external data representation (xdr)

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Mobile Computing & Software Engineering Lab

External Data Representation (XDR)

Prof. Chuan-Ming LiuComputer Science and Information Engineering

National Taipei University of TechnologyTaipei, TAIWAN

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IntroductionThis chapter examines a de facto standard for external data representation and presentation as well as a set of library procedures used to perform data conversionThis chapter describes the general motivations for using an external data representation and the details of one particular implementation

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Introduction (cont.)The next chapter shows how an external data representation standard helps simplify client and server communicationIt illustrates how a standard makes it possible to use a single, uniform remote access mechanism for client-server communication

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Representation of DataFig. 20.1 shows little endian (if it stores the LSB of an integer at the lowest memory address) and big endian representation for 32-bit integers Programmers who create client and server software must contend with data representation because both endpoints must agree on the exact representation for all data sent across the communication channel between them; otherwise, conversion is required

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Figure 20.1

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Asynchronous Conversion and the N-Squared Problem

Designs that follow the paradigm of converting directly from one representation to the server’s representation are said to use asymmetric data conversion because only one side needs to perform conversion

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Asynchronous Conversion and the N-Squared Problem

If client-server software is designed to convert between the client’s native data representation and the server’s native data representation asymmetrically, the number of versions of the software grows as the square of the number of architectures (N-Squared Conversion Problem)

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Network Standard Byte OrderThe TCP/IP protocols use the same representation for all the data sent across a network in a protocol header (headers represent integer data in a standard, machine-independent form)

Such designs employ symmetric data conversion

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Network Standard Byte Order (cont.)

Symmetric data representation has an important consequence: only one version of protocol software is neededThe standard representation used for data traversing the network is known as the external data representation

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The chief advantage lies in flexibility: neither the client and the server needs to understand the architecture of the otherThe total programming effort required will be proportional to the number of machine architecturesThe chief disadvantage of symmetric conversion is computation overhead even if the client and the server both operate at the same architecture

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Because the external representation may add information to the data or align it with word boundaries, the conversion may result in a larger stream of bytes than necessaryMost programmers agree that using symmetric conversion is worthwhile

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It simplifies programming, reduces errors, and increases interoperability among programsIt also makes network management and debugging easier because the network manager can interpret the contents of packets without knowing the architectures of the sending and receiving machines

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A De Facto Standard External Data Representation

XDR, Sun’s eXternal Data Representation has become a de facto standard for most client-server applicationsXDR specifies data formats for most of the data types that clients and servers exchange.For example, XDR specifies that 32-bit binary integers should be represented in “big endian”order

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XDR Data TypesFig. 20.2 lists the data types for which XDR defines a standard representationXDR allows the programmer to compose aggregate types from other typesFor example, XDR allows an array of structure, each of which can have multiple fields that can be an array, structure, or unionThus, XDR provides representation for most of the structure that a C programmer can specify

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Figure 20.2

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Implicit TypesThe XDR standard specifies how a data object should be encoded for each of the data types listed in Fig. 20.2.However, the encodings contain only the data items and do not contain additional bits to identify their types.Thus, clients and servers using XDR must agree on the exact format of messages they will exchange.

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Software Support for Using XDR

Programmers perform the conversions:Sending: insert a function call in the code to convert each data item in a message to external form Receiving: and insert a function call to convert each data item to internal form when a message arrives

To eliminate potential conversion errors, an implementation of XDR includes library routines that perform the necessary conversions

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XDR Library RoutinesXDR library routines for a given machine can convert data items from the computer’s native representation to the XDR standard representation and vice versa.Most implementations of XDR use a buffer paradigm that allows a programmer to create a complete message in XDR form.

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Building a Message One Piece at a Time

Buffer paradigmallocate a buffer large enough to hold the external representation of a message and to add item (i.e., fields) one at a timeIn Linux, xdrmem_create

allocates a buffer in a memory and inform XDR that it intends to compose an external representation in itinitializes the memory so it represents an XDR stream that can be used to encode or decode

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Building a Message One Piece at a Time (cont.)

