Link Layer

Introduction

• Terminology

• Hosts and Routers are nodes

• Communication channels that connect adjacent nodes along communication path are link

  • Wired links

  • Wireless Links

  • LAN

• Layer-2 packet: frame, encapsulates datagram

• Data-link layer has the responsibility of transferring the upper layer’s datagram from one node to physically adjacent node over a link

Context

• A datagram is transferred by different link protocols over different links:

  • e.g., Ethernet on first link, frame relay on intermediate links, Wi-Fi on last link

• Each link protocol provides different services

  • e.g., may or may not provide RDT (Reliable Data Transmission) over link

• Transportation analogy

  • trip from KC to Paris

    • Uber: KC to KCI Airport

    • Plan: KCI to CHI

    • Plane: CHI to CDG

    • Train: CDG to Hotel

  • tourist = datagram

  • transport segment = communication link

  • transportation mode = link layer protocol

  • travel agent = routing algorithm

Services

• Framing, link access:

  • encapsulate datagram into frame, adding header, trailer

    • channel access if shared medium

    • "MAC" addresses used in frame headers to identify source, destination

      • different from IP address

• Reliable delivery between adjacent nodes

  • Acknowledgements (Ack)

  • Rarely used on low bit-error link (fiver, some twisted pair)

  • Wireless links: high error rates

• Premature Q: Why both link-level and end-end reliability?

  • Answer later in the course

• flow control:

  • pacing between adjacent sending and receiving nodes

• error detection

  • errors caused by signal attenuation, noise

  • receiver detects presence of errors:

    • signals sender for retransmission or drops frame

• error correction:

  • receiver identifies and corrects bit error(s) without resorting to retransmission

• half-duplex and full-duplex

  • with half duplex, nodes at both ends of link can transmit, but not at same time

Where is the link layer implemented

• In each and every host

• Link layer implemented in “adaptor” (aka network interface card - NIC) or on a chip

  • Ethernet card, 802.11 card; Ethernet chipset

  • Implements link and physical layers

• Attaches into host’s system buses

• Combination of hardware, firmware, and software

Adaptors Communicating

• Sending Side

  • Encapsulates datagram in frame

  • Adds error checking bits, RDT (Reliable Data Transmission - Ack), flow control, etc.

Receiving Side

• Looks for errors, RDT, flow control, etc.

• Extracts datagram, passes to upper layer at receiving side

Error Detection

• EDC:: Error detection and Correction bits (redundancy)

• D:: data protected by error checking, may include header fields

• Error detection not 100% reliable

  • protocol may miss some errors, but rarely

  • larger EDC field yields better detection and correction

Parity Checking

• Single Bit Parity:: detect single bit errors

• Two-Dimensional Bit Parity:: detect and correct single bit errors

MAC Addresses

• 32-bit IP address

  • network-layer address for interface

  • used for layer 3 (network layer) forwarding (later in course)

• MAC (or LAN or physical) address

  • function: used 'locally' to get frame from one interface to another physically connected interface (same network, in IP-addressing sense)

  • 48 bits MAC address (for most LANs) burned in NIC ROM, also sometimes software settable

  • e.g.: 1A-2F-BB-76-09-AD or 1A:2F:BB:76:09:AD

LAN Addresses

• Each adapter on LAN has unique LAN address and is meaningful only on a link (LAN)

• MAC address allocation is administered by IEEE (Institute of Electrical and Electronics Engineers - www.ieee.org)

• Manufacturer buys portion of MAC address space (to assure uniqueness)

  • 1A-2F-BB-76-09-AD

• Analogy

  • MAC address: like Social Security Number

  • IP address: like postal address

• MAC flat address -> portability

  • can move LAN card from one LAN to another

• IP hierarchical address not portable

  • address depends on IP subnet to which node is attached

ARP: Address Resolution Protocol

• Q: How to determine interface’s MAC address, knowing its IP address?

• ARP table: Each IP node (host, router) on LAN has table

  • IP/MAC address mappings for some LAN nodes:

  • TTL (Time to Live): Time after which address mapping will be forgotten (typically 20 min)

ARP Protocol - Same LAN

• A wants to send a packet to B

  • B’s MAC address not in A’s ARP table

• A broadcasts ARP query packet, containing B’s IP address

  • destination MAC address = FF-FF-FF-FF-FF-FF

  • all nodes on LAN receive ARP query

• B receives ARP packet, 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 (times out)

  • soft state: information that times out (goes away) unless refreshed

• ARP is “plug-and-play”

  • nodes create their ARP tables without intervention from a network administrator

Addressing - Routing to another LAN

• Walkthrough: send datagram from A to B via R

  • focus on addressing - at IP (datagram) and MAC layer (frame)

  • assume A knows B’s IP address

  • assume A knows IP address of first hop router, R (how?)

