lpwan Working Group A. Minaburo Internet-Draft Acklio Intended status: Informational L. Toutain Expires:September 5, 2018January 3, 2019 Institut MINES TELECOM; IMT AtlantiqueMarch 04,R. Andreasen Universidad de Buenos Aires July 02, 2018 LPWAN Static Context Header Compression (SCHC) for CoAPdraft-ietf-lpwan-coap-static-context-hc-03draft-ietf-lpwan-coap-static-context-hc-04 Abstract This draft defines the way SCHC header compression can be applied to CoAP headers. CoAP header structure differs from IPv6 and UDP protocols since the CoAPHeader isuse a flexible header with a variable number of options themself of a variable length. Another important difference is the asymmetry in the headerinformationformat usedforin request and response messages.This draft takes into accountMost of thefact that a thing can playcompression mechanisms have been introduced in [I-D.ietf-lpwan-ipv6-static-context-hc], this document explains how to use therole of a CoAP client, a CoAP client or both roles.SCHC compression for CoAP. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire onSeptember 5, 2018.January 3, 2019. Copyright Notice Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . .23 2.CoAP Compressing . . . .SCHC Compression Process . . . . . . . . . . . . . . . . . . 3 3.Compression ofCoAPheader fieldsCompression with SCHC . . . . . . . . . . . . . . . . . 43.1.4. Compression of CoAPversion field (2 bits) .header fields . . . . . . . . . . . . . .4 3.2.6 4.1. CoAPtypeversion field . . . . . . . . . . . . . . . . . . .. . 5 3.3.6 4.2. CoAPtoken lengthtype field . . . . . . . . . . . . . . . . .5 3.4.. . . . 6 4.3. CoAP code field . . . . . . . . . . . . . . . . . . . . . 63.5.4.4. CoAP Message ID field . . . . . . . . . . . . . . . . . .8 3.6.6 4.5. CoAP Tokenfieldfields . . . . . . . . . . . . . . . . . . . .9 4.7 5. CoAP options . . . . . . . . . . . . . . . . . . . . . . . .9 4.1. CoAP option Content-format field. . . . . . . . . . . . . 9 4.2.7 5.1. CoAPoptionContent and Acceptfield . . .options. . . . . . . . . . . . .. 10 4.3.7 5.2. CoAP option Max-Age field, CoAP option Uri-Host and Uri- Port fields . . . . . . . . . . . . . . . . . . . . . . .11 5.7 5.3. CoAP option Uri-Path and Uri-Query fields . . . . . . . . 8 5.3.1. Variable length Uri-Path and Uri-Query . .11 5.1.. . . . . 8 5.3.2. Variable number of path or query elements . . . . . . 9 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields . . . . . .12 5.2.. . . . . . . . . . . . . . . . . . . 9 5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and Location-Query fields . . . . . . . . . . . . . . . .139 6. Other RFCs . . . . . . . . . . . . . . . . . . . . . . . . .139 6.1. Block . . . . . . . . . . . . . . . . . . . . . . . . . .139 6.2. Observe . . . . . . . . . . . . . . . . . . . . . . . . .1310 6.3. No-Response . . . . . . . . . . . . . . . . . . . . . . .13 7. Protocol analysis10 6.4. Time Scale . . . . . . . . . . . . . . . . . . . . . .13 8.. 10 6.5. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. Examples of CoAP header compression . . . . . . . . . . . . .14 8.1.12 7.1. Mandatory header with CON message . . . . . . . . . . . .14 8.2.12 7.2. Complete exchange . . . . . . . . . . . . . . . . . . . .16 9.13 7.3. OSCORE Compression . . . . . . . . . . . . . . . . . . . 14 7.4. Example OSCORE Compression . . . . . . . . . . . . . . . 17 8. Normative References . . . . . . . . . . . . . . . . . . . .1722 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .1822 1. Introduction CoAP [rfc7252] is an implementation of the REST architecture for constrained devices.A Gateway between CoAP and HTTP can be easily built since both protocols uses the same address space (URL), caching mechanisms and methods.Nevertheless, if limited, the size of a CoAP header may be too large for LPWAN constraints and some compression may be needed to reduce the header size.[I-D.toutain-lpwan-ipv6-static-context-hc][I-D.ietf-lpwan-ipv6-static-context-hc] defines a header compression mechanism for LPWAN network based on a static context. The context is said static since the field description composing the Rules and the context are not learned during the packet exchanges but are previously defined. The context(s) is(are) known by both ends before transmission. A context is composed of a set of rules that are referenced by Rule IDs (identifiers). A rule contains an ordered list of the fields descriptions containing a field ID(FID)(FID), its length (FL) and its positionwhen repeated,(FP), a direction indicator (DI) (upstream, downstream and bidirectional) and some associated Target Values(TV) which are expected in(TV). Target Value indicates themessage header.value that can be expected. TV can also be a list of values. A Matching Operator (MO) is associated to each header field description. The rule is selected if all the MOs fit theTVs.TVs for all fields. In that case, a Compression/Decompression Action (CDA) associated to each field defines the link between the compressed and decompressed value for each of the header fields.This document describes howCompression results mainly in 4 actions: send the field value, send nothing, send less significant bits of a field, send an index. Values sent are called Compression Residues and follows the rule ID. 2. SCHC Compression Process The SCHC Compression rules can be applied to CoAP flows. SCHC Compression of the CoAP header may be done in conjunction with the above layers (IPv6/UDP) orindependantly. 