Data link and physical layers - Network and Performance ...fourmaux/ARes/AResC5v62_en_4.pdfOSI data link + physical layers ˘layers underlying TCP/IP Olivier Fourmaux ([email protected])

Ethernet architecturePoint-to-point architecture

Network Architecture (NetArch) 5/5 :Data link and physical layers

Olivier Fourmaux ([email protected])

Version 6.2

Olivier Fourmaux ([email protected]) Network Architecture (NetArch) 5/5 : Data link and physical layers


NetArch: course 5/5 outline

1 Ethernet architecturetechnologyhardwareIEEE standards

2 Point-to-point architectureHDLCPPP: basic mechanismsPPP: usages



Underlying technologies and OSI model

Presentation

Application

Session

Transport

Network

Data link

Physical

7

6

5

4

3

2

1

Interface Interface

Host A

APDU

Presentation

Application

Session

Transport

Network

Data link

Physical

Host B

Network Network

Data link Data link

Physical Physical

Router Router

Application protocol

Presentation protocol

Transport protocol

Session protocol



OSI: the data link layer

The data link layer carries bits over a medium using a particulartransmission technique. The associated functions are:

framing and error controlsequencing, reliability, and flow controlthree technology types for the data link layer:

point-to-pointmultipoint without broadcast à U.E. RTELmultipoint with broadcast (shared medium)

Routing process

Router

Frames here

Packets here

2 2 2 3

2

3

Data link layer process

Transmission line to a router

Data link protocol



OSI: physical layer

The physical layer is associated with signal transmission:specification of the medium and the signaling

bit encoding, baseband or passband transmissioncharacteristics of the electrical, optical, radio, signals. . .characteristics of the media:

impedance of electrical cables, attenuation, max. length. . .multimode, monomode fiber optics. . .connector shapes, conduit colors. . .

Copper core

Insulating material

Braided outer conductor

Protective plastic covering

erehpsonoI

Earth's surface Earth's surface

(a) (b)

Ground wave

Jacket (plastic) Core Cladding

Sheath Jacket

Cladding (glass)

Core (glass)

(a) (b)



Underlying technologies and TCP/IP

Ethernet

HTTP

TCP UDP

PPP

DNSSNMP

ATM MPLS

SMTP

IP

OSI data link + physical layers ∼ layers underlying TCP/IP



technologyhardwareIEEE standards







Ethernet: introduction

Until the 1990’s: many LAN technologies

Ethernet, Token Ring, FDDI, ATM...

Today (for wired networks): LAN = Ethernet

Ethernet is to local networks what Internet is toplanetary-scale networks

why?

appeared first (mid-1970’s)simpledecentralizedautoconfigurable

à economical and adaptable




Ethernet: where?

...

... ... ...

... ...

......




Ethernet: various flavors

Different types of Ethernet. . .

two topologies:

bus, star

various media:

coaxial cables, twisted pair, fiber optics

wide range of bandwidths:

10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps, 40 Gbps, 100 Gbps

. . . but all on the same basis:

LAN addresses

frame structure

connectionless service with no reliability guarantees

generally, baseband (digital) transmission




Ethernet: LAN addresses

Interface addresses (6 bytes, hexadecimal notation) à identifier

also called:

Ethernet addressesphysical addressesMAC (Media Access Control) addresses

flat address space, administered by the IEEE

h

00:10:A4:87:BF:1A

00:04:76:21:25:EA

00:04:76:21:27:8E

00:20:ED:87:FD:E6

00:10:A4:86:2D:0B

00:78:77:0A:91:84

00:19:78:27:94:1A




Ethernet: frame structure (1)

Delimiting the frame:beginning

preambledetecting the sending of a framesynchronization to the sender’s clockmark the start of the frame (8th byte)

endno current during the interframe gap: IFS (Inter FrameSpacing)

8 octets 6 octets 6 octets 2 46 − 1500 max octets 4 octets

101010101010....1011Préambule Adresse

sourceType Données CRC

Adressedestination

3 octets 3 octets

Codeunique

Codeconstructeur

1: Diffusion ou multipoint0: Adresse individuelle




Ethernet: frame structure (2)

Destination and source addresses

an interface only accepts frames that are addressed to it

Ethernet type (Ethertype) > 1500:

0x0800 = DoD Internet 0x0806 = ARP

0x0801 = X.75 Internet 0x8035 = RAP

0x0802 = NBS Internet 0x8098 = Appletalk

0x0803 = ECMA Internet 0x86DD = IPv6...

Data:

MTU (Maximum Transfer Unit) = 1500 bytes

minimum size = 46 bytes plus with, if necessary, the additionof padding bytes (sent at the network layer)

CRC-32 (Cyclic Redundancy Check), generator polynomial:G(x) = x32 +x26 +x23 +x22 +x16 +x12 +x11 +x10 +x8 +x7 +x5 +x4 +x2 +x +1




Ethernet: service

Service offered to the network layer:

connectionlessdatagram service (just like IP or UDP)no setup exchange prior to sending data

no reliability guaranteeserror control (and discard without warning)no error correctionno acknowledgements

the sender does not know if the data has been deliveredno flow control (except in switches)no receiver windowloss detection at the higher layers (e.g., TCP)

à simplicity




Ethernet: transmission

Baseband

direct sending of digital signals

Manchester coding for 10 Mbps bandwidth

20 Mhz bandwidth required (1B/2B)

for higher bandwidths, 4B/5B (FDDI), 8B/10B (FiberChannel), 64B/66B and various encapsulations (FR, ATM,SONET...)

