Chapter_05_1

The Network layer, or OSI Layer 3, provides services to exchange the individual pieces of data over the network between identified end devices. To accomplish this end-to-end transport, Layer 3 uses four basic processes:

• Addressing
• Encapsulation
• Routing
• Decapsulation

The animation in the figure demonstrates the exchange of data.

Addressing

First, the Network layer must provide a mechanism for addressing these end devices. If individual pieces of data are to be directed to an end device, that device must have a unique address. In an IPv4 network, when this address is added to a device, the device is then referred to as a host.

Encapsulation

Second, the Network layer must provide encapsulation. Not only must the devices be identified with an address, the individual pieces - the Network layer PDUs - must also contain these addresses. During the encapsulation process, Layer 3 receives the Layer 4 PDU and adds a Layer 3 header, or label, to create the Layer 3 PDU. When referring to the Network layer, we call this PDU a packet. When a packet is created, the header must contain, among other information, the address of the host to which it is being sent. This address is referred to as the destination address. The Layer 3 header also contains the address of the originating host. This address is called the source address.

After the Network layer completes its encapsulation process, the packet is sent down to the Data Link layer to be prepared for transportation over the media.

Routing

Next, the Network layer must provide services to direct these packets to their destination host. The source and destination hosts are not always connected to the same network. In fact, the packet might have to travel through many different networks. Along the way, each packet must be guided through the network to reach its final destination. Intermediary devices that connect the networks are called routers. The role of the router is to select paths for and direct packets toward their destination. This process is known as routing.

During the routing through an internetwork, the packet may traverse many intermediary devices. Each route that a packet takes to reach the next device is called a hop. As the packet is forwarded, its contents (the Transport layer PDU), remain intact until the destination host is reached.

Decapsulation

Finally, the packet arrives at the destination host and is processed at Layer 3. The host examines the destination address to verify that the packet was addressed to this device. If the address is correct, the packet is decapsulated by the Network layer and the Layer 4 PDU contained in the packet is passed up to the appropriate service at Transport layer.

Unlike the Transport layer (OSI Layer 4), which manages the data transport between the processes running on each end host, Network layer protocols specify the packet structure and processing used to carry the data from one host to another host. Operating without regard to the application data carried in each packet allows the Network layer to carry packets for multiple types of communications between multiple hosts.

Network Layer Protocols

Protocols implemented at the Network layer that carry user data include:

Internet Protocol version 4 (IPv4)

Internet Protocol version 6 (IPv6)

Novell Internetwork Packet Exchange (IPX)

AppleTalk

Connectionless Network Service (CLNS/DECNet)

The Internet Protocol (IPv4 and IPv6) is the most widely-used Layer 3 data carrying protocol and will be the focus of this course. Discussion of the other protocols will be minimal.

IPv4 basic characteristics:

Connectionless - No connection is established before sending data packets.

Best Effort (unreliable) - No overhead is used to guarantee packet delivery.

Media Independent - Operates independently of the medium carrying the data.

As shown in the figure, an IPv4 protocol defines many different fields in the packet header. These fields contain binary values that the IPv4 services reference as they forward packets across the network.

This course will consider these 6 key fields:

• IP Source Address
• IP Destination Address
• Time-to-Live (TTL)
• Type-of-Service (ToS)
• Protocol
• Fragment Offset

Key IPv4 Header Fields

IP Destination Address

The IP Destination Address field contains a 32-bit binary value that represents the packet destination Network layer host address.

IP Source Address

The IP Source Address field contains a 32-bit binary value that represents the packet source Network layer host address.

Time-to-Live

The Time-to-Live (TTL) is an 8-bit binary value that indicates the remaining "life" of the packet. The TTL value is decreased by at least one each time the packet is processed by a router (that is, each hop). When the value becomes zero, the router discards or drops the packet and it is removed from the network data flow. This mechanism prevents packets that cannot reach their destination from being forwarded indefinitely between routers in a routing loop. If routing loops were permitted to continue, the network would become congested with data packets that will never reach their destination. Decrementing the TTL value at each hop ensures that it eventually becomes zero and that the packet with the expired TTL field will be dropped.

Protocol

This 8-bit binary value indicates the data payload type that the packet is carrying. The Protocol field enables the Network layer to pass the data to the appropriate upper-layer protocol.

Example values are:

01 ICMP

06 TCP

17 UDP

Type-of-Service

The Type-of-Service field contains an 8-bit binary value that is used to determine the priority of each packet. This value enables a Quality-of-Service (QoS) mechanism to be applied to high priority packets, such as those carrying telephony voice data. The router processing the packets can be configured to decide which packet it is to forward first based on the Type-of-Service value.

Fragment Offset

As mentioned earlier, a router may have to fragment a packet when forwarding it from one medium to another medium that has a smaller MTU. When fragmentation occurs, the IPv4 packet uses the Fragment Offset field and the MF flag in the IP header to reconstruct the packet when it arrives at the destination host. The fragment offset field identifies the order in which to place the packet fragment in the reconstruction.

More Fragments flag

The More Fragments (MF) flag is a single bit in the Flag field used with the Fragment Offset for the fragmentation and reconstruction of packets. The More Fragments flag bit is set, it means that it is not the last fragment of a packet. When a receiving host sees a packet arrive with the MF = 1, it examines the Fragment Offset to see where this fragment is to be placed in the reconstructed packet. When a receiving host receives a frame with the MF = 0 and a non-zero value in the Fragment offset, it places that fragment as the last part of the reconstructed packet. An unfragmented packet has all zero fragmentation information (MF = 0, fragment offset =0).

Don't Fragment flag

The Don't Fragment (DF) flag is a single bit in the Flag field that indicates that fragmentation of the packet is not allowed. If the Don't Fragment flag bit is set, then fragmentation of this packet is NOT permitted. If a router needs to fragment a packet to allow it to be passed downward to the Data Link layer but the DF bit is set to 1, then the router will discard this packet.

Other IPv4 Header Fields

Version - Contains the IP version number (4)

Header Length (IHL) - Specifies the size of the packet header.

Packet Length - This field gives the entire packet size, including header and data, in bytes.

Identification - This field is primarily used for uniquely identifying fragments of an original IP packet

- The checksum field is used for error checking the packet header.

The routing table stores information about connected and remote networks. Connected networks are directly attached to one of the router interfaces. These interfaces are the gateways for the hosts on different local networks. Remote networks are networks that are not directly connected to the router. Routes to these networks can be manually configured on the router by the network administrator or learned automatically using dynamic routing protocols.

Routes in a routing table have three main features:

• Destination network
• Next-hop
• Metric

Routes to remote networks with the associated next hops can be manually configured on the router. This is known as static routing. A default route can also be statically configured.