Midterm Review

In class

Closed Book

One 8.5” by 11” sheet of paper permitted (single side)

Calculators Recommended

Packet Switching: Statistical Multiplexing

Sequence of A & B packets does not have fixed pattern statistical multiplexing.

In TDM each host gets same slot in revolving TDM frame.

A

B

C10 MbsEthernet

1.5 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link

Packet Switching versus Circuit Switching

• Circuit Switching

– Network resources (e.g., bandwidth) divided into “pieces” for allocation

– Resource piece idle if not used by owning call (no sharing)

– NOT efficient !

• Packet Switching:

– Great for bursty data

– Excessive congestion: packet delay and loss

– protocols needed for reliable data transfer, congestion control

Datagram Packet Switching

• Each packet is independently switched

– Each packet header contains destination address which determines next hop

– Routes may change during session

• No resources are pre-allocated (reserved) in advance

• Example: IP networks

Internet structure: network of networks

• “Tier-3” ISPs and local ISPs

– last hop (“access”) network (closest to end systems)

– Tier-3: Turkish Telecom, Minnesota Regional Network

Tier 1 ISP

Tier 1 ISP

Tier 1 ISP

NAP

Tier-2 ISPTier-2 ISP

Tier-2 ISP Tier-2 ISP

Tier-2 ISP

localISPlocal

ISPlocalISP

localISP

localISP Tier 3

ISP

localISP

localISP

localISP

Local and tier- 3 ISPs are customers ofhigher tier ISPsconnecting them to rest of Internet

Four sources of packet delay

• 1. processing:

– check bit errors

– determine output link

A

B

propagation

transmission

processingqueueing

• 2. queueing

– time waiting at output link for transmission

– depends on congestion level of router

Delay in packet-switched networks

3. Transmission delay:

• R=link bandwidth (bps)

• L=packet length (bits)

• time to send bits into link = L/R

4. Propagation delay:

• d = length of physical link

• s = propagation speed in medium (~2x108 m/sec)

• propagation delay = d/s

A

B

propagation

transmission

processingqueueing

Note: s and R are very different quantities!

Internet protocol stack• application: supporting network

applications

– FTP, SMTP, STTP

• transport: host-host data transfer

– TCP, UDP

• network: routing of datagrams from source to destination

– IP, routing protocols

• link: data transfer between neighboring network elements

– PPP, Ethernet

• physical: bits “on the wire”

application

transport

network

link

physical

HTTP connections

Nonpersistent HTTP

• At most one object is sent over a TCP connection.

• HTTP/1.0 uses nonpersistent HTTP

Persistent HTTP

• Multiple objects can be sent over single TCP connection between client and server.

• HTTP/1.1 uses persistent connections in default mode

• HTTP Message, Format, Response, Methods• HTTP cookies

Response Time of HTTP

Nonpersistent HTTP issues:

• requires 2 RTTs per object

• OS must work and allocate host resources for each TCP connection

• but browsers often open parallel TCP connections to fetch referenced objects

Persistent HTTP

• server leaves connection open after sending response

• subsequent HTTP messages between same client/server are sent over connection

Persistent without pipelining:

• client issues new request only when previous response has been received

• one RTT for each referenced object

Persistent with pipelining:

• default in HTTP/1.1

• client sends requests as soon as it encounters a referenced object

• as little as one RTT for all the referenced objects

FTP: separate control, data connections

• FTP client contacts FTP server at port 21, specifying TCP as transport protocol

• Client obtains authorization over control connection

• Client browses remote directory by sending commands over control connection.

• When server receives a command for a file transfer, the server opens a TCP data connection to client

• After transferring one file, server closes connection.

FTPclient

FTPserver

TCP control connection

port 21

TCP data connectionport 20

• Server opens a second TCP data connection to transfer another file.

• Control connection: “out of band”

• FTP server maintains “state”: current directory, earlier authentication

Electronic Mail: SMTP [RFC 2821]• uses TCP to reliably transfer

email message from client to

server, port 25

• direct transfer: sending

server to receiving server

user mailbox

outgoing message queue

mailserver

useragent

useragent

useragent

mailserver

useragent

useragent

mailserver

useragent

SMTP

SMTP

DNS name servers

• no server has all name-to-IP address mappings

local name servers:

– each ISP, company has local (default) name server

– host DNS query first goes to local name server

authoritative name server:

– for a host: stores that host’s IP address, name

– can perform name/address translation for that host’s name

Why not centralize DNS?

• single point of failure

• traffic volume

• distant centralized database

• maintenance

doesn’t scale!

DNS example

Root name server:

• may not know authoritative name server

• may know intermediate name server: who to contact to find authoritative name server

requesting hostsurf.eurecom.fr

www.cs.nwu.edu

root name server

local name serverdns.eurecom.fr

1

23

4 5

6

authoritative name serverdns.cs.nwu.edu

intermediate name serverdns.nwu.edu

7

8

DNS: iterated queries

recursive query:

• puts burden of name resolution on contacted name server

• heavy load?

iterated query:

• contacted server replies with name of server to contact

• “I don’t know this name, but ask this server”

requesting hostsurf.eurecom.fr

gaia.cs.umass.edu

root name server

local name serverdns.eurecom.fr

1

23

4

5 6

authoritative name serverdns.cs.umass.edu

intermediate name serverdns.umass.edu

7

8

iterated query

Web caches (proxy server)

• user sets browser: Web accesses via cache

• browser sends all HTTP requests to cache

– object in cache: cache returns object

– else cache requests object from origin server, then returns object to client

• Why web caching?

