SPL/2010 Protocols 1

SPL/2010 Application Level Protocol Design ● atomic units used by protocol: "messages" ● encoding ● reusable, protocol independent, TCP server, ● LinePrinting protocol implementation 2

SPL/2010 Protocol Definition ● set of rules, governing the communication details between two parties (processes) ● different forms and levels; ● protocols for exchange bits across a wire ● protocols governing administration of super computers. ● application level protocols - define interaction between computer applications 3

SPL/2010 Protocol Communication Rules ● syntax : how do we phrase the information we exchange. ● semantics : what actions/response for information received. ● synchronization : whose turn it is to speak (given the above defined semantics). 4

SPL/2010 Protocols Skeleton ● all protocols follow a simple skeleton. ● exchange information using messages, which define the syntax. ● difference between protocols: syntax used for messages, and semantics of protocol. 5

SPL/2010 Protocol Initialization (hand-shake) ● communication begins when party sends initiation message to other party. ● synchronization - each party sends one message in a round robin fashion. 6

SPL/2010 TCP 3-Way Handshake ● Establish/ tear down TCP socket connections ● computers attempting to communicate can negotiate network TCP socket connection ● both ends can initiate and negotiate separate TCP socket connections at the same time 7

SPL/2010 TCP 3-Way Handshake (SYN,SYN-ACK,ACK) 8

SPL/2010 ● A sends a SYNchronize packet to B ● B receives A's SYN ● B sends a SYNchronize-ACKnowledgement ● A receives B's SYN-ACK ● A sends ACKnowledge ● B receives ACK. ● TCP socket connection is ESTABLISHED. 9

SPL/2010 HTTP (Hyper Text Transfer Protocol) ● exchanging special text files over the network. ● brief (not complete) protocol description: ● synchronization: client initiates connection, sends single request, receive reply from server. ● syntax: text based, see rfc2616. ● semantics: server either sends to the client the page asked for, or returns an error. 10

SPL/2010 What next? ● syntax and semantics aspects of protocols. ● assume: synchronization works in round robin, i.e., each party sends one message at a time. 11

SPL/2010 Message Format ● Protocol syntax: message is the atomic unit of data exchanged throughout the protocol. ● message = letter ● concentrate on the delivery mechanism. 12

SPL/2010 Framing ● streaming protocols - TCP ● separate between different messages ● all messages are sent on the same stream, one after the other, ● receiver should distinguish between different messages. ● Solution: message framing - taking the content of the message, and encapsulating it in a frame (letter - envelop). 13

SPL/2010 Framing – what is it good for? ● sender and receiver agree on the framing method beforehand ● framing is part of message format/protocol ● enable receiver to discover in a stream of bytes where message starts/ends 14

SPL/2010 Framing – how? ● Simple framing protocol for strings: ● special FRAMING character (e.g., a line break). ● each message is framed by two FRAMING characters at beginning and end. ● message will not contain a FRAMING character ● framing protocol by adding a special tag at start and end. ● message can be framed using / strings. ● avoid having / in message body. 15

SPL/2010 Framing – how? ● framing protocol by employing a variable length message format ● special tag to mark start of a frame ● message contains information on message's length 16

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SPL/2010 Textual data ● Many protocols exchange data in textual form ● strings of characters, in character encoding, (UTF-8) ● very easy to document/debug - print messages ● Limitation: difficult to send non-textual data. – how do we send a picture? video? audio file? 18

SPL/2010 Binary Data ● non-textual data is called binary data. ● all data is eventually encoded in "binary" format, as a sequence of bits ● "binary data" = data that cannot be encoded as a readable string of characters? 19

SPL/2010 Binary Data ● Sending binary data in raw binary format in a stream protocol is dangerous. ● may contain any byte sequence, may corrupt framing protocol. ● Devising a variable length message format. 20

SPL/2010 Base64 Encoding Binary Data encode binary data using encoding algorithm ● Base64 encoding - encodes binary data into a string ● Convert every 2 bytes sequence from the binary data into 3 ASCII characters. ● used by many "standard" protocols ( to encode file attachments of any type of data). 21

SPL/2010 Encoding using Poco ● In C++, Poco library includes module for encoding/decoding byte arrays into/from Base64 encoded ASCII data. ● functionality is modeled as a stream "filter" ● performs encode/decode on all data flowing through the stream ● classes Base64Encoder / Base64Decoder. 22

