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COM 360
1
Chapter 4
Internetworking
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Interconnecting Networks
Problem: There’s more than one network
• Problems of Heterogeneity and Scale
– Heterogeneity- users on one type of network want
to be able to communicate with users on other types
of networks.
– Internet Protocol(IP) and how it is used to build
heterogeneous, scalable networks.
– Principle of Routing- finding loop-free paths
– The problem of the growth of the Internet- going
from IPv4 to IPv6
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Simple Internetworking
• What is an internetwork?
• Internetworks or internets (with lower case i) are large,
highly heterogeneous networks with reasonably
efficient routing.
• They are a collection of networks that are
interconnected to provide host-to-host packet delivery
service.
• With a capital ‘I’ the Internet refers to the global
Internetwork.
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What is an Internetwork?
• What is the difference between networks, subnets
and internets?
– A network is a directly connected or switched network,
which uses a single technology (802.5, Ethernet, or ATM)
and represents a physical network.
– A subnet uses single IP address to denote multiple
physical addresses.
– An internet is a collection of networks or logical
networks, built out of a collection of physical networks.
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A Simple Internetwork
• An internetwork is referred to as a “network of
networks” because it is made up of many smaller
networks.
• For example, an internetwork can connect Ethernets,
FDDI rings and Point-to-links (See next slide)
• The nodes that connect them are called routers (and
sometimes gateways)
• The Internet Protocol is the tool used to build
heterogenous internetworks.
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A Simple Internetwork
Netw ork 1 (Ethernet)
H1
H2
H7
H3
R3
H8
Netw ork 4
(point-to-point)
Netw ork 2 (Ethernet)
R1
R2
H4
Netw ork 3 (FDDI)
H5
HN = host
H6
Rn = router
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Internet Protocol (IP)
• IP is the tool used to build scalable,
heterogeneous internetworks.
• Originally called the Kahn-Cerf protocol after
its inventors.
• IP runs on all the hosts and routers and defines
the infrastructure that allows them to function
as a single network.
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A Simple Internetwork
H1
H8
TCP
R1
IP
ETH
R2
IP
ETH
R3
IP
FDDI
FDDI
IP
PPP
PPP
TCP
IP
ETH
ETH
Shows Protocol layers used to connect H1 to H8
ETH is the Protocol that runs over the Ethernet
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Service Model
• When you build an internet, start by defining the
service model, or the host-to-host services that you
want to provide, over each of the underlying physical
networks.
– An addressing scheme, which provides a way to identify all
hosts in the internet
– A datagram (connectionless) model of data delivery.
• This service model is called best effort, because
although IP makes every effort to deliver datagrams, it
makes no guarantees.
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Datagram Delivery
• IP datagram is fundamental to the Internet
Protocol
• A datagram is a type of packet that is sent in a
connectionless manner over a network.
• Every datagram carries enough information to let
the network forward the packet to its destination.
• No set up mechanism is needed – just send it and
the network tries to get it to its destination.
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Best Effort Delivery
• If something goes wrong and a packet gets lost,
corrupted or misdelivered, or in any way fails to
reach its destination, the network does nothing.
It is called unreliable service.
• Best-effort, connectionless service is the
simplest service for an internetwork.
• Keeping the routers as simple as possible was
one of the original design goals of IP.
• The ability of IP to “run over anything” is its
most important characteristic.
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Data Transmission and Frames
•
•
•
•
•
•
IP internet layer:
Constructs datagram
Determines next hop
Hands to network interface layer
Network interface layer:
Binds next hop address to hardware address
Prepares datagram for transmission
But ... hardware frame doesn't understand IP; how
is datagram transmitted?
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Datagram Encapsulation
• Network interface layer encapsulates IP
datagram as data area in hardware frame
• Hardware ignores IP datagram format
• Standards for encapsulation describe details
• Standard defines data type for IP datagram,
as well as others (e.g., ARP)
• Receiving protocol stack interprets data area
based on frame type
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Encapsulation in a Hardware
Frame
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Transmission Across an Internet
Each router in the path from the source to
the destination:
 Unencapsulates incoming datagram from
frame
 Processes datagram - determines next hop
Encapsulates datagram in outgoing frame
 Datagram may be encapsulated in different
hardware format at each hop
• Datagram itself is (almost!) unchanged
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Datagram Transmission
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Datagram Delivery
• A datagram is fundamental to the IP portocol
• A datagram is sent in a connectionless manner
over a network
• “Best effort” – if something goes wrong, the
network does nothing.
• Simples type of service- keeping routers simple
was one of the design goals
• Ability of IP to “run over anything”- main
advantage (even a network of carrier pigeons!??)
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IP Packets
• Part of the IP service model is the type of
packets it can carry.
• IP datagram consists of a header followed
by the number of bytes of data.
• These are usually represented by 32-bit
words, where the top word and the leftmost
words are transmitted first.
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IPv4 Packet Header
0
4
Version
8
HLen
16
TOS
31
Length
Ident
TTL
19
Flags
Protocol
Offset
Checksum
SourceAddr
DestinationAddr
Options (variable)
Pad
(variable)
Data
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Packet Format
• There are some subtle features of this simple
model:
– The Version field specifies the current IP version,
called IPv4. Putting it first makes it easy to define
everything else.
– HLEN specifies the length of the header (about 5
words or 20 bytes).
– TOS- is the Type of Service field
– The LENGTH field (in bytes)- length of datagram,
including the header
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Packet Format
• TTL- is the Time to Live field, set to a specific
number (64 is current default) which the routers would
then decrement, until it reached 0. It purpose is to
discard packets that have been circling around and to
discard them.
• Protocol field identifies the higher level protocol (TCP,
UDP) to which this packet should be passed.
• Checksum- add the entire header and take the ones
complement of the result.
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Packet Format
• SourceAddr- Source Address –enables a
recipient to reply
• DestinationAddr - Destination Address –
this is key to the delivery of the datagram
• IP defines its own global address space,
independent of the physical network
• There are also optional fields, which are
rarely used.
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Fragmentation and Reassembly
• Each network technology has its own
maximum packet size:
– (Ethernet –1500 bytes, FDDI- 4500 bytes)
• Two choices for the IP service model:
– Make sure all IP datagams are small enough or
– Provide a means by which packets can be
fragmented and reassembled, when they are too big
to be sent though a network technology
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Fragmentation and Reassembly
• Every network has a maximum transmission
unit (MTU), which is the largest IP datagram
that it can carry in a frame.
• This value is smaller than the largest network
packet size, because it must fit into the
payload of the data link layer frame.
• When a host sends a datagram it can choose
any size. A reasonable choice is the MTU of
the network to which it is directly attached.
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Datagram Fragmentation
• Router uses local MTU to compute the size of each
fragment and puts part of the original data in each
fragment and rest of the information in the header.
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Fragmentation and Reassembly
• Fragmentation will be necessary if the path to the
destination includes a destination with a smaller
MTU.
• Fragmentation typically occurs in a router (in IPv4).
• To enable the fragments to be reassembled at the
receiver, each datagram carries the same identifier in
the ident field.
• The unique identifier is chosen by the sender.
• If all fragments do not arrive at the receiver, it
discards all datagram fragments and does not attempt
to recover them.
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Fragment Loss
 IP may drop fragment
 What happens to original datagram?




Destination drops entire original datagram
How does destination identify lost fragment?
Sets timer with each fragment
If timer expires before all fragments arrive,
fragment assumed lost
 Datagram dropped
 Source (application layer protocol) assumed
to retransmit
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IP Datagram Traversing the Sequence of
Physical Networks
H1
R1
R1
ETH IP (1400)
R2
R2
FDDI IP (1400)
R3
R3
H8
PPP IP (512)
ETH IP (512)
PPP IP (512)
ETH IP (512)
PPP IP (376)
ETH IP (376)
This is what happens when H1 sends a datagram to H8.
Assume 1500 bytes for an Ethernet, 4500 for FDDI, 532 for
PPP. The datagram is broken into 3 fragments at router 2,
which are then forwarded.
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Fragments
• Each fragment is a self-contained IP datagram that is
transmitted over physical networks, independent of the
other fragments.
• Each IP datagram is re-encapsulated for each physical
network over which it travels.
• Fragmentation is done in 8 byte chunks.
• The router sets the M bit in the FLAGS field to
indicate there are more fragments, and sets the
OFFSET field to zero to indicate the first part of the
datagram.
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Header Fields Used in IP
Fragmentation
(a)
Start of header
Ident = x
0 Offset = 0
Rest of header
1400 data bytes
a) Unfragmented packet
Start of header
Ident = x
1 Offset = 0
Rest of header
512 data bytes
(b)
Start of header
b) Fragmented packets
Ident = x
1 Offset = 64
Rest of header
512 data bytes
Start of header
Ident = x
0 Offset = 128
Rest of header
376 data bytes
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Fragmentation
• Fragmentation is done in such a way that it could be
repeated if a fragment arrived at another network
with an even smaller MTU.
• The fragments are easily reassembled independent
of the order in which they are received.
• Reassembly is done at the receiving host and not at
each router. Why?
• (See p. 243-247 for reassembly code.)
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Path MTU Discovery
• IP reassembly is not a simple process and should be
avoided. ( For example, if a fragment is lost, the
receiver still tries to reassemble the whole datatgram
until it finally must discard it.)
• Instead, hosts are encouraged to perform “path
MTU discovery” by sending packets small enough
to go through the path with the smallest MTU form
sender to receiver. It first sends large datagrams, and
if they are not successful, then is sends smaller ones,
until it discover the smallest MTU from sender to
receiver.
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Global Addresses
• Global uniqueness is the first property of an
addressing scheme.
• Ethernet addresses are flat and without structure.
• IP addresses are hierarchical and are made up of
several parts that correspond to parts of the
network.
• IP addresses consist of a network part and a host
part.
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Addresses For the Internet
 One difference between an internet and a physical
network is that an internet is an abstraction
imagined by its designers and created by software.
• Designers choose addresses, packet formats, and
delivery techniques independent of the hardware.
 One key aspect of virtual network is single, uniform
address format
 Each address must be unique
 Can't use hardware addresses because different
technologies have different address formats.
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IP Addressing Scheme
 Addressing in TCP/IP is specified by the Internet
Protocol (IP)
 Each host is assigned a 32-bit number
(4 octets, separated by “dots”) –referred to as
dotted octet ( e.g. 216.72.32.10)
 Called the IP address or Internet address
 Unique across entire Internet
• Different from a domain name: linux.sjcny.edu
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IP Address Hierarchy
 Each IP address is divided into a prefix and a suffix
 Prefix identifies network to which computer is
attached
 Suffix identifies computer within that network
 Each physical network is assigned a unique network
number
 Address format makes routing efficient
 Each computer is assigned a unique address
 Network assignments are coordinated globally but
suffixes can be assigned locally.
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IP Addresses
• The network part of the IP address identifies
the network to which the host is attached
• All hosts attached to the same network have
the same network part in their IP address.
• The host part or suffix, identifies each host
uniquely on that network.
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Classes of IP Addresses
 Designers chose a compromise - multiple address
formats that allow both large and small prefixes
 Original scheme called classful IP addressing,
divided the IP address space into 3 primary
classes, where each class had a different size prefix
and suffix
 Each format is called an address class
• Class of an address is identified by first four bits
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IP Addressing
 Octet (8-bit) boundaries are used to partition an address into