Bit stream 1 0 0 0 0 1 0 1 1 1 1

Binary encoding

Manchester encoding

(a)

(b)




Medium access protocole

Direct sender-receiver connections à see following slidesShared links:

channel sharing protocolsstatic bandwidth share (R/N per sender)

frequency division multiplexing (FDM)time division multiplexing (TDM)

resource sharing protocols (taking-turns protocols)deterministinc bandwidth share (R per sender)

pollingtoken-passing

random access protocolsstatistical bandwidth sharing (R per sender) but collisions arepossible

ALOHACSMA à Ethernet




ALOHA

University of Hawaii, 1970

datagram based radio networkrandom access protocol, completely decentralizedif collision, retransmission after a random wait time

User

Time

A

B

C

D

E

picture from Tanenbaum A. S. Computer Networks 3rd edition




CSMA

Improvement over the random approach

carrier sensing: CSMA (Carrier Sense Multiple Access)à wait before sendingcollision detection: CSMA/CD (CSMA with CollisionDetection) à retransmissions

example with a mini frame size of 64 bytesT detection: 64 bytes at 10 Mbps = 512/107 = 51, 2µsecT prop. max: 2 * 2500m at 2.108ms−1 = 25µsec + 8 ∗ trepet

Packet starts at time 0A B A B

Packet almost at B at τ - ∋

Collision at time τ

A B

Noise burst gets back to A at 2τ

A B

(a) (b)

(c) (d)




Ethernet access protocol (1)

How interfaces function:

start sending at any moment: time not discretized

no sending if activity is detected on the channel: CSMA

stop sending if other activity is detected: /CD

wait a (growing) random amount of time beforeretransmission: TBEB (Truncated Binary ExponentialBackoff)




Ethernet access protocol (2)

Protocol steps carried out by interfaces:

1 build and store the frame

2 if activity detected, wait for end of signal

3 wait for 96 bit IFS (without detecting a signal)

4 start transmission

1 if collision is detected

1 stop transmitting2 32 bit jam sequence3 exponential backoff (for the nth consecutive collision) of

int(rand()*2min(10,n))* 512 bits (exponential backoff phase)then return to step 2.

2 if not, continue transmitting until the end




Ethernet: 10Base5

bandwidth: 10 Mbps

topology: extended bus, with hosts connected viatransceivers that are attached to a coaxial cable by a bluecable (51.2µs max between 2 hosts à 4 repeaters + 2500mmax.)

medium: yellow coax., 500m max., and 2 50Ω terminatingresistors




Ethernet: 10Base2

bandwidth: 10 Mbps

topology: extended bus (51.2µs max à 4 repeaters and 925mmax. between 2 stations)

medium: black coaxial cable of 200m (185m max) and 30hosts max per segment, BNC T-Connecters and 50Ωterminating resistors




Ethernet: 10BaseT

bandwidth: 10 Mbpstopology: star based at a hubmedium: twisted pair, 100m max. (UTP3), RJ45connectorsaccess:

half duplex à CSMA/CDmultiple cascading hubs possible (51.2µs max)

full duplex à point-to-point bidirectional simultaneous(without collisions)

activity detection (Link Pulse every 16±8 ms)




Ethernet: 100BaseTX (Fast Ethernet, 1995)

bandwidth: 100 Mbps

topology: hub based star

medium: twisted pair, 100m max (UTP5), RJ45 connectors

coding: 4B/5B (FDDI)access:

half duplex à CSMA/CD, minimum 64 bytes2 hubs can be linked (but 5.12µs max: 210m max)length limits and corporate networks... see switches

full duplex à point-to-point simultaneous bidirectional

activity detection (Fast Link Pulse: 33 pulses/∼16 ms)FLP consists of 16 bits for autonegotiation

detection of possible speeds, modes, and available mechanisms

many variants:100BaseT4: 4 UTP3 twisted pairs (no full duplex)100BaseFX: 2 optical fibers (400m MMF, 20km SMF)




Ethernet: 1000BaseT (Gigabit Ethernet, 1998)

bandwidth: 1000 Mbps (1 Gbps)

topology: hub based star

medium: twisted pair, 100m max., UTP5+ (4 pairs)

coding: 8B/10B (= Fiber Channel 1G)

access:half duplex à CSMA/CD, min. 512 bytes (extend themedium if necessary) à 4.01µs rather than 0.512µs!

2 hubs can be connected (still 210m max)performance? carrier extension plus burst

full duplex à point-to-point simultaneous bidirectional

many variants:

1000BaseCX : 2 shielded twisted pairs (STPs): 25m1000BaseSX : 850nm multimode optical fiber (MMF): 500m1000BaseLX : 1300nm MMF and singlemode fiber (SMF): 5km




Ethernet: 10GBaseT (10Gigabit Ethernet, 2002)

bandwidth: 10 Gbps

topology: switch based star

medium:optical fiber:

MMF and SMF (from 65m to 40km max.)multiplex SONET/SDH : OC192 (10Gbit/s)

twisted pair:

100m max. on Cat. 6e (FTP 500MHz), 6a (UTP 500MHz)and 7 (STP 600MHz)

access: full duplex only (no more CSMA/CD)

coding: 64B/66B (= Fiber Channel 10G)




Ethernet: 40/100G (100Gigabit Ethernet, 2009)

bandwidth: 40/100 Gbps

topology: switch based star

medium:optical fiber:

100m on OM3 MMF40km max. on SMFmultiplexed SONET/SDH: OC768 (40Gbit/s)

twisted pair:

a few meters (backplane)

access: only full duplex




Broadcast networks

IP transport:

address resolution

encapsulation format

Multiple access on a shared medium: implicit broadcast

A

B

C




ARP: Address Resolution Protocol

Explicit broadcast (use of a broadcast address)

@IP−A

@IP−B

@IP−C@H−A

@H−C

@H−B

Broadcast @H (@IP−B)?

