Universidad Publica de Valencia_T2_encaminamiento

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Transmisin de Datos Multimedia http://www.grc.upv.es/docencia/tdm Master IC 2007/2008

Tema 2:

Aspectos de encaminamiento

Tema 2:


Algoritmos bsicos de encaminamiento Link state

Distance Vector

Encaminamiento en Internet RIP

OSPF BGP

Multi-Protocol Label Switching (MPLS).

IP multicast


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0111

value in arrivingpackets header

routing algorithm

local forwarding table

header value output link

0100010101111001

3221

Interplay between routing, forwarding

Computer Networking: A TopDown Approach Featuring the

Internet,3rd edition.

Jim Kurose, Keith RossAddison-Wesley, July 2004.


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3

u

yx

wvz

2

2

13

1

1

2

5

3

5

Graph: G = (N,E)

N = set of routers = { u, v, w, x, y, z }

E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

Graph abstraction

Remark: Graph abstraction is useful in other network contexts

Example: P2P, where N is set of peers and E is set of TCP connections


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Graph abstraction: costs

u

yx

wv

z2

2

13

1

1

2

53

5 c(x,x) = cost of link (x,x)

- e.g., c(w,z) = 5

cost could always be 1, or

inversely related to bandwidth,

or inversely related tocongestion

Cost of path (x1, x2, x3,, xp) = c(x1,x2) + c(x2,x3) + + c(xp-1,xp)

Question: Whats the least-cost path between u and z ?

Routing algorithm: algorithm that finds least-cost path


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Routing Algorithm classification

Global or decentralized information?

Global:

all routers have complete topology, link costinfo

link state algorithms

Decentralized: router knows physically-connected

neighbors, link costs to neighbors

iterative process of computation, exchangeof info with neighbors

distance vector algorithms

Static or dynamic?

Static:

routes change slowly over time

Dynamic:

routes change more quickly periodic update

in response to link costchanges


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A Link-State Routing Algorithm: Dijsktras Algorithm

net topology, link costs known to allnodes

accomplished via link statebroadcast

all nodes have same info computes least cost paths from one

node (source) to all other nodes

gives forwarding table forthat node

iterative: after k iterations, know leastcost path to k destinations

Notation:

c(x,y): link cost from node x to y; = if notdirect neighbors

D(v): current value of cost of path from source todestination v

p(v):predecessor node along path from source to

v

N': set of nodes whose least cost path definitivelyknown

1 Initialization:

2 N' = {u}3 for all nodes v4 if v adjacent to u5 then D(v) = c(u,v)6 else D(v) = 7

8 Loop9 find w not in N' such that D(w) is a minimum10 add w to N'11 update D(v) for all v adjacent to w and not in N' :12 D(v) = min( D(v), D(w) + c(w,v) )13 /* new cost to v is either old cost to v or known

14 shortest path cost to w plus cost from w to v */15 until all nodes in N'


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Dijkstras algorithm example

A F

B

D E

C2

2

2

3

1

1

1

3

5

step SPT D(b), P(b) D(c), P(c) D(d), P(d) D(e), P(e) D(f), P(f)

0 A 2, A 5, A 1, A ~ ~

5

B C D E F


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A F

B

D E

C2

2

2

3

1

1

1

3

5


0 A 2, A 5, A 1, A ~ ~1 AD 2, A 4, D 2, D ~

5

B C D E F


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A F

B

D E

C2

2

2

3

1

1

1

3

5


0 A 2, A 5, A 1, A ~ ~

1 AD 2, A 4, D 2, D ~

2 ADE 2, A 3, E 4, E

5

B C D E F


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0


A F

B

D E

C2

2

2

3

1

1

1

3

5


0 A 2, A 5, A 1, A ~ ~

1 AD 2, A 4, D 2, D ~2 ADE 2, A 3, E 4, E

3 ADEB 3, E 4, E

5

B C D E F


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1

1


A F

B

D E

C2

2

2

3

1

1

1

3

5


0 A 2, A 5, A 1, A ~ ~

1 AD 2, A 4, D 2, D ~2 ADE 2, A 3, E 4, E

3 ADEB 3, E 4, E

4 ADEBC 4, E

5

B C D E F


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1

2


A F

B

D E

C2

2

2

3

1

1

1

3

5


0 A 2, A 5, A 1, A ~ ~

1 AD 2, A 4, D 2, D ~2 ADE 2, A 3, E 4, E

3 ADEB 3, E 4, E

4 ADEBC 4, E

5

B C D E F


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1

3


A

ED

CB

F

Resulting shortest-path tree from A:

B

DE

C

F

(A,B)

(A,D)(A,D)

(A,D)

(A,D)

destination link

Resulting forwarding table in A:


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Dijkstras algorithm, discussion

Algorithm complexity: n nodes

each iteration: need to check all nodes, w, not in N

n(n+1)/2 comparisons: O(n2)

more efficient implementations possible: O(nlogn)Oscillations possible:

e.g., link cost = amount of carried traffic

A

D

C

B1 1+e

e0

e

1 1

0 0

A

DC

B

2+e 0

00 1+e 1

A

DC

B

0 2+e

1+e1 0 0

A

DC

B

2+e 0

e0 1+e 1

initially recompute

routing

recompute recompute


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Distance Vector Algorithm

Bellman-Ford Equation (dynamic programming)

