Transcript Slide 1

Chapter 6: Objectives
 Explain how network layer protocols and services support
communications across data networks.
 Explain how routers enable end-to-end connectivity in a small to
medium-sized business network.
 Determine the appropriate device to route traffic in a small to
medium-sized business network.
 Configure a router with basic configurations.
The Network Layer
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
3
Encapsulation
DATA
SEGMENT DATA
S.P / D.P. / S.N. / Ack # / …
DATA (SEGMENT)
PACKET
IPv / HLEN / Flag / S. IP / D. IP / …
Frame Header
FRAME
DATA
(PACKET)
Trailer
111010110101011100001001011010101010010101010101101101010001010101010110101010
Functions of
the Network
Layer
IP
IP
 The network layer, or OSI Layer 3, provides services to allow end devices to
exchange data across the network.
 The network layer uses four basic processes:
 Addressing end devices
 Encapsulation
 Routing
 De-encapsulation
Network Layer Protocols
 Common Network Layer Protocols
 Internet Protocol version 4 (IPv4)
 Internet Protocol version 6 (IPv6)
 Legacy Network Layer Protocols
 Novell Internetwork Packet Exchange (IPX)
 AppleTalk
 Connectionless Network Service (CLNS/DECNet)
Characteristics of IPv4
 Connectionless:
 No connection is established before sending data packets.
 Best effort delivery:
 No additional overhead is used to guarantee packet delivery.
 Makes it unreliable …?
 Media independent:
 Operates independently of the medium carrying the data.
Connectionless Service = Postal Service
Connectionless Service
Best Effort Delivery = Unreliable
Best Effort Delivery = Unreliable
 IP is unreliable because it doesn’t have the capability to manage,
and recover from, undelivered or corrupt packets.
 TCP (if used) will manage the transmission reliability.
 It also makes for a smaller IP header.
 Less overhead = less delay in delivery = very fast.
IPv4 Media Independent
 IP doesn’t care what type of media the packet is carried on.
MTU
The
The outgoing
outgoing link
link has
has a
a
large enough
MTU
but to
I
smaller
MTU so
I have
don’t reconstruct
fragment
the packets.
packets.
It is my job to reconstruct
the packets.
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
The Network layer does consider the maximum size of PDU that each medium can
transport.
 This is referred to as the Maximum Transmission Unit (MTU).
The Network layer determines how large to create the packets.
 Routers may need to split up a packet when forwarding it from one media to a
media with a smaller MTU.
 This process is called fragmenting the packet or fragmentation.
This is similar to segmenting at the Transport layer but happens at the Network layer.
IPv4 Packet
IPv4 Packet
IP Header
Data (Payload)
 IPv4 has been in use since 1983 when it was deployed on the
Advanced Research Projects Agency Network (ARPANET).
 An IPv4 packet has two parts:
 IP Header - Identifies the packet characteristics.
 Payload - Contains the Layer 4 segment information and the
actual data.
IPv4 Header – Significant Fields
Byte 1
Version
Byte 2
IP Header
Length
Byte 3
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Byte 4
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
IPv4 Header – Validation Fields
Byte 1
Version
Byte 2
IP Header
Length
Byte 3
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Byte 4
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Sample IPv4 Packet
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Version (4 bits)
– Indicates the version of IP currently used.
– 0100 = 4 and therefore IPv4
– 0110 = 6 and therefore IPv6
Padding
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
IP Header Length (4 bits)
– Identifies the number of 32-bit words in the header.
– The IHL value varies due to the Options and Padding fields.
– The minimum value for this field is 5 (i.e., 5×32 = 160 bits =
20 bytes) and the maximum value is 15 (i.e., 15×32 = 480
bits = 60 bytes).
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Differentiated Services (8 bits)
– Formerly called the Type of Service (ToS) field.
– The field is used to determine the priority of each packet.
– First 6 bits identify the Differentiated Services Code Point (DSCP) value for QoS.
– Last 2 bits identify the explicit congestion notification (ECN) value used to prevent
dropped packets during times of network congestion.
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Total Length (16 bits)
– Sometimes referred to as the Packet Length.
– Defines the entire packet (fragment) size, including header and data, in bytes.
– The minimum length packet is 20 bytes (20-byte header + 0 bytes data) and the
maximum is 65,535 bytes. .
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
A router may have to fragment
a packet
Padding
when forwarding it from one medium to
another medium that has a smaller MTU.
Options (optional)
When this happens, fragmentation
occurs and the IPv4 packet uses the
following 3 fields to keep track of the
fragments
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Identification (16 bits)
– Field uniquely identifies the fragment of an
original IP packet.
