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.