computer networks prof. ashok k agrawala · framing – bit stuffing . link layer . stuffing done...

CSMC 417

Computer Networks Prof. Ashok K Agrawala

© 2012 Ashok Agrawala

Based on J.F Kurose and K.W. Ross

Set 5

1 Network Layer

Link Layer 5-2

Chapter 5: Link layer our goals: understand principles behind link layer

services: error detection, correction sharing a broadcast channel: multiple access link layer addressing local area networks: Ethernet, VLANs

instantiation, implementation of various link layer technologies

Link Layer 5-3

Link layer, LANs: outline

5.1 introduction, services 5.2 error detection,

correction 5.3 multiple access

protocols 5.4 LANs addressing, ARP Ethernet switches VLANS

5.5 link virtualization: MPLS

5.6 data center networking

5.7 a day in the life of a web request

Link Layer 5-4

Link layer: introduction terminology: hosts and routers: nodes communication channels that

connect adjacent nodes along communication path: links wired links wireless links LANs

layer-2 packet: frame, encapsulates datagram

data-link layer has responsibility of

transferring datagram from one node to physically adjacent node over a link

global ISP

Link Layer 5-5

Link layer: context

datagram transferred by different link protocols over different links: e.g., Ethernet on first link,

frame relay on intermediate links, 802.11 on last link

each link protocol provides different services e.g., may or may not

provide rdt over link

transportation analogy: trip from Princeton to Lausanne

limo: Princeton to JFK plane: JFK to Geneva train: Geneva to Lausanne

tourist = datagram transport segment =

communication link transportation mode = link

layer protocol travel agent = routing

algorithm

Link Layer 5-6

Link layer services framing, link access: encapsulate datagram into frame, adding header, trailer channel access if shared medium “MAC” addresses used in frame headers to identify

source, dest • different from IP address!

reliable delivery between adjacent nodes we learned how to do this already (chapter 3)! seldom used on low bit-error link (fiber, some twisted

pair) wireless links: high error rates

• Q: why both link-level and end-end reliability?

Link Layer 5-7

flow control: pacing between adjacent sending and receiving nodes

error detection: errors caused by signal attenuation, noise. receiver detects presence of errors:

• signals sender for retransmission or drops frame

error correction: receiver identifies and corrects bit error(s) without resorting to

retransmission

half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit, but not

at same time

Link layer services (more)

Link Layer 5-8

Where is the link layer implemented? in each and every host link layer implemented in

“adaptor” (aka network interface card NIC) or on a chip Ethernet card, 802.11

card; Ethernet chipset implements link, physical

layer attaches into host’s system

buses combination of hardware,

software, firmware

controller

physical transmission

cpu memory

host bus (e.g., PCI)

network adapter card

application transport network

link

link physical

Link Layer 5-9

Adaptors communicating

sending side: encapsulates datagram in

frame adds error checking bits,

rdt, flow control, etc.

receiving side looks for errors, rdt,

flow control, etc extracts datagram, passes

to upper layer at receiving side

controller controller

sending host receiving host

datagram datagram

datagram

frame

Link Layer 5-10








Link Layer 5-11

Error detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable!

• protocol may miss some errors, but rarely • larger EDC field yields better detection and correction

otherwise

Link Layer 5-12

Parity checking

single bit parity: detect single bit

errors

two-dimensional bit parity: detect and correct single bit errors

0 0

Link Layer 5-13

Internet checksum (review)

sender: treat segment contents

as sequence of 16-bit integers

checksum: addition (1’s complement sum) of segment contents

sender puts checksum value into UDP checksum field

receiver: compute checksum of

received segment check if computed

checksum equals checksum field value: NO - error detected YES - no error detected.

But maybe errors nonetheless?

goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only)

Link Layer 5-14

Cyclic redundancy check more powerful error-detection coding view data bits, D, as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that

exactly divisible by G (modulo 2) receiver knows G, divides by G. If non-zero remainder:

error detected! can detect all burst errors less than r+1 bits

widely used in practice (Ethernet, 802.11 WiFi, ATM)

Link Layer 5-15

CRC example

want: D.2r XOR R = nG

equivalently: D.2r = nG XOR R

equivalently: if we divide D.2r by

G, want remainder R to satisfy:

R = remainder[ ] D.2r G

1001 101110000 1001

1

101

01000

000 1010 1001

010 000

100 000 1000 0000 1000

D G

R

r = 3

Cyclic Redundancy Check Have to maximize the probability of

detecting the errors using a small number of additional bits.

Based on powerful mathematical formulations – theory of finite fields

Consider (n+1) bits as n degree polynomial

Message M(x) represented as polynomial

Divisor C(x) of degree k Send P(x) as (n+1) bits +k bits such that

P(x) is exactly divisible by C(x)

Link Layer 16

3 2

7 4 3 1

( ) 1( )

C x x xM x x x x x

= + +

= + + +

CRC Basis

Use modulo 2 arithmetic Any Polynomial B(x) can be divided by a divisor

polynomial C(x) if B(x) is of higher degree than C(x)

Any polynomial B(x) can be divided once by a divisor polynomial C(x) if they are of the same degree

The remainder obtained when B(x) is divided by C(x) is obtained by subtracting C(x) from B(x)

To subtract C(x) from B(x) we simply perform the exclusive-OR operation on each pair of matching coefficients.

Link Layer 17

Error Detection – CRCs (1) Adds bits so that transmitted frame viewed as a polynomial is evenly

divisible by a generator polynomial

Link Layer

Start by adding 0s to frame and try dividing

Offset by any reminder to make it evenly divisible

5-18

Cyclic Redundancy Check

Link Layer 19

All single bit errors – if xk and x0 terms are nonzero

All double-bit errors – as long as C(x) has a factor with at least three terms

Any odd number of errors as long as C(x) has (x+1) as a factor

Any burst error of length k bits

Common CRC Polynomials

CRC C(x)

CRC-8 x8 + x2 + X1 + 1

CRC-10 x10 + x9 + x5 + x4 + x1 + 1

CRC-12 x12 + x11 + x3 + x2 + 1

CRC-16 x16 + x15 + x2 + 1

CRC-CCITT x16 + x12 + x5 + 1

CRC-32 x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x1 + 1

Link Layer 20

Error Detection – CRCs (2)

Link Layer

Based on standard polynomials: Ex: Ethernet 32-bit CRC is defined by:

Computed with simple shift/XOR circuits

Stronger detection than checksums: E.g., can detect all double bit errors Not vulnerable to systematic errors

Framing Methods

Link Layer

Byte count » Flag bytes with byte stuffing » Flag bits with bit stuffing » Physical layer coding violations

• Use non-data symbol to indicate frame

Framing Break sequence of bits into a frame Typically implemented by the network adaptor

Sentinel-based Delineate frame with special pattern (e.g., 01111110)

Problem: what if special patterns occurs within frame? Solution: escaping the special characters

• E.g., sender always inserts a 0 after five 1s • … and receiver always removes a 0 appearing after five 1s • Bit Stuffing

Similar to escaping special characters in C programs

01111110 01111110 Frame contents

23 Link Layer

Bit Oriented Protocols

Frame – a collection of bits No Byte boundary

SDLC – Synchronous Data Link Control IBM

HDLC – High-Level Data Link Control ISO Standard

Link Layer 24

HDLC Frame Format

Framing (Continued)

Counter-based Include the payload length in the header … instead of putting a sentinel at the end Problem: what if the count field gets corrupted?