  • assume A knows R’s MAC address (how?)

    

• A creates IP datagram with IP source A, destination B

• A creates link-layer frame with R’s MAC address as destination address, frame contains A-to-B IP datagram !

  

• frame sent from A to R

• frame received at R, datagram removed, passed up to IP

  

• R forwards datagram with IP source A, destination B

• R creates link-layer frame with B’s MAC address as destination address, frame contains A-to-B IP datagram

  

• R forwards datagram with IP source A, destination B

• R creates link-layer frame with B’s MAC address as destination address, frame contains A-to-B IP datagram

  

Multiple Access Links

• Two types of "links:

  • Point-to-point

    • PPP for dial-up access

    • point-to-point link between between two routers for instance

  • broadcast (shared wire or medium)

    • old-fashioned Ethernet

    • 802.11 wireless LAN

Multiple Access Protocols

• single shared broadcast channel

• two or more simultaneous transmissions by nodes: interference

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

• multiple access protocol

  • distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit

  • Communication about channel sharing must use channel itself

    • no out-of-band channel for coordination

An Ideal Multiple Access Protocol

• Given: broadcast channel of rate R bps

• desiderata:

  • 1. when one node wants to transmit, it can send at rate R

  • 2. when M nodes want to transmit, each can send at average rate R/M

  • 3. Flly decentralized

    • no special node to coordinate transmissions

    • no synchronization of clocks, slots

  • 4. simple

MAC protocols: Taxonomy

• Three Broad Classes:

  • Channel partitioning

    • divide channel into smaller "pieces" (time slots, frequency, code)

    • allocate piece to node for exclusive use

  • random access

    • channel not divided, allow collisions

    • "recover" from collisions

  • "taking turns"

    • nodes take turns, but nodes with more to send can take longer turns

Channel Partitioning MAC protocols

• TDMA:: Time Division Multiple Access

  • Access to channel in “rounds”

  • Each station gets fixed length slot (length = packet transmission time) in each round

  • Unused slots go idle

  • example: 6-station LAN, 1, 3, 4 have packets to send, slots 2, 5, 6 idle

    

• FDMA:: Frequency Division Multiple Access

  • Channel spectrum divided into frequency bands

  • Each station assigned fixed frequency band

  • Unused transmission time in frequency bands go idle

  • example: 6-station LAN, 1,3,4 have packet to send, frequency bands, 2,5,6 idle

Random Access Protocols

• When node has packet to send

  • transmit at full channel data rate R

  • no a priori coordination among nodes

• two or more transmitting nodes → “collision”,

• random access MAC protocol specifies:

  • how to detect collisions

  • how to recover from collisions (e.g., via delayed retransmissions)

• Examples of random-access MAC protocols:

  • CSMA, CSMA/CD (Ethernet), CSMA/CA (Wi-FI)

CSMA (Carrier Sense Multiple Access)

• CSMA: listen before transmit: if channel sensed idle: transmit entire frame; if channel sensed busy, defer transmission * human analogy: don’t interrupt others!

CSMA Collisions

• Collisions can still occur propagation delay means two nodes may not hear each other’s transmission

• collision:: entire packet transmission time wasted

  • distance & propagation delay play role in determining collision probability

CSMA/CD (Collision Detection)

• CSMA/CD: carrier sensing, deferral as in CSMA

  • collisions detected within short time

  • colliding transmissions aborted, reducing channel wastage

• collision detection:

  • easy in wired LANs: measure signal strengths, compare transmitted, received signals

  • difficult in wireless LANs: received signal strength overwhelmed by local transmission strength

    

Ethernet CSMA/CD Algorithm

1. NIC receives datagram from network layer, creates frame

2. If NIC senses channel idle, starts frame transmission. If NIC senses channel busy, waits until channel idle, then transmits

3. If NIC transmits entire frame without detecting another transmission, NIC is done with frame!

4. If NIC detects another transmission while transmitting, aborts and sends jam signal

5. After aborting, NIC enters binary (exponential) backoff

after m^th collision, NIC chooses K at random from {0,1,2,…,2^(m-1)}. NIC waits K\*512 bit times, returns to Step 2

“Taking turns” MAC protocols

• Channel Partitioning MAC protocols:

  • share channel efficiently and fairly at high load

  • inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node!

• Random Access MAC protocols

  • efficient at low load: single node can fully utilize channel

  • high load: collision overhead

• “Taking turns” protocols

  • look for best of both worlds!