2.independently. The SCHC adaptation layers as described in [I-D.ietf-lpwan-ipv6-static-context-hc] may be used as as shown in the Figure 1. ^ +------------+ ^ +------------+ ^ +------------+ | | CoAP | | | CoAP | inner | | CoAP | | +------------+ v +------------+ x | OSCORE | | | UDP | | DTLS | outer | +------------+ | +------------+ +------------+ | | UDP | | | IPv6 | | UDP | | +------------+ v +------------+ +------------+ | | IPv6 | | IPv6 | v +------------+ +------------+ Figure 1: rule scope for CoAP Figure 1 shows some examples for CoAPCompressingarchitecture and the SCHC rule's scope. A rule can covers all headers from IPv6 to CoAP, SCHC C/D is done in the device and at the LPWAN boundary. If an end-to- end encryption mechanisms is used between the device and the application. CoAP must be compressed independently of the other layers. The rule ID and the compression residue are encrypted using a mechanism such as DTLS. Only the other end can decipher the information. Layers below may also be compressed using other SCHC rules (this is out of the scope of this document). OSCORE [I-D.ietf-core-object-security] can also define 2 rules to compress the CoAP message. A first rule focuses on the inner header and is end to end, a second rule may compress the outer header and the layer above. SCHC C/D for inner header is done by both ends, SCHC C/D for outer header and other headers is done between the device and the LPWAN boundary. 3. CoAP Compression with SCHC CoAP differs from IPv6 and UDP protocols on the following aspects: o IPv6 and UDP are symmetrical protocols. The same fields are found in the request and in the response, only the location in the header may vary (e.g. source and destination fields). A CoAP request is different from a response. For example, the URI-path option is mandatory in the request and is not found in the response, a request may contain an Accept option and the response aContent-formatContent option. [I-D.ietf-lpwan-ipv6-static-context-hc] defines the use of a message direction (DI) when processing the rule which allows the description of message header format in both directions. o Even when a field is "symmetric" (i.e. found in both directions) the values carried in each direction are different.For instance the Type field will contain a CON value in the request andCombined with aACK or RST valuematching list in theresponse. Exploiting the asymmetry in compressionTV, this will allow tosend no bit inreduce thecompressed requestrange of expected values in a particular direction and therefore reduce the size of asinglecompression residue. For instance, if a client sends only CON request, the type can be elided by compression and the answer may use one bitinto carry either theanswer.ACK or RST type. Same behavior can be applied to the CoAP Code field(O.OX(0.0X code are present in the request and Y.ZZ in the answer). The direction allows to split in two parts the possible values for each direction. o In IPv6 and UDP header fields have a fixed size. In CoAP, Token size may vary from 0 to 8 bytes, length is given by a field in the header. More systematically, the CoAP options are described using the Type-Length-Value. [I-D.ietf-lpwan-ipv6-static-context-hc] offers the possibility to define a function for the Field Length in the Field Description. o In CoAP headers, a field can be duplicated several times, for instances, elements of an URI (path or queries). The position defined in a rule, associated to a Field ID, can be used to identify the proper element. [I-D.ietf-lpwan-ipv6-static-context-hc] allows a Field id to appears several times in the rule, the Field Position (FP) removes ambiguities for the matching operation. o Field size defined in the CoAP protocol can be to large regarding LPWAN traffic constraints. This is particularly true for the message ID field or Token field. The use of MSB MO can be used to reduce the information carried on LPWANs. o CoAP also obeys to the client/server paradigm and the compression rate can be different if the request is issued from an LPWAN node or from an non LPWAN device. For instance aThing (ES)Device (Dev) aware of LPWAN constraints can generate a 1 byte token, but a regular CoAP client will certainly send a larger token to the Thing. SCHC compression will not modify the values to offer a better compression rate. Nevertheless a proxy placed before the compressor may change some field values to offer a better compression rate and maintain the necessary context for interoperability with existing CoAP implementations.o In IPv6 and UDP header fields have a fixed size. In CoAP, Token size may vary from 0 to 8 bytes, length is given by a field in the header. More systematically, the CoAP options are described using the Type-Length-Value. When applying SCHC header compression. By sending compressed field information following the rule order, SCHC offers a serialization/deserialization mechanism. Since a field exists to indicate the token length there is no ambiguity. For options, the rule indicates also the expected options found the int CoAP header. Therefore only the length is needed to recognize an option. The length will be sent using the same CoAP encoding (size less than 12 are directly sent, higher values use the escape mechanisms defined by [rfc7252]). Delta Type is omitted, the value will be recovered by the decompressor. This reduces the option length of 4, 12 or 20 bits regarding the original size of the delta type encoding in the option. o In CoAP headers a field can be duplicated several times, for instances, elements of an URI (path or queries) or accepted formats. The position defined in a rule, associated to a Field ID, can be used to identify the proper element. 3.4. Compression of CoAP header fields This section discusses of the compression of the different CoAP header fields.These are just examples. The compression should take into account the nature of the traffic and not only the field values. Next chapter will define some compression rules for some common exchanges. 