16 - layer 4€¦ · tpdu: transport protocol data unit nesting of the pdus: frame header packet...

Lehrstuhl für Informatik 4

Kommunikation und verteilte Systeme

Chapter 3.6: Layer 4

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 2Chapter 3.6: Layer 4

For what do we need a Transport Layer for?

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

Similarity to layer 3:

• Connection-oriented and connectionless data transmission

• Addressing

• Flow control




Simple Transport Protocol

TPDU: Transport Protocol Data Unit

Nesting of the PDUs:

Frame header

Packet header

TPDU header

TPDU payload

Packet payloadFrame payload

FCS

DISCONNECT

RECEIVE

SEND

CONNECT

LISTEN

Operation

DISCONNECTION REQ. (DR)

--

DATA

CONNECTION REQ. (CR)

--

Sent TPDU

Attempt of a connection termination

Block until a Data TPDU arrives

Data transmission

Attempt of a connection establishment

Block until a process tries to establish a connection

Meaning




Simple transport protocol

Sender (Client)Receiver (Server)

LISTENCONNECT

RECEIVESEND

DISCONNECT

DISCONNECT

CONNECT




Sounds easy…

… is however nevertheless quite complicated: the network can loose packets, store packets, duplicate packets.

Solution:

• Sequence numbers• Three-Way Handshake

A connection establishment consists of three parts:

� The Connection Request,

� The confirmation of the receiver that the connection establishment is accepted,

� 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.

REJECT (ACK=y)

ACK (seq=y, ACK=x)

ACK (seq=x+1, ACK=y)

Host 1 Host 2

CR (seq=x)

Host 1 Host 2

CR (seq=x)

ACK (seq=y, ACK=x)

Duplicate

Connection Establishment




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

Flow Control and Buffer Assignment




A waits for buffer space

B waits that A sends dataDanger of deadlock; send Control TPDUs in regular time intervals or make more use of timeouts

Dynamic Buffer Assignment

B informs A about 4 buffer places←<ack = 6, buf = 4>16

A sends TPDU 6; A is blocked (no more buffer)→<seq = 6, data = m6>→14

B acknowledges TPDU 5 and 6, but sender remains blocked←<ack = 6, buf = 0>←15

B informs A about 1 buffer place←<ack = 4, buf = 1>←11

B acknowledges TPDU 2, 3, 4 - sender is stopped (no buffer)←<ack = 4, buf = 0>←10

B informs A about 2 buffer places←<ack = 4, buf = 2>←12

A sends TPDU 5→<seq = 5, data = m5>→13



A sends TPDU 2, TPDU is lost<seq = 2, data = m2>→5

B acknowledges TPDU 0 and 1, buffer places are reduced to 3←<ack = 1, buf = 3>←6



A repeats to send TPDU 2→<seq = 2, data = m2>→9

B grants 4 buffer places for A and waits for TPDU 0←<ack = 15, buf = 4>←2

A asks for 8 places in B’s buffer→<request 8 buffers>→1

BMessageAack/seq ∈{0, …, 15}

Timeout for TPDU 2 }




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 ⇒ timer Host 1 Host 2

DR

DR

ACK

send DR and start

timer

send DR and start

timer

terminate connection


send confirmation

Host 1 Host 2

DR

DR

ACK

send DR and start

timer

send DR and start

timer

Timeout



ACK

DR

DR

send DR and start

timer

send DR and start

timer



Timeout

send DR and start

timerDR

send DR and start

timerDR

DR

DR

send DR and start

timer

send DR and start

timer

Timeout


N Timeouts


Timeout

send DR and start

timer

DR

::

DR

DR

Connection Termination

send confirmation

send confirmation




Implementation Example

Simple transport protocol, which covers the following TPDU types:

Control data for the management of the window sizeCREDIT

Data transmissionDATA

Answer to CLEAR REQUESTCLEAR CONFIRMATION

Attempt of a connection terminationCLEAR REQUEST

Answer to CALL REQUESTCALL ACCEPTED

Attempt of a connection establishmentCALL REQUEST

MeaningTPDU





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.












Implementation example
























Representation of the protocol as finite state machine





Representation of protocol as finite State Machine (graphical form)




Multiplexing

The multiplexing serves for better utilization of network connections. There are two cases:

(a) Upward Multiplexing: different transport connections are multiplexed on one network connection to avoid overhead

(b) Downward Multiplexing: a transport connection is distributed on several network connections to achieve a better network utilization

(a) (b)

16 - layer 4€¦ · tpdu: transport protocol data unit nesting of the pdus: frame header packet...

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