coms/csee 4140 networking laboratory lecture 11

COMS/CSEE 4140 Networking Laboratory

Lecture 11

Salman Abdul BasetSpring 2008

2

Announcements Prelab 10 and Lab report 9 due next week

before your lab slot Weekly project reports due before Friday

5pm

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Agenda Multicast

why multicast? multicast groups terminology (S,G) notation

Addressing IGMP Source and core-based trees PIM

PIM-DM PIM-SM

4

Many applications transmit the same data at one time to multiple receivers

Broadcasts of radio or video (streaming) Video conferencing (interactive video) Shared applications (data, file transfer)

A network “must” have mechanisms to support such applications in an efficient manner

Applications with Multiple Receivers

5

Multicasting Multicast communications refers to one-to-

many or many-to-many communications.

IP Multicasting refers to the implementation of multicast communication in the Internet

Multicast is driven by receivers: Receivers indicate interest in receiving data

Unicast Broadcast Multicast

6

Multicasting over a Packet Network Without support for multicast at the

network layer:

Multiple copies of the same message is transmitted on the same link

7

Multicasting over a Packet Network With support for multicast at the

network layer:

• Requires a set of mechanisms: (1) Packet forwarding can send multiple copies of same packet(2) Multicast routing algorithm which builds a spanning tree (dynamically)

Why IP Multicast?

Cost-efficient distribution of data Timely distribution of data Robust distribution of data

“Data” here could be Files Streamed Audio or Video

Source: Internet2 workshop

Cost Efficient Data Distribution This is the core of the streaming case.

Unicast streaming is just too expensive. This is true either on the commodity

Internet or on the Intranet. Multicast is especially compelling for

video.

Question: Is multicast a good model for YouTube? Live video streaming?


Timely Distribution of Data An argument for multicast in financial

services. Though reliability is an issue.

In unicast, it takes time to send packets separately to each receiver. Even if cost is not a problem, time may be. 100 Mb/s link, takes 22 hours to distribute a

100 MB file t0 10,000 machines. With multicast, it will take eight seconds.

Alternate solution is to have huge upstream bw But…

Multicasting is compelling here if timely distribution is important. Source: Internet2 workshop

Robust Distribution of Data Handle sudden large increases in load

streaming data distribution

Multicast was designed to handle sudden large increases in load.


12

Multicast Groups The set of receivers for a multicast

transmission is called a multicast group A multicast group is identified by a multicast

address A user that wants to receive multicast

transmissions joins the corresponding multicast group, and becomes a member of that group

After a user joins, the network builds the necessary routing paths so that the user receives the data sent to the multicast group

13

IP Multicasting: Key questions What IP address / Ethernet address to

send multicast packets to? How to manage groups? How to route multicast packets?

14Network Interface

IP IP Multicast

UDPTCP

Socket Layer

Stream Sockets Datagram Sockets Multicast Sockets

User Layer

The IP Protocol Stack IP Multicasting only supports UDP as

higher layer There is no multicast TCP !

Essential Multicast Terminology

source = origin of multicast streammulticast address = an IP address in the Class D range (224.0.0.0 –

239.255.255.255), used to refer to multiple recipients. A multicast address is also called a multicast group or channel.

multicast stream = stream of IP packets with multicast address for IP destination address. All multicast uses UDP packets.

receiver(s) = recipient(s) of multicast streamtree = the path taken by multicast data. Routing loops are not allowed, so

there is always a unique series of branches between the root of the tree and the receivers.

IP source = IP unicast addrEthernet source = MAC addr

IP destination = IP multicast addr Ethernet dest = MAC addr

source

Multicast stream

receivers

e.g., video server


(S,G) notation For every multicast source there must

be two pieces of information: the source IP address, S, and the group address, G. These correspond to the sender and

receiver addresses in unicast. This is generally expressed as (S,G). Also commonly used is (*,G) - every source

for a particular group.


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Multicast Protocols

Senders

Delivery tree

Receivers

Group Management Protocol - enables hosts to dynamically join/leave multicast groups. Receivers send group membership reports to the nearest router.

