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Overview Of The Application Layer

ashish — Mon, 03 Aug 2009 04:41:13 +0000

Computer networks are inherently insecure. To keep information secret, it must be encrypted. Encryption protocols fall into two general classes: secret key (e.g. DES, IDEA), and public key (e.g. RSA). Using these protocols is straight-forward; the hard part is key management.
In addition to providing secrecy, cryptographic protocols can also provide authentication. Finally, cryptography can also be used to allow messages to be signed in such a way that the sender cannot repudiate them after they have been sent. Naming in the Internet uses a distributed database system, DNS. Domain Name Server(DNS) holds records with IP addresses, mail exchanges, and other information. By querying a DNS server, a process can map an Internet domain name onto the IP address used to communicate with that domain.
As networks grow larger, they become harder to manage. For this reason, special network management systems and protocols have been devised, the most popular of which is SNMP. This protocol allows managers to communicate with agents inside devices to read out their status and issue commands to them.
Four major network applications are electronic mail, USENET news, the World Wide Web, and multimedia. Most email systems use the mail system defined in RFCs 821 and 822. Messages sent in this system use system ASCII headers to define message properties. These messages are sent using SMTP. Two systems for securing email exist, PGP and PEM.
USENET news consists of thousands of newsgroups on all manner of topics. People can join newsgroups locally, and can then post messages all over the world using the NNTP protocol, which has some resemblance to SMTP.
The World Wide Web is a system for linking up hypertext documents. Each document is a page written in HTML, possible with hyperlinks to other documents. A browser can display a document by establishing a TCP connection to its server, asking for the document, and then closing the connection. When a hyperlink is selected by the user, that document can also be fetched in the same way. In this manner, documents all over the world are linked together in a giant web.
Multimedia is the rising star in the networking firmament. It allows audio and video to be digitized and transported electronically for display. Most multimedia projects use the MPEG standards and transmit the data over ATM connections.

Overview Of The Transport Layer

ashish — Mon, 03 Aug 2009 04:34:31 +0000

The transport layer is the key to understanding layered protocols. It provides various services, the most important of which is an end-to-end, reliable, connection-oriented byte stream from sender to receiver. It is accessed through service primitives that permit the establishment, use and release of connection.
Transport protocols must be able to do connection management over unreliable networks. Connection establishment is complicated by the existence of delayed duplicate packets that can reappear at inopportune moments. To deal with them, three-way handshakes are needed to establish connections. Releasing a connection is easier than establishing one, but is still far from trivial due to the two-army problem.
Even when the network layer is completely reliable, the transport layer has plenty of work to do. It must handle all the service primitives, manage connections and timers, and allocate and utilize credits.
The main Internet transport protocol is TCP. It uses a 20-byte header on all segments. Segments can be fragmented by routers within the Internet, so hosts must be prepared to do reassembly. A great deal of work has gone into optimizing TCP performance, using algorithms from Nagle, Clark, Jacobson, Karn and others.
ATM has four protocols in the AAL layer. All of them break messages into cells at the source and reassemble the cells into messages at the destination. The CS and SAR sublayers add their own headers and trailers in various ways, leaving from 44 to 48 bytes of cell payload.
Network performance is typically dominated by protocol and TPDU processing overhead, and the situation gets worse at higher speeds. Protocols should be designed to minimize the number of TPDUs, context switches, and times each TPDU is copied. For gigabit networks, simple protocols using rate, rather than credit, flow control are called for.

Overview Of The Medium Access Sublayer

ashish — Mon, 03 Aug 2009 04:27:57 +0000

Some networks have a single channel that is used for all communication. In these networks, the key design issue is the allocation of this channel among the competing stations wishing to use it. Numerous channel allocation algorithms have been devised like :

- FDM : Dedicate a frequency band to each station.
- TDM : Dedicate a time slot to each station.
- Pure ALOHA : Unsynchronized transmission at any instant.
- Slotted ALOHA : Random transmission in well defined time slots.
- 1-persistent CSMA : Standard carrier sense multiple access.
- Nonpersistent CSMA : Random delay when channel is sensed busy.
- P-persistent CSMA : CSMA, but with probability of p of persisting.
- CSMA/CD : CSMA. but abort on detecting a collision.
- Bit Map : Round robin scheduling using a bit map.
- Binary countdown : Highest numbered ready station goes next.
- Tree walk : Reduced contention by selective enabling.
- Wavelength division : A dynamic FDM scheme for fiber.
- MACA, MACAW : Wireless LAN protocols.
- GSM : FDM plus TDM for cellular radio.
- CDPD : Packet radio within an AMPS channel.
- CDMA : Everybody speak at once but in different language.
- Ethernet : CSMA/CD with binary exponential backoff.
- Token bus : Logical ring on a physical bus.
- Token Ring : Capture the token to send a frame.
- DQDB : Distributed queuing on a two-bus MAN.
- FDDI : Fiber-optic token ring.
- HIPPI : Crossbar using 50-100 twisted pairs.
- Fibre channel : Crossbar using fiber optics.
- SPADE : FDM with dynamic channel allocation.
- ACTS : TDM with centralized slot allocation.
- Binder : TDM with ALOHA when slot owner is not interested.
- Crowther : ALOHA with slot owner getting to keep it.
- Roberts : Channel time reserved in advance by ALOHA.

