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ICNRG Working Group                                          C. Tschudin
Internet-Draft                                       University of Basel
Intended status: Informational                                   C. Wood
Expires: October 6, 2016                                      PARC, Inc.
                                                          April 04, 2016


                    File-Like ICN Collection (FLIC)
                      draft-tschudin-icnrg-flic-00

Abstract

   This document describes a bare bones "index table"-approach for
   organizing a set of ICN data objects into a large, File-Like ICN
   Collection (FLIC).

   At the core of this collection is a so called manifest which acts as
   the collection's root node.  The manifest contains an index table
   with pointers, each pointer being a hash value pointing to either a
   final data block or another index table node.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 6, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  FLIC as a Distributed Data Structure  . . . . . . . . . .   2
     1.2.  Design goals  . . . . . . . . . . . . . . . . . . . . . .   3
   2.  File-Like ICN Collection (FLIC) Format  . . . . . . . . . . .   4
     2.1.  Use of hash-valued pointers . . . . . . . . . . . . . . .   4
     2.2.  Creating a FLIC data structure  . . . . . . . . . . . . .   5
     2.3.  Reconstructing the collection's data  . . . . . . . . . .   6
     2.4.  Metadata in HashGroups  . . . . . . . . . . . . . . . . .   7
     2.5.  Locating FLIC leaf and manifest nodes . . . . . . . . . .   8
   3.  Advanced uses of FLIC manifests . . . . . . . . . . . . . . .   9
     3.1.  Seeking . . . . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Block-level de-duplification  . . . . . . . . . . . . . .  10
     3.3.  Growing ICN collections . . . . . . . . . . . . . . . . .  10
     3.4.  Re-publishing a FLIC under a new name . . . . . . . . . .  11
     3.5.  Data Chunks of variable size  . . . . . . . . . . . . . .  11
   4.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.1.  Encoding for CCNx1.0  . . . . . . . . . . . . . . . . . .  12
     4.2.  Encoding for NDN  . . . . . . . . . . . . . . . . . . . .  12
     5.1.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

1.1.  FLIC as a Distributed Data Structure

   One figure


















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                            root manifest
    .------------------------------------.
    | optional name:                     |
    |   /icn/name/of/this/flic           |
    |                                    |
    | HashGroup (HG):                    |
    |   optional metadata:               |
    |     data size, entry size          |
    |     overall digest, locator, etc.  |    .------.
    |   hash-valued data pointer -----------> | data |
    |     ...                            |    `------'  sub manifest
    |   hash-valued manifest pointer ------.     .------------------.
    |                                    |  `--> |                ----->
    | optional additional HashGroups ..  |       |                ----->
    |                                    |       `------------------'
    | optional signature                 |
    `------------------------------------'

         Figure 1: A FLIC manifest and its directed acyclic graph

1.2.  Design goals

   o  Copy the proven UNIX inode concept:

      *  index tables and memory pointers

   o  Adaption to ICN:

      *  hash values instead of block numbers, unique with high
         probability

   o  Advantages (over non-manifest collections):

      *  single root manifest signature covers all elements of the full
         collection, including intermediate sub manifests

      *  eliminate reference to chunk numbering schemata (hash values
         only)

      *  supports block-level deduplification (can lead to a directed
         acyclic graph, or DAG, instead of a tree)

   o  Limitations

      *  All data leafs must be present at manifest creation time
         (otherwise one cannot compute the pointers)

   o  Potential extensions (for study):



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      *  Enhance the manifest such that it can serve as a "database
         cursor" or as a cursor over a time series, e.g. having entries
         for "previous" and "next" collections.

2.  File-Like ICN Collection (FLIC) Format

   We first give the FLIC format in EBN notation:

      ManifestMsg := Name? HashGroup+
      HashGroup   := MetaData? (DataPointer | ManifestPtr)+
      DataPointer := HashValue
      ManifestPtr := HashValue
      HashValue   := OCTET[32]

      MetaData    := Property*
      Property    := Locator | DataSize | EntrySize | BlockSize |
                     DataDigest | TreeDepth | ...