Define: dx(y) := cost of least-cost path from x to y

Then

where min is taken over all neighbors v of x

vdx(y) = min {c(x,v) + dv(y) }


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Bellman-Ford example

u

yx

wv

z2

2

1 3

1

1

2

53

5

Clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3

du(z) = min { c(u,v) + dv(z),

c(u,x) + dx(z),c(u,w) + dw(z) }= min {2 + 5,

1 + 3,

5 + 3} = 4

Node that achieves minimum is nexthop in shortest path forwarding table

B-F equation says:


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Node x maintains the Distance vector: Dx = [Dx(y): y N ]

Where Dx(y) = estimate of least cost from x to y

Node x also maintains its neighbors distance vectors

For each neighbor v, x maintainsDv = [Dv(y): y N ]

Node x knows the cost to each neighbor v: c(x,v)


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Distance vector algorithm

Basic idea:

Each node periodically sends its own distance vector estimate toneighbors

When a node x receives new DV estimate from neighbor, it updatesits own DV using B-F equation:

Under minor, natural conditions, the estimate Dx(y) converge to theactual least cost dx(y)

Dx(y) minv{c(x,v) + Dv(y)} for each node y N


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Iterative, asynchronous: each localiteration caused by:

local link cost change

DV update message fromneighbor

Distributed:

each node notifies neighborsonly when its DV changes

neighbors then notify their

neighbors if necessary

waitfor (change in local linkcost of msg from neighbor)

recomputeestimates

if DV to any dest has

changed, notifyneighbors

Each node:


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0

x y z

x

yz

0 2 7

from

cost to

from

from

x y z

xyz

0 2 3

from

cost tox y z

x

yz

0 2 3

from

cost to

x y z

xyz

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

0 2 3

from

cost to

x y z

xy

z

0 2 3

from

cost tox y z

xy

z

0 2 7

from

cost tox y z

xy

z

7 1 0

cost to

2 0 1

2 0 17 1 0

2 0 17 1 0

2 0 1

3 1 0

2 0 13 1 0

2 0 1

3 1 0

2 0 1

3 1 0

time

x z12

7

y

node x table

node y table

node z table

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}= min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) + Dy(z), c(x,z) + Dz(z)}

= min{2+1 , 7+0} = 3


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Comparison of LS and DV algorithms

Message complexity

LS: with n nodes, E links, O(nE)msgs sent

DV: exchange betweenneighbors only

convergence time varies

Speed of Convergence

LS: O(n2) algorithm requiresO(nE) msgs

may have oscillations DV: convergence time varies

may be routing loops

count-to-infinity problem

Robustness: what happens if routermalfunctions?

LS:

node can advertise incorrect linkcost

each node computes only itsown table

DV:

DV node can advertise incorrectpath cost

each nodes table used byothers

error propagate thru network


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Tema 2:


Tema 2:


Algoritmos bsicos de encaminamiento Link state Distance Vector


OSPF

BGP


IP multicast


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Hierarchical Routing

Our routing study thus far - idealization all routers identical

network flat

not true in practice

scale: with 200 million destinations:

cant store all dests in routing tables!

routing table exchange would swamp links!

administrative autonomy

internet = network of networks

each network admin may want to control routing in its own network


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Hierarchical Routing

Gateway router

Direct link to router in another AS

aggregate routers into regions,autonomous systems (AS) routers in same AS run same routing protocol

intra-AS routing protocol

routers in different AS can run different intra-AS routing protocol

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3b

1d

3a

1c 2aAS3

AS1

AS21a

2c

2b

1b

Intra-ASRoutingalgorithm

Inter-ASRouting

algorithm

Forwardingtable

3c

Interconnected ASes

Forwarding table is configured byboth intra- and inter-AS routingalgorithm

Intra-AS sets entries forinternal dests

Inter-AS & Intra-As sets

entries for external dests

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3b

1d

3a

1c2aAS3

AS1

AS21a

2c2b

1b

3c

Inter-AS tasks

Suppose router in AS1 receivesdatagram for which dest is outside of

AS1

Router should forward packettowards one of the gateway

routers, but which one?

AS1 needs:

to learn which dests arereachable through AS2 andwhich through AS3

to propagate this reachabilityinfo to all routers in AS1

Job of inter-AS routing!

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Learn from inter-ASprotocol that subnetx is reachable viamultiple gateways

Use routing infofrom intra-AS

protocol to determinecosts of least-cost

paths to eachof the gateways

Hot potato routing:Choose the gateway

that has thesmallest least cost

Determine fromforwarding table theinterface I that leads

to least-cost gateway.Enter (x,I) in

forwarding table

Example: Choosing among multiple ASes

Now suppose AS1 learns from the inter-AS protocol that subnet x isreachable from AS3 and from AS2.

To configure forwarding table, router 1d must determine towardswhich gateway it should forward packets for dest x.

This is also the job on inter-AS routing protocol!

Hot potato routing: send packet towards closest of two routers.

