OSI Reference Model Application Layer Presentation Layer Session Layer Application Protocols Chapter 3: Internet Protocols Chapter 2: Computer Networks Transport Layer Network Layer Data Link Layer Physical Layer Page 1
Layer 5: Session Layer Layer 5 is the lowest of the application orientated layers; it controls dialogs, i.e. the exchange of related information: Synchronization of partner instances by synchronization points: data can have been transferred correctly but have to be nevertheless partially retransmitted. (Crash of a sender in the mid of the data transmission process.) Therefore, synchronization points can be set on layer 5 at arbitrary times of the communication process. If a connection breaks down, not the entire data transmission has to be repeated; the transmission can remount at the last synchronization point. Dialog management during half duplex transmission: layer 5 controls the order in which the communication partners are allowed to send their data. Connection establishment, data transmission, and connection termination for layer 5 to 7. Use of different tokens for the assignment of transmission authorizations, for connection termination, and for the setting synchronization points. Page 2
Layer 6: Presentation Layer Layer 6 hides the use of different data structures or differences in their internal representation The same meaning of the data with the sender and the receiver is guaranteed Adapt character codes ASCII 7-bit American Standard Code for Information Interchange EBCDIC 8-bit Extended Binary Coded Digital Interchange Code Adapt number notation 32/40/56/64 bits Little Endian (byte 0 of a word is right) vs. Big Endian (byte 0 is left) Abstract Syntax Notation One, ASN.1 as transfer syntax Substantial tasks of layer 6: 1.) Negotiation of the transfer syntax 2.) Mapping of the own data to the transfer syntax 3.) and further data compression, data encryption (source coding) Page 3
Codes Source coding generally converts the representation of messages into a sequence of code words Efficient coding Remove redundancies Data compression Codes are meaningful only if they are clearly decodable, i.e. each sequence of characters, which consists of code words, can be divided definitely into a sequence of code words. In communication, immediately decodable codes are important, i.e. character sequences from code words can be decoded definitely from the beginning of the character sequence word by word, without considering following characters. Prefix code: no code word may be a prefix of another. Example: C = {0, 10, 011, 11111} is a definite code, but not immediately decodable To each definite code, an immediately decodable code exists, which is not longer. Page 4
Information Theory What is information? Definition: The mean information content (entropy) of a character is defined by N p log i= 1 i a p i = N p log i= 1 i a 1 p i with N - Number of different characters p i - Frequency of a character i (i=1,, N) A -Basis In a transferred sense: The entropy indicates, how surprised we are, which character comes next. Example 1: Given: 4 characters All N=4 characters are equivalent frequently (p i = 0.25 i) 4 Entropy: 0,25log 4 = log 4 = 2[ bit] 2 2 = i 1 There does not exist a better coding as with 2 bits per character Example 2: Given: 4 characters The first character has the frequency p 1 =1, thus is p 2 = p 3 = p 4 = 0 Entropy: 1 1 log 1+ lim3* p log = 0 + 0 = 0 2 a p 0 The entropy is 0 [bit], i.e. because anyway only character 1 is transferred, we did not even code and transfer it. p Page 5
Huffman Code Lehrstuhl für Informatik 4 The entropy indicates how many bits at least are needed for coding. A good approximation to that theoretical minimum (for mean code word length) is the use of a binary tree. The characters which are to be coded are at the leafs. Huffman code (a prefix code) Precondition: the frequency of the occurrence of all characters is well-known. Principle: more frequently arising characters are coded shorter than rarer ones 1.) List all characters as well as their frequencies 2.) Select the two list elements with the smallest frequency and remove them from the list 3.) Make them the leafs of a tree, whereby the probabilities for both elements are being added; place the tree into the list 4.) Repeat steps 2 and 3, until the list contains only one element 5.) Mark all edges: Father left son with 0 Father right son with 1 The code words result from the path from the root to the leafs Page 6
Huffman Code - Example The characters A, B, C, D and E are given with the probabilities p(a) = 0.27, p(b) = 0.36, p(c) = 0.16, p(d) = 0.14, p(e) = 0.07 Entropy: 2,13 4 p(adceb) = 1.00 0 1 2 p(ced) = 0.37 0 1 p(c) = 0.16 1 p(ed) = 0.21 0 1 3 p(ab) = 0.63 0 1 p(a) = 0.27 p(b) = 0.36 p(e) = 0.07 p(d) = 0.14 Resulting Code Words: w(a) = 10, w(b) = 11, w(c) = 00, w(d) = 011, w(e) = 010 Page 7
