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Versions: 00                                                            
ICNRG Working Group                                          C. Tschudin
Internet-Draft                                       University of Basel
Intended status: Informational                                   C. Wood
Expires: December 27, 2017               University of California Irvine
                                                           June 25, 2017


                    File-Like ICN Collection (FLIC)
                          draft-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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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 December 27, 2017.

Copyright Notice

   Copyright (c) 2017 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 . . . . . . . . . . . . . . .   5
     2.2.  Creating a FLIC data structure  . . . . . . . . . . . . .   5
     2.3.  Reconstructing the collection's data  . . . . . . . . . .   7
     2.4.  Metadata in HashGroups  . . . . . . . . . . . . . . . . .   8
     2.5.  Locating FLIC leaf and manifest nodes . . . . . . . . . .   9
   3.  Advanced uses of FLIC manifests . . . . . . . . . . . . . . .  10
     3.1.  Seeking . . . . . . . . . . . . . . . . . . . . . . . . .  10
     3.2.  Block-level de-duplication  . . . . . . . . . . . . . . .  11
     3.3.  Growing ICN collections . . . . . . . . . . . . . . . . .  11
     3.4.  Re-publishing a FLIC under a new name . . . . . . . . . .  12
     3.5.  Data Chunks of variable size  . . . . . . . . . . . . . .  12
   4.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Example Encoding for CCNx1.0  . . . . . . . . . . . . . .  13
     4.2.  Example Encoding for NDN  . . . . . . . . . . . . . . . .  13
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     6.2.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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:               |
    |     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 de-duplication (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? (SizeDataPtr | SizeManifestPtr)+
     BlockHashGroup := MetaData? SizePerPtr (DataPtr | ManifestPtr)+

     DataPtr := HashValue
     ManifestPtr := HashValue
     SizeDataPtr := Size HashValue
     SizeManifestPtr := Size HashValue

     SizePerPtr    := Size
     HashValue     := See {{CCNxMessages}}
     Size          := OCTET[8]

     MetaData    := Property*
     Property    := Locator | OverallByteCount | OverallDataDigest | ...

   Description:

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

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

   o  A BlockHashGroup (BHG) consists of a sequence of data or manifest
      pointers and a mandatory field that lists the total size of each
      pointer.  These HashGroups should be used when each pointer
      (except the last) contains an identical number of application
      bytes.

   o  Sizes are 64-bit unsigned integers.

   o  Data and manifest pointers are cryptographic HashValues encoded
      according to the mechanism listed in [CCNxMessages].
      Specifically, a HashValue specifies the cryptographic hash
      algorithm and the actual digest.

   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).



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   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
   hard disk 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.  Each
   pointer explicitly indicates the amount of application data bytes
   contained by the referred object.  For example, the size of a data
   pointer (to a leaf) represents the size of the leaf's content object
   payload.  Conversely, the size of a manifest pointer represents the
   total size of all pointers contained in that manifest.

   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.

2.2.  Creating a FLIC data structure

   Starting from the original content, the corresponding byte array is
   sliced into chunks.  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.




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   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.

   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.









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   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
   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.


























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   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 overall hash
   digest (this is akin to a Merkle hash).  The intent of such metadata
   is to enable an in-network retriever to 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.









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  * 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.

  * OverallByteCount - indicates the total number of *application
    data bytes* contained in a single HashGroup. This does not include
    bytes consumed by child manifests. This value is equal to the sum of
    all pointer sizes contained in the HashGroup.

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

   BlockHashGroups contain a mandatory piece of metadata called the
   SizePerPtr.  This value indicates the total number of application
   bytes contained within each pointer in the hash group _except for the
   last pointer._ Normal HashGroups do not require this piece of
   metadata; Instead, each pointer includes their size explicitly.

