OBJ 1.1 X Flashcards
OSI Model Layers
- Physical
- Data link
- Network
- Transport
- Session
- Presentation
- Application
Ethernet Header
Fields in a frame used to identify source and destination MAC addresses, protocol type, and error detection.
TCP (transmission control protocol)
Protocol in the TCP/IP suite operating at the transport layer to provide connection-oriented, guaranteed delivery of packets.
UDP (user datagram protocol)
Protocol in the TCP/IP suite operating at the transport layer to provide connectionless, non-guaranteed communication.
TCP Flags
Field in the header of a TCP segment designating the connection state, such as SYN, ACK, or FIN.
Internet Protocol (IP) Header
Fields in a datagram used to identify source and destination IP addresses, protocol type, and other layer 3 properties.
MTU (Maximum transmission unit)
Maximum size in bytes of a frame’s payload. If the payload cannot be encapsulated within a single frame at the Data Link layer, it must be fragmented.
- Physical Layer
Transmission of bits across the network.
- Data link layer
Packages data into frames and transmitting those frames on the network, performing error detection/correction, and uniquely identifying network devices with an address (MAC), and flow control
- Network layer
Forwards traffic (routing) with logical address. (example: IP address IPv4 or IPv6)
- Transport layer
Dividing line between upper and lower layers of the OSI model. Data is sent as segments.
- Session layer
Think of a session as a conversation that must be kept separate from others to prevent intermingling of the data. Setting up, maintaining, and tearing down sessions.
- Presentation layer
Responsible for formatting the data exchanged and securing that data with proper encryption.
- Application layer
Provides application-level services. The layer where the users communicate with the computer.
- Physical layer (How are bits represented on the medium?)
Electrical voltage ((copper wiring) or light (fiber optics) represents 1’s and 0’s (bits) If 0 volts, then 0 is represented. If +/-5 volts, then 1 is represented.
Transition modulation: If it changed during the clock cycle, then a 1 is represented, otherwise, a 0.
- Physical layer (How are the cables wired?)
TIA/EIA-568-B is standard wiring for RJ-45 cables and ports. Crossover cables use T-568A and T-568B. Straight thru cables typically use T-568 B on both ends, but could use T-568A on both.
- Physical layer (How are the cables connected?)
Layer 1 devices view networks from a physical topology perspective. Bus, Ring, Star, Hub-and-Spoke, Full Mesh, Partial Mesh
- Physical layer (How is communication synchronized?)
Asynchronous: uses start bits and stop bits to indicate when transmissions occur from sender to receiver
Synchronous: Uses a reference clock to coordinate the transmissions by both sender and receiver.
- Physical layer (examples)
Cables (ethernet, fiber optic)
Radio frequencies (Wi-Fi, bluetooth)
Infrastructure devices (hubs, wireless access points, media converters)
- Data link layer (physical addressing)
Physical addressing
uses 48-bit address assigned to a network interface card by manufacturer. First 24 bits is the vendor code, second 24-bits is a unique value.
- Data link layer (logical topology)
Layer 2 devices view networks logically (ring, bus, star, mesh, hub-and-spoke)
- Data link layer examples
Network Interface Cards, Bridges, Switches
- Network Layer (Logical addressing)
Numerous routed protocols were used for logical addressing over the years. (AppleTalk, IPX, IP) Only IP remains dominant. IP v4, IP v6
- Network Layer (how data should be forwarded or routed?)
Packet switching (routing) where data is divided into packets and forwarded.
Circuit switching - dedicated communication link is established between two devices.