   Description:

   o  The core of a manifest is the sequence of "hash groups".

   o  A HashGroup (HG) consists of a sequence of data or manifest
      pointers.

   o  Data as well as manifest pointers are SHA256 digests (32 Bytes);
      their encoding assigns them distinct types.

   o  A HashGroup can contain a metadata section to help a reader to
      optimize content retrieval (block size of leaf nodes, total size,
      overall digest etc).

   o  None of the ICN objects used in FLIC are allowed to be chunked,
      including the (sub-) manifests.  The smallest possible complete
      manifest contains one HashGroup with one pointer to an ICN object.

2.1.  Use of hash-valued pointers

   FLIC's tree data structure is a generalized index table as it is
   known from file systems.  The pointers, which in an OS typically are
   harddisk block numbers, are replaced by hash values of other ICN
   objects.  These ICN objects contain either other manifest nodes, or
   leaf nodes.  Leafs contain the actual data of the collection.

   FLIC makes use of "nameless ICN object" where the network is tasked
   with fetching an object based on its digest only.  The interest for
   such an object consists of a routing hint (locator) plus the given
   digest value.



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2.2.  Creating a FLIC data structure

   Starting from the original content, the corresponding byte array is
   sliced into chunks (of equal size if blocksize is present in the
   metadata section, except for the last chunk).  Each chunk is encoded
   as a data object, according the ICN suite.  For each resulting data
   object, the hash value is computed.  Groups of consecutive objects
   are formed and the corresponding hash values collected in manifests,
   which are also encoded.  The hash values of the manifest objects
   replace the hash values of the covered leaf nodes, thus reducing the
   number of hash values.  This process of hash value collection and
   replacement is repeated until only one (root) manifest is left.

   data1 <-- h1  -  -  -  -  -  -  -  -  -  -  -  - \
   data2 <-- h2 \                                    root mfst
   ...            mfst 1 <-- hN+1  \                /
   dataJ <-- hJ /                    mfst2 <-- hN+2
   ...                              /
   dataN <-- hN  -  -  -  -  -  -  /

   Of special interest are "skewed trees" where a pointer to a manifest
   may only appear as last pointer of (sub-) manifests.  Such a tree
   becomes a sequential list of manifests with a maximum of datapointers
   per manifest packet.  Beside the tree shape we also show this data
   structure in form of packet content where D stands for a data pointer
   and M is the hash of a manifest packet.

   data1   <-- h1  -  -  -  -  -  -  -  -  root mfst
   ...                                    /
   dataJ-1 <-- hJ-1                      /
   dataJ   <-- hJ  -  -  mfst1 <-- hN+1 /
   ...                /
   dataN   <-- hN  - /

   DDDDDDM--> DDDDDDM--> ....... DDDDDDM--> DDDDDDD

   A pseudo code description for producing a skewed tree follows below.














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   Input:
       Application data D of size |D| (bytes)
       Block size B (in bytes)
   Output:
       FLIC root node R
   Algo:
       n = number of leaf nodes = ceil(|D| / B)
       k = number of (encoded) hash values fitting in a block of size B
       H[1..n] = array of hash values
         initialized with the data hash values for data chunks 1..n
       While n > k do
         a)  create manifest M with a HashGroup
         b)  append to the HashGroup in M all hash values H[n-k+1..n]
         c)  n = n - k + 1
         d)  H[n] = manifest hash value of M
       Create root manifest R with a HashGroup
       Add to the HashGroup of R all hash values H[1..n]
       Optionally: add name to R, sign manifest R
       Output R

