【正文】 characters only header body blank line 2: Application Layer 57 Message format: multimedia extensions ? MIME: multimedia mail extension, RFC 2045, 2056 ? additional lines in msg header declare MIME content type From: To: Subject: Picture of yummy crepe. MIMEVersion: ContentTransferEncoding: base64 ContentType: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data multimedia data type, subtype, parameter declaration method used to encode data MIME version encoded data 2: Application Layer 58 Mail access protocols ? SMTP: delivery/storage to receiver’s server ? Mail access protocol: retrieval from server ? POP: Post Office Protocol [RFC 1939] ? authorization (agent server) and download ? IMAP: Inter Mail Access Protocol [RFC 1730] ? more features (more plex) ? manipulation of stored msgs on server ? HTTP: Hotmail , Yahoo! Mail, etc. user agent sender’s mail server user agent SMTP SMTP access protocol receiver’s mail server 2: Application Layer 59 POP3 protocol authorization phase ? client mands: ? user: declare username ? pass: password ? server responses ? +OK ? ERR transaction phase, client: ? list: list message numbers ? retr: retrieve message by number ? dele: delete ? quit C: list S: 1 498 S: 2 912 S: . C: retr 1 S: message 1 contents S: . C: dele 1 C: retr 2 S: message 1 contents S: . C: dele 2 C: quit S: +OK POP3 server signing off S: +OK POP3 server ready C: user bob S: +OK C: pass hungry S: +OK user successfully logged on 2: Application Layer 60 POP3 (more) and IMAP More about POP3 ? Previous example uses “download and delete” mode. ? Bob cannot reread if he changes client ? “Downloadandkeep”: copies of messages on different clients ? POP3 is stateless across sessions IMAP ? Keep all messages in one place: the server ? Allows user to anize messages in folders ? IMAP keeps user state across sessions: ? names of folders and mappings between message IDs and folder name 2: Application Layer 61 Chapter 2: Application layer ? Principles of work applications ? Web and HTTP ? FTP ? Electronic Mail ? SMTP, POP3, IMAP ? DNS ? P2P file sharing ? Socket programming with TCP ? Socket programming with UDP ? Building a Web server 2: Application Layer 62 DNS: Domain Name System People: many identifiers: ? SSN, name, passport Inter hosts, routers: ? IP address (32 bit) used for addressing datagrams ? “name”, ., used by humans Q: map between IP addresses and name ? Domain Name System: ? distributed database implemented in hierarchy of many name servers ? applicationlayer protocol host, routers, name servers to municate to resolve names (address/name translation) ? note: core Inter function, implemented as applicationlayer protocol ? plexity at work’s “edge” 2: Application Layer 63 DNS Why not centralize DNS? ? single point of failure ? traffic volume ? distant centralized database ? maintenance doesn’t scale! DNS services ? Hostname to IP address translation ? Host aliasing ? Canonical and alias names ? Mail server aliasing ? Load distribution ? Replicated Web servers: set of IP addresses for one canonical name 2: Application Layer 64 Root DNS Servers DNS servers DNS servers edu DNS servers DNS servers DNS servers DNS servers DNS servers DNS servers Distributed, Hierarchical Database Client wants IP for 。 1st approx: ? Client queries a root server to find DNS server ? Client queries DNS server to get DNS server ? Client queries DNS server to get IP address for 2: Application Layer 65 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 13 root name servers worldwide b USCISI Marina del Rey, CA l ICANN Los Angeles, CA e NASA Mt View, CA f Inter Software C. Palo Alto, CA (and 17 other locations) i Autonomica, Stockholm (plus 3 other locations) k RIPE London (also Amsterdam, Frankfurt) m WIDE Tokyo a Verisign, Dulles, VA c Cogent, Herndon, VA (also Los Angeles) d U Maryland College Park, MD g US DoD Vienna, VA h ARL Aberdeen, MD j Verisign, ( 11 locations) 2: Application Layer 66 TLD and Authoritative Servers ? Toplevel domain (TLD) servers: responsible for , , , edu, etc, and all toplevel country domains uk, fr, ca, jp. ? Network solutions maintains servers for TLD ? Educause for edu TLD ? Authoritative DNS servers: anization’s DNS servers, providing authoritative hostname to IP mappings for anization’s servers (., Web and mail). ? Can be maintained by anization or service provider 2: Application Layer 67 Local Name Server ? Does not strictly belong to hierarchy ? Each ISP (residential ISP, pany, university) has one. ?Also called “default name server” ? When a host makes a DNS query, query is sent to its local DNS server ? Acts as a proxy, forwards query into hierarchy. 2: Application Layer 68 requesting host root DNS server local DNS server 1 2 3 4 5 6 authoritative DNS server 7 8 TLD DNS server Example ? Host at wants IP address for 2: Application Layer 69 requesting host root DNS server local DNS server 1 2 4 5 6 authoritative DNS server 7 8 TLD DNS server 3 Recursive queries recursive query: ? puts burden of name resolution on contacted name server