Performance

Teoría de las Comunicacionesa.k.a “Redes”

2 Cuatrimestre 2011

“El Santo Grial de las redes de computadores es diseñar una red que tenga la flexibilidad y el bajo costo de la Internet, pero que ofrezca las

garantías de calidad de servicio extremo a extremo de la red telefónica.”

S. Keshav: 'An Engineering Approach to Computer Networking‘, 1997

Calidad de Servicio en Internet

Performance

4

Chapter 1

Foundation

Computer Networks: A Systems Approach, 5eLarry L. Peterson and Bruce S. Davie

Copyright © 2010, Elsevier Inc. All rights Reserved

Performance

Bandwidth Width of the frequency band Number of bits per second that can be transmitted

over a communication link

1 Mbps: 1 x 106 bits/second = 1x220 bits/sec 1 x 10-6 seconds to transmit each bit or

imagine that a timeline, now each bit occupies 1 micro second space.

On a 2 Mbps link the width is 0.5 micro second.

Smaller the width more will be transmission per unit time.

Performance Metrics

Bandwidth (throughput) data transmitted per time unit link versus end-to-end notation

• KB = 210 bytes !!!!!!!• Mbps = 106 bits per second !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Latency (delay) time to send message from point A to point B one-way versus round-trip time (RTT), one-two-delay

Bandwidth

Bits transmitted at a particular bandwidth can be regarded as having some width: (a) bits transmitted at 1Mbps (each bit 1 μs wide); (b) bits transmitted at 2Mbps (each bit 0.5 μs wide).

Performance

Latency = Propagation + transmit + queue Propagation = distance/speed of light . β Transmit = size/bandwidth

One bit transmission => propagation is important Large bytes transmission => bandwidth is

important

β = 1 in the vacuum, β < 1 in the media in the vacuum, usually denoted by c, is a physical

constant , 300.000 km/sec

Delay X Bandwidth

We think the channel between a pair of processes as a hollow pipe

Latency (delay) length of the pipe and bandwidth the width of the pipe

Delay of 50 ms and bandwidth of 45 Mbps 50 x 10-3 seconds x 45 x 106 bits/second 2.25 x 106 bits = 280 KB data.

Network as a pipe

Delay x Bandwidth Product ( 2) Amount of data “in flight” or “in the

pipe” Example: 100ms x 45Mbps = 560KB

Bandwidth

Delay

Delay X Bandwidth

Relative importance of bandwidth and latency depends on application For large file transfer, bandwidth is critical For small messages (HTTP, NFS, etc.),

latency is critical Variance in latency (jitter) can also affect

some applications (e.g., audio/video conferencing)

Delay X Bandwidth

How many bits the sender must transmit before the first bit arrives at the receiver if the sender keeps the pipe full

Takes another one-way latency to receive a response from the receiver

If the sender does not fill the pipe—send a whole delay × bandwidth product’s worth of data before it stops to wait for a signal—the sender will not fully utilize the network

Delay X Bandwidth

Infinite bandwidth RTT dominates Throughput = TransferSize / TransferTime TransferTime = RTT + 1/Bandwidth x

TransferSize Its all relative

1-MB file to 1-Gbps link looks like a 1-KB packet to 1-Mbps link

Relationship between bandwidth and latency

A 1-MB file would fill the 1-Mbps link 80 times, but only fill the 1-Gbps link 1/12 of one time

Bandwidth versus Latency

Relative importance 1-byte: 1ms vs 100ms dominates 1Mbps vs

100Mbps 25MB: 1Mbps vs 100Mbps dominates 1ms vs 100ms

Infinite bandwidth RTT dominates

• Throughput = TransferSize / TransferTime• TransferTime = RTT + 1/Bandwidth x TransferSize

1-MB file to 1-Gbps link as 1-KB packet to 1-Mbps link

Algunas de los slides siguientes estan basadas en :

Redes de computadores: un enfoque descendente basado en Internet, 2ª edición.Jim Kurose, Keith Ross

Nota sobre el uso de estas diapositivas ppt:Proporcionamos estas diapositivas de forma gratuita para todos (profesores, estudiantes, lectores). Se encuentran en formato PowerPoint, por lo que puede añadir, modificar y borrar diapositivas (incluida la presente) y su contenido según sus necesidades. Evidentemente, significan un gran trabajo por nuestra parte. A cambio, sólo pedimos para su uso: Que mencione la fuente si usa estas diapositivas (por ejemplo, en clase), sin alterar su contenido de forma considerable (¡nos gustaría que la gente usara nuestro libro!). Que indique que dichas diapositivas son una adaptación o copia de las nuestras y que muestre el copyright de nuestro material si cuelga las mismas en un sitio web, sin alterar su contenido de forma considerable.¡Gracias y disfrute! JFK/KWR

Copyright 1996-2002.J.F Kurose y K.W. Ross.Todos los derechos reservados.