FDM and TDM are efficient when the number of stations is small and the traffic is continous.
ALOHA protocol, with and without slotting and control, has been proposed when the number of stations is large and variable.
BINARY COUNTDOWN completely eliminates contention.
TREE WALK reduces contention by dynamically dividing the stations into two disjoint groups, one of which is permitted to transmit and one of which is not.
WIRELESS LANs have their own problems and solutions. The biggest problem is caused by hidden stations, so CSMA does not work. MACA attempts to stimulate transmissions around the destination, to make CSMA work better.
GSM, CDPD and CDMA are widely used for mobile computers and telephones.
The IEEE 802 LANs are : CSMA/CD, TOKEN BUS, and TOKEN RING. Each of these has its unique advantages and disadvantages, and each has found its own user community.
FDDI, FAST ETHERNET, HIPPI, and FIBER CHANNEL offer bandwidth in the 100 Mbps range and up.

Quick Tech Tip: Layer 2 Tunneling protocol : L2TP

ashish — Thu, 23 Jul 2009 13:49:58 +0000

Layer Two Tunneling Protocol (L2TP) is an extension of the Point-to-Point Tunneling Protocol (PPTP) used by an Internet service provider (ISP) to enable the operation of a virtual private network (VPN) over the Internet.
The two end components that make up L2TP are the L2TP Access Concentrator (LAC) which is the device that physically terminates a call and the L2TP Network Server (LNS), which is the device that terminates and possibly authenticates the PPP stream. Once a tunnel is established, the network traffic between the peers is bidirectional. To be useful for networking, higher level protocols are then run through the L2TP tunnel. To facilitate this L2TP session (or call) is established within the tunnel for each higher-level protocol such as PPP. Either the LAC or LNS may initiate sessions. The traffic for each session is isolated by L2TP, so it is possible to set up multiple virtual networks across a single tunnel.
The packets exchanged within an L2TP tunnel are either categorized as control
packets or data packets. L2TP provides reliability features for the control packets, but no reliability for data packets. Reliability, if desired, must be provided by the nested protocols running within each session of the L2TP tunnel.
An L2TP tunnel can extend across an entire PPP session or only across one segment of a two-segment session. This can be represented by four different tunneling models :
- Voluntary Tunnel model : a tunnel is created by the user, typically by the use of an L2TP enabled client which is called the LAC client. The user will send L2TP packets to the Internet Service Provider (ISP) which will forward them on to the LNS.
- Compulsory tunnel model-incoming call: a tunnel is created between ISP LAC and the LNS home gateway.
- Compulsory tunnel model-remote dial the home gateway (LNS) initiates a tunnel to an ISP (LAC) (outgoing call) and instructs the ISP to place a local call to the PPP enabled client which is the remote user.
- L2TP Multi-hop connection : It is a way of redirecting L2TP traffic on behalf of client LACs and LNSs. A Multi-hop connection is established using an L2TP Multi-hop gateway. A tunnel is established from a client LAC to the L2TP Multi-hop gateway and then another tunnel is established between the L2TP Multi-hop gateway and a target LNS. L2TP traffic between client LAC and LNS is redirected to each other through the gateway.

Quick Tech Tip: Point-to-point tunneling protocol – PPTP

ashish — Thu, 23 Jul 2009 13:47:30 +0000

Overview of Point-to-point Protocol:

The Point-to-Point Protocol (PPP) provides a standard method for transporting multi-protocol datagrams over point-to-point links. PPP was originally emerged as an encapsulation protocol for transporting IP traffic between two peers.PPP is comprised of the following main components:
* Encapsulation: A method for encapsulating multi-protocol datagrams.
* Link Control Protocol: The LCP is used to automatically agree upon the encapsulation format options, handle varying limits on sizes of packets, detect a looped-back link and other common misconfiguration errors, and terminate the link.
* Network Control Protocol: An extensible Link Control Protocol (LCP) for establishing, configuring, and testing and managing the data-link connections.
* Configuration: Easy and self configuration mechanisms using Link Control Protocol. This mechanism is also used by other control protocols such as Network Control Protocols (NCPs).

Introduction TO PPTP :

PPTP packages data within PPP packets, then encapsulates the PPP packets within IP packets (datagrams) for transmission through an Internet-based VPN tunnel. PPTP supports data encryption and compression of these packets.
The PPTP protocol is designed to perform the following tasks:
* Query the status of Comm Servers
* Provide In-Band management
* Allocate channels and place outgoing calls
* Notify NT Server on incoming calls
* Transmit and Receive User Data with flow control in both directions
* Notify NT Server on disconnected calls.

PPTP-based Internet remote access VPNs are by far the most common form of PPTP VPN. In this environment, VPN tunnels are created via the following two-step process:
1. The PPTP client connects to their ISP using PPP dial-up networking.
2. Via the broker device (described earlier), PPTP creates a TCP control connection between the VPN client and VPN server to establish a tunnel.

Once the VPN tunnel is established, PPTP supports two types of information flow:
* control messages for managing and eventually tearing down the VPN connection. Control messages pass directly between VPN client and server.
* data packets that pass through the tunnel, to or from the VPN client.

PPTP also supports VPN connectivity via a LAN.
PPTP supports authentication, encryption, and packet filtering.

Though PPTP remains a popular choice for VPNs, one drawback of PPTP is its failure to choose a single standard for authentication and encryption. Two products that both fully comply with the PPTP specification may be totally incompatible with each other if they encrypt data differently.