chapter 5 network layer

Chapter 5Network Layer

CIS 81 Networking Fundamentals

Rick Graziani

Cabrillo College

graziani@cabrillo.edu

Spring 2010

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This Presentation

For a copy of this presentation and access to my web site for other CCNA, CCNP, and Wireless resources please email me for a username and password. Email: graziani@cabrillo.edu Web Site: www.cabrillo.edu/~rgraziani

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Note

This presentation is not in the order of the book or online curriculum. This presentation also contains information beyond the curriculum.

Network Layer Overview

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Network Layer

IPv4

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Application Header + data

IP Header

0 15 16 31 4-bit

Version 4-bit

Header Length

8-bit Type Of Service (TOS)

16-bit Total Length (in bytes)

16-bit Identification

3-bit Flags

13-bit Fragment Offset

8 bit Time To Live

TTL

8-bit Protocol

16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Data

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IP

IP

IP IP

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Focus on Transport LayerIP

IP

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Network Layer

The Network layer (Layer 3) provides services to exchange the data over the network between identified end devices.

Layer 3 uses four basic processes: Addressing Encapsulation Routing Decapsulation

0 15 16 31 4-bit

Version 4-bit

Header Length

8-bit Type Of Service (TOS)

16-bit Total Length (in bytes)

16-bit Identification

3-bit Flags

13-bit Fragment Offset

8 bit Time To Live

TTL

8-bit Protocol

16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Data

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Addressing

0 15 16 31 4-bit

Version 4-bit

Header Length

8-bit Type Of Service (TOS)

16-bit Total Length (in bytes)

16-bit Identification

3-bit Flags

13-bit Fragment Offset

8 bit Time To Live

TTL

8-bit Protocol

16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Data

172.16.3.10192.168.100.99 Source IP = 192.168.100.99

Destination IP = 172.16.3.10

Destination IP = 192.168.100.99

Source IP = 172.16.3.10

What would be the Source IP Address and Destination IP Address of a Packet from the client to the server?

What would be the Source IP Address and Destination IP Address of a Packet from the server to the client?

More later!

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DataHTTP Header

TCP Header

IP Header

Data Link Header

Data Link Trailer

IP PacketData Link Header

Data Link Trailer

IP PacketData Link Header

Data Link Trailer

IP PacketData Link Header

Data Link Trailer

IP PacketData Link Header

Data Link Trailer

IP PacketData Link Header

Data Link Trailer

IP PacketData Link Header

Data Link Trailer

DataHTTP Header

TCP Header

IP Header

Data Link Header

Data Link Trailer

Encapsulation and Decapsulation

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Decapsulation

Destination Arrival packet processed at Layer 3. Destination address examined. If the address is correct segment is passed up to the appropriate

service at Transport layer.

Is the Destination IP Address of this

packet my IP Address?

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Routing

Routers examine Layer 3 Destination IP addresses to forward packets. Search their routing tables. Send the packet to the next-hop router or host if on that network

0 15 16 31 4-bit

Version 4-bit

Header Length

8-bit Type Of Service (TOS)

16-bit Total Length (in bytes)

16-bit Identification

3-bit Flags

13-bit Fragment Offset

8 bit Time To Live

TTL

8-bit Protocol

16-bit Header Checksum

32-bit Source IP Address

32-bit Destination IP Address

Options (if any)

Data

172.16.3.10192.168.100.99

Source IP = 192.168.100.99

Destination IP = 172.16.3.10

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Network Layer Protocols

The Internet Protocol (IPv4 and IPv6) is the most widely-used Layer 3 data carrying protocol and will be the focus of this course.

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IPv4 basic characteristics…

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Connectionless

IP does not notify the destination host. Which layer 4 protocol on the sending host will establish a connection?

TCP: A connection-oriented protocol. Which layer 4 protocol on the sending host will not establish a connection?

UDP: A connectionless protocol.

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Best Effort Service (unreliable)

Layer 3 (IP) Speed over reliability

Unreliable: Does not have the capability or responsibility to manage, and recover from, undelivered or corrupt packets.

Who does? TCP at the end-to-end hosts

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Responsibility of the OSI Data Link layer to take an IP packet and prepare it for transmission over the communications medium.

Transport of IP packets is not limited to any particular medium. May need to fragment the packet if it is too many bits (later).

Media Independent

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IP Header

IP Destination Address 32-bit binary value that represents the packet destination

Network layer host address. IP Source Address

32-bit binary value that represents the packet source Network layer host address.

