Transcript Week10
NETS 3303 Networked Systems Multicast Björn Landfeldt School of Information Technologies 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 Björn Landfeldt School of Information Technologies 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 Björn Landfeldt School of Information Technologies 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 Björn Landfeldt School of Information Technologies Applications • One-to-many or many-to-many – – – – Distributed games TV broadcast Video conferences Group telephone call • IPv4 not built for this Björn Landfeldt School of Information Technologies Unicast Björn Landfeldt School of Information Technologies 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) Björn Landfeldt School of Information Technologies Multicast • Instead of many unicast flows – Let routers build hierarchy – Tree structure • Multicast group: – Everyone interconnected – Everything “broadcasted” within group Björn Landfeldt School of Information Technologies Multicast mrouter mrouter mrouter Björn Landfeldt School of Information Technologies Some broadcasting sites today are limited to a maximum number of users, why do you think? Björn Landfeldt School of Information Technologies Multicast • Because they use unicast. Multicast is not yet widely implemented in the Internet Björn Landfeldt School of Information Technologies 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 Björn Landfeldt School of Information Technologies • Tree structure IGMP – 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) Björn Landfeldt School of Information Technologies LAN • IGMP not needed for LAN, why? Björn Landfeldt School of Information Technologies LAN • On a LAN, the Multicast address can simply be converted into a corresponding link layer address and be broadcasted. Björn Landfeldt School of Information Technologies 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 Björn Landfeldt School of Information Technologies 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) Björn Landfeldt School of Information Technologies Flood and Prune Björn Landfeldt School of Information Technologies 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 Björn Landfeldt School of Information Technologies Is there anything wrong with these approaches? Björn Landfeldt School of Information Technologies 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. Björn Landfeldt School of Information Technologies 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 Björn Landfeldt School of Information Technologies BGMP Domain C Root Domain B Domain D Domain A BGMP Join MOSPF enabled Routing Björn Landfeldt School of Information Technologies 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 Björn Landfeldt School of Information Technologies 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 Björn Landfeldt School of Information Technologies 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? Björn Landfeldt School of Information Technologies Unwanted traffic • What negative effects can someone sending high volume traffic to a multicast group have? • What can prevent this? Björn Landfeldt School of Information Technologies 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 Björn Landfeldt School of Information Technologies 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) Björn Landfeldt School of Information Technologies Reading • IETF RFCs 1075, 1112, 1584, 2357 Björn Landfeldt School of Information Technologies