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Intra-AS Routing

Also known as Interior Gateway Protocols (IGP) Most common Intra-AS routing protocols:

RIP: Routing Information Protocol

OSPF: Open Shortest Path First

IGRP: Interior Gateway Routing Protocol (Cisco proprietary)

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RIP ( Routing Information Protocol)

Distance vector algorithm Included in BSD-UNIX Distribution in 1982

Distance metric: # of hops (max = 15 hops)

DC

BA

u v

w

x

yz

destination hopsu 1v 2w 2

x 3y 3z 2

From router A to subsets:

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RIP advertisements

Distance vectors: exchanged among neighbors every 30 sec viaResponse Message (also called advertisement)

Each advertisement: list of up to 25 destination nets within AS

Table processing:

RIP routing tables managed by application-level process called route-d(daemon)

advertisements sent in UDP packets, periodically repeated

physical

link

network forwarding(IP) table

Transprt(UDP)

routed

physical

link

network(IP)

Transprt(UDP)

routed

forwardingtable

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RIP: Link Failure and Recovery

If no advertisement heard after 180 sec --> neighbor/link declareddead

routes via neighbor invalidated

new advertisements sent to neighbors

neighbors in turn send out new advertisements (if tables changed) link failure info quickly propagates to entire net

poison reverse used to prevent ping-pong loops (infinite distance = 16hops)

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OSPF (Open Shortest Path First)

open: publicly available Uses Link State algorithm

LS packet dissemination

Topology map at each node

Route computation using Dijkstras algorithm

OSPF advertisement carries one entry per neighbor router

Advertisements disseminated to entire AS (via flooding)

Carried in OSPF messages directly over IP (rather than TCP or UDP

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OSPF advanced features (not in RIP)

Security: all OSPF messages authenticated (to prevent maliciousintrusion)

Multiple same-cost paths allowed (only one path in RIP)

For each link, multiple cost metrics for different TOS (e.g., satellite

link cost set low for best effort; high for real time) Integrated uni- and multicast support:

Multicast OSPF (MOSPF) uses same topology data base as OSPF

Hierarchical OSPF in large domains.

008


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Hierarchical OSPF

Two-level hierarchy: local area,backbone.

Link-state advertisements onlyin area

each nodes has detailed area

topology; only know direction(shortest path) to nets in otherareas.

Area border routers: summarizedistances to nets in own area,

advertise to other Area Border routers. Backbone routers: run OSPF routing

limited to backbone.

Boundary routers: connect to otherASs.

008


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Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): thede facto standard BGP provides each AS a means to:

1. Obtain subnet reachability information from neighboring ASs.

2. Propagate the reachability information to all routers internal to the AS.

3. Determine good routes to subnets based on reachability informationand policy.

Allows a subnet to advertise its existence to rest of the Internet: Iam here

008


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BGP basics

Pairs of routers (BGP peers) exchange routing info over semi-permanent TCP connections: BGP sessions

Note that BGP sessions do not correspond to physical links.

When AS2 advertises a prefix to AS1, AS2 is promising it will

forward any datagrams destined to that prefix towards the prefix. AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c

2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

008


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BGP route selection

Router may learn about more than 1 route to some prefix. Routermust select route.

Elimination rules:

Local preference value attribute: policy decision

Shortest AS-PATH Closest NEXT-HOP router: hot potato routing

Additional criteria

2008

ff


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Why different Intra- and Inter-AS routing ?

Policy: Inter-AS: admin wants control over how its traffic routed, who routes

through its net.

Intra-AS: single admin, so no policy decisions needed

Scale: hierarchical routing saves table size, reduced update traffic

Performance:

Intra-AS: can focus on performance

Inter-AS: policy may dominate over performance

T 2

T 2


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Tema 2:


Algoritmos bsicos de encaminamiento

Link state Distance Vector


OSPF

BGP


IP multicast

2008

MPLS Th M ti ti


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MPLS - The Motivation

IP Protocol Suite - the most predominant networking technology.Voice & Data convergence on a single network infrastructure.

Continual increase in number of users.

Demand for higher connection speeds.

Increase in traffic volumes.

Ever-increasing number of ISP networks.

MPLS Working Groups and Standards Standardized by the IETF - currently in Draft stage.

MPLS recommendations are done by IP players for IP services

MPLS core components are generic

MPLS doesnt use specific technology process

2008

MPLS d ISO d l


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MPLS and ISO model

PPP

Physical (Optical - Electrical) 1

2

IP 3

4

Applications7to

5

FrameRelay

ATM (*)

TCP UDP

PPP Frame Relay ATM (*)

MPLS

(*) ATM overlay model(without addressing and

P-NNI) is considered asan ISO layer 2 protocol.

IETF main goal is that

when a layer is added,no modification isneeded on the existinglayers.

All new protocol mustbe backwardcompatible

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Some MPLS Terms


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Some MPLS Terms...

LER - Label Edge Router LSR - Label Switch Router

FEC - Forward Equivalence Class

Label - Associates a packet to a FEC

Label Stack - Multiple labels containing information on how a packetis forwarded.

Shim - Header containing a Label Stack

Label Switch Path - path that a packet follows for a specific FEC LDP - Label Distribution Protocol, used to distribute Label

information between MPLS-aware network devices

Label Swapping - manipulation of labels to forward packets towards

the destination.

