1 Review of Previous Lecture r Principles of network applications m App architectures m App requirements r Web and HTTP m Non-persistent & persistent Pipeling m Messages, cookies m Web cashing r FTP

2 Outline r Electronic Mail m SMTP, POP3, IMAP r DNS r P2P file sharing

3 Electronic Mail One of the Internet killer apps Asynchronous app Three major components: r user agents r mail servers r simple mail transfer protocol: SMTP User Agent r a.k.a. “mail reader” r composing, editing, reading mail messages r e.g., Eudora, Outlook, elm, Netscape Messenger r outgoing, incoming messages stored on server user mailbox outgoing message queue mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP

4 Electronic Mail: mail servers Mail Servers r mailbox contains incoming messages for user r message queue of outgoing (to be sent) mail messages r SMTP protocol between mail servers to send messages m client: sending mail server m “server”: receiving mail server r Example  m If the sending mail server cannot deliver the message, it is queued mail server user agent user agent user agent mail server user agent user agent mail server user agent SMTP

5 Scenario: Alice sends message to Bob 1) Alice uses UA to compose message and “to” 2) Alice’s UA sends message to her mail server; message placed in message queue 3) Client side of SMTP opens TCP connection with Bob’s mail server -> there are no intermediate servers! 4) SMTP client sends Alice’s message over the TCP connection -> if there are more messages – they are sent via a persistent TCP connection 5) Bob’s mail server places the message in Bob’s mailbox 6) Bob invokes his user agent to read message user agent mail server mail server user agent

6 Try SMTP interaction for yourself:  telnet servername 25 r see 220 reply from server r enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send without using client (reader)

7 Sample SMTP interaction r S: 220 server_host_name r C: HELO client_host_name r S: 250 Hello client_host_name, pleased to meet you r C: MAIL FROM: r S: 250 Sender ok r C: RCPT TO: r S: 250 Recipient ok r C: DATA r S: 354 Enter mail, end with "." on a line by itself r C: Hello Bob, r C: how are you doing? r C:. r S: 250 Message accepted for delivery r C: QUIT r S: 221 server_host_name closing connection

8 Comparison with HTTP r HTTP: pull; SMTP: push r both use persistent TCP connections r both have ASCII command/response interaction, status codes Handling documents with text and images: r HTTP: each object encapsulated in its own response msg r SMTP: multiple objects sent in multipart msg

9 SMTP and Mail access protocols Questions: Why does Alice needs an intermediate mail server? Why does Bob use a different protocol? user agent sender’s mail server user agent SMTP access protocol receiver’s mail server

10 Mail access protocols r SMTP: delivery/storage to receiver’s server r Mail access protocol: retrieval from server m POP: Post Office Protocol [RFC 1939] authorization (agent server) and download m IMAP: Internet Mail Access Protocol [RFC 1730] more features (more complex) manipulation of stored msgs on server m HTTP: Hotmail, Yahoo! Mail, etc. user agent sender’s mail server user agent SMTP access protocol receiver’s mail server

11 POP3 protocol C: telnet mailserver 110 authorization phase r client commands:  user: declare username  pass: password r server responses m +OK  -ERR transaction phase, client:  list: list message numbers  retr: retrieve message by number  dele: delete r quit C: list S: S: S:. C: retr 1 S: S:. C: dele 1 C: retr 2 S: 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

12 POP3 (more) and IMAP More about POP3 r Previous example uses “download and delete” mode. r Bob cannot re-read e- mail if he changes client r “Download-and-keep”: copies of messages on different clients r POP3 is stateless across sessions IMAP r Keep all messages in one place: the server r Allows user to organize messages in folders r IMAP keeps user state across sessions: m names of folders and mappings between message IDs and folder name

13 Outline r Electronic Mail m SMTP, POP3, IMAP r DNS r P2P file sharing

14 DNS: Domain Name System People: many identifiers: m SSN, name, passport # Internet hosts, routers: m IP address (32 bit) - used for addressing datagrams m “name”, e.g., ww.yahoo.com - used by humans Q: map between IP addresses and name ? Domain Name System: r distributed database implemented in hierarchy of many name servers r application-layer protocol (udp on port 53) enables host, routers, name servers to communicate to resolve names (address/name translation) m note: core Internet function, implemented as application-layer protocol m complexity at network’s “edge”

15 DNS Why not centralize DNS? r single point of failure r traffic volume r distant centralized database r maintenance doesn’t scale! DNS services r Hostname to IP address translation m E.g., r Host aliasing m Canonical and alias names m E.g., dell.com r Mail server aliasing m E.g., r Load distribution m Replicated Web servers: set of IP addresses for one canonical name m E.g., cnn.com

16 Root DNS Servers com DNS servers org DNS serversedu 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: r Client queries a root server to find com DNS server r Client queries com DNS server to get amazon.com DNS server r Client queries amazon.com DNS server to get IP address for

