1. Introduction to Communication Networks
CSEN 503
Introduction to Communication Networks
Amr El Mougy
Rana Eisa
Hana Medhat
**Slides are attributed to J. F. Kurose

2. Roadmap: Application layer
Web caches (proxy server)
DNS
2: Application Layer

3. Web caches (proxy server)
Goal: satisfy client request without involving origin server
user sets browser: Web accesses via cache
browser sends all HTTP requests to cache
object in cache: cache returns object
else cache requests object from origin server, then returns object to client
origin
server
HTTP response
Proxy
server
HTTP request
client
HTTP request
HTTP response
client
origin
server
2: Application Layer

4. More about Web caching cache acts as both client and server
typically cache is installed by ISP (university, company, residential ISP)
Why Web caching?
reduce response time for client request
reduce traffic on an institution’s access link.
Internet dense with caches: enables “poor” content providers to effectively deliver content (but so does P2P file sharing)
2: Application Layer

5. Caching example Assumptions Consequences origin servers
average object size = 1,000,000 bits
avg. request rate from institution’s browsers to origin servers = 15 req/sec
delay from internet router to any origin server and back to router = 2 sec
Consequences
utilization on LAN = (15 requests/sec) . (1Mbits/request)/(100Mbps) = 15%
utilization on access link = (15 requests/sec) . (1Mbits/request)/(15Mbps) = 100%
total delay = Internet delay + access delay + LAN delay
= 2 sec + minutes + milliseconds
public
Internet
15 Mbps
access link
institutional
network
100 Mbps LAN
institutional
cache
2: Application Layer

6. Caching example (cont)
origin
servers
possible solution
increase bandwidth of access link to, say, 100 Mbps
consequence
utilization on LAN = 15%
utilization on access link = 15%
Total delay = Internet delay + access delay + LAN delay
= 2 sec + msecs + msecs
often a costly upgrade
public
Internet
100 Mbps
access link
institutional
network
100 Mbps LAN
institutional
cache
2: Application Layer

7. Caching example (cont)
possible solution: install cache
suppose hit rate is 0.4
consequence
40% requests will be satisfied almost immediately
60% requests satisfied by origin server
utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec)
total avg delay = Internet delay + access delay + LAN delay = .6*(2) secs + .4*milliseconds < 1.4 secs
origin
servers
public
Internet
15 Mbps
access link
institutional
network
100 Mbps LAN
institutional
cache
2: Application Layer

8. If-modified-since: <date> If-modified-since: <date>
Conditional GET
cache
server
Goal: don’t send object if cache has up-to-date cached version
cache: specify date of cached copy in HTTP request
If-modified-since: <date>
server: response contains no object if cached copy is up-to-date:
HTTP/ Not Modified
HTTP request msg
If-modified-since: <date>
object
not
modified
HTTP response
HTTP/1.0
304 Not Modified
HTTP request msg
If-modified-since: <date>
object
modified
HTTP response
HTTP/ OK
<data>
2: Application Layer

9. DNS: Domain Name System
People: many identifiers:
SSN, name, passport #
Internet hosts, routers:
IP address (32 bit) - used for addressing datagrams
“name”, e.g., ww.yahoo.com - 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)
note: core Internet function, implemented as application- layer protocol
complexity at network’s “edge”
2: Application Layer

10. DNS DNS services Hostname-to-IP-address translation host aliasing
Canonical, alias names
mail server aliasing
load distribution
replicated Web servers: set of IP addresses for one canonical name
Why not centralize DNS?
single point of failure
traffic volume
distant centralized database
Maintenance: update huge DB
doesn’t scale!
2: Application Layer

11. 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
2: Application Layer

12. 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
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also LA)
d U Maryland College Park, MD
g US DoD Vienna, VA
h ARL Aberdeen, MD
j Verisign, ( 21 locations)
k RIPE London (also 16 other locations)
i Autonomica, Stockholm (plus other locations)
e NASA Mt View, CA
f Internet Software C. Palo Alto, CA (and 36 other locations)
m WIDE Tokyo (also Seoul, Paris, SF)
13 root name servers worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
2: Application Layer

13. 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, mail).
can be maintained by organization or service provider
2: Application Layer

14. Local Name Server does not strictly belong to hierarchy
each ISP (residential ISP, company, university) has one.
also called “default name server”
when host makes DNS query, query is sent to its local DNS server
acts as proxy, forwards query into hierarchy
2: Application Layer

15. DNS name resolution example
root DNS server 2
Host at cis.poly.edu wants IP address for gaia.cs.umass.edu 3
TLD DNS server 4 5
iterated query:
contacted server replies with name of server to contact
“I don’t know this name, but ask this server”
local DNS server
dns.poly.edu 7 6 1 8
authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu
gaia.cs.umass.edu
2: Application Layer

16. DNS name resolution example
requesting host
cis.poly.edu
gaia.cs.umass.edu
root DNS server
local DNS server
dns.poly.edu 1 2 4 5 6
authoritative DNS server
dns.cs.umass.edu 7 8
TLD DNS server 3
recursive query:
puts burden of name resolution on contacted name server
heavy load?
2: Application Layer

17. DNS: caching and updating records
once (any) name server learns mapping, it caches mapping
cache entries timeout (disappear) after some time (often 48 hours)
TLD servers typically cached in local name servers
Thus root name servers not often visited
2: Application Layer

18. DNS Problem
Consider the network diagram on the right depicting three hosts H1, H2 and H3, as well as three levels of hierarchical DNS-Servers. The three hosts perform a large number of domain-name lookups over the course of a day.
When a given DNS-request is in the zone of a name-server, this means the server can resolve the domain-name directly, in other words it doesn't have to propagate the request to the next level. Imagine for example host H3 is issuing DNS-requests to DNS3. We say that the probability (Pz) of that request being in DNS3's zone is 17% when the expected number of requests DNS3 can resolve directly is 0.17 of the total number of requests. This in turn means that 0.83 of all the requests to DNS3 have to be propagated to DNS2.
Furthermore, DNS2 and DNS3 perform caching. If for example the hit- rate of a certain cache (denoted CHR) is 1/10, this means that on the average, 1 in 10 requests can be resolved from that server's cache (the other 9 have to be resolved by propagation).
Consider the table containing the probabilities and CHRs for the three name-servers. What is the overall average RTT for domain-name lookups, if RTT1, RTT2 and RTT3 denote the round-trip time from Hx to servers DNS1, DNS2 and DNS3 respectively?
Server Pz
CHR
DNS1
100% -
DNS2
43%
2/7
DNS3
17%
4/5

19. Solution
Suppose 100 requests are sent to DNS3, consider the following:
Of the 100 requests, how many will be resolved at server DNS3?
n3 = 17 + (4/5) (100-17) = (1)
Of the 100 requests, how many will be resolved at DNS2? Here we need to remember, that out of the 100 original packets, only some will be propagated to DNS2:
i2 = (100 - n3) = 16.6
where i2 is the number of requests propagated to DNS2. The number of requests found in DNS2 zone out of the i2 packets:
z2= i2 *(43/100) = 7.138
The number of requests resolved at DNS2 can now be calculated as follows:
n2 = z2 + (2/7)*(i2-z2) = (2/7) * ( ) =9.84 (2)
Since DNS1 has Pz = 100%, all the remaining packets will be resolved at DNS1, namely:
n1 = i1 = n3 - n2 (3)
Now we can calculate the weighted-average of the total RTT:
RTT =(1/100)* (RTT1*n1 + RTT2*n2 + RTT3* n3) (4)