multicasting in mpls network christy gnanapragasam christy@sce.carleton.ca nov 08, 2003

Multicasting In MPLS Network

Christy Gnanapragasamchristy@sce.carleton.ca

Nov 08, 2003

July 25, 2003AONL & EION 2

Outline• Introduction and Background• Literature survey

– IP Multicasting – Multicasting in MPLS

• Difficulties in supporting• A New approach (ERM2)• Extension to ERM2 (ERM2-Ext) (Own

work)• Performance analysis• Summary

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Introduction and Background

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• Advantages of multicasting– Reduction in network resource consumption– Source Link stress

• Example applications:– Audio/video distribution– Push applications– Audio/video conferencing

• Requirements of this applications (QoS):– Bandwidth, bounded delay and low loss rate

Introduction

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• There is no standardized algorithms for multicasting in MPLS

• However, the power of MPLS can provide QoS for IP multicast in MPLS.

• Problem arise, when mapping L3 multicast trees in to L2 LSPs.– Flood and prune– Source/shared trees– Uni/bi directional trees– Encapsulated multicast

Difficulties in supporting IP multicast in MPLS

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ERM2• ERM:

– Easy to implement and it requires simple modification to current IP mulicasting protocols– But, LSRs need to participate in routing process

• ERM2:– In MPLS-TE, Until a PLR has a backup path available, the PLR must clear relevant four flags in the

corresponding RRO sub-object

– Local Protection available flag• PLR should set this when a backup path is available. • If no established backup path or it is in DOWN state, PLR should clear this flag and should send a updated RESV message

– Local Protection in use flag• PLR Should set this flag if it redirecting traffic into the backup path else should be clear

– Node Protection flag• PLR should set this flag if backup path is protected against the failure of immediate downstream node.

– BW Protection flag• PLR should set this flag if the backup path offers a BW guarantee.

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• Difficulties:

– LSP Design • Multicast tree requires point-to-multipoint LSP setup, but only point-

to-point LSP setup is supported by MPLS. • Volatile LSPs

– Dynamic behavior of group members– Singling overhead and over consumed labels

– Traffic aggregation:• Unicast traffics are normally aggregated if ingress and egress LERs

are same for scalability. • Not possible in multicast traffic.

– Coexistence of Layer 2 and Layer 3 forwarding in LSRs.• Two cases:

– Switched over from shared tree to source based tree– Same label for unicast and multicast traffic

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• Edge Router Multicasting

• In this approach, trees are formed by branching at Edge router.

Solution to mentioned problems

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Advantages of ERM• ERM converts the multicast flow into multiple unicast

flow

• Simplifies the LSP setup: – Diverging nodes are only LERs– No need for point-to-multipoint LSPs.

• Makes multicast flow aggregatable:– Unicast and multicast traffic can be treaded in a same way and

scalability will not be compromised

• Relaxes the requirements at Core Routers:– No modification needed at LSRs

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ERM in depth

• ERM concept can be used to build trees in MPLS by slightly modify existing IP multicast routing protocols.– PIM_SM and CBT uses Rendezvous Point

and explicit Join messages– But need some modifications needed

• Select LER as RP of the tree• Allow a sub tree to join at only LER

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• We can add two more techniques that can optimize the end results.– Where to join if there are multiple choices.– Dynamic tree optimization

• Advantages.– Minimize the End-to-End delay– Minimize the bandwidth consumption– Minimize the packet processing (L2->L3->L2)

Modification to ERM2 (own work)

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RESULT

LER1• ERM2 illustrations

LER6

LER5

LER4

LER3

LER2

MM

R2

QUARY

R1

JOIN

ADD

S

PRUNESUBTRACT

LER1

LER6

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RESULT

LER1

• ERM2-Ext Joining choice

LER6

LER5

LER4

LER3

LER2

MM

R2

QUARY

R1

SLER1 3

LER5 0

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LER1

• ERM2-Ext: Dynamic tree optimization

LER6

LER5

LER4

LER3

LER2

MM

R2

R1

S

1) Unwanted packet processing

(L2->L3->L2)

3) Extra bandwidth wastage (5 link -

>4link)

2) ETE hop delay is larger than it should

be (5->4)

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• Challenges:

– Triggering the optimization algorithm• Optimization can be triggered when a LER does not have any directly

connected receivers and it has one or more downstream peer LERs.

– Optimization• Which nodes should be refreshed (rearranged)• Where should they join (who is the parent)• Which order those nodes should be rearranged

Introducing a simple algorithm would optimize the tree and the performance

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Snapshot of a tree

R

All of these nodes are LERs and they

all have directly attached Receivers

5

4

5

3

3

5 6

3 45

5

3

2

Which nodes need to be refreshed:

children

Where should they join:

Easy solution: join them in parent node of the leaving LER

1) Which children to refresh first

2) Find the potential candidates (LERs)

MM

RefreshTrigger (list of children)

Refresh (potential candidates)

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• Find a order: which children need to be refreshed first to obtain optimal result.– Chose the one has less cost to source (root)– This new child also be part of the potential candidate for next child.

• Finding the potential candidates– Which is a sub-set of on-tree LERs– Sub-tree should exclude

• Leaving LER• Children of leaving LER

– What about descendant of leaving node• Should be excluded

– Why?

