]> Ericsson

daniel.migault@ericsson.com

Concordia University

maryam.hatami@mail.concordia.ca

Concordia University

sandra.cespedes@concordia.ca

Concordia University

william.atwood@concordia.ca

Ericsson

harold.liu@ericsson.com

LMU

guggemos@nm.ifi.lmu.de

Universitaet Bremen TZI

cabo@tzi.org

Google LLC

dschinazi.ietf@gmail.com

Security IPsecme Internet-Draft This document specifies Diet-ESP, a compression mechanism for control information in IPsec/ESP communications. The compression is expressed through the Static Context Header Compression architecture.

Requirements notation The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Introduction The Encapsulating Security Payload (ESP) protocol is part of the IPsec suite of protocols and can provide confidentiality, data origin authentication, integrity, anti-replay, and traffic flow confidentiality. The set of services ESP provides depends on the Security Association (SA) parameters negotiated between devices. An ESP packet is composed of the ESP Header, the ESP Payload Data, the ESP Trailer, and the Integrity Check Value (ICV). ESP has two modes of operation: Transport and Tunnel. In Transport mode, the ESP Payload Data consists of the payload of the original IP packet; the ESP Header is inserted after the original IP packet header. In Tunnel mode, commonly used for VPNs, the ESP Header is placed after an outer IP header and before the inner IP packet headers of the original datagram. This ensures both the original IP headers and payload are protected. Consequently, the ESP Data Payload field contains either the payload from the original IP packet or the fully-encapsulated IP packet, in transport mode or tunnel mode, respectively. The ESP Trailer, placed at the end of the ESP Payload Data, includes fields such as Padding and Pad Length to ensure proper alignment, and Next Header to indicate the protocol following the ESP header. The ICV, calculated over the ESP Header, ESP Data Payload, and ESP Trailer, is appended after the ESP Trailer to ensure packet integrity. For a simplified overview of ESP, readers are referred to Minimal ESP . While ESP is effective in securing traffic, compression can reduce packet sizes, enhancing performance in networks with limited bandwidth. In such environments, reducing the size of transmitted packets is essential to improve efficiency. This document defines Diet-ESP, a protocol that includes compression/decompression (C/D) of the various structures processed by ESP. These C/D are expressed through the Static Context Header Compression and Fragmentation (SCHC) framework . The structure of the ESP packet to be compressed is shown in .

The Three compressors described in this specification The document outlines the three compressors utilized in Diet-ESP, which are detailed as follows: Inner IP Compression (IIPC): This process pertains to the compression and decompression of the Header of the Inner IP packet (IIP), i.e., the original packet to be protected by ESP. For outbound packets, after IIPC, ESP incorporates the compressed Header and the Payload into the ESP Data Payload of a Clear Text ESP packet (CTE) (refer to ). In the case of inbound packets, decompression occurs after the compressed Header is retrieved from the ESP Payload Data within the CTE. Clear Text ESP Compression (CTEC): This process pertains to the compression and decompression of the segment of the ESP packet that is destined for encryption. This encompasses the ESP Data Payload and the ESP Trailer, which includes the Padding, Pad Length, and Next Header fields, as illustrated in . At this stage, only the latter fields are eligible for compression. For outbound packets, ESP subsequently encrypts the compressed CTE packet. For inbound packets, decompression takes place following the decryption process of the ESP. Encrypted ESP Compression (EEC): This process pertains to the compression and decompression of the Encrypted ESP packet (EE), which consists of the ESP Header, the encrypted payload, and the Integrity Check Value (ICV). Since neither the encrypted payload nor the ICV can be compressed, only the ESP Header, specifically the SPI and SN fields, are subject to compression.

The scope of SCHC in this specification SCHC offers a mechanism for header compression as well as an optional fragmentation feature. SCHC facilitates the compression and decompression of headers by utilizing a common context that may encompass multiple Rules. Each Rule is designed to correspond with specific values or ranges of values within the header fields. When a Rule is successfully matched, the corresponding header fields are substituted with the Rule ID and the Compression Residue. The Compression Residue for the packet header is the concatenation of the non-empty residues for each field of the header. The Compression Residue is directly followed by the packet payload and an optional padding to ensure byte alignment. This document utilizes SCHC as a practical means to illustrate the capability to compress and decompress a structured payload. It is important to note that any elements of SCHC that pertain to aspects other than compression or decompression, such as fragmentation, fall outside the purview of this document. The reference to SCHC herein is solely for descriptive purposes related to compression and decompression, and it is not anticipated that the general SCHC framework will be integrated into the ESP implementation. The structured payloads addressed in this specification pertain to internal structures managed by ESP for the processing of an IP packet. Consequently, the compression and decompression processes outlined in this document represent supplementary steps for the ESP stack in handling the ESP packet.

Diet-ESP Rules and Context In IPsec, the process of encryption or decryption between IPsec peers necessitates a common context known as a Security Association (SA). More broadly, the SA encompasses all essential parameters required by the ESP to handle both inbound and outbound packets. SAs are unidirectional. Furthermore, IPsec can link both outbound and inbound IP packets to the SA through Traffic Selectors (TS) or Security Parameter Index (SPI). This capability allows IPsec to uniquely associate outbound and inbound packets with a specific context (SA), which contains all pertinent information for IPsec processing. This document adopts a comparable methodology for compression and decompression, ensuring that the SA includes all necessary parameters to create the unique Rule applicable for compressing or decompressing each structured payload. This guarantees that each SA in Diet-ESP is linked to a single Rule, thereby allowing the Rule ID in Diet-ESP to be empty. If future implementations support differentiated compression for multiple flows over the same SA, the Rule ID may not be omitted. The Rule associated with each structured payload is generated based on specific parameters referred to in this document as Attributes for Rule Generation (AfRG) (see for a more detailed description). These AfRGs are negotiated through IKEv2 , and in such cases, they are likely already included in the SA. Any additional missing AfRGs are negotiated via .

