Communication protocol - Wikipedia
In computer networking, the Link Control Protocol (LCP) forms part of the Point-to -Point Protocol (PPP), within the family of Internet protocols. In setting up PPP. High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the . timing out waiting for a positive acknowledge packet, the sender can retransmit the failed frame. .. The relationship between the DISC (disconnect) command and the RD (request disconnect). PPP Link Control Protocol (LCP) Phase. PPP Outgoing LCP Note: The terms local machine and peer do not imply a client-server relationship.
The selection of the next protocol is accomplished by extending the message with a protocol selector for each layer. The data received has to be evaluated in the context of the progress of the conversation, a protocol therefore must include rules describing the context.
These kind of rules are said to express the syntax of the communication. Other rules determine whether the data is meaningful for the context in which the exchange takes place.
These kind of rules are said to express the semantics of the communication. Messages are sent and received on communicating systems to establish communication. Protocols should therefore specify rules governing the transmission. In general, much of the following should be addressed: The bitstrings are divided in fields and each field carries information relevant to the protocol.
Conceptually the bitstring is divided into two parts called the header and the payload. The actual message is carried in the payload.
The header area contains the fields with relevance to the operation of the protocol. Bitstrings longer than the maximum transmission unit MTU are divided in pieces of appropriate size. The addresses are carried in the header area of the bitstrings, allowing the receivers to determine whether the bitstrings are of interest and should be processed or should be ignored. A connection between a sender and a receiver can be identified using an address pair sender address, receiver address.
Usually some address values have special meanings. An all-1s address could be taken to mean an addressing of all stations on the network, so sending to this address would result in a broadcast on the local network. The rules describing the meanings of the address value are collectively called an addressing scheme. This is referred to as address mapping. On the Internet, the networks are connected using routers. The interconnection of networks through routers is called internetworking.
Detection of transmission errors Error detection is necessary on networks where data corruption is possible.
In a common approach, CRCs of the data area are added to the end of packets, making it possible for the receiver to detect differences caused by corruption. The receiver rejects the packets on CRC differences and arranges somehow for retransmission. Acknowledgements are sent from receivers back to their respective senders. To cope with this, under some protocols, a sender may expect an acknowledgement of correct reception from the receiver within a certain amount of time.
Thus, on timeoutsthe sender may need to retransmit the information. Exceeding the retry limit is considered an error. This is known as media access control.
Arrangements have to be made to accommodate the case of collision or contention where two parties respectively simultaneously transmit or wish to transmit. As a result, pieces may arrive out of sequence. Retransmissions can result in duplicate pieces. By marking the pieces with sequence information at the sender, the receiver can determine what was lost or duplicated, ask for necessary retransmissions and reassemble the original message.
Flow control can be implemented by messaging from receiver to sender. Design of complex protocols often involves decomposition into simpler, cooperating protocols.
Such a set of cooperating protocols is sometimes called a protocol family or a protocol suite,  within a conceptual framework. Communicating systems operate concurrently. An important aspect of concurrent programming is the synchronization of software for receiving and transmitting messages of communication in proper sequencing.
Concurrent programming has traditionally been a topic in operating systems theory texts. Mealy and Moore machines are in use as design tools in digital electronics systems encountered in the form of hardware used in telecommunication or electronic devices in general. In analogy, a transfer mechanism of a protocol is comparable to a central processing unit CPU.
The framework introduces rules that allow the programmer to design cooperating protocols independently of one another. Protocols are to computer communication what programming languages are to computation. In modern protocol design, protocols are layered to form a protocol stack. Layering is a design principle which divides the protocol design task into smaller steps, each of which accomplishes a specific part, interacting with the other parts of the protocol only in a small number of well-defined ways.
Layering allows the parts of a protocol to be designed and tested without a combinatorial explosion of cases, keeping each design relatively simple. The communication protocols in use in the Internet are designed to function in diverse and complex settings. This model was expanded to four layers by additional protocols.
Link Control Protocol
However, the Internet protocol development has not focussed on the principle of layering as mandatory recipe for communication, rather it evolved as a convenient description of modularity and protocol cooperation. A different model is the OSI seven layer modelwhich was developed internationally as a rigorous reference model for general communication, with much stricter rules of protocol interaction and a rigorous layering concept of functionality.
Typically, application software is built upon a robust data transport layer. Underlying this transport layer is a datagram delivery and routing mechanism that is typically connectionless in the Internet. Packet relaying across networks happens over another layer that involves only network link technologies, which are often specific to certain physical layer technologies, such as Ethernet.