3.1.4.1. CoAP version field(2 bits)This field is bidirectional andcanmust be elided during the SCHC compression, since it always contains the same value.It appears only in first position. FID FL FP DI Value MO CDA Sent ver 2 1 bi 1 equal not-sent 3.2.In the future, if new version of CoAPtype field This field can be managed bidirectionally or unidirectionally.Several strategies canare defined, new rules ID will beapplied to thisdefined avoiding ambiguities between versions. 4.2. CoAP type fieldregarding the values used: o If the ES is a client or a Server[rfc7252] defines 4 types of messages: CON, NON, ACK andnon confirmable messageRST. The latter two ones areused, the transmission of the Type field can be avoided: * Pos is always 1, * DI can either be "uplink" if the ES isaCoAP client or "downlink" if the ES is a CoAP server, or "bidirectional" * TV is set toresponse of thevalue, * MO is set to "equal" * CDA is set to "not-sent". FID FL FP DI Target Value MO CDA Sent type 2 1 bi NON equal not-sent otwo first ones. If theES is eitherdevice plays aclient orspecific role, aServer and confirmable message are used, the DIrule canbe used to elide the type onexploit these property with therequestmapping list: [CON, NON] for one direction andcompress it[ACK, RST] for the other direction. Compression residue is reduced to 1bit on the response.bit. Theexample above shows the rule for a ES acting as a client, directions need tofield must bereversed for a ES acting as a server. FID FL FP DI TV MO CDA Sent type 2 1 up CON equal not-sent type 2 1 dw [ACK,RST] match-mapping mapping-sent [1] o Otherwiseelided ifthe ES is acting simultaneously asfor instance a clientand a server and the rule handle these two traffics, Type field must be sent uncompressed. FID FL FP DI TV MO CDA Sent type 2 1 bi ignore send-value [2] 3.3. CoAP token length field This fieldisbi-directional. Several strategies can be applied to this field regarding the values: o no tokensending only NON ora wellknown length, the transmission can be avoided. A special care must be taken, ifCONmessages are acknowledged with an empty ACK message.messages. Inthat case the token is not always present. FID FL FP DI TV MO CDA Sent TKL 4 1 bi value ignore send-value [4] o If the length is changing from one message to an other, the Token Length fieldany case, a rule must besent. If the Token length can be limited, then only the least significant bits havedefined tobe sent. The example below allows values between 0 and 3. FID FL FP DI TV MO CDA Sent TKL 4 1 bi 0x0 MSB(2) LSB(2) [2] o otherwise the field value hascarry RST tobe sent. FID FL FP DI TV MO CDA Sent TKL 4 1 bi ignore value-sent [4] 3.4.a client. 4.3. CoAP code fieldThis field is bidirectional, but compression can be enhanced using DI.The compression of the CoAPCodecode fielddefines a tricky way to ensure compatibility with HTTP values. Nevertheless only 21 values are defined by [rfc7252] compared tofollows the same principle as for the255 possible values. +------+------------------------------+-----------+ | Code | Description | Mapping | +------+------------------------------+-----------+ | 0.00 | | 0x00 | | 0.01 | GET | 0x01 | | 0.02 | POST | 0x02 | | 0.03 | PUT | 0x03 | | 0.04 | DELETE | 0x04 | | 0.05 | FETCH | 0x05 | | 0.06 | PATCH | 0x06 | | 0.07 | iPATCH | 0x07 | | 2.01 | Created | 0x08 | | 2.02 | Deleted | 0x09 | | 2.03 | Valid | 0x0A | | 2.04 | Changed | 0x0B | | 2.05 | Content | 0x0C | | 4.00 | Bad Request | 0x0D | | 4.01 | Unauthorized | 0x0E | | 4.02 | Bad Option | 0x0F | | 4.03 | Forbidden | 0x10 | | 4.04 | Not Found | 0x11 | | 4.05 | Method Not Allowed | 0x12 | | 4.06 | Not Acceptable | 0x13 | | 4.12 | Precondition Failed | 0x14 | | 4.13 | Request Entity Too Large | 0x15 | | 4.15 | Unsupported Content-Format | 0x16 | | 5.00 | Internal Server Error | 0x17 | | 5.01 | Not Implemented | 0x18 | | 5.02 | Bad Gateway | 0x19 | | 5.03 | Service Unavailable | 0x1A | | 5.04 | Gateway Timeout | 0x1B | | 5.05 | Proxying Not Supported | 0x1C | +------+------------------------------+-----------+ Figure 1: Example ofCoAPcode mapping Figure 1 givestype field. If the device plays apossible mapping, it can be changed to add new codes or reduced if some values are never used by both ends. It could efficiently be coded on 5 bits. Even ifspecific role, thenumberset of code values can beincrease with other RFC, implementations may use a limited number of values, which can help to reduce the number of bits sent onsplit in two parts, theLPWAN. The number of code may vary over time, some newrequest codesmay be introduced or some applications use a limited number of values. The client andwith theserver do not use0 class and thesameresponse values.This asymmetry can be exploited to reduce the size sent on the LPWAN. The field can be treated differently in upstream than in downstream.If theThing isdevice implement only aclient an entry can be set onCoAP client, theuplink message with arequest codematching for 0.0X values and another for downlink values for Y.ZZ codes. It is the opposite if the thing is a server. If the ES always sends or receives requests with the same method, the Code fieldcan beelided. The entry below shows a rule for a client sending only GET request. FID FL FP DI TV MO CDA Sent code 8 1 up GET equal not-sent Ifreduced to theclient may send different methods, a matching-list can be applied. For table Figure 1, 3 bits are necessary, but it could be less if fewer methods are used. Example below gives an example whereset of request theESclient isa server and receives only GET and POST requests. FID FL FP DI Target Value MO CDA Sent code 8 1 dw [0.01, 0.02] match-mapping mapping-sent [1] The same approach can be appliedable toresponses. 