Chamada remota de procedimento Problemas

Transcrição

1 Camada de Transporte Protocolo Protocol Stack Basic Protocols Formatting Standard Protocols Curso de Redes de Computadores, 2003 DCC - ICEx - UFMG Prof. José Marcos Silva Nogueira 1 2 Conceitos é mais que apenas um protocolo É um mecanismo popular para estruturar sistemas distribuídos Request Blocked É baseado na semântica de uma chamada local de procedimento O programa de aplicação faz uma chamada a um procedimento não se preocupando se ele está local ou remotamente situado Blocked Reply Computing Blocked 3 4 Problemas A rede situada entre o processo chamador e o chamado tem muito mais propriedades complexas que o bastidor de um computador Os computadores nos quais os processos executam podem ter diferenças significativas de arquitetura de formatos de representação de dados 5 6

2 Um protocolo que Cuida das mensagens trocadas entre os processos cliente e servidor Trata das potencialmente indesejáveis propriedades das redes subjacentes 7 8 Suporte de linguagem de programação e de compilador para Ajuntar os argumentos em uma mensagem de requisição no lado cliente Traduzir a mensagem de volta em argumentos no lado servidor Mecanismo completo Caller (client) Return value Request Reply protocol Callee (server) Return value Request Reply protocol Fazer o mesmo com o valor de retorno do procedimento (compilador ) 9 10 Pode-ver o sendo composto de uma pilha de microprotocolos Cada um trata de um problema e tem um algoritmo São completos e auto-contidos Não são protocolos padronizados Microprotocolos BLAST: fragmenta e remonta mensagens grandes CHAN: sincroniza mensagens de requisição e resposta SELECT: despacha requisições aos processos corretos 11 12

3 Bulk Transfer (BLAST) Como transformar uma rede que trata de mensagens de tamanho pequenos em um serviço que entrega mensagens de tamanhos muito maiores? 1k 32k, por exemplo. Uso de técnicas de fragmentação e remontagem Diferentemente da AAL e IP, o Blast tenta recuperar fragmentos perdidos, usando retransmissão Blast não garante entrega, entretanto. Bulk Transfer (BLAST) Strategy selective retransmission aka partial acknowledgements SRR selective retransmission request Sender Fragment 1 Fragment 2 Fragment 3 Fragment 4 Fragment 5 Fragment 6 SRR Fragment 3 Fragment 5 SRR Receiver Sender: BLAST Details after sending all fragments, set timer DONE if receive SRR, send missing fragments and reset DONE if timer DONE expires, free fragments BLAST Details (cont) Receiver: when first fragments arrives, set timer LAST_FRAG when all fragments present, reassemble and pass up four exceptional conditions: if last fragment arrives but message not complete send SRR and set timer RETRY if timer LAST_FRAG expires send SRR and set timer RETRY if timer RETRY expires for first or second time send SRR and set timer RETRY if timer RETRY expires a third time give up and free partial message BLAST Header Format MID identify this message must protect against wrap around TYPE = DATA or SRR NumFrags indicates number of fragments FragMask distinguishes among fragments if Type=DATA, identifies this fragment if Type=SRR, identifies missing fragments ProtNum MID Length NumFrags Type FragMask Data Request/Reply (CHAN) Guarantees message delivery Guarantees just one copy delivery Synchronizes client with server Supports at-most-once semantics A logical request/reply channel between pair of participants At one instant, only one message transition 17 18

4 Request/Reply (CHAN) CHAN Details Simple case Request ACK Reply ACK Implicit Acks Request 1 Reply 1 Request 2 Reply 2 Lost message (request, reply, or ACK) set RETRANSMIT timer use message id (MID) field to distinguish Slow (long running) server client periodically sends are you alive probe, or server periodically sends I m alive notice Want to support multiple outstanding calls use channel id (CID) field to distinguish Machines crash and reboot use boot id (BID) field to distinguish CHAN Header Format typedef struct { u_short Type; /* REQ, REP, ACK, PROBE */ u_short CID; /* unique channel id */ int MID; /* unique message id */ int BID; /* unique boot id */ int Length; /* length of message */ int ProtNum; /* high-level protocol */ } ChanHdr; typedef struct { u_char type; /* CLIENT or SERVER */ u_char status; /* BUSY or IDLE */ int retries; /* number of retries */ int timeout; /* timeout value */ XkReturn ret_val; /* return value */ Msg *request; /* request message */ Msg *reply; /* reply message */ Semaphore reply_sem; /* client semaphore */ int mid; /* message id */ int bid; /* boot id */ } ChanState; Dispatch to appropriate procedure Synchronous counterpart to UDP Dispatcher (SELECT) xpush Caller SELECT CHAN xcall xcall xdemux Address Space for Procedures flat: unique id for each possible procedure hierarchical: program + procedure number Callee xcalldemux SELECT xcalldemux CHAN xpush xdemux Simple Stack SELECT CHAN Sun IP implements BLAST-equivalent except no selective retransmit Sun implements CHAN-equivalent except not at-most-once Sun UDP IP BLAST IP UDP + Sun implement SELECT-equivalent UDP dispatches to program (ports bound to programs) Sun dispatches to procedure within program ETH ETH 30 32

5 Sun Header Format Formatting XID (transaction id) is similar to CHAN s MID does not remember last XID it serviced Problem if client retransmits request while reply is in transit 0 31 XID MsgType = CALL Version = 2 Program Version Procedure Credentials (variable) Verifier (variable) Data 0 31 XID MsgType = REPLY Status = ACCEPTED Data Marshalling (encoding) application into messages Unmarshalling (decoding) messages into application encoding Message Message Message decoding Data types we consider integers floats strings arrays structs Formatting encoding Types of we do not consider images video multimedia documents Message Message Message decoding Difficulties Representation of base types floating point: IEEE 754 versus non-standard integer: big-endian versus little-endian (e.g., 34,677,374) Big-endian Little-endian (2) (126) (17) 00 0 (34) High address Compiler layout of structures (34) (17) (126) (2) Low address Data types Taxonomy base types (e.g., ints, floats); must convert flat types (e.g., structures, arrays); must pack complex types (e.g., pointers); must linearize Conversion Strategy canonical intermediate form receiver-makes-right (an N x N solution) Marshaller structure Taxonomy (cont) Tagged versus untagged type = INT Stubs compiled interpreted len = 4 value = Call P Code Marshalled arguments Interface descriptor for Procedure P Stub compiler Specification Code P Marshalled arguments Message 37 38

6 external Data Representation (XDR) Defined by Sun for use with Sun C type system (without function pointers) Canonical intermediate form Untagged (except array length) Compiled s Abstract Syntax Notation One (ASN-1) An ISO standard Essentially the C type system Canonical intermediate form Tagged Compiled or interpreted s BER: Basic Encoding Rules type (tag, length, value) length type length value type length value value INT 4 4-byte integer 39 41