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The specs behind the specs part 1

Β· 32 min read
Sebastian Weddmark Olsson

"Abstract Syntax Notation One (ASN.1) is a standard interface description language for defining data structures that can be serialized and deserialized in a cross-platform way." - Wikipedia

Introduction

Today you'll read about a specific language used to describe many of the messages in the telecom specifications. It will be a deep-dive into technical parts, so I imagine you could just use the blog post when you want to look up different parts without fully reading it.

In wgtwo we use this language for some specific telco messages (such as SIGTRAN layers TCAP/MAP/CAP, as well as S1AP, NGAP and probably some more). They are defined directly in some of the telecom specifications, and because of that it is possible to use them to send messages between different telecom cores.

This will probably be a two piece blog post as there is another interface describing language which is not ASN.1, and this is already a very long post. The other specification is used in Diameter dictionaries, but I'll spare those for now. It has already taken me about half a year to finish up this article.

There might be some Erlang specific paragraphs here and there, but this blog post is mainly about ASN.1 as a specification, which can be used in any language supporting it. For Erlang specifics I came across this blog post written by Viacheslav Katsuba, and because it is build into Erlang by default I recommend the APIs as well.

DISCLAIMER: SEVERE HEADACHE MIGHT FOLLOW.

Abstract Syntax Notation One

Abstract Syntax Notation One (ASN.1 for short) provides a high level description of messages. It abstracts the language implementations from the protocol design.

It was initially used by Open Systems Interconnection (OSI) to describe email messages but is used by many other applications especially within telecommunications and cryptography.

You might have heard of similar such abstract syntax notations used for interface definitions such as Google Protocol Buffers, or Facebook's Apache Thrift, but those languages have not been managed by a standardization organization, so the owning corporations could (in theory) make breaking changes or change the license or even remove the language definitions overnight.

Anyway, back to ASN.1

The first ASN.1 standardization came out 1984, and there have been many improvements since, for instance with the 1994 update which added extended functionality for telecommunication technologies.

"Long live ASN.1!" - Olivier Dubuisson from the best book that I've read on the subject. (How many ASN.1 books are there you might wonder? Actually there were more books than I expected on the subject, but to make it perfectly clear: I did only read the one.)

Off-topic but a bit of a fun fact I got from reading the book which I didn't know about before is that 'little Endian' and 'big Endian', which are used to denote if the bitstring should be read from leftmost or rightmost bit, actually originates from the 1726 best-seller Gulliver's travels.

The how and why​

ASN.1 builds on the following ideas:

  • Data structures to be transmitted should be described regardless of programming language used transmitting or receiving them.
  • The notation should allow building complex data types from basic types, and be able to do so recursively.
  • The notation must be formal to prevent ambiguities.

That said, ASN.1 is not an abstract syntax in itself, but a language to describe abstract syntaxes.

There are currently four main ASN.1 specifications (listed below), as well as at least one specification per encoding rule (listed in the last section).

ITU-T noASN.1 specifications
X.680Abstract Syntax Notation One (ASN.1): Specification of basic notation
X.681Abstract Syntax Notation One (ASN.1): Information object specification
X.682Abstract Syntax Notation One (ASN.1): Constraint specification
X.683Abstract Syntax Notation One (ASN.1): Parameterization of ASN.1 specifications

Nitty gritty

Modules​

The purpose of an ASN.1 module is to name a collection of types and/or value definitions.

It consist of a module reference and an optional object identifier together with the declaration of the DEFINITIONS type definition. Note that even though the object identifier is optional, it is considered bad practice to leave it out. The reason for it being optional is for backward compatibility; it was not part of the original ASN.1 specification. The DEFINITIONS keyword usually comes together with the BEGIN and END keywords so multiple definitions can be done. (What else is the point of a module if not to make a collection...).

The Erlang ASN.1 compiler requires each module to be in a separate file, but generally one ASN.1 file could contain many modules. Usual file endings are .asn and .asn1. One trick that can be used to circumvent this Erlang specific problem is to list multiple ASN.1 files in a new file ending with set.asn.

One example of a file with many modules exist in the CAP specification 3GPP TS 29.078

The ASN.1 template for a module

ModuleReference ObjectIdentifier
DEFINITIONS ::= BEGIN

END

as seen in an example

CAP-operationcodes {itu-t(0) identified-organization(4) etsi(0) mobileDomain(0) umts-network(1)
modules(3) cap-operationcodes(53) version8(7)}

DEFINITIONS ::= BEGIN

For information about the object identifier see the types section

Importing from other modules​

Importing types, values and other structures from other modules can be done with the IMPORTS and FROM keywords in the beginning of the module body. The IMPORTS keyword ends with a single semicolon ;, and the different imported definitions are comma-separated.

The meaning of the optional IMPLICIT TAGS keywords I'll handle later.