Terminology

ESP Header Compression:

A method to reduce the size of ESP headers and trailer using predefined compression rules and contexts to improve efficiency.

ESP Trailer:

A set of fields added at the end of the ESP payload, including Padding, Pad Length, and Next Header, used to ensure alignment and indicate the next protocol.

Inner IP C/D (IIPC):

Process that compresses/decompresses the inner IP packet headers.

Clear Text ESP C/D (CTEC):

Process that compresses/decompresses all fields that will later be encrypted by ESP, which include the ESP Data Payload and ESP Trailer.

Encrypted ESP C/D (EEC):

Process that compresses/decompresses ESP fields not encrypted by ESP.

Security Parameter Index (SPI):

As defined in .

Sequence Number (SN):

As defined in .

Static Context Header Compression (SCHC):

A framework for header compression designed for LPWANs, as defined in .

Static Context Header Compression Rules (SCHC Rules):

As defined in

RuleID:

A unique identifier for each Rule part of the Diet-ESP context.

SCHC Parameters:

A set of predefined values used for SCHC compression and decompression, ensuring byte alignment and proper packet formatting based on the SCHC profile.

Traffic Selector (TS):

A set of parameters (e.g., IP address range, port range, and protocol) used to define which traffic should be protected by a specific Security Association (SA).

It is assumed that the reader is familiar with other SCHC terminology defined in , , and eventually .

Diet-ESP Integration into the IPsec Stack depicts the incorporation of Diet-ESP within the IPsec framework. IPsec requires that both endpoints agree on a shared context known as the Security Association (SA). This SA is established via IKEv2 and encompasses all Attributes for Rule Generation (AfRG) (refer to ) essential for formulating the Rules for each compressor defined in , specifically the Inner IP packet Compressor (IIPC), the Clear Text ESP Compressor (CTEC), and the Encrypted ESP Compressor (EEC). When an Inner IP packet (IIP) is received, IPsec identifies the SA linked to that packet. Upon the ESP determining the IIPC Rule from the AfRG contained within the SA, the IIPC separates the IIP into Header and Payload, and compresses the Header. The compressed Header is composed of RuleID, Compressed Residue, and an optional padding field. The original payload of the IIP is then appended after the compressed header (IIPC: C{Header}, Payload). Subsequently, ESP constructs the Clear Text ESP packet (CTE). The CTEC Rule is derived from the AfRG of the SA, allowing for the compression of the CTE (CTEC: C {C{Header}, Payload, ET}, where ET represents the ESP Trailer). Then, ESP encrypts the ESP Data Payload, computes the Integrity Check Value (ICV), and forms the Encrypted ESP packet (EE). The EE Rule is derived from the AfRG of the SA, and then utilized to compress the EE (C {EH, C{C{Header}, Payload, ET}}, ICV}, where EH represents the ESP Header). The resulting compressed ESP extension is integrated into an IP packet and transmitted as outbound traffic. For inbound traffic, the endpoint extracts the Security Parameter Index (SPI) from the compressed EE, along with any other selectors from the packet, to conduct a lookup for the SA. As outlined in , since the SPI is derived from a potentially compressed ESP Header, there may be instances where the endpoint must explore multiple options, potentially leading to several lookups or, in the worst-case scenario, multiple signature verifications (see for a more detailed discussion). Once the SA is retrieved, the ESP accesses the AfRG to ascertain the EEC Rule and proceeds to decompress the EE. The ESP verifies the signature prior to decryption. Following this, the CTEC Rule is derived from the AfRG of the SA, allowing for the subsequent decompression. Finally, ESP extracts the Data Payload from the CTE packet, retrieves the IIPC Rule from the AfRG of the SA, and decompresses the Header. Note that implementations MAY differ from the architectural description but it is assumed that the output will be the same.

SCHC Parameters for Diet-ESP The SCHC Packet is always in the form:

The RuleID is a unique identifier for each SCHC Rule. It is included in packets to ensure the receiver applies the correct decompression rule, maintaining consistency in packet processing. Note that the Rule ID does not need to be explicitly agreed upon and can be defined independently by each party. Furthermore, indicates that the way the RuleID is sent is left open to the profile specification. The RuleID in Diet-ESP is expressed as 1 byte but it can be elided as there is a unique Rule determined for the compressors. The 1 byte may be used in future implementations to support multiple flows over the same SA. SCHC padding in SCHC serves the purpose of aligning data to a designated boundary, which is typically byte-aligned or aligned to 8 bits. This document presumes that this field is not utilized in CET and EEC. This document outlines a simpler form of padding for byte-alignment, as detailed in section . Such alignment is essential to ensure that encryption is applied to data that is byte-aligned. The rationale for employing a padding method other than SCHC Padding is to accommodate the length of the compressed ESP Payload Data. Another variable required for the C/D in Diet-ESP is the Maximum Packet Size (MAX_PACKET_SIZE) determined by the specific IPsec ESP configuration and the underlying transport, but it is typically aligned with the network’s MTU. The size constraints are optimized based on the available link capacity and negotiated parameters between endpoints.

