Network Layer: IP Protocols and Routing

Module 8: Network Layer

• Module 8 is on network layer
• Introduces IP protocols for reliable network communications, IPv4 and IPv6 packet header fields.
• Explains how network devices use routing tables to direct packets

Network Layer Characteristics

• Network Layer provides services for end devices to exchange data
• IPv4 and IPv6 are key communication protocols in the network layer
• Contains four operations: addressing end-points, encapsulation, routing and de-encapsulation
• Encapsulates the transport layer segment
• Utilizes an IPv4 or IPv6 packet that does not affect the layer 4 segment
• An IP packet is examined by layer 3 devices as it crosses the network
• IP addressing does not change from origin to destination
• NAT will change the address, the topic will be discussed in another module

IP Characteristics

• Intended to have low overhead
• Can be described as connectionless, minimal service, and multimedia independent

Connectionless (Connectionless)

• IP does not establish any connection with the destination before sending the packet
• Control information like synchronizing and confirmations are not needed
• The destination receives the packet when it arrives but no previous notifications are sent via IP
• If connection oriented traffic is needed there is another protocol to handle it, usually TCP

Best Effort

• IP provides the best effort
• Does not guarantee packet delivery
• Overhead gets reduced because there is no mechanism to re send information that is not received
• IP does not need to know operational status of the other device, and nor if it received the packet

Media Independence

• IP is not reliable

• Cannot manage packets if not delivered or corrupted

• Cannot retransmit after errors

• Cannot align packets sequentially

• Depends on other protocols for these functions

• IP is media independent

• Not related to the type of required frame in the data link layer

• Can be sent on any media type

• Network layer establishes the MTU ( Maximum Transmission Unit ) sizes

• Network Layer receives MTU's from data link layers for configured interfaces

• Fragmentation occurs when Layer 3 divides the IPv4 packet into smaller units

• Fragmentation increases latency

• IPv6 does not fragment packets

• When a router handles a data transfer from ethernet to a WAN, there is an MTU size decrease

IPv4 Packet Header

• A primary communication protocol for the network layer
• Packet Headers help determine the route for data packet and ensure it is sent to the destination
• Handles the information processing for network layers

IPv4 Packet Header Fields

• Exists in binary which contain information

• Diagram is read left to right, 4 bytes at a time

• Version field: 4 bits set to 0100 to indicate IPv4

• Differentiated Services field: Used for QoS, an 8 bit field to determine traffic priority

• Total Length field: Specifies total packet length in bytes, including header and data

• Identification field: A 16-bit value used to track fragments of a packet if fragmentation occurs

• Flags field: Contains control flags for fragmentation, such as "Don't Fragment" and "More Fragments"

• Fragment Offset field: Indicates the position of the fragment within the original packet

• Time-to-Live (TTL) field: An 8-bit field that limits the lifespan of a packet in the network to prevent routing loops

• Protocol field: Specifies the next-level protocol (ICMP, TCP, UDP) used in the data portion

• Header Checksum field: 16-bit field used to detect corruption in the IPv4 header

• Source IP Address field: 32-bit IP address of the sending host

• Destination IP Address field: 32-bit IP address of the receiving host

IPv4 Packet - Wireshark Examples

• IPv4 Ethernet Packets will be shown in Wireshark software
• Control and general data information will be outlined
• Packet differences will be made apparent

IPv6 Packet Limitations

• IPv4 has 3 limitations:
• IPv4 addresses are becoming depleted
• Lack of end-to-end connectivity, which relies on NAT and private addresses to allow it to survive
• Increase in network complexity. NAT acts as a band aid and creates network issues

Introduction to IPv6

• IPv6 was developed by the IETF (Internet Engineering Task Force)
• It overcomes limitations of IPv4
• Delivers improvements like bigger address directions from 32b to 128b
• Improved packet management, which simplifies headers
• Eliminates need for NAT. Given how many addresses there are, there is no need for internal private addresses to be assigned
• There are 4 billion IPv4 Addresses
• There are 340 Undecillion IPv6 Addresses

IPv6 Packet Header

• Headers simplify and are not necessarily small

• Header is set at 40 Bytes in length

• Several IPv4 fields were removed to improve performance

• Version field: 4-bit field set to 0110 indicating IPv6

• Traffic Class field: Used for QoS (Quality of Service) similar to IPv4

• Flow Label field: 20-bit field that informs network devices how to handle specific traffic flows consistently

• Payload Length: 16-bit field indicating length of the data portion or payload of the IPv6 packet

• Next Header: Identifies the protocol in the data portion (ICMP, TCP, UDP, etc.)

• Hop Limit: Replaces the TTL field in IPv4 to prevent routing loops

• Source Address: 128-bit source IPv6 address

• Destination Address: 128-bit destination IPv6 address

IPv6 Packet Header Continued

• IPv6 Packets also may contain extension headers (EH)
• These deliver optional network layer information and is positioned between the IPv6 header
• Supports Fragmentation to help support mobility

IPv6 Packets Wireshark Example

• Covers ethernet IPv6 packets shown in Wireshark
• Highlights differences among packets and control information

Host Routing

• Packets always start at source
• Each host device has it's own routing table
• Hosts can send packets to itself(127.0.0.1 and ::1) and local networks behind the same LAN

Host Forwarding Decision Cont.

• Original device needs to determine its local or remote destinations
• IPv4 uses its own IP Address with subnet masks and a destination address
• IPv6 uses network addresses and prefixes that are pushed by the local router
• Local traffic gets disconnected at the host interface and can then be handled by an intermediary device
• Remote traffic gets forwarded directly to the default gateway

Default Gateway

• A Layer 3 switch can be a viable default gateway
• The IP address must match the range of the LAN
• It is able to accept LAN data and traffic outside of it
• Must route to other networks

Gateways Cont.

• Hosts know of the default gateway through DHCP in IPv4
• IPv6 delivers it through router solicitation, although can be configured manually
• LAN devices need the DGW router for the intention of sending remote data

Host Routing Tables

• In Windows, Route Print and netstat-r showcase the PC routing table
• The two commands show three sections, including interfaces and IPv4/v6 routing tables

Router Packet Forwarding

• Routers receive the frame of the host device
• The destination IPv4 address is inspected to search the routing table for best match

IP Routing Table

• Routes have three different types:
• Connect Directly: This enables static allocation of routes
• Remote is a route where routers do not need a direct connection Manual allocation of the route would be a static route Dynamic use would enable the use of dynamic routing protocols for shared information
• A default route will forward a traffic to a predetermined address when one does not exist in the table

Static Routing

• Must be adjusted by administrator depending if there is a topology change
• The best type of topology is a small, nonredundant network
• Often used along with dynamic routing protocol for predetermination