CAP-datatypes {itu-t(0) identified-organization(4) etsi(0) mobileDomain(0) umts-network(1) modules(3) cap-datatypes(52) version8(7)}
DEFINITIONS IMPLICIT TAGS ::= BEGIN

IMPORTS

Duration,
Integer4,
Interval,
LegID,
ServiceKey
FROM CS1-DataTypes {itu-t(0) identified-organization(4) etsi(0) inDomain(1) in-network(1)
modules(0) cs1-datatypes(2) version1(0)}

BothwayThroughConnectionInd,
CriticalityType,
MiscCallInfo
FROM CS2-datatypes {itu-t(0) identified-organization(4) etsi(0) inDomain(1) in-network(1)
cs2(20) modules(0) in-cs2-datatypes(0) version1(0)}

-- ...more imports...
; -- IMPORTS end here

END -- CAP-datatypes ends here --

-- CAP-errortypes module starts here --
CAP-errortypes {itu-t(0) identified-organization(4) etsi(0) mobileDomain(0) umts-network(1) modules(3) cap-errortypes(51) version8(7)}
DEFINITIONS IMPLICIT TAGS ::= BEGIN


END -- CAP-errortypes ends here --

The type definitions Duration and LegID above are imported from CS1-DataTypes module, while MiscCallInfo comes from the CS2-datatypes module.

Exporting from a module​

Exports from a module are done in a similar fashion.

If the EXPORT keyword is not used in a module, the ASN.1 compilers should export all values and types from the module. It's the same as specifying EXPORTS ALL;.

CAP-GPRS-ReferenceNumber {itu-t(0) identified-organization(4) etsi(0) mobileDomain(0)
umts-network(1) modules(3) cap-dialogueInformation(111) version8(7)}

DEFINITIONS ::= BEGIN

EXPORTS
id-CAP-GPRS-ReferenceNumber,
cAP-GPRS-ReferenceNumber-Abstract-Syntax;

IMPORTS

Integer4
FROM CS1-DataTypes {itu-t(0) identified-organization(4) etsi(0) inDomain(1) in-network(1)
modules(0) cs1-datatypes(2) version1(0)}
;

END

Commenting​

As can be seen in the above example one can enter comments into the ASN.1. Comments starts with double dash -- and ends with either a newline or another --, whichever comes first.

Assignments and naming​

The rules specify that type references must start with an uppercase letter and may not end with a dash -. It may also only contain upper- and lower-case letters, digits or dashes -. The syntax for a type assignment is

TypeRef ::= TypeDefinition

For instance

InvokeIdType ::= INTEGER (-128..127)

CancelArg ::= CHOICE {
invokeID [0] InvokeID,
allRequests [1] NULL
}

Duration ::= INTEGER (-2..86400)

Integer4 ::= INTEGER (0..2147483647)

Interval ::= INTEGER (-1..60000)

InvokeID ::= InvokeIdType

LegID ::= CHOICE {
sendingSideID [0] LegType,
-- used in operations sent from SCF to SSF
receivingSideID [1] LegType
-- used in operations sent from SSF to SCF
}

LegType ::= OCTET STRING (SIZE(1))

ServiceKey ::= Integer4

Value references have a similar syntax as type references except that value references must start with a lower-case letter, and also carry the values type.

Syntax

valueRef Type ::= value

For example

leg1 LegType ::= '01'H
leg2 LegType ::= '02'H

highLayerCompatibilityLength INTEGER ::= 2
minAChBillingChargingLength INTEGER ::= 0

Types

Now when we have talked a bit about naming references, and how to assign values and types I'll go over which built-in types exist, and how to create new types.

There are some common types, each consists of a type reference and a tag number. The tag number is used to identify it when sending the type in the network. The universal tags are specified in ITU-T X.680

Here is a list of the most common types

TypeUniversal Tag Number
BOOLEAN1
INTEGER2
BIT STRING3
OCTET STRING4
NULL5
OBJECT IDENTIFIER6
EXTERNAL8
REAL9
ENUMERATED10
UTF8String12
TIME14
SEQUENCE (OF)16
SET (OF)17
NumericString18
IA5String22
UTCTime23
GeneralizedTime24
VisibleString26
DATE31
TIME-OF-DAY32
DATE-TIME33
DURATION34
CHOICE*
SELECTION*

The common types can be divided into simple and structured types. Structured types are the composition of multiple types (so called component types) using one of the following types and keywords SEQUENCE, SEQUENCE OF, SET, SET OF, CHOICE, and/or SELECTION. Note that CHOICE and SELECTION does not [need to] have their own universal tags, due to those are consisting of other types.

Basic types​

BOOLEAN​

The BOOLEAN type takes values TRUE or FALSE.

AudibleIndicator ::= CHOICE {
tone BOOLEAN,
burstList [1] BurstList
}

Here the value tone of the composit type AudibleIndicator is of type BOOLEAN. Note: It was one of the cleanest example I could find of a BOOLEAN in the ASN.1 files we use, because Telco often use a special "trick" when it comes to booleans in order to save bandwidth, i.e. the NULL type.

NULL​

The NULL type is basically a placeholder, where the recognition of a value is important but the actual value is not.

In 3GPP it is similar to BOOLEAN in the sense that a defined NULL value is considered TRUE and if the value is missing it is considered FALSE. The reason for this is that when sent over the network with BER encoding, it will take no space because BOOLEAN is always of length 1 but NULL is always length 0, i.e. NULL does not contain a value.

