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Application Layer – DNS – Mail –
• History of WWW
• Introduction to HTTP
– Persistent vs non-persistent
– Caching/Proxies
– Requests/Responses
University of
• Still at Application Layer
– Streaming
University of
• Remember back to URLs
– We use DNS to resolve the URL to an absolute location
• Briefly mentioned IP addresses before
– For now, just consider them to be unique numerical identifiers
• 8.8.8.8 Google Public DNS server
• 203.2.218.208 – abc.com.au
Aside: Conceptually an IP addresses should uniquely identify a
socket/jack (or wireless interface) on a computer
– Often not the case today
– We will see why when look at the network layer in Week 11
Domain Name System – DNS
University of
• DNS is essentially the technology behind mapping
host.domain.com to an IP address.
• Four elements comprise the DNS:
– Domain name space: DNS uses a tree-structured name space to
identify resources on the Internet.
– DNS database: Each node/leaf in the name space tree names a set
of information that is contained in a resource record (RR). The
collection of all RRs is organized into a distributed database.
– Name servers: Server programs that hold information about a
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.
Domain Name System – DNS
University of
• Domain names:
– are not case sensitive
– can have up to 63 characters per constituent
– can have up to 255 chars per path
– can be internationalised (since 1999) – caused security problems
• Naming conventions usually follow either organisational or
physical boundaries e.g.,
– au.ibm.com / uk.ibm.com (for email)
– ibm.com.au / ibm.co.uk (for web)
Domain name characteristics
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Conceptual division of DNS
University of
TN 4th 7-1
• The same followed
within country TLDs
But exceptions…
• abc.net.au is not a
network provider
• Many new starting 2014
– .accenture
– .calvinklein
Top-level domains
University of
Resource Records
University of
TN 6th 7-4
Resource Record Example
University of
TN 6th 7-5
names ends
constituent
e.g., .com
Name server zones
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TN 6th 7-4
– DNS namespace is divided into overlapping zones. The name servers are
authoritative for that zone.
• usually two name servers for a zone
– 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.
– There are 13 “root names servers” globally
– a “root server” may be a cluster of geographically dispersed servers
• F-ROOT 252 sites; J-ROOT 162 sites
4/13/22 12
Name server zones
University of
• Top-level domain DNS servers: responsible for com, org, net, edu,
etc, and all top-level country domains uk, fr, au, jp.
– Examples include: Network Solutions maintains servers for com; and
Educause for edu
• Authoritative DNS servers: organizations DNS servers, providing
authoritative hostname to IP mappings for organizations servers
(e.g., Web, mail).
– Can be maintained by the organization itself or service provider.
• Local DNS server: Typically, each ISP (residential ISP, company,
university) has a “default name server” which handles DNS queries
– Returns cached value if one exists
– Otherwise, acts as proxy, and forwards the request up the query hierarchy
Types of name servers
University of
• A resolver client asks the local DNS for the domain to IP
– if answer is known by the local DNS, then it sends the answer.
– if answer is not known, then the local DNS queries up the hierarchy
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.
4/13/22 14
Resolving a query
University of
Example Resolver Query
University of
TN 5th 7-6
(“recursive”)
(“iterative”)(not examinable)
• Hard-coding mappings
– Unix: /etc/hosts
– Windows: C:\Windows\System32\drivers\etc\hosts
• Example:
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
University of
• No security in original design
– DNS spoofing
– DNS flooding
• Solutions
– Root signing
DNS Security
University of
University of
• 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
– RFC 821 (transmission)
– RFC 822 (message format)
– RFC 2821 and RFC 2822 (revised versions of earlier RFCs)
• Architecture and Services
– User agents (UA’s/ MUA’s)
• allow user to read and send email
– Message transfer agents (MTA’s)
• transport messages from source – destination
Email services and architecture
University of
Email services and architecture
University of
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
https://blog.mailchannels.com/what-is-an-smtp-relay-service
• Basic functions:
– compose, report, display, dispose
• Envelope and contents:
– encapsulation of transport related information
– user agent control info
– for human recipient
• User must provide message, destination, optional other
parameters
• Addressing scheme
User agent (mail program)
University of
Mail Message Format
University of
Payload format – analogous to HTML
To: Cc: Bcc:
From: Date: Subject:
Message-Id: In-Reply-To: References:
Reply-To: Sender: Return-Path:
Keywords: Received:
Message Header Fields
University of
Headers the user
usually sees
Headers the user
usually doesn’t see.
