Volume 1 • Issue 4 Winter 1999 Introduction to Industrial Ethernet, Part 2. Part 1 was featured in Issue 3, the Fall 1999. If you would like a copy, please send your request to EXTENSION [email protected] A Technical Supplement to control NETWORK INTRODUCTION TO THE INTERNET PROTOCOL How does IP impact control networks? By George Thomas, Contemporary Controls Application TELNET FTP SMTP DNS SNMP DHCP INTRODUCTION Presentation Session The push to incorporate Industrial RIP Ethernet or even “plain vanilla” Ethernet into control networks Transport RTP Transmission User Diagram implies that by making that choice OSPF RTCP Control Protocol Protocol completes the selection process. As mentioned in a previous article, Ethernet II and IEEE 802.3 are IGMP ICMP strictly data link layer technologies Internet Protocol which do not guarantee the deliv- Network ery of messages over a network or between networks. Protocol stacks ARP such as TCP/IP or SPX/IPX provide that functionality and without them Ethernet would be useless. With Data Link Ethernet ARCNET Token Ring FDDI the immense interest in the Physical Internet and the potential of at- taching control networks to the to guarantee the delivery of mes- Figure 1. The TCP/IP stack is actually a Internet, the protocol stack of sages. Above TCP is the applica- set of protocols. IP resides at the choice is TCP/IP because it pro- network layer of the OSI Reference tion layer. The services of the Model shown on the left. vides the foundation for the presentation and session layers of Internet. This article addresses the OSI Reference Model are in- lump these fields into Ethernet issues related to the IP portion of corporated into the application header, data and trailer fields The the TCP/IP stack as it applies to layer. Therefore, the reference IP data sits above the data link control networks. model for TCP/IP-based systems layer and its data, called a data- actually consists of only five gram, is inserted into the data field layers. Technologies such as THE TCP/IP STACK of the Ethernet frame. The data- Ethernet II and IEEE 802.3 reside gram has it own header and data Actually TCP/IP is a set of proto- at the lower data link and physical fields but no trailer field. Above cols defined by a series of RFCs layers of the same model. the IP layer is the transport layer (request for comments) that have where TCP and User Datagram evolved over the years. In general Protocol (UDP) reside. Data from the Internet Protocol (IP) is used DATA ENCAPSULATION this layer is likewise applied to the to route messages between net- The data sent over wires is repre- data portion of the IP datagram. works and, therefore, properly sented as frames. An Ethernet II TCP applies segments while UDP resides at the network layer of frame consists of a preamble, applies datagrams. Both TCP and the OSI Reference Model. source and destination addresses, UDP have headers as well. Finally Transmission Control Protocol type field, data field and a frame above the transport layer is the (TCP) sits on top of IP and is used sequence check field. You can application layer which needs to 1 USER DATA works thereby corrupting data- grams. It is not the responsibility of APPLICATION IP to guarantee the delivery of mes- APPL sages and, therefore, IP is frequent- HEADER USER DATA ly termed an unreliable delivery service. That may be a little harsh TCP of a criticism of IP but it is the re- sponsibility of the transport layer TCP APPLICATION DATA HEADER and not the network layer to guar- TCP SEGMENT antee end-to-end message delivery. IP IP is simply responsible for the ad- IP TCP dressing and routing of datagrams. HEADER HEADER APPLICATION DATA IP DATAGRAM ETHERNET DRIVER ROUTERS AND HOSTS ETHERNET IP TCP ETHERNET Unlike repeaters that operate at the HEADER HEADER HEADER APPLICATION DATA TRAILER ETHERNET physical layer and bridges that ETHERNET FRAME operate at the data link layer, routers operate at the network 46 TO 1500 BYTES layer. A router is used to intercon- nect two networks together to Figure 2. The wrapping of data into the sends datagrams. Finally, the appli- form an internet. An internet is a data field of the next immediate lower cation sends data. To further add general term used to denote a col- layer is called encapsulation. to the confusion, the terms packet lection of networks. It is not to be and frame are sometimes used in- confused with the Internet which terchangeably. is the public network that requires insert its own data into the data strict addressing standards in order portion of the transport layer as for different systems to communi- well as its own header. THE INTERNET PROTOCOL cate. With a control network, we The Internet Protocol provides the may want to keep it completely This application data is simply re- basic unit of data transfer, provides private and not connect it to the ferred to as data since there is no addressing, routing and fragmenta- Internet or the corporate internet defined structure in terms of the tion. The Internet Protocol resides (sometimes called an Intranet) but TCP/IP stack. That is why if two at the network layer and sends and if we do we will need a router. application data structures are dif- receives blocks of data called data- This is being mentioned here ferent, communication between grams received from upper layer because IP is a routable protocol these applications will not be ef- software. IP feeds these datagrams and routers are used to implement fective even with strict adherence to its attached data link layer the protocol. to TCP/IP standards. which sends and receives these datagrams as a series of packets. A datagram is analogous to a first- The end-to-end devices on the in- This wrapping of data within the class letter sent in the Post. In ternet are called hosts. If two hosts data field of the next immediate general, it will reach its destination are on the same local network, then lower layer of the protocol stack is but there is no formal acknowl- messages are routed directly involv- called encapsulation while the un- edgement that the letter was re- ing no routers. If the two hosts are wrapping of the same data at the ceived like there would be with on different networks, a router must receiving side is called demulti- either registered or certified mail. pass the message. This is called in- plexing. In order to reduce confu- IP utilizes a “best effort” or “con- direct routing. sion on what is the actual data we nectionless” delivery service will say that frames are sent over between source and destination IP ADDRESSING the data link layer. The IP sends addresses. It is connectionless out datagrams to the data link because there was no formal The IP is responsible for source layer in the form of packets. A session established between the and destination addresses and its packet can be a datagram or a source and destination before the structure is defined in RFC 761. fragment of a datagram. The TCP data was sent. Packets can be lost IPv4 is the most common version sends segments while the UDP as they traverse the network or net- of addressing and it uses 32-bit ad- 2 dressing. The newer IPv6 calls for shown as a decimal number from If the first two bits of the first byte 128-bit addressing and was devel- 0 to 255. Therefore, an IP address are a “10,” then this is a class B oped because the explosive growth is usually represented as address. With class B addresses of the Internet will soon deplete XXX.XXX.XXX.XXX. This address the first two bytes identify the the inventory of possible 32-bit ad- can be shown as a binary or hexa- network and the remaining two dresses. IPv6 will not be discussed decimal number as well but the bytes identify the host. This pro- here since there is ample confu- decimal-dot-decimal notation is the vides a slightly more reasonable sion in simply discussing 32-bit IP most popular. Therefore, the range 65,534 host addresses. addressing. of addresses is from 0.0.0.0 to 255.255.255.255. An example of an address would be 128.8.120.5 but If the first three bits of the first An IP address must not only looking at the address it is hard to byte are a “110,” then this is a address a particular host but a par- tell which is the network address class C address. With class C ad- ticular network as well. The IP and which is the host address. dresses the first three bytes identi- address must not be confused with fy the network and the remaining the Ethernet II address which is a byte identifies the host. This pro- 48-bit address sometimes called There are five classes of IP ad- vides a reasonable 254 hosts. the MAC address. The MAC dresses: A, B, C, D, E. Class D is address is used to facilitate com- for multicasting, a message from munication only at the data link one host to many hosts, and class Class D and class E addresses can layer. The IP address facilitates E is reserved for experiments. That be identified in the same way. A communication over networks and leaves classes A, B and C which class D address has a leading bit must be universally recognized, are the most important.