Computer Networking:

a Top-Down Approach (8th ed.) :

Notes of "Select" Lectures

Chapter 4 Network Layer - data plane

The network layer can be decomposed into two parts:

• the data plane (数据平面)

• the per-router functions in the network layer

• determine how a datagram (that is, a network-layer packet) arriving on one of a router's input links is forwarded to one of that router's output links

• the control plane (控制平面)

• the network-wide logic

• controls how a datagram is routed among routers along an end-to-end path from the source host to the destination host

4.1 Network Layer Overview

Forwarding versus routing; data plane, control plane; network service model.

There is a piece of the network layer in each and every host and router in the network.

• H1 is sending information to H2.

• The network layer in H1 takes segments from the transport layer in H1, encapsulates each segment into a datagram, and then sends the datagrams to its nearby router, R1.

• At the receiving host, H2, the network layer receives the datagrams from its nearby router R2, extracts the transport-layer segments, and delivers the segments up to the transport layer at H2.

• Routers:

• the primary data-plane role of each router is to forward datagrams from its input links to its output links;

• the primary role of the network control plane is to coordinate these local, per-router forwarding actions so that datagrams are ultimately transferred end-to-end, along paths of routers between source and destination hosts.

4.1.1 Forwarding and Routing: The Network Data and Control Planes

The primary role of the network layer: to move packets from a sending host to a receiving host.

    Two important network-layer functions:

• Forwarding (转发): the router-local action of transferring a packet from an input link interface to the appropriate output link interface

• Routing (路由选择): the network-wide process that determines the end-to-end paths that packets take from source to destination.

• Routing algorithms (路由选择算法).

    Forwarding table (转发表): a key element in every network router.

• A router forwards a packet by examining the value of one or more fields in the arriving packet's header, and then using these header values to index into its forwarding table.

• The value stored in the forwarding table entry for those values indicates the outgoing link interface at that router to which that packet is to be forwarded.

Control Plane: The Traditional Approach

A routing algorithm runs in each and every router and both forwarding and routing functions are contained within a router.

Control Plane: The SDN Approach

• A physically separate, remote controller computes and distributes the forwarding tables to be used by each and every router.

• At the heart of software-defined networking (SDN, 软件定义网络), where the network is "software-defined" because the controller that computes forwarding tables and interacts with routers is implemented in software.

4.1.2 Network Service Model

The network service model (网络服务模型) defines the characteristics of end-to-end delivery of packets between sending and receiving hosts.

Some possible services that the network layer could provide could include:

• Guaranteed delivery.

• Guaranteed delivery with bounded delay.

• In-order packet delivery.

• Guaranteed minimal bandwidth.

• Security.

The Internet's network layer provides a single service, known as best-effort service (尽力而为服务).

With best-effort service, packets are

• neither guaranteed to be received in the order in which they were sent,

• nor is their eventual delivery even guaranteed.

There is

• no guarantee on the end-to-end delay

• nor is there a minimal bandwidth guarantee.

The Internet's basic best-effort service model combined with adequate bandwidth provisioning (带宽供给) and bandwidth-adaptive application-level protocols have arguably proven to be more than "good enough" to enable an amazing range of applications.

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来源： https://www.cnblogs.com/zjnu/p/kurose_ross_4.html