Multicast Routing Protocol - enables routers to build a delivery tree between the sender(s) and receivers of a multicast group.

Group Management Protocol (e.g. IGMP)

Multicast Routing Protocol (e.g. PIM-SM)

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Agenda Multicast



PIM-DM PIM-SM

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Multicast Addressing Multicast address

IP addresses in the range 224.0.0.0 - 239.255.255.255 (class D address)

Multicast group: same as multicast address Every host (interface) can join and leave a

multicast group dynamically no access control

Every IP datagram send to a multicast group is transmitted to all members of the group no security, no “floor control” Sender does not need to be a member of the group

The IP Multicast service is unreliable

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Multicast Addressing

Class D 1 multicast group id28 bits

01 1

• All Class D addresses are multicast addresses:

Class From To

D 224.0.0.0 239.255.255.255

• Multicast addresses are dynamically assigned.

• An IP datagram sent to a multicast address is forwarded to everyone who has joined the multicast group

• If an application is terminated, the multicast address is (implicitly) released.

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Types of Multicast Addresses RFC 3171

http://www.iana.org/assignments/multicast-addresses Examples:

224.0.0.0 - 224.0.0.255 (224.0.0/24) - reserved & not forwarded

224.0.0.1 - All local hosts 224.0.0.2 - All local routers 224.0.0.4 - DVMRP 224.0.0.5 - OSPF 224.0.0.9 - RIP2 224.0.0.13 – PIM 224.0.0.22 – IGMP

232.0.0.0 - 232.255.255.255 (232/8) - SSM 239.0.0.0 - 239.255.255.255 (239/8) - Administrative Scoping

(RFC 2365) Site-local scope: 239.253.0.0/16 Organization-local scope: 239.192.0.0/14

“Ordinary” multicasts don’t have to request a multicast address from IANA.

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Multicast Address Translation In Ethernet MAC addresses, a multicast

address is identified by setting the lowest bit of the “most left byte”

-------1-------- -------- -------- -------- --------

Not all Ethernet cards can filter multicast addresses in hardware- Then: Filtering is done in

software by device driver.

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Multicast Address Mapping

0000000100000000 01011110 0------- EthernetAddress

1110xxxx x------- -------- -------- Class DIP Address

Identifes

Class DIgnored 23-bit

address

-------- --------

Ethernet Addresses

with 01:00:5e in the

first 3 bytes are

reserved for IP

multicast

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Agenda Multicast



PIM-DM PIM-SM

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Multicast Protocols

Senders

Delivery tree

Receivers





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IGMP (RFC 1112) The Internet Group Management Protocol

(IGMP) is a simple protocol for the support of IP multicast.

IGMP operates on a physical network (e.g., single Ethernet Segment.

IGMP is used by multicast routers to keep track of membership in a multicast group.

Support for: Joining a multicast group Query membership Send membership reports

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A host sends an IGMP report when it joins a multicast group Multiple processes on a host can join. A report is sent only for the first process.

No report is sent when a process leaves a group A multicast router regularly multicasts an IGMP

query to all hosts (group address is set to 0.0.0.0). A host responds to an IGMP query with an IGMP

report.

Multicast router keeps a table of the multicast groups that have joined hosts. The router only forwards a packet, if there is a host still joined.

IGMP Protocol

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IGMP Packet Format IGMP messages are only 8 bytes long

Ethernet Header IP headerIGMP

Message

8 bytes20 bytes14 bytes

Type Max resp. time

Multicast group address

Checksum

IGMPv20x11 Membership query (group addr: 0.0.0.0)0x16 Membership report0x17 Leave group

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IGMP Protocol

IGMP query

IGMP Report

R1Ethernet

H1 H2

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IGMP Protocol

R1Ethernet

IGMP general queryIGMP group address = 0.0.0.0Destination IP address = 224.0.0.1Source IP address = router's IP address

IGMP membership reportIGMP group address = group addressDestination IP address= group addressSource IP address = host's IP address

H1 H2

IGMP group-specific queryIGMP group address = group addressDestination IP address = group addressSource IP address = router's IP address

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Networks with multiple multicast routers Only one router

responds to IGMP queries (Querier) Router with

smallest IP address becomes the querier on a network.

