| ICN Research Group | C. Gundogan |
| Internet-Draft | TC. Schmidt |
| Intended status: Experimental | HAW Hamburg |
| Expires: April 25, 2019 | M. Waehlisch |
| link-lab & FU Berlin | |
| C. Scherb | |
| C. Marxer | |
| C. Tschudin | |
| University of Basel | |
| October 22, 2018 |
ICN Adaptation to LowPAN Networks (ICN LoWPAN)
draft-irtf-icnrg-icnlowpan-00
In this document, a convergence layer for CCNx and NDN over IEEE 802.15.4 LoWPAN networks is defined. A new frame format is specified to adapt CCNx and NDN packets to the small MTU size of IEEE 802.15.4. For that, syntactic and semantic changes to the TLV-based header formats are described. To support compatibility with other LoWPAN technologies that may coexist on a wireless medium, the dispatching scheme provided by 6LoWPAN is extended to include new dispatch types for CCNx and NDN. Additionally, the link fragmentation component of the 6LoWPAN dispatching framework is applied to ICN chunks. Basic improvements in efficiency are advised by stateless and stateful compression schemes.
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Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved.
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The Internet of Things (IoT) has been identified as a promising deployment area for Information Centric Networks (ICN), as infrastructureless access to content, resilient forwarding, and in-network data replication have shown noteable advantages over the traditional host-to-host approach on the Internet [NDN-EXP1], [NDN-EXP2]. Recent studies [NDN-MAC] have shown that an appropriate mapping to link layer technologies has a large impact on the practical performance of an ICN. This will be even more relevant in the context of IoT communication where nodes often exchange messages via low-power wireless links under lossy conditions. In this memo, we address the base adaptation of data chunks to such link layers for the ICN flavors NDN [NDN] and CCNx.
The IEEE 802.15.4 [ieee802.15.4] link layer is used in low-power and lossy networks (see LLN in [RFC7228]), in which devices are typically battery-operated and constrained in resources. Characteristics of LLNs include an unreliable environment, low bandwidth transmissions, and increased latencies. IEEE 802.15.4 admits a maximum physical layer packet size of 127 octets. The maximum frame header size is 25 octets, which leaves 102 octets for the payload. IEEE 802.15.4 security features further reduce this payload length by up to 21 octets, yielding a net of 81 octets for CCNx or NDN packet headers, signatures and content.
6LoWPAN [RFC4944][RFC6282] is a convergence layer that provides frame formats, header compression and link fragmentation for IPv6 packets in IEEE 802.15.4 networks. The 6LoWPAN adaptation introduces a dispatching framework that prepends further information to 6LoWPAN packets, including a protocol identifier for IEEE 802.15.4 payload and meta information about link fragmentation.
Prevalent Type-Length-Value (TLV) based packet formats such as in CCNx and NDN are designed to be generic and extensible. This leads to header verbosity which is inappropriate in constrained environments of IEEE 802.15.4 links. This document presents ICN LoWPAN, a convergence layer for IEEE 802.15.4 motivated by 6LoWPAN that compresses packet headers of CCNx as well as NDN and allows for an increased payload size per packet. Additionally by reusing the dispatching framwork defined by 6LoWPAN, compatibility between coexisting wireless networks of competing technologies is enabled. This also allows to reuse the link fragmentation scheme specified by 6LoWPAN for ICN LoWPAN.
ICN LoWPAN utilizes a more space efficient representation of CCNx and NDN packet formats. This syntactic change is described for CCNx and NDN separately, as the header formats and TLV encodings differ largely. For further reductions, default header values suitable for constrained IoT networks are selected in order to elide corresponding TLVs.
In a typical IoT scenario (see Figure 1), embedded devices are interconnected via quasi-stationary infrastructure whith a border router (BR) interconnecting the constrained LoWPAN networks via some Gateway with the public Internet. In ICN based IoT networks, Interest and Data messages transparently travel through the BR up and down between a Gateway and the embedded devices within the constrained LoWPANs.
|Gateway Services|
-------------------------
|
,--------,
| |
| BR |
| |
'--------'
LoWPAN
O O
O
O O embedded
O O O devices
O O
Figure 1: IoT Stub Network
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]. The use of the term, "silently ignore" is not defined in RFC 2119. However, the term is used in this document and can be similarly construed.
This document uses the terminology of [RFC7476], [RFC7927], and [RFC7945] for ICN entities.
