number for itself. Nobody is trying to find it, However the servers have to have well-known numbers, so that people can open connections to them and start sending them commands. The official port numbers for each program are given in "Assigned Numbers".

 

Note that a connection is actually described by a set of 4 numbers: the Internet address at each end, and the tCP port number at each end. Every datagram has all four of those numbers in it. (The Internet addresses are in the IP header, and the TCP port numbers are in the TCP header.) In order to keep things straight, no two connections can have the same set of numbers. However it is enough for any one number to be different, For example, it is perfectly possible for two different users on a machine to be sending files to the same other machine, This could result in connections with the following parameters:

 

Internet addresses TCP ports

connection I 12864194, 128.64,7 1234, 21

connection 2 128.6.4.194, 128,6.4.7 1235, 21

 

Since the same machines are involved, the Internet addresses are the same. Since they are both doing file transfers, one end of the connection involves the well-known port number for FTP. The only thing that differs is the port number for the program that {he users are running. That's enough of a difference Generally. at least one end of the connection asks the network software to assign ~t a port number that is guaranteed to be unique. Normally, it's the user's end. since the server has to use a well-known number.

 

Now that we know how to open connections, let's get back to The applications programs. As mentioned earlier, once TCP has opened a connection, we have something that might as well be a simple wire. All the hard parts are handled by TCP and IP. However we still need some agreement as to what we

send over this connection, in effect this is simply an agreement on what set of commands the application will understand, and the format in which they are to be sent. Generally, what is sent is a combination of commands and data. They use context to differentiate. For example, the mail protocol works like this: Your mail program opens a connection to the mail server at the other end. Your program gives it your machine's name, the sender of the message, and the recipients you want it sent to. It then sends a command saying that it is starting the message, At that point, the other end stops treating what it sees as commands, and starts accepting the message. Your end then starts sending the text of the message. At the end of the message, a special mark is sent (a dot in the first column). After that, both ends understand that your program is again sending commands. This is the simplest way to do things, and the one that most applications use.

 

File transfer is somewhat more complex. The file transfer protocol involves two different connections, h starts out just like mail. The user's program sends commands like "log me in as this user", "here is my password", "send me the file with this name". However once the command to send data is sent, a second connection is opened for the data itself. [t would certainly be possible to send the data on the same connection, as mail[ does. However file transfers often take a long time. The designers of the file transfer protocol wanted to allow the user to continue issuing commands while the transfer is going on. For example, the user might make an inquiry, or he might abort the transfer. Thus the designers felt it was best to use a separate connection for the data and leave the original command connection for commands. (it is also possible to open command connections to two different computers, and tell them to send a file from one to the other. In that case, the data couldn't go over the command connection,)


Remote terminal connections use another mechanism still. For remote Iogins, there is just one connection, h normally sends data. When it is necessary to send a command (e.g. to set the terminal type or to change some mode), a special character is used to indicate that the next character is a command. If the user happens to type that special character as data, two of them are sent,

 

We are not going to describe the application protocols in detail in this document, lCs better to read the RFC's yourself. However there are a couple of common conventions used by applications that will be described here. First, the common network representation: TCP/IP is intended to be usable on any computer. Unfortunately, not all computers agree on how data is represented. There are differences in character codes (ASCII vs. EBCDIC), in end of line conventions (carriage return, line feed. or a representation using counts), and in whether terminals expect characters to be sent individually or a line at a time In order to allow computers of different kinds to communicate, each applications protocol defines a standard representation. Note that TCP and lP do not care about the representation. TCP simply sends octets However the programs at both ends have to agree on how the octets are to be interpreted The RFC for each application specifies the standard representation for that application. Normally it is "net ASCII", This uses ASCII characters, with end of line denoted by a carriage return followed by a line feed. For remote login, there is also a definition of a "standard terminal", which turns out to be a half-duplex terminal with echoing happening on the local machine. Most applications a[so make provisions for die two computers to agree on other representations that they may find more convenient. For example, PDP-10'S have 36-bit words, There is a way that two PDP-10'S can agree to send a 36-bit binary file. Similarly, two systems that prefer full-duplex terminal conversations can agree on that. However each application has a standard representation, which every machine must support.

