Download Slide 1

Chapter 5
Network Layer
CIS 81 Networking Fundamentals
Rick Graziani
Cabrillo College
[email protected]
Last Updated: 3/9/2008
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: [email protected]
 Web Site: www.cabrillo.edu/~rgraziani
2
Note
 This presentation is not in the order of the book or online curriculum.
 This presentation also contains information beyond the curriculum.
3
Network Layer Overview
Network Layer
IPv4
5
0
4-bit
Version
15 16
4-bit
Header
Length
8-bit Type Of
Service
(TOS)
31
16-bit Total Length (in bytes)
3-bit
Flags
16-bit Identification
13-bit Fragment Offset
IP Header
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
Application
Header + data
6
IP
IP
IP
IP
7
Focus on Transport Layer
IP
IP
8
0
4-bit
Version
Network Layer
15 16
4-bit
Header
Length
8-bit Type Of
Service
(TOS)
16-bit Total Length (in bytes)
3-bit
Flags
16-bit Identification
8 bit Time To Live
TTL
31
8-bit Protocol
13-bit Fragment Offset
16-bit Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Data
 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
9
Addressing
192.168.100.99
172.16.3.10
Source IP = 192.168.100.99
Destination IP = 172.16.3.10
Source IP = 172.16.3.10
Destination IP = 192.168.100.99
 Source IP Address
 Destination IP Address
 More later!
0
4-bit
Version
15 16
4-bit
Header
Length
8-bit Type Of
Service
(TOS)
16-bit Total Length (in bytes)
3-bit
Flags
16-bit Identification
8 bit Time To Live
TTL
31
8-bit Protocol
13-bit Fragment Offset
16-bit Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Data
10
Encapsulation and Decapsulation
IP
Header
Data Link
Header
IP Packet
Data Link
Trailer
Data Link
Header
IP Packet
Data Link
Trailer
Data Link
Header
IP Packet
Data Link
Trailer
Data Link
Header
IP
Header
TCP
Header
TCP
Header
HTTP
Header
Data Link
Trailer
Data Link
Header
HTTP
Header
Data
Data
Data Link
Trailer
11
Decapsulation
Application
Header + data
 When the packet arrives at the destination host and is processed at
Layer 3.
 The host examines the destination address to verify that the packet
was addressed to this device.
 If the address is correct, the packet is decapsulated by the Network
layer and the Layer 4 PDU contained in the packet is passed up to
the appropriate service at Transport layer.
12
0
4-bit
Version
Routing
15 16
4-bit
Header
Length
8-bit Type Of
Service
(TOS)
16-bit Total Length (in bytes)
3-bit
Flags
16-bit Identification
8 bit Time To Live
TTL
31
8-bit Protocol
13-bit Fragment Offset
16-bit Header Checksum
32-bit Source IP Address
32-bit Destination IP Address
Options (if any)
Data
192.168.100.99
Source IP = 192.168.100.99
Destination IP = 172.16.3.10
172.16.3.10
 Routers examine Layer 3 Destination IP addresses to forward packets.
 Search their routing tables for a match with a network address.
 Send the packet on to the next-hop router.
 Last router in path forwards the router to the host matching the
Destination IP address of the packet.
13
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.
14
IPv4 basic characteristics:
15
Connectionless
 IP packets are sent without notifying the end host that they are coming.
 TCP: A connection-oriented protocol does requires a connection to be
established prior to sending TCP segments.
 UDP: A connectionless protocol does not require a session to be
established.
16
Best Effort Service (unreliable)
 The mission of Layer 3 is to transport the packets between the hosts
while placing as little burden on the network as possible.
 Speed over reliability
 Layer 3 is not concerned with or even aware of the type of data
contained inside of a packet.
 This responsibility is the role of the upper layers as required.
 Unreliable: IP does not have the capability or responsibility to
manage, and recover from, undelivered or corrupt packets.
 TCP’s responsibility at the end-to-end hosts
17
Media Independent
 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.
 In some cases a router will need to split up a packet when
forwarding it from one media to a media with a smaller MTU.
 fragmenting the packet or fragmentation.
18
TCP MSS defines the
maximum size of the data
in the TCP segment.