Frequency of Characters and Character Sequences (English language) Letters Digrams Trigrams E 13,05 TH 3,16 THE 4,72 T 9,02 IN 1,54 ING 1,42 O 8,21 ER 1,33 AND 1,13 A 7,81 RE 1,30 ION 1,00 N 7,28 AN 1,08 ENT 0,98 I 6,77 HE 1,08 FOR 0,76 R 6,64 AR 1,02 TIO 0,75 S 6,46 EN 1,02 ERE 0,69 H 5,85 TI 1,02 HER 0,68 D 4,11 TE 0,98 ATE 0,66 L 3,60 AT 0,88 VER 0,63 C 2,93 ON 0,84 TER 0,62 F 2,88 HA 0,84 THA 0,62 U 2,77 OU 0,72 ATI 0,59 M 2,62 IT 0,71 HAT 0,55 P 2,15 ES 0,69 ERS 0,54 Y 1,51 ST 0,68 HIS 0,52 W 1,49 OR 0,68 RES 0,50 G 1,39 NT 0,67 ILL 0,47 B 1,28 HI 0,66 ARE 0,46 V 1,00 EA 0,64 CON 0,45 K 0,42 VE 0,64 NCE 0,43 X 0,30 CO 0,59 ALL 0,44 J 0,23 DE 0,55 EVE 0,44 Q 0,14 RA 0,55 ITH 0,44 Z 0,09 RO 0,55 TED 0,44 Codes like the Huffman code are not limited necessarily to individual characters. It can be more meaningful (depending on the application) to code directly whole character strings example: the English language. Page 8
Arithmetic Coding Characteristics: Achieves optimality (coding rate) as the Huffman coding Difference to Huffman: the entire data stream has an assigned probability, which consists of the probabilities of the contained characters. Coding a character takes place with consideration of all previous characters. The data are coded as an interval of real numbers between 0 and 1. Each value within the interval can be used as code word. The minimum length of the code is determined by the assigned probability. Disadvantage: the data stream can be decoded only as a whole. Page 9
Arithmetic Coding: Example Code data ACAB with p A = 0.5, p B = 0.2, p C = 0.3 0 p A = 0.5 pb = 0.2 p C = 0.3 0.5 0.7 1 0 p AA = 0.25 p AB = 0.1p AC = 0.15 p BA p BB p BC p CA p CB p CC 0.25 0.35 0.5 0.6 0.68 0.7 0.85 0.91 1 p ACA = 0.075 p ACB = 0.03 p ACC = 0.045 0.35 0.425 0.455 0.5 0.35 p ACAA = 0.0375 p ACAB = 0.015 p ACAC = 0.0225 0.3875 0.4025 0.425 ACAB can be coded by each binary number from the interval [0.3875, 0.4025), rounded up to log 2 (p ACAB ) = 6.06 i.e. 7 bit, e.g. 0.0110010 Page 10
Layer 7: Application Layer Collection of often used communication services Identification of communication partners Detection of the availability of communication partners Authentication Negotiation of the grade of the transmission quality Synchronization of cooperating applications Page 11
Application Protocols in the TCP/IP Reference Model File Transfer E-Mail Network Management WWW Virtual Terminal Name Service File Transfer HTTP FTP Telnet SMTP DNS SNMP TFTP Internet protocols TCP UDP ARP RARP IP ICMP IGMP Layer 1/2 Ethernet Token Ring Token Bus Wireless LAN Page 12
Application Protocols in the TCP/IP Reference Model Protocols of the application layer are common communication services Protocols of the application layer are defined for special purposes and specify Thetypes of the sent messages Thesyntax of the message types Thesemantics of the message types Rules for definition, when and how an application process sends a message resp. responses to it Usually: Client/Server structure. Processes on the application layer are using TCP(UDP)/IP-Sockets Page 13
DNS - Domain Name System Top level Domain de rwth-aachen IP addresses are difficult to remember for humans, but computers can deal with them perfectly. Symbolic names are simpler for humans to handle, but computers can unfortunately not deal with them. informatik metatron.informatik.rwth-aachen.de 137.226.12.221 Page 14
DNS - Concept 1. DNS manages the mapping of logical computer names to IP addresses (and further services) 2. DNS is a distributed database, i.e. the individual segments are subject to local control 3. The structure of the used name space of the database shows the administrative organization of the Internet 4. Data of each local area are available by means of a Client/Server architecture in the entire network 5. Robustness and speed of the system are being achieved by replication and caching of the naming data 6. Main components: Name Server: Server which manages information about a part of the database Resolver: Client which requests naming information from the server Resolver Resolver Request Response Name Name Server Server Page 15
DNS - Architecture User Program User Request User Response Resolver Requests Responses Remote Name Server Shared Database References Update References Master Files Name Server Responses Requests Remote Resolver Administrative Requests Administrative Responses Remote Name Server Page 16
Structure of the Database For structuring of all information: the database can be represented as a tree Each node of the tree is marked with a label, which identifies it relatively to the father node Each (internal) node is root of a sub-tree Each of those sub-trees represents a domain Each domain can be divided into sub-domains Domain com edu gov mil se de Sub-domain Oxford rwth-aachen cs informatik Generic Countries Page 17