One router forwards multicast packets to the network (Forwarder).

Ethernet

Host

Querier Forwarder

Multicast Network

IGMP query

Multicastpacket

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Agenda Multicast



PIM-DM PIM-SM

33

Multicast Protocols

Senders

Delivery tree

Receivers





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Multicast Routing Protocols Goal: Build a spanning tree between all

members of a multicast group

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Multicast routing as a graph problem

S Problem: Embed a tree

such that all multicast group members are connected by the tree

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(c) Minimum-cost tree

S Problem: Embed a tree such

that all multicast group members are connected by the tree

Solution 1: Shortest Path Tree or source-based tree Build a tree that minimizes the path cost from the source to each receiver Good tree if there is a single sender If there are multiple senders, need one

tree per sender Easy to compute

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S Problem: Embed a tree such

that all multicast group members are connected by the tree

Solution 2: Minimum-Cost Tree Build a tree that minimizes the total cost of the edges Good solution if there are multiple

senders Very expensive to compute (not

practical for more than 30 nodes)

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Multicast routing in practice Routing Protocols implement one of two

approaches:1. Source Based Tree:

– Essentially implements Solution 1. – Builds one shortest path tree for each sender– Tree is built from receiver to the sender reverse shortest

path / reverse path forwarding (RPF)

2. Core-based Tree: – Build a single distribution tree that is shared by all senders– Does not use Solution 2 (because it is too expensive)– Selects one router as a “core” (also called “rendezvous

point”)– All receivers build a shortest path to the core reverse

shortest path / reverse path forwarding

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Reverse Path Forwarding (RPF) RPF builds a shortest path tree in a distributed fashion by taking

advantage of the unicast routing tables. Main concept: Given the address of the root of the tree (e.g., the

sending host), a router selects as its upstream neighbor in the tree the router which is the next-hop neighbor for forwarding unicast packets to the root.

This concept leads to a reverse shortest path from any router to the sending host. The union of reverse shortest paths builds a reverse shortest path tree.

RPF Forwarding:Forward a packet only if it is receives from an RPF neighbor

H1

Source

R1

R3

R4

R2

R5

RPFinterface

for H1

RPFneighbor of R3

for H1

Unicast routing table of router R3: Destination Next Hop H1 …

R2

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Multicast Routing table Routing table entries for source-based trees

and for core-based trees are different Source-based tree: (Source, Group) or (S, G)

entry. Core-based tree: (*, G) entry.

Source IP address

Multicast group

Incoming interface(RPF interface)

Outgoing interface

list

S1 G1 I1 I2, I3

* G2 I2 I1, I3

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Multicast routing in practice Routing algorithms in practice implement

one of two approaches:

1. Source Based Tree Tree: – Establish a reverse path to the source

2. Core-based Tree: – Establish a reverse path to the core router

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Building a source-based tree Set routing

tables according to RPF forwarding

Flood-and-Prune

H2

H3

H5H4

H1

Source

R1

R2

R6

R4

R5

R3

R8R7

joined

joined

IGMP

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H2

H3

H5H4

H1

Source

R1

R2

R6

R4

R5

R3

R8R7

joined

joined

Building a source-based tree Set routing

tables according to RPF forwarding

Flood-and-Prune

Flood= Forward packets that arrive on RPF interface on all non-RPF interfaces

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H2

H3

H5H4

H1

Source

R1

R2

R6

R4

R5

R3

R8R7

joined

joined

Building a source-based tree Set routing tables

according to RPF forwarding

Flood-and-Prune

Flood= Forward packets on all non-RPF interfaces

Receiver drops packets not received on RPF interface

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H2

H3

H5H4

H1

Source

R1

R2

R6

R4

R5

R3

R8R7

Prune

joined

joined

Prune

Prune

PrunePrune

Prune

Prune

Prune

Prune

Prun

e

Prune

Prune

Building a source-based tree Set routing tables

according to RPF forwarding

Flood-and-Prune

Prune= Send a prune message when a packet is received on a non-RPF interface or when there are no receivers downstreamPrune message disables routing table entry

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Pruning Prune message temporarily disables a routing

table entry Effect: Removes a link from the multicast tree No multicast messages are sent on a pruned link Prune message is sent in response to a multicast packet Question: Why is routing table only temporarily disabled?

