1

1

Ethernet

broadband or baseband signallingHub and repeatersEthernet switches and bridges: Ethernet packets Ethernet topologies

Bus, star, tree.

Carrier-Sense Multiple Access with Collision Detection (CSMA/CD)All nodes are continuously ‘listening’ to the medium for packets that are addressed to them.Packets frames

Prefix: hardware timing purposesthe destination address, the sending address;length of data (46—1,500 bytes), data of variable length,checksum

2

2

Ethernet

Packet collisioncarrier sensing: not enough

Collision detection• Sender’s responsibility to detect

Minimum packet length in collision detectionSend jamming signal, delay and try againDelay time is selected using binary exponential back-off

A B

A B

Message almost there at time T whenB starts – collision!

time = 0

time = T’

time = 2T

A B

3

3

Middleware

Application programming interfaces (APIs)Examples of middleware services

Remote data access (RDA) -- SQL access to server-based DBMS.Remote procedure calls (RPC)Remote method invocation (RMI)

Today’s topicsInterprocess communication (IPC);Java API for Internet transport protocolsExternal data representation and marshalling

Applications, services

Middlewarelayers

request-reply protocol

marshalling and external data representation

UDP and TCP

Thischapter

RMI and RPC

4

4

InterProcess Communication

two basic modelsshared memorymessage passing

two basic operations on messages Send (to, message)Receive (from, message)

A typical communication system

5

5

Example of Persistent Asyn. Comm.: email system

Types of communicationPersistent communication – Stores message until communicated to userTransient communication – Stored only when sending and receiving processes are alive

• Transport level protocols provide transient communication

Asynchronous – Sender continues after sending messageSynchronous – Sender blocks until message is stored at receiver's local buffer, delivered to receiver or processed by receiver

6

6

c) Transient asynchronous communication: UDP, one-way RPCs.

d) Receipt-based transient synchronous communication

e) Delivery-based transient synchronous communication at message delivery: Asyn. RPCs

f) Response-based transient synchronous communication: PRCs, RMIs.

7

7

IPC mechanisms

Pipesprocesses must be related through a common ancestorimpossible in a distributed environment

shared memory Sockets

4.4BSD UnixJava

message queues: Message-oriented Middleware (MOM)

message

agreed portany port

socketsocket

Internet address = 138.37.88.249Internet address = 138.37.94.248

other ports

client server

8

8

Socket Types in 4.4BSD UNIX

Stream socket: provides for the bidirectional, reliable, sequenced, and unduplicated flow of data without record boundary.

Very similar to pipeDatagram socket: supports bidirectional flow of data which is not promised to be sequenced, reliable, or unduplicated

Reliability guaranteed by high-level apps.Most widely used in name service, time service.

A raw socket: provides users access to the underlying communication protocols which support socket abstractions.

Not for general usersSequenced packet socket: No protocol for this typ

9

9

Socket creation: socket()

communication domains local domain: at the same machineinternet domain: InternetXNS domain: XEROX Network Systems

s = socket(domain, type, protocol); A system calldomain: AF_UNIX, AF_INET, or AF_NStype: SOCK_STREAM, SOCK_DGRAM, etcprotocol: TCP or UDP. Auto selected if 0Return a socket descriptor (a small integer for later reference)Ex: s = socket(AF_INET, SOCK_STREAM, 0);

Creation Failure: ReasonsLack of memory (ENOBUFS)Unknown protocol (EPROTONOSUPPORT) Socket without supporting protocol (EPROTOTYPE)

10

10

Binding Names: bind()

Socket created without a name (i.e. port address)Process has no way to access it.

System call: bind(s, address, len)s: socket descriptoraddress: <local address, local port> or a path namelen: the length of the address.

Connection Establishment

Asymmetric, involving a server and a clientServer: create bind listen acceptClient: create bind connectconnect(s, address, len)

• s: socket descriptor• address: server address• len: the length of the address

11

11

Connection FailureTimeout (ETIMEDOUT)

Server down or network corrupt

Connection refused (ECONNREFUSED)Server not ready yet

Network down or server downoperational errors are returned by the underlying communication system

Unknown hostoperational errors returned by intermediate gateways or switching nodes.

System Call: listen()listen(s, max_num)

s: socket descriptormax_num: the maximum number of outstanding connections which may be queued awaiting acceptance by the server processIf the queue is full, a connection will be ignored (instead of refused). Why?

12

12

System call: accept()

newsock = accept(s, from-addr, len)s: socket descriptorfrom-addr: to store the address of the client

• Usually a pointer, could be nulllen: length of from-addrReturn a new socket.Usually block the callerCannot select the client to be accepted.

Data TransferOnce a connection is established, data flow may begin.write(s, buf, sizeof (buf));

send(s, buf, sizeof (buf), flags);read(s, buf, sizeof (buf));

recv(s, buf, sizeof (buf), flags); Flags: provide more features

13

13

Discarding Socketsclose(s)

Sockets which promises reliable transmission will still attempt transfer data.

