Address Resolution Protocol

ARP and RARP Address Translation

Address Resolution Protocol (ARP) provides a completely different function to the network than Reverse Address Resolution Protocol (RARP). ARP is used to resolve the ethernet address of a NIC from an IP address in order to construct an ethernet packet around an IP data packet. This must happen in order to send any data across the network. Reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network.

Address Resolution Protocol (ARP)

In an earlier section, there was an example where a chat program was written to communicate between

two servers. To send data, the user (Tom) would type text into a dialog box, hit send and the following

happened:

1. The program passed Tom's typed text in a buffer, to the socket.

2. The data was put inside a TCP data packet with a TCP header added to the data. This header

    contained a source and destination port number along with some other information and a

    checksum.

3. The TCP packet was be placed inside an IP data packet with a source and destination IP address

    along with some other data for network management.

4. The IP data packet was placed inside an ethernet data packet. This data packet includes the

    destination and source address of the network interface cards (NIC) on the two computers. The

    address here is the hardware address of the respective cards and is called the MAC address.

5. The ethernet packet was transmitted over the network line.

6. With a direct connection between the two computers, the network interface card on the intended

     machine, recognized its address and grabbed the data.

7. The IP data packet was extracted from the ethernet data packet.

8. The TCP data packet was extracted from the IP data packet.

9. The data was extracted from the TCP packet and the program displayed the retrieved data (text) in

     the text display window for the intended recipient to read.

In step 4 above, the IP data was going to be placed inside an ethernet data packet, but the computer

constructing the packet does not have the ethernet address of the recipient's computer. The computer that is sending the data, in order to create the ethernet part of the packet, must get the ethernet hardware (MAC) address of the computer with the intended IP address. This must be accomplished before the ethernet packet can be constructed. The ethernet device driver software on the receiving computer is not programmed to look at IP addresses encased in the ethernet packet. If it did, the protocols could not be independent and changes to one would affect the other. This is where address resolution protocol (ARP) is used. Tom's computer sends a network broadcast asking the computer that has the recipient's IP address to send it's ethernet address. This is done by broadcasting. The ethernet destination is set with all bits on so all ethernet cards on the network will receive the data packet. The ARP message consists of an ethernet header and ARP packet. The ethernet header contains:

1. A 6 byte ethernet destination address.

2. A 6 byte ethernet source address.

3. A 2 byte frame type. The frame type is 0806 hexadecimal for ARP and 8035 for RARP

The encapsulated ARP data packet contains the following:

1. Type of hardware address (2 bytes). 1=ethernet.

2. Type of protocol address being mapped( 2 bytes). 0800H (hexadecimal) = IP address.

3. Byte size of the hardware address (1 byte). 6

4. Byte size of the protocol address (1 byte). 4

5. Type of operation. 1 = ARP request, 2=ARP reply, 3=RARP request, 4=RARP reply.

6. The sender's ethernet address (6 bytes)

7. The sender's IP address (4 bytes)

8. The recipient's ethernet address (6 bytes)

9. The recipient's IP address (4 bytes)

When the ARP reply is sent, the recipient's ethernet address is left blank.

In order to increase the efficiency of the network and not tie up bandwidth doing ARP broadcasting, each computer keeps a table of IP addresses and matching ethernet addresses in memory. This is called ARP cache. Before sending a broadcast, the sending computer will check to see if the information is in it's ARP cache. If it is it will complete the ethernet data packet without an ARP broadcast. Each entry normally lasts 20 minutes after it is created. RFC 1122 specifies that it should be possible to configure the ARP cache timeout value on the host. To examine the cache on a Windows, UNIX, or Linux computer type "arp -a".

If the receiving host is on another network, the sending computer will go through its route table and determine the correct router (A router should be between two or more networks) to send to, and it will substitute the ethernet address of the router in the ethernet message. The encased IP address will still have the intended IP address. When the router gets the message, it looks at the IP data to tell where to send the data next. If the recipient is on a network the router is connected to, it will do the ARP resolution either using it's ARP buffer cache or broadcasting.

