OAM for LPWAN using Static Context Header Compression (SCHC)

Introduction With IP protocols now generalizing to constrained networks, users expect to be able to Operate, Administer and Maintain (OAM) such networks with the familiar tools and protocols they already use on less constrained networks. However, this requires a little care, because OAM traffic adds load to the network, and LPWANs could easily be overwhelmed by it. LPWANs’ salient characteristics are described in . This document specifies ways to compress such OAM traffic over LPWANs, or to eschew it altogether. OAM uses specific messages sent into the data plane to probe a network. Most often, these messages do not carry meaningful data. Instead, network metrics are inferred from analysing the OAM traffic. For example, such traffic is used

to detect if a host is up or down,
to measure the Round-Trip Time (RTT) and its variation over time,
to determine avalaible bandwidth, or
to learn the path used by packets to reach a destination.

The primitive functionalities of OAM are achieved with the ICMPv6 protocol . ICMPv6 messages are transported over IPv6 . ICMPv6 defines a generic message format, used to inform the source of IPv6 packets of errors during packet delivery.. instantiates 4 such error messages: Destination Unreachable, Packet Too Big, Time Exceeded and Parameter Problem. also defines the Echo Request and Echo Reply messages, which provide support for the ping application. Other ICMPv6 messages are defined in other RFCs, such as an extended format of the same messages and other messages used by the Neighbor Discovery Protocol . This document focuses on using Static Context Header Compression (SCHC) to compress messages that need to be transmitted over the LPWAN network, and on having the LPWAN gateway proxying the Device to save it the unwanted traffic. More specifically, this document describes recommended compression of ICMPv6/IPv6 messages (including header fields and structured payload) and extends SCHC by specifying new surrogate behavior, addressing four scenarios:

OAM reachability messages coming from the internet: the core SCHC acts as a proxy and may decide to respond by itself, thereby acting as a surrogate to the Device.
OAM messages initiated by LPWAN Devices: they can be anticipated and sent in their SCHC-compressed form like regular Device traffic.
OAM error messages returned from the internet after an LPWAN Device transmission. The core SCHC forwards a compressed version of the error message to the Device.
traffic coming from the internet that would generate an error at the Device: if it can detect the situation, the core SCHC responds with an ICMPv6 error message, acting as a surrogate to the Device.

Terminology This draft re-uses the Terminology defined in . 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.

Use cases In the LPWAN architecture, we can distinguish the following cases:

The Device is the originator of an Echo Request message, and therefore the destination of the Echo Reply message. These messages are compressed/decompressed by the device and by the core SCHC using SCHC rules that match the ICMPv6 fields.
The Device is the destination of an Echo Request message, and therefore the purported source of an Echo Reply message. The core SCHC can either forward the SCHC-compressed Echo Request message to the Device, or proxy the Device by answering with an Echo Reply message on its behalf, in order to spare the constrained link. The proxy answer can be related to the Device observed activity.
The Device is the (purported) source of an ICMP error message, mainly in response to an incorrect incoming IPv6 message. In this case, as much as possible, the core SCHC should act as a proxy and originate the ICMP Destination Unreachable message, so that the Device and the LPWAN network are protected from this unwanted traffic.
the Device is the destination of the ICMPv6 message, mainly in response to a packet sent by the Device to the network that generates an error. In this case, we want the ICMPv6 message to reach the Device, and this document describes in what SCHC compression should be applied.

These cases are further described in .

This section defines ICMPv6 fields that can be compressed by SCHC. {{RFC4443}} defines several formats regarding the type of the ICMPv6 message. From them, several fields can be extracted. Note that names listed here are just informative and lisibility, the Field ID identifiers are specified int augmentation of the YANG Data Model (cf. ): These fields are present in all the messages:

ICMPv6 Type indicates the fields present in the message.
ICMPv6 Code is related to the ICMPv6 type and has not impact on the message format.
ICMPv6 Checksum covers the ICMPv6 message and part of the IPv6 header to protect against errors.
ICMPv6 Payload contains either part of the message at the orgine of the error or some data in the case of ping.

The other fields depends of the message type:

ICMPv6 MTU is used by Packet Too Big message (type = 2) to carry the MTU expected by a node rejecting the packet forwarding
ICMPv6 Pointer is used by Parameter Problem message to indicate the position of a detected error in the original message
ICMPv6 Identifier and ICMPv6 Sequence Number are used by ping echo (type 128) and reply (type 129) messages.

ICMPv6 is the support for several protocols to configure nodes. These protocols defines new types and may add optionnal information after the ICMPv6 header

A Device may send an Echo Request message to check the availability of the network and of the host running the Application. If a ping request is generated by a Device, then SCHC compression applies. The format of an ICMPv6 Echo Request message is described in , with Type=128 and Code=0.

