Transcript Chapter 19

Chapter 19
Binding Protocol Addresses (ARP)
Chapter 20
IP Datagrams and Datagram
Forwarding
Chapter 19
Topics
• 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 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 Techniques
• Table Lookup
• Closed-Form Computation
• Message Exchange Address Resolution
Table 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 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
Resolution
• 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)
• 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 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 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 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 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
Boundary
• 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: 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:
Topics
•
•
•
•
•
•
•
Virtual Packets
IP Datagram
Datagram Forwarding
Routing Tables
Next Hop Addresses
The IP Datagram Header
Best-Effort Delivery
Virtual 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 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 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 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 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 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 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 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 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?