Application Layer Chapter 7 DNS Domain Name System Electronic Mail The Web Streaming Audio and Video Content Delivery Revised: August 2011
The Application Layer Uses transport services to build distributed applications Application Transport Network Link Physical
DNS Domain Name System DNS is a world-wide distributed directory service. DNS delegates the responsibility of assigning domain names and mapping those names to internet resources by designating authoritative name servers for each domain (zone). Net admins may delegate authority over subdomains within their allocated name space to other name servers. This provides a distributed and fault tolerant service that was designed to avoid a single large central database. Internet has two primary name-spaces: 1. domain name hierarchy (DNS addresses) 2. Internet Protocol Address spaces (IPv4 and IPv6) The DNS resolves high-level human readable names for computers to low-level IP addresses DNS name space» Domain Resource records» Name servers»
The DNS Name Space (1) DNS namespace is hierarchical from the root down Different parts delegated to different organizations Can register under multiple top-level domains Absolute domain names always end in a period, relative domain names do not Relative names have to be interpreted in a context to uniquely determine their meaning 250+ Top Level Domains: 1. Generic 2. Countries Generic Second Level Domains run by Registrars appointed by ICANN The computer robot.cs.washington.edu Question: What is a domain?
The DNS Name Space (2) The Generic top-level domains are controlled by ICANN who appoints registrars to run them ICANN = Internet Corporation for Assigned Names and Numbers This one was controversial To create a new domain, permission is required of the domain in which it will be included. Naming follows organizational boundaries, not physical networks.
Domain Resource Records (1) Every domain (all levels) has its own DNS database that is comprised of Resource Records. Each Resource Record is a 5 tuple: Domain_Name, TTL, Class, Type, Value (page 616) The key resource records in the namespace are IP addresses (A/AAAA) and name servers (NS), but there are others too (e.g., MX)
Domain Resource Records (2) Name server IP addresses of computers CNAME and PTR are aliases; CNAME allows the same IP address to have multiple names; PTR enables reverse lookups Mail gateways A portion of a possible DNS database for cs.vu.nl. Question: What is cs.vu.nl s web server s name and IP address? Question: What is a reverse lookup?
Name Servers (1) DNS name space divided into overlapping zones. Admins determine zone boundaries. Each zone has one or more name servers. Name servers contain data for portions of the name space called zones (circled). One zone
Name Servers (2) Finding the IP address for a given hostname is called resolution and is done with the DNS protocol. Resolution: Computer requests local name server to resolve Local name server asks the root name server Root returns the name server for a lower zone Continue down zones until a name server can answer DNS protocol: Runs on UDP port 53, retransmits lost messages Caches name server answers for better performance
Name Servers (3) Example of a computer looking up the IP for a name
Electronic Mail Architecture and services» The user agent» Message formats» Message transfer» Final delivery»
Architecture and Services (1) The key components and steps (numbered) to send email User Agent = Email Reader; a program that supports composing, receiving, and replying to messages. Uses Simple Mail Transfer Protocol (SMTP) over TCP (port 25) 1. Mail submission is method by which user agents send messages into the email system for delivery 2. Message transfer between MTAs 3. Final delivery to receiving User Agent Architecture of the email system Question: Where is the user s mailbox located?
Architecture and Services (2) Envelope Question: Who knows why this is funny for Tanenbaum? Message (= header and body) Paper mail Electronic mail
The User Agent What users see interface elements of a typical user agent Email address format: user@dns-address
Message Formats (1) Header fields related to message transport; headers are readable ASCII text
Message Formats (2) Other header fields useful for user agents
Message Formats (3) Originally only ASCII text could be sent by email. Multipurpose Internet Mail Extensions (MIME) was developed to send messages with richer content (non-latin alphabets, Chinese, Japanese) as well as audio, images, or binary documents or programs. MIME adds structures to the message body and also defines encoding rules for the transfer of non-ascii messages. MIME header fields used to describe what content is in the body of the message Message Headers added by MIME
Message Formats (4) Common MIME content types and subtypes
Message Formats (5) Putting it all together: a multipart message containing HTML and audio alternatives. One part (HTML) Another (audio) Question: What does this email message do?
