Computer Networks and Internets, 5e By Douglas E. Comer

Computer Networks and Internets, 5e By Douglas E. Comer
Lecture PowerPoints By Lami Kaya, © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Chapter 23 Support Protocols and Technologies
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Topics Covered 23.1 Introduction 23.2 Address Resolution
23.3 The Address Resolution Protocol (ARP) 23.4 ARP Message Format 23.5 ARP Encapsulation 23.6 ARP Caching and Message Processing 23.7 The Conceptual Address Boundary 23.8 Internet Control Message Protocol (ICMP) 23.9 ICMP Message Format and Encapsulation Protocol Software, Parameters, and Configuration Dynamic Host Configuration Protocol (DHCP) © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Topics Covered 23.12 DHCP Protocol Operation and Optimizations
DHCP Message Format Indirect DHCP Server Access Through a Relay Network Address Translation (NAT) NAT Operation and Private Addresses Transport-Layer NAT (NAPT) NAT and Servers NAT Software and Systems for Use at Home © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.1 Introduction This chapter Future chapters
continues the discussion of internetworking by introducing four key support technologies: address binding error reporting bootstrapping address translation Each technology handles a small problem When combined with other protocols each makes a significant contribution to the overall functionality Future chapters extend the discussion of internetworking by focusing on transport layer protocols and Internet routing protocols © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.2 Address Resolution The principle is:
A crucial step of the forwarding process requires a translation: forwarding uses IP addresses a frame transmitted must contain the MAC address of the next hop IP must translate the next-hop IP address to a MAC address The principle is: IP addresses are abstractions provided by protocol software Network does not know how to locate a computer from its IP address the next-hop address must be translated to an equivalent MAC address Translation from a computer's IP address to an equivalent hardware address is known as address resolution And an IP address is said to be resolved to the correct MAC address Address resolution is local to a network © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.2 Address Resolution One computer can resolve the address of another computer only if both computers attach to the same physical network A computer never resolves the address of a computer on a remote network Address resolution is always restricted to a single network. For example, consider the simple internet in Figure 23.1 © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.3 The Address Resolution Protocol (ARP)
What algorithm does software use to translate? The answer depends on the protocol and hardware addressing here we are only concerned with the resolution of IP Most hardware has adopted the 48-bit Ethernet In Ethernet: Address Resolution Protocol (ARP) Consider Figure 23.2 Suppose B needs to resolve the IP address of C B broadcasts a request that says: “I'm looking for the MAC address of a computer that has IP address C” The broadcast only travels across one network An ARP request message reaches all computers on a network When C receives a copy of the request along other hosts Only C sends a directed reply back to B that says: “I'm the computer with IP address C, and my MAC address is M” © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.3 The Address Resolution Protocol (ARP)

23.4 ARP Message Format Rather than restricting ARP to IP and Ethernet
The standard describes a general form for ARP messages It specifies how the format is adapted for each type of protocol Choosing a fixed size for a hardware address is not suitable New network technologies might be invented that have addresses larger than the size chosen The designers included a fixed-size field at the beginning of an ARP message to specify the size of the hardware addresses being used For example, when ARP is used with an Ethernet the hardware address length is set to 6 octets because an Ethernet address is 48 bits long © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.4 ARP Message Format To increase the generality of ARP
the designers also included an address length field ARP protocol can be used to bind an arbitrary high-level address to an arbitrary hardware address In practice, the generality of ARP is seldom used most implementations of ARP are used to bind IP addresses to Ethernet addresses Figure 23.3 illustrates the format of an ARP message when the protocol is used with an IP version 4 address (4 octets) and Ethernet hardware address (6 octets) each line of the figure corresponds to 32 bits of an ARP message © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.4 ARP Message Format HARDWARE ADDRESS TYPE PROTOCOL ADDRESS TYPE
16-bit field that specifies the type of hardware address being used the value is 1 for Ethernet PROTOCOL ADDRESS TYPE 16-bit field that specifies the type of protocol address being used the value is 0x0800 for IPv4 HADDR LEN 8-bit integer that specifies the size of a hardware address in bytes PADDR LEN 8-bit integer that specifies the size of a protocol address in bytes OPERATION 16-bit field that specifies whether the message request (the field contains 1) or response (the field contains 2) © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.4 ARP Message Format SENDER HADDR SENDER PADDR TARGET HADDR
HADDR LEN bytes for the sender's hardware address SENDER PADDR PADDR LEN bytes for the sender's protocol address TARGET HADDR HADDR LEN bytes for the target's hardware address TARGET PADDR PADDR LEN bytes for the target's protocol address © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.4 ARP Message Format As Figure 23.3 shows
An ARP message contains fields for two address bindings one binding to the sender other to the intended recipient, ARP calls it target When a request is sent the sender does not know the target's hardware address (that is the information being requested) therefore, field TARGET HADDR in an ARP request can be filled with zeroes (0s) because the contents are not used In a response the target binding refers to the initial computer that sent the request Thus, the target address pair in a response serves no purpose the inclusion of the target fields has survived from an early version of the protocol © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.5 ARP Encapsulation When it travels across a physical network
an ARP message is encapsulated in a hardware frame An ARP message is treated as data being transported the network does not parse the ARP message or interpret fields Figure 23.4 illustrates ARP encapsulation in an Ethernet frame © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.5 ARP Encapsulation The type field in the frame header specifies that the frame contains an ARP message A sender must assign the appropriate value to the type field before transmitting the frame And a receiver must examine the type field in each incoming frame Ethernet uses type field 0x806 to denote an ARP message The same value is used for both ARP requests/ responses Frame type does not distinguish between types of ARP messages A receiver must examine the OPERATION field in the message to determine whether an incoming message is a request or a response © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.6 ARP Caching and Message Processing
Sending an ARP request for each datagram is inefficient Three (3) frames traverse the network for each datagram (an ARP request, ARP response, and the data datagram itself) Most communications involve a sequence of packets a sender is likely to repeat the exchange many times To reduce network traffic ARP software extracts and saves the information from a response so it can be used for subsequent packets The software does not keep the information indefinitely Instead, ARP maintains a small table of bindings in memory ARP manages the table as a cache an entry is replaced when a response arrives the oldest entry is removed whenever the table runs out of space or after an entry has not been updated for a long period of time ARP starts by searching the cache when it needs to bind an address © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

