Communication Networks (6)

Web caches (proxy server)

1. browser sends all HTTP requests to cache
2. object in cache: cache returns object
3. else cache requests object from origin server, then returns object to client

Conditional GET

1. send HTTP request with If-modified-since: <date>
2. server response HTTP/1.0 304 Not Modified if data not modified ever since
3. server response HTTP/1.0 200 OK <data> otherwise

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Chapter 2

Electronic mail

major components:

• user agents
• mail servers
• SMTP: Simple Mail Transfer Protocol

Electronic Mail: SMTP [RFC 2821]

SMTP: protocol for exchanging email messages

• use TCP, port 25
• direct transfer
• three phases of transfer
  • handshaking (greeting)
  • transfer of messages
  • closure
• command/response interaction like HTTP
  • commands: ASCII text (7-bit)
  • response: status code and phrase

Sample SMTP interaction

S: 220 hamburger.edu 
C: HELO crepes.fr 
S: 250  Hello crepes.fr, pleased to meet you 
C: MAIL FROM: <alice@crepes.fr> 
S: 250 alice@crepes.fr... Sender ok 
C: RCPT TO: <bob@hamburger.edu> 
S: 250 bob@hamburger.edu ... Recipient ok 
C: DATA 
S: 354 Enter mail, end with "." on a line by itself 
C: Do you like ketchup? 
C: How about pickles? 
C: . 
S: 250 Message accepted for delivery 
C: QUIT 
S: 221 hamburger.edu closing connection

(end with CRLF.CRLF)

Mail message format

RFC 822: standard for text message format

• header lines, e.g.,
  To:
  From:
  Subject:
  (different from SMTP MAIL FROM)
• Body: the “message” (ASCII characters only)

Mail access protocols

mail access protocol: retrieval from server

protocol	description
POP	Post Office Protocol [RFC 1939]: authorization, download
IMAP	Internet Mail Access Protocol [RFC 1730]: more features, including manipulation of stored messages on server
HTTP	gmail, Hotmail, Yahoo! Mail, etc.

DNS: domain name system

Domain Name System:

• distributed database (in hierarchy of name servers)
• application-layer protocol: hosts, name servers communicate to resolve names (address/name translation)

steps:

• client wants IP for www.amazon.com; 1st approximation:
• client queries 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

DNS name servers

Top-level domain (TLD) servers:

• com, org, net, edu, …
• top-level country domains: uk, fr, …

Authoritative DNS servers:

• organization’s named hosts
• maintained by organization or service provider

Local DNS name server (cached)

• does not strictly belong to hierarchy
• each ISP has one

DNS name resolution example

Diterated query

Name server caches mappings untiel expired (time of TTL)

DNS records

Distributed database storing resource records (RR).

RR Format: (name, value, type, ttl)

type	name	value
A	hostname	IP address
NS	domain	hostname of authoritative name server for this domain
CNAME	alias name	is canonical(real) name
MX	hostname	mail server

DNS protocol