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PACKET TA'L'ii"'''''''''''''' Brian Lloyd WB6RON 19200 Tilford Way Germantown MD 20874 THE MURKY WORLD OF PROTOCOLS Packet radio is gellmg ready to go through a major change. Many people involved wi th the development have discovered that AX.25 level 2 is not capable of supporting the growth , and we now need same sort of Layer 3, or networking, protocol. I have received a number of requests asking for a better explanation of what is going on in this area. This is the first in an ongoing series on networks and how they relate to your packet radio statio n. In order to facilitate discussion of networks, the Inte rnationa l Standards Organiza tion (ISO) came out with a recommendation for the description of networks . This became known as the reference model of Open System Interconnection (OSI) or the "sevenlayer network" model , Let's quickly go through the layers and their general meanings. Layer Definitions The PHYSICAL layer (layer 1) does those things having to do with the physical data transport. Items such as modem s, cables , data rates. and interface specif ications all fall into this layer. The LINK layer (layer 2) embodies whatever protocols and techniques are necessary to deliver packets from one point to another within the network, often in an error-free fashion . The NETWORK layer (layer 3) handles the routing ot packets from their original source to the final destination. The TRANSPORT layer (tayer 4) ensures the proper end-to-end delivery of packets from source to destination . The SESSION layer (layer 5) is responsible lor keeping the packets from one session separate from those of another. For exampie the SESSION layer would ensure that my file transfer does not get mixed up with my keyboard aso even Ihough they are going from the same source to the same destination. The PR ES ENTATION layer (layer 6) handles generic opera- 80 73AmaleurRadio · January, 1988 Latest in Digital Hamming nons such as ASCII 10 Baudot conversions and data encryption. The APPLICATION layer (layer 7) provides a way for programs and users to access the network in an orderly fashion. ISO Model and Current Packet Radio For most of us in packet radio, the physical layer means an narrow bandwidth FM signal in the 2-m band with a peak deviation of 3 kHz modulated with an AFSK signal using 1200· and 2400-Hz tones. The bit stream is encoded using Non-Return Zero Invert (NRZI) and sent at the rate of 1200 bps. The linl( layer is the High-level Data Link Control (HOLG) frame format with its built-in error detection in the form of a Cyclical Redundancy Check (CRG). The link protocol is very simple. II the CRC shows that a packet is in error, the packet is discarded. Note that while this ensures that bad data won't get delivered to the destination, it does not ensure that good data will get delivered. The network layer is our digipeat er specification, When you establish a connection, you must tell the TNC the precise route 10 use to get to the destination . This is called strict- source routing . The transport service is provided by AX.25 Level 2. If a packet is lost somewhere between the source and destination the transport service (AX.25) will cause that packet to be resent. Notice that the transport service will ensure receipt of packets end-to-end and it is not absolutely necessary that the link layer ensure delivery of data , Th e session layer is ra th er nebulous, but someone could make an argument t hat the stream switch is used to keep sessions with multiple stations separate and could therefore be considered by some to be a fo rm of session protocol. The presentation layer is again rather difficult to locate in our TNCs, but I guess that we could consider the TNC commands FI LTER, LFAOO, and LCOK to be a form of translation. The application layer is represented by TNC's command processor and the interface to your terminal. Questions about AX _25 Ok, ok, , can hear the screams of protest alrea dy. I know what many of you are going to say: AX.25 is really only a link-layer protocol and that the addition of a "layer 3" atop our existing AX.25 "layer 2" protocol is, in fact , what all this networking hoo-rah is about. I agree that AX.25 is supposed to be only a "layer 2" protocol, but that is not how it is used now. AX.25 makes sure that the packets are delivered from endpoint to end-point and therefore qualifies as a transport protocol. Someone once said to me, " If it looks like a duck, quacks like a duck, and swims like a duck, then it is probably a duck ." "AX.25 is supposed to be only a layer-2 protocol. rr Let's dig a tune further into some of the complaints that are surfacing. First, as Tom Clark W31WI pointed out a while back in his famous , "But wait, there's more, " ar ticle, trying to push packets throu gh more than about two digipeaters is a fruitless activity. Besides , the eight-digipeater limit and being required to specify the entire route is also annoying and very limiting. So the real complaint is that most of you do not like the existing link, network, and transport protocols and would