ece 5595 week 1. outline vocabulary protocols reference models types of service the client-server...

ECE 5595 Week 1

Outline

• Vocabulary• Protocols • Reference Models• Types of service• The client-server model• Sockets• Delay• Delay-bandwidth product

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Vocabulary (1)

• Network, Internetwork, WAN, MAN, CAN, LAN, PAN

• Stations, hosts, systems• Subnet, Segment, Link• Router, bridge, switch, hub, repeater• Switching• Point-to-Point (P2P) segment, Shared

segment• Contention, Collisions

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Vocabulary (2)

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Vocabulary (3)

• Simplex, Half-duplex, Full-duplex• Unicast, Broadcast, Multicast• Topology: the “shape” or pattern

– Physical and logical topology may be different • Mesh: P2P links between pairs of hosts

– A fully connected mesh of N devices requires N*(N-1)/2 separate P2P links, i.e., its “Big O” is N squared.

5Fully connected Partially connected

Vocabulary (4)• Bus or backbone: Big O is N• Ring: Big O is N• Star (or hub and spoke): Big O is N

– Physically the result of collapsing the backbone or ring into a “hub” in a communications closet

– Each host is a “home run” to the closet – The result is called “structured wiring”– Often used with bus (Ethernet) or ring (FDDI)

logical topologies

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Vocabulary (5)

Bus or backbone Collapsed backbone

Ring Collapsed ring

hub

hub

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Vocabulary (6)

• Multiplexing, De-multiplexing: • Statistical multiplexing: relying on the

network-usage statistics to allow oversubscription of a network resource

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Outline


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Protocols (1)

• The network provides communication services

• Software objects called protocols implement these services– The networking problem is complex– The protocol objects are layered to split up the

problem– “Protocol” also describes the operation and

messages exchanged by one protocol object and its peer object in another device

• Networking creates patterns of interactions between protocol objects

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Protocols (2)

• Each protocol object has two different interfaces– Service interface: The interface to a higher-

layer protocol object on the same system • It defines the operations that the higher-layer protocol

object can perform on this protocol object • A protocol accepts its Service Data Unit (SDU) or

payload at the service interface– Peer-to-peer interface: the message

interaction between this protocol object and its peer on another system• Peer-to-peer communication is indirect (or virtual)

except at the hardware layer• A protocol sends Protocol Data Units (PDUs) on this

interface11

Protocols (3)

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Protocols (4)

• A protocol object may provide service to multiple, higher-layer, protocol objects (via multiplexing)– As the PDU is created from the SDU a tag (or

address) is added to the PDU to distinguish between the various higher-layer customers• This allows the SDU to be extracted at this layer’s peer

and handed back to the peer of the higher-layer customer

• The PDU may also include information to support fragmentation and re-assembly, error detection or correction, flow control, or any other service characteristics that the protocol object provides to its customers 13

Protocols (5)

• The PDU for a protocol object is created from the SDU by encapsulating the SDU (= the payload) between a header and a (optional) trailer – The header and optional trailer provide the

additional information needed to support the service provided by the protocol object to its customers

– The SDU contents and structure are opaque to the protocol object and the encapsulation process – the payload is just a bunch of bytes

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Protocols (6)

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Protocols (7)

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Outline


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Reference Models (1)

• A reference model is the formal name for a protocol suite – a collection of protocols and layer definitions that provide network service

• We discuss 2 models– The 7-layer, Open Systems Interconnection (OSI)

model created primarily by the phone companies– The hybrid model derived from the 5-layer

Transmission Control Protocol (TCP) / Internet Protocol (IP) model created by the Internet Engineering Task Force (IETF)

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Reference Models (2): OSI

voltages, cabling

framing

routing

end-to-end boundary

synchronization

“endian” issues

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Reference Models (3): TCP/IP

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Layer 3 is often called the network layer and Layer 2 the data-link layer – borrowing from the OSI model


• Layer 1 = Physical Layer – Actually moves bits to its peer

• Layer 2 = Network Interface (aka Data Link) Layer– Creates frames from the Internet layer payload

• Having a unit of bytes called a frame permits checksums, for example, allows a header to be applied, etc.