CancelArg {PARAMETERS-BOUND : bound} ::= CHOICE {
invokeID [0] InvokeID,
allRequests [1] NULL,
callSegmentToCancel [2] CallSegmentToCancel {bound}
}

in this example allRequests can be defined (then only the tag is transmitted) or not at all.

INTEGER​

INTEGER takes any of the infinite set of integer values. It can also have the additional notation that names some of the values.

GSMMAPOperationLocalvalue ::= INTEGER{
updateLocation (2),
cancelLocation (3),
provideRoamingNumber (4),
noteSubscriberDataModified (5),
resumeCallHandling (6),
insertSubscriberData (7),
-- rest of the named integers --
}
localvalue1 GSMMAPOperationLocalvalue ::= updateLocation
localvalue2 GSMMAPOperationLocalvalue ::= 2
localvalue3 GSMMAPOperationLocalvalue ::= -55413459

are all valid GSMMAPOperationLocalvalues.

ENUMERATED​

ENUMERATED has the same interpretation as INTEGER but will hold specific values only.

RequestedInformationType ::= ENUMERATED {
callAttemptElapsedTime(0),
callStopTime(1),
callConnectedElapsedTime(2),
calledAddress(3),
releaseCause(30)
}
reqInfoType1 RequestedInformationType ::= callAttemptElapsedTime
reqInfoType2 RequestedInformationType ::= 0

are both valid values of RequestInformationType, while this is not:

notValidReqInfoType RequestedInformationType ::= 4

BIT STRING​

BIT STRING takes values that are a sequence of zero or more bits. It can also take an additional notation that name certain bits in the bit sequence.

DeferredLocationEventType ::= BIT STRING {
msAvailable (0) ,
enteringIntoArea (1),
leavingFromArea (2),
beingInsideArea (3) ,
periodicLDR (4)
} (SIZE (1..16))
eventType1 DeferredLocationEventType ::= (msAvailable, beingInsideArea)
eventType2 DeferredLocationEventType ::= '10010'B
eventType3 DeferredLocationEventType ::= '12'H

are all valid value definitions of the same bit sequence where the first and third bits are set, and no other bits are set. The B stands for binary representation and H for hexadecimal representation.

The SIZE is a constraint on the type defining it to be of a specific length. This keyword comes as an extra notation for many of the STRING types below (as well as some of the other types).

OCTET STRING​

Type OCTET STRING takes values that are an ordered sequence of zero or more (eight-bit) octets.

MM-Code ::= OCTET STRING (SIZE (1))

In the same manner as BIT STRING both values below are valid instances of MM-Code:

iMSI-Attach1 MM-Code ::= '00000010'B
iMSI-Attach2 MM-Code ::= '02'H

while

notValidIMSI-Attach MM-Code ::= '10010'B

is not considered a valid value due to it not being a multiple of eight bits.

OBJECT IDENTIFIER​

The OBJECT IDENTIFIER type (shortened OID) names information objects such as ASN.1 modules. The named information object is a node on an object identifier tree that is managed at the international level.

ETSI for instance is managed by ITU-T itu-t(0) identified-organization(4) etsi(0)

ETSI OID tree

and as can see in the Modules example version 8 of cap-datatypes is part of ETSI. CAP-datatypes {itu-t(0) identified-organization(4) etsi(0) mobileDomain(0) umts-network(1) modules(3) cap-datatypes(52) version8(7)}

Other root arcs

RootOrganization
0ITU-T
1ISO
2joint-iso-itu-t

The labels are optional and the reference could also be written as {0 4 0 0 1 3 52 7}. Only positive integers are allowed including zero (0).

Another example comes from the CAP-object-identifiers module in ETSI 129.078.

tc-Messages OBJECT IDENTIFIER ::=
{itu-t recommendation q 773 modules(2) messages(1) version3(3)}

id-CAP OBJECT IDENTIFIER ::=
{itu-t(0) identified-organization(4) etsi(0) mobileDomain(0)
umts-network(1) cap4(22)}

id-ac OBJECT IDENTIFIER ::= {id-CAP ac(3)}

id-ac is a child of the id-CAP object identifier.

One could lookup object identifiers by visiting this amazing page (oidref.com).

EXTERNAL​

EXTERNAL represents a value that does not need to be specified as a ASN.1 type. It carries information on how the data should be interpreted.

{% raw %}```asn.1 Unidirectional {OPERATION:Invokable, OPERATION:Returnable} ::= SEQUENCE { dialoguePortion DialoguePortion OPTIONAL, components ComponentPortion{{Invokable}, {Returnable}} }

DialoguePortion ::= [APPLICATION 11] EXPLICIT EXTERNAL


Here the value `dialoguePortion` will have tag 11 if specified, it is
then up to the application to decide how to deal with the value.

### REAL

Values of the type `REAL` will take a triplet of numbers (m, b, e),
where m is the mantissa (a signed number), b the base (2 or 10), and e
the exponent (a signed number).