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MPLS Architecture


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Routing protocol OSPF OSPF OSPF

AttributesPrecedence

Local tableLabel table Local table Local table

LSP (Label-Switched Paths)Label swapping Label removal

Classification

Label assignment

Ingress

Node

Core

Node

Egress

Node

Label SwitchLayer 2

Layer 1

Layer 2

Layer 1

Layer 2

Layer 1

Layer 2

Layer 1

Layer 2

Layer 1

MPLS Architecture

FEC table Local table Local table Local table

Forward Equivalence Class

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MPLS process


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Label swapping

Label removal

Classification

Label assignment

Label swapping

Label removal

Classification

Label assignment

OSPF / RIP / IS-IS

Label Switch Path

Label table

Ingress

Node

Core

Node

Egress

Node

Layer 2

Layer 1

Layer 2

Layer 1

Layer 2

Layer 1

Precedence

Label table Label table

Layer 2

Layer 1

Layer 2

Layer 1

FEC FEC FEC

MPLS process

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MPLS Cloud


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LSR

LER

LSR

LER

IP PacketIP Packet w/ Label

L3 RoutingL3 Routing

Label SwappingLabel Swapping

LER: Label Edge Router

LERLER

L3 RoutingL3 Routing

L3 Routing

MPLS Cloud

LSR: Label Switch(ing) Router

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FEC Classification


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Ingress Label FEC Egress Label

6 138.120.6/24 - xxxx 9

Ingress Label AttributeFEC Egress LabelIngress Label FEC Egress Label

6 138.120.6/24 - xxxx 9

Attribute

A

6 138.120.6/24 - xxxx 12B

FECs are manually initiated by the operator

A FEC is associated to at least one Label

FEC Classification

A packet can be mapped to a particular FEC based on the following criteria: destination IP address,

source IP address,

TCP/UDP port,

in case of inter AS-MPLS, Source-AS and Dest-AS, class of service,

application used,

any combination of the previous criteria.

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What is a Label?


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L2 Type L2 TypePort PortIngress Label Egress LabelFEC

ATM 1-1 12 (i.e. 4/65) F1 22 (i.e. 5/65)3-4ATM

ATM 1-1 15 (i.e. 0/25) F4 9 (i.e. 101) 5-1FR

Gig Eth 5-1 7 F1 22 (i.e. 4/65)3-4ATM

What is a Label?

A label is a short, fixed length, locally significant identifier used toidentify a FEC.

The label can be identified by the L2 technology identifier (e.g.VPI/VCI for ATM, DLCI for FR or MPLS label for PPP/Ethernet).

MPLS Label Assignment Schemes Topology Driven

Label assignment in response to routing protocols (OSPF and BGP) updates

Control Driven Label assignment in response to RSVP, CR-LDP requests

Traffic Driven Label assignment in response to flow detection & triggering

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The MPLS Shim Header


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Label

(20 bits)

Exp

(3 bits)

S

(1 bit)

TTL

(8bits)

Label : Label value (0 to 15 are reserved for special use)

Exp : Experimental Use

S : Bottom of Stack (set to 1 for the last entry in the label)TTL : Time To Live

The MPLS Shim Header

The Label (Shim Header) is represented as a sequence of Label Stack Entry

Each Label Stack Entry is coded by 4 bytes (32 bits) as described 20 Bits is reserved for the Label Identifier (also named Label)

Based on the contents of the label a swap,push(impose) orpop(dispose) operation can be performed on the packet's label stack

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Label Switched Path


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Label Switched Path

5 12

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 138.120 312

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 138.120 x4

53

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 x 138.120

MPLS switch

MPLS switch

MPLS switch

MPLS switch1

2

3

1 2

3

1

2

3

4

1

2

3

138.120

192.168127.20

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Hop by Hop IP forwarding


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MPLS switch

MPLS switch

MPLS switch

MPLS switch1

2

3

1 2

3

1

2

3

4

1

2

3

138.120

192.168127.20

138.120.6.12

138.12

0.6.12

138.120.6.12138.120.6.1

2

138.120.6.12

138.12

0.6.12

??

138.120.6.12

Default3

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 x None

??

138.12

0.6.12

Default Default

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 None 3

??

138.12

0.6.12 ??

138.120.6.12

Default

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 None x4

??138.120.6.1

2

Hop by Hop IP forwarding


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MPLS Label Distribution Protocol


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LDP - a set of procedures by which one LSR informs the other ofthe FEC-to-Label binding it has made.

Currently, several protocols used as Label Distribution Protocol(LDP) are available:

RSVP-TE (MPLS extension) LDP and CR-LDP

BGP-4 MPLS extensions

Label Distribution schemes

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Downstream stream on demand


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Downstream stream on demand

Mapping12

Mapping5

5 12

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 138.120 312

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 138.120 x4

53

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 x 138.120

Reques

t138.1

20

Request138.120

MPLS switch

MPLS switch

MPLS switch

MPLS switch1

2

3

1 2

3

1

2

3

4

1

2

3

138.120

192.168127.20

The label is requested by the

upstream node and thedownstream node defines the label

used.

007/2008

Unsolicited Downstream


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Unsolicited Downstream

MPLS switch

MPLS switch

MPLS switch

MPLS switch1

2

3

1 2

3

1

2

3

4

1

2

3

138.120

192.168127.20

Mapping12

Mapping5

5 1212

5

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 138.120 3

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 138.120 x4

3

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 x 138.120

The downstream node defines

the label and advertises it to theupstream node.

