Chapter 19 Binding Protocol Addresses (ARP) Chapter 20 IP Datagrams and Datagram Forwarding

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?