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| \lstnewenvironment{bash}[1][]{\lstset{style=listingStyle,language=bash,#1}}{} |
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| \usepackage[colorlinks=true, |
| linkcolor={clrlink}, |
| citecolor={clrlink}, |
| urlcolor={clrurl}, |
| pdfauthor={Lev Walkin}, |
| pdftitle={Using the Open Source ASN.1 Compiler}, |
| pdfkeywords={ASN.1,asn1c,compiler}, |
| bookmarksopen,bookmarksopenlevel=1, |
| pdffitwindow, |
| xetex |
| ]{hyperref} |
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| \begin{document} |
| |
| \title{Using the Open Source ASN.1 Compiler\\ |
| \vspace*{0.4cm} |
| \Large Documentation for asn1c version \asnver{}} |
| \author{Lev Walkin <\href{mailto:vlm@lionet.info?Subject=asn1c}{vlm@lionet.info}>} |
| |
| \pagestyle{fancy} |
| \fancyhead[L]{\leftmark} |
| \fancyhead[R]{\href{http://lionet.info/asn1c}{asn1c-\asnver}} |
| \maketitle |
| |
| \tableofcontents{} |
| |
| \part{Using the ASN.1 Compiler} |
| |
| |
| \chapter{Introduction to the ASN.1 Compiler} |
| |
| The purpose of the ASN.1 compiler is to convert the specifications |
| in ASN.1 notation into some other language. At this moment, only C |
| and C++ target languages are supported, the latter is in upward compatibility |
| mode. |
| |
| The compiler reads the specification and emits a series of target |
| language structures (C structs, unions, enums) describing the corresponding |
| ASN.1 types. The compiler also creates the code which allows automatic |
| serialization and deserialization of these structures using several |
| standardized encoding rules (BER, DER, XER, PER). |
| |
| For example, suppose the following ASN.1 module is given% |
| \footnote{Part \ref{par:ASN.1-Basics} provides a quick reference |
| on the ASN.1 notation.}: |
| \begin{asn} |
| RectangleTest DEFINITIONS ::= BEGIN |
| |
| Rectangle ::= SEQUENCE { |
| height INTEGER, -- Height of the rectangle |
| width INTEGER -- Width of the rectangle |
| } |
| |
| END |
| \end{asn} |
| The compiler would read this ASN.1 definition and produce the following |
| C type: |
| \begin{codesample} |
| typedef struct Rectangle_s { |
| long height; |
| long width; |
| } Rectangle_t; |
| \end{codesample} |
| It would also create the code for converting this structure into platform-independent |
| wire representation (a serializer API) and the decoder of such wire |
| representation back into local, machine-specific type (a deserializer |
| API). |
| |
| |
| \section{Quick start with asn1c} |
| |
| After building and installing the compiler, the \emph{asn1c} |
| command may be used to compile the ASN.1 modules% |
| \footnote{This is probably \textbf{not} what you want to try out right now. Read through the rest of this chapter and check the Section~\ref{sec:Command-line-options} |
| to find out about \textbf{-P} and \textbf{-R} options.% |
| }: |
| \begin{bash} |
| asn1c %\emph{<modules.asn1>}% |
| \end{bash} |
| If several ASN.1 modules contain interdependencies, all of the files |
| must be specified altogether: |
| \begin{bash} |
| asn1c %\emph{<module1.asn1> <module2.asn1> ...}% |
| \end{bash} |
| The compiler \textbf{-E} and \textbf{-EF} options are used for testing |
| the parser and the semantic fixer, respectively. These options will |
| instruct the compiler to dump out the parsed (and fixed, if \textbf{-F} |
| is involved) ASN.1 specification as it was understood |
| by the compiler. It might be useful to check whether a particular |
| syntactic construct is properly supported by the compiler. |
| \begin{bash} |
| asn1c %\textbf{-EF} \emph{<module-to-test.asn1>}% |
| \end{bash} |
| The \textbf{-P} option is used to dump the compiled output on the |
| screen instead of creating a bunch of .c and .h files on disk in the |
| current directory. You would probably want to start with \textbf{-P} |
| option instead of creating a mess in your current directory. Another |
| option, \textbf{-R}, asks compiler to only generate the files which |
| need to be generated, and supress linking in the numerous support |
| files. |
| |
| Print the compiled output instead of creating multiple source files: |
| \begin{bash} |
| asn1c %\textbf{-P} \emph{<module-to-compile-and-print.asn1>}% |
| \end{bash} |
| |
| \clearpage{} |
| \section{Recognizing compiler output} |
| |
| The asn1c compiler produces a number of files: |
| \begin{itemize} |
| \item A set of .c and .h files for each type defined |
| in the ASN.1 specification. These files will be named similarly to |
| the ASN.1 types (\emph{Rectangle.c} and \emph{Rectangle.h} for the |
| RectangleTest ASN.1 module defined in the beginning of this document). |
| \item A set of helper .c and .h files which contain the generic encoders, |
| decoders and other useful routines. There will be quite a few of them, some |
| of them are not even always necessary, but the overall amount of code |
| after compilation will be rather small anyway. |
| \item A \emph{converter-sample.c} file containing the \emph{int main()} function with a fully functioning decoder. It can convert a given PDU between BER, XER and possibly OER and PER (if -gen-OER or -gen-PER options to asn1c were in effect). At some point you will want to replace this file with your own file containing the \emph{int main()} function. |
| \item A \emph{Makefile.am.sample} file mentioning all the files created |
| at the earlier steps. This file is suitable for either automake suite |
| or the plain `make` utility. Just rename it into \emph{Makefile}. |
| \end{itemize} |
| It is possible to compile everything with just a couple of instructions: |
| \begin{bash} |
| asn1c -pdu=%\emph{Rectangle}% *.asn1 |
| make -f Makefile.am.sample # If you use `make` |
| \end{bash} |
| or |
| \begin{bash} |
| asn1c *.asn1 |
| cc -I. -DPDU=%\emph{Rectangle}% -o rectangle.exe *.c # ... or like this |
| \end{bash} |
| Refer to the Chapter \ref{cha:Step-by-step-examples} for a sample |
| \emph{int main()} function if you want some custom logic and not satisfied |
