COMP30023 – Computer Systems
Application Layer – DNS – Mail – Streaming
Dr 4/13/22
Copyright By PowCoder代写 加微信 powcoder
History of WWW
Introduction to HTTP
– Persistentvsnon-persistent
– Caching/Proxies
Requests/Responses Cookies
© 4/13 University of /22 Melbourne
Still at Application Layer
– Streaming
© 4/13 University of /22 Melbourne
Remember back to URLs
Briefly mentioned IP addresses before
Domain Name System – DNS
– WeuseDNStoresolvetheURLtoanabsolutelocation
– Fornow,justconsiderthemtobeuniquenumericalidentifiers • 8.8.8.8GooglePublicDNSserver
• 203.2.218.208–abc.com.au
Aside: Conceptually an IP addresses should uniquely identify a socket/jack (or wireless interface) on a computer
– Oftennotthecasetoday
– WewillseewhywhenlookatthenetworklayerinWeek11
© 4/13 University of /22 Melbourne
DNS is essentially the technology behind mapping host.domain.com to an IP address.
Domain Name System – DNS
– Domainnamespace:DNSusesatree-structurednamespaceto
Four elements comprise the DNS: identify resources on the Internet.
– DNSdatabase:Eachnode/leafinthenamespacetreenamesaset of information that is contained in a resource record (RR). The collection of all RRs is organized into a distributed database.
– Nameservers:Serverprogramsthatholdinformationabouta portion of the domain name tree structure and the associated RRs.
Resolvers: These are programs that extract information from name servers in response to client requests.
© 4/13 University of /22 Melbourne
Domain names:
– are not casesensitive
– canhaveupto63charactersperconstituent
– canhaveupto255charsperpath
– canbeinternationalised(since1999)–causedsecurityproblems
Naming conventions usually follow either organisational or physical boundaries e.g.,
– au.ibm.com/uk.ibm.com(foremail) – ibm.com.au/ibm.co.uk(forweb)
Domain name characteristics
© 4/13 University of /22 Melbourne
Conceptual division of DNS namespace
TN 4th 7-1
© 4/13 University of /22 Melbourne
Top-level domains
But exceptions…
The same followed within country TLDs
abc.net.au isnota network provider
– .accenture
Many new starting 2014 – .calvinklein
4/13/2U2niversity of Melbourne
Resource Records
TN 6th 7-4
© 4/13 University of
Absolute domain names ends in a ‘.’
Relative domain names end in a constituent e.g., .com
Resource Record Example
TN 6th 7-5
© University of
4/13/ Melbourne 22
Name server zones
TN 6th 7-4
© 4/13 University of
Name server zones
– DNS namespace is divided into overlapping zones. The name servers are authoritative for that zone.
• usuallytwonameserversforazone
– Name servers are arranged in a hierarchical manner extending from a
set of root servers
• Root name servers:
– The root servers form the authoritative cluster for enquiries. The root servers are contacted by a local name server that can not resolve name.
– Thereare13“rootnamesservers”globally
– a “root server” may be a cluster of geographically dispersed servers
• F-ROOT252sites;J-ROOT162sites
4/13/22 © University of 12 Melbourne
Types of name servers
• Top-level domain DNS servers: responsible for com, org, net, edu, etc, and all top-level country domains uk, fr, au, jp.
– Examplesinclude:NetworkSolutionsmaintainsserversforcom;and Educause for edu
Authoritative DNS servers: organizations DNS servers, providing authoritative hostname to IP mappings for organizations servers (e.g., Web, mail).
– Canbemaintainedbytheorganizationitselforserviceprovider.
• Local DNS server: Typically, each ISP (residential ISP, company, university) has a “default name server” which handles DNS queries
– Returnscachedvalueifoneexists
– Otherwise,actsasproxy,andforwardstherequestupthequeryhierarchy
© 4/13 University of
A resolver client asks the local DNS for the domain to IP mapping:
– ifanswerisknownbythelocalDNS,thenitsendstheanswer.
– ifanswerisnotknown,thenthelocalDNSqueriesupthehierarchy to the top level (root) DNS for the domain and then relays the answer to the resolver client.
Queries are subject to timers to avoid longer than necessary response times.
Resolving a query
4/13/22 © University of 14 Melbourne
Example Resolver Query
(“iterative”) (not examinable)
(“recursive”)
TN 5th 7-6
© 4/13 University of
127.0.0.1 localhost
127.0.1.1 username-VirtualBox
# ad blocking
0.0.0.0 pagead2.googlesyndication.com 0.0.0.0 static.adsafeprotected.com
Local DNS Providers
HOSTS File
– Unix: /etc/hosts
Hard-coding mappings
– Windows: C:\Windows\System32\drivers\etc\hosts
© 4/13 University of
DNS flooding
DNS Security
– DNSspoofing
No security in original design
– Rootsigning
© 4/13 University of
© 4/13 University of /22 Melbourne
Email services and architecture
• Email has a long heritage (since 1960s)
In this time, evolutionary steps in infrastructure and standards have been taken.
