Computer Networks with Internet Technology William Stallings Chapter

Computer Networks with Internet Technology William Stallings Chapter 04 Modern Applications

Hypertext Transfer Protocol HTTP • Underlying protocol of the World Wide Web • Not a protocol for transferring hypertext —For transmitting information with efficiency necessary for hypertext jumps • Can transfer plain text, hypertext, audio, images, and Internet accessible information

HTTP Overview • Transaction oriented client/server protocol • Usually between Web browser (clinet) and Web server • Uses TCP connections • Stateless —Each transaction treated independently —Each new TCP connection for each transaction —Terminate connection when transaction complete

Key Terms • • • Cache Client Connection Entity Gateway Message Origin server Proxy Resource Server Tunnel User agent

Figure 4. 1 Examples of HTTP Operation

Figure 4. 2 Intermediate HTTP Systems

HTTP Messages • Requests — Client to server • Responses — Server to client • • Request line Response line General header Request header Response header Entity body

Figure 4. 3 HTTP Message Structure

General Header Fields • • Cache control Connection Data Forwarded Keep alive MIME version Pragma Upgrade

Request Methods • Request-Line = Method <SP> Request_URL <SP> HTTP-Version <CRLF> • Methods: — — — — Options Get Head Post Put Patch Copy Move Delete Link Unlink Trace Wrapped Extension-method

Request Header Field • • • • Accept charset Accept encoding Accept language Authorization From Host If modified since Proxy authentication Range Referrer Unless User agent

Response Messages • Status line followed by one or more general, response and entity headers, followed by optional entity body • Status-Line = HTTP-Version <SP> Status-Code <SP> Reason-Phrase <CRLF>

Status Codes • • • Informational Successful Redirection Client error Server error

Response Header Fields • • • Location Proxy authentication Public Retry after Server WWW-Authenticate

Entity Header Fields • • • Allow Content encoding Content language Content length Content MD 5 Content range Content type Content version Derived from • • Expires Last modified Link Title Transfer encoding URL header Extension header

Entity Body • Arbitrary sequence of octets • HTTP transfers any type of data including: —text —binary data —audio —images —video • Interpretation of data determined by header fields —Content encoding, content type, transfer encoding

Internet Directory Services DNS • Directory lookup service • Provides mapping between host name and numerical address • Essential to functioning of Internet • RFCs 1034 and 1035. • Four elements — Domain name space • Tree-structured — DNS database • Each node and leaf in name space tree structure names set of information (e. g. , IP address, type of resource) in resource record — Name servers • Servers that hold information about portion of tree — Resolvers • Programs that extract information from name servers

Domain Names • 32 -bit IP address uniquely identifies devices • Two components — Network number — Host address • Problems — Routers devise routes based on network number — Can’t hold table of every network and path — Networks group to simplify routing — 32 -bit address usually written as four decimal numbers — Effective for computer processing — Not convenient for users • Problems are addressed by concept of domain — Group of networks are under control of single entity — Organized hierarchically — Names assigned reflect organization

Domain Name Example • • edu is college-level U. S. educational institutions mit. edu is M. I. T. lcs. mit. edu is Laboratory for Computer Science at M. I. T. Eventually get to leaf nodes — Identify specific hosts — Hosts assigned Internet (IP) addresses — Internet-wide organization assigns domain names • Delegated down the hierarchy • mit. edu, has four IP addresses: 18. 7. 21. 77, 18. 7. 21. 69, 18. 7. 21. 70, and 18. 7. 21. 110 • Subordinate domain lcs. mit. edu has IP address 18. 26. 0. 36

Figure 4. 4 Portion of Internet Domain Tree

DNS Database • Variable-depth unlimited levels hierarchy for names —Delimited by period (. ) • Distributed database • Distribution controlled by database —Thousands of separately managed zones • DNS servers provide name-to-address directory service for network applications

Resource Record • Domain name — Human readable form — Series of labels of alphanumeric characters or hyphens — Each pair separated by period • Type — E. g. A = Host address, MX = Mail exchange • Class — Usually IN, for Internet • Time to live — How long to hold result in local cache — Zero means don’t cache • Rdata field length • Rdata — Description of resource — For A type, Rdata is 32 -bit IP address

Figure 4. 5 DNS Resource Record Format

DNS Operation • User program requests IP address for domain name • Resolver module in local host or local ISP formulates query for local name server — In same domain as resolver • Local name server checks for name in local database or cache — If so, returns IP address to requestor — Otherwise, query other available name servers • Starting down from root of DNS tree or as high up as possible • Local name server caches reply — Depending on Time to live field • User program given IP address or error message • DNS name servers automatically send out updates to other relevant name servers as conditions warrant