If the binding is present in the cache ARP uses the binding without transmitting a request If the binding is not present in the cache ARP broadcasts a request waits for a response updates the cache and then proceeds to use the binding The cache is only updated when an ARP message arrives (either a request or a response) Algorithm 23.1 outlines the procedure for handling an incoming ARP message © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Check the text book for details of the algorithm For optimization, it is necessary to know two facts: Most computer communication involves two-way traffic if a message from A to B, probability is high that a reply will be from B back to A Each address binding requires memory a computer cannot store an arbitrary number of address bindings The first fact explains why extracting the sender's address binding optimizes ARP performance © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.7 The Conceptual Address Boundary
ARP provides an important conceptual boundary between MAC addresses and IP addresses: ARP hides the details of hardware addressing It allows higher layers of software to use IP addresses There is an important conceptual boundary imposed between the network interface layer and all higher layers Figure 23.5 illustrates the addressing boundary © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.7 The Conceptual Address Boundary

23.8 Internet Control Message Protocol (ICMP)
IP defines a best-effort communication service datagrams can be lost, duplicated, delayed, or delivered out of order It seems best-effort service does not need error detection! But IP attempts to avoid errors and to report problems when they occur We have already seen one example of error detection in IP: Header checksum is used to detect transmission errors When a host creates an IP datagram the host includes a checksum that covers the entire header Whenever a datagram is received the checksum is verified to ensure that the header arrived intact The IP header contains a TIME TO LIVE field used to prevent a datagram from circulating forever if the forwarding tables in routers incorrectly introduce a circular path © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

Response to a checksum error is following: Datagram must be discarded immediately without more processing The receiver cannot trust any fields in the datagram header because the receiver cannot know which bits were altered The receiver cannot send an error message back to the sender because the receiver cannot trust the source address in the header Thus, the receiver has no option but to discard the damaged datagram IP includes a companion protocol, ICMP It is used to report errors back to the original source IP and ICMP are co-dependent IP depends on ICMP to report errors and ICMP uses IP to carry error messages Many ICMP messages have been defined Figure 23.6 lists key ICMP messages and their purpose © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

