*Transport Layer Concepts

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OSI and Protocol Stack

*Packet Encapsulation The data is sent down the protocol stack Each layer adds to the data by adding headers

*OSI Model Layers

Physical Layer : how the physical bits are sent, details all the electrical and physical specifications for devices

Link Layer : set of rules that determine when a device can send data over a particular medium

Network Layer : handles the movement of packets, i.e. Logical addressing, Routing & Path Determination

Transport Layer : accepts the data from a session and formats the data into segments for transport across the network

Session Layer : establishes, manages and terminates communication sessions

Presentation Layer : translates data formats, Encryption and compression are performed at this level.

Application Layer : interfaces directly with the application programs running on the devices

*Transport Layer

*Transport Layer

*UDP User Datagram ProtocolSimple Data Transmission Model , basically an application interface to IP. It adds no reliability, flow-control, or error recovery to IP. It simply serves as a mux/demux for sending and receiving datagramsNO handshake between the communicating hosts

Datagrams can occasionally be dropped and subsequently not re-sentarrive at destination in the wrong orderArrive without integrity of data

ADVANTAGE : Fast (Ideal for real-time data transmission)

DISADVANTAGE : Unreliable, no account for the data lost

*UDP HeaderSource Port : UDP port number of the process on the sending deviceDest. Port : UDP port number of the process on the destination device Length : Size of Packet ( Header + Data )UDP Checksum : Verifies integrity of UDP header only

32 Bit

*UDPUse : Two critical systems utilise the UDP transport protocol precisely due to it's speed :

1. Domain Name System (DNS) which uses UDP for simple requests and replies to resolve hostnames on the Internet.2. Dynamic Host Configuration Protocol (DHCP) used to assign dynamic IP addresses to hosts

UDP is used for applications like Voice over IP and Online gaming where not always 100% of the data is required to function

*TCP Transmission Control ProtocolProvides a reliable, ordered delivery of data packets from one application on one system to another application on another system

Detects errors, initiates retransmissions, and controls the flow of information

Protocol Steps: 1. Set up connection2. Transfer data3. Close connection

ADVANTAGE : flow control and error correction

DISADVANTAGE : Low speed

*TCP HeaderAcknowledgement Number is the sequence number that the receiver expects to receive nextOffset is the number of 32-bit words in the TCP header Flags : Urgent (URG), Acknowledgement (ACK), Push (PSH), Reset connection (RST), Synchronous (SYN), Finish (FIN), Congestion Window Reduced (CWD) and ECN-Echo (ECE)Checksum is used to verify the integrity of the TCP header and dataUrgent Pointer is used when urgent data is being sent

*TCPUses :

1. Used by Domain Name System (DNS) for larger messages, especially zone transfers as reliability of the data is much more important in these cases2. Used by Border Gateway Protocol (BGP) to maintain the table of IP networks3. Ideal for Emails and FTP

*Session Comparison

*TCP : ConnectionHostClientSend SYN seq=xReceive SYN+ACK segmentSend ACK y+1Receive SYN segmentSend SYN seq=y, ACK x+1Receive ACK segmentHostClientSend FIN seq=xReceive FIN + ACK segmentSend ACK y+1Receive FIN segmentSend ACK x+1Receive ACK segmentEstablishing a TCP ConnectionClosing a TCP ConnectionReceive ACK segmentSend FIN seq=y, ACK x+1

*TCP : Data transferHostClientSend Packet 1Start TimerRetransmit Packet1Start TimerPacket should arrive ACK should be sentACK would normallyArrive at this timeReceive Packet 1Send AXK 1Time ExpiresReceive ACK 1Cancel TimerPacket LostTimerTimer

Comparison:*

*SCTPIn order to address the limitations of TCP and UDP the Signaling Transport (SIGTRAN) working group in the IETF developed SCTP

Places messages and control information into separate chunks

Chunk Data or Control - each identified by a chunk header

Multi-Streaming : Refers to the capability of SCTP to transmit several independent streams of chunks in parallelEg : transmitting web page images together with the web page text Features : Improved error detection Validation and acknowledgment mechanisms protect against flooding attacks and provide notification of duplicated or missing data chunks Path selection and monitoring select a primary data transmission path and test the connectivity of the transmission path.

*SCTP HeaderConsists of two basic sections:1. The common header, which occupies the first 12 bytes and is highlighted in blue2. The data chunks, which occupy the remaining portion of the packet.Each chunk has a type identifier that is one byte long yielding, at most, 255 different chunk types

Bits072431+0Source portDestination port32Verification tag64Checksum96Chunk 1 typeChunk 1 length128Chunk 1 dataChunk N typeChunk N lengthChunk N data

Mapping Layer 4 to Applications*

Questionnaire:In the OSI model, as a data packet moves from the lower to the upper layers, headers are AddedRemovedRe-arrangedmodifiedIn the OSI model, what is the main function of the transport layer?node-to-node deliveryprocess-to-process message deliverySynchronizationprocess-to-process message delivery_______ is a process-to-process protocol that adds only port addresses, checksum error control, and length information to the data from the upper layer.TCPUDPIPNone of the abovePackets in the IP layer are called _________ .segmentsDatagramsFramesNone of the above*

Thank You*

****Connectionconnection must be established before data is transferredReliableData guaranteed to eventually arrive with no errors