Domain Names The name of a domain consists of the sequence of labels (separated by. ) beginning with the root of the domain and going up to the root of the whole tree In the leaf nodes the IP addresses associated with the names given by the label sequence are being stored de rwth-aachen informatik logical name: metatron.informatik.rwth-aachen.de metatron Associated IP address: 137.226.12.221 Page 18
Administration of a Domain Each domain can be managed by another organization The responsible organization can split a domain into sub-domains and delegate the responsibility for them to other organizations The father domain manages pointers to the roots of the sub-domains to be able to forward requests to them The name of a domain corresponds to the domain name of the root node Managed by the Network Information Center edu com gov mil Berkeley Managed by the UC Berkeley (domain berkeley.edu) Page 19
Index of the Database The names of the domains serve as index for the database Each computer in the network has a domain name which refers to further information concerning the computer ca or nv oakland ba rinkon la The data associated with a domain name are stored in socalled Resource Records (RR) IP address: 192.2.18.44 Page 20
Domain Name Aliases Computers can have one or more secondary names, so-called Domain Name Aliases Aliases are pointers of one domain name to another one (canonical domain name) us ca or nv ba la mailhub oakland rinkon IP address: 192.2.18.44 No IP address is stored, but a logical name: rinkon.ba.ca.us. Page 21
Name Space Lehrstuhl für Informatik 4 The reverse tree represents the Domain Name Space The depth of the tree is limited to 127 levels Domain names can have up to 63 characters A label of the length 0 is reserved for the root node ( ) TheFully Qualified Domain Name (FQDN) is the absolute domain name, which is declared with reference to the root of the tree Example: informatik.rwth-aachen.de. Domain names which are declared not with reference to the root of the tree, but with reference to another domain, are called relative domain names Page 22
Domains Lehrstuhl für Informatik 4 A domain consists of all computers whose domain name is within the domain Leafs of the tree represent individual computers and refer to network addresses, hardware information and mail routing information Internal nodes of the tree can describe both a computer and a domain Domains are denoted often relatively or regarding their level: Top-Level Domain: child of the root node First Level Domain: child of the root node (top-level domain) Second Level Domain: child of a first level of domain etc. Page 23
Top Level Domains Originally the name space was divided into seven top-level domains: 1. com: commercial organizations 2. edu: educational organizations 3. gov: government organizations 4. mil: military organizations 5. net: network organizations 6. org: non-commercial organizations 7. int: international organizations Additionally, each country got its own top-level domain The name space was extended in the meantime by further top-level domains Within the individual top-level domains, different conventions for name structuring are given: Australia: edu.au, com.au, etc. UK: co.uk (for commercial organizations), ac.uk (for academic organizations), etc. Germany: completely unstructured Page 24
Name Servers and Zones Information about the name space are stored in name servers Name Servers manage the whole information for a certain part of the name space; this part is called zone The information about a zone is loaded either from a file or from another name server The name server has the authority for the zone A name server can be responsible for several zones Page 25
Domains and Zones Domain and zone are different concepts: com org edu edu zone berkeley berkeley.edu zone nwu purdue purdue.edu zone edu domain Delegation Zones are (except within the lowest levels of the tree) smaller than domains, therefore name servers have to manage less name information Page 26
Zones There are no guidelines how domains are divided into zones. Each domain can select a dividing for itself. Some zones (e.g. edu) do not manage IP addresses. As information they only store references to other zones Page 27
Name Resolution Generally mapping of names to addresses The term Name Resolution also designates the process, in which a name server searches the name space for data, for which he is not responsible For the searching, a name server needs the domain name and the addresses of the root name servers A name server can ask a root name server for each name in the name space Root name servers know the responsible servers for each top-level domain On request, a root name server can return names and addresses of name servers responsible for the top-level domain of the searched name The top level name server again manages references to name servers which are responsible for the second level domain If additional information is missing, each search begins with the root name servers Page 28