If we assume that one rule will be devoted to compressing Echo Request messages, then Type and Code are known in the rule to be 128 and 0 and can therefore be elided with the not-sent CDA. Checksum can be reconstructed with the compute-checksum CDA and therefore is not transmitted. states that Identifier and Sequence Number are meant to “aid in matching Echo Replies to this Echo Request” and that they “may be zero”. Data is “zero or more bytes of arbitrary data”. For constrained devices or networks, we recommend that Identifier be zero, Sequence Number be a counter on 3 bits, and Data be zero bytes (absent). Therefore, Identifier is elided with the not-sent CDA, Sequence Number is transmitted on 3 bits with the LSB CDA and no Data is transmitted. The transmission cost of the Echo Request message is therefore the size of the Rule Id + 3 bits. The rule ID length can be chosen to avoid adding padding. When the destination receives the Echo Request message, it will respond back with a Echo Reply message. This message bears the same format as the Echo Request message but with Type = 129 (see ). states that the Identifier, Sequence Number and Data fields of the Echo Reply message shall contain the same values as the invoking Echo Request message. Therefore, a rule shall be used similar to that used for compressing the Echo Request message.

The following rule gives an example of a SCHC compression. The type can be elided if the direction is taken into account. Identifier is ignored and generated as 0 at decompression. This implies that only one single ping can be launched at any given time on a device. Finally, only the least significant 8 bits of the sequence number are sent on the LPWAN, allowing a serie of 255 consecutive pings. Example of compression rule for a ping from the device

Field	FL	FP	DI	Value	Matching Operator	CDA	Sent bits
IPv6 Headers description
ICMPv6 Type	8	1	Up	128	equal	not-sent
ICMPv6 Type	8	1	Dw	129	equal	not-sent
ICMPv6 Code	8	1	Bi	0	equal	not-sent
ICMPv6 Identifier	16	1	Bi	0	ignore	not-sent
ICMPv6 Sequence	16	1	Bi	0	MSB(13)	LSB	3

NOTE: Add an example where the Payload is also compressed.

If the Device is ping’ed (i.e., is the destination of an Echo Request message), the device receives the compress message and generate an Echo. In that case, the fields sequence number and identifier cannot be compressed if the source is not aware of the compression scheme. But the default behavior is to avoid propagating the Echo Request message over the LPWAN. This is done by proxying the ping request on the core SCHC. This requires to introduce a new processing when the rule is selected. The selection of a compression rule triggers the compression and sends the SCHC packet to the other end. Specifying an Action, change this behavior. In our case, being processed by the compressor, the packet description is processed by a ping proxy. Since the rule is used for the selection, so CDAs are not necessary and set to "not-sent". The ping-proxy takes a parameter in second, gives the interval during which the device is considered active. During this interval, the proxy-ping echoes ping requests, after this duration, the ping request will be discarded. The resulting behavior is shown on and described below:

| Echo Request, Code=0 | l t | | | |<---------------------------| i i | | | | | f m | | | |--------------------------->| e e X | | | Echo Reply, Code=0 | r | | | | | | | | | | | Echo Request, Code=0 | | | |O---------------------------| | | | | | | | | ]]> NOTE: Do we add a proxy-ping-or-sent with instead a not answering send the compress packet to the device ?

The following rule shows an example of a compression rule for pinging a device. Example of compression rule for a ping to a device

Field	FL	FP	DI	Value	Matching Operator	CDA	Sent bits
Action: proxy-ping(300)
IPv6 Headers description
ICMPv6 Type	8	1	Dw	128	equal	not-sent
ICMPv6 Code	8	1	Bi	0	equal	not-sent
ICMPv6 Identifier	16	1	Bi	0	ignore	not-sent
ICMPv6 Sequence	16	1	Bi	0	MSB(13)	LSB	3

In this example, type and code are elided, the identifer has to be sent, and the sequence number is limited to one byte.

As stated in , a node should generate an ICMPv6 message in response to an IPv6 packet that is malformed or which cannot be processed due to some incorrect field value. The general intent of this document is to spare both the Device and the LPWAN network this un-necessary traffic. The incorrect packets should be caught at the core SCHC and the ICMPv6 notification should be sent back from there.

| | | | ICMPv6 Port Unreachable | | | | | | | | | ]]> shows an example of an IPv6 packet trying to reach a Device. Let's assume that no rule matches the incoming packet (i.e. there is no co-compression rule) Instead of sending the packet over the LPWAN and having this packet rejected by the Device, the core SCHC issues an ICMPv6 error message “Destination Unreachable” (Type 1) with Code 1 (“Port Unreachable”) on behalf of the Device. In that case the SCHC C/D MAY act as a router (i.e. it MUST have a routable IPv6 address to generate an ICMPv6 message). When compressing a packet containing an IPv6 header, no compression rules are found and:

if a rule contains some extension headers, a parameter problem may be generated (type 4),
no rule contains the IPv6 device address found in the incoming packet, a no route to destination ICMPv6 message (type 0, code 3) may be generated,
a device IPv6 address is found, but no port matches, a port unreachable ICMPv6 message (type 0, code 4) may be generated,
if the incoming packet is too large for any of the fragmentation rules, an ICMPv6 Message Too big MAY be generated with the largest size allowaed by the fragmentation rules.

