From Slides Flashcards
TCP/IP Layer Schematic
Types of servers
- File Server
- Print Server
- Communications Server
- Telephony Server
- Fax Server
- Web Server
SNMP
Simple Network Management Protocol
Symbolic Name
Names that users are familiar with, like john.doe@brooklyn.edu
www.google.com
Directory Services
Used to find the addresses of a named destination user:
convert email or URL to IP address, NPA address, or TCP/UDP port #
Domain Name System (DNS)
Total directory system in a TCP/IP suite
Original 6 high-level domains
com, edu, gov, mil, net, org
Domain Name Server
- Associated with each institution network is a host that runs an AP known as the Domain Name Server.
- Associated with the DNS server is a Directory Information Base (DIB) which contains all thedirectory related information for that institution.
- Each host has a client process known as the Name Resolver which communicates the DNS server
- Name resolver can cache some symbolic - IP mappings, if it doens’t find then it goes to DNS over TCP/IP
Foreign name server
DNS server outside of the autonomous network (?) that is queries if the local name server cannot resolve the current DNS name
DNS Referral
If the destination is not listed within the DIB, the local
DNS server creates its own request message (a
referral) which it forwards to another server.
DNS Resolvers: TCP vs. UDP
Resolvers use UDP for single queries and TCP for
group queries. (why??)
Recursive vs. Iterave DNS resolution techniques
- Recursive Technique:
Query another name server for the desired result
and then send the result back to A. - Iterative Technique:
Return to A the address of the next server (C) to
whom the request should be sent. A then sends
out a new DNS request to C.
In exchanges between name servers, either the
iterative or recursive technique may be used. For
requests sent by a name resolver , the recursive
technique is used. (Resolver doesn’t go out to the internet, just communicates with DNS server)
Name
Cache
Typically, each local name server will maintain a Name
Cache of the most recently referred names.
DNS Message Format
- Identifier:
Assigned by that sender so that it can match queries
and responses.
• Query Response:
Indicates whether this message is a query or
response.
• Opcode:
- standard query,
- an inverse query,
- a server status request.
• Authoritative Answer:
if responding name server is an authority
• Truncated:
if response message was truncated due to length
• Recursion Desired:
If set, directs the server to pursue the query
recursively.
• Recursion Available:
if recursive query support is available
• Response Code:
- no error,
- format error,
- server failure,
- name error,
- not implemented,
- refused
• Qdcount, Ancount, Nscount, ARcount:
number of RRs in the question/ answer/ authority/
additional records section (zero or more).
DNS Question Section Format
Query Type: the type of question (e.g.,whether the question refers to a machine name or a mail address).
**QUERY CLASS** specifies the class of the query, typically Internet (**IN**)
Query types in image
NVT
Nework Virtual Terminal - common paradigm so client and server stations can talk to each other. An NVT is an imaginary device with a well-defined set of characteristics. Using the VTP, aconnection is set up between a terminal user and a remote host. Both sides generate data and control signals in their native language. Each side translates its native data and control signals into those of the NVT and translates incoming NVT traffic into its native data and control signals. Normally, all data is transferred as 7-bit ASCII character strings.
Option requests can be initiated by either side using
WILL, WON’T, DO and DON’T commands.
TELNET
allows remote access to a station
Original ARPANET TELNET
Dumb terminals needed to gain access to remote servers
VTP Concept
Phases of operation (uses TCP)
Connection management:
- Connection request and termination
- Telnet uses TCP
! Negotiation:
- To determine a mutually agreeable set of
characteristics
- NVT has a wide range of capabilities and features
- Real terminal is more limited
- NVT has options, such as line length
! Control:
- Exchange of control information and commands
e.g., end of line, interrupt process
! Data:
- Transfer of data between two correspondents
Principles of TELNET Options Negatiation
! Either side can initiate negotiation
! Can ask that an option be enabled or that a currently
enabled option be disabled.
! A request to enable an option may always be rejected.
! A request to disenable an option must always be
accepted.
! Options are not enabled until the negotiation is
complete.
! Never negotiate about something that is already true
TELNET Negotiation messages
Examples:
IAC, SB, WILL (DO), ‘0’, SE (send in 8-bit binary)
IAC, SB, DO (WILL), ‘0’ SE (accept)
IAC, SB, DON’T (WON’T), ‘0’ SE (refuse)
(Note: parentheses indicate receiver initiated option
negotiation)
IAC: Begins command sequence
SB: Begins options negotiation string
SE: End option negotiation string
FTP
provides access from a client to a remote file
server across a TCP/IP internet connection
Uses TWO TCP/IP connections: Data and Control
FTP Options
! FTP assumes files are objects in mass storage:
- Files share some properties regardless of machine
- Files are uniquely identified by symbolic names
- Files have owners and protection mechanisms
against unauthorized access
- Files may be created, read from (copied from),
written into, or deleted (within protection rules)
! To support specific computers and operating systems,
FTP can negotiate options in three dimensions:
- Data type,
- File type,
- Transfer mode
! Systems programmer on each system determines:
- How a particular file can be mapped to a standard
file type using one of the standard data types,
and transferred using a standard mode such that
it is useful at the destination.