Where I am going.

Where I came from.

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Sending hosts generates the value for TTL. Common operating system TTL values are:

UNIX: 255 Linux: 64 or 255 depending upon vendor and version Microsoft Windows 95: 32 Microsoft Vista: 128

IP’s TTL – Time To Live field

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IP’s TTL – Time To Live field

Decremented by each router. If the router decrements the TTL field to 0, it will then drop the packet. What is the advantage to decrementing the TTL by each router and dropping

the packet if it is 0? So IP packets can not travel around the Internet forever, from router to

router.

Decrement by 1, if 0 drop the packet.

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IP’s Protocol Field

Protocol field enables the Network layer to pass the data to the appropriate upper-layer protocol.

Example values are: 01 ICMP 06 TCP 17 UDP

Protocol = 06 TCP

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IP’s ToS Field

Type-of-Service is used to determine the priority of each packet. Enables Quality-of-Service (QoS) mechanism for high priority traffic. What types of traffic might a network administrator need to give priority to?

Traffic that cannot accept any delays. VoIP Streaming video

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TCP MSS = 1460Data = 1460 bytes

20 bytes 20 bytes 1460 bytes

1500 bytes

Determining TCP MTU The default Ethernet MTU value

for a PC is 1500 bytes. (curriculum says MSS)

Typical Maximum Segment Size (MSS) of a TCP segment is 1460 bytes. The number of bytes of data.

TCP MSS defines the maximum size of the data in the TCP segment.

Ethernet MTU defines the maximum size of the data in the Ethernet frame.

The host using Ethernet, MTU of 1500 octets so I will set my MSS to 1460.

IP Fragmentation

How much is enough?

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IP Fragmentation

A router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU. If Don’t Fragment flag set, it will not fragment packet, but discard it.

Data = 1480 bytesIP

Data = 500IP

Data = 480IP

IP Packet Fragments

Original IP Packet

IP Header = 20 bytes

Data = 500IP

Data = 520L2 L2This packet is too big to go over my serial link all at once. I need to break it

into smaller fragmentsData = 520L2 L2

Data = 500L2 L2

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IP Fragmentation

When fragmentation occurs, it does not get reconstructed until it reaches the host. This takes processing time. Fragment Offset field identifies the order

IP Packet

IP Packet

IP Packet

IP Packet

IP Packet

IP Packet

IP Packet

IP Packet

IP Packet

IP PacketIP Packet

Network link with larger MTU

Network link with smaller MTU

Network link with larger MTU

The outgoing link has a smaller MTU so I have to

fragment the packets.

The outgoing link has a large enough MTU but I

don’t reconstruct packets.

It is my job to reconstruct the packets.

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Path MTU Discovery

Path MTU Discovery (Not discussed here, but is important) RFC 1191 (RFC1191) Path MTU Discovery and Filtering ICMP

Marc Slemko Link on CIS 81 web page

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Other IPv4 fields

Version - Contains the IP version number (4) Header Length (IHL) - Specifies the size of the packet header. Packet Length - This field gives the entire packet size, including header and

data, in bytes. Identification - This field is primarily used for uniquely identifying fragments

of an original IP packet Header Checksum - The checksum field is used for error checking the

packet header. Options - There is provision for additional fields in the IPv4 header to

provide other services but these are rarely used.

Host and Network Addresses

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IP Addresses – First look

Host IP addresses are IP addresses assigned to end devices such as: Client computers Server computers Network Printers Router interfaces

Note: the /16 refers to the subnet mask, which will be discussed later. Note: Intermediary devices such as a switch may have an IP address to allow

the network administrator to Telnet to the device for remote management.

Kiwi Airliners - Network Address 172.16.0.0/16

172.16.10.100/16

172.16.10.55/16

172.16.10.3/16

172.16.20.77/16

172.16.20.96/16

172.16.20.103/16

172.16.30.39/16

172.16.30.10/16

172.16.30.111/16

172.16.40.123/16

172.16.40.51/16

172.16.40.29/16

172.16.1.1/16

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IP Addresses – First look

Host IP addresses are members of a group of addresses call the Network Address

IANA (Internet Assigned Numbers Authority) have the responsibility to allocate network addresses.

A company or individual needing a network addresses typically goes to their ISP

ISPs then allocate network addresses to their customers. More detail in the next chapter.