Reverse Address Resolution Protocol (RARP)

As mentioned earlier, reverse address resolution protocol (RARP) is used for diskless computers to determine their IP address using the network. The RARP message format is very similar to the ARP format. When the booting computer sends the broadcast ARP request, it places its own hardware address in both the sending and receiving fields in the encapsulated ARP data packet. The RARP server will fill in the correct sending and receiving IP addresses in its response to the message. This way the booting computer will know its IP address when it gets the message from the RARP server.

IEEE 802 Standard

The Data Link Layer and IEEE

When we talk about Local Area Network (LAN) technology the IEEE 802 standard may be heard. This

standard defines networking connections for the interface card and the physical connections, describing how they are done. The 802 standards were published by the Institute of Electrical and Electronics Engineers (IEEE). The 802.3 standard is called ethernet, but the IEEE standards do not define the exact original true ethernet standard that is common today. There is a great deal of confusion caused by this. There are several types of common ethernet frames. Many network cards support more than one type.

The ethernet standard data encapsulation method is defined by RFC 894. RFC 1042 defines the IP to link

layer data encapsulation for networks using the IEEE 802 standards. The 802 standards define the two

lowest levels of the seven layer network model and primarily deal with the control of access to the

network media. The network media is the physical means of carrying the data such as network cable. The

control of access to the media is called media access control (MAC). The 802 standards are listed below:

•  802.1 - Internetworking
•  802.2 - Logical Link Control *
•  802.3 - Ethernet or CSMA/CD, Carrier-Sense Multiple Access with Collision detection LAN *
•  802.4 - Token-Bus LAN *
•  802.5 - Token Ring LAN *
•  802.6 - Metropolitan Area Network (MAN)
•  802.7 - Broadband Technical Advisory Group
•  802.8 - Fiber-Optic Technical Advisory Group
•  802.9 - Integrated Voice/Data Networks
•  802.10 - Network Security
•  802.11 - Wireless Networks
•  802.12 - Demand Priority Access LAN, 100 Base VG-AnyLAN

*The Ones with stars should be remembered in order for network certification testing.

Network Access Methods

There are various methods of managing access to a network. If all network stations tried to talk at once, the messages would become unintelligible, and no communication could occur. Therefore a method of being sure that stations coordinate the sending of messages must be achieved. There are several methods listed below which have various advantages and disadvantages.

 Contention

1.  Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) - Used by Ethernet
2.  Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA)

•  Token Passing - A token is passed from one computer to another, which provides transmission permission.
•  Demand Priority - Describes a method where intelligent hubs control data transmission. A computer will send a demand signal to the hub indicating that it wants to transmit. The hub sill respond with an acknowledgement that will allow the computer to transmit. The hub will allow computers to transmit in turn. An example of a demand priority network is 100VG-AnyLAN (IEEE 802.12). It uses a star-bus topology.
•  Polling - A central controller, also called the primary device will poll computers, called secondary devices, to find out if they have data to transmit. Of so the central controller will allow them to transmit for a limited time, then the next device is polled.

Token passing performs better when the network has a lot of traffic, while ethernet which uses CSMA/CD is generally faster but loses performance when the network has a lot of traffic. CSMA/CD is basically a method that allows network stations to transmit any time they want. They, however, sense the network line and detect if another station has transmitted at the same time they did. This is called a collision. If a collision happened, the stations involved will retransmit at a later, randomly set time in hopes of avoiding another collision.

IP to link layer encapsulation

The requirements for IP to link layer encapsulation for hosts on a Ethernet network

•  All hosts must be able to send and receive packets defined by RFC 894.
•  All hosts should be able to receive a mix of packets defined by RFC 894 and RFC 1042.
•  All hosts may be able to send RDC 1042 defined packets.

