]> ICN Adaptation to LoWPAN Networks (ICN LoWPAN) HAW Hamburg
Berliner Tor 7 Hamburg D-20099 Germany +4940428758067 cenk.guendogan@haw-hamburg.de http://inet.haw-hamburg.de/members/cenk-gundogan
HAW Hamburg
Berliner Tor 7 Hamburg D-20099 Germany t.schmidt@haw-hamburg.de http://inet.haw-hamburg.de/members/schmidt
link-lab & FU Berlin
Hoenower Str. 35 Berlin D-10318 Germany mw@link-lab.net http://www.inf.fu-berlin.de/~waehl
University of Basel
Spiegelgasse 1 Basel 4051 Switzerland christopher.scherb@unibas.ch
University of Basel
Spiegelgasse 1 Basel 4051 Switzerland claudio.marxer@unibas.ch
University of Basel
Spiegelgasse 1 Basel 4051 Switzerland christian.tschudin@unibas.ch
ICN Research Group example This document defines a convergence layer for CCNx and NDN over IEEE 802.15.4 LoWPAN networks. 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 fragmentation component of the 6LoWPAN dispatching framework is applied to ICN chunks. In its second part, the document defines stateless and stateful compression schemes to improve efficiency on constrained links. Stateless compression reduces TLV expressions to static header fields for common use cases. Stateful compression schemes elide state local to the LoWPAN and replace names in data packets by short local identifiers. This document is a product of the IRTF Information-Centric Networking Research Group (ICNRG).
Introduction 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 demonstrated notable advantages over the traditional host-to-host approach on the Internet , . Recent studies 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 and CCNx , . The IEEE 802.15.4 link layer is used in low-power and lossy networks (see LLN in ), 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 bytes. The maximum frame header size is 25 bytes, which leaves 102 bytes for the payload. IEEE 802.15.4 security features further reduce this payload length by up to 21 bytes, yielding a net of 81 bytes for CCNx or NDN packet headers, signatures and content. 6LoWPAN , is a convergence layer that provides frame formats, header compression and adaptation layer 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 payload and meta information about 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. ICN LoWPAN compresses packet headers of CCNx as well as NDN and allows for an increased effective payload size per packet. Additionally, reusing the dispatching framework defined by 6LoWPAN enables compatibility between coexisting wireless deployments of competing network technologies. This also allows to reuse the adaptation layer fragmentation scheme specified by 6LoWPAN for ICN LoWPAN. ICN LoWPAN defines 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 notably. For further reductions, default header values suitable for constrained IoT networks are selected in order to elide corresponding TLVs. Experimental evaluations of the ICN LoWPAN header compression schemes in illustrate a reduced message overhead, a shortened message airtime, and an overall decline in power consumption for typical Class 2 devices compared to uncompressed ICN messages. In a typical IoT scenario (see ), embedded devices are interconnected via a quasi-stationary infrastructure using a border router (BR) that connects the constrained LoWPAN network by some Gateway with the public Internet. In ICN based IoT networks, non-local Interest and Data messages transparently travel through the BR up and down between a Gateway and the embedded devices situated in the constrained LoWPAN.
IoT Stub Network
The document has received fruitful reviews by members of the ICN community and the research group (see Acknowledgments) for a period of two years. It is the consensus of ICNRG that this document should be published in the IRTF Stream of the RFC series. This document does not constitute an IETF standard.
Terminology 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 . 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 , , and for ICN entities. The following terms are used in the document and defined as follows:
ICN LoWPAN:
Information-Centric Networking over Low-power Wireless Personal Area Network
LLN:
Low-Power and Lossy Network
CCNx:
Content-Centric Networking Architecture
NDN:
Named Data Networking Architecture
byte:
synonym for octet
nibble:
synonym for 4 bits
time-value:
a time offset measured in seconds
time-code:
an 8-bit encoded time-value
Overview of ICN LoWPAN
Link-Layer Convergence 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 .
ICN LoWPAN convergence layer for IEEE 802.15.4 Application | |----------------|-| | NDN / CCNx | |-|----------------| | NDN / CCNx | | | ,--------------, | | | NDN / CCNx | |----------------|-| |-|--------------|-| |-|----------------| | ICN LoWPAN | | | | ICN LoWPAN | | | | ICN LoWPAN | |----------------|-| |-|--------------|-| |-|----------------| | Link-Layer | | | | Link-Layer | | | | Link-Layer | '----------------|-' '-|--------------|-' '-|----------------' '--------' '---------' ]]>
of this document defines the convergence layer for IEEE 802.15.4.
Stateless Header Compression 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 variably structured 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 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 range of certain header fields. 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) for which the type, length and the value fields are elided.
Compression using a compact bit field - bits encode the inclusion of TLVs.
Stateless TLV compression for NDN is defined in . defines the stateless TLV compression for CCNx. The extensibility of this compression is described in and allows future documents to update the compression rules outlined in this manuscript.
Stateful Header Compression ICN LoWPAN further employs two orthogonal stateful compression schemes for packet size reductions which are defined in . These mechanisms rely on shared contexts that are either distributed and maintained in the entire LoWPAN, or are generated on-demand hop-wise on a particular Interest-data path. The shared context identification is defined in . The hop-wise name compression "en-route" is specified in .
IEEE 802.15.4 Adaptation
LoWPAN Encapsulation The IEEE 802.15.4 frame header does not provide a protocol identifier for its payload. This causes problems of misinterpreting frames when several network layers coexist on the same link. To mitigate errors, 6LoWPAN defines dispatches as encapsulation headers for IEEE 802.15.4 frames (see ). Multiple LoWPAN encapsulation headers can precede the actual payload and each encapsulation header is identified by a dispatch type. 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 TBD1 (1111 TBD1 (0xFTBD1)) for ICN LoWPAN. The base dispatch format () is used and extended by CCNx and NDN in and .