20 octets 20 octets
1460 octets
Ethernet MTU defines the
maximum size of the data in
the Ethernet frame.
TCP MSS = 1460
Data = 1460 octets
1500 octets
The host using Ethernet, MTU of 1500
octets so I will set my MSS to 1460.
Determining TCP MTU
 Typically, an end system uses the "outgoing interface MTU" minus 40 as its
reported MSS.
 For example, an TCP over IP over Ethernet MSS value is 1460 (1500 - 40 =
1460).
 When a host (usually a PC) initiates a TCP session with a server, it negotiates
the TCP segment size by using the maximum segment size (MSS) option field in
the TCP SYN packet. (curriculum say IP segment).
 The value of the MSS field is determined by the maximum transmission unit
(MTU) configuration on the host.
 The default Ethernet MTU value for a PC is 1500 bytes. (curriculum says MSS)19
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.
20
IP’s TTL – Time To Live field
 When a packet is first generated a value is entered into the TTL field.
 Originally, the TTL field was the number of seconds, but this was difficult to
implement and rarely supported.
 Now, the TTL is now set to a specific value which is then decremented by each
router.
21
IP’s TTL – Time To Live field
Decrement by 1, if 0
drop the packet.
 If the router decrements the TTL field to 0, it will then drop the packet (unless
the packet is destined specifically for the router, I.e. ping, telnet, etc.).
 Common operating system TTL values are:
 UNIX: 255
 Linux: 64 or 255 depending upon vendor and version
 Microsoft Windows 95: 32
 Other Microsoft Windows operating systems: 128
22
http://www.switch.ch/docs/ttl_default.html
TTL Overview - Disclaimer:
The following list is a best effort overview of some widely used TCP/IP stacks. The information was provided by vendors and
many helpful system administrators. We would like to thank all these contributors for their precious help ! SWITCH
cannot, however, take any responsibility that the provided information is correct. Furthermore, SWITCH cannot be made
liable for any damage that may arise by the use of this information.
+-------------------------------+-------+---------+---------+
| OS Version
|"safe" | tcp_ttl | udp_ttl |
+-------------------------------+-------+---------+---------+
AIX
n
60
DEC Pathworks V5
n
30
FreeBSD 2.1R
y
64
HP/UX 9.0x
n
30
HP/UX 10.01
y
64
Irix 5.3
y
60
Irix 6.x
y
60
Linux
y
64
MacOS/MacTCP 2.0.x
y
60
60
OS/2 TCP/IP 3.0
y
64
OSF/1 V3.2A
n
60
Solaris 2.x
y
255
SunOS 4.1.3/4.1.4
y
60
Ultrix V4.1/V4.2A
n
60
VMS/Multinet
y
64
VMS/TCPware
y
60
VMS/Wollongong 1.1.1.1
n
128
VMS/UCX (latest rel.)
y
128
MS WfW
n
32
32
MS Windows 95
n
32
MS Windows NT 3.51
n
32
MS Windows NT 4.0
y
128
30
30
64
30
64
60
60
64
64
30
255
60
30
64
64
30
128
32
32
128
Assigned Numbers (RFC
1700, J. Reynolds, J.
Postel, October 1994):
IP TIME TO LIVE
PARAMETER
The current
recommended default
time to live (TTL)
for the Internet
Protocol (IP) is 64.
Safe: TCP and UDP
initial TTL values
should be set to a
"safe" value of at
least 60 today.
23
IP’s TTL – Time To Live field
Decrement by 1, if 0
drop the packet.
 The idea behind the TTL field is that IP packets can not travel around the
Internet forever, from router to router.
 Eventually, the packet’s TTL which reach 0 and be dropped by the router, even
if there is a routing loop somewhere in the network.
24
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
25
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 such
as;
 VoIP
 Streaming video
 For ToS to be used:
 Hosts set ToS field (can be an intermediary device such as a switch)
 Routers must be configured to examine ToS
26
IP Fragmentation
Original IP
Packet
IP
Data = 1480 bytes
IP Header = 20 bytes
IP Packet
Fragments
IP
Data = 500
IP
Data = 500
IP
L2
Data = 480
Data = 500
L2
 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.
 Fragment Offset field and More Fragments flag is used to reconstruct the
packet at the destination host.
