The Network Layer - MODULO 8 Flashcards
What network layer provides
The network layer, or OSI Layer 3, provides services to allow end devices to exchange data
across networks.
Who are principle network layer protocols
, IP version 4 (IPv4) and IP version 6 (IPv6)
What is others 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).
To accomplish end-to-end communications across network boundaries, network layer
protocols perform four basic operations:
Addressing end devices
Encapsulation
Routing
De-encapsulation
Addressing end devices
End devices must be configured with a unique IP
address for identification on the network.
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.
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.
NOTES
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.
Describes IP encapsulation
IP encapsulates the transport layer (the layer just above the network layer) segment or
other data by adding an IP header. The IP header is used to deliver the packet to the
destination host.
The figure illustrates how the transport layer PDU is encapsulated by the network layer
PDU to create an IP packet.
The process of encapsulating affects the other layers
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.
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.
Note: NAT is discussed in later modules.
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.
Characteristics of IP
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.
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.
Connectionless
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. The figure summarizes this key point.
Connectionless data communications work on the same principle.
As shown in the figure, IP requires no initial exchange of control information to establish an
end-to-end connection before packets are forwarded.
Best Effort
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, nor are they aware 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 are, in fact,
received. The figure illustrates the unreliable or best-effort delivery characteristic of the
IP protocol.
Media Independent
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.
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.
IP operates independently of the media that carry the data at lower layers of the protocol
stack.
As shown in the figure, IP packets can be communicated as electronic signals over copper
cable, as optical signals over fiber, or wirelessly as radio signals.
How does the OSI data link layer contribute to the process of determining the maximum size of IP packets for transmission, and what impact does this have on network performance, particularly regarding fragmentation and latency?
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.
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.
This characteristic 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.
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.
IPv6 packets cannot be fragmented by the router.
IPv4 Packet Header
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.
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.
IPv4 Packet Header Fields
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.
The IP protocol header diagram in the figure identifies the fields of an IPv4 packet.
Significant fields in the IPv4 header include the following:
Version – Contains a 4-bit binary value set to 0100 that identifies this as na IPv4
packet.
* Differentiated Service sor DiffServ (DS) – Formerly called the type of service (ToS)
field, the DS field is na 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.
* Time to Live (TTL) – TTL contains na 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 na 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.
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.
* 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.
* Destination IPv4 Address – This contains a 32-bit binary value that represents the
destination IPv4 address of the Packet. The destination IPv4 is a unicast, multicast
or Broadcast address.
What are the primary functions of the source and destination IP address fields in an IPv4 packet, and how do the Identification, Flags, and Fragment Offset fields facilitate the handling of packet fragmentation by routers?
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.
The Internet Header Length (IHL), Total Length, and Header Checksum fields are used to
identify and validate the packet.
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 and are beyond the scope of this module.
Limitations of IPv4
IPv4 is still in use today. This topic is about IPv6, which will eventually replace IPv4.
To better understand why you need to know the IPv6 protocol, it helps to know the
limitations of IPv4 and the advantages of IPv6.
Through the years, additional protocols and processes have been developed to address
new challenges. However, even with changes, IPv4 still has three major issues:
* 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.
* 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.
* 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.
IPv6 Overview
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 activity 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.
Improvements that IPv6 provides include the following:
Increased address space - IPv6 addresses are based on 128-bit hierarchical
addressing as opposed to IPv4 with 32 bits.
* Improved packet handling - The IPv6 header has been simplified with fewer
fields.
* Eliminates the need for NAT - With such a large number of public IPv6
addresses, NAT between a private IPv4 address and a public IPv4 is not needed.
This avoids some of the NAT-induced problems experienced by applications that
require end-to-end connectivity.
The 32-bit IPv4 address space provides approximately 4,294,967,296 unique addresses.
IPv6 address space provides 340,282,366,920,938,463,463,374,607,431,768,211,456, or
340 undecillion addresses.
This is roughly equivalent to every grain of sand on Earth.
The figure provides a visual to compare the IPv4 and IPv6 address space.
IPv4 Packet Header Fields in the IPv6
Packet Header
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, as highlighted in the figure.
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.