007/2008

Edge LSR Features


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Routing protocols FEC Classification

Initiates LSP setup for Downstream On Demand method

Adaptation of non-MPLS data to MPLS data

Layer 2 translation for MPLS data

Terminated MPLS-VPN

At least one LDP protocol

Edge LSR is counted into the TTL count as a regular router

007/2008

Core LSR Features


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Routing protocols Propagates Downstream On Demand method (request and mapping)

Layer 2 translation

High speed label forwarding/switching

At least one LDP protocol

007/2008

MPLS Advantages


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Simplified Forwarding Efficient Explicit Routing

Traffic Engineering

QoS Routing

Mappings from IP Packet to Forwarding Equivalence Class (FEC)

Partitioning of Functionality

Common Operation over Packet and Cell media

Tema 2:

Tema 2:


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Aspectos de encaminamientoAspectos de encaminamiento

Algoritmos bsicos de encaminamiento Link state

Distance Vector


OSPF

BGP


IP multicast

2007/2008

Multicast = Efficient Data Distribution


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Src Src

2007/2008

Why Multicast ?


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Need for efficient one-to-many delivery of same dataApplications:

News/sports/stock/weather updates

Distance learning

Configuration, routing updates, service location Pointcast-type push apps

Teleconferencing (audio, video, shared whiteboard, text editor)

Distributed interactive gaming or simulations

Email distribution lists Content distribution; Software distribution

Web-cache updates

Database replication

2007/2008

Why Not Broadcast or Unicast?


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1

Broadcast: Send a copy to every machine on the net

Simple, but inefficient

All nodes must process packet even if they dont care

Wastes more CPU cycles of slower machines (broadcast radiation) Network loops lead to broadcast storms

Replicated Unicast:

Sender sends a copy to each receiver in turn

Receivers need to register or sender must be pre-configured Sender is focal point of all control traffic

Reliability => per-receiver state, separate sessions/processes at sender

2007/2008

Multicast Apps Characteristics


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2

Number of (simultaneous) senders to the group The size of the groups

Number of members (receivers)

Geographic extent or scope

Diameter of the group measured in router hops The longevity of the group

Number of aggregate packets/second

The peak/average used by source

Level of human interactivity

Lecture mode vs interactive

Data-only (eg database replication) vs multimedia

2007/2008

Reliable Multicast vs. Unreliable Multicast


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3

When a multicast message is sent by a process, the runtimesupport of the multicast mechanism is responsible for delivering themessage to each process currently in the multicast group.

As each participating process may be on a separate host, due to

factors such as failures of network links and/or network hosts,routing delays, and differences in software and hardware, the timebetween when a message is sent and when it is received may varyamong the recipient processes.

Moreover, a message may not be received by one or more of theprocesses at all.

2007/2008

Classification of multicasting mechanisms in terms of messagedelivery


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Unreliable multicast: The arrival of the correct message at each process is not guaranteed.

Reliable multicast:

Guarantees that each message is eventually delivered in a non-

corrupted form to each process in the group. The definition of reliable multicast requires that each participating

process receives exactly one copy of each message sent. It doesnot put any restriction of the order the messages delivered.

Reliable multicast can be further classified based on the order of thedelivery of the messages: unordered, FIFO, causal order, atomicorder.

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Classification of reliable multicast -- unordered


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An unordered reliable multicast system guarantees the safe deliveryof each message, but it provides no guarantee on the delivery orderof the messages.

Example: Processes P1, P2, and P3 have formed a multicast group.

Three messages, m1, m2, m3 have been sent to the group. Anunordered reliable multicast system may deliver the messages toeach of the three processes in any of these:

m1-m2-m3,

m1-m3-m2,m2-m1-m3,

m2-m3-m1,

m3-m1-m2,

m3-m2-m1

C2007/2008

Classification of reliable multicast - FIFO


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If process P sent messages mi and mj, in that order, then eachprocess in the multicast group will be delivered the messages miand mj, in that order.

Note that FIFO multicast places no restriction on the delivery order

among messages sent by different processes. For example, P1sends messages m11 then m12, and P2 sends messages m21 thenm22. It is possible for different processes to receive any of thefollowing orders:

m11-m12-m21-m22,m11-m21-m12-m22,

m11-m21-m22-m12,

m21-m11-m12-m22

m21-m11-m22-m12m21-m22-m11-m12.

C2007/2008

Classification of reliable multicast Causal order


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If message mi causes (results in) the occurrence of message mj,then mi will be delivered to each process prior to mj. Messages miand mj are said to have a causal or happen-before relationship.

For example, P1 sends a message m1, to which P2 replies with a

multicast message m2. Since m2 is triggered by m1, the twomessages share a causal relationship of m1-> m2. A causal-ordermulticast message system ensures that these two messages will bedelivered to each of the processes in the order of m1- m2.

C2007/2008

Classification of reliable multicast Atomic order


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In an atomic-order multicast system, all messages are guaranteedto be delivered to each participant in the exact same order. Notethat the delivery order does not have to be FIFO or causal, butmust be identical for each process.

Example: P1 sends m1, P2 sends m2, and P3 sends m3.

An atomic system will guarantee that the messages will be deliveredto each process in only one of the six orders:

m1-m2- m3, m1- m3- m2, m2- m1-m3,

m2-m3-m1, m3-m1- m2, m3-m2-m1.