| with the supplied \emph{converter-sample.c}. |
| |
| \clearpage{} |
| \section{\label{sec:Command-line-options}Command line options} |
| |
| The following table summarizes the asn1c command line options. |
| |
| \renewcommand{\arraystretch}{1.33} |
| \begin{longtable}{lp{4in}} |
| \textbf{Stage Selection Options} & \textbf{Description}\\ |
| \midrule |
| {\ttfamily -E} & {\small Stop after the parsing stage and print the reconstructed ASN.1 |
| specification code to the standard output.}\\ |
| {\ttfamily -F} & {\small Used together with \texttt{-E}, instructs the compiler to stop after |
| the ASN.1 syntax tree fixing stage and dump the reconstructed ASN.1 |
| specification to the standard output.}\\ |
| {\ttfamily -P} & {\small Dump the compiled output to the standard output instead of |
| creating the target language files on disk.}\\ |
| {\ttfamily -R} & {\small Restrict the compiler to generate only the ASN.1 tables, omitting the usual support code.}\\ |
| {\ttfamily -S~\emph{<directory>}} & {\small Use the specified directory with ASN.1 skeleton files.}\\ |
| {\ttfamily -X} & {\small Generate the XML DTD for the specified ASN.1 modules.}\\\\ |
| \textbf{Warning Options} & \textbf{Description}\\ |
| \midrule |
| {\ttfamily -Werror} & {\small Treat warnings as errors; abort if any warning is produced.}\\ |
| {\ttfamily -Wdebug-lexer} & {\small Enable lexer debugging during the ASN.1 parsing stage.}\\ |
| {\ttfamily -Wdebug-fixer} & {\small Enable ASN.1 syntax tree fixer debugging during the fixing stage.}\\ |
| {\ttfamily -Wdebug-compiler} & {\small Enable debugging during the actual compile time.}\\ \\ |
| \textbf{Language Options} & \textbf{Description}\\ |
| \midrule |
| {\ttfamily -fbless-SIZE} & {\small Allow SIZE() constraint for INTEGER, ENUMERATED, and other types for which this constraint is normally prohibited by the standard. |
| This is a violation of an ASN.1 standard and compiler may fail to produce the meaningful code.}\\ |
| {\ttfamily -fcompound-names} & {\small Use complex names for C structures. Using complex names prevents |
| name clashes in case the module reuses the same identifiers in multiple |
| contexts.}\\ |
| {\ttfamily -findirect-choice} & {\small When generating code for a CHOICE type, compile the CHOICE |
| members as indirect pointers instead of declaring them inline. Consider |
| using this option together with \texttt{-fno-include-deps} |
| to prevent circular references.}\\ |
| {\ttfamily -fincludes-quoted} & {\small Generate \#include lines in "double" instead of <angle> quotes.}\\ |
| {\ttfamily -fknown-extern-type=\emph{<name>}} & {\small Pretend the specified type is known. The compiler will assume |
| the target language source files for the given type have been provided |
| manually. }\\ |
| {\ttfamily -fline-refs} & {\small Include ASN.1 module's line numbers in generated code comments.}\\ |
| {\ttfamily -fno-constraints} & {\small Do not generate ASN.1 subtype constraint checking code. This |
| may produce a shorter executable.}\\ |
| {\ttfamily -fno-include-deps} & {\small Do not generate courtesy \#include lines for non-critical dependencies.}\\ |
| {\ttfamily -funnamed-unions} & {\small Enable unnamed unions in the definitions of target language's structures.}\\ |
| {\ttfamily -fwide-types} & {\small Use the wide integer types (INTEGER\_t, REAL\_t) instead of machine's native data types (long, double). }\\\\ |
| \textbf{Codecs Generation Options} & \textbf{Description}\\ |
| \midrule |
| {\ttfamily -gen-OER} & {\small Generate the Octet Encoding Rules (OER) support code.}\\ |
| {\ttfamily -gen-PER} & {\small Generate the Packed Encoding Rules (PER) support code.}\\ |
| {\ttfamily -pdu=\{\textbf{all}|\textbf{auto}|\emph{Type}\}} & {\small Create a PDU table for specified types, or discover the Protocol Data Units automatically. |
| In case of \texttt{-pdu=\textbf{all}}, all ASN.1 types defined in all modules wil form a PDU table. In case of \texttt{-pdu=\textbf{auto}}, all types not referenced by any other type will form a PDU table. If \texttt{\emph{Type}} is an ASN.1 type identifier, it is added to a PDU table. The last form may be specified multiple times.}\\ \\ |
| \textbf{Output Options} & \textbf{Description}\\ |
| \midrule |
| {\ttfamily -print-constraints} & {\small When \texttt{-EF} are also specified, this option forces the compiler |
| to explain its internal understanding of subtype constraints.}\\ |
| {\ttfamily -print-lines} & {\small Generate \texttt{``-{}- \#line''} comments |
| in \texttt{-E} output.}\\ |
| \end{longtable} |
| \renewcommand{\arraystretch}{1} |
| |
| |
| \chapter{Using the ASN.1 Compiler} |
| |
| |
| \section[Invoking the helper code]{Invoking the ASN.1 helper code} |
| |
| First of all, you should include one or more header files into your |
| application. Typically, it is enough to include the header file of |
| the main PDU type. For our Rectangle module, including the Rectangle.h |
| file is sufficient: |
| \begin{codesample} |
| #include <Rectangle.h> |
| \end{codesample} |
| The header files defines the C structure corresponding to the ASN.1 |
| definition of a rectangle and the declaration of the ASN.1 type descriptor, |
| which is used as an argument to most of the functions provided by |
| the ASN.1 module. For example, here is the code which frees the Rectangle\_t |
| structure: |
| \begin{codesample} |
| Rectangle_t *rect = ...; |
| |
| ASN_STRUCT_FREE(asn_DEF_Rectangle, rect); |
| \end{codesample} |
| This code defines a \emph{rect} pointer which points to the Rectangle\_t |
| structure which needs to be freed. The second line uses the generic |
| ASN\_STRUCT\_FREE() macro which invokes the memory deallocation routine |
| created specifically for this Rectangle\_t structure. |
| The \emph{asn\_DEF\_Rectangle} is the type descriptor which holds |
| a collection of routines and operations defined for the Rectangle\_t structure. |
| |
| The following member functions of the asn\_DEF\_Rectangle type descriptor |