Who sends prune messages? A router with no group members in its local network and no

connection to other routers (sent on RPF interface) A router with no group members in its local network which has

received a prune message on all non-RPF interfaces (sent on RPF interface)

A router with group members which has received a packet from a non-RPF neighbor (to non-RPF neighbor)

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H2

H3

H5H4

H1

Source

R1

R2

R6

R4

R5

R3

R8R7

joined

joined

Gra

ft

Graft

joined

Building a source-based tree When a

receiver joins, one needs to re-activate a pruned routing table entry

GraftingSending a Graft message disables prune, and re-activates routing table entry.

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H2

H3

H5H4

H1

Source

R1

R2

R6

R5

R3

R8R7

joined

joined

Join

JoinJoin

Join

R4

Alternative method for building a source-based tree This only

works if the receiver knows the source

Explicit-Join Receiver sends

a Join message to RPF neighbor

Join message creates (S,G) routing table entry

Join message is passed on

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H2

H3

H5H4

H1

Source

R1

R2

R6

R4

R5

R3

R8R7

joined

joined

Join

Join

joined

Core

Join

Join

Join

Building a core-based tree One route is

the core

Receiver sends a Join message to RPF neighbor with respect to core

Join message creates (*, G) routing table entry

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H2

H3

H5H4

H1

Source

R1

R2

R6

R4

R5

R3

R8R7

joined

joined joined

Core

Building a core-based tree Source sends

data to the core

Core forwards data according to routing table entry

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Agenda Multicast



PIM-DM PIM-SM

52

Multicast routing protocols in the Internet Distance Vector Multicast Routing Protocol (DVMRP):

First multicast routing protocol Implements flood-and-prune

Multicast Open Shortest Path First (MOSPF): Multicast extensions to OSPF. Each router calculates a

shortest-path tree based on link state database Not widely used

Core Based Tree (CBT): First core-based tree routing protocol

Protocol Independent Multicast (PIM): Runs in two modes: PIM Dense Mode (PIM-DM) and PIM

Sparse Mode (PIM-SM). PIM-DM builds source-based trees using flood-and-prune PIM-SM builds core-based trees as well as source-based trees

with explicit joins.

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PIM Messages (PIM version 2)Version

(= 2)Type Reserved Checksum

32 bit

Message type specific part

PIM-DM messages Type PIM-DM PIM-SM Description

Hello 0 Similar to periodic keep-alive

Register 1 Sent by designated router to rendezvous point.

Register-Stop 2

Join/Prune 3 Creates/negates forwarding state

Bootstrap 4

Assert 5

Graft 6 Reinstate a previously pruned branch

Graft-Ack 7

Candidate-RP-Advertisement

8

• Encapsulated in IP datagrams with protocol number 103.

• PIM messages can be sent as unicast or multicast packet

• 224.0.0.13 is reserved as the ALL-PIM-Routers group

54

PIM-DM: PIM Dense Mode PIM-DM

implements flood-and-prune

Orange packet: Multicast packet (=Data)

Blue packet: PIM message

H3H2

S1Source

R1

R3

joined

R4

R2

src: S1dest: G

src: S1dest: G

src: S1dest: G

src: S1dest: G

src: S1dest: G

prune (S1, G)

I1

I1 I3

55

PIM-SM: PIM Sparse Mode Core is called

rendezvous-point (RP)

Receivers know RP (statically configured or dynamically elected)

When receiver joins, a Join message is sent to RP on RPF. H3H2

S1Source

R3

joined

R4

H3H2

S1Source

R3

joined

R4

I1

I1 I3

I1

I2 I3

(a) PIM-SM: H2 joins (b PIM-DM: H3 joins

RP

R1

R2 R5

IGMP

joined

join (*, G)

join (*, G)

IGMP

join (*, G)

I1

I1 I3

RP

R5

R1

R2

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PIM-SM: PIM Sparse Mode Host H3 joins:

Join message is only forwarded until the first router that is part of the core-based tree.

