ICN "Begin-End" Hop by Hop Fragmentation
draft-mosko-icnrg-beginendfragment-01
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draft-mosko-icnrg-beginendfragment-01
ICNRG M. Mosko
Internet-Draft PARC
Intended status: Experimental C. Tschudin
Expires: January 9, 2017 University of Basel
July 8, 2016
ICN "Begin-End" Hop by Hop Fragmentation
draft-mosko-icnrg-beginendfragment-01
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
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 9, 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Abstract Description of the Begin-End-Fragment Protocol . . . 4
2.1. Initialization . . . . . . . . . . . . . . . . . . . . . 5
2.1.1. Examples . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Sender Protocol . . . . . . . . . . . . . . . . . . . . . 7
2.3. Receiver Protocol . . . . . . . . . . . . . . . . . . . . 8
3. Ethernet as a common use case . . . . . . . . . . . . . . . . 8
4. CCNx 1.0 Fragment Protocol Description . . . . . . . . . . . 9
4.1. Initialization examples . . . . . . . . . . . . . . . . . 11
4.2. Example . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.3. CCNx 1.0 Frame Packing . . . . . . . . . . . . . . . . . 16
5. NDN Fragment Protocol Description . . . . . . . . . . . . . . 17
5.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 18
5.2. NDN Frame Packing . . . . . . . . . . . . . . . . . . . . 19
5.3. Assigned Numbers for NDN Begin-End fragmentation . . . . 19
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
7.1. CCNx Packet Type Registry . . . . . . . . . . . . . . . . 19
7.2. CCNx Message Registry . . . . . . . . . . . . . . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
9.1. Normative References . . . . . . . . . . . . . . . . . . 20
9.2. Informative References . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
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
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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
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 OK.
2. RESET messages carry a sequence number denoted as N_LOCAL[peer]
and N_REMOTE[peer], for the messages in each direction. The
sequence number MAY be a positive random number.
3. A node tracks its FragSeqNo as FSN_LOCAL[peer] and the next
expected FragSeqno from its peer as FSN_REMOTE[peer].
4. We will drop the [peer] subscript from the state variables in
the following with the understanding that this protocol executes
per pair of peers.
5. If S_LOCAL or S_REMOTE is in INIT state, the node MUST only send
RESET or RESET_ACK messages. Note that two sides need to both
be in the same set of states. One side could be in OK/OK and
another in INIT/OK. In that case the side in OK/OK may begin
sending data to the peer, though the peer cannot reply with data
until it progresses to OK/OK.
6. Upon initialization, a peer sets its FSN_LOCAL and FSN_REMOTE to
0, and both S_LOCAL and S_REMOTE to INIT. It sets N_LOCAL to a
new sequence number (i.e. a positive random number) and N_REMOTE
to 0.
7. A first peer, called A, sends {RESET, N_LOCAL} to its peer,
called B.
8. Upon receiving a message {RESET, N}:
1. If N does not equal N_REMOTE, set N_REMOTE to N, set both
FSN_LOCAL and FSN_REMOTE to 0. If S_REMOTE is not INIT, set
S_LOCAL to INIT. Set S_REMOTE to OK. The node MUST discard
any partially received packet, and MUST clear its current
partially transmitted packet (if any), though it MAY re-
queue it.
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2. Send {RESET_ACK, N} to the peer
9. Upon receiving a message {RESET_ACK, N}:
1. If N does not equal N_LOCAL, discard the message.
2. Else, set N_LOCAL to OK. If this sets both N_LOCAL and
N_REMOTE to OK, this node may begin sending data. It
cancels any pending RESET timer.
10. Upon sending a first RESET message, the sending node starts a
retransmit timer for the peer. It begins at RESET_TIMOUT (50
msec).
11. 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
the new timeout. If it is not less than MAX_TIMEOUT, it reports
an error for the peer.
2.1.1. Examples
The first example, Figure 1 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.
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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.
|---|-----|-----|-----| |-----|-----|-----|
| 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 | | | | |OK | 7 0 | 0 0 |
| 6 | | | | >-{RESET, 5}---------> | | | |
| 7 | | | | | OK| 5 | 0 0 |
| 8 | | | | <-----{RESET_ACK, 5}-< | | | |
| 9 |OK | | | <-------------{data}-< | | | |
| 10| | | | >-{data}-------------> | | | |
|---|-----|-----|-----| |-----|-----|-----|
Figure 1
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.
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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.
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.
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.
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If using other Ethernet encapsulations, such as 802.1AE MacSec, one
could use a security identifier in place of the {source, destination,
ethertype} tuple.
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
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allows for variable length sequence numbers. In the example shown
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_REMOTE value /
+---------------+---------------+---------------+---------------+
/ Optional CCNx ValidationAlgorithm TLV /
+---------------+---------------+---------------+---------------+
/ Optional CCNx ValidationPayload TLV (ValidationAlg required) /
+---------------+---------------+---------------+---------------+
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_REMOTE value /
+---------------+---------------+---------------+---------------+
/ Optional CCNx ValidationAlgorithm TLV /
+---------------+---------------+---------------+---------------+
/ Optional CCNx ValidationPayload TLV (ValidationAlg required) /
+---------------+---------------+---------------+---------------+
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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
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small Interests queued back-to-back, they could be encapsulated in a
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)]
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"
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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.
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.
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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
6. Acknowledgements
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
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>.
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[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>.
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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