| are of interest: |
| \begin{description} |
| \item [{ber\_decoder}] This is the generic \emph{restartable}% |
| \footnote{Restartability mean that if the decoder encounters the end of the buffer it may be invoked again with the rest of the |
| buffer to continue decoding.} |
| BER decoder (Basic Encoding Rules). This decoder would create and/or |
| fill the target structure for you. See Section~\ref{sub:Decoding-BER}. |
| \item [{der\_encoder}] This is the generic DER encoder (Distinguished Encoding |
| Rules). This encoder will take the target structure and encode it |
| into a series of bytes. See Section~\ref{sub:Encoding-DER}. NOTE: |
| DER encoding is a subset of BER. Any BER decoder should be able to |
| handle DER input. |
| \item [{xer\_decoder}] This is the generic XER decoder. It takes both BASIC-XER |
| or CANONICAL-XER encodings and deserializes the data into a local, |
| machine-dependent representation. See Section~\ref{sub:Decoding-XER}. |
| \item [{xer\_encoder}] This is the XER encoder (XML Encoding Rules). This |
| encoder will take the target structure and represent it as an XML |
| (text) document using either BASIC-XER or CANONICAL-XER encoding rules. |
| See Section~\ref{sub:Encoding-XER}. |
| \item [{uper\_decoder}] This is the Unaligned PER decoder. |
| \item [{uper\_encoder}] This is the Unaligned Basic PER encoder. This encoder |
| will take the target structure and encode it into a series of bytes. |
| \item [{check\_constraints}] Check that the contents of the target structure |
| are semantically valid and constrained to appropriate implicit or |
| explicit subtype constraints. See Section~\ref{sub:Validating-the-target}. |
| \item [{print\_struct}] This function convert the contents of the passed |
| target structure into human readable form. This form is not formal |
| and cannot be converted back into the structure, but it may turn out |
| to be useful for debugging or quick-n-dirty printing. See Section~\ref{sub:Printing-the-target}. |
| \item [{free\_struct}] This is a generic disposal which frees the target |
| structure. See Section~\ref{sub:Freeing-the-target}. |
| \end{description} |
| Each of the above function takes the type descriptor (\emph{asn\_DEF\_\ldots{}}) |
| and the target structure (\emph{rect}, in the above example). |
| |
| |
| \subsection{\label{sub:Decoding-BER}Decoding BER} |
| |
| The Basic Encoding Rules describe the most widely used (by the ASN.1 |
| community) way to encode and decode a given structure in a machine-independent |
| way. Several other encoding rules (CER, DER) define a more restrictive |
| versions of BER, so the generic BER parser is also capable of decoding |
| the data encoded by CER and DER encoders. The opposite is not true. |
| |
| \emph{The ASN.1 compiler provides the generic BER decoder which is |
| capable of decoding BER, CER and DER encoded data.} |
| |
| The decoder is restartable (stream-oriented), which means that in |
| case the buffer has less data than it is expected, the decoder will |
| process whatever there is available and ask for more data to be provided. |
| Please note that the decoder may actually process less data than it |
| was given in the buffer, which means that you must be able to make |
| the next buffer contain the unprocessed part of the previous buffer. |
| |
| Suppose, you have two buffers of encoded data: 100 bytes and 200 bytes. |
| \begin{itemize} |
| \item You can concatenate these buffers and feed the BER decoder with 300 |
| bytes of data, or |
| \item You can feed it the first buffer of 100 bytes of data, realize that |
| the ber\_decoder consumed only 95 bytes from it and later feed the |
| decoder with 205 bytes buffer which consists of 5 unprocessed bytes |
| from the first buffer and the additional 200 bytes from the second |
| buffer. |
| \end{itemize} |
| This is not as convenient as it could be (the BER encoder could |
| consume the whole 100 bytes and keep these 5 bytes in some temporary |
| storage), but in case of existing stream based processing it might |
| actually fit well into existing algorithm. Suggestions are welcome. |
| |
| Here is the simplest example of BER decoding: |
| |
| \begin{codesample} |
| Rectangle_t * |
| simple_deserializer(const void *buffer, size_t buf_size) { |
| asn_dec_rval_t rval; |
| Rectangle_t *%$\underbracket{\textrm{\listingfont rect = 0}}$%; /* %\textbf{\color{red}Note this 0\footnote{Forgetting to properly initialize the pointer to a destination structure is a major source of support requests.}!}% */ |
| |
| rval = %\textbf{asn\_DEF\_Rectangle.ber\_decoder}%(0, |
| &asn_DEF_Rectangle, |
| (void **) %$\underbracket{\textrm{\listingfont \&rect}}$%, /* Decoder %\emph{moves}% the pointer */ |
| buffer, buf_size, 0); |
| |
| if(rval%\textbf{.code}% == RC_OK) { |
| return rect; /* Decoding succeeded */ |
| } else { |
| /* Free partially decoded rect */ |
| ASN_STRUCT_FREE(asn_DEF_Rectangle, rect); |
| return 0; |
| } |
| } |
| \end{codesample} |
| The code above defines a function, \emph{simple\_deserializer}, which |
| takes a buffer and its length and is expected to return a pointer |
| to the Rectangle\_t structure. Inside, it tries to convert the bytes |
| passed into the target structure (rect) using the BER decoder and |
| returns the rect pointer afterwards. If the structure cannot be deserialized, |
| it frees the memory which might be left allocated by the unfinished |
| \emph{ber\_decoder} routine and returns 0 (no data). (This \textbf{freeing |
| is necessary} because the ber\_decoder is a restartable procedure, |
| and may fail just because there is more data needs to be provided |
| before decoding could be finalized). The code above obviously does |
| not take into account the way the \emph{ber\_decoder()} failed, so |
| the freeing is necessary because the part of the buffer may already |
| be decoded into the structure by the time something goes wrong. |
| |
| A little less wordy would be to invoke a globally available \emph{ber\_decode()} |
