EECS 122 Introduction to Computer Networks Multicast Computer Science Division Department of Electrical Engineering and Computer Sciences University of California Berkeley Berkeley CA 94720 1776 Katz Stoica F04 Barriers to Multicast Hard to change IP multicast means change to IP details of multicast were very hard to get right Not always consistent with ISP economic model charging done at edge but single packet from edge can explode into millions of packets within network Troublesome security model Anyone can send to a group Denial of service attacks on known groups Katz Stoica F04 2 Application Layer Multicast ALM Let the hosts do all the special work only require unicast from infrastructure Basic idea hosts do the copying of packets set up tree between hosts Example Narada Yang hua et al 2000 Small group sizes hundreds of nodes Typical application chat Katz Stoica F04 3 Narada End System Multicast Gatech Stanford Stan1 Stan2 CMU Berk1 Berk2 Berkeley Overlay Tree Gatech Stan1 Stan2 CMU Berk1 Berk2 Katz Stoica F04 4 Algorithmic Challenge Choosing replication forwarding points among hosts how do the hosts know about each other and know which hosts should forward to other hosts Katz Stoica F04 5 Advantages of ALM No need for changes to IP or routers No need for ISP cooperation End hosts can prevent other hosts from sending Easy to implement reliability use hop by hop retransmissions Katz Stoica F04 6 Performance Concerns Stretch ratio of latency in the overlay to latency in the underlying network Stress number of duplicate packets sent over the same physical link Katz Stoica F04 7 Performance Concerns Gatech Delay from CMU to Berk1 increases Stan1 Stan2 CMU Berk1 Duplicate Packets Bandwidth Wastage Gatech Stanford Berk2 Stan1 Stan2 CMU Berk1 Berkeley Berk2 Katz Stoica F04 8 Single Sender Multicast Many problems with IP multicast disappear if each group is associated with a single source Hosts joining multicast group can send join messages to source this sets up delivery tree no worry about root being in wrong place This solves several problems better security and charging model simple algorithm Katz Stoica F04 9 Example Group members M1 M2 M3 source M1 M2 M3 control join messages data Katz Stoica F04 10 What s Wrong with SSM Multiple sources can set up group per source or source can serve as relay for other senders Algorithm trivial So why isn t SSM the answer multicast no longer serves as rendezvous ok for broadcast apps not good for meeting apps Katz Stoica F04 11 What Do You Need to Know DVRMP CBT SSM How they compare Katz Stoica F04 12 EECS 122 Introduction to Computer Networks DNS and WWW Computer Science Division Department of Electrical Engineering and Computer Sciences University of California Berkeley Berkeley CA 94720 1776 Katz Stoica F04 Internet Names Addresses Names e g ariachne berkeley edu human usable labels for machines conforms to organizational structure Addresses e g 169 229 131 109 router usable labels for machines conforms to network structure How do you map from one to another Domain Name System DNS Katz Stoica F04 14 DNS History Initially all host addess mappings were in a file called hosts txt in etc hosts Changes were submitted to SRI by email New versions of hosts txt ftp d periodically from SRI An administrator could pick names at their discretion As the internet grew this system broke down because SRI couldn t handled the load Names were not unique Many hosts had inaccurate copies of hosts txt Internet growth was threatened Domain Name System DNS was born Katz Stoica F04 15 Basic DNS Features Hierarchical namespace As opposed to original flat namespace Distributed storage architecture As opposed to centralized storage plus replication Client server interaction on UDP Port 53 But can use TCP if desired Katz Stoica F04 16 Naming Hierarchy root edu berkeley eecs gov com mil org net uk Top Level Domains are at the top Depth of tree is arbitrary limit 128 Domains are subtrees fr etc mit sims argus E g edu berkeley edu eecs berkeley edu Name collisions avoided E g berkeley edu and berkeley com can coexist but uniqueness is job of domain 17 Katz Stoica F04 Host names are administered hierarchically root edu berkeley eecs argus sims com gov mil org net uk fr mit A zone corresponds to an administrative authority that is responsible for that portion of the hierarchy eecs controls names x eecs berkeley edu berkeley controls names x berkeley edu and y sims berkeley edu Katz Stoica F04 18 Server Hierarchy Each server has authority over a portion of the hierarchy A server maintains only a subset of all names Each server contains all the records for the hosts in its zone might be replicated for robustness Each server needs to know other servers that are responsible for the other portions of the hierarchy Every server knows the root Root server knows about all top level domains Katz Stoica F04 19 DNS Name Servers Local name servers Each ISP company has local default name server Host DNS query first goes to local name server Authoritative name servers For a host stores that host s name IP address Can perform name address translation for that host s name Can also do IP to name translation but won t discuss Katz Stoica F04 20 DNS Root Name Servers Contacted by local name server that can not resolve name Root name server Contacts authoritative name server if name mapping not known Gets mapping Returns mapping to local name server Dozen root name servers worldwide Katz Stoica F04 21 Simple DNS Example root name server Host whistler cs cmu edu wants IP address of www berkeley edu 1 Contacts its local DNS server mango srv cs cmu edu 2 mango srv cs cmu edu contacts root name server if necessary 2 5 3 4 3 Root name server contacts authoritative name server local name server authorititive name server ns1 berkeley edu if necessarymango srv cs cmu edu ns1 berkeley edu 1 6 requesting host www berkeley edu whistler cs cmu edu Katz Stoica F04 22 Example of Recursive DNS Query root name server Root name server May not know authoritative name server May know intermediate name server who to contact to find authoritative name server Recursive query 2 local name server 6 7 Puts burden of name resolution on contacted name mango srv cs cmu edu server 1 8 Heavy load requesting host 3 intermediate name server edu server 4 5 authoritative name server ns1 berkeley edu whistler cs cmu edu www berkeley edu Katz Stoica F04 23 Example of Iterated DNS Query Iterated query root name server
View Full Document
Unlocking...