File Transfer Protocol (FTP)

Transcription

1 COM 362 Computer Networks I. FTP Lecture Outline Lec 4: Applicatin Layer: FTP, and P2P Applications 2. Electronic Mail:, POP3, IMAP 3. P2P Applications Prof. Dr. Halûk Gümüşkaya Computing Engineering Department Tuesday, March 8, File Transfer Protocol (FTP) Download/upload files between a client and One of the first Internet protocols More complex than (we`ll see later) Control connection Separate data connection Sends passwords in plain ASCII text Eavesdropper can recover passwords Fatal flaw, turned off at a lot of sites Replaced with scp, sftp instead FTP: the file transfer protocol at host FTP interface FTP client local file system file transfer FTP remote file system transfer file to/from remote host client/ model client: side that initiates transfer (either to/from remote) : remote host ftp: RFC 959 ftp : port 2 (TCP control connection) port 20 (TCP data connection) 3 4

2 FTP Client/Server FTP: separate control, data connections User client file system Client Program User Interface Client protocol interpreter Client data transfer function Port 2 Port 20 Server Program Server protocol interpreter Server data transfer function file system 5 FTP client contacts FTP at port 2, using TCP client authorized over control connection client browses remote directory, sends commands over control connection when receives file transfer command, opens 2 nd TCP data connection (for file) to client after transferring one file, closes data connection FTP client TCP control connection, port 2 TCP data connection, port 20 FTP opens another TCP data connection to transfer another file control connection: out of band FTP maintains state : current directory, earlier authentication 6 FTP commands, responses FTP Session Example sample commands: sent as ASCII text over control channel USER name PASS password LIST return list of file in current directory RETR filename retrieves (gets) file STOR filename stores (puts) file onto remote host sample return codes status code and phrase (as in HTTP) 33 Username OK, password required 25 data connection already open; transfer starting 425 Can t open data connection 452 Error writing file 7 >ftp Connected to (vsftpd.2.) User ( haluk 33 Please specify the password. Password: 230 Login successful. ftp> ls 200 PORT command successful. Consider using PASV. 50 Here comes the directory listing. public_html 226 Directory send OK. ftp: 3 bytes received in 0.0Seconds.30Kbytes/sec. ftp> cd public_html 250 Directory successfully changed. ftp> get Queue.dat 200 PORT command successful. Consider using PASV. 50 Opening BINARY mode data connection for Queue.dat (30 bytes). 226 File send OK. ftp: 30 bytes received in 0.00Seconds Kbytes/sec. ftp> quit 8

3 . FTP 2. Electronic Mail, POP3, IMAP 3. P2P Applications E E is transferred from one host to another using the Simple Mail Transfer Protocol () Like HTTP, has a similar ASCII command and reply set to transfer messages between machines Think of a set of request strings and reply strings sent over the network transfers occur between: sending host and dedicated e dedicated e s They do not occur between receiving hosts and e s POP or IMAP protocols are used to receive e-s 9 0 Direct Mode Relay Mode Direct mode: Relay mode: Sending e from jack@town.com to jill@hill.com town. com Messages E Server Sending e from jack@town.com to jill@hill.com town. com E Server E Server Responses for hill.com for town.com for hill.com town.com first finds IP address for hill.com e using DNS request (type=ms) town.com opens TCP connection on port 25 and initiates protocol to transfer e message town.com is configured to send all e messages through a local e The local e buffers e messages and forwards them to other e s open relays can be used for spamming 2

4 Electronic Mail outgoing message queue Electronic Mail: s Three major components: s s simple transfer protocol: User Agent a.k.a. reader composing, editing, reading messages e.g., Outlook, Thunderbird, iphone client outgoing, incoming messages stored on box 3 Mail Servers box contains incoming messages for message queue of outgoing (to be sent) messages protocol between s to send e messages client: sending : receiving 4 Electronic Mail: [RFC 282] Scenario: Alice sends message to Bob uses TCP to reliably transfer e message from client to, port 25 direct transfer: sending to receiving three phases of transfer handshaking (greeting) transfer of messages closure command/response interaction commands: ASCII text response: status code and phrase messages must be in 7-bit ASCII ) Alice uses UA to compose message and to bob@someschool.edu 2) Alice s UA sends message to her ; message placed in message queue 3) Client side of opens TCP connection with Bob s 2 3 Alice s 4 4) client sends Alice s message over the TCP connection 5) Bob s places the message in Bob s box 6) Bob invokes his to read message 6 5 Bob s 5 6

