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CSE524: Lecture 5

Data-link Layer (Part 2)

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Administrative

• Reading assignment– Chapter 5.5-5.11

– Due Wednesday, Oct. 8

• Homework #2– Due Monday, Oct. 15

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Data-link layer

• Last class– Data-link layer overview

– Data-link layer functions

• This class– Specific data-link layers and devices

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DL: Specific data-link layers and devices

• Specific data-link layers– Ethernet (802.3)

– Token Ring (802.5)

– WiFi (802.11)

– PPP

– ATM

– X.25

– Frame relay

• Specific data-link layer devices– Hubs

– Bridges

– Switches

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DL: Ethernet

“dominant” LAN technology: • First practical local area network, built at Xerox PARC in 70’s• Cheap: $20 for 100Mbs!• Simpler, cheaper than token LANs and ATM• Kept up with speed race: 10, 100, 1000 Mbps

Metcalfe’s Ethernetsketch

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DL: Ethernet

• Carrier sense– Check to see if active transmission

• Collision detect– Sender checks for collision; wait and retry

• Adaptive randomized wait to avoid collisions

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

– Preamble – 8 bytes• 7 bytes with pattern 10101010 followed by one byte with pattern 10101011• Used to synchronize receiver, sender clock rates

– Src/Dst Address – 6 bytes• Globally unique, allocated to manufacturers• All adapters listening receive frame, discard if not destined for itself

– Type – 2 bytes• Indicates higher layer (network) protocol being used, mostly IP these days• http://www.cavebear.com/CaveBear/Ethernet/type.html

– Data – 46 to 1500 bytes– CRC – 4 bytes

• Checked at receiver, dropped if doesn’t match• CRC-32 (x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1)

DL: Ethernet Frame Structure

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DL: Ethernet: uses CSMA/CD

if packetthen { A: sense channel

if idlethen {

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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DL: Ethernet CSMA/CD

Packet?

Sense Carrier

Discard Packet

Send Detect Collision

b=CalcBackoff(); wait(b);

attempts++;

No

Yes

attempts < 16

attempts == 16

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DL: Ethernet Backoff Calculation

• If deterministic delay after collision, collision will occur again in lockstep

• If random delay with fixed mean– Few senders needless waiting

– Too many senders too many collisions

• Exponentially increasing random delay– Infer senders from # of collisions

– More senders increase wait time

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DL: Ethernet’s CSMA/CD (more)

Exponential 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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DL: Ethernet CSMA/CD and Packet Size

• What if two people sent really small packets– How do you find

collision?

– Must have a minimum packet size

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DL: Ethernet Collision Detect & Packet Size

• Min packet length > 2x max prop delay– If A, B are at opposite sides of link, and B starts one link prop

delay after A

• Jam signal– Jam network for 32-48 bits after collision, then stop sending

– Ensures that everyone notices collision

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DL: Propagation delay & packet size

• Propagation delay• 1Km, c in cable = 60% * c in vacuum = 1.8 x 10^8 m/s

– 1000/1.8 x 10^8 ~= 5 x 10^-6 = 5us

– 5us * 3Mbps = 15bits in flight!

• Modern 10Mb Ethernet {– 2.5km=maximum network segment length, 10Mbps

– ~= 12.5us delay

– +introduced repeaters (max 5 segments)

– worst case – 51.2us round trip time!

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DL: Minimum packet size

• Slot time = 51.2us = 512bits in flight– After this amount, sender is guaranteed sole access to link

– 51.2us = slot time for backoff

• What about scaling? 100Mbit, 1Gbit...• Make network smaller?

– Solution for 100BaseT

• Make min pkt size larger?– 512bits @ 1Gbps = 512ns

– 512ns * 1.8 * 10^8 = 92meters

– Gigabit ethernet uses collision extension for small pkts

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DL: Ethernet Problems

• Ethernet unstable at high loads– Peak utilization = 1/e = 37%

• Peak throughput worse with– More hosts – more collisions needed to identify single sender

– Smaller packet sizes – more frequent arbitration

– Longer links – collisions take longer to observe, more wasted bandwidth

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DL: 10Base2 Ethernet

• 10: 10Mbps; 2: under 200 meters max cable length• thin coaxial cable in a bus topology

• repeaters used to connect up to multiple segments• repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

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DL: 10BaseT and 100BaseT Ethernet

• 10/100 Mbps rate; latter called “fast ethernet”

• T stands for Twisted Pair cabling

• Nodes connected to hubs or switches in a “star topology”

• Max distance from node to Hub is 100 meters

• Smart hubs– Disconnect “jabbering adapter

– Managed hubs (monitoring, statistics)

