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ICNRG                                                           M. Mosko
Internet-Draft                                                      PARC
Intended status: Experimental                                C. Tschudin
Expires: June 16, 2017                               University of Basel
                                                       December 13, 2016


                ICN "Begin-End" Hop by Hop Fragmentation
                 draft-mosko-icnrg-beginendfragment-02

Abstract

   This document describes a simple hop-by-hop fragmentation scheme for
   ICN and mappings to the CCNx 1.0 and NDN packet formats, called
   "begin-end fragmentation".  This scheme may be used at Layer 3 when
   ICN packets are used natively over a Layer 2 media which does not
   reorder packets.

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
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   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 June 16, 2017.

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
   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



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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   2.  Abstract Description of the Begin-End-Fragment Protocol  . . .  5
     2.1.  Initialization . . . . . . . . . . . . . . . . . . . . . .  6
       2.1.1.  Examples . . . . . . . . . . . . . . . . . . . . . . . 11
     2.2.  Sender Protocol  . . . . . . . . . . . . . . . . . . . . . 12
     2.3.  Receiver Protocol  . . . . . . . . . . . . . . . . . . . . 12
   3.  Ethernet as a common use case  . . . . . . . . . . . . . . . . 14
   4.  CCNx 1.0 Fragment Protocol Description . . . . . . . . . . . . 15
     4.1.  Initialization examples  . . . . . . . . . . . . . . . . . 17
     4.2.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.3.  CCNx 1.0 Frame Packing . . . . . . . . . . . . . . . . . . 22
   5.  NDN Fragment Protocol Description  . . . . . . . . . . . . . . 24
     5.1.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . 25
     5.2.  NDN Frame Packing  . . . . . . . . . . . . . . . . . . . . 25
     5.3.  Assigned Numbers for NDN Begin-End fragmentation . . . . . 25
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
     7.1.  CCNx Packet Type Registry  . . . . . . . . . . . . . . . . 27
     7.2.  CCNx Message Registry  . . . . . . . . . . . . . . . . . . 27
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 29
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31





















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1.  Introduction

   In the past, there were two known hop-by-hop fragmentation schemes
   for ICN packets: The one described in NDNLP [NDNLP] as "indexed
   fragmentation" and the one implemented in CCN-lite [CCNLite], using
   the old ccnb encoding.  In a first part, this document describes a
   third, hop-by-hop fragmentation protocol in an encoding-neutral way.
   In a second part, we show mappings of this "begin-end fragmentation
   scheme" to the CCNx Messages in TLV Format [CCNMessages] and the NDN
   TLV [NDN] encoding.  Thirdly, possible extensions and their encodings
   are discussed, for example reporting link reliability or link ARQ
   schemes such as windowing protocols.

   The proposed hop-by-hop "begin-end fragments" scheme may be used at
   Layer 3 when large ICN messages are to be natively sent over a Layer
   2 media with a small MTU.  In cases where ICN packets are carried
   over an existing Layer 3 protocol, such as IP, the Information
   Centric Network SHOULD use that protocol's native fragmentation.

   This proposed fragmentation scheme is an adaptation of PPP Multilink
   PPP Multilink [RFC1990] fragmentation between peers identified by
   their Layer 2 identity.  It is appropriate for standard Layer 2 media
   that guarantee in-order packet delivery.

   Definitions:

   o  (Network Protocol) Packet: A layer 3 ICN datagram, such as a
      Content Object or Interest, which is too large to be transmitted
      over a given L2 technology.

   o  Fragment: The datagram containing all serialized data fields
      required by the proposed fragmentation protocol.  Depending on the
      mapping, the fragment will contain these fragment protocol
      specific data but also, for example, a CCNx fixed header, optional
      Per-Hop-Headers and/or and validator fields like checksums or
      signatures.

   o  Fragment Header: The serialized CONTROL data structures of the
      proposed fragmentation protocol plus mapping specific bits.

   o  Fragment Data (or payload): The portion of the original Network
      Protocol Packet that is carried in the Fragment.

   o  Frame: A layer-2 frame in which the Fragment will be transferred.

   Fragments are represented as 32-bit wide words using ASCII art.
   Because of the Type-Length-Value encoding used (TLV) and optional
   fields or sizes, there is no concise way to represent all



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   possibilities.  We use the convention that ASCII art fields enclosed
   by vertical bars "|" represent exact bit widths.  Fields with a
   forward slash "/" are variable bit widths, which we typically pad out
   to word alignment for picture readability.

   TODO -- we have not adopted the Requirements Language yet.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].







































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2.  Abstract Description of the Begin-End-Fragment Protocol

   A Fragment is defined as the following fields (plus any additional
   fields required by the wire formatting in which fragments are
   encoded):

 Fragment       = FragProtocolID FragProtocolData
 FragProtocolID = <BeginEndFragmentsProtocolID> | (other protocols' ids)
 FragProtocolData = BeginEndFragmentsData | (other protocols' data)
 BeginEndFragmentsData = Flags FragSeqNo FragLength FragData
 Flags          = B / E / BE / I
 B              = <begin flag>
 E              = <end flag>
 BE             = <begin and end flag>
 I              = <idle flag>
 FragSeqNo      = 1*OCTETS
 FragLength     = <Octets of fragment data>
 FragData       = <Continuous octets (portion of Packet)>

   The fragmentation protocol is run between a sender and a "peer",
   which can be one or more, potentially passive, receivers.  They
   execute first an Initialization Protocol (Section 2.1), then use a
   Sender Protocol (Section 2.2) and Receiver Protocol (Section 2.3) to
   exchange frames.