like to relegate AX,25 to the job of being a link protocol, a job it was originally designed to do. Or; I'll buy that. But, it you do that, you need a new networking and transport protocol. Therein lies the furor-how to best accompusn this task. Datagrams and Virtual Circuits In order to go much further, you need to understand some other concepts. A major debate to date has raged on the differences between a datagram-based network and a virtual-circuit network , These differences have a great deal to do with how the network and transport protocols do their jobs. Datagram and virtual circuit (VC) netwo rks may work in a sim ilar fashion or they may be very different. The definition of a datagram network is that every packet contains the tun source and destination addresses (that means that our existing implementation ofAX.25 is a datagram. based network and transport protocol with strict-source routing). This allows the network the option of routing each packet independently. The analogy used most often is that of the post office. You take your tile or message , break it up into packets (letters), and transmit (mail) each one separately. By virtue of this technique there is no guarantee that the network (post office) will keep the packets (letters) in the same order so that it becomes the job of the transport service (the recipient of the mail) to reassemble them into the proper order. As a consequence the transport layer does more of the work allowing the network layer to be simpler. On the other hand we c an construct a VC network and use the analogy ot the phone system. An exchange begins with a call request packet (dial the call) traveling through the netwo rk. At each node in the network that particular " call " is assigned a Logical Channel Identifier (LCI). From then on each packet ccntains the Lei rather than the address and every packet takes th e same route th roug h th e network much like you are assigned a channel from the beginning of a phone call until you hang up again (or are disconnected) . As a result of this a VC network tends to have a more complex network layer and a simpler transport layer. Both approaches have their cresses and minuses . There are two advantages of a vc-baeeo network: • It switches packets faster because it doesn't have to look up the route for each packet. That's done at the time of the cali-request packet. • The full address isn't needed in every packet so header overhead is less. Datagrams also offer two advantages: • If a switch should fail other switches can easily bypass the failed switch and keep the packets moving. Any packets lost when the switch failed are retransmitted by the source transport service when the loss is finally detected. • Each packet is treated separately so a switch does not need to " remember" anything about the
past (no "channels"). Most of the memory in the switch can be used for buffering without concern for wh e re packets come from o r where they are going. Thus, the key diffe rence between the two approaches is that a virtual circui t switch can support only a limited number of connections and will " disconnect" all connections passing through it if it fails. In the commercial world wh ere extremely rel iable hardware is the rule, this usually is not a problem. On the other hand, a datag ram networ k hand les changes in and damage to the network much better than a VC network. Packet Networks Today Currently there are three networking packages in amateur packet radio, all based on datagrams. These three net working packages are NET/ROM , TEX NET, and NET .EXE . A fou rt h, COSI, was announced about a year ago but has not been released. It uses virtual circuits and is based on some of the ISO and CC ITT pro tocol recommendations. I will talk abou t it in another column when the software is final Iyavailable. NET/ROM is available in the form of a replace ment ROM for the TNC-2s and is intended to essentially replace digipeaters. TEXNET is a set of software that runs on a special piece 01 hardware calted a Nod e Con trol Processor (NCP) designed by the members of the Texas Packet Radio Society. NET.EXE is an implementation of the international standard Internet Protocol Suite (TCP/IP) and runs on PC clones, the Commodore Amiga, the Apple Macintosh , and most small UNIX systems. Which one of these packages or approaches is best? The answer is simple: it depends upon your application and interest. Before you decide wh ich nelworking package is best, you need to decide what services you want the network 10 provide. NET/ROM To date all amateur packet operations have been based on the concept of a user sitting at a terminal conversing with another user or with a SSS. The other concept is that of a computer conversing with another computer. If you are satisfied with rag-chewing and using the local BSS then very little needs to be changed. In that case, NET/ROM will probably appeal most to you since it is by far the simplest Change to our existing network. NETfROM nodes connect to each other using AX.25 and then converse with each other using their own internal protocol. You use a N