Goal: satisfy client request without involving origin server

client

Proxyserver

client

HTTP request

HTTP request

HTTP response

HTTP response

HTTP request

HTTP response

origin server

origin server

Caching example (3)Install cache

• suppose hit rate is .4

Consequence

• 40% requests will be satisfied almost immediately

• 60% requests satisfied by origin server

• utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec)

• total delay = Internet delay + access delay + LAN delay

= .6*2 sec + .6*.01 secs + milliseconds < 1.3 secs

originservers

public Internet

institutionalnetwork 10 Mbps LAN

1.5 Mbps access link

institutionalcache

Transport Layer

• Transport-layer services

• Multiplexing and demultiplexing

• Connectionless transport: UDP

• Principles of reliable data transfer

• TCP

– Segment structures

– Flow control

– Congestion control

Demultiplexing• UDP socket identified

by two-tuple:

(dest IP address, dest port number)

• When host receives UDP segment:

– checks destination port number in segment

– directs UDP segment to socket with that port number

• TCP socket identified by 4-tuple:

– source IP address

– source port number

– dest IP address

– dest port number

• recv host uses all four values to direct segment to appropriate socket

UDP: User Datagram Protocol [RFC 768]

Why is there a UDP?

• no connection establishment (which can add delay)

• simple: no connection state at sender, receiver

• small segment header

• no congestion control: UDP can blast away as fast as desired

source port # dest port #

32 bits

Applicationdata

(message)

UDP segment format

length checksum

UDP checksum

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:

• addition of all segment contents + checksum

• check if all bits are 1:

– NO - error detected

– YES - no error detected. But maybe errors nonetheless? More later ….

Goal: detect “errors” (e.g., flipped bits) in transmitted segment

0110010110110100

Addition:1’s complement sum:

0110010101001111

1’s complement sum:Addition:

Go-Back-NSender:

• k-bit seq # in pkt header

• “window” of up to N, consecutive unack’ed pkts allowed

• ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK”

– may deceive duplicate ACKs (see receiver)

• Single timer for all in-flight pkts

• timeout(n): retransmit pkt n and all higher seq # pkts in window

Selective Repeat

• receiver individually acknowledges all correctly received pkts

– buffers pkts, as needed, for eventual in-order delivery to upper layer

• sender only resends pkts for which ACK not received

– sender timer for each unACKed pkt

• sender window

– N consecutive seq #’s

– again limits seq #s of sent, unACKed pkts

Selective repeat: sender, receiver windows

TCP segment structure

source port # dest port #

32 bits

applicationdata

(variable length)

sequence number

acknowledgement numberReceive window

Urg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now(generally not used)

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby bytes of data(not segments!)

Internetchecksum

(as in UDP)

26

Approaches towards congestion control

End-end congestion control:

• no explicit feedback from network

• congestion inferred from end-system observed loss, delay

• approach taken by TCP

Network-assisted congestion control:

• routers provide feedback to end systems

– single bit indicating congestion (SNA, DECbit, TCP/IP ECN, ATM)

– explicit rate sender should send at

Two broad approaches towards congestion control:

27

TCP Slow Start (more)

• When connection begins, increase rate exponentially until first loss event:

– double CongWin every RTT

– done by incrementing CongWin for every ACK received

• Summary: initial rate is slow but ramps up exponentially fast

Host A

one segment

RT

T

Host B

time

two segments

four segments

28

TCP sender congestion control

Event State TCP Sender Action Commentary

ACK receipt for previously unacked data

Slow Start (SS)

CongWin = CongWin + MSS, If (CongWin > Threshold) set state to “Congestion Avoidance”

Resulting in a doubling of CongWin every RTT

ACK receipt for previously unacked data

CongestionAvoidance (CA)

CongWin = CongWin+MSS * (MSS/CongWin)

Additive increase, resulting in increase of CongWin by 1 MSS every RTT

Loss event detected by triple duplicate ACK

SS or CA Threshold = CongWin/2, CongWin = Threshold,Set state to “Congestion Avoidance”

Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS.

Timeout SS or CA Threshold = CongWin/2, CongWin = 1 MSS,Set state to “Slow Start”

Enter slow start

Duplicate ACK

SS or CA Increment duplicate ACK count for segment being acked

CongWin and Threshold not changed

29

Why is TCP fair?Two competing sessions:

• Additive increase gives slope of 1, as throughout increases

• multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughput

Con

nect

ion

2 th

roug

h pu t

congestion avoidance: additive increaseloss: decrease window by factor of 2

congestion avoidance: additive increaseloss: decrease window by factor of 2