MM responsibilities:

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Find the potential candidates for joining the tree

R

5

4

5

3

3

5 6

3 45

5

3

2

MM

MM does not know the relationship between nodes, it just know the distance from

source

To exclude descendants, may be we can use the height for

now

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Bandwidth consumption

Bandwidth consumption

0

20

40

60

80

100

120

140

1 12 23 34 45 56 67 78 89 100

Time

BW

link utilization(ERM2-EXT)

link Utilization(ERM2)

link utilization (ERM2-Ext_completed)

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End to End DelayETE Delay (num of hops)

0

1

2

3

4

5

6

1 12 23 34 45 56 67 78 89 100

Time

nu

m h

op

s

ETE delayERM2_Ext(time-average)

ETE delay ERM2(time_average)

ETE delay_ERM2-Ext-completed(timeaverage)

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Summary• ERM2 is a simple algorithm that supports the

multicasting in MPLS.• It excludes the need for point-to-multipoint LSP setup• It excludes the participation of LSR in tree or tree

building

• ERM2-Ext: for more optimization– Choice of parent when multiple joining points are possible– Dynamically refreshing the tree

• Two new messages– RefeshTriggring and Refresh

– Advantages• Minimize the bandwidth consumption• Minimize the end to end delay• Minimize the processing overhead (L2->L3->L2)

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References• [1] Boudani, A and Cousin, B.;”A New Approach to Construct Multicast Trees in MPLS

Network,” Computers and Communications, 2002. Proceedings. ISCC 2002. Seventh International Symposium on, 1-4 July 2002 Pages: 913 – 919.

• Baijian Yang and Mohapatra, P.” Edge router multicasting with MPLS traffic engineering,” Networks, 2002. ICON 2002. 10th IEEE International Conference on, 27-30 Aug. 2002 Pages: 43 – 48

• Young-Kyu Oh, Dong-Keun Kim, Hun-Je Yoen, Mi-sun Do and Jaiyong Lee,” Scalable MPLS multicast using label aggregation in Internet broadcasting systems,” Telecommunications, 2003. ICT 2003. 10th International Conference on, Volume: 1, 23 Feb.-1 March 2003.Pages:273 - 280 Vol.1

• Zhongshan Zhang, Keping Long, Wendong Wang, Shiduan Cheng, “The new mechanism for MPLS supporting IP multicast,”, Circuits and Systems, 2000. IEEE APCCAS 2000. The 2000 IEEE Asia-Pacific Conference on, 4-6 Dec. 2000 Pages: 247 – 250

• M. Ramalho. Intra- and Inter-domain multicast routing protocols: A survey and taxonomy. IEEE Communications Survey sand Tutorials, 3(1):2–25, first Quarter 2000

• Almeroth, K.C., “The evolution of multicast: from the MBone to interdomain multicast to Internet2 deployment,“ Network, IEEE , Volume: 14 , Issue: 1 , Jan.-Feb. 2000, Pages:10 – 20

Thank You

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Xcast solution

• Explicit multicast [Boivie & al.]

• Xcast packet – explicit list of group members – list of unicast addresses in Xcast packet header

• No multicast routing table– standard unicast routing protocol– no additionnal entry

• No management protocol

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MPLS Multicast Tree

• uses LSPs between branching nodes

• no multicast states into no-branching nodes

• enhances scalability of MPLS domain for multicast trafic

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Branching nodes

• Multicast trees have: – few branching nodes– many non-branching

nodes

• A branching node– a router with distinct next

hops for a multicast tree

from REUNITE study

from our study

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Xcast problems

• Membership management– delegated to another protocol

• multicast source knows somehow the IP address of every member of the multicast group

• When size of multicast group increases – size of destinations list increases

• packet processing time increases• packet length increases

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Multicasting in MPLSIn designing a MPLS multicast protocol, we must

deal with two problems.1. Creating multicast tree.2. Allocating and distributing labels.Definitely, LDP must deal with the second

problem. However, creating Multicast tree can be based on:– Topology driven: Map the L3 tree in the routing

protocol into L2.– Request driven: By signaling– Traffic driven: Map the L3 tree in the routing

protocol into L2 when data arrive.

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Xcast packet forwarding

A B

C

D

E

F

d1

d3, G

d2, G

s1

d1 : E

dest : next hop

d4, G

d2 : Cd3 : Dd4 : D

G : d1,d2,d3

s1->G : Xcast d2, d3, d4 <data>

grp: list of dest

s1->G : Xcast d3, d4 <data>

s1->d2 <data>

- Lookup for every destinations into Xcast packet

- Forward a copy of the packet (with an appropriated destination list) for every distinct next hop

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MPLS domain

Multicast tree over MPLS• In a MPLS domain, usually

– packets is forwarded over the LSP ending at the edge LSR associated to the packet destination

– paths followed by multicast packets will not be efficient

ingress LSR egress LSR

LSP

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SEM principles

• Use of the next branching node address as Destination address of IP packet– no specific treatment into routers which are

between branching nodes

• SEM packet header encapsulates Multicast group address– processed by branching nodes– increasing of packet length is low

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Segmentation for XCast

• Overhead of Xcast increases with the increasing of the number of destinations– best tradeoff between path-MTU length and

maximum number of destinations into a Xcast packet

– proposition of GXcast protocol: a generalization of Xcast

• partition of the set of destinations based on the optmized size of the parts

• how to find the best partition [still an open question]

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IGMP is only used between hosts and first hop router; for router-to-router distribution, a routing protocol is used to build and maintain group distribution trees

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MOSPF, which is basically the OSPF routing protocol with extensions, makes use of the SPF (Shortest Path First) algorithm. All the other multicast protocols can be split into two types: "broadcast and prune" and "explicit join." Broadcast and prune protocols (DVMRP, PIM-DM) typically build source-specific distribution trees (fig. A) while explicit join protocols (PIM-SM, CBT) build shared trees (fig. B) from a Rendezvous Point (RP).