Networking Stuff Flashcards
Differentiating factors of networks - two transmission types
Broadcast
Point to point
IP
Provides a“best-effort”service to route datagrams from source host to destination host. These hosts may be on same network or on different networks.
Connectionless model
Design goals:
services should be independent of router technologies
transport layer should be shielded from number, type and topology of
routers
network addressing should use a uniform numbering plan
Internet control message protocol
The ICMP Internet control message protocol is used for testing and
monitoring ambient conditions between hosts and routers.
application layer
The Application layer protocols define:
Types of messages exchanged: e.g., request, response
Message syntax: what fields in messages and how fields are delineated
Message semantics: meaning of information in fields
Rules for when and how processes send and respond to messages
DNS – Domain Name System
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.
name servers
Top-level domain DNS servers: responsible for com, org, net, edu, etc,
and all top-level country domains uk, fr, au, jp. Example 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 organization or service provider.
Local DNS server: does not strictly belong to hierarchy. Typically, each
ISP (residential ISP, company, university) has a “default name server”
which handles DNS queries – a query is sent to its local DNS server acts
as proxy, forwards query into hierarchy
resolving query
A resolver client asks the local DNS for the domain to IP mapping:
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.
Essentially, this is a recursive query mode. Queries are subject to timers to
avoid longer than necessary response times.
mail has 2 agents
user and message transfer
User agent (or mail program)
Basic functions: compose, report, display, dispose
Envelope and contents: encapsulation of transport related
information
Header and body: header - user agent control info; body for human
recipient
User must provide message, destination, optional other parameters
Addressing scheme user@dns-address
SMTP – Simple Message Transfer Protocol
uses TCP to reliably transfer email message from client to server, port
25
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
Applications Email
MIME – Multipurpose Internet Mail Extentsions
In the early days of email, messages were in English and used only ASCII -
RFC 822 reflects these simple constraints. In time, the inadequacy of
RFC822 became apparent: eg., other language requirements and
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
Message transfer and access protocol
Transfer
SMTP: delivery/storage to receiver’s server
Delivery
Local
POP3 : Post Office Protocol; authorization (agent – server) and
download
IMAP: Internet Mail Access Protocol; more features (more complex);
provides for the manipulation of stored messages on server
HTTP: gmail, Hotmail, Yahoo! Mail, etc.
POP3 – Post Office Protocol
Three states of a POP3 transaction - Authorisation - Transactions - Update Syntax - USER / PASS - LIST - RETR / DELE - QUIT (update) Issue: “download and delete” mode does not allow messages to be re-read.
IMAP – Internet Message Access Protocol
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.
HTTP – Hypertext Transfer Protocol
- Client initiates TCP connection (creates socket) to server, port 80
- Server accepts TCP connection from client
- HTTP messages (application-layer protocol messages) exchanged
between browser (HTTP client) and Web server (HTTP server) - TCP connection closed
Steps that occur when a link is selected:
- Browser determines the URL
- Browser asks DNS for the IP address of the server
- DNS replies
- The browser makes a TCP connection
- Sends HTTP request for the page
- Server sends the page as HTTP response
- Browser fetches other URLs as needed
- The browser displays the page
- The TCP connections are released
