Lecture 23


Reminders: Programming Project 5 (reassigned
Homework Problem 5.48) due on Thursday.
Homework 7 due next Tuesday.
Questions?
Tuesday, November 15
CS 475 Networks - Lecture 23
1
Outline
Chapter 7 - End-to-End Data
7.1 Presentation Formatting
7.2 Multimedia Data
7.3 Summary
Tuesday, November 15
CS 475 Networks - Lecture 23
2
Introduction
A sender and receiver must agree to a message
or presentation format. How do we send
traditional (integers, floating-point numbers,
characters, arrays, structures) and multimedia
(audio, image, video) data?
We may want to use data compression to encode
the data as efficiently as possible for network
transmission. However, compression and
decompression processing places increased
demands on the end hosts.
Tuesday, November 15
CS 475 Networks - Lecture 23
3
Presentation Formatting
Data must be encoded into a form suitable for
transmission over the network. It is decoded at
the receiver. (Encoding/decoding are also known
as marshalling/unmarshalling.)
Tuesday, November 15
CS 475 Networks - Lecture 23
4
Presentation Formatting
Encoding is necessary because computers
represent data in different ways. For example, the
native size of an integer may differ between
computers and languages. Computers also have
different endianess.
Tuesday, November 15
CS 475 Networks - Lecture 23
5
Taxonomy – Data Types
Marshalling systems differ in the data types they
support. At the lowest level is support for the
base types (integers, floats, characters).
At the next level is support for flat types –
structures and arrays.
At the highest level is support for complex types
that contain pointers. The pointer can not be
simply copied across. The data pointed to must
also be copied. This is known as flattening or
serialization.
Tuesday, November 15
CS 475 Networks - Lecture 23
6
Taxonomy – Data Types
Argument marshalling: converting,
packing and linearizing
Tuesday, November 15
CS 475 Networks - Lecture 23
7
Taxonomy – Conversion Strategy
Some systems convert all data to canonical
intermediate form. For example a system may
require that all integers be sent in big-endian
regardless of the endianess of either the source
or receiver.
The alternative approach is receiver-makes-right
in which the sender transmits data in its native
form. The receiver is responsible for translating
the data to its own local form.
Tuesday, November 15
CS 475 Networks - Lecture 23
8
Taxonomy - Tags
Tags can be sent with the data to indicate the
type and length of a data item. A tag can also be
used to indicate the endianess of the data in a
receiver-makes-right system.
If tags are not used then the receiver must be
programmed to expect data of a particular type in
a particular order.
A 32-bit integer encoded in a tagged message
Tuesday, November 15
CS 475 Networks - Lecture 23
9
Taxonomy - Stubs
Stubs are typically used to implement argument
marshalling in support of RPC. They may be
either compiled or interpreted. Compiled stubs
are more efficient while interpreted stubs are
more flexible.
Interpreted stubs translate data according to a
data description file. Changing the data
description file does not require recompilation.
Tuesday, November 15
CS 475 Networks - Lecture 23
10
Taxonomy - Stubs
A stub compiler produces stubs
from an interface description.
Tuesday, November 15
CS 475 Networks - Lecture 23
11
Examples - XDR
External Data Representation (XDR) is used with
SunRPC for argument marshalling.

It supports the entire C type system except for function
pointers.

It uses a canonical intermediate form.

It does not use tags (except to indicate array lengths)

It uses compiled stubs.
Tuesday, November 15
CS 475 Networks - Lecture 23
12
Examples - XDR
Example XDR encoding of a structure containing
a 32-bit integer, a 7 character string, and a 3
element 32-bit integer array.
Tuesday, November 15
CS 475 Networks - Lecture 23
13
Examples - ASN.1
Abstract Syntax Notation One (ASN.1) is an ISO
standard that defines a representation (the Basic
Encoding Rules or BER) for data sent over a
network.
It supports all C types except function pointers,
uses a canonical intermediate form, and it uses
tags. The stubs can be either compiled or
interpreted.
Tuesday, November 15
CS 475 Networks - Lecture 23
14
Examples - ASN.1
Compound types (structures) can be created by
means of nesting in ASN.1/BER.
ASN.1/BER encoding of a 32-bit integer. The
value is preceded by TYPE and LENGTH tags.
Tuesday, November 15
CS 475 Networks - Lecture 23
15
Examples - ASN.1
ASN.1/BER LENGTH tags can be of variable size.
If the data item is less than 128 bytes in length,
the LENGTH tag is a single byte. If the data item
is longer than 127 bytes, that LENGTH tag is
multiple bytes long. The MSB of the leading byte
is one. The remaining 7 bits indicate the length of
the LENGTH tag.
Tuesday, November 15
CS 475 Networks - Lecture 23
16
Examples - NDR
Network Data Representation (NDR) uses
receiver makes right. It supports the C type
system. Data items are not tagged. Compiled
stubs are generated from the Interface
Description Language (IDL).
NDR inserts an architecture tag (shown below) at
the front of each message so that the receiver
knows how to translate the data from source
format to a local format.
Tuesday, November 15
CS 475 Networks - Lecture 23
17
Markup Languages (XML)
In XML all data are tagged and represented using
text.
<?xml version=”1.0”?>
<employee>
<name>John Doe</name>
<title>Head Bottle Washer</title>
<id>123456789</id>
<hiredate>
<day>5</day>
<month>June</month>
<year>1986</year>
</hiredate>
</employee>
Tuesday, November 15
CS 475 Networks - Lecture 23
18
Markup Languages (XML)
The XML data types and compound data type
formats are defined in an XML Schema Definition
(XSD) file. XSDs are also written in XML.
XML messages are easy to read and edit. They
are naturally portable between architectures.
They are inefficient (compared to binary
representations of data) and slow to parse.
Tuesday, November 15
CS 475 Networks - Lecture 23
19
In-Class Exercise


Write a C++ program that writes your name (first and
last names), your age (as an integer) and the first 5
digits of π (3.1415) to memory using XDR (refer to the
xdr man page). Normally this block of memory would
then be sent across the network to another computer.
Simulate this by copying the memory block to a new
location (use memcpy) and then have your program
read the values from the new location and display
them to standard output.
Email your program to the instructor. It is due by the
beginning of the Thursday's class.
Tuesday, November 15
CS 475 Networks - Lecture 23
20