Chapter 2: Application layer
 2.1 Principles of
network applications
 2.2 Web and HTTP
 2.3 FTP
 2.4 Electronic Mail

SMTP, POP3, IMAP
 2.5 DNS
 2.6 P2P applications
 2.7 Socket programming
with TCP
 2.8 Socket programming
with UDP
 2.9 Building a Web
server
2: Application Layer
1
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
2
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
doesn’t scale!
2: Application Layer
3
Distributed, Hierarchical Database
Root DNS Servers
com DNS servers
yahoo.com
amazon.com
DNS servers DNS servers
org DNS servers
pbs.org
DNS servers
edu DNS servers
poly.edu
umass.edu
DNS serversDNS servers
Client wants IP for www.amazon.com; 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 www.amazon.com
2: Application Layer
4
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)
e NASA Mt View, CA
f Internet Software C. Palo Alto,
k RIPE London (also 16 other locations)
i Autonomica, Stockholm (plus
28 other locations)
m WIDE Tokyo (also Seoul,
Paris, SF)
CA (and 36 other locations)
13 root name
servers worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
2: Application Layer
5
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
6
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
7
DNS name
resolution example
root DNS server
2
 Host at cis.poly.edu
3
wants IP address for
gaia.cs.umass.edu
iterated query:
 contacted server
replies with name of
server to contact
 “I don’t know this
name, but ask this
server”
TLD DNS server
4
5
local DNS server
dns.poly.edu
1
8
requesting host
7
6
authoritative DNS server
dns.cs.umass.edu
cis.poly.edu
gaia.cs.umass.edu
2: Application Layer
8
DNS name
resolution example
recursive query:
root DNS server
2
 puts burden of name
resolution on
contacted name
server
 heavy load?
3
7
6
TLD DNS server
local DNS server
dns.poly.edu
1
5
4
8
requesting host
authoritative DNS server
dns.cs.umass.edu
cis.poly.edu
gaia.cs.umass.edu
2: Application Layer
9
DNS: caching and updating records
 once (any) name server learns mapping, it caches
mapping
 cache entries timeout (disappear) after some
time
 TLD servers typically cached in local name
servers
• Thus root name servers not often visited
 update/notify mechanisms under design by IETF
 RFC 2136

http://www.ietf.org/html.charters/dnsind-charter.html
2: Application Layer
10
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name,
 Type=A
 name is hostname
 value is IP address
 Type=NS
 name is domain (e.g.
foo.com)
 value is hostname of
authoritative name
server for this domain
value, type, ttl)
 Type=CNAME
 name is alias name for some
“canonical” (the real) name
www.ibm.com is really
servereast.backup2.ibm.com

value is canonical name
 Type=MX
 value is name of mailserver
associated with name
2: Application Layer
11
DNS protocol, messages
DNS protocol : query and reply messages, both with
same message format
msg header
 identification: 16 bit #
for query, reply to query
uses same #
 flags:
 query or reply
 recursion desired
 recursion available
 reply is authoritative
2: Application Layer
12
DNS protocol, messages
Name, type fields
for a query
RRs in response
to query
records for
authoritative servers
additional “helpful”
info that may be used
2: Application Layer
13
SIP: Session Initiation Protocol
[RFC 3261]
SIP long-term vision:
 all telephone calls, video conference calls
take place over Internet
 people are identified by names or e-mail
addresses, rather than by phone numbers
 you can reach callee, no matter where
callee roams, no matter what IP device
callee is currently using
Multimedia
Networking
7-14
SIP Services
 Setting up a call, SIP
provides mechanisms ..
 for caller to let
callee know she
wants to establish a
call
 so caller, callee can
agree on media type,
encoding
 to end call
Multimedia
Networking
 determine current IP
address of callee:

maps mnemonic
identifier to current IP
address
 call management:
 add new media streams
during call
 change encoding during
call
 invite others
 transfer, hold calls
7-15
Alice
Setting up a call to known IP
address
Alice’s SIP invite
Bob
167.180.112.24
INVITE bob
@193.64.2
10.89
c=IN IP4 16
7.180.112.2
4
m=audio 38
060 RTP/A
VP 0
193.64.210.89
port 5060
port 5060
Bob's
terminal rings
200 OK
.210.89
c=IN IP4 193.64
RTP/AVP 3
3
m=audio 4875
ACK
message indicates her
port number, IP address,
encoding she prefers to
receive (PCM ulaw)
Bob’s 200 OK message
indicates his port number,
IP address, preferred
encoding (GSM)

port 5060
SIP messages can be
sent over TCP or UDP;
here sent over RTP/UDP.

m Law audio
port 38060
GSM

port 48753
default
is 5060.
Multimedia
time
Networking
time
SIP port number
7-16
Setting up a call (more)
 codec negotiation:
suppose Bob doesn’t
have PCM ulaw
encoder.
 Bob will instead reply
with 606 Not
Acceptable Reply,
listing his encoders
Alice can then send
new INVITE
message, advertising
different encoder

Multimedia
Networking
 rejecting a call
Bob can reject with
replies “busy,”
“gone,” “payment
required,”
“forbidden”
 media can be sent over
RTP or some other
protocol

7-17
Example of SIP message
INVITE sip:bob@domain.com SIP/2.0
Via: SIP/2.0/UDP 167.180.112.24
From: sip:alice@hereway.com
To: sip:bob@domain.com
Call-ID: a2e3a@pigeon.hereway.com
Content-Type: application/sdp
Content-Length: 885
c=IN IP4 167.180.112.24
m=audio 38060 RTP/AVP 0
Notes:
 HTTP message syntax
 sdp = session description protocol
 Call-ID is unique for every call.
Multimedia
Networking
 Here we don’t know
Bob’s IP address.
 intermediate SIP
servers needed.
 Alice sends, receives
SIP messages using
SIP default port 506
Alice specifies in
header that SIP client
sends, receives SIP
messages over UDP

7-18
Name translation and user
location
 caller wants to call
callee, but only has
callee’s name or e-mail
address.
 need to get IP address
of callee’s current
host:



user moves around
DHCP protocol
user has different IP
devices (PC, PDA, car
device)
Multimedia
Networking
 result can be based on:
 time of day (work, home)
 caller (don’t want boss to
call you at home)
 status of callee (calls sent
to voicemail when callee is
already talking to
someone)
Service provided by SIP
servers:
 SIP registrar server
 SIP proxy server
7-19
SIP Registrar

when Bob starts SIP client, client sends SIP
REGISTER message to Bob’s registrar server
(similar function needed by Instant Messaging)
Register Message:
REGISTER sip:domain.com SIP/2.0
Via: SIP/2.0/UDP 193.64.210.89
From: sip:bob@domain.com
To: sip:bob@domain.com
Expires: 3600
Multimedia
Networking
7-20
SIP Proxy
 Alice sends invite message to her proxy
server

contains address sip:bob@domain.com
 proxy responsible for routing SIP
messages to callee

possibly through multiple proxies.
 callee sends response back through the
same set of proxies.
 proxy returns SIP response message to
Alice
contains Bob’s IP address
Multimedia

Networking
7-21
Example
Caller jim@umass.edu
with places a
call to keith@upenn.edu
SIP registrar
upenn.edu
2
SIP proxy
umass.edu
SIP
registrar
eurecom.fr
3
4
(1) Jim sends INVITE
message to umass SIP
1
5
7
proxy. (2) Proxy forwards
8
request to upenn
6
registrar server.
9
(3) upenn server returns
SIP client
197.87.54.21
SIP client
redirect response,
indicating that it should 217.123.56.89
try keith@eurecom.fr
(4) umass proxy sends INVITE to eurecom registrar. (5) eurecom
registrar forwards INVITE to 197.87.54.21, which is running keith’s SIP
client. (6-8) SIP response sent back (9) media sent directly
between clients.
Note:
also a SIP ack message, which is not shown.
Multimedia
Networking
7-22