ChapterChapter 1919Binding Protocol Addresses (ARP)Binding Protocol Addresses (ARP)

Chapter 20Chapter 20IP Datagrams and Datagram IP Datagrams and Datagram

ForwardingForwarding

Chapter 19Chapter 19TopicsTopics

• Protocol Addresses and Packet Delivery• Address Resolution Techniques

– Table Lookup– Closed-Form Computation– Message Exchange

• ARP Message Delivery and Format• Sending ARP Messages• Caching ARP Responses• Incoming ARP Messages• Layering: The Protocol Address Boundary

The ProblemThe Problem

• IP provides the illusion of a single, large network.• However, an internet really consists of many networks

possibly using incompatible hardware.• IP addresses are virtual- They are maintained by

software and hardware doesn’t understand them.• Before protocol software can send a packet across a

physical network, the software must translate the IP address of the destination computer to the equivalent hardware address.

• Three Address Resolution Techniques– Table Lookup– Mathematical Function– Distributed Computation

Address Resolution TechniquesAddress Resolution Techniques

• Table Lookup

• Closed-Form Computation

• Message Exchange Address Resolution

Table LookupTable Lookup

• Used most often in WANs• Data structure contains

address bindings.• One address binding

table per physical network.

• direct indexing improves efficiency in large networks.– The suffix is used as an

index into an array.

Closed –Form ComputationClosed –Form Computation

• Used with network technologies that use static physical addresses.

• Uses a mathematical function to map an IP address to a hardware address.

• In networks with configurable addresses the addresses may be chosen to optimize the translation. – Ex: make the IP suffix equal to the computers

hardware address.• Hardware_address = ip_address & 0xff

Message Exchange Address Message Exchange Address ResolutionResolution

• Most often use in LANs with static addressing• The computer that needs to resolve an address sends a message

across a network and receives a reply. – Request carries an IP address – Reply carries the corresponding hardware address.

• Two possible designs depend on where the request is sent.– One or more servers

• A message is sent to one of the servers which send a reply.

– Each computer participates by answering the resolution requests for its address.

• Computer broadcasts a request on the network. Every machine receives the request and examines the IP address, but only the computer with that address responds.

Address Resolution Protocol (ARP)Address Resolution Protocol (ARP)

• The standard used by TCP/IP to resolve addresses.

• Defines two message types: a request and a response.

• Specifies exactly how ARP messages should be sent – In a hardware frame

– Requests are broadcast to all computers

– Responses are sent directly back to the requesting computer.

ARP Message FormatARP Message Format

• Describes the general form for ARP messages and how to determine the details for each network hardware.

• Variable protocol and hardware address length add generallity– Adaptable to future address lengths

– Most often used to bind IP addresses to Ethernet addresses • HADDR LEN = 6 (octets) PADDR LEN = 4 (octets)

Sending ARP MessagesSending ARP Messages

• When sent, an ARP message is encapsulated in a hardware frame.– Treated as data– The network does not

examine the contents

• The ARP message is identified in the type field of the frame header– Ex: Ethernet specifies type

hex 0x806– Request or Response?

Caching ARP ResponsesCaching ARP Responses

• Message exchange is inefficient– Three packets are sent for each ARP message.– Computer communication is usually repeated many

times.

• To reduce network traffic ARP maintains a small table of bindings as a cache.– The cache is searched while performing a binding– If the binding is found no request is transmitted.

Incoming ARP MessagesIncoming ARP Messages

• When a message arrives protocol specifies two steps.

– Extract the sender’s address binding and check cache.– Examine the message’s OPERATION field.

• Request or Response?• If response, the receiver must have sent a request• If request, compare the TARGET PADDR to the local protocol address and

send a response.

• To optimize the process the computer replies and adds the binding to its cache for later use.– Computer communication is bidirectional

• There is a high probability that this response will be followed by another reply.

• This eliminates the need for the responding computer to issue an ARP request.

Layering: The Protocol Address Layering: The Protocol Address BoundaryBoundary

• Recall the TCP/IP layering model.

• A conceptual boundary between the network interface layer and higher layers hides physical addressing.– Protocol Address Boundary– Above this boundary

applications and protocol software use protocol addresses exclusively.

Chapter 19: SummaryChapter 19: Summary

• Hardware does not recognize IP addresses so hardware addresses must be resolved.

• There are three methods of resolving addresses which depend on hardware.– Table Lookup, Computation, Message Request

• TCP/IP suite uses Address Resolution Protocol (ARP)– ARP defines message formats, rules of exchange, and how

these messages should be processed.

• Questions?

Chapter 20:Chapter 20:TopicsTopics

• Virtual Packets

• IP Datagram

• Datagram Forwarding

• Routing Tables

• Next Hop Addresses

• The IP Datagram Header

• Best-Effort Delivery

Virtual PacketsVirtual Packets

• Goal: Universal packet communication system

• Problem: an internet consists of heterogeneous networks.– Hardware frames may be incompatible

between networks.– Routers can not route them from one network

to the next because frame formats differ.

Virtual PacketsVirtual Packets

• Solution: IP defines an internet packet format (Universal, Virtual Packet)– Independent of underlying hardware.– Can be transferred intact

• Virtual-created and handled entirely by software.

• Universal-Every host or router in an internet contains this protocol software.

The IP DatagramThe IP Datagram

• TCP/IP protocol packet• Same general format as hardware frame• Flexible

– Size determined by the sending application (1-64K octets)• More throughput, less overhead

• Similar to a frame header, the datagram header also contains Sending and Destination IP address for routing across an internet.

Datagram ForwardingDatagram Forwarding

• Datagrams traverse a path from source to destination through routers.– Routers extract the destination address and determine the next

hop.– Each IP router maintains a routing table.– ex: Net 2 to Net 4– Each entry is a network, not a host (reduces table size)

Routing TablesRouting Tables

• An actual table contains– IP address network prefix– Mask– Next Hop

• Routing-The process of selecting a next hop

• Routers compute the Boolean “and” of the mask and destination address.– Efficient extraction

• Compares to the tables destination field.

• Routers are assigned one IP address per network.

Next Hop AddressesNext Hop Addresses

• After the router determines the next hop the datagram is sent directly to the next router. – The destination address remains unchanged.– Address binding is used to resolve the next

routers hardware address.

The IP Datagram HeaderThe IP Datagram Header

• Specifies characteristics of the datagram.• VERS-protocol version number (currently 4)• SERVICE TYPE-specifies either a minimal delay or maximal

throughput route• TOTAL LENGTH-the total number of octets including data and

header• TIME TO LIVE-prevents a datagram from traveling forever in a loop• IP OPTIONS may be present therefore header length is variable

Best-Effort DeliveryBest-Effort Delivery

• Best effort describes the service IP offers.– It does not guarantee that

• Datagram duplication• Delayed or out-of-order delivery• Corruption of data• Datagram loss

– Will not occur

• IP is not responsible for these problems– Physical networks may cause the problems– Software using IP must solve them

Chapter 20 SummaryChapter 20 Summary

• Virtual Packets are used by IP software to send messages.– Datagrams in TCP/IP

• Routers use routing tables to forward datagrams through an internet.

• When sending to the next hop, next hop addresses do not replace the destination address in the datagram header.

• IP datagram headers specify characteristics of the datagram for routing.

• Because an internet consists of many types of hardware IP can not guarantee certain problems will not occur. – These problems must be resolved by other layers.

• Questions?