Module 8 Network Layer

Hop

Each router a packet crosses to reach the destination host.

Addressing end devices

End devices must be configured with a unique IP address for identification on the network.

IPv4 header: 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.

Media Independent

Operation is independent of the medium (i.e., copper, fiber-optic, or wireless) carrying the data.

Transport layer

The OSI layer that sends segments to be encapsulated in an IPv4 or IPv6 packet.

Best Effort

The delivery method does not guarantee that the packet will be delivered fully without errors.

Data Link Layer

The layer that is responsible for taking an IP packet and preparing it for transmission over the communications medium.

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.

IPv4 and IPv6

The only two network layer protocols that are commonly implemented.

IPV4 Packet Header

Used to ensure that this packet is delivered to its next stop on the way to its destination end device.

Connectionless

There is no connection with the destination established before sending data packets.

Version

Contains a 4-bit binary value set to 0100 that identifies this as an IPv4 packet.

Best Effort

IP is inherently unreliable because packet delivery is not guaranteed.

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.

IPv4 address depletion, lack of end-to-end connectivity, increased network complexity.

The 3 major issues with IPv4

Fragmentation

The term for splitting up an IP packet when forwarding it from one medium to another medium with a smaller MTU.

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.

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.

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.

Advantage of encapsulating data layer by layer

enables the services at the different layers to develop and scale without affecting the other layers.