Stallings William Data and Computer Communications
Chapter 19 Distributed Applications
Abstract Syntax Notation One ASN.1
Used to define format of PDUs Representation of distributed information Representation of operations performed on transmitted data
Terms Relevant to ANS.1
Abstract Syntax Data Type Encoding
Describes generic structure of data Named set of values Sequence of octets used to represent data value Mapping from one syntax to another Way data represented in bit patterns while in transit
Encoding Rules
Transfer Syntax
Use of Abstract and Transfer Syntaxes
ASN.1 Concepts
Module definition
Structured definition of a data structure using ASN.1 Name of module used as abstract syntax name
Form of Modules
<modulereference>DEFINITIONS::= BEGIN EXPORTS IMPORTS AssignmentsList End EXPORTS IMPORTS
Definitions which may be imported by other modules Definitions to be imported from other modules Type assignments, value assignments, macro definitions <name>::=<description>
AssignmentList
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 Built in type consists of all upper case letters
Abstract Data Types
Collection of values Simple Structured Tagged Other
Atomic No components
Has components Derived from other types Include CHOICE and ANY types (see later)
Tag Classes (1)
Every data type (except CHOICE and ANY) has associated tag Universal
Generally useful Application independent Defined in standard e.g. Boolean, Integer, Real
Tag Classes (2)
Application wide Context specific Private
Relevant to particular application Relevant to particular application Applicable in limited context User defined
CHOICE and ANY
Data types without tags CHOICE ANY
When value assigned, type also assigned Type assigned at run time List of alternative known types Only one type used to create value Arbitrary value Arbitrary type
Subtypes (1)
Derived from parent type Restricted subset of values May be nested Single value subtype
Contained subtype
Explicit listing of all valid values Used to form new subtype from existing subtypes Includes all values of subtypes it contains Real and Integer only Specify endpoints of range
Value range subtype
Subtypes (2)
Permitted alphabet constraint Size constrained
Only character string All values that can be constructed using subalphabet Limits number of items in type e.g. number of bits in bit type Applied to SEQUENCE, SEQUENCE OF, SET, SET OF, CHOICE Only values from parent that satisfy one or more contraints
Inner type constraint
PDU Example (part 1)
PDU Example (part 2)
PDU Example (part 3)
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 multivendor networks Covering:
Services Protocols Management information base (MIB)
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
View entire network as unified architecture Active elements provide regular feedback
Key Elements
Management station or manager Agent Management information base Network management protocol
Management Station
Stand alone system or part of shared system Interface for human network manager Set of management applications Interface to monitor and control network Translate manager's requirements into monitoring and control of remote elements Data base of network management information extracted from managed entities
Data analysis Fault recovery
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 information
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 Objects standardized across class of system
Bridge, router etc.
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 TCP/IP
Protocol Capabilities
Get Set Notify
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 intermediate manager
Example of Distributed Network Management Configuration
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 1993, revised 1996
RFC 19011908
SNMP v2 (1)
Framework on which network management applications can be built Protocol used to exchange management information Each player maintains local MIB At least one system responsible for management
Houses management applications Structure defined in standard e.g fault management, performance monitoring, accounting
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 Reduces management overhead
SNMP v2 Managed Configuration
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 scalars (tables) only
Protocol Operation
Exchange of messages Outer message header deals with security Seven types of PDU
SNMP v2 PDU Formats
SNMP v3
Addresses security issues of SNMP v1/2 RFC 25702575 Proposed standard January 1998 Defines overall architecture and security capability To be used with SNMP v2
SNMP v3 Services
Authentication
Part of UserBased Security (UBS) Assures that message:
Came from identified source Has not been altered Has not been delayed or replayed
Privacy
Access control
Encrypted messages using DES Can configure agents to provide a number of levels of access to MIB Access to information Limit operations
Electronic Mail
Most heavily used application on any network Simple Mail Transfer Protocol (SMTP) Multipurpose Internet Mail Extension (MIME)
Delivery of other types of data Voice, images, video clips TCP/IP Delivery of simple text messages
SMTP
RFC 821 Not concerned with format of messages or data SMTP uses info written on envelope of mail Does not look at contents Except:
Message body Message header Covered in RFC 822 (see later)
Standardize message character set to 7 bit ASCII Add log info to start of message
Shows path taken
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 UNIX)
Mail Message Contents
Each queued message has:
Message text
RFC 822 header with message envelope and list of recipients Message body, by composed user 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
A list of mail destinations
If BCCs indicated, user agent needs to prepare correct message format
SMTP Sender
Takes message from queue Transmits to proper destination host
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 deleted
Via SMTP transaction Over one or more TCP connections to port 25
Optimization
If message destined for multiple users on a given host, it is sent only once If multiple messages ready for given host, a single TCP connection can be used
Saves overhead of setting up and dropping connection Delivery to users handled at destination host
Possible Errors
Host unreachable Host out of operation TCP connection fail during transfer Sender can requeue mail
Give up after a period
Faulty destination address
User error Target user changed address Redirect if possible Inform user if not
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 Error indication delivery not guaranteed Generally considered reliable
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
Indicates mail has arrived at host, not user
SMTP Forwarding
Mostly direct transfer from sender host to receiver host May go through intermediate machine via forwarding capability
Sender can specify route Target user may have moved
Conversation
SMTP limited to conversation between sender and receiver Main function is to transfer messages Rest of mail handling beyond scope of SMTP
May differ between systems
SMTP Mail Flow
SMTP System Overview
Commands and responses between sender and receiver Initiative with sender Sender sends commands to receiver e.g. HELO<SP><domain><CRLF> Each command generates exactly one reply e.g. 250 requested mail action ok; completed
Establishes TCP connection
SMTP Replies
Leading digit indicates category
Positive completion reply (2xx) Positive intermediate reply (3xx) Transient negative completion reply (4xx) Permanent negative completion reply (5xx)
Operation Phases
Connection setup Exchange of commandresponse pairs Connection termination
Connection Setup
Sender opens TCP connection with receiver Once connected, receiver identifies itself Sender identifies itself
HELO 220 <domain> service ready
Receiver accepts sender's identification If mail service not available, step 2 above becomes:
421 service not available 250 OK
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 DATA command transfers message text
Separate reply for each recipient
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 reply to QUIT
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 colon followed by arguments
Example Message
Date:Tue, 16 Jan 1996 10:37:17 (EST) From: "William Stallings" <[e-mail]> Subject:The syntax of RFC 822 To: [e-mail] Cc: Jones@Yetanother_host.com This is the main text, delimited from the header by a blank line.
Multipurpose Internet Mail Extension (MIME)
Extension to RFC822 SMTP can not transmit executables
Not standardized
Uuencode and other schemes are available
Can not transmit text including international characters (e.g. , , , , , , ) 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 space, etc. Need 8 bit ASCII
Overview of MIME
Five new m...