Chapter 8- Network Layer
IPv6 address space provides
340,282,366,920,938,463,463,374,607,431,768,211,456, or 340 undecillion (10 raised to the power of 36) addresses. This is roughly equivalent to every grain of sand on Earth.
The 32-bit IPv4 address space provides approximately
4,294,967,296 unique addresses. There are 4 billion IPv4 addresses
In contrast, the simplified IPv6 header shown the next figure consists of a fixed length header of
40 octets (largely due to the length of the source and destination IPv6 addresses). The IPv6 simplified header allows for more efficient processing of IPv6 headers.
To accomplish end-to-end communications across network boundaries, network layer protocols perform four basic operations:
Addressing end devices Encapsulation Routing De-encapsulation
Which delivery method does not guarantee that the packet will be delivered fully without errors?
Best effort
These are the basic characteristics of IP:
Connectionless - There is no connection with the destination established before sending data packets. Best Effort - IP is inherently unreliable because packet delivery is not guaranteed. Media Independent - Operation is independent of the medium (i.e., copper, fiber-optic, or wireless) carrying the data.
Version
Contains a 4-bit binary value set to 0100 that identifies this as an IPv4 packet.
Which layer is responsible for taking an IP packet and preparing it for transmission over the communications medium?
Data Link Layer
The routing table of the router contains network route entries listing all the possible known network destinations. The routing table stores three types of route entries:
Directly-connected networks Remote networks Default route
Basic configuration only requires the network administrator to enable the directly connected networks within the dynamic routing protocol. The dynamic routing protocol will automatically do as follows:
Discover remote networks Maintain up-to-date routing information Choose the best path to destination networks Attempt to find a new best path if the current path is no longer available
Addressing end devices
End devices must be configured with a unique IP address for identification on the network.
Differentiated Services or DiffServ (DS)
Formerly called the type of service (ToS) field, the DS field is an 8-bit field used to determine the priority of each packet. The six most significant bits of the DiffServ field are the differentiated services code point (DSCP) bits and the last two bits are the explicit congestion notification (ECN) bits.
What is the term for splitting up an IP packet when forwarding it from one medium to another medium with a smaller MTU?
Fragmentation
Which field is used to detect corruption in the IPv4 header?
Header Checksum
Which three options are major issues associated with IPv4?
IP address depletion Increased network complexity and internet routing table expansion lack of end-to-end connectivity
In the early 1990s, the Internet Engineering Task Force (IETF) grew concerned about the issues with IPv4 and began to look for a replacement. This led to the development of
IP version 6 (IPv6). IPv6 overcomes the limitations of IPv4 and is a powerful enhancement with features that better suit current and foreseeable network demands.
IPv4 still has three major issues:
IPv4 address depletion Lack of end-to-end connectivity Increased network complexity
IPv4 address depletion
IPv4 has a limited number of unique public 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.
The show ip route privileged EXEC mode command is used to view the
IPv4 routing table on a Cisco IOS router. At the beginning of each routing table entry is a code that is used to identify the type of route or how the route was learned.
Increased address space
IPv6 addresses are based on 128-bit hierarchical addressing as opposed to IPv4 with 32 bits.
Whether a packet is destined for a local host or a remote host is determined by the source end device. The source end device determines whether the destination IP address is on the same network that the source device itself is on. The method of determination varies by IP version:
In IPv4 - The source device uses its own subnet mask along with its own IPv4 address and the destination IPv4 address to make this determination. In IPv6 - The local router advertises the local network address (prefix) to all devices on the network.
Improvements that IPv6 provides include the following:
Increased address space Improved packet handling Eliminates the need for NAT
Which two options are improvements provided by IPv6 as compared to IPv4? (Choose two.)
Increased the IP address space uses a simpler header to provide improved packet handling
On a network, a default gateway is usually a router with these features:
It has a local IP address in the same address range as other hosts on the local network. It can accept data into the local network and forward data out of the local network. It routes traffic to other networks.
Another role of the network layer is to direct packets between hosts. A host can send a packet to the following:
Itself - A host can ping itself by sending a packet to a special IPv4 address of 127.0.0.1 or an IPv6 address ::1, which is referred to as the loopback interface. Pinging the loopback interface tests the TCP/IP protocol stack on the host. Local host - This is a destination host that is on the same local network as the sending host. The source and destination hosts share the same network address. Remote host - This is a destination host on a remote network. The source and destination hosts do not share the same network address.
Common route sources (codes) include these:
L - Directly connected local interface IP address C - Directly connected network S - Static route was manually configured by an administrator O - OSPF D - EIGRP
Default route
Like a host, most routers also include a default route entry, a gateway of last resort. The default route is used when there is no better (longer) match in the IP routing table.
