OSI Model, TCP/IP

Tom Kelliher, CS 318

Feb. 18, 2000

Administrivia

Announcements

Project 1 due Monday.

Assignment

From Last Time

Communication protocols, the OSI 7-layer model.

Outline

1. A closer look at the OSI layers.

2. Introduction to TCP/IP.

Coming Up

A closer look at the TCP/IP layers.

Communication Protocols

Aside: Designing a new Internet application: TCP vs. UDP.

Layers of the OSI Model

Physical Layer

• Switching levels, transmission rate
• Duplex
• Type of connector, cable

Ethernet:

• 10BaseT (UTP, 10Base2 (thinnet), 10Base5 (thicknet), 10BaseF (fiber)
• 10, 100Mbps

ATM:

• Fiber, SONET, Cat 5 UTP
• 45, 100, 155, 622Mbps; 2.5Gbps

Data Link Layer

• Partition the bit (packet) stream into frames
• Append checksums for error detection
• ``Guarantees'' that a frame makes ``one hop'', not that it's seen at the higher layers

Network Layer

• Routing multi-hop messages
• Connection-Oriented: X.25
• Connectionless: IP

Transport Layer

• End-To-End (possibly multiple-hops) reliability
• Partition the message stream into packets
• Message reassembly if built on a connectionless network layer protocol
• Connection-Oriented: TCP
• Connectionless: UDP

Session Layer

• Additional reliability features
• Often, not implemented (because it's thought of as the user process)

Presentation Layer

``Library''-type functionality

• Compression
• Encryption
• Character code conversion

Application Layer

User- System-Level utilities:

• ftp
• telnet, rlogin
• SMTP
• NFS
• DNS

Introduction to TCP/IP

Some of the protocols:

• Link layer --- NIC, device driver
• Network layer --- Routing, ``hop-by-hop'' messages
• Transport layer --- ``end-to-end'' messages

Message transmission example:

``Gluing'' Networks Together

• Repeaters/Hubs --- physical layer
• Bridges/Switches --- data link layer (ethernet)
• Routers --- network layer (protocol specific)
• Gateways --- protocol conversion

IP Addresses

Form:

• 32-bit, dotted decimal
• different from ethernet address (ARP, RARP)
• Class A: 0, 7 bit netid, 24 bit hostid

  0.0.0.0 to 127.255.255.255

• Class B: 10, 14 bit netid, 16 bit hostid

  128.0.0.0 to 191.255.255.255

• Class C: 110, 21 bit netid, 8 bit hostid

  192.0.0.0 to 223.255.255.255

• Domain Name System

Encapsulation

1. Data
2. Application layer
3. TCP/UDP layer --- TCP frame
4. IP layer --- IP datagram
5. Ethernet layer --- ethernet frame: 46--1500 bytes (MTU)

Demultiplexing

Process Communication

How do processes on separate machines communicate?

• Client/Server
• ``Well-known'' addresses
• Multiple telnet clients