| function instead of dereferencing the asn\_DEF\_Rectangle type descriptor: |
| \begin{codesample} |
| rval = ber_decode(0, &asn_DEF_Rectangle, (void **)&rect, buffer, buf_size); |
| \end{codesample} |
| Note that the initial (asn\_DEF\_Rectangle.ber\_decoder) reference |
| is gone, and also the last argument (0) is no longer necessary. |
| |
| These two ways of BER decoder invocations are fully equivalent. |
| |
| The BER de\emph{coder} may fail because of (\emph{the following RC\_\ldots{} |
| codes are defined in ber\_decoder.h}): |
| \begin{itemize} |
| \item RC\_WMORE: There is more data expected than it is provided (stream |
| mode continuation feature); |
| \item RC\_FAIL: General failure to decode the buffer; |
| \item \ldots{} other codes may be defined as well. |
| \end{itemize} |
| Together with the return code (.code) the asn\_dec\_rval\_t type contains |
| the number of bytes which is consumed from the buffer. In the previous |
| hypothetical example of two buffers (of 100 and 200 bytes), the first |
| call to ber\_decode() would return with .code = RC\_WMORE and .consumed |
| = 95. The .consumed field of the BER decoder return value is \textbf{always} |
| valid, even if the decoder succeeds or fails with any other return |
| code. |
| |
| Look into ber\_decoder.h for the precise definition of ber\_decode() |
| and related types. |
| |
| |
| \subsection{\label{sub:Encoding-DER}Encoding DER} |
| |
| The Distinguished Encoding Rules is the \emph{canonical} variant of |
| BER encoding rules. The DER is best suited to encode the structures |
| where all the lengths are known beforehand. This is probably exactly |
| how you want to encode: either after a BER decoding or after a manual |
| fill-up, the target structure contains the data which size is implicitly |
| known before encoding. Among other uses, the DER encoding is used |
| to encode X.509 certificates. |
| |
| As with BER decoder, the DER encoder may be invoked either directly |
| from the ASN.1 type descriptor (asn\_DEF\_Rectangle) or from the stand-alone |
| function, which is somewhat simpler: |
| \begin{codesample} |
| /* |
| * This is the serializer itself. |
| * It supplies the DER encoder with the |
| * pointer to the custom output function. |
| */ |
| ssize_t |
| simple_serializer(FILE *ostream, Rectangle_t *rect) { |
| asn_enc_rval_t er; /* Encoder return value */ |
| |
| er = der_encode(&asn_DEF_Rect, rect, write_stream, ostream); |
| if(er%\textbf{.encoded}% == -1) { |
| fprintf(stderr, "Cannot encode %\%%s: %\%%s\n", |
| er%\textbf{.failed\_type}%->name, strerror(errno)); |
| return -1; |
| } else { |
| /* Return the number of bytes */ |
| return er.encoded; |
| } |
| } |
| \end{codesample} |
| As you see, the DER encoder does not write into some sort of buffer |
| or something. It just invokes the custom function (possible, multiple |
| times) which would save the data into appropriate storage. The optional |
| argument \emph{app\_key} is opaque for the DER encoder code and just |
| used by \emph{\_write\_stream()} as the pointer to the appropriate |
| output stream to be used. |
| |
| If the custom write function is not given (passed as 0), then the |
| DER encoder will essentially do the same thing (i.~e., encode the data) |
| but no callbacks will be invoked (so the data goes nowhere). It may |
| prove useful to determine the size of the structure's encoding before |
| actually doing the encoding% |
| \footnote{It is actually faster too: the encoder might skip over some computations |
| which aren't important for the size determination.% |
| }. |
| |
| Look into der\_encoder.h for the precise definition of der\_encode() |
| and related types. |
| |
| |
| \subsection{\label{sub:Encoding-XER}Encoding XER} |
| |
| The XER stands for XML Encoding Rules, where XML, in turn, is eXtensible |
| Markup Language, a text-based format for information exchange. The |
| encoder routine API comes in two flavors: stdio-based and callback-based. |
| With the callback-based encoder, the encoding process is very similar |
| to the DER one, described in Section~\ref{sub:Encoding-DER}. The |
| following example uses the definition of write\_stream() from up there. |
| \begin{codesample} |
| /* |
| * This procedure generates the XML document |
| * by invoking the XER encoder. |
| * NOTE: Do not copy this code verbatim! |
| * If the stdio output is necessary, |
| * use the xer_fprint() procedure instead. |
| * See Section~%\ref{sub:Printing-the-target}%. |
| */ |
| int |
| print_as_XML(FILE *ostream, Rectangle_t *rect) { |
| asn_enc_rval_t er; /* Encoder return value */ |
| |
| er = xer_encode(&asn_DEF_Rectangle, rect, |
| XER_F_BASIC, /* BASIC-XER or CANONICAL-XER */ |
| write_stream, ostream); |
| |
| return (er.encoded == -1) ? -1 : 0; |
| } |
| \end{codesample} |
| Look into xer\_encoder.h for the precise definition of xer\_encode() |
| and related types. |
| |
| See Section~\ref{sub:Printing-the-target} for the example of stdio-based |
| XML encoder and other pretty-printing suggestions. |
| |
| |
| \subsection{\label{sub:Decoding-XER}Decoding XER} |
| |
| The data encoded using the XER rules can be subsequently decoded using |
| the xer\_decode() API call: |
| \begin{codesample} |
| Rectangle_t * |
| XML_to_Rectangle(const void *buffer, size_t buf_size) { |
| asn_dec_rval_t rval; |
| Rectangle_t *%$\underbracket{\textrm{\listingfont rect = 0}}$%; /* %\textbf{\color{red}Note this 0\footnote{Forgetting to properly initialize the pointer to a destination structure is a major source of support requests.}!}% */ |
| |
| rval = xer_decode(0, &asn_DEF_Rectangle, (void **)&rect, buffer, buf_size); |
| |
| if(rval%\textbf{.code}% == RC_OK) { |
| return rect; /* Decoding succeeded */ |
| } else { |
| /* Free partially decoded rect */ |
| ASN_STRUCT_FREE(asn_DEF_Rectangle, rect); |
| return 0; |
| } |
| } |
| \end{codesample} |
| The decoder takes both BASIC-XER and CANONICAL-XER encodings. |
| |
| The decoder shares its data consumption properties with BER decoder; |