   Obtaining with each manifest a maximum of data pointers is beneficial
   for keeping the download pipeline filled.  On the other hand, this
   tree doesn't support well random access to arbitrary byte positions:
   All data pointers coming before that offset have to be fetched before
   locating the block of interest.  For random access, binary trees
   (where both subtrees of a node cover half of the content bytes) are
   better suited.  This can be combined with the "skewed tree" approach:
   Manifests of intermediate nodes are filled with data pointers except
   for the last two slots.  The second last slot points to a manifest
   for the "first half" of the left content, the last slots then points
   to a manifest for the rest.

   root manifest=     DDDDDMM
              ____________/  \_____
             /                     \
             DDDDDMM                DDDDDMM
          _______/  \              _____/  \
         /           \            /         \
         DDDDDDD      DDDDDDD     DDDDDDD    DDDDDDD

   This can be generalized to k-ary trees by allocating k pointers per
   manifest instead of 2.

2.3.  Reconstructing the collection's data

   To fetch the data associated with a given FLIC (sub-) manifest, the
   receiver sequentially works through all entries found in the
   HashGroups and issues corresponding hash-based interests.  In case of



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   a data hash pointer, the received content object is appended.  In
   case of a manifest hash pointer, this procedure is called recursively
   for the received manifest.  In other words, the collection data is
   represented as the concatenation of data leaves from this _pre-order_
   "depth-first search" (DFS) traversal strategy of the manifest tree.
   (Currently, pre-order DFS is the only supported traversal strategy.)
   This procedure works regardless of the tree's shape.

   A pseudo code description for fetching is below.

   Input:
       Root manifest R
   Output:
       Application data D
   Algo:
       global D = []
       DFS(R)
       Output D

   where:

   procedure DFS(M)
   {
   L:
     H = sequence of hash valued pointers of M
     foreach p in H do:
       if p is a data pointer then
         data = lookup(p)
         Append data to D
       else
         M = lookup(p)
         if p is last element in H then
           goto L;   // tail recursion
         DFS(M)
   }

   The above DFS code works for FLIC manifest trees of arbitrary shape.
   In case of a skewed tree, no recursion is needed and a single
   instance of the DFS procedure suffices (i.e., one uses tail
   recursion).

2.4.  Metadata in HashGroups

   In FLIC, metadata is linked to HashGroups and permits to inform the
   FLIC retriever about properties of the data that is covered by this
   hash group.  Examples are overall data bytes or the size per entry.
   The intent of such metadata is to enable an in-network retriever to




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   optimize its operation - other attributes linked to the collection as
   a whole (author, copyright, etc.) is out of scope.

   The list of available metadata is below.

* Locator - provides a new routing hint (name prefix) where the
  chunks of this hash group can be retrieved from. The default is to
  use the locator of the root manifest.

* DataSize - indicates the total number of *application data bytes*
  contained in a single HashGroup. This does not include bytes consumed
  by child manifests.

* EntrySize - indicates the number of *application data bytes* contained
  by a single entry (pointer) in a HashGroup.

* BlockSize - indicates the size of each data and manifest node used
  when producing each entry of the HashGroup.

* DataDigest - expresses the overall digest of all application data
  contained in the HashGroup.

* Tree Depth - expresses the depth of each entry in the HashGroup and
  allows a receiver to predict the amount of memory needed when
  traversing this tree.

   To give an example of how the DataSize and EntrySize values are used,
   consider the following example.  Let HG be a HashGroup with n
   entries, DataSize S, and EntrySize E.  It is required that the _first
   (n - 1) entries of HG_ "contain" E bytes of application data and that
   the last (nth) entry has at most E bytes of application data.  Thus,
   the following inequality always holds:

      n * E <= S

   We will give an example of how to use these sizes for seeking in
   Section 3.1.

