The Hacker's Encyclopedia 1998

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==Phrack Inc.== Volume Three, Issue Thirty-Three, File 8 of 13 A TCP/IP Tutorial : Behind The Internet Part One of Two September 12, 1991 by The Not Table of Contents 1. Introduction 2. TCP/IP Overview 3. Ethernet 4. ARP 1. Introduction This tutorial contains only one view of the salient points of TCP/IP, and therefore it is the "bare bones" of TCP/IP technology. It omits the history of development and funding, the business case for its use, and its future as compared to ISO OSI. Indeed, a great deal of technical information is also omitted. What remains is a minimum of information that must be understood by the professional working in a TCP/IP environment. These professionals include the systems administrator, the systems programmer, and the network manager. This tutorial uses examples from the UNIX TCP/IP environment, however the main points apply across all implementations of TCP/IP. Note that the purpose of this memo is explanation, not definition. If any question arises about the correct specification of a protocol, please refer to the actual standards defining RFC. The next section is an overview of TCP/IP, followed by detailed descriptions of individual components. 2. TCP/IP Overview The generic term "TCP/IP" usually means anything and everything related to the specific protocols of TCP and IP. It can include other protocols, applications, and even the network medium. A sample of these protocols are: UDP, ARP, and ICMP. A sample of these applications are: TELNET, FTP, and rcp. A more accurate term is "internet technology". A network that uses internet technology is called an "internet". 2.1 Basic Structure To understand this technology you must first understand the following logical structure: ---------------------------- | network applications | | | |... \ | / .. \ | / ...| | ----- ----- | | |TCP| |UDP| | | ----- ----- | | \ / | | -------- | | | IP | | | ----- -*------ | | |ARP| | | | ----- | | | \ | | | ------ | | |ENET| | | ---@-- | ----------|----------------- | ----------------------o--------- Ethernet Cable Figure 1. Basic TCP/IP Network Node This is the logical structure of the layered protocols inside a computer on an internet. Each computer that can communicate using internet technology has such a logical structure. It is this logical structure that determines the behavior of the computer on the internet. The boxes represent processing of the data as it passes through the computer, and the lines connecting boxes show the path of data. The horizontal line at the bottom represents the Ethernet cable; the "o" is the transceiver. The "*" is the IP address and the "@" is the Ethernet address. Understanding this logical structure is essential to understanding internet technology; it is referred to throughout this tutorial. 2.2 Terminology The name of a unit of data that flows through an internet is dependent upon where it exists in the protocol stack. In summary: if it is on an Ethernet it is called an Ethernet frame; if it is between the Ethernet driver and the IP module it is called a IP packet; if it is between the IP module and the UDP module it is called a UDP datagram; if it is between the IP module and the TCP module it is called a TCP segment (more generally, a transport message); and if it is in a network application it is called a application message. These definitions are imperfect. Actual definitions vary from one publication to the next. More specific definitions can be found in RFC 1122, section 1.3.3. A driver is software that communicates directly with the network interface hardware. A module is software that communicates with a driver, with network applications, or with another module. The terms driver, module, Ethernet frame, IP packet, UDP datagram, TCP message, and application message are used where appropriate throughout this tutorial. 2.3 Flow of Data Let's follow the data as it flows down through the protocol stack shown in Figure 1. For an application that uses TCP (Transmission Control Protocol), data passes between the application and the TCP module. For applications that use UDP (User Datagram Protocol), data passes between the application and the UDP module. FTP (File Transfer Protocol) is a typical application that uses TCP. Its protocol stack in this example is FTP/TCP/IP/ENET. SNMP (Simple Network Management Protocol) is an application that uses UDP. Its protocol stack in this example is SNMP/UDP/IP/ENET. The TCP module, UDP module, and the Ethernet driver are n-to-1 multiplexers. As multiplexers they switch many inputs to one output. They are also 1-to-n de-multiplexers. As de-multiplexers they switch one input to many outputs according to the type field in the protocol header. 