Figure 4. 6 DNS Name Resolution

Server Hierarchy • Name servers operated by any organization that has domain • Each name server holds subset of name space (a zone) — One or more (or all) subdomains within domain — Authoritative • This name server maintains accurate data for this portion hierarchy • Can extend to any depth • 13 root name servers share responsibility for top level zones — Replication prevents root server bottleneck • Individual root servers are busy • Internet Software Consortium server (F) answers almost 300 million DNS requests daily (www. isc. org/services/public/F-root-server. html) • Typically, single queries carried over UDP • Queries for group of names carried over TCP

Name Resolution • Resolver knows name and address of local DNS server • If resolver does not have name in cache, it sends DNS query to local server • Either returns address or after querying one or more other servers • Server (A) forwards request to server (B) — If B has name in cache or database, it can return result — If not, B can — Query another name server and send result back to A • Recursive — Tell A address of next server (C) to ask • A then asks to C • Iterative • Server exchanges use can either • Name resolvers use recursive

Figure 4. 7 DNS Message Format

DNS Message Fields - Header • Header always present — Identifier to match queries and responses. — Query Response: is message query or response — Opcode: Standard, inverse query (address to name), or server status request — Authoritative Answer — Truncated: was response truncated • Requestor will use TCP to resend query — Recursion Desired — Recursion Available — Response Code: e. g. no error, format error, refused — QDcount: entries in question section (zero or more) — ANcount: RRs in answer section (zero or more) — NScount: RRs in authority section (zero or more) — ARcount: RRs in additional records section (zero or more)

DNS Message Fields – Question and Answer • If present, question typically contains only one entry • Domain Name — Sequence of labels • Length octet followed by that number of octets • Terminates with the zero length octet for null label of root • Query Type — Values include all values valid for Type field in RR format plus general codes that match more than one type of RR • Query Class: typically Internet. • Answer section contains RRs that answer question — Authority section contains RRs that point toward an authoritative name server — Additional records section contains RRs that relate to query but not strictly answers

Session Initiation Protocol Overview • RFC 3261 • Application-level control protocol — Setting up, modifying, and terminating real-time sessions — Enable Internet telephony, (voice over IP, Vo. IP — Supports single or multimedia session, including teleconferencing • Facets of SIP — User location: Users can access application features from remote locations — User availability: Willingness of called party to communicate — User capabilities: Media and parameters to be used — Session setup: Point-to-point and multiparty calls — Session management: Transfer and termination, modifying session parameters, and invoking services

Session Initiation Protocol Features • Based on HTTP-like request/response transaction model • Client request invokes function on server — At least one response • Uses most HTTP header fields, encoding rules, and status codes — Readable format for displaying information • Uses concepts similar to recursive and iterative searches of DNS • Incorporates Session Description Protocol (SDP) — Defines session content using types similar to MIME

Components and Protocols (1) • Client — Sends requests and receives responses — User agent clients and proxies are clients • Server — Receives requests and sends back responses — Proxies, user agent servers, redirect servers, and registrars • User Agent — In every SIP end station • User agent client (UAC): Issues requests • User agent server (UAS): Receives requests and reponds • Redirect Server — Determines address of called device — Like iterative searches in DNS

Components and Protocols (2) • Proxy Server — Server and client — Makes requests for other clients • Routing • Enforcing policy • Like recursive searches in DNS • Registrar — Server that accepts REGISTER requests — Places information it receives in requests inlocation service for domain • SIP address, associated IP address of device • Location Service — Used by redirect or proxy server to obtain information about a callee's possible location(s) — Maintains database of SIP-address/IP-address mappings

Components and Protocols (3) • Servers defined (RFC 3261) as logical devices • Implemented as separate servers or combined into single application • Proxy servers may act as redirect servers — Need to consult location service database • May be on proxy server or not • Communication between proxy server and location service beyond scope of SIP standard • Proxy consults DNS server to find target domain proxy • SIP typically runs on UDP for performance — Own reliability mechanisms — May also use TCP — May use Transport Layer Security (TLS) protocol for secure connection

Session Description Protocol SDP • RFC 2327 • SIP invites participants to session • SDP-encoded body of SIP message contains information about what media encodings (e. g. , voice, video) parties can and will use • Then data transmission begins, using appropriate transport protocol —Real-Time Transport Protocol (RTP) • Participants can make changes to session parameters using SIP