3.5.process. All the response codes should be compressed with a SCHC rule. 4.4. CoAP Message ID field This field isbidirectional. Message IDbidirectional and is used to manage acknowledgments. Server memorizes the value fortwo purposes: o To acknowledgea EXCHANGE_LIFETIME period (by default 247 seconds) for CONmessage with an ACK. o To avoid duplicate messages. In LPWAN, since a message can be received by several radio gateway, some LPWAN technologies includemessages and asequence number in L2 to avoid duplicate frames. Therefore if the message does not need to be acknowledged (NON or RST message), the Message ID field can be avoided. FID FL FP DI TV MO CDA Sent Mid 8 1 bi ignore not-sent The decompressor must generateNON_LIFETIME period (by default 145 seconds) for NON messages. During that period, avalue. [[Note; check id this field is not used by OSCOAP .]] To optimize information sent on the LPWAN, shorter values may be used duringserver receiving theexchange, butsame Message IDvalues generated a common CoAP implementationvalue willnot take into account this limitation. Beforeprocess thecompression,message has aproxy mayretransmission. After this period, it will beneeded to reduce the size. FID FL FP DI TV MO CDA Sent Mid 8 1 bi 0x0000 MSB(12) LSB(4) [4] Otherwise if no compression is possible,processed as a new messages. In case thefield has to be sent FID FL FP DI TV MO CDA Sent Mid 8 1 bi ignore value-sent [8] 3.6. CoAP Token field This field is bi-directional. TokenDevice isused to identify transactions and varies from one transaction to another. Therefore, it is usually necessary to senda client, thevaluesize of thetokenmessage ID fieldon the LPWAN network. The optimization will occur by using small values. Common CoAP implementationsmaygeneratethe too largetokens, even if shorter tokens could be usedregarding theLPWAN characteristics. A proxynumber of messages sent. Client may use only small message ID values, for instance 4 bit long. Therefore a MSB can beneededused toreducelimit the size of thetoken before compression. The size ofcompression residue. In case thecompress token sentDevice isknown byacombinationserver, client may be located outside of theToken Length fieldLPWAN area and view therule entry. For instance, withdevice as a regular device connected to theentry below: FID FL FP DI TV MO CDA Sent tkl 4 1 bi 2 equal not-sent token 8 1 bi 0x00 MSB(12) LSB(4) [4]internet. Theuncompressed token is 2 bytes long, but the compressed sizeclient willbe 4 bits. 4. CoAP options 4.1. CoAP option Content-formatgenerate Message ID using the 16 bits space offered by this field.This field is unidirectional and must notA CoAP proxy can be setto bidirectional in a rule entry. It is used only bybefore theserverSCHC C/D toinformreduce theclient aboutvalue of thepayload typeMessage ID, to allow its compression with the MSB matching operator and LSB CDA. 4.5. CoAP Token fields Token isnever founddefined through two CoAP fields, Token Length inclient requests. If single value is expected bytheclient, the TV contains that value and MO is set to "equal"mandatory header and Token Value directly following theCDFmandatory CoAP header. Token Length isset to "not-sent". The examples below describe the rules for an ES actingprocessed as aserver. FID FL FP DI TV MO CDA Sent content 16 1 up value equal not-senttradition protocol field. Ifseveral possiblethe valueare expected byremains theclient, a matching-list can be used. FID FL FP DI TV MO CDA Sent content 16 1 up [50, 41] match-mapping mapping-sent [1] Otherwisesame during all thevaluetransaction, the size can besent.The value-sent CDFstored in thecompressor do not send the option typecontext and elided during thedecompressor reconstructtransmission. Otherwise itregardingwill have to thepositionsend as a compression residue. Token Value size should not be defined directly in therule. FID FL FP DI TV MO CDA Sent content 16 1 up ignore value-sent [0-16] 4.2.rule in the Field Length (FL). Instead a specific function designed as "TKL" must be used. This function informs the SCHC C/D that the length of this field has to be read from the Token Length field. 5. CoAPoptionoptions 5.1. CoAP Content and Accept options. These fieldThis field isare both unidirectional and must not be set to bidirectional in a rule entry.ItIf single value isused onlyexpected by theclient to inform of the possible payload type and is never found in server response. The number of accept options is not limited andclient, it canvary regarding the usage. Tobeselected a rule must contain the exact number about accept options with their positions. Since the orderstored inwhich the Accept value are sent,theposition order can be modified. The rule below FID FL FP DITVMO CDA Sent accept 16 1 up 41 egal not-sent accept 16 2 up 50 egal not-sent will be selected only if two accept options are inand elided during theCoAP headertransmission. Otherwise, ifthis order. The rule below: FID FL FP DI TV MO CDA Sent accept 16 0 up 41 egal not-sent accept 16 0 up 50 egal not-sent will accept a-only CoAP messages with 2 accept options, but the order will not influence the rule selection. The decompression will reconstruct the header regardingseveral possible values are expected by therule order. Otherwiseclient, a matching-listcanshould beappliedused to limit thedifferent values, in that case the order is important to recoversize of theappropriate value andresidue. If not theposition must be clearly indicate. FID FL FP DI TV MO CDA Sent accept 16 1 up [50,41] match-mapping mapping-sent [1] accept 16 2 up [50,61] match-mapping mapping-sent [1] accept 16 3 up [61,71] match-mapping mapping-sent [1] Finally,possible, theoption canvalue as to beexplicitly sent. FID FL FP DI TV MO CDA Sent accept 1 up ignore value-sent 4.3.sent as a residue (fixed or variable length). 