   The initialization protocol uses a reliable messages exchange to
   reset the the FragSeqNo to 0 on both peers.  This ensures that when
   one or another peer restarts both peers will reset their state.

   The sender breaks a packet P (typically an Interest- or Content
   packet in the embedding wire encoding) into one or more fragments
   which are tagged with monotonically increasing sequence numbers.  The
   B, E and BE flags are used to signal the start of a fragment series
   (B), its end (E), or a single fragment (BE) for the given packet P.

   It is advisable that the 'B' fragment contains enough information in
   its Fragment Data to let the receiver know the total length of the
   packet to be reconstructed (and size of the reassembly buffer to be
   allocated) and the type of the expected packet.

   The receivers listen to the fragment stream and reconstruct from a
   valid fragment series the original packet, and reject fragments with
   invalid sequence numbers, flags, or validation data.

   The 'I' flag allows the sender to send idle frames that do not
   contain any Fragment Data, but do increment the fragment sequence
   number.  This is useful on lossy links to indicate that the sender is
   past the end of the previous packet in case the 'E' fragment was



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   lost.  Moreover, as a possible extension of the protocol, this allows
   for periodic keepalives, measuring for example link quality when
   there is no other traffic to send.

2.1.  Initialization

   Whenever a peer begins operation, it must reliably reset the sequence
   number space.  If the underlying link already ensures a complete
   reset of both peers, that method MAY be used.  Otherwise, the method
   presented here SHOULD be used.

   1.   For each peer, a node tracks its local state, S_LOCAL[peer], and
        its peer's state S_REMOTE[peer].  A state may be INIT or Sync or
        OK, though only certain combinations are possible.

   2.   A node tracks its FragSeqNo as FSN_LOCAL[peer] and the next
        expected FragSeqno from its peer as FSN_REMOTE[peer].

   3.   A node remembers two numbers N_LOCAL[peer] and N_REMOTE[peer].
        N_LOCAL is the node's own sequence number and N_REMOTE is the
        one learned from a peer.  These two numbers identify the current
        reboot of a node and serve as computation identifiers.  They may
        be sequence numbers or random numbers or taken from some other
        source that is unlikely to repeat reboot to reboot.

   4.   A RESET message carries a nodes N_LOCAL[peer] value and informs
        the peer that it should reset it's fragment sequence numbers.

   5.   Upon receiving a RESET, a node will set N_REMOTE[peer] to the
        RESET number and will send a RESETACK message to acknowledge the
        RESET and to inform the peer of it's own N_LOCAL[peer] number.
        Thus a RESET mesage carries one value and a RESETACK carries two
        values.

   6.   We will drop the [peer] subscript from the state variables in
        the following with the understanding that this protocol executes
        per pair of peers.

   7.   If a node is not in the OK/OK state, it MUST send only RESET or
        RESETACK messages.

   8.   Upon sending a first RESET message, the sending node starts a
        retransmit timer for the peer.  It begins at RESET_TIMOUT (50
        msec).

   9.   At each RESET timeout, a node sets a new timeout as twice the
        previous timeout.  If it is less than MAX_TIMEOUT (4 seconds),
        it sends {RESET, N_LOCAL} again and starts a reset timer with



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        the new timeout.

   10.  Upon receiving a valid RESETACK that matches a node's N_LOCAL,
        the RESET timeout is set back to RESET_TIMEOUT.

   The ccnx-beginendfragment-sim [python-sim] software package is a
   Python simulator of the sequence number reset algorithm.  It uses
   simple discrete event simulator to validate the reset algorithm under
   various conditions of delay and packet loss and node reboots.

   Figures 1 - 3 show the reset state machine.  In the figures, we use
   short abbreviations for compactness.  N_L is N_LOCAL, N_R is
   N_REMOTE, FSN_L is FSN_LOCAL, FSN_R is FSN_REMOTE.  We also
   abbreviate RESET as RST and RESTACK as RSTACK.  Double line boxes
   indicate a potentially blocking state that responds to external
   events (timeouts or packet reception).  Single line boxes indicate
   transient states the perform some operation.

   Figure 1 shows the initial state of a system as REBOOT.  REBOOT state
   will ignore any received packets and initialize the state variables.
   After rebooting, a node will be in the INIT, INIT state.  From here,
   it may either receive a RESET message from its peer or send its own
   RESET message.  In the first case, it proceeds to X2 (the slave
   state), in the second case it proceeds to X1 (the master state).

   Figure 2 shows the master state, where the system sends out a RESET
   before receiving a RESET from a peer.  In state SYNC, INIT, the node
   is waiting for either a RESETACK or a RESET from its peer.  If it
   does not receive either before its timeout, it will double its
   timeout and send another RESET

   Figure 3 shows the slave state, where the system has received a RESET
   from its peer before it has sent its own RESET.  This puts the node
   in the INIT, OK state.  Typically, a node will immediately send its
   own RESET message, though the RESETACK its sends has the same effect.
   This puts the node in the SYNC, OK state.  One of three events can
   happen: a timeout to send another RESET message, receive a RESETACK
   or receive another RESET.  Upon receiving a valid RESETACK, the node
   can move to the OK, OK state.