H3H2

S1Source

R3

joined

R4

I1

I1 I3

RP

R1

R2 R5

IGMP

join (*, G)

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H3H2

S1Source

R3

joined

R4

src: S1dest: G

src: S1dest: G

I1

I1 I3

RP

R1

R2 R5

src: S1dest: G

src: S1dest: G

H3H2

S1Source

R3

joined

R4

src: S1dest: G

src: S1dest: G

I1

I1 I3

(a) PIM-SM: Register message to RP (b) PIM-SM: Register Stop message to S1

RP

R1

R2 R5

join (S1,G)

register(S1, G)

src: S1dest: G

src: S1dest: G

src: S1dest: G

src: S1dest: G

register stop (S1,G)

register(S1, G)

src: S1dest: G

PIM-SM: Data transmission Source sends

multicast packet to RP Packet is attached to

an RP Register message

When packet reaches RP, it is forwarded in the tree

Also: RP sends a Join message on reverse path to S1

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H3H2

S1Source

R3

joined

R4

src: S1dest: G

src: S1dest: G

I1

I1 I3

RP

R1

R2 R5

src: S1dest: G

src: S1dest: G

H3H2

S1Source

R3

joined

R4

src: S1dest: G

src: S1dest: G

I1

I1 I3

(b) PIM-SM: Register Stop message to S1

RP

R1

R2 R5

src: S1dest: G

src: S1dest: G

src: S1dest: G

register(S1, G)

src: S1dest: G

PIM-SM: Data transmission When Join

messages reaches R1, it sends a native multicast packet to the RP (in addition to the packet attached to the register message)

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H3H2

S1Source

R3

joined

R4

src: S1dest: G

src: S1dest: G

I1

I1 I3

RP

R1

R2 R5

src: S1dest: G

src: S1dest: G

H3H2

S1Source

R3

joined

R4

src: S1dest: G

src: S1dest: G

I1

I1 I3

(b) PIM-SM: Register Stop message to S1

RP

R1

R2 R5

src: S1dest: G

src: S1dest: G

src: S1dest: G

register stop (S1,G)

register(S1, G)

src: S1dest: G

PIM-SM: Data transmission When RP receives

native multicast packet it sends a register stop message to R1. This message stops the transmission of register messages from R1.

60

H3H2

S1Source

R3

joined

R4

I1

I1 I3

(a) PIM-SM: R3 switches to a SPT

RP

R1

R2 R5

join (S1,G)

join (S1,G)

src: S1dest: G

src: S1dest: G

src: S1dest: G

src: S1dest: G

src: S1dest: G

PIM-SM: Switching to source-based tree When data to receivers

exceeds a threshold, routers switch to a source-based tree

This is done by sending an explicit join message to the source

There may be duplicate packets being sent for some time

61

PIM-SM: Switching to source-based tree When data arrives

from source (as opposed to RP), a Prune message is sent to the RPT

Now: data is forwarded only along the shortest-path tree

src: S1dest: G

H3H2

S1Source

R3

joined

R4

I1

I1 I3

RP

R1

R2 R5

(b) PIM-SM: Data follows a SPT

src: S1dest: G

prune (S,G,RPT)

src: S1dest: G

prune (S,G,RPT)

src: S1dest: G

62

Conclusion Multicast Addressing IGMP Source and core-based trees PIM (SM vs. DM)

coms/csee 4140 networking laboratory lecture 11

Documents

multicast packets

multicast tcp

pmagendamulticastwhy

multicast groupsterminologys

origin of multicast

multicast transmissions

multicast groupip multicasting

multicast addressa user