– Sends frames to the physical layer for transmission

– Standards often describe a combination of layer 2 on a specific layer 1 medium

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• Layer 3 = Internet (aka Network) layer– Performs internetworking role

• Forwards packets between layer 2 networks• This layer is end-to-end but multiple peer-to-peer hops

may be needed to reach the destination• Creates a virtual, uniform network on top of

heterogeneous layer-2 network technologies– Creates packets from data sent by transport layer

• May perform fragmentation in the TCP/IP protocol suite– Sends packets to layer 2 to be sent through the

data link layer

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• Layer 4 = Transport layer– Lowest layer that is completely end-to-end

• The peers are the endpoints rather than the intermediate hops

– Provides service characteristics required by the application layer• Reliable stream service: Transmission Control Protocol

(TCP)• Best-effort message service: User Datagram Protocol

(UDP)

• Layer 5 = Application layer• Other layers are added as necessary• Not every object has protocol objects at

every layer– The protocols, the flows between the protocol

objects and the patterns that they form are what we study

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Outline


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Types of service (1)

• Connection-oriented vs. Connectionless– Connection-oriented requires setup prior to use

but has more predictable characteristics• Reliable vs. Unreliable

– Reliable service uses Acknowledgments and Timeouts as its fundamental mechanisms

• Datagram or message versus Stream– Datagram: connectionless, unreliable, many-to-

many, sequence of discrete messages of finite length (UDP)

– Stream: connection-oriented, reliable, one-to-one, sequence of individual bytes, arbitrary length (TCP)

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Outline


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Client-Server (1)

• A server starts first and waits to be contacted– It does not know which clients will contact it – It continues to run after servicing one client

• A client starts second and initiates the connection– It must know what server to contact– It initiates contact only when necessary

• A computer can run– One or more clients of the same type, or multiple

clients of different types, etc. – One or more servers for providing one or more

services– A combination

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Client-Server (2)

• An iterative server handles one request at a time

• A concurrent server handles multiple requests at a time with N+1 threads used for N requests– A main process handles receiving the requests

and handing them off to child processes• An application may have both server and

client relationships to other applications– Circular relationships can lead to a deadlock – This is an example of a loop – and loops are a

pattern that we must avoid at every layer30

Client-Server (3)

• How does a client identify a server? – In TCP/IP an application service is identified in two

steps:• Identify the hardware interface where the service can be

reached: an IP address• Identify which service at that interface: a TCP, or UDP

port number– The IP address is also used to locate the interface

on the network at layer 3– Typically a Domain Name Service (DNS) name is

specified and that is translated by DNS into an IP address

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Outline


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Sockets (1)

• A networking API called Sockets was distributed as part of the original UNIX

• An application creates a socket, and then invokes functions to adjust the socket details and to pass data through the socket– The socket descriptor returned by the socket call

is an argument to those other functions– The result it that each function call is relatively

simple, but it takes a sequence of function calls to actually pass data

– Address to name, and byte reordering routines are also provided

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Sockets (2)Name Used By Description

accept server Accept an incoming connection

bind server Specify IP address and protocol port

close either Terminate communication

connect client Connect to a remote application (active open)

getpeername server Obtain client’s IP address

getsocketopt server Obtain current options for a socket

listen server Prepare socket for use by a server (passive open)

recv either Receive incoming data or message

recvmsg either Receive data (message transport)

recvfrom either Receive a message and sender’s address

send (write) either Send outgoing data or message

sendmsg either Send outgoing data (message transport)

sendto either Send outgoing message (variant of sendmsg)

setsocketopt either Change socket options

shutdown either Terminate a connection (in one direction, i.e., half-close)

socket either Create a socket for use by the other functions

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Sockets (3)

Illustration of stream socket function calls

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Outline


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Performance (1)

• There are several important measures of network performance– Throughput or capacity: data transferred per unit

time• Throughput is limited by the slowest link traversed

– Delay or Latency: time required to complete some step of network activity; there are several different components of delay that are of interest

– Jitter or variability: the statistical description of how the delay changes

• A user computes throughput as follows– Throughput = amount of data transferred / time it

took37

Performance (2)

• We are surprised when the throughput doesn’t match the data rate of the network, but data rate is an ideal number– Protocols have overhead such as headers– Helper protocols such as DNS consume time– Call setup for a connection consumes time– In general, the other desirable characteristics of a

data communication come at the expense of throughput• Congestion avoidance, reliability, etc.

– Finally, the nominal data rate assumes the network is full of data all of the time• This may not be true due to protocol limitations or

configuration 38

Performance (3)

• Latency (delay)– Specifically the time between when the first bit is

placed on the wire and when the last bit leaves the wire at the other end

• We discuss three elements of delay– Transmit time or delay (not considered in the

textbook)– Propagation delay– Queuing delay (we include the textbook’s

switching delay and access delay in this category)– We ignore the text’s server delay as a non-

network delay

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Performance (4)

• Transmit time for a message is the time needed to put that message into the network– It is also the time for the message to transit past a

single point• Transmit time = message size / data rate

– For a typical file on a LAN this is the predominant delay component so we mistakenly believe this is the only component under all circumstances

• Transmit delay is based on data rate – so we can throw money at it– i.e., buy a higher data-rate network

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Performance (5)• Propagation delay is the time for an event to

travel from point A to point B– Event: start of a bit, end of a bit, etc.

