CCNA-1. Chapter4. OSI Transport layer.

The Transport layer primary responsibilities:

• Tracking the individual communication between applications on the source and destination hosts
• Segmenting data and managing each piece
• Reassembling the segments into streams of application data
• Identifying the different applications

Some protocols at the Transport layer provide additional functions:

• Connection-oriented conversations (data for a communication between the two applications can be closely managed)
• Reliable delivery (source device retransmit any data that is lost)
• Ordered data reconstruction (ensure that segments are reassembled into the proper order)
• Flow control (sending application reduce the rate of data flow)

UDP (RFC 768) transport protocol that provide low overhead (8 bytes )data delivery. PDU - datagram. Datagrams sent "best effort"  
 Applications that use UDP:

• Domain Name System (DNS)
• Video Streaming
• Voice over IP (VoIP)

TCP (RFC 793) incurs additional overhead (20 bytes), same order delivery, reliable delivery, and flow control. PDU - segment.  
TCP segment fields values:

• Source port - TCP session on a device that opened connection - normally a random value above 1023
• Destination port - Identifies upper layer protocol or apptlication on remote site
• Sequence number - Specifies the number of the last octet (byte) in a segment (+1 for each byte of data sent from the client)
• Acknowledgement Number - Specifies the next octet expected by reciever (equal to the received sequence value plus 1)
• Header Length - specifies the length of segment header in bytes
• Reserved - set to 0
• Code bits - used in session management and in treatment of segments.
  
  • URG - Urgent pointer field significant
  • ACK - Acknowledgement field significant
  • PSH - Push function
  • RST - Reset the connection
  • SYN - Synchronize sequence numbers
  • FIN - No more data from sender
• Window - how many bytes must be recieved before sending acknowlegement (determined during the session startup)
• Checksum - used for error checking of header and data
• Urgent - onty used with an URG (Urgent) code bit
• Options - Optional information

Applications that use TCP:

• Web Browsers
• E-mail
• File Transfers

Socket - combination of the Transport layer port number and the Network layer IP address. Types of port numbers (Port numbers are assigned by The Internet Assigned Numbers Authority (IANA)).:

• Well Known Ports (0 to 1023) - these numbers are reserved for services and applications.
  
  • TCP: FTP-21 SMTP-25 Telnet-23 HTTP-80 POP3-110, IRC(Internet Relay Chat)-194, HTTPS-443
  • UDP:  RIP - 520 TFTP- 69, DHCP- 67
  • TCP\UDP: DNS - 53, SNMP - 161, AOL Instant manager,IRC - 531
• Registered Ports (1024 to 49151) - assigned to user processes or applications. May be used dynamically as source port.
  
  • TCP: MSN Messenger-1863 Cisco SCCP(VoIP)-2000 Alternate HTTP-8008 Alternate HTTP-8080
  • UDP: RADIUS Authentication Protocol - 1812, RTP(Voice and video transport protocol) - 5004, SIP(Voip) - 5060
  • TCP\UDP: MS SQL - 1433, WAP(MMS) - 2948
• Dynamic or Private Ports (49152 to 65535) -known as Ephemeral Ports, assigned dynamically as source port.

TCP connection establishment (the three way hanshake):

• Establishes that the destination device is present on the network: client sends an initial sequence value (ISN) (SYN=1)
• Verifies active destination service and if  destination port number accept requests: reciever send acknowledgement value plus its own synchronizing sequence value (SYN=1, ACK=1)
• Informs that client intends to establish a communication session: client responds with an acknowledgement value  (ACK=1)

TCP connection termination:

• When the client has no more data to send in the stream, it sends a segment with the FIN flag set. (FIN=1)
• The server sends an ACK to acknowledge the receipt of the FIN to terminate the session from client to server. (ACK=1)
• The server sends a FIN to the client, to terminate the server to client session. (FIN=1)
• The client responds with an ACK to acknowledge the FIN from the server. (ACK=1)

Security can be added to the data network by:

• Denying the establishment of TCP sessions
• Only allowing sessions to be established for specific services
• Only allowing traffic as a part of already established sessions

Selective Acknowledgements - an optional feature. If both hosts support Selective Acknowledgements, destination acknowledge bytes in discontinuous segments (less than window size) and the host would only need to retransmit the missing data. TCP flow conrol mechanisms (RFC 2581):

• Delay of retransmiting if the acknowledge not recieved (if network is congested - acknowledge can be not lost but delayed)
• Reducing Window Size - require that received segments be acknowledged more frequently.

TCP connection states:

• LISTEN - The local connection is waiting for a connection request from any remote device.
• ESTABLISHED The connection is open, data may be exchanged through the connection.
• TIME-WAIT The local connection is waiting a default period of time after sending a connection termination request before closing the connection. This is a normal condition, and will normally last between 30 - 120 seconds.
• CLOSE-WAIT The connection is closed, but is waiting for a termination request from the local user.
• SYN-SENT The local connection is waiting for a response after sending a connection request.
• SYN_RECEIVED The local connection is waiting for a confirming connection request acknowledgment.