A router can learn about remote networks in one of two ways:
Manually - Remote networks are manually entered into the route table using static routes. Dynamically - Remote routes are automatically learned using a dynamic routing protocol.
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 IPv4 address. However, because the public IPv4 address is shared, the IPv4 address of an internal network host is hidden. This can be problematic for technologies that require end-to-end connectivity.
Dynamic routing protocols include
OSPF and Enhanced Interior Gateway Routing Protocol (EIGRP).
Which field includes common values such as ICMP (1), TCP (6), and UDP (17)?
Protocol
IP encapsulates the transport layer segment or other data by adding an IP header. The IP header is used to deliver the packet to the destination host.
an IP header. The IP header is used to deliver the packet to the destination host.
Local hosts can reach each other and share information without the need for
any additional devices. If a host is sending a packet to a device that is configured with the same IP network as the host device, the packet is simply forwarded out of the host interface, through the intermediary device, and to the destination device directly.
The OSI data link layer is responsible for taking an IP packet and preparing it for transmission over the communications medium. This means that the delivery of IP packets is not limited to
any particular medium.
An IPv4 packet header consists of fields containing important information about the packet. These fields contain
binary numbers which are examined by the Layer 3 process.
It is common for some routers to use a combination of
both static routes and a dynamic routing protocol.
the IPv6 header?
consists of 40 octets it contains 8 header fields
A default gateway is required to send traffic outside of the local network. Traffic cannot be forwarded outside the local network if there is no
default gateway, the default gateway address is not configured, or the default gateway is down.
A host routing table will typically include a default gateway. In IPv4, the host receives the IPv4 address of the default gateway either
dynamically from Dynamic Host Configuration Protocol (DHCP) or configured manually.
IPv4 is one of the primary network layer communication protocols. The IPv4 packet header is used to
ensure that this packet is delivered to its next stop on the way to its destination end device.
Routers implement routing protocols to route packets between networks. The routing performed by these intermediary devices
examines the network layer addressing in the packet header. In all cases, the data portion of the packet, that is, the encapsulated transport layer PDU or other data, remains unchanged during the network layer processes.
An IPv6 packet may also contain
extension headers (EH), which provide optional network layer information. Extension headers are optional and are placed between the IPv6 header and the payload. EHs are used for fragmentation, security, to support mobility and more.
Network layer protocols
forward transport layer PDUs between hosts
Unlike IPv4, routers do not
fragment routed IPv6 packets.
IPv6 packets cannot be
fragmented by the router.
IP operates
independently of the media that carry the data at lower layers of the protocol stack. IP packets can be communicated as electronic signals over copper cable, as optical signals over fiber, or wirelessly as radio signals.
When a host sends a packet to another host
it consults its routing table to determine where to send the packet. If the destination host is on a remote network, the packet is forwarded to the default gateway, which is usually the local router.
Static routes are route entries that are
manually configured. The static route includes the remote network address and the IP address of the next hop router.
If there is a change in the network topology, the static route is not automatically updated and must be
manually reconfigured
Unlike the transport layer (OSI Layer 4), which manages the data transport between the processes running on each host
network layer communication protocols (i.e., IPv4 and IPv6) specify the packet structure and processing used to carry the data from one host to another host. Operating without regard to the data carried in each packet allows the network layer to carry packets for multiple types of communications between multiple hosts.
IP is connectionless, meaning that
no dedicated end-to-end connection is created by IP before data is sent. Connectionless communication is conceptually similar to sending a letter to someone without notifying the recipient in advance. IP requires no initial exchange of control information to establish an end-to-end connection before packets are forwarded.
Devices that are beyond the local network segment are known as
remote hosts. When a source device sends a packet to a remote destination device, then the help of routers and routing is needed. Routing is the process of identifying the best path to a destination. The router connected to the local network segment is referred to as the default gateway.
A dynamic routing protocol allows the routers to automatically learn about
remote networks, including a default route, from other routers. Routers that use dynamic routing protocols automatically share routing information with other routers and compensate for any topology changes without involving the network administrator. If there is a change in the network topology, routers share this information using the dynamic routing protocol and automatically update their routing tables.
The default gateway is the network device (i.e., router or Layer 3 switch) that can
route traffic to other networks. If you use the analogy that a network is like a room, then the default gateway is like a doorway. If you want to get to another room or network, you need to find the doorway.