• Propagation delay = distance / propagation_velocity

– For most network media the propagation velocity is a fraction of the speed of light

• In general – one cannot buy lower propagation delay– May be able to switch from a geo-synch satellite

to a land-line in some circumstances– Protocol design must work around expected delay

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Performance (6)

• Queuing delay is time spent in network equipment– In store and forward switching you store the entire

inbound frame before forwarding on the output port• As a minimum this adds another transmit time for the

frame– A newly arrived frame once it is switched into an

output queue must wait for all previous frames to be transmitted, i.e., the transmit time for all of those frames

– Variation in overall delay is called jitter and that is often the result of variations in queuing delay• To minimize jitter we want smaller atomic units of data,

(often called frames or packets) but that compromises efficiency with a poor ratio of header to payload

– Increases in data rate also reduce queuing delay

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Performance (7)

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Outline


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Delay-bandwidth (1)

• A school (file) full of people (bits) must be moved to a new school

• A fleet of taxicabs carries people 4 at a time (a frame) to the new school

• The cabs enter the highway at a fixed rate (the data rate = frame rate x bits per frame)– Controlled by a policeman at the intersection, for

example

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Delay-bandwidth (2)

• All cabs travel the same route at the speed limit (the propagation velocity) and experience the same delays at intersections (queuing delays)

• Time to transfer the whole school will be the time between when the first and the last cab leave the parking lot (transmit time), plus the time it takes for the last cab (= time for any cab) to make the trip (propagation delay and queuing delay)

• Does the rate at which cabs enter the highway have any bearing on how far a cab travels? No!– The data rate does not affect the propagation

delay

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Delay-bandwidth (3)

• Assume we could send 20-person buses at the same interval – but otherwise follow the same rules - we have 5 times higher data rate. Does the bus get there any faster than a cab? No– The data rate does not affect the propagation

delay• Will the transfer be completed faster with

the buses? Yes, but it won’t be 5 times faster – It will be close to 5 if the new school is just down

the street (minimum propagation delay so the reduction in transmit time is the predominant effect)

– It will be much less significant a savings if the new school is across the country

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Delay-bandwidth (4)

• What if we only had one taxi and we had to wait for it to get back before we could send it again?

• To achieve the fastest transfer we must have enough taxis so we can keep sending them at the data rate until the first one gets back and can be sent again

• Assume we send a cab every 2 minutes (data rate) and they take 120 minutes (round-trip delay) to get back1 cab

x 120 minutes 60 cabs2 minutes

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Delay-bandwidth (5)

• Amount of data “in the pipe” or “in flight” is determined by the delay-bandwidth product– It is really the delay-”data rate” product

• Round-Trip Time (RTT) is used because acknowledgement is involved – One-way-delay * data rate will have been

received and one-way-delay * data rate will still be in the pipe before the acknowledgement gets back to the sender that the first data has arrived

• For the school analogy – 30 cabs on the way out, 30 cabs on the way back at any time

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Delay-bandwidth (6)

Sender

One-way Delay = 100

Receiver

Assume sender can use the full bandwidth and has 200 * data_rate of data

t=0

t=50

t=100first unit of data arrives; first acknowledgement starts back

50th time unit of data arrives; 50 acks on the way back

t=150

t=200100th time unit of data arrives; 100 acks on the way back; sender sees ack for first unit of data

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Delay-bandwidth (7)

• Delay used in computation is often just the round trip propagation delay– Usually computed in a WAN setting where the

propagation delay dominates– Queuing delay is often estimated or ignored– Transmit delay component would be for a single

frame, and a single ACK in the other direction: usually too small to matter in a WAN

• A sender must buffer the RTT delay-data_rate product amount of data to keep the pipe full if there is any requirement to resend lost data

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Delay-bandwidth (8)

Sender

One-way Delay = 100

Receiver

Assume sender can use the full data_rate and is only allowed 50 * data_rate unacknowledged due to buffer limitations

t=0

t=50

t=150

t=100

t=200

t=225

t=250

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ece 5595 week 1. outline vocabulary protocols reference models types of service the client-server...

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