Lecture 3 - Multicast · Lecture 3: Multicast Goals: Understand the abstract idea with multicast and its benefits Get some insight into some applications using multicast. Understand

TSIN02 - Internetworking

© 2004 Image Coding Group, Linköpings Universitet

Lecture 3: Multicast

Literature:Fouruzan: p. 108- Multicast Addresses

Fouruzan: ch 10: IGMP

RFC3170: IP Multicast Applications Challenges and Solutions

RFC1112, RFC2236, RFC3376 (IGMP ver. 1,2,3)

(Multicast routing Fouruzan: p. 409- not part of this lecture)


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Lecture 3: MulticastGoals:

Understand the abstract idea with multicast and its benefits

Get some insight into some applications using multicast.

Understand the IETF multicast architecture

Multicast addressing

Multicasting on a LAN: the IGMP protocol

IGMP evolution

Layer-2 relation

Gain some understanding regarding some emergingtechniques

Address allocation

Reliable multicast


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Lecture 3: MulticastOutline:

Applications (one-to-many, many-to-many)

Multicast architecture

Multicast addressing

LAN mechanisms (IGMP v. 2)

Support in layer 2 (IGMP snooping)

IGMP ver. 3

Emerging techniques

Address allocation (Which addresses can I use?)

Reliable multicast


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Communication Forms

One-to-one:

In most applications the peer sends data exclusivelyto the receiver. Examples: Web-traffic, Video-on-demand etc.

One-to-all:

What is all? (ans: the local network)

Requesting a service from a host with unknownIP#/MAC. (E.g., DHCP, (R)ARP, SLP)

Sending small pieces of information which arefundamental to most hosts. (E.g., NTP, routinginfo)


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Communication Forms cont.

One-to-many:

Streaming TV/lectures/radio etcPush media and announcementsDistributed requests (e.g., DB-queries)Mass distribution of filesMany-to-many

Multimedia conferencing

Synchronized resources

Concurrent processing


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Major reasons why preferable over broadcast:Network may replicate packets and prune vastregions not interested in receiving packets

Save network bandwidth

Save packet processing cycles.

Communication Forms cont.

Packet replication

Pruned areas

sender


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Application Example #1

TV over IP

Access network

(turned off)

Per channel:Potentially a very large group.Just one sender.

Operator in control. Works todayZaptime important!


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Videoconferencing

Every receiver also a sender.Many WAN:s/operators involved. How to buildthe tree over domain boundaries?Inter domain multicast routing needed.

Scenario not yet globally deployable!


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Multiuser Gaming

Server

Normally not a many-to-many application!Tough upholding synchronism between clientsSensitive to cheating if all clients have access tothe whole world stateScales badly, The total amount of user input willat some point surpass the data needed per userto render a scene.Though shared audio etc could benefit


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Application Needs

...from an application’s point of view

Receivers form a group

Group identification scheme (scope?)

We want to form groups dynamically.Need API for telling OS we’re interested inreceiving/sending data for a group

Security: Authentication, data integrity, privacyand anonymity.

How to find information on existing groups?announcements, web-pages, predefined?

Reliability


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IP Multicast architecture

Two models:

1) ISM – Internet Standard Multicast

Many-to-many. Everyone (also non-group-members) can send to a group (*,G)

A host need to communicate with gateway tostart receiving packets for group (*,G) (IGMPver. 1 & 2)

2) SSM – Source Specific Multicast

Specific support for one-to-many. A group canbe identified via the pair (S,G) where S is asender’s host IP#.

Communication with gateway via IGMP ver. 3


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IPv4 Multicast Addressing

In IPv4 the “G” in the id is part of the normal hostaddress space. The multicast “subnet” is :

224.0.0.0/4

I.e., Addresses 224.0.0.0 to 239.255.255.255

Some of these addresses are statically allocated byIANA and some ranges have predefined use.


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IPv4 Multicast Addressing cont.

