5 Data Link Layer Data Link Layer Self-learning, Interconnecting - - PDF document

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5 Data Link Layer Data Link Layer Self-learning, Interconnecting - - PDF document

Mar 23, 2023 385 likes •446 views

Data Link Layer Data Link Layer Hubs Switch physical- layer (dumb) repeaters: bits coming in one link go out all other links at same link-layer device: smarter than hubs, take rate active role all nodes connected to hub can

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SLIDE 1

5

Data Link Layer

Hubs

physical-layer (“dumb”) repeaters:

 bits coming in one link go out all other links at same

rate

 all nodes connected to hub can collide with one

another

 no frame buffering  no CSMA/CD at hub: host NICs detect collisions

twisted pair hub

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Data Link Layer

Switch

 link-layer device: smarter than hubs, take

active role

 store, forward Ethernet frames  examine incoming frame’s MAC address,

selectively forward frame to one-or-more

  • utgoing 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

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Data Link Layer

Switch: allows 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’ simultaneously, without collisions

 not possible with dumb hub

A A’ B B’ C C’ switch with six interfaces (1,2,3,4,5,6) 1 2 3 4 5 6

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Data Link Layer

Switch Table

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 Q: how does switch know that

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!  Q: how are entries created,

maintained in switch table?

 something like a routing

protocol?

A A’ B B’ C C’ switch with six interfaces (1,2,3,4,5,6) 1 2 3 4 5 6

Data Link Layer

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

A A’ B B’ C C’ 1 2 3 4 5 6 A A’

Source: A Dest: A’

MAC addr interface TTL

Switch table (initially empty)

A 1 60

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Data Link Layer

Switch: frame filtering/forwarding

When frame received:

  • 1. record link associated with sending host
  • 2. index switch table using MAC dest address
  • 3. if entry found for destination

then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood

forward on all but the interface

  • n which the frame arrived

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SLIDE 2

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Data Link Layer

Self-learning, forwarding: example

A A’ B B’ C C’ 1 2 3 4 5 6 A A’

Source: A Dest: A’

MAC addr interface TTL

Switch table (initially empty)

A 1 60

A A’ A A’ A A’ A A’ A A’  frame destination

unknown: flood

A’ A  destination A

location known:

A’ 4 60

selective send

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Data Link Layer

Interconnecting switches

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switches can be connected together

A B

 Q: sending from A to G - how does S1 know to

forward frame destined to F via S4 and S3?

 A: self learning! (works exactly the same as in

single-switch case!) S1 C D E F S2 S4 S3 H I G

Data Link Layer

Self-learning multi-switch example

Suppose C sends frame to I, I responds to C

 Q: show switch tables and packet forwarding in S1,

S2, S3, S4 A B S1 C D E F S2 S4 S3 H I G 1 2

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Data Link Layer

Institutional network

to external network router IP subnet mail server web server

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Data Link Layer

Switches vs. Routers

 both store-and-forward devices

 routers: network layer devices (examine network layer

headers)

 switches are link layer devices

 routers maintain routing tables, implement routing

algorithms

 switches maintain switch tables, implement

filtering, learning algorithms

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Data Link Layer

Switches vs. Routers (2)

Switch Router Pro Con Pro Con

  • Plug-and-

play

  • Broadcast

storms

  • Topology

restricted to spanning tree

  • No

broadcast storms

  • More

general topologies

  • Not plug-

and-play

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SLIDE 3

7

Data Link Layer

Link Layer

 1 Introduction and

services

 2 Error detection

and correction

 3 Multiple access

protocols

 4 Link-layer

Addressing

 5 Ethernet  6 Link-layer switches  7 Link virtualization:

ATM

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Data Link Layer

Virtualization of networks

 Virtualization of resources: powerful

abstraction in systems engineering

 Computing examples: virtual memory,

virtual devices

 Virtual machines: e.g., java  IBM VM os from 1960’s/70’s

 Layering of abstractions: don’t sweat the

details of the lower layer, only deal with lower layers abstractly

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Data Link Layer

Connecting different networks is a challenge

1974: multiple unconnected nets

 ARPAnet  data-over-cable networks  packet satellite network

(Aloha)

 packet radio network

differing in:

 addressing conventions  packet formats  error recovery  routing

ARPAnet satellite net

"A Protocol for Packet Network Intercommunication",

  • V. Cerf, R. Kahn, IEEE Transactions on Communications,

May, 1974, pp. 637-648.

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Data Link Layer

The idea of the Internet

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ARPAnet satellite net gateway

(1) Add a Gateway:

 “embed internetwork packets in

local packet format or extract them”

 route (at internetwork level) to

next gateway

(2) Add Internetwork layer (IP):

 addressing: internetwork

appears as single, uniform entity, despite underlying local network heterogeneity

 network of networks Data Link Layer

Central to the Internet: Virtualizing

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The new layer (IP) makes everything homogeneous at internetwork layer underlying local network technology => to a homogeneous link layer service that delivers an IP datagram from one host to another host

  • “invisible” at internetwork layer.
  • Looks like a link layer technology to IP!
  • cable
  • satellite
  • 56K telephone modem
  • today: ATM

Data Link Layer

To IP, no difference!

Ethernet PSTN Switch Switch

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SLIDE 4

8

Data Link Layer

ATM

 ATM separate networks in their own right

 different service models, addressing, routing

from Internet  viewed by Internet as logical link connecting

IP routers

 just like dialup link is really part of separate

network (telephone network)  ATM: of technical interest in their own right

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Data Link Layer

Asynchronous Transfer Mode: ATM

 1990’s/00 standard for high-speed (155Mbps to

622 Mbps and higher) Broadband Integrated Service Digital Network architecture

 Goal: integrated, end-end transport of carry voice,

video, data

 meeting timing/QoS requirements of voice, video

(versus Internet best-effort model)

 “next generation” telephony: technical roots in

telephone world

 packet-switching using virtual circuits (fixed

length packets, called “cells”)

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Data Link Layer

ATM architecture

 adaptation layer: only at edge of ATM network

 data segmentation/reassembly  roughly analogous to Internet transport layer

 ATM layer: “network” layer

 cell switching, routing

 physical layer

physical ATM AAL physical ATM AAL physical ATM physical ATM end system end system switch switch

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Data Link Layer

ATM Adaptation Layer (AAL)

 ATM Adaptation Layer (AAL): “adapts” upper

layers (IP or native ATM applications) to ATM layer below

 AAL present only in end systems, not in switches  AAL layer segment (header/trailer fields, data)

fragmented across multiple ATM cells

 analogy: TCP segment in many IP packets

physical ATM AAL physical ATM AAL physical ATM physical ATM end system end system switch switch

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Data Link Layer

ATM: network or link layer?

Vision: end-to-end transport: “ATM from desktop to desktop”

 ATM is a network

technology Reality: used to connect IP backbone routers

 “IP over ATM”  ATM as switched

link layer, connecting IP routers

ATM network IP network

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Data Link Layer

IP-Over-ATM

Classic IP only

 3 “networks” (e.g.,

LAN segments)

 MAC (802.3) and IP

addresses IP over ATM

 replace “network”

(e.g., LAN segment) with ATM network

 ATM addresses, IP

addresses ATM network Ethernet LANs Ethernet LANs

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SLIDE 5

9

Data Link Layer

IP-Over-ATM

AAL ATM phy phy Eth IP ATM phy ATM phy app transport IP AAL ATM phy app transport IP Eth phy

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Data Link Layer

Homework #4

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