C2007/2008

IP Multicast Architecture


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Hosts

Routers

Service modelService model

Host-to-router protocol

(IGMP)

Multicast routing protocols

(various)

IC2007/2008

IP Multicast model: RFC 1112


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Message sent to multicast group (of receivers) Senders need not be group members

A group identified by a single group address

Use group address instead of destination address in IP packet sent togroup

Groups can have any size;

Group members can be located anywhere on the Internet

Group membership is not explicitly known

Receivers can join/leave at will

Packets are not duplicated or delivered to destinations outside thegroup

Distribution tree constructed for delivery of packets

No more than one copy of packet appears on any subnet

Packets delivered only to interested receivers => multicast deliverytree changes dynamically

Network has to actively discover paths between senders and receivers

IC2007/2008

IP Multicast Addresses


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Class D IP addresses 224.0.0.0 239.255.255.255

Address allocation: Well-known (reserved) multicast addresses, assigned by IANA:

224.0.0.x and 224.0.1.x Transient multicast addresses, assigned andreclaimed dynamically, e.g., by sdr program

Each multicast address represents a group of arbitrary size, called ahost group

There is no structure within class D address space like subnetting=> flat address space

1 1 1 0 Group ID

IC2007/2008

IP Multicast Service


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2

Sending Uses normal IP-Send operation, with an IP multicast address specified

as the destination

Must provide sending application a way to: Specify outgoing network interface, if >1 available

Specify IP time-to-live (TTL) on outgoing packet Enable/disable loop-back if the sending host is/isn't a member of the

destination group on the outgoing interface

Receiving Two new operations

Join-IP-Multicast-Group(group-address, interface)

Leave-IP-Multicast-Group(group-address, interface)

Receive multicast packets for joined groups via normal IP-Receive

operation

IC2007/2008

Link-Layer Transmission/Reception


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Transmission IP multicast packet is transmitted as a link-layer multicast, on those

links that support multicast

Link-layer destination address is determined by an algorithm specific tothe type of link

Reception

Necessary steps are taken to receive desired multicasts on a particularlink, such as modifying address reception filters on LAN interfaces

Multicast routers must be able to receive all IP multicasts on a link,

without knowing in advance which groups will be used

IC2007/2008

Using Link-Layer Multicast Addresses


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Ethernet and other LANs using 802 addresses: Direct mapping! Simpler than unicast! No ARP etc.

32 class D addresses may map to one MAC address

Special OUI for IETF: 0x01-00-5E.

No mapping needed for point-to-point links

LAN multicast address

0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 1 0 1 1 1 1 0 0

1 1 1 0 28 bits

23 bits

IP multicast address

Group bit

rIC2007/2008

Multicast over LANs & Scoping


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Multicasts are flooded across MAC-layer bridges along a spanningtree

But flooding may steal sending opportunity for non-member stationswhich want to transmit

Almost like broadcast!

Scope: How far do transmissions propagate?

Implicit scoping: Reserved Mcast addresses => dont leave subnet.

Also called link-local addresses

TTL-based scoping:

Multicast routers have a configured TTL threshold

Multicast datagram dropped if TTL


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TTL expanding-ring search to reach or find a nearby subset of agroup

Rings can be nested, but not overlapping

s

1

2

3

rIC2007/2008



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Hosts

Routers

Service model

HostHost--toto--router protocolrouter protocol(IGMP)(IGMP)

Multicast routing protocols

(various)

erIC2007/2008

Internet Group Management Protocol


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IGMP: signaling protocol to establish, maintain, remove groups ona subnet.

Objective: keep router up-to-date with group membership of entireLAN

Routers need not know who all the members are, only that membersexist

Each host keeps track of which mcast groups are subscribed to

Socket API informs IGMP process of all joins


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How IGMP Works (cont.)


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When a hosts timer for group G expires, it sends a MembershipReport to group G, with TTL = 1

Other members of G hear the report and stop (suppress) theirtimers

Routers hear all reports, and time out non-responding groups

Q

G G G G

Routers:

Hosts:

erIC2007/2008

How IGMP Works (cont.)


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Normal case: only one report message per group present is sent inresponse to a query

Query interval is typically 60-90 seconds

When a host first joins a group, it sends immediate reports, instead

of waiting for a query

IGMPv2: Hosts may send a Leave group message to all routers(224.0.0.2) address

Querier responds with a Group-specific Query message: see if anygroup members are present

Lower leave latency

terIC2007/2008

The Java Basic Multicast API


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At the transport layer, the basic multicast supported by Java is anextension of UDP (the User Datagram Protocol)

For the basic multicast, Java provides a set of classes which areclosely related to the datagram socket API classes

terIC2007/2008

Datagram - recap


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a byte array

a DatagramPacket object

receiver's

address

a DatagramSocket

object

sender process

a byte array

a DatagramPacket object

a DatagramSocket

object

receiver process

send

receive

object reference

data flow

terIC2007/2008

The Java Basic Multicast API - 2


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There are four major classes in the API, the first three of which wehave already seen in the context of datagram sockets.

InetAddress: In the datagram socket API, this class represents theIP address of the sender or receiver. In multicasting, this classcan be used to identify a multicast group.

DatagramPacket: As with datagram sockets, an object of this classrepresents an actual datagram; in multicast, a DatagramPacketobject represents a packet of data sent to all participants orreceived by each participant in a multicast group.

DatagramSocket: In the datagram socket API, this class representsa socket through which a process may send or receive data.

MulticastSocket : A MulticastSocket is a DatagramSocket, withadditional capabilities for joining and leaving a multicast group. Anobject of the multicast datagram socket class can be used for

sending and receiving multicast packets. In the Java API, aMulticastSocket object is bound to a port address, e.g. 3456, andmethods of the object allows for the joining and leaving of amulticast address, e.g. 239.1.2.3

terIC2007/2008

Joining a multicast group

To join a multicast group at IP address mand UDP portp, a


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MulticastSocket object must be instantiated withp, then the objectsjoinGroup method can be invoked specifying the address m:

// join a Multicast group at IP address

// 239.1.2.3 and port 3456

InetAddress group =

InetAddress.getByName("239.1.2.3"); MulticastSocket s =

new MulticastSocket(3456); s.joinGroup(group);

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Sending to a multicast group


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A multicast message can be sent using syntax similar with the datagramsocket API.

