IHA præsentation1 Text Based Protocols Lesson 3. IHA præsentation2 Outline for today Text Based Protocols Augmented Backus-Naur (ABNF) Motivation The.

IHA præsentation 1

Text Based Protocols

Lesson 3

IHA præsentation 2

Outline for today

• Text Based Protocols

• Augmented Backus-Naur (ABNF)

• Motivation• The Language• Usage

• Text-based vs. Binary encoding

IHA præsentation 3

Augmented Backus-Naur Form

• A metalanguage• Based on Backus-Naur Form

• Describes a formal system of a language to be used as a bidirectional communication protocol

• Many variants of Backus-Naur

• Extended (EBNF)

• Augmented (ABNF)

IHA præsentation 4

Augmented Backus-Naur FormatThe Language

IHA præsentation 5

ABNF – Text-based Syntax

name = elements CR LF

An ABNF Specification is a set of derivation rules.

A rule is defined by the following sequence:

name of the rule one or more rule names

separates the name from the definition of the rule

Rules resolve into a string of terminal values, sometimes called characters

IHA præsentation 6


rulename = “abc”

Example

ABNF strings are case insensitive. Hence:

rulename = “abc”

and

rulename = “aBc”

will match “Abc”, abC”, ABc”…………

IHA præsentation 7

ABNF – Operators

A rule may be defined by listing a sequence of rule names. For example:

Concatenation: Rule1 Rule2

foo = %x61 ; a

bar = %x62 ; b

mumble = foo bar foo

The rule <mumble> matches the lowercase string “aba”

A semicolon starts a comment that continues to the end of line

individually specified character – makes the rule case sensitive

IHA præsentation 8

ABNF – Operators

A rule may be defined by a list of alternative rules by a forward slash (“/”):

Alternatives: Rule1 / Rule2

foo / bar

will accept <foo> or <bar>

Value Range Alternatives: %c##-##

A range of alternative numeric values can be specified compactly using a dash(”-”) to indicate the range of alternative values

DIGIT = %x30-39

IHA præsentation 9

ABNF – Operators

Additional alternatives may be added to a rule through the use of =/ s that the ruleset:

Incremental Alternatives: Rule1 =/ Rule2

ruleset = alt1 / alt2

ruleset =/ alt3

ruleset =/ alt4 / alt5

is the same as specifying

ruleset = alt1 / alt2 / alt3 / alt4 / alt5

IHA præsentation 10

ABNF – Operators

Elements enclosed in parentheses are treated as a single element. Thus,

Sequence Group: (Rule1 Rule2)

elem (foo / bar) blat

matches (elem foo blat) or (elem bar blat), and

elem foo / bar blat

matches (elem foo) or (bar blat)


ABNF – Operators

The operator “*” preceding an element indicates repetition. The full form is:

Variable Repetition: *Rule

<a>*<b>element

where <a> and <b> are optional decimal values, indicating at least <a> and at most <b> occurrences of the element

Specific Repetition: nRule

A rule of the form:

<n>element

is equivalent to

<n>*<n>element


ABNF – Operators

Examples - Variable Repetition: *Rule

Example - Specific Repetition: nRule

2DIGIT is a 2-digit3ALPHA is a string of three alphabetic characters

Default values are 0 and infinity so that

*<element> allows any number, including zero1*<element> requires at least one3*3<element> allows exactly 31*2<element> allows one or two.


ABNF – Operators

Square brackets enclose an optional element sequence:

Optional Sequence: [Rule]

[foo bar]

is equivalent to:

*1(foo bar)


ABNF – Operators

Operator Precedence

1. Rule name, prose-val, Terminal value

2. Comment

3. Value Range

4. Repetition

5. Grouping, Optional

6. Concatenation

7. Alternative


ABNF – Example (from wikipedia)

postal-address = name-part street zip-part

name-part = *(personal-part SP) last-name [SP suffix] CRLFname-part =/ personal-part CRLF

personal-part = first-name / (initial ".")first-name = *ALPHAinitial = ALPHAlast-name = *ALPHAsuffix = ("Jr." / "Sr." / 1*("I" / "V" / "X"))

street = [apt SP] house-num SP street-name CRLFapt = 1*4DIGIThouse-num = 1*8(DIGIT / ALPHA)street-name = 1*VCHAR

zip-part = town-name "," SP state 1*2SP zip-code CRLFtown-name = 1*(ALPHA / SP)state = 2ALPHAzip-code = 5DIGIT ["-" 4DIGIT]


ABNF – Exercise 1

Write a grammar to accept the following input:

“I would like to fly from ___ to ___ (please/thanks)”

where the following cities are allowed:

“paris”, “new york”, “dublin”

and

“please” or “thanks” is an optional extra


ABNF - Excise 2

Specify, using ABNF, the syntax for a directory path, like

users/smith/file or

users/smith/WWW/file

with none, one or more directory names, followed by a file name.