5.2. CoAP option Max-Age field, CoAP option Uri-Host and Uri-Port fields This field is unidirectional and must not be set to bidirectional in a rule entry. It is used only by the server to inform of the caching duration and is never found in client requests. If the duration is known by both ends, value can be elided on the LPWAN. A matching list can be used if somewellknownwell-known values are defined. Otherwisethe option length and value canthese options should be senton the LPWAN. [[note: we can reduce (or createas anew option) the unit to minute, second is small for LPWAN ]] 5.residue (fixed or variable length). 5.3. CoAP option Uri-Path and Uri-Query fields This fields are unidirectional and must not be set to bidirectional in a rule entry. They are used only by the client to access to a specific resource and are never found in serverresponse. The Matching Operator behavior has not changed, but the value must takeresponses. Uri-Path and Uri-Query elements are a repeatable options, the Field Position (FP) gives the positionvalue, ifin theentrypath. A Mapping list can be used to reduce size of variable Paths or Queries. In that case, to optimize the compression, several elements can be regrouped into a single entry. Numbering of elements do not change, MO comparison isrepeated :set with the first element of the matching. FID FL FP DI TV MO CDASentURI-Path 1 upfoo["/a/b", equal not-sent "/c/d"] URI-Path23 upbar equal not-sentignore value-sent Figure 2:Position entry. For instance, the rulecomplex path example In Figure 2matches with /foo/bar, buta single bit residue can be used to code one of the 2 paths. If regrouping was not/bar/ foo.allowed, a 2 bits residue whould have been needed. 5.3.1. Variable length Uri-Path and Uri-Query When the length isnot clearly indicated inknown at therule,rule creation, thevalue lengthField Length must besent withset to variable, and thefield data, which means for CoAPunit is set tosend directlybytes. The MSB MO can be apply to a Uri-Path or Uri-Query element. Since MSB value is given in bit, theCoAP option with lengthsize must always be a multiple of 8 bits and the LSB CDA must not carry any value. The length sent at the beginning of a variable length residue indicates the size of the LSB in bytes. For instance for a CoMi path /c/X6?k="eth0" the rule can be set to: FID FL FP DI TV MO CDASentURI-Path 1 upc"c" equal not-sent URI-Path 2 up ignore value-sent URI-Query 1 upk="k=" MSB (16) LSB Figure 3: CoMi URI compression Figure 3 shows the parsing and the compression of the URI. where c is not sent. The second element is sent with the length (i.e. 0x2 X 6) followed by the query option (i.e. 0x05 "eth0").A Mapping list can be used to reduce size5.3.2. Variable number ofvariable Pathspath orQueries. In that case, to optimize the compression, severalquery elementscan be regrouped into a single entry. NumberingThe number ofelements do not change, MO comparisonUri-path or Uri-Query element in a rule isset withfixed at thefirst element ofrule creation time. If thematching. FID FL FP DI TV MO CDA Sent URI-Path 1 up {0:"/c/c", equal not-sent 1:"/c/d" URI-Path 3 up ignore value-sent URI-Query 1 up k= MSB (16) LSB Figure 4: complex path example For instance,number varies, several rules should be created to cover all thefollowing Path /foo/bar/variable/stable can leadspossibilities. Another possibilities is to define therule defined Figure 4. 5.1.length of Uri-Path to variable and send a compression residue with a length of 0 to indicate that this Uri-Path is empty. This add 4 bits to the compression residue. 5.4. CoAP option Size1, Size2, Proxy-URI and Proxy-Scheme fields These fields are unidirectional and must not be set to bidirectional in a rule entry. They are used only by the client to access to a specific resource and are never found in server response. If the field value must be sent, TV is not set, MO is set to "ignore" and CDF is set to "value-sent. A mapping can also be used. Otherwise the TV is set to the value, MO is set to "equal" and CDF is set to "not-sent"5.2.5.5. CoAP option ETag, If-Match, If-None-Match, Location-Path and Location-Query fields These fields are unidirectional. These fields values cannot be stored in a rule entry. They must always be sent with therequest. [[Can include OSCOAP Object security in that category ]]compression residues. 6. Other RFCs 6.1. Block Blockoption should be avoided in LPWAN. The minimum size of 16 bytes can be incompatible with some LPWAN technologies. [[Note: do we recommand LPWAN[rfc7959] allows a fragmentationsinceat thesmallest value of 16CoAP level. SCHC includes also a fragmentation protocol. They are compatible. If a block option istoo big?]]used, its content must be sent as a compression residue. 6.2. Observe [rfc7641] defines the Observe option. The TV is not set, MO is set to "ignore" and the CDF is set to "value-sent". SCHC does not limit the maximum size for this option (3 bytes). To reduce the transmission size either theThingdevice implementation should limit the value increase or a proxy canbe used limitmodify theincrease.incrementation. Since RST message may be sent to inform a server that the client do not require Observe response, a rule must allow the transmission of this message. 6.3. No-Response [rfc7967] defines an No-Response option limiting the responses made by a server to a request. If the value is not known by both ends, then TV is set to this value, MO is set to "equal" and CDF is set to"not- sent"."not-sent". Otherwise, if the value is changing over time, TV is not set, MO is set to "ignore" andCDFCDA to "value-sent". A matching list can also be used to reduce the size. 6.4. Time Scale Time scale [I-D.toutain-core-time-scale] option allows a client to inform the server that it is in a slow network and that message ID should be kept for a duration given by the option. If the value is not known by both ends, then TV is set to this value, MO is set to "equal" and CDA is set to "not-sent". Otherwise, if the value is changing over time, TV is not set, MO is set to "ignore" and CDA to "value-sent". A matching list can also be used to reduce the size. 6.5. OSCORE OSCORE [I-D.ietf-core-object-security] defines end-to-end protection for CoAP messages. This section describes how SCHC rules can be applied to compress OSCORE-protected messages. 