prefix and suffix
Class A, B and C are primary classes
Used for ordinary host addressing
Class D is used for multicast, a limited form of broadcast
Internet hosts join a multicast group
Packets are delivered to all members of group
Routers manage delivery of single packet from source to all
members of multicast group
Used for MBone (multicast backbone)
Class E is reserved ( for future use)
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Computing the Class of an
Address
 IP software computes the class of the
destination address when it receives a packet.
 IP addresses are self-identifying because the
class can be computed directly from the first few
bits of the address
 The first 4 (leading) bits of the address denote
the class:
– Class A begins with 0
– Class B begins with 10
– Class C begins with 110
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Computing the Class of an Address
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IP Address Classes
Prefix designates the network, suffix designates the host.
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Primary IP Address Classes
7
(a)
0
24
Netw ork
Host
14
(b)
1
0
16
Netw ork
1
1
0
B) Class B
Host
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(c)
a) Class A
Netw ork
8
Host
C) Class C
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Dotted Decimal Notation
 Class A, B and C all break between prefix and suffix
on byte boundary
 Dotted decimal notation is a convention for
representing 32-bit internet addresses in decimal
 Convert each byte of address into decimal; separate
octet by periods ("dots'')
 Dotted decimal notation treats each octet as an
unsigned binary integer
 Smallest value is 0.0.0.0 and largest is
255.255.255.255
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Dotted Decimal Notation
What would SJC’s address be in binary ( 216.73.32.0)?
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Classes and Dotted Decimal Notation
 While dotted decimal makes separating network address
from host address easier, determining class is not so
obvious
 Look at first dotted decimal number, and use this table to
calculate the class:
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Division of Address Space
• Addressing scheme is flexible and allows networks of
various sizes to be accommodated efficiently
• Original idea was that Internet would consist of small
number of wide area networks (Class A), a few site (or
campus) sized (Class B) networks, and a large number
of LANs (Class C)
• Additional flexibility was needed and removed some
of the distinction between classes present in this
“classful” scheme.
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Division of Address Space
 IP Class scheme does not yield equal number of
networks in each class
 Class A:
 First bit must be 0
 7 remaining bits identify Class A net
 27 (= 128) possible class A nets
 Number of bits allocated to a prefix or suffix
determines how many unique numbers can be
assigned
 A prefix of n bits allows 2n unique network numbers,
while a suffix of n bits allows 2n hosts number on a
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given network
Number of Networks and Hosts
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Authority for Addresses
• An organization obtains unique network
numbers from an Internet Service Provider
(ISP), which coordinates with the Internet
Assigned Number Authority. A network
administrator can assign prefixes in a
private internet.
• (See Internic, ICANN, Educause, etc.)
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Insufficient Addresses
 Large organizations may not be able to get as many
addresses in the Internet as they need
 Example - UPS needs addresses for millions of
computers
 One solution - set up private internet and allocate
addresses from entire 32-bit address space
• Others do not use all their assigned addresses
• For example, SUNY Stony Brook has a Class B
license but probably only uses 3000-40,000
• of its 216 addresses (65,536 possible).
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A Classful Addressing Scheme
 Select address class (usually class C) for each
network depending on expected number of hosts:
 Chosen by the internet service provider for the
internet
 Chosen by the network administrator in a private
network
 Assign network numbers from appropriate classes
 Assign host suffixes to form internet addresses for
all hosts
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Classful Addressing with Private internets
• Consider an organization with a private TCP/IP with 4
networks, connected by routers.
 A prefix is chosen denoting the class (A,B,C) depending on
the size of the network;
 In the next example, there is one Class A network
(prefix 10), two class B prefixes (128.100 and 128.211) and
one class C (192.5.48).
 The IP address assigned to the host begins with the prefix
assigned to the host's physical network
 Suffixes, which are assigned by the local network
administrator, can be arbitrary numbers, often chosen
sequentially.
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Classful Addressing with Private
internets
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Subnet and Classless Addressing
• Two mechanism were invented to overcome the
addressing limitations:
– 1.
– 2.
Subnet addressing
Classless addressing
• These are so closely related that they can be
thought of as a single abstraction: instead of
having 3 distinct address classes, allow the
division between prefix and suffix to occur on an
arbitrary bit boundary.
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Address Masks
• How can an IP address be divided at
an arbitrary boundary?
• It requires an additional piece of
information to be stored with each
address. This information specifies the
exact boundary between the network
prefix and the host suffix.
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Address Masks
• To use classless or subnet addressing the
routers must store 2 pieces of information:
– the 32 bit address and
– another 32 bit value that specifies the boundary
between the prefix and suffix.
• This second value is called the called the
subnet mask and 1 bits mark the network prefix
and zero bits mark the host portion. This
makes computation efficient.
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Routers and Addresses
• Routers compare the network prefix portion of
the address to a value in their routing tables.
• Suppose a router is given a destination address,
D and a pair (A,M) that represents the 32 bit
address and the 32 bit subnet mask.
• To make the comparison, the router tests the
logical "and" condition to set the host bits of
address D to zero and then compares the result
with the network prefix A:
A == ( D & M)
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Routers and Addresses
• For example consider this 32 bit mask:
(255.255.0.0 in decimal)
11111111 11111111 00000000 00000000
and the network prefix (128.10.0.0 in decimal):
10000000 00001010 00000000 00000000
• Now consider the 32 bit destination address 128.10.2.3
which has the binary equivalent of
10000000 00001010 00000010 00000011
• The logical "and" between the destination address and the
address mask produces the result:
10000000 00001010 00000000 00000000
• which is equal to the prefix 128.10.0.0
60
CIDR (Classless Interdomain
Routing) Notation
• Inside the computer each address mask
is stored as a 32 bit value in binary,
which is then expressed in dotted octet
notation.
• The new CIDR notation append a slash
and the size of the mask in decimal
notation:
For example 128.10.0.0/16
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CIDR Address Block Example
• Suppose an ISP has a single Class B license
128.211.00.0. Using a classful address scheme,
he/she can only assign the prefix to one customer,
who can have up to 216 host addresses.
• Using CIDR, the ISP could assign the entire prefix
to a single organization by using 128.211.0.0/16
• Or he could partition the address into three pieces
(two of them big enough for 2 customers with 12
computers each and the remainder available for
future use.
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CIDR Address Block Example
• One customer could be assigned
128.211.0.16/28
• and the other could be assigned
128.211.0.32/28
• Both customers have the same mask size (28
bits), but the prefixes differ and each has a
unique prefix. More importantly the ISP
retains most of the addresses, which can then
be assigned to other customers.
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CIDR Host Address
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Special IP Addresses
IP assigned a set of addresses that are reserved and never
assigned to hosts:
Network Address
• IP reserves host address zero and uses it to denote a
network. (For example,128.211.0.0 is a Class B network)
Direct Broadcast Address
• It is formed by adding a suffix consisting of all 1's to the
network prefix (For example, 128.211.111.111)
Limited Broadcast Address
A broadcast on a local physical network (or limited to a "single
wire") is used during system startup by a computer that
does not yet know the network number. The address with all
1's is a limited broadcast.
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Special IP Addresses
This Computer Address
• A computer needs to know its own IP address to
send or receive internet packets. The TCP/IP
protocol allows a computer to obtain its address
automatically but strangely enough, when using
these startup protocols the computer cannot
supply a correct IP source address. To handle
such cases, IP reserves the address that
consists of all zeroes to mean "this computer".
66
Special IP Addresses
Loopback Address
• A loopback address is used to test network
applications. IP reserves the network prefix 127 for
use with loopback and programmers usually use
the host number 1 (forming the address 127.0.0.1)
for loopback testing.
• During loopback no packets actually leave the
machine - the IP software forwards packets from
one application program to another on the same
computer. Therefore the loopback address never
appears in a packet traveling across the network.
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Summary of Special IP Addresses
• Special addresses are reserved and should
never be assigned to host computers.
• Each special address is restricted to certain
uses.
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Datagram Forwarding in IP
• Forwarding is the process of taking a
packet from an input and sending it out on
the appropriate output.
• Routing is the process of building the tables
that allow the correct output for a packet to
be determined.
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Bridges, Switches, Routers
• All forward messages form one link to another.
• Bridges are data link-level nodes and forward
frames from one link to another (in a LAN).
• Switches are network layer nodes, which forward
packets in a switched network.
• Routers are internet-level nodes which forward
datagrams from one network to another.
• Bridges and switches are often called “Layer2
Switches, meaning above the physical and below
the internet layer.
70
Datagram Forwarding in IP
Main ideas needed to forward IP packets:
• Every IP datagram contains the IP address of the destination
host.
• The network part of the IP address uniquely identifies a single
physical network on the larger Internet.
• All hosts and routers that share the same network part of their
address are connected to the same physical network and can
communicate by sending frames over that network.
• Every physical network that is part of the Internet has at least
one router that is also connected to at least one other network
and can exchange packets with hosts or routers on either
network.
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Forwarding An IP Datagram
• IP packets are sent from a source to a destination host,
possibly passing through several routers.
• Any node (host or router) tries to determine if it is
connected to the same physical network as the
destination, by comparing the network address part of
the destination address with the network address part of
each interface address. ( Hosts have one address,
routers have two or more, since they are connected to
multiple networks.)
• If there is a match, the destination is on the same
network and the packet is delivered.
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Forwarding An IP Datagram
• If the node is not connected to the same physical
network as the destination, it sends the datagram
to a router, called the next hop router.
• The router finds the correct next hop by
consulting its forwarding or routing table.
• The table is primarily a list of (NetworkNum,
NextHop) pairs.
• There is usually a default router if none of the
entries match the destination’s network number.
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Forwarding Algorithm
if (Destination NetworkNum = = NetworkNum
of one of my interfaces)
deliver packet to destination over the interface
else if (Destination NetworkNum is in my
forwarding table)
deliver packet to NextHop router
else
deliver packet to default router.
74
Forwarding Example
Netw ork 1 (Ethernet)
H1
H2
H7