Message Transfer (1) Messages are transferred with SMTP (Simple Mail Transfer Protocol) Readable (4 byte long ASCII) text commands Submission from user agent to MTA on port 587 If message cannot be delivered, an error report containing the first part of the undeliverable msg is returned to the sender One MTA to the next MTA on port 25 usually using TCP Recipient s mailbox is within the destination MTA The destination MTA is the @dns-address part of the user s email address user@dns-address is the user s mailbox within the destination MTA Other protocols for final delivery (IMAP, POP3, webmail) IMAP/POP3/webmail get email messages from the mailbox on the user s mail server to the user s email user agent on the user s machine. Webmail systems use web protocols: E.g., Gmail, Hotmail, Yahoo!mail
Sending a message: From Alice to Bob SMTP commands are marked [pink] Message Transfer (2) C: = from client (sender) S: == from server (receiver) HELO Hello message RCPT ID email recipient(s) EHLO Hello msg for clients wanting to use an extension (next slide)... (rest of message)... Question: Does the Sender (client) or the Receiver (server) start the dialog? If the server is first, how does it know when to talk?
Message Transfer (3) Common SMTP extensions (not in simple example)
Final Delivery (1) User agent uses protocol like IMAP for final delivery Has commands to manipulate folders / messages [right] Alternatively, a Web interface (with proprietary protocol) might be used
The World Wide Web (WWW) Standards body for the web is the W3C (World Wide Web Consortium; see http://www.w3.org) Primary web protocol is HTTP, which is a simple text-based protocol that runs over TCP in the general case. Architectural overview» Static Web pages» Dynamic pages and Web applications» HTTP HyperText Transfer Protocol» The mobile Web» Web search»
Architectural Overview (1) HTTP transfers pages from servers to browsers Question: Where is the User in this picture?
Architectural Overview (2) Pages are named with URLs (Uniform Resource Locators) Example: http://www.phdcomics.com/comics.php Protocol Server Page on server Our focus Common URL protocols
Architectural Overview (3) Steps a client (browser) takes to follow a hyperlink: Determine the protocol (HTTP) Ask DNS for the IP address of server Make a TCP connection to server (port 80 or 443) Send request for the page; server sends it back Fetch other URLs as needed to display the page Close idle TCP connections Steps a server takes to serve pages: Accept a TCP connection from client Get page request and map it to a resource (e.g., file name) Get the resource (e.g., file from disk) Send contents of the resource to the client. Release idle TCP connections
Architectural Overview (4) -- Clients When a server returns a page, it also returns some additional info about that page. This info includes the MIME type of the page that instructs the client s browser how to process that info. If the MIME type is not one of the built-in ones, then the browser consults its table of MIME types which associates MIME types with Third-Party-provided viewers. Two types of viewers: 1) plug in or 2) helper app. Content type is identified by MIME types Browser takes the appropriate action to display Plug-ins / helper apps extend browser for new types Installed as an extension to the browser (same process as the browser) Question: Where did we see MIME before? Complete program running as a separate process
Architectural Overview (5) -- Servers Web servers are optimized to be able to quickly service numerous requests To scale performance, Web servers can use: Caching to optimize disk retrievals, multiple threads for parallel processing, and a front end to orchestrate
Architectural Overview (6) Server steps, revisited: 1. Resolve name of Web page requested Incoming request may not contain the actual name of file or program as a literal string 2. Perform access control on the Web page Authenticate client to determine if (s)he can access material 3. Check the cache Dynamic pages cannot be cached; only current cached content returned (if so, skip step 4) 4. Fetch requested page from disk or run program 5. Determine the rest of the response that accompany the contents of the page 6. Return the response to the client 7. Make an entry in the server log
Architectural Overview (7) -- cookies Even though HTTP uses TCP, it does NOT have any concept of a login session so each HTTP request has no memory nor context. Cookies provide a memory or context. Examples: pay-for-view web sites; e-commerce; customized web portals like Yahoo! Cookies can be abused to learn a user s browsing habits. Cookies support stateful client/server interactions Cookies are a small (4KB or less) named string that the server associates with a browser Browsers store the server-provided cookie for a time period within their browser directory on the client s disk. Cookies persist across browser invocations to that server -- unless the user disables cookies. Server sends cookies (state) with page response Client stores cookies across page fetches Client sends cookies back to server with requests Example of a browser directory within a client s disk
Static Web Pages (1) Static Web pages are simply files Have the same contents for each viewing Can be visually rich and interactive nonetheless: HTML that mixes text and images Forms that gather user input Style sheets that tailor presentation Vector graphics, videos, and more (over)...