What are they for?
• SMTP uses TCP to reliably transfer email message from
client to server, default port 25
• 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
SMTP – Simple Message Transfer
University of
Transfer protocol – analogous to HTTP
SMTP – Interaction Example
University of
Note: many
back-and-forth
exchanges.
Slow on modern
networks, where
latency is larger
than serialization
Modern approach
of one header
• 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:
– other language requirements
– alternative message content type (audio/images)
• MIME has 5 additional message headers:
– MIME-Version: identifies the MIME version
– Content-Description: human readable describing contents
– Content-Id: unique identifier
– Content-Transfer-Encoding: how body is wrapped for transmission
– Content-Type: type and format of content
MIME – Multipurpose Internet Mail
Extensions
University of
MIME – Content-Types
University of
• Transfer
– SMTP: delivery/storage to receiver’s server
• Delivery
– POP3 : Post Office Protocol; authorization (agent – server) and
– IMAP: Internet Mail Access Protocol; more features (more complex);
provides for the manipulation of stored messages on server
– HTTP: gmail, Hotmail, Yahoo! Mail, etc.
Message Transfer & Access
University of
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) Current case: notebook/PC/phone is not an MTA.
Receiving Mail: Local vs Remote
University of
Possibly Intermittent
connection
TN 4th 7-15
• Three states of a POP3 transaction
– Authorisation
– Transactions
– USER / PASS
– RETR / DELE
– QUIT (update)
• Issue: “download and delete” mode does not allow
messages to be re-read.
POP3 – Post Office Protocol
University of
©4/13/22 31
University of
• IMAP keeps user state across sessions.
– Retain mailbox contents online (server) and allow manipulation of
online and offline messages and mailbox folders
– Implications of server infrastructure to support high volume of IMAP
users. This implies storage projections by the provider, and hence
limitations.
IMAP – Internet Message Access
University of
• 40% of internet download traffic is streaming (video, audio)
– Not necessarily 40% of core traffic, due to caching
• WebSockets (ws://… and wss://…)
– Multiplexing full-duplex channels over one TCP connection
– Everything on TCP port 80 or 443 (HTTP/HTTPS) to pass firewalls
– Unlike HTTP, doesn’t require client to keep requesting. streaming
• RTP + RTCP/RTSP (Real Time [Control/Streaming] Protocol)
– RTP uses a playback buffer to deliver packets with the same spacing as
they were sent, undoing network jitter
– RTCP Monitors delays, adapts video coding rate to available capacity
– RTSP provides play/record/pause services
• Real Time Messaging Protocol (RTMP, used by Flash)
University of
And finally…
University of
• The slides were based on slides prepared by ,
based on material developed previously by: ,
, , and .
• 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
Acknowledgement
University of
https://ebookcentral.proquest.com/lib/unimelb/detail.action?docID=6481879
• Non-examinable material
• Removed from course, but still useful reference
University of
• Example: new start-up Network Utopia
• Register name networkuptopia.com at DNS registrar
(e.g., Network Solutions)
– provide names, IP addresses of authoritative name server (primary
and secondary)
– registrar inserts two RRs into com TLD server:
• (networkutopia.com, dns1.networkutopia.com, NS)
• (dns1.networkutopia.com, 212.212.212.1, A)
– create authoritative server:
• Type A record for www.networkuptopia.com;
• Type MX record for networkutopia.com
Inserting records into DNS
University of
• Using DNS query tools:
– nslookup
Example of DNS query tools
University of
Application Layer – DNS – Mail – Streaming
Domain Name System – DNS
Domain Name System – DNS (2)
Domain name characteristics
Conceptual division of DNS namespace
Top-level domains
Resource Records
Resource Record Example
Name server zones
Name server zones (2)
Types of name servers
Resolving a query
Example Resolver Query
HOSTS File
DNS Security
Email services and architecture
Email services and architecture (2)
User agent (mail program)
Mail Message Format
Message Header Fields
SMTP – Simple Message Transfer Protocol
SMTP – Interaction Example
MIME – Multipurpose Internet Mail Extensions
MIME – Content-Types
Message Transfer & Access
Receiving Mail: Local vs Remote
POP3 – Post Office Protocol
IMAP – Internet Message Access Protocol
And finally…
Acknowledgement
Inserting records into DNS
Example of DNS query tools
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