The following terms are used in the document and defined as follows:
ICN LoWPAN provides a convergence layer that maps ICN packets onto constrained link-layer technologies. This includes features such as link-layer fragmentation, protocol separation on the link-layer level, and link-layer address mappings. The stack traversal is visualized in Figure 2.
Device 1 Device 2
,------------------, Router ,------------------,
| Application . | __________________ | ,-> Application |
|----------------|-| | NDN / CCNx | |-|----------------|
| NDN / CCNx | | | ,--------------, | | | NDN / CCNx |
|----------------|-| |-|--------------|-| |-|----------------|
| ICN LoWPAN | | | | ICN LoWPAN | | | | ICN LoWPAN |
|----------------|-| |-|--------------|-| |-|----------------|
| Link-Layer | | | | Link-Layer | | | | Link-Layer |
'----------------|-' '-|--------------|-' '-|----------------'
'--------' '---------'
Figure 2: ICN LoWPAN convergence layer for IEEE 802.15.4
Section 4 of this document defines the convergence layer for IEEE 802.15.4.
ICN LoWPAN also defines a stateless header compression scheme with the main purpose of reducing header overhead of ICN packets. This is of particular importance for link-layers with small MTUs. The stateless compression does not require pre-configuration of global state.
The CCNx and NDN header formats are composed of Type-Length-Value (TLV) fields to encode header data. The advantage of TLVs is its native support of variable-sized data. The main disadvantage of TLVs is the verbosity that results from storing the type and length of the encoded data.
The stateless header compression scheme makes use of compact bit fields to indicate the presence of mandatory and optional TLVs in the uncompressed packet. The order of set bits in the bit fields corresponds to the order of each TLV in the packet. Further compression is achieved by specifying default values and reducing the codomain of certain header fields.
Figure 3 demonstrates the stateless header compression idea. In this example, the first type of the first TLV is removed and the corresponding bit in the bit field is set. The second TLV represents a fixed-length TLV (e.g. the Nonce TLV in NDN), so that the type and the length fields are removed. The third TLV represents a boolean TLV (e.g. the MustBeFresh selector in NDN) and is missing the type, length and the value field.
+---+---+---+---+---+---+---+---+
| 1 | 0 | 1 | 0 | 0 | 0 | 0 | 1 | Bit field
+---+---+---+---+---+---+---+---+
| | |
,--' '-----------, '- boolean
| |
+-------+--------------+-------------+
| LEN | VALUE | VALUE |
+-------+--------------+-------------+
Figure 3: Compression using a compact bit field to encode context information.
ICN LoWPAN further employs 2 stateful compression schemes to enhance size reductions. These mechanisms rely on shared contexts that are either distributed and maintained in the whole LoWPAN, or are generated on-demand for a particular Interest-data path.
A context identifier (CID) is a 1-octet wide number that refers to a particular conceptual context between network devices and MAY be used to replace frequently appearing information, like name prefixes, suffixes, or meta information, such as Interest lifetime.
The initial distribution and maintenance of shared context is out of scope. Frames containing unknown or invalid CIDs are silently discarded.
In CCNx and NDN, Name TLVs are included in Interest messages, and they return in data messages. Returning Name TLVs either equal to the original Name TLV, or they contain the original Name TLV as a prefix. ICN LoWPAN reduces this duplication in responses by replacing Name TLVs with 1-octet wide HopIDs. While an Interest is forwarded, each hop generates an ephemeral HopID that is tied to a PIT entry. Each HopID MUST be unique within the local PIT and only exist during the lifetime of a PIT entry. To maintain HopIDs, the local PIT is extended by two new columns: HIDi (inbound HopIDs) and HIDo (outbound HopIDs).
PIT of B PIT Extension PIT of C PIT Extension
+--------+------++------+------+ +--------+------++------+------+
| Prefix | Face || HIDi | HIDo | | Prefix | Face || HIDi | HIDo |
+========+======++======+======+ +========+======++======+======+
| /p0 | F_A || h_A | h_B | | /p0 | F_A || h_A | |
+--------+------++------+------+ +--------+------++------+------+
^ | ^
store | '----------------------, ,---' store
| send v |
,---, /p0, h_A ,---, /p0, h_B ,---,
| A | ------------------------> | B | ------------------------> | C |
'---' '---' '---'
Figure 4: Setting compression state en-route (Interest).