Attributes for Rule Generation The list of attributes for the Rule Generation (AfRG) is shown in . These attributes are used to express the various compressions that operate at the IIPC, CTEC, and EEC. As outlined in , this specification does not detail the process by which the AfRG are established between peers. Instead, such negotiations are addressed in . However, the AfRG can be classified into two distinct categories. The first category encompasses AfRG that are negotiated through a specific IKEv2 extension tailored for the negotiation of AfRG linked to a particular profile, the Diet-ESP profile in this context. The AfRG referenced in in this category are: the DSCP Compression/Decompression Action (CDA) dscp_cda, the ECN CDA ecn_cda, the Flow Label CDA flow_label_cda, the ESP alignment alignment, the ESP SPI Least Significant Bits (LSB) esp_spi_lsb, and the ESP Sequence Number LSB esp_sn_lsb. The second category pertains to AfRG that are negotiated through IKEv2 exchanges or extensions that are not specifically designed for compression purposes. This category includes AfRG associated with TS, as identified in , which are the TS IP Version ts_ip_version, the TS IP Source Start ts_ip_src_start, the TS IP Source End ts_ip_src_end, the TS IP Destination Start ts_ip_dst_start, the TS IP Destination End ts_ip_dst_end, the TS Protocol ts_proto, the TS Port Source Start ts_port_src_start, the TS Port Source End ts_port_src_end, the TS Port Destination Start ts_port_dst_start, and the TS Port Destination End ts_port_dst_end. These AfRG are derived from the Traffic Selectors established through TSi/TSr payloads during the IKEv2 CREATE_CHILD_SA exchange, as described in . The AfRG IPsec Mode designated as ipsec_mode in is determined by the presence or absence of the USE_TRANSPORT_MODE Notify Payload during the CREATE_CHILD_SA exchange, as detailed in . The AfRG Tunnel IP designated as tunnel_ip in is obtained from the IP address of the IKE messages exchanged during the CREATE_CHILD_SA process, as noted in . The AfRGs designated as ESP Encryption Algorithm esp_encr and ESP Security Parameter Index (SPI) esp_spi in are established through the SAi2/SAr2 payloads during the CREATE_CHILD_SA exchange, while the AfRG designated as ESP Sequence Number esp_sn in is initialized upon the creation of the Child SA and incremented for each subsequent ESP message. The DSCP values identified as dscp_list in are established through the DSCP Notify Payload . The ability to derive the IIP Compressor Rules for the internal IP packet from the agreed Traffic Selectors is indicated by the variable iipc_profile. Variable Possible Values Reference Compressor iipc_profile "iipc_diet-esp", "iipc_not_compressed" ThisRFC N/A dscp_cda "not_compressed", "lower", "sa" ThisRFC IIPC ecn_cda "not_compressed", "lower" ThisRFC IIPC flow_label_cda "not_compressed", "lower", "generated", "zero" ThisRFC IIPC ts_ip_version "IPv4-only", "IPv6-only" RFC7296 IIPC ts_ip_src_start IPv4 or IPv6 address RFC7296 IIPC ts_ip_src_end IPv4 or IPv6 address RFC7296 IIPC ts_ip_dst_start IPv4 or IPv6 address RFC7296 IIPC ts_ip_dst_end IPv4 or IPv6 address RFC7296 IIPC ts_proto TCP, UDP, UDP-Lite, SCTP, ANY, ... RFC7296 IIPC ts_port_src_start Port number RFC7296 IIPC ts_port_src_end Port number RFC7296 IIPC ts_port_dst_start Port number RFC7296 IIPC ts_port_dst_end Port number RFC7296 IIPC dscp_list list of DSCP numbers RFCYYYY IIPC alignment "8 bit", "16 bit", "32 bit", "64 bit" ThisRFC CTEC esp_trailer "Mandatory", "Optional" ThisRFC CTEC ipsec_mode "Tunnel", "Transport" RFC4301 CTEC tunnel_ip IPv4 or IPv6 address RFC4301 CTEC esp_encr ESP Encryption Algorithm RFC4301 CTEC esp_spi ESP SPI RFC4301 EEC esp_spi_lsb 0-32 ThisRFC EEC esp_sn ESP Sequence Number RFC4301 EEC esp_sn_lsb 0-32 ThisRFC EEC Any variable starting with "ts_" is associated with the Traffic Selectors (TSi/TSr) of the SA. The notation is introduced by this specification but the definitions of the variables are in and . The Traffic Selectors may result in a quite complex expression, and this specification restricts that complexity. This specification restricts the resulting TSi/TSr to a single type of IP address (IPv4 or IPv6), a single protocol (e.g., UDP, TCP, or ANY), a single port range for source and destination. This specification presumes that the Traffic Selectors can be articulated as a result of CREATE_CHILD_SA with only one Traffic Selector in both TSi and TSr payloads (as described in ). The TS Type MUST be either TS_IPV4_ADDR_RANGE or TS_IPV6_ADDR_RANGE. Let the resulting Traffic Selectors TSi/TSr be expressed via the Traffic Selector structure defined in . We designate the local TS the TS - either TSi or TSr - sent by the local peer. Conversely we designate as remote TS the TS - either TSi or TSr - sent by the remote peer. The details of each parameter are the following:

iipc_profile:

designates the behavior of the IIPC layer. When set to "iipc_not_compressed" IIPC is not performed. This specification describes IIPC that corresponds to the "iipc_diet-esp" profile.

flow_label_cda:

indicates the Flow Label CDA, that is how the Flow Label field of the inner IPv6 packet or the Identification field of the inner IPv4 packet is compressed / decompressed - See for more information. In a nutshell, "not_compressed" indicates that Flow Label (resp. Identification) is not compressed. "lower" (using compute-*) indicates the value is read from the outer IP header - eventually with some adaptations when inner IP packet and outer IP packets have different versions. "generated" indicates that the field is generated by the receiving party. In that case, the decompressed value may take a different value compared to its original value. "zero" indicates the field is set to zero.