String msg = "a multicast message.";

InetAddress group =

InetAddress.getByName("239.1.2.3");

MulticastSocket s =

new MulticastSocket(3456);

s.joinGroup(group); // optional

DatagramPacket hi =new DatagramPacket(msg.getBytes( ),

msg.length( ),group, 3456);

s.send(hi);

terIC2007/2008

Receiving messages sent to a multicast group

h h d l h


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A process that has joined a multicast group may receive messages sent to the groupusing syntax similar to receiving data using a datagram socket API.

byte[] buf = new byte[1000];

InetAddress group =

InetAddress.getByName("239.1.2.3");

MulticastSocket s =

new MulticastSocket(3456);

s.joinGroup(group);

DatagramPacket recv =

new DatagramPacket(buf,buf.length);

s.receive(recv);

terIC2007/2008

Leaving a multicast group

A l lti t b i ki th l G


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A process may leave a multicast group by invoking the leaveGroupmethod of a MulticastSocket object, specifying the multicastaddress of the group.

s.leaveGroup(group);

terIC2007/2008

Setting the time-to-live

Th ti t d t t lti t f


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The runtime support needs to propagate a multicast message froma host to a neighboring host in an algorithm which, when executedproperly, will eventually deliver the message to all the participants.

Under some anomalous circumstances, however, it is possible thatthe algorithm which controls the propagation does not terminateproperly, resulting in a packet circulating in the network indefinitely.

Indefinite message propagation causes unnecessary overhead onthe systems and the network.

To avoid this occurrence, it is recommended that a time to liveparameter be set with each multicast datagram.

The time-to-live (ttl) parameter, when set, limits the count ofnetwork links or hops that the packet will be forwarded on thenetwork.

as

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Th d d ttl tti


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The recommended ttl settings are: 0 if the multicast is restricted to processes on the same host

1 if the multicast is restricted to processes on the same subnet

32 if the multicast is restricted to processes on the same site

64 if the multicast is restricted to is processes on the same region 128 is if the multicast is restricted to processes on the same

continent

255 is the multicast is unrestricted

as

terIC2007/2008


String msg = "Hello everyone!";

dd


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InetAddress group =

InetAddress.getByName("239.1.2.3");MulticastSocket s = new MulticastSocket(3456);s.setTimeToLive(1);

// set time-to-live to 1 hop

DatagramPacket hi =

new DatagramPacket(msg.getBytes( ),msg.length( ),group, 3456);

s.send(hi);

The value specified for the ttlmust be in the range 0


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To join a group, you use the setsockopt() kernel service call with anew parameter. The new parameter is the ip_mreq structure:

The imr_multiaddr field specifies the multicast group you want tojoin. It is the same format as the sin_addr field in the sockaddr_instructure. The imr_interface field lets you choose a particular hostinterface. This is similar to a bind(), which lets you specify the host

interface (or leave the host option wide open with an INADDR_ANYvalue).

/************************************************************/

/*** The ip_mreq structure for selecting a multicast addr ***/

/************************************************************/

struct ip_mreq{

struct in_addr imr_multiaddr; /* known multicast group */

struct in_addr imr_interface; /* network interface */

} ;

Mas

terIC2007/2008

The C version: Joining Multicast Groups

The following code snippet shows you how to join a group using thei t t It t th i i t f fi ld t INADDR ANY


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The following code snippet shows you how to join a group using theip_mreq structure. It sets the imr_interface field to INADDR_ANYmerely for demonstration. Do not use it unless you have only oneinterface on your host; the results can be unpredictable .

/************************************************************/

/*** Join a multicast group ***/

/************************************************************/

const char *GroupID = "224.0.0.10";

struct ip_mreq mreq;

if ( inet_aton(GroupID, &mreq.imr_multiaddr) == 0 )

panic("address (%s) bad", GroupID);

mreq.imr_interface.s_addr = INADDR_ANY;

if ( setsockopt(sd, SOL_IP, IP_ADD_MEMBERSHIP,&mreq,sizeof(mreq))!= 0)

panic("Join multicast failed");

/************************************************************/

/*** Drop a multicast group ***/

/************************************************************/

if ( setsockopt(sd, SOL_IP, IP_DROP_MEMBERSHIP, &mreq, sizeof(mreq)) != 0 )

panic("Drop multicast failed");

Mas

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Hosts

Routers

Service model

Host-to-router protocol

(IGMP)

Multicast routing

protocols

Mas

terIC2007/2008

Multicast Routing

Basic objective build distribution tree for multicast packets

Th l f th di t ib ti t th b t t i i t l t


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The leaves of the distribution tree are the subnets containing at leastone group member (detected by IGMP)

Multicast service model makes it hard

Anonymity

Dynamic join/leave

Mas

terIC2007/2008

Simple Multicast Routing Techniques

Flood and prune

Begin by flooding traffic to entire network


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Begin by flooding traffic to entire network

Prune branches with no receivers

Examples: DVMRP, PIM-DM

Unwanted state where there are no receivers

Link-state multicast protocols

Routers advertise groups for which they have receivers to entirenetwork

Compute trees on demand

Example: MOSPF

Unwanted state where there are no senders

Mas

terIC2007/2008

How to Flood Efficiently ?

