1. CS 5565 Network Architecture and Protocols
Lecture 7
Godmar Back

2. Announcements Problem Set 1 due Feb 18 Project 1A due Feb 19
Submission website will be up soon
Required Reading: research paper, see website
CS 5565 Spring 2009
5/31/2018

3. Application Protocols
Part 1: HTTP

4. Mashups
Web applications that combine and mix data from different sources
CS 5565 Spring 2009
5/31/2018

5. Base Server http://www.lib.edu Source A
Mash-Up Server
Source B
HTML
Source C
Client Browser
Base Page A C B
Server-side Mash-Up

6. Base Server http://www.lib.edu Source A
HTML+JavaScript
Proxy
Source B
XMLHttpRequest
Source C
Client Browser
Base Page A C B
Proxy-Based Mash-Up

7. Base Server http://www.lib.edu Source B http://opac.lib.edu/sourceB
HTML+JavaScript
Same Origin Restriction
Same Domain Restriction
Source A
XMLHttpRequest
+ XML/Text
XMLHttpRequest
+ XML/Text
No Domain Restriction
Client Browser
Base Page
Source C A B C B
(via hidden frame)
<SCRIPT> + JSON
Client-side Mash-Up

8. HTTP & JSON
In modern web applications, HTTP is increasingly initiated from code
Either directly, e.g., XMLHttpRequest
Or indirectly, via SCRIPT/JSON
Example:
callThisFunction({" ":{"bib_key":" ",
"info_url": "
"preview_url": "
preview":"noview", "embeddable":false}});
CS 5565 Spring 2009
5/31/2018

9. HTTP & AJAX
AJAX Frameworks no longer build entire web pages from an HTML description
Rather: bootstrap small amount of code, then dynamically load and update page “piece-by-piece”
Will explore this in HTTP Wireshark Lab
CS 5565 Spring 2009
5/31/2018

10. Goal – Use of AJAX Page 1 Page 2 Page 3 Conventional Navigation Model
Submit button pressed
Submit button pressed
Page 1
Servlet1
Servlet1-1
Update portion of page
HTML Page returned
Page 2
Submit button pressed
Mouse Hover
Servlet 2
Servlet
1-2
Page 3
HTML Page returned
Update Portion of Page
Popularity of ajax over recent years because of advtgs
AJAX is the technology responsible for the Web 2.0 hype.
Many companies have started using AJAX for their projects because of the notable advantages that it offers in terms of user-interaction
Our first goal was to implement the Edition Builder as a web application because
Web-applications can be accessed by any
user with a computer and an Internet connection.
Moreover , web applications can be updated and maintained without installation of software
on the client side. Hence, deployment of changes and updates can be done
transparently from the user and in an expedited fashion.
Problems in conventional navigation model
Disruptive page refreshes after each user-interaction
Confusing because of hyper-links and page layout
AJAX
Asynchronous JavaScript and XML
Incremental update of pages (No page refreshes)
Higher levels of interactivity and responsiveness
Explain Working of AJAX
An AJAX application has an intermediate layer between the user and the server called the
AJAX engine. Each user interaction results in a call to the AJAX engine. The AJAX engine
decides, whether a call requires action from the server. If the AJAX engine itself can service
a call, it does so without resorting to communication with the server. In this way, unnecessary
http requests to the server are avoided and user-interaction can proceed smoothly with
other components that do not require server-processing at that moment. Thus, due to the
interposition of the AJAX engine, a user interacts with the application without interruptions
Conventional Navigation Model
AJAX Navigation Model

11. Rich User Interaction with AJAX
Ad hoc AJAX programming problems
Browser Incompatibility
JavaScript development
Scripting language
History of ad hoc usage causing un-maintainable code
Developers responsible for low level details
AJAX Frameworks
Provide libraries, API for above problems
Save development time and effort
Variety of frameworks to choose from
Diff frameworks evaluation was a part of this wrk

12. Server centric vs. Client centric AJAX frameworks
Browser
Browser
Generated HTML+ JavaScript
Client Side AJAX Engine
Application
HTML+ JavaScript
Client Side AJAX Engine
Dom
Dom
Server Side AJAX Engine
Web Application
Web Application
Scraps of sample code that you find on the Web and that you can cut and paste into your own application, to comprehensive frameworks. Just above the level of code scraps are individual JavaScript components that present an API for a specific purpose. Some of these represent UI widgets that will appear on a Web page, like the Dojo Button, and others are for solving lower-level, plumbing-related issues like communicating with the server or manipulating the browser DOM (for example the Prototype APIs).
Fundamental choices
Give examples of toolkits that follow either model
Explain dom
If you look at the diagrams in Figure 2 below, you can see that in the server-centric model, the application programmer develops the entire application in what appears to the programmer to be server-side code. Of course, DHTML and JavaScript are involved when the application runs, but the framework does not expose these to the programmer. The framework provides both a server-side and client-side Ajax engine. The server-side engine generates HTML and JavaScript that is served into the browser along with a client-side engine, which provides APIs used by the generated code to manipulate the browser DOM, receive events, and push them back to the server-side engine, which communicates events back to the application.
A client-side framework, on the other hand, expects the application programmer to write both the server side of the application and the part that will run in the browser. The client-side code is served into the browser along with the Ajax engine, which performs the same intercession functions between the DOM and the application code that it does in the server-centric model
An advantage of client side frameworks is that developers can leverage JavaScript skills to
combine features from various frameworks and create a complex application. But, client side
frameworks entail significant amount of JavaScript programming, which can still give rise
to unmaintainable JavaScript code. Moreover, with client-centric AJAX frameworks, the
programmer needs to take the responsibility of sending client state to the server periodically.
If the client fails to send state information to the server then the server may not be able to
retain consistency of the application based on the client-state.
Server
Server
Server-Centric
Client-Centric
Framework code
Application Code
Generated Code