There are also three special values it can take `PLUS-INFINITY`, 0,
and `MINUS-INFINITY`.

```asn.1
theBestRealValue REAL ::= (123, 10, -2) -- 1.23
maxValue REAL ::= PLUS-INFINITY

String types​

I feel like most of the string types are the same, except that they all take diffrent character sets. I've already described BIT STRING and OCTET STRING which both operate the bit set, but there is a lot of others that operate over character sets.

TypeTagCharacter set regex/comment
UTF8String12Synonymous with UniversalString at abstract level
NumericString18[0-9 ]
PrintableString19[A-Za-z0-9'()+,./:=? -]
TelexString (T61String)20ISOReg reg. #6, #87, #102, #103, #106, #107, #126, #144, #150, #153, #156, #164, #165, #168 + space,delete
VideotexString21ISOReg reg. #1, #13, #72, #73, #87, #89, #102, #108, #126, #128, #129, #144, #150, #153, #164, #165, #168 + space,delete
IA5String22ISOReg reg. #1, #6 + space,delete
GraphicString25ISOReg graphical sets (called 'G') + space
VisibleString (ISO646String)26ISOReg reg. #6 + space
GeneralString27ISOReg graphical sets (called 'G'), control characters (called 'C') + space,delete
UniversalString28[ISO10646-1]
BMPString30Basic Multilingual Plane; subtype of UniversalString

ISOReg is a pretty good source, it reference most of the registers but not all of them as far as I can see.

I'll list some examples of string types found in our ASN.1 files:

AMFNameUTF8String ::= UTF8String (SIZE(1..150, ...))

DirectoryString ::= CHOICE {
teletexString TeletexString (SIZE (1..maxSize)),
printableString PrintableString (SIZE (1..maxSize)),
universalString UniversalString (SIZE (1..maxSize)),
bmpString BMPString (SIZE (1..maxSize))
-- utf8String UTF8String (SIZE (1..maxSize))
}

DisplayInformation ::= IA5String (SIZE (minDisplayInformationLength..maxDisplayInformationLength))

IA5String is used to represent ISO 646 (IA5; International Alphabet 5) characters. The entire character set contains precisely 128 characters and are generally equivalent to the first 128 characters of the ASCII alphabet.

There are multiple formats for the values of UniversalString, BMPString and UTF8String types. One could either specify a quadruple with {group, plane, row, cell} for the character needed, or an array of defined values (strings).

An example from Dubuisson:

latinCapitalLetterA UniversalString ::= {0,0,0,65}
greekCapitalLetterSigma UniversalString ::= {0,0,3,145}

my-string UniversalString ::= {
"This is a capital A: ", latinCapitalLetterA,
", and a capital alpha: ", greekCapitalLetterAlpha,
"; try and spot the difference!"}

And X.680 gives us yet another example

IMPORTS
BasicLatin, greekCapitalLetterSigma
FROM ASN1-CHARACTER-MODULE
{ joint-iso-itu-t asn1(1) specification(0) modules(0) iso10646(0) };

MyAlphabet ::= UniversalString (FROM (BasicLatin | greekCapitalLetterSigma))

mystring MyAlphabet ::= { "abc" , greekCapitalLetterSigma , "def" }

Time types​

The UTCTime and GeneralizedTime types are actually specified as VisibleString.

UTCTime format is "YYMMDD" for date followed by "hhmm" or "hhmmss" for time, ending with either "z", "-hhmm" or "+hhmm" for time offset.

Specifying "2021-12-14 04:32 CET" in UTCTime

"2112140332Z"
"2112140432+0100"

GeneralizedTime gives a bit more flexibility with regards to the format.

It consists of a calendar date of format "YYYYMMDD", followed by either "hh", "hhmm", "hhmmss" and optional parts ".[0-9]+", and optionally ending with the coordinated universal time character "z" or the time offset in hours/minutes "-hhmm" or "+hhmm".

These are the same, but one with higher precision and in local time.

"2021121403.54Z" -- 3.54 hours after midnight
"20211214043227.981935+0100" -- 3 hours, 32 minutes, 27 seconds, 981935 microseconds

DATE, TIME-OF-DAY, DATE-TIME and DURATION was introduced after the third generation of ISO 8601 was released 2004.

They are defined as subsets of TIME.

DATE ::= [UNIVERSAL 31] IMPLICIT TIME
(SETTINGS "Basic=Date Date=YMD Year=Basic")
TIME-OF-DAY ::= [UNIVERSAL 32] IMPLICIT TIME
(SETTINGS "Basic=Time Time=HMS Local-or-UTC=L")
DATE-TIME ::= [UNIVERSAL 33] IMPLICIT TIME
(SETTINGS "Basic=Date-Time Date=YMD Year=Basic Time=HMS Local-or-UTC=L")
DURATION ::= [UNIVERSAL 34] IMPLICIT TIME
(SETTINGS "Basic=Interval Interval-type=D")

As I can find no real world examples from our ASN.1-files, I'm forced to make-up examples of these

date1 DATE ::= "211214"
time1 TIME-OF-DAY ::= "043227"
date-time1 DATE-TIME ::= "211214043227"
duration1 DURATION ::= "P0Y29M0DT0H0M0S" -- 29 months to an accuracy of 1 second

Values of type DURATION starts with "P" followed by alot of different optional parts and formats. If the time designation is used it should start with a "T" to keep months and minutes separate.

duration2 DURATION ::= "P2MT2M" -- 2 months and 2 minutes
