An Introduction to TCP/IP
Contents
1. PING HTTP DNS Application TCP UDP Ta L TAL Transport Network Xf Data Link An Introduction to TCP IP rabbit com 15 5 1 IP IP provides communication between hosts on different kinds of networks i e different data link imple mentations such as Ethenet and Token Ring It is a connectionless unreliable packet delivery service Connectionless means that there is no handshaking each packet is independent of any other packet It is unreliable because there is no guarantee that a packet gets delivered higher level protocols must deal with that 5 1 1 IP Address IP defines an addressing scheme that is independent of the underlying physical address e g 48 bit MAC address IP specifies a unique 32 bit number for each host on a network This number is known as the Internet Protocol Address the IP Address or the Internet Address These terms are interchangeable Each packet sent across the internet contains the IP address of the source of the packet and the IP address of its destination For routing efficiency the IP address is considered in two parts the prefix which identifies the physical network and the suffix which identifies a computer on the network A unique prefix is needed for each network in an internet For the global Internet network numbers are obtained from Internet Service Pro viders ISPs ISPs coordinate with a central organization called the Internet Assigne2. Local Network Computers 2 3 Frames Bits flowing across the Ethernet are grouped into structures called frames A frame must be between 46 and 1500 bytes in size An Ethernet frame has four parts 1 A Preamble of 8 bytes that helps synchronize the circuitry thus allowing small bit rate differences between sender and receiver 2 A Header of 14 bytes that contains a 6 byte destination address 6 byte source address and a 2 byte type field 3 A Data area of variable length that along with the header is passed to the IP layer aka the Network layer 4 A Trailer of 4 bytes that contains a CRC to guard against corrupted frames If the destination address is all 1 bits it defines a broadcast frame and all systems on the local network process the frame There are also multicast frames A subset of systems can form a multicast group that has an address that does not match any other system on the network All systems in a particular subset pro cess a packet with a destination address that matches their subset A system can belong to any number of subsets A system may put its interface s into promiscuous mode and process all frames sent across its Ethernet This is known as sniffing the ether It is used for network debugging and spying 4 rabbit com Ethernet Basics 2 3 1 Collisions In a star shaped bus topology all systems have access to the network at any time Before sending data a system must determine if t
3. Dynamic C An Introduction to TCP IP For Embedded System Designers 019 0074 e 070720 J The latest revision of this manual is available on the Rabbit Semiconductor Web site www rabbit com for free unregistered download An Introduction to TCP IP Part Number 019 0074 J 070720 Printed in U S A 2006 Rabbit Semiconductor Inc 7 All rights reserved No part of the contents of this manual may be reproduced or transmitted in any form or by any means without the express written permission of Rabbit Semiconductor Permission is granted to make one or more copies as long as the copyright page contained therein is included These copies of the manuals may not be let or sold for any reason without the express written permission of Rabbit Semiconductor Rabbit Semiconductor reserves the right to make changes and improvements to its products without providing notice Trademarks Rabbit and Dynamic C are registered trademarks of Rabbit Semiconductor Windows is a registered trademark of Microsoft Corporation Semiconductor Table of Contents Chapter 1 Introduction 1 Chapter 2 Ethernet Basics 3 2 14 Ethernet Gel Li pie gn i app ps 3 2 2 dels ME en ERA N b AR sds Oe RE EE SS a a a 3 de GE EE OE OE EE EE a veer 4 2 35 oe EA E E N A a Ana a b 4 DSM SERE e A Es SR RTS 5 Chapter 3 Networks 7 Bil ECAN ED EE OR A DE EG GE DE DE EE EE errr 7 3 1 1 Repeaters and Brid es ii i er EE SE E EE A A A ae T 32
4. The Ethernet can use either a bus or star topology A bus topology attaches all devices in sequence on a single cable In a star topology all devices are wired directly to a central hub 10Base T uses a combination called a star shaped bus topology because while the attached devices can share all data coming in on the cable the actual wiring is in a star shape The access method used by the Ethernet is called Carrier Sense Multiple Access with Collision Detect CSMA CD This is a contention protocol meaning it is a set of rules to follow when there is competition for shared resources 2 1 Ethernet Address All Ethernet interfaces have a unique 48 bit address that is supplied by the manufacturer It is called the Ethernet address also known as the MAC address for Media Access Control Ethernet enabled Rabbit boards store this value in Flash Memory EEPROM that is programmed at the factory If you need unique Ethernet addresses for some product you are making you can obtain them from the IEEE Registration Authority Run the utility program Samples tcpip display MAC c to read the MAC address of a Rabbit based board The utility is included with Dynamic C starting with version 7 04 2 2 Physical Connections A Realtek RTL8019 10Base T interface chip provides a 10 Mbps Ethernet connection This chip is used on many Ethernet enabled Rabbit boards The corresponding port can be connected directly to an Ethernet network By using hubs
5. 3 5 1 1 Port Numbers Port numbers are the mechanism for identifying particular client and server applications Servers select a port to wait for a connection Most services have well known port numbers For example HTTP uses port 80 When a web browser the client requests a web page it specifies port 80 when contacting the server Clients usually have ephemeral port numbers since they exist only as long as the session lasts Some of the common well known TCP port numbers are listed in the table below Port Number Listening Application 7 Echo request 20 21 File Transfer Protocol FTP 23 Telnet 25 Simple Mail Transfer Protocol SMTP 53 Domain Name Server 80 HTTP Server An Introduction to TCP IP rabbit com 11 12 rabbit com Networks Semiconductor 4 NETWORK PROTOCOL LAYERS Computers on a network communicate in agreed upon ways called protocols The complexity of network ing protocol software calls for the problem to be divided into smaller pieces A layering model aids this division and provides the conceptual basis for understanding how software protocols together with hard ware devices provide a powerful communication system 4 1 Layering Models In the early days of networking before the rise of the ubiquitous Internet the International Organization for Standardization ISO developed a layering model whose terminology persists today Table 4 1 ISO 7 Layer Refere