Base dispatch format for ICN LoWPAN
C: Compression
0:
The message is uncompressed.
1:
The message is compressed.
P: Protocol
0:
The included protocol is NDN.
1:
The included protocol is CCNx.
M: Message Type
0:
The payload contains an Interest message.
1:
The payload contains a Data message.
ICN LoWPAN frames with compressed CCNx and NDN messages (C=1) use the extended dispatch format in .
Extended dispatch format for compressed ICN LoWPAN
CID: Context Identifier
0:
No context identifiers are present.
1:
Context identifier(s) are present (see ).
EXT: Extension
0:
No extension bytes are present.
1:
Extension byte(s) are present (see ).
The encapsulation format for ICN LoWPAN is displayed in .
LoWPAN Encapsulation with ICN-LoWPAN
IEEE 802.15.4:
The IEEE 802.15.4 header.
RFC4944 Disp.:
Optional additional dispatches defined in
Page:
Page Switch. TBD1 for ICN LoWPAN.
ICN LoWPAN:
Dispatches as defined in and .
Payload:
The actual (un-)compressed CCNx or NDN message.
Dispatch Extensions Extension bytes allow for the extensibility of the initial compression rule set. The base format for an extension byte is depicted in .
Base format for dispatch extensions.
EXT: Extension
0:
No other extension byte follows.
1:
A further extension byte follows.
Extension bytes are numbered according to their order. Future documents MUST follow the naming scheme EXT_0, EXT_1, ..., when updating or referring to a specific dispatch extension byte. Amendments that require an exchange of configurational parameters between devices SHOULD use manifests to encode structured data in a well-defined format, as, e.g., outlined in .
Adaptation Layer Fragmentation Small payload sizes in the LoWPAN require fragmentation for various network layers. Therefore, 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 this fragmentation handling of . The Fragmentation LoWPAN header can encapsulate other dispatch headers. The order of dispatch types is defined in . shows the fragmentation scheme. The reassembled ICN LoWPAN frame does not contain any fragmentation headers and is depicted in .
Fragmentation scheme
Reassembled ICN LoWPAN frame
The 6LoWPAN Fragment Forwarding (6FF) is an alternative approach that enables forwarding of fragments without reassembling packets on every intermediate hop. By reusing the 6LoWPAN dispatching framework, 6FF integrates into ICN LoWPAN as seamless as the conventional hop-wise fragmentation. Experimental evaluations , however, suggest that a more refined integration can increase the cache utilization of forwarders on a request path.
Space-efficient Message Encoding for NDN
TLV Encoding The NDN packet format consists of TLV fields using the TLV encoding that is described in . Type and length fields are of variable size, where numbers greater than 252 are encoded using multiple bytes. If the type or length number is less than 253, then that number is encoded into the actual type or length field. If the number is greater or equals 253 and fits into 2 bytes, then the type or length field is set to 253 and the number is encoded in the next following 2 bytes in network byte order, i.e., from the most significant byte (MSB) to the least significant byte (LSB). If the number is greater than 2 bytes and fits into 4 bytes, then the type or length field is set to 254 and the number is encoded in the subsequent 4 bytes in network byte order. For larger numbers, the type or length field is set to 255 and the number is encoded in the subsequent 8 bytes in network byte order. In this specification, compressed NDN TLVs encode type and length fields using self-delimiting numeric values (SDNVs) commonly known from DTN protocols. Instead of using the first byte as a marker for the number of following bytes, SDNVs use a single bit to indicate subsequent bytes. NDN TLV encoding compared to SDNVs.
Value NDN TLV encoding SDNV encoding
0 0x00 0x00
127 0x7F 0x7F
128 0x80 0x81 0x00
253 0xFD 0x00 0xFD 0x81 0x7D
2^14 - 1 0xFD 0x3F 0xFF 0xFF 0x7F
2^14 0xFD 0x40 0x00 0x81 0x80 0x00
2^16 0xFE 0x00 0x01 0x00 0x00 0x84 0x80 0x00
2^21 - 1 0xFE 0x00 0x1F 0xFF 0xFF 0xFF 0xFF 0x7F
2^21 0xFE 0x00 0x20 0x00 0x00 0x81 0x80 0x80 0x00
2^28 - 1 0xFE 0x0F 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0x7F
2^28 0xFE 0x1F 0x00 0x00 0x00 0x81 0x80 0x80 0x80 0x00
2^32 0xFF 0x00 0x00 0x00 0x01 0x00 0x00 0x00 0x00 0x90 0x80 0x80 0x80 0x00
2^35 - 1 0xFF 0x00 0x00 0x00 0x07 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0x7F
2^35 0xFF 0x00 0x00 0x00 0x08 0x00 0x00 0x00 0x00 0x81 0x80 0x80 0x80 0x80 0x00
compares the required bytes for encoding a few selected values using the NDN TLV encoding and SDNVs. For values up to 127, both methods require a single byte. Values in the range [128;252] encode as one byte for the NDN TLV scheme, while SDNVs require two bytes. Starting at value 253, SDNVs require a less or equal amount of bytes compared to the NDN TLV encoding.