Iterative Name Resolution Request Name Name server server Response Resolver Resolver Request for address of girigiri.gbrmpa.gov.au Reference to au name server Request for address of girigiri.gbrmpa.gov.au Reference to gov.au name server Request for address of girigiri.gbrmpa.gov.au Reference to gbrmpa.gov.au name server Request for address of girigiri.gbrmpa.gov.au Address of girigiri.gbrmpa.gov.au root root name name server server au au name name server server gov.au gov.au name name server server gbrmpa.gov.au gbrmpa.gov.au name name server server au gov nz sg edu sa ips gbrmpa Page 29
Recursive Resolution Distinction between recursive and iterative requests resp. recursive and iterative name resolution In case of recursive resolution, a resolver sends a recursive inquiry to a name server The name server must answer either with the searched information or an error message, i.e. the name server may not refer to another name server If the addressed name server is not responsible for the searched information, it must contact other name servers The name server can start a recursive or iterative inquiry; usually it will use an iterative inquiry With the inquiry, the name server tries to shorten the resolution process by directing the inquiry to the most suitable name server regarding the searched information (i.e. if known, a server on a lower level is contacted instead of the root name server) Page 30
Root Name Server Requests to which a name server cannot answer, are handed upward in the tree Name server on the upper levels are heavily loaded Inquiries, which go into another zone, often run over the root name server Thus, the root name server must always be available Therefore: replication - there are 13 instances of the root name server, more or less distributed over the whole world Problem: very central placement of the servers! Page 31
Mapping of Addresses to Names Information in the database is indicated by names Mapping of a name to an address is simple Mapping of an address onto a name is more difficult to realize (complete search of name space) Solution: Place a special area in the name space, which uses addresses as label; the in-addr.arpa domain Nodes in this domain are marked in accordance with the usual notation for IP addresses (four octets separated by points) The in-addr.arpa domain has 256 sub-domains, each of which again having 256 sub-domains, On the fourth level, the appropriate resource records are assigned with the octet, which refers to the domain name of the computer or the network with the indicated address The IP address appears backwards because it is read beginning with the leaf node (IP address: 15.16.192.152 => sub-domain: 152.192.16.15.in-addr.arpa) Page 32
Mapping of Addresses to Names arpa in-addr 0 15 255 0 16 255 0 192 255 0 255 152 hostname winnie.corp.hp.com Page 33
Caching & Time to Live Caching is the process of buffering name information in a name server not responsible for those information. In further requests these information are present and the name resolution process can be speeded up Stored are not only information about the requested hosts, but additionally all information about other name servers used in the resolution process TheTime to Live (TTL) indicates how long data are allowed to be buffered The TTL guarantees that no outdated information is used Small TTL gives a high consistency Large TTL gives a faster resolution of a name Page 34
DNS Protocol Lehrstuhl für Informatik 4 DNS defines only one protocol format, which is used both for inquiries and for responses: Identification: 16 bits for the definite identification of an inquiry, to match requests and responses Flag: 4 Bit, marking of (1) request/response, (2) authorative/not authorative, (3) iterative/recursive, (4) recursion possible Number of : Indication of the contained number of inquiries resp. data records Questions: Names to be resolved Answers: Resource records to the previous inquiry Authority: Identification of passed responsible name servers Additional information: further data to the inquiry. If the name searched is only an alias, the belonging resource record for the correct name is placed here Identification Number of Questions Number of Authority RR Flag Number of Answers RR Number of Additional RR Questions (variable number of RR) Answers (variable number of RR) Authority (variable number of RR) Additional information (variable number of RR) Page 35
