In-Depth Notes on Network Layer – Data Plane

  • Overview of Network Layer

  • The Network Layer is responsible for transporting segments from sending to receiving hosts.

  • It encapsulates segments into datagrams at the sending side and delivers segments to the transport layer at the receiving side.

  • Network layer protocols exist in every host and router, examining header fields in all passing IP datagrams.

  • Key Functions of Network Layer

  • Forwarding: Moves packets from the router's input to the appropriate output.

    • Analogy: Getting through a single interchange during a trip.

  • Routing: Determines the path taken by packets from source to destination.

    • Analogy: Planning a trip from start to finish.

  • Data Plane vs Control Plane

  • Data Plane:

    • Local, per-router function.
    • Determines how datagrams arriving on router input are forwarded to output.
    • Involves forwarding functions.

  • Control Plane:

    • Network-wide logic for routing datagrams among routers from source to destination.
    • Includes traditional routing algorithms and software-defined networking (SDN).

  • Routing Algorithms and Forwarding

  • Each router contains individual routing algorithm components that communicate.

  • Routing algorithms compute values in the forwarding tables.

    • No routing protocols would be needed if human operators manually configured forwarding tables, leading to errors and slow responses.

  • Network Service Models

  • Models for the “channel” transporting datagrams include:

    • Guaranteed delivery
    • Guaranteed delivery with bounded delay
    • In-order datagram delivery for flows
    • Guaranteed minimum bandwidth and maximum jitter

  • Internet relies on a “best-effort” service model without guarantees of delivery, timing, or order.

  • Router Architecture

  • Routers execute routing algorithms/protocols (e.g., RIP, OSPF, BGP) and forward datagrams from incoming to outgoing links using high-speed switching fabric.

  • Two functions:

    • Run routing protocols
    • Forward datagrams

  • Input Port Functions

  • Receive bits at the physical layer and perform link layer protocol actions (e.g., Ethernet).

  • Forward datagrams based on header field values using a decentralized switching method (match + action).

  • Destination-Based Forwarding

  • Traditional forwarding based only on IP address destination.

  • Generalized forwarding can utilize any header field values.

  • Longest Prefix Matching

  • Technique used in forwarding tables to find the longest matching prefix for a destination address.

  • Often implemented using Ternary Content Addressable Memories (TCAMs) for fast access.

  • Switching Fabrics

  • Essential for transferring packets from input to output buffers with high speed.

  • Types include:

    1. Memory-based switching
    2. Bus-based switching
    3. Interconnection networks

  • Output Port Functions

  • Involves line termination, link layer protocols, switching fabrics, and datagram queuing.

  • Buffering necessary to manage instances when datagrams arrive faster than output rates, potentially leading to congestion and packet loss.

  • Buffering Strategies

  • RFC 3439 suggests buffering that equals typically RTT (round trip time) times link capacity.

  • Scheduling mechanisms (e.g., FIFO, Priority Scheduling, Round Robin) govern how packets are sent.

  • Internet Protocols

  • Major components of the network layer include:

    • IP Protocol: Manages IP addressing and datagram format.
    • Routing Protocols: Handle path selection.
    • ICMP Protocol: For error reporting and routing signaling.

  • IP Addressing

  • Utilizes a 32-bit identifier for hosts (dotted-decimal notation).

  • Subnets defined by the high-order bits, determined by subnet masks.

  • Special addresses include:

    • 0.0.0.0 (invalid address)
    • 127.0.0.1 (loopback address)
    • Private IP ranges (e.g., 192.168.0.0 to 192.168.255.255).

  • Dynamic Host Configuration Protocol (DHCP)

  • Allows hosts to dynamically obtain IP addresses from a server when joining a network, facilitating mobility and address reuse.

  • DHCP process involves discovery, offer, request, and acknowledgment messages.

  • Network Address Translation (NAT)

  • Enables a local network to use one IP address for external connections while having multiple internal addresses, conserving IP space and enhancing privacy.

  • Issues arise with NAT, such as end-to-end connectivity concerns and application compatibility.

  • IPv6 Development

  • Developed due to IPv4 address exhaustion, enabling larger address spaces and improved routing efficiency.

  • Key changes from IPv4 include fixed-length headers and no fragmentation at the router level.

  • Conclusion

  • The network layer encompasses essential services such as forwarding and routing, interfacing via IP protocols, and incorporates advancements through IPv6 and practices like NAT to address current network challenges.