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DL: Gbit Ethernet

• Use standard Ethernet frame format• Allows for point-to-point links and shared broadcast

channels• In shared mode, CSMA/CD is used; short distances

between nodes to be efficient• Uses hubs, called here “Buffered Distributors”• Full-Duplex at 1 Gbps for point-to-point links

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DL: Token Rings

• Packets broadcast around ring• Token “right to send” rotates around ring

– Fair, real-time bandwidth allocation• Every host holds token for limited time

• Higher latency when only one sender

– Higher bandwidth• Point to point links electrically simpler than bus

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DL: Token Passing: IEEE802.5 standard

• 4 Mbps • max token holding time: 10 ms (limits frame length)

• SD, ED mark start, end of packet • AC: access control byte:

– token bit: value 0 means token can be seized, value 1 means data follows FC – priority bits: priority of packet – reservation bits: station can write these bits to prevent stations with lower priority packet from seizing token

after token becomes free

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DL: Token Passing: IEEE802.5 standard

• FC: frame control used for monitoring and maintenance

• source, destination address: 48 bit physical address, as in Ethernet

• data: packet from network layer

• checksum: CRC

• FS: frame status: set by dest., read by sender – set to indicate destination up, frame copied OK from ring

– DLC-level ACKing

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DL: Why Did Ethernet Win?

• Failure modes– Token rings – network unusable– Ethernet – node detached

• Good performance in common case• Volume lower cost higher volume ….• Adaptable

– To higher bandwidths (vs. FDDI)– To switching (vs. ATM)

• Completely distributed, easy to maintain/administer• Easy incremental deployment• Cheap cabling, etc

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DL: IEEE 802.11 Wireless LAN

• Wireless LANs: untethered (often mobile) networking

• IEEE 802.11 standard:– MAC protocol

– unlicensed frequency spectrum: 900Mhz, 2.4Ghz

• Basic Service Set (BSS) (a.k.a. “cell”) contains:

– wireless hosts

– access point (AP): base station

• BSS’s combined to form distribution system (DS)

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DL: Ad Hoc Networks

• Ad hoc network: IEEE 802.11 stations can dynamically form network without AP

• Applications:– “laptop” meeting in conference room, car– interconnection of “personal” devices– battlefield

• IETF MANET (Mobile Ad hoc Networks) working group

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DL: IEEE 802.11 MAC Protocol: CSMA/CA

802.11 CSMA sender- if sense channel idle for DIFS sec. then transmit entire frame (no collision

detection)-if sense channel busy

then backoff (random, exponential)802.11 CSMA receiverif received OK return ACK after SIFS 802.11 CSMA others• NAV: Network Allocation

Vector• 802.11 frame has transmission time field• others (hearing data) defer access for NAV

time units

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DL: IEEE 802.11 MAC Protocol CSMA/CA

• Same as previous mode but with explicit channel reservation

– sender: send short RTS: request to send

– receiver: reply with short CTS: clear to send

• CTS reserves channel for sender, notifying (possibly hidden) stations

• avoid hidden station collisions

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DL: CSMA/CA

• RTS and CTS short:

– collisions less likely, of shorter duration

– end result similar to collision detection

• IEEE 802.11 alows:

– CSMA

– CSMA/CA: reservations

– Polling from AP

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DL: Point to Point Data Link Control

• one sender, one receiver, one link: easier than broadcast link:

– no Media Access Control

– no need for explicit MAC addressing

– e.g., dialup link, ISDN line

• popular point-to-point DLC protocols:

– PPP (point-to-point protocol)

– HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)

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DL: PPP Design Requirements

• http://www.rfc-editor.org/rfc/rfc1548.txt

• packet framing: encapsulation of network-layer datagram in data link frame

– carry network layer data of any network layer protocol (not just IP) at same time

– ability to demultiplex upwards

• bit transparency: must carry any bit pattern in the data field

• error detection: (no correction)

• connection liveness: detect, signal link failure to network layer

• network layer address negotiation: endpoint can learn/configure each other’s network address

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DL: PPP non-requirements

• no error correction/recovery• no flow control• out of order delivery OK • no need to support multipoint links (e.g., polling)

Error recovery, flow control, data re-ordering all relegated to higher layers!|

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DL: PPP Data Frame

• Flag: delimiter (framing)• Address: does nothing (only one option)• Control: does nothing; in the future possible multiple

control fields• Protocol: upper layer protocol to which frame delivered

(eg, PPP-LCP, IP, IPCP, etc) • info: upper layer data being carried• check: cyclic redundancy check for error detection

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DL: Byte stuffing in PPP

• “data transparency” requirement: data field must be allowed to include flag pattern <01111110>– Q: is received <01111110> data or flag?