6. Bridges LANs typically connect computers located in close physical proximity i e all the computers in a building Repeaters are used to join network segments when the distance spanned causes electrical signals to weaken Repeaters are basically amplifiers that work at the bit level they do not actively modify data that is amplified and sent to the next segment Like repeaters bridges are used to connect two LANs together Unlike repeaters bridges work at the frame level This is useful allowing bridges to detect and discard corrupted frames They can also perform frame filtering only forwarding a frame when necessary Both of these capabilities decrease network con gestion Bridged LANs can span arbitrary distances when using a satellite channel for the bridge The resulting net work is still considered a LAN and not a WAN An Introduction to TCP IP rabbit com 7 3 2 WAN To be considered a WAN a network must be able to connect an arbitrary number of sites across an arbi trary distance with an arbitrary number of computers at each site In addition it must have reasonable per formance no long delays and allow all of the computers connected to it to communicate simultaneously This is accomplished with packet switches Figure 3 1 WAN with 4 Switches Switch at Site 1 at Site 2 Switch Switch at Site 3 at Site 4 3 2 1 Packet Switches Packet switches are small computers with CPUs memory and I O devices They move
7. It takes two strings The first string is the setting to be changed and the second string is the value to change it to The configuration macros M IP ADDRESS MY_NETMASK MY_GATEWAY and MY_NAMESERVER can all be overridden by this function tcp config MV IP ADDRESS 10 10 6 101 Some of the tcp config functionality is duplicated by other Dynamic C TCP IP functions tcp config can override the macro MY IP ADDRESS and so can the sethostid function 26 rabbit com Dvnamic C TCP IP Implementation 6 2 Skeleton Program The following program is a general outline for a Dynamic C TCP IP program The first couple of defines set up the default IP configuration information The memmap line causes the program to compile as much code as it can in the extended code window The use line causes the compiler to compile in the Dynamic C TCP IP code using the configuration data provided above it define MY IP ADDRESS 10 10 6 101 define MY NETMASK 255 255 255 0 define MY GATEWAY 10 10 6 19 memmap xmem use dcrtcp lib main sock init for a tcp tick NULL To run this program start Dynamic C and open the SAMPLESVTCPIPVPINGME C file Edit the MY IP ADDRESS MY_NETMASK and MY GATEWAY macros to reflect the appropriate values for your device Run the program and try to run ping 10 10 6 101 from a command line on a computer on the same physical network replacing 10 10 6 101 with your value for MY IP ADDRES
8. WAN ar ER Ma ae ea EEEE A a A 8 32 1 PacketSwitches es os ik a a e 8 3 2 2 Forwarding a Packeteer aka Seg Ee iii Res WER GR KERSE Ry GER Ge EE avid EEES ED EINES EI DR Seg Ego Reg 8 sd EER OE EI EE EE Ee OE ENE 9 EE dB AR EE ER EE e ga 9 AA LE EE EE a ER NE EE EE 9 34 2 EA ERK a OE OE HE EO N N EE 9 A EGAIEWAVSE ie ae ra OS cates EE GE AT ea 10 3 35 Network Architeoture i GE ao se oe ee ee A a aa bee SR Ee 10 3 5 1 Client Server Networks ccccccscccsssccssccssseessecssecsseecsseeesecesssesseesseceseccseeeaecessecsseceseecseceseseeeeesseesesenseeess 10 Port Nun be eea a AE GE ED aa 11 Chapter 4 Network Protocol Lavers 13 4 1 Layering Model RE AE NEE i a p EE tates 13 4 2 TCP IP Protocol Stack i sitt e ali OE EE E RO iii ad feel 14 Chapter 5 TCP IP Protocols 15 EE teks cscs sb OE RE RE AE RR EE OR EN 16 5 TIP Address A N EE ee Ee ee ee 16 Sled IPAddress Classes EE EE EE EE Ee DR EE EG EE GE EE ee ed 16 De 13 Nelimaske ie ER ER a ee ee ee ee ee EE ta 16 5 14 Subnet Address EE a ER ES DE EE ER EG RE ENER S 17 5 15 Directed Broadcast Address i s esse ed GEE ER ER GER Eg dee ae 17 5 1 6 Limited Broadcast Address sies sissies Ee Reg EES GER ERGE REG ERG eg Eed GE weeg de Gees bee ee GE i aa ttanta ta 17 5 2 IP de ER EE OE EE RE ER N 17 SA RE SEE OE EE GE ER Ee ER Ee es ie ee ed sae 18 5 4 The Transport Layer RE RR ER OE EE RE a EO EN 19 DAT UDP a RE OE EE RE EE OE ON IG 19 DAD BEP EER DE EDE EES EE EE EE EE GE N EE GE EN
9. and routers a network can include a large number of computers A network might include all the computers in a particular building A local network can be connected to the Internet by means of a gateway The gateway is a computer that is connected both to the local network and to the Internet Data that must be sent out over the Internet are sent to the local network interface of the gateway and then the gateway sends them on to the Internet for routing to some other computer in the world Data coming in from the Internet are directed to the gateway which then sends them to the correct recipient on the local network An Introduction to TCP IP rabbit com 3 2 2 1 Cables Ethernet cables are similar to U S telephone plug cables except they have eight connectors For our pur poses there are two types of cables crossover and straight through In most instances the straight through cables are used It is necessary to use a crossover cable when two computers are connected directly without a hub for example if you want to connect your PC s Ethernet directly to the Rabbit Semi conductor TCP IP Development Board Some hubs have one input that can accept either a straight through or crossover cable depending on the position of a switch In this case make sure that the switch position and cable type agree Figure 2 1 Ethernet Network Ethernet To Internet gables Gateway
10. data Among other things such as IP addresses the header contains the following fields Sequence Number This 32 bit number contains either the sequence number of the first byte of data in this particular segment or the Initial Sequence Number ISN that identifies the first byte of data that will be sent for this particular connection The ISN is sent during the connection setup phase by setting the SYN control bit An ISN is chosen by both client and server The first byte of data sent by either side will be identified by the sequence num ber ISN 1 because the SYN control bit consumes a sequence number Figure 5 2 illustrates the three way handshake Figure 5 2 Synchronizing Sequence Numbers for TCP Connection Host A Host B Client SYN Segment Server ISN A ACK 0 SYN ACK Segment ISN B ACK A 1 ACK Segment Seq A 1 ACK B 1 The sequence number is used to ensure the data is reassembled in the proper order before being passed to an application protocol 20 rabbit com TCP IP Protocols Acknowledgement Number This 32 bit number is the other host s sequence number 1 of the last successfully received byte of data It is the sequence number of the next expected byte of data This field is only valid when the ACK control bit is set Since sending an ACK costs nothing because it and the Acknowledgement Number field are part of the header the ACK control bit is always set after a connection has been established T
11. delivery services not offered by TCP Like IP UDP is connectionless and works with datagrams 5 4 2 TCP TCP is a connection oriented transport service it provides end to end reliability resequencing and flow control TCP enables two hosts to establish a connection and exchange streams of data which are treated in bytes The delivery of data in the proper order is guaranteed TCP can detect errors or lost data and can trigger retransmission until the data is received complete and without errors 5 4 2 1 TCP Connection Socket A TCP connection is done with a 3 way handshake between a client and a server The following is a sim plified explanation of this process e The client asks for a connection by sending a TCP segment with the SYN control bit set e The server responds with its own SYN segment that includes identifying information that was sent by the client in the initial SYN segment e The client acknowledges the server s SYN segment The connection is then established and is uniquely identified by a 4 tuple called a socket or socket pair destination IP address destination port number source IP address source port number During the connection setup phase these values are entered in a table and saved for the duration of the connection An Introduction to TCP IP rabbit com 19 5 4 2 2 TCP Header Every TCP segment has a header The header comprises all necessary information for reliable complete delivery of
