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.
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- Trigraph, World Wide Web, Peer-to-peer, Hypertext Transfer Protocol, clie, application laye