CIS 203

03 : Traditional Applications

Terminal Access – TelnetHistory• Oldest Internet application• Demonstrated on four-node ARPANET deployed in

1969• Two years to expand protocol sufficiently to make it

useful and to work out bugs• First published version RFC 97

—"First Cut at a Proposed Telnet Protocol," February 1971

• 1983 final form issued as RFC 854 and RFC 855— (Get and study these RFCs – see last slide)

• Still useful Internet application• Also pioneering effort application-level protocol design• Basis of many newer protocols

—HTTP

Remote Terminal Access• Early motivation for networks was remote access

to interactive systems• Dumb terminals• Keyboard and screen with primitive comms

hardware• Stream of character data transmitted in each

direction• Local host computer or terminal controller

establish connection to remote host• Local user can use remote host• Hosts handle particular set of characteristics

Figure 3.1 (a) Telnet Operational Environment on Arpanet

Network Virtual Terminals• Virtual terminal protocol (VTP)• Transform characteristics of real terminal

into standardized form—Network virtual terminal (NVT)

• Imaginary device—Well defined set of characteristics

• Both sides generate data and control signals in native language

• Each side translates these to NVT and translates incoming NVT to native signals

Figure 3.2 Network Virtual Terminal Concept

Phases of operation• Connection management

—Connection request and termination—Telnet uses TCP

• Negotiation—To determine mutually agreeable set of characteristics —NVT has range of capabilities and features—Real terminal more limited—NVT has options, such as line length

• Control—Exchange of control information and commands —e.g., end of line, interrupt process

• Data—Transfer of data between two correspondents

• For Telnet, control and data conveyed in single stream

Current Use of Telnet• Original environment for Telnet little

relevance today• Still used and included in the TCP/IP suite• Availableon PCs for use over the Internet

—PC includes Telnet software —Telnet protocol and translation between PC

keyboard/display and NVT—Not GUI

• Services available include United States Library of Congress—locis.loc.gov

Figure 3.1(b) Telnet Operational Environment on Internet

Figure 3.3 (Incomplete) A Telnet Sessiontelnet locis.loc.govTrying 140.147.254.3...Connected to locis.loc.gov.Escape character is '^]'. L O C I S: LIBRARY OF CONGRESS INFORMATION SYSTEM To make a choice: type a number, then press ENTER 1 Copyright Information -- files available and up-to-date 2 Braille and Audio -- files frozen mid-August 1999 3 Federal Legislation -- files frozen December 1998 * * * * * * * * * * * * * * * The LC Catalog Files are available at: http://lcweb.loc.gov/catalog/ * * * * * * * * * * * * * * * 8 Searching Hours and Basic Search Commands 9 Library of Congress General Information 10 Library of Congress Fast Facts 12 Comments and Logoff Choice:

Telnet NVT• Lowest common denominator of existing systems

(at the time)• Includes options and negotiation for more

advanced equipment• Between two terminals, two processes, or

terminal and process• Server module and user module map

characteristics of terminal to NVT• NVT bi-directional character device with display

and keyboard• Keyboard can generate all 128 ASCII codes plus

user control signals

Telnet Transfer Protocol• Data sent half-duplex • Terminal-to-process, newline signifies end of user

input• Process-to-terminal, Telnet Go Ahead used • Underlying TCP full duplex

—Control signals sent any time regardless of current data direction

• Data sent as stream of 8-bit bytes—No other formatting

• Control signals and other non-data information sent as Telnet commands—Byte strings embedded in data stream—User control signals, commands between Telnet processes as

part of protocol and option negotiation and sub-negotiation

Interpret as Command (IAC)• Delimited by Interpret as Command (IAC) char (all 1s, FF, 255)

— If bit pattern 255 is part of the data stream, it must also be preceded by an IAC. i.e. IAC 255 or 255 255

• Commands 2 bytes long• Option negotiation commands 3 bytes long

— Third byte option identifier— Option sub-negotiation commands vary in length

• Begin 3-byte sequence (IAC SB option-id)• End 2-byte sequence (IAC SE)