• Causes receiver to think the frame ends at a different place Solution: catch later when doing error detection

• And wait for the next sentinel for the start of a new frame

25 Link Layer

DDCMP Frame Format

Framing – Byte count

Link Layer

Frame begins with a count of the number of bytes in it Simple, but difficult to resynchronize after an error

Error case

Expected case

Framing – Bit stuffing

Link Layer

Stuffing done at the bit level: Frame flag has six consecutive 1s (not shown) On transmit, after five 1s in the data, a 0 is added On receive, a 0 after five 1s is deleted

Transmitted bits with stuffing

Data bits

Framing – Byte stuffing

Link Layer

Special flag bytes delimit frames; occurrences of flags in the data must be stuffed (escaped) Longer, but easy to resynchronize after error

Stuffing examples

Frame format

Need to escape extra ESCAPE bytes too!

Byte-Oriented Protocols

Frame – a collection of bytes. Examples BISYNC – Binary Synchronous Communication – IBM DDCMP – Digital Data Communication Message

Protocol PPP – Point-to-Point

Sentinel Based – Use special character as marker BISYNC

• SYN and SOH • STX and ETX • DLE as escape character. - Character Stuffing

Link Layer 29

Frame Structure

Link Layer 30

PPP Frame Format

BISYNC Frame Format

Clock-Based Framing (SONET)

Clock-based Make each frame a fixed size No ambiguity about start and end of frame But, may be wasteful

Synchronous Optical Network (SONET) Slowest speed link STS-1 – 51.84 Mbps ( 810*8*8K) Frame – 9 rows of 90 bytes

• First 3 bytes of each row are overhead • First two bytes of a frame contain a special bit pattern – to

mark the start of the frame – check for it every 810 bytes

Link Layer 31

Sonet Frame

Link Layer 32

Elementary Data Link Protocols

Link Layer

Link layer environment » Utopian Simplex Protocol » Stop-and-Wait Protocol for Error-free channel » Stop-and-Wait Protocol for Noisy channel »

Link layer environment (1)

Link Layer

Commonly implemented as NICs and OS drivers; network layer (IP) is often OS software

Link layer environment (2) Link layer protocol implementations use library functions

See code (protocol.h) for more details

Link Layer

Group Library Function Description

Network layer

from_network_layer(&packet) to_network_layer(&packet) enable_network_layer() disable_network_layer()

Take a packet from network layer to send Deliver a received packet to network layer Let network cause “ready” events Prevent network “ready” events

Physical layer

from_physical_layer(&frame) to_physical_layer(&frame)

Get an incoming frame from physical layer Pass an outgoing frame to physical layer

Events & timers

wait_for_event(&event) start_timer(seq_nr) stop_timer(seq_nr) start_ack_timer() stop_ack_timer()

Wait for a packet / frame / timer event Start a countdown timer running Stop a countdown timer from running Start the ACK countdown timer Stop the ACK countdown timer

5-35

Protocol Definitions

Continued

Some definitions needed in the protocols to follow. These are located in the file protocol.h.

36 Link Layer

Protocol Definitions (ctd.)

Some definitions needed in the protocols to follow. These are located in the file protocol.h.

37 Link Layer

Utopian Simplex Protocol

Link Layer

An optimistic protocol (p1) to get us started Assumes no errors, and receiver as fast as sender Considers one-way data transfer

That’s it, no error or flow control …

Sender loops blasting frames Receiver loops eating frames }

PresenterPresentation NotesThis is essentially what Ethernet does – just blast packets and receive as quickly as possible

Utopian Simplex Protocol (1)

A utopian simplex protocol. . . .

Link Layer 5-39

Utopian Simplex Protocol (2) A utopian simplex protocol.

Link Layer 5-40

Reliable Transmission

Transfer frames without errors Error Correction Error Detection Discard frames with error

Acknowledgements and Timeouts Retransmission ARQ – Automatic Repeat Request

Link Layer 41

Stop and Wait with 1-bit Seq No

Link Layer 42

Stop and Wait Protocols

Simple Low Throughput One Frame per RTT

Increase throughput by having more frames in

flight Sliding Window Protocol

Link Layer 43

Stop and Wait

Link Layer 44

Duplicate Frames

Stop-and-Wait – Error-free channel

Link Layer

Protocol (p2) ensures sender can’t outpace receiver: Receiver returns a dummy frame (ack) when ready Only one frame out at a time – called stop-and-wait We added flow control!

Sender waits to for ack after passing frame to physical

layer

Receiver sends ack after passing frame to network layer

PresenterPresentation NotesThis is an intermediate protocol in which we’ve dealt with one issue (flow control) but not another (error control). It is instructive, but does not represent any real protocol (as it will deadlock if there are errors).

Stop-and-Wait – Noisy channel (1)

Link Layer

ARQ (Automatic Repeat reQuest) adds error control Receiver acks frames that are correctly delivered Sender sets timer and resends frame if no ack)

For correctness, frames and acks must be numbered Else receiver can’t tell retransmission (due to lost ack

or early timer) from new frame For stop-and-wait, 2 numbers (1 bit) are sufficient

PresenterPresentation NotesARQ is also called PAR (Positive Acknowledgement with Retransmission)


Link Layer

Sender loop (p3):

Send frame (or retransmission) Set timer for retransmission Wait for ack or timeout

If a good ack then set up for the next frame to send (else the old frame will be retransmitted)

{

PresenterPresentation NotesThis is essentially what 802.11 does – stop-and-wait to handle wireless errors. There are differences in the details: 802.11 uses a larger sequence number (12 bits) and gives up after a limited number of retransmissions (retries).


Link Layer

Receiver loop (p3):

Wait for a frame

If it’s new then take it and advance expected frame Ack current frame

Sliding Window Protocols

Link Layer

Sliding Window concept » One-bit Sliding Window » Go-Back-N » Selective Repeat »

Sliding Window concept (1)

Link Layer

Sender maintains window of frames it can send Needs to buffer them for possible retransmission Window advances with next acknowledgements

Receiver maintains window of frames it can receive Needs to keep buffer space for arrivals Window advances with in-order arrivals

Sliding Window Protocol

Sender assigns a sequence number –SeqNum Sender maintains three variables: Send Window Size – SWS Last Ack Received – LAR Last Frame Sent – LFS

Invariant LFS – LAR < SWS When ACK arrives sender movers LAR to the

right and thereby allowing the sender to transmit another frame

Associate a timer with each frame it transmits

Link Layer 51

Sliding Window Protocol

Receiver maintains three variables: Receiver Window Size – RWS Largest acceptable Frame Number – LAF Last Frame Received – LFR

Invariant LAF –LFR < RWR When frame with SEQNum arrives If SeqNum < LFR or SeqNum >LAF discard the

frame If LFR < SeqNum < LAF then accept the frame

SeqNumtoAck – largest seq no not yet acked. Send this as ack.

Link Layer 52

Sliding Window

Link Layer 53

Sliding Window on Sender

Sliding window on Receiver

Timeline


Link Layer

A sliding window advancing at the sender and receiver Ex: window size is 1, with a 3-bit sequence number.

At the start First frame is

sent

First frame is received

Sender gets first ack

Sender

Receiver


Link Layer

Larger windows enable pipelining for efficient link use Stop-and-wait (w=1) is inefficient for long links Best window (w) depends on bandwidth-delay (BD) Want w ≥ 2BD+1 to ensure high link utilization

Pipelining leads to different choices for errors/buffering We will consider Go-Back-N and Selective Repeat

PresenterPresentation NotesPipelining means that there are multiple frames outstanding in the network at any instant.