Hosts that support both must provide a means to configure the type of packet sent and the default must be

packets defined by RFC 894.

Ethernet and IEEE 802 Encapsulation formats

Ethernet (RFC 894) message format consists of:

1. 6 bytes of destination address.

2. 6 bytes of source address.

3. 2 bytes of message type which indicates the type of data being sent.

4. 46 to 1500 bytes of data.

5. 4 bytes of cyclic redundancy check (CRC) information.

IEEE 802 (RFC 1042) Message format consists of 3 sections plus data and CRC as follows:

•  802.3 Media Access Control section used to coordinate the sending of data between computers.

      1. 6 bytes of destination address.

      2. 6 bytes of source address.

      3. 2 bytes of length - The number of bytes that follow not including the CRC.

•  802.2 Logical Link control establishes service access points (SAPs) between computers.

      1. 1 byte destination service access point (DSAP).

      2. 1 byte source service access point (SSAP).

      3. 1 byte of control.

•  Sub Network Access Protocol (SNAP).

      1. 3 bytes of org code.

      2. 2 bytes of message type which indicates the type of data being sent.

•  38 to 1492 bytes of data.
•  4 bytes of cyclic redundancy check (CRC) information.

Some ethernet message types include:

•  0800 - IP datagram with length of 38 to 1492 bytes. 
•  0806 - ARP request or reply with 28 bytes and pad bytes that are used to make the frame long enough for the minimum length.
•  8035 - RARP request or reply of 28 bytes and pad bytes that are used to make the frame long enough for the minimum length.

These message types are the same for both formats above with the exception of the pad bytes. The pad bytes for the RFC 894 and RFC 1042 datagrams are of different lengths between the two message formats because the RFC 894 minimum message length is 46 bytes and the RFC 1042 minimum message length is 38 bytes. Also the two message formats above are distinguishable from each other. This is because the RFC 894 possible length values are exclusive of RFC 1042 possible type values.

Trailor Encapsulation

This is described in RFC 1122 and RFC 892, but this scheme is not used very often today. The trailer protocol [LINK:1] is a link-layer encapsulation method that rearranges the data contents of packets sent on the physical network. It may be used but only after it is verified that both the sending and receiving hosts support trailers. The verification is done for each host that is communicated with.

RFC 1122 states: "Only packets with specific size attributes are encapsulated using trailers, and typically only a small fraction of the packets being exchanged have these attributes. Thus, if a system using trailers exchanges packets with a system that does not, some packets disappear into a black hole while others are

delivered successfully."

Trailer negotiation is performed when ARP is used to discover the media access control (MAC) address

of the destination host. RFC 1122 states: "a host that wants to speak trailers will send an additional "trailer ARP reply" packet, i.e., an ARP reply that specifies the trailer encapsulation protocol type but otherwise has the format of a normal ARP reply. If a host configured to use trailers receives a trailer ARP reply message from a remote machine, it can add that machine to the list of machines that understand trailers, e.g., by marking the corresponding entry in the ARP cache."

Network Protocol Levels

You should be aware of the fact, that when talking about networking you will hear the word "protocol" all the
time. This is because protocols are sets of standards that define all operations within a network. They define how various operations are to be performed. They may even define how devices outside the network can interact with the network. Protocols define everything from basic networking data structures, to higher level application programs. They define various services and utility programs. Protocols operate at many layers of the network models described below. There are protocols considered to be transport protocols such as TCP and UDP. Other protocols work at the network layer of the OSI network model shown below, and some protocols work at several of the network layers.

RFCs

Protocols are outlined in Request for Comments (RFCs). At the end of this document is a list of protocols and associated RFC numbers.Protocols. Although RFCs define protocols not all RFCs define protocols but may define other requirements for the internet such as RFC 1543 which provides information about the preparation of RFCs. The following RFCs are very central to the TCP/IP protocol.