? ?

?




ARP over Ethernet

ARP messages are transported directly in Ethernet frames:

ARP request: destination address = broadcast(FF:FF:FF:FF:FF:FF), source = requester

ARP response: destination address = requester,source = responder

6 octets 6 octets 2 46 − 1500 max octets 4 octets

Adressesource

Type CRCAdresse

destinationARP

2 octets 2 octets 2 octets

HWtype

Prototype

Plen

1 1 Hlen octets Plen octets Hlen octets Plen octets

Sender HA Target HASender PA Target PAOper−ation

Hlen

1: request2: response

1: ethernet0x0800 : IP

(ethernet=6)(IP=4)




IP over Ethernet

Ethertype > 1500:

0x0800 = DoD Internet

Data:

MTU: maximum IP packet size = 1500 bytes

minimum size = 46 bytes (the IP packet can be smaller)

if necessary, add padding bytes8 octets 6 octets 6 octets 2 46 − 1500 max octets 4 octets

101010101010....1011Préambule

sourceCRC

destinationAdr. MAC Adr. MAC

Entête IP Données IP (bourrage)

08 00










Ethernet hub

physical layer entity (bit level)

multiport repeatera bit arriving on one interface is broadcast to the others

administration through SNMP, RMON...

interconnecting hubs

linearlyhierarchically with a federating hub...




Interconnecting hubs

in a multilevel system (several hubs)

LAN = the entire local network (collision domain)segment = the equipment connected to a hub

benefits:

4 increases connectivity4 increases redundancy (in case hubs break down)8 physical limitations (distance, number of machines. . . )8 reduces the bandwidth per host8 increases collisions (and so reduces overall bandwidth)




Ethernet bridge

link layer entity (frame level)

frame switchingfilters on the basis of destination addressan arriving frame is sent out on the destination port onlyrecord + CSMA/CD (entities without address)

benefits:

4 separation of collision domains4 multi-technology (10Base2 with 100BaseTX...)4 no more physical limitations




Bridge: filtering and forwarding

Filtering

determining whether to accept or reject a frame

Forwarding

choosing the output interface

à table:

LAN address Interface Time

00:10:A4:86:2D:0B 1 09: 32: 5500:04:76:21:27:8E 3 09: 32: 5500:04:76:21:1B:95 3 09: 32: 55... ... ...

algorithm, using the table:when a frame with @LAN dest arrives on If x, the table showsoutput port IF y:

if IF x = IF y then the frame comes from the destinationsegment à filterotherwise, send the frame on IF y à forwarding




Transparent bridging

Autonomous learning

table creation algorithm:1 table initially empty2 upon frame reception, insertion into the table of:

1 its @LAN source

2 its arrival interface3 its arrival time

3 the entry remains valid for a limited time only

note:

if @LAN dest is not in the table then broadcast (copy to theother interfaces, record + CMSA/CD)such bridges are called:

plug and play (no configuration required)transparent (they do not themselves have addresses)




Bridge: redundancy

multiple paths

4 backup path4 autoconfiguration8 loops (duplicate frames)

spanning tree protocol (STP)




Bridges: STP (Spanning Tree Protocol)

bridged LAN with redundancy ∼ graph (nodes = bridges)graph without loop = tree à construction of a spanning tree

each bridge has an ID number: the smallest becomes the rootexchange of BPDUs <id root, dst root, id snd,

num port>

disable the ports that are not on the shortest path to the root




Bridge or router

Bridge (layer 2): forwards frames

4 autoconfigurable4 forwarding performance8 all frames follow the same tree (SPF)8 global broadcasting

limited network size (→100 machines)simple functionality

Router (layer 3): forwards packets

4 no loops (TTL-limited forwarding)4 calculation of the best path (routing)8 manual configuration8 slower treatment of each PDU

large networks (1000→ machines)“intelligent” functionality: traffic isolation,filtering. . .




Ethernet switch

high performance bridge (layer 2)

many interfaces (∼ hub)high aggregate bandwidth à switching fabric

multi-bandwidth

10 Mbps, 100 Mbps, 1 Gbps...

full duplexpossible to avoid CSMA/CD (∼ point-to-point links)

100

1001000

10Workgroup Switch

Catalyst

CiscoSystems




“Store and forward” switching

Storing prior to forwarding the frame

∼ bridge functionality

storage of the complete frame (and CRC-32 calculation)

minimum latency LF/Ri (LF frame size, Ri output bandwidth)

Workgroup Switch

Catalyst

CiscoSystems




“Cut-through” switching

Direct transmission

∼ hub functionality

send as soon as the output buffer is empty

minimum latency = time to read the destination address

example: 100Mbps, 1518 frame à gain ∼ 120µs

no integrity check on the frame (CRC-32)

Workgroup Switch

Catalyst

CiscoSystems




Ethernet hierarchy (1)

Router

WAN




Ethernet hierarchy (2)

Additional functionality associated with full duplex:

flow control (back pressure)

avoid frame loss due to switch overloaddirect a PAUSE frame to the sender (IEEE 802.3x)

link aggregation (Ethernet trunk, NIC teaming, portchannel, port teaming, port trunking, link bundling,EtherChannel, Multi-link trunking, NIC bonding, networkbonding, Network Fault Tolerance...)

parallel use of several cables/ports to increase bandwidth andredundancysame switches, identical link bandwidthslate standardisation (IEEE 802.3ad in 2000, 802.1ax in 2008with the Link Aggregation Control Protocol)

virtualisation ...