12. ee RE Ee Rg eed f tie ese Gees 6 4 1 Opening and Closing iese see se ee Ge ee Re ee RR ek nanna nsara ee ee 642 WIND NE BE ME ET EE EE angeneee 64 3 Reading RE EE RE OE EE EE 6 4 4 Checksumis ssikkar IE OE EE RE Eiei 6 5 ABR OE AE N OE eaten eden ON 6 5 1 State Based Program Design sesse ee RR RA RA Re ee Re ee 6 5 2 Blocking vs Non Blocking ss ee ae RA Re Re RA Re Re ee 6 5 3 Blocking Macros iese e ee ee ee ee Re Re GR GR GR GR Ge AA Ge 6 6 Multitasking and TCP IP ees see se ee ge ee ke Re GR Ge Ge ee AR ee ee ke Chapter 7 References iv rabbit com Table of Contents 1 INTRODUCTION This manual is intended for embedded systems engineers and support professionals who are not familiar with basic networking concepts An overview of an Ethernet network and the TCP IP suite of protocols used to communicate across the network will be given The implementation details that are discussed in this manual pertain to versions of Dynamic C prior to 7 05 Improvements and additions to the TCP IP suite of protocols are fully documented in the Dynamic C TCP IP User Manual Vols 1 and 2 An Introduction to TCP IP rabbit com 1 rabbit com Introduction Semiconductor 2 ETHERNET BASICS TCP IP Transmission Control Protocol Internet Protocol is a set of protocols independent of the physical medium used to transmit data but most data transmission for Internet communication begins and ends with Ethernet frames
13. is 1460 bytes You may want to lower tcp MaxBufSize which defaults to 2048 bytes to reduce RAM usage It can be reduced to as little as 600 bytes tcp MaxBufSize will work slightly differently in Dynamic C versions 6 57 and higher In these later versions the buffer for the UDP socket will be tcp MaxBufSize 2 whichis twice as large as before 6 1 4 Delay a Connection Sometimes it is appropriate to accept a connection request when the resources to do so are not available This happens with web servers when web pages have several graphic images each requiring a separate socket The macro USE_RESERVEPORTS is defined by default It allows the use of the function tcp reserveport When a connection to the port specified in tcp_reserveport is attempted the 3 way handshaking is done even if there is not yet a socket available This is done by setting the window parameter in the TCP header to zero meaning I can take 0 bytes of data at this time The other side of the connection will wait until the value in the window parameter indicates that data can be sent When using tcp reserveport the 2MSL for Maximum Segment Lifetime waiting period for closing a socket is avoided Using the companion function tcp clearreserve causes the connection to the port to be done in the conventional way 6 1 5 Runtime Configuration Functions are provided to change configuration values at runtime The most general one is tcp config
14. state machine model An example of state based programming is SAM PLES TCPIP STATE C This program is a basic Web server that should work with most browsers It allows a single connection at a time but could easily be extended to allow multiple connections 6 5 2 Blocking vs Non Blocking The sock fastread and sock preread functions read as much data as is available in the buffers and return immediately Similarly sock fastwrite fills the buffers and returns the num ber of characters that were written When using these functions it is your responsibility to ensure that all of the data were written completely offset 0 while offset lt length bytes written sock fastwrite amp socket buffer offset length off set if bytes written lt 0 error handling l offset bytes written The other functions do not return until they have completed or there is an error If it is important to avoid blocking you can check the conditions of an operation to insure that it will not block sock mode socket TCP MODE ASCII if sock bytesready amp my socket 1 sock gets buffer MAX BUFFER In this case sock gets will not block because it will be called only when there is a complete new line terminated record to read An Introduction to TCP IP rabbit com 35 6 5 3 Blocking Macros To block at a certain point and wait for a condition DCRTCP LIB provides some macros to make this task easier In this
15. will cause DCRTCP LIB to try sending any pending data immediately If you know ahead of time that data will need to be sent immediately call sock flushnext on the socket This function will cause the next set of data written to the socket to be sent immediately and is more efficient than sock flush An Introduction to TCP IP rabbit com 31 6 3 3 2 Status Functions e sock bytesready e sock tbleft e sock established e sock tbsize e sock rbleft e sock tbused e sock rbsize e tcp tick e sock rbused When you supply tcp_tick witha pointer to a TCP socket it will first process the packets and then check to see if the socket has an established connection It returns a zero if the socket is no longer open because of an error condition or if the socket has been closed You can use this functionality after calling sock close on the socket to determine whether the socket is completely closed sock close amp my socket while tcp tick amp my socket check time out do idle work The status functions can be used to avoid blocking when using sock write and some of the other I O functions The following blocks of code illustrate a way of using the buffer management and socket management functions to avoid blocking The first block of code checks to make sure that there is enough room in the buffer before adding data with a blocking function The second makes sure that there is a string terminated wi
16. EE GE ee Ee ee 19 TCP Connection Socket iese ee ee ee RA Re RR AA ee ee ee ee ee ee ee ee ee 19 TCP Heiders SE OE EE De ee Ee 20 DA ICMP RE cava GR EE vos a 21 59 The ei DE AE ME RE EI ONE EE RA EE NE N N EE AR 22 DA DNS EE EO A A 22 DCRTCP LIB Implementation of DNS loose ea RA Re RA Re Re RA Re RA GR AR ee Re Re Re ee ee ee ee 22 An Introduction to TCP IP rabbit com iii Chapter 6 Dynamic C TCP IP Implementation 6 1 TCP IP Configuration Macros sse see see ee Re Re RA Re AR Re Re ee Re ee 6 1 1 IP Addresses Set Manually eise ee Re RR Re Re ee ee 6 1 2 IP Addresses Set Dynamically ss sea ee Re RA Re ER Re Re 6 1 3 Default Buffet Size ss REEN ESE e ESE ees ina ian ee Ee GR EE dee ke Poe dee 6 1 4 Delay a Connection 2 ss Re AA Re ee Re Re Re Re ee ee 6 1 5 Runtime Configuration see se se ee Ge ee Re ek ee GR Re Re GR Ge ke ee 6 2 Skeleton Program i s ske zar SERS SA ED GE RR pk ER RSG DR GE TA GREG Ee RR Ve ee Eb Ee NG deeg ees 6 3 POP Socket ees ee suave ee rd GE Ek Ee Ek ate DAE eviews dies eN MES de AE N EE EE i Example of Passive Open 6 mn eennenenenzannnzzznnznnznnnnnananznnnznnnnnnzznznna 6 3 2 Active Opetiss iii EE EO ER OE OE EE N 6 3 3 TCP Socket Functions se see see se ee Ge ee ee ee Re GR Re Re GR ee Be Geo ee EE EE EE eta Status Functions 0 sees ee ek ee RA RA ee Re Re Re Re ee Re Re Re ee Re ee ee es AE IE AE EE AE 64 UDP Interface sies ies
