程序代做CS代考 Java flex Week 7: Transport Layer Internet Technologies COMP90007 – cscodehelp代写

Week 7: Transport Layer Internet Technologies COMP90007
Lecturer: Semester 2, 2021
© University of Melbourne 2020
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Connection Release
n Asymmetric Disconnection
q Either party can issue a DISCONNECT, which results in DISCONNECT TPDU and transmission end in both directions
n Symmetric Disconnection
q Both parties issue DISCONNECT, closing only one direction at a time – allows flexibility to remain in receive mode
2

Connection Release (Cont.)
n Asymmetric vs Symmetric connection release types
n Asymmetric release may result in data loss hence symmetric release is more attractive
n Symmetric release works well where each process has a set amount of data to transmit and knows it has been sent
3

Generalizing the Connection Release Problem
n How do we decide the importance of the last message? Is it essential or not?
n No protocol exists which can resolve this ambiguity – Two-army problem shows pitfall of agreement
Attack? Attack?
4

Strategies for Connection Release
n 3 way handshake n Finite retry
n Timeouts
n Normal release sequence, initiated by transport user on Host 1
q DR=Disconnect Request
q Both DRs are ACKed by the other side
5

Connection Release (Error Cases)
n Error cases are handled with timers and retransmission
Final ACK lost, Host 2 times out
Lost DR causes retransmissions
Extreme: Many lost DRs cause both hosts to timeout
6

Addressing
n Specification of remote process to connect to is required at application and transport layers
n Addressing in transport layer is typically done
using Transport Service Access Points (TSAPs)
q on the Internet, a TSAP is commonly referred to as a port (e.g. port 80)
n Addressing in the network layer is typically done
using Network Service Access Points (NSAPs)
q on the Internet, the concept of an NSAP is commonly interpreted as simply an IP address
7

TSAPs, NSAPs and Transport Layer Connections Illustrated
8

Types of TSAP Allocation
1.
2.
3.
Static
q Well known services have standard allocated TSAPs/ports, which are embedded in OS
q
Directory Assistance – Port-mapper
A new service must register itself with the portmapper, giving both its service name and TSAP
Mediated
q A process server intercepts inbound connections and spawns requested server and attaches inbound connection
q cf. Unix /etc/(x)inetd
9

Programming using Sockets
n Sockets widely used for interconnections
q “Berkeley” sockets are predominant in internet applications
q Notion of “sockets” as transport endpoints
q Like the simple set plus SOCKET, BIND, and ACCEPT
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Recall Example Pseudo Code
Socket A_Socket = createSocket(“TCP”); connect(A_Socket, 128.255.16.0, 80); send(A_socket, “My first message!”); disconnect(A_socket);
… there is also a server component for this client that runs on another host…
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SERVER
CLIENT
socket() bind() listen () accept()
read() write()
close()
socket()
Connection request
connect()
write()
read() close()
Connection established
12

Let’s Look at the Code from the book (in a specific language)
Example from the book has more details but the essence is the same… This is the case in most languages…
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Socket Example – Server Side
Server code. . .
…
Assign address
Prepare for incoming connections
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Server Code Contd
Block waiting for the next connection
Read (receive) request
…..
The server can also create a new thread to handle the
connection on the new socket and go back to waiting for the next connection on the original socket…
15

An Example on Multi-Threading
ServerSocket serverSocket = new ServerSocket([parameters]);
While (true) {
Socket socket = serverSocket.accept();
MultiThreadMyServer server = new MultiThreadMyServer();
server.setMyService([some more parameters]);
server.setSocket(socket);
new Thread(server).start();
….
n (Code from OO Programming with Java; Chp. 14)
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Looking under the hood for Transport Layer Services…
n The most basic is actually connectionless: q Called: User Datagram Protocol (UDP)
q Does not add much to the Network Layer functionality
q TCP we just does the real-deal for this layer, reliability…
q For UDP: Just remove connection primitives to use it in a program
q UDP good for?:
n It is used for apps like video streaming/gaming regularly
q The reliability issue is left to?:
n the application layer… retransmission decisions as well as congestion control
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: UDP Client…
public static void main(String args[]) { ….
DatagramSocket mySocket = new DatagramSocket();
mySocket.send([data,address, etc parameters]);
… }
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Server Side: UDP Example Contd
public static void main(String args[]) { ….
DatagramSocket server = new DatagramSocket(port);
while (true) { server.receive([parameters]); …
}}
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