UNIT 4 The Transport Layer Design Issues 1

UNIT 4 The Transport Layer Design Issues 1

Transport Layer The ultimate goal/function of the transport layer is to provide efficient, reliable and cost effective service to it users - to the processes in the application layer 2

The Transport Service • Transport entity – is a hardware and/or software within the transport layer that does the function of the layer Eg: OS kernel, or NIC, library package in network applications • Services Provided to the Upper Layers Connection-oriented and connectionless PL, DLL , NL = Transport service provider SL, PL, AL = Transport service user 3

Services Provided to the Upper Layers The network, transport, and application layers. 4

Qo. S- Quality Of Service • Parameters Connection establishment delay – is the amount of time elapsing between a transport connection requested and the confirmation received by the user of the transport service. • Connection establishment failure probability – is the chance of a connection not being established. • Throughput – number of bytes of user data transferred per second measured over some interval. 5

Qo. S- Quality Of Service • • • Transit delay – time measured between message being sent by the transport user on the source machine and received by the transport user on the destination machine. Residual error ratio – the number of lost or garbled messages as a fraction of the total sent. Protection – specifies protection measures against unauthorized use of data. Priority – differentiate between high and low priprity connections. Resilience – probability of the transport layer itself terminating a connection due to congestion or internal problem 6

Option Negotiation ü Consider the case, the transport layer knows it cannot achieve the desired goal of throughput – 600 Mbps, ü But it can achieve a lower but still acceptable rate – 150 Mbps. ü It sends the lower/min acceptable rate to the remote machine asking to establish a connection. 7

ü Option Negotiation If the remote machine cannot handle the proposed value but can handle a value above the min, it may make a counteroffer. üBut, if it cannot handle any value above the min, it rejects the connection attempt. üFinally, originating transport user is informed : 8

Option Negotiation ü Connection established : values of parameters agreed upon. ü Connection rejected : reason ü This process is called option negotiation. ü Once agreed. . it remains for the entire connection setup. 9

Transport Service Primitives ü The transport service primitives allow transport users(app progms) to access the transport service. 10

Transport Service Primitives The primitives for a simple transport service. 11

Transport Service Primitives ü Consider an application with a server and a number of remote clients. ü Server – LISTEN – by calling a library procedure that makes a system call to block the server until a client tries to connect. 12

Transport Service Primitives ü Client (wants to talk to server) – CONNECT – the transport entity blocks the caller(client) by sending a packet to the server. ü Encapsulation of entities : 13

Transport Service Primitives (2) The nesting of TPDUs, packets, and frames. 14

Transport Service Primitives ü Client – CONNECT call causes a CONNECTION REQUEST TPDU to be sent to the server. ü Server – The transport entity unblocks the server and sends a CONNECTION ACCEPTED TPDU back to the client. ü Client – When this TPDU arrives, the client is unblocked and connection is established. 15

Transport Service Primitives ü When a connection is no longer needed, it must be released to free space. ü Disconnection – 2 variants : ü Asymmetric ü Symmetric 16

Transport Service Primitives ü Asymmetric : either transport user can issue a DISCONNECT primitive results in DISCONNECT TPDU being sent to the remote transport entity. (Telephone system) ü When this TPDU arrives - RECEIVE – connection is released ü Symmetric : each direction is closed separately that is When one side does a DISCONNECT that means it has no more 17

Transport Service Primitives ü Data to be sent-but is willing to accept data from its partner. ü Connection is released – when both sides have done DISCONNECT 18

Transport Service Primitives A state diagram for a simple connection management scheme. Transitions labeled in italics are caused by packet arrivals. The solid lines show the client's state sequence. The dashed lines show 19 the server's state sequence.