| please read the Section~\ref{sub:Decoding-BER} to know more. |
| |
| Look into xer\_decoder.h for the precise definition of xer\_decode() |
| and related types. |
| |
| |
| \subsection{\label{sub:Validating-the-target}Validating the target structure} |
| |
| Sometimes the target structure needs to be validated. For example, |
| if the structure was created by the application (as opposed to being |
| decoded from some external source), some important information required |
| by the ASN.1 specification might be missing. On the other hand, the |
| successful decoding of the data from some external source does not |
| necessarily mean that the data is fully valid either. It might well |
| be the case that the specification describes some subtype constraints |
| that were not taken into account during decoding, and it would actually |
| be useful to perform the last check when the data is ready to be encoded |
| or when the data has just been decoded to ensure its validity according |
| to some stricter rules. |
| |
| The asn\_check\_constraints() function checks the type for various |
| implicit and explicit constraints. It is recommended to use asn\_check\_constraints() |
| function after each decoding and before each encoding. |
| |
| Look into constraints.h for the precise definition of asn\_check\_constraints() |
| and related types. |
| |
| |
| \subsection{\label{sub:Printing-the-target}Printing the target structure} |
| |
| There are two ways to print the target structure: either invoke the |
| print\_struct member of the ASN.1 type descriptor, or using the asn\_fprint() |
| function, which is a simpler wrapper of the former: |
| \begin{codesample} |
| asn_fprint(stdout, &asn_DEF_Rectangle, rect); |
| \end{codesample} |
| Look into constr\_TYPE.h for the precise definition of asn\_fprint() |
| and related types. |
| |
| Another practical alternative to this custom format printing would |
| be to invoke XER encoder. The default BASIC-XER encoder performs reasonable |
| formatting for the output to be useful and human readable. To invoke |
| the XER decoder in a manner similar to asn\_fprint(), use the xer\_fprint() |
| call: |
| \begin{codesample} |
| xer_fprint(stdout, &asn_DEF_Rectangle, rect); |
| \end{codesample} |
| See Section~\ref{sub:Encoding-XER} for XML-related details. |
| |
| |
| \subsection{\label{sub:Freeing-the-target}Freeing the target structure} |
| |
| Freeing the structure is slightly more complex than it may seem to. |
| When the ASN.1 structure is freed, all the members of the structure |
| and their submembers are recursively freed as well. But it might not |
| be feasible to free the structure itself. Consider the following case: |
| \begin{codesample} |
| struct my_figure { /* The custom structure */ |
| int flags; /* <some custom member> */ |
| /* The type is generated by the ASN.1 compiler */ |
| Rectangle_t rect; |
| /* other members of the structure */ |
| }; |
| \end{codesample} |
| In this example, the application programmer defines a custom structure |
| with one ASN.1-derived member (rect). This member is not a reference |
| to the Rectangle\_t, but an in-place inclusion of the Rectangle\_t |
| structure. If the freeing is necessary, the usual procedure of freeing |
| everything must not be applied to the \&rect pointer itself, because |
| it does not point to the memory block directly allocated by the memory |
| allocation routine, but instead lies within a block allocated for |
| the my\_figure structure. |
| |
| To solve this problem, in addition to ASN\_STRUCT\_FREE macro, the asn1c |
| skeletons define the ASN\_STRUCT\_RESET macro which doesn't free the passed |
| pointer and instead resets the structure into the clean and safe state. |
| \begin{codesample} |
| /* %\textbf{1. Rectangle\_t is defined within my\_figure}% */ |
| struct my_figure { |
| Rectangle_t rect; |
| } *mf = ...; |
| /* |
| * Freeing the Rectangle_t |
| * without freeing the mf->rect area. |
| */ |
| ASN_STRUCT_RESET(asn_DEF_Rectangle, &mf->rect); |
| |
| /* %\textbf{2. Rectangle\_t is a stand-alone pointer}% */ |
| Rectangle_t *rect = ...; |
| /* |
| * Freeing the Rectangle_t |
| * and freeing the rect pointer. |
| */ |
| ASN_STRUCT_FREE(asn_DEF_Rectangle, &mf->rect); |
| \end{codesample} |
| It is safe to invoke both macros with the target structure pointer |
| set to 0 (NULL). In this case, the function will do nothing. |
| |
| \chapter{\label{cha:Step-by-step-examples}Step by step examples} |
| |
| |
| \section{A ``Rectangle'' Encoder} |
| |
| This example will help you create a simple BER and XER encoder of |
| a ``Rectangle'' type used throughout this document. |
| \begin{enumerate} |
| \item Create a file named \textbf{rectangle.asn1} with the following contents: |
| |
| \begin{asn} |
| RectangleModule1 DEFINITIONS ::= BEGIN |
| |
| Rectangle ::= SEQUENCE { |
| height INTEGER, |
| width INTEGER |
| } |
| |
| END |
| \end{asn} |
| \item Compile it into the set of .c and .h files using asn1c compiler \cite{ASN1C}: |
| |
| \begin{bash} |
| asn1c %\textbf{rectangle.asn1}% |
| \end{bash} |
| \item Alternatively, use the Online ASN.1 compiler \cite{AONL} by uploading |
| the \textbf{rectangle.asn1} file into the Web form and unpacking the |
| produced archive on your computer. |
| \item By this time, you should have gotten multiple files in the current |
| directory, including the \textbf{Rectangle.c} and \textbf{Rectangle.h}. |
| \item Create a main() routine which creates the Rectangle\_t structure in |
| memory and encodes it using BER and XER encoding rules. Let's name |
| the file \textbf{main.c}: |
| |
| \begin{codesample}[basicstyle=\scriptsize\listingfont] |
| #include <stdio.h> |
| #include <sys/types.h> |
| #include <Rectangle.h> /* Rectangle ASN.1 type */ |
| |
| /* Write the encoded output into some FILE stream. */ |
| static int write_out(const void *buffer, size_t size, void *app_key) { |
| FILE *out_fp = app_key; |
| size_t wrote = fwrite(buffer, 1, size, out_fp); |
| return (wrote == size) ? 0 : -1; |
