Network Working Group T. Socolofsky
`Request for Comments: 1180 C. Kale
` Spider Systems Limited
` January 1991
`
` A TCP/IP Tutorial
`
`Status of this Memo
`
` This RFC is a tutorial on the TCP/IP protocol suite, focusing
` particularly on the steps in forwarding an IP datagram from source
` host to destination host through a router. It does not specify an
` Internet standard. Distribution of this memo is unlimited.
`
`Table of Contents
`
` 1. Introduction................................................ 1
` 2. TCP/IP Overview............................................. 2
` 3. Ethernet.................................................... 8
` 4. ARP......................................................... 9
` 5. Internet Protocol........................................... 12
` 6. User Datagram Protocol...................................... 22
` 7. Transmission Control Protocol............................... 24
` 8. Network Applications........................................ 25
` 9. Other Information........................................... 27
` 10. References.................................................. 27
` 11. Relation to other RFCs...................................... 27
` 12. Security Considerations..................................... 27
` 13. Authors’ Addresses.......................................... 28
`
`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.
`
`Socolofsky & Kale [Page 1]
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`Page 1 of 28
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` 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
`
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` 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.
`
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` 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.
`
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` 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.
`
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` 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
`
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` 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.
`
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`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...".
`
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`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
`
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` 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:
`
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` ---------------------------------------
` |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|
` ----------------------------------
` TABLE 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.
`
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` 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.
`
`5. Internet Protocol
`
` The IP module is central to internet technology and the essence of IP
` is its route table. IP uses this in-memory table to make all
` decisions about routing an IP packet. The content of the route table
` is defined by the network administrator. Mistakes block
` communication.
`
` To understand how a route table is used is to understand
` internetworking. This understanding is necessary for the successful
` administration and maintenance of an IP network.
`
` The route table is best understood by first having an overview of
` routing, then learning about IP network addresses, and then looking
` at the details.
`
`5.1 Direct Routing
`
` The figure below is of a tiny internet with 3 computers: A, B, and C.
` Each computer has the same TCP/IP protocol stack as in Figure 1.
` Each computer’s Ethernet interface has its own Ethernet address.
` Each computer has an IP address assigned to the IP interface by the
` network manager, who also has assigned an IP network number to the
` Ethernet.
`
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` A B C
` | | |
` --o------o------o--
` Ethernet 1
` IP network "development"
`
` Figure 6. One IP Network
`
` When A sends an IP packet to B, the IP header contains A’s IP address
` as the source IP address, and the Ethernet header contains A’s
` Ethernet address as the source Ethernet address. Also, the IP header
` contains B’s IP address as the destination IP address and the
` Ethernet header contains B’s Ethernet address as the destination
` Ethernet address.
`
` ----------------------------------------
` |address source destination|
` ----------------------------------------
` |IP header A B |
` |Ethernet header A B |
` ----------------------------------------
` TABLE 5. Addresses in an Ethernet frame for an IP packet
` from A to B
`
` For this simple case, IP is overhead because the IP adds little to
` the service offered by Ethernet. However, IP does add cost: the
` extra CPU processing and network bandwidth to generate, transmit, and
` parse the IP header.
`
` When B’s IP module receives the IP packet from A, it checks the
` destination IP address against its own, looking for a match, then it
` passes the datagram to the upper-level protocol.
`
` This communication between A and B uses direct routing.
`
`5.2 Indirect Routing
`
` The figure below is a more realistic view of an internet. It is
` composed of 3 Ethernets and 3 IP networks connected by an IP-router
` called computer D. Each IP network has 4 computers; each computer
` has its own IP address and Ethernet address.
`
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` A B C ----D---- E F G
` | | | | | | | | |
` --o------o------o------o- | -o------o------o------o--
` Ethernet 1 | Ethernet 2
` IP network "development" | IP network "accounting"
` |
` |
` | H I J
` | | | |
` --o-----o------o------o--
` Ethernet 3
` IP network "factory"
`
` Figure 7. Three IP Networks; One internet
`
` Except for computer D, each computer has a TCP/IP protocol stack like
` that in Figure 1. Computer D is the IP-router; it is connected to
` all 3 networks and therefore has 3 IP addresses and 3 Ethernet
` addresses. Computer D has a TCP/IP protocol stack similar to that in
` Figure 3, except that it has 3 ARP modules and 3 Ethernet drivers
` instead of 2. Please note that computer D has only one IP module.
`
` The network manager has assigned a unique number, called an IP
` network number, to each of the Ethernets. The IP network numbers are
` not shown in this diagram, just the network names.
`
` When computer A sends an IP packet to computer B, the process is
` identical to the single network example above. Any communication
` between computers located on a single IP network matches the direct
` routing example discussed previously.
`
` When computer D and A communicate, it is direct communication. When
` computer D and E communicate, it is direct communication. When
` computer D and H communicate, it is direct communication. This is
` because each of these pairs of computers is on the same I