For stop-and-wait, imagine a link from Seattle to Amsterdam that has a 500ms round-trip time. It doesn’t matter how fast the link is in bits/sec, stop-and-wait will only let it run at two packets per second.

One-Bit Sliding Window (1) Transfers data in both directions with stop-and-wait Piggybacks acks on reverse data frames for efficiency Handles transmission errors, flow control, early timers

Link Layer . . .

{

Each node is sender and receiver (p4):

Prepare first frame

Launch it, and set timer

5-56

PresenterPresentation NotesShows a basic implementation of a sliding window for w=1 handling error/flow control for us to build on

One-Bit Sliding Window (2)

Link Layer

. . .

If a frame with new data then deliver it

Wait for frame or timeout

(Otherwise it was a timeout)

If an ack for last send then prepare for next data frame

Send next data frame or retransmit old one; ack the last data we received

5-57

Two scenarios show subtle interactions exist in p4: • Simultaneous start [right] causes correct but slow operation compared

to normal [left] due to duplicate transmissions.

Time

(a) Normal case (b) Correct, but poor performance

One-Bit Sliding Window (3)

Link Layer

Notation is (seq, ack, frame number). Asterisk indicates frame accepted by network layer .

5-58

Go-Back-N (1)

Link Layer

Receiver only accepts/acks frames that arrive in order: Discards frames that follow a missing/errored frame Sender times out and resends all outstanding frames

Go-Back-N (2)

Link Layer

Tradeoff made for Go-Back-N: Simple strategy for receiver; needs only 1 frame Wastes link bandwidth for errors with large

windows; entire window is retransmitted

Selective Repeat (1)

Link Layer

Receiver accepts frames anywhere in receive window Cumulative ack indicates highest in-order frame NAK (negative ack) causes sender retransmission of a

missing frame before a timeout resends window


Link Layer

Tradeoff made for Selective Repeat: More complex than Go-Back-N due to buffering at

receiver and multiple timers at sender More efficient use of link bandwidth as only lost

frames are resent (with low error rates)

Implemented as p6 (see code in book)

PresenterPresentation NotesWhat happens if the NAK is lost, or if multiple data frames are lost per round-trip time? The retransmission timer goes off and transmission restarts from the acknowledgement point. To be more tolerant of these situations more acknowledgement information needs to be returned (more NAKs, a bit vector of the frames received beyond the cumulative ack, etc.)


Link Layer

For correctness, we require: Sequence numbers (s) at least twice the window (w)

Originals Originals Retransmits Retransmits

Error case (s=8, w=7) – too few sequence numbers

Correct (s=8, w=4) – enough sequence numbers

New receive window overlaps old – retransmits ambiguous

New and old receive window don’t overlap – no ambiguity

Example Data Link Protocols

Link Layer

Packet over SONET » PPP (Point-to-Point Protocol) » ADSL (Asymmetric Digital Subscriber Loop) »

Packet over SONET

Link Layer

Packet over SONET is the method used to carry IP packets over SONET optical fiber links Uses PPP (Point-to-Point Protocol) for framing

Protocol stacks PPP frames may be split over SONET payloads

Packet over SONET (2)

PPP Features 1. Separate packets, error detection 2. Link Control Protocol 3. Network Control Protocol

Link Layer 5-66

PPP (1)

Link Layer

PPP (Point-to-Point Protocol) is a general method for delivering packets across links Framing uses a flag (0x7E) and byte stuffing “Unnumbered mode” (connectionless unacknow-

ledged service) is used to carry IP packets Errors are detected with a checksum

IP packet 0x21 for IPv4

PPP (2)

Link Layer

A link control protocol brings the PPP link up and down

State machine for link control

PPP – Point to Point Protocol (3)

The LCP frame types.

69 Link Layer

ADSL (1)

Link Layer

Widely used for broadband Internet over local loops ADSL runs from modem (customer) to DSLAM (ISP) IP packets are sent over PPP and AAL5/ATM (over)

ADSL (2)

Link Layer

PPP data is sent in AAL5 frames over ATM cells: ATM is a link layer that uses short, fixed-size cells (53

bytes); each cell has a virtual circuit identifier AAL5 is a format to send packets over ATM PPP frame is converted to a AAL5 frame (PPPoA)

AAL5 frame is divided into 48 byte pieces, each of which goes into one ATM cell with 5 header bytes

PresenterPresentation NotesRe ATM: ATM was a major technology in the 1990s that was hyped to win in the convergence of the Internet and telecommunications, but IP won instead. The short, fixed-size cells give flexibility (can mix voice and data without having the voice wait for a whole data packet). ATM is now used in niches such as ADSL and WAN links.

High-Level Data Link Control

Frame format for bit-oriented protocols.

72 Link Layer

High-Level Data Link Control (2)

Control field of (a) An information frame. (b) A supervisory frame. (c) An unnumbered frame.

73 Link Layer

The Data Link Layer in the Internet

A home personal computer acting as an internet host.

74 Link Layer

Link Layer 5-75








The MAC Sublayer

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D. Wetherall, 2011

Responsible for deciding who sends next on a multi-access link An important part of the link

layer, especially for LANs Physical Link

Network Transport Application

MAC is in here!

Link Layer 5-77

MAC protocols: taxonomy

three broad classes: channel partitioning

divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use

random access channel not divided, allow collisions “recover” from collisions

“taking turns” nodes take turns, but nodes with more to send can take longer

turns

Link Layer 5-78

Channel partitioning MAC protocols: TDMA

TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt

trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots

2,5,6 idle

1 3 4 1 3 4

6-slot frame

6-slot frame

Link Layer 5-79

FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6

idle

frequ

ency

ban

ds

FDM cable

Channel partitioning MAC protocols: FDMA

Link Layer 5-80

Multiple access protocols single shared broadcast channel two or more simultaneous transmissions by nodes:

interference collision if node receives two or more signals at the same

time multiple access protocol distributed algorithm that determines how nodes share

channel, i.e., determine when node can transmit communication about channel sharing must use channel itself!

no out-of-band channel for coordination

Link Layer 5-81

An ideal multiple access protocol

given: broadcast channel of rate R bps desiderata:

1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average

rate R/M 3. fully decentralized:

• no special node to coordinate transmissions • no synchronization of clocks, slots

4. simple

Link Layer 5-82

Multiple access links, protocols two types of “links”: point-to-point

PPP for dial-up access point-to-point link between Ethernet switch, host

broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 802.11 wireless LAN

shared wire (e.g., cabled Ethernet)

shared RF (e.g., 802.11 WiFi)

shared RF (satellite)

humans at a cocktail party

(shared air, acoustical)

Link Layer 5-83

Random access protocols when node has packet to send transmit at full channel data rate R. no a priori coordination among nodes

two or more transmitting nodes ➜ “collision”, random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed

retransmissions) examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA

ALOHA (1)

In pure ALOHA, frames are transmitted at completely arbitrary times

Collision Collision

Time

User A B C D E

Pure ALOHA (2)

Vulnerable period for the shaded frame.

