Operating Systems

1. Operating Systems Operating Systems Unit 10: Networking Distributed File Systems

2. Networking concepts • host • single computer • local CPU, memory, disks • link • connecting media • attributes: • bandwidth, latency COP 5994 - Operating Systems

3. Network Topologies COP 5994 - Operating Systems

4. Application Data Transport Communication Protocols COP 5994 - Operating Systems

5. Data Transport layers 4: Transport layer • End-to-end communication • Relies on network layer to determine proper path from one end of communication to the other 3: Network layer • Moving data between computers 2: Link layer • Provides an interface between the network layer and the underlying physical medium of the connection 1: Physical layer COP 5994 - Operating Systems

6. 2: Link Layer • first software layer on top of physical medium • unit: frame • responsibilities: • deliver frame • detect errors • correct transmission errors COP 5994 - Operating Systems

7. 2: IEEE 802.3 CSMA/CD (Ethernet) • Carrier Sense Multiple Access with Collision Detection protocol • transceiver tests delivery medium for availability • due to delays in medium, it is possible that multiple transceivers begin transmitting simultaneously • if transceivers detect collision: • continues to transmit bytes for a specific period of time to ensure that all transceivers become aware of the collision • waits for a random interval before attempting to retransmit COP 5994 - Operating Systems

8. 2: Token Ring • operates on ring networks • employs tokens to gain access to the transmission medium • token is empty frame that is circulated between machines COP 5994 - Operating Systems

9. 2: Token Ring example scenario 1/2 COP 5994 - Operating Systems

10. 2: Token Ring example scenario 2/2 COP 5994 - Operating Systems

11. 2: Fiber Distributed Data Interface (FDDI) • operates over fiber-optic cable • support more transfers at greater speeds over larger distance • built on two token rings • the second usually being reserved for backup COP 5994 - Operating Systems

12. 2: IEEE 802.11 (Wireless) • similar to Ethernet: • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • require each sender broadcast a Request to Send (RTS) to entire network • upon receiving an RTS • receiver broadcasts a Clear to Send (CTS) message to the entire network if the medium is available COP 5994 - Operating Systems

13. 3: Network Layer • routes packet to next host toward destination • determines next host • address information • network topology • link quality • strength of signal, error rate and interference • Interference is broadcast throughout networks using various router protocols, such as Routing Information Protocol (RIP) COP 5994 - Operating Systems

14. 3: Internet Protocol (IP or IPv4) 131.94.134.39 /24 255.255.255.0 131.94.134.255 131.94.134.1 • hosts have unique IP address • 32-bit number written as quad • network • first n bits of IP number, written as “/n” • 8 - class A, 16 - class B, 24 - class C • more than 24 - class D • netmask • 32 bit number with first n bits all 1, rest 0 • broadcast • network number (first n bits), rest all 1 • gateway IP COP 5994 - Operating Systems

15. 3: Internet Protocol (IP or IPv4) • host names are mapped to IP address via the Domain Name System (DNS) • need IP numbers of DNS servers • private IP numbers • used locally • Network Address Translation (NAT) COP 5994 - Operating Systems

16. 3: IPv4 packet COP 5994 - Operating Systems

17. 3: Internet Protocol version 6 (IPv6) • larger address space: 128 bit number • written as 8 groups of 4 hex digits 2001:468:701:3800:206:5bff:fe53:2a87 COP 5994 - Operating Systems

18. 3: IPv6 new features • quality of service • authentication and privacy • anycast address • used to send packet to one of a group • multicast • used to send packet in all of a group COP 5994 - Operating Systems

19. 4: Transport Layer • Connection-oriented approach • Hosts send each other control information through handshaking to set up a logical end-to-end connection • Imposes reliability on unreliable networks • Guarantees that data sent from sender will arrive at intended receiver undamaged and in correct sequence • Connectionless approach • Two hosts do not handshake before transmission • No guarantee that sent messages will be received in their original order, or at all COP 5994 - Operating Systems

20. 4: Transmission Control Protocol (TCP) • connection-oriented transmission protocol • guarantees that segments sent from a sender will arrive at the intended receiver undamaged and in correct sequence • handles error control, congestion control, and retransmission • allows protocols like HTTP and FTP to send information into network as simply and reliably as writing to a file on the local computer COP 5994 - Operating Systems

21. 4: User Datagram Protocol (UDP) • Connectionless User Datagram Protocol • provides minimum overhead necessary for the transport layer • no guarantee that UDP datagrams will reach their destination in their original order, or at all COP 5994 - Operating Systems

22. Application Layers • Application layer protocols • Specify rules that govern remote interprocess communication • Determine how processes should interact • protocols interact with remote resources • via Uniform Resource Identifier (URI) COP 5994 - Operating Systems

23. Application Layer protocols URL: Uniform Resource Locator • special form of URI syntax: protocol://host:port/path • Common protocols • ftp • http • smtp • ldap • sip COP 5994 - Operating Systems

24. Application Layer protocol: RPC Remote Procedure Call general mechanism to enable function invocation across host boundaries • extension of local function call model COP 5994 - Operating Systems