Evolution of the WWW World Wide Web (WWW) Access to linked documents, which are distributed over several computers in the Internet History of the WWW Origin: 1989 in the nuclear research laboratory CERN in Switzerland. Developed to exchange data, figures, etc. between a large number of geographically distributed project partners via Internet. First text-based version in 1990. First graphic interface (Mosaic) in February 1993, developed on to Netscape, Internet Explorer Standardization by the WWW consortium (http://www.w3.org). Page 36
Communication in the WWW The Client/Server model is used: Client (a Browser) Presents the actually loaded WWW page Permits navigating in the network (e.g. through clicking on a hyperlink) Offers a number of additional functions (e.g. external viewer or helper applications). Usually, a browser can also be used also for other services (e.g. FTP, e-mail, news, ). Server Process which manages WWW pages. Is addressed by the client e.g. through indication of an URL (Uniform Resource Locator = logical address of a web page). The server sends the requested page (or file) back to the client. Page 37
WWW, HTML, URL and HTTP WWW stands for World Wide Web and means the world-wide cross-linking of information and documents. The standard protocol used between a web server and a web client is the HyperText Transfer Protocol (HTTP). uses the TCP port 80 defines the allowed requests and responses is an ASCII protocol Each web page is addressed by a unique URL (Uniform Resource Locator) (e.g. http://www-i4.informatik.rwth-aachen.de/education/tcpip). The standard language for web documents is the HyperText Markup Language (HTML). Page 38
HTTP - Message Format command URL GET http://server.name/path/file.type protocol HTTP server domain name path name file name GET http:// www.informatik.rwth-aachen.de / info / general.html Instructions on a URL are GET: Load a web page HEAD: Load only the header of a web page PUT: Store a web page on the server POST: Append something to the request passed to the web server DELETE: Delete a web page Page 39
Loading of Web Pages DNS server Browser PC TCP/IP network WWW server Browser asks DNS for the IP address of the server DNS answers Browser opens a TCP connection to port 80 of the computer Browser sends the command GET /info/general.html WWW server sends back the file general.html Connection is terminated Page 40
Loading of Web Pages Example: Call of the URL http://www.informatik.rwth-aachen.de/material/general.html 1. The Browser determines the URL (which was clicked or typed). 2. The Browser asks the DNS for the IP address of the server www.informatik.rwthaachen.de. 3. DNS answers with 137.226.116.241. 4. The browser opens a TCP connection to port 80 of the computer 137.226.116.241 5. Afterwards, the browser sends the command GET /material/general.html 6. The WWW server sends back the file general.html. 7. The connection is terminated. 8. The browser analyzes the WWW page general.html and presents the text. 9. If necessary, each picture is reloaded over a new connection to the server (The address is included in the page general.html in form of an URL). Note! Step 9 applies only to HTTP/1.0! With the newer version HTTP/1.1 all referenced pictures are loaded before the connection termination (more efficiently for pages with many pictures). Page 41
HTTP Request Header method sp URL sp version cr lf header field name : value cr lf header field name : value cr lf : : header field name : calue cr lf cr lf Data Request line: necessary part, e.g. GET path/file.type Header lines: optionally, further information to the host/document, e.g. Host: www.rwth-aachen.de Accept-language: fr User-agent: Opera /6.0 Entity Body: optionally. Further data, if the Client transmits data (POST method) sp: space cr/lf: carriage return/line feed Page 42
HTTP Response Header version sp status code sp phrase cr lf header field name : value cr lf header field name : value cr lf : : header field name : value cr lf Status LINE: status code and phrase indicate the result of an inquiry and an associated message, e.g. 200 OK 400 Bad Request 404 Not Found cr lf Data Groups of status messages: 1xx: Only for information 2xx: Successful inquiry Entity Body: inquired data HEAD method: the server answers, but does not transmit the inquired data (debugging) 3xx: Further activities are necessary 4xx: Client error (syntax) 5xx: Server error Page 43
Proxy Server Lehrstuhl für Informatik 4 A Proxy is an intermediate entity used by several browsers. It takes over tasks of the browsers (complexity) and servers for more efficient page loading! HTTP Browser Proxy Server Internet Server e.g. HTTP Caching of WWW pages A proxy temporarily stores the pages loaded by browsers. If a page is requested by a browser which already is in the cache, the proxy controls whether the page has changed since storing it. If not, the page can be passed back from the cache. If yes, the page is normally loaded from the server and again stored in the cache, replacing the old version. Support when using additional protocols A browser enables also access to FTP, News, Gopher or telnet servers etc. Instead of implementing all protocols in the browser, it can be realized the proxy. The proxy then speaks HTTP with the browser and e.g. FTP with a FTP server. Integration into a Firewall The proxy can deny the access to certain web pages (e.g. in schools). Page 44