0 1 2 3 4 5 6 7 <--------- n bytes -------------> +-+-+-+-+-+-+-+-+--------------------------------- |0 0 0|h|k| n | Partial IV (if any) ... +-+-+-+-+-+-+-+-+--------------------------------- | | | <--------- CoAP OSCORE_piv ------------------> | <- 1 byte -> <------ s bytes -----> +------------+----------------------+-----------------------+ | s (if any) | kid context (if any) | kid (if any) ... | +------------+----------------------+-----------------------+ | | | | <------ CoAP OSCORE_kidctxt ----->|<-- CoAP OSCORE_kid -->| Figure 4: OSCORE Option The encoding of the OSCORE Option Value defined in Section 6.1 of [I-D.ietf-core-object-security] is repeated in Figure 4. The first byte is used for flags that specify the contents of the OSCORE option. The 3 most significant bits are reserved and always set to 0. Bit h, when set, indicates the presence of the kid context field in the option. Bit k, when set, indicates the presence of a kid field. The 3 least significant bits n indicate to length of the piv field in bytes, n = 0 taken to mean that no piv is present. After the flag byte follow the piv field, kid context field and kid field in order and if present; the length of the kid context field is encoded in the first byte denoting by s the length of the kid context in bytes. This draft recommends to implement a parser that is able to identify the OSCORE Option and the fields it contains - this makes it possible to do a preliminary processing of the message in preparation for regular SCHC compression. Conceptually, the OSCORE option can transmit up to 3 distinct pieces of information at a time: the piv, the kid context, and the kid. This draft proposes that the SCHC Parser split the contents of this option into 3 SCHC fields: o CoAP OSCORE_piv, o CoAP OSCORE_ctxt, o CoAP OSCORE_kid. These fields are superposed on the OSCORE Option format in Figure 4, and include the corresponding flag and size bits for each part of the option. Both the flag and size bits can be omitted by use of the MSB matching operator on each field. 7.Protocol analysis 8.Examples of CoAP header compression8.1.7.1. Mandatory header with CON message In this first scenario, the LPWAN compressor receives from outside client a POST message, which is immediately acknowledged by theThing.Device. For this simple scenario, the rules are described Figure 5. Rule ID 1 +-------------+--+--+--+------+---------+-------------++------------+ | Field |FL|FP|DI|Target| Match | CDA || Sent | | | | | |Value | Opera. | || [bits] | +-------------+--+--+--+------+---------+-------------++------------+ |CoAP version | | |bi| 01 |equal |not-sent || | |CoAP version | | |bi| 01 |equal |not-sent || | |CoAP Type | ||bi| |ignore |value-sent ||TT|dw| CON |equal |not-sent || | |CoAP Type | | |up|[ACK, | | || | | | | | | RST] |match-map|matching-sent|| T | |CoAP TKL | | |bi| 0 |equal |not-sent || | |CoAP Code | | |bi| ML1 |match-map|matching-sent|| CC CCC | |CoAP MID | | |bi| 0000 |MSB(7 ) |LSB(9) || M-ID| |CoAP Uri-Path| | |dw| path |equal 1 |not-sent || | +-------------+--+--+--+------+---------+-------------++------------+ Figure 5: CoAP Context to compress header without token The version and Token Length fields are elided. Code has shrunk to 5 bits usingthe matching list (as the one given Figure 1: 0.01 is value 0x01 and 2.05 is value 0x0c) Message-ID has shrunk to 9 bits to preserve alignment on byte boundary. The most significant bit must be set to 0 throughaCoAP proxy.matching list. Uri-Path contains a single element indicated in the matching operator. Figure 6 shows the time diagram of the exchange. ALPWANclient in the Application Server sends a CONmessage.request. It can go through a proxy which reduces the message ID to a smallest value, with at least the 9 most significant bits equal to 0. SCHC Compression reduces the header sending only the Type, a mapped code and the least 9 significant bits of Message ID.The receiver decompresses the header. . The CON message is a request, therefore the LC process to a dynamic mapping. When the ES receives the ACK message, this will not initiate locally a message ID mapping since it is a response. The LC receives the ACK and uncompressed it to restore the original value. Dynamic Mapping context lifetime follows the same rules as message ID duration. End System LPWA LCDevice LPWAN SCHC C/D | | | rule id=1 |<-------------------- |<-------------------| +-+-+--+----+------+ <------------------- |TTCC CCCM MMMM MMMM|CCCCCMMMMMMMMM | |1|0| 4|0.01|0x0034| +-+-+--+----+-------+ |0000 0010 0011 0100|00001000110100 | | 0xb4 p a t| |1|0| 1|0.01|0x0034 | | | | h | | 0xb4 p a t | | | +------+ | h | | | +------+ | | | | | | ---------------------->| rule id=1 | +-+-+--+----+--------+ |------------------->| |1|2| 0|2.05| 0x0034 | |TTCC CCCM MMMM MMMM|--------------------->TCCCCCMMMMMMMMM |---------------------> +-+-+--+----+--------+ |1001 1000 0011 0100|001100000110100 | +-+-+--+----+------+ | | |1|2| 0|2.05|0x0034| v v +-+-+--+----+------+ Figure 6: Compression with global addresses 7.2. Complete exchange In that example, the Thing is using CoMi and sends queries for 2 SID. CON MID=0x0012 | | POST | | Accept X | | /c/k=AS |------------------------>| | | | | |<------------------------| ACK MID=0x0012 | | 0.00 | | | | |<------------------------| CON | | MID=0X0034 | | Content-Format X ACK MID=0x0034 |------------------------>| 0.00 7.3. OSCORE Compression OSCORE aims to solve the problem of end-to-end encryption for CoAP messages, which are otherwise required to terminate their TLS or DTLS protection at the proxy, as discussed in Section 11.2 of [rfc7252]. CoAP proxies are men-in-the-middle, but not all of the information they have access to is necessary for their operation. The goal, therefore, is to hide as much of the message as possible while still enabling proxy operation. Conceptually this is achieved by splitting the CoAP message into an Inner Plaintext and Outer OSCORE Message. The Inner Plaintext contains sensible information which is not necessary for proxy operation. This, in turn, is the part of the message which can befurther optimized by setting some fields unidirectional,encrypted and need not be decrypted until it reaches its end destination. The Outer Message acts asdescribeda shell matching the format of a regular CoAP message, and includes all Options and information needed for proxy operation and caching. This decomposition is illustrated in Figure 7.Note that Type is no more sentCoAP options are sorted into one of 3 classes, each granted a specific type of protection by the protocol: o Class E: Enrypted options moved to the Inner Plaintext, o Class I: Intergrity-protected options included in thecompressed format, Compressed Code sizeAAD for the encryption of the Plaintext but otherwise left untouched innot changedthe Outer Message, o Class U: Unprotected options left untouched in the Outer Message. Additionally, the OSCORE Option is added as an Outer option, signaling thatexample (8 values are needed to code alltherequests and 21message is OSCORE protected. This option carries the information necessary tocode allretrieve theresponses inSecurity Context with which thematching list Figure 1) Rule ID 2 +-------------+--+--+--+------+---------+------------++------------+message was encrypted so that it may be correctly decrypted at the other end-point. Orignal CoAP Message +-+-+---+-------+---------------+ |v|t|tkl| code |Field |FL|FP|DI|Target| MOMsg Id. |CDA || Sent+-+-+---+-------+---------------+....+ | Token | +-------------------------------.....+ | Options (IEU) | . . . . +------+-------------------+ ||Value0xFF | +------+------------------------+ ||| [bits]|+-------------+--+--+--+------+---------+------------++------------+ |CoAP version| Payload ||bi|01 |equal |not-sent ||||CoAP Type| +-------------------------------+ / \ / \ / \ / \ Outer Header v v Plaintext +-+-+---+--------+---------------+ +-------+ |v|t|tkl|new code| Msg Id. ||dw|CON |equal |not-sent ||||CoAP Typecode | +-+-+---+--------+---------------+....+ +-------+-----......+ ||up| ACK |equal |not-sent ||Token ||CoAP TKL| Options (E) ||bi|0 |equal |not-sent ||+--------------------------------.....+ +-------+------.....+ ||CoAP CodeOptions (IU) | ||dw|ML2 |match-map|mapping-sent||CCCC COxFF ||CoAP Code. . +-------+-----------+ . OSCORE Option . | ||up|ML3 |match-map|mapping-sent||CCCC C+------+-------------------+ ||CoAP MIDPayload | ||bi|0000 |MSB(5) |LSB(11) || M-ID0xFF ||CoAP Uri-Path|||dw|path |equal 1 |not-sent |||+-------------+--+--+--+------+---------+------------++------------+ ML1 = {CON : 0, ACK:1} ML2 = {POST:0, 2.04:1, 0.00:3}+------+ +-------------------+ Figure 7:CoAP Context to compressOSCORE inner and outer headerwithout token 8.2. Complete exchangeform a CoAP message Figure 7 shows the message format for the OSCORE Message and Plaintext. Inthat example,theThingOuter Header, the original message code isusing CoMihidden andsends queries for 2 SID. CON MID=0x0012replaced by a default value (POST or FETCH) depending on whether the original message was a Request or a Response. The original message code is put into the first byte of the Plaintext. Following the message code come the class E options and if present the original message Payload preceded by its payload marker. The Plaintext is now encrypted by an AEAD algorithm which integrity protects Security Context parameters and eventually any class I options from the Outer Header. Currently no CoAP options are marked class I. The resulting Ciphertext becomes the new Payload of the OSCORE message, as illustrated in Figure 8. Outer Header +-+-+---+--------+---------------+ |v|t|tkl|new code| Msg Id. | +-+-+---+--------+---------------+....+ |POSTToken | +--------------------------------.....+ |Accept XOptions (IU) | . . . OSCORE Option . +------+-------------------+ |/c/k=AS |------------------------>|0xFF | +------+-------------------------+ | | ||<------------------------| ACK MID=0x0012Encrypted Inner Header and | |0.00Payload | | | +--------------------------------+ Figure 8: OSCORE message The SCHC Compression scheme consists of compressing both the Plaintext before encryption and the resulting OSCORE message after encryption, see Figure 9. This way compression reaches all fields of the original CoAP message. Outer Message OSCORE Plaintext +-+-+---+--------+---------------+ +-------+ |v|t|tkl|new code| Msg Id. ||<------------------------| CON| code |MID=0X0034+-+-+---+--------+---------------+....+ +-------+-----......+ | Token |Content-Format X ACK MID=0x0034 |------------------------>| 0.00 Rule ID 3 +--------------+--+--+--+------+--------+-----------++------------+|Field |FL|FP|DI|Target| MOOptions (E) |CDA || Sent+--------------------------------.....+ +-------+------.....