1 cse401n: computer networks lan address & arp ethernet basics

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CSE401N: COMPUTER NetworkS

LAN address & ARPEthernet Basics

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LAN technologies

Data link layer so far: services, error detection/correction, multiple

access

Next: LAN technologies addressing Ethernet hubs, bridges, switches 802.11 PPP ATM

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LAN Addresses and ARP

32-bit IP address: network-layer address used to get datagram to destination IP network

(recall IP network definition)

LAN (or MAC or physical or Ethernet) address:

used to get datagram from one interface to another physically-connected interface (same network)

48 bit MAC address (for most LANs) burned in the adapter ROM

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NIC or Network Adaptor

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LAN Addresses and ARPEach adapter on LAN has unique LAN address

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LAN Address (more)

MAC address allocation administered by IEEE manufacturer buys portion of MAC address space

(to assure uniqueness) Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address => portability

can move LAN card from one LAN to another

IP hierarchical address NOT portable depends on IP network to which node is attached

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Recall Earlier Routing Discussion

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

Starting at A, given IP datagram addressed to E:

look up net. address of E, find C

link layer send datagram to C inside link-layer frame

C’s MACaddr

A’s MACaddr

A’s IPaddr

E’s IPaddr

IP payload

datagramframe

frame source,dest address

datagram source,dest address

C

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ARP: Address Resolution Protocol

Each IP node (Host, Router) on LAN has ARP table

ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL>

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

Question: how to determineMAC address of Cknowing C’s IP address?

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ARP protocol A knows B's IP address, wants to learn physical

address of B A broadcasts ARP query pkt, containing B's IP

address all machines on LAN receive ARP query

B receives ARP packet, replies to A with its (B's) physical layer address

A caches (saves) IP-to-physical address pairs until information becomes old (times out) soft state: information that times out (goes

away) unless refreshed

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Address Resolution Protocol (ARP)

1. With TCP/IP networking, a data packet must contain both a destination MAC address and a destination IP address.

2. Some devices will keep tables that contain MAC addresses and IP addresses of other devices that are connected to the same LAN.

3. These are called Address Resolution Protocol (ARP) tables. 4. ARP tables are stored in RAM memory, where the cached

information is maintained automatically on each of the devices. 5. Each device on a network maintains its own ARP table. 6. When a network device wants to send data across the network, it

uses information provided by the ARP table. 7. When a source determines the IP address for a destination, it

then consults the ARP table in order to locate the MAC address for the destination.

8. If the source locates an entry in its table, destination IP address to destination MAC address, it will associate the IP address to the MAC address and then uses it to encapsulate the data.

>arp -a

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Address Resolution Protocol (ARP) 1. The computer that requires an IP and

MAC address pair broadcasts an ARP request.

2. All the other devices on the local area network analyze this request, and if one of the local devices matches the IP address of the request, it sends back an ARP reply that contains its IP-MAC pair.

3. Another method to send data to the address of a device that is on another network segment is to set up a default gateway.

4. If the receiving host is not on the same segment, the source host sends the data using the actual IP address of the destination and the MAC address of the router.

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ARP conversation

HEY - Everyone please listen! Will 128.213.1.5 please send me his/her Ethernet address?

not me

Hi Green! I’m 128.213.1.5, and my Ethernet address is 87:A2:15:35:02:C3

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RARP conversation

HEY - Everyone please listen! My Ethernet address is 22:BC:66:17:01:75.Does anyone know my IP address ?

not me

Hi Green! Your IP address is 128.213.1.17.

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Getting a datagram from source to dest.

IP datagram:

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

miscfields

sourceIP addr

destIP addr data

datagram remains unchanged, as it travels source to destination

addr fields of interest here

Dest. Net. next router Nhops

223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

routing table in A

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Starting at A, given IP datagram addressed to B:

look up net. address of B find B is on same net. as A link layer will send datagram

directly to B inside link-layer frame B and A are directly connected


223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

miscfields223.1.1.1223.1.1.3data

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

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223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2

Starting at A, dest. E: look up network address of E E on different network

A, E not directly attached routing table: next hop router

to E is 223.1.1.4 link layer sends datagram to

router 223.1.1.4 inside link-layer frame

datagram arrives at 223.1.1.4 continued…..

miscfields223.1.1.1223.1.2.3 data

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

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Arriving at 223.1.4, destined for 223.1.2.2

look up network address of E E on same network as

router’s interface 223.1.2.9 router, E directly

attached link layer sends datagram to

223.1.2.2 inside link-layer frame via interface 223.1.2.9

datagram arrives at 223.1.2.2!!! (hooray!)

miscfields223.1.1.1223.1.2.3 data network router Nhops interface

223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9

223.1.3 - 1 223.1.3.27

Dest. next

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

BE

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Routing to another LANwalkthrough: routing from A to B via R

In routing table at source Host, find router 111.111.111.110

In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc

A

RB

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A creates IP packet with source A, destination B A uses ARP to get R’s physical layer address for