• Protocol minimizes transmission overhead— No message headers

• Processing overhead is high• Must process stream one character at a time

— Data translation (between NVT and native)— Scan for Telnet commands

• Classic tradeoff between message and stream protocols

TCP Urgent Data Pointer• Tells receiver there is urgent data further

along data stream• TCP does not define what user about this• Receiving process will usually process

urgent data quickly• Prevents TCP buffering holding up urgent

data

Telnet Synch Mechanism• Allows user to communicate urgent command to server

process—E.g. Interrupt Process (IP) or Abort Output (AO) command

• Partially alleviates problems caused by time delays over network connections

• Consists of Telnet command Data Mark (DM) in TCP segment with TCP urgent notification

• Telnet sends urgent command such as AO, or IP, followed by DM sequence (IAC DM) as urgent data

• Receiver should immediately scan data stream for Telnet commands as normal—Discard all data—Telnet commands handled normally—Continues until DM found—Then processing returns to normal

Telnet Options• Enable two sides connection to use

capabilities beyond default NVT• Negotiation allows one side to request an

option—Otherside may accept or reject—If accepted, effective immediately—Negotiation any time after connection established—Usually as soon as connected

• Options not part of Telnet protocol specification

• Published in RFCs

Telnet Options Categories• Three major categories

—Change, enhance, or refine NVT characteristics • E.g. further definition of printer characteristics

—Change transfer protocol• E.g. Suppress Go Ahead option (3), making data transfer

protocol full duplex instead of half duplex

—Allow other information to be defined and passed over connection

• E.g. Status option (5) requests remote side to send status of all options negotiated

• Most options may be single sided—Two negotiations required for both sides to use such

options

Option Negotiation• Either side may initiate negotiation • Can ask that option be enabled or that

currently enabled option be disabled—You may always reject a request to enable an

option—You must always accept a request to disable an

option—Options are not enabled until the negotiation is

complete—Never negotiate (either request or respond)

about something that is already true; that is, do not initiate or respond to a request to initiate an option that is already in effect

WILL WONT DO DONT• Interpretation of commands depends on context (see

next slide)• Unambiguous if both sides make same request and

messages pass on network—A wishes B to implement option

• A issues DO • If B agrees, it responds with WILL

—B wishes to implement option on its (B’s) side, it issues WILL • If A agrees, it responds with DO

—Both A and B request B to implement option at about same time

• A issues DO and B issues WILL• Commands cross in network• Both sides receive response to their command

• May be sub-negotiation to define details

Figure 3.4 Negotiation Messages in Telnet

The Longevity of Telnet• Telnet is older than most of its users

— (But not most lecturers!)

• Telnet is simple—RFC 854 is 15 pages—HTTP (see later lecture) is 176 pages—Simple job done by simple protocol

• Telnet can evolve—Option negotiation was brilliant—Not common in IETF protocol designs until late 1980s—Enables Telnet to evolve to meet new demands without

endless new versions of basic protocol

• Currently over 100 RFCs on Telnet and its options—3% of the entire body of RFCs—Most recent RFC 2953, Telnet Encryption, September 2000

FILE TRANSFER—FTP• FTP evolved from an era of radically diverse systems• Has obsolete commands, transfer modes, and data

representations• Objectives:

— Promote sharing of files (computer programs and/or data)— Encourage indirect or implicit (via programs) use of remote computers— Shield user from variations in file storage systems among hosts— Transfer data reliably and efficiently

• File systems, rather than just files• Single file viewed as set of bits with name

— Trivial File Transfer Protocol (TFTP) does this— Send request header to read or write file with some name— Stream bits across— 11 pages to define

• FTP deals with metadata such as file pathnames, file organization, access control, and data representation— Accordingly, RFC 959 is 69 pages long

FTP Model• User FTP entity and Server FTP entity• Initiating host is user

—Chooses file name and options

• Server accepts or rejects request —Based on its file system protection and options requested— If accepted, server responsible for establishing and

managing transfer

• Operates on two levels (Figure 3.5)• Establish TCP connection

—Exchange control information (commands and replies)—Second TCP connection established for data transfer

• FTP user interface enables human user or program to access User FTP

FTP Commands• Specify parameters for data connection

—Data port, transfer mode, representation type, and structure

—Nature of file system operation• Store, retrieve, append, delete

• User data transfer protocol should "listen" on specified data port

• Server initiates data connection and data transfer• FTP uses Telnet protocol on control connection