¿Cómo se producen el retardo y la pérdida?

Paquetes encolados en los búferes de router: La tasa de llegada de paquetes al enlace excede la

capacidad de salida del enlace. Cola de paquetes esperando turno.

A

B

Paquetes en transmisión (retardo)

Paquetes encolados (retardo)

Búferes libres (disponibles): paquetes de llegada abandonados (pérdida) si no haybúferes libres

Cuatro fuentes de retardo de paquetes 1. Procesamiento nodal:

Comprueba errores de bit. Determina la salida del

enlace.

A

B

Propagación

Transmisión

Procesamientonodal Encolado

2. Encolado: Tiempo de espera

para un enlace de salida para la transmisión.

Depende del nivel de congestión del router.

Tipos de Retardo

Componentes del retardo extremo a extremo:Retardo de ProcesamientoRetardo de ColasRetardo de Transmisión Retardo de Propagación

Retardo de ProcesamientoTiempo requerido en analizar el

encabezado y decidir a dónde enviar el paquete (ej. decisión de enrutamiento) En un enrutador, dependerá del número de

entradas en la tabla de rutas, la implementación (estructuras de datos), el hardware, etc.

Puede incluir la verificación de errores

Retardo de Colas

Tiempo en que el paquete espera en un búfer hasta ser transmitido

El número de paquetes esperando en cola dependerá de la intensidad y la naturaleza del tráfico

Los algoritmos de colas en los enrutadores intentan adaptar estos retardos a ciertas preferencias, o imponer un uso equitativo

Retardo de Transmisión

El tiempo requerido para empujar todos los bits de un paquete a través del medio de transmisión

Para R=Tasa de bits, L=Longitud del paquete, d = delay o retardo:

d = L/R

Por ejemplo, para transmitir 1024 bits utilizando Fast Ethernet (100 Mbps):

d = 1024/1x10e8 = 10.24 micro segundos

Una vez que el bit es 'empujado' en el medio, el tiempo transcurrido en su propagación hasta el final del trayecto físico

La velocidad de propagación del enlace depende más que nada de la distancia medio físico Cercano a la velocidad de la luz en la mayoría de

los casos Para d = distancia, s = velocidad de propagación

Dp = d/s

Retardo de Propagación

Transmisión vs. Propagación

Puede ser confuso al principioConsiderar un ejemplo:

Dos enlaces de 100 Mbps. Fibra óptica de 1 KmVia Satélite, con una distancia de 30Km entre

base y satélite

Para dos paquetes del mismo tamaño, cuál tiene mayor retardo de transmisión? Y propagación?

Retardo en redes de conmutación de paquetes3. Retardo de transmisión: R=ancho de banda del

enlace (bps). L=longitud del paquete

(bits). Tiempo de envío de bits

hacia el enlace = L/R.

4. Retardo de propagación: d = longitud del enlace físico s = media de velocidad de

propagación (~2x108 m/sec) Retardo de propagación=d/s

A

B

Propagación

Transmisión

Procesamientonodal Encolado

Nota: ¡s y R son cantidades muy distintas!

Analogía de la caravana

Los coches se “propagan” a 100 km/h.

El peaje tarda 12 seg en servir a un coche (tiempo de transmisión).

Coche~bit; caravana ~ paquete.

P: ¿Cuánto tiempo transcurre hasta que la caravana se alinee ante el segundo peaje?

Tiempo para “soltar” toda la caravana pasando los peajes a la autopista = 12*10 = 120 seg.

Tiempo hasta que el último coche se propaga del primer al segundo peaje: 100km/(100km/h)= 1 h.

R: 62 minutos

PeajePeajeCaravana de10 coches

100 km

100 km

Analogía de la caravana

Ahora los coches se “propagan” a 1000 km/h.

El peaje tarda 1 min en servir un coche.

P: ¿Llegarán los coches al segundo peaje antes de que se sirva a todos en el primero?

¡Sí! Tras 7 min., el primer coche se encuentra en el segundo peaje y aún quedan tres coches en el primero.

¡El primer bit de paquete puede llegar al segundo router antes de que se haya transmitido el paquete por completo al primer router! Véase applet Ethernet en el

sitio Web de AWL.