November 12 85 CMSC417 Set 5

Link Layer 5-86

Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives transmit immediately

collision probability increases: frame sent at t0 collides with other frames sent in [t0-

1,t0+1]

Link Layer 5-87

Pure ALOHA efficiency P(success by given node) = P(node transmits) . P(no other node transmits in [t0-1,t0] . P(no other node transmits in [t0-1,t0] = p . (1-p)N-1 . (1-p)N-1 = p . (1-p)2(N-1) … choosing optimum p and then letting n

= 1/(2e) = .18

even worse than slotted Aloha!

Pure ALOHA (3)

Throughput versus offered traffic for ALOHA systems.


Link Layer 5-89

Slotted ALOHA

assumptions: all frames same size time divided into equal size

slots (time to transmit 1 frame)

nodes start to transmit only slot beginning

nodes are synchronized if 2 or more nodes transmit

in slot, all nodes detect collision

operation: when node obtains fresh

frame, transmits in next slot if no collision: node can send

new frame in next slot if collision: node retransmits

frame in each subsequent slot with prob. p until success

Link Layer 5-90

Pros: single active node can

continuously transmit at full rate of channel

highly decentralized: only slots in nodes need to be in sync

simple

Cons: collisions, wasting slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

Slotted ALOHA 1 1 1 1

2

3

2 2

3 3

node 1

node 2

node 3

C C C S S S E E E

Link Layer 5-91

suppose: N nodes with many frames to send, each transmits in slot with probability p

prob that given node has success in a slot = p(1-p)N-1

prob that any node has a success = Np(1-p)N-1

max efficiency: find p* that maximizes Np(1-p)N-1

for many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives:

max efficiency = 1/e = .37

efficiency: long-run fraction of successful slots (many nodes, all with many frames to send)

at best: channel used for useful transmissions 37% of time! !

Slotted ALOHA: efficiency

Link Layer 5-92

CSMA (carrier sense multiple access)

CSMA: listen before transmit: if channel sensed idle: transmit entire frame if channel sensed busy, defer transmission

human analogy: don’t interrupt others!

CSMA (1)


CSMA improves on ALOHA by sensing the channel! User doesn’t send if it senses someone else

Variations on what to do if the channel is busy: 1-persistent (greedy) sends as soon as idle Nonpersistent waits a random time then tries again p-persistent sends with probability p when idle

PresenterPresentation Notes802.11 uses a refinement of p-persistent CSMA

Persistent and Nonpersistent CSMA

Comparison of the channel utilization versus load for various random access protocols. November 12

94 CMSC417 Set 5

CSMA Collisions

Collisions can still occur: propagation delay means two nodes may not hear each other’s transmission

Collision: entire packet transmission time wasted

October 08 95 CMSC417 Set 11

Link Layer 5-96

CSMA/CD (collision detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage

collision detection: easy in wired LANs: measure signal strengths, compare

transmitted, received signals difficult in wireless LANs: received signal strength

overwhelmed by local transmission strength human analogy: the polite conversationalist

Link Layer 5-97

Ethernet CSMA/CD algorithm

1. NIC receives datagram from network layer, creates frame

2. If NIC senses channel idle, starts frame transmission. If NIC senses channel busy, waits until channel idle, then transmits.

3. If NIC transmits entire frame without detecting another transmission, NIC is done with frame !

4. If NIC detects another transmission while transmitting, aborts and sends jam signal

5. After aborting, NIC enters binary (exponential) backoff: after mth collision, NIC

chooses K at random from {0,1,2, …, 2m-1}. NIC waits K·512 bit times, returns to Step 2

longer backoff interval with more collisions

Link Layer 5-98

CSMA/CD efficiency

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

efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity

better performance than ALOHA: and simple, cheap, decentralized!

transprop /ttefficiency

511

+=

CSMA/CD Collision Detection


CSMA (3) – Collision Detection


CSMA/CD improvement is to detect/abort collisions Reduced contention times improve performance

Collision time is much shorter than frame time

Link Layer 5-101

“Taking turns” MAC protocols

channel partitioning MAC protocols: share channel efficiently and fairly at high load inefficient at low load: delay in channel access, 1/N

bandwidth allocated even if only 1 active node! random access MAC protocols efficient at low load: single node can fully utilize

channel high load: collision overhead

“taking turns” protocols look for best of both worlds!

Link Layer 5-102

polling: master node “invites”

slave nodes to transmit in turn

typically used with “dumb” slave devices

concerns: polling overhead latency single point of

failure (master)

master

slaves

poll

data

data


Link Layer 5-103

token passing: control token passed

from one node to next sequentially.

token message concerns: token overhead latency single point of failure

(token)

T

data

(nothing to send)

T


Collision-Free Protocols (1) – Bitmap


Collision-free protocols avoid collisions entirely Senders must know when it is their turn to send

The basic bit-map protocol: Sender set a bit in contention slot if they have data Senders send in turn; everyone knows who has data

Collision-Free Protocols– Countdown

CN5E by Tanenbaum & Wetherall, © Pearson Education-Prentice Hall and D.

Binary countdown improves on the bitmap protocol

Stations send their address in contention slot (log N bits instead of N bits) Medium ORs bits; stations

give up when they send a “0” but see a “1” Station that sees its full

address is next to send

Limited-Contention Protocols (1)


Idea is to divide stations into groups within which only a very small number are likely to want to send Avoids wastage due to idle periods and collisions

Already too many contenders for a good chance of one winner

Limited Contention Adaptive Tree Walk


Tree divides stations into groups (nodes) to poll Depth first search under nodes with poll collisions Start search at lower levels if >1 station expected

Level 0

Level 1

Level 2

Wavelength Division Multiple Access Protocols

Wavelength division multiple access.


Ethernet • Ethernet Cabling • Manchester Encoding • The Ethernet MAC Sublayer Protocol • The Binary Exponential Backoff Algorithm • Ethernet Performance • Switched Ethernet • Fast Ethernet • Gigabit Ethernet • IEEE 802.2: Logical Link Control • Retrospective on Ethernet


Link Layer 5-110

Ethernet “dominant” wired LAN technology: cheap $20 for NIC first widely used LAN technology simpler, cheaper than token LANs and ATM kept up with speed race: 10 Mbps – 10 Gbps

Metcalfe’s Ethernet sketch

Classic Ethernet– Physical Layer


One shared coaxial cable to which all hosts attached Up to 10 Mbps, with Manchester encoding Hosts ran the classic Ethernet protocol for access

Ethernet Transceiver and Adapter

Medium – 50 ohm cable

Taps 2.5 m apart Transceiver – can

send and receive Multiple segments can

be joined by repeaters – no more than 4

Max end-to-end distance – 2500 m

October 08 CMSC417 Set 11 112

Ethernet Uses CSMA/CD

Carrier sense: wait for link to be idle Channel idle: start transmitting

Channel busy: wait until idle

Collision detection: listen while transmitting No collision: transmission is complete

Collision: abort transmission, and send jam signal

Random access: exponential back-off After collision, wait a random time before trying again

After mth collision, choose K randomly from {0, …, 2m-1}

… and wait for K*512 bit times before trying again October 08

113 CMSC417 Set 11

Ethernet Cabling

The most common kinds of Ethernet cabling.


Ethernet Cabling (2)

Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.


Ethernet Cabling (3)

Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.


Ethernet Signalling

(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding. November 12

117 CMSC417 Set 5

Ethernet Frame Format

Preamble – For synchronizing Alternate 0 and 1

Address – 48 bit MAC Type – Id for higher

level protocol Length – up to 1500

bytes, with minimum of 46 bytes, of data

802.3 – Length for Type field.


