Lecture 8
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Reminder: Homework 3, Programming Project
2 due on Thursday.
Questions?
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Outline
Chapter 3 - Internetworking
3.1 Switching and Bridging
3.2 Basic Internetworking (IP)
3.3 Routing
3.4 Implementation and Performance
3.5 Summary
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Basic Internetworking (IP)
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Bridges and LAN switches from last section
have limited ability to scale and to handle
heterogeneity.
An internetwork (or just internet - with
lowercase i) is a logical network built out of a
collection of physical networks. Each physical
network uses one technology (e.g., Ethernet or
Wi-Fi).
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Basic Internetworking (IP)
An example
internet is
shown at right.
Routers or
gateways are
used to connect
different
physical
networks.
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Basic Internetworking (IP)
The figure above illustrates how host H1 and H8
are logically connected in an internet.
While switches operate only at the physical layer,
routers operate at the higher networking layer of
the protocol stack.
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Service Model
The Internet protocol (IP) service model is
unreliable (best-effort) and connectionless. This
simple model allows IP to run over almost any
physical link. It also keeps router design simple.
Reliable, connection-oriented services can use IP.
These services just need to be implemented at a
higher layer in the protocol stack.
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Service Model - Packet Format
The IP Version 4
packet format is
shown at right. The
HLen field is the size
of the header (number
of 32 bit words). The
Type Of Service
(TOS) field allows for
differentiated service.
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The IP packet is treated
as a collection of 32
byte words.
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Service Model - Packet Format
The Length fields contains the entire packet size
in bytes (up to 65,535 bytes). The Ident, Flags
and Offset support fragmentation and reassembly
and will be discussed shortly.
The Time To Live (TTL) field is decremented at
each router. When it reaches 0 the packet can be
dropped. It is intended to prevent an endlessly
circulating packet. An initial value of 64 is the
default.
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Service Model - Packet Format
The Protocol field is a demux key (TCP is 6, UDP
is 17). The Checksum is a 16 bit checksum over
the header. Packets with checksum errors are
dropped.
The SourceAddr and DestinationAddr are 32 bit
IP addresses (not MAC addresses!)
There may be any number of Options. These are
rarely used.
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Service Model - Fragmentation and
Reassembly
To allow sending IP packets over links with
different Maximum Transmission Units (MTUs) IP
allows packets to be fragmented while in route to
the destination.
Fig 3.17 IP packet fragmentation
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Service Model - Fragmentation and
Reassembly
The Ident field is the same in each
fragment (it is the same as in the
original packet). The M bit in the
Flags field is 1 except in the last
fragment. The Offset is the byte
offset divided by 8.(Fragmentation
is on an 8 byte boundary only.)
Notice that the fragments are not
reassembled until they reach the
destination.
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Global Addresses
IP addresses are globally unique. Each host has
a unique IP (actually each interface has a unique
IP, hosts with multiple NICs and routers will have
multiple IP addresses).
IP addresses are hierarchical. They consist of a
network part and a host part.
Fig 3.19 IP Addresses
(a) class A address
(b) class B address
(c) class C address
The network and host parts of the IP
address are not fixed-size.
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Datagram Forwarding in IP
Hosts or routers that have the same network part
are on the same physical network.
Every physical network connected to the Internet
contains at least one router.
The network part uniquely identifies a single
physical network on the Internet.
Every IP datagram contains the IP address of the
destination.
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Datagram Forwarding in IP
Every node in the path (including the source)
looks at the network part of the destination
address to determine if it is on the same physical
network as the destination.
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If the node is on the same physical network it
sends it directly to the destination
If not, it sends the packet to the next hop router
by consulting its forwarding table.
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Datagram Forwarding in IP
Conceptually
the forwarding
table for
router R2
could appear
as shown at
right.
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R2 forwarding table
Network NextHop
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R1
2
Int 1
3
Int 0
4
R3
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Datagram Forwarding in IP
A network may have multiple routers. If the
forwarding table does not contain an entry for a
desired network, the packet can be sent to the
default router (or gateway).
The forwarding table for many end nodes (hosts)
may contain only an entry for a single default
router.
Note that forwarding tables used in routers
contain entries for network addresses, not
individual hosts.
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Subnetting
The original IP classes (A, B, C) resulted in a lot
of wasted IP address. Every network, no matter
how small, required at least a class C network
address with up to 255 hosts.
A fairly large network of 1000 hosts would require
a class B network address leaving 64000 IP
addresses unusable.
Subnetting allows for much more efficient use of
the IP address space by allowing a network
number to be split and the pieces assigned to
different subnets.
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Subnetting
Subnetting works by
using a portion of the
host number as a
subnet ID. The subnet
ID and network number
together form a subnet
number (address).
A subnet mask is used to define which bits of an
address are part of the subnet address and which
bits are used to identify a host on the subnet.
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Subnetting
In the example at right a
subnet mask with 24
leading 1s allows a Class
B address to be subnetted
into 256 subnets with 255
hosts on each subnet.
All hosts on the same subnet have the same
subnet address and subnet mask.
Externally the subnetted network still appears as a
single Class B network.
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Subnetting
An example
subnetted
network is shown
at right. The top
two networks can
have up to 127
hosts. The
bottom network
can have up to
255 hosts.