Shutdown(s, how), where how is0: no more receiving1: no more sending2: no more receiving or sending

socket()

close()

read()

connect()

write()

client

socket()

bind()

listen()

accept()

accept()

read()

write()

close()

server

Start a thread

Wait for new connection

14

14

Java Sockets

close()

readUTF()

socket()

writeUTF()

client socket()

accept()

accept()

readUTF()

writeUTF()

close()

server

Start a thread

Wait for new connection

15

15

Java API for the Internet protocols

Transport address: IP address + port numberjava.net.InetAddress

host name: “java.sun.com”getHostAddress(): IP address string in textual presentation. getHostName(): the host name for this IP address.32-bit integers for port numberSocket types

UDP socketTCP socket

Static InetAddress.getByName(String host)

16

16

UDP socket

Data unit: datagramCalled datagram socketSocket must be bound to a local port and one IP address

Server: well-known portClient: any free one

issues related to datagram communicationMessage sizenon-blocking send operations and blocking receive operations Timeouts

applications use UDP: Domain Naming Service, Voice Over IP

17

17

UDP socket

Encapsulated in Class DatagramSocketDatagram: Class DatagramPacketDatagramSocket.send(DatagramPacket)DatagramSocket.receive(DatagramPacket)DatagramSocket.setSoTimeout(int timeout) DatagramSocket.connect(InetAddress, int port )public DatagramPacket(byte[] buf, int length, InetAddress address, int port)

18

18

TCP socket

TCP is a connection-oriented protocol, a connection is established first.Server listens connection requestClient asks for a connectionTwo types of TCP sockets: ordinary sockets and server socketsA client process constructs an ordinary socket and then it asks for a connection with the server.A server socket receives a connection request, it constructs an ordinary socket with an unused port number which completes the connection. No limit on data size.Streams: one in each direction

19

19

Some issues in stream communication

Matching of data itemsBlockingThreadsApplications use TCP: HTTP, FTP, Telnet, SMTP.ordinary TCP socket: Socketserver socket: ServerSocket

ServerSocket(int port), or ServerSocket.bind(SocketAddress, int port);Socket ServerSocket.accept();

InputStream and OutputStream classesAn example in textbook

20

20

Message-oriented Middleware (MOM)

Main featuresintermediate-term storage for messages: persistent !neither sender nor receiver is required to be activeMessage queue eliminate the need for programs to be logically connected: asynchronous !takes minutes

Only guarantee is that a message will be inserted in receivers’ queue. But no guarantees about when, or even if the message will actually be read Source queue & destination queueQueue managers: Message Queue Interface (MQI)Overlay network

21

21

Message BrokersIssue: message format

How to make sure the receiver understands sender’s message?

One format?Application are too diverse.

Act as an application level gatewayE.g. change delimiters at the end of records

Account Id, Trader Id, Price, Quantity, Date, Customer Id

Customer Id, Account Id, Trader Id, Price, Quantity, Date

Application A outputs:Application 1 inputs:

Application B outputs: Application 3 inputs:

ParseRules

Transform

Application 2 inputs:

Date, Customer Id, Account Id, Price, Quantity, Trader Id

Date, Price, Quantity, Trader Id

Date, Customer Id, Price, Quantity

22

22

Example: IBM MQSeries

QueuesLocal QueueRemote QueueAlias QueueModel QueueSome properties of local queues:

• Maximum Message Size• Maximum Queue Depth• Enable/Disable Put or Get• Persistent/Not Persistent• Local queues can generate events (messages) under

certain conditions (like queue full).

Queue managersChannels

Message Channels: message channel agent (MCA) MQI channels: client channels

23

23

Example: IBM MQSeries

Messagesdescriptor + application data

Four types of messages:request messagereply message.one-way messagereport message: generated by the Queue Manager. For example, Delivery confirmation.

Messages can have a “time-to-live”, called Expiry: Each queue manager can have a dead-letter queue.persistent or non-persistent messages Message Priority & Message CorrelatorSegmented Messages - allows ending of VERY LARGE messages (> 100 MB)A “reply to” address (the name of a Queue Manager and Queue).Messages are added and removed from queues in Units of Work

24

24

External data representation and marshalling

Heterogeneity: different representations for simple data items.Big Endian vs. Little EndianSolutions

transmitted in the sender’s format together with an indication of the format usedconverted to an agreed external format

An agreed standard for the representation: external data representationMarshaling vs. UnmarshallingCORBA’s Common Data Representation (CDR);Java’s object serialization;XML (Extensible Markup Language)

25

25

Java object serialization

Goal: supports the encoding of objects, and the objects reachable from them, into a stream of bytes and the complementary reconstruction of the object graph from the stream.the state of an objectsufficient information to reconstruct the objectthe receiving process has no knowledge about the types of objectsReference: handlesObjects to be serialized in the stream may support either the Serializable or the Externalizable interface.the “java.io.Serializable” interface

26

26

Java object serialization

Serializable objectsimplement the “java.io.Serializable”interfaceYou can implement one or more of the methods readObject(), writeObject() to custom serialization.

Externalizable objectsimplement the “java.io.Externalizable”interfacethe programmer takes full responsibility for the serialization and deserialization of objects.

java.io.ObjectOutputStream and java.io.ObjectInputStream

Serialization will preserve the state of all fields in the object graph except for fields marked transient or static or fields contained in superclasses that are not serializable.

27

27

Java object serialization

Visibility modifiers (e.g., private, protected, etc.) on fields do not affect serialization.

Any subclasses of a serializable class are serializable classes, and any data inside a serializable class are also serializable data.

Two important methods:readObject(), reads an object from the defined input streamwriteObject(), writes an object to an output stream.

Other methods that support the serialization and deserialization of primitive data types: readInt, readFloat, writeInt, writeFloat, etc.