SPL/2010 Encoding in Java ● iharder library. ● modeled as stream filters (wrappers around Input/Output Java streams). 23

SPL/2010 Encoding binary data ● advantage: any stream of bytes can be "framed" as ASCII data regardless of character encoding used by protocol. ● disadvantage - size of the message, increased by 50%. ● (we will use UTF-8 encoding scheme) 24

SPL/2010 Protocol and Server Separation 25

SPL/2010 Protocol and Server Separation code reuse is one of our design goals! ● generic implementation of server, which handles all communication details ● generic protocol interface: ● handles incoming messages ● implements protocol's semantics ● generates the reply messages. 26

SPL/2010 Protocol-Server Separation: protocol object ● protocol object is in charge of implementing expected behavior of our server: ● What actions should be performed upon the arrival of a request. ● requests may be correlated one to another, meaning protocol should save an appropriate state per client. 27

SPL/2010 Example: authenticated session ● protocols require user authentication (login), ● only authorized users can perform certain actions. ● protocol is statefull - serving requests of client can be in at least 2 distinct states: 1. authenticated (user has already logged in) 2. non-authenticated (user has not provided login). ● by state of the protocol object, behavior of protocol object is different 28

SPL/2010 Protocol and Server Separation separate different tasks server must perform. ● Accept new connections from new clients. ● Receive new bytes from connected clients. ● Parse incoming bytes from clients into messages ("de-serialization" / "unframing"). ● Dispatch message to right method on server side to execute requested operation. ● Send back an answer to a connected client after an action has been executed. 29

SPL/2010 a software architecture that separates tasks into separate interfaces 30

SPL/2010 ● The key participants in this architecture are: ● Tokenizer - syntax, tokenizing a stream of data into messages. ● MessagingProtocol – semantics, handling received messages and generating responses. 31

SPL/2010 ● implementations of interfaces: ● generic server ● MessageTokenizer ● LinePrinitingProtocol, 32

SPL/2010 Interfaces ● implement separation between protocol and server. Define: 1. message (can be encoded in various ways: Base64, XML, text). ● Our messages encoded as plain UTF-8 text. 2. framing of messages - delimiters between messages sent in stream. 3. protocol interface which handles each individual message. 33

SPL/2010 ConnectionHandler ● server accepted new connection from client. ● server creates ConnectionHandler - will handle all incoming messages from this client. ● ConnectionHandler - maintains state of connection for specific client ● Ex: user perform "login" - ConnectionHandler object remembers this in its state 34

SPL/2010 ConnectionHandler - Socket ● ConnectionHandler has access to Socket connecting server to client process. ● TCP server - Socket connection is viewed as a pair of InputStream and OutputStream. ● streams of bytes – client and the server exchange a bunch of bytes. 35

SPL/2010 Tokenizer - in charge of parsing a stream of bytes into a stream of messages ● Tokenizer interface: filter between Socket input stream and protocol ● Protocol accesses the input stream only through the tokenizer. ● instead of "seeing" a stream of bytes, it sees a stream of messages. ● Many libraries model such "filters" on streams as wrappers around a lower-level stream. ● OutputStreamWriter - wraps stream and performs encoding from one character encoding to another ● BufferedReader - adds a layer of buffering around a non-buffered input stream. 36

SPL/2010 Tokenizer ● splits incoming bytes from the socket into messages. ● For simplicity, we model the Tokenizer as an iterator… ● protocol will see the input stream from the socket as an iterator over messages (instead of an iterator over bytes). 37

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SPL/2010 Messaging Protocol ● protocol interface ● wraps together: socket and Tokenizer ● Pass incoming messages to MessagingProtocol - execute action requested by client. ● look at the message and decide on action ● decision may depend on the state ● Once the action is performed - answer back from the MessagingProtocol. 39

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SPL/2010 ● We use a String to pass data from Tokenizer to Protocol, and back from Protocol. ● Serialization/Deserialization (encode/decode parameters to/from Strings) performed by Protocol - and not by the Tokenizer. ● Tokenizer is only in charge of deframing (split bytes into messages). 41

SPL/2010 Implementations 42

SPL/2010 Connection Handler ● active object: ● handles one connection to one client for the whole period during which the client is connected ● (from the moment the connection is accepted, until one of the sides decides to close it). ● modeled as a Runnable class. 43