PeajePeajeCaravana de10 coches

100 km

100 km

Retardo nodal

dproc = retardo de proceso Normalmente unos pocos microsegundos o menos.

dcola = retardo de cola Depende de la congestión.

dtrans = retardo de transmisión = L/R, significativo para enlaces de baja velocidad.

dprop = retardo de propagación Desde unos pocos microsegundos hasta a cientos de

milisegundos.

proptranscolaprocnodal ddddd

Retardo de cola (repaso)

R = ancho de banda del enlace (bps).

L = longitud del paquete (bits).

a = media de tasa de llegada del paquete.

Intensidad de tráfico = La/R

La/R ~ 0: media de retardo de cola pequeño.

La/R -> 1: aumentan los retardos. La/R > 1: ¡Llega más “trabajo” del que

puede servirse, media de retardo infinita!

Media de retardo de cola

Perfomance del Protocolo de Ventana deslizante

Sea un Host A, que usa protocolo de ventana deslizante, si debe transmitir un archivo de unos 10 GB con un Host B

Window size = 64KB RTT de la red es de 1 segundo

Cual es la velocidad esperada con que el emisor envia datos?

64KB/s

Sliding Window

Host A

Host BACK

Window Size

Round-trip time

(1) RTT > Window size

ACK

Window Size

Round-trip time

(2) RTT = Window sizeACK

Window Size???

Utilización vs RTT normalizado

Fuente : Stallings W . Comunicaciones y Redes de Computadores- 7 Edición

Throughput protocolo ventana delizante

T < W/RTT !!!!!!!

One way delay

Fluctuación del retardo—“Jitter”

t

t

Emisor Transmite

Receptor Recibe

AA BB CC

AA BB CC

50 ms

Emisor Receptor

Red

50 ms 90 ms

Congestión

Retardo: 70 ms 20 ms (retardo: 70 ms, jitter: 40 ms)

Red vacía

Fin temas del primer parcial

Los slides que siguen los iremos viendo a lo largo de la segunda

parte de la materia

Network Measurement

Bandwidth Analysis

Why measure bandwidth?

Network congestion has increased tremendously. Bottlenecks are not always obvious. Measuring bandwidth may become more essential for service

providers as congestion increases. Measuring bandwidth enables us to improve current systems

as well as diagnosis network problems. Measuring bandwidth may be the key to observing what is

wrong with current protocol standards. In effect, measurement is a tool for research in general.

Why measure bandwidth?

Network congestion has increased tremendously. Bottlenecks are not always obvious. Measuring bandwidth may become more essential for service

providers as congestion increases. Measuring bandwidth enables us to improve current systems

as well as diagnosis network problems. Measuring bandwidth may be the key to observing what is

wrong with current protocol standards. In effect, measurement is a tool for research in general.

Why measure bandwidth?

Network congestion has increased tremendously. Bottlenecks are not always obvious. Measuring bandwidth may become more essential for service

providers as congestion increases. Measuring bandwidth enables us to improve current systems

as well as diagnosis network problems. Measuring bandwidth may be the key to observing what is

wrong with current protocol standards. In effect, measurement is a tool for research in general.

How we measure bandwidth

It’s really more complicated than a connection speed. We might want to look at capacity or we might be looking at

throughput or bandwidth congestion. We can observe packet loss, propagation delay, link capacity,

but some of this results in educated “guess work.” There are many theories and applications intended to

measure bandwidth and network statistics. For our purposes we will look at the three most common

utilities used: traceroute, ping, and pathchar. It really depends on what you are after!

traceroute

Written by Van Jacobsen in 1988 to solve persistent network problems.

Traceroute counts hops : roughly tracing the path of an IP packet from the client to the destination.

Traceroute does this by sending UDP packets with an extremely short TTL.

If all routing nodes in the path are working properly, an ICMP (Internet Connection Message Protocol) Time Exceeded message is sent (RFC 792).

traceroute

Traceroute utilizes the information encapsulated in the ICMP message to determine the source (the router that sent the packet).

We continue sending packets until we get an ICMP “host unreachable message” (this implies that we have reached the destination) or until the max number of hops has been reached.

traceroute disadvantages

Traceroute is a simple tool that is based on a few key ideas:1. All packets will be sent on the same paths (going).2. Consistent Routing (all packets will be routed back the same way).3. TCP/IP implementations supporting ICMP.

In reality, poor TCP/IP implmentation means that Traceroute is not dependable.