17. S The main function first initializes the DCRTCP LIB TCP IP stack with a call to sock_init This call initializes internal data structures and enables the Ethernet chip which will take a couple of sec onds with the RealTek chip At this point DCRTCP LIB is ready to handle incoming packets DCRTCP LIB processes incoming packets only when tcp tick is called Internally the functions tcp open udp open sock read sock write sock close and sock abort all call tcp tick Itis a good practice to make sure that tcp tick is called periodically in your program to insure that the system has had a chance to process packets When you ping your device the Ethernet chip temporarily stores the packet waiting for DCRTCP LIB to process it DCRTCP LIB removes the packet the next time tcp tick gets called and responds appropriately A rule of thumb is to call tcp tick around 10 times per second although slower or faster call rates should also work The Ethernet interface chip has a large buffer memory and TCP IP is adaptive to the data rates that both end of the connection can handle thus the system will generally keep working over a wide variety of tick rates A more difficult question is how much computing time is consumed by each call to tep_tick Rough numbers are less than a millisecond if there is nothing to do 10s of milliseconds for typical packet pro cessing and 100s of milliseconds under exceptional c
18. ader contains the ultimate destination of the IP datagram but it might go through several other IP addresses routers before reaching that destination Routing table entries are created when TCP IP initializes The entries can be updated manually by a net work administrator or automatically by employing a routing protocol such as Routing Information Proto col RIP Routing table entries provide needed information to each local host regarding how to communicate with remote networks and hosts When IP receives a packet from a higher level protocol like TCP or UDP the routing table is searched for the route that is the closest match to the destination IP address The most specific to the least specific route is in the following order e A route that matches the destination IP address host route e A route that matches the network ID of the destination IP address network route e The default route If a matching route is not found IP discards the datagram An Introduction to TCP IP rabbit com 17 IP provides several other services e Fragmentation IP packets may be divided into smaller packets This permits a large packet to travel across a network which only accepts smaller packets IP fragments and reassembles packets transparent to the higher layers e Timeouts Each IP packet has a Time To Live TTL field that is decremented every time a packet moves through a router If TTL reaches zero the packet is discarded e Opti
19. ass system The suffix bits are divided into a subnet ID and a host ID This makes sense for class A and B networks since no one attaches as many hosts to these networks as is allowed Whether to subnet and how many bits to use for the subnet ID is determined by the local network administrator of each network If subnetting is used then the netmask will have to reflect this fact On a class B network with subnetting the netmask would not be 255 255 0 0 The bits of the Host ID that were used for the subnet would need to be set in the netmask 5 1 5 Directed Broadcast Address IP defines a directed broadcast address for each physical network as all ones in the host ID part of the address The network ID and the subnet ID must be valid network and subnet values When a packet is sent to a network s broadcast address a single copy travels to the network and then the packet is sent to every host on that network or subnetwork 5 1 6 Limited Broadcast Address If the IP address is all ones 255 255 255 255 this is a limited broadcast address the packet is addressed to all hosts on the current sub network A router will not forward this type of broadcast to other sub net works 5 2 IP Routing Each IP datagram travels from its source to its destination by means of routers All hosts and routers on an internet contain IP protocol software and use a routing table to determine where to send a packet next The destination IP address in the IP he
20. complete packets using a technique called Store and Forward An incoming packet is stored in a memory buffer and the CPU is interrupted The processor examines the packet and forwards it to the appropriate place This strategy allows the switch to accept multiple packets simultaneously As the figure above illustrates WANs currently do not need to be symmetric 3 2 2 Forwarding a Packet A data structure contains the information that tells the switch where to send the packet next This is called a routing table The destination address in the packet header determines the routing table entry that is used to forward the packet It could be forwarded to a computer attached to the switch that is examining the packet or it could be to another switch in the WAN 8 rabbit com Networks 3 3 VPN VPNs are built on top of a publicly accessible infrastructure such as the Internet or the public telephone network They use some form of encryption and have strong user authentication Essentially a VPN is a form of WAN the difference is their ability to use public networks rather than private leased lines A VPN supports the same intranet services as a traditional WAN but also supports remote access service This is good for telecommuting as leased lines don t usually extend to private homes and travel destinations A remote VPN user can connect via an Internet Service Provider ISP in the usual way This eliminates long distance charges The user ca
21. d Number Authority IANA 5 1 2 IP Address Classes The first four bits of an IP address determine the class of the network The class specifies how many of the remaining bits belong to the prefix aka Network ID and to the suffix aka Host ID The first three classes A B and C are the primary network classes Class First 4 Bits Number Of Max Of Number Of Max Of Hosts Per Prefix Bits Networks Suffix Bits Network A Oxxx 7 128 24 16 777 216 B 10xx 14 16 384 16 65 536 c 110x 21 2 097 152 8 256 D 1110 Multicast E 1111 Reserved for future use When interacting with mere humans software uses dotted decimal notation each 8 bits is treated as an unsigned binary integer separated by periods IP reserves host address 0 to denote a network 140 211 0 0 denotes the network that was assigned the class B prefix 140 211 5 1 3 Netmasks Netmasks are used to identify which part of the address is the Network ID and which part is the Host ID This is done by a logical bitwise AND of the IP address and the netmask For class A networks the net mask is always 255 0 0 0 for class B networks it is 255 255 0 0 and for class C networks the netmask is 255 255 255 0 16 rabbit com TCP IP Protocols 5 1 4 Subnet Address All hosts are required to support subnet addressing While the IP address classes are the convention IP addresses are typically subnetted to smaller address sets that do not match the cl