SPL/2010 Connection Handler ● holds references to: ● TCP socket connected to the client, ● Tokenizer ● an instance of the MessagingProtocol. 44

SPL/2010 ● connection handler is generic, works with any implementation of a messaging protocol. ● assumes data exchanged between client and server is in form of encoded strings ● encoder passed to constructor as an Encoder interface. 45

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SPL/2010 What’s left? ● only need to implement: ● specific framing handler (tokenizer) ● specific protocol we wish to use. ● continue our line printing example… 47

SPL/2010 Message Tokenizer ● we use a framing method based on a single character delimiter. ● assume stream of messages, delimited by FRAMING = we will use the character '\0‘ 48

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SPL/2010 ● important part is connection termination and exception handling at any moment ● most of the code in low-level input/output and socket manipulation relates to error handling and connection termination. 50

SPL/2010 Line Printing Protocol ● implement a specific protocol on the server side. ● when receives a message, prints it on the server side screen and adds a line number. ● line number is the state of the protocol. ● each client has its own line number. Two clients connected at the same time will see each one its own version of the line number. ● when protocol processes a message, - sends back message to client: ": printed" + date-time value when the message was processed (on the server side). ● timestamp acknowledgments. 51

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SPL/2010 A Client ● before ConnectionHandler, review code of compatible TCP client for protocol we have just described. ● no new idea - it is similar to the TCP client we have reviewed in the previous section. 53

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SPL/2010 Concurrency Models of TCP Servers Server quality criteria: ● Scalability: capability to server a large number of concurrent clients. ● Low accept latency: acceptance wait time ● Low reply latency: reply wait time after message received. ● High efficiency: use little resources on the server (RAM, number of threads CPU usage). 55

SPL/2010 ● model the concurrency model of the server, ● define interface which controls concurrency application of each connection handler 56

SPL/2010 ● Given: ● Encoder ● Tokenizer ● Protocol ● ServerConcurrencyModel defined the MessagingServer 57

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SPL/2010 ● To obtain good quality, a TCP server will most often use multiple threads. ● 3 simple models of concurrency servers ● 3 implementations of preparing the ServerConcurrencyModel interface 59

SPL/2010 Server Model 1: Single Thread ● 1 thread for; ● accepting a new client ● dealing requests, by applying run method of the passive ConnectionHandler object. 60

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SPL/2010 Single Thread Model: Quality ● no scalability: at any given moment, it can serve at most one client. ● high accept latency: a second client must wait until first client disconnects ● low reply latency: all resources are concentrated on serving one client. ● Good efficiency: server uses exactly the resources needed to serve one client 62

SPL/2010 When is model appropriate? ● time to process a full connection from one client is guaranteed to remain small. ● Example: server provides date and time value on the server machine. ● sends one string to the client then disconnects. 63

SPL/2010 Server Model 2: Thread per Client ● assigns a new thread, for each connected client, by invoking the 'start' method over the runnable ConnectionHandler object. 64

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SPL/2010 Model Quality: Scalability ● server can serve several concurrent clients, up to max threads running in the process. ● RAM of the host is used - each thread allocates a stack and thus consumes RAM ● Approx threads become active in a single process. ● process does not defend itself – keeps creating new threads - dangerous for the host. 66

SPL/2010 Model Quality: Latency ● Low accept latency: time from one accept to the next ~ time to create a new thread – ● short compared to delay in incoming client connections. ● Reply latency: resources of the server are spread among concurrent connections. ● reasonable number of active connections (~hundreds), load requested relatively low in CPU and RAM, 67

SPL/2010 Model Quality: Efficiency ● Low efficiency: server creates full thread per connection, – connection may be bound to Input/Output operations. – ConnectionHandler thread will be blocked waiting for IO,,still use the resources of the thread (RAM and Thread). ● Reactor architecture … 68

SPL/2010 Server Model 3: Constant Number of Threads ● constant number of 10 threads (given by the Executor interface of Java) ● adding runnable ConnectionHandler object to task queue of a thread pool executor 69

SPL/2010 Model Quality ● avoids server causing host crash when too many clients connect at the same time ● up to N concurrent client connections - server behaves as "thread-per-connection" ● above N, accept latency will grow ● scalability is limited to amount of concurrent connections we believe we can support. 70