VLAN (1)

Generic cabling infrastructureà logical LAN configuration: Virtual LAN

Twisted pairto a hub

Office

Switch

Hub

Hub

Corridor

Cableduct





VLAN (2)

Configuration table in bridges and switches

assigns VLANsby portby LAN addressby protocol or layer 3 network

several VLANs per port for transit (Virtual STP)

E F G H

A B C D

I

J

K

L

M

N

O

E F G H

A B C D

I

J

K

L

M

N

OB1 B2 S1 S2

GW GW

GW

GW

W

G

G

G

G

W

WWWG

G W G G

W

WG

31

4

2











IEEE 802 standardization

Definition de l’architecture de standardisation:80

2 O

verv

iew

& A

rchi

tect

ure

802.3MAC

802.3

802.5MAC

802.5

802.11MAC

802.11

802.16MAC MAC

CSMA/CD Token ring WLAN WPAN WMAN

802.15

802.15

802.16PHY PHY PHY PHY PHY

802.1 Bridging

802.2 LLC

802.

10 S

ecur

ity

802.

1 M

anag

emen

t




802.1x

Some interesting standards:

802.1d MAC Bridges

STP...

802.1f MIB IEEE 802

802.1g MAC distant bridging

LAN interconnection of LAN with WAN technologies

802.1h MAC Bridging of Ethernet V2 in IEEE 802 LAN

802.1q Virtual Bridged LAN...




802.1q (1)

Adds a VLAN identifier to the frame:

Switching doneusing tags

Legacyframe

Taggedframe

Taggedframe

VLAN−awareswitch

VLAN−awareend domain

Legacyend domain

LegacyPC

VLAN−awarecore domain

VLAN−aware

PCpicture from Tanenbaum A. S. Computer Networks 4rd edition




802.1q (2)

Evolution of the Ethernet frame structure: 1522 bytes max!

only 802.1q enabled equipment exchanges the new frames

possibility to identify 4096 VLANs

3 pirority bits

802.3 Data PadCheck−

sumDestination

addressSourceaddress

802.1Q Data Pad

VLAN protocolID (0x8100)

Check−sum

Destinationaddress

Sourceaddress

Length

LengthTag

VLAN IdentifierCFI

Pri





802.2 bridging

Subdivision into two sublayers of the OSI link layer

LLC (Logical Link Control) sublayer

MAC (Medium Access Control) sublayer

à allows direct bridging of different IEEE 802 networks:Host A

Network

LLC

MAC

Physical

Pkt

Pkt

Pkt802.3

802.3 Pkt

Host B

BridgePkt

Pkt

Pkt802.4

802.4 Pkt802.3

802.3 Pkt

Pkt

802.3 Pkt

802.4 Pkt

802.4 Pkt

802.4 PktPkt

CSMA/CD LAN Token bus LANpicture from Tanenbaum A. S. Computer Networks 3rd edition




IEEE 802.2

Datalink

layer

Network layer

Physical layer Network

LLC

MAC

LLC

LLC

Packet

Packet

Packet MAC MAC

802.3 Length Data PadCheck−

sumDestination

addressSourceaddress

802.11 Seq. DataCheck−

sumAddress

1Address

2Address

3Address

4Framecontrol

Dur−ation

802.16 DataCheck−

sumLength Connection ID Header

CRCType EK0 E

CCI





IEEE 802.3: CSMA/CD

SNAP/LLC encapsulation:

8 octets 6 octets 6 octets 2 46 − 1500 max octets 4 octets

101010101010....1011Préambule

sourceCRC

destination

Données

Code

3 octets

AA AA

SNAP

DSAP SSAP

LLC

Type

2 octets1 1 1

Ctrl Organisation (Eth)03 00 00 00

TailleAdr. MAC Adr. MAC

Sous couche MAC

Sous couche LLC




IEEE 802.3: naming

802.3 1985 10Base5 thick coax 50Ω

802.3a 1988 10Base2 thin coax 50Ω

802.3b 1985 10Broad36 coaxial TV 75Ω

802.3i 1990 10BaseT on 2 UTP3 pairs

802.3j 1993 10BaseF on MM/SM fibers

802.3u 1995 100BaseT4 on 4 UTP3 pairs

802.3x/y 1997 100BaseT2 on 2 UTP5 pairs802.3z 1998 1000BaseX GBIC module

802.3ab 1999 1000BaseT on 4 UTP5 pairs

802.3ac 1998 VLAN for 802.3

802.3ad 2000 Trunking

802.3ae 2002 10GBaseX on MM/SM fibers

802.3af 2003 Power-over-Ethernet

802.3ah 2004 Ethernet in the First Mile

802.3an 2006 10GBASE-T on FTP6e or UTP7

802.3ap 2007 Backplane Ethernet

802.3av 2009 10 Gbits/s PHY EPON

802.3ba 2009 Ethernet 40 and 100 Gbits/s




IEEE 802.5: Token RingRing

interfaceStation

Unidirectional ring

1 bit delay

Ring interface

To station

From station

To station

From station

(a) (b) (c)

Destination address

Source address

ChecksumData

No limit 4Bytes 2 or 6 2 or 6

Starting delimiter

Access controlFrame control Ending delimiter

Frame status

SD AC ED FS

1 1

FC

1 1 1

SD AC ED

1 1 1

(a)

(b)




IEEE802.11: WLAN (Wireless Ethernet)

service zone: cell or BSS (Basic Service Set)

wireless stationsbase station or AP (Access Point) acting as an 802 bridge

MAC: CSMA/CA

...