Electronic Mail: E-Mail Early systems A simple file transmission took place, with the convention that the first line contains the address of the receiver of the file. Problems E-Mail to groups, structuring of the e-mail, delegation of the administration to a secretary, file editor as user interface, no mixed media Solution X.400 as standard for e-mail transfer. This specification was however too complex and badly designed. Generally accepted only became a simpler system, cobbled together by a handful of computer science students : the Simple Mail Transfer Protocol (SMTP). Page 45
Electronic Mail: E-Mail An e-mail system generally consists of two subsystems: User Agent (UA, normal e-mail program) Usually runs on the computer of the user and helps during the processing of e-mails Creation of new and answering of old e-mail Receipt and presentation of e-mail Administration of received e-mail Message Transfer Agent (MTA, e-mail server) Usually runs in the background (around the clock) Delivery of e-mail which is sent by User Agents Intermediate storage of messages for users or other Message Transfer Agents User User Agent User Agent User Agent Agent Message Transfer Agent Internet Page 46
Structure of an E-Mail For sending an e-mail, the following information is needed from the user: Message (usually normal text + attachments, e.g. word file, GIF image ) Destination address (in general in the form mailbox@location, e.g. thissen@informatik.rwth-aachen.de) Possibly additional parameters concerning e.g. priority or security E-Mail formats: two used standards RFC 2822 MIME (Multipurpose Internet Mail Extensions) With RFC 2822 an e-mail consists of a simple envelope (created by the Message Transfer Agent based on the data in the e-mail header), a set of header fields (each one line ASCII text), a blank line, and the actual message (Message Body). Page 47
E-Mail Header To: Cc: Bcc: Header From: Sender: Received: Lehrstuhl für Informatik 4 Meaning Address of the main receiver (possibly several receivers or also a mailing list) Carbon copy, e-mail addresses of less important receivers Blind carbon copy, a receiver which is not indicated to the other receivers Person who wrote the message Address of the actual sender of the message (possibly different to From person) One entry per Message Transfer Agent on the path to the receiver Return Path: Path back to the sender (usually only e-mail address of the sender) Date: Reply to: Transmission date and time E-Mail address to which answers are to be addressed Message-Id: Clear identification number of the e-mail (for later references) In-Reply-to: Message-Id of the message to which the answer is directed References: Other relevant Message-Ids Subject: One line to indicate the contents of the message (is presented the receiver) Page 48
E-Mail Header Lehrstuhl für Informatik 4 RFC 822: only suitably for messages of pure ASCII text without special characters. Nowadays demanded additionally: E-Mail in languages with special characters (e.g. French or German) E-Mail in languages not using the Latin alphabet (e.g. Russian) E-Mail in languages not at all using an alphabet (e.g. Japanese) E-Mail not completely consisting of pure text (e.g. audio or video) MIME keeps the RFC-2822 format, but additionally defines a structure in the Message Body (by using additional headers), and coding rules for non-ascii characters. Header Meaning MIME-Version: Content-Description: Content-Id: Content-Transfer- Encoding: Content-Type: Used version of MIME is marked String which describes the contents of the message Clear identifier for the contents Coding which was selected for the contents of the email (some networks understand e.g. only ASCII characters). Examples: base64, quoted-printable Type/Subtype regarding RFC 1521, e.g. text/plain, image/jpeg, multi-part/mixed Page 49
MIME MIME-Version: 1.0 Content-Type: MULTIPART/MIXED; BOUNDARY= "8323328-2120168431-824156555=:325" --8323328-2120168431-824156555=:325 Content-Type: TEXT/PLAIN; charset=us-ascii A picture is in the appendix --8323328-2120168431-824156555=:325 Content-Type: IMAGE/JPEG; name="picture.jpg" Content-Transfer-Encoding: BASE64 Content-ID: <PINE.LNX.3.91.960212212235.325B@localhost> Content-Description: /9j/4AAQSkZJRgABAQEAlgCWAAD/2wBDAAEBAQEBAQEBAQEBAQEBAQIBAQEBA QIBAQECAgICAgICAgIDAwQDAwMDAwICAwQDAwQEBAQEAgMFBQQEBQQEBAT/ 2wBDAQEBAQEBAQIBAQIEAwIDBAQEBA [ ] KKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAoooo AKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiig AooooAD//Z ---8323328-2120168431-824156555=:325 Page 50