Kiwi Airliners - Network Address 172.16.0.0/16

172.16.10.100/16

172.16.10.55/16

172.16.10.3/16

172.16.20.77/16

172.16.20.96/16

172.16.20.103/16

172.16.30.39/16

172.16.30.10/16

172.16.30.111/16

172.16.40.123/16

172.16.40.51/16

172.16.40.29/16

172.16.1.1/16

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IP Addresses – First look

Host IP Address Unique host IP address

Default Gateway A router which is used to forward packets out of the network. This is a host IP address on the router. Host IP address on the same network as the host.

The host only has to be aware of: Its own network address Default gateway IP address to reach all devices outside its own network

Network Address 172.16.0.0

172.16.10.100/16

172.16.10.55/16

172.16.10.3/16172.16.1.1/16

ISP

Internet

Network Address 192.168.1.0/30

192.168.1.2/30

192.168.1.1/30

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IP Addresses – First look

All hosts in the same network will typically have the same default gateway IP address.

Network Address 172.16.0.0

172.16.10.100/16

172.16.10.55/16

172.16.10.3/16172.16.1.1/16

ISP

Internet

Network Address 192.168.1.0/30

Gateway: 172.16.1.1

Gateway: 172.16.1.1

Gateway: 172.16.1.1

192.168.1.2/30

192.168.1.1/30

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Confirming IP Address, Default GatewayC:\> ipconfig

Windows IP ConfigurationEthernet adapter Local Area Connection: Connection-specific DNS Suffix . : IP Address. . . . . . . . . . . . : 172.16.10.100 Subnet Mask . . . . . . . . . . . : 255.255.0.0 Default Gateway . . . . . . . . . : 172.16.1.1

Root# ifconfigeth0 Link encap:Ethernet HWaddr 00:0F:20:CF:8B:42 inet addr:172.16.1.100 Bcast:172.16.255.255 Mask:255.255.0.0 UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:2472694671 errors:1 dropped:0 overruns:0 frame:0 TX packets:44641779 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:1761467179 (1679.8 Mb) TX bytes:2870928587 (2737.9 Mb) Interrupt:28

Linux: netstat –rn for default gateway information.

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Subnets

Networks can be subdivided into subnets. This provides for several benefits which we will discuss later. Networks can be grouped based on factors that include:

Geographic location, Purpose, Ownership

Kiwi Airliners - Network Address 172.16.0.0/16

172.16.10.100/24

172.16.10.55/24

172.16.10.3/24

172.16.20.77/24

172.16.20.96/24

172.16.20.103/24

172.16.30.39/24

172.16.30.10/24

172.16.30.111/24

172.16.40.123/24

172.16.40.51/24

172.16.40.29/24172.16.1.1/24

172.16.10.1/24 172.16.20.1/24 172.16.30.1/24 172.16.40.1/24

172.16.10.0/24 172.16.20.0/24 172.16.30.0/24 172.16.40.0/24

A Quick Look at Routing

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Routing – First Look

Routers know about: Directly connected networks (C):

Network addresses of its interfaces Remote networks

192.168.1.254/24

C 192.168.2.0/24 is direction connected, FastEthernet0/1

Network 192.168.2.0/24

Network 192.168.1.0/24

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Routers know about: Directly connected networks (C):

Network addresses of its interfaces When a router is configured with the IP address/mask on an interface the

router knows that it has an interface which is part of that network. This is just like a host that is configured with an IP address/mask. (coming)

192.168.1.254/24

C 192.168.2.0/24 is direction connected, FastEthernet0/1

Network 192.168.2.0/24

Network 192.168.1.0/24

Routing – First Look

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Routers learn about remote networks using: Static routes Dynamic Routing Protocol (R = RIP)

Routes in a routing table have three main features: Destination network Next-hop Metric

192.168.1.254/24

C 192.168.2.0/24 is direction connected, FastEthernet0/1

Network 192.168.2.0/24

Network 192.168.1.0/24

Routing – First Look

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Static routes Manually entered by the administrator

Dynamic Routing protocols Routers automatically learn about remote networks Ex: RIP, EIGRP, OSPF, IS-IS, BGP

192.168.1.254/24

C 192.168.2.0/24 is direction connected, FastEthernet0/1

Network 192.168.2.0/24

Network 192.168.1.0/24

Routing – First Look

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Host Routing Table

Hosts also have a local routing table. Usually only contains:

Its own network address (directly connected network) Default gateway IP address

Hosts usually do not have remote networks in their routing tables

netstat –r

or

route print

Chapter 5Network Layer

CIS 81 Networking Fundamentals

Rick Graziani

Cabrillo College

graziani@cabrillo.edu