dscp_cda:

indicates the DSCP CDA, that is how the DSCP values of the inner IP packet are compressed / decompressed - See for more information. In a nutshell, "not_compressed" indicates that DSCP are not compressed. "lower" (using compute-*) indicates the value is read from the outer IP header - eventually with some adaptations when inner IP packet and outer IP packets have different versions. "sa" indicates, compression is performed according to the DSCP values agreed by the SA (dscp_list).

ecn_cda:

indicates ECN CDA, that is how the ECN values of the inner IP packet are compressed / decompressed - See for more information. In a nutshell, "not_compressed" indicates that DSCP are not compressed. "lower" (using compute-*) indicates the value is read from the outer IP header - eventually with some adaptations when inner IP packet and outer IP packets have different versions.

ts_ip_version:

designates the TS IP version. Its value is set to "IPv4-only" when only IPv4 IP addresses are considered and to "IPv6-only" when only IPv6 addresses are considered. Practically, when IKEv2 is used, it means that the agreed TSi or TSr results only in a mutually exclusive combination of TS_IPV4_ADDR_RANGE or TS_IPV6_ADDR_RANGE payloads. If TS Type of the resulting TSi/TSr is set to TS_IPV4_ADDR_RANGE, ts_ip_version takes the value "IPv4-only". Respectively, if TS Type is set to TS_IPV6_ADDR_RANGE, ts_ip_version is set to "IPv6-only".

ts_ip_src_start:

designates the TS IP Source Start, that is the starting value range of source IP addresses of the inner packet and has the same meaning as the Starting Address field of the local TS.

ts_ip_src_end:

designates TS IP Source End, that is the high end value range of source IP addresses of the inner packet and has the same meaning as the Ending Address field of the local TS.

ts_ip_dst_start:

designates the TS IP Destination Start, that is the starting value range of destination IP addresses of the inner packet and has the same meaning as the Starting Address field of the remote TS.

ts_ip_dst_end:

designates the TS IP Destination End, that is the high end value range of destination IP addresses of the inner packet and has the same meaning as the Ending Address field of the remote TS.

ts_proto:

designates the TS Protocol, that is the Protocol ID of the resulting TSi/TSr. This profile considers the specific protocol values "TCP", "UDP", "UDP-Lite", "SCTP", and "ANY". The representation of "ANY" is given in .

ts_port_src_start:

designates the TS Port Source Start, that is the the starting value of the source port range of the inner packet and has the same meaning as the Start Port field of the local TS.

ts_port_src_end:

designates the TS Port Source End, that is the high end value range of the source port range of the inner packet and has the same meaning as the End Port field of the local TS.

ts_port_dst_start:

designates TS Port Destination Start, that is the starting value of the destination port range of the inner packet and has the same meaning as the Start Port field of the remote TS.

ts_port_dst_end:

designates TS Port Destination End, that is the high end value range of the destination port range of the inner packet and has the same meaning as the End Port field of the remote TS.

IP addresses and ports are defined as a range and compressed using the Least Significant Bits (LSB). For a range defined by start and end values, msb( start, end ) is defined as the function that returns the Most Significant Bits (MSB) that remains unchanged while the value evolves between start and end. Similarly, lsb( start, end ) is defined as the function that returns the LSB that changes while the value evolves between start and end. Finally, len( x ) is defined as the function that returns the number of bits of the bit array x.

dscp_list:

designates the list of DSCP values associated to the inner traffic - see for example . These are not Traffic Selectors, but the compression mandates that the packets take one of these listed DSCP values.

alignment:

designates the ESP alignment as defined by .

esp_trailer:

When configured to "Mandatory," it signifies that the implementation requires the ESP Trailer to include a Next Header and a Pad Len field, as outlined in . This requirement primarily aims to ensure compatibility with current hardware implementations of ESP, as detailed in . Conversely, if set to "Optional," it indicates that the implementation is capable of supporting the compression of the ESP Trailer.

ipsec_mode:

designates the IPsec Mode defined in . In this document, the possible values are "tunnel" for the Tunnel mode and "transport" for the Transport mode.

tunnel_ip:

designates the Tunnel IP address of the tunnel defined in . This field is only applicable when the Tunnel mode is used. That IP address can be an IPv4 or IPv6 address.

esp_encr:

designates the ESP Encryption Algorithm - also designated as Transform 1 in . The algorithm is needed to determine whether the ESP Payload Data needs to be aligned to some predefined block size and if the ESP Pad Length and Padding fields can be compressed. For the purpose of compression it is RECOMMENDED to use algorithms that already compressed their IV .

esp_spi:

designates the Security Parameter Index defined in .

esp_spi_lsb:

designates the LSB to be considered for the compressed SPI. A value of 32 for esp_spi_lsb will leave the SPI unchanged.

esp_sn:

designates the ESP Sequence Number (SN) field defined in .

esp_sn_lsb:

designates the LSB to be considered for the compressed SN. It works similarly to ESP SPI LSB (see esp_spi_lsb).