Using three different Traceroute implementations, to the same IP address, resulted in three different routes.

ping

One of the most widely distributed analysis tools. First released in 1980.

The UNIX version of ping is slightly more robust, allowing us to specify the testing data and modify the patterns.

ping, works by sending a single packet and waiting for the ICMP Echo response.

ping

ping puts its own Round Trip Time value on each packet so we are not left at the mercies of the router (as in traceroute).

ping also provides us with a diagnostic of ICMP messages, usually buried by the system.

ping is clearly a much different tool from traceroute, but it’s simplicity makes it more reliable.

ping is only useful for estimating bandwidth under certain conditions.

Pathchar

Also written by Van Jacobson, in 1997. pathchar attempts to improve upon traceroute by adding

mathematical analysis to the problems that occur in propagation.

Working in the same basic manner as traceroute, pathchar sends out packets and waits for the response. Only instead of one set of packets, it sends out several.

The difference being the analysis of the returned data. pathchar attempts to account for:

- loss rate- link capacity- propagation and queing delay

(Grossglauser, pg40)

pathchar

Taking into account the rtt from two nodes, say n and n – 1 we generate the following formula using Van Jacobson’s specifications:

But he assumes three things:

The error message is small enough to ignore (toss error_size/bandwidth out) The forward time is not big enough to worry about. If enough transmission groups are sent at least one will not have any queuing

delays.

And so, we get:

pathchar

In practice pathchar is not the easiest tool to use. It can be difficult to

implement and its output is often chaotic. A better implementation of pathchar was made by Bruce Mah, called pchar Here is an example of a particular node in a trace.

What we learn from pathchar

Pathchar’s focus is on the statistics of data loss and the analysis of delay.

Instead of capacity, we can look at data loss and latency. Using pathchar and traceroute, one is more likely to track

down the source of delay than to estimate bandwidth in the sense of capacity.

In a commercial sense, we can utilize this information to see where end users are running into difficulty.

In private application weak network components can be sorted out.

For our purposes, bandwidth congestion allows us to think intelligibly about improving network protocols and gives us some real world metric to diagnosis real world problems.

Conclusion

There are two things we can conclude from this: ICMP may need to be rewritten to facilitate better tools. Bandwidth Analysis is at it’s heart a simple idea.

Sources

Downey, Alan B. “Using pathchar to estimate Internet length characteristics.” http://www.acm.org/sigcomm/sigcomm99/papers/session7-1.pdf. 1999

Jacobson, Van. “pathchar – A Tool to Infer Characteristics of Internet Paths.” ftp://ftp.ee.lbl.gov/pathchar/msri-talk.pdf. April 21, 1997.

Postel, J. “RFC 792 Internet Control Message Protocol.” http://www.freesoft.org/CIE/RFC/792/index.htm. September, 1981.

Pérdida de paquetes

La cola (también conocida como búfer) que precede a un enlace en el búfer tiene una capacidad limitada.

Cuando un paquete llega a una cola llena, éste es abandonado (es decir, se pierde).

El paquete perdido puede o no ser retransmitido por un nodo anterior, por una fuente del sistema terminal.

Adicionales ( Importantes!)

Payload VozPayload Voz

TransporteTransporte

RedRed

EnlaceEnlace

FísicoFísico

G.711, G.729, G.723(.1)G.711, G.729, G.723(.1)

RTP/UDPRTP/UDP

IPIP

MLPPP/FR/ATM AAL1 MLPPP/FR/ATM AAL1

– – –– – –

Voz sobre IPProtocolos de señalización ( H.323, MGCP, SGCP, SIP etc)

Requerimientos de ancho de banda de los Codecs

Encoding/Compression

G.711 PCM A-Law/u-Law

G.726 ADPCM

G.727 E-ADPCM

G.729 CS-ACELP

G.728 LD-CELP

G.723.1 CELP

Resulting Bit Rate

64kbps (DS0)

16, 24, 32, 40 kbps

16, 24, 32, 40 kbps

8kbps

16kbps

6.3/5.3 kbps

Fluctuación del retardo—“Jitter”

t

t

Emisor Transmite

Receptor Recibe

AA BB CC

AA BB CC

50 ms

Emisor Receptor

Red

50 ms 90 ms

Congestión

Retardo: 70 ms 20 ms (retardo: 70 ms, jitter: 40 ms)

Red vacía

Reducción del Jitter

La principal causa de jitter es la congestión Se puede reducir el jitter añadiendo un retardo

adicional en el lado del receptor. Por ejemplo con un retardo de 70 20 ms se puede asegurar jitter 0 si se añade un retardo de 40 ms (90 0 ms).