 

An example application: SMTP

 

In order to give a bit better idea what is involved in the application protocols, I'm going to show an example of SMTP, which is the mail protocol. (SMTP is "simple mall transfer protocol.) We assume that a computer called TOPAZ.RUTGERS.EDU wants to send the following message.

 

Date: Sat, 27 Jun 87 13:26:31 EDT From: hedr ickot opaz. rut get s. edu To: levyored.rutgets,edu

Subject: meeting

 

Let's get together Monday at lpm.

 

First, note that the format of the message itself is described by an Internet standard (RFC g22). The standard specifies the fact that the message must be transmitted as net ASCII (i.e. it must be ASCII. with carriage return/linefeed to delimit lines). It also describes the general structure, as a group of header lines, then a blank line, and then the body of the message. Finally, it describes the syntax of the header lines in detail. Generally they consist of a keyword and then a value.

 

Note that the addressee is indicated as LEVY~'RED.RUTGERS.EDU. Initially, addresses were simply "person at machine". However recent standards have made things more flexible. There are now provisions for systems to handle other systems' mail. This can allow automatic forwarding on behalf of computers not connected to the Internet. It can be used to direct mail[ for a number of systems to one central mail server. Indeed there is no requirement that an actual computer by the name of RED.RUTGERS.EDU even exist. The name servers could be set up so that you mail to department names, and each department's mail is routed automatically to an appropriate computer. It is also possible that! the part be[ore the ~' is something other than a user name. It is possible [or programs to he set up to process mail.

There are also provisions to handle mailing lists, and generic names such as "postmaster" or "operator,,*

The way the message is to be sent to another system is described by RFC's 821 and 974. The program that is going to be doing the sending asks the name server several queries to determine where to route the message, The first query is to find out which machines handle mail for the name RED.RUTGERS.EDU. In this case, the server replies that REDRUTGERS.EDU handles its own mail. The program then asks for the address of RED.RUTGERS.EDU, which is 128.6.4.2, Then the mail program opens a TCP connection to port 25 on 128.6.4.2, Port 25 is the well-known socket used for receiving mail, Once this connection is established, the mail program starts sending commands. Here is a typical conversation, Each line is labeled as to whether it is from TOPAZ or RED. Note that TOPAZ initiated the connection:

 

B~D 220 I~ED ~UTGEi~S EDU SMTP Service at 29 Jun 87 05:17:18

TOPAZ 8ELO topaz, rutgers edu

 

NED 250 RED.RUTOERS.EDU - Hello, TOPAZ.~UTGERS.EDU

TOPAZ MAIL Fro~:<hedriokOtopaz.rutgers edu>

RED 250 MAIL accepted

 

TOPAZ nCPT To:<levy@red rutgers~edu>

 

I~I;D 250 ~eoipient accepted

 

TOPAZ DATA

 

I~ED 3~4 Start mall input; end with <CRLF>.<CRLF>

 

TOPAZ Date: Sat, 27 Jun 87 13:26:32 EDT

TOPAZ Prom: hedrick~topaz rutgers.edu

TOPAZ TO: levy@red,rutgers.edu

TOPAZ Subject: meet ing

 

TOPAZ


TOPAZ Let's [et together NondaF at lpm,

 

TOPAZ .

 

RED a&o OK

 

TOPAZ QUIT

 

BED 221 RED.~UTGgB$.EDU $orvlce closing transmission channel

 