224. 0. 0. 0 – 224. 0. 0.255224. 0. 1. 0 – 224. 0. 1.255224. 0. 2. 0 – 224. 0.255. 0224. 1. 0. 0 – 224. 1.255.255224. 2. 0. 0 – 224. 2. 0.255224. 3. 0. 0 – 231.255.255.255232. 0. 0. 0 – 232.255.255.255233. 0. 0. 0 – 233.255.255.255234. 0. 0. 0 – 238.255.255.255239. 0. 0. 0 – 239.255.255.255

Excerpt from draft-ietf-mboned-rfc3171bis-01.txt: (expires July2004)


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Local Network Control Block224.0.0.0 Base Address (Reserved) [RFC1112,JBP]224.0.0.1 All Systems on this Subnet [RFC1112,JBP]224.0.0.2 All Routers on this Subnet [JBP]224.0.0.3 Unassigned [JBP]224.0.0.4 DVMRP Routers [RFC1075,JBP]224.0.0.5 OSPFIGP OSPFIGP All Routers [RFC2328,JXM1]224.0.0.6 OSPFIGP OSPFIGP Designated Routers [RFC2328,JXM1]224.0.0.7 ST Routers [RFC1190,KS14]224.0.0.8 ST Hosts [RFC1190,KS14]224.0.0.9 RIP2 Routers [RFC1723,GSM11]224.0.0.10 IGRP Routers [Farinacci]224.0.0.11 Mobile-Agents [Bill Simpson]224.0.0.12 DHCP Server / Relay Agent [RFC1884]224.0.0.13 All PIM Routers [Farinacci]224.0.0.14 RSVP-ENCAPSULATION [Braden]224.0.0.15 all-cbt-routers [Ballardie]224.0.0.16 designated-sbm [Baker]224.0.0.17 all-sbms [Baker]224.0.0.18 VRRP [Hinden]224.0.0.19 IPAllL1ISs [Przygienda]224.0.0.20 IPAllL2ISs [Przygienda]224.0.0.21 IPAllIntermediate Systems [Przygienda]224.0.0.22 IGMP [Deering]224.0.0.23 GLOBECAST-ID [Scannell]. . .


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Internetwork Control Block224.0.1.0 VMTP Managers Group [RFC1045,DRC3]224.0.1.1 NTP Network Time Protocol [RFC1119,DLM1]224.0.1.2 SGI-Dogfight [AXC]224.0.1.3 Rwhod [SXD]224.0.1.4 VNP [DRC3] 224.0.1.5 Artificial Horizons – Aviator [BXF]224.0.1.6 NSS - Name Service Server [BXS2]224.0.1.7 AUDIONEWS - Audio News Multicast [MXF2]224.0.1.8 SUN NIS+ Information Service [CXM3]224.0.1.9 MTP Multicast Transport Protocol [SXA]224.0.1.10 IETF-1-LOW-AUDIO [SC3]224.0.1.11 IETF-1-AUDIO [SC3]224.0.1.12 IETF-1-VIDEO [SC3]224.0.1.13 IETF-2-LOW-AUDIO [SC3]224.0.1.14 IETF-2-AUDIO [SC3]224.0.1.15 IETF-2-VIDEO [SC3]224.0.1.16 MUSIC-SERVICE [Guido van Rossum]224.0.1.17 SEANET-TELEMETRY [Andrew Maffei]224.0.1.18 SEANET-IMAGE [Andrew Maffei]224.0.1.19 MLOADD [Braden]224.0.1.20 any private experiment [JBP]224.0.1.21 DVMRP on MOSPF [John Moy]. . .


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Sending an multicast datagram

For sending a datagram to a multicast group we usenormal UDP where the destination address is set tothe multicast address.

ver

8 bits 8 bits

DS

.

.

.

8 bits 8 bits

hlen Total Length

Identifaction (16bits) Frag. offsetflags

TTL Protocol (17) Header checksum

Source IP address

Destination IP address

Source port address Destination port address

UDP total length UDP Checksum

Data

multicastaddressgoes here!

UDP encapsulation in IP datagram


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IGMP – Internet Group Management Protocol

Just for group membership communication between ahost and a router

Has nothing to do with multicast routing

Keeps an updated list of active group listeners foreach connected LAN.

