The following is a summary of each topic in the chapter and some questions for your reflection.
What Did I Learn in this Module? (22.4.1)
• Network Layer Characteristics—The network layer, or OSI Layer 3, provides services to allow end devices to exchange data across networks. IPv4 and IPv6 are the principal network layer communication protocols. Other network layer protocols include routing protocols such as OSPF and messaging protocols such as ICMP.
Network layer protocols perform four operations: addressing end devices, encapsulation, routing, and de-encapsulation. 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 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. The IP header is examined by routers and Layer 3 switches as it travels across a network to its destination. 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 NAT for IPv4.
The basic characteristics of IP are that it is: connectionless, best effort, and media independent. IP is connectionless, meaning that no dedicated end-to-end connection is created by IP before data is sent. IP does not require additional fields in the header to maintain an established connection. This reduces the overhead of IP. 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. 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. One characteristic of the media that the network layer considers is the maximum size of the PDU that each medium can transport, or the MTU.
• IPv4 Packet—The IPv4 packet header is used to ensure that a packet is delivered to its next stop on the way to its destination end device. An IPv4 packet header consists of fields containing binary numbers which are examined by the Layer 3 process. Significant fields in the IPv4 header include: version, DS, TTL, protocol, header checksum, source IPv4 address, and destination IPv4 address.
The IHL, Total Length, and Header Checksum fields are used to identify and validate the packet. 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.
• IPv6 Packet—IPv4 has limitations, including: IPv4 address depletion, lack of end-to-end connectivity, and increased network complexity. IPv6 overcomes the limitations of IPv4. Improvements that IPv6 provides include the following: increased address space, improved packet handling, and it eliminates the need for NAT.
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 IPv6 simplified header fields include: version, traffic class, flow label, payload length, next header, hop limit, source IP address, and destination IP address. An IPv6 packet may also contain 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. Unlike IPv4, routers do not fragment routed IPv6 packets.
More protocols! I am beginning to understand that there was a great deal of work done to create these protocols. At the network layer, protocols handle addressing, encapsulating the data, routing the packets and then de-encapsulating the packets so they can be read. Marcy and Vincent are not IT professionals, so a working network can seem a bit mysterious to them. But it does help to know about what happens at the network layer of the OSI model. How can this knowledge help you to troubleshoot your own network?