17 DNS: Root name servers r contacted by local name server that can not resolve name r root name server: m contacts TLD server if name mapping not known TLD server contacts authoritative name server if name mapping not known m gets mapping m returns mapping to local name server 13 root name servers worldwide each server is actually a cluster of replicated servers 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 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)

18 TLD and Authoritative Servers r Top-level domain (TLD) servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp. m “Network solutions” maintains servers for com TLD m “Educause” for edu TLD r Authoritative DNS servers: organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web and mail). m Can be maintained by organization or service provider

19 Local Name Server r Does not strictly belong to hierarchy r Each ISP (residential ISP, company, university) has one. m Also called “default name server” When you connect to an ISP, you have to type the address of the default DNS server r When a host makes a DNS query, query is sent to its local DNS server m Acts as a proxy, forwards query into hierarchy.

20 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server Example r Host at cis.poly.edu wants IP address for gaia.cs.umass.edu

21 requesting host cis.poly.edu gaia.cs.umass.edu root DNS server local DNS server dns.poly.edu authoritative DNS server dns.cs.umass.edu 7 8 TLD DNS server 3 Recursive queries recursive query: r puts burden of name resolution on contacted name server iterated query: r contacted server replies with name of server to contact m Used in practice r “I don’t know this name, but ask this server”

22 DNS: caching and updating records r once (any) name server learns mapping, it caches mapping the DNS server can provide the desired IP address even if it is not authoritative for that hostname m cache entries timeout (disappear) after some time because hosts and mapping between host names and IP addresses are by no means permanent m TLD servers typically cached in local name servers Thus root name servers not often visited

23 DNS records DNS: distributed db storing resource records (RR) r Type=NS  name is domain (e.g. foo.com)  value is IP address of authoritative name server for this domain RR format: (name, value, type, ttl) r Type=A  name is hostname  value is IP address r Type=CNAME  name is alias name for some “cannonical” (the real) name is really servereast.backup2.ibm.com  value is cannonical name r Type=MX  value is name of mailserver associated with name

24 DNS protocol, messages DNS protocol : query and reply messages, both with same message format msg header r identification: 16 bit # for query, reply to query uses same # r flags: m query or reply m recursion desired m recursion available m reply is authoritative

25 DNS protocol, messages Name, type fields for a query (Name, Type) e.g., (ibm.com, CNAME) RRs in reponse to query (Type, Value, TTL) (CNAME, serv.bckup.ibm.com,5) records for authoritative servers additional “helpful” info that may be used e.g., (serv.bckup.ibm.com, ,A)

26 Inserting records into DNS r Example: just created startup “Network Utopia” r Register name networkuptopia.com at a registrar (e.g., “Network Solutions”) m Need to provide registrar with names and IP addresses of your authoritative name server (primary and secondary) m Registrar inserts two RRs into the com TLD server: (networkutopia.com, dns1.networkutopia.com, NS) (dns1.networkutopia.com, , A) r Put in authoritative server Type A record for and Type MX record for networkutopia.com r How do people get the IP address of your Web site?

27 Outline r Electronic Mail m SMTP, POP3, IMAP r DNS r P2P file sharing

28 P2P file sharing Example r Alice runs P2P client application on her notebook computer r Intermittently connects to Internet; gets new IP address for each connection r Asks for “Hey Jude” r Application displays other peers that have copy of Hey Jude. r Alice chooses one of the peers, Bob. r File is copied from Bob’s PC to Alice’s notebook: HTTP r While Alice downloads, other users uploading from Alice. r Alice’s peer is both a Web client and a transient Web server. All peers are servers = highly scalable!

29 P2P: centralized directory original “Napster” design 1) when peer connects, it informs central server: m IP address m content 2) Alice queries for “Hey Jude” 3) Alice requests file from Bob centralized directory server peers Alice Bob

30 P2P: problems with centralized directory r Single point of failure m if the directory server crashes, then the entire p2p application crashes r Performance bottleneck m a centralized server must maintain a huge database r Copyright infringement m Easy to shut down the directory servers by legal actions file transfer is decentralized, but locating content is highly decentralized

31 Query flooding: Gnutella r fully distributed m no central server r public domain protocol r many Gnutella clients implementing protocol overlay network: graph r edge between peer X and Y if there’s a TCP connection r all active peers and edges is overlay net r Edge is not a physical link r Given peer will typically be connected with < 10 overlay neighbors

32 Gnutella: protocol Query QueryHit Query QueryHit Query QueryHit File transfer: HTTP r Query message sent over existing TCP connections r peers forward Query message r QueryHit sent over reverse path Scalability: limited scope flooding

33 Gnutella: Peer joining 1. Joining peer X must find some other peer in Gnutella network: use list of candidate peers 2. X sequentially attempts to make TCP with peers on list until connection setup with Y 3. X sends Ping message to Y; Y forwards Ping message. 4. All peers receiving Ping message respond with Pong message 5. X receives many Pong messages. It can then setup additional TCP connections