tema 2: aspectos de encaminamiento

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

Tema 2: Aspectos de encaminamientoTema 2: Aspectos de encaminamiento

Algoritmos básicos 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 arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing, forwardingComputer Networking: A Top

Down Approach Featuring the Internet,

3rd edition. Jim Kurose, Keith Ross

Addison-Wesley, July 2004.

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u

yx

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z2

2

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1

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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

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z2

2

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1

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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 to congestion

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

Question: What’s 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 cost info “link state” algorithmsDecentralized: router knows physically-connected

neighbors, link costs to neighbors iterative process of computation,

exchange of info with neighbors “distance vector” algorithms

Static or dynamic?Static: routes change slowly over timeDynamic: routes change more quickly

periodic update in response to link cost

changes

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A Link-State Routing Algorithm: Dijsktra’s Algorithm

net topology, link costs known to all nodes

accomplished via “link state broadcast”

all nodes have same info computes least cost paths from

one node (‘source”) to all other nodes

gives forwarding table for that node

iterative: after k iterations, know least cost path to k destinations

Notation: c(x,y): link cost from node x to y; = ∞ if not

direct neighbors D(v): current value of cost of path from

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

source to v N': set of nodes whose least cost path

definitively known

1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 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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Dijkstra’s 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

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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 ~ ~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

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 ~ ~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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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 ~ ~1 AD 2, A 4, D 2, D ~2 ADE 2, A 3, E 4, E3 ADEB 3, E 4, E

5

B C D E F

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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 ~ ~1 AD 2, A 4, D 2, D ~2 ADE 2, A 3, E 4, E3 ADEB 3, E 4, E4 ADEBC 4, E

5

B C D E F

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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 ~ ~1 AD 2, A 4, D 2, D ~2 ADE 2, A 3, E 4, E3 ADEB 3, E 4, E4 ADEBC 4, E

5

B C D E F

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A

ED

CB

F

Resulting shortest-path tree from A:

BD

E

C

F

(A,B)(A,D)

(A,D)

(A,D)

(A,D)

destination link

Resulting forwarding table in A:

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Dijkstra’s 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

D

C

B2+e 0

001+e1

A

D

C

B0 2+e

1+e10 0

A

D

C

B2+e 0

e01+e1

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

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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 є 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 maintains

Dv = [Dv(y): y є 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 to neighbors When a node x receives new DV estimate from neighbor, it

updates its own DV using B-F equation:

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

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

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Iterative, asynchronous: each local iteration caused by: local link cost change DV update message from

neighbor Distributed:

each node notifies neighbors only when its DV changes

neighbors then notify their neighbors if necessary

wait for (change in local link cost of msg from neighbor)

recompute estimates

if DV to any dest has

changed, notify neighbors

Each node:

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x y z

xyz

0 2 7

∞ ∞ ∞∞ ∞ ∞

from

cost tofr

om

from

x y z

xyz

0 2 3

from

cost tox y z

xyz

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

xyz

0 2 3fr

om

cost tox y z

xyz

0 2 7

from

cost to

x y z

xyz

∞ ∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

2 0 17 1 0

2 0 13 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 between

neighbors only convergence time varies

Speed of Convergence LS: O(n2) algorithm requires

O(nE) msgs may have oscillations

DV: convergence time varies may be routing loops count-to-infinity problem

Robustness: what happens if router malfunctions? LS:

node can advertise incorrect link cost

each node computes only its own table

DV: DV node can advertise

incorrect path cost each node’s table used by

others – error propagate thru network

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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: can’t store all dest’s 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

1c2aAS3

AS1

AS21a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

Forwarding table is configured by both intra- and inter-AS routing algorithm Intra-AS sets entries for

internal dests Inter-AS & Intra-As sets

entries for external dests

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3b

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3a

1c2aAS3

AS1

AS21a

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1b

3c

Inter-AS tasks

Suppose router in AS1 receives datagram for which dest is outside of AS1 Router should forward

packet towards one of the gateway routers, but which one?

AS1 needs: to learn which dests are

reachable through AS2 and which through AS3

to propagate this reachability info to all routers in AS1

Job of inter-AS routing!

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Learn from inter-AS protocol that subnet x is reachable via multiple gateways

Use routing infofrom intra-AS

protocol to determine

costs of least-cost paths to each

of the gateways

Hot potato routing:Choose the

gatewaythat has the

smallest least cost

Determine fromforwarding table the interface 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 is reachable from AS3 and from AS2.