ABNF - Exercise 3

Specify the syntax of an e-mail header field with the following properties:

Name: .Weather.Values: .Sunny. or .Cloudy. or .Raining. or .Snowing.Optional parameters: ";" followed by parameter, "=" and integer

valueParameters: .temperature. and .humidity.

Examples:

Weather: Sunny; temperature=20; humidity=50

Weather: Cloudy


Augmented Backus-Naur FormatUsage



- HTTP- Browsing the internet

- Session Initiation Protocol- VoIP (IP telephony), IMS (IP Multimedia Subsystem)

- Session Description Protocol

Usage:


ABNF (VoIP Example)

SIP SIP

INVITE sip:e9-airport.mit.edu SIP/2.0

From: "Dennis Baron"<sip:[email protected]>;tag=1c41

To: sip:e9-airport.mit.edu

Call-Id: [email protected]

Cseq: 1 INVITE

Contact: "Dennis Baron"<sip:[email protected]>

Content-Type: application/sdp

Via: SIP/2.0/UDP 18.10.0.79

200 OK


VoIP (SIP Session)


Session Initiation Protocol

SIP-message = Request / Response

Request = Request-Line *( message-header ) CRLF [ message-body ]

Response = Status-Line *( message-header ) CRLF [ message-body ]


Session Initiation Protocol

Request-Line=Method SP Request-URI SP SIP-Version CRLF

Status-Line=SIP-Version SP Status-Code SP Reason-Phrase CRLF

Method= INVITEm / ACKm / OPTIONSm / BYEm / CANCELm / REGISTERm / INFOm / PRACKm / SUBSCRIBEm / NOTIFYm / UPDATEm / MESSAGEm / REFERm / PUBLISHm

/ extension-method

INVITEm=%x49.4E.56.49.54.45 ; INVITE in caps


Text-based vs. Binary Encoding



ABNF Specification ASN.1 Specification

Family = "Family"

CRLF *(Person)

"End of Family"

Person = "Person" CRLF

" Name: " 1*A CRLF

" Birthyear: " 4D CRLF

" Gender: "

("Male"/"Female") CRLF

" Status: "

("unmarried"/ "married"/

"divorced"/ "widow"/"widower" )

Family = SEQUENCE OF Person

Person := SEQUENCE {

name VisibleString,

birthyear INTEGER,

gender Gender,

status Status }

Gender := ENUMERATED {

male(0), female(1) }

Status := ENUMERATED {

unmarried(0), married(1), divorced(2),

widow(3), widower(4) }



Example of textual encoding Example of BER encoding

Family

Person

Name: John Smith

Birthyear: 1958

Gender: Male

Status: Married

Person

Name: Eliza Tennyson

Birthyear: 1959

Gender: Female

Status: Married

End of Family

30 34

30 16

1A 0A J o h n S m i t h

02 02 07 A6

0A 01 00

0A 01 01

30 1A

1A 0E E l i z a T e n n y s o n

02 02 07 A7

0A 01 01

0A 01 01

169 octets (excl. new lines)

18% efficiency compared to ASN.1 PER

54 octets

57% efficiency compared to ASN.1 PER


Text-based vs. Binary EncodingThe PER (unaligned variant) encoding of the same ASN.1 and the same data would be the following 31 octets:

00000010 (no of persons in family) 000011 10 (14 characters)

00001010 (10 characters) 100010 1 E

1001010 J 1101100 l

1 101111 o 1101001 i

11 01000 h 1 111010 z

110 1110 n 11 00001 a

0100 000 010 0000

10100 11 S 1010 100 T

110110 1 m 11001 01 e

1101001 i 110111 0 n

1110100 t 1101110 n

1 101000 h 1111001 y

00 000010 (2 octest) 1 110011 s

00 0011110 100110 (1958) 11 01111 o

0 (male) 110 1110 n

0 01 (married) 0000 0010 (2 bytes)

0000 01111010 0111 (1959)

1 (female)

001 (married)

IHA præsentation1 Text Based Protocols Lesson 3. IHA præsentation2 Outline for today Text Based Protocols Augmented Backus-Naur (ABNF) Motivation The.

Documents

rule foo bar

foo bar slide

element slide

abc slide

foo bar foo

bar blat slide

rule rules

language slide