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Flag (3 bits)
– This 3-bit field identifies how the packet is fragmented.
– It is used with the Fragment Offset and Identification
fields to help reconstruct the fragment into the original
packet.
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Fragment Offset (13 bits)
– Field identifies the order in which to place the packet
fragment in the reconstruction of the original
unfragmented packet.
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Time-to-Live (TTL) (8 bits)
Padding
– Used to limit the lifetime of a packet.
– It is specified in seconds but is commonly referred to as hop
count.
– The packet sender sets the initial TTL value and is decreased
by one each time the packet is processed by a router, or hop.
– If the TTL field decrements to zero, the router discards the
packet and sends an ICMP Time Exceeded message to the
source IP address.
– The traceroute command uses this field to identify the routers
used between the source and destination.
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Protocol (8 bits)
– Field indicates the data payload type that the packet is
carrying, which enables the network layer to pass the data
to the appropriate upper-layer protocol.
– Common values include ICMP (1), TCP (6), and UDP (17).
– Others: GRE (47), ESP (50), EIGRP (88), OSPF (89)
– http://www.iana.org/assignments/protocol-numbers/
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Header Checksum (8 bits)
– Field is used for error checking of the IP header.
– The checksum of the header is recalculated and
compared to the value in the checksum field.
– If the values do not match, the packet is discarded.
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Source IP Address (32 bits)
– Contains a 32-bit binary value that represents the
source IP address of the packet.
Version
IP Header
Length
Differentiated Services
Total Length
DSCP
ECN
Identification
Time-To-Live
Flag
Protocol
Fragment Offset
Header Checksum
Source IP Address
Destination IP Address
Options (optional)
Padding
Destination IP Address (32 bits)
– Contains a 32-bit binary value that represents the
destination IP address of the packet.
Sample IPv4 Headers
Sample IPv4 Headers
Sample IPv4 Headers
IPv6 Packet
IPv4
Limitations of IPv4
 Since 1983, IPv4 has been updated to address new challenges.
 However, even with changes, IPv4 still has three major issues:
 IP address depletion
 Internet routing table expansion
 Lack of end-to-end connectivity
IP Address Depletion
 IPv4 has a limited number of unique public IP addresses available.
 Although there are approximately 4 billion IPv4 addresses, the
increasing number of new IP-enabled devices, always-on
connections, and the potential growth of less-developed regions
have increased the need for more addresses.
Blocks Assigned in 1993
Blocks Assigned in 2000
Blocks Assigned in 2007
Blocks Assigned in 2010
IPv4 Address Depletion
 In October 2010, less than 5% of the public IPv4 addresses
remained unallocated.
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 Monday, January 31, 2011 IANA allocated two blocks of IPv4 address
space to APNIC, the RIR for the Asia Pacific region (39/8 and 106/8)
 This triggered a global policy to allocate the remaining IANA pool of 5
/8’s equally between the five RIRs.
 So, basically…
Internet Routing Table Expansion
 A routing table is used by routers to make best path determinations.
 As the number of servers (nodes) connected to the Internet
increases, so too does the number of network routes.
 These IPv4 routes consume a great deal of memory and processor
resources on Internet routers.
Lack of End-to-End Connectivity
NAT
192.168.1.0/24
RFC 1918 Private Address
Public IPv4 Address
 Network Address Translation (NAT) is a technology commonly
implemented within IPv4 networks.
 NAT provides a way for multiple devices to share a single public
IP address.
 However, because the public IP address is shared, the IP address of
an internal network host is hidden.
 This can be problematic for technologies that require end-to-end
connectivity.
IETF To The Rescue
 To address these problems, the IETF it implemented solutions to
solve these problems.
 Short Term solutions included:
 Subnetting
 Variable-length subnet masking (VLSM)
 Classless interdomain routing (CIDR)
 Supernetting
 Network Address Translation (NAT)
 Private Addresses
 However, its long term solution was IP version 6 (IPv6)
IPv6
 IPv6 overcomes the limitations and provides the following
improvements:
 Increased address space
 Improved packet handling
 Eliminates the need for NAT
 Integrated security
Increased Address Space
 The 32-bit IPv4 address space provides approximately
4,294,967,296 unique addresses.
 Of these, only 3.7 billion addresses are assignable, because the
IPv4 addressing system separates the addresses into classes,
and reserves addresses for multicasting, testing, and other
specific uses.
 IPv6 addresses are based on 128-bit hierarchical addressing as
opposed to IPv4 with 32 bits.
 340 undecillion addresses
 This dramatically increases the number of available IP
addresses.
