Networking: Basic Network Abstraction, Layering, and MAC Protocols, Summaries of Local Area Network (LAN)

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Networking

based on slides by Prof. Sirer, Bracy, Van Renesse, Ross, Kurose 1

Basic Network Abstraction

A process can create “endpoints”

Each endpoint has a unique address

Processes can receive messages on

endpoints

Processes can send messages to

endpoints

A message is a byte array

Network “protocol”

An agreement between processes about

the content of messages

■ Syntax: Layout of bits, bytes, fields, etc.

⬥message format

■ Semantics: What they mean

Examples:

■ HTTP “get” requests and responses

⬥HTML is part of the format

Excuse me, please, thank you, etc. in real life

Network Layering

The network abstraction is usually layered

■ (^) Each layer provides a service to layers above; relies on services from layers below

Example:

Application Layer HTTP/FTP/DNS; exchanges messages Transport Layer Transports messages; TCP (connection oriented)/ UDP; exchanges segments Network Layer Transports segments; IP; exchanges datagrams Link Layer Transports datagrams; Ethernet/WiFi; exchanges frames Physical Layer Trasports frames;wires, signal encoding, wireless; exchanges bits

destination application transport network link physical Hl Hn Ht M Hn Ht M Ht M

M

network link physical Hl Hn Ht M Hn Ht M (^) H Hn t M link physical

Encapsulation

application transport network link physical Hn Ht M

segment Ht

Hl Hn Ht M

message M

Ht M Hn

source

switch Hl Hn Ht M (^) H Hl Hn t M router

datagram

frame

Link Layer: Local Area Networking (LAN) and Ethernet Application Layer Transport Layer Network Layer Link Layer Physical Layer

Addressing

Each NIC has a MAC address

■ Media Access Control address

■ Unique!

■ 6 bytes long

■ Ethernet example: b8:e3:56:15:6a:

■ Address space managed by IEEE; first 24 bits identify

manufacturer

■ Does not change if the NIC moves

⬥ (^) Not true of IP address!

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

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 TDMA: time division multiple access ❖ (^) access to channel in "rounds" ❖ (^) each station gets fixed length slot (length = pkt trans time) in each round ❖ (^) unused slots go idle ❖ (^) example: 6-station LAN, 1,3,4 have frames, slots 2,5,6 idle

6-slot

frame

6-slot

frame

“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

■ 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)

T

data

(nothing

to send)

T

“Taking turns” MAC protocols

Example: Ethernet

1976, Metcalfe & Boggs at Xerox

⬥ (^) Later at 3COM

Based on the Aloha network in Hawaii

Named after the “ luminiferous ether”

Centered around a broadcast bus

Simple link-level protocol, scales pretty well

Tremendously successful

Still in widespread use

⬥ (^) many orders of magnitude increase in bandwidth since early versions

“CSMA/CD”

Carrier sense

■ Listen before you speak

Multiple access

■ Multiple hosts can access the network

Collision detect

■ Detect and respond to cases where two hosts

collide