duration3 DURATION ::= "P29M0DT0.00M" -- 29 months with accuracy of one-hundredth of a minute
duration4 DURATION ::= "P32W" -- 32 weeks

Structured types​

CHOICE​

The type CHOICE can take values from one of multiple types, CHOICE doesn't have it's own universal tag.

CancelArg ::= CHOICE {
invokeID [0] InvokeID,
allRequests [1] NULL
}

The value of type CancelArg will either of an InvokeID type, or a NULL type. It will be tagged [0] or [1] respectively, that is why CHOICE doesn't have it's own universal tag, as it is derived from ASN.1 specification.

SEQUENCE (OF)​

SEQUENCE and SEQUENCE OF are used for composing multiple types.

EventTypeSMS ::= ENUMERATED {
sms-CollectedInfo (1),
o-smsFailure (2),
o-smsSubmission (3),
sms-DeliveryRequested (11),
t-smsFailure (12),
t-smsDelivery (13)
}
MonitorMode ::= ENUMERATED {
interrupted (0),
notifyAndContinue (1),
transparent (2)
}

SMSEvent ::= SEQUENCE {
eventTypeSMS [0] EventTypeSMS,
monitorMode [1] MonitorMode
}

Tone ::= SEQUENCE {
toneID [0] Integer4,
duration [1] Integer4 OPTIONAL,
...
}

A value of the SMSEvent type have information on both EventTypeSMS and MonitorMode. The fixed number of fields in the SEQUENCE type are ordered. Context-specific tagging (e.g. the [0], [1], [2] stuff in the examples), is frequently applied for the structured types, but one could also utilize the keywords AUTOMATIC TAGGING in the module definition.

SEQUENCE OF on the other hand, holds an arbitrary number of fields of a single type.

FilterItem ::= CHOICE {
equality [0] AttributeValueAssertion,
substrings [1] SEQUENCE {
type ATTRIBUTE.&id({SupportedAttributes}),
strings SEQUENCE OF CHOICE {
initial [0] ATTRIBUTE.&Type({SupportedAttributes}{@substrings.type}),
any [1] ATTRIBUTE.&Type({SupportedAttributes}{@substrings.type}),
final [2] ATTRIBUTE.&Type({SupportedAttributes}{@substrings.type})
}
},
greaterOrEqual [2] AttributeValueAssertion,
lessOrEqual [3] AttributeValueAssertion,
present [4] AttributeType,
approximateMatch [5] AttributeValueAssertion,
extensibleMatch [6] MatchingRuleAssertion
}

In the quite complex example above we see that the type FilterItem is of type CHOICE and can take subtype called strings. strings is of type SEQUENCE OF CHOICE which means it can take a list of zero, one or more of initial, any or final. The example is quite complex because it also uses multiple parameterized values. see Automatic, Implicit, Explicit tags

We find another example in the DialoguePDUs module from Q.773 where the AARQ is of type SEQUENCE, and the third field user-information is an SEQUENCE OF EXTERNAL type.

AARQ-apdu ::= [APPLICATION 0] IMPLICIT SEQUENCE {
protocol-version
[0] IMPLICIT BIT STRING {version1(0)} DEFAULT {version1},
application-context-name [1] OBJECT IDENTIFIER,
user-information [30] IMPLICIT SEQUENCE OF EXTERNAL OPTIONAL
}

They are quite different in how they are used, but they are encoded in a similar way. Some languages represent SEQUENCE internally as a struct, and SEQUENCE OF as an array, but encoded they would look quite similar.

SET (OF)​

SET and SET OF are similar to SEQUENCE and SEQUENCE OF respectively. The difference is that the composite types are unordered.

From CAP-datatypes we find an example of a SET OF with a parameterized component specifying a size constraint.

GenericNumbers {PARAMETERS-BOUND : bound} ::= SET SIZE(1..bound.&numOfGenericNumbers) OF GenericNumber {bound}

Or an example of a value from the TCAP-Tools module in Q.775

cancelFailed ERROR ::= {
PARAMETER
SET {problem [0] CancelProblem,
invokeId [1] present < TCInvokeIdSet
}
}

SELECTION​

The SELECTION type < is used when one want's to obtain one of the possible subtypes of a CHOICE definition.

If we expand the previous example from the SET

cancel OPERATION ::= {
ARGUMENT present < TCInvokeIdSet
ERRORS {cancelFailed}
}

cancelFailed ERROR ::= {
PARAMETER
SET {problem [0] CancelProblem,
invokeId [1] present < TCInvokeIdSet
}
}

we see that the ARGUMENT type and the invokeId field take the type from the present field in the TCInvokeIdSet type.

the definition of TCInvokeIdSet is as follows

InvokeId ::= CHOICE {present  INTEGER,
absent NULL
}

TCInvokeIdSet ::= InvokeId(WITH COMPONENTS {
present (-128..127)
})

Thus invokeId and ARGUMENT fields will take integer values which are between -128 and 127.

Other concepts

DEFAULT and OPTIONAL keywords​

One can use the DEFAULT keyword in order to specify the default value.

CollectedDigits ::= SEQUENCE {
minimumNbOfDigits [0] INTEGER (1..16) DEFAULT 1,
maximumNbOfDigits [1] INTEGER (1..16),
endOfReplyDigit [2] OCTET STRING (SIZE (1..2)) OPTIONAL,
cancelDigit [3] OCTET STRING (SIZE (1..2)) OPTIONAL,
startDigit [4] OCTET STRING (SIZE (1..2)) OPTIONAL,
firstDigitTimeOut [5] INTEGER (1..127) OPTIONAL,
interDigitTimeOut [6] INTEGER (1..127) OPTIONAL,
errorTreatment [7] ErrorTreatment DEFAULT stdErrorAndInfo,