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Subnetting
A host now ANDs a destination IP with its own
subnet mask to see if the destination subnet is the
same as the source's subnet. If it is, the packet is
sent directly to the destination. If it is not, the
packet is sent to the default router.
Note that it is not necessary that the 1s in the
subnet mask be contiguous, but this is highly
recommended.
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Subnetting
Forwarding tables
for the internal
routers must also
include a subnet
mask.
Forwarding table for Router R1
Subnet #
128.96.34.0
128.96.34.128
128.96.33.0
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SubnetMask
255.255.255.128
255.255.255.128
255.255.255.0
Next
intf0
intf1
R2
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Classless Addressing
Classless Interdomain Routing (CIDR) drops the
classful addresses discussed previously and
allows the network part of an IP address to be any
length. In CIDR, network addresses are
represented using a /X after the network prefix.
192.4.16/24 would include all IP addresses in the
range 192.4.16.0 – 192.4.16.255 (256 IP
numbers), while 192.4.16/20 would include all IP
addresses in the range 192.4.16.0 – 192.4.31.255
(4096 IP numbers).
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Classless Addressing
Fig. 3.22 Route Aggregation
The ISP advertises a 21 bit CIDR address that is
used externally for routing. Routers within the ISP
route traffic to 8 different networks.
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Classless Addressing
With CIDR the network portion of an IP address
may match multiple entries in a router's
forwarding table. Forwarding is then based on the
“longest match”.
Entries for both 171.69/16 and 171.69.10/24
match an IP destination of 171.69.10.100. The
second entry is the longest, so the packet would
be routed to 171.69.10/24.
A packet addressed to 171.69.12.5 would be
routed to 171.69/16 (assuming that there are no
other matching entries).
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Address Translation (ARP)
The Address Resolution Protocol (ARP) enables a
host to dynamically construct a table (known as
the ARP table or cache) of IP addresses to
physical address mappings.
Since the mappings may change over time (NIC
card replacement) the entries time-out and are
removed periodically (15 minutes is typical).
ARP relies on the fact that most link-level
technologies support broadcast.
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Address Translation (ARP)
If a host wants to send an IP datagram to a node
on the same network it first checks its ARP cache
for a mapping. If there is no entry, the host
broadcasts an ARP query. The query contains
the target IP, the source IP and source link-layer
address.
The target adds or refreshes its ARP cache with a
source entry and then sends an ARP reply to the
source.
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Address Translation (ARP)
The ARP packet format used on Ethernet is
shown above. HLen and PLen are the lengths of
the hardware and protocol addresses in bits.
Operation is 1 for a query and 0 for a reply.
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Address Translation (ARP)
Wireshark
capture of an
ARP request
by 10.10.0.200
for 10.10.0.21
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Host Configuration (DHCP)
Ethernet addresses are hardwired into the
adaptor. IP addresses can not be hardwired
since all hosts on the same physical network must
have a common network address.
Most operating systems allow the IP address and
the IP address of the default router/ gateway to be
manually configured.
Manual configuration is time consuming and error
prone. The primary method of automatic
configuration is known as Dynamic Host
Configuration Protocol (DHCP).
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Host Configuration (DHCP)
A DHCP server sends out IP addresses to hosts
when they boot. The server can be configured to
always give the same IP address to the same
interface (based on its MAC address) or the
server can hand out IP addresses from a pool of
IP addresses.
The DHCP server might be a standard computer
server or a router. The DHCP server might also
be setup to provide other network services (DNS,
email, etc).
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Host Configuration (DHCP)
When a host on the network boots, it sends a
DHCPDISCOVER message to the IP broadcast
address (255.255.255.255). Routers will pass IP
broadcasts to other subnetworks (subnets) but not
to other networks. The DCHP server replies with
an IP address that can be used by the hosts.
It is possible to use DHCP relay agents so that
one DHCP server can provide IP addresses to
multiple networks.
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Host Configuration (DHCP)
A DHCP relay agent receives a
broadcast DHCPDISCOVER message
and sends a unicast to a DHCP server
on another network.
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DHCP packet format
(See the text for details.)
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Error Reporting (ICMP)
IP networks support a companion protocol –
Internet Control Message Protocol (ICMP) - that is
used for reporting errors back to the source host
whenever a router (or host) is unable to process a
datagram successfully (destination host
unreachable, failed reassembly, TTL
decremented to 0, checksum error, etc.)
An ICMP-Redirect control message can be used
to send a better route back to a host so that it can
update its forwarding table.
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Virtual Networks and Tunnels
Virtual circuit networks
can be used to create
virtual private networks
(VPNs) that act very
much like separate
private networks.
VPNs use a shared network to
create private networks.
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Virtual Networks and Tunnels
The Internet can be used to create VPNs via the
use of an IP tunnel. An IP tunnel is a virtual pointto-point link between two endpoint routers.
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Virtual Networks and Tunnels
If router R1 receives a packet containing an
address in network 2, it encapsulates the packet
in an IP packet destined for router R2. R2 strips
the added header and forwards the packet to
network 2.
VPNs can be used for security or to carry non-IP
packets across an IP network. Tunnels can also
be used to connect two routers that may have
special capabilities.
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In-class Exercises
Explore the following Unix commands:
1) arp
2) route
3) ping and traceroute (Unix)/tracert(Windows)
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