First, note that commands all use normal text. This is typical of the Internet standards. Many of the protocols use standard ASCII commands. This makes it easy to watch what is going on and to diagnose problems. For example, the mail program keeps a log of each conversation. If something goes wrong, the log file can simply be mailed to the postmaster. Since it is normal text, he can see what was going on, It also allows a human to interact directly with the mail server, for testing, (Some newer protocols are complex enough that this is not practical. The commands would have to have a syntax that would require a significant parser. Thus there is a tendency for newer protocols to use binary formats. Generally they are structured like C or Pascal record structures.) Second, note that the responses all begin with numbers. This is also typical of Internet protocols. The allowable responses are defined in the protocol. The numbers allow the user program to respond unambiguously. The rest of the response is text, which is normally for use by any human who may be watching or looking at a log, It has no effect on the operation of the programs. (However there is one point at which the protocol uses part of the text of the response.) The commands themselves simply allow the mail program on one end to tell the mail server the information it needs to know in order to deliver the message. In this case, the mail server could get the information by looking at the message itself. But for more complex cases, that would not be safe. Every session must begin with a HELO, which gives the name of the system that initiated the connection. Then the sender and recipients are specified, (There can be more than one RCPT command, if there are several recipients.) Finally the data itself is sent.


Note that the text of the message is terminated by a line containing just a period. (If such a line appears in the message, the period is doubled.) After the message is accepted, the sender can send another message, or terminate the session as in the example above,

 

Generally, there is a pattern to the response numbers. The protocol defines the specific set of responses that can be sent as answers to any given command. However programs that don't want to analyze them in detail can just look at the first digit. In general, responses that begin with a 2 indicate success, Those that begin with 3 indicate that some further action is needed, as shown above. 4 and 5 indicate errors. 4 is a "temporary" error, such as a disk filling. The message should be saved, and tried again later. 5 is a permanent error, such as a non-existent recipient. The message should be returned to the sender with an error message,

 

(For more details about the protocols mentioned in this section, see RFC's 821/822 for mail, RFC 959 for file transfer, and RFC's 854/855 for remote Logins. For the well-known port numbers, see the current edition of Assigned Numbers, and possibly RFC 814.)

 

Protocols other than TCP:

UDP and ICMP

 

$o far, we have described only connections that use TCP. Recall that TCP is responsible for breaking up messages into datagrams, and reassembling them properly. However in many applications, we have messages that will always fit in a single datagram. An example is name lookup. When a user attempts to make a connection to another system, he will generally specify the system by name, rather than Internet address. His system has to translate that name to an address before it can



do anything. Generally, only a few systems have the database used to translate names to addresses. So the user's system will want to send a query to one of the systems that has the database. This query is going to be very short, It will certainly fit in one datagram. So will the answer. Thus it seems silly to use TCP. Of course TCP does more than just break things up into datagrams. It also makes sure that the data arrives, resending datagrams where necessary. But for a question that fits in a single datagram, we don't need all the complexity of TCP to do this. If we don't get an answer after a few seconds, we can just ask again. For applications like this, there are alternatives to TCP,

 

The most common alternative is UDP ("user datagram protocol"). UDP is designed for applications where you don*t need to put sequences of datagrams together. It fits into the system much like TCP. There is a UDP header. The network software puts the UDP header on the front of your data, just as it would put a TCP header on the front of your data. Then UDP sends the data to IP, which adds the IP header, putting UDP's protocol number in the protocol field instead of TCP's protocol number. However UDP doesn't do as much as TCP does. It doesn't split data into multiple datagrams. It doesn't keep track of what it has sent so it can resend if necessary. About all that UDP provides is port numbers, so that several[ programs can use UDP at once. UDP port numbers are used just like TCP port numbers. There are well-known port numbers for servers that use UDP. Note that the UDP header is shorter than a TCP header. It still has source and destination port numbers, and a checksum, but that's about it. No sequence number, since it is not needed. UDP is used by the protocols that handle name lookups (see fEN 116, RFC 882, and RFC 883), and a number of similar protocols.

 

Another alternative protocol is ICMP ("Internet control message protocol"). ICMP is used for error messages, and

 


other messages intended for the TCP/IP software itself, rather than any particular user program. For example, if you attempt to connect to a host, your system may get back an ICMP message saying "host unreachable". ICMP can also be used to find out some information about the network, See RFC 792 for details of ICMP. ICMP is similar to UDP. in that it handles messages that fit in one datagram. However it is even simpler than UDP. it doesn't even have port numbers in its header, Since all ICMP messages are interpreted by the network software itself, no port numbers are needed to say where a ICMP message is supposed to go.