• Sender: adds (“stuffs”) extra < 01111110> byte after each < 01111110> data byte

• Receiver: – two 01111110 bytes in a row: discard first byte,

continue data reception– single 01111110: flag byte

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DL: Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

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DL: PPP Data Control Protocol

Before exchanging network-layer data, data link peers must

• configure PPP link (max. frame length, authentication)

• learn/configure network layer information

– for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

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DL: PPP and security

• Encrypt data at a layer below network layer– IPsec only works for IP packets

• PP2P, L2F, L2TP– Allow encryption at data-link layer

– Works for all network layer packets of all types using PPP

– Often tunneled using IP packets

– IP(Tunnel) | PP2P/L2F/L2TP | Original Network Packet

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DL/NL: Asynchronous Transfer Mode (ATM)

• 1980s/1990’s standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture

• Take strengths of IP, learn from its shortcomings– Packet switching good

– Packet switching without explicit network-level connections and reservations bad

– Design new network to address emerging applications while allowing for efficient support for non-real-time data applications

• 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 (fixed length packets, called “cells”) using virtual circuits

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DL/NL: ATM architecture (whole 9 yards)

• adaptation layer: only at edge of ATM network

– data segmentation/reassembly

– roughly analagous to Internet transport layer

• ATM layer: “network” layer

– cell switching, routing

• physical layer

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DL/NL: 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

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DL/NL: ATM Adaption Layer (AAL) [more]

Different versions of AAL layers, depending on ATM service class:

• AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation

• AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video

• AAL5: for data (eg, IP datagrams)

AAL PDU

ATM cell

User data

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DL/NL: AAL5

• AAL5: low overhead AAL used to carry IP datagrams– 4 byte cyclic redundancy check

– PAD ensures payload multiple of 48bytes

– large AAL5 data unit to be fragmented into 48-byte ATM cells

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DL/NL: ATM LayerService: transport cells across ATM network• analagous to IP network layer• very different services than IP network layer

NetworkArchitecture

Internet

ATM

ATM

ATM

ATM

ServiceModel

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constantrateguaranteedrateguaranteed minimumnone

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestionfeedback

no (inferredvia loss)nocongestionnocongestionyes

no

Guarantees ?

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DL/NL: ATM Layer: Virtual Circuits

• VC transport: cells carried on VC from source to dest– call setup, teardown for each call before data can flow– each packet carries VC identifier (not destination ID)– every switch on source-dest path maintain “state” for each

passing connection– link,switch resources (bandwidth, buffers) may be allocated

to VC: to get circuit-like perf.

• Permanent VCs (PVCs)– long lasting connections– typically: “permanent” route between to IP routers

• Switched VCs (SVC):– dynamically set up on per-call basis

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DL/NL: ATM VCs

• Advantages of ATM VC approach:

– QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter)

• Drawbacks of ATM VC approach:

– Inefficient support of datagram traffic

– one PVC between each source/dest pair does not scale (N*2 connections needed)

– SVC introduces call setup latency, processing overhead for short lived connections

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DL/NL: ATM Layer: ATM cell

• 5-byte ATM cell header

• 48-byte payload

– Why?: small payload -> short cell-creation delay for digitized voice

– halfway between 32 and 64 (compromise!)

Cell header

Cell format

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DL/NL: ATM cell header

• VCI: virtual channel ID– will change from link to link thru net

• PT: Payload type (e.g. RM cell versus data cell) • CLP: Cell Loss Priority bit

– CLP = 1 implies low priority cell, can be discarded if congestion

• HEC: Header Error Checksum– cyclic redundancy check

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DL: ATM: network or link layer?

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

– ATM is both a network technology and a data-link layer technology

Reality: used to connect IP backbone routers

– “IP over ATM”

– ATM as switched link layer, connecting IP routers

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DL: IP-Over-ATMClassic 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

ATMnetwork

EthernetLANs

EthernetLANs

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DL: IP-Over-ATM

Issues:

• IP datagrams into ATM AAL5 PDUs

• from IP addresses to ATM addresses– just like IP addresses

to 802.3 MAC addresses!