As in Figure 23.6, ICMP contains two message types: messages used to report errors messages used to obtain information For example Time Exceeded and Destination Unreachable are for reporting errors when a datagram cannot be delivered successfully A destination is unreachable if no route exists to the address A datagram times out if either the TTL count in the header expires or fragments of the datagram do not arrive before the timer expires Echo Request and Echo Reply do not correspond to an error Instead, they are used by the ping application to test connectivity When a host receives an echo request message ICMP software on a host or router sends an echo reply that carries the same data as the request © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.9 ICMP Message Format and Encapsulation
ICMP uses IP to transport each error message: when a router has an ICMP message to send it creates an IP datagram and encapsulates the ICMP message in it the ICMP message is placed in the payload area of the IP datagram the datagram is then forwarded as usual with the complete datagram being encapsulated in a frame for transmission Figure 23.7 (below) illustrates the two levels of encapsulation © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.9 ICMP Message Format and Encapsulation
ICMP messages do not have special priority They are forwarded like any other datagram, with one minor exception If an ICMP error message causes an error no error message is sent The reason should be clear: the designers wanted to avoid the Internet becoming congested carrying error messages about error messages © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.10 Protocol Software, Parameters, and Configuration
Once a host or router has been powered on, OS is started and the protocol software is initialized How does the protocol software in a host or router begin operation? For a router, the configuration manager must specify initial values for items such as the IP address for each network connection the protocol software to run and initial values for a forwarding table the configuration is saved, and a router loads the values during startup Host configuration usually uses a two-step process, known as bootstrapping A protocol was invented to allow a host to obtain multiple parameters with a single request, known as the Bootstrap Protocol (BOOTP) Currently, DHCP is used to take care of most configuration needed © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.11 Dynamic Host Configuration Protocol (DHCP)
Various mechanisms have been created to allow a host computer to obtain parameters An early mechanism known as the Reverse Address Resolution Protocol (RARP) allowed a computer to obtain an IP address from a server ICMP has Address Mask Request and Router Discovery messages can obtain the address mask used and the address of a router Each of the early mechanisms was used independently requests were broadcast and a host typically configured layers from lowest to highest DHCP allows a computer to join a new network and obtain an IP address automatically The concept has been termed plug-and-play networking © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

When a computer boots the client computer broadcasts a DHCP Request the server sends a DHCP Reply DHCP uses the term offer to denote the message a server sends and we say that the server is offering an address to the client We can configure a DHCP server to supply two types of addresses: permanently assigned addresses as provided by BOOTP or a pool of dynamic addresses to be allocated on demand Typically, a permanent address is assigned to a server, and a dynamic address is assigned to an arbitrary host In fact, addresses assigned on demand are not given out for an arbitrary length of time © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 32 32

DHCP issues a lease on the address for a finite period The use of leases allows a DHCP server to reclaim addresses When the lease expires the server places the address to the pool of available addresses this allows the address to be assigned to another computer When a lease expires, a host can choose to relinquish the address or renegotiate with DHCP to extend the lease Negotiation occurs concurrent with other activity Normally, DHCP approves each lease extension A computer continues to operate without any interruption However, a server may be configured to deny lease extension for administrative or technical reasons DHCP grants absolute control of leasing to a server If a server denies an extension request the host must stop using the address © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 33 33

23.12 DHCP Protocol Operation and Optimizations
DHCP includes several important details that optimize performance, such as Recovery from loss or duplication DHCP is designed to insure that missing or duplicate packets do not result in misconfiguration If no response is received a host retransmits its request If a duplicate response arrives a host ignores the extra copy Caching of a server address once a host finds a DHCP server the host caches the server's address Avoidance of synchronized flooding DCHP takes steps to prevent synchronized requests © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

DHCP Message Format DHCP adopted a slightly modified version of the BOOTP message format Figure 23.8 illustrates the DHCP message format OP specifies whether the message is a Request or a Response HTYPE and HLEN fields specify the network hardware type and the length of a hardware address FLAGS specifies whether it can receive broadcast or directed replies HOPS specifies how many servers forwarded the request TRANSACTION IDENTIFIER provides a value that a client can use to determine if an incoming response matches its request SECONDS ELAPSED specifies how many seconds have elapsed since the host began to boot Except for OPTIONS (OP), each field in a DHCP message has a fixed size © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

DHCP Message Format Later fields in the message are used in a response to carry information back to the host that sent a request if a host does not know its IP address, the server uses field YOUR IP ADDRESS to supply the value server uses fields SERVER IP ADDRESS and SERVER HOST NAME to give the host information about the location of a server ROUTER IP ADDRESS contains the IP address of a default router DHCP allows a computer to negotiate to find a boot image To do so, the host fills in field BOOT FILE NAME with a request The DHCP server does not send an image © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 37 37

23.14 Indirect DHCP Server Access Through a Relay
DHCP broadcasts on the local network to find a server DHCP does not require each individual network to have a server Instead, a DHCP relay agent forwards requests and responses between a client and the server At least one relay agent must be present on each network and the relay agent must be configured with the address of the appropriate DHCP server When the server responds the relay agent forwards the response to the client It may seem that using multiple relay agents is no better than using multiple DHCP servers © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.14 Indirect DHCP Server Access Through a Relay
Network managers prefer to manage multiple relay agents for two reasons First In a network with one DHCP server and multiple relay agents, administration of addresses is centralized into a single device Thus, a network manager does not need to interact with multiple devices to change the lease policy or determine the current status Second Many commercial routers contain a mechanism that provides DHCP relay service on all the networks to which the router attaches Relay agent facilities in a router are usually easy to configure and the configuration is unlikely to change © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 39 39