Name TLV Compression This Name TLV compression encodes length fields of two consecutive NameComponent TLVs into one byte, using a nibble for each. The most significant nibble indicates the length of an immediately following NameComponent TLV. The least significant nibble denotes the length of a subsequent NameComponent TLV. A length of 0 marks the end of the compressed Name TLV. The last length field of an encoded NameComponent is either 0x00 for a name with an even number of components, and 0xYF (Y > 0) if an odd number of components are present. This process limits the length of a NameComponent TLV to 15 bytes, but allows for an unlimited number of components. An example for this encoding is presented in .
Name TLV compression for /HAW/Room/481/Humid/99
Interest Messages
Uncompressed Interest Messages An uncompressed Interest message uses the base dispatch format (see ) and sets the C flag to 0 and the P as well as the M flag to 0 (). The Interest message is handed to the NDN network stack without modifications.
Dispatch format for uncompressed NDN Interest messages
Compressed Interest Messages The compressed Interest message uses the extended dispatch format () and sets the C flag to 1, the P flag to 0 and the M flag to 0. If an Interest message contains TLVs that are not mentioned in the following compression rules, then this message MUST be sent uncompressed. This specification assumes that a HopLimit TLV is part of the original Interest message. If such HopLimit TLV is not present, it will be inserted with a default value of DEFAULT_NDN_HOPLIMIT prior to the compression. In the default use case, the Interest message is compressed with the following minimal rule set:
  1. The Type field of the outermost MessageType TLV is removed.
  2. The Name TLV is compressed according to . For this, all NameComponents are expected to be of type GenericNameComponent with a length greater than 0. An ImplicitSha256DigestComponent or ParametersSha256DigestComponent MAY appear at the end of the name. In any other case, the message MUST be sent uncompressed.
  3. The Nonce TLV and InterestLifetime TLV are moved to the end of the compressed Interest as illustrated in . The InterestLifetime is encoded as described in . On decompression, this encoding may yield an Interestlifetime that is smaller than the original value.
  4. The Type and Length fields of Nonce TLV, HopLimit TLV and InterestLifetime TLV are elided. The Nonce value has a length of 4 bytes and the HopLimit value has a length of 1 byte. The compressed InterestLifetime () has a length of 1 byte. The presence of a Nonce and InterestLifetime TLV is deduced from the remaining length to parse. A remaining length of 1 indicates the presence of an InerestLifetime, a length of 4 indicates the presence of a nonce, and a length of 5 indicates the presence of both TLVs.
The compressed NDN LoWPAN Interest message is visualized in .
Compression of NDN LoWPAN Interest Message +---------+---------+ : FWDH T : FWDH L : FWDH V : : APM Lc : APM Vc : +---------+---------+---------+ +---------+---------+ : NONCE T : NONCE L : NONCE V : : NONCE V : +---------+---------+---------+ +---------+ : ILT T : ILT L : ILT V : : ILT Vc : +---------+---------+---------+ +---------+ : HPL T : HPL L : HPL V : +---------+---------+---------+ : APM T : APM L : APM V : +---------+---------+---------+ ]]>
Further TLV compression is indicated by the ICN LoWPAN dispatch in .
Dispatch format for compressed NDN Interest messages
CID: Context Identifier
See .
EXT: Extension
0:
No extension byte follows.
1:
Extension byte EXT_0 follows immediately. See .
PFX: CanBePrefix TLV
0:
The uncompressed message does not include a CanBePrefix TLV.
1:
The uncompressed message does include a CanBePrefix TLV and is removed from the compressed message.
FRE: MustBeFresh TLV
0:
The uncompressed message does not include a MustBeFresh TLV.
1:
The uncompressed message does include a MustBeFresh TLV and is removed from the compressed message.
FWD: ForwardingHint TLV
0:
The uncompressed message does not include a ForwardingHint TLV.
1:
The uncompressed message does include a ForwardingHint TLV. The Type field is removed from the compressed message. Further, all link delegation types and link preference types are removed. All included names are compressed according to . If any name is not compressible, the message MUST be sent uncompressed.
APM: ApplicationParameters TLV
0:
The uncompressed message does not include an ApplicationParameters TLV.
1:
The uncompressed message does include an ApplicationParameters TLV. The Type field is removed from the compressed message.
DIG: ImplicitSha256DigestComponent TLV
0:
The name does not include an ImplicitSha256DigestComponent as the last TLV.
1:
The name does include an ImplicitSha256DigestComponent as the last TLV. The Type and Length fields are omitted.
RSV: Reserved
Must be set to 0.
Dispatch Extension The EXT_0 byte follows the description in and is illustrated in .
EXT_0 format
NCS: Name Compression Strategy
00:
Names are compressed with the default name compression strategy (see ).
01:
Reserved.
10:
Reserved.
11:
Reserved.
RSV: Reserved
Must be set to 0.
EXT: Extension
0:
No extension byte follows.
1:
A further extension byte follows immediately.
Data Messages
Uncompressed Data Messages An uncompressed Data message uses the base dispatch format and sets the C flag to 0, the P flag to 0 and the M flag to 1 (). The Data message is handed to the NDN network stack without modifications.
Dispatch format for uncompressed NDN Data messages
Compressed Data Messages The compressed Data message uses the extended dispatch format () and sets the C as well as the M flags to 1. The P flag is set to 0. If a Data message contains TLVs that are not mentioned in the following compression rules, then this message MUST be sent uncompressed. By default, the Data message is compressed with the following base rule set:
  1. The Type field of the outermost MessageType TLV is removed.
  2. The Name TLV is compressed according to . For this, all NameComponents are expected to be of type GenericNameComponent and to have a length greater than 0. In any other case, the message MUST be sent uncompressed.