1 2 3 ... n 1 2 3 ... n \ | / | \ | | / ^ \ | | / | \ | | / | ------------- flow ---------------- flow |multiplexer| of |de-multiplexer| of ------------- data ---------------- data | | | | | v | | 1 1 Figure 2. n-to-1 multiplexer and 1-to-n de-multiplexer If an Ethernet frame comes up into the Ethernet driver off the network, the packet can be passed upwards to either the ARP (Address Resolution Protocol) module or to the IP (Internet Protocol) module. The value of the type field in the Ethernet frame determines whether the Ethernet frame is passed to the ARP or the IP module. If an IP packet comes up into IP, the unit of data is passed upwards to either TCP or UDP, as determined by the value of the protocol field in the IP header. If the UDP datagram comes up into UDP, the application message is passed upwards to the network application based on the value of the port field in the UDP header. If the TCP message comes up into TCP, the application message is passed upwards to the network application based on the value of the port field in the TCP header. The downwards multiplexing is simple to perform because from each starting point there is only the one downward path; each protocol module adds its header information so the packet can be de- multiplexed at the destination computer. Data passing out from the applications through either TCP or UDP converges on the IP module and is sent downwards through the lower network interface driver. Although internet technology supports many different network media, Ethernet is used for all examples in this tutorial because it is the most common physical network used under IP. The computer in Figure 1 has a single Ethernet connection. The 6-byte Ethernet address is unique for each interface on an Ethernet and is located at the lower interface of the Ethernet driver. The computer also has a 4-byte IP address. This address is located at the lower interface to the IP module. The IP address must be unique for an internet. A running computer always knows its own IP address and Ethernet address. 2.4 Two Network Interfaces If a computer is connected to 2 separate Ethernets it is as in Figure 3. ---------------------------- | network applications | | | |... \ | / .. \ | / ...| | ----- ----- | | |TCP| |UDP| | | ----- ----- | | \ / | | -------- | | | IP | | | ----- -*----*- ----- | | |ARP| | | |ARP| | | ----- | | ----- | | \ | | / | | ------ ------ | | |ENET| |ENET| | | ---@-- ---@-- | ----------|-------|--------- | | | ---o--------------------------- | Ethernet Cable 2 ---------------o---------- Ethernet Cable 1 Figure 3. TCP/IP Network Node on 2 Ethernets Please note that this computer has 2 Ethernet addresses and 2 IP addresses. It is seen from this structure that for computers with more than one physical network interface, the IP module is both a n-to-m multiplexer and an m-to-n de-multiplexer. 1 2 3 ... n 1 2 3 ... n \ | | / | \ | | / ^ \ | | / | \ | | / | ------------- flow ---------------- flow |multiplexer| of |de-multiplexer| of ------------- data ---------------- data / | | \ | / | | \ | / | | \ v / | | \ | 1 2 3 ... m 1 2 3 ... m Figure 4. n-to-m multiplexer and m-to-n de-multiplexer It performs this multiplexing in either direction to accommodate incoming and outgoing data. An IP module with more than 1 network interface is more complex than our original example in that it can forward data onto the next network. Data can arrive on any network interface and be sent out on any other. TCP UDP \ / \ / -------------- | IP | | | | --- | | / \ | | / v | -------------- / \ / \ data data comes in goes out here here Figure 5. Example of IP Forwarding a IP Packet The process of sending an IP packet out onto another network is called "forwarding" an IP packet. A computer that has been dedicated to the task of forwarding IP packets is called an "IP-router". As you can see from the figure, the forwarded IP packet never touches the TCP and UDP modules on the IP-router. Some IP-router implementations do not have a TCP or UDP module. 2.5 IP Creates a Single Logical Network The IP module is central to the success of internet technology. Each module or driver adds its header to the message as the message passes down through the protocol stack. Each module or driver strips the corresponding header from the message as the message climbs the protocol stack up towards the application. The IP header contains the IP address, which builds a single logical network from multiple physical networks. This interconnection of physical networks is the source of the name: internet. A set of interconnected physical networks that limit the range of an IP packet is called an "internet". 2.6 Physical Network Independence IP hides the underlying network hardware from the network applications. If you invent a new physical network, you can put it into service by implementing a new driver that connects to the internet underneath IP. Thus, the network applications remain intact and are not vulnerable to changes in hardware technology. 