| } |
| |
| int main(int ac, char **av) { |
| Rectangle_t *rectangle; /* Type to encode */ |
| asn_enc_rval_t ec; /* Encoder return value */ |
| |
| /* Allocate the Rectangle_t */ |
| rectangle = calloc(1, sizeof(Rectangle_t)); /* not malloc! */ |
| if(!rectangle) { |
| perror("calloc() failed"); |
| exit(1); |
| } |
| |
| /* Initialize the Rectangle members */ |
| rectangle->height = 42; /* any random value */ |
| rectangle->width = 23; /* any random value */ |
| |
| /* BER encode the data if filename is given */ |
| if(ac < 2) { |
| fprintf(stderr, "Specify filename for BER output\n"); |
| } else { |
| const char *filename = av[1]; |
| FILE *fp = fopen(filename, "wb"); /* for BER output */ |
| |
| if(!fp) { |
| perror(filename); |
| exit(1); |
| } |
| |
| /* Encode the Rectangle type as BER (DER) */ |
| ec = der_encode(&asn_DEF_Rectangle, rectangle, write_out, fp); |
| fclose(fp); |
| if(ec.encoded == -1) { |
| fprintf(stderr, "Could not encode Rectangle (at %\%%s)\n", |
| ec.failed_type ? ec.failed_type->name : "unknown"); |
| exit(1); |
| } else { |
| fprintf(stderr, "Created %\%%s with BER encoded Rectangle\n", filename); |
| } |
| } |
| |
| /* Also print the constructed Rectangle XER encoded (XML) */ |
| xer_fprint(stdout, &asn_DEF_Rectangle, rectangle); |
| |
| return 0; /* Encoding finished successfully */ |
| } |
| \end{codesample} |
| \item Compile all files together using C compiler (varies by platform): |
| |
| \begin{bash} |
| cc -I. -o %\textbf{\emph{rencode}} \emph{*.c}% |
| \end{bash} |
| \item Voila! You have just created the BER and XER encoder of a Rectangle |
| type, named \textbf{rencode}! |
| \end{enumerate} |
| |
| \section{\label{sec:A-Rectangle-Decoder}A ``Rectangle'' Decoder} |
| |
| This example will help you to create a simple BER decoder of a simple |
| ``Rectangle'' type used throughout this document. |
| \begin{enumerate} |
| \item Create a file named \textbf{rectangle.asn1} with the following contents: |
| |
| \begin{asn} |
| RectangleModule1 DEFINITIONS ::= BEGIN |
| |
| Rectangle ::= SEQUENCE { |
| height INTEGER, |
| width INTEGER |
| } |
| |
| END |
| \end{asn} |
| \item Compile it into the set of .c and .h files using asn1c compiler \cite{ASN1C}: |
| |
| \begin{bash} |
| asn1c %\textbf{rectangle.asn1}% |
| \end{bash} |
| \item Alternatively, use the Online ASN.1 compiler \cite{AONL} by uploading |
| the \textbf{rectangle.asn1} file into the Web form and unpacking the |
| produced archive on your computer. |
| \item By this time, you should have gotten multiple files in the current |
| directory, including the \textbf{Rectangle.c} and \textbf{Rectangle.h}. |
| \item Create a main() routine which takes the binary input file, decodes |
| it as it were a BER-encoded Rectangle type, and prints out the text |
| (XML) representation of the Rectangle type. Let's name the file \textbf{main.c}: |
| |
| \begin{codesample}[basicstyle=\scriptsize\listingfont] |
| #include <stdio.h> |
| #include <sys/types.h> |
| #include <Rectangle.h> /* Rectangle ASN.1 type */ |
| |
| int main(int ac, char **av) { |
| char buf[1024]; /* Temporary buffer */ |
| asn_dec_rval_t rval; /* Decoder return value */ |
| Rectangle_t *%$\underbracket{\textrm{\listingfont rectangle = 0}}$%; /* Type to decode. %\textbf{\color{red}Note this 0\footnote{Forgetting to properly initialize the pointer to a destination structure is a major source of support requests.}!}% */ |
| FILE *fp; /* Input file handler */ |
| size_t size; /* Number of bytes read */ |
| char *filename; /* Input file name */ |
| |
| /* Require a single filename argument */ |
| if(ac != 2) { |
| fprintf(stderr, "Usage: %\%%s <file.ber>\n", av[0]); |
| exit(1); |
| } else { |
| filename = av[1]; |
| } |
| |
| /* Open input file as read-only binary */ |
| fp = fopen(filename, "rb"); |
| if(!fp) { |
| perror(filename); |
| exit(1); |
| } |
| |
| /* Read up to the buffer size */ |
| size = fread(buf, 1, sizeof(buf), fp); |
| fclose(fp); |
| if(!size) { |
| fprintf(stderr, "%\%%s: Empty or broken\n", filename); |
| exit(1); |
| } |
| |
| /* Decode the input buffer as Rectangle type */ |
| rval = ber_decode(0, &asn_DEF_Rectangle, (void **)&rectangle, buf, size); |
| if(rval.code != RC_OK) { |
| fprintf(stderr, "%\%%s: Broken Rectangle encoding at byte %\%%ld\n", filename, (long)rval.consumed); |
| exit(1); |
| } |
| |
| /* Print the decoded Rectangle type as XML */ |
| xer_fprint(stdout, &asn_DEF_Rectangle, rectangle); |
| |
| return 0; /* Decoding finished successfully */ |
| } |
| \end{codesample} |
| \item Compile all files together using C compiler (varies by platform): |
| |
| \begin{bash} |
| cc -I. -o %\textbf{\emph{rdecode}} \emph{*.c}% |
| \end{bash} |
| \item Voila! You have just created the BER decoder of a Rectangle type, |
| named \textbf{rdecode}! |
| \end{enumerate} |
| |
| \chapter{Constraint validation examples} |
| |
| This chapter shows how to define ASN.1 constraints and use the generated |
| validation code. |
| |
| |
| \section{Adding constraints into ``Rectangle'' type} |
| |
| This example shows how to add basic constraints to the ASN.1 specification |
| and how to invoke the constraints validation code in your application. |
| \begin{enumerate} |
| \item Create a file named \textbf{rectangle.asn1} with the following contents: |
| |
| \begin{asn} |
| RectangleModuleWithConstraints DEFINITIONS ::= BEGIN |
| |
| Rectangle ::= SEQUENCE { |
| height INTEGER (0..100), -- Value range constraint |
| width INTEGER (0..MAX) -- Makes width non-negative |
| } |
| |
| END |
| \end{asn} |
| \item Compile the file according to procedures shown in the previous chapter. |
| \item Modify the Rectangle type processing routine (you can start with the |
| main() routine shown in the Section~\ref{sec:A-Rectangle-Decoder}) |
| by placing the following snippet of code \emph{before} encoding and/or |
| \emph{after} decoding the Rectangle type% |
| \footnote{Placing the constraint checking code \emph{before encoding} helps |
