Naming

1
Naming

• Chapter 4

2
Name Spaces (1)

• A general naming graph with a single root node.

3
Name Spaces (2)

• The general organization of the UNIX file system
  implementation on a logical disk of contiguous
  disk blocks.

4
Name Resolution (3)

• How does name resolution work in OS?
• Traverse the directory
• Poor performance? Why?
• Solution Look-aside cache what kind of info.?

5
Linking

• The concept of a symbolic link explained in a
  naming graph.
• Q1 How to differentiate hard links and symbolic
  links?

6
Hard links or Symbolic Links

• Hard links store the address of an entity,
  i-node?
• Symbolic links store the absolute pathname?

7
Mounting

• Purpose Merge different name spaces
• Mount (local) and Mounting points (for foreign
  name space)
• 3 Requirements to mount a foreign name space in a
  distributed system
• Access protocol, e.g., NFS protocol
• Server name
• Name of mounting point in the foreign name space
• Example nfs//oitlnx.seattleu.edu//home/zhuy

8
Mounting

• Mounting remote name spaces through a specific
  process protocol.

9
Mounting (2)

• Another approach DECs Global Name Service (GNS)
• Add a new root
• Make the existing root nodes as the new roots
  children
• Problem the new root must have a table mapping
  from old root nodes to new names, posing a
  scalability issue in the table size.?

10
Mounting (2)

• Organization of the DEC Global Name Service

11
Name Space Distribution

• Large-scale distributed systems
• Name space is in hierarchy
• Global layer stable, why? how? Availability?
  (replication?) Performance? (replication,
  client/proxy-side caching due to stability, no
  change?)
• Administrational layer by organization.
  Availability? (resource within the organization),
  performance? (critical), quick response?
  (high-performance, replicated servers, with
  client-side cache?)
• Managerial layer change. Low availability?

12
Name Space Distribution (1)

• An example partitioning of the DNS name space,
  including Internet-accessible files, into three
  layers.

13
Name Space Distribution (2)

• A comparison between name servers for
  implementing nodes from a large-scale name space
  partitioned into a global layer, as an
  administrational layer, and a managerial layer.

14
Name Resolution

• Iterative name resolution
• Recursive name resolution

15
Implementation of Name Resolution (1)

• The principle of iterative name resolution.

16
Implementation of Name Resolution (2)

• The principle of recursive name resolution.

17
Implementation of Name Resolution (3)

• Recursive name resolution of ltnl, vu, cs, ftpgt.
  Name servers cache intermediate results for
  subsequent lookups.

18
Iterative (I) vs. Recursive (R)

• Comparison
• Overload the global layer name servers. I or R?
• Caching at servers are more effective. I or R?
• Low latency?. I or R?
• Trouble shooting? I or R? know which server goes
  wrong?
• DNS name resolver? proxy sitting the entry/exit
  of the oganization, caching,
• DNS name servers stateful or stateless?

19
Implementation of Name Resolution (4)

• The comparison between recursive and iterative
  name resolution with respect to communication
  costs.

20
DNS Implementation

• DNS name space, only two layers global and
  administrative layers
• Each zone is implemented by a/multiple name
  server(s), primary name server ?zone transfer to
  secondary name servers
• A DNS database a collection of files

21
The DNS Name Space

• The most important types of resource records
  forming the contents of nodes in the DNS name
  space.

22
DNS Implementation (1)

• An excerpt from the DNS database for the zone
  cs.vu.nl.

23
DNS Implementation (2) refer to nodes in a
subdomain

• Part of the description for the vu.nl domain
  which contains the cs.vu.nl domain.
• Need to specify a name server for the subdomain
  by giving its domain name and IP address

24
The X.500 Name Space (1)

• A simple example of a X.500 directory entry using
  X.500 naming conventions.

25
The X.500 Name Space (2)

• Part of the directory information tree.

26
The X.500 Name Space (3)

• Two directory entries having Host_Name as RDN.

27
Naming versus Locating Entities

• Example 1 ftp server ftp.cs.seattleu.edu moved
  to another machine in the same domain
• Solution
• Problem?
• Example 2 ftp server ftp.cs.seattleu.edu move to
  another machine in another domain ftp.cs.uw.edu
• Solution
• Problem?