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     +-------------------------+
     |                         v
   +-------+  recv any       #===============#
   | Drop  | <-------------- H    Reboot     H
   +-------+                 #===============#
                               |
                               |
                               v
                             +---------------+
                             | N_L=N, N_R=0, |
                             | FSN_R=FSN_L=0 |
                             +---------------+
                               |
                               |
                               v
   .........  recv (RST M)   #===============#
   : To X2 : <-------------- H  Init, Init   H
   :.......:                 #===============#
                               |
                               |
                               v
                             .................
                             :     To X1     :
                             :...............:

                          Figure 1: Reboot State

























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                 ......        +-------------------+
                 : X1 : ---->  |  Send RST(N_L)    | <--------+
                 :....:        +-------------------+          |
                                       |                      |
                                       v                      |
                               +-------------------+   +--------------+
                               |    Start Timer    |   | Inc. Timeout |
                               +-------------------+   +--------------+
                                       |                      ^
                       recv(RST M)     v                      |
  +-------------------+        #===================#  timeout |
  |     N_R = M       | <----- H    Sync, Init     H ---------|
  +-------------------+        #===================#
        |                              | recv(RSTACK N,M)
        v                              v
     .........                 +-------------------+  no    +----------+
     : To X4 :                 |     If N=N_L      | -----> |   Drop   |
     :.......:                 +-------------------+        +----------+
                                       | yes
                                       v
                               +-------------------+
                               |   Cancel Timer    |
                               |   Reset Timeout   |
                               |      N_R = M      |
                               +-------------------+
                                       |
                                       v
                               +-------------------+
                               |       Send        |
                               | RSTACK(N_R, N_L)  | <+
                               +-------------------+  |
                                        |             | yes
                  recv RSTACK(N, M)     v             |
                .........      #===================#  |
                : To X6 : <--- H      OK, OK       H  |
                :.......:      #===================#  |
                                        | recv RST(M) |
                                        v             |
                .........  no  +-------------------+  |
                : To X3 : <--- |     If M=N_R      | -+
                :.......:      +-------------------+

                          Figure 2: Master State








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    ........       +-------------------+
    :  X2  : ----> |       N_R=M       | <----+
    :......:       +-------------------+      |
                       |                      | recv (RST M)
                       v                      |
                   +-------------------+    #===================#
                   |       Send        | -> H     Init, OK      H
                +> |  RSTAK(N_R, N_L)  |    #===================#
                |  +-------------------+      |
                |                             |
                |  +-------------------+      |
                |  |   Send RST(N_L)   | <----+
                |  |    Start Timer    | <------------------------+
                |  +-------------------+                          |
                |    |                                            |
                |    v                                     recv   |
                |  #====================================# (RST M) |
  +-------------+> H            Sync, OK                H --------+---+
  |             |  #====================================#         |   |
  |             |      | recv (RSTAK N,M)   | timeout             |   |
  |             |      v                    |                     |   |
  |  ........   |    +-----------------+    |      +---------------+  |
  |  :  X6  : --+--> |   If N=N_L      |    +----> | Inc. Timeout  |  |
  |  :......:   |no  +-----------------+           +---------------+  |
  |  +------+   |     | no        | yes                    +----------+
  |  | Drop | <-+-----+           v                        v
  |  +------+   |  +-------------------+           +---------------+
  |             |  |   Cancel Timer    |           | If M=N_REMOTE | yes
  |             |  |   Reset Timeout   |           |               | -+
  |             |  +-------------------+           +---------------+  |
  |             |      |                             | no             |
  |  .........  |      v                             v                |
  |  :   To  :  |    +-----------------+           +---------------+  |
  |  : OK,OK :<-+--- |  If M=M_REMOTE  |           | Cancel Timer  |  |
  |  :.......:  |yes +-----------------+           +---------------+  |
  |             |      | no                          |                |
  |             |      v                             |                |
  |             |  +-------------------+             |                |
  |  ........   +- |      N_R=M,       | <-----------+                |
  |  :  X3  : ---> |   FSN_R=FSN_L=0   |                              |
  |  :......:      +-------------------+                              |
  +--------------------- +    +-------------------+                   |
               ........  +--< |       Send        |                   |
               :  X4  : ----> |  RSTAK(N_R, N_L)  | <-----------------+
               ........       +-------------------+

                           Figure 3: Slave State




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2.1.1.  Examples

   The first example, Figure 4 shows an example message sequence diagram
   of the initialization process.  The "T" column is Time, The "S"
   column is for S_LOCAL and S_REMOTE, the N column is for N_LOCAL and
   N_REMOTE, and the FSN column is for FSN_LOCAL and FSN_REMOTE.  We
   show both peers, one on the left side of the figure and one on the
   right side; call them nodes "A" and "B".  We only show an entry in
   the table when state changes.

   o  T=0: A initializes and sets its state values as shown, picking the
      message sequence number 5.  It sends the message {RESET, 5} to B.
      It starts a RESET timer.

   o  T=2: Node B initializes, picking message sequence number 7.  It
      sends {RESET, 7} to A. It starts a RESET timer.