22. device that controls data flow between net works Generally a firewall is a first line defense against attacks from the outside world A firewall can be hardware based or software based A hardware based firewall is a special router with additional filter and management capabilities A software based firewall runs on top of the operating system and turns a PC into a firewall Conceptually firewalls can be categorized as Network layer aka Data Link layer or Application layer Network layer firewalls tend to be very fast They control traffic based on the source and destination addresses and port numbers using this information to decide whether to pass the data on or discard it Application layer firewalls do not allow traffic to flow directly between networks They are typically hosts running proxy servers Proxy servers can implement protocol specific security because they understand the application protocol being used For instance an application layer firewall can be configured to allow only e mail into and out of the local network it protects An Introduction to TCP IP rabbit com 9 3 4 3 Gateways A gateway performs routing functions The term default gateway is used to identify the router that con nects a LAN to an internet A gateway can do more than a router it also performs protocol conversions from one network to another 3 5 Network Architecture There are two network architectures widely used today peer to peer and client s
23. e de facto standard Protocols are sometimes referred to as protocol stacks or protocol suites A protocol stack is an appropriate term because it indicates the layered approach used to design the networking software Figure 4 1 Flow of Data Between Two Computers Using TCP IP Stacks Sender Receiver Application 1 Application Identical Message Identical Message Identical Message Data Link Data Link Identical Message Hardware Hardware Each host or router in the internet must run a protocol stack The details of the underlying physical connec tions are hidden by the software The sending software at each layer communicates with the corresponding layer at the receiving side through information stored in headers Each layer adds its header to the front of the message from the next higher layer The header is removed by the corresponding layer on the receiving side Virtual Connection Physical Connection 14 rabbit com Network Protocol Layers Semiconductor 5 TCP IP PROTOCOLS This chapter discusses the protocols available in the TCP IP protocol suite The following figure shows how they correspond to the 5 layer TCP IP Reference Model This is not a perfect one to one correspon dence for instance Internet Protocol IP uses the Address Resolution Protocol ARP but is shown here at the same layer in the stack Figure 5 1 TCP IP Protocol Flow BOOTP FIP DHCP SMTP TFTP
24. e either a lost message does not cause a system failure or is handled by the application Since UDP is a simple protocol and you have control over the retransmissions you can decide if you can trade low latency for high reliability Another advantage of UDP is the ability to broadcast packets to a number of computers on the same network When done prop erly broadcasts can reduce overall network traffic because information does not have to be duplicated when there are multiple destinations An Introduction to TCP IP rabbit com 33 6 4 1 Opening and Closing The udp open function takes a remote IP address and port number If they are set to a specific value all incoming and outgoing packets are filtered on that value i e you talk only to the one socket If the remote IP address is set to 1 it receives any packet and outgoing packets are broadcast If the remote IP address is set to 0 no outgoing packets may be sent until a packet has been received This first packet completes the socket filling in the remote IP address and port number with the return address of the incoming packet Multiple sockets can be opened on the same local port with the remote address set to 0 to accept multiple incoming connections from separate remote hosts When you are done communicating on a socket that was started with a 0 IP address you can close it with sock close and reopen to make it ready for another source 6 4 2 Writing The normal socket f
25. er software For example to resolve the domain name www rabbitsemiconductor com the following code available in SAMPLES TCP DNS C can be used define MV IP ADDRESS 10 10 6 101 define MY NETMASK 255 255 255 0 define MY GATEWAY 10 10 6 19 define MY NAMESERVER 10 10 6 19 memmap xmem use dertep lib main longword ip char buffer 20 SOC almiie 3 ip resolve www rabbitsemiconductor com if ip 0 printf couldn t find www rabbitsemiconductor com n else printf s is www rabbitsemiconductors address Nn lnet mtoa SUE LS aj p Your local name server is specified by the configuration macro MY_NAMESERVER Chances are that your local name server does not have the requested information so it queries the root server The root server will not know the IP address either but it will know where to find the name server that contains authorita 22 rabbit com TCP IP Protocols tive information for the com zone This information is returned to your local name server which then sends a query to the name server for the com zone Again this name server does not know the requested IP address but does know the local name server that handles rabbitsemiconductor com This information is sent back to your local name server who sends a final query to the local name server of rabbitsemiconduc tor com This local name server returns the requested IP address of www rabbitsemiconductor com to your local name ser
26. ers that are similar to the tcp listen call It is necessary to supply exact parameters for ina and port but the 1port parameter can be zero which tells DCRTCP LIB to select an unused port between 1024 and 65536 When you call tcp open Dynamic C tries to contact the other device to establish the connection tcp open will fail and return a zero if the connection could not be opened due to routing difficulties such as an inability to resolve the remote host s hardware address with ARP define MY IP ADDRESS 10 10 6 101 define MY NETMASK 255 255 255 0 define MY GATEWAY 10 10 6 19 define MY NAMESERVER 10 10 6 19 define WEBSITE www zweng com define FILE define PORT 80 memmap xmem use dcrtcp lib main int status tcp Socket s char buffer 2048 longword ip sock init ip resolve WEBSITE tcp open amp s 0 ip PORT NULL sock wait established amp s 0 NULL amp Status sock mode amp s TCP MODE ASCII sprintf buffer GET sVrin FILE sock puts amp s buffer while tcp tick amp s sock wait input amp s 0 NULL amp status if sock gets amp s buffer 2048 printf s n buffer return 0 30 rabbit com Dynamic C TCP IP Implementation sock err switch status case 1 foreign host closed printf User closed session n break case 1 time out printf nConnection timed out n break 6 3 3 TCP Socket Function