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:

   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:








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   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-duplication, 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 following
   expression shows how to compute the byte offset of the data pointed
   at by pointer P_i, offset_i.  In this formula, let P_i represent the
   Size value of the i-th pointer.

      offset_i = \sum_{i = 1}^{i - 1} P_i.size

   With this offset, seeking is done as follows:

   Input: seek_pos P, a FLIC manifest with a hash group having N entries
   Output: pointer index i and byte offset o, or out-of-range error
   Algo:
       offset = 0
       for i in 1..N do
           if (P < P_i.size)
               return (i, P - offset)
           offset += P_i.size
       return out-of-range

   Seeking in a BlockHashGroup is different since offsets can be quickly
   computed.  This is because the size of each pointer P_i except the
   last is equal to the SizePerPtr value.  For a BlockHashGroup with N
   pointers, OverallByteCount D, and SizePerPointer L, the size of P_i
   is equal to the following:

      D - ((i - 1) * L)



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   In a BlockHashGroup with k pointers, the size of P_k is equal to:

       D - L * (k - 1)

   Using these, the seeking algorithm can be thus simplified to the
   following:

   Input: seek_pos P, a FLIC manifest with a hash group having
          OverallByteCount S and SizePerPointer L.
   Output: pointer index i and byte offset o, or out-of-range error
   Algo:
       if (P > S)
           return out-of-range
       i = floor(P / L)
       if (i > N)
           return out-of-range # bad FLIC encoding
       o = P mod L
       return (i, o)

   Note: In both cases, 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-duplication

   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, the HashGroup can be
   used to still convey to a reader the length of the chunks at the
   manifest level.  (This can be computed based on the size of pointers,
   but the metadata field makes this determination simpler.)  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(SizePerPtr=4096)) (dataptr=h1))
         (hashgroup((metadata(SizePerPtr=1500)) (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.

4.1.  Example Encoding for CCNx1.0

 [FIXED_HEADER OCTET[8]]
 (ManifestMsg/T_MANIFEST
   (Name/T_NAME ...)
   (HashGroup/T_HASHGROUP
      (MetaData/T_HASHGROUP_METADATA
         (HGLocator/T_HASHGROUP_METADATA_LOCATOR (T_NAME ...))
         (HGOverallByteCount/T_HASHGROUP_METADATA_BYTECOUNT INT)
         (HGOverallDataDigest/T_HASHGROUP_METADATA_DATADIGEST OCTET[32])
      )
      (SizeDataPtr/T_HASHGROUP_SIZEDATAPTR OCTET[8] (T_HASH ...))
      (SizeMfstPtr/T_HASHGROUP_SIZEMANIFESTPTR OCTET[8] (T_HASH ...))
   )
   (BlockHashGroup/T_BLOCKHASHGROUP
      (MetaData/T_HASHGROUP_METADATA (...))
      (DataPtr/T_HASHGROUP_DATAPTR OCTET[32] (T_HASH ...))
      (MfstPtr/T_HASHGROUP_MANIFESTPTR OCTET[32] (T_HASH ...))
   )
 )

   Interest: name is locator, use objHashRestriction as selector.

4.2.  Example 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 ...))
           (OverallDataDigest/0xC4 OCTET[32])
           (OverallByteCount/0xC5 INT)
         )
         (DataPtr/0xC1 OCTET[8] OCTET[32])
         (MfstPtr/0xC2 OCTET[8] OCTET[32])
         (SizeDataPtr/0xC3 OCTET[32])
         (SizeMfstPtr/0xC4 OCTET[32])
       )
     )
     (SignatureInfo/0x16 ...)
     (SignatureValue/0x17 ...)
   )

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

5.  Security Considerations

   None.

6.  References

6.1.  Normative References

   [CCNxMessages]
              PARC, ., LinkedIn, ., and . PARC, "CCNx Messages in TLV
              Format", n.d., <https://datatracker.ietf.org/doc/draft-
              irtf-icnrg-ccnxmessages/>.

6.2.  URIs

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

Authors' Addresses

   Christian Tschudin
   University of Basel

   Email: christian.tschudin@unibas.ch



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   Christopher A. Wood
   University of California Irvine

   Email: woodc1@uci.edu















































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