Message switching (data is divided into messages, similar to packet switching, except these messages may be stored then forwarded
- Network layer (route discovery and selection)
Routers maintain a routing table to understand how to forward a packet based on destination IP address. Manually configured as a static route or dynamically through a routing protocol. (RIP, OSPF, EIGRP)
ICMP
Internet Control Message Protocol (ICMP) used to send error messages and operational information about an IP destination. Not regularly used by end-user applications, used in troubleshooting (ping and traceroute)
- Network Layer examples
Routers, Multilayer swtiches, IPv4 protocol, IPv6 protocol, ICMP
TCP (transmission control protocol) (L4)
Connection-oriented, reliable transport of segments, (if segment is dropped, protocol detects it and resends segment), acknowledgements received for successful communications, used for all network data that needs to be assured to get to its destination
UDP (user datagram protocol) (L4)
Connectionless protocol, unreliable transport of segments, (if dropped, sender is unaware), no retransmission, good for audio/video streaming, lower overhead for increased performance.
Windowing (L4)
Allows the clients to adjust the amount of data sent in each segment, continually adjusts to send more or less data per segment transmitted. Adjusts lower as number of retransmissions occur, adjusts upwards as retransmissions are eliminated.
Buffering (L4)
Devices, such as routers, allocate memory to store segments if bandwidth isn’t readily available. When available, it transmits the contents of the buffer, if the buffer overflows, segments will be dropped.
- Transport Layer examples
TCP, UPD, WAN Accelerators, Load Balancers, Firewalls
- Session Layer (set up, maintain, tear down)
Setting up - check user credentials, assign numbers to session to identify them, negotiate services needed for session, negotiate who begins sending data.
Maintaining - transfer the data, reestablish a disconnected session, acknowledging receipt of data
Tearing down a session - due to mutual agreement (after the transfer is done), due to the other party disconnecting the session
- Session layer examples
H.323 (used to setup, maintain, and tear down a voice/video connection)
NetBIOS (used by computers to share files over a network)
- Presentation Layer (Data formatting, Encryption)
Data formatting - formats data for proper compatibility between devices, ASCII, GIF, JPG, ensures data is readable by receiving system, provides proper data structures, negotiates data transfer syntax for the application layer (7)
Encryption - Used to scramble the data in transit to keep it secure from prying eyes.
- Presentation layer examples
HTML, XML, PHP, JAVASCRIPT, ASCII, EBCDIC, UNICODE, GIF, JPG, TIF, SVG, PNG, MGP, MOV, TLS, SSL
- Application layer (App services, Service Advertisement)
Application services - unite communicating components from more than one network application. Ex: file transfers and sharing, e-mail, remote access, network mgmt activities, client/server processes.
Service Advertisement - some apps send out announcements, states the services they offer on the network, some centrally register with Active Directory server instead (printers, file servers)
- Application layer examples
E-mail (POP3, IMAP, SMPT)
Web browsing (HTTP, HTTPS)
DNS< FTP< FTPS< TELNET< SSH< SNMP
Encapsulation
the process of putting headers (and sometimes trailers) around some data
Decapsulation
Action of removing the encapsulation that was applied.
If we move down the OSI layers from 7 to 1, we ENCAPSULATE data
If we move upward from layers 1 to 7 we DECAPSULATE data
Protocol data unit
a single unit of info transmitted within a computer network
Layer 1 - bits
Layer 2 - frames
Layer 3 - packets
Layer 4 - segments if TCP or datagrams if UDP
SYN (synchronization flag)
The most well-known flag in TCP communications because it is used to
synchronize the connection during the three-way handshake
ACK (or acknowledgement) flag
Used during the three-way handshake, but it is also used to acknowledge
the successful receipt of packets
FIN (or finished) packet
-Used to tear down the virtual connections created using the three-way
handshake and the SYN flag
-The FIN flag always appears when the last packets are exchanged
between a client and server and the host is ready to shutdown the
connection
RST (or reset) flag
▪ Used when a client or server receives a packet that it was not expecting
during the current connection
PSH (or PUSH) flag
Used to ensure that the data is given priority and is processed at the
sending or receiving ends
URG (or urgent) flag
It is like the Push flag and identifies incoming data as “urgent”
The main difference is PSH is used by a sender to indicate data with a
higher priority level where URG is sent to tell the recipient to process it
immediately and ignore anything else in queue