Diet-ESP SCHC Rule Table defines the SCHC compression rules for Diet-ESP in IPsec using specific Compression/Decompression Actions (CDAs) for this profile in addition to the ones defined in . These CDAs are either a refinement of the compute-* CDA or the result of a combined CDA. It specifies how various IPv6, UDP/TCP, and ESP header fields are compressed and decompressed. Field FL FP DI TV MO CDA Version 3 1 Bi ts_ipversion equal not-sent DSCP 6 1 Dw — ignore value-sent ECN 2 1 Dw — ignore value-sent Flow Label 20 1 Dw — ignore compute-* Payload Length 16 1 Bi — ignore compute-* Next Header 8 1 Bi ts_proto equal not-sent Hop Limit 8 1 Dw — elided compute-* Source Address 128 1 Bi msb(src_start, src_end) MSB LSB Destination Address 128 1 Bi msb(dst_start, dst_end) MSB LSB Source Port (UDP/TCP) 16 1 Bi msb(src_port_start, end) MSB LSB Destination Port (UDP/TCP) 16 1 Bi msb(dst_port_start, end) MSB LSB Checksum 16 1 Bi — ignore compute-* UDP Length 16 1 Bi — ignore compute-* ESP Padding - - - — compute-* elided Byte Alignment 8 1 Bi — compute-* send SPI 32 1 Dw — MSB(4 - esp_spi_lsb) LSB SN 32 1 Dw — MSB(4 - esp_sn_lsb) LSB Here are a few key features from the table: The IPv6 Header Compression includes the following fields: Version, DSCP, ECN, Flow Label, etc. The UDP/TCP Header Compression includes the following fields: Ports, Checksum, Length. The ESP Header Compression fields include the following: Padding, Sequence Number (SN), SPI. The Byte Alignment Padding is maintained to comply with the byte-aligned ESP Payload requirement. Here are the additional CDAs defined in this profile:

Diet-ESP for IPsec in Tunnel mode

Inner IP Compression (IIPC) When iipc_profile is set to "iipc_not_compressed", the packet is not compressed. When iipc_profile is set to "iipc_diet-esp", IIPC proceeds to the compression of the inner IP Packet composed of Header and Payload. In the latter case, the Header is compressed when ipsec_mode is set to "Tunnel" and not compressed otherwise. ts_ip_version determines how the IPv6 Header (resp. the IPv4 header) is compressed - see (resp. ). The SCHC packet illustrated in appends the padding at the end of the SCHC Packet. This approach presents notable challenges, including handling a Payload that lacks byte alignment. Furthermore, the absence of a specified Payload length would necessitate the inclusion of the padding length within the padding itself, which would require a particular padding construction akin to that utilized by the Padding ESP, thereby posing difficulties for hardware implementations. To address these issues, IIPC uses a pre-processing phase where the IIP is divided into two segments: the Header, which is subject to compression, and the Payload, which is not. Following this, the compression process applies the defined rule to the Header, resulting in a SCHC Packet (refer to ) that features an empty SCHC Payload field, with a padding field positioned after the Compression Residue. Ultimately, a post-processing phase merges the SCHC Packet with the Payload, allowing the compressor to produce an IIPC packet in the format outlined in .

|<-0..7 bits->|<---n bytes--->| +-+-+-+-+-+-+-+-+---------...---------+~~~~~~~~~~~~~+---------------+ | RuleID | Compression Residue | padding | Payload | +-+-+-+-+-+-+-+-+---------...---------+~~~~~~~~~~~~~+---------------+ |-------- Compressed Header ----------|--as needed--| ]]>

It is important to observe that the resulting packet is byte-aligned. Additionally, the division between Header and Payload can only occur because all the Header fields are of a fixed size.

Inner IP Payload Compression This section describes the compression of the inner IP Payload. The compression described herein only affects UDP, UDP-Lite, TCP or SCTP segments. The type of segment is specified in the IP Header. For UDP, UDP-Lite, TCP and SCTP segments, source ports, destination ports, and checksums are compressed. For source port (resp. destination port) only the least significant bits are sent. FL is set to 16 bits, TV is set to msb( ts_port_src_start, ts_port_src_end ) ( resp. ts_port_dst_start, ts_port_dst_end ), MO is set to "MSB" and CDA to "LSB". The checksum is elided, FL is set to 16 bits, TV is not set, MO is set to "ignore" and CDA is set to "checksum". This may result in decompressing a zero-checksum UDP packet with a valid checksum, but this has no impact as a valid checksum is universally accepted. For UDP or UDP-Lite the length field is elided. FL is set to 16, TV is not set, MO is set to "ignore".