The network layer, or OSI Layer 3, provides
services to allow end devices to exchange data across networks. IP version 4 (IPv4) and IP version 6 (IPv6) are the principle network layer communication protocols. Other network layer protocols include routing protocols such as Open Shortest Path First (OSPF) and messaging protocols such as Internet Control Message Protocol (ICMP)
The IP header is examined by Layer 3 devices (i.e., routers and Layer 3 switches) as it travels across a network to its destination. It is important to note, that
the IP addressing information remains the same from the time the packet leaves the source host until it arrives at the destination host, except when translated by the device performing Network Address Translation (NAT) for IPv4.
Other fields are used to reorder a fragmented packet. Specifically
the IPv4 packet uses Identification, Flags, and Fragment Offset fields to keep track of the fragments. A router may have to fragment an IPv4 packet when forwarding it from one medium to another with a smaller MTU. The Options and Padding fields are rarely used.
There is, however, one major characteristic of the media that the network layer considers:
the maximum size of the PDU that each medium can transport which is referred to as the maximum transmission unit (MTU). Part of the control communication between the data link layer and the network layer is the establishment of a maximum size for the packet. The data link layer passes the MTU value up to the network layer. The network layer then determines how large packets can be.
IP also does not require additional fields in the header to maintain an established connection. This process greatly reduces
the overhead of IP. However, with no pre-established end-to-end connection, senders are unaware whether destination devices are present and functional when sending packets, or if the destination receives the packet, or if the destination device is able to access and read the packet. The IP protocol does not guarantee that all packets that are delivered or even received. Other protocols manage delivery.
When a router is manually configured with a static route or learns about a remote network dynamically using a dynamic routing protocol
the remote network address and next hop address are entered into the IP routing table. if there is a change in the network topology, the routers will automatically adjust and attempt to find a new best path.
Having a default gateway configured creates a default route in the routing table of the PC. A default route is
the route or pathway your computer will take when it tries to contact a remote network.
On a Windows host,
the route print or netstat -r command can be used to display the host routing table. Both commands generate the same output. The output only displays the IPv4 route table.
In IPv6
the router advertises the default gateway address or the host can be configured manually.
One of the major design improvements of IPv6 over IPv4 is
the simplified IPv6 header. For example, the IPv4 header consists of a variable length header of 20 octets (up to 60 bytes if the Options field is used) and 12 basic header fields, not including the Options field and Padding field. For IPv6, some fields have remained the same, some fields have changed names and positions, and some IPv4 fields are no longer required (the flow label)
The two most commonly referenced fields are
the source and destination IP addresses. These fields identify where the packet is coming from and where it is going. Typically, these addresses do not change while travelling from the source to the destination.
With both IPv4 and IPv6, packets are always created at
the source host. The source host must be able to direct the packet to the destination host. To do this, host end devices create their own routing table.
The process of encapsulating data layer by layer enables the services at the different layers to develop and scale without affecting the other layers. This means
the transport layer segments can be readily packaged by IPv4 or IPv6 or by any new protocol that might be developed in the future.
If out-of-order packets are delivered, or packets are missing
then applications using the data, or upper layer services, must resolve these issues. This allows IP to function very efficiently. In the TCP/IP protocol suite, reliability is the role of the TCP protocol at the transport layer.
The Internet Header Length (IHL), Total Length, and Header Checksum fields are used
to identify and validate the packet.
Unreliable means that IP does not have the capability
to manage and recover from undelivered or corrupt packets. This is because while IP packets are sent with information about the location of delivery, they do not contain information that can be processed to inform the sender whether delivery was successful. Packets may arrive at the destination corrupted, out of sequence, or not at all. IP provides no capability for packet retransmissions if errors occur.
Static routing has the following characteristics:
A static route must be configured manually. The administrator needs to reconfigure a static route if there is a change in the topology and the static route is no longer viable. A static route is appropriate for a small network and when there are few or no redundant links. A static route is commonly used with a dynamic routing protocol for configuring a default route.
Time to Live (TTL)
TTL contains an 8-bit binary value that is used to limit the lifetime of a packet. The source device of the IPv4 packet sets the initial TTL value. It is decreased by one each time the packet is processed by a router. If the TTL field decrements to zero, the router discards the packet and sends an Internet Control Message Protocol (ICMP) Time Exceeded message to the source IP address. Because the router decrements the TTL of each packet, the router must also recalculate the Header Checksum.
Improved packet handling
The IPv6 header has been simplified with fewer fields.
What happens when a packet arrives on a router interface?
The router examines the destination IP address of the packet and searches its routing table to determine where to forward the packet. The routing table contains a list of all known network addresses (prefixes) and where to forward the packet. These entries are known as route entries or routes. The router will forward the packet using the best (longest) matching route entry.
