chapter 6: objectives explain how network layer protocols and services support communications across...

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

3

DataHTTP Header

TCP Header

IP Header

Data Link Header

Data Link Trailer

IP PacketData Link Header

Data Link Trailer


Data Link Trailer


Data Link Trailer


Data Link Trailer


Data Link Trailer


Data Link Trailer

DataHTTP Header

TCP Header

IP Header

Data Link Header

Data Link Trailer

Encapsulation and Decapsulation

Encapsulation

S.P / D.P. / S.N. / Ack # / … DATA

DATA

IPv / HLEN / Flag / S. IP / D. IP / … DATA (SEGMENT)

Frame Header DATA (PACKET) Trailer

111010110101011100001001011010101010010101010101101101010001010101010110101010

SEGMENT

PACKET

FRAME

Functions of the Network Layer

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

IP

IP

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 Network layer does consider the maximum size of PDU that each medium can transport. This is referred to as the Maximum Transmission Unit (MTU). 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.

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.

IPv4 Packet

IPv4 Packet

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.

IP Header Data (Payload)

IPv4 Header – Significant Fields

VersionIP Header

Length

Differentiated Services Total Length

DSCP ECN

Identification Flag Fragment Offset

Time-To-Live Protocol Header Checksum

Source IP Address

Destination IP Address

Options (optional)Padding

Byte 1 Byte 2 Byte 3 Byte 4

IPv4 Header – Validation Fields

VersionIP Header

Length

Differentiated Services Total Length

DSCP ECN



Source IP Address


Options (optional) Padding


Sample IPv4 Packet

VersionIP Header

Length

Differentiated Services

Total LengthDSCP ECN



Source IP Address



VersionIP Header

Length





Source IP Address



Version (4 bits)

– Indicates the version of IP currently used.

– 0100 = 4 and therefore IPv4


Version (4 bits)




VersionIP Header

Length





Source IP Address



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).

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).

VersionIP Header

Length





Source IP Address



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.

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.

VersionIP Header

Length





Source IP Address



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. .

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. .

VersionIP Header

Length





Source IP Address


Options (optional) PaddingA router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU.

When this happens, fragmentation occurs and the IPv4 packet uses the following 3 fields to keep track of the fragments

A router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU.

When this happens, fragmentation occurs and the IPv4 packet uses the following 3 fields to keep track of the fragments

VersionIP Header

Length





Source IP Address



Identification (16 bits)

– Field uniquely identifies the fragment of an original IP packet.

Identification (16 bits)

– Field uniquely identifies the fragment of an original IP packet.

VersionIP Header

Length





Source IP Address



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.

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.

VersionIP Header

Length





Source IP Address



Fragment Offset (13 bits)

– Field identifies the order in which to place the packet fragment in the reconstruction of the original unfragmented packet.

Fragment Offset (13 bits)

– Field identifies the order in which to place the packet fragment in the reconstruction of the original unfragmented packet.

VersionIP Header

Length





Source IP Address



Time-to-Live (TTL) (8 bits)

– 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.

Time-to-Live (TTL) (8 bits)

– 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.

VersionIP Header

Length





Source IP Address



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/

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/

http://www.iana.org/assignments/protocol-numbers/

http://www.iana.org/assignments/protocol-numbers/

VersionIP Header

Length





Source IP Address



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.

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.

VersionIP Header

Length





Source IP Address



Source IP Address (32 bits)

– Contains a 32-bit binary value that represents the source IP address of the packet.

Source IP Address (32 bits)

– Contains a 32-bit binary value that represents the source IP address of the packet.

VersionIP Header

Length





Source IP Address



Destination IP Address (32 bits)

– Contains a 32-bit binary value that represents the destination IP address of the packet.

Destination IP Address (32 bits)

– Contains a 32-bit binary value that represents the destination IP address of the packet.

Sample IPv4 Headers

IPv6 PacketIPv4

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

IPv4 Address Depletion In October 2010, less than 5% of the public IPv4 addresses

remained unallocated.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79

80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95

96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111

112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127

128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143

144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159

160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175

176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191

192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207

208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223

224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239

240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255

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

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.

192.168.1.0/24RFC 1918 Private Address

Public IPv4 Address

NATNAT

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

50 billion billion billion addresses for every person on earth

Number nameScientific Notation

Number of zeros

1 Thousand 103 1,000

1 Million 106 1,000,000

1 Billion 109 1,000,000,000

1 Trillion 1012 1,000,000,000,000

1 Quadrillion 1015 1,000,000,000,000,000

1 Quintillion 1018 1,000,000,000,000,000,000

1 Sextillion 1021 1,000,000,000,000,000,000,000

1 Septillion 1024 1,000,000,000,000,000,000,000,000

1 Octillion 1027 1,000,000,000,000,000,000,000,000,000

1 Nonillion 1030 1,000,000,000,000,000,000,000,000,000,000

1 Decillion 1033 1,000,000,000,000,000,000,000,000,000,000,000

1 Undecillion 1036 1,000,000,000,000,000,000,000,000,000,000,000,000

There are 4 billion IPv4 addresses

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

Version Traffic Class Flow Label

Payload LengthNext

HeaderHop Limit

Source IP Address



Sample IPv4 Packet

VersionTraffic Class

Flow Label

Payload LengthNext

HeaderHop Limit

Source IP Address



Flow Label

Payload LengthNext

HeaderHop Limit

Source IP Address


Version (4 bits)




Version (4 bits)





Flow Label

Payload LengthNext

HeaderHop Limit

Source 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.

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.


Flow Label

Payload LengthNext

HeaderHop Limit

Source 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.

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.


Flow Label

Payload LengthNext

HeaderHop Limit

Source 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

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


Flow Label

Payload LengthNext

HeaderHop Limit

Source 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.

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.


Flow Label

Payload LengthNext

HeaderHop Limit

Source 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.

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.


Flow Label

Payload LengthNext

HeaderHop Limit

Source IP Address


Source Address (128 bits)

– Field identifies the IPv6 address of the sending host.

Source Address (128 bits)

– Field identifies the IPv6 address of the sending host.


Flow Label

Payload LengthNext

HeaderHop Limit

Source IP Address


Destination Address (128 bits)

– Field identifies the IPv6 address of the receiving host.

Destination Address (128 bits)

– Field identifies the IPv6 address of the receiving host.

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.

chapter 6: objectives explain how network layer protocols and services support communications across...

Documents

ip packet network link

data packets

network layer slide

data networks

smaller mtu network

larger mtu network link

smaller ip header

outgoing link