• RFC 1122 - Defines host requirements of the TCP/IP suite of protocols covering the link, network (IP), and transport (TCP, UDP) layers.
• RFC 1123 - The companion RFC to 1122 covering requirements for internet hosts at the application layer
• RFC 1812 - Defines requirements for internet gateways which are IPv4 routers

Network Models

There are several network models which you may hear about but the one you will hear about most is the ISO network model described below. You should realize, however that there are others such as

• The internet layered protocol
• The TCP/IP 4 layered protocol
• The Microsoft networking protocol

If you don't like any of these models, feel free to invent your own along with your own networking scheme of

course, and add it to the list above. You can call it "The MyName Protocol". Ever wonder why networking can be so complex and confusing? Welcome to the world of free enterprise!

The ISO Network Model Standard

The International Standards Organization (ISO) has defined a standard called the Open Systems Interconnection (OSI) reference model. This is a seven layer architecture listed below. Each layer is considered to be responsible for a different part of the communications. This concept was developed to accommodate changes in technology. The layers are arranged here from the lower levels starting with the physical (hardware) to the higher levels.

1. Physical Layer - The actual hardware.

2. Data Link Layer - Data transfer method (802x ethernet). Puts data in frames and ensures error free

transmission. Also controls the timing of the network transmission. Adds frame type, address, and error

control information. IEEE divided this layer into the two following sublayers.

•  Logical Link control (LLC) - Maintains the Link between two computers by establishing Service Access Points (SAPs) which are a series of interface points. IEEE 802.2.
• Media Access Control (MAC) - Used to coordinate the sending of data between computers. The 802.3, 4, 5, and 12 standards apply to this layer. If you hear someone talking about the MAC address of a network card, they are referring to the hardware address of the card.

3. Network Layer - IP network protocol. Routes messages using the best path available.

4. Transport Layer - TCP, UDP. Ensures properly sequenced and error free transmission.

5. Session Layer - The user's interface to the network. Determines when the session is begun or opened, how long it is used, and when it is closed. Controls the transmission of data during the session. Supports security and name lookup enabling computers to locate each other.

6. Presentation Layer - ASCII or EBCDEC data syntax. Makes the type of data transparent to the layers

around it. Used to translate date to computer specific format such as byte ordering. It may include compression. It prepares the data, either for the network or the application depending on the direction it is

going.

7. Application Layer - Provides services software applications need. Provides the ability for user applications to interact with the network.

 

Many protocol stacks overlap the borders of the seven layer model by operating at multiple layers of the model. File Transport Protocol (FTP) and telnet both work at the application, presentation, and the session layers.

The Internet, TCP/IP, DOD Model

This model is sometimes called the DOD model since it was designed for the department of defense It is also

called the TCP/IP four layer protocol, or the internet protocol. It has the following layers:

1. Link - Device driver and interface card which maps to the data link and physical layer of the OSI model.

2. Network - Corresponds to the network layer of the OSI model and includes the IP, ICMP, and IGMP

protocols.

3. Transport - Corresponds to the transport layer and includes the TCP and UDP protocols.

4. Application - Corresponds to the OSI Session, Presentation and Application layers and includes FTP,

Telnet, ping, Rlogin, rsh, TFTP, SMTP, SNMP, DNS, your program, etc.

Please note the four layer TCP/IP protocol. Each layer has a set of data that it generates.

1. The Link layer corresponds to the hardware, including the device driver and interface card. The link layer

has data packets associated with it depending on the type of network being used such as ARCnet, Token

ring or ethernet. In our case, we will be talking about ethernet.

2. The network layer manages the movement of packets around the network and includes IP, ICMP, and

IGMP. It is responsible for making sure that packages reach their destinations, and if they don't, reporting

errors.

3. The transport layer is the mechanism used for two computers to exchange data with regards to software.

The two types of protocols that are the transport mechanisms are TCP and UDP. There are also other typesof protocols for systems other than TCP/IP but we will talk about TCP and UDP in this document.

