ITNv7 Lesson 8 - Network Layer

Routing

Service provided by the network layer. Used to direct the packets to a destination host on another network.

Connectionless

No connection with the destination is 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 carrying the data.

Encapsulation

The network layer adds IP header information. The PDU is now a packet

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. Then the resulting Layer 4 PDU is passed up to the appropriate service at the transport layer.

Maximum Transmission Unit (MTU)

The largest packet size supported on an interface.

Fragmentation

A router must split up an IPv4 packet when forwarding from one medium to another with a smaller MTU.

Internet Control Message Protocol (ICMP)

A TCP/IP protocol that is used by devices to communicate updates or error information to other devices.

Network Address Translation (NAT)

Translates the private IPv4 address to a public address for routing over the Internet.

Used to conserve IPv4 addresses - not needed with IPv6.

loopback interface

Virtual interface configured on the router utilized for maintenance/management functions

default gateway

Allows a switch to forward packets that originate on the switch to remote networks

Directly connected networks

These network route entries are active router interfaces. These routes are added when an interface is configured with an IP address and activated. They have no next hop, because they do not require going through another router to be reached.

Remote networks

These network route entries have a next hop IP address. The next hop IP address is the address of the router interface of the next device to be used to reach the destination network.. They can be manually configured or dynamically learned.

Default route

Gateway of last resort. Used when there is no better match in the routing table.

Static routes

Manually configured routes

Dynamic Routing Protocols

Allows routers to automatically learn about remote networks

OSPF (Open Shortest Path First)

A routing protocol that makes up for some of the limitations of RIP and can coexist with RIP on a network.

EIGRP (Enhanced Interior Gateway Routing Protocol)

Cisco proprietary routing protocol

Uses composite metric based on bandwidth, delay, load and reliability

Routing entry code L

Directly connected local interface IP address

Routing entry code C

Directly connected network

Routing entry code S

Static route manually configured by an administrator

Routing entry code O

OSPF

Routing entry code D

EIGRP

IP protocol

A connectionless protocol, considered unreliable in terms of end-to-end delivery.

It does not provide error control in the cases where receiving packets are out-of-order or in cases of missing packets. It relies on upper layer services, such as TCP, to resolve these issues.

Network layer services

Addressing

Encapsulating PDUs from the transport layer

Routing packets to the destination

De-encapsulation

Time to Live (TTL)

The maximum amount of time a packet is allowed to circulate through a network before it is destroyed

IPv6 simplified header advantages

Better routing efficiency and efficient packet handling for performance and forwarding-rate scalability·

No requirement for processing checksums·

Simplified and more efficient extension header mechanisms (as opposed to the IPv4 Options field)·

A Flow Label field for per-flow processing with no need to open the transport inner packet to identify the various traffic flows

Loopback test

Verifies that the host NIC, drivers, and TCP/IP stack are functioning.

IPv4 header fields

Version, IHL, Type of Service, Total Length, Flags, Fragment offset, TTL (Time to Live), Protocol, Header Checksum, Source IPv4 Address, Destination IPv4 Address, IP Options, Padding and the Data Payload.

IPv6 header fields

Version, traffic class, flow label, payload length, next header, hop limit, source address, destination address

How does a router choose the best path?

If there is a route for the destination network in the routing table, the router forwards the packet using that information.

If there are two or more possible routes to the same destination, the router chooses the path with the lower metric value.

Protocol field in IPv4 header

identifies the upper-layer protocol the packet is carrying.

Version field in IPv4 header

identifies the IP version

Differential Services field in IPv4 header

used for setting packet priority

Identification field in IPv4 header

used to reorder fragmented packets.

Next hop IP address.

The address of the router interface of the next device to be used to reach the destination network.

Directly-connected and local routes have no next hop, because they do not require going through another router to be reached.

Flow Label in IPv6 header

A 20-bit field that provides a special service for real-time applications. This field can be used to inform routers and switches to maintain the same path for the packet flow so that packets will not be reordered.

Local Routing Table

Used by hosts to ensure that network layer packets are directed to the correct destination network. This local table typically contains a route to the loopback interface, a route to the network that the host is connected to, and a local default route, which represents the route that packets must take to reach all remote network addresses.

What is the likely issue if a and computer can access devices on its own network, but cannot access devices on other networks?

incorrectly configured default gateway

What IPv4 header field identifies the upper layer protocol carried in the packet?

protocol