Transcript IP: Datagram and Addressing

NAT, DHCP
Autonomous System
Network Protocols and Standards
Winter 2007-2008
Jan 29, 2008
CS573: Network Protocols and Standards
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IPv4
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IP Datagram Format
IPv4 Addressing
ARP and RARP
IP Routing Basics
Subnetting and Supernetting
ICMP
Network Address Translation (NAT)
Dynamic Addressing
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Private Networks
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Private networks have no “direct” connection
to the Internet
Blocks of addresses have been reserved for
the private networks (RFC 1918)
Blocks in different classes
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10.0.0.0 – 10.255.255.255 (1 class A)
172.16.0.0 – 172.31.255.255 (16 class B)
192.168.0.0 – 192.168.255.255 (256 class C)
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Purpose
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Internet
Host 1
Host 2
IP1
Firewall
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IP2
Host N
Protected
Network
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Machines in the
protected network can
access the Internet
normally
Packets coming from
the protected network
all appear to be coming
from IP1
Addresses in the
protected network are
in the private range
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Implementation
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Hosts inside the private network are configured to
use the firewall (IP2) as their gateway
The firewall rewrites the IP datagram header for the
outbound packets, replacing the source IP with IP1
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All packets “seem” to be coming from IP1
The destination IP in the packets received from the
Internet is IP1; it is rewritten replacing IP1 with the
IP address of the internal destination
Problem: How to figure out what is the right
destination in the private network?
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Demultiplexing Incoming Packets
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There is not enough information in the IP
header to demultiplex incoming packets
It is necessary to use information from the
higher layers (transport layer)
Common transport layers: TCP and UDP
Transport layer has the concept of port which
identifies which process in the host should
finally get the packet
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Ports
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16-bit numbers identifying
which process should get the
packet
UDP and TCP ports exist in
different spaces
Each packet carries two port
numbers
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Telnet
FTP
TCP
The source port of the
process which generated it
in the source host
The destination port of the
process which should get it
at the destination
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UDP
IP
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Implementation (revisited)
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Upon receiving an outbound packet from a host in
the private network, the firewall:
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Rewrites the source IP with its own IP (IP1)
Generates a local source port and rewrites the source port in
the packet as this port and makes a record of it
Upon receiving an inbound packet from the Internet,
the firewall checks whether the destination port in
the packet is in the list of local ports:
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If not, the packet is dropped
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Can not initiate connections from outside!
If yes, the firewall knows where to send this packet
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IPv4

