Download TCP/IP Protocol Suite

Ch. 9 – Address Allocation,
Resolution, and Packet
Forwarding (TCP/IP)
TCP/IP
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Most of the introduction to TCP/IP will be covered in
Chapter 11.
IP Addressing
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Static
Dynamic
Static IP Addressing
• You have to go to each
individual device
– Meticulous records must
be kept
– No duplicate IP addresses
Dynamic Addressing
• Dynamic Host Configuration Protocol (DHCP)
– Successor to BOOTP
– Allows host to obtain an IP address quickly and dynamically
– Uses a defined range of IP address
DHCP
DHCP – Getting more than the IP Address
BOOTP
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BOOTP (Bootstrap Protocol)
Provides IP address, subnet mask, default gateway IP
address and DNS IP address.
Disadvantage:
• BOOTP is not a dynamic configuration protocol (like
DHCP).
• When a client requests an IP address the BOOTP server
looks up its MAC address in a table to find the IP address.
• This binding is predetermined.
• What if the computer is moved to another subnet/network?
• Use DHCP!
The ARP Table
• The ARP table is stored in area of Random-Access Memory on each
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host.
Such an area of memory is often called a cache. The ARP table is
often referred to as an ARP cache.
Entries in the ARP table “age out.” They are removed from the table
after a period of inactivity.
Aging Out
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For Microsoft Windows hosts:
– Initial mappings have a 2-minute time-to-live.
– An entry that is used twice in 2 minutes is
automatically given a 10-minute time-to-live.
Using a default gateway
• If the destination IP address is not on the same subnet (or network), a
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computer must use the services of a router.
Routers are sometimes called gateways for this reason.
Sending computer checks for a default gateway in its TCP/IP
configuration.
If no default gateway is installed, the sending computer cannot send
the message.
198.189.232.1
Domain Names and IP Addresses
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Many times we communicate with other hosts using
domain names such as www.cisco.com
Hosts and routers route packets using IP addresses, NOT
domain names.
The host must translate the domain name to an IP
address.
The host will have the DNS Server do this translation for it.
The Domain Name System (abbreviated DNS) is an
Internet directory service.
DNS is how domain names are translated into IP
addresses, and DNS also controls email delivery.
If your computer cannot access DNS, your web browser
will not be able to find web sites, and you will not be able to
receive or send email.
Domain Names and IP Addresses
We usually use domain names,
www.cisco.com, but the IP
packets are sent using the IP
address, 198.133.219.25.
Data link destination address
Data link source address Other data link fields
IP Destination Address
IP Source Address Other IP fields and data
198.133.219.25
Exploring the Domain Name Space
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DNS software is generally
made up of two elements:
the actual name server,
and something called a
resolver.
The name server responds
to browser requests by
supplying name-to-address
conversions.
When it doesn't know the
answer, the resolver will ask
another name server for the
information.
Exploring the Domain Name Space
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To see how it works, let's go
back to the domain-namespace inverted tree.
When you type in a URL,
your browser sends a request
to the closest name server.
If that server has ever fielded
a request for the same host
name (within a time period
set by the administrator to
prevent passing old
information), it will locate the
information in its cache and
reply.
Exploring the Domain Name Space
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If the name server is
unfamiliar with the domain
name, the resolver will
attempt to "solve" the
problem by asking a server
farther up the tree.
If that doesn't work, the
second server will ask yet
another - until it finds one
that knows.
(When a server can supply
an answer without asking
another, it's known as an
authoritative server.)
Exploring the Domain Name Space
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Once the information is located, it's
passed back to your browser, and
you're sent on your merry way.
Usually this process occurs quickly,
but occasionally it can take an
excruciatingly long time (like 15
seconds).
Exploring the Domain
Name Space
• In the worst cases, you'll get a
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dialog box that says the domain
name doesn't exist - even though
you know it does.
This happens because the
authoritative server is slow replying
to the first, and your computer gets
tired of waiting so it times-out (drops
the connection).
But if you try again, there's a good
chance it will work, because the
authoritative server has had enough
time to reply, and your name server
has stored the information in its
cache.
Proxy ARP – (from cisco.com)
• http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note0
9186a0080094adb.shtml
• This link and PDF is on my Semester 1 web page.
Introduction
• This document explains the concept of proxy Address Resolution
Protocol (ARP).
• Proxy ARP is the technique in which one host, usually a router,
answers ARP requests intended for another machine.
• By "faking" its identity, the router accepts responsibility for routing
packets to the "real" destination.