BSS

AP



HDLCPPP: basic mechanismsPPP: usages







Direct communication between two entities

Basic functionalities of point-to-point communications:

framing

functionalities similar to those found in the transport layer arealso possible (except congestion control):

error controlflow controlsequencing (numbering)sliding windowreliability (acknowledgements and retransmissions)

For transporting data:

no address resolution

encapsulation format




Point-to-point data link layer

Point-to-point service defined in the OSI data link layer

Characteristics:homogeneous interface technology

Routing process

Router

Frames here

Packets here

2 2 2 3

2

3

Data link layer process

Transmission line to a router

Data link protocol

various transmission unitsbits, bytes, cells...

various neighboring layerscan have layers between this layer and the physical layercan have multiple active elements (multiplexers, modems,bridges, switches, routers, application gateways. . . )




Point-to-point data link: where?

...

... ... ...

... ...

......










HDLC: protocol family

Most protocols from the data link layer are related to HDLC:

SDLC (Synchronous Data Link Control) from IBM for SNA

ADCCP (Advance Data Communication Control Procedure)ANSI version of SDLC

HDLC (High-level Data Link Control) ISO version of SDLC

LAP (Link Access Procedure) ITU version of HDLC

LAP-B for X25LAP-D for ISDNLAP-F for Frame Relay ...

PPP (Point-to-Point Protocol) IETF standard

These protocols function over a wide variety of physical mediathat enable transmission of bits (or other signals) between twohosts.




HDLC: structure

Breakdown into bits or bytes

a (flag) used for delimitation à (01111110)2

series of bits: 01111110 (bit stuffing protection)byte of value 0x7E (byte stuffing protection)

3 frame types (control):

Information: data transmission with a sliding window (max.7 unacknowledged frames)

Supervisory: flow control, non-piggybacked ACK, NACK,request for selective retransmission. . .

Unumbered: for internal control in the data link layer

0 1 1 1 1 1 1 0 0 1 1 1 1 1 1 0Address Control Data Checksum

Bits 8 8 8 > 0 16 8





IP over serial line

SLIP (Serial Line Internet Protocol)

character oriented, delimited with the character 0xC0basic: no control, no negotiation

PPP ...

PC

Routing process

Router

Modems

Internet provider's office

User's home

Client process using TCP/IP

Modem

Dial-up telephone line

TCP/IP connection using SLIP or PPP











PPP: (Point-to-Point Protocol, RFC 1661)

General protocol à many features

multi-protocol

transports a variety of layer 3 traffic, not just IPoperates over a variety of technologies, not just serial lines

negotiation

adaptation to the medium (detection and correction of errors,protection of modems’ control codes, header compression. . . )automatic client configuration

Couche réseau (3)

PAP

CHAPCouche liaison (2)

Compression d’entete

PPP

IP

Adaptation au support

Couche physique (1) ou assimilée...

authentification données négociation

LCP

NCP




PPP: protocol structure

Simple encapsulation: adds 2 bytes (compressible to 1)Protocol: indicates the type of information transported

LCP: control protocol at the link layernegotation of parameters of the underlying medium(compression, frame size. . . )

PAP and CHAP: authentication protocolsNCP: control protocol at the network layer

negotation of parameters of the protocol being transported(addressing. . . ) à specific to each network layer protocol thatis supported

IP, AppleTalk, IPX, IPv6...Payload: contains the frame data

MRU (Maximum Receive Unit) negotiable (default: 1500 B)padding if the underlying medium requires it

Flag01111110

Flag01111110

Address11111111

ProtocolControl00000011

Payload Checksum

Bytes 1 1 2 (1)1 Variable 2 1




PPP: protocols carried

Valeur Description

0x0001 Padding protocol

0x0021 IP

0x0029 AppelTalk

0x002B IPX

0x002D/2F TCP/IP w/ Van Jacobson header compression

0x0057 IPv6

0x0281 MPLS

0x8021 IPCP: IP configuration

0x8029 ATCP: AppleTalk configuration

0x802B IPXCP: IPX configuration

0x8057 IPV6CP: IPv6 configuration

0x8281 MPLS configuration

0xC021 LCP: Link Control Protocol

0xC023 PAP: Password Authentification Protocol

0xC025 LQR: Link Quality Report

0xC223 CHAP: Challenge Handshake Authentification Protocol




PPP: classical encapsulation

Similar to an HDLC frame for a bytestream:

a binary flag: 0111 1110 (0x7E)address (1 byte): 1111 1111 (0xFF, broadcast)

there is only one receiver (point-to-point)control (1 octet):

reliable link à no control: 0000 0011 (0x03, UI frame, seeRFC 1662)

optimisation: field suppression Address and Control

unreliable link à sequencing control (see HDLC, UA andSABME frames, see RFC 1663)