Static Web Pages (2) Progression of features through HTML 5.0
Dynamic Pages & Web Applications (1) Applications can now run inside the browser or the server with the web providing the user interface. Users do not need to install separate apps and user data can be accessed from different computers; this is a form of cloud computing Dynamic pages are generated by programs running at the server (with a database) and/or the client E.g., PHP, JSP, and CGI at server; JavaScript at client Microsoft s Active Server Pages.NET is a proprietary version of PHP and JSP CGI = Common Gateway Interface; JSP = JavaServer Pages Pages vary each time like using an application Question: What does a form of cloud computing mean?
Dynamic Pages & Web Applications (2) Web page that gets form input and calls a server program PHP server program that creates a custom Web page PHP calls Resulting Web page (for inputs Barbara and 32 )
Dynamic Pages & Web Applications (3) Client-side processing occurs inside the user s browser. Technologies: 1) JavaScript, 2) VBScript, 3) Java applets, and 4) Microsoft ActiveX controls (see pages 677-678) JavaScript program produces result page in the browser First page with form, gets input and calls program above
Dynamic Pages & Web Applications (4) Client-side and Server-side scripting look the similar in that they both embed code in HTML files, they are processed totally differently. The difference between server and client programs Server-side scripting with PHP After user clicks on submit button, the browser collects info into a long string and sends it to server as a request for PHP Page. The server loads the PHP file and executes the PHP script to produce a new HTML page that is sent to the browser to display. Client-side scripting with JavaScript When the user clicks on the submit button the browser interprets a JavaScript function contained on the page. All of the work is done locally within the browser. Question: when is it preferable to use server-side and when preferable to use client-side scripting?
Dynamic Pages & Web Applications (5) AJAX (Asynchronous Javascript and XML) is a set of technologies that work together that work together to enable Web applications that are as responsive and powerful as traditional desktop applications. Web applications use a set of technologies that work together, e.g. AJAX: HTML: present information as pages. DOM: change parts of pages while they are viewed. DOM (Document Object Model) model of HTML page that is accessible to programs (e.g., an API to change parts of a page) XML: let programs exchange data with the server. XML (extensible Markup Language) is a language for specifying structural content W3C created XML to allow web content to be structured for automated processing Asynchronous way to send and retrieve XML data. SOAP (Simple Object access Protocol) language/systemindependent way to do RPC between programs JavaScript as a language to bind all this together.
Dynamic Pages & Web Applications (6) The DOM (Document Object Model) tree represents Web pages as a structure that programs can alter
Dynamic Pages & Web Applications (7) XML captures document structure HTML is concerned with presentation. Example of XML: Note: XHTML is HTML defined in terms of XML
Dynamic Pages & Web Applications (8) Web application developers have a library of technologies available to them to create web content:
HTTP (1) HTTP is the primary protocol of the web. HTTP (HyperText Transfer Protocol) is a requestresponse protocol that runs on top of TCP Fetches pages from server to client Server usually runs on port 80 Headers are given in readable ASCII Content is described with MIME types Protocol has support for pipelining requests Protocol has support for caching
HTTP (2) HTTP 1.0 connection was set up and released for every response HTTP 1.1 Enables connection reuse HTTP uses persistent connections to improve performance One connection for each request HTTP 1.0 Sequential requests on one connection HTTP 1.1 Pipelined requests on one connection HTTP 1.1
HTTP (3) HTTP has several request methods. Requests server to Fetch a MIME-encoded page Used to send input data (forms) to a server program Both GET and POST are used for SOAP web services
HTTP (4) Response codes tell the client how the request fared:
HTTP (5) A request line (e.g., GET, POST) may be optionally followed by additional lines with more information, called request headers. Responses also optionally may have response headers. Many headers carry key information: Function Browser capabilities (client server) Caching related (mixed directions) Browser context (client server) Content delivery (server client) Example Headers User-Agent, Accept, Accept-Charset, Accept- Encoding, Accept-Language If-Modified-Since, If-None-Match, Date, Last- Modified, Expires, Cache-Control, ETag Cookie, Referer, Authorization, Host Content-Encoding, Content-Length, Content-Type, Content-Language, Content-Range, Set-Cookie