• Proxy ARP can help machines on a subnet reach remote subnets
without configuring routing or a default gateway.
Prerequisites
• This document requires an understanding of the ARP and Ethernet
environment.
How Does Proxy ARP Work?
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Below is an example of how proxy ARP works:
How Does Proxy ARP Work?
• The Host A (172.16.10.100) on Subnet A needs to send packets to
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Host D (172.16.20.200) on Subnet B.
As shown in the diagram above, Host A has a /16 subnet mask.
What this means is that Host A believes that it is directly connected to
all of network 172.16.0.0.
When Host A needs to communicate with any devices it believes are
directly connected, it will send an ARP request to the destination.
Therefore, when Host A needs to send a packet to Host D, Host A
believes that Host D is directly connected, so it sends an ARP request
to Host D.
To reach Host D (172.16.20.200), Host A needs the MAC address of
Host D.
Therefore, Host A broadcasts an ARP request on Subnet A, as below:
Sender's MAC Address
Sender's IP Address
Target MAC Address
Target IP Address
00-00-0c-94-36-aa
172.16.10.100
00-00-00-00-00-00
172.16.20.200
How Does Proxy ARP Work?
• In above ARP request, Host A (172.16.10.100) is requesting that Host
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D (172.16.20.200) send its MAC address.
The above ARP request packet is then encapsulated in an Ethernet
frame with Host A's MAC address as the source address and a
broadcast (FFFF.FFFF.FFFF) as the destination address.
Since the ARP request is a broadcast, it reaches all the nodes in the
Subnet A, including the router's e0 interface, but does not reach Host
D.
The broadcast will not reach Host D because routers, by default, do not
forward broadcasts.
Since the router knows that the target address (172.16.20.200) is on
another subnet and can reach Host D, it will reply with its own MAC
address to Host A.
Sender's MAC Address
Sender's IP Address
Target MAC Address
Target IP Address
00-00-0c-94-36-ab
172.16.20.200
00-00-0c-94-36-aa
172.16.10.100
How Does Proxy ARP Work?
• Above is the Proxy ARP reply that the router sends to Host A.
• The proxy ARP reply packet is encapsulated in an Ethernet frame with
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router's MAC address as the source address and Host A's MAC
address as the destination address.
The ARP replies are always unicast to the original requester.
On receiving this ARP reply, Host A updates its ARP table as below:
IP Address
MAC Address
172.16.20.200
00-00-0c-94-36-ab
How Does Proxy ARP Work?
• From now on Host A will forward all the packets that it wants to reach
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172.16.20.200 (Host D) to the MAC address 00-00-0c-94-36-ab
(router).
Since the router knows how to reach Host D, the router forwards the
packet to Host D.
The ARP cache on the hosts in Subnet A is populated with the MAC
address of the router for all the hosts on the Subnet B.
Hence all packets destined to Subnet B are sent to the router.
The router forwards those packets to the hosts in Subnet B.
The ARP cache of Host A is given below:
IP Address
MAC Address
172.16.20.200
00-00-0c-94-36-ab
172.16.20.100
00-00-0c-94-36-ab
172.16.10.99
00-00-0c-94-36-ab
172.16.10.200
00-00-0c-94-36-bb
Note: Multiple IP addresses are mapped to a single MAC address (the
router's MAC address), indicating that proxy ARP is in use.
Routers and the Network Layer
Network Layer
• Routers
– Pass data between networks
– Use Layer 3 addresses (logical)
– Can make intelligent decisions regarding best path
Assigned by Network
Administrator
Assigned by NIC
manufacturer
Network Segmentation
172.30.10.0/24
10.25.0.0/16
172.30.10.11/24
172.30.10.13/24
10.25.7.11/16
10.25.3.13/16
172.30.10.10/24
172.30.10.12/24
10.25.1.10/16
10.25.3.12/16
172.30.10.1/24
10.25.1.1/16
Since the interface where the router connects
to a network is considered to be part of that
network, the interface where the router
connects to network A has an IP address of A1.
Routers and Data Relaying
• You want to send data from one network to another.
• The router:
– Strips off the data link header, carried by the frame
– Examines the network layer address to determine the destination
network
– Consults its routing table to determine which interface it will use to
send the data
Router Interface
Each router interface must have a separate
network (or subnetwork) address.
Includes subnet mask.
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The network layer provides best-effort end-to-end packet delivery across
interconnected networks.
The network layer uses the IP routing table to send packets from the source
network to the destination network.
After the router determines which path to use, it proceeds with forwarding the
packet.
It takes the packet that it accepted on one interface and forwards it to another
interface or port that reflects the best path to the packet's destination.