2.5.  Locating FLIC leaf and manifest nodes

   The optional name of a manifest is a mere decoration and has no
   locator functionality at all: All objects pointed to by a manifest
   are retrieved from the location where the manifest itself was
   obtained from (which is not necessarily its name).  Example:







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   Objects:
     manifest(name=/a/b/c, ptr=h1, ptr=hN)  - has hash h0
     nameless(data1)                        - has hash h1
     ...
     nameless(dataN)                        - has hash hN

   Query for the manifest:
     interest(name=/the/locator/hint, implicitDigest=h0)

   In this example, the name "/a/b/c" does NOT override "/the/locator/
   hint" i.e., after having obtained the manifest, the retriever will
   issue requests for

     interest(name=/the/locator/hint, implicitDigest=h1)
     ...
     interest(name=/the/locator/hint, implicitDigest=hN)

   Using the locator metadata entry, this behavior can be changed:

   Objects:
     manifest(name=/a/b/c,
              hashgroup(loc=/x/y/z, ptr=h1)
              hashgroup(ptr=h2)             - has hash h0
     nameless(data1)                        - has hash h1
     nameless(data2)                        - has hash h2

   Queries:
     interest(name=/the/locator/hint, implicitDigest=h0)
     interest(name=/x/y/z, implicitDigest=h1)
     interest(name=/the/locator/hint, implicitDigest=h2)

3.  Advanced uses of FLIC manifests

   The FLIC mechanics has uses cases beyond keeping together a set of
   data objects, such as: seeking, block-level de-duplification, re-
   publishing under a new name, growing ICN collections, and supporting
   FLICs with different block sizes.

3.1.  Seeking

   Fast seeking (without having to sequentially fetch all content) works
   by skipping over entries for which we know their size.  The byte
   offset of the data pointed at by pointer P_i (in a HashGroup with
   EntrySize E and DataSize S) is computed as follows:

      offset = (i * E)





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   Recall that the total size of each pointer (except the last) is equal
   to the EntrySize.  If P_n is the last pointer in a HashGroup, its
   size is calculated as

      size = S - ((n - 1) * E)

   With these formulas, seeking is done as follows:

   Input: seek_pos P, a FLIC manifest with a HashGroup having
          EntrySize E, DataSize S and N entries
   Output: blockindex i and offset o, or out-of-range error
   Algo:
      if (P >= S)
         return out-of-range
      i = floor(P / E)
      if (i >= N)
         return out-of-range # bad FLIC encoding
      o = S - (i * E)
      return (i, o)

   Note: If the pointer at position i is a manifest pointer, this
   algorithm has to be called once more, seeking to seek_pos o inside
   that manifest.

3.2.  Block-level de-duplification

   Consider a huge file, e.g. an ISO image of a DVD or program in binary
   form, that had previously been FLIC-ed but now needs to be patched.
   In this case, all existing encoded ICN chunks can remain in the
   repository while only the chunks for the patch itself is added to a
   new manifest data structure, as is shown in the picture below.  For
   example, the venti [1] archival file system of Plan9 uses this
   technique.

   old_mfst -  - > h1 --> oldData1  <-- h1 < -  -  new_mfst
            \  - > h2 --> oldData2  <-- h2 < -  - /
             \            replace3  <-- h5 < -  -/
              \- > h3 --> oldData3              /
               \ > h4 --> oldData4  <-- h4 < - /

3.3.  Growing ICN collections

   A log file, for example, grows over time.  Instead of having to re-
   FLIC the grown file it suffices to construct a new manifest with a
   manifest pointer to the old root manifest plus the sequence of data
   hash pointers for the new data (or additional sub-manifests if
   necessary).  Note that this tree will not be skewed (anymore).




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   old data < -  -  -  mfst_old <-- h_old -  - mfst_new
                                               /
   new data1 <-- h_1 -  -  -  -  -  -  -  -  -/
   new data2                                 /
   ...                                      /
   new dataN <-- h_N -  -  -  -  -  -  -  -/

3.4.  Re-publishing a FLIC under a new name

   It can happen that a publisher's namespace is part of a service
   provider's prefix.  When switching provider, the publisher may want
   to republish the old data under a new name.  This can easily be
   achieved with a single nameless root manifest for the large FLIC plus
   arbitrarily many per-name manifests (which are signed by whomever
   wants to publish this data):

   data < - nameless_mfst() <-- h  < - mfst(/com/parc/east/the/flic)
                                   < - mfst(/com/parc/west/old/the/flic)
                                   < - mfst(/internet/archive/flic234)

   Note that the hash computation (of h) only requires reading the
   nameless root manifest, not the entire FLIC.

