[Ch6] Link Layer

Link Layer Services

Link Layer

• 1 subnet & 4 L2-switch (no IP addr, no MAC addr)

Link Layer (Layer 2)

• nodes: hosts and routers
• links: communication channels that connect adjacent nodes along communiation path - wired & wireless links
• layer-2 packet == frame that encapsulates datagram

Data Link Layer (Layer 2) has responsibility of transferring datagram from one node to physically adjacent node over a link (each node has L3)

 

Where is the Link Layer Implemented

• Link layer is implemented in each and every host in adapter (aka. network interface card; NIC)
  • Ethernet card(802.11)
  • implements link, physical layer
• Link layer is combination of hardware, software, firmware

 

Link Layer Services

Framing

• encapsulate datagram into frame, adding header (src/dst MAC addr)

Link Access

• channel access if shared medium

Reliable delivery btw adjacent nodes

• flow control: pacing btw sending & receiving nodes
• error detection: receiver detects presense of errors and signals sender for retransmission or drops frame
• error correction: receiver identifies and corrects bit errors without resorting to retransmission

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Multiple Access Link

Multiple Access Link: 2 Typs of Link

Point-to-Point networks (typically WAN)

• pairs of host (or routers) directly connected
• ex) p2p link btw Ethernet switch and host

Broadcast networks (typically LAN)

• all hosts (or routers) share a single communication channel
• ex) old fashioned Ethernet, Wi-Fi, Satellite networks, HFC (Hybrid fiber-coaxial cable)

• L2 Switch: Buffer O -> 포트마다 다른 BW 가지는 링크 연결 가능
• Hub: Buffer X -> flooding, 모든 포트에 연결된 링크의 BW가 동일해야 함

Multiple Access Protocol

Due to a single shared broadcast channel:

👉 two or more simultaneous transmissions by nodes: interference

👉 collision if node receives two or more signals at the same time

👉 Why we need Multiple Access Protocol

 

Multiple Access Protocol

• distributed algorithm that determines how nodes share channel (i.e. determine when node can transmit)
• channel particioning
  • by time (TDMA), frequency (FDMA) or code (CDMA)
• random access (dynamic)
  • Collision Detection (CD): CSMA/CD used in Ethernet (IEEE 802.3)
  • Collision Avoidance (CA): CSMA/CA used in 802.11 (Wi-Fi)
• taking turns
  • bluetooth, FDDI, token ring

Collision Detection (CSMA/CD)

• transmission time (L/R) 동안 listen -> collision detection 가능

Collision Avoidance (CSMA/CA)

• CTS, RTS 보내서 충돌이 일어나지 않도록 선점
• AP 하고만 통신 -> detection 불가 (Hidden node problem)

Hidden node problem: wireless network에서 두 노드 A, B가 공통으로 통신이 가능한 노드 P가 있는데 A와 B는 통신 거리(radio range) 밖에 있거나 두 노드 사이에 장애물이 있어 서로의 신호가 P에서 충돌하는 것을 감지 (Collision Detection)할 수 없는 현상

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MAC addresses and ARP

IEEE 802 Standard

IEEE 802

• a family of standards for LANs which defines
  • Logical Link Control (LLC) - OS
  • Several Media Access Control (MAC) sub-layers - HW, NIC

 

MAC Address (or LAN address)

32-bit IPv4 address

• Network Layer (L3) address for interface used for layer 3 forwarding

MAC address (== LAN or physical or Ethernet address)

• 48-bit MAC address burned in NIC
• permanent, unique, IEEE manages the MAC address space
• no hierarchical structure (same MAC address can be used no matter where the computer goes)
• 16진수로 표현 (ex. FF-FF-FF-FF-FF-FF: broadcast MAC addr)

• 두 호스트를 연결하는 e2e path에서 경유하는 라우터들은 모두 동일한 src IP addr와 dst IP addr를 보고 라우팅을 결정한다.
• 어떤 NIC 카드에 할당된 L3 IP 주소는 위치에 따라 변하는 주소이나, L2 MAC 주소는 변하지 않는다.
• IP address = postal address / MAC address = social security number

ARP: Address Resolution Protocol

ARP Table

• each IP node (host, router) on LAN has ARP table
  • 특정 NIC에 설정된 IP address와 MAC address의 매핑 테이블
  • <IP addr; MAC addr; TTL>
  • TTL (Time to Live): time after which address mapping will be forgotten (typically 20 min)

In case of Same LAN (A wants to send datagram to B & B's MAC address not in A's ARP table)

• A broadcasts ARP query packet, containing B's IP address
  • broadcast: dest MAc address = FF-FF-FF-FF-FF-FF
  • all nodes on LAN receive ARP query
• B receives ARP pkt and replies to A with its (B's) MAC address
  • frame sent to A's MAC address (unicast)
• A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (time out)
  • <IP addr; MAC addr; TTL>
• ARP is plug-and-play
  • nodes create their ARP table without intervention from network administrator

In case of Routing to another LAN

• walkthrough: send datagram from A to B via R
  • 두 host를 연결하는 e2e path에서 한 hop을 지날 때 2계층 주소는 계속 바뀌지만 3계층 주소는 동일