| to make sure the data is correct and within constraints before |
| sharing the data with anyone else. |
| Placing the constraint checking code \emph{after decoding} helps to make sure |
| the application got the valid contents before making use of it.% |
| }: |
| |
| \begin{codesample} |
| int ret; /* Return value */ |
| char errbuf[128]; /* Buffer for error message */ |
| size_t errlen = sizeof(errbuf); /* Size of the buffer */ |
| |
| /* ... here goes the Rectangle %\emph{decoding}% code ... */ |
| |
| ret = asn_check_constraints(&asn_DEF_Rectangle, rectangle, errbuf, &errlen); |
| /* assert(errlen < sizeof(errbuf)); // you may rely on that */ |
| if(ret) { |
| fprintf(stderr, "Constraint validation failed: %\%%s\n", |
| errbuf /* errbuf is properly nul-terminated */ |
| ); |
| /* exit(...); // Replace with appropriate action */ |
| } |
| |
| /* ... here goes the Rectangle %\emph{encoding}% code ... */ |
| \end{codesample} |
| \item Compile the resulting C code as shown in the previous chapters. |
| \item Try to test the constraints checking code by assigning integer value |
| 101 to the \textbf{.height} member of the Rectangle structure, or |
| a negative value to the \textbf{.width} member. In either case, the |
| program should print ``Constraint validation failed'' message, followed |
| by a short explanation why validation did not succeed. |
| \item Done. |
| \end{enumerate} |
| |
| \part{\label{par:ASN.1-Basics}ASN.1 Basics} |
| |
| |
| \chapter{\label{cha:Abstract-Syntax-Notation:}Abstract Syntax Notation: ASN.1} |
| |
| \emph{This chapter defines some basic ASN.1 concepts and describes |
| several most widely used types. It is by no means an authoritative |
| or complete reference. For more complete ASN.1 description, please |
| refer to Olivier Dubuisson's book \cite{Dub00} or the ASN.1 body |
| of standards itself \cite{ITU-T/ASN.1}.} |
| |
| The Abstract Syntax Notation One is used to formally describe the |
| data transmitted across the network. Two communicating parties may employ |
| different formats of their native data types (e.~g., different number |
| of bits for the native integer type), thus it is important to have |
| a way to describe the data in a manner which is independent from the |
| particular machine's representation. |
| The ASN.1 specifications are used to achieve the following: |
| \begin{itemize} |
| \item The specification expressed in the ASN.1 notation is a formal and |
| precise way to communicate the structure of data to human readers; |
| \item The ASN.1 specifications may be used as input for automatic compilers |
| which produce the code for some target language (C, C++, Java, etc) |
| to encode and decode the data according to some encoding formats. |
| Several such encoding formats (called Transfer Encoding Rules) |
| have been defined by the ASN.1 standard. |
| \end{itemize} |
| Consider the following example: |
| \begin{asn} |
| Rectangle ::= SEQUENCE { |
| height INTEGER, |
| width INTEGER |
| } |
| \end{asn} |
| This ASN.1 specification describes a constructed type, \emph{Rectangle}, |
| containing two integer fields. This specification may tell the reader |
| that there exists this kind of data structure and that some entity |
| may be prepared to send or receive it. The question on \emph{how} |
| that entity is going to send or receive the \emph{encoded data} is |
| outside the scope of ASN.1. For example, this data structure may be |
| encoded according to some encoding rules and sent to the destination |
| using the TCP protocol. The ASN.1 specifies several ways of encoding |
| (or ``serializing'', or ``marshaling'') the data: BER, PER, XER |
| and others, including CER and DER derivatives from BER. |
| |
| The complete specification must be wrapped in a module, which looks |
| like this: |
| \begin{asn} |
| RectangleModule1 |
| { iso org(3) dod(6) internet(1) private(4) |
| enterprise(1) spelio(9363) software(1) |
| asn1c(5) docs(2) rectangle(1) 1 } |
| DEFINITIONS AUTOMATIC TAGS ::= |
| BEGIN |
| |
| -- This is a comment which describes nothing. |
| Rectangle ::= SEQUENCE { |
| height INTEGER, -- Height of the rectangle |
| width INTEGER -- Width of the rectangle |
| } |
| |
| END |
| \end{asn} |
| The module header consists of module name (RectangleModule1), the |
| module object identifier (\{...\}), a keyword ``DEFINITIONS'', a |
| set of module flags (AUTOMATIC TAGS) and ``::= BEGIN''. The module |
| ends with an ``END'' statement. |
| |
| |
| \section{Some of the ASN.1 Basic Types} |
| |
| |
| \subsection{The BOOLEAN type} |
| |
| The BOOLEAN type models the simple binary TRUE/FALSE, YES/NO, ON/OFF |
| or a similar kind of two-way choice. |
| |
| |
| \subsection{The INTEGER type} |
| |
| The INTEGER type is a signed natural number type without any restrictions |
| on its size. If the automatic checking on INTEGER value bounds are |
| necessary, the subtype constraints must be used. |
| \begin{asn} |
| SimpleInteger ::= INTEGER |
| |
| -- An integer with a very limited range |
| SmallPositiveInt ::= INTEGER (0..127) |
| |
| -- Integer, negative |
| NegativeInt ::= INTEGER (MIN..0) |
| \end{asn} |
| |
| \subsection{The ENUMERATED type} |
| |
| The ENUMERATED type is semantically equivalent to the INTEGER type |
| with some integer values explicitly named. |
| \begin{asn} |
| FruitId ::= ENUMERATED { apple(1), orange(2) } |
| |
| -- The numbers in braces are optional, |
| -- the enumeration can be performed |
| -- automatically by the compiler |
| ComputerOSType ::= ENUMERATED { |
| FreeBSD, -- acquires value 0 |
| Windows, -- acquires value 1 |
| Solaris(5), -- remains 5 |
| Linux, -- becomes 6 |
| MacOS -- becomes 7 |
| } |
| \end{asn} |
| |
| \subsection{The OCTET STRING type} |
| |
| This type models the sequence of 8-bit bytes. This may be used to |
| transmit some opaque data or data serialized by other types of encoders |
| (e.~g., video file, photo picture, etc). |
| |
| \subsection{The OBJECT IDENTIFIER type} |
| |
| The OBJECT IDENTIFIER is used to represent the unique identifier of |