 

Keeping track of names and information: the domain system

 

As we indicated earlier, the network software generally needs a 32-bit Internet address in order to open a connection or send a datagram. However users prefer to deal with computer names rather than numbers. Thus there ~s a database that allows the software to look up a name and find the corresponding number. \When the Internet was small, this was easy. Each system would have a file that listed all of the other systems, giving both their name and number. There are now too many computers for this approach to be practical. Thus these files have been replaced by a set of name servers that keep track of host names and the corresponding Internet addresses. (In fact these servers are somewhat more general than that. This is just one kind of information stored in the domain system.) Note that a set of interlocking servers are used, rather than a single central one. There are now so many different institutions connected to the Internet that it would be impractical for them to notify a central authority whenever they installed or moved a computer, Thus naming authority is delegated to individual institutions. The name servers form a tree, corresponding to institutional structure. The names

themselves follow a similar structure. A typical example is the name BORAX.LCS.MIT.EDU. This is a computer at the Laboratory for Computer Science (LCS) at MIT. In order to find its Internet address, you might potentially have to consult 4 different servers. First, you would ask a central server (called the root) where the EDU server is. EDU is a server that keeps track of educational institutions. The root server would give you the names and Internet addresses of several servers for EDU. (There are several servers at each level, to allow for the possibly that one might be down.) You would then ask EDU where the server for MIT is. Again, it would give you names and Internet addresses of several servers for MIT. Generally, not all of those servers would be at MIT, to allow for the possibility of a general power failure at MIT. Then you would ask MIT where the server for LCS is, and finally you would ask one of the LES servers about BORAX. The final result would be the Internet address for BORAX,LCS.MIT.EDU. Each of these levels is referred to as a "domain". The entire name. BORAX.LCS.MIT.EDU, is called a "domain name'*. (So are the names of the higher-level domains, such as LCS.MIT.EDU, MIT.EDU, and EDU.)

 

Fortunately, you don't really have to go through all of this most of the time, First of all, the root name servers also happen to be the name servers for the top-level domains such as EDU, Thus a single query to a root server will get you to MIT. Second. software generally remembers answers that it got before, So once we look up a name at LCS,MIT.EDU. our software remembers where to find servers for LCS.MIT,EDU, MIT.EDU, and EDU, It also remembers the translation of BORAX,LCS.MIT.EDU. Each of these pieces of information has a "time to live" associated with it. Typically this is a few days. After that, the information expires and has to be looked up again. This allows institutions to change things.


The domain system is not limited to finding out Internet addresses. Each domain name is a node in a database. The node can have records that define a number of different properties. Examples are Internet address, computer type, and a list of services provided by a computer. A program can ask for a specific piece of information, or all information about a given name. It is possible for a node in the database to be marked as an "alias" (or nickname) for another node. It is also possible to use the domain system to store information about users, mailing lists, or other objects.

 

There is an Internet standard defining the operation of these databases, as well as the protocols used to make queries of them. Every network utility has to be able to make such queries, since this is now the official way to evaluate host names. Generally utilities will talk to a server on their own system. This server will take care of contacting the other servers for them. This keeps down the amount of code that has to be in each application program.

 

The domain system is particularly important for handling computer mail. There are entry types to define what computer handles mail for a given name. to specify where an individual is to receive mail, and to define mailing lists,

 

(See RFC's 882, 883, and 973 for specifications of the domain system. RFC 974 defines the use of the domain system in sending mail.)

 

Routing

 

The description above indicated that the IP implementation is responsible for getting datagrams to the destination indicated by the destination address, but little was said about how this would be done. The task of finding how to get a datagram to its destination is referred to as '*routing". In fact many of the


details depend upon the particular implementation, However some general things can be said.