  3. The MetaInfo TLV Type and Length fields are elided from the compressed Data message.
  4. The FreshnessPeriod TLV MUST be moved to the end of the compressed Data message. Type and Length fields are elided and the value is encoded as described in as a 1-byte time-code. If the freshness period is not a valid time-value, then the message MUST be sent uncompressed in order to preserve the security envelope of the Data message. The presence of a FreshnessPeriod TLV is deduced from the remaining one byte length to parse.
  5. The Type fields of the SignatureInfo TLV, SignatureType TLV and SignatureValue TLV are removed.
The compressed NDN LoWPAN Data message is visualized in .
Compression of NDN LoWPAN Data Message +---------+---------+ : FrPr T : FrPr L : FrPr V : : CONT Lc : CONT V : +---------+---------+---------+ +---------+---------+ : FBID T : FBID L : FBID V : | Sig Lc | +---------+---------+---------+ +---------+---------+ : CONT T : CONT L : CONT V : | SInf Lc | SInf Vc | +---------+---------+---------+ +---------+---------+ | Sig T | Sig L | | SVal Lc | SVal Vc | +---------+---------+---------+ +---------+---------+ | SInf T | SInf L | SInf V | : FrPr Vc : +---------+---------+---------+ +---------+ | SVal T | SVal L | SVal V | +---------+---------+---------+ ]]>
Further TLV compression is indicated by the ICN LoWPAN dispatch in .
Dispatch format for compressed NDN Data messages
CID: Context Identifier
See .
EXT: Extension
0:
No extension byte follows.
1:
Extension byte EXT_0 follows immediately. See .
FBI: FinalBlockId TLV
0:
The uncompressed message does not include a FinalBlockId TLV.
1:
The uncompressed message does include a FinalBlockId and it is encoded according to . If the FinalBlockId TLV is not compressible, then the message MUST be sent uncompressed.
CON: ContentType TLV
0:
The uncompressed message does not include a ContentType TLV.
1:
The uncompressed message does include a ContentType TLV. The Type field is removed from the compressed message.
KLO: KeyLocator TLV
0:
If the included SignatureType requires a KeyLocator TLV, then the KeyLocator represents a name and is compressed according to . If the name is not compressible, then the message MUST be sent uncompressed.
1:
If the included SignatureType requires a KeyLocator TLV, then the KeyLocator represents a KeyDigest. The Type field of this KeyDigest is removed.
RSV: Reserved
Must be set to 0.
Dispatch Extension The EXT_0 byte follows the description in and is illustrated in .
EXT_0 format
NCS: Name Compression Strategy
00:
Names are compressed with the default name compression strategy (see ).
01:
Reserved.
10:
Reserved.
11:
Reserved.
RSV: Reserved
Must be set to 0.
EXT: Extension
0:
No extension byte follows.
1:
A further extension byte follows immediately.
Space-efficient Message Encoding for CCNx
TLV Encoding The generic CCNx TLV encoding is described in . Type and Length fields attain the common fixed length of 2 bytes. The TLV encoding for CCNx LoWPAN is changed to the more space efficient encoding described in . Hence NDN and CCNx use the same compressed format for writing TLVs.
Name TLV Compression Name TLVs are compressed using the scheme already defined in for NDN. If a Name TLV contains T_IPID, T_APP, or organizational TLVs, then the name remains uncompressed.
Interest Messages
Uncompressed Interest Messages An uncompressed Interest message uses the base dispatch format (see ) and sets the C as well as the M flag to 0. The P flag is set to 1 (). The Interest message is handed to the CCNx network stack without modifications.
Dispatch format for uncompressed CCNx Interest messages
Compressed Interest Messages The compressed Interest message uses the extended dispatch format () and sets the C and P flags to 1. The M flag is set to 0. If an Interest message contains TLVs that are not mentioned in the following compression rules, then this message MUST be sent uncompressed. In the default use case, the Interest message is compressed with the following minimal rule set:
  1. The Type and Length fields of the CCNx Message TLV are elided and are obtained from the Fixed Header on decompression.
The compressed CCNx LoWPAN Interest message is visualized in .
Compression of CCNx LoWPAN Interest Message : KIDR Vc : +---------+---------+---------+ +---------+ : KIDR T : KIDR L : KIDR V : : OBHR Vc : +---------+---------+---------+ +---------+---------+ : OBHR T : OBHR L : OBHR V : : PAYL Lc : PAYL V : +---------+---------+---------+ +---------+---------+ : PAYL T : PAYL L : PAYL V : : VALG Lc : VALG Vc : +---------+---------+---------+ +---------+---------+ : VALG T : VALG L : VALG V : : VPAY Lc : VPAY V : +---------+---------+---------+ +---------+---------+ : VPAY T : VPAY L : VPAY V : +---------+---------+---------+ ]]>
Further TLV compression is indicated by the ICN LoWPAN dispatch in .
Dispatch format for compressed CCNx Interest messages
CID: Context Identifier
See .
EXT: Extension
0:
No extension byte follows.
1:
Extension byte EXT_0 follows immediately. See .
VER: CCNx protocol version in the fixed header
0:
The Version field equals 1 and is removed from the fixed header.
1:
The Version field appears in the fixed header.
FLG: Flags field in the fixed header
0:
The Flags field equals 0 and is removed from the Interest message.
1:
The Flags field appears in the fixed header.
PTY: PacketType field in the fixed header
0:
The PacketType field is elided and assumed to be PT_INTEREST.
1:
The PacketType field is elided and assumed to be PT_RETURN.
HPL: HopLimit field in the fixed header
0:
The HopLimit field appears in the fixed header.
1:
The HopLimit field is elided and assumed to be 1.
FRS: Reserved field in the fixed header
0:
The Reserved field appears in the fixed header.
1:
The Reserved field is elided and assumed to be 0.
