Presentation Formatting

1
Presentation Formatting
2
Overview

• Marshalling (encoding) application data into
  messages
• Unmarshalling (decoding) messages into
  application data

3
Data Types

• Data types we consider
• integers
• floating point numbers
• character strings
• arrays
• structures
• Types of data we do not consider (now)
• images
• video
• multimedia documents

4
Difficulties

• Representation of base types
• floating point IEEE 754 versus non-standard
• integer big-endian versus little-endian
• Compiler layout of structures

(0)
(0)
(0)
(2)
big endian
00000010
00000000
00000000
00000000
(2)
(0)
(0)
(0)
little endian
00000010
00000000
00000000
00000000
0
1
2
3
Low
High
Address
address
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Taxonomy

• Data types
• base types (e.g., ints, floats) must convert
• flat types (e.g., structures, arrays) must pack
• complex types (e.g., pointers) must linearize
• Conversion Strategy
• canonical intermediate form
• receiver-makes-right (an N x N solution)
• N machine architectures must be able to handle N
  representations

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Marshalling
Application Data Structure
Argument Marshaller
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Tagged versus Untagged data

• Stubs
• compiled
• interpreted
• CORBA

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Examples

• XDR eXternal Data Representation
• Defined by Sun for use with SunRPC
• C type system (without function pointers)
• Canonical intermediate form
• Untagged (except array length)
• Compiled stubs

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• define MAXNAME 256
• define MAXLIST 100
• struct item
• int count
• char nameMAXNAME
• int listMAXLIST
• bool_t
• xdr_item(XDR xdrs, struct item ptr)
• return(xdr_int(xdrs, ptr-gtcount)
• xdr_string(xdrs, ptr-gtname,MAXNAME)
• xdr_array(xdrs, ptr-gtlist, ptr-gtcount, MAXLIST,
  sizeof(int), xdr_int))

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XDR
70MBps attainable on 175MHz Alpha
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Abstract Syntax Notation ASN.1

• (tag-8 bits, length-8bits, value)
• Used by Simple Network Management Protocol (SNMP)
• Canonical Intermediate form
• Length0x82 01 01257
• Each call sends 3 bytes

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NDR Network Data Representation

• Defined by DCE (CORBA)
• Essentially the C type system
• Receiver-makes-right (architecture tag)
• Individual data items untagged
• Compiled stubs from Interface Definition Language
  (IDL)

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4-byte architecture definition tag

• IntegrRep
• 0 big-endian
• 1 little-endian
• CharRep
• 0 ASCII
• 1 EBCDIC
• FloatRep
• 0 IEEE 754
• 1 VAX
• 2 Cray
• 3 IBM

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Remote Procedure Calls
15
Overview
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Mechanics
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Address Space for Procedures

• Flat unique id for each possible procedure
• Hierarchical program procedure within program

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SunRPC

• UDP SunRPC implement
• UDP dispatches to program (ports bound to
  programs)
• SunRPC dispatches to procedure w/in program

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Implementation

• Port Mapper program exists at well known UDP port
  (111)
• The Port Mapper translates program numbers (32
  bits) to UDP port numbers
• The 32 bit procedure number is then used to make
  the remote call
• NFS read procedure 6

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SunRPC Header Format
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(No Transcript)
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RPC Lab

• Write code to implement open, read and write
  commands to remote file
• You pass off against your own code
• The TA code is there to give you an example of
  how things might work
• Can be implemented on top of IP if TCP isnt
  complete

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Overview
Server
Client
Main() calls rpcopen(file)
RPC creates message and sends To TCP
Pop decodes RPC_OPEN, calls fdopen(file)
fd Handle
1 12
14 3
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Overview
Server
Client
Main() calls rpcread(Handle, buf, len)
RPC creates message and sends To TCP Waits on
semaphore
Pop decodes RPC_READ calls rvalread(fd,tmpb,len)

rpcpop copies data from message into buf,
signals semaphore
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Server
Client
Overview
Main() calls rpcwrite(Handle, buf, len)
RPC creates message and sends To TCP, waits on
semaphore
Pop decodes RPC_WRITE calls rvalwrite(fd,tmpb,le
n)
rpcpop signals semaphore
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Details

• Passoff
• Open remote file
• Read from within file
• Read beyond end (you should only return amount
  actually available)
• Write beyond end
• Read total data
• Copy new version of test file before each run.