27. erver In peer to peer net works each workstation has the same capabilities and responsibilities These networks are usually less expensive and simpler to design than client server networks but they do not offer the same performance with heavy traffic 3 5 1 Client Server Networks The client server paradigm requires some computers to be dedicated to serving other computers A server application waits for a client application to initiate contact Table 3 1 Summary of Differences between Client and Server Software Client Software Server Software An arbitrary application program that becomes A special purpose privileged program a client when a remote service is desired It also dedicated to providing one service It can performs other local computations handle multiple remote clients at the same time Actively initiates contact Passively waits for contact Invoked when the system boots and executes Invoked by a user and executes for one session f through manv sessions Capable of accessing multiple services as Accepts contact from an arbitrarv number of needed but actively contacts only one remote clients but offers a single service or a fixed set server at a time of services Can require powerful hardware and a sophisticated operating system depending on how many clients are being served Does not require special hardware or a sophisticated operating system 10 rabbit com Networks
28. he Acknowledgement Number ensures that the TCP segment arrived at its destination Control Bits This 6 bit field comprises the following l bit flags left to right URG Makes the Urgent Pointer field significant ACK Makes the Acknowledgement Number field significant PSH The Push Function causes TCP to promptly deliver data RST Reset the connection SYN Synchronize sequence numbers FIN No more data from sender but can still receive data Window Size This 16 bit number states how much data the receiving end of the TCP connection will allow The sending end of the TCP connection must stop and wait for an acknowledgement after it has sent the amount of data allowed Checksum This 16 bit number is the one s complement of the one s complement sum of all bytes in the TCP header any data that is in the segment and part of the IP packet A checksum can only detect some errors not all and cannot correct any 5 4 3 ICMP Internet Control Message Protocol is a set of messages that communicate errors and other conditions that require attention ICMP messages delivered in IP datagrams are usually acted on by either IP TCP or UDP Some ICMP messages are returned to application protocols A common use of ICMP is pinging a host The Ping command Packet INternet Groper is a utility that determines whether a specific IP address is accessible It sends an ICMP echo request and waits for a reply Ping can be used to trans
29. he network is free or if it is already sending a frame If a frame is already being sent a system will wait Two systems can listen on the network and hear silence and then proceed to send data at the same time This is called a collision Ethernet hardware has collision detection sensors to take care of this problem This is the Collision Detect CD part of CSMA CD The colliding data is ignored and the systems involved will wait a random amount of time before resending their data An Introduction to TCP IP rabbit com 5 rabbit com Ethernet Basics Semiconductor 3 NETWORKS A network is a system of hardware and software put together for the purpose of communication and resource sharing A network includes transmission hardware devices to interconnect transmission media and to control transmissions and software to decode and format data as well as to detect and correct prob lems There are several types of networks in use today This chapter will focus on three of them e LAN Local Area Network e WAN Wide Area Network e VPN Virtual Private Network 3 1 LAN The most widely deployed type of network LANs were designed as an alternative to the more expensive point to point connection A LAN has high throughput for relatively low cost LANs often rely on shared media usually a cable for connecting many computers This reduces cost The computers take turns using the cable to send data 3 1 1 Repeaters and
30. ircumstances An Introduction to TCP IP rabbit com 27 6 3 TCP Socket For Dynamic C version 6 57 and above each socket must have an associated tcp Socket of 145 bytes oraudp Socket of 62 bytes The I O buffers are in extended memory For earlier versions of Dynamic C each socket must have a tcp Socket data structure that holds the socket state and I O buffers These structures are by default around 4200 bytes each The majority of this space is used by the input and output buffers There are two ways to open a TCP socket passive or active 6 3 1 Passive Open To wait for someone to contact your device open a socket with tcp listen This type of open is commonly used for Internet servers that listen on a well known port like 80 for HTTP You supply tcp listen witha pointer toa tcp Socket data structure the local port number others will be contacting on your device and the IP address and port number that are valid for the device If you want to be able to accept connections from any IP address or any port number set one or both to zero To handle multiple simultaneous connections each new connection will require its own tcp Socket structure and a separate call to tcp listen but using the same local port number 1port value The tcp listen call will immediately return and you must poll for the incoming connection You can use the sock wait established macro which will call tcp tick and block until the con nection i
31. mit a series of packets to measure average round trip times and packet loss per centages An Introduction to TCP IP rabbit com 21 5 5 The Application Layer There are many applications available in the TCP IP suite of protocols Some of the most useful ones are for sending mail SMTP transferring files FTP and displaying web pages HTTP These applications are discussed in detail in the TCP IP User Manual vols 1 and 2 Another important application layer protocol is the Domain Name System DNS Domain names are sig nificant because they guide users to where they want to go on the Internet 5 5 1 DNS The Domain Name System is a distributed database of domain name and IP address bindings A domain name is simply an alphanumeric character string separated into segments by periods It represents a spe cific and unique place in the domain name space DNS makes it possible for us to use identifiers such as rabbit com to refer to an IP address on the Internet Name servers contain information on some segment of the DNS and make that information available to clients who are called resolvers 5 5 1 1 DCRTCP LIB Implementation of DNS The resolve function in DCRTCP LIB immediately converts a dotted decimal IP address to its cor responding binary IP address and returns this value If resolve is passed a domain name a series of queries take place between the computer that called resolve and computers running name serv
32. n then initiate a tunnel request to the destination server The server authenticates the user and creates the other end of the tunnel VPN software encrypts the data packages it in an IP packet for compatibility with the Internet and sends it through the tunnel where it is decrypted at the other end There are several tunneling protocols available IP security IPsec Point to Point Tunneling Protocol PPTP and Layer 2 Tunneling Protocol L2TP 3 4 Network Devices Some network devices repeaters bridges and switches were discussed in the previous sections These are all dedicated hardware devices Network devices can also be non dedicated systems running network soft ware 3 4 1 Routers A router is a hardware device that connects two or more networks Routers are the primary backbone device of the Internet connecting different network technologies into a seamless whole Each router is assigned two or more IP addresses because each IP address contains a prefix that specifies a physical net work Before a packet is passed to the routing software it is examined If it is corrupted it is discarded If it is not corrupted a routing table is consulted to determine where to send it next By default routers do not propagate broadcast packets see Directed Broadcast Address on page 17 A router can be configured to pass certain types of broadcasts 3 4 2 Firewalls A firewall is a computer router or some other communications