 

First, it is necessary to understand the model on which IP is based, lP assumes that a system is attached to some local network, We assume that the system can send datagrams to any other system on its own network. (In the case of Ethernet, it simply finds the Ethernet address of the destination system, and puts the datagram out on the Ethernet.) The problem comes when a system is asked to send a datagram to a system on a different network. This problem is handled by gateways, A gateway is a system that connects a network with one or more other networks. Gateways are often normal computers that happen to have more than one network interface. For example, we have a Unix machine that has two different Ethernet interfaces. Thus it is connected to networks 128.6,4 and 128.6,3, This machine can act as a gateway between those two networks. The software on that machine must be set up so that it wig forward datagrams from one network to the other. That is, if a machine on network 128.6.4 sends a datagram to the gateway, and the datagram is addressed to a machine on network 128,6.3, the gateway will forward the datagram to the destination. Major communications centers often have gateways that connect a number of different networks. (In many cases, special-purpose gateway systems provide better performance or reliability than general-purpose systems acting as gateways. A number of vendors sell such systems.)

 

Routing in lP is based entirely upon the network number of the destination address. Each computer has a table of network numbers. For each network number, a gateway is listed. This is the gateway to be used to get to that network. Note that the gateway doesn't have to connect directly to the network. It just has to be the best place to go to get there. For example at Rutgets, our interface to NSFnet is at the John yon Neuman Supercomputer Center (JvNC). Our connection to JvNC is via


a high-speed serial line connected to a gateway whose address is 128.6.3.12. Systems on net 128.6.3 will list 128.6.3.12 as the gateway for many off-campus networks. However systems on net 128.6.4 will list 128.6.4.1 as the gateway to those same off-campus networks. 128.6.4.1 is the gateway between networks 128.6.4 and 128.6.3, so it is the first step in getting to JvNC,

 

When a computer wants to send a datagram, it first checks to see if the destination address is on the system's own local network. If so, the datagram can be sent directly. Otherwise, the system expects to find an entry for the network that the destination address is on. The datagram is sent to the gateway listed in that entry. This table can get quite big. For example, the Internet now includes several hundred individual networks. Thus various strategies have been developed to reduce the size of the routing table. One strategy is to depend upon "default routes". Often, there is only one gateway out of a network. This gateway might connect a local Ethernet to a campus-wide backbone network. In that case, we don't need to have a separate entry for every network in the world. We simply define that gateway as a "default". When no specific route is found for a datagram, the datagram is sent to the default gateway. A default gateway can even be used when there are several gateways on a network. There are provisions for gateways to send a message saying "I'm not the best gateway -- use this one instead." (the message is sent via ICMP. See RFC 792.) Most network software is designed to use these messages to add entries to their routing tables. Suppose network 128.6.4 has two gateways, 128.6.4.59 and 128.6.4.1. 128,6.4.59 leads to several other internal Rutgers networks. 128,6.4.1 leads indirectly to the NSFnet. Suppose we set 128.6.4.59 as a default gateway, and have no other routing table entries. Now what happens when we need to send a datagram to MIT? MIT is network 18. Since we have no entry for network 18, the datagram will be sent to the default,

 

128.6.4.59. As it happens, this gateway is the wrong one. So it would] forward the datagram to 128.6.4.1. But it will also send back an error saying in effect: "to get to network 18, use 128.6.4.1". Our software will then add an entry to the routing table. Any future datagrams to MIT will then go directly to 128.6.4.1. (The error message is sent using the ICMP protocol. The message type is called "ICMP redirect.")

 

Most IP experts recommend that individual computers should not try to keep track of the entire network. Instead, they should start with default gateways, and let the gateways tell them the routes, as just described. However this doesn't say how the gateways should find out about the routes. The gateways can't depend upon this strategy. They have to have fairly complete routing tables. For this, some sort of routing protocol is needed. A routing protocol is simply a technique for the gateways to find each other, and keep up to date about the best way to get to every network. RFC 1009 contains a review of gateway design and routing. However rip.doc is probably a better introduction to the subject. It contains some tutorial material, and a detailed description of the most commonly-used routing protocol.