A router forwards a packet from source (S) iffit arrives via theshortest path from the router back to S


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shortest path from the router back to S

Reverse path check!

Packet is replicated out all but the incoming interface

Reverse shortest paths easy to compute

just use info in DVrouting tables

DV gives shortest reverse paths

Efficient if costs are symmetric

xxyy

tt

SS

a

zz

Forward packets that arriveon shortest path from t to S(assume symmetric routes)

Mas

terIC2007/2008

Problem

Flooding can cause a given packet to be sent multiple times over the same

link: can filter better than this!


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ca te bette t a t s

Solution: Reverse Path Broadcasting

xx yy

zz

SS

a

b

duplicate packet

MasterIC2007/2008

Reverse Path Broadcasting (RPB)

Basic idea: forward a packet from S only on child links for S

Child link of router x for source S: link that has x as parent on the


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Child link of router x for source S: link that has x as parent on theshortest path from the link to S

xx yy

zz

SS

a

b

5 6

child link of xfor S

forward onlyto child link

MasterIC2007/2008

How to Find Child Links?

Routing updates !

If z tells x that it can reach S through y


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If z tells x that it can reach S through y,

and if the cost of this path is >= the cost of the path from z to Sthrough x,

then x knows that the link to z is a child link

In case of tie, lower address wins

xx yy

zz

SS

a

b

5 6

child link of xfor S

forward only

to child link

MasterIC2007/2008

Truncated RPB

This is still a broadcast algorithm the traffic goes everywhere lousy filtering!


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y g

First order solution: Truncated RPB

Don't forward traffic onto networks with no receivers

Identify leaves

Leaf links are the child links that no other router uses to reach source S

Use periodic updates of form: this is my next-link to source S

If child is not the next-link for anyone, it is a leaf

Detect group membership in leaf (IGMP)

-MasterIC2007/2008

Reverse Path Multicast (RPM)

Prune back transmission so that only absolutely necessary linkscarry traffic


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10

y

Use on-demand pruning so that router group state scales withnumber of active groups

xx yy

tt

SS

vv bbaa

aa

bb

data messageprune message

-MasterIC2007/2008

Basic RPM Idea

Prune (Source,Group) at leaf if no members

Send Non-Membership Report (NMR) up the tree


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p p ( ) p

If all children of router R prune (S,G)

Propagate prune for (S,G) to parent R

On timeout: Prune dropped

Flow is reinstated

Down stream routers re-prune

Note: this is a soft-state approach Grafting: Explicitly reinstate sub-tree when

IGMP detects new members at leaf, or when a child asks for a graft.

-MasterIC2007/2008

Putting it together: Topology


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G G

S

G

-MasterIC2007/2008

Flood with Truncated Broadcast

G G


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S

G

-MasterIC2007/2008

Pruning

G G


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S

Prune (s,g)

Prune (s,g)

G

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Grafting

G G


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Graft (s,g)

Graft (s,g)S

G

G

Report (g)

a-Ma

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After Grafting Complete

G G


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G

G

a-Ma

sterIC2007/2008

Reliable Multicast: The Goal

Implement reliability on top of IP multicast

Why is this hard ?


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Sender cannot keep state for unknown number of dynamic receivers

Remember open & dynamic group semantic?

Algorithms like TCP that estimate path properties such as RTT and

congestion window dont generalize to trees. Remember: TCP is only for a unicast session!

Has to address (N)ACK implosions

a-Ma

sterIC2007/2008

Implosion

Packet 1 is lost All 4 receivers request a resend


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R1

S

R3 R4

R2

21

R1

S

R3 R4

R2

Resend request

ia-Ma

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Retransmission

Re-transmitter

Options: sender, other receivers


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How to retransmit

Unicast, multicast, scoped multicast, retransmission group,

Problem: retransmissions (aka repairs) may reach destinations thatdont require a retransmission

A.k.a exposure problem

Solution: subcast the re-transmission only to receivers that need it.

ia-Ma

sterIC2007/2008

Why Subcast? Exposure problem

Packet 1 does not reach R1;Receiver 1 requests a resend

Packet 1 resent to all 4 receivers


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R1

S

R3 R4

R2

21

R1

S

R3 R4

R2

q

1

1

Resend request Resent packet

dia-Ma

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Ideal Recovery Model

Packet 1 reaches R1 but is lostbefore reaching other Receivers

Only one receiver sends NACK tothe nearest S or R with packet


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S

R3 R4

R2

2

1

S

R3 R4

R2

g p

Resend request

1 1

Resent packet

Repair sent

only to

those thatneed packet

R1 R1

dia-Ma

sterIC2007/2008

Reliable Multicast Transport: Issues

Retransmission can make reliable multicast as inefficient as

replicated unicast (N)ACK-implosion if all destinations ack at once


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(N)ACK-implosion if all destinations ack at once

Crying baby: a bad link affects entire group

Heterogeneity: receivers, links, group sizes

Anonymous/Open/Dynamic Group Model:

Source does not know # of destinations, and destinations may vanish

Multicast applications do not need strong reliability of the typeprovided by TCP.

Can tolerate some reordering, delay, etc

Universidad Publica de Valencia_T2_encaminamiento

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