CCNA-1. Chapter3. Application layer functionality and protocols

Executable program components:

• application (provide people with a way to create messages)
• service (establish an interface to the network, can support  multiple protocols, may use different protocols in 1 program)
• protocol (provide the rules and formats/types of messages that govern how data is directed, treated, structured)

Application layer protocols specify:

• The processes that occur at end of the communication: what has to happen to the data and how the PDU is to be structured.
• The types of messages: requests, acknowledgements, data messages, status messages and error messages.
• The syntax of the message: this gives the expected order of information (fields) in a message.
• The meaning of the fields within specific message types has to constant so the services can correctly act.
• The message dialogs: which messages elicit which responses so the correct services are invoked and the data transfer occurs.

Types of applications:

• 
   network aware (implement the application layer protocols and are able to communicate directly with the lower layers)
  
  • Example: Web browsers, e-mail client
• not network aware (need the assistance of application layer services (alg.exe and etc.) to use network resources)
  
  • Example: File transfer, network printer spooling

Network architecture models:

• Client/Server model
• Peer-to-Peer Model  (computers in network  share resources  without dedicated server. In large P2P hard to enforce security)

DNS server record types:

• A - an end device address
• NS - an authoritative name server
• CNAME - the canonical name (or Fully Qualified Domain Name) for an alias; used when multiple services have the single network address but each service has its own entry in DNS
• MX - mail exchange record; maps a domain name to a list of mail exchange servers for that domain

Authoritative DNS server - server has resource records that correspond to its level in the domain hierarchy. DNS resolve process: Client-LocalDNS-RootDNS-regionalDNS(.com;.ru)-lowerlevelDNS Common HTTP message types:

• GET (request for data, respond - HTTP/1.1 200 OK, and a requested file, an error message, or some other information)
• POST (includes the data in the message sent to the server)
• PUT (uploads resources or content to the web server)

E-mail processes:

• Mail User Agent (MUA) - allows messages to be sent and places received messages into the client's mailbox
• Mail Transfer Agent (MTA) - forward e-mail. If a recipient mailbox on the local server -to the MDA if not - to another MTA
• Mail Delivery Agent (MDA) - receives mail, places to mailboxes, resolve virus scanning, spam filtering, return-receipt handling.

E-mail protocol types,, functions, commands:

• Simple Mail Transfer Protocol (SMTP) TCP 25. Message formats, command strings used to send e-mail from client or server.
  
  • Functions:
    
    • session initiation
    • mail transaction
    • forwarding mail
    • verifying mailbox names
    • expanding mailing lists
    • opening and closing exchanges.
  • Commands:
    
    • HELO - identifies the SMTP client process to the SMTP server process
    • EHLO - Is a newer version of HELO, which includes services extensions
    • MAIL FROM - Identifies the sender
    • RCPT TO - Identifies the recipient
    • DATA - Identifies the body of the message
• Post Office Protocol (POP) TCP 110. Used to receive e-mail messages from an e-mail server.
• Internet Message Access Protocol (IMAP). TCP 143. Allows a central server to provide remote access to e-mail messages
• Propriatary protocols (IBM's Lotus Notes, Novell's Groupwise, or Microsoft's Exchange have their own internal e-mail format)

File Transfer Protocol (FTP) - allow download (pull) or upload (push) file between a client and a server. FTP requires two connections:

• Connection to the server on TCP port 21. Control traffic, consisting of client commands and server replies.
• Connection to the server over TCP port 20. Actual file transfer, created every time there is a file transferred.

DHCP messages:

• DHCP DISCOVER (broadcast to identify any available DHCP servers on the network)
• DHCP OFFER, (server reply  with an assigned IP address, subnet mask, DNS server, default gateway, duration of the lease)
• DHCP REQUEST (broadcast packet that identifies DHCP server if >1, can request an address that previously been allocated)
• DHCP ACK (acknowledges IP address client requested or offered by the server, is still valid and the lease is finalized)
• DHCP NAK (Offer is no longer valid if a time-out or another client allocating the lease. Selection process must begin again)

Server Message Block (SMB)-client/server file sharing request-response protocol. Provide access to server resources as if they local. SMB messages functions:

• Start, authenticate, and terminate sessions
• Control file and printer access
• Allow an application to send or receive messages to or from another device

Telnet provides a standard method of emulating text-based terminal devices over the data network. provides access to the server command line interface (CLI).Connection using Telnet is called a Virtual Terminal (VTY) session. Telnet command first byte - Interpret as Command (IAC) character. IAC defines the next byte as a command rather than text. Telnet protocol commands include:

• Are You There (AYT) - Lets the user request that something appear on the terminal screen to indicate that the VTY session is active.
• Erase Line (EL) - Deletes all text from the current line.
• Interrupt Process (IP) - Suspends, interrupts, aborts, or terminates the process to which the Virtual Terminal is connected. For example, if a user started a program on the Telnet server via the VTY, he or she could send an IP command to stop the program.