E-Mail over POP3 and SMTP Simple Mail Transfer Protocol (SMTP) Sending e-mails over a TCP connection (port 25) User User Agent User Agent User Agent Agent SMTP is a simple ASCII protocol Without checksums, without encryption Receiving machine is the server and begins with the communication If the server is ready for receiving, it signals this to the client. This sends the information from whom the e-mail comes and who the receiver is. If the receiver is known to the server, the client sends the message, the server confirms the receipt. Agent Post Office Protocol version 3 (POP3) Get e-mails from the server over a TCP connection, port 110 Commands for logging in and out, message download, deleting messages on the server (maybe without transferring them to the client) Only copies e-mails of the remote server to the local system User User Agent User Agent User Agent Message Transfer Agent Message Transfer Agent Internet Internet Page 51
E-Mail over POP3 and SMTP User 1: writes an e-mail Client 1 (UA 1): formats the e-mail, produces the receiver list, and sends the e-mail to its mail server (MTA 1) Server 1 (MTA 1): Sets up a connection to the SMTP server (MTA 2) of the receiver and sends a copy of the e-mail Server (MTA 2): Produces the header of the e-mail and places the e-mail into the appropriate mailbox Client 2 (UA 2): sets up a connection to the mail server and authenticates itself with username and password (unencrypted!) Server (MTA 2): sends the e-mail to the client Client 2 (UA 2): formats the e-mail User 2: reads the e-mail User User Agent User Agent Agent User Agent Message Transfer Agent Internet Message Transfer Agent User User Agent User Agent Agent POP3 User Agent SMTP SMTP Page 52
SMTP - Command Sequence Communication between partners (from abc.com to beta.edu) in text form of the following kind: /* Receiver is ready/* S: 220 <beta.edu> Service Ready C: HELO <abc.com> /* Identification of the sender/* S: 250 <beta.edu> OK /* Server announces itself */ C: MAIL FROM:<Krogull@abc.com> /* Sender of the e-mail */ S: 250 OK /* Sending is permitted */ C: RCPT TO:<Bolke@beta.edu> /* Receiver of the e-mail */ S: 250 OK /* Receiver known */ C: DATA /* The data are following */ S: 354 Start mail inputs; end with <crlf>.<crlf> on a line by itself C: From: Krogull@. <crlf>.<crlf> /* Transfer of the whole e-mail, including all headers */ S: 250 OK C: QUIT /* Terminating the connection */ S: 221 <beta.edu> Server Closing S = server, receiving MTA / C = Client, sending MTA Page 53
POP3 Get e-mails from the server by means of POP3: Client (UA) POP3 Server (MTA) Authorizing phase: USER name PASS string PC TCP/IP network TCP connection port 110 Greetings Commands Replies Transaction phase: STAT LIST [msg] RETR msg DELE msg NOOP RSET QUIT Minimal protocol with only two command types: Copy e-mails to the local computer Delete e-mails from the server Page 54
POP3 Protocol Authorizing phase user identifies the user pass is its password +OK or -ERR are possible server answers Transaction phase list for the listing of the message numbers and the message sizes retr to requesting a message by its number dele deletes the appropriate message S: +OK POP3 server ready C: user alice S: +OK C: pass hungry S: +OK user successfully logged in 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 2 contents> S:. C: dele 2 C: quit S: +OK Page 55
IMAP as POP3 Variant Enhancement of POP3: IMAP (Interactive Mail Access Protocol) TCP connection over port 143 E-Mails are not downloaded and stored locally, but remain on the server The client performs all actions remotely. This is suitable for users who need access to their e-mails from different hosts Protocols are more complex than with POP3: set up and manage remote mailboxes Meanwhile also many operators of web pages offer email services: gmx, web.de, yahoo, Here finally again HTTP serves as protocol for the access to the e-mails. The management is similar as with IMAP, only that the client is integrated into the web server. Page 56
Conclusion Lehrstuhl für Informatik 4 IP is the core protocol which enables the Internet IPv4 still in use Lots of helper protocols, e.g. RSVP for connection-oriented communication, DHCP for mobile devices, IPv6 would make easier lots of things, but migration is hard TCP and UDP as two different transport protocols TCP is connection-oriented, UDP is connectionless Several other protocols like RTP to fill the gap between them for today s needs Application protocols Nothing to do with physical communication Dealing more with contents of communication All using on the client/server paradigm Page 57