   o  T=3: Node A receives {RESET, 7}.  It sets S_REMOTE to OK and
      records N_REMOTE as 7.  It sets FSN_LOCAL and FSN_REMOTE to 0.

   o  T=4: Node A sends {RESET_ACK, 7} to node B.

   o  T=5: Node B reeives {RESET_ACK, 7}, so it sets S_LOCAL to OK.  It
      cancels its RESET timer.

   o  T=6: Node A's RESET timer expires and it re-sends {RESET, 5} to B.

   o  T=7: Node B receives {RESET, 5}.  It records N_REMOTE as 5 and
      sets S_REMOTE to OK.  At this point, Node B is in OK/OK state and
      may begin sending data.

   o  T=8: Node B sends {RESET_ACK, 5}.

   o  T=9: Node B begins sending data to A.

   o  T=9: Node A reveives {RESET_ACK, 5} and sets S_LOCAL to OK.  It is
      now in OK/OK state and may begin sending data to B.

   o  T=10: Node A begins sending data to B.












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   |---|-----|-----|-----|                        |-----|-----|-----|
   | T |  S  |  N  | FSN |                        |  S  |  N  | FSN |
   |   | L R | L R | L R |                        | L R | L R | L R |
   |---|-----|-----|-----|                        |-----|-----|-----|
   | 0 | I I | 5 0 | 0 0 | >-{RESET, 5}--------->X|    not ready    |
   |   |     |     |     |                        |     |     |     |
   | 2 |     |     |     | <---------{RESET, 7}-< | I I | 7 0 | 0 0 |
   | 3 |   OK| 5 7 | 0 0 |                        |     |     |     |
   | 4 |     |     |     | >-{RESET_ACK, 7, 5}--> |     |     |     |
   | 5 |     |     |     |                        |OK   | 7 0 | 0 0 |
   | 6 |     |     |     | >-{RESET, 5}---------> |     |     |     |
   | 7 |     |     |     |                        |   OK|   5 | 0 0 |
   | 8 |     |     |     | <---{RESET_ACK, 5,7}-< |     |     |     |
   | 9 |OK   |     |     | <-------------{data}-< |     |     |     |
   | 10|     |     |     | >-{data}-------------> |     |     |     |
   |---|-----|-----|-----|                        |-----|-----|-----|

                                 Figure 4

2.2.  Sender Protocol

   1.  A sender maintains a separate state machine for each peer.

   2.  When a peering is established, the FragSequenceNumber begins at
       0.

   3.  After sending a Fragment, FragSequenceNumber is incremented by
       one.

   4.  In the first fragment for a packet, set the B bit to '1'.

   5.  In the last fragment for a packet, set the E bit to '1'.

   6.  Both the B and E bits must be set to '1' for a single fragment.

   7.  If both the B and E and I bits are not set, the fragment is in
       the middle of a series.

   8.  When not sending a fragment (with fragment data), the sender may
       send an Idle fragment with only the 'I' bit set.  This indicates
       that the sender has no packet to send.  Idle frames may only be
       sent in between E and B frames.

2.3.  Receiver Protocol

   1.  A receiver maintains one reassembly queue per sender.





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   2.  Discard Idle fragments.

   3.  Discard fragments until a 'B' fragment is received.  Store the
       received sequence number for this sender.

   4.  If an out-of-order fragment is received next, discard the
       reassembly buffer and go to step (2).

   5.  Continue receiving in-order fragments until the first 'E'
       fragment.  At this time, the fragmented packet is fully re-
       assembled and may be passed on to the next layer.

   6.  The receiver cannot assume it will receive the 'E' fragment or a
       subsequent 'I' frame, so it should use a timeout mechanism
       appropriate to the link to release preallocated memory resources.




































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3.  Ethernet as a common use case

   We expect that Ethernet will be the most common L2 technology where
   the proposed ICN fragmentation will be used, therefore we briefly
   elaborate on how fragmentation functions with the broadcast and
   multi-protocol nature of Ethernet.

   When the fragmentation protocol is used with Ethernet, each
   participant uses the tuple {source mac, destination mac, ethertype}
   to identify a send or receive buffer and FragSequenceNumber number
   space.

   If the fragmentation protocol is using a group address destination,
   each group address is considered a "peer" with its own
   FragSequenceNumber.  For example, the MAC address 0x01005E0017AA on
   EtherType 0x0801 is the CCNx assigned group address for its
   224.0.23.170 IP multicast address.  Each sender would maintain a
   FragSequenceNumber for that peer.  Each receiver would maintain a
   separate reassembly buffer for that group address based on the sender
   and ethertype.

   If using other Ethernet encapsulations, such as 802.1AE MacSec, one
   could use a security identifier in place of the {source, destination,
   ethertype} tuple.



























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4.  CCNx 1.0 Fragment Protocol Description

   The hop-by-hop fragmentation protocol introduces a new CCNx 1.0
   Packet Type called PT_FRAG and uses new fields in the Fixed Header.
   The hop-by-hop headers of a CCNx 1.0 fragment may be used for
   purposes like link quality reporting or a reliable ARQ scheme, which
   are out-of-scope of this document.

   We describe a basic hop-by-hop fragmentation header, using bits in
   the Fixed Header for the fragment encoding.  We also describe an
   extended version with variable sequence number size that puts the
   fragmentation header in the body of the CCNx message.  This allows
   the fragmentation header to be signed or covered by a MIC.  The
   extended encoding sets the 'X' flag to 1 in the Fixed Header,
   otherwise it is the basic encoding.