23.15 Network Address Translation (NAT)
The Internet has expanded and addresses became scarce subnet and classless addressing (CIDR) were introduced to help conserve addresses Another mechanism was invented that allows multiple computers at a site to share a single, globally valid IP address, known as Network Address Translation (NAT) NAT provides transparent communication a host in the Internet always appears to receive communication from a single computer rather than from one of many computers at the site NAT runs as an in-line service It must be placed on the connection between the Internet and a site Most implementations embed NAT in another device such as a Wi-Fi wireless access point or an Internet router © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 40 40

23.16 NAT Operation and Private Addresses
The goal of NAT is to provide an illusion When viewed from the Internet the site appears to consist of a single host computer that has been assigned a valid IP address all datagrams sent from the site appear to originate from one host and all datagrams sent to the site appear to be sent to one host When viewed from a host in the site the Internet appears to accept and route private addresses A single IP address cannot be assigned to multiple computers if two or more computers use the same address conflicts arise because multiple computers will respond to an ARP request © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

NAT solves the problem by using two (2) types of addresses The NAT device itself is assigned a single globally-valid IP address as if the NAT device were a host on the Internet Each computer at the site is assigned a unique private address also known as a nonroutable address Figure (below) lists address blocks that the IETF has designated as private © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 43 43

Private addressing is only used inside a site Before a datagram from the site can be allowed onto the Internet NAT must translate the private IP into a globally valid IP address NAT must translate the globally valid IP address in an incoming packet to a private address before transferring a datagram to a host at the site Basic function of NAT is a two-way translation, such as the source address translation as a datagram passes from the site to the Internet and the destination address translation as a datagram passes from the Internet to the site © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 44 44

Most implementations of NAT use a translation table to store the information needed to rewrite addresses Figure (below) shows a translation table that corresponds to the address mapping in Figure 23.11 © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 46 46

23.17 Transport-Layer NAT (NAPT)
NAT handles situations in which each host at a site communicates with a unique server in the Internet If two hosts at the site attempt to communicate with remote server X, the translation table will contain multiple entries for X and NAT will not be able to route incoming datagrams Basic NAT also fails in some situations, such as Consider problems when two or more applications running on a given host at a site attempt simultaneous communication with different destinations on the Internet © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

A variation of NAT, called Network Address and Port Translation (NAPT) avoids such problems: It allows a site to have arbitrary numbers of applications running on arbitrary hosts all communicating simultaneously with arbitrary destinations throughout the Internet most networking professionals assume the term NAT means NAPT Applications use protocol port numbers to distinguish among services In addition to a table of source and destination addresses NAPT uses port numbers to associate each datagram with a TCP or UDP flow © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 48 48

Instead of stopping at the IP-layer NAPT operates on transport-layer headers NAPT entries contain a 4-tuple of source and destination IP addresses and protocol port numbers To avoid a conflict NAPT must choose an alternative TCP source port for the connections Figure (below) shows one possibility (web-server) © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved. 49 49

NAT and Servers NAT system builds a translation table automatically by watching outgoing traffic and establishing a new mapping whenever an application at the site initiates communication Automatic table construction does not work well for communication initiated from the Internet to the site For example, if multiple computers at a site each run a web server the NAT device cannot know which computer should receive an incoming web connection A variant of NAT called Twice NAT has been created to allow a site to run multiple servers: Twice NAT arranges for the NAT system to interact with the site's DNS server © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.19 NAT Software and Systems for Use at Home
NAT is especially useful at a residence or small business that has a broadband connection it allows a set of computers to share the connection without requiring the customer to purchase additional IP addresses A NAT software can make a PC act as a NAT device Also, dedicated NAT hardware systems are available at low cost Such systems are usually called wireless routers The terminology is slightly misleading because such routers also provide wired connections for host computers Figure illustrates how such a router is connected © 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.

23.19 NAT Software and Systems for Use at Home

Computer Networks and Internets, 5e By Douglas E. Comer

Similar presentations

Presentation on theme: "Computer Networks and Internets, 5e By Douglas E. Comer"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Computer Networks and Internets, 5e By Douglas E. Comer

Similar presentations

Presentation on theme: "Computer Networks and Internets, 5e By Douglas E. Comer"— Presentation transcript:

Similar presentations

About project

Feedback