PAY: Optional Payload TLV
0:
The Payload TLV is absent.
1:
The Payload TLV is present and the type field is elided.
ILT: Optional Hop-By-Hop InterestLifetime TLV

 See for further details on the ordering of hop-by-hop TLVs.
0:
No InterestLifetime TLV is present in the Interest message.
1:
An InterestLifetime TLV is present with a fixed length of 1 byte and is encoded as described in . The type and length fields are elided.
MGH: Optional Hop-By-Hop MessageHash TLV

 See for further details on the ordering of hop-by-hop TLVs. This TLV is expected to contain a T_SHA-256 TLV. If another hash is contained, then the Interest MUST be sent uncompressed.
0:
The MessageHash TLV is absent.
1:
A T_SHA-256 TLV is present and the type as well as the length fields are removed. The length field is assumed to represent 32 bytes. The outer Message Hash TLV is omitted.
KIR: Optional KeyIdRestriction TLV

 This TLV is expected to contain a T_SHA-256 TLV. If another hash is contained, then the Interest MUST be sent uncompressed.
0:
The KeyIdRestriction TLV is absent.
1:
A T_SHA-256 TLV is present and the type as well as the length fields are removed. The length field is assumed to represent 32 bytes. The outer KeyIdRestriction TLV is omitted.
CHR: Optional ContentObjectHashRestriction TLV

 This TLV is expected to contain a T_SHA-256 TLV. If another hash is contained, then the Interest MUST be sent uncompressed.
0:
The ContentObjectHashRestriction TLV is absent.
1:
A T_SHA-256 TLV is present and the type as well as the length fields are removed. The length field is assumed to represent 32 bytes. The outer ContentObjectHashRestriction TLV is omitted.
VAL: Optional ValidationAlgorithm and ValidationPayload TLVs
0:
No validation related TLVs are present in the Interest message.
1:
Validation related TLVs are present in the Interest message. An additional byte follows immediately that handles validation related TLV compressions and is described in .
Hop-By-Hop Header TLVs Compression Hop-By-Hop Header TLVs are unordered. For an Interest message, two optional Hop-By-Hop Header TLVs are defined in , but several more can be defined in higher level specifications. For the compression specified in the previous section, the Hop-By-Hop TLVs are ordered as follows:
  1. Interest Lifetime TLV
  2. Message Hash TLV
Note: Other Hop-By-Hop Header TLVs than those two remain uncompressed in the encoded message and they appear in the same order as in the original message, but after the Interest Lifetime TLV and Message Hash TLV.
Validation
Dispatch for Interset Validations
ValidationALg: Optional ValidationAlgorithm TLV
0000:
An uncompressed ValidationAlgorithm TLV is included.
0001:
A T_CRC32C ValidationAlgorithm TLV is assumed, but no ValidationAlgorithm TLV is included.
0010:
A T_CRC32C ValidationAlgorithm TLV is assumed, but no ValidationAlgorithm TLV is included. Additionally, a Sigtime TLV is inlined without a type and a length field.
0011:
A T_HMAC-SHA256 ValidationAlgorithm TLV is assumed, but no ValidationAlgorithm TLV is included.
0100:
A T_HMAC-SHA256 ValidationAlgorithm TLV is assumed, but no ValidationAlgorithm TLV is included. Additionally, a Sigtime TLV is inlined without a type and a length field.
0101:
Reserved.
0110:
Reserved.
0111:
Reserved.
1000:
Reserved.
1001:
Reserved.
1010:
Reserved.
1011:
Reserved.
1100:
Reserved.
1101:
Reserved.
1110:
Reserved.
1111:
Reserved.
KeyID: Optional KeyID TLV within the ValidationAlgorithm TLV
00:
The KeyId TLV is absent.
01:
The KeyId TLV is present and uncompressed.
10:
A T_SHA-256 TLV is present and the type field as well as the length fields are removed. The length field is assumed to represent 32 bytes. The outer KeyId TLV is omitted.
11:
A T_SHA-512 TLV is present and the type field as well as the length fields are removed. The length field is assumed to represent 64 bytes. The outer KeyId TLV is omitted.
RSV: Reserved
Must be set to 0.
The ValidationPayload TLV is present if the ValidationAlgorithm TLV is present. The type field is omitted.
Dispatch Extension The EXT_0 byte follows the description in and is illustrated in .
EXT_0 format
NCS: Name Compression Strategy
00:
Names are compressed with the default name compression strategy (see ).
01:
Reserved.
10:
Reserved.
11:
Reserved.
RSV: Reserved
Must be set to 0.
EXT: Extension
0:
No extension byte follows.
1:
A further extension byte follows immediately.
Content Objects
Uncompressed Content Objects An uncompressed Content object uses the base dispatch format (see ) and sets the C flag to 0, the P and M flags to 1 (). The Content object is handed to the CCNx network stack without modifications.
Dispatch format for uncompressed CCNx Content objects
Compressed Content Objects The compressed Content object uses the extended dispatch format () and sets the C, P, as well as the M flag to 1. If a Content object contains TLVs that are not mentioned in the following compression rules, then this message MUST be sent uncompressed. By default, the Content object is compressed with the following base rule set:
  1. The PacketType field is elided from the Fixed Header.
  2. The Type and Length fields of the CCNx Message TLV are elided and are obtained from the Fixed Header on decompression.
The compressed CCNx LoWPAN Data message is visualized in .