2.7 Interoperability If two computers on an internet can communicate, they are said to "interoperate"; if an implementation of internet technology is good, it is said to have "interoperability". Users of general-purpose computers benefit from the installation of an internet because of the interoperability in computers on the market. Generally, when you buy a computer, it will interoperate. If the computer does not have interoperability, and interoperability can not be added, it occupies a rare and special niche in the market. 2.8 After the Overview With the background set, we will answer the following questions: When sending out an IP packet, how is the destination Ethernet address determined? How does IP know which of multiple lower network interfaces to use when sending out an IP packet? How does a client on one computer reach the server on another? Why do both TCP and UDP exist, instead of just one or the other? What network applications are available? These will be explained, in turn, after an Ethernet refresher. 3. Ethernet This section is a short review of Ethernet technology. An Ethernet frame contains the destination address, source address, type field, and data. An Ethernet address is 6 bytes. Every device has its own Ethernet address and listens for Ethernet frames with that destination address. All devices also listen for Ethernet frames with a wild- card destination address of "FF-FF-FF-FF-FF-FF" (in hexadecimal), called a "broadcast" address. Ethernet uses CSMA/CD (Carrier Sense and Multiple Access with Collision Detection). CSMA/CD means that all devices communicate on a single medium, that only one can transmit at a time, and that they can all receive simultaneously. If 2 devices try to transmit at the same instant, the transmit collision is detected, and both devices wait a random (but short) period before trying to transmit again. 3.1 A Human Analogy A good analogy of Ethernet technology is a group of people talking in a small, completely dark room. In this analogy, the physical network medium is sound waves on air in the room instead of electrical signals on a coaxial cable. Each person can hear the words when another is talking (Carrier Sense). Everyone in the room has equal capability to talk (Multiple Access), but none of them give lengthy speeches because they are polite. If a person is impolite, he is asked to leave the room (i.e., thrown off the net). No one talks while another is speaking. But if two people start speaking at the same instant, each of them know this because each hears something they haven't said (Collision Detection). When these two people notice this condition, they wait for a moment, then one begins talking. The other hears the talking and waits for the first to finish before beginning his own speech. Each person has an unique name (unique Ethernet address) to avoid confusion. Every time one of them talks, he prefaces the message with the name of the person he is talking to and with his own name (Ethernet destination and source address, respectively), i.e., "Hello Jane, this is Jack, ..blah blah blah...". If the sender wants to talk to everyone he might say "everyone" (broadcast address), i.e., "Hello Everyone, this is Jack, ..blah blah blah...". 4. ARP When sending out an IP packet, how is the destination Ethernet address determined? ARP (Address Resolution Protocol) is used to translate IP addresses to Ethernet addresses. The translation is done only for outgoing IP packets, because this is when the IP header and the Ethernet header are created. 4.1 ARP Table for Address Translation The translation is performed with a table look-up. The table, called the ARP table, is stored in memory and contains a row for each computer. There is a column for IP address and a column for Ethernet address. When translating an IP address to an Ethernet address, the table is searched for a matching IP address. The following is a simplified ARP table: ------------------------------------ |IP address Ethernet address | ------------------------------------ |223.1.2.1 08-00-39-00-2F-C3| |223.1.2.3 08-00-5A-21-A7-22| |223.1.2.4 08-00-10-99-AC-54| ------------------------------------ TABLE 1. Example ARP Table The human convention when writing out the 4-byte IP address is each byte in decimal and separating bytes with a period. When writing out the 6-byte Ethernet address, the conventions are each byte in hexadecimal and separating bytes with either a minus sign or a colon. The ARP table is necessary because the IP address and Ethernet address are selected independently; you can not use an algorithm to translate IP address to Ethernet address. The IP address is selected by the network manager based on the location of the computer on the internet. When the computer is moved to a different part of an internet, its IP address must be changed. The Ethernet address is selected by the manufacturer based on the Ethernet address space licensed by the manufacturer. When the Ethernet hardware interface board changes, the Ethernet address changes. 