Layer 4 - Transport Layer

Transcription

1 Layer 4 - Transport Layer Core of the protocol hierarchy: Network-independent, reliable and economical data transfer Tasks of the transport layer: Connection-oriented or connectionless data transfer Addressing of a certain communication process on a computer Error handling, error detection, error correction Flow control Multiplexing 1. Upward - several transport connections to one network connection 2. Downward - one transport connection to several network connections (e.g. if not sufficient capacity of a network connection is given) Quality guarantees Page 1

2 For what do we need a Transport Layer for? Similarity to layer 3: Connection-oriented and connectionless data transmission Addressing Flow control Is layer 4 redundant? No, because: The network layer belongs to the network carrier: A user does not have any control over network details and problems Hiding of technical details from higher layers Addressing of a destination process End-to-end view of a data flow Generally: Improvement of the quality of the data communication on layer 3 Page 2

3 Simple Transport Protocol Nesting of the PDUs: TPDU: Transport Protocol Data Unit Frame header Packet header TPDU TPDU payload header Packet payload Frame payload FCS Operation LISTEN CONNECT SEND RECEIVE DISCONNECT Sent TPDU -- CONNECTION REQ. (CR) DATA -- DISCONNECTION REQ. () Meaning Block until a process tries to establish a connection Attempt of a connection establishment Data transmission Block until a Data TPDU arrives Attempt of a connection termination Page 3

4 Simple transport protocol LISTEN CONNECT CONNECT RECEIVE SEND DISCONNECT DISCONNECT Receiver (Server) Sender (Client) Page 4

5 Sounds easy Lehrstuhl für Informatik 4 Connection Establishment is however nevertheless quite complicated: the network can lose packets, store packets, duplicate packets. Solution: Sequence numbers Duplicate Three-Way Handshake A connection establishment consists of three parts: The Connection Request, The confirmation of the receiver that the connection establishment is accepted, Host 1 Host 2 The confirmation of the sender that the confirmation of the receiver was received and the sending request is still given. A compromise between reliability and complexity is to be obtained by this principle. CR (seq=x) ACK (seq=y, ACK=x) ACK (seq=x+1, ACK=y) CR (seq=x) ACK (seq=y, ACK=x) REJECT (ACK=y) Host 1 Host 2 Page 5

6 Flow Control and Buffer Assignment The flow control is similar to the one on layer 2, only considering the whole network: Larger number of connections Buffering of TPDUs Apart from losses, also permutations of packets are possible Dynamic buffer management (request for buffer at the communication partner) Principle: Sliding Window with variable window size: Adaptation of buffer size considering receiver and network Page 6

7 Dynamic Buffer Assignment ack/seq {0,, 15} 1 2 A Message <request 8 buffers> <ack = 15, buf = 4> B A asks for 8 places in B s buffer B grants 4 buffer places for A and waits for TPDU 0 3 <seq = 0, data = m0> A sends TPDU 0 4 <seq = 1, data = m1> A sends TPDU 1 5 <seq = 2, data = m2> A sends TPDU 2, TPDU is lost 6 <ack = 1, buf = 3> B acknowledges TPDU 0 and 1, buffer places are reduced to 3 7 <seq = 3, data = m3> A sends TPDU 3 Timeout for TPDU 2 } 8 9 <seq = 4, data = m4> <seq = 2, data = m2> A sends TPDU 4 A repeats to send TPDU 2 10 <ack = 4, buf = 0> B acknowledges TPDU 2, 3, 4 - sender is stopped (no buffer) 11 <ack = 4, buf = 1> B informs A about 1 buffer place 12 <ack = 4, buf = 2> B informs A about 2 buffer places 13 <seq = 5, data = m5> A sends TPDU 5 14 <seq = 6, data = m6> A sends TPDU 6; A is blocked (no more buffer) 15 <ack = 6, buf = 0> B acknowledges TPDU 5 and 6, but sender remains blocked 16 <ack = 6, buf = 4> B informs A about 4 buffer places A waits for buffer space Chapter B waits 3.6: that A Layer sends 4 data Danger of deadlock; send Control TPDUs in regular time intervals Page 7

8 Connection Termination 2 variants: asymmetrically, as with the telephone: abrupt break symmetrically, i.e. each participant terminates his direction The symmetrical variant is more complex, again TPDUs can be lost send terminate connection send confirmation ACK send terminate connection Host 1 Host 2 send Timeout send terminate connection send confirmation ACK send send terminate connection send terminate connection send confirmation ACK send Timeout terminate connection Host 1 Host 2 send Timeout send : : N Timeouts terminate connection send Timeout terminate connection Page 8

9 Implementation Example Simple transport protocol, which covers the following TPDU types: TPDU CALL REQUEST CALL ACCEPTED CLEAR REQUEST CLEAR CONFIRMATION DATA CREDIT Meaning Attempt of a connection establishment Answer to CALL REQUEST Attempt of a connection termination Answer to CLEAR REQUEST Data transmission Control data for the management of the window size Page 9

10 Implementation Example A connection can have one of the following states: Idle no connection is established. Waiting a CONNECT took place, CALL REQUEST is sent. Queued the CALL REQUEST was received but not yet processed. Established connection establishment finished. Sending wait for transmission permission. Receiving the receiver executed a RECEIVE. Disconnecting local call of DISCONNECT. Page 10

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19 Representation of the protocol as finite state machine Page 19

20 Implementation Example Representation of protocol as finite State Machine (graphical form) Page 20