Link Layer 5-119

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

dest. address

source address

data (payload) CRC preamble

type

Link Layer 5-120

Ethernet frame structure (more) addresses: 6 byte source, destination MAC addresses if adapter receives frame with matching destination

address, or with broadcast address (e.g. ARP packet), it passes data in frame to network layer protocol

otherwise, adapter discards frame type: indicates higher layer protocol (mostly IP but

others possible, e.g., Novell IPX, AppleTalk) CRC: cyclic redundancy check at receiver error detected: frame is dropped

dest. address

source address

data (payload) CRC preamble

type

Link Layer 5-121

Ethernet: unreliable, connectionless

connectionless: no handshaking between sending and receiving NICs

unreliable: receiving NIC doesnt send acks or nacks to sending NIC data in dropped frames recovered only if initial

sender uses higher layer rdt (e.g., TCP), otherwise dropped data lost

Ethernet’s MAC protocol: unslotted CSMA/CD wth binary backoff

Link Layer 5-122

802.3 Ethernet standards: link & physical layers

many different Ethernet standards common MAC protocol and frame format different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps,

10G bps different physical layer media: fiber, cable


link physical

MAC protocol and frame format

100BASE-TX

100BASE-T4

100BASE-FX 100BASE-T2

100BASE-SX 100BASE-BX

fiber physical layer copper (twister pair) physical layer

Classic Ethernet (2) – MAC


MAC protocol is 1-persistent CSMA/CD Random delay (backoff) after collision is computed with BEB (Binary Exponential Backoff) Frame format is still used with modern Ethernet.

Ethernet (DIX)

IEEE 802.

3

Ethernet Transmitter Algorithm

P-persistent Transmit with prob p

when line goes idle Ethernet uses 1-

persistent algorithm On Collision- 32 bit jamming

sequence Stops transmitting Runt frame – 64 synch

+ 32 bit jamming sequence

Min frame size – 64 bytes – 46 + 14 + 4

For 2500 m line with up to 4 repeaters max round trip delay – 51.2µs

Exponential Backoff In steps of 51.2µs Wait k* rand (0,.. 2k-1)


IEEE 802.2: Logical Link Control

(a) Position of LLC. (b) Protocol formats. November 12

125 CMSC417 Set 5

Limitations on Ethernet Length

Latency depends on physical length of link Time to propagate a packet from one end to the other

Suppose A sends a packet at time t And B sees an idle line at a time just before t+d … so B happily starts transmitting a packet

B detects a collision, and sends jamming signal But A doesn’t see collision till t+2d

latency d A B


Limitations on Ethernet Length

A needs to wait for time 2d to detect collision So, A should keep transmitting during this period … and keep an eye out for a possible collision

Imposes restrictions on Ethernet Maximum length of the wire: 2500 meters Minimum length of the packet: 512 bits (64 bytes)

latency d A B


Ethernet MAC Sublayer Protocol (2)

Collision detection can take as long as 2 .τNovember 12

128 CMSC417 Set 5

Worst Case Secnario


A sends a frame at time t

Frame arrives at B at t+d

B begins transmitting at t+d and collides with A’s Frame

B’s 32 bit frame arrives at A at t+2d

Ethernet Performance k stations, always ready to transmit Assume constant retransmission probability p Probability, A that some station acquires channel during a given slot 𝐴 = 𝑘𝑘(1 − 𝑘)𝑘−1 Optimal value of p – differentiate w.r.t. p and equate to 0

• 𝑘 = 1𝑘 then optimal 𝐴 = [𝑘−1

𝑘]𝑘−1

• As k → ∞ 𝑘 → 1𝑒

Probability that Contention Interval is exactly j slots is 𝐴(1 − 𝐴)𝑗−1 So, the mean number of slots per contention

∑ 𝑗𝐴(1 − 𝐴)𝑗−1 = 1𝐴

∞𝑗=0

Slot duration = 2𝜏 Mean contention interval 𝑤 = 2𝜏

𝐴 …. Mean number of cont. slots = e

If frame transmission time = P

Channel efficiency = 𝑃𝑃+2𝜏/𝐴

= 11+2𝐵𝐵𝑒/𝑐𝑐

Where F=Frame length, B=network Bandwidth, L = cable length

November 12 CMSC417 Set 5 130

Ethernet Performance Efficiency of Ethernet at 10 Mbps with 512-bit slot times.


Ethernet Repeaters

Up to 4 Repeaters Cables 10Base 5

• 10Mbps • Baseband • 500 Meters

10Base2 10BaseT

• Twisted Pair 100 M Category 5 (Cat 5)

• 10 M and 100 M • Twisted Pair


Hubs: Physical-Layer Repeaters

Hubs are physical-layer repeaters Bits coming from one link go out all other links At the same rate, with no frame buffering No CSMA/CD at hub: adapters detect

collisions

twisted pair

hub


Interconnecting with Hubs

Backbone hub interconnects LAN segments All packets seen everywhere, forming one large

collision domain Can’t interconnect Ethernets of different speeds

hub hub hub

hub


Switch

Link layer device Stores and forwards Ethernet frames Examines frame header and selectively

forwards frame based on MAC dest address 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


Switched/Fast Ethernet (1)


Hubs wire all lines into a single CSMA/CD domain Switches isolate each port to a separate domain

• Much greater throughput for multiple ports • No need for CSMA/CD with full-duplex lines



Switches can be wired to computers, hubs and switches Hubs concentrate traffic from computers More on how to switch frames the in 4.8

Switch

Twisted pair Switch ports

Hub

Switch: Traffic Isolation Switch breaks subnet into LAN segments Switch filters packets

Same-LAN-segment frames not usually forwarded onto other LAN segments

Segments become separate collision domains

hub hub hub

switch

collision domain collision domain

collision domain




Fast Ethernet extended Ethernet from 10 to 100 Mbps Twisted pair (with Cat 5) dominated the market

Gigabit Ethernet (1)

A two-station Ethernet

Gigabit / 10 Gigabit Ethernet (1)


Switched Gigabit Ethernet is now the garden variety With full-duplex lines between computers/switches

Gigabit / 10 Gigabit Ethernet (1)


Gigabit Ethernet is commonly run over twisted pair

10 Gigabit Ethernet is being deployed where needed

40/100 Gigabit Ethernet is under development

Benefits of Ethernet

Easy to administer and maintain Inexpensive Increasingly higher speed

Moved from shared media to switches Change everything except the frame format A good general lesson for evolving the Internet


cable headend

CMTS

ISP

cable modem termination system

multiple 40Mbps downstream (broadcast) channels single CMTS transmits into channels

multiple 30 Mbps upstream channels multiple access: all users contend for certain upstream

channel time slots (others assigned)

Cable access network

cable modem splitter

…

…

Internet frames,TV channels, control transmitted downstream at different frequencies

upstream Internet frames, TV control, transmitted upstream at different frequencies in time slots

Link Layer 5-145

DOCSIS: data over cable service interface spec FDM over upstream, downstream frequency channels TDM upstream: some slots assigned, some have contention downstream MAP frame: assigns upstream slots request for upstream slots (and data) transmitted

random access (binary backoff) in selected slots

MAP frame for Interval [t1, t2]