In this situation, we assume that a Device has been configured to send information to a server on the Internet. If this server becomes no longer accessible, an ICMPv6 message will be generated back towards the Device by either an intermediate router or the destination. This information can be useful to the Device, for example for reducing the reporting rate in case of periodic reporting of data. Therefore, we compress the ICMPv6 message using SCHC and forward it to the Device over the LPWAN. We also introduce new MO and CDA that can be used to test the presence and/or compress the returning payload.

|----------------------X | | | |<--------------------- | |<~~~~~~~~~~|------------| ICMPv6 Host unreachable | |SCHC compressed ICMPv6 | payload: IPv6 packet | |payload: compressed IPv6| | | | | | ]]> illustrates this behavior. The ICMPv6 error message is compressed as described in and forwarded over the LPWAN to the Device. The SCHC returning message contains the SCHC residue of the ICMPv6 message and MAY contain the compressed original message contained in the ICMP message. The compression can be done by the core SCHC by reversing the direction as if this message was issued by the device.

The ICMPv6 error messages defined in contain the fields shown in .

states that Type can take the values 1 to 4, and Code can be set to values between 0 and 6. Value is unused for the Destination Unreachable and Time Exceeded messages. It contains the MTU for the Packet Too Big message and a pointer to the byte causing the error for the Parameter Error message. Therefore, Value is never expected to be greater than 1280 in LPWAN networks. The payload is viewed as a field. An unsued field MUST not appear in the compressoin rules. The source address of the message SHOULD be "ignore", since it can be initiated by any router on the path. The following generic rule can therefore be used to compress all ICMPv6 error messages as defined today. More specific rules can also be defined to achieve better compression of some error messages. The Type field can be associated to a matching list [1, 2, 3, 4] and is therefore compressed down to 2 bits. Code can be reduced to 3 bits using the LSB CDA. Value can be sent on 11 bits using the LSB CDA, but if the Device is known to send smaller packets, then the size of this field can be further reduced. The first rule example just sends the ICMP type and code as residue to the device. Example of compression rule for a ICMP error to a device

Field	FL	FP	DI	Value	Matching Operator	CDA	Sent bits
IPv6 Headers description
ICMPv6 Type	8	1	Dw	128	equal	not-sent
ICMPv6 Code	8	1	Dw	[0,1,2,3,4,5,6]	match-mapping	mapping-sent	3
ICMPv6 Payload	var	1	Dw	0	ignore	not-sent

The second rule example also only sends the ICMP type and code as residue to the device, but it introduces the new MO "rev-rule-match". This MO will check if a rule matches the payload. Example of compression rule for a ICMP error to a device

Field	FL	FP	DI	Value	Matching Operator	CDA
IPv6 Headers description
ICMPv6 Type	8	1	Dw	128	equal	not-sent
ICMPv6 Code	8	1	Dw	[0,1,2,3,4,5,6]	match-mapping	mapping-sent
ICMPv6 Payload	var	1	Dw	0	rev-rule-match	not-sent

By , the rest of the ICMPv6 message must contain as much as possible of the IPv6 offending (invoking) packet that triggered this ICMPv6 error message. This information is used to try and identify the SCHC rule that was used to decompress the offending IPv6 packet. If the rule can be found then the Rule Id is added at the end of the compressed ICMPv6 message. Otherwise the compressed packet ends with the compressed Value field. The third rule example also sends the ICMP type, code and the compresssed payload as residue. It can be noted that this field is identified as "variable" in the rule which will introduce a size before the IPv6 compressed header. Example of compression rule for a ICMP error to a device

Field	FL	FP	DI	Value	Matching Operator	CDA	Sent bits
IPv6 Headers description
ICMPv6 Type	8	1	Dw	128	equal	not-sent
ICMPv6 Code	8	1	Dw	[0,1,2,3,4,5,6]	match-mapping	mapping-sent
ICMPv6 Payload	var	1	Dw	0	rev-rule-match	rev-compress-sent	(compressed IPv6 header*9) + 4 or +12

LT: do we add packet too big, for instance if a fragmentation rule cannot handle a size larger than 1280?