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



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
IP Datagram Format
IPv4 Addressing
ARP and RARP
IP Routing Basics
Subnetting and Supernetting
ICMP
Network Address Translation (NAT)
Dynamic Addressing
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BOOTP
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Alternative to RARP
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RARP operates at a low level, requesting
direct access to the network hardware
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Difficult for an application programmer to build
a server
RARP gives “only” the IP address
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BOOTP
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Devised to allow a machine to obtain:
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Its IP address
Address of a router
Subnet mask to use
Address of a name server
Can be implemented with an application
program
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Uses UDP/IP for communication
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BOOTP
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Reliability in communication is based on
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UDP checksum
Timeout and retransmissions
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To minimize collisions among many clients, use
random timeouts
Increase timeouts with each retransmission
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Starting with the interval 0-4 seconds
Doubling interval each retransmission up to 60s
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BOOTP Message Format
0
8
OP
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16
24
31 bits
HTYPE
HLEN
HOPS
Transaction ID
Seconds
Unused
Client IP Address
Your IP Address
Server IP Address
Router IP Address
Client Hardware Address (16 octets)
Server Hostname (64 octets)
Boot File Name (128 octets)
Vendor-specific area (64 octets)
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BOOTP Message
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Field OP
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HTYPE and HLEN
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Client passes 0 in this field; BOOTP server increments it if the
request is passed to another server across a router
Transaction ID
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Hardware type and address length (For Ethernet, HTYPE is 1 and
HLEN is 6)
HOPS
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Specifies whether a request(1) or reply(2)
Contains an integer that machines use to match requests with
responses
Seconds
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Number of seconds since the client started to boot
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BOOTP Message
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Remaining fields in the message
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To allow the greatest flexibility
Clients fill in as much information as they know;
unknown fields are set to zero
Example
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If server IP or server hostname are non-zero, only
the server with matching address/name will
answer the request
If they are zero, any server that receives the
request will reply
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BOOTP Message Format
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BOOTP can be used by a client that already
knows its IP address (e.g., to obtain boot file
information)
A client that knows its IP address places it in
the client IP address field; other clients set
this field to zero
If the client’s IP address in the request
message is zero, a server returns the client IP
address in the “your IP address” field
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DHCP
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Dynamic Host Configuration Protocol
RARP and BOOTP designed for relatively static
environment
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Each host a permanent network connection
Manager creates a BOOTP configuration file specifying
BOOTP parameters for each host
Manager configures server with mapping of host identifier to
IP address
New Requirements
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Portable computers
Number of computers exceeds available IP host addresses
(although not all will be up and running at the same time)
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DHCP
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DHCP allows:
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Manual configuration
Automatic configuration
Managers let DHCP server assign a
permanent address when a computer first
attaches to the network
Dynamic configuration
Loaning IP addresses for a limited time
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IP Routing Protocols
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IP Routing
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Autonomous System
Domain
Intra-domain Routing
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Inter-domain Routing
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Interior Gateway Protocols
Exterior Gateway Protocols
IP Multicast Routing
MPLS
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Routing in the Internet
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Routing Algorithms
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Routing Protocols
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Bellman-Ford
Dijkstra
Distance Vector
Link State
Routing Hierarchy
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Interior Gateway Protocols (RIP, OSPF, IGRP)
Exterior Gateway Protocols (EGP, BGP, CIDR, Policy Routing)
Multicasting (IGMP)
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Internet from the start
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First, there was ARPANET
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Routers had complete information about all the possible
destinations – core routers
GGP (gateway-to-gateway) protocol was used for routing – a
distance vector protocol
R
R
H
R
R
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H
H
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Internet from the start
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Then, LANs were connected to ARPANET
ARPANET
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LAN
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R
LAN
R
Core Routers
LAN
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Internet from the start
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Problems with above configuration:
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Routing overhead increased with the number of
connected routers
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Number of routes increased with the number of
connected segments
Frequency of routing exchanges increased
Higher likelihood that something went wrong somewhere
requiring updates
Number of different types of routers increased
Slow deployment of new versions of routing
algorithms
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Internet from the start
Backbone Network
R1
Core Router
Local Network
R2
Local Network
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R3
Local Network
R4
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Local Network
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Autonomous System
Backbone Network
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AS
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Core Routers
AS
AS: Autonomous System
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Autonomous System
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What is an autonomous system?
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A set of routers and networks under the
same administration. Examples:
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A single router directly connecting one local
network to the Internet
A corporate network linking several local
networks through a corporate backbone
A set of client networks served by a single ISP
NOTE: From a routing point of view, all
parts of an AS must remain connected
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Autonomous System
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Internal connectivity within the AS means:
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All routers must be connected
Parts of network connected through core AS (yes,
core is an AS!) cannot form an AS
All routers must exchange routing information in
order to maintain the connectivity (normally
achieved by using a single routing protocol)
Routers inside an AS are called “interior
gateway” and the protocol they use is called
Interior Gateway Protocol (IGP)
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Autonomous System
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In 1982, the IGP of choice was GGP
IGPs in use today are:
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RIP
OSPF
IGRP
Each AS is identified by a 16-bit number
Number is assigned by the numbering
authorities
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Autonomous System: Benefits
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Routing overhead is lower
Network management becomes easy
Easier computation of new routes
Distribution of new software versions is
easier
Failing elements can be isolated easily
AS use an Exterior Gateway Protocol to
exchange information about reachability
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