Figure 4. 8 SIP Components and Protocols

Uniform Resource Identifier URI • Identifies SIP resource — User of online service — Appearance on multiline phone — Mailbox on messaging system — Telephone number at gateway service — Group (such as "sales" or "helpdesk") in an organization • Format based on email address formats — user@domain — sip: bob@biloxi. com • May also include password, port number, and related parameters • If secure transmission required, use "sips: “ — SIP messages are transported over TLS • URI is generic identifier for resource on Internet — URL, for Web addresses is type of URI

Figure 4. 9 SIP Call Setup Attempt Scenario

Figure 4. 10 SIP Presence Example

Figure 4. 11 SIP Registration and Notification Example

Figure 4. 12 SIP Successful Call Setup

SIP Messages • Requests and responses • Difference between types in first line • Request — Method: nature of request — Request-URI: where request should be sent • Response has response code • All messages include header — Number of lines • Beginning with header label • Message can also contain body e. g. SDP media description

SIP Messages - Requests • Methods —REGISTER: notify SIP network of IP address and URLs for which it would like to receive calls —INVITE: establish session between user agents —ACK: Confirms reliable message exchanges —CANCEL: Terminates pending request, but does not undo completed call —BYE: Terminates session between two users in conference —OPTIONS: Solicits information about callee capabilities

SIP Message Request Example INVITE sip: bob@biloxi. com SIP/2. 0 Via: SIP/2. 0/UDP 12. 26. 17. 91: 5060 Max-Forwards: 70 To: Bob <sip: bob@biloxi. com> From: Alice <sip: alice@atlanta. com>; tag=1928301774 Call-ID: a 84 b 4 c 76 e 66710@12. 26. 17. 91 CSeq: 314159 INVITE Contact: <sip: alice@atlanta. com> Content-Type: application/sdp Content-Length: 142

SIP Messages - Response • Provisional (1 xx): Request received and being processed • Success (2 xx): Action successfully received, understood, and accepted • Redirection (3 xx): Further action needed • Client Error (4 xx): Request contains bad syntax or cannot be fulfilled at this server • Server Error (5 xx): Server failed to fulfill apparently valid request • Global Failure (6 xx): Request cannot be fulfilled at any server

SIP Response Example SIP/2. 0 200 OK Via: SIP/2. 0/UDP server 10. biloxi. com Via: SIP/2. 0/UDP bigbox 3. site 3. atlanta. com Via: SIP/2. 0/UDP 12. 26. 17. 91: 5060 To: Bob <sip: bob@biloxi. com>; tag=a 6 c 85 cf From: Alice <sip: alice@atlanta. com>; tag=1928301774 Call-ID: a 84 b 4 c 76 e 66710@12. 26. 17. 91 CSeq: 314159 INVITE Contact: <sip: bob@biloxi. com> Content-Type: application/sdp Content-Length: 131

SDP Information • Media streams — Session can include multiple streams of differing content — Currently defines audio, video, data, control, and application • Addresses — Destination addresses, — May be multicast • Ports — For each stream • Payload types — For each media stream type • Start and stop times — For broadcast sessions e. g. television or radio program • Originator — For broadcast sessions

Sockets • 1980 s UNIX Berkeley Sockets Interface • Socket enables communications between client and server process • Connection-oriented or connectionless • Endpoint in communication • Client socket in one computer uses address to call server socket on another computer • Once appropriate sockets engaged, can exchange data • Server sockets keep TCP or UDP port open • Once connected server switches dialogue to different port

Sockets API (1) • Sockets can be constructed from within program in most languages • Berkeley Sockets Interface is de facto standard API — Windows Sockets (Win. Sock) based on Berkeley • TCP and UDP header includes source port and destination port fields — Identify respective users (applications) • IPv 4 and IPv 6 header includes source address and destination address fields — Identify host systems • Port value with IP address forms socket — Unique throughout Internet • When used as API, socket is identified by triple — Protocol, local-address (IP), local-process (port)

Sockets API • Sockets API recognizes three types of sockets —Stream sockets for TCP, connection-oriented reliable —Datagram sockets for UDP, connectionless —Raw sockets • Direct access to lower layer protocols, e. g. IP and ICMP

Socket Interface Calls • • Gethostname Gethostbyname Setup Connect —Client • Listen/accept —Server • Send • Receive • Close

Figure 4. 13 Socket System Calls for Connection-Oriented Protocol

Required Reading • • • Stallings chapter 4 RFCs WWW Consortium Loads of books and web sites on sockets E. g. Comer, D. E. and Stevens, D. L. Internetworking with TCP/IP Volume III, Prentice Hall —Comes in three versions: • Windows Sockets • BSD Sockets • AT&T TLI (not sockets)