Introduction to Peer-to-Peer Networking

Peer to peer is an approach to computer networking where all computers share equivalent responsibility for processing data. Peer-to-peer networking (also known simply as peer networking) differs from client-server networking, where certain devices have responsibility for providing or "serving" data and other devices consume or otherwise act as "clients" of those servers.

Characteristics of Peer-to-Peer Network

- A P2P network can be an ad hoc connection—a couple of computers connected via a Universal Serial Bus to transfer files.
- A P2P network also can be a permanent infrastructure that links a half-dozen computers in a small office over copper wires.
- A P2P network can be a network on a much grander scale in which special protocols and applications set up direct relationships among users over the Internet.
- P2P software systems like Kazaa and Napster rank amongst the most popular software applications ever.
- P2P technologies promise to radically change the future of networking. P2P file sharing software has also created much controversy over legality and "fair use."
- A P2P network implements search and data transfer protocols above the Internet Protocol (IP).
- To access a P2P network, users simply download and install a suitable P2P client application.

A P2P setup can facilitate the following:
- Sharing of file by all the users of the network.
- Telephony.
- Media streaming - both audio and video.
- Community/discussion forums.

Types of P2P Networks

The P2P networks are normally classified as either ‘pure’ or ‘hybrid’ types.
Pure Networks
- Peers have got to be equals – no single node can supersede or dictate terms over another.
- There is no requirement of a central server and as such, no possibilty of a client-server correlation.
- There is also no need for a central router.

Hybrid P2P Networks
- This type of network needs to have a central server that can store data on all the peers and deliver it whenever asked to do so.
- The route terminals are treated as addresses, each one of which can be referenced by a specfic set of indices.
- Since the central server is not supposed to have any kind of resources available to it, the peers themselves are required to host all the resources. As and when required, a peer informs the central server about the type of resource to be shared and the details of the peer/s who should be allowed to share it.

Benefits of P2P Networks

1. Efficient use of resources.
- Unused bandwidth, storage, processing power at the edge of the network.
2. Scalability
- Consumers of resources also donate resources.
- Aggregate resources grow naturally with utilization.
3. Reliability
- Replicas.
- Geographic distribution.
- No single point of failure.
4. Ease of administration
- Nodes self organize.
- No need to deploy servers to satisfy demand (c.f. scalability).
- Built-in fault tolerance, replication, and load balancing.

POP - Post Office Protocol

Post Office Protocol (POP) is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. The Post Office Protocol (POP) allows you to fetch email that is waiting in a mail server mailbox. POP defines a number of operations for how to access and store email on your server.
It works in conjunction with the SMTP (Simple Mail Transfer Protocol), which provides the message transport services required to move mail from one system to another.

Purpose of POP

If somebody sends you an email it usually cannot be delivered directly to your computer. The message is stored in a place where you can pick it up easily. Your ISP (Internet Service Provider) does this job. It receives the message for you and keeps it until you download it. Now, POP, the Post Office Protocol is what allows you to retrieve mail from your ISP. This is also about all the Post Office Protocol is good for.

What POP allows you to do?

Things that can be done via the POP include:
- Retrieve mail from an ISP and delete it on the server.
- Retrieve mail from an ISP but not delete it on the server.
- Ask whether new mail has arrived but not retrieve it.
- Peek at a few lines of a message to see whether it is worth retrieving.

POP3 is an open Internet standard. The common POP3 commands and responses are :
- getwelcome(): Gets the greeting from the server.
- user(username): Login with a username. If valid username, server will respond with request for password.
- pass_ (password): Send password. If valid, server response will be two numbers, message count and mailbox size.
- stat(): Get the mailbox status. Response is two numbers, message count and mailbox size.
- list([message]): Get list of messages. An option "message" gets information on a specific message.
- retr(message): Get message number "message".
- dele(message): Delete message number "message".
- rset(): Remove all deleted message markings.
- noop(): No operation. Do nothing. Really. Needed in unusual programming situations.
- quit(): Quit. Commits all changes, unlocks the mailbox, and ends the server connection.
- top(message, lines): Gets just the first "lines" number of lines of message number "message". Useful on low bandwidth lines to get just the first part of long messages.
- uidl([message]): Gets a unique id list -- a message digest including unique ids. The option gets the unique id for the specific message "message".

Dynamic Trunking Protocol (DTP)

The Dynamic Trunking Protocol (DTP) is a proprietary networking protocol developed by Cisco Systems for the purpose of negotiating trunking on a link between two VLAN-aware switches, and for negotiating the type of trunking encapsulation to be used. It works on the Layer 2 of the OSI model. If a port can become a trunk, it may also have the ability to trunk automatically, and in some cases even negotiate what type of trunking to use on the port. DTP provides this ability to negotiate the trunking method with the other device.
There are a couple of other potential issues that arise when you start trunking.
- The first issue is that both ends of a trunk cable had better agree they're trunking, or they're going to be interpreting trunk frames as normal frames. To resolve this, Cisco came up with a protocol for switches to communicate intentions. The first version of it was VTP, VLAN Trunking Protocol, which worked with ISL. The newer version works with 802.1q as well, and is called Dynamic Trunking Protocol (DTP).
- The second issue is creating VLAN's.

Switch port modes

- auto : causes the port to passively be willing to convert to trunking. The port will not trunk unless the neighbor is set to on or desirable . This is the default mode.
- on : forces the link into permanent trunking, even if the neighbor doesn't agree.
- off : forces the link to permanently not trunk, even if the neighbor doesn't agree.
- desirable : causes the port to actively attempt to become a trunk, subject to neighbor agreement.
- nonegotiate : forces the port to permanently trunk but not send DTP frames.

Protocol Structure of DTP

On a Catalyst set-based switch, the syntax for setting up a link as a trunk is:

set trunk mod_num/port_num [on | desirable | auto | nonegotiate] [isl | dot1q | negotiate] [vlan_range]
Use this command to set the specified port or ports to trunking.