XDR External Data Representation

1
XDRExternal Data Representation
Process A
Process B
XDR Encode/Decode
XDR Encode/Decode
Transport
Transport
2
XDR as a case study

• Sun RPC uses XDR.
• A good example of a layer.
• Interesting API.
• Powerful paradigm for creation of complex data
  structures.

3
XDR

• XDR provides a service associated with the OSI
  Presentation Layer.
• Common data representation
• Library (not part of the O.S.).
• Easy to port to new architectures.
• Independence from transport layer.

4
Data Conversion

• Asymmetric Data Conversion
• client always converts to the servers data
  representation.
• Symmetric Data Conversion
• both client and server convert to some standard
  representation.

5
XDR Data Types

• boolean
• char
• short
• int
• long
• float
• double

• enumeration
• structure
• string
• fixed length array (1-D)
• variable length array (1-D)
• union
• opaque data

6
Implicit vs. Explicit Typing

• Explicit Typing means that each piece of data
  includes information about the type.
• Implicit typing means that the sender and
  receiver must agree on the order and type of all
  data.
• XDR uses Implicit Typing

7
XDR Programming

• Most XDR implementations are based on a buffer
  paradigm.
• A buffer is allocated that is large enough to
  hold an entire message.
• Individual data items are added to the buffer one
  at a time.
• XDR buffers can be attached to a file, pipe,
  socket or memory.

8
Conversion Terminology

• Converting from local representation to XDR
  representation is called Encoding.
• Converting from XDR representation to local
  representation is called Decoding.

Sender
Receiver
DECODE
ENCODE
XDR
9
XDR Buffer Creation

• There are a number of buffer creation functions
  in the XDR library.
• xdrmem_create
• destination for encoding -or- source for
  decoding is a chunk of memory.
• xdrstdio_create
• destination for encoding -or- source for decoding
  is a file descriptor.

10
XDR filters

• The XDR library includes an extensive set of
  filters that perform encoding/decoding
  operations.
• Each XDR stream includes an attribute that
  determines the specific operation that will be
  performed by a filter (encoding or decoding).

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XDR Filters

• The filter to encode/decode an integer is called
  xdr_int
• bool_t xdr_int( XDR xdrs, int ip)
• the return value indicates success or failure.
• xdrs is a pointer to an XDR stream
• ip is a pointer to an integer.

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xdr_int()

• If the XDR stream operation is XDR_ENCODE
• int count
• XDR xstream
• xdr_int(xstream, count)
• will convert (encode) the value of count to the
  integer representation used by XDR (big-endian)
  and put the result on the XDR stream.

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xdr_int()

• If the XDR stream operation is XDR_DECODE
• int count
• XDR xstream
• xdr_int(xstream, count)
• will get an XDR integer from the stream, convert
  (decode) the value to local integer
  representation and put the result in count.

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xdr_int()
Source Encodes
int count xdr_int(xstream,count)
Destintation decodes
int count xdr_int(xstream,count)
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Complex Data Filters

• The XDR library includes a set of filters
  designed to translate complex C data structures
  to and from XDR representation.
• Many of these filters make use of the simpler
  filters to convert individual components.

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xdr_array()

• The xdr_array() filter provides support for
  encoding/decoding a variable length array.
• bool_t xdr_array( XDR xdrs, char arrp,
• u_int sizep, u_int maxsize, u_int elsize,
  xdrproc_t elproc)
• sizep is a pointer to the size of the array.
• elsize is the size of each element (in bytes).
• elproc is a pointer to a function that can
  encode/decode individual array elements.

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xdr_array()
Source Array -gt
0
1
2
3
elproc()
Destination Array -gt
0
1
2
3
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Inside xdr_array()
encode/decode the number of elements in the array

• xdr_int(xdrs,sizep)
• for (i0iltsizepi)
• elproc(xdrs,arrpelsizei)

encode/decode each array element.
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xdr_string()

• the string conversion filter is a little
  different since XDR strings have a maximum size.
• bool_t xdr_string( XDR xdrs,
• char string, u_int maxsize)

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Problem!!

• We want to send an array of strings between
  processes.
• What is the problem (using xdr_array)?
• What is a possible solution?

21
Distributed Program Design
Typical Sockets Approach

• Communication-Oriented Design
• Design protocol first.
• Build programs that adhere to the protocol.
• Application-Oriented Design
• Build application(s).
• Divide programs up and add communication
  protocols.

RPC
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RPCRemote Procedure Call

• Call a procedure (subroutine) that is running on
  another machine.
• Issues
• identifying and accessing the remote procedure
• parameters
• return value

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Server
Client
blah, blah, blah bar foo(a,b) blah, blah,
blah
int foo(int x, int y ) if (xgt100) return(y-2)
else if (xgt10) return(y-x) else
return(xy)
protocol
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Sun RPC

• There are a number of popular RPC specifications.
• Sun RPC (ONC RPC) is widely used.
• NFS (Network File System) is RPC based.
• Rich set of support tools.