HopIDs are included in Interests and stored on the next hop with the resulting PIT entry in the HIDi column. The HopID is replaced with a newly generated local HopID before the Interest is forwarded. This new HopID is stored in the HIDo column of the local PIT (see Figure 4).
PIT of B PIT Extension PIT of C PIT Extension
+--------+------++------+------+ +--------+------++------+------+
| Prefix | Face || HIDi | HIDo | | Prefix | Face || HIDi | HIDo |
+========+======++======+======+ +========+======++======+======+
| /p0 | F_A || h_A | h_B | | /p0 | F_A || h_A | |
+--------+------++------+------+ +--------+------++------+------+
| ^ |
send | '----------------------, ,---' send
v match | v
,---, h_A ,---, h_B ,---,
| A | <------------------------ | B | <------------------------ | C |
'---' '---' '---'
Figure 5: Eliding Name TLVs using en-route state (data).
Responses include HopIDs that were obtained from Interests. If the returning Name TLV equals the original Name TLV, then the name is elided fully. Otherwise, the distinct suffix is included along with the HopID. When a response is forwarded, the contained HopID is extracted and used to match against the correct PIT entry by performing a lookup on the HIDo column. The HopID is then replaced with the corresponding HopID from the HIDi column before forwarding the reponse (Figure 5).
The IEEE 802.15.4 frame header does not provide a protocol identifier for its payload. This causes problems of misinterpreting frames when several networks coexist on the same link layer. To mitigate errors, 6LoWPAN defines dispatches as encapsulation headers for IEEE 802.15.4 frames (see Section 5 of [RFC4944]). Multiple LoWPAN encapsulation headers can prepend the actual payload and each encapsulation header is identified by a dispatch type.
[RFC8025] further specifies dispatch pages to switch between different contexts. When a LoWPAN parser encounters a Page switch LoWPAN encapsulation header, then all following encapsulation headers are interpreted by using a dispatch table as specified by the Page switch header. Page 0 and page 1 are reserved for 6LoWPAN. This document uses page 2 (1111 0010 (0xF2)) for NDN and page 3 (1111 0011 (0xF3)) for CCNx.
The base dispatch format (Figure 6) is used and extended by CCNx and NDN in Section 5 and Section 6.
0 1 2 ... 7
+---+---+-----------------------+
| C | M | |
+---+---+-----------------------+
Figure 6: Base dispatch format for NDN
The encapsulation format for ICN LoWPAN identifying an NDN Interest message is exemplarily displayed in Figure 7.
+---------------+------------+--------+----------------+-------+
| IEEE 802.15.4 | Dispatches | Page 2 | NDN Dispatches | Payl. /
+---------------+------------+--------+----------------+-------+
Figure 7: LoWPAN Encapsulation of NDN Interest with ICN LoWPAN
Section 5.3 of [RFC4944] defines a protocol independent fragmentation dispatch type, a fragmentation header for the first fragment and a separate fragmentation header for subsequent fragments. ICN LoWPAN adopts the fragmentation handling of [RFC4944].
The Fragmentation LoWPAN header can encapsulate other dispatch headers. The order of dispatch types is adopted from [RFC4944]. Figure 8 shows the fragmentation scheme. The reassembled ICN LoWPAN frame does not contain any fragmentation headers and is depicted in Figure 9.
+---------------+-----------+--------+----------------+-------------+
| IEEE 802.15.4 | Frag. 1st | Page 2 | ICN LoWPAN | Payload ... /
+---------------+-----------+--------+----------------+-------------+
+---------------+-----------+-------------+
| IEEE 802.15.4 | Frag. 2nd | ... Payload /
+---------------+-----------+-------------+
.
.
.
+---------------+-----------+-------------+
| IEEE 802.15.4 | Frag. Nth | ... Payload /
+---------------+-----------+-------------+
Figure 8: Fragmentation scheme
+---------------+--------+----------------+---------+
| IEEE 802.15.4 | Page 2 | ICN LoWPAN | Payload /
+---------------+--------+----------------+---------+
Figure 9: Reassembled ICN LoWPAN frame
A CID is appended to the last ICN LoWPAN dispatch octet. Multiple CIDs are chained together, whereas the most significant bit indicates the presence of a subsequent CID (Figure 10).
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|1| CID | --> |1| CID | --> |0| CID |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
Figure 10: Multiple 1-octet wide context identifiers.