 

Details about Internet addresses: subnets and broadcasting

 

AS indicated earlier, Internet addresses are 32-bit numbers, normally written as 4 octets (in decimal), e.g. 128,6.4.7. There are actually 3 different types of address. The problem is that the address has to indicate both the network and the host within the network. It was felt that eventually there would be lots of networks. Many of them would be small but probably 24 bits would be needed to represent all the IP networks. It was also felt that some very big networks might need 24 bits to represent all of their hosts. This would seem to lead to 48 bit

addresses. But the designers really wanted to use 32 bit addresses. So they adopted a kludge. The assumption is that most of the networks will be small. So they set up three different ranges of address. Addresses beginning with I to 126 use only the first octet for the network number. The other direct octets are available for the host number. Thus 24 bits are available for hosts. These numbers are used for large networks. But there can only be 126 of these very big networks. The Arpanet is one, and there are a few large commercial networks. But few normal organizations get one of these "class A" addresses. For normal large organizations, "class B" addresses are used. Class B addresses use the first two octets for the network number. Thus network numbers are 128,I through 191,254. (We avoid 0 and 255, for reasons that we see below, We also avoid addresses beginning with 127, because that is used by some systems for special purposes.) The last two octets are available for host addresses, giving 16 bits of host address. This allows for 64516 computers, which should be enough for most organizations. (It is possible to get more than one class B address, if you nan out.) Finally, class C addresses use three octets, in the range 192.1.1 to 223.254.254. These allow only 254 hosts on each network, but there can be lots of these networks. Addresses above 223 are reserved for future use, as class D and E (which are currently not defined).

 

Many large organizations find it convenient to divide their network number into "subnets". For example, Rutgers has been assigned a class B address, 128.6. We find it convenient in use the third octet of the address to indicate which Ethernet a host is on. This division has no significance outside of Rutgers. A computer at another institution would treat all datagrams addressed to 128.6 the same way. They would not look at the third octet of the address. Thus computers outside Rutgers would not have different routes for 128.6.4 or

128.6.5. But inside Rutgers, we treat 1286.4 and 128.6.5 as

 

separate networks, In effect, gateways inside Burgers have separate entries for each Burgers subnet, whereas gateways outside Rutgers just have one entry for 128.6, Note that we could do exactly the same thing by using a separate class C address for each Ethernet. As far as Rutgets is concerned, it would be just as convenient for us to have a number of class C addresses. However using class C addresses would make things inconvenient for the rest of the world. Every institution that wanted to talk to us would have to have a separate entry for each one of our networks. If every institution did this, there would be far too many networks for any reasonable gateway to keep track of. By subdividing a class B network, we hide our internal structure from everyone else, and save them trouble. This subnet strategy requires special provisions in the network software. It is described in RFC 950,

 

0 and 255 have special meanings. 0 is reserved for machines that don't know their address, In certain circumstances it is possible for a machine not to know the number of the network it is on, or even its own host address. For example, 0.0.0.23 would be a machine that knew it was host number 23, but didn't know on what network.

 

255 is used for "broadcast". A broadcast is a message that you want every system on the network to see. Broadcasts are used in some situations where you don't know who to talk to. For example, suppose you need to look up a host name and get its Internet address. Sometimes you don't know the address of the nearest name server. In that case, you might send the request as a broadcast. There are also cases where a number of systems are interested in information. It is then ]less expensive to send a single broadcast than to send datagrams individually to each host that is interested in the information, In order to send a broadcast, you use an address that is made by using your network address, with all ones in the part of the address where the host number goes. For example, ii you are

on network 128.6.4, you would use 128.6.4.255 for broadcasts How this ~s actually implemented depends upon the medium, h is not possible to send broadcasts on the Arpanet, or on point to point lines. However it is possible on an Ethernet. If you use an Ethernet address with all its bits on (all ones), every machine on the Ethernet is supposed to look at II,at datagram.

 

Although the official broadcast address for network 128.6.4 is now 128.6.4.255, there are some other addresses that may be treated as broadcasts by certain implementations. For convenience, the standard also allows 255.285.255.255 to be used. This refers to all hosts on the local network. It is often simpler to use 255.255,255.255 instead of finding out the network number for the local network and forming a broadcast address such as 128.6.4.255. In addition, certain older implementations may use 0 instead of 255 to form the broadcast address. Such implementations would use 128.6.4.0 instead of 128.6.4.255 as the broadcast address on network 128.6.4. Finally, certain older implementations may not understand about subnets. Thus they consider the network number to he 128.6. In that case, they will assume a broadcast address of 128.6.255.255 or 128~6.0.0. Until support for broadcasts is implemented properly, it can be a somewhat dangerous feature to use.

