TCP/IP 3 way handshake

TCP uses a handshaking technique to open connections. It is referred to as 3-way handshaking or as "SYN-SYN-ACK". This mechanism is designed so that two systems attempting to initiate a connection for communication can negotiate one connection at a time independently of each other.

CISCO CCNA ICMP PING REQUEST

The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet protocol suite. It is chiefly used by networked computers' operating systems to send error messages—indicating, for instance, that a requested service is not available or that a host or router could not be reached.

ICMP differs in purpose from TCP and UDP in that it is not used to send and receive data between end systems. It is usually not used directly by user network applications, with some notable exceptions being the ping tool and traceroute.

CISCO CCNA MAC ARP TABLE

ARP and the ARP table

How do we find out what the MAC address is?

Workstation A wants to talk to workstation B - workstation A puts an ARP request onto the wire, which happens to be broadcast. Essentially what it 's saying is - Who has workstation B 's MAC address? Of course, because it's a broadcast, every workstation on the network hears it. Does everybody respond? Well what happens is that C hears that A is looking for the MAC address of workstation B. C knows that it is not workstation B and therefore does not respond to the broadcast. The broadcast, the ARP request, goes out to every workstation but the only workstation that will reply is Workstation B with an ARP reply.

In other words: Workstation A says "Who has the MAC address of workstation B" and although all the workstations hear the question, only B replies and says "I've got the MAC address of workstation B and this is what it is..." So the ARP reply sends back the MAC address to workstation A and each of these machines start building an ARP table.
What is ARP?

ARP is the Address Resolution Protocol and its job is to match MAC address to IP address and obviously vice versa - to match IP addresses to MAC addresses.

In workstation A 's ARP table, we have stored the information on workstation B and it 's MAC address. If workstation A talks to workstation C, we'll have the same information for workstation C stored on the ARP table.

Workstation B replied to workstation A and therefore will have stored the relevant information. If workstation B also talked to workstation D recently, it will have workstation D 's IP address and MAC address stored in the ARP table.

arp -a

Run "arp - a" which will report on all the information it has stored on the ARP table during the usage of the network for that workstation.

The ARP table is a dynamic system table, it is built-up at the time of communicating with the different workstations. After a period of time, if workstation A no longer speaks to workstation B the system will age out the entry in the ARP table.

Now lets complete the circle of logic: If after the entry for workstation B has been aged of the ARP table on workstation A, and once again workstation A again wants to talk to workstation B? Workstation A will once again put out ARP request to broadcast by saying, "Who has the MAC address of workstation B" And again, B will reply saying, "I have it and here it is." Again the information will be stored back into the ARP table of workstation A. Doing arp with the '-an' options will enable you to see all the machines that you've communicated with prior to the aging period being reached.
arping

In some versions of the Linux operating system there 's a command called arping. arping is a MAC based ping program, which has the job to determine whether the IP address for a particular MAC address is already being used on the network.

Unicast services such as ssh, telnet and ftp do not use broadcast mechanisms to communicate. As a result they are less noisy and more efficient on the network.

TCP / IP - An animated discussion

Transmission Control Protocol/Internet Protocol, the suite of communications protocols used to connect hosts on the Internet. TCP/IP uses several protocols, the two main ones being TCP and IP. TCP/IP is built into the UNIX operating system and is used by the Internet, making it the de facto standard for transmitting data over networks. Even network operating systems that have their own protocols, such as Netware, also support TCP/IP.

OSI Model Training Tutorial

OSI Model Training Tutorial-1



OSI Model Training Tutorial-2



Layer 1 - Physical
Physical layer defines the cable or physical medium itself, e.g., thinnet, thicknet, unshielded twisted pairs (UTP). All media are functionally equivalent. The main difference is in convenience and cost of installation and maintenance. Converters from one media to another operate at this level.

Layer 2 - Data Link
Data Link layer defines the format of data on the network. A network data frame, aka packet, includes checksum, source and destination address, and data. The largest packet that can be sent through a data link layer defines the Maximum Transmission Unit (MTU). The data link layer handles the physical and logical connections to the packet's destination, using a network interface. A host connected to an Ethernet would have an Ethernet interface to handle connections to the outside world, and a loopback interface to send packets to itself.

Ethernet addresses a host using a unique, 48-bit address called its Ethernet address or Media Access Control (MAC) address. MAC addresses are usually represented as six colon-separated pairs of hex digits, e.g., 8:0:20:11:ac:85. This number is unique and is associated with a particular Ethernet device. Hosts with multiple network interfaces should use the same MAC address on each. The data link layer's protocol-specific header specifies the MAC address of the packet's source and destination. When a packet is sent to all hosts (broadcast), a special MAC address (ff:ff:ff:ff:ff:ff) is used.

Layer 3 - Network
NFS uses Internetwork Protocol (IP) as its network layer interface. IP is responsible for routing, directing datagrams from one network to another. The network layer may have to break large datagrams, larger than MTU, into smaller packets and host receiving the packet will have to reassemble the fragmented datagram. The Internetwork Protocol identifies each host with a 32-bit IP address. IP addresses are written as four dot-separated decimal numbers between 0 and 255, e.g., 129.79.16.40. The leading 1-3 bytes of the IP identify the network and the remaining bytes identifies the host on that network. The network portion of the IP is assigned by InterNIC Registration Services, under the contract to the National Science Foundation, and the host portion of the IP is assigned by the local network administrators. For large sites, the first two bytes represents the network portion of the IP, and the third and fourth bytes identify the subnet and host respectively.

Even though IP packets are addressed using IP addresses, hardware addresses must be used to actually transport data from one host to another. The Address Resolution Protocol (ARP) is used to map the IP address to it hardware address.

Layer 4 - Transport
Transport layer subdivides user-buffer into network-buffer sized datagrams and enforces desired transmission control. Two transport protocols, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP), sits at the transport layer. Reliability and speed are the primary difference between these two protocols. TCP establishes connections between two hosts on the network through 'sockets' which are determined by the IP address and port number. TCP keeps track of the packet delivery order and the packets that must be resent. Maintaining this information for each connection makes TCP a stateful protocol. UDP on the other hand provides a low overhead transmission service, but with less error checking. NFS is built on top of UDP because of its speed and statelessness. Statelessness simplifies the crash recovery.

Layer 5 - Session
The session protocol defines the format of the data sent over the connections. The NFS uses the Remote Procedure Call (RPC) for its session protocol. RPC may be built on either TCP or UDP. Login sessions uses TCP whereas NFS and broadcast use UDP.

Layer 6 - Presentation
External Data Representation (XDR) sits at the presentation level. It converts local representation of data to its canonical form and vice versa. The canonical uses a standard byte ordering and structure packing convention, independent of the host.

Layer 7 - Application
Provides network services to the end-users. Mail, ftp, telnet, DNS, NIS, NFS are examples of network applications.

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CCNA-OSI

The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design.

It was developed as part of the Open Systems Interconnection (OSI) initiative and is sometimes known as the OSI seven layer model. From top to bottom, the OSI Model consists of the Application, Presentation, Session, Transport, Network, Data Link, and Physical layers. A layer is a collection of related functions that provides services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path.

CCNA - Getting to Know Frame Relay

Frame Relay Terminology: Long Distance Non-persistent Connections
VC: Virtual Circuit (Switched is Permanent)
DTE: Data Terminal Equipment
DCE: Data Communication Equipment
DLCI: Data Link Connection Identifier
NBMA: Non Broadcast Multiaccess
LMI: Local Management Interface
LAPF: Link Access Procedure Frame Bearer Services