1. EECS122 - UCB 1 CS 194: Distributed Systems Communication Protocols, RPC Computer Science Division Department of Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley, CA 94720-1776

2. ISO OSI Reference Model for Layers Application Presentation Session Transport Network Datalink Physical

3. Mapping Layers onto Routers and Hosts –Lower three layers are implemented everywhere –Next four layers are implemented only at hosts Application Presentation Session Transport Network Datalink Physical Application Presentation Session Transport Network Datalink Physical Network Datalink Physical Physical medium Host AHost B Router

4. Encapsulation A layer can use only the service provided by the layer immediate below it Each layer may change and add a header to data packet –higher layer’s header is treated as payload data

5. OSI Model Concepts Service – says what a layer does Interface – says how to access the service Protocol – says how is the service implemented –A set of rules and formats that govern the communication between two peers

6. Layering: Internet Universal Internet layer: Internet has only IP at the Internet layer Many options for modules above IP Many options for modules below IP Internet Net access/ Physical Transport Application IP LAN Packet radio TCPUDP TelnetFTPDNS

7. Internet’s Hourglass Physical Layer Datalink Layer Network Layer Application Layer

8. Physical Layer Service: move information between two systems connected by a physical link Interface: specifies how to send a bit Protocol: coding scheme used to represent a bit, voltage levels, duration of a bit Examples: coaxial cable, optical fiber links; transmitters, receivers Adaptor Signal Network Application Transport Link Physical

9. Datalink Layer Service: –Framing (attach frame separators) –Send data frames between peers –Medium access: arbitrate the access to common physical media –Error detection and correction Interface: send a data unit (packet) to a machine connected to the same physical media Protocol: layer addresses, implement Medium Access Control (MAC) (e.g., CSMA/CD)… Network Application Transport Link Physical

10. Network Layer Service: –Deliver a packet to specified network destination –Perform segmentation/reassembly –Others Packet scheduling Buffer management Interface: send a packet to a specified destination Protocol: define global unique addresses; construct routing tables Network Application Transport Link Physical

11. Datagram (Packet) Switching Host A Host B Host E Host D Host C router 1 router 2 router 3 router 4 router 5 router 6 router 7 Network Application Transport Link Physical

12. Datagram (Packet) Switching Host A Host B Host E Host D Host C router 1 router 2 router 3 router 4 router 5 router 6 router 7 Network Application Transport Link Physical

13. Transport Layer Services: –Multiplex multiple transport connections to one network connection –Provide an error-free and flow-controlled end-to-end connection –Split one transport connection in multiple network connections Interface: send a packet to specify destination Protocols: implement reliability and flow control Examples: TCP and UDP Network Application Transport Link Physical

14. End-to-End View Process A sends a packet to process B 16.25.31.10 128.15.11.12 Proc. A (port 10) Proc. B (port 7) Internet

15. End-to-End Layering View 16.25.31.10128.15.11.12 Proc. A (port 10) Internet Proc. B (port 7) Transport Network Datalink Physical Proc. A (port 10) Proc. B (port 7) Transport Network Datalink Physical data 16.25.31.10128.15.11.12data107 data107 16.25.31.10128.15.11.12 data 107 7 Internet 16.25.31.10 128.15.11.12

16. Client-Server TCP a)Normal operation of TCP. b)Transactional TCP. 2-4

17. Conventional Procedure Call a)Parameter passing in a local procedure call: the stack before the call to read b)The stack while the called procedure is active

18. Example: Local Procedure Call. n = sum(4, 7);. sum(i, j) int i, j; { return (i+j); } Machine Process

19. Example: Remote Procedure Call. n = sum(4, 7);. sum(i, j) int i, j; { return (i+j); } Client Process sum 4 7 message OS Server Process sum 4 7 message OS Stubs

20. Client and Server Stubs Principle of RPC between a client and server program.

21. Steps of a Remote Procedure Call 1.Client procedure calls client stub in normal way 2.Client stub builds message, calls local OS 3.Client's OS sends message to remote OS 4.Remote OS gives message to server stub 5.Server stub unpacks parameters, calls server 6.Server does work, returns result to the stub 7.Server stub packs it in message, calls local OS 8.Server's OS sends message to client's OS 9.Client's OS gives message to client stub 10.Stub unpacks result, returns to client

22. Parameter Passing Server and client may encode parameters differently –E.g., big endian vs. little endian How to send parameters “call-by-reference”? –Basically do “call-by-copy/restore” –Woks when there is an array of fixed size –How about arbitrary data structures?

23. Different Encodings a)Original message on the Pentium b)The message after receipt on the SPARC c)The message after being inverted. The little numbers in boxes indicate the address of each byte

24. Parameter Specification and Stub Generation a)A procedure b)The corresponding message.

25. Binding a Client to a Server Client-to-server binding in DCE. 2-15

26. Doors The principle of using doors as IPC mechanism.

27. RPC Semantics in the Presence of Failures The client is unable to locate the server The request message from the client to server is lost The reply message from the client is lost The server crashes after sending a request The client crashes after sending a request

28. Client is Unable to Locate Server Causes: server down, different version of server binary, … Fixes –Return -1 to indicate failure (in Unix use errno to indicate failure type) What if -1 is a legal return value? –Use exceptions Transparency is lost

29. Lost Request Message Easiest to deal with Just retransmit the message! If multiple message are lost then –“client is unable to locate server” error

30. Lost Reply Message Far more difficult to deal with: client doesn’t know what happened at server –Did server execute the procedure or not? Possible fixes –Retransmit the request Only works if operation is idempontent: it’s fine to execute it twice –What if operation not idempotent? Assign unique sequence numbers to every request

31. Server Crashes Two cases –Crash after execution –Crash before execution Three possible semantics –At least once semantics Client keeps trying until it gets a reply –At most once semantics Client gives up on failure –Exactly once semantics Can this be correctly implemented?

32. Client Crashes Let’s the server computation orphan Orphans can –Waste CPU cycles –Lock files –Client reboots and it gets the old reply immediately

33. Client Crashes: Possible Solutions Extermination: –Client keeps a log, reads it when reboots, and kills the orphan –Disadvantage: high overhead to maintain the log Reincarnation: –Divide times in epochs –Client broadcasts epoch when reboots –Upon hearing a new epoch servers kills the orphans –Disadvantage: doesn’t solve problem when network partitioned Expiration: –Each RPC is given a lease T to finish computation –If it does not, it needs to ask for another lease –If client reboots after T sec all orphans are gone –Problem: what is a good value of T?

34. RPC Semantics: Discussion The original goal: provide the same semantics as a local call Impossible to achieve in a distributed system –Dealing with remote failures fundamentally affects transparency Ideal interface: balance the easy of use with making visible the errors to users

35. Asynchronous RPC (1) a)The interconnection between client and server in a traditional RPC b)The interaction using asynchronous RPC 2-12

36. Asynchronous RPC (2) A client and server interacting through two asynchronous RPCs