HTTP (6) HTTP has built-in support to help clients identify when they can safely reuse pages HTTP caching checks to see if the browser has a known fresh copy, and if not if the server has updated the page HTTP uses two strategies: 1) cache page validation (step 2), 2) conditional GET asks server if the cached copy is still valid Uses a collection of headers for the checks Can include further levels of caching (e.g., proxy caching)
The Mobile Web Enhancement/extensions made to support web access for small handhelds and other mobile devices. Mobiles (phones, tablets) are challenging as clients: Relatively small screens Limited input capabilities, lengthy input. Network bandwidth is limited Connectivity may be intermittent. Computing power is limited Strategies to handle them: Content: servers provide mobile-friendly versions; transcoding can also be used Protocols: no real need for specialized protocols; HTTP with header compression sufficient
Web Search Search has proved hugely popular, in tandem with advertising that has proved hugely profitable A simple interface for users to navigate the Web Search engine requires: Content from all sites, accessed by crawling. Follow links to new pages, but beware programs. Each web search engine obtains its database of pages by doing web crawling a systematic traversal of all pages and links Deep web: Web content that search engines cannot catalog (e.g., dynamic content), which is therefore hidden Indexing, which benefits from known and discovered structure (such as XML) to increase relevance
Streaming Audio and Video Real time and streamed multimedia traffic. Real time multimedia is usually carried using RTP Streamed multimedia may be carried by RTP or HTTP Real Time Streaming Protocol proprietary (RealNetworks) variant of RTCP RTCP (real time control protocol) controls remote multimedia playout Audio and video have become key types of traffic, e.g., voice over IP, and video streaming. Digital audio» Digital video» Streaming stored media» Streaming live media» Real-time conferencing»
Digital Audio (1) review ADC (Analog-to-Digital Converter using Nyquist s Sampling Theorem) produces digital audio from a microphone Telephone: 8000 8-bit samples/second (64 Kbps); computer audio is usually better quality (e.g., 16 bit) ADC Continuous audio (sine wave) Digital audio (sampled, 4-bit quantized)
Digital Audio (2) -- review Digital audio is typically compressed before it is sent Lossy encoders (like AAC) exploit human perception Large compression ratios (can be >10X) Sensitivity of the ear varies with frequency A loud tone can mask nearby tones
Digital Video (1) When the human eyes sees something, the image is retained for some milliseconds before decaying. If a sequence of images is shown at 50 images/sec, the eye does not notice that it is looking at discrete images. All video systems exploit this principle to produce moving pictures. Video is digitized as pixels (sampled, quantized) TV quality: 640x480 pixels, 24-bit per pixel color, 30 frames/sec Leveraging interlacing odd number scan lines and even number cumulatively sent sequentially at 60 frames/sec to fool the eye HDTV: 1280X720; Blu-ray: 1080X720 Video is sent compressed due to its large bandwidth Lossy compression exploits human perception E.g., JPEG for still images, MPEG, H.264 for video Large compression ratios (often 50X for video) Video is normally > 1 Mbps, versus >10 kbps for speech and >100 kbps for music Question: Why is music s bandwidth higher than speech s?
Digital Video (2) JPEG = Joint Photographic Experts Group JPEG lossy compression sequence for one image: Step 1 Step 2 Step 3 Step 5 Subsequent slides show each step in the JPEG compression sequence Question: What does lossy mean?
Digital Video (3) Step 1: Pixels are mapped to luminance/chrominance (matrix: YCbCr) color space and chrominance is sub-sampled Y = 16 +.26R +.5G +.09B (luminance); Cb = 128 +.15R -.29G -.44B (chrominance) Cr = 128 +.44R -.37G +.07B (chrominance) The eye is less sensitive to chrominance Input 24-bit RGB pixels 8-bit luminance pixels 8-bit chrominances for every 4 pixels
Digital Video (4) Step 2: Each component block is transformed to spatial frequencies with DCT (Discrete Cosine Transformation) Captures the key image features One component block of the Y matrix Transformed block (i.e., the DTC coefficients)
Digital Video (5) Step 3: DCT coefficients are quantized by dividing by thresholds; reduces bits in higher spatial frequencies Top left element is differenced over blocks (Step 4) Input / Thresholds = Output
Digital Video (6) Step 5: The block is run-length encoded in a zig-zag order. Then it is Huffman coded before sending (Step 6) Order in which the block coefficients are sent
Digital Video (7) MPEG = Motion Picture Experts Group; following describes MPEG-1 MPEG compresses over a sequence of frames, further using motion tracking to remove temporal redundancy I (Intra-coded) frames are self-contained P (Predictive) frames use block motion predictions B (Bidirectional) frames may base prediction on future frame Three consecutive frames with stationary and moving components Who has viewed I-frames in Netflix or other video playout during fast forwards?
