network protocols handbook.pdf

55Protocols GuideTCP/IP - Transport Layer ProtocolsProtocol NameRDP : Reliable Data ProtocolProtocol DescriptionRDP is a connection-oriented transport protocol designed to efficientlysupport the bulk transfer of data for such host monitoringand control applications as loading/dumping and remotedebugging. It attempts to provide only those services necessary,in order to be efficient in operation and small in size. The keyfunctions of RDP are as follows:RDP will provide a full-duplex communications channel betweenthe two ports of each transport connection.RDP will attempt to reliably deliver all user messages and willreport a failure to the user if it cannot deliver a message. RDPextends the datagram service of IP to include reliable delivery.RDP will attempt to detect and discard all damaged and duplicatesegments. It will use a checksum and sequence number ineach segment header to achieve this goal.RDP will optionally provide sequenced delivery of segments.Sequenced delivery of segments must be specified when theconnection is established.RDP will acknowledge segments received out of sequence, asthey arrive. This will free up resources on the sending side.RDP supports a much simpler set of functions than TCP. Theflow control, buffering, and connection management schemesof RDP are considerably simpler. The goal is a protocol that canbe easily and efficiently implemented and that will serve a rangeof applications.RDP functions can also be subset to further reduce the size ofa particular implementation. For example, a target processorrequiring down-loading from another host might implement anRDP module supporting only the passive Open function and asingle connection. The module might also choose not to implementout-of-sequence acknowledgements.Protocol Structure1 2 3 4 5 6 8 16bitSYN ACK EAK RST NUL 0 Ver No Header LengthSource PortDestination PortData LengthSequence NumberAcknowledgement NumberChecksumVariable header area …Control flagsThe 8 control bits are divided as follows:.SYN The SYN bit indicates a synchronization segmentis present.ACK The ACK bit indicates the acknowledgmentnumber in the header is valid.EACK The EACK bit indicates an extended acknowledgesegment is present.RST The RST bit indicates the packet is a reset segment.NUL The NUL bit indicates the packet is a null segment.0: The value of this field must be zero.Ver no: version number; current version is 2.Header lengthThe length of the RDP header.Source PortsSource address to identify the processes that originatedthe communication. The combination of the portidentifiers with the source and destination addressesin the network access protocol header serves to fullyqualify the connection and constitutes the connectionidentifier. This permits RDP to distinguish multiple connectionsbetween two hosts.Destination PortsDestination address to identify the processes targetedin the communication.Data LengthThe length in octets of the data in this segment. Thedata length does not include the RDP header.Sequence numberThe sequence number of this segment.Acknowledgement numberIf the ACK bit is set in the header, this is the sequencenumber of the segment that the sender of this segmentlast received correctly and in sequence. Once a connectionis established this should always be sent.ChecksumThe checksum to ensure integrityVariable Header AreaThis area is used to transmit parameters for the SYNand EACK segments.Related protocolsUDP, RUDP, IP, TCP, ICMP