Simplistic network layer service. No addressmanagement, no session management, no reliabledata delivery, no security support, no synchronism

IP

ICMPIGMP

ARP RARP


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IGMP – Message format (IGMP v. 2)

Type Maximumresponse time checksum

Group Address

8 bits 8 bits 16 bits

Message types:

Membership query (0x11)sent by router on 224.0.0.0. If Group Address field is0.0.0.0 we have a general query. Otherwise we have aspecial query.

Membership Report (0x16)sent by host when first time joining or when routerqueries.

Leave Report (0x17)sent by host when leaving a group. (Not in IGMP ver. 1)


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IGMP – Communication Example

Host / Router communication

Host RouterJoin multicast group (*,G)

Membership report senton groupaddr G. Router allocates table space and

sets a timer to y secs. (default)And again...

General membershipquery sent to 224.0.0.1

Membership report senton groupaddr G.

Wait a randomamount of time

Leave group (*,G)Leave group messagesent on 224.0.0.2

Several group-specificmembership queries sent on224.0.0.1

A general membership query isroutinely sent every 125s (default)

Wait a randomamount of time

Router must check if there aremore hosts in group. Sends 2(default)G-S membership queries with 1s.(default) apart.


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IGMP – Pitfalls taken care of

What if a “join group” message gets lost?

Join Group messages should be sent at least twice

In certain applications a quick tear-down of a group feedis highly prioritized.

IGMP ver. 2 introduced “leave group”. When routersees this it sends a group specific membership querya couple of times to give other group members thechance to reestablish the groups existance.

What if there are thousands of members in a group anda router sends a query message?

Hosts must delay a random amount of time to see ifanother host answers.


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IGMP – Host state diagram

Non-Member

DelayingMember Idle Member

Each host hasone of threestates withrespect to anymulticastaddress

Leave Groupsend leave if flag set

Report Receivedstop timerclear flag

Timer Expiredsend reportset flag

Query Receivedstart timer

Join Groupsend reportset flagstart timer

Leave Groupstop timersend leave

if flag set

Query Receivedrestart timer ifmaximum

responsetime < current

timer

The flag indicates if host isthe the last one sendingthe membership report.


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Link-level Multicast

Layer-2 networks can be very large with manynodes. How does multicast work on e.g., Ethernet?

Ethernet uses MAC-addressing (6 bytes):

xxxxxxx1 xxxxxxxx xxxxxxxx xyyyyyyy yyyyyyyy yyyyyyy

All addresses with the low-order bit in the high-order byte set is an Ethernet multicast address

IP-multicast addresses is mapped onto MACaddresses wherex = 0000000(1)00000000010111100 andy = 23 low-order bits of the IPv4 multicast address

Hence frames may arrive at an interface which thehost is not really interested in.


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Link-layer Multicast cont.

How does a typical Ethernet-network handle thoseframes?

Dumb switches broadcast all multicast frames!

More advanced switches may use a technique called“IGMP snooping” to filter out group join and leavemessages.

Hosts and switches which are VLAN enabled mayuse the GRMP (GARP Multicast RegistrationProtocol) defined in IEEE 802.1Q.

NOTE: We have free access to all IEEE standards within theUniversity domain! Download from

ieeexplore.ieee.org


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IGMP ver. 3 (RFC3376)

Adds a new message type

Version 3 Membership Report (0x22)sent by host when joining one or more source specificmulticast groups

Type = 0x22 Reserved checksum

Reserved


Number of Group Records [M]

Group Record [1]

Group Record [2]

Group Record [M]

.

.

.


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IGMP ver. 3 cont.

Record Type Aux Data Len Number of Sources (N)

Multicast Address


Source Address [1]Source Address [2]

.

.Source Address [N]

Auxillary Data (may not currently be used)

Group Record Internal Format:

Where Record Type tells if the (S,G) pairs are included or excluded fromthe interface’s multicast filter. Also the membership query messageformat has been updated to include a specific source list.


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More on Multicast Addresses

Multicast addresses are not that many. Some of themwe want to use locally.