H3
R3
Netw ork 4
(point-to-point)
Netw ork 2 (Ethernet)
R1
H8
Suppose H1 wants to send
a datagram to H2- on same
network sends directly.
What about H1 to H8?
(R1, R2, R3)
R2
Routing table for R2
H4
Netw ork 3 (FDDI)
NetworkNum NextHop
H5
H6
1
R3
2
R1
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Forwarding Tables
• Simple enough to be manually configured
• Usually built by routing protocol
• Routers contain tables that list only a set of
network numbers, not all the hosts.
Sometimes they also contain interface
information.
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Routers and IP Addressing
 IP address depends on network address
 What about routers - connected to two networks?
 IP address specifies an interface, or network attachment
point, not a computer
 Router has multiple IP addresses - one for each interface
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Principle of Scalability
• An important principle of building a scalable is to
reduce the amount of information stored in each
node.
• Most common way to do that is a hierarchical
aggregation. IP uses a 2 level hierarchy, with
networks at the top level and nodes at the bottom.
• Information is aggregated by letting routers deal only
with reaching the right network, and the information
that a router needs is represented by a single piece of
information.
78
Router Implementation
• Control processor is responsible for running
the routing protocols.
• The switching fabric transfers packets from
one port to another.
• Routers differ from switches:
– Must handle variable length packets
79
Block Diagram of a Router
Control
processor
Switching
fabric
Input
port
Output
port
80
Address Translation (ARP)
• IP addresses are virtual because they are maintained by
software
• Neither LAN nor WAN hardware understands the
relationship between
– an IP address prefix and a network nor
– an IP address suffix and a particular computer
• Upper levels of protocol stack use protocol addresses
• Network hardware must use hardware address for
eventual delivery
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Address Translation (ARP)
•
Protocol address must be translated into
hardware address for delivery and there are three
methods:
1. Used with WAN hardware- uses table look up
2. Uses a mathematical function to translate the
addresses
3. Uses a distributed system in which two
computers exchange messages
82
Protocol Addresses and Packet
Delivery
• An application places the data in a packet, which
contains the protocol address of the destination
• Software in the host or router uses the destination
address to select the next hop for the packet and
transfers the packet.
• Both the next hop and the destination address are
IP addresses.
• But there is a problem with this!
83
Protocol Addresses and Packet
Delivery
• Problem: The physical hardware does not
understand IP addressing and addresses in a frame
must be physical addresses.
• Solution: Protocol addresses of next hop must be
translated into hardware addresses
84
Address Translation
•
Address Translation
Upper levels use only protocol addresses
–
–
•
"Virtual network" addressing scheme
Hides hardware details
Translation occurs at data link layer
–
–
Upper layer hands down protocol address of
destination
Data link layer translates into hardware address
for use by hardware layer
85
Address Resolution
•
Finding or mapping or translating hardware
address for protocol address:
–
•
•
•
Called address resolution
Data link layer resolves protocol address to
hardware address
Resolution is local to a network
Network component only resolves address for
other components on same network
86
Address Resolution
87
Address Resolution Techniques
Three techniques are used for address resolution depending on the protocol:
•
Table lookup
–
–
•
Bindings or mappings are stored in a table memory with protocol
address as key
Data link layer looks up protocol address to find hardware address
Closed-form computation
–
–
–
–
•
Protocol address based on hardware address
Data link layer computes the hardware address from protocol address
by using basic Boolean and arithmetic operatons
Simply choose hardware address to be hostid and any host can
determine hardware address as:
hardware_address = ip_address & 0xff
Dynamic Message Exchange
–
–
Network messages used for "just-in-time" resolution
Data link layer sends message requesting hardware address;
destination responds with its hardware address
88
Address Resolution
•
•
•
•
•
•
•
A resolves protocol address for B for protocol messages from
an application on A sent to an application on B
A does not resolve a protocol address for F
Through the internet layer, A delivers to F by routing through
R1 and R2
A resolves R1 hardware address and sends packet to router.
Software on R1 resolves the address for R2
R2 receives the packet and resolves the address for F
Network layer on A passes packet containing destination
protocol address F for delivery to R1 which forwards it to R2
and then to F
89
Address Translation (ARP)
• IP datagrams contain IP addresses, but the host or router
to which it is sent only understands network addresses.
• Need to translate the IP addresses to link-level addresses.
• One solution is to map an IP address to a physical
address by encoding the host’s physical address into the
host part of the IP address.
• More general solution is for each host to maintain a table
of address pairs and to map an IP address to a physical
address.
• Better approach – each host dynamically learns the
contents of the table using the network and ARP.
90
Address Translation (ARP)
• Goal of Address Resolution Protocol (ARP) is to
enable each host on the network to build a table
of mappings between IP addresses and data link
level addresses.
• Set of mappings stored in a host is called the
ARP cache or ARP table.
• ARP takes advantage of the fact that many
technologies support broadcast.
91
ARP Packet Format
0
8
16
Hardware type = 1
HLen = 48
PLen = 32
31
ProtocolType = 0x0800
Operation
SourceHardwareAddr (bytes 0 ― 3)
SourceHardwareAddr (bytes 4― 5)
SourceProtocolAddr (bytes 0― 1)
SourceProtocolAddr (bytes 2 ― 3)
TargetHardwareAddr (bytes 0― 1)
TargetHardwareAddr (bytes 2― 5)
TargetProtocolAddr (bytes 0 ― 3)
Used to map IP addresses into Ethernet Addresses
92
ARP Message Delivery
•
•
•
•
•
•
•
•
ARP request message dropped into hardware frame and
broadcast
Uses separate protocol type in hardware frame (ethernet =
806)
Sender inserts IP address into message and broadcast
Every other computer examines request
Computer whose IP address is in request responds, others
discard it
Puts hardware address in response
Unicasts to sender
Original requester can then extract hardware address and send
IP packet to destination
93
ARP Message Exchange
94
ATMARP
• ARP procedure will not work with an ATM network
because it depends on the fact that ARP packets can be
broadcast to all hosts on a single network.
• On solution is to use LAN emulation, which makes an
ATM network behave like a shared media LAN.
• LAN can be inefficient in a wide area ATM network.
• There is a different ARP procedure called ATMARP
that does not depend on LAN emulation or broadcast.
• ATMARP relies on a server to resolve addresses.
95
Logical IP Subnets
• A large ATM can be subdivided into several smaller
subnets, which behaves like a single network.
• All nodes on the same subnet have the same IP
network number and can communicate directly.
• An advantage of the LIS model is that we can
connect a large number of hosts and routers to a big
ATM network with out necessarily giving them all
addresses from the same IP network.
• This makes it easier to manage address assignment
and improves scalability
96
Logical IP Subnets
R
10.0.0.2
12.0.0.3
10.0.0.1
12.0.0.5
H1
LIS 10
LIS 12
H2
ATM netw ork
An example of an ATP network divided into two LIS.
One has an IP address of 10 and the other is 12.
97
Summary of Basic IP Mechanisms
•
Heterogeneity-IP defines best effort service based
on unreliable datagrams:
1. Uses a common packet format with fragmentation and
reassembly
2. Uses a common global address space and ARP for
identifying all hosts
•
Scalability-IP hierarchical aggregation reduces the
amount of information needed to forward packets.
IP addresses are partitioned into network and host
components. Packets are routed first to a network
and then delivered to the correct host on that
network.
98
Host Configuration (DHCP)
• Ethernet addresses are configured into the NIC card
by the manufacturer and assures that these addresses
are unique.
• IP addresses, by contrast, must be unique on an
internetwork, and also must reflect the structure of
that network with a network part and a host part.
• A host also needs the address of a default router- the
place to which it can send packets.
• Dynamic Host Configuration Protocol (DHCP)
99
Host Configuration (DHCP)
•
Most operating systems provide a way to manually
configure the IP information needed by a host, but
there are disadvantages to this:
1. This is a lot of work
2. It is error prone, since every host must get a unique number
•
•
Usually automated methods are required, using a
protocol called Dynamic Host Configuration Protocol
(DHCP).
There is at least one DHCP server that is the central
repository for the host configuration information.
100
Host Configuration (DHCP)
• DHCP relies on a server that is responsible for
providing configuration information to hosts.
• Configuration information for each host is stored
in the server and automatically retrieved when it is
booted or connected to the network.
• Administrator can assign addresses or allow the
DHCP server to maintain an available pool of
addresses that it provides to hosts on demand.
101
Host Configuration (DHCP)
• First problem faced by DHCP server is that of server
discovery.
• To contact an DHCP server, a newly booted or attached
host sends a DHCPDISCOVER message to a special IP
address (25.255.255.255) that is an IP broadcast address.
• It is received by all hosts an routers on the network.
(Routers do not forward these packets beyond this
network.)
• The server would reply to the host and the other nodes
would ignore it.
102
Relay Agent
• Since requiring a DHCP server on every network
would need a large number of servers, the DHCP
uses the concept of a relay agent.
• There is at least one relay agent on each network
and it is configured with just one piece of
information- the IP address of the DHCP server.
• When it receives a DHCPDISCOVER message, it
unicasts to the DHCP server and waits for the
response which it sends back to the requesting
103
client.
DHCP
Unicast to server
DHCP
relay
Other netw orks
DHCP
server
Broadcast
Host
A DHCP relay agent receives a broadcast DHCPDISCOVER
message from a host and sends a unicast DHCPDISCOVER
message to the DHCP server.
104
DHCP Packet
• A DHCP packet is actually sent using a protocol called
UDP (User Datagram Protocol) that runs over IP.
• The UDP packet provides a demultiplexing key that
says “This is a DHCP packet.”
• Client puts its address in the chaddr field.
• DHCP server responds by filling in the yiaddr field
(“your” IP address). These addresses are “leased” and
the host needs to renew the lease if it is still connected.
• Other information such as the default router can be
included in the options field.
105
DHCP Packet Format
Operation
HType
HLen
Hops
Xid
Secs
Flags
ciaddr
yiaddr
siaddr
giaddr
chaddr (16 bytes)
sname (64 bytes)
file (128 bytes)
options
106
DHCP Management
• By allowing network managers to configure a range
of IP addresses per network rather than one IP
address per host, DHCP improves the manageability
of the network.
• DHCP may also introduce some more complexity to
the network since it makes binding between physical
hosts and IP addresses more dynamic.
• This makes the manager’s job more difficult when it
is necessary to locate a malfunctioning host.
107
Error Reporting (ICMP)
• How does the Internet treat errors?
• IP drops datagrams when a fragment fails to arrive at a
destination.
• It has a companion protocol, called the Internet
Control Message Protocol (ICMP), that defines a
collection of error messages that are sent back to the
source when an router or host is unable to process a
datagram successfully.
• Examples: host is unreachable, TTL is 0, header
checksum failed, etc.
108
ICMP Messages
109
Error Reporting (ICMP)
• ICMP also defines other control messages
that a router can send back to a source host.
• ICMP-Redirect tells the source host that
there is a better route to the destination.
• The source host adds this new route to its
forwarding table and uses it for future
datagrams addressed to that destination.
110
ICMP Transport
 ICMP uses IP to transport an error message
 Router creates the datagram and encapsulates
the ICMP message in the datagram.
111
Using ICMP Messages to
Test Reachability
 An internet host, A, is reachable from another host,
B, if datagrams can be delivered from A to B
 The ping program tests reachability - sends
datagram from B to A that A echoes back to B
 Ping uses ICMP echo request and echo reply
messages
• Internet layer includes code to reply to incoming
ICMP echo request messages
112
Using ICMP To Trace a Route
 List of all routers on path from A to B is called the route from A







