Chapter 19 Distributed Applications

William Stallings

Data and Computer

Communications

Chapter 19

Abstract Syntax Notation One

ASN.1

 _{Used to define format of PDUs}

 _{Representation of distributed information}  _{Representation of operations performed}

Terms Relevant to ANS.1

 _{Abstract Syntax}

 _{Describes generic structure of data}  _{Data Type}

 _{Named set of values}  _Encoding

 _{Sequence of octets used to represent data}

value

 _{Encoding Rules}

 _{Mapping from one syntax to another}  _{Transfer Syntax}

 _{Way data represented in bit patterns while}

ASN.1 Concepts

 _{Module definition}

 _{Structured definition of a data structure}

using ASN.1

 _{Name of module used as abstract}

Form of Modules

 _{<modulereference>DEFINITIONS::=} BEGIN EXPORTS IMPORTS AssignmentsList End  _EXPORTS

 _{Definitions which may be imported by other} modules

 IMPORTS

 _{Definitions to be imported from other modules}  _{AssignmentList}

 _{Type assignments, value assignments, macro} definitions

Lexical Conventions

 _{Layout not significant}

 _{Comments delimited by pair of hyphens (--) at}

start and pair of hyphens or end of line end of comment

 _{Identifiers, type references and module names}

consist of upper and lower case letters, digits and hyphens

 _{Identifier starts with lower case letter}

 _{Type reference or module name begins with}

upper case letter

Abstract Data Types

 _{Collection of values}  _Simple

 _Atomic

 _{No components}  _Structured

 _{Has components}  _Tagged

 _{Derived from other types}  _Other

Tag Classes (1)

 _{Every data type (except CHOICE and ANY)}

has associated tag

 _Universal

 _{Generally useful}

 _{Application independent}  _{Defined in standard}

Tag Classes (2)

 _{Application wide}

 _{Relevant to particular application}  _{Context specific}

 _{Relevant to particular application}  _{Applicable in limited context}

 _Private

CHOICE and ANY

 _{Data types without tags}

 _{When value assigned, type also}

assigned

 _{Type assigned at run time}  _CHOICE

 _{List of alternative known types}

 _{Only one type used to create value}  _ANY

Subtypes (1)

 _{Derived from parent type}  _{Restricted subset of values}  _{May be nested}

 _{Single value subtype}

 _{Explicit listing of all valid values}

 _{Contained subtype}

 _{Used to form new subtype from existing subtypes}  _{Includes all values of subtypes it contains}

 _{Value range subtype}

 _{Real and Integer only}

Subtypes (2)

 _{Permitted alphabet constraint}  _{Only character string}

 _{All values that can be constructed using}

sub-alphabet

 _{Size constrained}

 _{Limits number of items in type}  _{e.g. number of bits in bit type}  _{Inner type constraint}

 _{Applied to SEQUENCE, SEQUENCE OF, SET,}

SET OF, CHOICE

 _{Only values from parent that satisfy one or}

Network Management - SNMP

 _{Simple Network Management Protocol}  _{Networks are becoming indispensable}

 _{More complexity makes failure more likely}  _{Require automatic network management}

tools

 _{Standards required to allow multi-vendor}

networks

 _Covering:  _Services  _Protocols

Network Management Systems

 _{Collection of tools for network}

management

 _{Single operator interface}

 _{Powerful, user friendly command set}

 _{Performing most or all management tasks}  _{Minimal amount of separate equipment}

 _{i.e. use existing equipment}

Key Elements

 _{Management station or manager}  _Agent

Management Station

 _{Stand alone system or part of shared system}

 _{Interface for human network manager}

 _{Set of management applications}

 _{Data analysis}

 _{Fault recovery}

 _{Interface to monitor and control network}

 _{Translate manager’s requirements into}

monitoring and control of remote elements

 _{Data base of network management}

Agent

 _{Hosts, bridges, hubs, routers equipped}

with agent software

 _{Allow them to be managed from}

management station

 _{Respond to requests for information}  _{Respond to requests for action}

 _{Asynchronously supply unsolicited}

Management Information Base

 MIB

 _{Representation of network resources as}

objects

 _{Each object a variable representing one}

aspect of managed object

 _{MIB is collection of access points at agent}

for management of station

Network Management Protocol

 _{Link between management station and}

agent

 _{TCP/IP uses SNMP}

 _{OSI uses Common Management}

Information Protocol (CMIP)

 _{SNMPv2 (enhanced SNMP) for OSI and}

Protocol Capabilities

 _Get  _Set

Management Layout

 _{May be centralized in simple network}  _{May be distributed in large, complex}

network

 _{Multiple management servers}  _{Each manages pool of agents}

 _{Management may be delegated to}

SNMP v1

 _{August 1988 SNMP specification issued}  _{Stand alone management stations and}

bridges, routers workstations etc supplied with agents

 _{Defines limited, easily implemented MIB of}

scalar variables and two dimensional tables

 _{Streamlined protocol}  _{Limited functionality}  _{Lack of security}

SNMP v2 (1)

