EECS 122: Introduction to Computer Networks Multicast

1
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

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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

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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 lt hundreds of nodes
• Typical application chat

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Narada End System Multicast
Stanford
Gatech
Stan1
Stan2
CMU
Berk1
Berk2
Berkeley
Overlay Tree
Stan1
Gatech

Stan2
CMU
Berk1
Berk2
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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

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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

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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

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Performance Concerns
Delay from CMU to Berk1 increases
Stan1
Gatech

Stan2
CMU
Berk2
Berk1
Duplicate Packets Bandwidth Wastage
Stanford
Gatech
Stan1
Stan2
CMU

Berk1
Berk2

Berkeley
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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

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Example

• Group members M1, M2, M3

source
M1
M2
M3
control (join) messages
data
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Whats Wrong with SSM?

• Multiple sources?
• can set up group per source, or...
• source can serve as relay for other senders
• Algorithm?
• trivial
• So, why isnt SSM the answer?
• multicast no longer serves as rendezvous
• ok for broadcast apps, not good for meeting
  apps

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What Do You Need to Know?

• DVRMP
• CBT
• SSM
• How they compare

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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

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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)

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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 ftpd periodically from
  SRI
• An administrator could pick names at their
  discretion
• As the internet grew this system broke down
  because
• SRI couldnt 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

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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

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Naming Hierarchy
root
edu
gov
mil
net
uk
fr
etc.
com
org

• Top Level Domains are at the top
• Depth of tree is arbitrary (limit 128)
• Domains are subtrees
• 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

mit
berkeley
eecs
sims
argus
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Host names are administered hierarchically
root
root
edu
gov
mil
net
uk
fr
edu
gov
mil
net
uk
fr
com
org
com
org
mit
berkeley
berkeley

• 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

eecs
eecs
sims
sims
argus
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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

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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 hosts (name, IP address)
• Can perform name/address translation for that
  hosts name
• Can also do IP to name translation, but wont
  discuss

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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

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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
• 3. Root name server contacts authoritative name
  server, ns1.berkeley.edu, if necessary

2
4
3
5
authorititive name server ns1.berkeley.edu
1
6
requesting host whistler.cs.cmu.edu
www.berkeley.edu
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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
• Puts burden of name resolution on contacted name
  server
• Heavy load?

6
2
3
7
5
4
1
8
authoritative name server ns1.berkeley.edu
requesting host whistler.cs.cmu.edu
www.berkeley.edu
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Example of Iterated DNS Query

• Iterated query
• Contacted server replies with name of server to
  contact
• I dont know this name, but ask this server

root name server
iterated query
2
3
4
5
7
6
1
8
authoritative name server ns1.berkeley.edu
requesting host whistler.cs.cmu.edu
www.berkeley.edu
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DNS Records

• Four fields (name, value, type, TTL)
• Type A
• name hostname
• value IP address
• Type NS
• name domain
• value name of dns server for domain

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DNS Records (contd)

• Type CNAME
• name hostname
• value canonical name
• Type MX
• name domain in email address
• value canonical name of mail server

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DNS as Indirection Service

• Can refer to machines by name, not address
• not only easier for humans
• also allows machines to change IP addresses
  without having to change way you refer to machine
• Can refer to machines by alias
• www.berkeley.edu can be generic web server
• but DNS can point this to particular machine that
  can change over time
• But, this flexibility applies only within domain!

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Special Topics

• DNS caching
• Improve performance by saving results of previous
  lookups
• DNS hacks
• Return records based on requesting IP address
• dynamic DNS
• Allows remote updating of IP address for mobile
  hosts
• DNS politics (ICANN) and branding battles

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Important Properties of DNS

• Administrative delegation and distributed server
  architecture results in
• Easy unique naming
• Fate sharing for network failures
• Reasonable trust model

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The Web

• A distributed database of pages
• Core components
• Servers store files and execute remote commands
• Browsers retrieve and display pages
• URLs way to refer to pages
• Need a protocol to transfer information between
  clients and servers
• HTTP