•
to B
traceroute uses UDP (User Datagram Protocol) to nonexistent port and TTL field to find route via expanding ring
search
Sends ICMP echo messages with increasing TTL
Router that decrements TTL to 0 sends ICMP time exceeded
message, with router's address as source address
First, with TTL 1, gets to first router, which discards and sends
time exceeded message
Next, with TTL 1, gets through first router to second router
Continue until message from destination received
traceroute must accommodate varying network delays
113
Must also accommodate dynamically changing routes
Using ICMP For Path MTU Discovery
 Fragmentation should be avoided
 How can source configure outgoing datagrams to
avoid fragmentation?
 Source determines path MTU - smallest network
MTU on path from source to destination
 Source probes path using IP datagrams with don't
fragment flag
 Router responds with ICMP fragmentation required
message
• Source sends smaller probes until destination
114
reached
Virtual Networks and Tunnels
• On most internets, it is possible for nodes to
communicate with other nodes on different
networks.
• There are situations, where controlled
connectivity s required- these are virtual private
networks (VPN).
• Communication is restricted to take place only
among these sites (often of a corporation),
providing security.
115
Virtual Private Networks
• To make a private network virtual, the
leased transmission lines, that are not
shared, are replaced by some sort of shared
network.
• A Virtual Circuit is a reasonable
replacement because it provides a logical
point-to-point connection between two sites.
116
Virtual Private Networks
C
a) Two separate
private networks
Physical links
A
B
Corporation X private netw ork
K
L
M
Corporation Y private netw ork
(a)
b) Two virtual
private networks
sharing common
switches
K
C
L
Physical links
A
M
B
Virtual circuits
(b)
117
Virtual Private Networks and
Tunnels
• Two separate corporations may migrate to a virtual
circuit network.
• The limited connectivity of a private network is
maintained, but since the networks share switches, we
say that two virtual private networks have been
created.
• An ATM or Frame Relay can provide the connectivity
or an IP network can be used by providing a tunnel.
118
Tunnels
• An IP tunnel is a virtual point-to-point link
between a pair of nodes that are separated
by an arbitrary number of networks.
• This virtual link is created within the router
at the entrance by providing it with the IP
address of the router at the far end of the
tunnel.
119
Routing Through a Tunnel
• When a router at the entrance wants to send a
packet over this virtual link, it encapsulates the a
packet inside an IP datagram.
• The destination address is the address of the router
at the end of the tunnel, and the source address is
the router at the entrance.
• The virtual link, looks similar to a normal link in
the routing table.
120
A Tunnel Through an Internetwork
1
0
Netw ork 1
R1
virtual
Internetw ork
R2
Netw ork 2
10.0.0.1
IP header,
Destination = 2.x
IP header,
Destination = 10.0.0.1
IP header,
Destination = 2.x
IP payload
IP header,
Destination = 2.x
IP payload
IP payload
R1 has two physical interfaces: Interface 0 connects to
Network 1, interface 1 connects to the Internetwork and is the
121
default. It also has a virtual interface to the tunnel.
Tunneling
• Suppose a tunnel has been configured from R1 to R2
and assigned a virtual interface number of 0. The
forwarding table might look like this:
NetworkNum
NextHop
1
Interface 0
2
Virtual Interface 0
Default
Interface 1
122
Tunneling Example
• Suppose R1 receives a packet from Network 1
that is addressed to network 2.
• To send it out on the virtual interface, the router
adds an IP header addressed to R2 and then
proceeds to forward the packet as I it had been
received.
• R2’s address is 10.0.0.1 since the network
number of this address is 10 not 1 or 2
• When R2 receives the packet it removes the IP
header and processes it.
123
Why Tunnels?
• Why create a tunnel?
– Greater security- it becomes a private link across a public
network.
– R1 and R2 have properties like multicast routing and by
connecting them with a tunnel, all these routers appear to be
connected. This is how the MBone (multicast backbone ) is
built.
– Tunnels can carry packets from protocols other than IP across
an IP network. As long as the routers can handle other
protocols, the IP tunnel looks to them like a point-to-point link
over which they can send non-IP packets.
– Tunnels also provide a mechanism by which we can force a
packet to be delivered to a particular place.
124
Disadvantages of Tunnels
• It increases the length of packets causing a
waste of bandwidth for short packets.
• Routers at the endpoints must also do more
work as they add and remove tunnel
headers.
• There is also a management cost to set up
the tunnels and and make sure they are
correctly handled by the routing protocols.
125
Routing
• A switch or router needs to be able to look at a packet’s
destination address and then to determine which of the
output ports is the best one for that destination.
• In datagram networks, including IP networks, routing is
an issue for every packet.
• In virtual circuits routing is an issue only for the
connection request packets; all subsequent packets follow
the same path.
• The switch makes a decision by consulting a forwarding
table.
126
Forwarding and Routing
• The fundamental problem of routing is “How do
switches and routers acquire the information in their
forwarding tables?”
• Forwarding consists of taking a packet, consulting a
table and sending the packet in the direction
determined by the table. This is a relatively simple and
well-defined process performed locally at a node.
• Routing is the process by which the forwarding tables
are built. This depends on complex distributed
algorithms that continue to evolve.
127
Forwarding and Routing Tables
• Forwarding table and routing table are sometimes used
interchangeably but there is a distinction.
• The forwarding table is used when a packet is being
forwarded and must contain enough information to
accomplish that task. This requires that a row in the table
must contain the mapping from a network number to an
outgoing interface and some MAC information, such as
the Ethernet address of the next hop.
• The routing table, built up by the routing algorithm as a
precursor to the forwarding table, contains mappings
from network numbers to next hop and information about
how this was learned.
128
Forwarding and Routing Tables
• There are reasons for implementing these tables as
separate data structures:
– The forwarding table needs to be structured to optimize
the process of looking up a network number when
forwarding a packet.
– The routing table needs to be optimized for calculating
changes in topology.
– The forwarding table is sometimes implemented in
specialized hardware, but this is rarely done with the
routing table.
129
Routing and Forwarding Tables
Network
Number
10
Next
Hop
171.69.245.1
0
a) A Routing Table
Network Interface MAC
Address
Number
10
if0
8:0:2b:e4:b:1:2
b) A Forwarding Table- the MAC
Address is provided by the Address
Resolution Protocol (ARP)
130
Scalability
• Key question in building a mechanism for the
Internet is “Does this solution scale?”
• The answer for the previous algorithms and
protocols is “NO”, since they are designed for
networks of modest size…(< 100) nodes.
• These do serve as building blocks for a
hierarchical infrastructure that is used in the
Internet today.
131
Domains
• These protocols collectively are called intradomain
routing protocols or interior gateway protocols(IGPs).
• A routing domain is an internet in which all the routers
are under the same administrative control (e.g. Single
campus or single ISP)
• For now, we are considering the problem of routing in a
small to midsize network, not the full Internet.
132
Network as a Graph
• Routing in essence is a problem of graph
theory.
• The nodes may be hosts, switches, routers
or networks.
• The edges of the graph correspond to the
network links. Each edge has an associated
cost, which indicates the desirability of
sending traffic over that link.
133
Network Represented as a Graph
A
3
4
C
6
1
2
1
B
9
E
F
1
D
134
The Routing Problem
• The basic problem of routing is finding the lowest
cost path between any two nodes, where the cost
of a path equals the sum of the cost of all the
edges on the path.
• For a simple path calculate all the shortest paths
and store them on each node.
• Such a static approach has shortcomings:
– It does not deal with node or link failures
– It does not consider the addition of new nodes or links
– It implies that edge costs do not change
135
Routing Protocols
• Routing is achieved by running protocols among
the nodes. These protocols provide a distributed,
dynamic way to solve the problem of finding the
lowest cost path in the presence of link and node
failures and changing edge costs.
• It is difficult to make centralized solutions
scalable so the widely used protocols are
distributed and are areas of challenges and
research.
136
Distributed Protocols
• Distributed algorithms raise the possibility that two
routers will at one instant have different ideas about the
shortest path to some destination.
• Packets can become stuck in a loop if each router thinks
the other one is closer to the destination. This
discrepancy must be resolved as soon as possible.
• Assume the edge costs in a network are known.
• The two main classes of routing protocols are distance
vector and link state.
137
Distance Vector (RIP)
• RIP ( Routing Information Protocol) dynamically
builds a routing table using the distance vector
algorithm.
• The idea behind the distance vector algorithm is that
each node constructs a one dimensional array (vector)
containing the distances (costs) to all other nodes and
distributes that vector to its immediate neighbors.
• Each node knows the cost of its directly connected
neighbors.
• A link that is down is assigned an infinite cost.
138
Distance Vector Routing
• In the next graph, the cost of each link is set to 1,
so that the least cost path is simply the one with
the fewest hops.
• We represent each node’s knowledge as a table.
• Note that each node only “knows” the information
in on row of the table (the one in the left column
that bears its name)
• The global view is not available at any single
point in the network.
139
Distance Vector Routing
B
C
A
D
E
F
G
140
Global View of Initial Distances
Information
Stored at
Node
Distance to Reach Node
A
B
C
D
E
F
G
A
0
1
1