Para el retardo adicional el receptor ha de tener un buffer suficientemente grande.

En algunas aplicaciones no es posible añadir mucho retardo pues esto reduce la interactividad. Ej.: videoconferencia, telefonía por Internet

PSTN

PSTN

Wireless access network

“voz paquetizada” one-way delay aceptable : 0-150 ms (ITU-T G.114)

Latencia=transmisión + propagación + queuing delay

• Queuing delay: variable, se necesita acotar

=> Assume 10* hops, per hop queuing delay < 5 ms (*Vern Paxson’s results: 8-12 hops typical across U.S.)

Bandwidth Estimation:Metrics Mesurement Techniques and Tools

By Ravi Prasad, Constantinos Dovrolis, Margaret Murray and Kc Claffy

IEEE Network, Nov/Dec 2003

Variable size packet probing

Measures capacity of each hop Measure RTT Limit packet propogation by TTL Uses ICMP to measure RTT until that

hop

Variable size packet probing

RTT includes: Serialization delay

• Delay to send packet of length L across channel of capacity C = L/C

Propogation delay• Time taken to traverse the link

Queuing delay• Delay in routers/Switches

Variable size packet probing

Send multiple packets and calculate minimum RTT Assumption: for minimum RTT no queuing

delay

RTT has two terms Delay independent of packet size = α Based on packet size

LC

LLT

i

ki

ki

1

)(

i

k ki C1

1where

Variable size packet probing

1

1

iiiC

i

k ki C1

1

packet size vs RTT

Packet pair dispersion probing

Measures end-to-end capacity

iinout C

L,max

C

L

C

L

C

L

iHiiHir

,...0,...1,0 minmax

Problems

Assumption that no other traffic exists is not real

Existing traffic can increase/decrease the estimate

Solution? Send multiple pairs and get a statistical

estimate• Does not always yield a correct estimate

Self-loading periodic streams

Measures end-to-end available bandwidth

Sends k packets at different rates Receiver notifies the “one way delay

trends” If stream rate greater than available b/w

One way delay will grow large Else

Packets will not make the one way delay large

One way delay

Train dispersion probing

Similar to packet pair dispersion probing Instead of sending just two packets

send a train of packets Calculate the average dispersion rate

Taxonomy of estimation toolsPer-hop capacity estimation

tools

Pathchar First tool to implement

Clink On routing instability collects data along all

paths Until one path provides statistically

significant estimate Pchar

Uses linear regression algorithms

Taxonomy of estimation toolsend-to-end capacity

estimation tools

BProbe Uses packet pair dispersion Uses variable sized packets to improve efficiency Access needed only in sender side, uses ICMP

messages

Nettimer Uses sophisticated “kernel density algorithm” to

provide better accuracy

Pathrate Sprobe

Available bandwidth estimation tools CProbe

Measures dispersion of a train of eight maximum sized packets

It measures dispersion rate and not available bandwidth

Dispersion rate depends on all links of the path and the train’s initial rate

Available b/w depends only on tight link of the path Pathload

Implements SLoPS Used UDP and requires access at both ends Reports range

• Center represents the average• Range represents values during mesurement period

TCP throughput and BTC measurement tools Treno

emulates TCP sends UDP packets to receiver

Replies with ICMP port unreachable Does not require access to remote end ICMP rate limited

• Accuracy of Treno affected

Cap More accurate than Treno Uses UDP for TCP data and ACK Requires access at both ends

Intrusiveness

If probe packets comparable to available b/w

VPS are non intrusive One packet per RTT

PPTD tools create bursts which last only for a very short duration Only a small fraction of available b/w used

BTC tools are intrusive They capture all b/w for a specific duration

Teoría de Colas

Notación Kendall

A/B/m/K/NA: distribución del tiempo de entrada de paquetes.B: distribución del tiempo de servicio de paquetes.

Ej.: M (Markov) , D (Determinístico), G (General)m: cantidad de servidores para procesar el

trabajo.K: capacidad máxima de paquetes (cola +

servicio). (Si es ∞ se omite.)N:disciplina de cola [FIFO, LIFO, Random, Priority].

(Si se omite es FIFO.)

Cola M/M/1

Es una cola en la que la entrada de los paquetes es un proceso de Poisson y el tiempo de servicio de paquetes es exponencial.

Hay un único servidor. La cola tiene capacidad infinita. (L(t)=0)