Chapter 6 - Network Layer Flashcards
What are the basic processes of the Network layer?
Addressing end devices, encapsulation, routing and de-encapsulation.
What does the addressing end devices process do?
End devices need to be configured with a unique IP address for identification on a network.
What does the encapsulation process do?
The network layer encapsulates the protocol data unit (PDU) from the transport layer into a packet. This process adds IP header information like the IP addresses of the source and destination devices.
What does the routing process do?
It directs packets to a destination host on another network.
What is the routers role in the routing process?
To travel to other networks a packet must be processed by a router. The routers role is to select the best path and direct packets toward their destination in a process called routing. A packet may cross many intermediary devices before reaching the destination host. Each router a packet crosses to get to its destination is called a hop.
What does the de-encapsulation process do?
When a packet arrives at the network layer of the destination host, the host checks the IP header. If the destination IP in the header matches it’s own then it removes the IP header from the packet. Once de-encapsulated the layer 4 PDU is passed up to the appropriate service at the transport layer.
What are the commonly implemented network layer protocols?
Internet Protocol version 4 (IPv4)
Internet Protocol version 6 (IPv6)
What does transport layer encapsulation do?
It adds a segment header so that the segments of data can be reassembled at the destination.
What does network layer encapsulation do?
The transport layer encapsulation is encapsulated further by adding a header so packets can be routed through complex networks and reach their destination. In TCP/IP based networks the network layer PDU is the IP packet.
What is the benefit of encapsulating data layer by layer?
It enables the services at each layer to develop and scale without affecting the other layers. For instance the transport layer segments can be packaged by IPv4, IPv6, or any other network protocols that may be developed in the future.
What are the characteristics of IP?
IP was designed as a protocol with low overhead, it provides only the functions that are necessary to send a packet from source to destination. The characteristics of IP are:
Connectionless, best effort, and medium independent
What does the connectionless aspect of IP mean?
No dedicated end-to-end connection is created before the data is sent. IP requires no initial exchange of control information to establish an end-to-end connection before forwarding packets. IP also does not need to add additional fields to the header in order to maintain an established connection. This helps greatly cut down on the overhead of IP. However this also means that senders are unaware whether destination devices are present and functional when sending packets. They are also not aware if the destination receives the packet, or if they were able to access and read the packet.
What does the best effort aspect of IP mean?
The IP protocol does not guarantee that all delivered packets are received. This makes the IP protocol inherently unreliable. IP does not have the ability to manage and recover undelivered or corrupt packets. This is because IP packets contain no information that can be processed to inform the sender whether delivery was successful. Other layers are responsible for ensuring data integrity, which allows IP to function very efficiently. In TCP/IP the transport layer is responsible for reliability.
What does the media independent aspect of IP mean?
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. The data link layer is responsible for taking an IP packet and preparing it for transmission, which means IP packets are not limited to any particular medium.
What aspect of the media does the network layer consider?
Although the network layer is media independent, it is responsible for the maximum size of the PDU that each medium can transport. This characteristic is referred to as the maximum transmission unit (MTU). The establishment of the maximum packet size is part of the control communication between the network layer and the data link layer. The data link layer passes the MTU value up to the network layer, and the network layer determines how large packets can be.
What is the router’s goal when a destination has a smaller MTU than a source medium?
It splits up the packet into smaller pieces. This process is called fragmenting the packet or fragmentation.
What are some significant fields in the IPv4 packet? What information do they carry?
Version - Contains a 4-bit binary number set to 0100 that identifies this as an IP version 4 packet
Differentiated Services or DiffServ (DS) - Formerly called the Type of Service (ToS) field, it’s an 8-bit field used to determine the priority of each packet. The 6 most significant bits are Differentiated Services Code Point (DSCP) and the last two bits are the Explicit Congestion Notification (ECN) bits.
Time-to-Live (TTL) - Contains an 8-bit binary value that is used to limit the lifetime of the packet. The packet sender sets the initial TTL value, and it is decreased by one each time the packet is processed by a router. If the TTL field decrements to 0, the router discards the packet and sends an Internet Control Message Protocol (ICMP) Time exceeded message to the source IP address. This field is important because it eliminates packets that may be caught in a loop, so that they don’t continue on forever.
Protocol - Used to identify the next level protocol. This 8-bit binary value indicates the data payload type that the packet is carrying enabling the network layer to pass the data to the appropriate upper-layer protocol. Common values include ICMP (1), TCP (6), and UDP (17).