Layering provides opportunities to exchange technologies when needed, for example, protocols are often stacked in a tunneling arrangement to accommodate connection of dissimilar networks. Protocol layering[ edit ] Figure 3. Message flows using a protocol suite.Why Most Relationships Fail - Weekly Wisdom Episode 12
Black loops show the actual messaging loops, red loops are the effective communication between layers enabled by the lower layers. Protocol layering now forms the basis of protocol design. The Internet protocol suite consists of the following layers: Computations deal with algorithms and data and communication involves protocols and messages, so the analog of a data flow diagram is some kind of message flow diagram.
The systems both make use of the same protocol suite. The vertical flows and protocols are in system and the horizontal message flows and protocols are between systems. The message flows are governed by rules, and data formats specified by protocols.
The blue lines therefore mark the boundaries of the horizontal protocol layers. The vertical protocols are not layered because they don't obey the protocol layering principle which states that a layered protocol is designed so that layer n at the destination receives exactly the same object sent by layer n at the source.
High-Level Data Link Control - Wikipedia
The horizontal protocols are layered protocols and all belong to the protocol suite. Layered protocols allow the protocol designer to concentrate on one layer at a time, without worrying about how other layers perform. This can be achieved using a technique called Encapsulation. The pieces contain a header area and a data area. The data in the header area identifies the source and the destination on the network of the packet, the protocol, and other data meaningful to the protocol like CRC's of the data to be sent, data length, and a timestamp.
The result is that at the lowest level the piece looks like this: This rule therefore ensures that the protocol layering principle holds and effectively virtualizes all but the lowest transmission lines, so for this reason some message flows are coloured red in figure 3. To ensure both sides use the same protocol, the pieces also carry data identifying the protocol in their header. The design of the protocol layering and the network or Internet architecture are interrelated, so one cannot be designed without the other.
The Internet offers universal interconnection, which means that any pair of computers connected to the Internet is allowed to communicate. Each computer is identified by an address on the Internet. Modified frames, however, are always distinguishable from standard frames. Troubleshooting a Serial Interface Go to the course online to perform this practice activity. These topics are discussed next. Establishing a PPP Session 3. Link establishment and configuration negotiation: This phase is complete when the receiving router sends a configuration-acknowledgment frame back to the router initiating the connection.
Link quality determination optional: The LCP tests the link to determine whether the link quality is sufficient to bring up network layer protocols. The LCP can delay transmission of network layer protocol information until this phase is complete. Network layer protocol configuration negotiation: After the LCP has finished the link quality determination phase, the appropriate NCP can separately configure the network layer protocols, and bring them up and take them down at any time.
If the LCP closes the link, it informs the network layer protocols so that they can take appropriate action. Figure Establishing a PPP Session The link remains configured for communications until explicit LCP or NCP frames close the link, or until some external event occurs such as an inactivity timer expiring, or an administrator intervening.
The LCP can terminate the link at any time. This is usually done when one of the routers requests termination, but can happen because of a physical event, such as the loss of a carrier or the expiration of an idle-period timer. Link-termination frames terminate a link Terminate-Request and Terminate-Ack. This phase must complete successfully, before any network layer packets can be exchanged. During link establishment, the LCP opens the connection and negotiates the configuration parameters.
The link establishment process starts with the initiating device sending a Configure-Request frame to the responder. The Configure-Request frame includes a variable number of configuration options needed to set up on the link.
The value of this object is meaningful only when the link has reached the open state ifOperStatus is up. The actual values of the parameters may be changed when the link is brought up via the LCP options negotiation mechanism.
Changing this object will have effect when the link is next restarted. In effect, this is the ACC Map that is required in order to ensure that the local modem will successfully receive all characters. In effect, this is the ACC Map that is required in order to ensure that all characters can be successfully transmitted through the local modem.
If false 1 then this negotiation is not performed. In any event, the local node will comply with any magic number negotiations attempted by the remote node, per the PPP specification. Regardless of the value of this object, the local node will comply with any FCS size negotiations initiated by the remote node, per the PPP specification. No effort is made to define good or bad, nor the policy used to determine it.
If disabled 1 then this negotiation is not performed.
Setting this object to the value disabled 1 has the effect of invalidating the corresponding entry in the pppLqrConfigTable object. It is an implementation-specific matter as to whether the agent removes an invalidated entry from the table. Accordingly, management stations must be prepared to receive tabular information from agents that corresponds to entries not currently in use. Assuming that this group has been implemented, a PPP link will have an entry in this table if and only if LQR Quality Monitoring has been successfully negotiated for said link.
The format of the packet is as described in the LQM Protocol specificiation. The LQR packet is stored in network byte order. PPP Tests -- The extensions to the interface table in RFC define a -- table through which the network manager can instruct the -- managed object to perform various tests of the interface. This -- is the ifExtnsTestTable. It returns failed 7 if the -- response did not properly return.
The definition of "transmission -- error" in this context is left to the discretion of the -- implementor. Acknowledgements This document was produced by the PPP working group.
In addition to the working group, the author wishes to thank the following individuals for their comments and contributions: This represents a security risk.