The first scheme is a cost benefit approach
that regulates buffer utilization in overlay network routers. The second scheme is a cost benefit approach
that regulates capacity utilization of the overlay links. We have implemented the buffer utilization cost-benefit
scheme in the ns2 simulation toolkit and in the Spread group communication system. We discovered that keeping
track of buffers is more successful than keeping track of link utilization since the link utilization is very hard to
measure on the overlay network, it changes rapidly, and is hard to approximate.
- New replication protocol: We have developed a general replication engine
that allows consistent ordering of actions in a network that is prone to
partitions and crashes.
Our method places instances of our replication engine strategically in
the network. The replicas maintain consistent state and recover from a wide range of possible faults.
We came up with a method to create new replicas and eliminate replicas while the system is operational without compromising consistency and without requiring complete connectivity. This is a work in progress, but when completed, it will allow our cost-benefit algorithms to determine how many replicas to keep in the system and possibly where to situate them.
We implemented a prototype of a Postgres Interceptor that allows us to apply our replication engine to the PosgreSQL database server.
Existing applications may seemlesly use our interceptor layer that gives them exactly the Postgres interface, while the Postgres database
server sees our interceptor as a regular client. This enables us to replicate a database for existing applications without any change
to both the database and the application.
Our plan for FY 2002 includes the following:
- Network level resiliency: Algorithmic design and simulations - focusing on gravitational flow.
- Data level resiliency: Enhancing our replication technology and making it dynamic. Investigating
new ways to replicate large scale systems.
- Cost benefit decision making: Algorithmic design - figuring out the specific interfaces and
formulas for the network-level resiliency and the data-level resiliency domains.
- Overlay network infrastructure: partial implementation of a core system providing the
overlay network abstraction.
Global Flow Control for Wide Area Overlay Networks: A Cost-Benefit Approach
Technical Report CNDS-2001-3
This paper presents a flow control for multi-sender multi-group
multicast and unicast in wide area overlay networks.
The protocol is analytically grounded and achieves real world goals,
such as simplicity, fairness and minimal resource usage.
Flows are regulated based on the "opportunity" costs of
network resources used and the benefit provided by the flow.
In contrast to existing window-based flow
control schemes, we avoid end-to-end per sender or per group
feedback by looking only at
the state of the virtual links between participating nodes. This produces
control traffic proportional only to the number of overlay network
links and independent of the number of groups, senders or receivers.
We show the effectiveness of the resulting protocol
through simulations and validate the simulations with live Internet experiments.
This is an updated version of Technical Report CNDS-2001-1
Flow Control for Many-to-Many Multicast: A Cost-Benefit Approach
Technical Report CNDS-2001-1
We present a protocol
that is analytically grounded, yet also achieves real world goals,
such as simplicity, fairness and minimal resource usage. We base our
flow control protocol on the Cost-Benefit algorithmic framework for
resource management. We base decisions on the "opportunity" costs of
network resources, comparing the cost of each individual resource to
the benefit it provides. As opposed to existing window-based flow
control schemes, we avoid end-to-end feedback by basing decisions on
the state of the links between participating nodes. This produces
control traffic proportional only to the number of overlay network
links and independent of the number of groups.
The cost-benefit decision making framework was used in the mod_backhand
load-balancing module for the Apache web server. The framework was used to decide which servers
should respond to each web request. The mod_backhand module has grown to be used in
over 10,000 websites and is included in the SuSE 7.0 and Caldera OpenLinux 3.2 linux distributions.
We have pre-released Wackamole which is a software tool that
allows N-Way Fail Over for IP Addresses in a cluster.
Wackamole runs as root on each of the cluster's machines.
It uses the membership notifications provided by the Spread toolkit
to generate a consistent state that is agreed upon
among all of the connected Wackamole instances. Wackamole uses this
knowledge to ensure that all of the public IP addresses served by the
cluster will be covered by exactly one Wackamole instance. We are planning on an
open-source release in the very near future.
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Johns Hopkins University
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