Source IPv4 Address - Contains a 32-bit value that represents the source IPv4 address of the packet. The source IPv4 is always a unicast address.
Destination IPv4 Address- Contains a 32-bit value that represents the destination IPv4 address of the packet. This can be a unicast, multicast or broadcast address.
What fields are used to identify and validate the packet?
The Internet Header Length (IHL), Total Length, and Header Checksum fields.
What fields are used to reorder a fragmented packet?
The Identification, Flags, and Fragment Offset fields.
What are some limitations of IPv4?
IP Address Depletion - IPv4 has a limited amount of unique public IPv4 addresses available. There are approximately 4 billion, but the increasing number of new IP-enabled devices, always on connections, and the potential growth of less developed regions have increased the need for more addresses.
Internet Routing Table Expansion - A routing table is used by routers to make best path determinations. As the number of servers with internet access increase, so do the number of network routes. These routes consume a great deal of memory and processor resources on internet routers.
Lack of end-to-end connectivity - Network Address Translation (NAT) is a technology commonly implemented within IPv4 networks. NAT provides a way for multiple devices to share a single public IPv4 address. However, because the public IPv4 address is shared, the IPv4 address of an internal network host is hidden. This can be problematic for technologies that require end-to-end connectivity.
How does IPv6 improve upon IPv4?
Increased address space - IPv6 addresses are based on 128-bit hierarchical addressing as opposed to IPv4 with 32-bits. The number of IP addresses is about 340 undecillion addresses, roughly equivalent to the number of grains of sand on earth.
Improved Packet Handling - The IPv6 header has been simplified with fewer fields.
Eliminates the need for NAT - With such a large number of public IPv6 addresses, NAT between a private IPv6 address and a public IPv6 is not needed. This avoids some of the NAT-induced application problems experienced by applications requiring end-to-end connectivity.
How does IPv6 simplify the IPv4 header?
The IPv4 header consists of 20 octets (up to 60 bytes if the option field is used) and 12 basic header fields, not including the Options field and the Padding field. In contrast the simplified IPv6 header consists of 40 octets (largely due to the length of the source and destination IPv6 addresses) and 8 header fields (3 IPv4 basic header fields and 5 additional header fields).
What are the advantages of the new IPv6 header field?
- Simplified Header format for efficient packet handling.
- Larger payload for increased throughput and transport efficiency.
- Hierarchical network architecture for routing efficiency.
- Auto-configuration for addresses
- Elimination of need for network address translation (NAT) between private and public addresses.
What are the significant fields in the IPv6 header?
Version - This field contains a 4-bit binary value set to 0110 that identifies this as an IP version 6 packet.
Traffic Class - This 8-bit field is equivalent to the IPv4 Differentiated Services (DS) field.
Flow Label - This 20-bit field suggests that all packets with the same flow label receive the same type of handling by routers.
Payload Length - This 16-bit field indicates the length of the data portion or payload of the IPv6 packet.
Next Header - This 8-bit field is equivalent to the IPv4 Protocol field. It indicates the data payload type that the packet is carrying, enabling the network layer to pass the data to the appropriate upper-layer protocol.
Hop Limit - This 8-bit field replaces the IPv4 TTL field. This value is decremented by a value of 1 by each router that forwards the packet. When the counter reaches 0, the packet is discarded, and an ICMPv6 Time Exceeded message is forwarded to the sending host, indicating that the packet did not reach its destination because the hop limit was exceeded.
Source IPv6 Address - This 128-bit field identifies the IPv6 address of the sending host.
Destination IPv6 Address - This 128-bit field identifies the IPv6 address of the receiving host.
Extension Headers (EH) - Provide optional network layer information. Not required, but EHs are used for fragmentation, security, to support mobility and more.
Where can a host send packets to?
Itself - A host can ping itself by sending a packet to a special IPv4 address of 127.0.0.1, which is referred to as the loopback interface. Pinging the loopback interface tests the TCP/IP protocol stack on the host.
Local Host - A host on the same local network as the sending host. The hosts share the same network address.
Remote Host - A host on a remote network. The hosts do not share the same network address.
What determines whether a packet is destined for a local host or remote host?
The IPv4 address and subnet mask combination of the source device compared to the IPv4 address and subnet mask of the destination device.
What is the default gateway?
The network device that can route traffic to other networks. It is the router that can route traffic out of the local network. It can take data in and forward data out. A default gateway has a local IP address in the same address range as other hosts on the network.
How does a host receive the IPv4 address of a default gateway?
Either dynamically from Dynamic Host Configuration Protocol (DHCP) or configured manually.
