Inter-Process Communication Flashcards
What are the characteristics of IPC?
- Message Passing
- Possible synchronisation of two processes
- Synchronous and Asynchronous communication
- Queue associated with each message destination
- Reliability
- Ordering
- Unicast/Multicast Communication
What is meant by message passing?
Passing message between two processes via send and receive
What is synchronous communication?
Sender and receiver synchronise (send and receive are blocking operations)
What is asynchronous communication?
Send is non-blocking, sender is allowed to continue
What does the sender and receiver processes do with regards to queues?
Sending process adds message to remote queue
Receiver process removes message from the remote queue
What is the destination of messages?
Sent to Internet Address, Local Port
What is meant by reliability?
The guaranteed delivery despite a ‘reasonable’ number of packets being dropped or lost
What is meant by integrity?
Messages arriving uncorrupted and without duplication
What is meant by ordering?
Some applications require sender order
What is unicast communication?
Communication from one process to a single other process e.g. socket communication
What is multicast communication?
Communication from one process to a group of processes e.g. publish/subscribe message model
What is IP?
Internet Protocol
What is the purpose of IP?
To deliver data packets to network devices
What type of address is used by IP to deliver data packets?
Logical IP addresses consisting of a network part and a host part
It is routable - can forward packets to another network
What is an internet?
2 or more connected TCP/IP networks that can be reached by routing
What is TCP?
Transmission Control Protocol
Is TCP connection- or connectionless- orientated?
Connection-orientated protocol
Ensures packet delivery is possibly by establishing connection first with receiving device
Does TCP resend packets if they don’t arrive?
Yes
When is the connection closed by TCP?
When packets have been successfully delivered
Unrecoverable error has occured
Is TCP unicast/multicast?
Unicast
Used for one-to-one connectioons
Which applications rely on TCP?
Web browser
FTP
SMTP - Simple Mail Transfer Protocol
What is UDP?
User Datagram Protocol
Is UDP connection- or connectionless-orientated?
Connectionless-orientated protocol
Used when the overhead of a connection isn’t required
Doesn’t guarantee delivery - places packet on the network via IP and forgets about it
How may a programmer deal with confirming delivery using UDP?
Applications can use expected replies
If no reply without a certain time, application either resends the packet or deals with packet loss
What applications use UDP?
DNS, used to looks up domain names/IP addresses
Voice and video applications normally
What is a socket address?
Message destination are defined as socket addresses
Consist of port number and internet address
Data is transmitted between sockets
How do sockets work in Datagram Communication (UDP)?
Connectionless protocol
Message-passing abstraction
Each time a datagram is sent, also requires local socket descriptor and receiving sockets address
How do sockets work in Stream Communication (TCP)?
Connection protocol
Stream abstraction
Connection is established between two sockets - one listens for connection, other asks for connection
Once connected, allows for two-way transmission
What is Stream communication used for?
Remote login
File Transfer
Building blocks for producer-consumer communication - produced, queued, and consumed
What is the size limit of UDP?
64kb
What is the size limit of TCP?
No size limit
Which (TCP or UDP) is more reliable?
TCP, guaranteed to be received in sent order
Which (TCP or UDP) is less complex?
UDP, less overheads
Often used for simple applications over LAN
What are the message size parameters for UDP? (3)
Receiver specifies an array of bytes of a size
Large messages may be shortened (IP allows 2^16)
Most environments have restrictions (8K)
What happens when UDP sends and receives?
Non-blocking sends
Blocking receives
Can timeouts be set on sockets using UDP
Yes, it is not always appropriate to wait indefintely
Can UDP receives receive from any or only a specific origin?
No specification of origin for messages
Receive method returns the Internet Address and Local Port of the sender (can check where it came from)
Can applications using UDP check to ensure for successful transmission?
Yes
What 2 main issues does UDP suffer from?
Omission issues (integrity) and ordering problems (validity)
What does the class DatagramPacket constructor require (2)?
DatagramPacket (byte[] buf, int length, InetAddress address, int port) - used for sending packets to specified port on specified host
DatagramPacket (byte[] buf, int length) - used for receiving packets of specified length
What does the class DatagramSocket constructor require?
DatagramSocket(int port) - socket bound to specified port on localhost
DatagramSocket() - socket bound to any available port
In TCP Stream Communication, how is the size of data for transmission determined?
Determined by the underlying TCP stream before transmission
How does TCP attempt to provide information regarding lost messages?
Uses an acknowledgement scheme
How does TCP attempt to deal with message duplication and ordering?
Message IDs associated with each IP packet
Recipient can detect and reject duplicates or reorder messages
What is involved in the TCP failure model? (2)
Integrity - Checksums to detect and reject corrupt packets and sequence numbers to detect and reject duplicate packets
Validity - Times and retransmissions to deal with lost packets
In TCP, what is a ServerSocket?
A class, created for listening, accept method gets a connect request from the queue
In TCP, what is a Socket?
A class, used by the client to, specifying the DNS and port.
Provides methods getInputStream and getOutputStream
What are I/O Streams?
Input sources or output destination
Some simply pass data, others modify data
Input reads data, output sends data, one item at a time
What are Data Streams?
Support binary I/O of primitive data types and String values
Mostly used DataInputStream and DataOutputStream
If data items are mapped, what has happened to them?
Data items are then represented by agreed data structures before transmission
If computers are of the same type, does data mapping need to occur?
No, conversion can be omittted
What is the alternative to data mapping?
Can be transmitted in native form using architectural identifier
What happens to data items during marshalling?
Data items are collected and assembled into a suitable form for transmission
What is unmarshalling?
Disassembling the data items on arrival
What does marshalling consist of? (2)
Flattening of structured data into a sequence of basic data items
Translation of those data items into the external data representation
How can marshalling be done ‘by hand?’
Sending program explicitly converts items to external format
How can marshalling be done automatically?
Generated from a specification of data types
What is the object serialisation (binary representation) approach?
implements Serializable, which flattens, then sends???
What is the XML (textual representation) approach?
Tags used, describes logical structure of data and associated attribute-value pair
What is the request-reply protocol based on? (3)
doOperation
getRequest
sendReply
How can delivery failure occur? (3)
Messages can be dropped by senders, receives, network gateways
Networks may become partitioned
Processes may fail
What should doOperation do to allow for times of server failings?
Wait for a timeout
What are the 3 RPC exchange protocols?
R protocol - request
RR protocol - request-reply
RRA protocol - request-reply-acknowledge
(client-server-client)
What can doOperation do after a timeout? (2)
Return and indicate it has failed - not usual since timeout may have been due to request or reply being lost
Send repeatedly until it succeeds or reasonably sure delay is due to lack of response from server
With RPC protocols, what is designed to happened with duplicate messages?
If server receives message more than once, protocol is designed to recognise it and filter it out
With RPC protocols, what is designed to happen with lost reply messages?
If server has already sent reply when it receives duplicate requests it may need to execute the operation again to obtain the result
With RPC protocols, what is ‘history’?
A history contains the structure of a reply already sent, and if retransmission is needed, it can be used rather than re-execution of the operation