Protocol and Payload: PPP encapsulation

Checksum (2 bytes): CRC 16 bits

another binary flag: 0111 1110

Flag01111110

Flag01111110

Address11111111

ProtocolControl00000011

Payload Checksum

Bytes 1 1 2 (1)1 Variable 2 1




PPP: flag protection

Deux types de liaison point-a-point:

synchronous (bits: the flag is the sequence 0111 1110)bit stuffing protection

a 0 bit is added after every five 1 bits01111110111110 à 0111110101111100

asynchronous (bytes, the flag has the value 0x7E)byte stuffing protection

escape byte with value 0x7D:0x7E à 0x7D 0x5E

0x7D à 0x7D 0x5D

special byte values for active control of the asynchronousconnection (correspond to ASCII codes < 32), same protectionprinciple:

0x11 (XON: restart the transfer) à 0x7D 0x31

0x13 (XOFF: stop the transfer) à 0x7D 0x33

à The available bandwidth is variable!




PPP: negotiation

Structure of a typical PPP negotiation frame:

0 7 15 bit 31code identifier length

data

code: indicates the type of negotiation

identifier: associates queries with responses

length: total frame size, with the LCP header

allows one to eliminate eventual padding bytes

data: parameters of the negotiation

Negotiation begins at the start of the connection




PPP: negotiation frames

Val. Code Description LCP NCP

1 Configure-Request change default values [4] [4]

2 Configure-Ack receiver accepts all changes [4] [4]

3 Configure-Nak values refused, others proposed [4] [4]

4 Configure-Reject values nonnegotiable [4] [4]

5 Terminate-Request a host wishes to terminate [4] [4]

6 Terminate-Ack confirm termination [4] [4]

7 Code-Reject code unknown [4] [4]

8 Protocol-Reject protocol unknown [4]

9 Echo-Request request link state test [4]

10 Echo-Reply link state test reply [4]

11 Discard-Request silently discarded by the receiver [4]




LCP (Link Control Protocol)

Oversee the state of the link

PPP frame protocol field: 0xC021

initial negotiation when connection is opened

TLV style option definition

see RFC 1570 and RFC 1661format:

1 byte 1 byte (Length - 2) bytes

Type Length Value




LCP: option types

Val. Code Length Description

1 MRU 4 maximum size of received frames

2 ACCM 6 table of transcoding values

3 authentification 4 type of auth. protocol

4 qualite 6 type of QoS mgt. protocol

5 Magic Number 6 negotiation of this value

7 compression protocol 2 1 byte protocol field

8 compression address 2 suppression of these fieldsand control

10 self-describing padding 3 padding parameter that can beautomatically eliminated

13 callback 3+ ...




PAP (Password Authentication Protocol, RFC 1334)

Once the connection is established and the LCP parametersnegotiatedà identity verification

protocol field of the PPP frame: 0xC023cleartext transmission of the identifier and password4 types of negotiation frame (Configure-Request,Configure-Ack, Configure-Nak ou Configure-Reject)format identical to LCP, code value:

1: authentication request:1 B (IdLgth) bytes 1 B (PassLgth) bytes

IdLgth Identifier PassLgth Password

2: positive acknowledgement:1 B (IdLgth) bytes

IdLgth Client message

3: negative acknowledgement (retransmission required):1 B (IdLgth) bytes

IdLgth Client message




CHAP (CHallenge Authentication Protocol, RFC 1334)

After LCP negotiation and during communicationà identity verification

PPP frame protocol field: 0xC223

the 2 endpoints share a secret key

4 types of negotiation frames (Configure-Request,Configure-Ack, Configure-Nak or Configure-Reject)format identical to LCP, value of the code field:

1: challenge (binary sequence sent):1 B (FieldLgth) bytes

FieldLgth binary sequence

2: response (sequence returned encrypted with the key

à certificate):1 B (CCLgth) bytes

CCLgth encrypted binary sequence

3: success: the received encrypted sequence and the onecalculated locally are identical4: failure (retransmission necessary)




RADIUS (Remote Auth. Dial-In User Service, RFC 2865)

Centralization of the information regarding a user:

AAA functions: Authentication, Authorization, andAccounting

identity verificationknowing one’s rights and access configurationtracking the user’s actions

client/server modelclient can connect to different access points of an ISP

client: access point of an ISP (PPP endpoint, or other proto.)server: supports a database of ISP users

ADSL

PPPoA PPP

CHAPPAP

RTC

Serveur RADIUS

FAI

10

1010




RADIUS: message

Connectionless service (UDP port 1812)

reliability handled at the application layer

format:

0 7 16 bit 31

code identifier length

authentifierdigest = MD5(code+id.+lgth.+auth req+param+secret)

parameters

typical exchange:Access-Request client message

username, encrypted passwordIP address of the access point, UDP portsession type (PPP, rlogin, telnet...)

Access-Accept RADIUS server resonselist of session attributes (address, servers. . . )

Access-Reject RADIUS server responsethe user is not in the database, or does not have access rights




NCP (Network Control Protocol)

After link configuration (LCP) and optional authentication (PAPor CHAP), configuration of the layer 3 protocols

one NCP per protocol that is carried:

IPCP for IPv4 configuration (RFC 1332)IPV6CP for IPv6 configuration (RFC 2472)ATCP for AppleTalk configuration (RFC 1378)IPXCP for IPX configuration (RFC 1552)OSINLCP for OSI protocol configuration (RFC 1377)...




