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– 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 connection-oriented service that provides flow control as well as reliability
TCP Protocols
• 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
• Source and destination ports• Sequence number• Acknowledgement number• Window – number of octets willing to accept• Code bits• Check Sum• Others• Data
UDP Segment Fields
• 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
• 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
• 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.