 

Because 0 and 255 are used for unknown and broadcast addresses, normal hosts should never be given addresses containing 0 or 255. Addresses should never begin with 0, 127, or any number above 223. Addresses violating these rules are sometimes referred to as "martians", because of rumors that the Central University of Mars is using network 225.

 

 

Datagram fragmentation and reassembly

 

TCP/IP is designed for use with many different kinds of network. Unfortunately, network designers do not agree about how big packets can be. Ethernet packets can be 1500 octets long. Arpanet packets have a maximum of around 1000 octets. Some very fast networks have much larger packet sizes, At first, you might think that IP should simply settle on the smallest possible size. Unfortunately, this would cause serious performance problems, When transferring large files, big packets are far more efficient than small ones, So we want to be able to use the largest packet size possible. But we also want to be able to handle networks with small limits. There are two provisions for this. First, TCP has the ability to "negotiate" about datagram size. When a TCP connection first opens, both ends can send the maximum datagram size they can handle. The smaller of these numbers is used for the rest of the connection, This allows two implementations that can handle big datagrams to use them, but also lets them talk to implementations that can't handle them. However this doesn't completely solve the problem. The most serious problem is that the two ends don't necessarily know about all of the steps in between. For example, when sending data between Rutgets and Berkeley, it is likely that both computers will be on Ethernets. Thus they will both be prepared to handle lS00-octet datagrams. However the connection will at some point end up going over the Arpanet. It can't handle packets of that size. For this reason, there are provisions to split datagrams up into pieces. (This is referred to as "fragmentatlon".) The IP header contains fields indicating the a datagram has been split, and enough information to [et the pieces be put back together. If a gateway connects an Ethernet to the Arpanet, it must be prepared to take 1500-octet

Ethernet packets and split them into pieces that will fit on the Arpanet. Furthermore, every host implementation of TCP/IP must be prepared to accept pieces and put them back together. This is referred to as "reassembly".

 

TCP/IP implementations differ in the approach they take to deciding on datagram size. It is fairly common for implementations to use 576-byte datagrams whenever they can't verify that the entire path is able to handle larger packets. This rather conservative strategy is used because of the number of implementatlons with bugs in the code to reassemble fragments. Implementors often try to avoid ever having fragmentation occur. Different implementors take different approaches to deciding when it is safe to use large datagrams. Some use them only for the local network. Others will use them for any network on the same campus. 576 bytes is a "sale" size, which every implementation must support.

 

Ethernet encapsulation: ARP

 

There was a brief discussion earlier about what IP datagrams look like on an Ethernet The discussion showed the Ethernet header and checksum. However it left one hole: It didn't say how to figure out what Ethernet address to use when you want to talk to a given internet address. In fact, there is a separate protocol for this. called ARP ("address resolution protocol"). (Note by the way that ARP is not an IP protocol. That is, the ARP datagrams do not have IP headers.) Suppose you are on system 128.6.4.194 and you want to connect to system

128,6.4.7. Your system will first verify that 128.6.4.7 is on the same network, so it can talk directly via Ethernet. Then it will look up 128.6.4.7 in its ARP table, to see if it already knows the Ethernet address. If so, it will stick on an Ethernet

header, and send the packet But suppose this system is not in

the ARP table. There is no way to send the packet, because

you need the Ethernet address. So it uses the AARP protocol to


send an AARP request, Essentially an AARP request says "l need the Ethernet address for 128,6.4.T'. Every system listens to AARP requests, When a system sees an AARP request for itself, it is required to respond. So 128.6.4.7 will see the request, and will respond with an AARP reply saying in effect "128.6.4.7 is 8:0:20:1:56:34". (Recall that Ethernet addresses are 48 bits. This is 6 octets. Ethernet addresses are conventionally shown in hex, using the punctuation shown.) Your system will save this information in its ARP table, so future packets will go directly. Most systems treat the AARP table as a cache, and clear entries in it ii they have not been used in a certain period of time.