A default route has a network address of
all zeroes. For example, the IPv4 network address is 0.0.0.0. A static route entry in the routing table begins with a code of S*
In the IPv6 packet header
The hop limit field replaces the IPv4 Time to Live Field.
Encapsulation
The network layer encapsulates the protocol data unit (PDU) from the transport layer into a packet. The encapsulation process adds IP header information, such as the IP address of the source (sending) and destination (receiving) hosts. The encapsulation process is performed by the source of the IP packet.
Routing
The network layer provides services to direct the packets to a destination host on another network. To travel to other networks, the packet must be processed by a router. The role of the router is to select the best path and direct packets toward the destination host in a process known as routing. A packet may cross many routers before reaching the destination host. Each router a packet crosses to reach the destination host is called a hop.
Directly-connected networks
These network route entries are active router interfaces. Routers add a directly connected route when an interface is configured with an IP address and is activated. Each router interface is connected to a different network segment.
Remote networks
These network route entries are connected to other routers. Routers learn about remote networks either by being explicitly configured by an administrator or by exchanging route information using a dynamic routing protocol.
Destination IPv6 Address
This 128-bit field identifies the IPv6 address of the receiving host.
Source IPv6 Address
This 128-bit field identifies the IPv6 address of the sending host.
Payload Length
This 16-bit field indicates the length of the data portion or payload of the IPv6 packet. This does not include the length of the IPv6 header, which is a fixed 40-byte header.
Flow Label
This 20-bit field suggests that all packets with the same flow label receive the same type of handling by routers.
Traffic Class
This 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field.
Next Header
This 8-bit 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.
Hop Limit
This 8-bit field replaces the IPv4 TTL field. This value is decremented by a value of 1 by each router that forwards the packet. When the counter reaches 0, the packet is discarded, and an ICMPv6 Time Exceeded message is forwarded to the sending host,. This indicates that the packet did not reach its destination because the hop limit was exceeded. Unlike IPv4, IPv6 does not include an IPv6 Header Checksum, because this function is performed at both the lower and upper layers. This means the checksum does not need to be recalculated by each router when it decrements the Hop Limit field, which also improves network performance.
Destination IPv4 Address
This contains a 32-bit binary value that represents the destination IPv4 address of the packet. The destination IPv4 address is a unicast, multicast, or broadcast address.
Source IPv4 Address
This contains a 32-bit binary value that represents the source IPv4 address of the packet. The source IPv4 address is always a unicast address.
Version
This field contains a 4-bit binary value set to 0110 that identifies this as an IP version 6 packet.
Protocol
This field is used to identify the next level protocol. This 8-bit binary value 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).
Header Checksum
This is used to detect corruption in the IPv4 header.
Which OSI layer sends segments to be encapsulated in an IPv4 or IPv6 packet?
Transport Layer
Significant fields in the IPv4 header include the following:
Version Differentiated Services or DiffServ (DS) Time to Live (TTL) Protocol Header Checksum Source IPv4 Address Destination IPv4 Address
The fields in the IPv6 packet header include the following:
Version Traffic Class Flow Label Next Header Hop Limit Source IPv6 Address Destination IPv6 Address
De-encapsulation
When the packet arrives at the network layer of the destination host, the host checks the IP header of the packet. If the destination IP address within the header matches its own IP address, the IP header is removed from the packet. After the packet is de-encapsulated by the network layer, the resulting Layer 4 PDU is passed up to the appropriate service at the transport layer. The de-encapsulation process is performed by the destination host of the IP packet.
Increased network complexity
While NAT has extended the lifespan of IPv4 it was only meant as a transition mechanism to IPv6. NAT in its various implementation creates additional complexity in the network, creating latency and making troubleshooting more difficult.
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.
IP was designed as a protocol with low overhead. It provides only the functions that are necessary to deliver a packet from
a source to a destination over an interconnected system of networks. The protocol was not designed to track and manage the flow of packets. These functions, if required, are performed by other protocols at other layers, primarily TCP at Layer 4.
A directly connected route is automatically created when a router interface is configured with IP address information and is
activated. The router adds two route entries with the codes C (i.e., the connected network) and L (i.e., the local interface IP address of the connected network). The route entries also identify the exit interface to use to reach the network.
In some cases, an intermediate device, usually a router, must split up an IPv4 packet
when forwarding it from one medium to another medium with a smaller MTU. This process is called fragmenting the packet, or fragmentation. Fragmentation causes latency.
The binary values of each field identify various settings of the IP packet. Protocol header diagrams
which are read left to right, and top down, provide a visual to refer to when discussing protocol fields.