1
1

B
1
0
1




C
1
1
0
1



D


1
0


1
E
1



0


F
1




0
1
G



1

1
0
141
Initial Routing Table at Node A
Destination
Cost
Next Hop
B
1
B
C
1
C
D

-
E
1
E
F
1
F
G

142
Routing At Node A
• Initially the routing table at each node reflects the
beliefs that a packet can reach a connected node in one
hop and that others are unreachable.
• The next step in distance-vector routing is that every
node sends a message to its directly connected
neighbors containing its list of distances.
• The router “learns” the new paths and can update its
table with the new costs for next hops.
• It takes only a few exchanges before each node has a
complete routing table.
143
Final Routing Table At Node A
Destination
Cost
Next Hop
B
1
B
C
1
C
D
2
C
E
1
E
F
1
F
G
2
F
144
Final Routing Tables
• The process of getting constant routing information to all
the nodes is called convergence.
• There is no one node in the network that has all the
information in this next table.
• Each node knows only the content of its own routing
table.
• This distributed algorithm enables all nodes to achieve a
consistent view of the network without a centralized
authority.
145
Final Distances Stored at Each Node
(Global View)
Information
Stored at
Node
Distance to Reach Node
A
B
C
D
E
F
G
A
0
1
1
2
1
1
2
B
1
0
1
2
2
2
3
C
1
1
0
1
2
2
2
D
2
2
1
0
3
2
1
E
1
2
2
3
0
2
3
F
1
2
2
2
2
0
1
G
2
3
2
1
3
1
0
146
Other Distance Vector Issues
• When does a given node send a routing
update to its neighbors?
– Periodic update – sends every so often (several
seconds to several minutes) even if nothing
changes. Lets others know it is still running.
– Triggered update- sent when a node receives an
update from a neighbor that causes a change in
its routing table.
147
Other Distance Vector Issues
• What happens when a link or node fails?
– The nodes, that notice the failure, send a new list of
distances to their neighbors and tables are updated.
• How does a node detect a failure?
– Nodes test links by sending control packets and wait
for an acknowledgement.
– Nodes determine a link is down when it does not
receive a periodic update.
148
Count to Infinity Problem
• Sometimes the network does not stabilize.
• ( See example p. 278) Each node advertises an
unreachable link and the hop count increases on each
router table in a cycle.
• Partial solution uses a relatively small number s an
approximation to infinity.
• Split horizon solution- when a node sends an update, it
does not include those it learned from a neighbor
back to the neighbor.
• These solutions do not work for large routing tables.
149
Routing Information Protocol
(RIP)
• Use is widespread since it was distributed with Berkely
Unix.
• It s also simple and based on the distance-vector
algorithm.
• Routing in internetworks differ slightly.
• In an internetwork, the goal of the routers is to learn
how to forward packets to other networks.
• Instead of advertising the cost of reaching other
routers, they advertise the cost of reaching other
networks.
150
Example Network Running RIP
1
4
A
B
2
5
C
3
D
6
Router C advertises to router A that it can reach networks 2,3
at a cost of 0 networks 5,6 at a cost of 1, and network 4 at a cost of 2.
151
RIP Packet Format
0
8
Command
16
Version
Family of net 1
31
Must be zero
Address of net 1
Address of net 1
Distance to net 1
Family of net 2
Address of net 2
Address of net 2
Distance to net 2
152
RIP
• RIP is a straightforward implementation of distancevector routing and one of the most widely used.
• Built on distance-vector algorithm.
• Routers running RIP send their advertisements every 30
seconds.
• A router sends an update message when its table
changes.
• RIP supports multiple address families, not just IP
• It tries to find the minimum hop route.
• Valid distances are 1-15, with 16 representing infinity,
which limits it t running on fairly small networks.
153
Link State (OSPF)
• Open Shortest Path First Protocol (OSPF) is the
most widely used link-state routing protocol.
• Link-state routing is the second major class of
intradomain routing protocols.
• Assumptions are similar to distance-vector routing.
Each node knows the state and the cost of the link to
its neighbors.
• Need to provide each node with enough information
to find the least cost path to any destination.
154
Link State (OSPF)
• Basic idea: Every node knows how to reach its
neighbor and if this knowledge is disseminated
to every node, then every node will have enough
knowledge of the network to build a complete
map of the network.
• This is a sufficient condition for finding the
shortest path to any point in the network.
155
Link State (OSPF)
• Link-state routing protocols rely on two
mechanism:
– Reliable dissemination of link-state information
– The calculation of routes from the sum of all
the accumulated link-state knowledge.
156
Reliable Flooding
• Reliable flooding is the process of making sure
that all the nodes participating in the routing
protocol get a copy of the link-state
information form all other nodes.
• Basic idea is for a node to send information out
on all of its directly connected links, with each
receiving node forwarding it out on all its links.
157
Reliable Flooding
• Each node creates an update packet, called a
link state packet (LSP) that contains the
following information:
– The ID of the node that created the LSP
– A list of directly connected neighbors of that
node, with the cost of each one
– A sequence number
– A time to live for this packet
158
Reliable Flooding
• First two ( node ID and list of neighbors) are needed to
enable route calculation
• Last two (sequence number and time to live (TTL) for
this packet) are needed to make the process of flooding
the packet to all nodes reliable.
• Reliability includes making sure that you have the most
recent copy of the information, since there may be
multiple contradictory LSPs.
• Making the flooding reliable is quite difficult.
159
Link State Packet Flooding
a) LSP arrives at
node X
X
A
C
B
b) X floods LSP
to A and C
c) A and C flood
LSP to B but
not X
d) Flooding is complete
D
X
A
C
B
(a)
X
A
C
B
(c)
D
(b)
D
X
A
C
B
D
(d)
160
Link State Packets
• Like RIP, each node generates LSPs:
– When a periodic timer expires
– When there is a change in topology
• The newest information must be flooded to all
nodes as quickly as possible, while old
information must be removed and not allowed to
circulate.
161
Goals For LSPs
• Minimize the total amount of routing traffic:
– Avoid generating LSPs unless necessary by using very long
timers. Assume messages saying “no change” do not need to
be sent often.
– Make sure that old information is replaced by newer
information by inserting sequence numbers. Each time a new
LSP packet is generated, increment the sequence number.
– The TTL value assures that old information is eventually
removed. ( When TTL=0 delete.)
162
Route Calculation
• When a node has a copy of the LSP from every
other node, it can compute a complete map for
the topology of the network.
• From this map it is able to determine the best
route to the destination using Dijkstra’s shortestpath algorithm. (See p. 280-283)
163
Dijkstra’s Shortest-Path
Algorithm
• Main idea:
– Start with a set of nodes (M) which contains this node s.
– Initialize the table of costs ( C(n)s) to other nodes using the
known costs to directly connected nodes.
– Look for the node reachable at the lowest cost C(w) and add
it to M.
– Update table of costs considering reaching nodes through w.
– Repeat until all nodes are included in M.
164
Link State Routing
B
5
A
3
10
C
11
2
D
See p. 287 – Steps for building the routing table for node D
165
Link State Routing Properties
• Advantages:
– It has been proven to stabilize quickly.
– It does not generate much traffic
– It responds quickly to topology changes or node
failures.
• Disadvantages:
– Amount of information stored at each node
(one LSP for every node in the network) can be
quite large
166
Distance –Vector vs Link State Routing
• In distance vector, each node talks only to its
directly connected neighbor, but tells them
everything it has learned (distance to all nodes).
• In link-state, each node talks to all other nodes,
but tells them only what it knows for sure (only
the state of its directly connected links)
167
Open Shortest Path First Protocol
(OSPF)
• “Open” – indicates that it is an open, nonproprietary standard created under the Internet
Engineering Task Force (IETF).
• “SPF” comes from an alternate name for linkstate routing.
• Internet Routing protocol based on the link state
algorithm in which every node constructs the
topography of the Internet and uses it to make
forwarding decisions.
168
Open Shortest Path First Protocol
(OSPF)
• Features added to basic link-state algorithm:
– Authentication of routing messages – all updates are
required to be authenticated, eliminating misconfiguration
and malicious users.
– Additional hierarchy- makes systems more scalable, b
allowing a domain to be partitioned into areas, allowing a
router to get to the right are rather than to every network
within the domain.
– Load balancing- allows multiple routes to the same place
to be assigned the same cost and will cause traffic to be
evenly distributed evenly over these routes.
169
OSPF Header Format
0
8
Version
16
Type
31
Message length
SourceAddr
AreaId
Checksum
Authentication type
Authentication
There are different types of OSPF messages, but all begin
With the same header: version =2, type may be 1-5,
Authentication= 0 if not required,1 for password, 2, for
170
Cryptographic checksum.
OSPF Link-State Advertisement
LS Age
Options
Link-state ID
Advertising router
Type=1
LS sequence number
LS checksum
Length
0 Flags
0
Number of links
Link ID
Link data
Link type
Num_TOS
Metric
Optional TOS information
More links
171
Metrics
• Ways to calculate the link costs or metrics:
– Assign a cost of 1 to all links – lowest cost route will be
the one with the least hops.
• Disadvantage:
– Does not distinguish links on a latency basis ( satellite link with
250ms latency looks just as good as a link with 1 ms latency)
– Does not distinguish links on a capacity basis ( 9.6Kbps link looks the
same as a 45 Mbps link)
– Does not distinguish between links based on their current load
(hardest and most complex problem)
– ARPANET routing Metrics
172
ARPANET Routing Metrics
• Measured the number of packets that were queued, waiting
to be transmitted on each link ( link with 10 packets was
assigned a larger cost). Did not work well…did not
consider bandwidth or latency.
• Second version “new routing mechanism” considered
bandwidth and latency and used delay rather than queue
length as a measure of load:
Delay = (DepartureTime – ArrivalTime)+TrasmissionTime + Latency
173
ARPANET Routing Metrics
• Still had problems:
– Worked well under light loads
– Under heavy loads a congested link advertised
