7 ICS132-TCP-IP

7 ICS132-TCP-IP - The Transport Layer The Transport Layer...

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The Transport Layer
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The Transport Layer Recall: The transport layer is responsible for providing the illusion of a reliable end-to- end connection between two hosts Functions of the transport layer include – End-to-end connection setup and tear-down – End-to-end flow control – End-to-end error control
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Contents Connection Control Flow and Error Control Multiplexing Transport Layer in the Internet
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1. Connection Control The transport layer must provide higher layers with the illusion of an end-to-end connection, especially in connectionless networks Required functions: Connection setup Connection tear-down
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1.1 Setting up a Transport Layer Connection When an application in one host wants to communicate with an application in another host, it must set up a transport layer connection to that application Setting up a connection is not as easy as it seems
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Setting up a Transport Layer Connection Naïve (bad) approach: Source host sends a connection setup packet to the destination host Destination host acknowledges the connection setup packet, and the connection is considered set up Host A Host B Time
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Problems with the Naïve approach Duplicate “connection request” or “accept connection” packets in the network How can CR and AC packets be duplicated in the first place, though?
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Duplicate Packet Problem Consider a network experiencing long delays (perhaps due to congestion) A R R R B New CR generated after the first one timed out First CR is just arriving to B after being delayed by congested routers Result: B thinks two connections have been requested by A
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Connection request with sequence number 0 is transmitted, but delayed within the network. Sender times out, and retransmits a duplicate request, which was then successfully received at the destination, and the connection was set up.
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Now, the delayed duplicate appears at the receiver. The receiver cannot tell if it is a duplicate, or it is a new connection request (with a sequence number wrapped around) only way for the receiver to tell if it is a duplicate is to keep history of all connection requests (This is not desirable. What if the receiver crashes and loses all history?)
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Solving the Duplicate Packet Problem Introduce sequence numbers and a 3-way handshake Sequence numbers must cycle through a large range of numbers to make sure there are never two packets in the network with the same sequence number
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Three-way Handshake: Case 1 Successful connection Host A Host B
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Three-way Handshake: Case 2 Duplicate Connection Request duplicate Host A Host B
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Three-way Handshake: Case 3 Dulicate CR and duplicate ACK duplicate Host A Host B duplicate
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1.2 Connection Tear-Down Two types of connection tear-down: Asymmetric Release: – Either host may terminate the connection Symmetric Release: – Both sides keep a unidirectional connection to the other – For each connection, the source tears it down when no more packets will be sent
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Problem #1: Loss of Data In asymmetric tear-down, data may be lost: Host A Host B Lost data
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7 ICS132-TCP-IP - The Transport Layer The Transport Layer...

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