WDM Multicasting via Optical Burst / Label Switching

By

Krishna Kishore Konakanchi

Fall 2001 10/23/01

Overview

• Introduction• Protocols requiring global knowledge of WDM

layer topology only• Algorithms to construct a M-cast Forest given

global knowledge of both WDM layer topology and multicast capability of the switches

• Re-routing protocol without global topology as well as multicast capability info

• Future work

Introduction

• Supporting IP Multicasting over WDM networks has many advantages including data rate and coding transparency for multicast

• Main issue - multicast in WDM without changing the semantics of IP

• All switches in WDM network are not MC i.e some are MI

• MC switch - can switch one incoming path onto many downstream

• MI switch - Incoming signal can be passed through to only one downstream

Layers 0-3 Characteristics (IP,ATM,SONET,WDM)

Network Factors

IP ATM SONET WDM

Data Protocol

Unit(PDU)

Varied –packet size

53 byte cell STS-1(45 Mbps)

Format Independent

Channel BW Varied(logic) Varied(logic) Fixed

(Physical)

Fixed

(Physical)

Protocol Layer Network

(layer 3)

Link

(layer 2)

Physical

(layer 1)

Optical

(layer 0)

Connection Type

Connection less

Connection Oriented

Connection Oriented

Connection Oriented

QOS Possible Yes NO Possible

Traditional

Role

Routing Switching Cross Connects

Cross Connects*

Technology Strength

Simple,easy - scale,robust

Multi service

Integration

High speed

Transport

High – speed

Transport* WDM Emerging Technology

Protocols with global WDM topology Knowledge

• Given a shortest path m-cast tree constructed by MOSPF these protocols form a m-cast forest avoiding branching at a MI nodes if present

• Protocol 1– Re-route to source : MI switch having more than 1

downstream sends request towards source to find alternate path. MC switch along the path acknowledges by establishing new LSP

– no new modification to the SPT

– no hiding of routes from IP layer

– WDM need not know detail functionality of IP layer

Protocols with global WDM topology Knowledge (cont.)

• Protocol 2– Re-route to Any : determines paths to nodes on the tree

other than any of its children whose costs are less than that of the path to source. Worst case is path to source

– Results in BW savings without substantial increase in cost

• Consider following scenario where– Source node : 1– MI Node : 2– Destinations : 3, 4 & 5

Shortest Path by MOSPF

1

2 3

54

Protocols 1 & 2 - Scenarios

1

2 3

54

1

2 3

5

4

Protocol 1 Protocol 2

Req.&Ack

Req.&Ack

Implementation & Results

• Protocols 1 & 2 were implemented in C & OPNET

• Additional cost of forest constructed by 2nd re-routing protocol when compared with original cost of tree is around 5% in a network with 20 - 40 switches , 50% of which are MI and 50% belong to a session

• 2nd protocol can cut down cost by half when compared to the 1st protocol.

Protocols with global WDM topology Knowledge and MC Info

• Approach 1– Construct forest based on any multicast tree– At each branching MI switch, remove all

downstream links except one, thus breaking the tree into base sub-tree and several superNodes

– Reconnect the base sub-tree with the closest possible super node without using any removed link

Protocols with global WDM topology Knowledge and MC Info

(cont.)• When its not possible to combine the base sub-tree

with any supernode , removed links are used to form new LSP’s

• Resulting forest will consist of multiple source routed trees.

• This is very similar to Re-route to Source.

KMB Tree heuristic to construct Mcast Tree

• Given a graph “G” (modeling a network) and set of multicast destination say “Z”

• The algorithm to construct the m-cast tree as follows :– Construct a complete directed distance graph

G1 = (V1,E1,c1)

– Find the minimum spanning tree T1 of G1

– Construct a sub-graph Gs of G by replacing each edge in T1 by its corresponding shortest path in G

KMB Tree heuristic to construct Mcast Tree (cont.)

– Find the minimum spanning tree Ts of Gs

– construct a Steiner Tree TH from TS by deleting edges in TS if necessary, so that all the leaves in TH are Steiner points.

• Worst Case Time complexity O(|S||V|2)

Approach 1 - Example

A

B

C

D

E F

G H

Approach 1 (cont.)

• From previous fig., the cost of the M-cast forest can be given as

2(AB + BC + CD) + DE + EG + EH + DF

• This approach is very relevant in cases where the higher layer is oblivious to the MI-nature of some of the nodes. E.g IP over WDM

Protocols with global WDM topology Knowledge and MC Info

(cont.)• Approach 2

– Every member switch including the source acts as a super node

– repeatedly combine the super-nodes into one until only one super node containing all members are left

– when finding shortest path bet. 2 super nodes, only paths without MI nodes are considered

– the downstream of a branching MI node may forward multicast data to other down-streams thought an OB/LSP

Approach 2 - Example

A

B

C

D

F

G H

E

E F

Approach 2 (cont.)

• The cost with this approach is given by

AB + BC + CD + 2(DE) + EG +EH+DF• Gives the near optimal solution to the problem of

multicasting in a network where some/many nodes do not have the

• It has been found that cost of forest using alg. 2 is much lower than the 1st

Distributed Re-routing protocol with neither Global Topology Nor MC

Info• Basic idea - local multicast forwarding cache• A branching MI switch sends a “purge” message

to all but one downstream switch• Each purged switch then floods a “grow” request

to its neighbors – direct grow : only neighbors already on the forest can

reply to the request

– indirect grow : if a neighbor is not on the forest then it can still relay the request

Advantages of this Protocol

• Uses only local information of WDM layer• Does not require any change to the IP multicasting

protocol• Uses DVMRP as the IP multicast protocol, which

is most widely used in the internet• Even if a MI switch is purged out, it can very

easily grow back into the forest with the grow schemes

Distributed Protocol - Repair Message

MC

Repair

Repair

Repair

Repair

MI

Repair

Repair

Purge

Purge

MC = Multicast-capable switchMI = Multicast-incapable switch

Repair Messages

Distributed Protocol - Purge Message

Purge

MC

X

Repair

Y

Repair

Z

Repair

MI

XY

Z

Purge

Repair

Purge Purge

MC = Multicast-capable switchMI = Multicast-incapable switch

Purge Messages

Grow

Grow

Repair & Purge

• MI can use randomly some heuristics to select down stream switch to send repair messages

• Purged switch without any attached member need not have to grow back to reduce signaling.

Grow Scheme

•Direct Grow Scheme

•Indirect Grow Scheme

•Neighbor not on multicast tree may replay grow request

Work to be done…

• The results obtained from the different methods discussed are not comparable due to mismatch of parameters used. They need to be generalized and have to be run with same parameters to compare the cost arrived at.

• The distributed protocol developed needs to be implemented and its performance evaluated in terms of BW and latency reduction.

Work to be done…(cont.)

• These results must be compared with the results obtained from the other approaches

• New heuristics for supporting MOSPF, protocols for PIM-SM and mechanisms to provide QoS in multicasting can also be given an insight.