4. The application layer refers to networking protocols that are used to support various services such as FTP, Telnet, BOOTP, etc. Note here to avoid confusion, that the application layer is generally referring to

protocols such as FTP, telnet, ping, and other programs designed for specific purposes which are governed

by a specific set of protocols defined with RFC's (request for comments). However a program that you

may write can define its own data structure to send between your client and server program so long as the

program you run on both the client and server machine understand your protocol. For example when your

program opens a socket to another machine, it is using TCP protocol, but the data you send depends on

how you structure it.

Data Encapsulation, a Critical concept to be understood

When starting with protocols that work at the upper layers of the network models, each set of data is wrapped inside the next lower layer protocol, similar to wrapping letters inside an envelope. The application creates the data, then the transport layer wraps that data inside its format, then the network layer wraps the data, and finally the link (ethernet) layer encapsulates the data and transmits it.

To continue, you should understand the definition of a client and server with regards to networking. If you are a server, you will provide services to a client, in much the same way as a private investigator would provide services to their clients. A client will contact the server, and ask for service, which the server will t are many clients, a server must be constantly ready to receive client requests, even though it may already be working with other clients. Usually the client program will operate on one computer, while the server program will operate on another computer, although programs can be written to be both a client and a server.

Lets say you write a client chat program and a server chat program to be used by two people to send messages between their machines. You run the server program on machine B, and the client program on machine A. Tom is on machine A and George is on machine B. George's machine is always ready to be contacted, but cannot initiate a contact. Therefore if George wants to talk to Tom, he cannot, until Tom contacts him. Tom, of course can initiate contact at any time. Now you decide to solve the problem and merge the functionality of the two programs into one, so both parties may contact the other. This program is now a client/server program which operates both as a client and a server. You write your code so when one side initiates contact, he will get a dialog box, and a dialog box will pop up on the other side. At the time contact is initiated, a socket is opened between the two machines and a virtual connection is established. The program will let the user (Tom) type text into the dialog window, and hit send. When the user hits send, roughly the following will happen.

1. Your program will pass Tom's typed text in a buffer, to the socket. This happens on machine A.

2. The underlying software (Code in a library called by a function your program used to send the data) supporting the socket puts the data inside a TCP data packet. This means that a TCP header will be          added to the data. This header contains a source and destination port number along with some other information

and a checksum. Deamon programs (Daemon definition at the bottom of this page) may also work at this

level to sort packages based on port number (hence the TCP wrapper program in UNIX and Linux).

3. The TCP packet will be placed inside an IP data packet with a source and destination IP address along  with some other data for network management. This may be done by a combination of your library  function, the operating system and supporting programs.

4. The IP data packet is placed inside an ethernet data packet. This data packet includes the destination and source address of the network interface cards (NIC) on the two computers. The address here is the   hardware address of the respective cards and is called the MAC address.

5. The ethernet packet is transmitted over the network line.

6. Assuming there is a direct connection between the two computers, the network interface card on machine

B, will recognize its MAC address and grab the data.

7. The IP data packet will be extracted from the ethernet data packet. A combination of deamons and the

operating system will perform this operation.

8. The TCP data packet will be extracted from the IP data packet. A combination of deamons, the operating

system, and libraries called by your program will perform this function.

9. The data will be extracted from the TCP packet. Your program will then display the retrieved data (text) in the text display window for George to read.

Be aware that for the sake of simplicity, we are excluding details such as error management, routing, and

identifying the hardware address of the NIC on the computer intended to receive the data. Also we are not

mentioning the possible rejection of service based on a packet's port number or sender's IP address.

A deamon program is a program that runs in the background on a computer operating system. It is used to

perform various tasks including server functions. It is usually started when the operating system is booted, but a user or administrator may be able to start or stop a daemon at any time.