interruptableAnnInd [8] BOOLEAN DEFAULT TRUE,
voiceInformation [9] BOOLEAN DEFAULT FALSE,
voiceBack [10] BOOLEAN DEFAULT FALSE
}

In this example we see the type CollectedDigits where most of the values are either DEFAULT or OPTIONAL. The only value that needs to be set is maximumNbOfDigits.

Classes​

One can also use informal object classes in order to specify and define values for general types.

OPERATION ::= CLASS {
&ArgumentType OPTIONAL,
&argumentTypeOptional BOOLEAN OPTIONAL,
&returnResult BOOLEAN DEFAULT TRUE,
&ResultType OPTIONAL,
&resultTypeOptional BOOLEAN OPTIONAL,
&Errors ERROR OPTIONAL,
&Linked OPERATION OPTIONAL,
&synchronous BOOLEAN DEFAULT FALSE,
&alwaysReturns BOOLEAN DEFAULT TRUE,
&InvokePriority Priority OPTIONAL,
&ResultPriority Priority OPTIONAL,
&operationCode Code UNIQUE OPTIONAL
}
WITH SYNTAX {
[ARGUMENT &ArgumentType
[OPTIONAL &argumentTypeOptional]]
[RESULT &ResultType
[OPTIONAL &resultTypeOptional]]
[RETURN RESULT &returnResult]
[ERRORS &Errors]
[LINKED &Linked]
[SYNCHRONOUS &synchronous]
[ALWAYS RESPONDS &alwaysReturns]
[INVOKE PRIORITY &InvokePriority]
[RESULT-PRIORITY &ResultPriority]
[CODE &operationCode]
}

provideRoutingInformation OPERATION ::= {
ARGUMENT RequestArgument
RESULT RoutingInformation
ERRORS
{invalidCalledNumber | subscriberNotReachable | calledBarred |
processingFailure}
LINKED {getCallingPartyAddress}
}

getCallingPartyAddress OPERATION ::= {
RESULT CallingPartyAddress
ERRORS {callingPartyAddressNotAvailable | processingFailure}
}

invalidCalledNumber ERROR ::= {CODE local:1}
subscriberNotReachable ERROR ::= {CODE local:2}

RequestArgument ::= SEQUENCE {
calledNumber IsdnNumber,
basicService BasicServiceIndicator OPTIONAL
}

RoutingInformation ::= CHOICE {
reroutingNumber [0] IMPLICIT IsdnNumber,
forwardedToNumber [1] IMPLICIT IsdnNumber
}

In the above OPERATION class, a syntax is defined for the class. This class and syntax can then be used to define specific values, for instance the provideRoutingInformation and getCallingPartyAddress values.

We also glimse another class in the above example which I didn't include. Can you find it?

Parameterized components​

Another way to make the specs more generalized is to use parameterized components. We've already seen a couple of examples of such, see the chapter for the NULL, EXTERNAL and SET (OF) types.

Let's look at the example from SET OF again.

GenericNumbers {PARAMETERS-BOUND : bound} ::= SET SIZE(1..bound.&numOfGenericNumbers) OF GenericNumber {bound}

GenericNumber {PARAMETERS-BOUND : bound} ::= OCTET STRING (SIZE(
bound.&minGenericNumberLength .. bound.&maxGenericNumberLength))

GenericNumbers take a parameter bound of the type PARAMETERS-BOUND as input.

PARAMETERS-BOUND is defined as a class with a lot of different integer values, so I've minimized the class a bit.

PARAMETERS-BOUND ::= CLASS {
--- a lot of other fields
&minGenericNumberLength INTEGER,
&maxGenericNumberLength INTEGER,
&numOfGenericNumbers INTEGER,
}

WITH SYNTAX {
--- a lot of other fields
MINIMUM-FOR-GENERIC-NUMBER &minGenericNumberLength
MAXIMUM-FOR-GENERIC-NUMBER &maxGenericNumberLength
NUM-OF-GENERIC-NUMBERS &numOfGenericNumbers
}

One could then specify different values using the PARAMETERS-BOUND class to reuse the GenericNumber and GenericNumbers types.

cAPSpecificBoundSet PARAMETERS-BOUND ::= {
--- again, lots of other values
MINIMUM-FOR-GENERIC-NUMBER 3
MAXIMUM-FOR-GENERIC-NUMBER 11
NUM-OF-GENERIC-NUMBERS 5
}

If one would pass cAPSpecificBoundSet to a value of type GenericNumbers, it would define an instance which holds 1-5 GenericNumbers of 3-11 octets.

Quite powerful if you have multiple definitions which uses similar structure.

Extensions​

Sometimes you will need to support different versions of a protocol (or someone else need to support different version, and you just need to read the types of the protocol), and maybe the new version need to extend some types in order to include more information. Then without redefining everything and copy the previous version of the type to the new version of the type one can use extension markers (syntax ...).

Take the QosMonitoringRequest type below as an example. In the first version of the protocol (NGAP if you really want to know), the QoSMonitoringRequest could take only enums ul, dl, or both. However, in a subsequent version it was extended with a new enum stop. With the extension marker ... the v1-compilers can handle the first three enums, and the v2-compilers can handle all enums from v1 but also the stop enum.

QosMonitoringRequest ::= ENUMERATED {
ul,
dl,
both,
...
} -- version 1

QosMonitoringRequest ::= ENUMERATED {
ul,
dl,
both,
...,
stop
} -- version 2

For enums, if there is yet a newer version, say version 3 of this type one, you should just add the new enum under the previous enums.

QosMonitoringRequest ::= ENUMERATED {
ul,
dl,
both,
