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Katz, Stoica F04
EE122: DNS and the Web
(after a little more multicast)
October 27, 2003
Katz, Stoica F04
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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3Katz, 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
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4Katz, Stoica F04
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
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Narada: End System Multicast
Stanford
CMU
Stan1
Stan2
Berk2
“Overlay” TreeGatech
Berk1
Berkeley
GatechStan1
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
Duplicate Packets:
Bandwidth Wastage
CMU
Stan1
Stan2
Berk2
Gatech
Berk1
Delay from CMU to
Berk1 increases
Stanford
Berkeley
GatechStan1
Stan2
CMU
Berk1
Berk2
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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) messagesdata
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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
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What Do You Need to Know?
DVRMP CBT SSM How they compare
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Today’s Lecture: 17
Network (IP)
Application
Transport
Link
Physical
2
7, 8, 9
10,11
17, 18, 19
14, 15, 16
21, 22, 23
25
6
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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 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
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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
“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
root
edu com gov mil org net uk fr
berkeley mit
eecs sims
argus
etc.
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Host names are administered hierarchically
root
edu com gov mil org net uk fr
berkeley mit
eecs sims
argus
root
edu com gov mil org net uk fr
berkeley
eecs sims 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
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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 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
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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
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
requesting hostwhistler.cs.cmu.edu
www.berkeley.edu
root name server
authorititive name serverns1.berkeley.edu
local name servermango.srv.cs.cmu.edu
1
23
4
5
6
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Example of Recursive DNS Query
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?
requesting hostwhistler.cs.cmu.edu
www.berkeley.edu
root name server
local name servermango.srv.cs.cmu.edu
1
23
4 5
6
authoritative name serverns1.berkeley.edu
intermediate name server(edu server)
7
8
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Example of Iterated DNS Query
Iterated query: Contacted server replies
with name of server to contact
“I don’t know this name, but ask this server”
requesting hostwhistler.cs.cmu.edu
www.berkeley.edu
root name server
local name servermango.srv.cs.cmu.edu
1
2
34
67
authoritative name serverns1.berkeley.edu
intermediate name server(edu server)
5
8
iterated query
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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 (cont’d)
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-name:port/directory-path/resource
Extend the idea of hierarchical namespaces to include anything in a file system- ftp://www.eecs.berkeley.edu/122/Lecture6/presentation.ppt
Extend to program executions as well…- http://us.f413.mail.yahoo.com/ym/ShowLetter?box=%40B%40Bul
k&MsgId=2604_1744106_29699_1123_1261_0_28917_3552_1289957100&Search=&Nhead=f&YY=31454&order=down&sort=date&pos=0&view=a&head=b
- 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 ServerTCP Syn
TCP syn + ack
TCP ack + HTTP GET
...
Establishconnection
Requestresponse
Client request
Closeconnection
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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; can’t satisfy the request
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Client Request
Steps to get the resource:
http://www.eecs.berkeley.edu/index.html
1. Use DNS to obtain the IP address of www.eecs.berkeley.edu
2. Send to an HTTP request:
GET /index.html HTTP/1.0
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Server Response
HTTP/1.0 200 OKContent-Type: text/htmlContent-Length: 1234Last-Modified: Mon, 19 Nov 2001 15:31:20 GMT<HTML><HEAD><TITLE>EECS Home Page</TITLE></HEAD>…</BODY></HTML>
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40Katz, Stoica F04
Example (from Kurose and Ross)
http://www.mylife.org/mypictures.htm After finding out the IP address of the host…1. http client initiates a TCP connection on :802. Client sends the get request via socket established in 13. Server sends the html file, which is encapsulated in its
response4. http server tells tcp to terminate connection5. http client receives the file and the browser parses it…
contains ten jpeg images6. Client repeats steps 1-4
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41Katz, Stoica F04
HTTP/1.0 Example
Client Server
Request image 1
Transfer image 1
Request image 2
Transfer image 2
Request text
Transfer text
Finish displaypage
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42Katz, Stoica F04
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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44Katz, Stoica F04
Web Proxies
Intermediaries between client and server
Client 1
Proxy Proxy Server
Client 2
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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47Katz, Stoica F04
Pipelined Requests/Responses
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
Client Server
Request 1Request 2Request 3
Transfer 1
Transfer 2
Transfer 3
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48Katz, Stoica F04
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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49Katz, Stoica F04
What’s 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....
