[Paper Review] On the Optimality of Separation between Caching and Delivery in General Cache Networks

Navid Naderializadeh, Mohammad Ali Maddah-Ali+1 more|arXiv (Cornell University)|Jan 20, 2017

Caching and Content Delivery5 references4 citations

TL;DR

This paper proposes a universal caching and multicast delivery scheme for general memoryless networks that achieves near-optimal delivery delay regardless of network topology. By separating cache placement and multicast message generation from network-dependent delivery, the scheme ensures optimal performance within a constant factor, even without knowing the network's capacity region.

ABSTRACT

We consider a system, containing a library of multiple files and a general memoryless communication network through which a server is connected to multiple users, each equipped with a local isolated cache of certain size that can be used to store part of the library. Each user will ask for one of the files in the library, which needs to be delivered by the server through the intermediate communication network. The objective is to design the cache placement (without prior knowledge of users' future requests) and the delivery phase in order to minimize the (normalized) delivery delay. We assume that the delivery phase consists of two steps: (1) generation of a set of multicast messages at the server, one for each subset of users, and (2) delivery of the multicast messages to the users. In this setting, we show that there exists a universal scheme for cache placement and multicast message generation, which is independent of the underlying communication network between the server and the users, and achieves the optimal delivery delay to within a constant factor for all memoryless networks. We prove this result, even though the capacity region of the underlying communication network is not known, even approximately. This result demonstrates that in the aforementioned setting, a separation between caching and multicast message generation on one hand, and delivering the multicast messages to the users on the other hand is approximately optimal. This result has the important practical implication that the prefetching can be done independent of network structure in the upcoming delivery phase.

Motivation & Objective

• To minimize normalized delivery delay in a cache network with multiple users and a server connected via a general memoryless communication network.
• To design a cache placement strategy independent of future user requests and network structure.
• To develop a multicast message generation and delivery protocol that works efficiently across arbitrary network topologies.
• To prove that separation between caching/multicast generation and network delivery is approximately optimal, even without knowledge of the network's capacity region.

Proposed method

• Designing a universal cache placement scheme that does not depend on the underlying network structure or future user requests.
• Generating multicast messages at the server for every subset of users, enabling efficient delivery over any memoryless network.
• Using a two-phase approach: (1) cache placement without knowledge of user demands, and (2) multicast message generation and delivery based on actual requests.
• Analyzing delivery delay under general network constraints, without requiring exact knowledge of the network's capacity region.
• Demonstrating that the proposed scheme achieves delivery delay within a constant factor of the optimal, regardless of network topology.
• Leveraging the structure of multicast delivery and universal caching to decouple network-specific design from caching and message generation.

Experimental results

Research questions

• RQ1 Can a universal caching and multicast delivery scheme be designed that performs near-optimally across all memoryless networks?
• RQ2 Is it possible to separate cache placement and multicast message generation from the network-specific delivery phase without significant performance loss?
• RQ3 To what extent can the performance of the delivery phase be guaranteed when the network's capacity region is unknown?
• RQ4 Does the proposed scheme achieve optimal delivery delay to within a constant factor, even without prior knowledge of network topology?
• RQ5 Can the separation between caching and delivery be proven approximately optimal in general cache networks with arbitrary memoryless communication links?

Key findings

• A universal scheme for cache placement and multicast message generation achieves delivery delay within a constant factor of the optimal for all memoryless networks.
• The separation between caching/multicast generation and network delivery is approximately optimal, even when the network's capacity region is unknown.
• The proposed scheme ensures optimal performance regardless of the underlying communication network structure.
• The scheme operates without prior knowledge of user requests during cache placement, yet achieves near-optimal delivery delay.
• The result holds despite the lack of approximate knowledge of the network's capacity region, demonstrating robustness and generality.
• The practical implication is that prefetching can be performed independently of network design, simplifying deployment in diverse network environments.