Introduction to Network Operating Systems Standards & Protocols Standards and Protocols Standards Organizations Standardization assures that different manufacturers and organizations can produce hardware and software that is universally consistent and, therefore, able to work together efficiently and reliably. There are numerous standards organizations which are important to networking. These include ANSI - American National Standards Institute (www.ansi.org). More than 1000 representatives from industry and government determine standards in electronics, chemical and nuclear engineering, health, safety and construction. ANSI is the United States representative to ISO. IEEE - Institute of Electrical and Electronics Engineers (www.ieee.org). An international society of engineering professionals. Some important networking standards from the IEEE are the Ethernet (802,3) and Wireless (802.11) LAN communication methods. It is also responsible for assigning Media Access Control (MAC) addresses to manufacturers of Ethernet devices. ISO - International Organization for Standardization (iso from the Greek word meaning ‘equal’)(www.iso.org). A collection of standards organizations representing 130 countries, headquartered in Geneva, Switzerland. There are about 12000 ISO standards, about 500 of which are computer-related. Of paramount importance to networking is the Open Standards Interface (OSI) model. ITU (CCITT) - International Telecommunications Union (formerly the Consultative Committee on International Telegraph and Telephony). A United Nations agency that regulates international telecommunications. Primarily concerned IETF - Internet Engineering Task Force (www.ietf.org). Responsible for the RFC1s which define all Internet protocols. There are also white papers to clarify the RFCs. ICANN - Internet Corporation for Assigned Names and Numbers (www.icann.org). Responsible for allocating IP addresses, Domain Names, managing the Domain Name System and root name servers. It has taken over tasks formerly done by the Internet Assigned Numbers Authority (INAN) and Network Solutions, Inc. 1Request for Comment. nos_text_standards_protocols.wpd NTC 12/15/04 1 Introduction to Network Operating Systems Standards & Protocols The OSI Model Moving a request or a response, reliably, from one host to another is a very complicated process. Here is an abbreviated list of tasks that are accomplished when communicating across the Internet. 1. Format the message so it is understood by the remote host 2. Break The message up into reasonably-sized units for transmission across the network. 3. Identify the requesting process within each unit 4. Establish communication with the remote host 5. Find the address of the remote host 6. Find a route to the remote host 7. Move the unit of data across each link, following the established route, from sender to receiver. 8. Reassemble the message at the receiving end. 9. Identify the intended recipient and forward the message to that process It would not make sense to try to implement all of these tasks (plus many others not mentioned) in one monolithic protocol responsible for all aspects of the communication. Instead, it makes more sense to break the total task up into a set of manageable ‘modules’ or layers. Each layer has a well-defined function and, more importantly, well defined interfaces with its adjacent layers. These layers are viewed as a vertical ‘stack’, and the purpose of each layer is to provide a service to the layer immediately above it. The Open Standards Interface (OSI) Model identifies seven layers, each of which is responsible for some small portion of the entire task outlined above. The actual implementation of the tasks at each layer is specified by a set of protocols2. The seven layers (with the way they are typically numbered) are 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical The tasks of each level are briefly outlined here: Physical Layer The physical layer specifies the media over which data is transmitted, and all aspects of physically moving signals from one host to another. This includes 2 In the case of the Internet, these protocols are developed, maintained and documented under the auspices of the IETF. nos_text_standards_protocols.wpd NTC 12/15/04 2 Introduction to Network Operating Systems Standards & Protocols specification of the cabling, NICs, and routing hardware, sets data transmission rates, electrical specifications, and monitors error rates. Specifications for this layer include Ethernet (802.3), Wireless (802.11), Asynchronous serial communication (RS-232/EIA-232), Modems (V.90, etc.) Data Link Layer The Data Link Layer is responsible for transmission of data, in the form of frames, across a single link of the network.3 It may detect (and sometimes correct) errors, provide flow control, and identifies computers on the network using physical addressing information. The Data Link Layer attaches a header containing control and addressing information. A trailer is also added containing error checking codes. Errors are corrected by retransmission. Specifications for this layer include Ethernet (802.3) and Address Resolution Protocol (ARP) Network Layer The Network Layer is responsible for transmission of data from the originating source host to the final destination host across a network made up of multiple segments. It organizes data into datagrams and It determines the route that the datagrams will take through the network. Routing is accomplished using logical (not physical) addresses. The Network Layer adds a header to each datagram containing logical addressing and other control information. The most important protocol used (by the Internet) for this layer is the Internet Protocol (IP). Also important is Novell’s IPX. Transport Layer The transport layer is responsible for transmission of data from the originating process to the destination process4. In the TCP/IP stack, it takes the original message and breaks it up into segments for transmission across the network. It provides reliability and flow control (transmission rate based on capabilities of the receiver) and assigns addressing information (port numbers) to the process which created the message. A header is added to each segment containing this addressing and other control information. 3 It is frequently possible, certainly in the case of the Internet, that numerous ‘links’ (sometimes referred to as ‘hops’) will need to be crossed in going from one host to another. 4 By ‘process’ we mean any of a number of programs which may be running on a single host. nos_text_standards_protocols.wpd NTC 12/15/04 3 Introduction to Network Operating Systems Standards & Protocols The Transport Control Protocol (TCP) and the User Datagram Protocol (UDP) implement this layer for the Internet. Novell uses SPX, and Windows peer-to- peer networks use NetBEUI (NetBIOS Extended User Interface) Session Layer The Session Layer establishes and maintains communication, called a session, between hosts. Presentation Layer The Presentation Layer provides translation and encryption/decryption as required by the network itself as well as differences between the hosts. It may also provide compression, graphics encoding and other services. MIME (Multiuser Internet Mail Extensions) is an example of a protocol that operates at the Presentation Layer. Application Layer The Application Layer allows software applications to access the network. It does not include actual applications (such as MS Word). It does include such services such as file transfer, file management, message handling for electronic mail. In the Internet implementation, many protocols are used at the Application layer, including HTTP, SMTP, FTP, SNMP, NFS, and DHCP. In the above description we refer to ‘headers’ that are added to each data unit by various layers of the OSI Model. It should be understood that every protocol which is running on the source host machine is mirrored by an identical protocol, referred to as a peer process, running on the destination host machine. It is the source host which adds the headers. The peer process at each layer on the destination machine removes the corresponding headers and uses the information appropriately. The Internet Most network implementations do not implement all seven layers of the OSI Model. Specifically, the Internet uses a set of protocols called the TCP/IP Protocol Suite5 which implements just five of the layers: Physical, Data Link, Network, Transport, and Application. The Presentation layer functions are performed by the Application layer 5 A Protocol Suite (sometimes called a Protocol Stack) is a set of protocols which are commonly used together in a particular networking implementation. nos_text_standards_protocols.wpd NTC 12/15/04 4 Introduction to Network Operating Systems Standards & Protocols protocols and the Session layer functions are split between Application layer and Transport layer protocols. Some of the most important protocols used to implement the Internet have been indicated above in the description of the OSI Model. LANs Many LANs today, even networks which never connect to the Internet, implement their networking functions using the TCP/IP protocol suite. Prior to the rise in popularity of the Internet LANs were frequently implemented using Novell’s Netware6, Apple’s AppleTalk, IBM’s SNA, and, on Windows machines, NetBIOS/NetBEUI. We briefly discuss the last of these here. NetBIOS (Network Basic Input/Output System) establishes a naming convention for hosts on the network