| any object, starting from the very root of the registration tree. |
| If your organization needs to uniquely identify something (a router, |
| a room, a person, a standard, or whatever), you are encouraged to |
| get your own identification subtree at \url{http://www.iana.org/protocols/forms.htm}. |
| |
| For example, the very first ASN.1 module in this Chapter (RectangleModule1) |
| has the following OBJECT IDENTIFIER: 1 3 6 1 4 1 9363 1 5 2 1 1. |
| \begin{asn} |
| ExampleOID ::= OBJECT IDENTIFIER |
| |
| rectangleModule1-oid ExampleOID |
| ::= { 1 3 6 1 4 1 9363 1 5 2 1 1 } |
| |
| -- An identifier of the Internet. |
| internet-id OBJECT IDENTIFIER |
| ::= { iso(1) identified-organization(3) |
| dod(6) internet(1) } |
| \end{asn} |
| As you see, names are optional. |
| |
| |
| \subsection{The RELATIVE-OID type} |
| |
| The RELATIVE-OID type has the semantics of a subtree of an OBJECT |
| IDENTIFIER. There may be no need to repeat the whole sequence of numbers |
| from the root of the registration tree where the only thing of interest |
| is some of the tree's subsequence. |
| \begin{asn} |
| this-document RELATIVE-OID ::= { docs(2) usage(1) } |
| |
| this-example RELATIVE-OID ::= { |
| this-document assorted-examples(0) this-example(1) } |
| \end{asn} |
| |
| \section{Some of the ASN.1 String Types} |
| |
| |
| \subsection{The IA5String type} |
| |
| This is essentially the ASCII, with 128 character codes available |
| (7 lower bits of an 8-bit byte). |
| |
| |
| \subsection{The UTF8String type} |
| |
| This is the character string which encodes the full Unicode range |
| (4 bytes) using multibyte character sequences. |
| |
| |
| \subsection{The NumericString type} |
| |
| This type represents the character string with the alphabet consisting |
| of numbers (``0'' to ``9'') and a space. |
| |
| |
| \subsection{The PrintableString type} |
| |
| The character string with the following alphabet: space, ``\textbf{'}'' |
| (single quote), ``\textbf{(}'', ``\textbf{)}'', ``\textbf{+}'', |
| ``\textbf{,}'' (comma), ``\textbf{-}'', ``\textbf{.}'', ``\textbf{/}'', |
| digits (``0'' to ``9''), ``\textbf{:}'', ``\textbf{=}'', ``\textbf{?}'', |
| upper-case and lower-case letters (``A'' to ``Z'' and ``a'' |
| to ``z''). |
| |
| |
| \subsection{The VisibleString type} |
| |
| The character string with the alphabet which is more or less a subset |
| of ASCII between the space and the ``\textbf{\textasciitilde{}}'' |
| symbol (tilde). |
| |
| Alternatively, the alphabet may be described as the PrintableString |
| alphabet presented earlier, plus the following characters: ``\textbf{!}'', |
| ``\textbf{``}'', ``\textbf{\#}'', ``\textbf{\$}'', ``\textbf{\%}'', |
| ``\textbf{\&}'', ``\textbf{*}'', ``\textbf{;}'', ``\textbf{<}'', |
| ``\textbf{>}'', ``\textbf{{[}}'', ``\textbf{\textbackslash{}}'', |
| ``\textbf{{]}}'', ``\textbf{\textasciicircum{}}'', ``\textbf{\_}'', |
| ``\textbf{`}`` (single left quote), ``\textbf{\{}'', ``\textbf{|}'', |
| ``\textbf{\}}'', ``\textbf{\textasciitilde{}}''. |
| |
| |
| \section{ASN.1 Constructed Types} |
| |
| |
| \subsection{The SEQUENCE type} |
| |
| This is an ordered collection of other simple or constructed types. |
| The SEQUENCE constructed type resembles the C ``struct'' statement. |
| \begin{asn} |
| Address ::= SEQUENCE { |
| -- The apartment number may be omitted |
| apartmentNumber NumericString OPTIONAL, |
| streetName PrintableString, |
| cityName PrintableString, |
| stateName PrintableString, |
| -- This one may be omitted too |
| zipNo NumericString OPTIONAL |
| } |
| \end{asn} |
| |
| \subsection{The SET type} |
| |
| This is a collection of other simple or constructed types. Ordering |
| is not important. The data may arrive in the order which is different |
| from the order of specification. Data is encoded in the order not |
| necessarily corresponding to the order of specification. |
| |
| |
| \subsection{The CHOICE type} |
| |
| This type is just a choice between the subtypes specified in it. The |
| CHOICE type contains at most one of the subtypes specified, and it |
| is always implicitly known which choice is being decoded or encoded. |
| This one resembles the C ``union'' statement. |
| |
| The following type defines a response code, which may be either an |
| integer code or a boolean ``true''/``false'' code. |
| \begin{asn} |
| ResponseCode ::= CHOICE { |
| intCode INTEGER, |
| boolCode BOOLEAN |
| } |
| \end{asn} |
| |
| \subsection{The SEQUENCE OF type} |
| |
| This one is the list (array) of simple or constructed types: |
| \begin{asn} |
| -- Example 1 |
| ManyIntegers ::= SEQUENCE OF INTEGER |
| |
| -- Example 2 |
| ManyRectangles ::= SEQUENCE OF Rectangle |
| |
| -- More complex example: |
| -- an array of structures defined in place. |
| ManyCircles ::= SEQUENCE OF SEQUENCE { |
| radius INTEGER |
| } |
| \end{asn} |
| |
| \subsection{The SET OF type} |
| |
| The SET OF type models the bag of structures. It resembles the SEQUENCE |
| OF type, but the order is not important. The elements may arrive |
| in the order which is not necessarily the same as the in-memory order |
| on the remote machines. |
| \begin{asn} |
| -- A set of structures defined elsewhere |
| SetOfApples :: SET OF Apple |
| |
| -- Set of integers encoding the kind of a fruit |
| FruitBag ::= SET OF ENUMERATED { apple, orange } |
| \end{asn} |
| \begin{thebibliography}{ITU-T/ASN.1} |
| \bibitem[ASN1C]{ASN1C}The Open Source ASN.1 Compiler. \url{http://lionet.info/asn1c} |
| |
| \bibitem[AONL]{AONL}Online ASN.1 Compiler. \url{http://lionet.info/asn1c/asn1c.cgi} |
| |
| \bibitem[Dub00]{Dub00}Olivier Dubuisson --- \emph{ASN.1 Communication |
| between heterogeneous systems} --- Morgan Kaufmann Publishers, 2000. |
| \url{http://asn1.elibel.tm.fr/en/book/}. ISBN:0-12-6333361-0. |
| |
| \bibitem[ITU-T/ASN.1]{ITU-T/ASN.1}ITU-T Study Group 17 --- Languages |
| for Telecommunication Systems \url{http://www.itu.int/ITU-T/studygroups/com17/languages/} |
| \end{thebibliography} |
| |
| \end{document} |