The HopID is included as the very first CID. To distinguish the HopID from a typical LoWPAN-local CID, the 1st bit MUST be set (Figure 11). This yields 64 distinct HopIDs. If this range (0..63) is exhausted, the messages MUST be sent without en-route state compression until new HopIDs are available.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| X | 1 | HopID |
+---+---+---+---+---+---+---+---+
Figure 11: Context Identifier as HopID.
The NDN packet format consists of TLV fields using the TLV encoding that is described in [NDN-PACKET-SPEC]. Type and length fields are of variable size, where numbers greater than 252 are encoded using multiple octets. Figure 12 shows the NDN TLV encoding scheme.
If the type or length number is less than 253, then that number is encoded into the actual type or length field (Figure 12 a). If the number is greater or equals 253 and fits into 2 octets, then the type or lengh field is set to 253 and the number is encoded in the next following 2 octets in network byte order, i.e., from the most significant byte (MSB) to the least significant byte (LSB) (Figure 12 b). If the number is greater than 2 octets and fits into 4 octets, then the type or length field is set to 254 and the number is encoded in the subsequent 4 octets in network byte order (Figure 12 c). For greater numbers, the type or length field is set to 255 and the number is encoded in the subsequent 8 octets in network byte order (Figure 12 d).
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
a) | < 253 |
+-+-+-+-+-+-+-+-+
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
b) | 253 | MSB LSB |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 254 | MSB /
c) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSB |
+-+-+-+-+-+-+-+-+
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 255 | MSB /
+-+-+-+-+-+-+-+-+ +
d) | /
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LSB |
+-+-+-+-+-+-+-+-+
Figure 12: NDN TLV encoding scheme
In this document, compressed NDN TLVs make use of a different TLV scheme that puts more emphasis on size reduction. Instead of using the first octet as a marker for the number of following octets, the compressed NDN TLV scheme uses a method to chain a variable number of octets together. If an octet equals 255 (0xFF), then the following octet will also be interpreted. The actual value of a chain equals the sum of all links.
If the type or length number is less than 255, then that number is encoded into the actual type or length field (Figure 13 a). If the type or length number (X) fits into 2 octets, then the first octet is set to 255 and the subsequent octet equals X mod 255 (Figure 13 b). Following this scheme, a variable-sized number (X) is encoded using multiple octets of 255 with a trailing octet containing X mod 255 (Figure 13 c).
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
a) | < 255 (X) | = X
+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
b) | 255 | < 255 (X) | = 255 + X
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+-+-+-.....-+-+-+-+-+-+-+-+-+-+-+
c) | 255 | 255 | < 255 (X) | = (N * 255) + X
+-+-+-+-+-+-+-+-+-+-+-.....-+-+-+-+-+-+-+-+-+-+-+
(N - 1)
Figure 13: Compressed NDN TLV encoding scheme
This Name TLV compression encodes length fields of two consecutive NameComponent TLVs into one octet, using 4 bits each. This process limits the length of a NameComponent TLV to 15 octets. A length of 0 marks the end of the compressed Name TLV.
Name: /HAW/Room/481/Humid/99
0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 1 1|0 1 0 0| H | A | W |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R | o | o | m |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 1 1|0 1 0 1| 4 | 8 | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| H | u | m | i |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| d |0 0 1 0|0 0 0 0| 9 | 9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: Name TLV compression for /HAW/Room/481/Humid/99
An uncompressed Interest message uses the base dispatch format (see Figure 6) and sets the C as well as the M flag to 0 (Figure 15). resv MUST be set to 0. The Interest message is handed to the NDN network stack without modifications.
0 1 2 ... 7
+---+---+-----------------------+
| 0 | 0 | resv |
+---+---+-----------------------+
Figure 15: Dispatch format for uncompressed NDN Interest messages
The compressed Interest message uses the base dispatch format and sets the C flag to 1 and the M flag to 0. By default, the Interest message is compressed with the following base rule set: Figure 16.
Further TLV compression is indicated by the ICN LoWPAN dispatch in
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 0 |DIG|PFX|FRE|FWD|PRM|CID|
+---+---+---+---+---+---+---+---+
Figure 16: Dispatch format for compressed NDN Interest messages
An uncompressed Data message uses the base dispatch format and sets the C flag to 0 and the M flag to 1 (Figure 17). resv MUST be set to 0. The Data message is handed to the NDN network stack without modifications.