   The "hop-by-hop fixed header" follows the normal conventions: The
   Version, PacketLength, and HeaderLength fields are as per CCNx
   Messages in TLV Format [CCNMessages].  The PacketType is set to
   PT_FRAG.  However, in the packet-type dependent fields, we reserve 4
   bits for flags and 20 bits for a sequence number.

   The "message part" of the CCNx 1.0 fragment carries the fragment data
   in its own TLV block.  The message part may also contain standard
   CCNx validation algorithm and validation bits in subsequent TLV
   blocks.  In this way, the fragment can be covered by a CRC32C
   checksum or stronger validation methods.

   The new TLV type T_FRAGMENT wraps the fragment bytes.  It is a top-
   level message TLV, similar to T_INTEREST or T_OBJECT.

   For sequence number reset, the new TLV types T_FRAG_RESET and
   T_FRAG_RESET_ACK carry the initialization message sequence number.
   They MUST appear in an Idle (I) frame.  A packet MUST NOT have
   T_FRAGMENT if it has one of these fields.  A single packet may have
   both a T_FRAG_RESET and a T_FRAG_RESET_ACK.  These fields appear in
   the CCNx message section.  It is RECOMMENDED to use a 64-bit sequence
   number, though an implementation MAY use any length appropriate to
   the media.

   T_FRAG_RESET and T_FRAG_RESET_ACK may be in either basic header or
   extended header packets.  In extended header format, the
   T_FRAG_HEADER element MUST be length 1, which conveys only the flags.








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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |    Version    |    PT_FRAG    |         PacketLength          |
   +---------------+---------------+---------------+---------------+
   |X|B|E|I|          FragSequenceNumber           | HeaderLength  |
   +---------------+---------------+---------------+---------------+
   / Optional Hop-by-hop header in TLV format                      /
   +---------------+---------------+---------------+---------------+
   | Fragment Data TLV                                             /
   +---------------+---------------+---------------+---------------+
   / Optional CCNx ValidationAlgorithm TLV                         /
   +---------------+---------------+---------------+---------------+
   / Optional CCNx ValidationPayload TLV (ValidationAlg required)  /
   +---------------+---------------+---------------+---------------+

   o  X: Extended Format (X=0 shown above)

   o  B: Begin flag.

   o  E: End flag.

   o  I: Idle flag.

   o  FragSequenceNumber: a 20-bit sequence number to identify the
      fragment (see below).

   The FragSequenceNumber follows Serial Number Arithmetic [RFC1982] for
   a 20-bit serial number.  This means we have 19 bits of "valid"
   sequence number space, or 524,288 fragments.  The packets per second
   for a 10 Gbps link with 1500 bytes Ethernet frames is 833,333 packets
   per second.  Therefore, the 20-bit sequence number space allows for
   629 milliseconds of frames.

   In CCNx 1.0, the maximum encapsulated length is 64 KB -- which
   requires under 50 PT_FRAG frames of 1500 bytes, depending on the
   HeaderLength and validation options.  So the valid sequence number
   space (when e.g. used over Ethernet) is approximately 10,500 maximum
   size (network protocol) packets.

   If a PT_FRAG packet has optional hop-by-hop headers, the
   implementation should pass the fragment to the appropriate subsystem
   to process those headers before discarding the fragment.

   The Extended encoding (X=1) moves the fragment header fields (= flags
   and sequence number) to the CCNx packet's message part, so they are
   covered by any ValidationAlgorithm used on the packet.  It also
   allows for variable length sequence numbers.  In the example shown



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   below, there is a 7-byte (56-bit) sequence number.

   The Extended encoding also allows different fragmentation protocols
   to co-exist by changing the opening TLV type from T_FRAG_HEADER to a
   new type.

   The first 8 bytes of the first fragment are the FixedHeader of the
   encapsulated Packet, so one may learn the overall length of the
   Packet from that FixedHeader.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |    Version    |    PT_FRAG    |         PacketLength          |
   +---------------+---------------+---------------+---------------+
   |1|0|0|0|                 Reserved              | HeaderLength  |
   +---------------+---------------+---------------+---------------+
   / Optional Hop-by-hop header in TLV format                      /
   +---------------+---------------+---------------+---------------+
   |          T_FRAG_HEADER        |               8               |
   +---------------+---------------+---------------+---------------+
   |1|B|E|I|0|0|0|0|           FragSequenceNumber                  /
   /                                                               /
   +---------------+---------------+---------------+---------------+
   | Fragment Data TLV                                             /
   +---------------+---------------+---------------+---------------+
   / Optional CCNx ValidationAlgorithm TLV                         /
   +---------------+---------------+---------------+---------------+
   / Optional CCNx ValidationPayload TLV (ValidationAlg required)  /
   +---------------+---------------+---------------+---------------+

4.1.  Initialization examples

   This section presents examples of initialization packets.

