ATMnetwork

EthernetLANs

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DL: Datagram journey in IP-over-ATM

• at ATM network source edge:

– IP layer finds mapping between IP, ATM dest address (using ARP)

– passes datagram to AAL5

– AAL5 encapsulates data, segments to cells, passes to ATM layer

• ATM network: moves cell along VC to destination

• at ATM network destination edge:

– AAL5 reassembles cells into original datagram

– if CRC OK, datgram is passed to IP

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DL: ATM and “IP switching”

• Start with ATM switch– Rip out ATM signaling– Rip out ATM routing– Add IP routing software– Add Flow classifier to map IP flow to underlying ATM virtual circuit

• Operation– Upon arrival of first packet in flow

• Assign IP flow to a virtual circuit ID (label) using IP routing software• Label locally significant

– Subsequent flows• Switched in hardware after flow classified• IP packet forwarding done as label index lookup O(1) versus IP route lookup O(log

n)

– http://pnewman.org/papers/infocom96.pdf– Subsequent versions add label or flow ID distribution to downstream nodes

• Akin to local routing algorithm

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DL: ATM and “IP switching”

• Later generalized as MPLS (multi-protocol label switching)– “Layer 2 ½”

– Not tied to ATM

– Extensible to IPv6

– http://www.rfc-editor.org/rfc/rfc3031.txt

• Used as a tool for traffic engineering– http://www.rfc-editor.org/rfc/rfc2702.txt

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DL: X.25 and Frame Relay

Like ATM:• wide area network technologies • virtual circuit oriented • origins in telephony world• can be used to carry IP datagrams

– can thus be viewed as Link Layers by IP protocol

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DL: X.25

• X.25 builds VC between source and destination for each user connection

• Per-hop control along path

– error control (with retransmissions) on each hop using LAP-B

• variant of the HDLC protocol

• developed when bit error rates over long-haul copper links were orders of magnitude higher

– per-hop flow control using credits

• congestion arising at intermediate node propagates to previous node on path

• back to source via back pressure

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DL: IP versus X.25

• X.25: reliable in-sequence end-end delivery from end-to-end– “intelligence in the network”

– built for dumb terminals accessing mainframes

• IP: unreliable, out-of-sequence end-end delivery– “intelligence in the endpoints”

• gigabit routers: limited processing possible• 2000: IP wins

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DL: Frame Relay

• Designed in late ‘80s, widely deployed in the ‘90s• Frame relay service:

– no error control– no flow control– End-to-end congestion control

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DL: Frame Relay (more)

• Designed to interconnect corporate customer LANs– typically permanent VC’s: “pipe” carrying

aggregate traffic between two routers

– switched VC’s: as in ATM• corporate customer leases FR service from public

Frame Relay network (eg, Sprint, ATT)

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DL: Frame Relay (more)

• Flag bits, 01111110, delimit frame

• address:

– 10 bit VC ID field

– 3 congestion control bits

• FECN: forward explicit congestion notification (frame experienced congestion on path)

• BECN: congestion on reverse path

• DE: discard eligibility

– Precursor to IP DiffServ and ECN

addressflags data CRC flags

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DL: Frame Relay -VC Rate Control

• Committed Information Rate (CIR)– defined, “guaranteed” for each VC– negotiated at VC set up time– customer pays based on CIR

• DE bit: Discard Eligibility bit – Edge FR switch measures traffic rate for each VC; marks DE

bit– DE = 0: high priority, rate compliant frame; deliver at “all

costs”– DE = 1: low priority, eligible for discard when congestion– Precursor to IP DiffServ– Can be used to support higher layer QoS mechanisms

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DL: Interconnecting LANs

Q: Why not just one big LAN? • Limited amount of supportable traffic: on single LAN,

all stations must share bandwidth • limited length: 802.3 specifies maximum cable length • large “collision domain” (can collide with many

stations)• limited number of stations: 802.5 have token passing

delays at each station

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DL: Hubs

• Effectively a physical layer device– Multi-port repeater

– Repeater operating at bit level

– Repeat received bits on one interface to all other interfaces

• Hubs can be arranged in a hierarchy (or multi-tier design), with backbone hub at its top

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DL: Hubs (more)

• Each connected LAN referred to as LAN segment

• Hubs do not isolate collision domains: node may collide with any node residing at any segment in LAN

• Hub Advantages:

– simple, inexpensive device

– Multi-tier provides graceful degradation: portions of the LAN continue to operate if one hub malfunctions

– extends maximum distance between node pairs (100m per Hub)

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DL: Hub limitations

• single collision domain results in no increase in max throughput– multi-tier throughput same as single segment

throughput• individual LAN restrictions pose limits on number of

nodes in same collision domain and on total allowed geographical coverage

• cannot connect different Ethernet types (e.g., 10BaseT and 100baseT)

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DL: Bridges

• Link Layer devices: operate on Ethernet frames, examining frame header and selectively forwarding frame based on its destination

• Bridge isolates collision domains since it buffers frames• When frame is to be forwarded on segment, bridge uses

CSMA/CD to access segment and transmit

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DL: Bridges (more)

• Bridge advantages:– Isolates collision domains resulting in higher total max

throughput, and does not limit the number of nodes nor geographical coverage

– Can connect different type Ethernet since it is a store and forward device

– Transparent: no need for any change to hosts LAN adapters

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DL: Bridges: frame filtering, forwarding• Bridges filter packets

– same-LAN -segment frames not forwarded onto other LAN segments

• Forwarding: – how to know which LAN segment on which to forward

frame?