Residences with cable modems

Downstream channel i

Upstream channel j

t1 t2

Assigned minislots containing cable modem upstream data frames

Minislots containing minislots request frames

cable headend

CMTS

Cable access network

Link Layer 5-146

Summary of MAC protocols

channel partitioning, by time, frequency or code Time Division, Frequency Division

random access (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD carrier sensing: easy in some technologies (wire), hard

in others (wireless) CSMA/CD used in Ethernet CSMA/CA used in 802.11

taking turns polling from central site, token passing bluetooth, FDDI, token ring

Link Layer 5-147








Link Layer 5-148

MAC addresses and ARP

32-bit IP address: network-layer address for interface used for layer 3 (network layer) forwarding

MAC (or LAN or physical or Ethernet) address: function: used ‘locally” to get frame from one interface to

another physically-connected interface (same network, in IP-addressing sense)

48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable

e.g.: 1A-2F-BB-76-09-AD

hexadecimal (base 16) notation (each “number” represents 4 bits)

Link Layer 5-149

LAN addresses and ARP each adapter on LAN has unique LAN address

adapter

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN (wired or wireless)

Link Layer 5-150

LAN addresses (more)

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

(to assure uniqueness) analogy: MAC address: like Social Security Number IP address: like postal address

MAC flat address ➜ portability can move LAN card from one LAN to another

IP hierarchical address not portable address depends on IP subnet to which node is

attached

Link Layer 5-151

ARP: address resolution protocol

ARP table: each IP node (host, router) on LAN has 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 determine interface’s MAC address, knowing its IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137.196.7.23

137.196.7.78

137.196.7.14

137.196.7.88

Link Layer 5-152

ARP protocol: same LAN A wants to send datagram

to B B’s MAC address not in

A’s ARP table. A broadcasts ARP query

packet, containing B's IP address dest MAC address = FF-FF-

FF-FF-FF-FF all nodes on LAN receive

ARP query B receives ARP packet,

replies to A with its (B's) MAC address frame sent to A’s MAC

address (unicast)

A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state: information that

times out (goes away) unless refreshed

ARP is “plug-and-play”: nodes create their ARP

tables without intervention from net administrator

Link Layer 5-153

walkthrough: send datagram from A to B via R focus on addressing – at IP (datagram) and MAC layer (frame) assume A knows B’s IP address assume A knows IP address of first hop router, R (how?) assume A knows R’s MAC address (how?)

Addressing: routing to another LAN

R

1A-23-F9-CD-06-9B 222.222.222.220

111.111.111.110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111 74-29-9C-E8-FF-55

A

222.222.222.222 49-BD-D2-C7-56-2A

222.222.222.221 88-B2-2F-54-1A-0F

B

R

1A-23-F9-CD-06-9B 222.222.222.220

111.111.111.110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111 74-29-9C-E8-FF-55

A

222.222.222.222 49-BD-D2-C7-56-2A

222.222.222.221 88-B2-2F-54-1A-0F

B

Link Layer 5-154


IP Eth Phy

IP src: 111.111.111.111 IP dest: 222.222.222.222

A creates IP datagram with IP source A, destination B A creates link-layer frame with R's MAC address as dest, frame

contains A-to-B IP datagram

MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B

R

1A-23-F9-CD-06-9B 222.222.222.220

111.111.111.110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111 74-29-9C-E8-FF-55

A

222.222.222.222 49-BD-D2-C7-56-2A

222.222.222.221 88-B2-2F-54-1A-0F

B

Link Layer 5-155


IP Eth Phy

frame sent from A to R

IP Eth Phy

frame received at R, datagram removed, passed up to IP

MAC src: 74-29-9C-E8-FF-55 MAC dest: E6-E9-00-17-BB-4B

IP src: 111.111.111.111 IP dest: 222.222.222.222

IP src: 111.111.111.111 IP dest: 222.222.222.222

R

1A-23-F9-CD-06-9B 222.222.222.220

111.111.111.110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111 74-29-9C-E8-FF-55

A

222.222.222.222 49-BD-D2-C7-56-2A

222.222.222.221 88-B2-2F-54-1A-0F

B

Link Layer 5-156


IP src: 111.111.111.111 IP dest: 222.222.222.222

R forwards datagram with IP source A, destination B R creates link-layer frame with B's MAC address as dest, frame


MAC src: 1A-23-F9-CD-06-9B MAC dest: 49-BD-D2-C7-56-2A

IP Eth Phy

IP Eth Phy

R

1A-23-F9-CD-06-9B 222.222.222.220

111.111.111.110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111 74-29-9C-E8-FF-55

A

222.222.222.222 49-BD-D2-C7-56-2A

222.222.222.221 88-B2-2F-54-1A-0F

B

Link Layer 5-157

Addressing: routing to another LAN R forwards datagram with IP source A, destination B R creates link-layer frame with B's MAC address as dest, frame


IP src: 111.111.111.111 IP dest: 222.222.222.222


IP Eth Phy

IP Eth Phy

R

1A-23-F9-CD-06-9B 222.222.222.220

111.111.111.110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111 74-29-9C-E8-FF-55

A

222.222.222.222 49-BD-D2-C7-56-2A

222.222.222.221 88-B2-2F-54-1A-0F

B

Link Layer 5-158

Addressing: routing to another LAN R forwards datagram with IP source A, destination B R creates link-layer frame with B's MAC address as dest, frame


IP src: 111.111.111.111 IP dest: 222.222.222.222


IP

Eth Phy

Link Layer 5-159








Link Layer 5-160

Ethernet switch link-layer device: takes an active role store, forward Ethernet frames examine incoming frame’s MAC address,

selectively forward frame to one-or-more outgoing 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

Link Layer 5-161

Switch: 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’

can transmit simultaneously, without collisions switch with six interfaces

(1,2,3,4,5,6)

A

A’

B

B’ C

C’

1 2

3 4 5

6

Link Layer 5-162

Switch forwarding table

Q: how does switch know A’ reachable via interface 4, B’ reachable via interface 5?

switch with six interfaces (1,2,3,4,5,6)

A

A’

B

B’ C

C’

1 2

3 4 5

6 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’

1 2

3 4 5

6

Link Layer 5-163

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’

Source: A Dest: A’

MAC addr interface TTL Switch table

(initially empty) A 1 60

Link Layer 5-164

Switch: frame filtering/forwarding

when frame received at switch: 1. record incoming link, MAC address of sending host 2. index switch table using MAC destination address 3. if entry found for destination

then { if destination on segment from which frame arrived

then drop frame else forward frame on interface indicated by entry } else flood /* forward on all interfaces except arriving interface */

A

A’

B

B’ C

C’

1 2

3 4 5

6

Link Layer 5-165

Self-learning, forwarding: example 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, A’, locaton unknown: flood

A’ A

destination A location known:

A’ 4 60

selectively send on just one link

Link Layer 5-166

Interconnecting switches

switches can be connected together

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!)

A

B

S1

C D

E

F S2

S4

S3

H I

G

Link Layer 5-167

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

Link Layer 5-168

Institutional network

to external network

router

IP subnet

mail server

web server

Link Layer 5-169

Switches vs. routers

both are store-and-forward: routers: network-layer

devices (examine network-layer headers)

switches: link-layer devices (examine link-layer headers)

both have forwarding tables: routers: compute tables

using routing algorithms, IP addresses

switches: learn forwarding table using flooding, learning, MAC addresses


link physical

network link

physical

link physical

switch

datagram


link physical

frame

frame

frame datagram

Link Layer 5-170

VLANs: motivation consider: CS user moves office to

EE, but wants connect to CS switch?

single broadcast domain: all layer-2 broadcast

traffic (ARP, DHCP, unknown location of destination MAC address) must cross entire LAN

security/privacy, efficiency issues

Computer Science Electrical

Engineering

Computer Engineering

Link Layer 5-171

VLANs port-based VLAN: switch ports

grouped (by switch management software) so that single physical switch ……

switch(es) supporting VLAN capabilities can be configured to define multiple virtual LANS over single physical LAN infrastructure.