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Sun RPC Organization
Remote Program
Shared Global Data
Procedure 1
Procedure 2
Procedure 3
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Procedure Arguments

• To reduce the complexity of interface
  specification Sun RPC includes support for a
  single argument to a remote procedure.
• Typically the single argument is a structure that
  contains a number of values.
• Newer versions can handle multiple args.

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Procedure Identification

• Each procedure is identified by
• Hostname (IP Address)
• Program identifier (32 bit integer)
• Procedure identifier (32 bit integer)

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Procedure Identification

• Each procedure is identified by
• Hostname (IP Address)
• Program identifier (32 bit integer)
• Procedure identifier (32 bit integer)
• Program Version identifier
• for testing and migration.

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Program Identifiers

• Each remote program has a unique ID.
• Sun divided up the IDs
• 0x00000000 - 0x1fffffff
• 0x20000000 - 0x3fffffff
• 0x40000000 - 0x5fffffff
• 0x60000000 - 0xffffffff

Sun Sys Admin Transient Reserved
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Procedure Identifiers Program Version Numbers

• Procedure Identifiers usually start at 1 and are
  numbered sequentially
• Version Numbers typically start at 1 and are
  numbered sequentially.

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Iterative Server

• Sun RPC specifies that at most one remote
  procedure within a program can be invoked at any
  given time.
• If a 2nd procedure is called the caller blocks
  until the 1st procedure has completed.
• This is useful for applications that may share
  data among procedures.
• Example database - to avoid insert/delete/modify
  collisions.

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Communication Semantics

• To act like a local procedure (exactly one
  invocation per call) - a reliable transport (TCP)
  is necessary.
• Sun RPC does not support reliable call semantics.
• At Least Once Semantics
• Zero or More Semantics

If the procedure returns
No reply
33
Dynamic Port Mapping

• Servers typically do not use well known protocol
  ports.
• Clients known the Program ID (and host).
• A port lookup service runs on each host that
  contains RPC servers.
• RPC servers register themselves with the port
  mapper

34
The portmapper

• Each system which will support RPC servers runs
  a port mapper server that provides a central
  registry for RPC services.
• Servers tell the port mapper what services they
  offer.
• Clients ask a remote port mapper for the port
  number corresponsing to Remote Program ID.

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More on the portmapper

• The portmapper is itself an RPC server!
• The portmapper is available on a well-known port
  (111).

36
Sun RPC Programming

• The RPC library is a collection of tools for
  automating the creation of RPC clients and
  servers.
• RPC clients are processes that call remote
  procedures.
• RPC servers are processes that include
  procedure(s) that can be called by clients.

37
RPC Programming

• RPC library
• XDR routines
• RPC run time library
• call rpc service
• register with portmapper
• dispatch incoming request to correct procedure
• Program Generator

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RPC Run-time Library

• High- and Low-level functions that can be used by
  clients and servers.
• High-level functions provide simple access to RPC
  services.

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High-level Client Library

• int callrpc( char host,
• u_long prognum,
• u_long versnum,
• u_long procnum,
• xdrproc_t inproc,
• char in,
• xdrproc_t outproc,
• char out)

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High-Level Server Library

• int registerrpc(
• u_long prognum,
• u_long versnum,
• u_long procnum,
• char (procname)()
• xdrproc_t inproc,
• xdrproc_t outproc)

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High-Level Server Library (cont.)

• void svc_run()
• svc_run() is a dispatcher.
• A dispatcher waits for incoming connections and
  invokes the appropriate function to handle each
  incoming request.

42
High-Level Library Limitation

• The High-Level RPC library calls support UDP only
  (no TCP).
• You must use lower-level RPC library functions to
  use TCP.
• The High-Level library calls do not support any
  kind of authentication.

43
Low-level RPC Library

• Full control over all IPC options
• TCP UDP
• Timeout values
• Asynchronous procedure calls
• Multi-tasking Servers
• Broadcasting

44
RPCGEN

• There is a tool for automating the creation of
  RPC clients and servers.
• The program rpcgen does most of the work for you.
• The input to rpcgen is a protocol definition in
  the form of a list of remote procedures and
  parameter types.

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RPCGEN
Protocol Description
Input File
rpcgen
Client Stubs
XDR filters
header file
Server skeleton
C Source Code
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rpcgen Output Files

• gt rpcgen foo.x
• foo_clnt.c (client stubs)
• foo_svc.c (server main)
• foo_xdr.c (xdr filters)
• foo.h (shared header file)

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Client Creation

• gt gcc -o fooclient foomain.c foo_clnt.c foo_xdr.c
• foomain.c is the client main() (and possibly
  other function) that call rpc services via the
  client stub functions in foo_clnt.c
• The client stubs use the xdr functions.

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Server Creation

• gcc -o fooserver fooservices.c foo_svc.c
  foo_xdr.c
• fooservices.c contains the definitions of the
  actual remote procedures.

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Example Protocol Definition

• struct twonums
• int a
• int b
• program UIDPROG
• version UIDVERS
• int RGETUID(stringlt20gt) 1
• string RGETLOGIN( int ) 2
• int RADD(twonums) 3
• 1
• 0x20000001

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