33. nce Model Name of Layer Purpose of Layer Layer 7 Application Specifies how a particular application uses a network Layer 6 Presentation Specifies how to represent data Specifies how to establish communication with a remote Laver 5 Session svstem Layer 4 Transport Specifies how to reliably handle data transfer ayers Network Specifies addressing assignments and how packets are forwarded Laa Data Link Specifies the organization of data into frames and how to send frames over a network Layer 1 Physical Specifies the basic network hardware The 7 layer model has been revised to the 5 layer TCP IP reference model to meet the current needs of protocol designers Table 4 2 TCP IP 5 Layer Reference Model Name of Layer Purpose of Layer Layer 5 Application Specifies how a particular application uses a network Layer 4 Transport Specifies how to ensure reliable transport of data Layer 3 Internet Specifies packet format and routing Layer 2 Network Specifies frame organization and transmittal Layer 1 Physical Specifies the basic network hardware An Introduction to TCP IP rabbit com 13 4 2 TCP IP Protocol Stack TCP IP is the protocol suite upon which all Internet communication is based Different vendors have developed other networking protocols but even most network operating systems with their own protocols such as Netware support TCP IP It has become th
34. ocket After calling sock recv init on the socket you should not use sock_getc sock read orsock fastread 6 4 4 Checksums There is an optional checksum field inside the UDP header This field verifies only the header portion of the packet and doesn t cover anv part of the data This feature can be disabled on a reliable network where the application has the ability to detect transmission errors Disabling the UDP checksum can increase the performance of UDP packets moving through DCRTCP LIB This feature can be modified by sock mode s UDP MODE CHK enable checksums sock mode s UDP MODE NOCHK disable checksums 34 rabbit com Dynamic C TCP IP Implementation 6 5 Program Design When designing your program you must place some thought into how it will be structured If you plan on using the state based approach you need to select the appropriate functions 6 5 1 State Based Program Design One strategy for designing your program with Dynamic C is to create a state machine within a function where you pass it the socket This method allows you to handle multiple sockets without the services of a multitasking kernel This is the way the HTTP LIB functions are organized see HTTP in the Dynamic C TCP IP User s Manual The general states are waiting to be initialized waiting for a connection a bunch of connected states and waiting for the socket to be closed Many of the common Internet protocols fit well into this
35. of the world the default gate way 4 The IP address of the local DNS server for the host s network This is only necessary if DNS backups are needed MY IP ADDRESS MY NETMASK MY GATEWAY and MY NAMESERVER respectively correspond to these four critical addresses 6 1 2 IP Addresses Set Dynamically The macro USE_DHCP enables the Dynamic Host Configuration Protocol DHCP If this option is enabled a DHCP client contacts a DHCP server for the values of MY_IP_ADDRESS MY_NETMASK MY GATEWAY and MY_NAMESERVER DHCP servers are usually centrally located on a local network and operated by the network administrator An Introduction to TCP IP rabbit com 25 6 1 3 Default Buffer Size There are two macros used to define the size of the buffer that is used for UDP datagram reads and TCP packet reads and writes tcp MaxBufSize and SOCK BUF SIZE tcp MaxBufSize is deprecated in Dynamic C version 6 57 and higher and is being kept for backwards compatibility It has been replaced by SOCK_BUF_ SIZE If SOCK BUF SIZE is 4096 bytes the UDP buffer is 4096 bytes the TCP read buffer is 2048 bytes and the TCP write buffer is 2048 bytes In Dynamic C versions 6 56 and earlier tcp MaxBuf Size determines the size of the input and output buffers for TCP IP sockets The sizeof tcp Socket will be about 200 bytes more than double tcp MaxBufSize The optimum value for local Ethernet connections is greater than the Maximum Segment Size MSS The MSS
36. ons IP allows a packet s sender to set requirements on the path the packet takes through the net work source route the route taken by a packet may be traced record route and packets may be labeled with security features 5 3 ARP The Address Resolution Protocol is used to translate virtual addresses to physical ones The network hard ware does not understand the software maintained IP addresses IP uses ARP to translate the 32 bit IP address to a physical address that matches the addressing scheme of the underlying hardware for Ethernet the 48 bit MAC address There are three general addressing strategies 1 Table lookup 1 Translation performed by a mathematical function 2 Message exchange TCP IP can use any of the three ARP employs the third strategy message exchange ARP defines a request and a response A request message is placed in a hardware frame e g an Ethernet frame and broadcast to all computers on the network Only the computer whose IP address matches the request sends a response 18 rabbit com TCP IP Protocols 5 4 The Transport Layer There are two primary transport layer protocols Transmission Control Protocol TCP and User Datagram Protocol UDP They provide end to end communication services for applications 5 4 1 UDP This is a minimal service over IP adding only optional checksumming of data and multiplexing by port number UDP is often used by applications that need multicast or broadcast
37. program fragment sock wait established is used to block the program until a connection is established Notice the time out second parameter value of zero This tells Dynamic C to never time out Associated with these macros is a sock_ err label to jump to when there is an error If you supply a pointer to a status integer it will set the status to an error code Valid error codes are 1 for time out and 1 for a reset connection tcp open amp s 0 ip PORT NULL sock wait established amp s 0 NULL amp status Ms sock err switch status case 1 foreign host closed printf User closed session n break case 1 time out printf nConnection timed out n break 6 6 Multitasking and TCP IP The TCP IP engine may be used with the L C OS real time kernel The line use ucos2 lib must appear before the line use dcrtcp lib 36 rabbit com Dynamic C TCP IP Implementation Semiconductor 7 REFERENCES 1 A two part article Introduction to TCP IP in Embedded Systems Programming discusses issues related to programming embedded systems http www embedded com internet 99 12 991 2ial htm 2 Ethereal is a good free program for viewing network traffic It works under various Unix operating sys tems and under Windows http www ethereal com 3 Computer Networks and Internets Douglas E Comer Published by Prentice Hall ISBN 0 13 239070 1 This book gives an excellent high level desc