0 1 2 ... 7
+---+---+-----------------------+
| 0 | 1 | resv |
+---+---+-----------------------+
Figure 17: Dispatch format for uncompressed NDN Data messages
The compressed Data message uses the base dispatch format and sets the C flag as well as the M flag to 1. By default, the Data message is compressed with the following base rule set: Figure 18.
Further TLV compression is indicated by the ICN LoWPAN dispatch in
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 | 1 |DIG|FBI|CON| SIG |CID|
+---+---+---+---+---+---+---+---+
Figure 18: Dispatch format for compressed NDN Data messages
The CCNx TLV encoding is described in [I-D.irtf-icnrg-ccnxmessages]. Type and Length fields attain the common fixed length of 2 octets.
In this document, the TLV encoding is changed to the more space efficient encoding described in Section 5.1. Type and Length fields MUST be encoded as in Figure 13.
Name TLVs are compressed using the same approach outlined in Section 5.2. If a Name TLV contains T_IPID, T_APP, or organizational TLVs, then the name remains uncompressed.
An uncompressed Interest message uses the base dispatch format (see Figure 6) and sets the C as well as the M flag to 0 (Figure 19). resv MUST be set to 0. The Interest message is handed to the CCNx network stack without modifications.
0 1 2 ... 7
+---+---+-----------------------+
| 0 | 0 | resv |
+---+---+-----------------------+
Figure 19: Dispatch format for uncompressed CCNx Interest messages
The compressed Interest message uses the base dispatch format and sets the C flag to 1 and the M flag to 0. By default, the Interest message is compressed with the following base rule set: Figure 20.
Further TLV compression is indicated by the ICN LoWPAN dispatch in
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 1 | 0 |FLG| HBH |PTY|HPL|FRS|MSG|PAY|VAL|EXT| RESVD |CID|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 20: Dispatch format for compressed CCNx Interest messages
Hop-By-Hop Header TLVs are unordered. For an Interest message, two optional Hop-By-Hop Header TLVs are defined in [I-D.irtf-icnrg-ccnxmessages], but several more can be defined in higher level specifications. For better compression, an ordering of Hop-By-Hop TLVs is required as follows:
With this ordering in place, Type fields are elided from the Interest Lifetime TLV and the Message Hash TLV.
Note: If the original Interest message includes Hop-By-Hop Header TLVs that follow a different ordering, then they remain uncompressed.
0 1 2 3 4 5 6 7
+-------+-------+-------+-------+-------+-------+-------+-------+
| IntLifetime | MsgHash | Reserved |
+-------+-------+-------+-------+-------+-------+-------+-------+
Figure 21: Dispatch for HBH Compression
0 1 2 3 4 5 6 7
+-------+-------+-------+-------+-------+-------+-------+-------+
| KeyIDRestr | CObHRestr | Reserved |
+-------+-------+-------+-------+-------+-------+-------+-------+
Figure 22: Dispatch for Interest Messages
0 1 2 3 4 5 6 7 8
+-------+-------+-------+-------+-------+-------+-------+-------+
| ValidationAlg | KeyID | Reserved |
+-------+-------+-------+-------+-------+-------+-------+-------+
Figure 23: Dispatch for Interset Validations
The ValidationPayload TLV is present if the ValidationAlgorithm TLV is present. The type field is omitted.
An uncompressed Content object uses the base dispatch format (see Figure 6) and sets the C flag to 0 and the M flag to 1 (Figure 24). resv MUST be set to 0. The Content object is handed to the CCNx network stack without modifications.
0 1 2 ... 7
+---+---+-----------------------+
| 0 | 1 | resv |
+---+---+-----------------------+
Figure 24: Dispatch format for uncompressed CCNx Content objects
The compressed Content object uses the base dispatch format and sets the C flag as well as the M flag to 1. By default, the Content object is compressed with the following base rule set: Figure 25.
Further TLV compression is indicated by the ICN LoWPAN dispatch in
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| 1 | 1 |FLG| HBH |FRS|MSG|PAY|VAL|EXT| RESVD |CID|
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Figure 25: Dispatch format for compressed CCNx Content objects
Hop-By-Hop Header TLVs are unordered. For a Content Object message, two optional Hop-By-Hop Header TLVs are defined in [I-D.irtf-icnrg-ccnxmessages], but several more can be defined in higher level specifications. For better compression, an ordering of Hop-By-Hop TLVs is required as follows:
With this ordering in place, Type fields are elided from the Recommended Cache Time TLV and Message Hash TLV.
