Applications and Reliable Transport BasicsOverview• Lookup and shutdown• Applications: power of reliable sockets- Telnet- Web/HTTP- BitTorrent- Skype• Challenges of reliable transport• Next lecture: Reliable transport detailsAddress lookup details• getaddrinfo notes:- Can specify port as service name or number (e.g., "80" or"http", allows possibility of dynamically looking up port)- May return multiple addresses (chained with ai next field)- You must free structure with freeaddrinfo• Other useful functions to know about- getnameinfo – Lookup hostname based on address- inet ntop – convert IPv4 or 6 address to printable form- inetpton – convert string to IPv4 or 6 addressEOF in more detail• Simple client-server application- Client sends request- Server reads request, sends response- Client reads response• What happens when you’re done?- Client wants server to read EOF to say request is done- But still needs to be able to read server reply – fd is notclosed!shutdown system call• int shutdown (int fd, int how);- Shuts down a socket w/o closing file descriptor- how: 0 = reading, 1 = writing, 2 = both- Note: Applies to socket, not descriptor—so copies ofdescriptor (through dup or fork) affected- Note 2: With TCP, can’t detect if other side shuts forreading, so in practice 1 is only useful how argument• Play with uc from Lab 1 to see this- Pressing Ctrl-D at beginning of line sends EOF- uc reads EOF from standard input, uses shutdown to writeEOF to TCP socketReview• The Internet is a network of networks• Hourglass/narrow waist:host-host delivery- IP protocol to deliver packets tohosts- Transport above for service/process demultiplexing- Link layer below for a single hop• Sockets are system call interface for communication…FTPTCPUDPIPNET1NET2NETnHTTP NV TFTPApplications• Simple socket abstraction is very powerful- Applications can use them in many ways- We’ll go through four examples• “the intelligence is end to end rather than hiddenin the network.”– [RFC 1958] (architectural principles doc)• Two basic questions for an application- What do you send?- Whom do you send it to?An Reliable Socket View of the Worldclient serverInternet• Telnet: simple text stream• HTTP: downloading documents• BitTorrent: swarming large documents• Skype: IP telephony/lookupTelnet• Server listens on port 23• On connection, provides a login prompt• Allows insecure remote logins• Telnet client is a text interface- Allows session management- Handles terminal control signals, etc.• What matters most for performance?client serverInternetWeb/HTTP• Server listens on port 80• On connection, waits for a request• Protocol (but not data) is in ASCII• Sends response, maybe closes connection (clientcan ask it to stay open)Parsing a URLhttp://cs144.s cs.stanford.edu/labs/sc .htmlFileHostProtocolHTTP Request Formatmethod URL version ←↓header field name value ←↓header field name value ←↓←↓requestheadersbodyblank line• Request types: GET, POST, HEAD, PUT, DELETE• A URL given to browser: http://localhost:8000/• Resulting request: GET / HTTP/1.1- Someday, requests will contain the full URL not just pathA Browser RequestGET / HTTP/1.1Host: localhost:8000User-Agent: Mozilla/5.0 (Macinto ...Accept: text/xml,application/xm ...Accept-Language: en-us,en;q=0.5Accept-Encoding: gzip,deflateAccept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7Keep-Alive: 300Connection: keep-alive[LiveHTTPHeaders for Firefox is good way to see this]HTTP Response Formatversion status code phrase ←↓header field name value ←↓header field name value ←↓←↓statusheadersbodyblank line• 1xx codes: Informational• 2xx codes: Successes• 3xx codes: Redirection• 4xx codes: Client Error• 5xx codes: Server Errorwww.scs.stanford.edu ResponseHTTP/1.1 200 OKDate: Thu, 03 Apr 2008 21:09:57 GMTServer: ApacheLast-Modified: Thu, 03 Jan 2008 01:05:54 GMTETag: "a577678157a762e3d46afd4038d9a35bf2ae9386"Accept-Ranges: bytesContent-Length: 3586Connection: closeContent-Type: text/html<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN""http://www.w3.org/TR/html4/strict.dtd">...HTTP Performance• What matters most for performance?• Depends on types of requests- Lots of small requests (loading a page)- Big request (fetching a download)• Require different solutionsSmall Requests• Latency matters• Governed by round-trip-time (RTT) between hosts• Two major causes:- Opening a TCP connection(TCP requires 7 messages even with no data, including oneround-trip even before you can send request)- Actually sending request and receiving response• Mitigate first cost with persistent connectionsBrowser Request, RevisitedGET / HTTP/1.1Host: localhost:8000User-Agent: Mozilla/5.0 (Macinto ...Accept: text/xml,application/xm ...Accept-Language: en-us,en;q=0.5Accept-Encoding: gzip,deflateAccept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7Keep-Alive: 300Connection: keep-aliveBig Requests• Problem is throughput on edge link• Use an HTTP proxy cache (or cache locally)- Can also improve latency!clientsserverInternetproxycacheStale Caches• Items in the cache can go stale (you don’t want toread yesterday’s paper)• Cache needs a way to conditionally ask for adocument• Issue a conditional GET (If-modified-sinceheader)- Server can reply with a 304 Not ModifiedGET / HTTP/1.1Host: cs144.scs.stanford.eduIf-modified-since: Wed, 02 Apr 2008 08:00:00 GMTClient-Server vs. Peer-to-Peer• Server can be a bottleneck- Download time can scale O(n) with n clients- Scaling up server bandwidth can be expensive (CDNs,lecture 12)- Slashdotting/flash crowds• Peer-to-peer: get a bunch of end-hosts to distributecontent collaboratively• A common peer-to-peer challenge is finding whomto collaborate withBitTorrent• Torrent file (.torrent) describes file to download- Names tracker, server tracking who is participating- File length, piece length, SHA1 hashes of pieces- Additional metadata (who created torrent, etc.)- Also specifies tracker• Client contacts tracker, starts communicating withpeersd8:announce47:http://vip.tracker.thepiratebay.org:80/announce7:comment20:THANKS FOR SEEDING!10:created by13:uTorrent/177013:creationdatei1207365453e8:encoding5:UTF-84:infod6:lengthi839833600e4:name18:XXX XXXXXXXXX.avi12:piece lengthi1048576e6:pieces16020:U ....Pieces and Sub-pieces• BitTorrent breaks up a file into N pieces- For throughput, pieces are large: 256KB–1MB- For latency, broken into
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