To configure forwarding table, router 1d must determine towards which 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 hops u 1 v 2 w 2 x 3 y 3 z 2

From router A to subsets:

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RIP advertisements

Distance vectors: exchanged among neighbors every 30 sec via Response 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 declared dead 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 = 16 hops)

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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 Dijkstra’s 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 malicious intrusion)

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.

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Hierarchical OSPF

Two-level hierarchy: local area, backbone. Link-state advertisements

only in area each nodes has detailed

area topology; only know direction (shortest path) to nets in other areas.

Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.

Backbone routers: run OSPF routing limited to backbone.

Boundary routers: connect to other AS’s.

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Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de 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

information and policy. Allows a subnet to advertise its existence to rest of the

Internet: “I am here”

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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

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

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BGP route selection

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

Elimination rules: Local preference value attribute: policy decision Shortest AS-PATH Closest NEXT-HOP router: hot potato routing Additional criteria

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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

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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 doesn’t use specific technology process

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MPLS and ISO model

PPP

Physical (Optical - Electrical) 1

2

IP 3

4

Applications7to5

FrameRelay

ATM (*)

TCP UDP

PPP Frame Relay ATM (*)

MPLS

(*) ATM overlay model (without addressing and P-NNI) is considered as an ISO layer 2 protocol.

IETF main goal is that when a layer is added, no modification is needed on the existing layers.All new protocol must be backward compatible

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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 packet is 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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Routing protocol OSPF OSPF OSPF

Attributes Precedence

Local tableLabel table Local table Local table

LSP (Label-Switched Paths) Label swapping Label removal

ClassificationLabel assignment

IngressNode

CoreNode

EgressNode

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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Label swapping

Label removal


Label swapping

Label removal


OSPF / RIP / IS-IS

Label Switch Path

Label table

IngressNode

CoreNode

EgressNode

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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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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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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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 to identify 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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Label(20 bits)

Exp(3 bits)

S(1 bit)

TTL(8bits)

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

Exp : Experimental UseS : 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) or pop (dispose) operation can be performed on the packet's label stack

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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

41

2

3

138.120

192.168127.20

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MPLS switch

MPLS switch

MPLS switch

MPLS switch1

2

3

1 2

3

1

2

3

41

2

3

138.120

192.168127.20138.120.6.12

138.120.6.12138.120.6.12

138.120.6.12 138.120.6.12

138.120.6.12

??138.120.6.12

Default3

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 x None

??

138.120.6.12

Default Default

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 None 3

??

138.120.6.12 ??138.120.6.12

Default

IngressInterface

IngressLabel

FEC EgressInterface

EgressLabel

1 None x4

??138.120.6.12

Hop by Hop IP forwarding

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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

41

2

3

138.120

192.168127.20

138.120.6.12

138.120.6.12

138.120.6.12

138.120.6.12 138.120.6.12

IP forwarding using Label Switch Path

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MPLS Label Distribution Protocol

LDP - a set of procedures by which one LSR informs the other of the 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

Mapping 12Mapping 5

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

Request 138.120Request 138.120

MPLS switch

MPLS switch

MPLS switch

MPLS switch1

2

3

1 2

3

1

2

3

41

2

3

138.120

192.168127.20

The label is requested by the upstream node and the downstream node defines the label used.

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Unsolicited Downstream

MPLS switch

MPLS switch

MPLS switch

MPLS switch1

2

3

1 2

3

1

2

3

41

2

3

138.120

192.168127.20

Mapping 12Mapping 5

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 the upstream node.

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Edge LSR Features

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

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Core LSR Features

Routing protocols Propagates Downstream On Demand method (request and

mapping) Layer 2 translation High speed label forwarding/switching At least one LDP protocol

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MPLS Advantages

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

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Multicast = Efficient Data Distribution

Src Src

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Why Multicast ?

Need for efficient one-to-many delivery of same data Applications:

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

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Why Not Broadcast or Unicast?

Broadcast: Send a copy to every machine on the net Simple, but inefficient All nodes must process packet even if they don’t 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

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Multicast Apps Characteristics

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

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Reliable Multicast vs. Unreliable Multicast

When a multicast message is sent by a process, the runtime support of the multicast mechanism is responsible for delivering the message 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 time between when a message is sent and when it is received may vary among the recipient processes.

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

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Classification of multicasting mechanisms in terms of message delivery

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 does not put any restriction of the order the messages delivered.