Streaming Stored Media (1) A simple method to stream stored media, e.g., for video on demand, is to fetch the video as a file download But has large startup delay, except for short files Video-on-Demand Playout can start after the file has downloaded
Streaming Stored Media (2) Effective streaming starts the playout during transport Using RTSP (Real-Time Streaming Protocol) or the RTCP (RTP s Real-Time Control Protocol) Question: What protocol is being used in step 5?
Streaming Stored Media (3) Key problem: how to handle transmission errors Strategy Advantage (PRO) Disadvantage (CON) Use reliable transport (TCP) Add FEC (e.g., parity packet) Repairs all errors Repairs most errors Increases jitter significantly Increases overhead, decoding complexity and jitter Interleave media Masks most errors Slightly increases decoding complexity and jitter Interleave media: mixing up the order of the media before transmission and unmixing it on reception. In this way if a packet (or a burst of packets) is lost, the loss will be spread out over time by the unmixing. Question: Who remembers what FEC is?
Streaming Stored Media (4) Parity packet can repair one lost packet in a group of N Decoding is delayed for N packets An approach for FEC using multimedia
Streaming Stored Media (5) Interleaving spreads nearby media samples over different transmissions to reduce the impact of loss Packet stream Media samples Loss reduces temporal resolution; doesn t leave a gap
Streaming Stored Media (6) Key problem: media may not arrive in time for playout due to variable bandwidth and loss/retransmissions Client buffers media to absorb jitter; we still need to pick an achievable media rate Safety margin, to avoid a stall Can pause server (or go ahead and save to disk)
Streaming Stored Media (7) RTSP commands Sent from player to server to adjust streaming
Streaming Live Media (1) Streaming live media is similar to the stored case plus: Can t stream faster than live rate to get ahead Usually need larger buffer to absorb jitter Often have many users viewing at the same time UDP with multicast greatly improves efficiency. When not available, many TCP connections are used instead. For very many users, content distribution is used [later section]
Streaming Live Media (2) With multicast streaming media, parity is effective Clients can each lose a different packet and recover
Streaming Live Media (2) Comparatively easy today for anyone to set up an Internet radio station. In this example, recorded content is either stored on a media server as podcasts for subsequent streaming or sent to for live streaming. Production side of a student radio station. As before
Real-Time Conferencing (1) Real-time conferencing has two or more connected live media streams, e.g., voice over IP, Skype video call Key issue over live streaming is low (interactive) latency Want to reduce delay to near propagation Benefits from network support, e.g., QoS Or, benefits from ample bandwidth (no congestion)
Real-Time Conferencing (2) H.323 was issued in 1996 for real time conferencing. It was a popular approach until it became supplanted by the IETF s SIP, which was VASTLY simpler and cheaper to use and produced equivalent results. H.323 architecture for Internet telephony supports calls between Internet computers and PSTN phones. VoIP call Internet/PSTN Internet Gatekeeper controls calls for LAN hosts
Real-Time Conferencing (3) H.323 protocol stack: Call is digital audio/video over RTP/UDP/IP Call setup is handled by other protocols (Q.931 etc.)
Real-Time Conferencing (4) Logical channels that make up an H.323 call
Real-Time Conferencing (5) SIP (Session Initiation Protocol) is an alternative to H.323 to set up real-time calls Simple, text-based protocol with URLs for addresses Data is carried with RTP / RTCP as before
Real-Time Conferencing (6) Setting up a call with the SIP three-way handshake Proxy server lets a remote callee be connected Call data flows directly between caller/callee
Real-Time Conferencing (7) YES Corrected by Instructor Comparison of H.323 and SIP.