5 Try interaction for yourself: telnet name 25 see 220 reply from enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands above lets you send e without using e client (reader) interaction with.fatih.edu.tr C: >telnet.fatih.edu.tr 25 S: 220 s.fatih.edu.tr E Postfix C: HELO fatih S: 250 s.fatih.edu.tr C: MAIL FROM: haluk@fatih.edu.tr S: 250 Ok C: RCPT TO: haluk@fatih.edu.tr S: 250 Ok C: DATA S: 354 Please start input. C: This is a test message C:. S: 250 Mail queued for delivery. C: QUIT S: 22 Closing connection. Good bye. 7 C: Connection to host lost. 8 : final words Mail message format uses persistent connections requires message (header & body) to be in 7-bit ASCII uses CRLF.CRLF to determine end of message Comparison with HTTP: HTTP: pull protocol : push protocol both have ASCII command/response interaction, status codes HTTP: each object encapsulated in its own response msg : multiple objects sent in multipart msg : protocol for exchanging e msgs RFC 822: standard for text message format: header lines, e.g., To: From: Subject: different MAIL FROM, RCPT TO: commands! body the message, ASCII characters only header body blank line 9 20

6 Message format: multimedia extensions MIME types Content-Type: type/subtype; parameters MIME (Multipurpose Internet Mail Extention): multimedia extension, RFC 2045, 2056 additional lines in msg header declare MIME content type MIME version method used to encode data multimedia data type, subtype, parameter declaration encoded data From: To: Subject: Picture of yummy crepe. MIME-Version:.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data base64 encoded data Text example subtypes: plain, html Image example subtypes: jpeg, gif Audio example subtypes: basic (8-bit mu-law encoded), 32kadpcm (32 kbps coding) Video example subtypes: mpeg, quicktime Application other data that must be processed by reader before viewable example subtypes: msword, octet-stream 2 22 Mail access protocols sender s receiver s access protocol (e.g., POP, IMAP) : delivery/storage to receiver s Mail access protocol: retrieval from POP: Post Office Protocol [RFC 939] authorization ( <-->) and download IMAP: Internet Mail Access Protocol [RFC 730] more features including manipulation of stored msgs on HTTP: g, Hot, Yahoo! Mail, etc. POP3 protocol authorization phase client commands: : declare name pass: password responses +OK -ERR transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete quit C: > telnet Server 0 S: +OK POP3 ready C: bob S: +OK C: pass hungry S: +OK successfully logged on C: list S: 498 S: 2 92 S:. C: retr S: <message contents> S:. C: dele C: retr 2 S: <message contents> S:. C: dele 2 C: quit S: +OK POP3 signing off 23 24

7 POP3 (more) and IMAP More about POP3 previous example uses POP3 download and delete mode Bob cannot re-read e- if he changes client POP3 download-andkeep : copies of messages on different clients POP3 is stateless across sessions IMAP keeps all messages in one place: at allows to organize messages in folders keeps state across sessions: names of folders and mappings between message IDs and folder name. FTP 2. Electronic Mail, POP3, IMAP 3. P2P Applications Pure P2P architecture File distribution: client- vs P2P no always-on arbitrary end systems directly communicate peers are intermittently connected and change IP addresses Question: how much time to distribute file (size F) from one to N peers? peer upload/download capacity is limited resource u s : upload capacity examples: file distribution (BitTorrent) Streaming (KanKan) VoIP (Skype) file, size F u N d N u s u d u 2 d 2 network (with abundant bandwidth) d i u i d i : peer i download capacity u i : peer i upload capacity 27 28

8 File distribution time: client- transmission: must sequentially send (upload) N file copies: time to send one copy: F/u s time to send N copies: NF/u s client: each client must download file copy d min = min client download rate min client download time: F/d min time to distribute F to N clients using client- approach F u s network D c-s > max{nf/u s,,f/d min } d i u i increases linearly in N File distribution time: P2P transmission: must upload at least one copy F u s time to send one copy: F/u s client: each client must download file copy min client download time: F/d min clients: as aggregate must download NF bits time to distribute F to N clients using P2P approach network max upload rate (limiting max download rate) is u s + u i D P2P > max{f/u s,,f/d min,,nf/(u s + u i )} increases linearly in N but so does this, as each peer brings service capacity d i u i Client- vs. P2P: example client upload rate = u, F/u = hour, u s = 0u, d min u s Minimum Distribution Time P2P Client-Server N P2P file distribution: BitTorrent popular P2P protocol for file distribution 30 millions installations, as of 2008.Q file divided into 256Kb chunks peers in torrent send/receive file chunks tracker: tracks peers torrent: group of peers participating in torrent exchanging chunks of a file Alice arrives obtains list of peers from tracker and begins exchanging file chunks with peers in torrent trading chunks 3 32