Note: If the original Content Object message includes Hop-By-Hop Header TLVs with a different ordering, then they remain uncompressed.
0 1 2 3 4 5 6 7
+-------+-------+-------+-------+-------+-------+-------+-------+
| RCT | MsgHash | Reserved |
+-------+-------+-------+-------+-------+-------+-------+-------+
Figure 26: Dispatch for HBH Compression
0 1 2 3 4 5 6 7
+-------+-------+-------+-------+-------+-------+-------+-------+
| PayloadType |ExpTime| Reserved |
+-------+-------+-------+-------+-------+-------+-------+-------+
Figure 27: Dispatch for Message TLVs
Main memory is typically a constrained resource of constrained networked devices. Fragmentation as described in this memo preserves fragments and purges them only after a packet is reassembled. This scheme is able to handle fragments for distinctive packets simultaneously, which can lead to overflowing packet buffers that cannot hold all necessary fragments for packet reassembly. Users are thus urged to make use of appropriate buffer replacement strategies for fragments.
The stateful header compression generates ephemeral HopIDs for incoming and outgoing Interests and consumes them on returning Data packets. Forged Interests can deplete the number of available HopIDs, thus leading to a denial of compression service for subsequent content requests.
To further alleviate the problems caused by forged fragments or Interest initiations, proper security mechanisms for accessing the link-layer may strengthen robustness in real deployments.
This document makes use of Page 2 from the existing paging dispatches in [RFC8025].
| [ieee802.15.4] | IEEE Computer Society, "IEEE Std. 802.15.4-2015", April 2016. |
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC4944] | Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007. |
| [RFC6282] | Hui, J. and P. Thubert, "Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based Networks", RFC 6282, DOI 10.17487/RFC6282, September 2011. |
In the following a theoretical evaluation is given to estimate the gains of ICN LoWPAN compared to uncompressed CCNx and NDN messages.
We assume that n is the number of name components, comps_n denotes the sum of n name component lengths. We also assume that the length of each name component is lower than 16 bytes. The length of the content is given by clen. The lengths of TLV components is specific to the CCNx or NDN encoding and outlined below.
The NDN TLV encoding has variable-sized TLV fields. For simplicity, the 1 octet form of each TLV component is assumed. A typical TLV component therefore is of size 2 (type field + length field) + the actual value.
Figure 28 depicts the size requirements for a basic, uncompressed NDN Interest containing a CanBePrefix TLV, a MustBeFresh TLV, a InterestLifetime TLV set to 4 seconds and a HopLimit TLV set to 6. Numbers below represent the amount of octets.
------------------------------------,
Interest TLV = 2 |
---------------------, |
Name | 2 + |
NameComponents = 2n + |
| comps_n |
---------------------' = 21 + 2n + comps_n
CanBePrefix = 2 |
MustBeFresh = 2 |
Nonce = 6 |
InterestLifetime = 4 |
HopLimit = 3 |
------------------------------------'
Figure 28: Estimated size of an uncompressed NDN Interest
Figure 29 depicts the size requirements after compression.
------------------------------------,
Dispatch Page Switch = 1 |
NDN Interset Dispatch = 1 |
Interest TLV = 1 |
-----------------------, |
Name | = 9 + n/2 + comps_n
NameComponents = n/2 + |
| comps_n |
-----------------------' |
Nonce = 4 |
InterestLifetime = 2 |
------------------------------------'
Figure 29: Estimated size of a compressed NDN Interest
The size difference is:
12 + 1.5n octets.
For the name /DE/HH/HAW/BT7, the total size gain is 18 octets, which is 46% of the uncompressed packet.
Figure 30 depicts the size requirements for a basic, uncompressed NDN Data containing a FreshnessPeriod as MetaInfo. A FreshnessPeriod of 1 minute is assumed. The value is thereby encoded using 2 octets. An HMACWithSha256 is assumed as signature. The key locator is assumed to contain a Name TLV of length klen.
------------------------------------,
Data TLV = 2 |
---------------------, |
Name | 2 + |
NameComponents = 2n + |
| comps_n |
---------------------' |
---------------------, |
MetaInfo | |
FreshnessPeriod = 6 = 53 + 2n + comps_n +
| | clen + klen
---------------------' |
Content = 2 + clen |
---------------------, |
SignatureInfo | |
SignatureType | |
KeyLocator = 41 + klen |
SignatureValue | |
DigestSha256 | |
---------------------' |
------------------------------------'
Figure 30: Estimated size of an uncompressed NDN Data
Figure 31 depicts the size requirements for the compressed version of the above Data packet.