• Token Passing:

  • control token passed from one node to next sequentially

  • token message

  • concerns:

    • token overhead

    • latency

    • single point of failure (token)

      

Summary of MAC protocols

• Channel Partitioning, by time, frequency or code

  • Time Division, Frequency Division

• Random Access (dynamic)

  • carrier sensing: easy in some technologies (wire), hard in others (wireless)

  • CSMA'

  • CSMA/CD used in Ethernet (802.3)

  • CSMA/CA used in Wi-Fi (802.11)

• Taking Turns

  • Polling from central site, token passing

  • Bluetooth, Wi-Fi, token ring

Ethernet

Ethernet

• “dominant” wired LAN technology:

  • single chip, multiple speeds

  • first widely used LAN technology

  • simpler, cheap

  • kept up with speed race: 10 Mbps-10Gbps

Physical Topology

• bus: popular through mid 90s

  • all nodes in same collision domain (can collide with each other)

• Star: prevails today

  • with hub (still all nodes in same collision domain)

  • active switch in center

    • each “spoke” runs a (separate) Ethernet protocol (nodes do not collide withe each other)

      

Frame Structure

• Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame

  

• Preamble:

  • 7 bytes with pattern 10101010 followed by one byte with pattern 10101011

  • Used to synchronize receiver, sender clock rates

• addresses: 6 byte source, destination MAC addresses

  • if adapter receives frame with matching destination address, or with broadcast address (e.g., ARP packet), it passes data in frame to network layer protocol

  • otherwise, adapter discards frame

• Type: indicates higher layer protocol (mostly IP but others possible, e.g., ARP, Novell IPX, AppleTalk)

• CRC: cyclic redundancy check at receiver

  • error detected: frame is dropped

Unreliable, Connectionless

• Connectionless: no handshaking between sending and receiving NICs

• Unreliable: receiving NIC doesn’t send Acks or NAcks to sending NIC

  • data in dropped frames recovered only if initial sender uses higher layer RDT (Reliable Data Transmission) (e.g., TCP), otherwise dropped data lost

• ethernet’s MAC protocol: CSMA/CD with binary backoff

802.3 Ethernet Standards: Link & Physical Layers

• many different Ethernet Standards

  • common MAC protocol and frame format

  • different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps, 40 Gbps

  • different physical layer media: fiber, cable

    

Ethernet Switch

• Link-layer device: takes an active role

  • store, forward Ethernet frames

  • examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment

• Transparent

  • hosts are unaware of presence of switches

• plug-and-play, self-learning

  • switches do not need to be configured

Switch: Multiple simultaneous transmissions

• hosts have dedicated, direct connection to switch

• switches buffer packets

• Ethernet protocol used on each incoming link, but no collisions; full duplex

  • each link is its own collision domain

• switching: A-to-A’ and B-to-B’ can transmit simultaneously, without collisions

Switch Forwarding Table

• Q: how does switch know A’ reachable via interface 4, B’ reachable via interface 5?

  • A: each switch has a switch table, each entry:

    • (MAC address of host, interface to reach host, time stamp)

    • looks like a routing table!

• How are entries created, maintained in switch table?

  • something like a routing protocol?

Switch Self-Learning

• Switch learns which hosts can be reached through which interfaces

  • when frame received, switch “learns” location of sender: incoming LAN segment

  • records sender/location pair in switch table

    

Switch: Frame Filtering/Forwarding

Interconnecting Switches

Self-Learning Multi-Switch Example

Institutional Network

Switches vs. Routers

• Both are store-and-forward:

  • routers: network-layer devices (examine network-layer headers)

  • switches: link-layer devices (examine link-layer headers)

• Both have forwarding tables

  • routers: compute tables using routing algorithms, IP addresses

  • switches: learn forwarding table using flooding, learning, MAC addresses

    

VLANs

Motivation

• Consider:

  • CS user moves office to EE, but wants connect to CS switch?

  • Single broadcast domain:

    • all layer-2 broadcast traffic (ARP, DHCP, unknown location of destination MAC address) must cross entire LAN

    • Security/privacy, efficiency issues

      

VLANs

• Virtual Local Area Network

  • Switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANs over single physical LAN infrastructure.

• Port-base VLAN: switch ports grouped (by switch management software) so that single physical switch operates as multiple virtual switches

Port-based VLAN

• Traffic Isolation:: frames to/from ports 1-8 can only reach ports 1-8

  • can also define VLAN based on MAC addresses of endpoints, rather than switch port

• Dynamic membership: ports can be dynamically assigned among VLANs

• forwarding between VLANS: done via routing (just as with separate switches)

  • In practice vendors sell combined switches plus routers

VLANs spanning multiple switches

• Trunk port:: carries frames between VLANS defined over multiple physical switches

  • frames forwarded within VLAN between switches can’t be 802.3 frames (must carry VLAN ID info)

  • 802.1q protocol adds/removes additional header fields for frames forwarded between trunk ports