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

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Classification of reliable multicast -- unordered

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

Example: Processes P1, P2, and P3 have formed a multicast group. Three messages, m1, m2, m3 have been sent to the group. An unordered reliable multicast system may deliver the messages to each 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

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Classification of reliable multicast - FIFO

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

Note that FIFO multicast places no restriction on the delivery order among messages sent by different processes. For example, P1 sends messages m11 then m12, and P2 sends messages m21 then m22. It is possible for different processes to receive any of the following orders:

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

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Classification of reliable multicast – Causal order

If message mi causes (results in) the occurrence of message mj, then mi will be delivered to each process prior to mj. Messages mi and 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 two messages share a causal relationship of m1-> m2. A causal-order multicast message system ensures that these two messages will be delivered to each of the processes in the order of m1- m2.

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Classification of reliable multicast – Atomic order

In an atomic-order multicast system, all messages are guaranteed to be delivered to each participant in the exact same order. Note that the delivery order does not have to be FIFO or causal, but must 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 delivered to 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.

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IP Multicast Architecture

Hosts

Routers

Service modelService model

Host-to-router protocol(IGMP)

Multicast routing protocols(various)

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IP Multicast model: RFC 1112

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 to group

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 the group 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

delivery tree changes dynamically Network has to actively discover paths between senders and

receivers

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IP Multicast Addresses

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 and reclaimed dynamically, e.g., by “sdr” program

Each multicast address represents a group of arbitrary size, called a “host group”

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

1 1 1 0 Group ID

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IP Multicast Service

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

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Link-Layer Transmission/Reception

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 to the type of link Reception

Necessary steps are taken to receive desired multicasts on a particular link, 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

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Using Link-Layer Multicast Addresses

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

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Multicast over LANs & Scoping

Multicasts are flooded across MAC-layer bridges along a spanning tree But flooding may steal sending opportunity for non-member

stations which want to transmit Almost like broadcast!

Scope: How far do transmissions propagate? Implicit scoping: Reserved Mcast addresses => don’t leave

subnet. Also called “link-local” addresses

TTL-based scoping: Multicast routers have a configured TTL threshold Multicast datagram dropped if TTL <= TTL threshold Useful as a blanket parameter.

Administrative scoping: Use a portion of class D address space (239.0.0.0 thru

239.255.255.255) Truly local to admin domain; address reuse possible. In IPv6, scoping is an internal attribute of an IPv6 multicast address

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Multicast Scope Control – Small TTLs

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

Rings can be nested, but not overlapping

s

1

2

3

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Hosts

Routers

Service model

Host-to-router protocolHost-to-router protocol(IGMP)(IGMP)

Multicast routing protocols(various)

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Internet Group Management Protocol

IGMP: “signaling” protocol to establish, maintain, remove groups on a subnet.

Objective: keep router up-to-date with group membership of entire LAN Routers need not know who all the members are, only that

members exist 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

On each link, one router is elected the “querier” Querier periodically sends a Membership Query message to

the all-systems group (224.0.0.1), with TTL = 1 On receipt, hosts start random timers (between 0 and 10

seconds) for each multicast group to which they belong

QRouters:

Hosts:

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How IGMP Works (cont.)

When a host’s timer for group G expires, it sends a Membership Report to group G, with TTL = 1

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

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

Q

G G G G

Routers:

Hosts:

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How IGMP Works (cont.)

Normal case: only one report message per group present is sent in response 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

any group members are present Lower leave latency

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The Java Basic Multicast API

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

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

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Datagram - recap

a by te a rra y

a D a ta g ra m Pa ck e t o bje ct

r e c e i ve r 'sa d d r e s s

a D a ta g ra m S o ck e t o bje ct

s e nde r pro c e s s

a by te a rra y

a D a ta g ra m Pa ck e t o bje ct

a D a ta g ra m S o ck e t o bje ct

re c e ive r pro c e s s

s e n d

re ce iv e

o bje ct re f e re n ce

da ta f lo w

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The Java Basic Multicast API - 2

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

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

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

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

MulticastSocket : A MulticastSocket is a DatagramSocket, with additional capabilities for joining and leaving a multicast group. An object of the multicast datagram socket class can be used for sending and receiving multicast packets. In the Java API, a MulticastSocket object is bound to a port address, e.g. 3456, and methods of the object allows for the joining and leaving of a multicast address, e.g. 239.1.2.3

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Joining a multicast group

To join a multicast group at IP address m and UDP port p, a MulticastSocket object must be instantiated with p, then the object’s joinGroup 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

A multicast message can be sent using syntax similar with the datagram socket 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);

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Receiving messages sent to a multicast group

A process that has joined a multicast group may receive messages sent to the group using 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);

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Leaving a multicast group

A process may leave a multicast group by invoking the leaveGroup method of a MulticastSocket object, specifying the multicast address of the group.

s.leaveGroup(group);

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Setting the “time-to-live”

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

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

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

To avoid this occurrence, it is recommended that a “time to live” parameter be set with each multicast datagram.