Much more information later in the presentation on “The Routing Table
Structure.”
Path Switching and Packet Forwarding
192.168.1.0/24
.1
e0
192.168.1.10/24
X
RTA
192.168.2.0/24
.1
.2
s0
s0
RTB
Data Link Header
Data link destination address
Data link source address Other data link fields
192.168.3.0/24
.1
.2
s1
s0
RTC
Y
192.168.4.0/24
.1
e0
192.168.4.10/24
IP (Network layer) Packet
IP Destination Address
IP Source Address Other IP fields and data
Data Link Frame = Data Link Header + IP Packet
Path Switching
• Host X has a packet(s) to send to Host Y
• A router generally relays a packet from one data link to another, using two basic
functions:
1. a path determination function - Routing
2. a switching function – Packet Forwarding
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Let’s go through all of the stages these routers use to route and switch this
packet.
See if you can identify these two functions at each router.
Note: Data link addresses have been abbreviated.
X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
00-10
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
RTB
Data link source address Other data link fields
0A-10
192.168.3.0/24
.1
.2
s0
s0
RTC
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
From Host X to Router RTA
• Host X begins by encapsulating the IP packet into a data link frame (in this case
Ethernet) with RTA’s Ethernet 0 interface’s MAC address as the data link
destination address.
• How does Host X know to forward to packet to RTA and not directly to Host Y?
How does Host X know or get RTA’s Ethernet address?
– Remember, it looks at the packet’s destination ip address does an AND
operation and compares it to its own ip address and subnet mask.
– It determines if the two ip addresses are on the same subnet or not.
– If the are on the same subnet, it looks for the destination ip address of the
packet in its ARP cache. – sending out an ARP request if it is not there.
– If they are on different subnets, it looks for the ip address of the default
gateway in its ARP cache – sending out an ARP request if it is not there.
• If you do not remember, be sure to review our previous presentation, “ARP –
The Process and the Protocol”
X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
0B-31
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
192.168.3.0/24
.1
.2
s0
s0
RTB
Data link source address Other data link fields
00-20
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
3
RTA ARP Cache
IP Address
MAC Address
192.168.2.2
0B-31
RTC
2
RTA Routing Table
Network
Hops Next-hop-ip Exit-interface
192.168.1.0/24 0
Dir.Conn.
e0
192.168.2.0/24 0
Dir.Conn
e1
192.168.3.0/24 1
192.168.2.2
e1
192.168.4.0/24 2
192.168.2.2
e1
RTA to RTB
1. RTA looks up the IP destination address in its routing table.
• 192.168.4.0/24 has next-hop-ip address of 192.168.2.2 and an exit-interface of
e1.
• Since the exit interface is on an Ethernet network, RTA must resolve the nexthop-ip address with a destination MAC address.
2. RTA looks up the next-hop-ip address of 192.168.2.2 in its ARP cache.
• If the entry was not in the ARP cache, the RTA would need to send an ARP
request out e1. RTB would send back an ARP reply, so RTA can update its ARP
cache with an entry for 192.168.2.2.
X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
0B-31
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
192.168.3.0/24
.1
.2
s0
s0
RTB
Data link source address Other data link fields
00-20
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
3
RTA ARP Cache
IP Address
MAC Address
192.168.2.2
0B-31
RTC
2
RTA Routing Table
Network
Hops Next-hop-ip Exit-interface
192.168.1.0/24 0
Dir.Conn.
e0
192.168.2.0/24 0
Dir.Conn
e1
192.168.3.0/24 1
192.168.2.2
e1
192.168.4.0/24 2
192.168.2.2
e1
RTA to RTB (continued)
3. Data link destination address and frame encapsulation
• After finding the entry for the next-hop-ip address 192.168.2.2 in its ARP cache,
RTA uses the MAC address for the destination MAC address in the reencapsulated Ethernet frame.
The frame is now forwarded out Ethernet 1 (as specified in RTA’s routing table.
• Notice, that the IP Addresses did not change.
• Also notice that the Routing table was used to find the next-hop ip address,
used for the data link address and exit interface, to forward the packet in a new
data link frame.
X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
RTB
192.168.3.0/24
.1
.2
s0
s0
Data link source address Other data link fields
FFFF
RTC
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
2
Network
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
RTB Routing Table
Hops Next-hop-ip Exit-interface
1
192.168.2.1
e0
0
Dir.Conn
e0
0
Dir.Conn
s0
1
192.168.3.2
s0
RTB to RTC
1. RTB looks up the IP destination address in its routing table.
• 192.168.4.0/24 has next-hop-ip address of 192.168.3.2 and an exit-interface of
s0 (serial 0).
