Handshake 2 transfer of messages 3 closure smtp uses

1. handshake, 2. transfer of messages, 3. closure
❒
SMTP uses persistent connections: sending mail server sends all its messages
to the receiving mail server over one TCP connection
❒
Email Scenario:
user
agent
mail
server
mail
server
user
agent
1
2
3
4
5
6
Send mail
Rcv mail

2: Application Layer 32
SMTP:
Comparison with HTTP:
❒
HTTP: pull
❒ SMTP: push
❒ both have ASCII command/response interaction, status codes
❒ HTTP: each object encapsulated in its own response msg
❒
SMTP: multipleobjects sent in multipart msg
Protocol Design Issue:
-
Pull vs. Push vs. Hybrid (spectrum)
-
how far do we push/pull
-
Issues & factors to analyze:
-
access pattern, delay, object dynamics, …

2: Application Layer 33
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 file sharing
❒
2.7 Socket programming with
TCP
❒
2.8 Socket programming with
UDP
❒ 2.9 Building a Web server

2: Application Layer 34
DNS: Domain Name System
Internet identifiers for hosts,
routers:

IP address used for addressing
datagrams

“name”, e.g., ww.yahoo.com - used
by humans
Q: map between IP addresses and
name?
Domain NameSystem:
❒ distributed database implemented in
hierarchy of many name servers
❒ application-layer protocol host, routers,
name servers to communicate to resolve
names (address/nametranslation)

note: coreInternet function,
implemented as application-layer
protocol

complexity at network’s “edge”

2: Application Layer 35
DNS
Why not centralizeDNS?
❒
single point of failure
❒
traffic volume
❒
distant centralized database=
delays
❒
maintenance
doesn’t scale!
DNS services
❒
hostnameto IP address
translation
❒
host aliasing

Canonical, alias names
❒
mail server aliasing
❒
load distribution

replicated Web servers: set of
IP addresses for one canonical
name

2: Application Layer 36
Root DNS Servers
com DNS servers
org DNS servers edu DNS servers
poly.edu
DNS servers
umass.edu
DNS servers
yahoo.com
DNS servers
amazon.com
DNS servers
pbs.org
DNS servers
Distributed, Hierarchical Database
Client wants IP for ; 1
st
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 37
DNS: Root nameservers
❒ contacted by local name server that can not resolve name
❒ root name server:

contacts authoritativenameserver if name mapping not known

gets mapping

returns mapping to local name server
13 root name servers
worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA
e NASA Mt View, CA
f Internet Software C. Palo
Alto,
CA (and 36 other locations)
i Autonomica, Stockholm (plus
28 other locations)
k RIPE London (also 16 other locations)
m WIDE Tokyo (also Seoul,
Paris, SF)
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)

2: Application Layer 38
TLD and AuthoritativeServers
❒
I. Top-level domain (TLD) servers:

responsiblefor com, org, net, edu, etc, and all top-level country
domains uk, fr, ca, jp.