a very high cost causing traffic to move off that
link leaving it idle and then advertising a very
low cost, creating instability.
– Range of link values was much too large
174
ARPANET Routing Metrics
• Third version- “revised ARPANET routing
metric” addressed the problems:
– Major changes were to compress the dynamic range of
the metric, to account for the link type, and to smooth
the variation of the metric with time.
– Smoothing was achieved by transforming the delay
measurement to a link utilization. There was a hard
measurement as to how much the metric could change
form one cycle to the next.
175
ARPANET Routing
225
9.6-Kbps satellite link
9.6-Kbps terrestrial link
140
56-Kbps satellite link
56-Kbps terrestrial link
90
75
60
30
25%
50%
Utilization
75%
100%
Revised ARPANET routing metric versus link utilization
176
Monitoring Routing Behavior
• How well does the system work?
• Study was conducted in 1995 by Vern Paxon using
the Unix traceroute to study, 40,000 routes
between 37 Internet sites.
• He found 1 in 30 encountered serious end to end
problems, which lasted about 30 seconds.
• His overall conclusion was that the Internet
routing is becoming less and less predictable over
time.
177
Routing For Mobile Hosts
• In IP addressing and routing there is an assumption about
that lack of mobility of hosts.
• A host address consists of a network number and a host
part and the network number indicates which network
the host is attached to.
• What happens if a host is disconnected from one network
and connected to another one?
• If we don’t change the IP address of the host it becomes
unreachable.
• A Solution - Provide the host with a new address when it
connects to a new network, using DHCP.
178
Mobile IP
• Suppose the user is using an application, while roaming
and changing from Network A to Network B. Changing
the address won’t allow the application to keep working.
• Mobile IP solves this problem. Mobility support requires
at least one router, the home agent. The mobile host is
assumed to have a permanent IP address, called its home
address, equal to its home agent.
• A second router, the foreign agent, is usually required.
This is the router to which the mobile node attaches when
it is away from its home network.
179
Mobile IP
• Both home and foreign agents periodically announce
their presence on the networks to which they are
attached.
• A mobile host also may solicit an advertisement when it
attaches to a new network. This enables it to learn the
address of its home agent before it leaves the home
network.
• When it attaches to the new network, it registers with
the foreign agent providing the address of its home
agent.
• The foreign agent then contacts the home agent
180
providing a care-of-address.
Mobile Host and Mobility Agents
Sending host
Home agent
(10.0.0.3)
Foreign agent
(12.0.0.6)
Internetw ork
Home netw ork
(netw ork 10)
Mobile host
(10.0.0.9)
181
Packet Delivery to Mobile Host
• Any host that ties to send a packet to the
mobile host will send it with a destination
address equal to the home address of that
node.
• Normal IP addressing will cause it to arrive
on the home network of the mobile node on
which the home agent is sitting.
182
Problems of Packet Delivery To a
Mobile Node
• How does the home agent intercept a packet that is
destined for a mobile node? Uses Proxy ARP
• How does the home agent then deliver the packet to the
foreign agent? Uses tunneling
• How does the foreign agent deliver the packet to the
mobile node? Uses the hardware address of mobile
node.
• What about traffic in the opposite direction? Normal
delivery (except if both are mobile).
• ( See details of possible solutions pp.297-298)
183
Route Optimization in Mobile IP
• Routes from sending node to mobile node can be
suboptimal (like on a cell phone).
• In general the goal is to deliver packets as directly
as possible without passing through a home agent
– called “triangle routing problem”, since path
from sender to mobile node via a home agent
takes two sides of a triangle rather than the third
side, which is the direct path.
184
Triangle Routing
• Basic idea is to let the sending node know the care-ofaddress of the mobile node.
• The sending node can then create its own tunnel to the
foreign agent.
• If sender can learn the care-of-address and create its own
tunnel, the route can be optimized.
• If the route is not optimized, the home agent send a
binding update message back to the source.
• The source creates an entry in a “binding cache”,
consisting of mappings from mobile node addresses to
care-of-addresses.
• Problem – binding cache can become outdated
185
Mobile Routing
• Mobile routing provides some security challenges and
areas of active research:
• An attacker wishing to intercept packets destined for
other nodes in the network could contact the home agent
for that node and announce itself as the new foreign
agent for that node. Preventing this requires
authentication mechanisms.
• Security and performance aspect require some other
routing algorithms ( for example, to avoid passing
through an un-trusted network)
• There is also the problem of “ad hoc” mobile networksenabling a group of mobile nodes to form a network
186
without fixed nodes – an area of research.
Global Internet
We have seen how to:
• Connect a heterogeneous collection of
networks to create an internetwork and
• Use the simple hierarchy of the IP address
to make routing somewhat scalable.
• Today’s Internet has tens of thousands of
networks connected to it and routers cannot
“know” all that information
187
Global Internet
• The Internet is not just random interconnections of Ethernets,
but reflects the “shape” of many organizations
• In the 1990’s this shape was rather simple, but with time it has
grown much more complex.
• The original topology consisted of “end users” sites (e.g.
Stanford Univ.) that connected to “service provider” networks
(e.g. BARRNET).
• Many providers served limited geographic regions and were
called regional networks.
• Regional networks were connected to a nation wide backbone,
funded by NSF – called NSFNET.
188
Tree Structure of the Internet in 1990
NSFNET backbone
Stanford
ISU
BARRNET
MidNet
regional
■■■
Westnet
regional
regional
Berkeley
PARC
UNM
NCAR
UNL
KU
UA
189
Global Internet
• This topology consists of end user sites connected
to service provider networks
• Each provider and end user was an administratively
independent entity.
• As a result, different networks had different routing
protocols and acted as a single autonomous system
(AS).
• The fact that the Internet has a structure is useful in
terms of scalability and address utilization.
• Subnetting deals with address utilization.
190
Global Internet
• Global Scalability has two issues:
• Scalability of routing – need to find ways of
minimizing the number of network numbers
that get carried around in routing protocols
and stored in routing tables of routers.
• Address utilization- need to make sure the IP
address space does not get consumed too
quickly.
191
Subnetting
• The original intent of IP addresses was that the network
part would uniquely identify exactly one physical
network. But this has drawbacks:
– For every network, the site needs at least a class C address.
– For every network with more than 255 hosts they need a class
B address.
– A network with two nodes uses an entire class C address
wasting 253 addresses; one with a class B address wastes over
65,000 addresses.
• This is an example of address assignment inefficiency.
192
Subnetting
• Assigning one network number per physical
network uses up address space too quickly.
• Assigning many network numbers has
another drawback for routing. State
information is stored in a node to build
forwarding tables that tell a router how to
reach other networks. The more network
numbers the bigger the routing tables get.
193
Subnetting
• Subnetting provides an elegantly simple way to reduce
the total number of network numbers that are assigned.
• Idea: take a single IP network number and allocate the
IP addresses with that number to several physical
networks, which are now called subnets.
• Subnets should be close to each other.
• A single network number can be shared by configuring
all the nodes on each subnet with a subnet mask.
194
Subnet Masks
• With simple IP addresses, all the hosts on the
same network have the same network number.
• The subnet mask introduces a subnet numberall hosts on the same physical network will have
the same subnet number.
• A subnet mask introduces another level of
hierarchy to the IP address.
• We now think of the address of having 3 parts: a
network part, a subnet part and a host part.
195
Subnet Masks
• Suppose we want to share a single class B address
among several physical networks.
• We could use a subnet mask of 255.255.255.0 so
that the first 24 bits (255s- all 1s) are the network
number and the lower 8 bits (0- all zeros).
• In a class B address, the first 16 bits identify the
network so we can think of the address as having 3
parts: a network part, a subnet part, and a host part.
• We have used the host part now to represent the
subnet and host.
196
Subnet Addressing
Netw ork number
Host number
Class B address
111111111111111111111111
00000000
Subnet mask (255.255.255.0)
Netw ork number
Subnet ID
Host ID
Subnetted address
197
An Example of Subnetting
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.0
128.96.34.15
128.96.34.1
R1
H1
Subnet mask: 255.255.255.128
Subnet number: 128.96.34.128
128.96.34.130
128.96.34.139
128.96.34.129
H3
R2
H2
128.96.33.1
128.96.33.14
Subnet mask: 255.255.255.0
Subnet number: 128.96.33.0
198
Subnetting Example
• Host is configured with both IP address and
subnet mask.
• All hosts on a given subnet have the same mask.
• Bitwise AND of these defines the subnet number
of the host and all other hosts on the same
subnet.
199
Forwarding Table with
Subnetting
The router ANDs the destination address with the subnet
mask and if it matches, it forwards the packet to the next
hop router indicated. Otherwise it is sent to the default
router.
Subnet Number Subnet Mask
NextHop
128.96.34.0
255.255.25.128
Interface 0
128.96.34.128
255.255.25.128
Interface 1
128.96.33.0
255.255.25.0
R2
200
Subnetting and Scalability
• Subnetting solves the scalability problem:
– It improves address assignment efficiency by
not requiring a new class B or C address every
time we add a new physical network.
– It helps aggregate information – a complex
physical network can look like a single
network, reducing the amount of information
that the routers need to store.
201
Classless Routing (CIDR)
• Classsless Interdomain Routing (“cider”) is a
technique that addresses two scaling concerns
in the Internet:
– growth of backbone routing tables as more network
numbers need to be stored in them and