—FTP user protocol or FTP server protocol may implement Telnet rules directly

—FTP user protocol or FTP server protocol may use existing Telnet module

Figure 3.5FTP Model

Figure 3.6Overview of an FTP Transfer

Options• FTP assumes files are objects in mass storage

—Share some properties regardless of machine—Files uniquely identified by symbolic names—Files have owners and protection against unauthorized

access—Files may be created, read from (copied from), written into,

or deleted (within protection rules)

• To support specific computers and operating systems, FTP can negotiate options in three dimensions—Datatype, file type, and transfer mode

• Systems programmer on each system determines—How particular file can be mapped to standard file type—Using one of standard data types—Transferred using standard mode—Such that it is useful at destination

Data Types• ASCII, EBCDIC, image, and logical byte size• Text files normally stored as character string

— 8-bit ASCII on most machines— If ASCII option used, no character code conversion required at

either end in most cases— EBCDIC appropriate if both machines IBM hosts

• ASCII or EBCDIC files may have further line or page printer specification— Nonprint: Suitable for files not destined for a line printer— Telnet formatting: Embedded control characters— Character control formatting: Formatting conventions from

FORTRAN • Image transfer is bit-by-bit replication of file from the source

machine on the target machine• Logical byte size type used when data unit size must be

preserved— Specifies byte size (need not be 8 bits)

File Types• File structure, record structure, and page structure• To promote convenient, efficient interface to file system• Not possible to address idiosyncrasies of all operating

systems • File structure

—String of bytes (defined by data type option) that terminates in an end of file marker

—Most transfers use this type

• Record structure—Sequence of records—Causes transmission of individual records, separated by

standard End of Record marker for specified data type

• Page structure—For files not stored contiguously on disk—Page structure needs to be preserved

Figure 3.7 FTP File Types

Transmission Modes – Stream Mode• Optimise use of network• Stream mode (default)

—Raw data sent—Least computational burden on user and

server systems—No restriction on file type

• Record-structure files, 2-byte control code for EOR and EOF

Transmission Modes – Block Mode• Provides for restarting failed or interrupted transfer

—Source encapsulates data into blocks (see Figure 3.8a)

• Block begins with two field header • Descriptor may indicate zero of more of:

—Last block in record: If record structure used each record consists of one or more blocks

—Last block in file—Suspect data: data may contain errors

• Not intended for error control within FTP• Allows sites to exchanging data (e.g., seismic or weather) to send

and receive all data despite local errors (such as "magnetic tape read errors"), but showcertain portions are suspect

—Restart marker: marks checkpoint in data stream• Receiver marks corresponding position and returns this• May restart from last correctly received marker

• Count field indicates total length of data block in bytes

Transmission Modes – Compressed Mode• Allows source to squeeze sequences of

same character into a shorter coded sequence—Uncompressed data—Replicated byte: Up to 63 of specified bytes—Filler string: Up to 63 of filler characters

inserted at destination—Escape sequence: Byte of all zeros followed by

descriptor code byte, as in block mode

Figure 3.8 FTP Transmission Mode Formats

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)—Delivery of other types of data—Voice, images, video clips

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

• 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, 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 prepare correct message format

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

deleted

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 dropping

connection

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—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

Figure 3.9 SMTP Mail Flow

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;

completed

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 command-response pairs• Connection termination

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:—421 service not available

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

reply to QUIT

Format for Text MessagesRFC 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 MessageDate:Tue, 16 Jan 1996 10:37:17 (EST)From: “William Stallings” <ws@host.com>Subject:The syntax of RFC 822To: Smith@otherhost.comCc: Jones@Yet-another_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

—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 space, etc.

Overview of MIME• Five new message header fields

—MIME version—Content type—Content transfer encoding—Content Id—Content Description

• Number of content formats defines• Transfer encoding defined

Content Types• Text body• Multipart

—Mixed, Parallel, Alternative, Digest

• Message—RFC 822, Partial, External-body

• Image—jpeg, gif

• Video—mpeg

• Audio—Basic

• Application—Postscript—octet stream

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—Named nonstandard encoding

Figure 3.10 Printable Encoding of Binary Data into Radix-64 Format

Required Reading• Stallings chapter 03• RFCs from www.rfc-editor.org