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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |    Version    |    PT_FRAG    |         PacketLength          |
   +---------------+---------------+---------------+---------------+
   |0|0|0|1|                0                      | HeaderLength  |
   +---------------+---------------+---------------+---------------+
   / Optional Hop-by-hop header in TLV format                      /
   +---------------+---------------+---------------+---------------+
   | T_FRAG_RESET (optional)       |               8               |
   +---------------+---------------+---------------+---------------+
   / 64-bit N_LOCAL value                                          /
   +---------------+---------------+---------------+---------------+
   | T_FRAG_RESET_ACK (optional)   |               8               |
   +---------------+---------------+---------------+---------------+
   / 64-bit N_LOCAL value                                          /
   / 64-bit N_REMOTE value                                         /
   +---------------+---------------+---------------+---------------+
   / Optional CCNx ValidationAlgorithm TLV                         /
   +---------------+---------------+---------------+---------------+
   / Optional CCNx ValidationPayload TLV (ValidationAlg required)  /
   +---------------+---------------+---------------+---------------+





























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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |    Version    |    PT_FRAG    |         PacketLength          |
   +---------------+---------------+---------------+---------------+
   |1|0|0|0|                 Reserved              | HeaderLength  |
   +---------------+---------------+---------------+---------------+
   / Optional Hop-by-hop header in TLV format                      /
   +---------------+---------------+---------------+---------------+
   |          T_FRAG_HEADER        |               1               |
   +---------------+---------------+---------------+---------------+
   |1|0|0|1|0|0|0|0|
   +---------------+---------------+---------------+---------------+
   | T_FRAG_RESET (optional)       |               8               |
   +---------------+---------------+---------------+---------------+
   / 64-bit N_LOCAL value                                          /
   +---------------+---------------+---------------+---------------+
   | T_FRAG_RESET_ACK (optional)   |               8               |
   +---------------+---------------+---------------+---------------+
   / 64-bit N_LOCAL value                                          /
   / 64-bit N_REMOTE value                                         /
   +---------------+---------------+---------------+---------------+
   / Optional CCNx ValidationAlgorithm TLV                         /
   +---------------+---------------+---------------+---------------+
   / Optional CCNx ValidationPayload TLV (ValidationAlg required)  /
   +---------------+---------------+---------------+---------------+

4.2.  Example

   We present a complete example of the basic fragment encoding for a
   2KB Content Object for 1500 byte frames according to the protocol
   described in this draft (with clear X-flag).  The original 2KB packet
   also has an RSA signature, but this cannot easily be used for
   integrity checking as the receiver may not have the appropriate key
   and it is an expensive operation.  We therefore chose to use a CRC32C
   validator on each fragment.  The Content Object has the name ccnx:/
   abcd.  First, the original 2000 byte packet is shown in entirety.














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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |       1       |       2       |              2000             |
   +---------------+---------------+---------------+---------------+
   |            Reserved           |     Flags     |       20      |
   +---------------+---------------+---------------+---------------+
   |         T_CACHETIME           |               8               |
   +---------------+---------------+---------------+---------------+
   |                                                               |
   |                    Recommended Cache Time                     |
   +---------------+---------------+---------------+---------------+
   |       T_CONTENTOBJECT         |              1508             |
   +---------------+---------------+---------------+---------------+
   |            T_NAME             |               8               |
   +---------------+---------------+---------------+---------------+
   |         T_NAMESEGMENT         |               4               |
   +---------------+---------------+---------------+---------------+
   |       a                b              c              d        |
   +---------------+---------------+---------------+---------------+
   |           T_PAYLOAD           |              1492             |
   +---------------+---------------+---------------+---------------+
   /                        Payload Contents                       /
   +---------------+---------------+---------------+---------------+
   |       T_VALIDATION_ALG        |              204              |
   +---------------+---------------+---------------+---------------+
   |          T_RSA-SHA256         |              200              |
   +---------------+---------------+---------------+---------------+
   |             T_KEYID           |               32              |
   +---------------+---------------+---------------+---------------+
   /                            KeyId                              /
   /---------------+---------------+-------------------------------+
   |          T_PUBLICKEY          |              160              |
   +---------------+---------------+---------------+---------------+
   /                Public Key (DER encoded SPKI)                  /
   +---------------+---------------+---------------+---------------+
   |     T_VALIDATION_PAYLOAD      |              256              |
   +---------------+---------------+---------------+---------------+
   /                          RSA Signature                        /
   +---------------+---------------+---------------+---------------+

   The 2000 byte packet will be fragmented into two pieces.  In the
   first fragment, there is 28 bytes of overhead (fixed header 8,
   T_STD_FRAGMENT 4, validation 16), so the fragment's payload size is
   1472 bytes.  In the second packet, the T_FRAGMENT block carries the
   remaining data 528 bytes, hence the overall packet size is 556 bytes
   due to the same 28 bytes of overhead.  We used FragSequenceNumber "0"
   and "1" for the two fragments.