Inner IPv6 packet Headers Compression The version field is elided, FL is set to 3, TV is set to ts_ipversion, MO is set to "equal" and CDA is set to "not-sent". Traffic Class is composed of the 6 bit DSCP and 2 bit ECN. The compression of DSCP and ECN are defined independently. DSCP values are compressed according to the dscp_cda value: If dscp_cda is set to "not_compressed", the DSCP values are included in the inner IP header. FL is set to 6 bits, TV is not set, MO is set to "ignore", CDA is set to "sent-value". If dscp_cda is set to "lower", the DSCP field is elided and its value is copied from the Tunnel IP header. FL is set to 6 bits, TV is not set, MO is set to "ignore", CDA is set to "lower". If dscp_cda is set to "sa", DSCP is compressed according to the DSCP values of the SA. If dscp_list contains a single element, the DSCP is elided, FL is set to 6 bits, TV is set to dscp_list[0], MO is set to "equal" and CDA is set to "not-sent". If dscp_list contains more than one DSCP value, FL is set to 6 bits, TV is set to dscp_list, MO is set to "match-mapping" and the CDA is set to "mapping-sent". For ECN, FL is set to 2 bits, TV is not set, MO is set to ignore and CDA is set to "value-sent". ECN values are compressed according to the ecn_cda value: If ecn_cda is set to "not_compressed", the ECN field is included in the inner IP header. FL is set to 2 bits, TV is not set, MO is set to "ignore", CDA is set to "sent-value". If ecn_cda is set to "lower", the ECN value is elided and the ECN value is copied in the outer IP header. FL is set to 2 bits, TV is not set, MO is set to "ignore", CDA is set to "lower". Flow label is compressed according to the flow_label_cda value: If flow_label_cda is set to "not_compressed", the Flow label is included in the IPv6 Header. FL is set to 20 bits, TV is not set, MO is set to "ignore", and CDA is set to "sent-value". If flow_label_cda is set to "lower", the Flow Label is elided and read from the outer IP Header (using compute-*(See )). FL is set to 20 bits, TV is not set, MO is set to "ignore", and CDA is set to "lower". If the outer IP header is an IPv4 header, only the 16 LSB of the Flow Label are inserted into the IPv4 Header. At the decompression, the 4 MSB of the Flow Label are set to 0. If flow_label_cda is set to "generated", the Flow Label is elided and the Flow Label is then re-generated (using compute-*) at the decompression (See ). The resulting Flow Label differs from the initial value. FL is set to 20, TV is not set, MO is set to "ignore" and CDA is set to "generated". If flow_label_cda is set to "zero", the Flow Label is elided and set to 0 at decompression. A 0 value indicates no flow label is present. Fl is set to 20 bits, TV is set to 0, MO is set to "equal" and CDA is set to "not-sent". Payload Length is elided and determined from the Tunnel IP Header Payload Length as well as the decompressed Payload. FL is set to 16 bits, TV is not set, MO is set to "ignore", CDA is set to "lower". Next Header is compressed according to ts_proto: If ts_proto is the single value 0, Next Header is not compressed. FL is set to 8 bits, TV is not set, MO is set to "ignore", CDA is set to "sent-value". If ts_proto is a single non zero value, Next Header is compressed. FL is set to 8 bits, TV is set to ts_proto, MO is set to "equal" and CDA is set to "not-sent". The IPv6 Hop Limit is read from the Tunnel IP Header Hop Limit. FL is set to 8 bits, TV is not set, MO is set to "ignore" and CDA is set to "lower." The source and destination IPv6 addresses are compressed using MSB. In both cases, FL is set to 128, TV is respectively set to msb(ts_ip_src_start, ts_ip_src_ed) or msb(ts_ip_dst_start, ts_ip_dst_end), the MO is set to "MSB," and the CDA is set to "LSB."

Inner IPv4 packet Header Compression The fields Version, DSCP, ECN, Source Address and Destination Address are compressed as described for IPv6 in . The field Total Length (16 bits) is compressed similarly to the IPv6 field Payload Length. The field Identification (16 bits) is compressed similarly to the IPv6 field Flow Label. If the tunnel IP Header is an IPv6 Header, the Identification is placed as the LSB of the IPv6 Header and the 4 remaining MSB are set to 0. The field Time to Live is compressed similarly to the IPv6 Hop Limit field. The Protocol field is compressed similarly to the last IPv6 Next Header field. The Internet Header Length (IHL) is not compressed, FL is set to 4 bits, TV is not set, MO is set to ignore and CDA is set to "value-sent". The IPv4 Header checksum is elided. FL is set to 16, TV is omitted, MO is set to "ignore," and CDA is set to "checksum."

Clear Text ESP Compression (CTEC) Once the Inner IP Packet has undergone compression as outlined in , the resulting compressed packet comprises a specific number of bytes. To construct the Clear Text ESP Packet, it is necessary to ascertain the ESP Payload Data, the Next Header, the ESP Pad Length, and the ESP Padding fields. When esp_trailer is set to "Mandatory", the Next Header and the ESP Pad Length fields are present. Such requirement is usually expected to remain compatible with hardware implementations of ESP. The ESP Pad Length value is determined to meet the required alignment. When alignment is set to "8bit", Pad Length is set to 0 and the Padding field is empty. Conversely, when esp_trailer is set to "Optional", the Next Header Pad Length and Padding are generated as follows: In transport mode, the IP header of the inner packet remains not compressed during the IIPC phase, and the ESP Payload Data is derived from the inner packet. Conversely, in tunnel mode, the ESP Payload Data encompasses the entirety of the packet generated by the IIPC. In transport mode, the Next Header field is obtained from either the inner IP Header or the Security Association, as specified in or . In tunnel mode, the Next Header is elided, as it is determined by ts_ip_version. FL is set to 8 bit, TV is set to IPv4 or IPv6 depending on ts_ip_version, MO is set to "equal" and CDA is set to "not-sent". The ESP Pad Length and ESP Padding fields are omitted only when ESP alignment has been selected to "8bit" and esp_encr does not necessitate a specific block size alignment, or if that block size is one byte. This is represented by setting FL to (Pad Length + 1) * 8 bits, leaving TV unset, configuring MO to "ignore," and designating CDA as padding. The ESP Padding and ESP Pad Length may vary from their decompressed counterparts. However, since the IPsec process removes the padding, these variations do not affect packet processing. When esp_encr requires a specific block size, the ESP Pad Length and ESP Padding fields remain not compressed.

Encrypted ESP Compression (EEC) Once the Clear Text Packet has undergone compression as outlined in , the resulting CTEC is encrypted. The header fields once the encrypted ESP packet is formed are the SPI and SN. To facilitate the processes of compression and decompression, this specification requires that the compressed ESP Header is byte-aligned. This requirement is satisfied by ensuring that the sum of esp_spi_lsb and esp_sn_lsb MUST be a multiple of 8. SPI is compressed to its LSB. FL is set to 32 bits, TV is not set, MO is set to "MSB( 4 - esp_spi_lsb)" and CDA is set to "LSB". SN is compressed to their LSB, similarly to the SPI. FL is set to 32 bits, TV is not set, MO is set to "MSB( 4 - esp_sn_lsb)" and CDA is set to "LSB".

