The transport layer is the basis of any hierarchy of protocols. Its mission is to provide reliable and efficient transmission of data from source to destination, regardless of the physical network or networks that are currently used. The transport layer of the OSI and TCP / IP reference model is a layer between the application layer (ie, the application layer of the OSI model sessions) and the network layer, as shown in Figure 1.1. The general role of this layer and other layers is enabling communication layers above and below the layer (in this case the application layer from the network layer). The specific role of this layer is to accept application data transmitter and receiver application submission thereof, wherein controlled transmission, controls and corrects transmission errors and ensure delivery. Support for the transport layer is usually implemented at the level of computer operating systems, but I transport layer is not explicitly defined, but is realized through support for protocols that layer, (Jevremovic and Veinovic, 2007)..
Figure 1.1: Comparison of OSI and TCP / IP models reviewing protocols for individual layers
Although the OSI reference model and TCP / IP enables the development of different transport protocols, is still in use, depending on user requirements often Transmission Control Protocol (TCP) or User Datagram Protocol (UDP). TCP is Connection oriented protocol that establishes a logical connection between the processes in the network. Performs functions of primary data, addressing and multiplexing, flow control, connection control, function and safety priorities. Enables reliable transmission from end to end, in the form of a continuous stream of data. UDP is beskonekcijski oriented service that transmits data in the form of datagrams. Unlike the TCP protocol does not provide reliable data transmission, and on the reliability of transmission, if necessary, take care of the application itself. It basically serves as an application interface to IP, [1].
SCTP, transmission control protocol sequence (Stream Control Transmission Protocol) is a newer transport protocol. The motivation for the creation of SCTP protocol appeared in the recent increase in the number of applications for which the TCP protocol is limited in many respects. TCP provides reliable transmission of data, but a strict order, while many applications need to transfer without the maintenance of order, and some will be satisfied with partial lined data. In both cases, the head-of-line blocking the TCP protocol causes unnecessary delay. Furthermore, the transmission of information in the form of current octet is often negative because applications need to add tags to delimit blocks of data message and to use the push mechanism to ensure the transmission of messages in a timely manner. The limited scope of TCP sockets complicates enabling high availability of data transmission using multisystem computer. TCP is relatively vulnerable to denial of service (DoS) attacks such as SYN attacks ence.
Transfer PSTN signaling over IP networks is an application for which all these limitations were relevant. While this application was the direct motivation for the development of SCTP's, and many other applications have adopted SCTP as a good response to their requests, (Stewart and Xie, 2000.). SCTP protocol is a reliable transport protocol that operates on top of the network with connectionless data transfer, such as IP. Provides users with many services, which will be presented in detail and explain the essay.
2nd SCTP PROTOCOL
Transmission control protocol sequence (SCTP??) is a newer IP transmission protocol transport layer developed within Signaling Transport (SIGTRAN) group members IETF. SIGTRAN group was defined by the SCTP ??standard in 2000. in RFC 2960, (Stewart and Xie, 2000.). Originally served as a support for signaling the'' Voice over IP'' (VoIP) applications. Since then, the generalized and brings some improvements compared to TCP, including security and robustness. SCTP is a reliable protocol focused on the message, the possibilities vi?esustavnosti and vi?esljednosti. As shown in Figure 2.1, the protocol allows end nodes communicate via multiple addresses, often due to the existence of multiple network interfaces, [4].
Slika 2.1: SCTP arhitektura
Today SCTP ??can be used as a transport protocol for applications that require monitoring and detection of loss of session, that SCTP ??associations, as shown in Figure 2.2. The concept of association is taken from RFC 2960, (Stewart and Xie, 2000).., Representing the proceeds SCTP ??communication between two equal cells. SCTP mechanisms of detection losing sessions actively monitor CONNeCTION session for these types of applications. Its design includes adequate controls to avoid congestion by directing traffic to all nodes in the network and attacks by masking (one system illegally uses the identity of another system). The main differences compared to the TCP protocol are vi?esustavnost (multihoming) and vi?esljednost (multistreaming).
SCTP uses vi?esustavnost Troubleshooter. One of istorazinskih address is designated as the default destination address. This default address with the default source address makes SCTP ??primary path. When packet loss on the primary path, use the alternative destination address. Results of study of the performance vi?esustavnosti showed that the reduced latency packet if there is packet loss.
Figure 2.1 shows that the connection between two endpoints set of one-way flows. Vi?esljednost has the option of removing'''' Head of Line (HOL) effect of TCP, for which packages are in the queue, waiting for the recognition of all the segments. In an environment in which it operates vi?esljednost, packets from different streams operate independently. SCTP uses selective acknowledgment (SACK) to denote the transmission sequence number (TSN) each received block of data, known as a chunk. Safety SCTP ??provides four-sided adjustment (4-way handshake) during initialization of the new association. COOKIE mechanism used during setup times to determine endpoints. Cookie identifies possible theft address. While there is a connection, Heartbeat messages are exchanged periodically to determine the availability of the offered IP address.
Application programming interface (API) allows SCTP??'s application for controlling the flow numbers and addresses for advertising during initialization. Applications can also pinpoint the lifetime package, editing and context identifier for each message sent. The identifier is used in the context of SCTP ??and when providing feedback to the application.
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