The Action is a new attribute in the rule. When a rule matching the packet is selected, the action is applied first and indicates if the regular compression based on CDA should be applied.

shows the augmentation of the Data Model defined in This YANG module extends Field ID identities to includes fields contained in ICMPv6 header. Note that the ICMPv6 payload is parsed to the specific field "fid-icmpv6-payload" It also defines two new Most identities:

mo-rev-rule-match: The value contained in the Field Value matches a rule. The direction used for matching isthe opposite of the incoming message: UP becomes DOWN and DOWN becomes UP. This MO can be used to test if the Payload contained in the ICMPv6 message matches a rule. This means that the original packet, at the origine of the ICMPv6 message, may have been generated from the SCHC decompression.
mo-rule-match: The value contained in the Target Value matches a rule. The direction is the one of the incoming message. This MO is not used for ICMPv6 messages, but since it can be used in other situations, it has been included in the Data Model.

The Field Value may be compressed by a rule. The result SHOULD be included in the SCHC message as a variable length residue. It contains the Rule ID used by the compression, the residue, the payload and some padding bits since the variable length init is in bytes.

cda-rev-compress-sent: The direction used for compression is the opposite of the incoming message: UP becomes DOWN and DOWN becomes UP.
cda-compress-sent: The direction used for compression is the same as for the incoming message.

WG List: Editor: Laurent Toutain Editor: Ana Minaburo "; description " Copyright (c) 2021 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Simplified BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info). This version of this YANG module is part of RFC XXXX (https://www.rfc-editor.org/info/rfcXXXX); see the RFC itself for full legal notices. 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 (RFC 2119) (RFC 8174) when, and only when, they appear in all capitals, as shown here. ************************************************************************* This module extends the ietf-schc module to include the compound-ack behavior for Ack On Error as defined in RFC YYYY. It introduces a new leaf for Ack on Error defining the format of the SCHC Ack and add the possibility to send several bitmaps in a single answer."; revision 2024-01-19 { description "Initial version for RFC YYYY "; reference "RFC YYYY: OAM"; } identity fid-icmpv6-base-type { base schc:fid-base-type; description "Field IP base type for ICMPv6 headers described in RFC 4443"; reference "RFC 4443 Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification"; } // ICMPv6 Fields identity fid-icmpv6-type { base schc:fid-icmpv6-base-type; description "ICMPv6 code field present in all ICMPv6 messages."; } identity fid-icmpv6-code { base schc:fid-icmpv6-base-type; description "ICMPv6 code field present in all ICMPv6 messages."; } identity fid-icmpv6-checksum { base schc:fid-icmpv6-base-type; description "ICMPv6 checksum field present in all ICMPv6 messages."; } identity fid-icmpv6-mtu { base schc:fid-icmpv6-base-type; description "ICMPv6 MTU, present in Packet Too Big message."; } identity fid-icmpv6-pointer { base schc:fid-icmpv6-base-type; description "ICMPv6 Pointer, present in Parameter Problem message."; } identity fid-icmpv6-identifier { base schc:fid-icmpv6-base-type; description "ICMPv6 identifier field, present in Echo Request/Reply message."; } identity fid-icmpv6-sequence { base schc:fid-icmpv6-base-type; description "ICMPv6 sequence number field, present in Echo Request/Reply message."; } identity fid-icmpv6-payload { base schc:fid-icmpv6-base-type; description "ICMPv6 payload following ICMPv6 header. If payload is empty, this field exists with a length of 0."; } // MO and CDA identity mo-rule-match { base schc:mo-base-type; description "Macthing operator return true, if the TV matches a rule keeping UP and DOWN direction." ; } identity mo-rev-rule-match { base schc:mo-base-type; description "Macthing operator return true, if the TV matches a rule reversing UP and DOWN direction." ; } identity cda-compress-sent { base schc:mo-base-type; description "Send a compressed version of TV keeping UP and DOWN direction." ; } identity cda-rev-compress-sent { base schc:mo-base-type; description "Send a compressed version of TV reversing UP and DOWN direction." ; } // Proxy actions identity proxy-schc-message{ description "Define how the message is proxied after compression."; } identity proxy-none { base proxy-schc-message; description "The message is not proxied and sent to L2, default behavior of RFC 8724."; } identity proxy-pingv6 { base proxy-schc-message; description "The message is processed by an ping6 proxy."; } typedef proxy-type { type identityref { base proxy-schc-message; } description "The type used in rules to define an action."; } // SCHC rule augment "/schc:schc/schc:rule/schc:nature/schc:compression" { leaf proxy-behavior { type schc-oam:proxy-type; default "schc-oam:proxy-none"; description "Entity proxying the SCHC message."; } list proxy-behavior-value { key "index"; uses schc:tv-struct; description "Parameters associated to the proxy action."; } description "Leaves added to SCHC rules for proxy."; } } ]]>

flood the return path with ICMP error messages.

TODO

The following people have been co-authors of precursor versions of this draft. Their contribution is deeply appreciated and acknowledged.

Arunprabhu Kandasamy (Acklio)
Diego Dujovne (Universidad Diego Portales)
Juan Carlos Zuniga (Cisco)