1 Solution the local DNS server update the
entry. The client-side cached copy should be
invalidated
2 Solution add a symbolic link into the old
DNS name server. Performance issue. For mobile
entities, even worse, need to update frequently
the entry!
28
Naming versus Locating Entities

• Direct, single level mapping between names and
  addresses.
• Two-level mapping using identities.

29
Solution to Locating an Entity

• Broadcasting
• Address Resolution Protocol (ARP)
• Problem scalability issue
• Multicasting
• IP-level multicasting
• Efficient than Broadcasting, but hard to deploy
• Multicast tree, multicast group
• Functions address resolution (identifier ? IP)
  find nearby replicas
• Overlay multicasting easy to deploy, lower
  performance

30
Approach to Locating Mobile Entities

• Forwarding pointers
• When an entity moves from A to B, it leaves
  behind a reference to its new location at B
• Problem
• Result in long chain of forwarding pointers ?
  expensive locating
• All intermediate locations in a chain have to be
  available (maintain the pointers and avoid broken
  chain)
• Solution short-cut the chain
• Send the response directly back to the initiator
  or along the reverse path of the forwarding path?

31
Forwarding Pointers (1)

• The principle of forwarding pointers using
  (proxy, skeleton) pairs.

32
Forwarding Pointers (2)

• Redirecting a forwarding pointer, by storing a
  shortcut in a proxy.

33
Home-Based Approaches

• A popular approach to support mobile entities in
  large-scale networks
• Home location keep track of the current location
  of an entity
• New requests to the entity is first forwarded to
  its home location which tunnels the request to
  its current location

34
Home-Based Approaches

• The principle of Mobile IP.

35
Hierarchical Location Scheme

• Domains, directory nodes
• Location record pointing to next-level domain in
  which the entity is located
• multiple records for multiple replicas of an
  entity

36
Hierarchical Approaches (1)

• Hierarchical organization of a location service
  into domains, each having an associated directory
  node.

37
Hierarchical Approaches (2)

• An example of storing information of an entity
  having two addresses in different leaf domains.

38
Hierarchical Approaches (3)

• Looking up a location in a hierarchically
  organized location service.

39
Hierarchical Approaches (4)

• An insert request is forwarded to the first node
  that knows about entity E.
• A chain of forwarding pointers to the leaf node
  is created.

40
Pointer Caches

• Hierarchical approach performance issue?
• Solution cache the unchanged part ? Pointer
  caches

41
Pointer Caches (1)

• Caching a reference to a directory node of the
  lowest-level domain in which an entity will
  reside most of the time.

42
Pointer Caches (2)

• A cache entry that needs to be invalidated
  because it returns a nonlocal address, while such
  an address is available.

43
Scalability Issues

• The scalability issues related to uniformly
  placing subnodes of
• partitioned root node across the network covered
  by a location service.
• Root nodes tend to be overloaded

44
Discussion

• Problem Do you know how a wireless network works
  where mobile entities mover around?
• If a mobile PC keeps moving around, faraway from
  its home station, do we have other option to
  solve this problem without keeping the forwarding
  pointer in the home base station?

45
Discussion

• Base stations, Hand-off, broadcast, transmission
  range, energy efficient routing
• Two-layer solution
• First, locating its current base station with its
  identifier
• Second, go to its current base station for
  address
• Why not keep identifier ? address one step?

46
The Problem of Unreferenced Objects

• An example of a graph representing objects
  containing references to each other.

47
Reference Counting (1)

• The problem of maintaining a proper reference
  count in the presence of unreliable communication.

48
Reference Counting (2)

• Copying a reference to another process and
  incrementing the counter too late
• A solution.

49
Advanced Referencing Counting (1)

• The initial assignment of weights in weighted
  reference counting
• Weight assignment when creating a new reference.

50
Advanced Referencing Counting (2)

• Weight assignment when copying a reference.

51
Advanced Referencing Counting (3)

• Creating an indirection when the partial weight
  of a reference has reached 1.

52
Advanced Referencing Counting (4)

• Creating and copying a remote reference in
  generation reference counting.

53
Tracing in Groups (1)

• Initial marking of skeletons.

54
Tracing in Groups (2)

• After local propagation in each process.

55
Tracing in Groups (3)

• Final marking.