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Uniform Record Locator

• protocol//host-nameport/directory-path/resource
• Extend the idea of hierarchical namespaces to
  include anything in a file system
• ftp//www.eecs.berkeley.edu/122/Lecture6/presentat
  ion.ppt
• Extend to program executions as well
• http//us.f413.mail.yahoo.com/ym/ShowLetter?box4
  0B40BulkMsgId2604_1744106_29699_1123_1261_0_289
  17_3552_1289957100SearchNheadfYY31454order
  downsortdatepos0viewaheadb
• Server side processing can be incorporated in the
  name

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Web and DNS

• URLs use hostnames
• Thus, content names are tied to specific hosts
• This is bad!
• URNs are one proposal to achieve persistence

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Hyper Text Transfer Protocol

• Client-server architecture
• Synchronous request/reply protocol
• Runs over TCP, Port 80
• Stateless
• Uses unicast

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Big Picture
Client
Server
TCP Syn
Establish connection
TCP syn ack
Client request
TCP ack HTTP GET
Request response
Close connection
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Hyper Text Transfer Protocol Commands

• GET transfer resource from given URL
• HEAD GET resource metadata (headers) only
• PUT store/modify resource under given URL
• DELETE remove resource
• POST provide input for a process identified by
  the given URL (usually used to post CGI
  parameters)

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Response Codes

• 1x informational
• 2x success
• 3x redirection
• 4x client error in request
• 5x server error cant satisfy the request

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Client Request

• Steps to get the resource http//www.eecs.berke
  ley.edu/index.html
• Use DNS to obtain the IP address of
  www.eecs.berkeley.edu
• Send to an HTTP request

GET /index.html HTTP/1.0
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Server Response
HTTP/1.0 200 OK Content-Type text/html Content-Le
ngth 1234 Last-Modified Mon, 19 Nov 2001
153120 GMT ltHTMLgt ltHEADgt ltTITLEgtEECS Home
Pagelt/TITLEgt lt/HEADgt lt/BODYgt lt/HTMLgt
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Example (from Kurose and Ross)

• http//www.mylife.org/mypictures.htm
• After finding out the IP address of the host
• http client initiates a TCP connection on 80
• Client sends the get request via socket
  established in 1
• Server sends the html file, which is encapsulated
  in its response
• http server tells tcp to terminate connection
• http client receives the file and the browser
  parses itcontains ten jpeg images
• Client repeats steps 1-4

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HTTP/1.0 Example
Server
Client
Request image 1
Transfer image 1
Request image 2
Transfer image 2
Request text
Transfer text
Finish display page
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HHTP/1.0 Performance

• Create a new TCP connection for each resource
• Large number of embedded objects in a web page
• Many short lived connections
• TCP transfer
• Too slow for small object
• May never exit slow-start phase
• Connections may be set up in parallel (5 is
  default in most browsers)

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HTTP/1.0 Caching

• Exploit locality of reference
• A modifier to the GET request
• If-modified-since return a not modified
  response if resource was not modified since
  specified time
• A response header
• Expires specify to the client for how long it
  is safe to cache the resource
• A request directive
• No-cache ignore all caches and get resource
  directly from server
• These features can be best taken advantage of
  with HTTP proxies
• Locality of reference increases if many clients
  share a proxy

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Web Proxies

• Intermediaries between client and server

Client 1
Client 2
Proxy
Proxy
Server
Client N
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HTTP/1.1 (1996)

• Performance
• Persistent connections
• Pipelined requests/responses
• Support for virtual hosting
• Efficient caching support
• Network Cache assumed more explicitly in the
  design
• Gives more control to the server on how it wants
  data cached

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Persistent Connections

• Allow multiple transfers over one connection
• Avoid multiple TCP connection setups
• Avoid multiple TCP slow starts

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Pipelined Requests/Responses
Server
Client

• Buffer requests and responses to reduce the
  number of packets
• Multiple requests can be contained in one TCP
  segment
• Note order of responses has to be maintained

Request 1
Request 2
Request 3
Transfer 1
Transfer 2
Transfer 3
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What You Need to Know

• DNS record types, and how they are used
• HTTP basics (and essential differences between
  1.0 and 1.1)

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Whats the Moral of this Story?

• QoS and IP Multicast
• interesting algorithmic and architectural issues
• thousands of academic papers
• ubiquitous in routers, but not deployed by ISPs
• little or no impact on end users
• DNS and the Web
• no research papers on topic before deployment
• really boring designs
• they changed the world....