IPCP (Internet Protocol Control Protocol)

PPP frame protocol field: 0x8021

4 types of negotiation frames (Configure-Request,Configure-Ack, Configure-Nak or Configure-Reject)

format identical to LCP, value of code field:2: header compression

2 bytes for the type of compression (0x002d for VanJacobson; 0x0061 for extended, RFC 2507; 0x0003 forROHC, RObust Header Compression, RFC 3241)1 byte for the max number of compressed connections1 byte to indicate the presence of a connection number

3: 4 byte client IP address4: permanent IP address (home address)129: primary DNS server IP address130: primary NBNS server IP address131: secondary DNS server IP address132: secondary NBNS server IP address




TCP/IP header compression

PPP must make efficient use of low bandwidth links

interactive TCP/IP connection (telnet...)

Nagle’s algorithmlarge headersexample:

0020 ea 14 81 cf 00 15 52 40 18 65 52 65 10 0e 50 10

0010 00 28 b5 8f 40 00 40 06 0d be 84 e3 3d 7a cb 10

trame 1 (A−>B)

trame 2 (A−>B)

0000 45 00

0000 45 00

0010 00 29 b5 8e 40 00 40 06 0f 78 84 e3 3d 7a cb 10

0020 ea 14 81 cf 00 15 52 40 18 64 52 65 10 0d 50 18

0030 ff ff bb 7b 00 00 64

0030 ff ff aa 5f 00 00




Differences between two segments

32 bits (4 octets)

D F

M F

HlenVer TOS Packet Lenght

Identifier Frag. offset

TTL Protocol Header checksum

IP source address

IP destination address

Destination portSource port

Sequence number

Acknowledgment number

Checksum Urgent data ptr

Rcv window sizeHlen

P S H

F I N

S Y N

R S T

U R G

A C K




Van Jacobson algorithm

TCP/IP header compression algorithm (RFC 1144)

classic headers preserved for SYN, RST, and FIN (protocolfield value 0x0021)then, compression:

entire packet sent with a connection identifier (0x002F):for synchronization (first complete packet)for negative ACK or sequence numbers (error)

difference between two headers (0x002D):connection identifierstate maintained at each endpointonly the fields that have changed are sentthe difference is usually encoded with one byte

SYNFINRST

Emetteur Récepteur

inef.

Comp.

0x002D

UDP/IP

TCP/IP

UDP/IP

TCP/IP 0x002F

0x0021




Compressed IP header

données

8 bits

C I P S A W U

delta fenetredelta acquittement

id. connexion

pointeur urgent

delta séquencedelta id. IP

checksum TCP

Only the first byte and theTCP checksum are required(3 B min.)

1st byte: presence of fieldsC bit: connection ID present

not sent if same as previous

U bit: copyW bit: window delta

negative two’s complement

S/A bits: seq./acq. delta

no negative values

I bit: IP ID delta

absent = +1

P bit: copy PUSH TCP bit

deltas encoded in 1 to 2 B

1 byte:0x01 to 0xFF3 B: 0x000100 to 0x00FFFF

TCP checksum: copy




Error detection

TCP checsum used to validate the reconstituted packet:

checksum TCPnon valide

Hôte A Hôte B

RTOrejet

S=10

S=10

S=12

S=17

S=15

S=15

S=12

S reconstitué

002F| S=12......

002D|S+3

002D|S+3

002D|S+2

002D|S+2










POS (Packet Over SONET)

PPP Over SONET/SDH (RFC 2615)

PPP initially for low bandwidth PSTN links

also adapted to telecom high bandwidth linkshierarchy of SONET/SDH multiplexing

OC-3c/STM-1 (155 Mbps)OC-12c/STM-4c (622 Mbps)OC-48c/STM-16c (2.5 Gbps)OC-192c/STM-64c (10 Gbps)OC-768c/STM-256c (40 Gbps)

PPP over synchronous byte-based links

∼ byte-oriented serial connections

goal: closeness to the fiberPOS simplifies the IP/ATM/SONET approach

MPLS/POS more flexible (Traffic Eng.)




POS (2)

POS

ATM




PPPoA (PPP Over ATM/AAL5, RFC 2364)

Telecom operators’ last mile is usually ATM

Use of AAL 5 ATM connections

no more HDLC framingadaptation of CPCS PDU AAL 5 frames

padding (multiples of 48 bytes)

two RFC 1483 encapsulations:VC-multiplexed PPP

the endpoints know that they are transporting PPP

LLC encapsulated PPP

protocol identification for each PDU




PPPoA: encapsulations

VCMUX

Protocol Identifier(8 or 16 bits)

...PPP information field PPP payload

...

PAD ( 0 - 47 bytes)

CPCS-UU (1 byte)CPI (1 byte)

Length (2 bytes) CPCS-PDUCRC (4 bytes) Trailer

SNAP/LLC

Destination SAP (0xFE)Source SAP (0xFE) LLC header

Frame Type = UI (0x03)

NLPID = PPP (0xCF)

Protocol Identifier(8 or 16 bits)

...PPP information field PPP payload

...

PAD ( 0 - 47 bytes)

CPCS-UU (1 byte)CPI (1 byte)

Length (2 bytes) CPCS-PDUCRC (4 bytes) Trailer




PPPoA: remarks

Avantages:

dissociates the ADSL/ATM provider from the ISPper-session authentication (PAP and CHAP)user supervision (RADIUS)

per-session billing of usersoverbooking and timed disconnections

attribution of an IP address to the clientsecuring access without ATM-level management

ATM signaling too complex: use of PVCVPNs handled by PPP tunnels (no end-to-end PVC)

adaptable to network evolutionaggregation routers (high density of PPP terminations)

Disadvantages:

one connection per PVCoverall coplexity of the solution (IP, PPP, AAA, ATM...)1 IP address à NPAT limits the applications




PPPoA over ADSL

Client

Multiplexer

OF OF OF OF

DSL Access

ATMATM ATMATM

PPP

IP

TCP

HTTP

IP

HTTP

TCP

IP

PPP

TelTel

?