Compression of CCNx LoWPAN Data Message : EXPT Vc : +---------+---------+---------+ +---------+---------+ : PTYP T : PTYP L : PTYP V : : PAYL Lc : PAYL V : +---------+---------+---------+ +---------+---------+ : EXPT T : EXPT L : EXPT V : : VALG Lc : VALG Vc : +---------+---------+---------+ +---------+---------+ : PAYL T : PAYL L : PAYL V : : VPAY Lc : VPAY V : +---------+---------+---------+ +---------+---------+ : VALG T : VALG L : VALG V : +---------+---------+---------+ : VPAY T : VPAY L : VPAY V : +---------+---------+---------+ ]]>
Further TLV compression is indicated by the ICN LoWPAN dispatch in .
Dispatch format for compressed CCNx Content objects
CID: Context Identifier
See .
EXT: Extension
0:
No extension byte follows.
1:
Extension byte EXT_0 follows immediately. See .
VER: CCNx protocol version in the fixed header
0:
The Version field equals 1 and is removed from the fixed header.
1:
The Version field appears in the fixed header.
FLG: Flags field in the fixed header
See .
FRS: Reserved field in the fixed header
See .
PAY: Optional Payload TLV
See .
RCT: Optional Hop-By-Hop RecommendedCacheTime TLV
0:
The Recommended Cache Time TLV is absent.
1:
The Recommended Cache Time TLV is present and the type as well as the length fields are elided.
MGH: Optional Hop-By-Hop MessageHash TLV

 See for further details on the ordering of hop-by-hop TLVs. This TLV is expected to contain a T_SHA-256 TLV. If another hash is contained, then the Content Object MUST be sent uncompressed.
0:
The MessageHash TLV is absent.
1:
A T_SHA-256 TLV is present and the type as well as the length fields are removed. The length field is assumed to represent 32 bytes. The outer Message Hash TLV is omitted.
PLTYP: Optional PayloadType TLV
00:
The PayloadType TLV is absent.
01:
The PayloadType TLV is absent and T_PAYLOADTYPE_DATA is assumed.
10:
The PayloadType TLV is absent and T_PAYLOADTYPE_KEY is assumed.
11:
The PayloadType TLV is present and uncompressed.
EXP: Optional ExpiryTime TLV
0:
The ExpiryTime TLV is absent.
1:
The ExpiryTime TLV is present and the type as well as the length fields are elided.
VAL: Optional ValidationAlgorithm and ValidationPayload TLVs
See .
RSV: Reserved
Must be set to 0.
Hop-By-Hop Header TLVs Compression Hop-By-Hop Header TLVs are unordered. For a Content Object message, two optional Hop-By-Hop Header TLVs are defined in , but several more can be defined in higher level specifications. For the compression specified in the previous section, the Hop-By-Hop TLVs are ordered as follows:
  1. Recommended Cache Time TLV
  2. Message Hash TLV
Note: Other Hop-By-Hop Header TLVs than those two remain uncompressed in the encoded message and they appear in the same order as in the original message, but after the Recommended Cache Time TLV and Message Hash TLV.
Dispatch Extension The EXT_0 byte follows the description in and is illustrated in .
EXT_0 format
NCS: Name Compression Strategy
00:
Names are compressed with the default name compression strategy (see ).
01:
Reserved.
10:
Reserved.
11:
Reserved.
RSV: Reserved
Must be set to 0.
EXT: Extension
0:
No extension byte follows.
1:
A further extension byte follows immediately.
Compressed Time Encoding This document adopts the 8-bit compact time representation for relative time values described in with the constant factor C set to C := 1/32. Valid time offsets in CCNx and NDN reach from a few milliseconds (e.g., lifetime of low-latency Interests) to several years (e.g., content freshness periods in caches). Therefore, this document adds two modifications to the compression algorithm. The first modification is the inclusion of a subnormal form for time-codes with exponent 0 to provide an increased precision and a gradual underflow for the smallest numbers. The formula is changed as follows (a := mantissa; b := exponent):
Subnormal (b == 0):
(0 + a/8) * 2 * C
Normalized (b > 0):
(1 + a/8) * 2^b * C (see )
This configuration allows for the following ranges:
Minimum subnormal number:
0 seconds
2nd minimum subnormal number:
~0.007812 seconds
Maximum subnormal number:
~0.054688 seconds
Minimum normalized number:
~0.062500 seconds
2nd minimum normalized number:
~0.070312 seconds
Maximum normalized number:
~3.99 years
The second modification only applies to uncompressible time offsets that are outside any security envelope. An invalid time-value MUST be set to the largest valid time-value that is smaller than the invalid input value before compression.
Stateful Header Compression Stateful header compression in ICN LoWPAN enables packet size reductions in two ways. First, common information that is shared throughout the local LoWPAN may be memorized in context state at all nodes and omitted from communication. Second, redundancy in a single Interest-data exchange may be removed from ICN stateful forwarding on a hop-by-hop bases and memorized in en-route state tables.
LoWPAN-local State A context identifier (CID) is a byte that refers to a particular conceptual context between network devices and MAY be used to replace frequently appearing information, such as name prefixes, suffixes, or meta information, such as Interest lifetime.
Context Identifier.
The 7-bit ContextID is a locally-scoped unique identifier that represents contextual state shared between sender and receiver of the corresponding frame (see ). If set the most significant bit indicates the presence of another, subsequent ContextID byte (see ). Context state shared between senders and receivers is removed from the compressed packet prior to sending, and reinserted after reception prior to passing to the upper stack. The actual information in a context and how it is encoded are out of scope of this document. The initial distribution and maintenance of shared context is out of scope of this document. Frames containing unknown or invalid CIDs MUST be silently discarded.