4 Data Types supported in FTP
- ASCII (most common, for text files)
- EBCDIC (only appropriate if both machines are IBM hosts)
- Image (bit by bit replication, useful for arbitrary files between similar machines with same operating systems)
- Logical byte size (specifies byte size other than 8 bits, useful for executable files that have to be interpreted on the new machine)
3 File Types used in FTP
File Structure (most common, assumes that file is a string of bytes, terminating with EOF marker)
Record Structure (useful for checkpointing and error detection – file is treated as a stream of records, separated by standards End of Record market)
Page Structure (used for files not stored contiguously on disk, wher page structure needs to be maintained)
3 FTP transmission modes
Modes used to optimize the use of the network
- Stream mode (default): least burdensome, no restriction on file type
- Block mode: allows restarting of failed or interrupted transfer. Has a descriptor that can indicate: last block in a record, last block in file, suspect data, restart marker
- Compressed Mode: allows source to squeeze sequences of same character into a shorter coded sequence
TFTP
Uses UDP
Uses Stop-and-Wait protocol for file transfer (as opposed to sliding window, every message has to be acknowledged)
Data blocks are all 512 octets, except last byte
Only 5 Message types
SMTP Spooling Area
intermediary between email applications and SMTP message sending
Mail handler/email gateway/forwarder/exploder
Server creates mailboxes for incoming mail, users access this
Post Office Protocol (POP)
POP allows a user’s mailbox to reside on a computer
that runs a mail server.
A mail server accepts a messages from an arbitrary
sender, while the POP server only allows a user
access to their mailbox only after the user enters
authentication information.
SMTP Limitations
- SMTP cannot transmit binary files.
- SMTP is limited to 7-bit ASCII textual data
- SMTP servers reject mail messages over a certain size
- SMTP gateways that translate between ASCII and
EBCDIC do not use a consistent set of mappings
MIME content types
- text
- multipart (generally used for executables)
- …
URL
Uniform resource locator
HTTP
- Protocol for transmitting info with the efficiency necessary for making hypertext jumps
- Transaction-oriented, client-server
- Uses TCP
- Stateless: each transaction is treated independently (since each connection accesses a different server, more efficient to stop old connection)
URI
Uniform resource identifier - generic WWW identifier. More general than URL, which designates access protocol and specific internet address
HTTP Proxy
Security intermediary on client side of firewall
Determines whether the origin server is safe, if so, delivers requests via authenticated connection
Acts as both a server (to the orginal requesting user) and a client (to the internet)
HTTP Gateway
Security intermediary on the server side of a firewall
Receives HTTP requests, determines whether they should be sent out to the internet
Non-HTTP server
HTTP Tunnel
Relay point between two TCP connections
External server can establish and maintain an authenticated connection for HTTP transactions
Once tunnel is established, the tunnel is not actively filtering requests
(Servers cannot use cache when they are acting as a tunnel)
HTTP Message Fields
Request Line: Identifies the message type and the requested resource.
Request-Line= Method SP Request-URL SP HTTP-Version CRLF
Status Line:
Provides status information about this response.
Status-Line= HTTP-Version SP Status-Code SP
Reason-Phrase CRLF
General-Header:
Contains fields that are applicable to both request and
response messages, but which do not apply to the entity
being transferred.
Request-Header:
Contains information about the request and the client.
Response-Header:
Contains information about the response.
Entity-Header:
Contains information about the resource identified by
the request and information about the entity body.
Entity-Body:
The body of the message.
HTTP General Header Fields
Cache-Control
Connection
Date
Forwarded
Keep-Alive
MIME-Version
Pragma
Upgrade
HTTP Request Methods
Options
Get
Head
Post
Put
Patch
Copy/Move
Delete
HTTP Request Header Fields
Function as request modifiers
Accept
Accept-Charset
Accept-Encoding
Accept-Language
Authorization
From
Host
HTTP Entity Header Fields
Provides optional information about the entity body or if no body is present, about the resource identified by the request.
Jitter
Inter-arrival delay between packets
Time delay buffer
Compensates for jitter, ensures that VOIP packets arrive at the right pace
Continuous data source
Fixed-size packets are generated at fixed intervals.
This characterizes applications that are constantly
generating data, have few redundancies, and that are
too important to compress in a lossy way.
Examples are air traffic control radar and real-time
simulations
On/off source:
The source alternates between periods when fixedsize packets are generated at fixed intervals and
periods of inactivity.
A voice source, such as in telephony or audio
conferencing, fits this profile.
Variable packet size
The source generates variable-length packets at
uniform intervals.
An example is digitized video in which different
frames may experience different compression ratios
for the same output quality level.
Soft real-time applications
- can tolerate the loss of some portion of the
communicated data. - impose fewer requirements on the network
- focus on maximizing network utilization, even at
the cost of some lost or misordered packets.
Hard real-time applications
- have zero loss tolerance.
- a deterministic upper bound on jitter and high
reliability take precedence over network
utilization considerations.
SIP
Application-level protocol, for setting up, modifying, and terminating real-time sessions between participants over an IP data network
Generally runs over UDP
Connection-oriented
supports single-media and multimedia sessions including teleconferencing
5 facets of SIP communications
User location
User availability
User capabilities (media and media parameters)
Session setup
Session managememt
