1. Standards Basics

2. Standards
Standards are rules of operation that are followed by most or all vendors
Standards allow hardware and software from different vendors to work together
Competition among vendors brings lower prices and feature-rich products

3. Messages Standards work through message exchange Message App HTTP App
Trans
TCP
Trans
Int IP
Int IP
Int DL
PPP DL ? DL
Phy
Modem
Phy ?
Phy
User PC
Router
Webserver

4. Message Structure Message is a long string of bits
Show 32 bits or some other number per line
Begins with Bit 0
In this example,
First line has bits 0 through 31
Second line has bits 32 through 63
Bit 0
TCP Segment
Bit 31
Source Port # (16)
Destination Port # (16)
Sequence Number (32 bits)
Acknowledgement Number (32 bits)
Hdr Len
(4)
Reserved (6)
Flags (6)
Window Size (16)
More fields follow …..

5. Message Structure Message is a long string of bits
Divided logically into sections called fields (source port field is bits 0 to 15, …)
TCP Segment
Bit 0
Bit 31
Source Port # (16)
Destination Port # (16)
Sequence Number (32 bits)
Acknowledgement Number (32 bits)
Hdr Len
(4)
Reserved (6)
Flags (6)
Window Size (16)
TCP Checksum (16)
Urgent Pointer (16)
Options (if any)
PAD
Data Field

6. Message Structure Message is a long string of bits
IP Packet message structure:
Bit 0
Bit 31
Version
(4)
Hdr Len
(4)
TOS (8)
Total Length in bytes (16)
Indication (16 bits)
Flags (3)
Fragment Offset (13)
Time to Live (8)
Protocol (8)
Header Checksum (16)
Source IP Address
Destination IP Address
Options (if any)
PAD
Data Field

7. Octets Lengths are often given in groups of eight bits called octets
In computer science, a group of eight bits is called a byte
Same meaning but different names

8. Message Structure Header Fields Initial fields in a message
Before the data field
Source and destination address, error handling information, etc.
Like address and greeting in a written letter
Header Fields
Data Field
Header Field

9. Message Structure Data Field
The information to be delivered to the peer process on the other system
Usually far longer than the header and trailer
Just as body of letter usually is far longer than the opening and closing
Data Field
Trailer Fields
(if there are any)
Header
Fields

10. Message Structure Trailer Fields Fields following the data field
Usually only present at the data link layer
Data Field
Trailer Fields
(if there are any)
Header
Fields

11. Message Structures Not All Fields are Always Present
Header alone may be sufficient in a supervisory message
Header plus data field for delivering data
Sometimes all three
Header
Data Field
Header
Trailer
Data Field
Header

12. Message Time Diagrams Standards also govern message timing
When each side may transmit
For example, webserver may not send an HTTP response message until it receives a request
Browser
Webserver
HTTP Request
Message
Time
HTTP Response
Message

13. Standards for Telephone Webserver Access
Layer
Standard
Application
HyperText Transport Protocol (HTTP)
Transport
Transmission Control Protocol (TCP)
Internet
Internet Protocol (IP); Messages are packets
Data Link
Point-to-Point Protocol (PPP); Messages are frames
Physical
Modem, telephone standards

14. Protocols
A protocol is a standard for communication between peer processes, that is, processes at the same layer, but on different machines
E.g., HTTP: Browser and webserver application programs are at the same layer but on different machines
Message
App
HTTP
App

15. Protocols
A protocol is a standard for communication between peer processes, that is, processes at the same layer, but on different machines
TCP, IP, and PPP all have “protocol” as their final “P;” they are all protocols
TCP (Transmission Control Protocol) is the protocol governing communication between transport layer processes on two hosts
Message
Trans
TCP
Trans