En-route State In CCNx and NDN, Name TLVs are included in Interest messages, and they return in data messages. Returning Name TLVs either equal the original Name TLV, or they contain the original Name TLV as a prefix. ICN LoWPAN reduces this redundancy in responses by replacing Name TLVs with single bytes that represent link-local HopIDs. HopIDs are carried as Context Identifiers (see ) of link-local scope as shown in .
Context Identifier as HopID.
A HopID is valid if not all ID bits are set to zero and invalid otherwise. This yields 127 distinct HopIDs. If this range (1...127) is exhausted, the messages MUST be sent without en-route state compression until new HopIDs are available. An ICN LoWPAN node that forwards without replacing the name by a HopID (without en-route compression) MUST invalidate the HopID by setting all ID-bits to zero. While an Interest is traversing, a forwarder generates an ephemeral HopID that is tied to a PIT entry. Each HopID MUST be unique within the local PIT and only exists 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). 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 ).
Setting compression state en-route (Interest). | B | ------------------------> | C | '---' '---' '---' ]]>
Responses include HopIDs that were obtained from Interests. If the returning Name TLV equals the original Name TLV, then the name is entirely elided. Otherwise, only the matching name prefix is elided and the distinct name 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 prior to forwarding the response ().
Eliding Name TLVs using en-route state (data).
It should be noted that each forwarder of an Interest in an ICN LoWPAN network can individually decide whether to participate in en-route compression or not. However, an ICN LoWPAN node SHOULD use en-route compression whenever the stateful compression mechanism is activated. Note also that the extensions of the PIT data structure are required only at ICN LoWPAN nodes, while regular NDN/CCNx forwarders outside of an ICN LoWPAN domain do not need to implement these extensions.
Integrating Stateful Header Compression A CID appears whenever the CID flag is set (see ). The CID is appended to the last ICN LoWPAN dispatch byte as shown in .
LoWPAN Encapsulation with ICN LoWPAN and CIDs
Multiple CIDs are chained together, with the most significant bit indicating the presence of a subsequent CID (). This allows to use multiple shared contexts in compressed messages. The HopID is always included as the very first CID.
Chaining of context identifiers. |1| CID | --> |0| CID | +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+ ]]>
ICN LoWPAN Constants and Variables This is a summary of all ICN LoWPAN constants and variables.
DEFAULT_NDN_HOPLIMIT:
255
Implementation Report and Guidance The ICN LoWPAN scheme defined in this document has been implemented as an extension of the NDN/CCNx software stack in its IoT version on RIOT . An experimental evaluation for NDN over ICN LOWPAN with varying configurations has been performed in . Energy profilings and processing time measurements indicate significant energy savings, while amortized costs for processing show no penalties.
Preferred Configuration The header compression performance depends on certain aspects and configurations. It works best for the following cases:
  • Signed time offsets compress as per without the need for rounding.
  • Contextual state (e.g., prefixes) is distributed, such that long names can be elided from Interest and data messages.
  • Frequently used TLV type numbers for CCNx and NDN stay in the lower range (< 255).
Name components are of GenericNameComponent type and are limited to a length of 15 bytes to enable compression for all messages.
Further Experimental Deployments An investigation of ICN LoWPAN in large-scale deployments with varying traffic patterns using larger samples of the different board types available remains as future work. This document will be revised to progress it to the Standards Track, once sufficient operational experience has been acquired. Experience reports are encouraged, particularly in the following areas:
  • The name compression scheme () is optimized for short name components of GenericNameComponent type. An empirical study on name lengths in different deployments of selected use cases, such as smart home, smart city, and industrial IoT can provide meaningful reports on necessary name component types and lengths. A conclusive outcome helps to understand whether and how extension mechanisms are needed (). As a preliminary analysis, investigates the effectiveness of the proposed compression scheme with URLs obtained from the WWW. Studies on CoAP deployments can offer additional insights on naming schemes in the IoT.
  • The fragmentation scheme () inherited from 6LoWPAN allows for a transparent, hop-wise reassembly of CCNx or NDN packets. Fragment forwarding with selective fragment recovery can improve the end-to-end latency and reliability, while it reduces buffer requirements on forwarders. Initial evaluations () show that a naive integration of these upcoming fragmentation features into ICN LoWPAN renders the hop-wise content replication inoperative, since Interest and data messages are reassembled end-to-end. More deployment experiences are necessary to gauge the feasibility of different fragmentation schemes in ICN LoWPAN.
  • Context state () holds information that is shared between a set of devices in a LoWPAN. Fixed name prefixes and suffixes are good candidates to be distributed to all nodes in order to elide them from request and response messages. More experience and a deeper inspection of currently available and upcoming protocol features is necessary to identify other protocol fields.
  • The distribution and synchronization of contextual state can potentially be adopted from , but requires further evaluations. While 6LoWPAN uses the Neighbor Discovery protocol to disseminate state, CCNx and NDN deployments are missing out on a standard mechanism to bootstrap and manage configurations.
  • The stateful en-route compression () supports a limited number of 127 distinct HopIDs that can be simultaneously in use on a single node. Complex deployment scenarios that make use of multiple, concurrent requests can provide a better insight on the number of open requests stored in the Pending Interest Table of memory-constrained devices. This number can serve as an upper-bound and determines whether the HopID length needs to be resized to fit more HopIDs to the cost of additional header overhead.
  • Multiple implementations that generate and deploy the compression options of this memo in different ways will also add to the experience and understanding of the benefits and limitations of the proposed schemes. Different reports can help to illuminate on the complexity of implementing ICN LoWPAN for constrained devices, as well as on maintaining interoperability with other implementations.
