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
non persistent http
reconnect before each message
persistent - stays connected until you manually close
Client side content – plugins and helpers
Plugins - integrated software module which executes inside the browser,
direct access to online context
Helper - separate program which can be instantiated by the browser, but
can only access local cache of file content
application/pdf
application/msword
Server side processing – a summary
- Accept TCP Connection from client (browser)
- Identify the file requested
- Get the specified file from the local disk storage
- Send the file to the client
- Release the TCP connection
cookies
Cookies have 5 fields: domain, path, content, expiry, security
TCP/IP model
Application - telnet, smtp, ftp
Transport - udp, tcp
Internet - ip
Network - ethernet
Transport layer
The primary function of the transport layer is to provide reliable
cost-effective data transport from source to destination, independent of
physical or data networks.
The Transport layer services provide interfaces between the Application
layer and the Network layer. The Transport layer entities (the hardware or
software which actually does the work eg. OS kernel, processes, NIC) can
exist in multiple locations.
Services provided a logical communication between application processes
running on different hosts:
Connection-oriented: connection establishment, data transfer,
connection release (TCP)
Connectionless: data transfer (UDP)
Abstract representation of messages sent to and from transport
entities
Transport Protocol Data Unit (TPDU)
If the transport and network layers are so similar, why are there two layers?
If the transport and network layers are so similar, why are there two layers?
Transport layer code runs entirely on hosts
Network layer code runs almost entirely on routers
Transport layer can fix reliability problems caused by the Network
layer eg. delayed, lost or duplicated packets
switches
Switches operate at the data link layer (layer 2) and sometimes the network layer (layer 3) of the OSI Reference Model and therefore support any packet protocol.
Transport vs Data-link protocols
Transport services are implemented as protocols used between transport
entities.
Transport services have some similarities with data link protocols
error control
sequencing
flow control
Transport services have some differences with data link protocols
physical network vs physical/data/network
addressing
connection establishment
storage
OSI model
Aus Psych Society Takes New Depressed Patients Application Presentation Session Transport Network Datalink Physical
how to address in network vs transport
addressing in the Transport layer is typically done using port numbers
(eg port 80).
cf. Unix /etc/services, www.iana.org (well known ports)
a process server intercepts inbound connections and spawns requested
server and attaches inbound connection
cf. Unix /etc/(x)inetd
addressing in the network later is typically done using the IP address
ports
0-65535 Ports are classified into 3 segments: Well Known Ports (0-1023) 21 FTP 22 SSH 23 Telnet 25 SMTP 80 HTTP 110 POP3 119 NNTP Registered Ports (1024-49151) Dynamic Ports (49152-65535)