38. ription of networks and their interfaces 4 TCP IP Illustrated Volume 1 The Protocols W Richard Stevens Published by Addison Wesley ISBN 0 20 163346 9 This book gives many useful low level details about TCP IP UDP and ICMP An Introduction to TCP IP rabbit com 37 38 rabbit com References
39. s There are many functions that can be applied to an open TCP socket The socket functions listed in the fol lowing sections are explained in the Dynamic C TCP IP User Manual Vol 1 They fall into three main categories control status and I O 6 3 3 1 Control Functions e sock abort e sock flushnext e sock close e tcp listen e sock flush e tcp open tcp open and tcp listen have already been explained in the active and passive sections sock close should be called when you want to end a connection A call to sock_close may not immediately close the connection because it may take some time to send the request to end the connec tion and receive the acknowledgements If you want to be sure that the connection is completely closed before continuing your program you can call tcp tick with the address of the socket When tcp tick returns zero then the socket is completely closed Please note that if there is data left to be read on the socket the socket will not completely close There may be some reason that you want to cancel an open connection In this case you can call sock abort This function will cause a TCP reset to be sent to the other end and other future pack ets sent on this connection will be ignored For performance reasons data may not be immediately sent from a socket to its destination If your appli cation requires the data to be sent immediately you can call sock flush This function
40. s established or you can manually poll the socket using sock established 28 rabbit com Dynamic C TCP IP Implementation 6 3 1 1 Example of Passive Open The following example waits for a connection on port 7 and echoes back each line as you enter it To test this program change the configuration information and start it running From a connected PC telnet to the device using port 7 define MY IP ADDRESS 10 10 6 101 define MY NETMASK 255 255 255 0 define MY GATEWAY 10 10 6 19 memmap xmem use dcrtcp lib define PORT 7 tcp Socket echosock main char buffer 2048 int status sock init while 1 tcp listen amp echosock PORT 0 0 NULL 0 sock wait established amp echosock 0 NULL amp status printf Receiving incoming connection n sock mode amp echosock TCP MODE ASCII while tcp tick amp echosock sock wait input amp echosock 0 NULL amp status if sock gets amp echosock buffer 2048 sock puts amp echosock buffer sock err switch status case 1 foreign host closed printf User closed session n break case 1 time out printf nConnection timed out n break An Introduction to TCP IP rabbit com 29 6 3 2 Active Open When your Web browser retrieves a page it is actively opening one or more connections to the Web server s passively opened sockets To actively open a connection call tcp open which uses parame t
41. th a new line in the buffer or that the buffer is full before calling sock gets if sock tbleft amp my socket size sock write amp my socket buffer size or sock mode amp my_socket TCP MODE ASCII if sock bytesready amp my socket 1 sock gets buffer MAX BUFFER 32 rabbit com Dynamic C TCP IP Implementation 6 3 3 3 VO Functions e sock fastread e sock putc e sock fastwrite e sock puts e sock getc e sock read e sock gets e sock write e sock preread There are two modes of reading and writing to TCP sockets ASCII and binary By default a socket is opened in binary mode but you can change that with a call to sock mode When a socket is in ASCII mode DCRTCP LIB assumes that the data is an ASCII stream with record boundaries on the newline characters for some of the functions This behavior means sock bytesready will return gt 0 only when a complete newline terminated string is in the buffer or the buffer is full The sock puts function will automatically place a newline character at the end of a string and sock gets will strip the newline character When in binary mode do not use sock_gets 6 4 UDP Interface e sock fastread e sock read e sock fastwrite e sock recv e sock getc e sock recv from e sock gets e sock recv init e sock putc e sock write e sock puts e udp open The UDP protocol is useful when sending messages wher
42. unctions you used for writing to a TCP socket will work for a UDP socket but since UDP is a significantly different service the result could be different Each atomic write sock putc sock puts sock write orsock fastwrite places its data into a single UDP packet Since UDP does not guarantee delivery or ordering of packets the data received may be different either in order or content than the data sent 6 4 3 Reading There are two ways to read packets using DERTCP LIB The first method uses the normal sock getc sock gets sock read andsock fastread functions These func tions will read the data as it came into the socket which is not necessarily the data that was written to the socket The second mode of operation for reading uses sock_recv_init sock_recv and sock recv from The sock recv_init function installs a large buffer area that gets divided into smaller buffers Whenever a datagram arrives DCRTCP LIB stuffs that datagram into one of these new buffers sock recv scans the buffers for any datagrams received by that socket If there is a datagram the length is returned and the user buffer is filled otherwise it returns zero The sock recv from function works like sock recv but it allows you to record the IP address where the datagram originated If you want to reply you can open a new UDP socket with the IP address modified by sock recv from There is no way to send UDP packets without a s
43. ver who then passes it to your computer An Introduction to TCP IP rabbit com 23 24 rabbit com TCP IP Protocols Semiconductor 6 DYNAMIC C TCP IP IMPLEMENTATION The Dynamic C TCP IP protocol suite is contained in a number of Dynamic C libraries The main library file is DCRTCP LIB IP version 4 is supported not version 6 This chapter will describe the configuration macros and the functions used to initialize and drive TCP IP The implementation details that are discussed here pertain to versions of Dynamic C prior to 7 05 Improvements and additions to the TCP IP suite of protocols are fully documented in the Dynamic C TCP IP User Manual Vols 1 and 2 6 1 TCP IP Configuration Macros TCP IP can be configured by defining configuration macros at compile time by using the tcp config function and other functions at runtime or by using the Dynamic Host Configuration Protocol DHCP Some ISPs require that the user provide them with a MAC address from the controller Run the utility program discussed in Section 2 1 to display the MAC address 6 1 1 IP Addresses Set Manually Four pieces of information are needed by any host on a network 1 The IP address of the host e g the TCP IP Development Board 2 The part of the IP address that distinguishes machines on the host s network from machines on other networks the netmask 3 The IP address of the router that connects the host s network to the rest
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