The early experimental Mbone (Multimedia backbone)used TTL-scoping: (the time-to-live field in IP header)

1 local (traverses no router)

2 – 31 site (never leaves institution or university)

63 region

127 world

I.e., routers was instructed to drop multicast packetsdepending on the TTL value.


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Administrative Scoping

TTL-scoping is not the preferred way while itcomplicates dynamic address allocation and is notsuited for intersected scopes etc.

The range 239.0.0.0/8 is reserved for so calledAdministrative scopes. Routers decide based on thegroup address whether to forward packets.Organizations etc. might use the reserved subrange

Organization local scope: 239.192.0.0/14

and decide for themselves how these addresses areto be used. A large organization might further wantto divide this address space into ranges used byvarious sub-scopes.


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Scope restrictions

Administrative scopes has some natural restrictions

Must be connected. I.e., there must exist a routebetween any two hosts part of a scope.

Must be convex. I.e., the route between any twohosts must not cross the scope boundary.

Two intersecting scopes should have disjunctaddress ranges in case a route within one scopegoes through the other.

Any scope boundary is also a boundary for alocal scope using the range 239.255.0.0/16.


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In this example scopes Z2 and Z4 might use thesame address range, but Z1 and Z3 need to usedifferent ranges (and not the same as Z2/Z4).

Adminstrative Scope Example

L5

L1 L2 L3

L4

Z1Z2

Z3

Z1: top level scopeLi : local scopesZ

2– Z

4:sub-scopes

Z4

L6


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Scoping and Relative Addresses

Given an administrative scope’s address range thelast 256 addresses are assigned by IANA. E.g., forthe IPv4 local scope we will have:

Local Session Announcements will hence always use239.255.255.255. MADCAP 239.255.255.254 etc.


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Multicast addresses on-demand

“I have developed this new fancy multi-partymultimedia application. How to get suitable multicastaddresses dynamically?”

Answer: RFC2730

Multicast Address Dynamic Client Allocation Protocol

Given a scope we can contact a MADCAP serverand lease an address for a given time. Leases maybe renewed and can be actively released.

If we don’t know server’s host address we can issuea DISCOVER message over the scope’s relativemulticast address no 1.


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Scope Discovery

How can we find out which scopes are available?Answer: RFC2776

Multicast-Scope Zone Announcement Protocol (MZAP):

Routers on the border of a scope (=zone) runs the protocol. Such arouter is called ZBR (Zone Boundary Router).

For every scope the ZBR is a border for it regularly transmits ZoneAnnouncement Messages (ZAMs) to the local scope MZAP multicastaddress (239.255.255.252). These messages are then flooded to alllocal scopes within the announced scope.

Announcements contain a Zone ID and address range, but also astring description of the zone. Example: “Department of ElectricalEngineering” “liu.se”

MZAP can detect misconfigurations.


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Reliable Multicast (RM)

How to use the best-effort network-layer multicast todistribute data reliably? (I.e., everything arrivessooner or later in the correct order at all receivers.)

Still an active research topic!

Three main solutions:

1) NACK-based: Receivers requests retransmission whena packet seems missing

2) ACK-based: Every packet is acknowledged byevery receiver. Sender resends on time-out

3) FEC-based: Redundancy is added in the form of a of aforward-error (or rather erasure)-correcting code.

See RFC2887 for a nice introduction!


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RM – NACK Based Approach

Receivers sends back a NACK when a packetdoesn’t arrive in time.

Problem: NACK-implosion at sender.

Solution 1: Don’t send a NACK immediately, but waita random amount to see (in the data stream) if anyother receiver has initiated a retransmission

Solution 2: Build in NACK aggregation in routers.NACK(3)

NACK(3)

NACK(4)

NACK(3)

NACK(3)NACK(3-4)

Packet 4lost here

Datasender

Packet 3lost here


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RM – ACK Based Approach

Every receiver acknowledges received content as inTCP. For this to scale the receivers need to form an“ACK-tree” in which the ACK:s are aggregated (justas in the NACK-case described earlier)

The ACK-tree could be the same as the multicasttree. This however needs new functionality inrouters

Receivers dynamically form a tree separate fromthe multicast tree and send ACK:s to their parentsonly. Parents might even react to late ACK:s andresend data to children themselves.


