NETS 3303 Networked Systems

Björn LandfeldtSchool of Information Technologies

NETS 3303Networked Systems

Multicast


Today!

• We will learn what multicast is why it is used and how it works.

• We will tie it to QoS and see why it is a corner stone of Multimedia delivery


Outcomes

• Understand why multicast is important (necessary)

• Understand how transport layer mechanisms fit into MCAST

• Knowing about some of the protocols and their features

• Knowing limitations and remedies


Cast?

• Unicast – flow from one host to another host

• Broadcast – flow from one host to all local hosts

• Directed Broadcast – flow from one host to all hosts on a foreign network

• Multicast – flow between hosts in a group


Applications

• One-to-many or many-to-many– Distributed games– TV broadcast– Video conferences– Group telephone call

• IPv4 not built for this


Unicast


Unicast

• Assume 100.000.000 people watch cricket on broadcast TV

• If every connection each have copy of match – unicast

• Each connection 1.5 Mbps => Total BW 150.000.000.000.000 bps for the match

• Does this scale to HDTV O(N2)


Multicast

• Instead of many unicast flows– Let routers build hierarchy– Tree structure

• Multicast group:– Everyone interconnected– Everything “broadcasted” within group


Multicast

mrouter

mrouter

mrouter


Some broadcasting sites today are

limited to a maximum number of users, why

do you think?


Multicast

• Because they use unicast. Multicast is not yet widely implemented in the Internet


Grouping• Multicast IP address range

– 224.0.0.0 – 239.255.255.255– Addresses not unique like unicast– One address shared by group– Some addresses reserved e.g. 224.0.0.1 (all hosts)

• Applicable to– LAN– WAN

• Special routing needed


IGMP• Tree structure

– Leaf – Host, intermediate – routers– Leaf initiated join and leave– Leaf can send to group without joining

• Inter network communication– Special multicast routers– Controlled by IGMP– Simple, 2 message types

• IGMPQUERY (from router)• IGMPREPORT (from leaf)


LAN

• IGMP not needed for LAN, why?


LAN

• On a LAN, the Multicast address can simply be converted into a corresponding link layer address and be broadcasted.


LAN• Need for address translation IP-MAC• IANA reserved space

– 00:00:5e:00:00:00 - 00:00:5e:ff:ff:ff (lower half for multicast)

– Ethernet: multicast -> set first byte 01 so

– Multicast MAC range =– 01:00:5e:00:00:00 - 01:00:5e:7f:ff:ff

• Set last 23 bits equal to IP address• Not 1-1 mapping, IP address filtering needed


Multicast Routing

• Flood and prune protocols– Sender floods network– Router rejects all incoming packets except link towards source

– Router floods all links except link towards source

– If traffic not desired, return prune message

– Example, Distance Vector Multicast Routing Protocol (DVMRP)


Flood and Prune


Multicast Routing

• Distance Vector Protocols– Example, MOSPF and extended OSPF– Distribute table of distances to all routers

– From received tables, derive own table

– Table decides on forwarding path


Is there anything wrong with these

approaches?


Multicast Routing

• They do not scale– Every router that has no participating host has to keep state of group to prune

– Deploying this on a global scale is insane.


BGMP

• Do not keep state of hosts• Build shared tree of domains• Use other routing protocol within domain

• Multicast Address-Set Claim (MASC)– Allocate group addresses to domains– Distribute knowledge to other domains using BGMP


BGMP

Domain ADomain A

Domain DDomain D

Domain CRoot

Domain CRoot

Domain BDomain B

BGMP JoinMOSPF enabled Routing


Scoping• Multicast addresses scarce • Scoping allows reuse• Method 1, set TTL field in IP header

– Simple– crude

• Method 2, administrative scoping– Set rules in routing tables– More refined– Difficult, more knowledge required


MBONE

• No multicast backbone routers– Overlaid experimental network

• Software typically runs on SPARCs– Most common mrouted

• IP Tunnelling between mcast islands– Normal routing protocols useless

• TTL scoping


Reliable multicast• Problems

– Fate sharing•If one unicast host fails the session fails. What if there are three multicast hosts and one fails?

– Performance•Reservations made on lowest or average connection?

– Centralised – Distributed retransmissions?•If one host needs few and another needs many retransmissions are they still the same session?


Unwanted traffic

• What negative effects can someone sending high volume traffic to a multicast group have?

• What can prevent this?


Unwanted Traffic

• Low-bandwidth links can get saturated. This can cause:– Packet loss or extensive delays– High costs (expensive links)

• The answer is QoS management


QoS Routing• Reliable Transport protocols

– Retransmissions do not scale– Alternative, FEC

• Resource reservations for multicast– Dynamic memberships– Scale across AS domains– Allow for heterogeneity (links, hosts)– Allow for different levels (differentiated cost)


Reading

• IETF RFCs 1075, 1112, 1584, 2357

NETS 3303 Networked Systems

Documents

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