   This example points out the problem of HashGroups having locator
   metadata elements: A retriever would be urged to follow these hints
   which are "hardcoded" deep inside the FLIC but might have become
   outdated.  We therefore recommend to name FLIC manifests only at the
   highest level (where these names have no locator function).  Child
   nodes in a FLIC manifest should not be named as these names serve no
   purpose except retrieving a sub-tree's manifest by name, if would be
   required.

3.5.  Data Chunks of variable size

   If chunks do not have regular (block) sizes, and therefore manifests
   do not have consistent entry sizes, the HashGroup can be used to
   still convey to a reader the length of the chunks at the manifest
   level.  Example use cases would be chunks each carrying a single
   ASCII line as entered by a user or a database with variable length
   records mapped to chunks.

   M = (manifest
         (hashgroup((metadata(blocksize=12)) (dataptr=h1))
         (hashgroup((metadata(blocksize=1)) (dataptr=h2))
         ...
       )





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4.  Encoding

   We express the packet encoding of manifests in a symbolic expression
   style in order to show the TLV structure and the chosen type values.
   In this notation, a TLV's type is a combination of "SymbolicName/
   Tvalue", Length is not shown and Values are sub-expressions.
   Moreover, we populate the data structure with all possible entries
   and omit repetition.  An abbreviated example for the NDN Interest
   packet would be:

   (Interest/0x5
     (Name/0x7 (NameComp=0x8 ...) ...)
     (Selector/0x9 ...)
     (Nonce/0xA BLOB)
     (Scope/0xB INT)
     (InterestLifeTime/0xC INT)
   )

4.1.  Encoding for CCNx1.0

   [FIXED_HEADER OCTET[8]]
   (ManifestMsg/0x6
     (Name/0x0 ...)
     (HashGroup/0x1
        (MetaData/0x1
           (HGLocator/0x0 (NameComp/0x ...))
           (HGDataSize/0x2 INT)
           (HGEntrySize/0x3 INT)
           (HGBlockSize/0x4 INT)
           (HGDataDigest/0x5 OCTET[32])
           (HGTreeDepth/0x6 INT)
        )
        (DataPtr/0x2 OCTET[32])
        (MfstPtr/0x3 OCTET[32])
     )
   )

   Interest: name is locator, use objHashRestriction as selector.

4.2.  Encoding for NDN

   The assigned NDN content type value for FLIC manifests is 1024
   (0x400).








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   (Data/0x6
     (Name/0x7 ...)
     (MetaInfo/0x14
       (ContentType/0x18 0x0400)
     )
     (Content/0x15
       (HashGroup/0xC0
         (MetaInfo/0x14
           (LocatorNm/0xC3 (NameComp/0x8 ...))
           (TotalHash/0xC4 OCTET[32])
           (TotalSize/0xC5 INT)
           (BlockSize/0xC6 INT)
           (TreeDepth/0xC7 INT)
         )
         (DataPtr/0xC1 OCTET[32])
         (MfstPtr/0xC2 OCTET[32])
       )
     )
     (SignatureInfo/0x16 ...)
     (SignatureValue/0x17 ...)
   )

   Interest: name is locator, use implicitDigest name component as
   selector.

5.  References

5.1.  URIs

   [1] http://plan9.bell-labs.com/sys/doc/venti/venti.pdf

Authors' Addresses

   Christian Tschudin
   University of Basel

   Email: christian.tschudin@unibas.ch


   Christopher A. Wood
   PARC, Inc.

   Email: christopher.wood@parc.com








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