The declarations and calls needed to create an XDR stream using C are shown in Fig. 20.2aOnce a program has created an XDR stream, it can call individual XDR conversion routines to convert native data objects into external form

For example, a program invokes xdr_int by passing it a pointer to an XDR stream and a pointer to an integer, as shown in Fig. 20.2bFig. 20.3 illustrates how the call to xdr_intshown in the sample code adds 4 bytes of data to the XDR stream

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Figure 20.2a

#include <rpc/xdr.h> //size of memory for encoding#define BUFSIZE 4000

XDR *xdrs; //pointer to an XDR “stream”char buf[BUFSIZE];//memory area to hold XDR data

xdrmem_create(xdrs, buf, BUFSIZE, XDR_ENCODE);

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Figure 20.2bint i; //integer in native representation… //assume stream initialized for ENCODEi = 260; //assign integer value to be convertedxdr_int(xdrs, &i); //convert integer and append to stream

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Figure 20.3

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Conversion Routines in the XDR Library

The table in Fig. 20.4 lists the XDR conversion routinesThe receiving application must calls xdrmem_create to create a memory buffer that will hold an XDR stream, and places the incoming message in a buffer area

For example, if the receiver has established an XDR stream used for input (i.e., the call specified XDR_DECODE), it can extract a 32-bit integer and convert it to the native representation by calling xdr_int, as shown in Fig. 20.4

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Figure 20.4

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Conversion Routines in the XDR Library (cont.)

Unlike the conversion, htons and ntohs that Linux provides, individual XDR conversion routines do not specify the direction of conversion.Instead, the direction is specified when creating the XDR stream.

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XDR Stream, I/O, and TCPThe code fragments above create an XDR stream associated with a buffer in memoryAfter items have been converted to external form and placed in the buffer, the application must call an I/O function like write to send it across a TCP connectionIt is possible to arrange to have XDR conversion routines send data across a TCP connection automatically each time they convert a data item to external form

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XDR Stream, I/O, and TCP (cont.)

An application program first creates a TCP socket, and then calls fdopen to attach a standard I/O stream to the socketThe application calls xdrstdio_create to create an XDR stream and connect it to the existing I/O descriptorEach time the application calls an XDR conversion routine, the conversion automatically performs a buffered read or writeoperation using the underlying descriptor

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Records, Record Boundaries, and Datagram I/O

The XDR mechanism works well when connected to a TCP socket because both XDR and TCP implementations use the stream abstractionTo make XDR work with UDP, the alterative design provides an application with a record-oriented interfaceTo use the record-oriented interface, a program calls function xdrrec_create when creating a record-oriented XDR stream

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Records, Record Boundaries, and Datagram I/O (cont.)

The call includes two arguments, inproc and outproc, that specify an input procedure and an output procedureWhen converting to external form, each conversion routine check the bufferIf the buffer becomes full, the conversion routine calls outproc to send the existing buffer contents and make space for the new data

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Each time the application calls a conversion routine to convert from external form to the native representation, the routine checks the buffer to see if it contains dataIf the buffer is empty, the conversion routine calls inproc to obtain more data

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To use XDR with UDP, an application creates a record-oriented XDR streamIt arranges for the input and output procedures associated with the stream to call read and write (or recv and send)Record-oriented streams allow an application to mark record boundaries

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SummaryClient-server interaction can be asymmetricor symmetricAsymmetric conversion requires either the client or the server to convert between its own representation and the other machine’s native representationSymmetric conversions uses a standard network representation and requires both the client and server to convert between the network standard and the local representation

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Summary (cont.)Client and server programs can use XDR routines to convert data to external form before sending it and to internal form after receiving itThe conversion routine can be associated with input and output using TCP or UDPAlthough some protocols in the TCP/IP suite use ASN.1 (Abstract Syntax Notation One, by ISO) representation, most application programmers use XDR

external data representation (xdr)

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