Transcript Cisco – Chapters 12-14 - YSU Computer Science & Information

Cisco – Chapters 12-14
Layers 4, 5, and 6
More Details
Transport Layer - 4
• Assures data reliably travels end-to-end across the
often vast network path
• Performs multiple functions to provide "quality of
service”
• Guarantees Flow control
• Provides End-to-end control using sliding
windows
• Provides reliability through sequencing numbers
and acknowledgements
Layer 4 Protocols
• TCP - supplies a virtual circuit between end-user
applications
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connection-oriented
reliable
divides outgoing messages into segments
reassembles messages at the destination station
re-sends anything not received
reassembles messages from incoming segments.
Layer 4 Protocols
• UDP --
transports data unreliably between
hosts.
*unreliable
*connectionless
*transmits messages (called user datagrams)
*provides no software checking for message delivery
(unreliable)
*does not reassemble incoming messages
*uses no acknowledgements
*provides no flow control
TCP/IP
• Combination of two individual protocols TCP and IP
– IP is a Layer 3 protocol - a connectionless
service that provides best-effort delivery across
a network
– TCP is a Layer 4 protocol - a connectionoriented service that provides flow control as
well as reliability
TCP Protocols
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HTTP – internet transmission rules
FTP – file transfer
SMTP -- mail
DNS – domain name
UPD Protocols
• TFTP – trivial file transfer
• DNS – domain name
• SNMP - monitor and control network
devices, and manage configurations,
statistics collection, performance, and
security.
• DHCP – assigns IP addresses
TCP Segment Fields
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Source and destination ports
Sequence number
Acknowledgement number
Window – number of octets willing to accept
Code bits
Check Sum
Others
Data
UDP Segment Fields
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Source and destination address
Length
Check Sum
Data
– Note: No acknowledgements, no guaranteed
delivery, no error control; used for tasks that do
not require assembling segments in sequence
Port (Socket) Numbers
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21 FTP
23 Telnet
25 SMTP
53 DNS
69 TFTP
161 SNMP
Port Ranges
• Numbers below 255 - for public
applications
• Numbers from 255-1023 - assigned to
companies for marketable applications
• Numbers above 1023 - are unregulated
TCP Three-way Handshake
• segments are encapsulated in packets
• Packets travel connectionless paths
through internetworks
• Sequence and acknowledgement numbers
necessary for TCP to track segments
• Synchronizes a connection at both ends
before data is transferred.
PAR
• the source sends a packet, starts a timer, and
waits for an acknowledgement before
sending the next packet.
• If the timer expires before the source
receives an acknowledgement, the source
retransmits the packet and starts the timer
over again.
Windows
• Size (number of windows determines the
amount of data that you can transmit at one
time before receiving an acknowledgment
from the destination.
• After a host transmits the window-sized
number of bytes, the host must receive an
acknowledgment before it can transmit
again
Windowing
• Expectational acknowledgments
– the acknowledgment number refers to the octet
that is next expected.
• The "sliding" part, of sliding window
– the window size is negotiated dynamically
during the TCP session.
• This results in inefficient use of bandwidth
by the hosts.
Chapter 13
Layer 5 -- Session
Session Layer Tasks
• Accounting
• Conversation control, that is, determining
who can talk when
• Session parameter negotiation.
Interhost Communications
• Establishes, manages and terminates
communications between applications
– two-way alternate communication - TWA
• Taking turns – uses a data token
– two-way simultaneous communication - TWS
• Both can transmit at same time – collisions result
– Checkpoints
• orderly initiation and termination of the
conversation.
Dual Roles
• Server and Client
• Dialogue Control
– Determining which role host is playing at any given
moment
• Dialogue Separation
– orderly initiation, termination, and managing of
communication
• Major and minor synchronizations – see slide
Layer 5 Protocols
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Network File System (NFS)
Structured Query Language (SQL)
Remote Procedure Call (RPC)
X-Window System
AppleTalk Session Protocol (ASP)
Digital Network Architecture Session
Control Protocol (DNA SCP)
Chapter 14 – Layer 6
Presentation Layer
Layer 6 Tasks
• Pass-through protocol for information from
adjacent layers
• Allows communication between
applications on diverse computer systems in
transparent manner
• Concerned with the format and
representation of data
• Concerned with data structures
Layer 6
• Responsible for presenting data in a form
that the receiving device can understand
– data formatting (presentation)
• EBCDIC – mainframe text
• ASCII – PC text
– Layer 6 translates between two formats
– data encryption (coding)
– data compression (bit substitution)
Layer 6 Graphic Formats
• PICT - a picture format used to transfer
QuickDraw graphics between programs on the
MAC operating system
• TIFF (Tagged Image File Format) - a format for
high-resolution, bit-mapped images
• JPEG (Joint Photographic Experts Group) graphic format used most often to compress still
images of complex pictures and photographs
• GIF (Graphic Image Format) – still images
Layer 6 Movie and Sound Formats
• MIDI (Musical Instrument Digital
Interface) - for digitized music
• MPEG (Motion Picture Experts Group) standard for the compression and coding of
motion video for CDs and digital storage
• QuickTime - a standard that handles
audio and video for programs on a MAC
operating system
More Formats
• HTML – displays web pages – uses tags
Data Encryption & Compression
• Encryption - Protects information during its
transmission
– encryption key is used to encrypt the data at its source
and then to decrypt the data at its destination
• Compression – Uses algorithms (complex
mathematical formulas) to shrink the size of the
files.
– The algorithm searches each file for repeating bit
patterns, and then replaces them with a token. A token
is a much shorter bit pattern that represents the long
pattern.