– The potential for the 32-bit address space to be
exhausted – (often due to inefficiency)
– Inefficiency arises because the classes force
assigning addresses in fixed “chunks”
• Subnetting helps but does not solve the
problem.
202
CIDR
• CIDR tries to balance the need to minimize the number
of routes that a router needs to know against the need to
hand out addresses efficiently.
• To do this CIDR aggregates routes, and lets us use a
single entry in a forwarding table to tell how to reach a
lot of networks.
• It does this by breaking the rigid boundaries between
address classes.
• All we need now is a routing protocol to deal with
“classless” addresses, which means that it must
understand a network number of any length.
• Modern routing protocols, such as BGP-4 do that. 203
Modern Routing Protocols
• In the Border Gateway Protocol (BGP-4) routing
protocol, network numbers are represented by
(length, value) pairs, similar to the (mask, value)
subnet pairs.
• The length give the number of bits in the network
prefix .
• Assigning addresses carefully so that all
corporations/campuses connected to a provider share
a common prefix aggregates routes even more.
204
Route Aggregation With CIDR
Corporation X
(11000000000001000001)
Border gatew ay
(advertises path to
11000000000001)
Regional netw ork
Corporation Y
(11000000000001000000)
Both corporations are reachable through the same
provider network and can advertise a single route to both.
205
IP Forwarding Revisited
• IP forwarding finds the network number in the packet
and then looks it up in a forwarding table.
• Problem with using CIDR- prefixes may be any length
form 2 to 32 bits.
• Some addresses may match more than one prefix in
the table. The rule is “longest match”.
• Efficiently finding the longest match is an area of
active research.
206
Interdomain Routing (BGP)
• Basic principle behind autonomous systems (AS)
or domains is providing ways to aggregate
information in a large internet, improving
scalability.
• Two parts of the routing problem:
– Routing within a single autonomous system
(intradomain)
– Routing between autonomous systems or domains
(interdomain)
207
A Network With Two Autonomous Systems
R1
R3
R2
Autonomous system 1
R4
Autonomous system 2
R5
•Each AS can run its own
Intradomain routing protocol.
•The Interdomain routing problem
Is reduced to sharing reachability
Information with one another.
R6
208
Border Routers
• Default routes also reduce the amount of
routing information.
• Router connecting to the Internet, usually at
the boundary between the AS and the
Internet, is called a border router.
• To send packets to the Internet they go to
the border router – the default route, which
eventually connects to a backbone.
209
Interdomain Routing Protocols
• Exterior Gateway Protocol (EGP)- forced a
treelike topology on the Internet, with a
single backbone and AS connected as
parents and children- not as peers.
• Replaced by the Border Gateway Protocol
(BGP), which assumes an arbitrarily
interconnected set of AS.
210
Today’s Multibackbone Internet
Large corporation
“Consumer”ISP
Peering
point
Backbone service provider
“Consumer”ISP
Large corporation
Peering
point
“Consumer”ISP
Small
corporation
Today’s Internet consists of service provider networks, operated by
private companies, rather than the government, and sites are
connected to each other in arbitrary ways. Providers connect to one
another at “peering points”.
211
Types of AS
• Local traffic – originates at or terminates on nodes
within the AS (autonomous systems)
• Transit Traffic- passes through an AS
• Types of AS:
– Stub AS – only a single connection to one other AS and carries
only local traffic
– Multihomed AS – connection to more than one AS- but does
not carry transit traffic (e.g. corporation)
– Transit AS- has connections to more than one other AS and
carries both local and transit traffic, such as a backbone.
212
Interdomain Routing
• Interdomain routing is hard:
– Matter of scale – backbone router must be able to forward any
packet anywhere in the Internet. Needs routing table to provide
a match for any valid IP address ( with about 140,000 prefixes)
– Autonomous nature of the domains- each with their own
protocols.
– Issue of trust – don’t always want to trust routes from others
– Need to support flexible policies, such as prevention of transit
traffic.
213
Border Gateway Protocol
• BGP does not belong to either of the other
protocol classes (distance vector or link state).
• BGP advertises complete paths as an enumerated
list of Ass to reach a particular network.
• The administrator picks at least one node to be the
“BGP speaker” for the AS and to exchange
reachability information.
• In addition there might be border gateways, which
are routers through which packets enter and leave
the AS.
214
Border Gateway Protocol
• An important job of BGP is to prevent looping paths, which is
done by carrying complete path information. If an AS “sees” itself
in the path, it does not use it.
• AS numbers carried in the BGP must be unique. They are 16 bit
numbers, assigned by a central authority, allowing for 65,000 ASs.
• An AS will only advertise routes that it considers good enough for
itself.
• BGP speakers can withdraw routes if a link goes down.
• BGP is designed for CIDR and writes addresses as 194.4.16/20 for
a 20 bit prefix.
• BGP runs on top of TCP- the reliable transport protocol.
215
Example of a Network Running
BGP
Customer P
(AS 4)
128.96
192.4.153
Customer Q
(AS 5)
192.4.32
192.4.3
Customer R
(AS 6)
192.12.69
Customer S
(AS 7)
192.4.54
192.4.23
Regional provider A
(AS 2)
Backbone netw ork
(AS 1)
Regional provider B
(AS 3)
Assume providers are transit networks and the customer networks
are stubs. A BGP speaker for the AS of provider A (AS2) can
advertise reachability information for customers P and Q
216
BGP-4 Update Packet Format
0
15
Unfeasible routes
length
Withdraw n routes
(variable)
Total path
attribute length
Path attributes
(variable)
Netw ork layer
reachability info
(variable)
217
Building Scalable Networks
• How does all this help build scalable
networks?
– Number of nodes participating in the BGP
protocol represents the number of AS, which s
much smaller than the number of networks.
– Finding a route is finding a path to the right
border router
218
Intergrating Interdomain and
Intradomain Routing
• How do all the other routers get the information?
• For Stubs router “injects” a default route.
• Border routers inject specific routes they have
learned from outside the AS.
• Routers in the backbone have too much
information and use a variation of BGP, called
interior BGP (IBGP) to distribute the information
that is learned.
219
Routing Areas
• An area is a set of routers that are
administratively configured to exchange linkstate information with each other. The backbone
is known as area 0.
• A router that is a member of both the backbone
area and a non-backbone area is a border router
(ABR). These are distinct form the routers at an
edge of an AS.
220
A Domain Divided Into Areas
Area 3
Area 1
Area 0
R9
R7
R8
R3
R1
R4
R2
Area 2
R6
R5
221
Routing Areas
• How does a router in one area determine the right next
hop for a packet in another area?
• Imagine the path in three parts:
– It travels from source network to the backbone area.
– Then it travels from backbone to destination network.
• The area border routers summarize the routing
information that they have learned and make it available
to other areas.
• Areas make a trade off between scalability and
optimality of routing and it forces all packets to travel
via the backbone area even if a shorter path is available.
222
IP Version 6 (IPv6)
• Motivation for new IP version is to deal
with scaling problems.
• Subnetting and CIDR have helped with the
address depletion problem as well as the
growth of routing table information needed
in routers.
• There is need for an address space greater
than 32 bits.
223
Historical Perspective
• IETF began looking at extending the IP
address space in 1991.
• Since the IP is contained in every header,
increasing it changes the packet header.
• Effort to redefine it was called IP Next
Generation (IPng), now called IPv6.
• Current version is IPv4. (The number 5 was
used for an experimental protocol.)
224
Historical Perspective
• In addition to scalable routing and addressing,
some of the other “wish list” were:
– Support for real-time services and multimedia
– Security support
– Autoconfiguration ( ability of hosts to automatically
configure themselves with IP address, domain name)
– Enhanced routing functionality
– 128 bit addresses
– Plan to move from current to new version
– Proposal called Simple Internet Protocol Plus (SIPP)
225
Addresses and Routing
• IPv6 provides 128 bit address space, instead of
the 32 bit address of IPv4.
• IPv6 can potentially address 3.4 x 1028 nodes or
approximately 1500 addresses per square foot
of the earth’s surface ( which should be
sufficient even when toasters on Venus have IP
addresses!)
226
Address Space Allocation
• IPv6 addresses do not have classes, but the
address space is still divided based on the leading
bits. ( See list of prefixes p. 321)
• IPv4 class A,B,C are all contained in the prefix
001.
• Aggregatable Global Unicast Addresses are like
IPv4 classless addresses, only longer.
• Some addresses (NSAP) are reserved for ISO
protocols and IPX Novel network-layer protocols.
227
Address Space Allocation
• Idea behind “link local use”addresses is to enable a host
to construct an address that will work on the network to
which it is connected without being concerned about
global uniqueness of the address.
• Site local use addresses are intended to allow a valid
address to be constructed on a site, that is not connected
to the larger Internet, where global uniqueness is not an
issue.
• Multicast address space severs the same function as
class D addresses.
• A node may be assigned an IPv4 compatible IPv6
address by zero-extending its 32 bit address to 128 bits.
228
Address Prefix Assignments
IPv6 addresses are classless with the leading bits specifying
the different uses. IPv4 A,B,C classes are contained in the
“Everything else” range.
Prefix
00…0 ( 128 bits)
00.. 1 ( 128 bits)
1111 1111
1111 1110 10
1111 1110 11
Everything else
Use
Unspecified
Loopback
Multicast Addresses
Link Local Unicast
Site Local Unicast
Global Unicast
229
Address Notation
• There is special notation for writing IPv6 addresses. The
standard is x: x: x: x: x: x: x: x: where each x: is a
hexadecimal representation of a 16 bit piece of the
address, (for example:
47CD:1234:4422:AC02:0022:1234:A456:0124
• Any IPv6 address can be written using this notation
• An address with a large number of contiguous 0’s can be
compressed by omitting the zeros
47CD:0000:0000:0000:0000:0000:A456:0124
can be written as 47CD:: A456:0124
230
Transition From IPv4 to IPv6