...,
stop,
half
} -- imaginary version 3

For SEQUENCE, SET and CHOICE one could either do the same, or to keep the versions separated, add another extension mark with the new fields in between. One could also add version brackets to group the extensions and highlight the differences.

Ax ::= SEQUENCE {
a INTEGER (250..253),
b BOOLEAN,
c CHOICE {
d INTEGER, -- version 1
...,
[[
e BOOLEAN,
f IA5String
]],
... -- version 2
},
..., -- version 1
[[
g NumericString (SIZE(3)),
h BOOLEAN OPTIONAL
]],
..., -- version 2
i BMPString OPTIONAL, -- version 3
j PrintableString OPTIONAL -- version 3
}

Only the types ENUMERATED, SEQUENCE, SET and CHOICE, as well as subtype constraints, and object and value sets can be extended.

Automatic, Implicit, Explicit tags​

When an value is transmitted all ambiguities need to be removed. That is why every type needs to have an unique identifier, called a tag.

The section DEFAULT and OPTIONAL keywords has a good example I will explain.

CollectedDigits ::= SEQUENCE {
minimumNbOfDigits [0] INTEGER (1..16) DEFAULT 1,
maximumNbOfDigits [1] INTEGER (1..16),
endOfReplyDigit [2] OCTET STRING (SIZE (1..2)) OPTIONAL,
cancelDigit [3] OCTET STRING (SIZE (1..2)) OPTIONAL,
startDigit [4] OCTET STRING (SIZE (1..2)) OPTIONAL,
firstDigitTimeOut [5] INTEGER (1..127) OPTIONAL,
interDigitTimeOut [6] INTEGER (1..127) OPTIONAL,
errorTreatment [7] ErrorTreatment DEFAULT stdErrorAndInfo,
interruptableAnnInd [8] BOOLEAN DEFAULT TRUE,
voiceInformation [9] BOOLEAN DEFAULT FALSE,
voiceBack [10] BOOLEAN DEFAULT FALSE
}

In this sequence we can see that many of the values are optional and some have defaults, only the value maximumNbOfDigits is mandatory.

When the sending side transmits a value of CollectedDigits type, the receiving side will get a sequence of the values mentioned. With BER-encoding each defined value will have an identifier, a length (of the value transmitted) and the value. This is called a Tag-Length-Value or TLV for short.

All basic types already has an universal tag as stated in the types table, but the composite types does not. (If not tagged, how would it see the difference between endOfReplyDigit, cancelDigit and startDigit in the example above?)

RoutingInformation ::= CHOICE {
reroutingNumber [0] IMPLICIT IsdnNumber,
forwardedToNumber [1] IMPLICIT IsdnNumber
}

IsdnNumber ::= SEQUENCE {
typeOfAddress TypeOfAddress,
digits TelephonyString
}

TypeOfAddress ::= ENUMERATED {national(0), international(1), private(2)}

TelephonyString ::=
IA5String
(FROM ("0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" | "*" |
"#"))(SIZE (1..15))

A value of RoutingInformation in this example will with BER encoding only transmit the tag [0] or [1] (and of course the length and value). It counts on that the receiving part has the same version of the ASN.1 and knows the abstract syntax. If it wouldn't have been IMPLICIT, then it would have sent either tags [0] or [1] followed by the tags for TypeOfAddress (ENUMERATED) [10], and tags for TelephonyString (IA5String) [4].

One can specify IMPLICIT and EXPLICIT tagging on module basis, where the DEFINITIONS are assigned.

DEFINITIONS IMPLICIT TAGS ::= BEGIN

Instead of writing all the tags self (explicitly in both cases), one can instead use the keywords AUTOMATED TAGS.

DEFINITIONS AUTOMATIC TAGS ::= BEGIN

This will add tags to the composit types that doesn't have them (explicitly) set.

Deprecations and discouragements​

Some things have been deprecated from earlier ASN.1 specifications, and use of these are strongly discouraged.

ANY​

First out is the ANY type, which could take the form of any value. It's like an unrestricted CHOICE type.

Invoke ::= SEQUENCE {
invokeID InvokeIdType,
linkedID [0] InvokeIdType OPTIONAL,
operationCode MAP-OPERATION,
parameter InvokeParameter OPTIONAL
}
InvokeParameter ::= ANY

Problem with this is that the ASN.1 compiler does not have a formal way of knowing which values are approved. The use of ANY also usually meant it was coupled with some other value, for instance the operationCode in the example above.

To make this link one could have used DEFINED BY specifying which field the ANY type is coupled with. The drawback with this solution is that it still has an ambiguous meaning, and the types are of no use for the application designer.

ExtensionField ::= SEQUENCE {
type INTEGER,
-- shall identify the value of an EXTENSION type
criticality ENUMERATED {
ignore(0),
abort(1)
} DEFAULT ignore,
value [1] ANY DEFINED BY type
}

Instead the concept of informal object classes and parameterized components were introduced.

Macros​

Macros were removed because they were poorly documented and too general. Because of this they were hard to implement and automate in the compilers. They follow the BNF notation.

OPERATION MACRO ::=
BEGIN
TYPE NOTATION ::= Parameter Result Errors LinkedOperations