Virtual Local Area Network 1 8

9

16 10 2

7

…

Electrical Engineering (VLAN ports 1-8)

Computer Science (VLAN ports 9-15)

15

…


…

1

8 2

7 9

16 10

15

…


… operates as multiple virtual switches

Link Layer 5-172

Port-based VLAN

1

8

9

16 10 2

7

…



15

…

traffic isolation: frames to/from ports 1-8 can only reach ports 1-8 can also define VLAN based on

MAC addresses of endpoints, rather than switch port

dynamic membership: ports can be dynamically assigned among VLANs

router

forwarding between VLANS: done via routing (just as with separate switches) in practice vendors sell combined

switches plus routers

Link Layer 5-173

VLANS spanning multiple switches

trunk port: carries frames between VLANS defined over multiple physical switches frames forwarded within VLAN between switches can’t be vanilla

802.1 frames (must carry VLAN ID info) 802.1q protocol adds/removed additional header fields for frames

forwarded between trunk ports

1

8

9

10 2

7

…



15

…

2

7 3

Ports 2,3,5 belong to EE VLAN Ports 4,6,7,8 belong to CS VLAN

5

4 6 8 16

1

Link Layer 5-174

type

2-byte Tag Protocol Identifier (value: 81-00)

Tag Control Information (12 bit VLAN ID field, 3 bit priority field like IP TOS)

Recomputed CRC

802.1Q VLAN frame format

802.1 frame

802.1Q frame

dest. address

source address data (payload) CRC preamble

dest. address

source address preamble data (payload) CRC

type

Link Layer 5-175








Link Layer 5-176

Multiprotocol label switching (MPLS)

initial goal: high-speed IP forwarding using fixed length label (instead of IP address) fast lookup using fixed length identifier (rather than

shortest prefix matching) borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address!

PPP or Ethernet header IP header remainder of link-layer frame MPLS header

label Exp S TTL

20 3 1 5

Link Layer 5-177

MPLS capable routers

a.k.a. label-switched router forward packets to outgoing interface based only on

label value (don’t inspect IP address) MPLS forwarding table distinct from IP forwarding tables

flexibility: MPLS forwarding decisions can differ from those of IP use destination and source addresses to route flows to

same destination differently (traffic engineering) re-route flows quickly if link fails: pre-computed backup

paths (useful for VoIP)

Link Layer 5-178

R2

D R3

R5

A

R6

MPLS versus IP paths

IP router IP routing: path to destination determined

by destination address alone

R4

Link Layer 5-179

R2

D R3 R4

R5

A

R6

MPLS versus IP paths

IP-only router

IP routing: path to destination determined by destination address alone

MPLS and IP router

MPLS routing: path to destination can be based on source and dest. address fast reroute: precompute backup routes in

case of link failure

entry router (R4) can use different MPLS routes to A based, e.g., on source address

Link Layer 5-180

MPLS signaling modify OSPF, IS-IS link-state flooding protocols to

carry info used by MPLS routing, e.g., link bandwidth, amount of “reserved” link bandwidth

D R4

R5

A

R6

entry MPLS router uses RSVP-TE signaling protocol to set up MPLS forwarding at downstream routers

modified link state flooding

RSVP-TE

Link Layer 5-181

R1 R2

D R3 R4

R5 0

1 0 0

A

R6

in out out label label dest interface

6 - A 0


10 6 A 1 12 9 D 0


10 A 0 12 D 0

1


8 6 A 0

0

8 A 1

MPLS forwarding tables

Link Layer 5-182








Link Layer 5-183

Data center networks

10’s to 100’s of thousands of hosts, often closely coupled, in close proximity: e-business (e.g. Amazon) content-servers (e.g., YouTube, Akamai, Apple, Microsoft) search engines, data mining (e.g., Google)

challenges: multiple applications, each

serving massive numbers of clients

managing/balancing load, avoiding processing, networking, data bottlenecks

Inside a 40-ft Microsoft container, Chicago data center

Link Layer 5-184

Server racks

TOR switches

Tier-1 switches

Tier-2 switches

Load balancer

Load balancer

B

1 2 3 4 5 6 7 8

A C

Border router

Access router

Internet

Data center networks load balancer: application-layer routing receives external client requests directs workload within data center returns results to external client (hiding data

center internals from client)

Server racks

TOR switches

Tier-1 switches

Tier-2 switches

1 2 3 4 5 6 7 8

Data center networks rich interconnection among switches, racks: increased throughput between racks (multiple routing

paths possible) increased reliability via redundancy

Link Layer 5-186








Link Layer 5-187

Synthesis: a day in the life of a web request

journey down protocol stack complete! application, transport, network, link

putting-it-all-together: synthesis! goal: identify, review, understand protocols (at all

layers) involved in seemingly simple scenario: requesting www page

scenario: student attaches laptop to campus network, requests/receives www.google.com

Link Layer 5-188

A day in the life: scenario

Comcast network 68.80.0.0/13

Google’s network 64.233.160.0/19 64.233.169.105

web server

DNS server

school network 68.80.2.0/24

web page

browser

router (runs DHCP)

Link Layer 5-189

A day in the life… connecting to the Internet

connecting laptop needs to get its own IP address, addr of first-hop router, addr of DNS server: use DHCP

DHCP UDP

IP Eth Phy

DHCP

DHCP

DHCP

DHCP

DHCP

DHCP UDP

IP Eth Phy

DHCP

DHCP

DHCP

DHCP DHCP

DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.3 Ethernet

Ethernet frame broadcast

(dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server

Ethernet demuxed to IP demuxed, UDP demuxed to DHCP

router (runs DHCP)

Link Layer 5-190

DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server

DHCP UDP

IP Eth Phy

DHCP

DHCP

DHCP

DHCP

DHCP UDP

IP Eth Phy

DHCP

DHCP

DHCP

DHCP

DHCP

encapsulation at DHCP server, frame forwarded (switch learning) through LAN, demultiplexing at client

Client now has IP address, knows name & addr of DNS server, IP address of its first-hop router

DHCP client receives DHCP ACK reply

A day in the life… connecting to the Internet

router (runs DHCP)

Link Layer 5-191

A day in the life… ARP (before DNS, before HTTP)

before sending HTTP request, need IP address of www.google.com: DNS

DNS UDP

IP Eth Phy

DNS

DNS

DNS

DNS query created, encapsulated in UDP, encapsulated in IP, encapsulated in Eth. To send frame to router, need MAC address of router interface: ARP

ARP query broadcast, received by router, which replies with ARP reply giving MAC address of router interface

client now knows MAC address of first hop router, so can now send frame containing DNS query

ARP query

Eth Phy

ARP

ARP

ARP reply

router (runs DHCP)

Link Layer 5-192

DNS UDP

IP Eth Phy

DNS

DNS

DNS

DNS

DNS

IP datagram containing DNS query forwarded via LAN switch from client to 1st hop router