Diet-ESP Compression for IPsec in Transport mode The transport mode mostly differs from the Tunnel mode in that the IP header of the packet is not encrypted. As a result, the IP Payload is compressed as described in . The IP header is not compressed. The byte alignment of the Compressed Payload is performed as described in . The Clear Text ESP Compression is performed as described in except for the Next Header Field, which is compressed as described in .

IANA Considerations We request the IANA to create a new registry for the IIPC Profile

We request IANA to create the following registries for the "diet-esp" IIPC Profile.

Security Considerations The security considerations encompass those outlined in ESP as well as those pertaining to SCHC . When employing ESP in Tunnel Mode, the complete inner IP packet is safeguarded against man-in-the-middle attacks through cryptographic means, rendering it virtually impossible for an attacker to alter any fields associated with either the inner IP header or the inner IP payload. This level of protection is achieved by configuring the Flow Label CDA Value to "not_compressed", the DSCP CDA Value to either "not_compressed" or "sa", and the ECN CDA Value to "not_compressed". However, this protection is compromised if the Flow Label CDA Value, DSCP CDA Value, or ECN CDA Values are set to "lower." In such cases, the values from the inner packet for the respective fields will be derived from the outer IP header, leaving these fields unprotected. It is important to note that even the Authentication Header does not provide protection for these fields. When associated with a CDA value of "lower," the level of protection is akin to that provided in Transport mode. This vulnerability could be exploited by an attacker within an untrusted domain, potentially disrupting load balancing strategies that rely on the Flow Label . More broadly, when the Flow Label CDA Value is set to "lower," the relevant Flow Label Security Considerations from apply. Additionally, an attacker may manipulate the DSCP values to diminish the priority of specific packets, resulting in packets from the same flow having varying priorities, traversing different paths, and introducing additional latency to applications, thereby facilitating DDoS attacks. Consequently, these fields remain unprotected and are susceptible to modification during transmission, which may also be regarded as an acceptable risk. When the Flow Label CDA Value is designated as "generated" or "zero," an attacker is unable to exploit the Flow Label field in any manner. The inner packet received is anticipated to possess a Flow Label distinct from that of the original encapsulated packet. However, the Flow Label is assigned by the receiving gateway. When the value is set to "zero," it is known, whereas when it is designated as "generated," it must be produced in accordance with the specifications outlined in . The DSCP CDA Value is assigned as "sa" when DSCP values are linked to Security Associations (SAs), but it should not be utilized when all DSCP values are encompassed within a single SA. In such instances, "not_compressed" is recommended. The encryption algorithm must adhere to the guidelines provided in to guarantee contemporary cryptographic protection. The least significant bits (LSB) of the ESP Security Parameter Index (SPI) determine the number of bits allocated to the SPI. An acceptable quantity of LSB must ensure that the peer possesses a sufficient number of SPIs, which is contingent upon the implementation of the SA lookup employed. If a peer relies solely on the SPI fields for SA lookup, then the number of LSB to consider must be sufficiently large to include the number of SPIs. A peer may compress its SPIs differently, in which case a incoming packet may have its SPI compressed to X bits while another packet may have its SPI compressed to Y bits. The operator must be cognizant that if multiple LSB values are permissible for each type of SA lookup, then multiple SA lookups and signature verifications may be required. It is advisable for a peer to ascertain the LSB associated with an incoming packet in a deterministic manner. The ESP SN LSB must be established in a manner that allows the receiving peer to clearly ascertain the sequence number of the IPsec packet. If this requirement is not met, it will lead to an invalid signature verification, resulting in the rejection of the packet. Furthermore, the LSB should have the capacity to accommodate the maximum number of packets that may be in transit simultaneously. This approach will guarantee that the last packet received is correctly linked to the corresponding sequence number. The ESP extension for IPv6 (and similarly for IPv4) requires a 64-bit (or 32-bit) alignment. Choosing alternative alignment values may result in a packet that fails to comply with this alignment requirement, potentially leading to rejection. The necessity for such alignment in IPv6 extensions arises from the fact that the length field in an IPv6 header extension is defined in terms of 64-bit words, making proper alignment essential for accurate packet parsing. Parsing of ESP does not present complications, as it is compatible with IPv6; the ESP extension is processed exclusively by the terminal IPsec peers and not by intermediary nodes. Furthermore, the ESP extension lacks a dedicated length field. Instead, its length is determined by the IPv6 Header Length field, which is measured in bytes, along with the starting position of the ESP header extension. Consequently, it remains entirely feasible to parse an ESP extension that is not aligned to 64 bits. The same principle is applicable to IPv4.

Acknowledgements We extend our gratitude to Laurent Toutain, Ana Carolina Minaburo, Pascal Thubert, and Alexandre Pelov for their guidance on SCHC and their valuable insights concerning the implementation of OpenSCHC . Additionally, we express our appreciation to Robert Moskowitz for his inspiration in coining the term "Diet-ESP," derived from Diet-HIP, as well as to Samita Chakrabart, Tero Kivinen, Michael Richardson, and Valery Smyslov for their enduring support. The authors also wish to acknowledge the assistance provided by Mitacs through the Mitacs Accelerate program.

&RFC2119; &RFC8174; &RFC4303; &RFC4301; &RFC8724; &RFC7296; &I-D.ietf-ipsecme-ikev2-diet-esp-extension; &I-D.mglt-ipsecme-dscp-np; &RFC8750; &RFC6437; &RFC8221; &RFC9333; &RFC8376; &I-D.ietf-schc-architecture; &RFC4302; &RFC6438;

Appendix This appendix provides the details of the SCHC rules defined for Diet-ESP compression, alongside an explanation and illustrative examples for both Tunnel and Transport modes.