ET/ RO M network by connecting to your local NET/ ROM node, telling it to connect to the NET/RO M node closest to the destination, and then telling that NETfROM node to connect to the destination (three connect commands are always required). I n this m a n n e r NETfROM re places the digipeaters and remove s the requirement for strict-source routing. However, you still need to know where your local NET/ROM node is and what NET/ROM node is nearest your destination. "Any TNC-2 can be turned into a NET/ROM node." Internally. NET/ROM uses datagrams. The N ET f RO Ms share information about what other NET/ROMs are in the net work. Eac h NET/ROM maintains a lisl of all the other nodes in the network and will attempt to route packets around a failed node should the need arise. There is no concept of a logical connection except at the edges of the network where the end-user TNCs connecl. NETfROM has a transport (layer 4) protocol that provides reliable service up to the point where users' TNCs connect and therefore is not a true transport protocol. A true transport protocol is end-to-end. Perhaps what For further information on the protocols discussed in this column: NET/ROM: Software 2000, Inc. 11 27 Hetrick Avenue Arroyo Grande CA 93420 (804) 489-1977 NET/ADM has can be called an edqe-to-ecqe protocol, meaning it provides reliable service to the edge of the network where the user connects. Any TNC-2 can be turned into a NETfROM node. All you need to do is remove the old TNC program ROM and insert a new ROM lrom Software 2000. Once this is done the TNC is no longer useful as an end-user TNC- it becomes a packet switch only. For NET/ROM to work welt you must have two or more TNCs and an appropriate number of NET/ ROMs (extra ROMs used in this manner are available at a lower price). The TNCs are connected together at the asynchronous (terminal) ports. This provides the potential for multi-frequency packet swnchrnq. a must for efficient operation. (Next month I will discuss why repeating packets on the same frequency is a no-no and why simplex digipeaters should be abolished). T eXNET This package comes next in the area o f interest and services. TEXNET is also a datagram service but it has no end-to-end or edge-to-edge transport-like protocol. For keyboard-type operation this is not a real problem because the user can act as a transport protocol and restart a session should he/she become disconnected. TEXNET becomes really interesting when you discover that it is providing services such as lhe network BBS, the conference bridge (round-table type of operation), and the weather-service interface. Finally something more than the venerable BSSI As far as I can tell, the major drawback to TEXNET (if it has one) is that it runs only on a special Node Control Processor TEXNET: Texas Packet RadioSociety,lnc. PO Box 831566 Richardson TX 75083 NET.EXE: Tucson Amateur Packet Radio Corp. POBox 22888 Tucson AZ 85734 (NCP) designed by the Texas Packet Radio Society. I have no idea how available it is, but I surmise that it is not as easy to get as a TNC-2 or a PC clone. NET .EXE The last nelworking package I wil l discuss this month is NET.EXE written by Phil Karn KA9Q. Unlike the other two packages, it makes a fairly radical break from traditional packet in that it does not make an attempt 10 use a standard TNC for the user interface. Instead it requires a more powertut personal computer such as a PC clone. It uses the international standard Internet Protocol Suite otherwise known as TCP/IP (named for the most significant protocols in the suite, the Transmission Control Protocol and the Internet Protocol). tP is the network layer and is a true dalagram protocol. TCP is the transport service that handles lhe end-to-end request for retransmission 01 lost packets and resequences out-of-sequence packets. The result is a network that looks like a VC to the user or application but has the advantages of a datagram network. NET.EXE doesn't just stop there either. It provides th ree applications: the File Transfer Protocol or FTP, the Simple Mail Transfer Protocol or SMTP, and a terminal-to-terminal and tarminal-lo-host protocol catted TELNET. In addi tion to providing these services, NET.EXE is lully multi-user and will support multiple FTP, TELNET, and SMTP sessions concurrently. The distribution of NET.EXE comes with full technical documenlation and source code so that you can make your own changes and enhancements. In Conclusion Aren't these protocols mutually exclusive? If you choose one do you have to forego the advantages of the others? Not necessari1y. However, this leads us into the realm of internet working (interconnecting two or more dissimilar networks), a subject I will discuss in detail in a subsequent column. I will give you a hint though: notice that the network protocol in TCP/ IP is called the Internet Protocol. That's it for this month. Next month I wilt be talking about improvements to lhe physical layer: modems, radios, and channel access techniques. See you thenlQJ' 73AmaieurRadio • January,1988 81
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