Network-Coding Multicast NetworksWith QoS Guarantees

Yuanzhe Xuan and Chin-Tau Lea, Senior Member, IEEE

IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 19, NO. 1, FEBRUARY 2011

Speaker: Lin-You Wu

Outline

I. INTRODUCTION

II. OPTIMAL ROUTING FORMULATION

III. SOLVING THE OPTIMAL ROUTING

PROBLEM

IV. NUMERICAL RESULTS

V. CONCLUSION

I. Introduction

• It is well known that without admission control, network congestion is bound to occur.

• However, to implement admission control is difficult in IP-based networks, which are constructed out of the end-to-end principle.

• Even if routers can perform admission control internally, the path computation and the state updating activities required for setting up and tearing down each flow will overwhelm the network.

• A new QoS architecture, called a nonblocking network, has been proposed recently, and it requires no internal admission control and can still offer hard QoS guarantees.

• In this architecture, as long as each edge node admits not more than a specified amount of traffic, the network will never experience link congestion.

• For multicast networks, the main problem with this approach is low throughput.

• Multicast architectures with hard QoS intentions can be divided into two types.

• One performs multicast at the network layer [Fig. 1(a)], and multicast is done by the routers (IP or MPLS type).

• The other, like a content distribution network (CDN), performs multicast at the application layer [Fig. 1(b)], and multicast is done by the servers.

• Data transmission in both architectures consists of two parts:

• Transmission in the backbone network– covers a long distance– bandwidth is more expensive

• Transmission between a client and its local server– handle by LANs– bandwidth is relatively ample

• Local data transmissions can also be tackled with the P2P technology as in a hybrid P2P network.

• The focus of this paper will be on the QoS guarantees in the backbone network.

• It is well known that without admission control, congestion inside a network is bound to occur, but to implement admission control in a high-speed IP-based network is difficult.

• One reason is that IP-based networks are constructed out of the end-to-end principle and major signaling protocols.– meaning that a signaling message’s semantics can

only be interpreted by the signaling servers located at the edge of the network [see Fig. 1(a)].

• Another reason is that even if every node understands the semantics of a signaling message, as it is the case in Fig. 1(b)

• where each node is a server, the activities of checking bandwidth availability and setting up the paths for each flow can overwhelm the network.

• A new QoS architecture has been proposed recently that requires no admission control inside the network and can still guarantee the congestion-free property.

• It applies to both shortest-path-routing (IP-like) and explicit-routing (MPLS-like) networks.

• The most salient property of the network is the following:

• As long as the traffic of the ingress and egress directions admitted by edge node I is less than ai

and bi respectively, the network will be congestion-free and none of its links will experience overflow.

• Suppose that the network in Fig. 1(a) is a nonblocking network with ai= bi =900 Mb/s for all edge routers.

• Suppose also that each edge router connects to three video servers, and each server is allocated 300 Mb/s (even allocations are not a requirement).

• Routing in a conventional network is based on the assumption that the traffic matrix T = {t ij} is given, where tij represents the traffic rate from edge node I to edge node J .

• However, ai and bi are given in a nonblocking network (this pattern is called a hose-model pattern.)

• For a unicast network, this means that only the row and column sums of T are known (ai =Σj tij

and bi= Σj tji) ,but not its tij.

• For a multicast network, the relationship between the traffic matrix T and(ai, bi) is more complicated (see Section II-A).

• Finding an efficient routing algorithm for a nonblocking network is not a simple task because there are infinite traffic matrices that can satisfy the constraint (ai, bi) , and a feasible routing scheme must guarantee the congestion-free property for all of them.

• The task becomes even harder if the network needs to support multicast traffic.

• As far as a nonblocking network is concerned, the most significant benefit of network coding is that it allows us to treat a multicast connection with q destinations as q unicast connections in formulating the flow optimization problem.

• we are able to prove two important results in this paper. Both results apply to explicit-routing and shortest-path routing networks.

1.The optimal paths between a source–destination pair in a nonblocking unicast network are also the optimal paths for the pair in a nonblocking multicast network with network coding.

2.An immediate consequence of the result of 1) is that a nonblocking multicast network can admit the same amount of traffic as in a nonblocking unicast network.

II. OPTIMAL ROUTING FORMULATION

• The formulation of the optimal routing problem of a nonblocking multicast network with network coding is given in this section.

• The discussion applies to both explicit routing (MPLS-like) and shortest-path routing (IP-like) networks.

A. Traffic Unevenness

• A network can be described as a directed G(V,E) where V is the set of vertices and E is the set of links.