Illustrative Examples of Diet-ESP in Tunnel Mode This section provides a structured example of how Diet-ESP operates in Tunnel Mode. The example includes Attributes for Rule Generation (AfRG), SCHC rules, the Inner IP packet (IIP), the compression process, and the decompression process.

Json representation in Tunnel mode In Tunnel Mode, the full inner IP packet is compressed before encryption, making it more efficient for VPN scenarios. The "iipc_diet-esp" profile is used to indicate compression of the inner packet. The IPv6 Source and Destination Addresses are compressed using "MSB", sending only the variable parts while keeping the most significant bits. UDP Source and Destination Ports are compressed with "LSB", reducing their size. "compute-length" removes the UDP Length field, and "checksum" omits the UDP checksum, which is recalculated at decompression. ESP SPI and Sequence Number are compressed using "LSB" with an MSB mask. The "not-sent" CDA is used for Next Header fields in both IPv6 and ESP, as their values are predictable.

Attributes for Rule Generation (AfRG) The SCHC rules for Tunnel Mode are derived from the following AfRG: IPsec Mode: Tunnel Traffic Selector IP Version: IPv6-only Traffic Selector Source Address: 2001:db8::1234 Traffic Selector Destination Address: 2001:db8::5678 DSCP CDA: Lower ECN CDA: Lower ESP SPI Compression: LSB ESP SN Compression: LSB

Diet-ESP Compression The rules for the IIPC, CTEC, and EEC layers are defined as IIPC to compress IPv6 headers and UDP headers, CTEC to compress ESP Trailer fields, and EEC to compress ESP SPI and Sequence Number. The IIPC original packet before compression consists of: IPv6 Source Address: 2001:db8::1234 IPv6 Destination Address: 2001:db8::5678 UDP Source Port: 5001 UDP Destination Port: 4500 UDP Length: 16 bytes ESP Payload Data Each compressor applies the rule selected by the SA as follows: IIPC: UDP Header Compression UDP ports are compressed using the LSB technique. UDP Length is removed (computed at decompression). UDP Checksum is omitted. IIPC: IPv6 Header Compression Source and Destination Addresses are compressed using MSB. Next Header field is omitted. CTEC: ESP Trailer Compression Pad Length and Padding are omitted. Next Header is omitted. EEC: ESP Header Compression SPI and SN are compressed using LSB. Compressed Packet Output The final compressed packet consists of the compressed ESP header, IIPC compressed data, and payload.

Diet-ESP Decompression The decompression process reverses these steps: EEC: ESP Header Decompression SPI and SN are reconstructed using the LSB values. CTEC: ESP Trailer Decompression (Optional) IIPC: IPv6 Header Decompression ESP Next Header and Padding fields are restored. IPv6 Source and Destination Addresses are restored. IIPC: UDP Header Decompression UDP ports are restored using the decompressed LSB values. UDP Length and Checksum are recalculated.

Illustrative Examples of Diet-ESP in Transport Mode This section follows the same structure as Tunnel Mode but applies to Transport Mode, where the IP header remains not compressed.

Json representation in Transport mode In Transport Mode, since the IP header remains not compressed, only the UDP payload and ESP fields are compressed. The new IANA-defined CDAs are applied as follows: "checksum" is used for the UDP checksum, meaning it is recalculated at decompression rather than transmitted. "compute-length" eliminates the UDP Length field, as it can be inferred from the payload size. "padding" ensures ESP padding aligns with 8-bit boundaries. "not-sent" is applied to the IPv6 and ESP Next Header fields because their values are predictable. The UDP Source and Destination Ports use "LSB" compression, reducing overhead by sending only the least significant bits. The ESP SPI and Sequence Number are compressed similarly using "LSB" with an MSB mask. Since the IP header is retained, the Source and Destination IPv6 Addresses are fully transmitted using "sent-value".

Attributes for Rule Generation (AfRG) The SCHC rules for Transport Mode are derived from the following AfRG: IPsec Mode: Transport Traffic Selector IP Version: IPv6-only Traffic Selector Source Address: 2001:db8::1001 Traffic Selector Destination Address: 2001:db8::2002 DSCP CDA: Lower ECN CDA: Lower ESP SPI Compression: LSB ESP SN Compression: LSB

Inner IP Packet (IIP) The original packet before compression consists of: IPv6 Source Address: 2001:db8::1001 IPv6 Destination Address: 2001:db8::2002 UDP Source Port: 1234 UDP Destination Port: 5678 UDP Length: 18 bytes ESP Payload Data

Diet-ESP Compression IIPC: UDP Header Compression UDP ports are compressed using the LSB technique. UDP Length is removed (computed at decompression). UDP Checksum is omitted. CTEC: ESP Trailer Compression Pad Length and Padding are omitted. Next Header is omitted. EEC: ESP Header Compression SPI and SN are compressed using LSB. Compressed Packet Output The final compressed packet consists of the compressed ESP header, IIPC compressed data, and payload.

Diet-ESP Decompression The decompression process mirrors the compression steps, restoring SPI, SN, UDP headers, ESP Next Header, and Padding fields.

GitHub Repository: Diet-ESP SCHC Implementation The source code for the implementation of the Diet-ESP profile, including the compression and decompression logic using the SCHC rules, is available on GitHub. Access the code at the following link: GitHub Repository: Diet-ESP SCHC Implementation This repository contains the rule definitions, examples, and source code for implementing and testing the Diet-ESP profile. Refer to the README file for setup instructions and usage guidelines.