 _{Framework on which network management}

applications can be built

 _{e.g fault management, performance}

monitoring, accounting

 _{Protocol used to exchange management}

information

 _{Each player maintains local MIB}  _{Structure defined in standard}

 _{At least one system responsible for}

management

SNPM v2 (2)

 Support central or distributed management  _{In distributes system, some elements}

operate as manager and agent

 _{Exchanges use SNMP v2 protocol}

 _{Simple request/response protocol}  _{Typically uses UDP}

 _{Ongoing reliable connection not}

required

SNMP v2

Managed

Structure of

Management Information

 _SMI

 _{Defines general framework with which MIB}

defined and constructed

 _{Identifies data types}

 _{How resources are represented and}

named

 _{Encourages simplicity and extensibility}  _{Scalars and two dimensional arrays of}

Protocol Operation

 _{Exchange of messages}

SNMP v3

 _{Addresses security issues of SNMP v1/2}  _{RFC 2570-2575}

 _{Proposed standard January 1998}

 _{Defines overall architecture and security}

capability

SNMP v3 Services

 _{Authentication}

 _{Part of User-Based Security (UBS)}  _{Assures that message:}

 _{Came from identified source}  _{Has not been altered}

 _{Has not been delayed or replayed}

 _Privacy

 _{Encrypted messages using DES}

 _{Access control}

 _{Can configure agents to provide a number of}

levels of access to MIB

Electronic Mail

 _{Most heavily used application on any}

network

 _{Simple Mail Transfer Protocol (SMTP)}  _TCP/IP

 _{Delivery of simple text messages}

 _{Multi-purpose Internet Mail Extension}

(MIME)

SMTP

 _{RFC 821}

 _{Not concerned with format of messages or}

data

 _{Covered in RFC 822 (see later)}

 _{SMTP uses info written on envelope of mail}

 _{Message header}

 _{Does not look at contents}

 _{Message body}

 _Except:

 _{Standardize message character set to 7 bit ASCII}  _{Add log info to start of message}

Basic Operation

 Mail created by user agent program (mail

client)

 _{Message consists of:}

 _{Header containing recipient’s address}

and other info

 _{Body containing user data}

 _{Messages queued and sent as input to}

SMTP sender program

 _{Typically a server process (daemon on}

Mail Message Contents

 _{Each queued message has:}

 _{Message text}

 _{RFC 822 header with message envelope and}

list of recipients

 _{Message body, composed by user}  _{A list of mail destinations}

 _{Derived by user agent from header}  _{May be listed in header}

 _{May require expansion of mailing lists}

 _{May need replacement of mnemonic names}

with mailbox names

 _{If BCCs indicated, user agent needs to}

SMTP Sender

 _{Takes message from queue}

 _{Transmits to proper destination host}

 _{Via SMTP transaction}

 _{Over one or more TCP connections to port 25}

 _{Host may have multiple senders active}

 _{Host should be able to create receivers on}

demand

 _{When delivery complete, sender deletes}

destination from list for that message

 _{When all destinations processed, message is}

Optimization

 _{If message destined for multiple users on}

a given host, it is sent only once

 _{Delivery to users handled at destination}

host

 _{If multiple messages ready for given host,}

a single TCP connection can be used

 _{Saves overhead of setting up and}

Possible Errors

 _{Host unreachable}

 _{Host out of operation}

 _{TCP connection fail during transfer}  _{Sender can re-queue mail}

 _{Give up after a period}  _{Faulty destination address}

 _{User error}

 _{Target user changed address}  _{Redirect if possible}

SMTP Protocol - Reliability

 _{Used to transfer messages from sender to}

receiver over TCP connection

 _{Attempts to provide reliable service}

 _{No guarantee to recover lost messages}  _{No end to end acknowledgement to}

originator

SMTP Receiver

 Accepts arriving message

 _{Places in user mailbox or copies to outgoing}

queue for forwarding

 _{Receiver must:}

 _{Verify local mail destinations}  _{Deal with errors}

 _Transmission

 _{Lack of disk space}

 _{Sender responsible for message until}

receiver confirm complete transfer

SMTP Forwarding

 _{Mostly direct transfer from sender host to}

receiver host

 _{May go through intermediate machine via}

forwarding capability

 _{Sender can specify route}

Conversation

 _{SMTP limited to conversation between}

sender and receiver

 _{Main function is to transfer messages}  _{Rest of mail handling beyond scope of}