Gnutella P2P protocol defines five different packet types:

• ping - for device discovery
• pong - as a reply to a ping
• query - for file location
• query hit - as a reply to a query
• push - as a download request

CCNA-1. Chapter2. Communicating over network

Communication methods common elements:

• sender
• reciever
• channel

Communication types:

• traditional computer data
• interactive voice
• video
• entertainment products

Segmentation (brake communication in to pieces)

• multiplexing (separate conversations can be transmitted together)
• increase the reliability of network communications
• added complexity

Local Area Network (LAN) - individual network usually administered by a single organization, spans a single geographical area, providing services and applications to people within a common organizational structure(business, campus, region). Wide Area Networks (WANs) - networks that connect LANs in geographically separated locations. Networking protocol suites (group of protocols that work cooperatively) describe:

• The format or structure of the message
• The method by which networking devices share information about pathways with other networks
• How and when error and system messages are passed between devices
• The setup and termination of data transfer sessions

Layered model:

• Assists in protocol design
• Fosters competition because products from different vendors can work together.
• Prevents technology or capability changes in one layer from affecting other layers above and below.
• Provides a common language to describe networking functions and capabilities.

Standards organizations that ratify networking protocols:

• Institute of Electrical and Electronics Engineers (IEEE)
• Internet Engineering Task Force (IETF)

Requests for Comments (RFCs) - publicly-available set of documents that contain specification of data communications protocols, resources that describe the use of the protocols, contain technical and organizational documents about the Internet, including the technical specifications and policy documents produced by the Internet Engineering Task Force (IETF). Protocol Data Unit (PDU) (the form that a piece of data takes at any layer) types:

• Data (The general term for the PDU used at the Application layer)
• Segment  (Transport Layer PDU)
• Packet (Internetwork Layer PDU)
• Frame (Network Access Layer PDU)
• Bits (PDU used when physically transmitting data over the medium)

TCP\IP model:

• Applicaton (represent data, encoding, diallog control)
• Transport (support communication of diverce devices across diverce networks)
• Internet (determine the best path through the network)
• Network Access (controls the hardware devices and media that make up the network)

OSI model (describes encoding, formatting, segmenting, encapsulating data for transmission over the network):

• Application (means for end-to-end connectivity between individuals)
  
  • Functions: Provide interfaces between applications
  • Protocols: DNS, HTTP, SMTP, Telnet, FTP, SMB
• Presentation (representation of data between the Application layers, standartization of data formats between systems)
  
  • Functions: Data coding and conversion for destination application interpretation, (de)compression, (de)encryption.
  • Protocols: QuickTime, MPEG, GIF, JPEG, TIFF.
• Session (provide services for Presentation of managing data exchange and diallogue organization)
  
  • Functions: Initiate dialogs, keep them active, restart idle\disrupted sessions
  • Protocols:NetBIOS
• Transport (manages the data transport between the processes running on each end host)
  
  • Functions: Conversation Multiplexing, segment/reassemble data, error recovery.
  • Protocols:TCP, UDP
• Network (specify the packet structure and processing used to carry the data from one host to another host.)
  
  • Functions: Address and route messages through an internetwork
  • Protocols: IP, IPX, AppleTalk
• Data Link (methods of exchanging data frames)
  
  • Function: Framing, MAC(data processing based on physical requirements, physical addressing), Error Detection
  • Protocols: Ethernet, PPP, HLDC, Frame Relay, ATM, GPRS
• Physical (mechanical, electrical,functional and procedural means to activate/maintain/deactivate physical connections)
  
  • Function: The physical components, Data encoding, Signaling
  • Protocols: Ethernet, Bluetooth, WiFI

CCNA-1. Chapter1. Living in a network centric world

Internet provides new forms of communications:

• Instant Messaging (Developed from Internet Relay Chat (IRC) services. Features: file transfer, voice, video communication, received immediately.
• Weblogs (blogs) (Communicate thoughts to a global audience without technical knowledge of web design)
• Wikis Wikis (Groups of people can edit and view together. businesses use wikis as their internal collaboration tool.
• Podcasting

Networks must meet four basic characteristics:

• Fault tolerance,
• Scalability,
• Quality of service (to overcome congestions)
• Security (avoid unauthorized disclosure,theft, modification, Denial of Service of information)

Types of networks architecture

• Circuit Switched Connection-oriented Networks(message consistency,can be charged by time,no fault tolerance ,hard scalability)
• Packet-switched Connectionless Networks(fault tolerance, scalability)

Data network consists of:

• Devices
• Medium
• The digital messages
• Rules (protocols)

Components of network architecture:

• Programmed services and protocols that move messages across the netwok
• Technologies that support network communications

Network – interconnected devices capable of carrying many different types of communications.
Intranets - private networks in use by just one company, enable businesses to communicate and perform transactions among global employee and branch locations (used by employees)  
Extranets(extended internetworks) - part of intranet that provide suppliers, vendors, and customers limited access to corporate data to check order status, inventory, and parts lists. (used by customers)
Internetwork - interconnection of 2 or more networks  
Internet - most well-known publicly accessible internetwork belonging to Internet Service Providers (ISPs), uses IP protocol.