 

Note by the way that AARP requests must be sent as

"broadcasts". There is no way that an ARP request can be sent directiy to the right system. After all. the whole reason for sending an ARP request is that you don't know tile

Ethernet address. So an Ethernet address of all ones is used,

i.e, ff:ff:ff:ff:ff:ff. By convention, every machine on the

Ethernet is required to pay attention to packets with this as an

address So every system sees every ARP requests. They all

look to see whether the request is for their own address. If so, they respond. If not, they could just ignore it, (Some hosts will use ARP requests to update their knowledge about other hosts on the network, even if the request isn't for them.) Note that packets whose IP address indicates broadcast (e.g.

255.255.255,255 or 128.6.4,255) are also sent with an

Ethernet address that is all ones.

 

Getting more information

 

This directory contains documents describing the major protocols. There are literally hundreds of documents, so we have chosen the ones that seem most important. Internet standards are called RFC's. RFC stands for Request for Comment. A proposed standard is initially issued as a

proposal and given an RFC number. When it is finally accepted, it is added to Official Internet Protocols, but it is still referred to by the RFC number. We have also included [two IEN's. (IEN's used to be a separate classification for more informal documents. This classification no longer exists -RFC's are now used for all official Internet documents, and a mailing fist is used for more informal reports.) The convention is that whenever an RFC is revised, the revised version gets a new number. This is fine for most purposes, but it causes problems with two documents: Assigned Numbers and Official Internet Protocols. These documents are being revised all the time, so the RFC number keeps changing, You will have to look in rfc-index.txt to find the number of the latest edition. Anyone who is seriously interested in TCP/IP should read the RFC describing IP (7911. RFC 1009 is also useful. It is a specification for gateways to be used by NSFnet. As such, it contains an overview of a lot of the TCP/IP technology. You should probably also read the description of at least one of the application protocols, just to get a feel for the way things work, Mail is probably a good one (821/8221. TCP (793) is of course a very basic specification, However the spec is fairly complex, so you should only read this when you have the time and patience to think about it carefully. Fortunately, the author of the major RFC's (Jon Postel) is a very good writer. The TCP RFC is far easier to read than you would expect, given the complexity of what it is describing. You can look at the other RFC's as you become curious about their subject matter, Here is a list~ of the documents you are more likely to want~

 

rfc-Index list of all RFC's

 

riel012 somewhat fuller list of all RFC's

 

rfcl011 Official Protocols. It's useful to scan this to see what tasks protocols have been built for. This defines which RFC's are actual standards, as opposed to requests for comments.


rfcl010 Assigned Numbers. If you are working with TCP/IP, you will probably want a hardcopy of this as a reference. It's not very exciting to read. h lists all the officially defined well-known ports and lots of other things.

 

rfcl009 NSFnet gateway specifications. A good overview of IP routing and gateway technology.

 

rfcl001/2 netBIOS: networking for PC's

 

rfc973 update on domains

 

rfc959 FTP (file transfer)

 

rfc950 subnets

 

rfc937 POP2: protocol for reading mail on PC's

 

rfc894 how IP is to be put on Ethernet, see also rfc825

 

rfc882/3 domains (the database used to go from host names to internet address and back -- also used to handle UUCP these days). See also rfc973

 

rfc854/5 telnet - protocol for remote Logins

 

rfc826 ARP - protocol for finding out Ethernet addresses rfc821/2 mail

 

rfc814 names and ports - general concepts behind well-known ports

 

rfc793 TCP rfc792 ICMP rfc791 IP rfc768 UDP



rip,doc details of the most commonly-used routing protocol

 

ien-116 old name server (still needed by several kinds of system}

 

ien-48 the Catcher model, general description of the philosophy behind TCP/IP The following documents are somewhat more specialized.

 

rfc8!3 window and acknowledgement strategies in TCP rfcS15 datagram reassembiy techniques rfcS16 fault isolation and resolution techniques rfc817 modularity and e[ficlency in implementation rfc879 tile maximum segment s~ze option in TCP rfc896 congestion control rfc827,888,904,975,985 EGP and related issues



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