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

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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

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String msg = "Hello everyone!"; 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 ttl must be in the range 0 <= ttl <= 255; an IllegalArgumentException will be thrown otherwise.

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The C version: Joining Multicast Groups

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

The imr_multiaddr field specifies the multicast group you want to join. It is the same format as the sin_addr field in the sockaddr_in structure. The imr_interface field lets you choose a particular host interface. This is similar to a bind(), which lets you specify the host interface (or leave the host option wide open with an INADDR_ANY value).

/************************************************************//*** 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 */} ;

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The C version: Joining Multicast Groups

The following code snippet shows you how to join a group using the ip_mreq structure. It sets the imr_interface field to INADDR_ANY merely for demonstration. Do not use it unless you have only one interface 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");

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Hosts

Routers

Service model

Host-to-router protocol(IGMP)

Multicast routing protocols

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Multicast Routing

Basic objective – build distribution tree for multicast packets The “leaves” of the distribution tree are the subnets containing

at least one group member (detected by IGMP)

Multicast service model makes it hard Anonymity Dynamic join/leave

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Simple Multicast Routing Techniques

Flood and prune 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

entire network Compute trees on demand Example: MOSPF Unwanted state where there are no senders

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How to Flood Efficiently ?

A router forwards a packet from source (S) iff it arrives via the 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

DV routing 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)

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Problem

Flooding can cause a given packet to be sent multiple times over the same link: can filter better than this!

Solution: Reverse Path Broadcasting

xx yy

zz

SS

a

b

duplicate packet

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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 shortest path from the link to S

xx yy

zz

SS

a

b

5 6

child link of xfor S

forward onlyto child link

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How to Find Child Links?

Routing updates ! 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

S through 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 onlyto child link

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Truncated RPB

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

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)

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Reverse Path Multicast (RPM)

Prune back transmission so that only absolutely necessary links carry traffic

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

xx yy

tt

SS

vv bbaa

aa bb

data messageprune message

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Basic RPM Idea

Prune (Source,Group) at leaf if no members Send Non-Membership Report (NMR) up the tree

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.

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Putting it together: Topology

G G

S

G

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Flood with Truncated Broadcast

G G

S

G

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Pruning

G G

S

Prune (s,g)

Prune (s,g)

G

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Graft (s,g)

Graft (s,g)

Grafting

G G

S

G

G

Report (g)

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After Grafting Complete

G G

S

G

G

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Reliable Multicast: The Goal

Implement reliability on top of IP multicast Why is this hard ?

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 don’t generalize to trees. Remember: TCP is only for a unicast session!

Has to address (N)ACK implosions

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R1

Implosion

S

R3 R4

R2

21

R1

S

R3 R4

R2

Packet 1 is lost All 4 receivers request a resend

Resend request

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Retransmission

Re-transmitter Options: sender, other receivers

How to retransmit Unicast, multicast, scoped multicast, retransmission group, …

Problem: retransmissions (aka repairs) may reach destinations that don’t require a retransmission A.k.a “exposure” problem Solution: subcast the re-transmission only to receivers that

need it.

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R1

Why Subcast? Exposure problem…

S

R3 R4

R2

21

R1

S

R3 R4

R2

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

Packet 1 resent to all 4 receivers

1

1

Resend request Resent packet

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Ideal Recovery Model

S

R3 R4

R2

2

1

S

R3 R4

R2

Packet 1 reaches R1 but is lost before reaching other Receivers

Only one receiver sends NACK to the nearest S or R with packet

Resend request

1 1Resent packet

Repair sent only to those that need packet

R1 R1

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Reliable Multicast Transport: Issues

Retransmission can make reliable multicast as inefficient as replicated unicast (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 type provided by TCP. Can tolerate some reordering, delay, etc

tema 2: aspectos de encaminamiento

Documents