25. Application Layer protocol: RPC COP 5994 - Operating Systems

26. RPC portmapper • common Unix implementation • translates RPC program numbers • TCP/IP port numbers • UDP/IP port numbers • common RPC programs • NIS • NFS COP 5994 - Operating Systems

27. Distributed File Systems • Networked file systems • Allow clients to access files stored on remote computers • Distributed file systems • Special examples of networked file systems that allow transparent access to remote files COP 5994 - Operating Systems

28. Distributed File System Concepts • location transparency • user is unaware of the physical location of a file within a distributed file system • user sees only a global file system • caching & consistency • Clients keep a local copy of a file and flush modified copies of it to the server from time to time • Because there are multiple copies of the same file, files can become inconsistent COP 5994 - Operating Systems

29. Distributed File System Concepts • scalability • Distributed file systems are designed to share information among large groups of computers • New computers should be able to be added to the distributed system easily • security • Ensuring secure communications • Access control • fault tolerance COP 5994 - Operating Systems

30. Distributed File System Concepts • server state • can be either stateful or stateless • stateful system • server keeps state information of the client requests so that subsequent access to the file is easier • stateless system • client must specify which file to access in each request COP 5994 - Operating Systems

31. Network File System • developed by Sun MicroSystems • early 1980s • dominant for Unix environments • Versions • NFS-3 • NFS-4, IETF standard COP 5994 - Operating Systems

32. NFS Architecture COP 5994 - Operating Systems

33. Network File System (NFS) • NFS versions 2 and version 3 • assume a stateless server implementation • if the server crashes: • client simply retries until the server responds, or • aborts • if server resumes, no need to rebuild state COP 5994 - Operating Systems

34. Network File System (NFS) • NFS-4 • Stateful • enables faster access to files • if the server crashes • all the state information of the client is lost • client needs to rebuild its state on the server before retrying COP 5994 - Operating Systems

35. NFS-4 Caching: Delegation • efficient client-caching scheme: • server temporarily transfers control of file to client • read delegation: no other client can write • write delegation: no other client can read or write • If another client requests a delegated file, the server will revoke the delegation and request that the original client flush the file back to server COP 5994 - Operating Systems

36. Andrew File System • developed by Carnegie Mellon University • after first names of A. Carnegie & A. Mellon • early 1980s • design goals: • large scale • secure • efficient • Versions: AFS-1, AFS-2, AFS-3 • basis for DFS of OSF, predecessor to Coda COP 5994 - Operating Systems

37. Andrew File System (AFS) • location transparency • all files appear as a branch of a traditional UNIX file system at each workstation • built on RPC • security: • uses Kerberos authentication server • access control lists for file/directories COP 5994 - Operating Systems

38. AFS Structure COP 5994 - Operating Systems

39. AFS caching • files are cached on clients local disk • notifies clients via callback that files are no longer valid • client must invalidate its file and request the most recent version COP 5994 - Operating Systems

40. Coda File System • developed by Andrew team • design goal: fault tolerance • mid 1980s COP 5994 - Operating Systems

41. Coda File System Concepts • Volumes • logical pieces of the file system • replicated physically across multiple file servers • Volume storage group (VSG) • servers that hold the same volume • Available volume storage group (AVSG) • reachable members of the VSG • Preferred Server • member of AVSG, currently serving files COP 5994 - Operating Systems

42. Coda volume structure COP 5994 - Operating Systems

43. Coda read/write scenario • to read, check cache for file • if found, done • else • get file from PS • get file versions from other members of AVSG • if conflict, then AVSG members agree on new versions, and update their copies • to write, • send file to all members of AVSG • track members of AVSG that have written file COP 5994 - Operating Systems

44. Coda inconsistency scenario COP 5994 - Operating Systems

45. Coda client disconnect scenario • When connected to Coda, clients cache files so they can be accessed when disconnected (hoarding stage) • When disconnected, clients enter the emulation stage where all file requests are serviced from the cache, if the file is resident (error otherwise) • When reconnected, file updates are sent to the server asynchronously (reintegration stage) COP 5994 - Operating Systems

46. Sprite File System • Part of Sprite distributed OS • Developed at UC Berkeley, mid 1990s • Sprite file system characteristics • Emulates a UNIX file system • even remote access to I/O devices • Every client has the exact same view of the hierarchy COP 5994 - Operating Systems

47. Sprite File System domains per client COP 5994 - Operating Systems

48. Sprite read scenario • to open a file, the client first checks its cache, then makes a request to server • if the server is unable to satisfy the request from its cache, it reads the data from disk • both caches retain a copy of file COP 5994 - Operating Systems

49. Sprite write scenario • client writes to its cache • updated pages are flushed to server every 30 seconds • lazy write-back strategy COP 5994 - Operating Systems

50. Sprite caching protocol • sequential write sharing • upon opening file for write, client checks file version with server • might cause reload of cached file from server • concurrent write sharing • if 2 clients open file for write, caching is turned disabled COP 5994 - Operating Systems