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y1 y2 y3 y5 = 0

y2 y3 y4 y6 = 0

y1 y3 y4 y7 = 0

RM – FEC Based ApproachIn general we can add redundancy to k bits of data obtaining n bitsof data. Any received k bits will enable us to recover the k bits oforiginal data. Example using a Hamming (7,4)

110 100 000 001Incoming Data Block:

Construct three parity blocks according to: ( = xor)p

1= x

1x

2x

3

p2= x

2x

3x

4

p3= x

1x

3x

4

110 100 000 001 010 101 111Outgoing:

We can now afford to loose any three of the above blocks!Solve the parity relation to the right after putting in theknown received bits (yi). For very large codes (large n) weneed some algebraic structure enabling fast reconstruction.

Hamming (7,4) is usually usedas an example of an block-codecapable of correcting one error(position unknown)


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RM – Layered Coding

Multicasting in general has a problem with congestioncontrol. What should sender do (or even know) when abranch suffers from overload?

Apart from various upstream solutions we could useseveral multicast streams, layers, and let receivers joindepending on traffic situation

Layering can be combined with FEC. In the example above any two Ai:swith different index need to be received to reconstruct A. We see that areceiver listening to all layers might have all data (A,B,C,D) at timeinstant (1) while a layer 1 and 2 receiver will have to wait till (2)

(1) (2)

C1

B3

C3

A1 B1

D5

D1

A5

C2

A3

D3

A2 B2

C5

D2

B5 D6 A6 C6 B6 D7 A7 C7 B7

C1

B4

C4

A1 B1 D1 C2

A4

D4

A2 B2 D2 C1

B3

C3

A1 B1 D1 C2

A3

D3

A2 B2 D2Layer 3:

Layer 2:

Layer 1:

time


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RM – The rmt Working GroupThere is a working group Reliable Multicast Transport(rmt) which are developing protocols:

Asynchronous Layered Coding (ALC)

Several multicast streams in different rates to avoidcongestion (Receiver joins the suitable ones)

Uses FEC-techniques

RFC is experimental

NACK-oriented reliable multicast (NORM)

Uses random back-off for NACK (truncated exponentialdistribution

Uses FEC

Is just a draft (expires May 2004)


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RM – The Ultimate Solution?From RFC3269:

Reliable Multicast Transport (RMT) protocols can beconstructed in a variety of ways, some of which willwork better for certain situations than others. It isbelieved that the requirements space for reliablemulticast transport is sufficiently diverse that no oneprotocol can meet all the requirements [RFC2887,(Sally Floyd et al)].

The working group RMT did some work on Generic RouterAssist (GRA) where small packet handling programs withaccess to buffers etc could be inserted into routers byapplications. In this way a generic solution could be had for allfuture multicast scenarios. GRA only made an appearance assome Internet-Drafts which seems all expired by now...


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IETF Working Groups

idmr - Inter-Domain Multicast Routing

IGMP ver. 2, 3

Various multicast routing protocols

mboned – MBONE Deployment

Zone Announcement Protocol (ZAP)

malloc – Multicast-Address Allocation

Multicast Address Dynamic Client AllocationProtocol (MADCAP)

rmt - Reliable Multicast Transport

Draft for NACK-based protocol: NORM

Experimental layered protocol: ALC


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SummaryMulticast can help in scaling up one-to-many and many-to-many applications.

Multicast addresses is part of the normal host addressspace. A group is either identified with a multicast addressor a multicast address plus source address(source-specific multicasting)

Hosts use IGMP to communicate with router to join andleave groups.

Multicast addresses may live in a scope. Scopes mayintersect and nest. Protocols for leasing multicastaddresses exist.

Using multicast for reliable file transfer can be done, butthere is no full IETF standard yet.

Documents

Lecture 3 - Multicast · Lecture 3: Multicast Goals: Understand the abstract idea with multicast and its benefits Get some insight into some applications using multicast. Understand