📌Network Layer

• transport segment from sending to receiving host

  • sender: encapsulates segments into datagrams, passes to link layer
  • receiver: delivers segments to transport layer protocol

• network layer protocols in every Internet device: hosts, routers

Network Layer 핵심 기능

Forwarding

HW; Data Plane 영역

• move packets from a router’s input link to appropriate router output link

Routing

SW; Control Plane 영역

• determine route taken by packets from source to destination

Data Plane & Control Plane

• Data Plane : 패킷 송수신 기능 담당. (forwarding 기능)
  • local, per-router function
  • determines how datagram arriving on router input port is forwarded to router output port
• Control Plane : 패킷 경로 설정, 관리 및 제어하는 기능 담당. (routing 기능)
  • network-wide logic
  • determines how datagram is routed among routers along end-end path from source host to destination host
  • 2가지 Control plane에 대한 접근:
    • 전통적 라우팅 알고리즘 : 라우터 안에서 forwarding, routing 모두 다 함
      • Per-router Control Plane
    • SDN(Software-defined network) : 오버헤드가 큰 routing을 따로 분리하여 remote-server에서 관리
      • Logically Centralized Control Plane

Per-router Control Plane

예전 방식

Individual routing algorithm components in each and every router interact in the control plane

Logically Centralized Control Plane

최근 방식

A remote controller computes, installs forwarding tables in routers by interacting with local control agents (CAs)

Network Layer Service Model

defines the characteristics of e2e transport of packets between sending & receiving hosts

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📌Router

Input Port : Data Plane

• Decentralized switching:
  • Given datagram, using header field(dest. IP addr.) values, lookup output port using forwarding table in input port memory (“match plus action”)
  • Goal: complete input port processing at ‘line speed’
  • Input port queuing: if datagrams arrive faster than forwarding rate into switch fabric (if the switch fabric is not fast enough, then input queueing delay or loss due to buffer overflow)

Destination-based Forwarding

Based only on destination IP address (traditional)

Generalized Forwarding

Based on any set of header field values

Switching Fabrics : Data Plane

• transfer packet from input buffer to appropriate output buffer
• switching rate: rate at which packets can be transfer from inputs to outputs
  • N inputs: switching rate N times line rate (R) desirable
  • often measured as multiple of input/output line rate (NR)

Switching via Memory

Switching via Bus

Switching via Interconnection (Crossbar)

Output Port

• Buffering required when datagrams arrive from fabric faster than link transmission rate. Drop policy: which datagrams to drop if no free buffers?

  • Datagrams can be lost due to congestion, lack of buffers

• Scheduling discipline chooses among queued datagrams for transmission

  • Priority scheduling – who gets best performance, network neutrality

Output Port Queueing → Buffering

Scheduling Mechanism

Priority

RR Scheduling

Weightened Fair Queuing (WFQ)