Increased Address Space
Number name
Scientific
Notation
1 Thousand
10
1 Million
10
1 Billion
10
1 Trillion
10
1 Quadrillion
10
1 Quintillion
10
1 Sextillion
10
1 Septillion
10
1 Octillion
10
1 Nonillion
10
1 Decillion
10
1 Undecillion
10
3
6
9
12
15
18
21
24
27
30
33
36
Number of zeros
1,000
1,000,000
1,000,000,000
There are 4 billion
IPv4 addresses
1,000,000,000,000
1,000,000,000,000,000
1,000,000,000,000,000,000
1,000,000,000,000,000,000,000
1,000,000,000,000,000,000,000,000
1,000,000,000,000,000,000,000,000,000
1,000,000,000,000,000,000,000,000,000,000
1,000,000,000,000,000,000,000,000,000,000,000
1,000,000,000,000,000,000,000,000,000,000,000,000
 50 billion billion billion addresses for every person on earth
There are 340
undecillion IPv6
addresses
Do we need this many addresses?
Improved
Packet
Handling
 The IPv6 header has been simplified with fewer fields.
 This improves packet handling by intermediate routers and also
provides support for extensions and options for increased
scalability/longevity.
IPv6 Header
Byte 1
Version
Byte 2
Byte 3
Traffic Class
Payload Length
Byte 4
Flow Label
Next
Header
Source IP Address
Destination IP Address
Hop Limit
Sample IPv4 Packet
Version
Traffic
Class
Flow Label
Payload Length
Next
Header
Source IP Address
Destination IP Address
Hop Limit
Version
Traffic
Class
Flow Label
Payload Length
Next
Header
Hop Limit
Source IP Address
Destination IP Address
Version (4 bits)
– Indicates the version of IP currently used.
– 0100 = 4 and therefore IPv4
– 0110 = 6 and therefore IPv6
Version
Traffic
Class
Flow Label
Payload Length
Next
Header
Hop Limit
Source IP Address
Destination IP Address
Traffic Class (8 bits)
– Field is equivalent to the
IPv4 Differentiated
Services (DS) field.
– It also contains a 6-bit
DSCP value used for QoS
and a 2-bit ECN used for
traffic congestion control.
Version
Traffic
Class
Flow Label
Payload Length
Next
Header
Hop Limit
Source IP Address
Destination IP Address
Flow Label (20 bits)
– Field provides a special
service for real-time
applications.
– It can be used to inform
routers and switches to
maintain the same path for
the packet flow so that
packets are not reordered.
Version
Traffic
Class
Flow Label
Payload Length
Next
Header
Hop Limit
Source IP Address
Destination IP Address
Payload Length (16 bits)
– Field is equivalent to the Total Length field
in the IPv4 header.
– It defines the entire packet (fragment) size,
including header and optional extensions
Version
Traffic
Class
Flow Label
Payload Length
Next
Header
Hop Limit
Source IP Address
Destination IP Address
Next Header (8 bits)
– Field is equivalent to the IPv4 Protocol
field.
– It indicates the data payload type that the
packet is carrying, enabling the network
layer to pass the data to the appropriate
upper-layer protocol.
– This field is also used if there are optional
extension headers added to the IPv6
packet.
Version
Traffic
Class
Flow Label
Payload Length
Next
Header
Hop Limit
Source IP Address
Destination IP Address
Hop Limit (8 bits)
– Field replaces the IPv4 TTL field.
– This value is decremented by one by each
router that forwards the packet.
– When the counter reaches 0 the packet is
discarded and an ICMPv6 message is
forwarded to the sending host, indicating
that the packet did not reach its
destination.
Version
Traffic
Class
Flow Label
Payload Length
Next
Header
Hop Limit
Source IP Address
Destination IP Address
Source Address (128 bits)
– Field identifies the IPv6
address of the sending host.
Version
Traffic
Class
Flow Label
Payload Length
Next
Header
Hop Limit
Source IP Address
Destination IP Address
Destination Address (128 bits)
– Field identifies the IPv6 address of
the receiving host.
Sample IPv6 Headers
Sample IPv6 Headers
Sample IPv6 Headers
Eliminates the Need for
NAT
 With such a large number of public IPv6 addresses, Network
Address Translation (NAT) is not needed.
 Customer sites, from the largest enterprises to single households,
can get a public IPv6 network address.
 This avoids some of the NAT-induced application problems
experienced by applications requiring end-to-end connectivity.
Integrated Security
 IPv6 natively supports authentication and privacy capabilities.
 With IPv4, additional features had to be implemented to do this.