111.111.111.110 A creates Ethernet frame with R's physical address as

dest, Ethernet frame contains A-to-B IP datagram A’s data link layer sends Ethernet frame R’s data link layer receives Ethernet frame R removes IP datagram from Ethernet frame, sees its

destined to B R uses ARP to get B’s physical layer address R creates frame containing A-to-B IP datagram sends to B

A

RB

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Ethernet (IEEE 802.3)“dominant” LAN technology: first widely deployed LAN technology simpler, cheaper than token ring, FDDI, and

ATM Lesson learned: KISS (Keep It Simple, Stupid)

kept up with speed race: 10, 100, 1000 Mbps

Metcalfe’s Ethernetsketch

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Ethernet(2)

First widely used LAN Technology Simpler than token ring, FDDI, or ATM Comply with new Technology and Speed

Can run over coaxial cable,or twisted pair, or fiber optics or radio link

10Mbps, 100Mbps, 1Gbps, 10Gbps. Ethernet Hardware(Hub/Bridge/Switch)

is widely available and cheap

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Ethernet and the OSI Model

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Repeaters: Layer1 Device

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Naming(MAC Address)

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802.3 Frame Structures

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Ethernet Frame Structures

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Ethernet Frame Structures

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Ethernet 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

8 6 6 2 46-1500 (including padding) 4

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Ethernet Frame Structure(2) Addresses: 6 bytes each

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

otherwise, adapter discards frame Type (2 bytes): indicates the higher layer

protocol, mostly IP but others may be supported (such as Novell IPX and AppleTalk)

Data (46-1500 bytes): MTU is 1500 bytes, MIN frame size = 46 + 18 = 64 bytes = 512 bits

CRC (4 bytes): checked at receiver, if error is detected, the frame is simply dropped

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Unreliable, connectionless service

Connectionless: No handshaking between sending and receiving adapter.

Unreliable: receiving adapter doesn’t send ACKs or NAKs to sending adapter stream of datagrams passed to network layer can have

gaps gaps will be filled if app is using TCP otherwise, app will see the gaps

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Ethernet: From Bit to Electrical Signal

Use Manchester encoding One voltage change per bit

For a “1”, a voltage from 1 to 0 For a “0”, a voltage from 0 to 1

Example

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MAC Rules & Collision Detection/Backoff

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MAC Rules and Collision Detection/Backoff

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Ethernet Timing

Slot time: amount of time required to travel between

the furthest points of the collision domain, collide with another transmission at the last possible instant, and then have the collision fragments return to the sending station and be detected.

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Interframe Spacing and Backoff

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The Basic MAC Mechanisms of Ethernet

get a packet from upper layer;K := 0; n :=0; // K: random wait time; n: no. of

collisionsrepeat: wait for K * 512 bit-time; while (network busy) wait; wait for 96 bit-time after detecting no signal; transmit and detect collision; if detect collision stop and transmit a 48-bit jam; n ++; m:= min(n, 10), where n is the number of

collisions choose K randomly from {0, 1, 2, …, 2m-1}. if n < 16 goto repeat else giveup

CSMA/CD + Exponential backoff

Question: Why exponential backoff?

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Ethernet: uses CSMA/CD

A: sense channel, if idle then {

transmit and monitor the channel; If detect another transmission then { abort and send jam signal;

update # collisions; delay as required by exponential backoff algorithm; goto A}

else {done with the frame; set collisions to zero}}

else {wait until ongoing transmission is over and goto A}

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Ethernet’s CSMA/CD (more)Jam Signal: make sure all other transmitters are aware

of collisionBit time: 0.1 microsec for 10 Mbps Ethernet; for

K=1023, wait time is about 50 msecExponential Backoff: Goal: adapt retransmission attempts to estimated

current load heavy load: random wait will be longer

first collision: choose K from {0,1}; delay is K x 512 bit transmission times

after second collision: choose K from {0,1,2,3}… after ten or more collisions, choose K from

{0,1,2,3,4,…,1023}

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Why 64 bytes min frame length?

10Base5 Ethernet: 10Mbps, max segment 500m, max 4 repeaters, max network diameter 2500m Repeater: physical layer device that amplifies and retransmits

bits it hears on one interface to its other interfaces, used to connect multiple segments

Round trip time (worst case collision detection time) about 50 microsec All frames must take more than 50 microsec to send so that

transmission is still taking place when the noise burst gets back to sender

With 10Mbps bandwidth, 1 bit time = 0.1 microsec minimum frame size at least 500 bits, choose 512 bits to

add some margin of safety As network speed goes up, the minimum frame length

must go up or the maximum cable length must come down E.g. for a 2500m 1Gbps LAN, minimum frame size should be

6400 bytes

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CSMA/CD efficiency

Tprop = max prop. time between 2 nodes in LAN ttrans = time to transmit max-size frame

Efficiency goes to 1 as tprop goes to 0 Efficiency goes to 1 as ttrans goes to infinity Much better than ALOHA, but still decentralized, simple, and cheap

transprop tt /51

1efficiency

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Parameters for 10Mbps Ethernet

1 cse401n: computer networks lan address & arp ethernet basics

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