SMTP

SMTP System Overview

 Commands and responses between sender

and receiver

 _{Initiative with sender}

 _{Establishes TCP connection}

 _{Sender sends commands to receiver}  _{e.g. HELO<SP><domain><CRLF>}

 _{Each command generates exactly one reply}  _{e.g. 250 requested mail action ok;}

SMTP Replies

 _{Leading digit indicates category}  _{Positive completion reply (2xx)}  _{Positive intermediate reply (3xx)}

 _{Transient negative completion reply}

(4xx)

 _{Permanent negative completion reply}

Operation Phases

 _{Connection setup}

Connection Setup

 _{Sender opens TCP connection with}

receiver

 _{Once connected, receiver identifies itself}  220 <domain> service ready

 _{Sender identifies itself}  _HELO

 Receiver accepts sender’s identification  _{250 OK}

 _{If mail service not available, step 2 above}

becomes:

Mail Transfer

 _{Sender may send one or more messages to}

receiver

 _{MAIL command identifies originator}

 _{Gives reverse path to used for error reporting}

 _{Receiver returns 250 OK or appropriate fail/error}

message

 _{One or more RCPT commands identifies}

recipients for the message

 _{Separate reply for each recipient}

 _{DATA command transfers message text}

 _{End of message indicated by line containing just period}

Closing Connection

 _{Two steps}

 _{Sender sends QUIT and waits for reply}  _{Then initiate TCP close operation}

 _{Receiver initiates TCP close after sending}

Format for Text Messages

RFC 882

 _{Message viewed as having envelope and}

contents

 _{Envelope contains information required to}

transmit and deliver message

 _{Message is sequence of lines of text}  _{Uses general memo framework}

 _{Header usually keyword followed by}

Example Message

Date:Tue, 16 Jan 1996 10:37:17 (EST)

From: “William Stallings” <ws@host.com> Subject:The syntax of RFC 822

To: Smith@otherhost.com

Cc: Jones@Yet-another_host.com

Multipurpose Internet Mail

Extension (MIME)

 _{Extension to RFC822}

 _{SMTP can not transmit executables}

 _{Uuencode and other schemes are available}  _{Not standardized}

 _{Can not transmit text including international characters}

(e.g. â, å, ä, è, é, ê, ë)

 _{Need 8 bit ASCII}

 _{Servers may reject mail over certain size}

 _{Translation between ASCII and EBCDIC not standard}

 _{SMTP gateways to X.400 can not handle none text data}

in X.400 messages

 _{Some SMTP implementations do not adhere to standard}  _{CRLF, truncate or wrap long lines, removal of white}

Overview of MIME

 _{Five new message header fields}  _{MIME version}

 _{Content type}

 _{Content transfer encoding}  _{Content Id}

 _{Content Description}

Content Types

 _{Text body}  _Multipart

 _{Mixed, Parallel, Alternative, Digest}  _Message

 _{RFC 822, Partial, External-body}  _Image

 _{jpeg, gif}  _Video  _mpeg  _Audio  _Basic  _Application  _Postscript

MIME Transfer Encodings

 _{Reliable delivery across wide largest range of}

environments

 _{Content transfer encoding field}

 _{Six values}

 _{Three (7bit, 8bit, binary) no encoding done}

 _{Provide info about nature of data}  _{Quoted-printable}

 _{Data largely printable ASCII characters}

 _{Non-printing characters represented by hex code}

 _Base64

 _{Maps arbitrary binary input onto printable output}

 _X-token

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,}

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}

Key Terms

 _Cache

 _Client

 _Connection  _Entity

 _Gateway

 _Message

 _{Origin server}  _Proxy

 _Resource  _Server  _Tunnel

HTTP Messages

 _Requests

 _{Client to server}

 _Responses

 _{Server to client}

 _{Request line}  _{Response line}

 _{General header}

 _{Request header}

 _{Response header}

General Header Fields

 _{Cache control}  _Connection

 _Data

 _Forwarded  _{Keep alive}

 _{MIME version}  _Pragma

Request Methods

Request Header Field

 _Accept

 _{Accept charset}

 _{Accept encoding}

 _{Accept language}

 _{Authorization}

 _From  _Host

 _{If modified since}

 _{Proxy authentication}

 _Range

 _Referrer

 _Unless

Response Messages

 _{Status line followed by one or more}

general, response and entity headers, followed by optional entity body

 _{Line = HTTP-Version <SP>}

Status Codes

 _{Informational}  _Successful

Response Header Fields

 _Location

 _{Proxy authentication}  _Public

 _{Retry after}  _Server

Entity Header Fields

 _Allow

 _{Content encoding}  _{Content language}  _{Content length}

 _{Content MD5}  _{Content range}  _{Content type}

 _{Content version}  _{Derived from}

 _Expires

 _{Last modified}  _Link

 _Title

 _{Transfer encoding}  _{URL 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}

Required Reading