Layer 3 Versus Layer 2 Packet Fragmentation Analysis

Packet Fragmentation: Layer 3 vs. Layer 2

As mentioned in the “Link-Layer Fragmentation and Interleaving” section, PPP, Frame Relay, ATM, and other WAN technologies support fragmenting packets (frames) at Layer 2. As you may have learned in basic networking classes, IP can also fragment packets at Layer 3. What are some advantages and disadvantages of fragmenting at Layer 3 versus fragmenting at Layer 2?

Advantages and Disadvantages

• Advantages (Layer 3 Fragmentation): Large packets can be transmitted at Layer 2 as fragmentation is done at Layer 3. This reduces the number of acknowledgment exchanges. Also, maintaining the order of fragments to ensure the order of transmission and reassembly is not required.
• Disadvantages (Layer 3 Fragmentation): When a packet is lost in transmission, the entire packet needs to be retransmitted, and delay-sensitive packets may suffer when congestion occurs.

Networking Terminology Definitions

Supplicant

A host that is requesting authentication and access to network resources.

Authentication and Authentication Server

Authenticator: A device that blocks or allows traffic to pass through its port. Authentication Server: A server that validates the credentials of the supplicant and notifies the authenticator.

EAP

EAP: Extensible Authentication Protocol

Discontiguous Subnet

A discontiguous subnet is two or more portions of a major network that are divided by another major network.

Routing Protocols

Interior Vs. Exterior Routing Protocols

Interior protocols are used within an autonomous system, whereas Exterior protocols operate between autonomous systems.

Autonomous System

A set of routers that presents a common routing policy to the internetwork.

Quality of Service (QoS)

QoS Policy

Quality of Service (QoS): Policy is a network definition of the specific levels of QoS that are assigned to different classes of traffic.

QoS Types

• Intserv: Guaranteed End-to-End, involves lots of signaling and is less scalable.
• Diffserv: Overcomes the drawback of Intserv; it is not guaranteed, is scalable, and involves network classification of QoS requirements.

VPN vs. VLAN

VPN: Tunnelling $\gg$ Encryption $\gg$ Secure connection. VLAN: Reduce broadcast tag/ID trunk line and separate broadcast domains.

Traffic Management

Traffic Shaping vs. Policing

• Traffic Shaping: Aims to avoid bottlenecks, queue bursts, and prevent overwhelming a downstream router.
• Policing: Ensures classified traffic meets requirements by dropping packets or changing packet content.

CIR

CIR: Committed Information Rate; the average bandwidth provided to the client/designer.

Subnetting and VPN Concentration

Basic Subnetting Concepts

Involves sub-networks, bits, subnet mask, block size, and the number of host IP ranges.

VPN Concentrator

VPN Concentrator: A single device used to handle a large number of VPNs, employing techniques to encrypt and authenticate, creating remote access or site-to-site VPNs.