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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |       1       |    PT_FRAG    |              1500             |
   +---------------+---------------+---------------+---------------+
   |0|1|0|0|                  0x00000              |       8       |
   +---------------+---------------+---------------+---------------+
   |          T_FRAGMENT           |             1472              |
   +---------------+---------------+---------------+---------------+
   |       1       |       2       |              2000             |
   +---------------+---------------+---------------+---------------+
   |            Reserved           |     Flags     |       20      |
   +---------------+---------------+---------------+---------------+
   |         T_CACHETIME           |               8               |
   +---------------+---------------+---------------+---------------+
   |                                                               |
   |                    Recommended Cache Time                     |
   +---------------+---------------+---------------+---------------+
   |       T_CONTENTOBJECT         |              1508             |
   +---------------+---------------+---------------+---------------+
   |            T_NAME             |               8               |
   +---------------+---------------+---------------+---------------+
   |         T_NAMESEGMENT         |               4               |
   +---------------+---------------+---------------+---------------+
   |       a                b              c              d        |
   +---------------+---------------+---------------+---------------+
   |           T_PAYLOAD           |              1492             |
   +---------------+---------------+---------------+---------------+
   /             Payload Contents  (1432 out of 1492 bytes)        /
   +---------------+---------------+---------------+---------------+
   |      T_VALIDATION_ALG         |               4               |
   +---------------+---------------+---------------+---------------+
   |            T_CRC32            |               0               |
   +---------------+---------------+---------------+---------------+
   |     T_VALIDATION_PAYLOAD      |               4               |
   +---------------+---------------+---------------+---------------+
   |                         CRC32C Value                          |
   +---------------+---------------+---------------+---------------+













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                       1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |       1       |    PT_FRAG    |              556              |
   +---------------+---------------+---------------+---------------+
   |0|0|1|0|                  0x00001              |       8       |
   +---------------+---------------+---------------+---------------+
   |          T_FRAGMENT           |             528               |
   +---------------+---------------+---------------+---------------+
   /                Payload Contents  (last 60 bytes)              /
   +---------------+---------------+---------------+---------------+
   |       T_VALIDATION_ALG        |              204              |
   +---------------+---------------+---------------+---------------+
   |          T_RSA-SHA256         |              200              |
   +---------------+---------------+---------------+---------------+
   |             T_KEYID           |               32              |
   +---------------+---------------+---------------+---------------+
   /                            KeyId                              /
   /---------------+---------------+-------------------------------+
   |          T_PUBLICKEY          |              160              |
   +---------------+---------------+---------------+---------------+
   /                Public Key (DER encoded SPKI)                  /
   +---------------+---------------+---------------+---------------+
   |     T_VALIDATION_PAYLOAD      |              256              |
   +---------------+---------------+---------------+---------------+
   /                          RSA Signature                        /
   +---------------+---------------+---------------+---------------+
   |      T_VALIDATION_ALG         |               4               |
   +---------------+---------------+---------------+---------------+
   |            T_CRC32            |               0               |
   +---------------+---------------+---------------+---------------+
   |     T_VALIDATION_PAYLOAD      |               4               |
   +---------------+---------------+---------------+---------------+
   |                         CRC32C Value                          |
   +---------------+---------------+---------------+---------------+

4.3.  CCNx 1.0 Frame Packing

   A sender/receiver pair may multiplex non-fragmentation frames on the
   same link.  For example, in CCNx 1.0, there may be some PacketType
   PT_FRAG frames and some plain PT_INTEREST or PT_CONTENTOBJECT frames
   on the same link between the same pairs.  PT_FRAG frames are
   considered independently of other frames between the pair.

   Because each CCNx 1.0 datagram with a Fixed Header has all
   information needed for framing, two peers may pack multiple CCNx 1.0
   datagrams in to one Layer 2 frame.  For example, if there are several
   small Interests queued back-to-back, they could be encapsulated in a



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   single Ethernet frame, up to the maximum Ethernet payload.

   At the extreme, a peer may use fragmentation for all packets and
   completely pack each Layer 2 frame.  The tail fragment would be cut
   off at whatever byte length fits the remaining Layer 2 frame.

   Example: Assume that the outgoing queue for a specific peer has the
   following four packets to be sent:

   interest1(200B)
   content1(3500B)
   interest2(200B)
   content2(500B)

   With 12 bytes of overhead per fragment these four packets could be
   fragmented and packeted into three Ethernet frames as:

   [frag(interest1,200B), frag(content1, 1276B)]
   [frag(content1, 1488B)]
   [frag(content1, 236B), frag(interest2, 200B), frag(content2, 500B)]































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5.  NDN Fragment Protocol Description

   The Begin-End fragmentation protocol described in this draft extends
   the NDN link protocol v2 packet format (NDNLPv2).  Note that this
   extension is not an official part of the NDN suite, at this point in
   time.

   NDNLPv2 packets have a start type NDNLP-TYPE which distinguishes them
   from the classic Interest and Data packets.  Inside the NDNLPv2 TLV
   structure, a sequence of NDNLPv2 header fields precede the payload
   (fragment data) which is introduced by the type value NND-FRAGMENT-
   TYPE.

          NDNpacket      ::= Interest | Data | NDNLP
          NDNLP          ::= NDNLP-TYPE TLV-LENGTH
                               NDNLPhdrFields*
                               NDNLPfragment?
          NDNLPhdrFields ::= BeginEndField | (other NDNLP header fields)
          BeginEndField  ::= BEGIN-END-FIELD-TYPE TLV-LENGTH
                               BYTE BYTE+
          NDNfragment    ::= NDN-FRAGMENT-TYPE TLV-LENGTH
                               BYTE+

   The extension for the "begin-end" fragmentation scheme relies on a
   new header field with type value NDN-BEGIN-END-FIELD-TYPE: The
   presence of this field marks a NDNLPv2 packet as a "begin-end"
   fragment.  The field's value is 1 to 8 bytes long and consists of 2
   flag bits (most-significant bits) plus a sequence number (remaining
   less-significant bits).