IP datagram forwarded from campus network into comcast network, routed (tables created by RIP, OSPF, IS-IS and/or BGP routing protocols) to DNS server

demux’ed to DNS server DNS server replies to client

with IP address of www.google.com

Comcast network 68.80.0.0/13

DNS server

DNS UDP

IP Eth Phy

DNS

DNS

DNS

DNS

A day in the life… using DNS

router (runs DHCP)

Link Layer 5-193

A day in the life…TCP connection carrying HTTP

HTTP TCP IP

Eth Phy

HTTP

to send HTTP request, client first opens TCP socket to web server

TCP SYN segment (step 1 in 3-way handshake) inter-domain routed to web server

TCP connection established! 64.233.169.105 web server

SYN

SYN

SYN

SYN

TCP IP

Eth Phy

SYN

SYN

SYN

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

SYNACK

web server responds with TCP SYNACK (step 2 in 3-way handshake)

router (runs DHCP)

Link Layer 5-194

A day in the life… HTTP request/reply HTTP TCP IP

Eth Phy

HTTP

HTTP request sent into TCP socket

IP datagram containing HTTP request routed to www.google.com

IP datagram containing HTTP reply routed back to client

64.233.169.105 web server

HTTP TCP IP

Eth Phy

web server responds with HTTP reply (containing web page)

HTTP

HTTP

HTTP HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

HTTP

web page finally (!!!) displayed

Link Layer 5-195

Chapter 5: Summary principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing

instantiation and implementation of various link layer technologies Ethernet switched LANS, VLANs virtualized networks as a link layer: MPLS

synthesis: a day in the life of a web request

Link Layer 5-196

Chapter 5: let’s take a breath

journey down protocol stack complete (except PHY)

solid understanding of networking principles, practice

….. could stop here …. but lots of interesting topics! wireless multimedia security network management

CSMC 417Chapter 5: Link layerLink layer, LANs: outlineLink layer: introductionLink layer: contextLink layer servicesLink layer services (more)Where is the link layer implemented?Adaptors communicatingLink layer, LANs: outlineError detectionParity checkingInternet checksum (review)Cyclic redundancy checkCRC exampleCyclic Redundancy CheckCRC BasisError Detection – CRCs (1) Cyclic Redundancy CheckCommon CRC PolynomialsError Detection – CRCs (2)Framing MethodsFramingBit Oriented ProtocolsFraming (Continued)Framing – Byte countFraming – Bit stuffingFraming – Byte stuffingByte-Oriented ProtocolsFrame StructureClock-Based Framing (SONET)Sonet FrameElementary Data Link ProtocolsLink layer environment (1)Link layer environment (2)Protocol DefinitionsProtocol �Definitions�(ctd.)Utopian Simplex ProtocolUtopian Simplex Protocol (1)Utopian Simplex Protocol (2)Reliable TransmissionStop and Wait with 1-bit Seq NoStop and Wait ProtocolsStop and WaitStop-and-Wait – Error-free channelStop-and-Wait – Noisy channel (1)Stop-and-Wait – Noisy channel (2)Stop-and-Wait – Noisy channel (3)Sliding Window ProtocolsSliding Window concept (1)Sliding Window ProtocolSliding Window ProtocolSliding WindowSliding Window concept (2)Sliding Window concept (3)One-Bit Sliding Window (1)One-Bit Sliding Window (2)One-Bit Sliding Window (3)Go-Back-N (1)Go-Back-N (2)Selective Repeat (1)Selective Repeat (2)Selective Repeat (3)Example Data Link ProtocolsPacket over SONETPacket over SONET (2)PPP (1)PPP (2)PPP – Point to Point Protocol (3)ADSL (1)ADSL (2)High-Level Data Link ControlHigh-Level Data Link Control (2)The Data Link Layer in the InternetLink layer, LANs: outlineThe MAC SublayerMAC protocols: taxonomyChannel partitioning MAC protocols: TDMAChannel partitioning MAC protocols: FDMAMultiple access protocolsAn ideal multiple access protocolMultiple access links, protocolsRandom access protocolsALOHA (1)Pure ALOHA (2)Pure (unslotted) ALOHAPure ALOHA efficiencyPure ALOHA (3)Slotted ALOHASlotted ALOHASlotted ALOHA: efficiencyCSMA (carrier sense multiple access)CSMA (1)Persistent and Nonpersistent CSMACSMA CollisionsCSMA/CD (collision detection)Ethernet CSMA/CD algorithmCSMA/CD efficiencyCSMA/CD Collision DetectionCSMA (3) – Collision Detection“Taking turns” MAC protocols“Taking turns” MAC protocols“Taking turns” MAC protocolsCollision-Free Protocols (1) – BitmapCollision-Free Protocols– CountdownLimited-Contention Protocols (1)Limited Contention�Adaptive Tree WalkWavelength Division Multiple Access ProtocolsEthernetEthernetClassic Ethernet– Physical LayerEthernet Transceiver and AdapterEthernet Uses CSMA/CDEthernet CablingEthernet Cabling (2)Ethernet Cabling (3)Ethernet SignallingEthernet Frame FormatEthernet frame structureEthernet frame structure (more)Ethernet: unreliable, connectionless802.3 Ethernet standards: link & physical layersClassic Ethernet (2) – MAC Ethernet Transmitter AlgorithmIEEE 802.2: Logical Link ControlLimitations on Ethernet LengthLimitations on Ethernet LengthEthernet MAC Sublayer Protocol (2)Worst Case SecnarioEthernet PerformanceEthernet PerformanceEthernet RepeatersHubs: Physical-Layer RepeatersInterconnecting with HubsSwitchSwitched/Fast Ethernet (1)Switched/Fast Ethernet (2)Switch: Traffic IsolationSwitched/Fast Ethernet (3)Gigabit Ethernet (1)Gigabit / 10 Gigabit Ethernet (1)Gigabit / 10 Gigabit Ethernet (1)Benefits of EthernetSlide Number 144Slide Number 145 Summary of MAC protocolsLink layer, LANs: outlineMAC addresses and ARPLAN addresses and ARPLAN addresses (more)ARP: address resolution protocolARP protocol: same LANAddressing: routing to another LANAddressing: routing to another LANAddressing: routing to another LANAddressing: routing to another LANAddressing: routing to another LANAddressing: routing to another LANLink layer, LANs: outlineEthernet switchSwitch: multiple simultaneous transmissionsSwitch forwarding tableSwitch: self-learningSwitch: frame filtering/forwardingSelf-learning, forwarding: exampleInterconnecting switchesSelf-learning multi-switch exampleInstitutional networkSwitches vs. routersVLANs: motivationVLANsPort-based VLANVLANS spanning multiple switchesSlide Number 174Link layer, LANs: outlineMultiprotocol label switching (MPLS)MPLS capable routersMPLS versus IP pathsMPLS versus IP pathsMPLS signalingMPLS forwarding tablesLink layer, LANs: outlineData center networks Slide Number 184Slide Number 185Link layer, LANs: outlineSynthesis: a day in the life of a web requestA day in the life: scenarioA day in the life… connecting to the InternetA day in the life… connecting to the InternetA day in the life… ARP (before DNS, before HTTP)A day in the life… using DNSA day in the life…TCP connection carrying HTTPA day in the life… HTTP request/reply Chapter 5: SummaryChapter 5: let’s take a breath

computer networks prof. ashok k agrawala · framing – bit stuffing . link layer . stuffing done...

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