9 P2P file distribution: BitTorrent peer joining torrent: has no chunks, but will accumulate them over time from other peers registers with tracker to get list of peers, connects to subset of peers ( neighbors ) while downloading, peer uploads chunks to other peers peer may change peers with whom it exchanges chunks churn: peers may come and go once peer has entire file, it may (selfishly) leave or (altruistically) remain in torrent 33 BitTorrent: requesting, sending file chunks requesting chunks: at any given time, different peers have different subsets of file chunks periodically, Alice asks each peer for list of chunks that they have Alice requests missing chunks from peers rarest first Which request to be responded first: tit-for-tat trading sending chunks: tit-for-tat Alice sends chunks to those 4 peers currently sending their chunks at highest rate other peers are choked by Alice (do not receive chunks from her) re-evaluate top 4 every0 secs every 30 secs: randomly select another peer, starts sending chunks optimistically unchoke this peer newly chosen peer may join top 4 Random selection allows new peers to get chunks, so they can start to trade Trading algorithm helps eliminate free-riding problem. 34 BitTorrent: tit-for-tat () Alice optimistically unchokes Bob (2) Alice becomes one of Bob s top-four providers; Bob reciprocates (3) Bob becomes one of Alice s top-four providers higher upload rate: find better trading partners, get file faster! P2P Case study: Skype inherently P2P: pairs of s communicate. proprietary application-layer protocol (inferred via reverse engineering) hierarchical overlay with SNs Index maps names to IP addresses; distributed over SNs Skype login Skype clients (SC) Supernode (SN) 35 36

10 Skype: making a call User starts Skype SC registers with SN o list of bootstrap SNs Skype login SC logs in (authenticate) Call: SC contacts SN with callee ID o SN contacts other SNs (unknown protocol, maybe flooding) to find addr of callee; returns addr to SC SC directly contacts callee Peers as relays Problem when both Alice and Bob are behind NATs. o NAT prevents an outside peer from initiating a call to insider peer Solution: o Using Alice s and Bob s SNs, Relay is chosen o Each peer initiates session with relay. o Peers can now communicate through NATs via relay Distributed Hash Table (DHT) How to implement a simple database in a P2P network? Hash table DHT paradigm Circular DHT and overlay networks Peer churn Simple Database Simple database with(key, value) pairs: key: human name; value: social security # Key Value John Washington Diana Louise Jones Xiaoming Liu Rakesh Gopal Linda Cohen Lisa Kobayashi key: movie title; value: IP address 39 40

11 Hash Table More convenient to store and search on numerical representation of key key = hash(original key) Original Key Key Value John Washington Diana Louise Jones Xiaoming Liu Rakesh Gopal Linda Cohen Lisa Kobayashi Distributed Hash Table (DHT) Distribute (key, value) pairs over millions of peers pairs are evenly distributed over peers Any peer can query database with a key database returns value for the key To resolve query, small number of messages exchanged among peers Each peer only knows about a small number of other peers Robust to peers coming and going (churn) 4 42 Assign key-value pairs to peers rule: assign key-value pair to the peer that has the closest ID. convention: closest is the immediate successor of the key. e.g., ID space {0,,2,3,,63} suppose 8 peers:,2,3,25,32,40,48,60 If key = 5, then assigned to peer 60 If key = 60, then assigned to peer 60 If key = 6, then assigned to peer Circular DHT each peer only aware of immediate successor and predecessor overlay network 44

12 Resolving a query 60 value 2 What is the value associated with key 53? Circular DHT with shortcuts value 2 25 What is the value for key O(N) messages on avgerage to resolve query, when there are N peers each peer keeps track of IP addresses of predecessor, successor, short cuts. reduced from 6 to 3 messages. possible to design shortcuts with O(log N) neighbors, O(log N) messages in query 46 Peer churn handling peer churn: peers may come and go (churn) each peer knows address of its successors each peer periodically pings its 4 2 successors to check aliveness 2 if immediate successor leaves, 5 choose next successor as new immediate successor 0 8 example: peer 5 abruptly leaves Peer churn handling peer churn: peers may come and go (churn) each peer knows address of its 5 3 two successors each peer periodically pings its 4 two successors to check aliveness 2 if immediate successor leaves, choose next successor as new immediate successor 0 8 example: peer 5 abruptly leaves peer 4 detects peer 5 s departure; makes 8 its immediate successor 4 asks 8 who its immediate successor is; makes 8 s immediate successor its second successor

13 Chapter 2: Summary Our study of network apps now complete! application architectures client- P2P application service requirements: reliability, bandwidth, delay Internet transport service model connection-oriented, reliable: TCP unreliable, datagrams: UDP Socket programming Specific protocols: DNS HTTP FTP, POP, IMAP P2P: BitTorren, Skype, DHT 49 Chapter 2: Summary Most importantly: learned about protocols typical request/reply message exchange: client requests info or service responds with data, status code message formats: headers: fields giving info about data data: info being communicated Important themes: control vs. data msgs in-band, out-of-band centralized vs. decentralized stateless vs. stateful reliable vs. unreliable msg transfer complexity at network edge 50