   For a sender/receiver pair and for a given direction, the value of
   the BeginEndField is of constant size.  But depending on the start
   configuration, different sizes can be chosen for operations, both in
   time and for the different directions.

   In the smallest possible setup (e.g. sensor network with very small
   MTUs), the BeginEndField can have a one-byte value (2 flag bits plus
   6 sequence number bits).  For Ethernet, it it is recommended to use a
   3-byte value (2 flag bits plus 22 sequence number bits).

   An idle fragmentation frame is encoded as a NDNLP packet with a
   Begin-End Field but no NDNfragment element.  Both the B- and the
   E-flags should be set to 1 in this case.

   The frist bytes of the first fragment are the outermost NDN TLV of
   the encapsulated Packet.  One may learn the overall length from the
   the outermost TLV length.




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5.1.  Example

   We present an example of the basic fragment encoding for a payload of
   size larger than 253 Bytes and less than 64KB.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   | type=NDNLP    | len=N+9                                       |
   +---------------+---------------+---------------+---------------+
   | type=BeginEnd | len=3         |B|E|        FragSequence...    |
   +---------------+---------------+---------------+---------------+
   | ..Number      | type=Fragment | len=N                         |
   +---------------+---------------+---------------+---------------+
   |               | FragmentData (N bytes) ...                    /
   +---------------+---------------+---------------+---------------+
   /                                                               /
   +---------------+---------------+---------------+---------------+
   /               |
   +---------------+

   o  B: Begin flag.

   o  E: End flag.

   o  FragSequenceNumber: a 22-bit sequence number to identify the
      fragment.

5.2.  NDN Frame Packing

   A sender/receiver pair may multiplex non-fragmentation frames on the
   same link.  For example, in NDN, there may be some NDNLP frames and
   some plain Interest or Data frames on the same link between the same
   pairs.  NDNLP frames are considered independently of other frames
   between the pair.

   NDNLP does not allow for frame packing: A frame contains only one out
   of the three Interest, Data and NDNLP packet types.

5.3.  Assigned Numbers for NDN Begin-End fragmentation

              NDNLP-TYPE            0x64   // official, might change
              BEGIN-END-FIELD-TYPE  0x5c   // inofficial
              NDN-FRAGMENT-TYPE     0x52   // official, might change







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6.  Acknowledgements


















































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7.  IANA Considerations

   This document adds new values to the CCNx Packet Type Registry and
   CCNx Message Registry.

7.1.  CCNx Packet Type Registry

   Value    CCNx Packet Type                Definition
   ----     ------------------------        ----------
   4        PT_FRAG                         See Section 4

7.2.  CCNx Message Registry

   Value    CCNx Message Type               Definition
   ----     ------------------------        ----------
   5        T_FRAGMENT                      See Section 4
   16       T_FRAG_HEADER                   See Section 4
   17       T_FRAG_RESET                    See Section 4
   18       T_FRAG_RESET_ACK                See Section 4
































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8.  Security Considerations

   This protocol has no security mechanisms and is vulnerable to
   injection attacks by other devices on the same physical link as the
   fragmentation peers.  One should use a secure Layer 2 protocol, such
   as 802.1AE (MacSec) to prevent such attacks.













































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9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/
              RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

9.2.  Informative References

   [CCNLite]  Mosko, M., Plass, M., and C. Tschudin, "CCN-Lite
              fragmentation", Summer 2012, <http://ccn-lite.net>.

   [CCNMessages]
              Mosko, M., Solis, I., and C. Wood, "CCNx Messages in TLV
              Format (Internet draft)", 2016, <http://tools.ietf.org/
              html/draft-irtf-icnrg-ccnxmessages-03>.

   [CCNx]     PARC, Inc., "CCNx Open Source", 2007,
              <http://www.ccnx.org>.

   [NDN]      "NDN specification Documentation, Release 0.1a2",
              March 2014, <http://www.named-data.net/wp-content/uploads/
              2013/11/packetformat.pdf>.

   [NDNLP]    Shi, J. and B. Zhang, "NDNLP: A Link Protocol for NDN",
              NDN Technical Report NDN-0006, July 2012,
              <http://www.named-data.net/techreports.html>.

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              DOI 10.17487/RFC1982, August 1996,
              <http://www.rfc-editor.org/info/rfc1982>.

   [RFC1990]  Sklower, K., Lloyd, B., McGregor, G., Carr, D., and T.
              Coradetti, "The PPP Multilink Protocol (MP)", RFC 1990,
              DOI 10.17487/RFC1990, August 1996,
              <http://www.rfc-editor.org/info/rfc1990>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <http://www.rfc-editor.org/info/rfc3552>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.



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   [python-sim]
              "ccnx-beginendfragment-sim", December 2016,
              <https://github.com/PARC/ccnx-beginendfragment-sim>.
















































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Authors' Addresses

   Marc Mosko
   PARC
   Palo Alto, California  94304
   USA

   Phone: +1 650-812-4405
   Email: marc.mosko@parc.com


   Christian Tschudin
   University of Basel
   Basel  4051
   Switzerland

   Phone: +41 61 267 0557
   Email: christian.tschudin@unibas.ch

































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