• Since the exit interface not on an Ethernet network, RTA does not need to
resolve the next-hop-ip address with a destination MAC address.
• Remember, serial interfaces do not have MAC addresses.
X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
RTB
192.168.3.0/24
.1
.2
s0
s0
Data link source address Other data link fields
FFFF
RTC
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
2
Network
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
RTB Routing Table
Hops Next-hop-ip Exit-interface
1
192.168.2.1
e0
0
Dir.Conn
e0
0
Dir.Conn
s0
1
192.168.3.2
s0
RTB to RTC
2. Data link destination address and frame encapsulation.
• When the interface is a point-to-point serial connection, the Routing Table
process does not even look at the next-hop IP address.
• Remember, a serial link is like a pipe - only one way in and only one way out.
• RTA now encapsulates the IP packet into the proper data link frame, using the
proper serial encapsulation (HDLC, PPP, etc.).
• The data link destination address is set to a broadcast, since there is only one
other end of the pipe and the frame is now forwarded out serial 0.
X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
0B-20
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
192.168.3.0/24
.1
.2
s0
s0
RTB
Data link source address Other data link fields
0C-22
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
3
RTC ARP Cache
IP Address
MAC Address
192.168.4.10
0B-20
RTC
2
Network
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
RTC Routing Table
Hops Next-hop-ip Exit-interface
2
192.168.3.1
s0
1
192.168.3.1
s0
0
Dir.Conn
s0
0
Dir.Conn
e0
RTC to Host Y
1. RTC looks up the IP destination address in its routing table.
• 192.168.4.0/24 is a directly connected network with an exit-interface of e0.
• RTC realizes that this destination ip address is on the same network as one of its
interfaces and it can sent the packet directly to the destination and not another
router.
• Since the exit interface is on an directly connected Ethernet network, RTC must
resolve the destination ip address with a destination MAC address.
2. RTC looks up the destination ip address of 192.168.4.10 in its ARP cache.
• If the entry was not in the ARP cache, the RTC would need to send an ARP
request out e0. Host Y would send back an ARP reply, so RTC can update its
ARP cache with an entry for 192.168.4.10.
X
192.168.1.0/24
.1
e0
192.168.1.10/24
00-10
0A-10
Data link destination address
0B-20
RTA
192.168.2.0/24
.1
.2
e1
e0
00-20
0B-31
192.168.3.0/24
.1
.2
s0
s0
RTB
Data link source address Other data link fields
0C-22
IP Destination Address
192.168.4.0/24
Y
.1
e0
192.168.4.10/24
0C-22
0B-20
IP Source Address Other IP fields and data
192.168.4.10 192.168.1.10
1
3
RTC ARP Cache
IP Address
MAC Address
192.168.4.10
0B-20
RTC
2
Network
192.168.1.0/24
192.168.2.0/24
192.168.3.0/24
192.168.4.0/24
RTC Routing Table
Hops Next-hop-ip Exit-interface
2
192.168.3.1
s0
1
192.168.3.1
s0
0
Dir.Conn
s0
0
Dir.Conn
e0
RTC to Host Y (continued)
3. Data link destination address and frame encapsulation
• After finding the entry for the destination ip address 192.168.4.10 in its ARP cache,
RTC uses the MAC address for the destination MAC address in the reencapsulated Ethernet frame.
The frame is now forwarded out Ethernet 0 (as specified in RTA’s routing table.
From Cisco on-line curriculum:
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When the router checks its routing table entries, it discovers that the best path to
destination Network 2 uses outgoing port To0, the interface to a token-ring LAN.
Although the lower-layer framing must change as the router passes packet traffic from
Ethernet on Network 1 to token-ring on Network 2, the Layer 3 addressing for source and
destination remains the same.
In the Figure, the destination address remains Network 2, Host 5, regardless of the
different lower-layer encapsulations.
From Cisco on-line curriculum:
• Routers enable LAN-to-WAN packet flow by keeping the end-to-end source and destination
addresses constant while encapsulating the packet in data link frames, as appropriate, for the next
hop along the path.
NOTE:
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Remember, when the interface is a point-to-point serial connection, the Routing Table
process does not even look at the next-hop IP address in the routing table, only the exitinterface.
Ch. 9 – Address Allocation,
Resolution, and Packet
Forwarding (TCP/IP)
CCNA 1 version 3
Rick Graziani
Cabrillo College
Spring 2005