? ?

?

Server

Telephone Company

ATM Switch

modem DSL

PPPoA

ISP

Internet

1 0




PPPoE (PPP Over Ethernet, RFC 2516)

Point-to-point over a shared link:

Ethernet has self-configuration: ARP, DHCP. . .... but no remote handling, nor AAA

Putting in place a point-to-point connection over EthernetEthertype values

0x8863 for discovery frames0x8864 for data frames

payload format for these Ethernet phase:0 4 7 16 bit 31

Ver Type Code Session id

Length

PPPoE payload...

Ver and Type = 0x01Code = 0x00 (data) and ... (discovery)Session id = flow identifier (with MAC addr.)Length = data length (remove padding)




PPPoE: discovery messages

Encapsulated in PPPoE frames (Ethertype = 0x8863)

Code field:

0x09: PADI (PPPoE Active Discovery Initiation) à broadcast0x07: PADO (PPPoE Active Discovery Offer) à proposition(with Session id)0x19: PADR (PPPoE Active Discovery Request) à selection0x65: PADS (PPPoE Active Discovery Session-confirmation)0xA7: PADT (PPPoE Active Discovery Terminate)

champ PPPoE payload (TLV avec caracteres codees UTF-8):0 16 bit 31

Tag type Tag length

Tag value ...

IRC name, name of last mile company, session identifier,validation cookie, error type




PPPoE: remarks

Avantages:

similar to those of PPPoAper-session authentification (PAP and CHAP)

in a LAN-type network type

user monitoring (RADIUS)billing session usersoverbooking and connections timing out

uuser without direct access ATM (briding)

many connections per PVCgiving an IP address to the client

maintain the point-to-point model on a shared medium

Drawbacks:

LAN technology subject to broadcast bursts

solution complexity (IP, PPP, AAA, ATM, LAN...)

1 IP address à NPAT still limits applications




PPPoE over ADSL

OFOF

DSL AccessMultiplexer

Client

ATMATM ATM

Brigde /

ATM

OFOF

IP

PPP

TCP

HTTP

IP

HTTP

TCP

IP

PPP

Tel Tel

TPTP

Eth Eth/LLC LLC

ISP

SNAP/LLC over ATM

PPPoE

modem DSL

Server

Telephone Company

ATM Switch

?

Internet

??

?

1 0




PPP tunnels

Carrying PPP frames

L2F (Layer 2 Forwarding)

proprietary to Cisco, Northern Telecom, and Shivafrom a Home Gateway to a Network Access Server

PPTP (Point-to-Point Tunneling Protocol)

proprietary to Microsoft, 3Com, Ascend, US Robotics, and ECITelematicsfrom a concentrator to an access server (software)

L2TP (Layer 2 Tunneling Protocol)

IETF standard




L2TP: architecture

Example: ADSL access

the ADSL access provider (AAP) manages the link up to anaccess concentrator (AC)how to reach the Internet service provider (ISP)?

AC at the ISP (serving a single ISP)the AAP manages IP configuration (delegated by the ISP)the ISP has access to each AC (too heavy)

creating of an tunnel from the AC to the ISPcarrying PPP across the network between the AAP and theISP

LAC LNSTelecomInternet

L2TP Acces Concentrator

ISP

L2TP Network Server

PPP L2TP

IP




L2TP (Layer 2 Tunneling Protocol, RFC 2661)

Two communication channels:

Control Channel: exchange of tunnel control messages,with a dedicated reliability and flow control protocolData Channel: PPP frames encapsulated in unreliable,unsecured L2TP messages

telecom networks (ATM, FR...)Internet (UDP port 1702)

+----------------------------+

| PPP Frames |

+----------------------------+ +---------------------------------+

| L2TP Data Messages | | L2TP Control Messages |

+----------------------------+ +---------------------------------+

| L2TP Data Channel (unrel.) | | L2TP Control Channel (reliable) |

+-----------------------------------------------------------------+

| Packet Transport (UDP, FR, ATM, etc.) |

+-----------------------------------------------------------------+




ADSL and L2TP

Brigde /

DSL Access

ATMATM

Client

Multiplexer

OF OF

ATM

TCP

L2TP

PPP

Eth/LLCEth

TP TP

Tel

IP

L2TP

IPTel IP

HTTP

IP

TCP

IP

PPP

LLC

UDP UDP

PPP

HTTP

ISP

Internet

?

?

?

?

?

?

Telephone Company

SNAP/LLC over ATM

PPPoE

modem DSL

Server

?

?

1 0

1 0




VPN (Virtual Private Network)

Virtual links between several entities

inexpensive private networks (shared infrastructure)generally Internet

can carry other protocolscan concatenate several technologies (ATM-TCP/IP. . . )

based on tunnels between different endpoints

no network accessisolated machine

security

IPSEC à U.E. ING


Documents

Data link and physical layers - Network and Performance ...fourmaux/ARes/AResC5v62_en_4.pdfOSI data link + physical layers ˘layers underlying TCP/IP Olivier Fourmaux ([email protected])