– looks like a routing problem (more shortly!)

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DL: Backbone Bridge

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DL: Interconnection Without Backbone

• Not recommended for two reasons:- single point of failure at Computer Science hub- all traffic between EE and SE must path over CS segment

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DL: Learning Bridges

• Basic idea: build cache of which nodes are downstream of which ports

• Monitor source address on all packets bridge forwards• Insert reverse path to source into cache• What to do with unknown sources?

– Flood network

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DL: Bridge Filtering

• bridges learn which hosts can be reached through which interfaces: maintain filtering tables

– when frame received, bridge “learns” location of sender: incoming LAN segment

– records sender location in filtering table

• filtering table entry:

– (Node LAN Address, Bridge Interface, Time Stamp)

– stale entries in Filtering Table dropped (TTL can be 60 minutes)

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DL: Bridge Filtering

• filtering procedure:if destination is on LAN on which frame was received

then drop the frame

else { lookup filtering table

if entry found for destinationthen forward the frame on interface indicated;else flood; /* forward on all but the interface on

which the frame arrived*/

}

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DL: Bridge Learning: example

Suppose C sends frame to D and D replies back with frame to C

• C sends frame, bridge has no info about D, so floods to both LANs

– bridge notes that C is on port 1

– frame ignored on upper LAN

– frame received by D

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DL: Bridge Learning: example

• D generates reply to C, sends

– bridge sees frame from D

– bridge notes that D is on interface 2

– bridge knows C on interface 1, so selectively forwards frame out via interface 1

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DL: Bridges Spanning Tree

• for increased reliability, desirable to have redundant, alternate paths from source to dest

• with multiple simultaneous paths, cycles result - bridges may multiply and forward frame forever

• solution: organize bridges in a spanning tree by disabling subset of interfaces

Disabled

75

DL: Switching

• Switches– “multi-port bridge”

– Each port acts as a bridge

– Each port determines MAC addresses connected to itself

– Master list within switch determines forwarding behavior

LAN 1 LAN 2

76

DL: Switched Network Advantages

• Higher link bandwidth– Point to point electrically simpler than bus

• Much greater aggregate bandwidth– Separate segments can send at once

– Data backplane of switches typically large to support simultaneous transfers amongst ports

• Challenge– Learning which packets to copy across links

– Avoiding forwarding loops

77

DL: Ethernet Switches

• layer 2 (frame) forwarding, filtering using LAN addresses

• Switching: A-to-B and A’-to-B’ simultaneously, no collisions

• large number of interfaces versus bridges (which typically have only two)

• often: individual hosts, star-connected into switch– Ethernet, but no collisions!

78

DL: Ethernet Switches

• cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame– slight reduction in latency

• combinations of shared/dedicated, 10/100/1000 Mbps interfaces

79

DL: Ethernet Switches (more)

Dedicated

Shared

80

DL: Bridges vs. Routers

• both store-and-forward devices– routers: network layer devices (examine network layer headers)– bridges are Link Layer devices

• routers maintain routing tables, implement routing algorithms• bridges maintain filtering tables, implement filtering,

learning and spanning tree algorithms

81

DL: Routers vs. Bridges

Bridges + and -

+ Bridge operation is simpler requiring less processing bandwidth

- Topologies are restricted with bridges: a spanning tree must be built to avoid cycles

- Bridges do not offer protection from broadcast storms (endless broadcasting by a host will be forwarded by a bridge)

82

DL: Routers vs. Bridges

Routers + and -+ arbitrary topologies can be supported, cycling is

limited by TTL counters (and good routing protocols)

+ provide firewall protection against broadcast storms- require IP address configuration (not plug and play)- require higher processing bandwidth

• bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

83

Data-link layer summary

• principles behind data link layer services:– error detection, correction

– sharing a broadcast channel: multiple access

– link layer addressing, ARP

• various link layer technologies– 802.3 Ethernet

– 802.5 Token-ring

– 802.11 LANs

– PPP

– ATM

– X.25, Frame Relay

– hubs, bridges, switches