Standards for Internet-enabled email are based on 2 RFC’s – RFC821(transmission)
– RFC822(messageformat)
– RFC2821andRFC2822(revisedversionsofearlierRFCs)
– Useragents(UA’s/MUA’s)
Architecture and Services
• allowusertoreadandsendemail
– Messagetransferagents(MTA’s)
• transportmessagesfromsource-destination
© 4/13 University of
Email services and architecture
There may be SMTP relays between the sender’s MTA and the receiver’s https://blog.mailchannels.com/what-is-an-smtp-relay-service
TN 6th 7-9
© 4/13 University of
User agent (mail program)
– compose,report,display,dispose
Basic functions:
• Envelope and contents:
– encapsulationoftransportrelatedinformation Header
– useragentcontrolinfo
– forhumanrecipient
• Addressing scheme
User must provide message, destination, optional other parameters
© 4/13 University of
Mail Message Format
Payload format – analogous to HT
© 4/13 University of
Message Header Fields
Message-Id:
In-Reply-To:
References:
Return-Path:
Headers the user usually sees
Headers the user usually doesn’t see.
What are they for?
© 4/13 University of
SMTP uses TCP to reliably transfer email message from client to server, default port 25
SMTP – Simple Message Transfer Protocol
Typically direct transfer: sending server to receiving server
Three phases of transfer (1) handshaking (greeting), (2) transfer of messages, and (3) closure
Command/response interaction: commands in ASCII text and response consists of status code and phrase
Messages must be in 7-bit ASCII
Transfer protocol – analogous to HTT
© 4/13 University of
ote: many ack-and-forth xchanges.
low on modern
etworks, where atency is larger
han serialization elay.
odern approach f one header
SMTP – Interaction Example
© 4/13 University of
In the early days of email, messages were in English and used only ASCII – RFC 822 reflects these simple constraints. In time, the limitations of RFC822 became clear:
– otherlanguagerequirements
alternative message content type (audio/images)
MIME – Multipurpose Internet Mail Extensions
MIME has 5 additional message headers:
– MIME-Version:identifiestheMIMEversion
– Content-Description:humanreadabledescribingcontents
– Content-Transfer-Encoding:howbodyiswrappedfortransmission
– Content-Type:typeandformatofcontent
Content-Id: unique identifier
© 4/13 University of
MIME – Content-Types
© 4/13 University of
Message Transfer & Access
– SMTP:delivery/storagetoreceiver’sserver
– POP3:PostOfficeProtocol;authorization(agent–server)and download
– HTTP:gmail,Hotmail,Yahoo!Mail,etc.
IMAP: Internet Mail Access Protocol; more features (more complex); provides for the manipulation of stored messages on server
© 4/13 University of
Receiving Mail: Local vs Remote
a) Sending and reading mail when the receiver has a permanent Internet connection and the user agent runs on the same machine as the message transfer agent (now rare).
b) Currentcase:notebook/PC/phoneisnotanMTA.
Possibly Intermittent connection
TN 4th 7-15
© 4/13 University of
Three states of a POP3 transaction – Authorisation
– Transactions – Update
POP3 – Post Office Protocol
– USER/PASS
RETR / DELE QUIT (update)
Issue: “download and delete” mode does not allow messages to be re-read.
© 4/13 University of
4/13/22 © University of 31 Melbourne
IMAP – Internet Message Access Protocol
– Retainmailboxcontentsonline(server)andallowmanipulationof
IMAP keeps user state across sessions. online and offline messages and mailbox folders
– ImplicationsofserverinfrastructuretosupporthighvolumeofIMAP users. This implies storage projections by the provider, and hence limitations.
© 4/13 University of
• 40% of internet download traffic is streaming (video, audio) – Notnecessarily40%ofcoretraffic,duetocaching
WebSockets (ws://… and wss://…)
– Multiplexingfull-duplexchannelsoveroneTCPconnection
– EverythingonTCPport80or443(HTTP/HTTPS)topassfirewalls
– Unlike HTTP, doesn’t require client to keep requesting. streaming
– RTPusesaplaybackbuffertodeliverpacketswiththesamespacingas
RTP + RTCP/RTSP (Real Time [Control/Streaming] Protocol)
they were sent, undoing network jitter
– RTCPMonitorsdelays,adaptsvideocodingratetoavailablecapacity
– RTSPprovidesplay/record/pauseservices
• Real Time Messaging Protocol (RTMP, used by Flash)
© 4/13 University of
And finally…
© 4/13 University of
The slides were based on slides prepared by , based on material developed previously by: , , , and .
Acknowledgement
Some of the images included in the notes were supplied as
part of the teaching resources accompanying the text books
listed on the previous slides.
– (And also) Computer Networks, 6th Edition, Tanenbaum A., Wetherall. D.
https://ebookcentral.proquest.com/lib/unimelb/detail.action?docID=6481879
Textbook Reference: Sections 2.3, 2.4, 2.6, and related topics on pp.199-210
© 4/13 University of /22 Melbourne
Non-examinable material
Removed from course, but still useful reference
© 4/13 University of /22 Melbourne
Inserting records into DNS
Example: new start-up Network Utopia
Register name networkuptopia.com at DNS registrar (e.g., Network Solutions)
– providenames,IPaddressesofauthoritativenameserver(primary and secondary)
– registrarinsertstwoRRsintocomTLDserver:
• (networkutopia.com,dns1.networkutopia.com,NS) • (dns1.networkutopia.com,212.212.212.1,A)
– createauthoritativeserver:
• TypeArecordforwww.networkuptopia.com; • TypeMXrecordfornetworkutopia.com
© 4/13 University of
– nslookup
Example of DNS query tools
Using DNS query tools:
© 4/13 University of
程序代写 CS代考 加微信: powcoder QQ: 1823890830 Email: powcoder@163.com