Content Delivery Majority of Internet bandwidth is currently used to deliver multimedia content. This resulted in greatly enhanced bandwidth deployments and Content Delivery Networks (CDN). CDN sets up a distributed collection of machines at locations inside the Internet and uses them to serve content to clients. This is used by big players. Alternative is a peer-to-peer (P2P) network architect, used by little players. This section also talks about other content delivery types for other data types. Delivery of content, especially Web and video, to users is a major component of Internet traffic Content and Internet traffic» Server farms and Web proxies» Content delivery networks» Peer-to-peer networks» Question: What does distributed collection of machines mean?
Content and Internet Traffic Internet traffic: 1. Shifts seismically (email FTP Web P2P video) Internet traffic changes quickly, not only in the details but in the overall makeup 2. Has many small/unpopular and few large/popular flows mice and elephants Important: note description of self-similar traffic on page 737 Zipf popularity distribution, 1/k Shows up as a line on log-log plot
Server Farms and Web Proxies (1) No matter how much bandwidth one machine has, it can only serve so many web requests before the load is too great. Server farm is a single logical machine comprised of many physical machines. Server farms enable large-scale Web servers: Front-end load-balances requests over servers Servers access the same backend database Question: What is the difference between logical and physical?
Server Farms and Web Proxies (2) Caching improves performance by shortening the response time and reducing the network load. Web proxy shares the cache among multiple users by actually fetching web requests on behalf of its users. This also enables an organization to control the content that is entering its AS. Proxy caches help organizations to scale the Web Caches server content over clients for performance Also implements organization policies (e.g., access) Question: Where is a web proxy usually physically located?
CDNs Content Delivery Networks (1) Server farms and web proxies help to build large sites and to improve web performance, but they are not sufficient for truly popular web sites that must serve content on a global scale. This is the role of a CDN. CDNs scale Web servers by having clients get content from a nearby CDN node (cache) Distributes copy of content to other CDN nodes and is a Name Server to resolve DNS queries
Content Delivery Networks (2) Directing clients to nearby CDN nodes with DNS: Client query returns local CDN node as response Local CDN node caches content for nearby clients and reduces load on the origin server
Content Delivery Networks (3) Akamai pioneered using DNS for content distribution and became the first major CDN and industry leader (page 745-6) Question: Where have we seen Akamai before? Origin server rewrites pages to serve content via CDN Traditional Web page on server Page that distributes content via CDN
Peer-to-Peer Networks (1) P2P is a file-sharing network that many computers use to pool their resources to jointly form a CDN together. The computers may simply be home computers. There is often no dedicated infrastructure and everyone participates in the task of distributing content, with no central point of control. P2P (Peer-to-Peer) is an alternative CDN architecture with no dedicated infrastructure (i.e., servers) Clients serve content to each other as peers Challenges when servers are removed: 1. How do peers find each other? 2. How do peers support rapid content downloads? 3. How do peers encourage each other to upload?
Peer-to-Peer Networks (2) Two common types of P2P technology: 1. BitTorrent protocol see pages 750 752 standard: www.bittorent.org 2. Distributed Hash Table (DHT) see pages 753-756 academic 1) BitTorrent lets peers download torrents, which are a content description Peers find each other via Tracker in torrent file» Tracker is a server with the list of all other peers uploading or downloading content Peers swap chunks (parts of content) with partners, preferring those who send most quickly [2] Many peers speed download; preference helps uploads [3]
Peer-to-Peer Networks (3) Distributed Hash Tables (DHTs) are a fully distributed index that scales to very many clients/entries Basic functionality of an index is to map a key to a value, much like a hash table Chord uses SHA-1 for its hash, just like in BitTorrent (and cryptography). It takes a variable length string and produces a random 160-bit number. It is used to: 1. Convert any IP address to a 160-bit number called a node identifier. 2. Convert a content name to a key Need to follow O(log N) path for N entries Each node maintains a finger table that has m entries, each pointing to a different actual node. Each entry has two fields: start and IP address of successor. Lookup process (next slide): requesting node sends a packet to its successor containing its IP address and the key it is looking for. The packet is propagated around the ring until it locates the successor to the node identifier being sought. That node checks to see if it has any info matching the key and, if so, returns it directly to the requesting node, whose IP address it has
Peer-to-Peer Networks (3) A Chord ring of 32 identifiers. Finger tables [at right, and as arcs] are used to navigate the ring. Example: path to look up 16 from 1 is 1 12 15 Identifier values are stored at predecessor Example: 32 node ID arranged in circle. Shaded ones are actual machines, Arcs show fingers and labels on arcs are table indices.
End Chapter 7