------------------------------------,
Dispatch Page Switch = 1 |
NDN Data Dispatch = 1 |
-----------------------, |
Name | = 38 + n/2 + comps_n +
NameComponents = n/2 + | clen + klen
| comps_n |
-----------------------' |
Content = 1 + clen |
KeyLocator = 1 + klen |
DigestSha256 = 32 |
FreshnessPeriod = 2 |
------------------------------------'
Figure 31: Estimated size of a compressed NDN Data
The size difference is:
15 + 1.5n octets.
For the name /DE/HH/HAW/BT7, the total size gain is 21 octets.
The CCNx TLV encoding defines a 2-octet encoding for type and length fields, summing up to 4 octets in total without a value.
Figure 32 depicts the size requirements for a basic, uncompressed CCNx Interest. No Hop-By-Hop TLVs are included and the protocol version as well as the reserved field are assumed to be 0. A KeyIdRestriction TLV with T_SHA-256 is included to limit the responses to Content Objects containing the specific key.
------------------------------------,
Fixed Header = 8 |
Message = 4 |
---------------------, |
Name | 4 + = 56 + 4n + comps_n
NameSegments = 4n + |
| comps_n |
---------------------' |
KeyIdRestriction = 40 |
------------------------------------'
Figure 32: Estimated size of an uncompressed CCNx Interest
Figure 33 depicts the size requirements after compression.
------------------------------------,
Dispatch Page Switch = 1 |
CCNx Interest Dispatch = 3 |
Fixed Header = 3 |
-----------------------, |
Name | = 39 + n/2 + comps_n
NameSegments = n/2 + |
| comps_n |
-----------------------' |
T_SHA-256 = 32 |
------------------------------------'
Figure 33: Estimated size of a compressed CCNx Interest
The size difference is:
17 + 3.5n octets.
For the name /DE/HH/HAW/BT7, the total size gain is 31 octets, which is 38% of the uncompressed packet.
Figure 34 depicts the size requirements for a basic, uncompressed CCNx Data containing an ExpiryTime Message TLV, an HMAC_SHA-256 signature, the signature time and a hash of the shared secret key.
------------------------------------,
Fixed Header = 8 |
Message = 4 |
---------------------, |
Name | 4 + |
NameSegments = 4n + |
| comps_n |
---------------------' |
ExpiryTime = 12 = 124 + 4n + comps_n + clen
Payload = 4 + clen |
---------------------, |
ValidationAlgorithm | |
T_HMAC-256 = 56 |
KeyId | |
SignatureTime | |
---------------------' |
ValidationPayload = 36 |
------------------------------------'
Figure 34: Estimated size of an uncompressed CCNx Data Object
Figure 35 depicts the size requirements for a basic, compressed CCNx Data.
------------------------------------,
Dispatch Page Switch = 1 |
CCNx Content Dispatch = 4 |
Fixed Header = 2 |
-----------------------, |
Name | |
NameSegments = n/2 + |
| comps_n = 91 + n/2 + comps_n + clen
-----------------------' |
ExpiryTime = 8 |
Payload = 1 + clen |
T_HMAC-SHA256 = 32 |
SignatureTime = 8 |
ValidationPayload = 34 |
------------------------------------'
Figure 35: Estimated size of a compressed CCNx Data Object
The size difference is:
33 + 3.5n octets.
For the name /DE/HH/HAW/BT7, the total size gain is 47 octets.
This work was stimulated by fruitful discussions in the ICNRG research group and the communities of RIOT and CCNlite. We would like to thank all active members for constructive thoughts and feedback. In particular, the authors would like to thank (in alphabetical order) Peter Kietzmann, Dirk Kutscher, Martine Lenders, +++. The hop-wise stateful name compression was brought up in a discussion by Dave Oran, which is gratefully acknowledged. Larger parts of this work are inspired by [RFC4944] and [RFC6282]. Special mentioning goes to Mark Mosko as well as G.Q. Wang and Ravi Ravindran as their previous work in [TLV-ENC-802.15.4] and [WIRE-FORMAT-CONSID] provided a good base for our discussions on stateless header compression mechanisms. This work was supported in part by the German Federal Ministry of Research and Education within the projects I3 and RAPstore.