4.2 Typical Translation Scenario During normal operation a network application, such as TELNET, sends an application message to TCP, then TCP sends the corresponding TCP message to the IP module. The destination IP address is known by the application, the TCP module, and the IP module. At this point the IP packet has been constructed and is ready to be given to the Ethernet driver, but first the destination Ethernet address must be determined. The ARP table is used to look-up the destination Ethernet address. 4.3 ARP Request/Response Pair But how does the ARP table get filled in the first place? The answer is that it is filled automatically by ARP on an "as-needed" basis. Two things happen when the ARP table can not be used to translate an address: 1. An ARP request packet with a broadcast Ethernet address is sent out on the network to every computer. 2. The outgoing IP packet is queued. Every computer's Ethernet interface receives the broadcast Ethernet frame. Each Ethernet driver examines the Type field in the Ethernet frame and passes the ARP packet to the ARP module. The ARP request packet says "If your IP address matches this target IP address, then please tell me your Ethernet address". An ARP request packet looks something like this: --------------------------------------- |Sender IP Address 223.1.2.1 | |Sender Enet Address 08-00-39-00-2F-C3| --------------------------------------- |Target IP Address 223.1.2.2 | |Target Enet Address <blank> | --------------------------------------- TABLE 2. Example ARP Request Each ARP module examines the IP address and if the Target IP address matches its own IP address, it sends a response directly to the source Ethernet address. The ARP response packet says "Yes, that target IP address is mine, let me give you my Ethernet address". An ARP response packet has the sender/target field contents swapped as compared to the request. It looks something like this: --------------------------------------- |Sender IP Address 223.1.2.2 | |Sender Enet Address 08-00-28-00-38-A9| --------------------------------------- |Target IP Address 223.1.2.1 | |Target Enet Address 08-00-39-00-2F-C3| --------------------------------------- TABLE 3. Example ARP Response The response is received by the original sender computer. The Ethernet driver looks at the Type field in the Ethernet frame then passes the ARP packet to the ARP module. The ARP module examines the ARP packet and adds the sender's IP and Ethernet addresses to its ARP table. The updated table now looks like this: ---------------------------------- |IP address Ethernet address | ---------------------------------- |223.1.2.1 08-00-39-00-2F-C3| |223.1.2.2 08-00-28-00-38-A9| |223.1.2.3 08-00-5A-21-A7-22| |223.1.2.4 08-00-10-99-AC-54| ---------------------------------- TA BLE 4. ARP Table after Response 4.4 Scenario Continued The new translation has now been installed automatically in the table, just milli-seconds after it was needed. As you remember from step 2 above, the outgoing IP packet was queued. Next, the IP address to Ethernet address translation is performed by look-up in the ARP table then the Ethernet frame is transmitted on the Ethernet. Therefore, with the new steps 3, 4, and 5, the scenario for the sender computer is: 1. An ARP request packet with a broadcast Ethernet address is sent out on the network to every computer. 2. The outgoing IP packet is queued. 3. The ARP response arrives with the IP-to-Ethernet address translation for the ARP table. 4. For the queued IP packet, the ARP table is used to translate the IP address to the Ethernet address. 5. The Ethernet frame is transmitted on the Ethernet. In summary, when the translation is missing from the ARP table, one IP packet is queued. The translation data is quickly filled in with ARP request/response and the queued IP packet is transmitted. Each computer has a separate ARP table for each of its Ethernet interfaces. If the target computer does not exist, there will be no ARP response and no entry in the ARP table. IP will discard outgoing IP packets sent to that address. The upper layer protocols can't tell the difference between a broken Ethernet and the absence of a computer with the target IP address. Some implementations of IP and ARP don't queue the IP packet while waiting for the ARP response. Instead the IP packet is discarded and the recovery from the IP packet loss is left to the TCP module or the UDP network application. This recovery is performed by time-out and retransmission. The retransmitted message is successfully sent out onto the network because the first copy of the message has already caused the ARP table to be filled. _______________________________________________________________________________