27
IP Fragmentation
IP Packet
IP Packet
Network link with
larger MTU
IP Packet
IP Packet
IP Packet
Network link with
smaller MTU
Network link with
larger MTU
IP Packet
IP Packet
IP Packet
IP Packet
IP Packet
IP Packet
 When fragmentation occurs, it does not get reconstructed until it
reaches the host.
 This takes processing time.
 Fragment Offset field identifies the order
28
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
29
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.
30
Host and Network Addresses
IP Addresses – First look
Kiwi Airliners - Network Address 172.16.0.0/16
172.16.10.100/16
172.16.20.77/16
172.16.10.55/16
172.16.20.96/16
172.16.1.1/16
172.16.10.3/16
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
 Host IP addresses are IP addresses assigned to end devices such as:
 Client computers
 Server computers
 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.
32
IP Addresses – First look
Kiwi Airliners - Network Address 172.16.0.0/16
172.16.10.100/16
172.16.20.77/16
172.16.10.55/16
172.16.20.96/16
172.16.1.1/16
172.16.10.3/16
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
 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.
33
IP Addresses – First look
Network Address 172.16.0.0
172.16.10.100/16
Network Address
192.168.1.0/30
172.16.10.55/16
ISP
Internet
192.168.1.2/30
192.168.1.1/30
172.16.1.1/16
172.16.10.3/16
 Default Gateway
 A router which is used to forward packets out of the network.
 This is a host IP address on the router.
 The default gateway IP address is typically a host IP address which is on the
same network as the host itself.
 The host only has to be aware of:
 Its own network address
 Default gateway IP address to reach all devices outside its own network
34
IP Addresses – First look
Network Address 172.16.0.0
172.16.10.100/16
Gateway: 172.16.1.1
172.16.10.55/16
Gateway: 172.16.1.1
172.16.10.3/16
Gateway: 172.16.1.1
Network Address
192.168.1.0/30
ISP
Internet
192.168.1.2/30
192.168.1.1/30
172.16.1.1/16
 All hosts in the same network will typically have the same default gateway IP
address.
35
Confirming IP Address, Default Gateway
C:\> ipconfig
Windows IP Configuration
Ethernet 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# ifconfig
eth0
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.
36
Subnets
Kiwi Airliners - Network Address 172.16.0.0/16
172.16.10.0/24
172.16.20.0/24
172.16.10.100/24
172.16.20.77/24
172.16.10.55/24
172.16.20.96/24
172.16.30.0/24
172.16.30.39/24
172.16.30.10/24
172.16.40.0/24
172.16.40.123/24
172.16.40.51/24
172.16.1.1/24
172.16.10.3/24
172.16.10.1/24
172.16.20.103/24
172.16.20.1/24
172.16.30.111/24
172.16.30.1/24
 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
172.16.40.29/24
172.16.40.1/24
37
A Quick Look at Routing
Routing – First Look
Network 192.168.1.0/24
Network 192.168.2.0/24
192.168.1.254/24
C 192.168.2.0/24 is direction connected, FastEthernet0/1
 Routers know about:
 Directly connected networks (C):
 Network addresses of its interfaces
 Remote networks
 Static routes
 Dynamic Routing Protocol (R = RIP)
39
Routing – First Look
Network 192.168.1.0/24
Network 192.168.2.0/24
192.168.1.254/24
C 192.168.2.0/24 is direction connected, FastEthernet0/1
 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)
40
Routing – First Look
Network 192.168.1.0/24
Network 192.168.2.0/24
192.168.1.254/24
C 192.168.2.0/24 is direction connected, FastEthernet0/1
 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
41
Routing – First Look
Network 192.168.1.0/24
Network 192.168.2.0/24
192.168.1.254/24
C 192.168.2.0/24 is direction connected, FastEthernet0/1
 Static routes
 Manually entered by the administrator
 Dynamic Routing protocols
 Routers automatically learn about remote networks
 Ex: RIP, EIGRP, OSPF, IS-IS, BGP
42
Host Routing Table
netstat –r
or
route print
 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
43
Chapter 5
Network Layer
CIS 81 Networking Fundamentals
Rick Graziani
Cabrillo College
[email protected]
Last Updated: 3/9/2008