Chapter 6
Encapsulation
- The network layer receives a protocol data unit (PDU) from the transport layer. In a process called encapsulation, the network layer adds IP header information, such as the IP address of the source (sending) and destination (receiving) hosts. After header information is added to the PDU, the PDU is called a packet.
What is an end device with a configured IP address?
A host
What are the basic characteristics of IP?
Connectionless Best Effort (unreliable) Media Independent
Version
Contains a 4-bit binary value identifying the IP packet version. For IPv4 packets, this field is always set to 0100.
What does the IPv4 packet consist of?
IP Header - Identifies the packet characteristics. Payload - Contains the Layer 4 segment information and the actual data.
Addressing end devices
In the same way that a phone has a unique telephone number, end devices must be configured with a unique IP address for identification on the network. An end device with a configured IP address is referred to as a host.
What the two most commonly implement network layer protocols?
Internet Protocol version 4 (IPv4) Internet Protocol version 6 (IPv6)
Legacy network layer protocols that are not widely used include:
Novell Internetwork Packet Exchange (IPX) AppleTalk Connectionless Network Service (CLNS/DECNet)
The network layer is on what OSI Layer?
OSI Layer 3
What does the network layer, or OSI Layer 3, provide? services to allow end devices to exchange data across the network. To accomplish this end-to-end transport, the network layer uses four basic processes:
Provides services to allow end devices to exchange data across the network.
What does Payload contain?
The Layer 4 segment information and the actual data.
Routing
The network layer provides services to direct 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 paths for and direct packets toward the destination host in a process known as routing. A packet may cross many intermediary devices before reaching the destination host. Each route the packet takes to reach the destination host is called a hop.
What is IP?
The network layer service implemented by the TCP/IP protocol suite. Was designed as a protocol with low overhead. 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.
Significant fields in the IPv4 header include:
Version Differentiated Services (DS) Time-to-Live (TTL) Protocol Source IP Address Destination IP Address
What are the four basic processes the Network layer uses?
Addressing end devices Encapsulation Routing De-encapsulation
ARPANET
Advanced Research Projects Agency Network
What does the IP Header do?
Identifies the packet characteristics.
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. This process of removing headers from lower layers is known as de-encapsulation. 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
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. The binary values of each field identify various settings of the IP packet.