• Internet is too big to have a “flag day” to
switch over from IPv4 to IPv6.
• IPv6 must be deployed incrementally so
that hosts and routers that understand only
IPv4 can function for as long as possible.
• Two mechanisms have been defined to help
in this transition:dual-stack and tunneling.
231
Transition From IPv4 to IPv6
• Dual stacks: IPv6 nodes run bothIPv4 and IPv6
and use the version field to decide which stack
should process the arriving packet.
• The basic tunneling technique in which an IP
packet is sent as the payload of another IP packet.
Tunneling is used to encapsulate an IPv6 within a
IPv4 header and is deciphered at the end point of
the tunnel.
232
Aggregatable Global Unicast
Addresses
• IPv6 must provide plain old unicast addressing,
in a way that supports the rapid rate of addition
of new hosts.
• At the heart of IPv6 is the address allocation
plan that determines how addresses beginning
with the 001 prefix will be assigned to service
providers, autonomous systems, network hosts,
and routers. Similar to CIDR in IPv4.
233
IPv6 Provider based Unicast Address
3
m
n
o
p
125- m- n- o- p
010
RegistryID
ProviderID
SubscriberID
SubnetID
InterfaceID
Natural divisions may be made on the basis of continents. For
example, the RegistryID might be an identifier assigned to a
European address registry, with different Ids assigned to other
continents or countries.
One problem might occur if a subscriber is connected to more
than one provider. Which prefix should be used? Could have 3
prefixes: one for subscribers of X only, one for Y only and one
for X and Y.
234
IPv6 Packet Format
• Header is simpler than IPv4 packet. It removes
unnecessary functionality from the protocol.
• Version field is set=6 for IPv6
• Traffic class and Flow Label fields relate to quality of
service issues.
• PAyloadLen gives the length of the packet.
• NextHeader field replaces the option and protocol
fields of IPv4. Options are included in a special
header following the IP header.
235
IPv6 Packet Header
0
Version
12
4
TrafficClass
PayloadLen
31
24
16
Flow Label
NextHeader
HopLimit
SourceAddress
DestinationAddress
Next header/data
236
IPv6 Packet Format
• Fragmentation is an optional header, which means that
IPv4 fragmentation fields are not included in the IPv6
header.
• HopLimit field is simply the TTL of IPv4, renamed to
reflect the way it is actually used.
• The bulk of the header is the source and destination
addresses (each 16 bytes or 128 bits long)
• IPv6 header is always 40 bytes long compared to the
IPv4 header which is 20 bytes, without the options.
237
IPv6 Options
• The way IPv6 handles options is an improvement over
IPv4. If IPv4 options were present.,every router had to
parse the entire options field at the end of the header.
• IPv6 treats options as an extension header, that must
appear in a specific order, so each router can determine
if any of the options are relevant. This is much more
efficient.
• The options headers, formatted as extensions, also
allows them to be of different lengths.
• NextHeader identifies the type of header to follow, or in
last header is the key to identify the higher-layer
238
protocol (e.g. TCP)
IPv6 Fragmentation Extension Header
0
8
NextHeader
16
Reserved
29
Offset
31
RES
M
Ident
Present if fragmentation is necessary. The NextHeader field of the
packet would be set=44 to indicate that the next header is a
fragmentation header. The NextHeader field of the fragmentation
header would contain a value describing the header that follows.
239
Autoconfiguration
• Every host connected to the Internet needs
to be configured with information such as
valid IP address, a subnet mask for the link
to which it attaches and the address of the
name server (DNS).
• One goal of IPv6 is to provide support for
autoconfiguration or “Plug and Play
operation”.
240
Autoconfiguration
• Autoconfiguration problem has two parts:
– Obtain an interface ID that is unique on the link to
which the host is attached.
– Obtain the correct address prefix for the subnet
241
Network Address Translation
• Another technology, called Network Address
Translation (NAT) is now in use and may delay the
switch to IPv6.
• Idea: all hosts that might communicate with one
another over the Internet, do not need to have
globally unique addresses. A host can be assigned a
“private address” that is not necessarily globally
unique, only unique in a limited scope.
• The class A network number 10 is used (originally
assigned to the ARPANET, which is no longer in use)
242
Address Routing Capabilities
• Another of IPv6’s extension headers is the
routing header. In its absence, routing for IPv6
is similar to IPv4 under CIDR.
• The routing header contains a list of IPv6
addresses that represent nodes or areas the
packet should visit en route to its destination.
243
Address Routing Capabilities
• To provide the ability to specify topological
entities rather than individual nodes, IPv6 defines
an anycast address.
• An anycast address is assigned to a set of
interfaces and packets sent to that address will go
to the “nearest” of these interfaces, with nearest
being determined by the routing protocols.
• The anycast address and the routing header will
provide enhanced routing support to mobile
hosts.
244
Other Features
• Primary motivation for the IPv6 is to support the
continued growth of the Internet.Once the header was to
be changed for the sake of addresses it opened the door
to other changes:
–
–
–
–
–
Autoconfiguration
Source-directed routing
Mobility
Network Security
A New Service model
• The main driving force for IPv6 is still the need for
larger addresses.
245
Multicast
• Multiaccess networks like Ethernet and token rings
implement multicast in hardware.
• Multicast can also be extended in software across an
internet.
• Multicast will also be supported in IPv6, with the
differences being restricted to the address format.
• Applications want to send a packet to more than one
destination, or to a multicast address.
• Internet can be implemented on top of networks that
support hardware multicast by extending the routing and
forwarding functions.
246
Multicast Service Model
• Service model for IP multicast:
– IP multicast uses the idea of a multicast group that
receivers may join ( for example, using the Internet to
distribute a pay-per-view movie).
– Each group has a specially assigned address, and
senders use that address as the destination for their
packets. ( Like IPv4 class D address.)
– Hosts join multicast groups using the Internet Group
Management Protocol (IGMP). They use this to notify
the router of their desire to receive packets sent to a
certain multicast group.
247
MultiProtocol Label Switching
• Multiprotocol Label switching (MPLS) tries
to combine some of the properties of virtual
circuit with the flexibility and robustness of
datagrams.
– MPLS relies on IP addresses and IP routing
protocols
– MPLS enabled routers also forward packets by
examining relatively short fixed-length labels,
with local scope.
248
MultiProtocol Label Switching
• Used to enable IP capabilities on devices that
cannot forward IP datagrams in the normal way.
• To forward IP packets along “explicit routes” –
precalculates routes that don’t mathc normal IP
routing.
• To support certain virtual private network
services.
• ( See pp. 340- 352 for details)
249
Summary
• IP tackles heterogenity by defining a simple,
common service model for an internetwork,
which is based on best-effort delivery of IP
datagrams.
• An important part of this model is the global
addressing scheme, which enables any two
nodes to uniquely identify each other and
exchange data.
• The ARP mechanism is used to translate global
IP addresses into local link-layer addresses.
250
Summary
• A critical aspect of the operation of an internet is the
determination of efficient routes. Internet routing algorithms
solve this: distance-vector and link-state. (RIP and OSPF)
• IP deals with major scaling issues: efficient use of address
space and the growth of routing tables as the Internet grows.
– Hierarchical IP address format – manage scale
– Subnetting- makes more efficient use of network numbers and
consolidates routing information
– CIDR- achieves further routing aggregation
– Autonomous systems(AS)- partition into inter and intradomain
routing, each of which is smaller than the total routing problem.
251
Summary
• These mechanisms are unable to keep up with
the growth of the Internet.
• A new address format will be needed (IPv6).
• IPv6 provides a 128 bit address with CIDRlike addressing and routing.
• See also
• http://playground.sun.com/pub/ipng/html/ipng-main.html
252
Figure 4.36
B
A
R1
R2
R4
R3
R6
R5
R7
C
253
B
Source
R1
R2
A
Figure 4.37
R5
R4
R3
R7
R6
C
Source
B
A
R1
R2
R4
R3
R5
R6
R7
C
B
A
R1
R2
R3
R4
C
R6
R5
R7
Source
254
Figure 4.38
RP
RP
Join
R3
R2
R4
R3
R2
R4
Join
R1
R5
R1
R5
(a)
(b)
RP
Join
RP
R3
R2
R3
R4
R2
Join
Join
R1
R5
(c)
R4
R1
R5
(d)
RP = Rendezvous point
Shared tree
Source-specific tree for source R1
255
Figure 4.39
RP
G
RP G
G
R3
R2
R4
RP G
G
R1
R5
G
Host
256
Figure 4.40
10.1.1/24
R3
1
0
R1
0
R2
Prefix
Interface
Prefix
Interface
10.1.1
0
10.1.1
1
10.3.3
0
10.3.3
0
■■■
10.3.3/24
R4
■■■
257
Figure 4.41ab
10.1.1/24
Label = 15, Prefix = 10.1.1
R3
1
0
R1
Prefix
0
R2
Interface
10.3.3/24
Label
Prefix
10.1.1
0
15
10.1.1
1
10.3.3
0
16
10.3.3
0
■■■
R4
Interface
■■■
(a)
10.1.1/24
R3
1
R1
Prefix
10.1.1
10.3.3
R2
0
Remote
Interface Label
0
15
0
16
0
10.3.3/24
R4
Label
Prefix
15
10.1.1
Interface
1
16
10.3.3
0
■■■
■■■
(b)
258
Figure 4.41c
Label = 24, Prefix = 10.1.1
10.1.1/24
R3
1
0
R1
0
R2
10.3.3/24
R4
Prefix
10. 1. 1
Interface
0
Remote
Label
15
10. 3. 3
0
16
Label Prefix
15 10.1.1
16
10.3.3
■■■
Interface
1
Remote
Label
24
0
■■■
(c)
259
Figure 4.42
(a)
ATM cell
header
GFC
VPI
VCI
PTI
CLP
HEC
DATA
Label
(b)
“ Shim “ header
(for PPP, Ethernet,
etc.)
PPP header
Label header
Layer 3 header
260
Figure 4.43
R6
R1
R5
R2
R3
R4
(a)
R6
R1
LSR1
LSR3
R5
R2
LSR2
R4
R3
(b)
261
Figure 4.44
R1
R6
R7
R3
R2
R4
R5
262
Figure 4.45
ATM cells arrive
ATM cells sent
Tail
Head
R2
Cells sent into
tunnel at head
R3
Tunneled data
arrives at tail
263
Figure 4.46
6. ATM cells sent
202
Tail
R2
R3
DL 101
TL DL 101
5. Demux label examined
4. Packet is forw arded to tail
264
Figure 4.47
VPN A / Site 2
VPN B / Site 2
VPN B / Site 1
Provider
netw ork
VPN A / Site 3
VPN A / Site 1
VPN B / Site 3
265
Figure 4.48
A
3
C
6
8
1
B
F
2
D
2
E
266
Figure 4.49
A
2
1
B
5
E
2
D
2
C
3
3
F
267
Figure 4.50
E
A
B
268
Figure 4.51
E
A
B
and
E
A
B
D
269
Figure 4.52
A
B
F
G
C
270
Figure 4.53
5
D
2
2
A
5
E
1
2
B
4
C
271
Figure 4.54
6
B
1
A
3
5
C
1
D
3
1
E
272
Figure 4.55
A
C
B
273
Figure 4.56
A
Provider P
1
2
Provider Q
4
B
3
Provider R
274
Figure 4.57
A
C
R1
RB
R2
Rest of Internet
D
B
275
Figure 4.58
C
A
B
276
Figure 4.59
R1
R2
D
R3
R4
R6
R5
R7
E
277
Figure 4.60
R8
R1
R7
S1
R2
R6
S2
R4
R5
278