 Hello and welcome to the session on stream control transmission protocol features. At the end of this session, students will be able to identify SCP as a new transport layer protocol. Students will be able to describe the purpose and the features of SCP. Transmission sequence number, that is TSN. The unit of data in TCP is a byte. Data transfer in TCP is controlled by numbering bytes using a sequence number. However, on the other hand, the unit of data in TCP is a byte of chunk which may or may not have a one-to-one relationship with the message coming from the process because of fragmentation. Data transfer in TCP is controlled by numbering the data chunks. SCP uses a transmission sequence number to number the data chunks. In other words, TSN in SCP plays the analogous role as a sequence number in TCP. TSNs are 32 bits long and randomly initialized between 0 to 2 raised to 32 minus 1. Each data chunk must carry the corresponding TSN in its header. In SCP, a data chunk is numbered using a TSN, stream identifier, that is SI. In TCP, there is only one stream in each connection. In SCP, there may be several streams in each association. Each stream in SCP needs to be identified using a stream identifier. Each data chunk must carry the SI in its header so that when it arrives at the destination, it can be properly placed in its stream. The SI is a 16-bit starting from 0. To distinguish between different streams, SCP uses an SI, that is stream identifier. Stream sequence number. When a data chunk arrives at the destination, it is delivered to the appropriate stream and in proper order. This means that in addition to an SI, SCP defines each data chunk in each stream with a stream sequence number. To distinguish between different data chunks belonging to the same stream, SCP uses SSNs. Packets. In TCP, a segment carries data and control information. The data are carried as a collection of bytes. Control information is defined by six control flags in the header. The design of SCP is totally different. Data are carried as a data chunk. Control information as control chunks. Control control chunks and data chunks can be packed together in a packet. A packet in SCP plays the same role as a segment in TCP. Figure shows the comparison between a segment in TCP and a packet in SCP. You must remember that TCP has segments and SCP has packets. Here let us briefly list the differences between an SCP packet and a TCP segment. The first difference. The control information in TCP is part of the header. The control information in SCP is included in the control chunks. There are several types of control chunks which is used for a different purpose. The second difference is that the data in the TCP segment is treated as one entity. An SCP packet can carry several data chunks which can belong to different streams. The third is the options section which can be part of the TCP segment does not exist in an SCP packet. Options in SCP are handled by defining new chunks types. The next difference is that the mandated part of the TCP header is 20 bytes while the general header in SCP is only 12 bytes. The SCP header is shorter due to the following reason. The first is that an SCP sequence number that is TSN belongs to each data chunk and hence is located in the chunks header. The second reason is that the acknowledgement number and window size are part of each control chunks. The third difference is that there is no need for a header length field because there are no options to make the length of the header variable. The SCP header length is fixed that is 12 bytes. The last difference is that there is no need for an urgent pointer in SCP. The fifth difference is that the checksum in TCP is 16 bits. In SCP it is 32 bits. The next difference is that the verification tag in TCP is an association identifier which does not exist in TCP. In TCP the combination of IP and port addresses define a connection. In SCP we have multi-homing using different IP addresses. A unique verification tag is needed to define each association. Next difference is that TCP includes only one sequence number in the header which defines the number of the first byte in the data section. An SCP packet can include several different data chunks. TSNs, IS and SSNs define each data chunks. The last difference is that some segments in TCP that carry control information such as SIN or affine segment need to consume one sequence number. Control chunks in SCP never use a TSN, IS or SSN number. These three identifiers belong only to data chunks not to the whole packet. Here you have to note that in SCP control information and data information are carried in separate chunks. In SCP we have data chunks, streams and packets. An association may send many packets. A packet may contain several chunks and chunks may belong to different streams. To make this term clear let us suppose that a process A needs to send 11 messages to process B in three streams. The first four messages are in the first stream, the second three messages are in the second stream and the last four messages are in the third stream. Even though a message is long can be carried by several data chunks. If we assume that each message fits into one data chunk therefore we have 11 data chunks in three streams. The application process delivers 11 messages to SCP where each message is a mark for the appropriate stream. Although the process could deliver one message from the first stream and then another from the second, here we assume that it delivers all messages belonging to the first stream, first and all messages belonging to the second stream next and finally all messages belonging to the last stream. Here we assume that the network allows only three data chunks per packet which means that we need four packets as shown in this diagram. Data chunks in stream 0 are carried in the first stream and part of second packet. Those in stream 1 are carried in the second and the third packet and those in stream 2 are carried in the third and fourth packet. You can see here in this diagram. Here note that each data chunk needs three identifiers TSN, SI and SSN as shown in the diagram. TSN is a cumulative number and used as well as for flow control and error control. SI defines the stream to which the chunk belongs. SSN defines the chunks ordered in a particular stream. Here in our example SSN starts from 0 for each stream. Here you have to note that data chunks are identified by three identifiers TSN, SI and SSN. TSN is a cumulative number identifying the association. SI defines the stream. SSN defines the chunk in the stream. Here pause the video, think and answer. In STP a data chunk is numbered using a TSN. The answer is A. Here is the reference. Thank you.