13. Application Protocols
Part 2: DNS

14. DNS: Domain Name System
People: many identifiers:
SSN, name, passport #
Internet hosts, routers:
IP address (32 bit) - used for addressing datagrams
“name”, e.g., - used by humans
Q: map between IP addresses and name ?
Domain Name System:
distributed database implemented in hierarchy of many name servers
application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) – not built into the network!
CS 5565 Spring 2009
5/31/2018

15. DNS Why not centralize DNS? single point of failure traffic volume
distant centralized database
maintenance
doesn’t scale!
DNS services
Hostname to IP address translation
Host aliasing
Canonical and alias names
Mail server aliasing
Load distribution
Replicated Web servers: set of IP addresses for one canonical name
CS 5565 Spring 2009
5/31/2018

16. Distributed, Hierarchical Database
Root DNS Servers
com DNS servers
org DNS servers
edu DNS servers
poly.edu
DNS servers
umass.edu
yahoo.com
amazon.com
pbs.org
Client wants IP for 1st approx:
Client queries a root server to find .com DNS server
Client queries .com DNS server to get amazon.com DNS server
Client queries amazon.com DNS server to get IP address for
CS 5565 Spring 2009
5/31/2018

17. Resource Types Type Name Value A Hostname www.cs.vt.edu IP Address
CNAME
Alias
rlogin.cs.vt.edu
Canonical Hostname
hn1.cs.vt.edu NS
Domainname
vt.edu
Authoritative Nameserver
nomen.cns.vt.edu MX
Mail Domain
cs.vt.edu
Mail Server
antispam.cs.vt.edu
CS 5565 Spring 2009
5/31/2018

18. DNS: Root Name Servers List published at Replicated using IP anycast
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also Los Angeles)
d U Maryland College Park, MD
g US DoD Vienna, VA
h ARL Aberdeen, MD
j Verisign, ( 11 locations)
13 root name servers worldwide
k RIPE London (also Amsterdam, Frankfurt)
i Autonomica, Stockholm (plus 3 other locations)
m WIDE Tokyo
e NASA Mt View, CA
f Internet Software C. Palo Alto, CA (and 17 other locations)
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
List published at
ftp://ftp.internic.net/domain/named.cache
See
Replicated using IP anycast
See Links section at website for more information
CS 5565 Spring 2009
5/31/2018

19. TLD and Authoritative Servers
Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp, ....
Network solutions maintains servers for com TLD
Educause for edu TLD
Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail).
Can be maintained by organization or service provider
CS 5565 Spring 2009
5/31/2018

20. Caching and Local Name Servers
Q: do we want every (of the 280 million) Internet hosts be contacting a dozen or so root servers all the time?
A: No. Caching is needed
Local Name Servers bundle queries by clients they serve and cache their results
CS 5565 Spring 2009
5/31/2018

21. Local Name Server Does not strictly belong to hierarchy
When a host makes a DNS query, query is sent to its local DNS server
Acts as a proxy, forwards query into hierarchy.
Each ISP (residential ISP, company, university) has one (or more.)
Also called “default name server” or resolver
Contacted by resolver library (libresolv.so, or part of libc – provides such functions as gethostbyname(), getaddrinfo() etc. – more on this later
CS 5565 Spring 2009
5/31/2018

22. Caching Records Aside: Local Name Server crucial resource
Once (any) name server learns mapping, it caches mapping
cache entries time out (disappear) after some time
86,400 seconds per day
TLD servers typically cached in local name servers
Thus root name servers not often visited
Aside: Local Name Server crucial resource
Proper cache management required
See Bernstein’s comments on caching in BIND (Berkeley Internet Name Domain) program
CS 5565 Spring 2009
5/31/2018

23. DNS Protocol: Query Types
Possible constellations:
Client’s resolver library talks to local name server
Local name server talks to other name servers: root server, TLD servers, …
Beauty of DNS Protocol: same protocol can be used for either constellation
Uses two query types
Recursive: “please resolve this name for me and send me the result”
Good for client, harder for server
Iterated: “please tell me what you know about the name – partial resolution is okay, I’ll ask the next server in hierarchy”
Easy for server, harder for client
CS 5565 Spring 2009
5/31/2018

24. Recursive vs. Iterative Queries
root DNS server
a.root-servers.net 2
TLD DNS server
a3.nstld.com 3
Host at
gback.cs.vt.edu
wants IP address for
godmar.stanford.edu
Sends recursive query to voodoo
Voodoo performs iterative queries 4 5
local DNS server
voodoo.slo.cs.vt.edu 7 6 1 8
authoritative DNS server
Argus.stanford.edu
requesting host
gback.cs.vt.edu
godmar.stanford.edu
CS 5565 Spring 2009
5/31/2018