[Notes on Unit-IV]: Computer Networks][By: ROHIT KUMAR]Syllabus:-Unit-IV•Transport Layer - Design issues, •Connection management, •Session Layer· - Design issues, •Remote procedure call. •Presentation Layer - Design issues, •Data compression techniques,•Cryptography •TCP - Window Management. Possible Conceptual Questions:-Question No-(1):What are various design issues of the Transport Layer?Answer: - Design issues of Transport Layer:- Transport layer is responsible for following issues:-a)Accepting message segments from the application layer and to divide into packets.b)End-to-End Delivery of the packetc)Combining packets into message segment at receiver side. d)Connection management.In other words transport layer is responsible for two tasks:- Transport and regulate the flow of information from source to destination, reliably and accurately.The end-to-end control: Sliding windows. Sequencing numbers.Acknowledgments.Segmentation.Multiplexing.Question No-(2): What are the differences between TCP and UDP? Also discuss various fields in TCP and UDP headers?Answer:- The TCP/IP protocol model at Layer 4 (transport layer) has two protocols - TCP and UDP.Both TCP and UDP use port (or socket) numbers to pass information to the upper layers.•TCP supplies a virtual circuit between end-user applications. •These are main characteristics of the TCP:
•Connection-oriented.•Reliable.•Divides outgoing messages into segments.•Reassembles messages at the destination station.•Re-sends anything not received.•Reassembles messages from incoming segments.In brief TCP is transport layer protocol that provides reliable full-duplex data transmission.TCP-Header:- Following are the fields in TCP-header.Source Port 16 bits-This is the port through which was packet was generated at source machine.Destination Port 16 bits-This is the port through which the packet is supposed to be received by destination machine.Sequence Number: 32 bits – Each TCP packet is assigned an identification number by transport layer. The sequence number field represents the sequence number of the first data octet in this segment (except when SYN is present).This field isused to ensure correct sequencing of the arriving dataAcknowledgment Number: 32 bits- This field contains the value of the next sequence number the sender of the segment is expecting to receive.HLEN:-Number of 32-bit words in the header.Checksum:-Ensure that the data has not been damaged during transmissionUrgent Pointer: Indicates the end of the urgent dataControl or Flag Bits: 8 bits ACK: Acknowledgment field significantRST: Reset the connectionSYN: Synchronize sequence numbersFIN: No more data from senderWindow: 16 bitsThe number of data octets beginning with the one indicated in the acknowledgment field which the sender of this segment is willing to accept.
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