Security Considerations Main memory is typically a scarce resource of constrained networked devices. Fragmentation as described in this memo preserves fragments and purges them only after a packet is reassembled, which requires a buffering of all fragments. 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. Implementers are thus urged to make use of appropriate buffer replacement strategies for fragments. Minimal fragment forwarding can potentially prevent fragment buffer saturation in forwarders. 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 protective mechanisms for accessing the link-layer should be deployed. IEEE 802.15.4, e.g., provides capabilities to protect frames and restrict them to a point-to-point link, or a group of devices.
IANA Considerations
Reserving Space in the 6LoWPAN Dispatch Type Field Registry IANA has assigned dispatch values of the 6LoWPAN Dispatch Type Field registry with Page TBD1 for ICN LoWPAN. represents updates to the registry. Dispatch types for NDN and CCNx with page TBD1.
Bit Pattern Page Header Type
00 000000 TBD1 Uncompressed NDN Interest messages
00 100000 TBD1 Uncompressed NDN Data messages
01 000000 TBD1 Uncompressed CCNx Interest messages
01 100000 TBD1 Uncompressed CCNx Content Object messages
10 0xxxxx TBD1 Compressed NDN Interest messages
10 1xxxxx TBD1 Compressed NDN Data messages
11 0xxxxx TBD1 Compressed CCNx Interest messages
11 1xxxxx TBD1 Compressed CCNx Content Object messages
References Normative References IEEE Std. 802.15.4-2015 Standard for Floating-Point Arithmetic Informative References CCN-lite: A lightweight CCNx and NDN implementation RIOT: an Open Source Operating System for Low-end Embedded Devices in the IoT INRIA HAW Hamburg INRIA and FU Berlin HAW Hamburg FU Berlin FU Berlin INRIA and FU Berlin HAW Hamburg FU Berlin Information Centric Networking in the IoT: Experiments with NDN in the Wild INRIA FU Berlin INRIA HAW Hamburg FU Berlin NDN, CoAP, and MQTT: A Comparative Measurement Study in the IoT HAW Hamburg HAW Hamburg FU Berlin FU Berlin HAW Hamburg FU Berlin The Need for a Name to MAC Address Mapping in NDN: Towards Quantifying the Resource Gain HAW Hamburg HAW Hamburg HAW Hamburg riot-os.org FU Berlin Connecting the Dots: Selective Fragment Recovery in ICNLoWPAN FU Berlin HAW Hamburg HAW Hamburg FU Berlin Networking Named Content NDN Packet Format Specification CCN and NDN TLV encodings in 802.15.4 packets CCN/NDN Protocol Wire Format and Functionality Considerations ICNLoWPAN -- Named-Data Networking in Low Power IoT Networks HAW Hamburg HAW Hamburg HAW Hamburg FU Berlin
Estimated Size Reduction 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.
NDN The NDN TLV encoding has variable-sized TLV fields. For simplicity, the 1 byte form of each TLV component is assumed. A typical TLV component therefore is of size 2 (type field + length field) + the actual value.
Interest 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 bytes.
Estimated size of an uncompressed NDN Interest
depicts the size requirements after compression.
Estimated size of a compressed NDN Interest
The size difference is: 11 + 1.5n bytes. For the name /DE/HH/HAW/BT7, the total size gain is 17 bytes, which is 43% of the uncompressed packet.
Data depicts the size requirements for a basic, uncompressed NDN Data containing a FreshnessPeriod as MetaInfo. A FreshnessPeriod of 1 minute is assumed and the value is encoded using 1 byte. An HMACWithSha256 is assumed as signature. The key locator is assumed to contain a Name TLV of length klen.
Estimated size of an uncompressed NDN Data
depicts the size requirements for the compressed version of the above Data packet.
Estimated size of a compressed NDN Data
The size difference is: 15 + 1.5n bytes. For the name /DE/HH/HAW/BT7, the total size gain is 21 bytes.
CCNx The CCNx TLV encoding defines a 2-byte encoding for type and length fields, summing up to 4 bytes in total without a value.
Interest depicts the size requirements for a basic, uncompressed CCNx Interest. No Hop-By-Hop TLVs are included, the protocol version is assumed to be 1 and the reserved field is assumed to be 0. A KeyIdRestriction TLV with T_SHA-256 is included to limit the responses to Content Objects containing the specific key.
Estimated size of an uncompressed CCNx Interest
depicts the size requirements after compression.
Estimated size of a compressed CCNx Interest
The size difference is: 18 + 3.5n bytes. For the name /DE/HH/HAW/BT7, the size is reduced by 53 bytes, which is 53% of the uncompressed packet.
Content Object depicts the size requirements for a basic, uncompressed CCNx Content Object containing an ExpiryTime Message TLV, an HMAC_SHA-256 signature, the signature time and a hash of the shared secret key. In the fixed header, the protocol version is assumed to be 1 and the reserved field is assumed to be 0
Estimated size of an uncompressed CCNx Content Object
depicts the size requirements for a basic, compressed CCNx Data.
Estimated size of a compressed CCNx Data Object
The size difference is: 35 + 3.5n bytes. For the name /DE/HH/HAW/BT7, the size is reduced by 70 bytes, which is 40% of the uncompressed packet containing a 4-byte payload.
Acknowledgments 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, Colin Perkins, Junxiao Shi. 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 and . Special mentioning goes to Mark Mosko as well as G.Q. Wang and Ravi Ravindran as their previous work in and provided a good base for our discussions on stateless header compression mechanisms. Many thanks also to Carsten Bormann and Lars Eggert, who contributed in-depth comments during the IRSG review. This work was supported in part by the German Federal Ministry of Research and Education within the projects I3 and RAPstore.