VALUE NOTATION ::= value (VALUE CHOICE { localValue INTEGER, globalValue OBJECT IDENTIFIER } )
Parameter ::= ArgKeyword NamedType | empty
ArgKeyword ::= "ARGUMENT" | "PARAMETER"
Result ::= "RESULT" ResultType | empty
Errors ::= "ERRORS" "{"ErrorNames"}" | empty
LinkedOperations ::= "LINKED" "{"LinkedOperationNames"}" | empty
ResultType ::= NamedType | empty
ErrorNames ::= ErrorList | empty
ErrorList ::= Error | ErrorList "," Error
Error ::= value (ERROR)
-- shall reference an error value
| type
-- shall reference an error type
-- if no error value is specified
LinkedOperationNames ::= OperationList | empty
OperationList ::= Operation | OperationList "," Operation
Operation ::= value (OPERATION)
-- shall reference an operation value
| type
-- shall reference an operation type if
-- no operation value is specified
NamedType ::= identifier type | type
END

The two needed fields of the macro are TYPE NOTATION and VALUE NOTATION. The rest of the fields are the value sequence defining what the macro should insert. Quotes define the string to insert (excluding the actual quotes), empty inserts nothing. identifier, value, or type are used to infer different things. Dubuisson has (yet again) a good chapter on this topic.

Also here one should use informal object classes and parameterized components instead of using macros.

Encodings

There are numerous codecs when transmitting the abstract syntax, all with different pros and cons.

Short nameLong name
BERBasic encoding rules
DERDistinguished encoding rules
CERCanonical encoding rules
PERPacked encoding rules
OEROctet encoding rules
XERXML encoding rules
EXERExtended XML encoding rules
JERJSON encoding rules

BER is the oldest encoding rule for ASN.1. It uses Tag-Length-Value format where all tags, lengths and values are multiples of octets. Because this was the first encoding rule it was named basic to indicate that there might be more standardized encoding rules in the future.

DER and CER are subsets of BER, which was added for developers of X.400 email and X.500 directory applications. It provides means to make sure bit strings are not altered during transfer. The main difference between DER and CER is that DER uses a definite-length format and CER an indefinite-length format, so CER is best used for applications that transfer a big amount of data.

PER is the most compact format, and used for bandwith conservation. It does not send the Tag of the TLV because the order in which components of the message occur is known. PER also does not send the Length of the TLV if the Value has a fixed length. It uses information from ASN.1 message description to eliminate redundant information from the Value portion. It can either be aligned to multiple of octets by padding each value with '0's, or unaligned (U-PER/Unaligned PER) which is more compact but take more time to decode.

OER was adapted from PER and uses an octet oriented format, so the length of all specified Tags, Lengths, and Values are padded to be multiples of 8 bits octets as in BER. OER is usually faster than both BER and PER with regards to encoding and decoding.

XER, CXER and EXER are used for transmitting XML format. CXER is used for transmitting data canonically, e.g. used by security applications. EXER or Extended XER is used when "stylish" features is wanted, and adds possibility to extend the encoder for instance when wanting to insert processing instructions or comments into the XML.

JER is used when transmitting JSON in accordance to the format specified in ECMA-404.

There are probably a bunch of others as well, but these are the ones that have a specification.

ITU-T numberName
X.690ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)
X.691ASN.1 encoding rules: Specification of Packed Encoding Rules (PER)
X.692ASN.1 encoding rules: Specification of Encoding Control Notation (ECN)
X.693ASN.1 encoding rules: XML Encoding Rules (XER)
X.694ASN.1 encoding rules: Mapping W3C XML schema definitions into ASN.1
X.695ASN.1 encoding rules: Registration and application of PER encoding instructions
X.696ASN.1 encoding rules: Specification of Octet Encoding Rules (OER)
X.697ASN.1 encoding rules: Specification of JavaScript Object Notation Encoding Rules (JER)

Final words

Congratulations for making through this blog post. You deserve a medal, and I hope this can help you understand the complexity and greatness of ASN.1.

We have gone through the basics of ASN.1, there are still a lot of things to be uncovered. You now understand the most common basic and structured types, as well as the main differences between the different encodings.

If I write a part 2 I will take you through the Diameter specs instead, which are much more straighforward.

Image from xkcd describing how I feel with ASN.1:

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