Standards Basics

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

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

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 …..

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

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

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

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

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

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

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

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

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

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

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