+ | Options (IU) | | OxFF | . . +-------+-----------+ . OSCORE Option . ||Value| +------+-------------------+ ||| [bits]Payload |+--------------+--+--+--+------+--------+-----------++------------+ |CoAP version| 0xFF ||bi| 01 |equal |not-sent ||||CoAP Type| +------+------------+ +-------------------+ ||up| CON |equal |not-sent ||Ciphertext |<---------\ ||CoAP Type| ||dw| ACK |equal |not-sent ||||CoAP TKLv +-------------------+ | +-----------------+ ||bi| 1 |equal |not-sent ||||CoAP Code| Inner SCHC ||up| POST |equal |not-sent ||v ||CoAP| Compression | +-----------------+ | +-----------------+ | Outer SCHC | | | | Compression | | v +-----------------+ | +-------+ | | |Rule ID| v | +-------+--+ +--------+ +------------+ | Residue | |Rule ID'| | Encryption | <--- +----------+--------+ +--------+--+ +------------+ | | | Residue' | | Payload | +-----------+-------+ | | | Ciphertext | +-------------------+ | | +-------------------+ Figure 9: OSCORE Compression Diagram 7.4. Example OSCORE Compression In what follows we present an example GET Request and consequent CONTENT Response and show a possible set of rules for the Inner and Outer SCHC Compression. We then show a dump of the results and contrast SCHC + OSCORE performance with SCHC + COAP performance. This gives an approximation to the cost of security with SCHC-OSCORE. Our first example CoAP message is the GET Request in Figure 10 Original message: ================= 0x4101000182bb74656d7065726174757265 Header: 0x4101 01 Ver 00 CON 0001 tkl 00000001 Request Code 1 "GET" 0x0001 = mid 0x82 = token Options: 0xbb74656d7065726174757265 Option 11: URI_PATH Value = temperature Original msg length: 17 bytes. Figure 10: CoAP GET Request Its corresponding response is the CONTENT Response in Figure 11. Original message: ================= 0x6145000182ff32332043 Header: 0x6145 01 Ver 10 ACK 0001 tkl 01000101 Successful Response Code 69 "2.05 Content" 0x0001 = mid 0x82 = token 0xFF Payload marker Payload: 0x32332043 Original msg length: 10 Figure 11: CoAP CONTENT Response The SCHC Rules for the Inner Compression include all fields that are already present in a regular CoAP message, what matters is the order of appearance and inclusion of only those CoAP fields that go into the Plaintext, Figure 12. Rule ID 0 +----------------+--+--+-----------+-----------+-----------++--------+ | Field |FP|DI| Target ||dw| 0.00 |equal |not-sentMO | CDA || Sent ||CoAP MID| ||bi| 0000 |MSB(8) |LSB ||MMMMMMMM||CoAP Token| Value ||up| |ignore |send-value ||TTTTTTTT||CoAP Uri-Path|| [bits] | +----------------+--+--+-----------+-----------+-----------++--------+ |CoAP Code ||dw| /c |equal|up| 1 | equal |not-sent || | |CoAPUri-query|Code ||dw| ML4 |equal 1 |not-sent ||P|dw|[69,132] | match-map |match-sent || c | |CoAPContentUri-Path | |up|temperature| equal |not-sent || ||up| X |equal|COAP Option-End | |dw| 0xFF | equal |not-sent || |+--------------+--+--+--+------+--------+-----------++------------++----------------+--+--+-----------+-----------+-----------++--------+ Figure 12: Inner SCHC Rules The Outer SCHC Rules (Figure 13) must process the OSCORE Options fields. Here we mask off the repeated bits (most importantly the flag and size bits) with the MSB Mathing Operator. Rule ID4 +--------------+--+--+--+------+--------+-----------++------------+0 +---------------+--+--+--------------+---------+-----------++------------+ | Field|FL|FP|DI|Target||FP|DI| Target | MO | CDA || Sent | | | | ||ValueValue | | || [bits] |+--------------+--+--+--+------+--------+-----------++------------++---------------+--+--+--------------+---------+-----------++------------+ |CoAP version |||bi| 01 |equal |not-sent || | |CoAP Type || |dw| CON|up| 0 |equal |not-sent || | |CoAP Type || |up| ACK|dw| 2 |equal |not-sent || | |CoAP TKL |||bi| 1 |equal |not-sent || | |CoAP Code || |dw| 2.05|up| 2 |equal |not-sent || | |CoAP Code || |up| 0.00|dw| 68 |equal |not-sent || | |CoAP MID |||bi| 0000|MSB(8)|MSB(12) |LSB||MMMMMMMM||MMMM | |CoAP Token | |bi| 0x80 |MSB(5) |LSB ||TTT ||dw| |ignore |send-value||TTTTTTTT|CoAP OSCORE_piv| |up| 0x0900 |MSB(12) |LSB ||PPPP | |COAPAcceptOSCORE_kid| |up|b'\x06client' |MSB(52) |LSB ||KKKK | |CoAP OSCORE_piv| |dw| b'' |equal |not-sent || | |COAP Option-End| |dw|X0xFF |equal |not-sent || |+--------------+--+--+--+------+---------+----------++------------+ alternative rule:+---------------+--+--+--------------+---------+-----------++------------+ Figure 13: Outer SCHC Rules Next we show a dump of the compressed message: Compressed message: ================== 0x00291287f0a5c4833760d170 0x00 = Rule ID4 +--------------+--+--+--+------+---------+-----------++------------+piv = 0x04 Compression residue: 0b0001 010 0100 0100 (15 bits -> 2 bytes with padding) mid tkn piv kid Payload 0xa1fc297120cdd8345c Compressed message length: 12 bytes Figure 14: SCHC-OSCORE Compressed GET Request Compressed message: ================== 0x0015f4de9cb814c96aed9b1d981a3a58 0x00 = Rule ID Compression residue: 0b0001 010 (7 bits -> 1 byte with padding) mid tkn Payload 0xfa6f4e5c0a64b576cd8ecc0d1d2c Compressed msg length: 16 bytes Figure 15: SCHC-OSCORE Compressed CONTENT Response For contrast, we compare these results with what would be obtained by SCHC compressing the original CoAP messages without protecting them with OSCORE. To do this, we compress the CoAP mesages according to the SCHC rules in Figure 16. Rule ID 1 +---------------+--+--+-----------+---------+-----------++------------+ | Field|FL|FP|DI|Target||FP|DI| Target | MO | CDA || Sent | | | | ||ValueValue | | || [bits] |+--------------+--+--+--+------+---------+-----------++------------++---------------+--+--+-----------+---------+-----------++------------+ |CoAP version |||bi| 01 |equal |not-sent || | |CoAP Type | |up| 0 |equal |not-sent || | |CoAP Type ||bi| ML1 |match-map|match-sent ||t|dw| 2 |equal |not-sent || | |CoAP TKL |||bi| 1 |equal |not-sent || | |CoAP Code |||up|ML2 |match-map|match-sent2 |equal |not-sent ||cc| |CoAP Code |||dw|ML3 |match-map|match-sent[69,132] |equal |not-sent ||cc| |CoAP MID |||bi| 0000|MSB(8)|MSB(12) |LSB||MMMMMMMM||MMMM | |CoAP Token || |dw| |ignore |send-value ||TTTTTTTT|bi| 0x80 |MSB(5) |LSB ||TTT | |CoAP Uri-Path || |dw| /c |equal 1 |not-sent || | |CoAP Uri-query| | |dw| ML4 |equal 1 |not-sent ||P | |CoAP Content | | |up| X |equal|up|temperature|equal |not-sent || | |COAPAccept | |Option-End| |dw|x0xFF |equal |not-sent || |+--------------+--+--+--+------+---------+-----------++------------+ ML1 {CON:0, ACK:1} ML2 {POST:0, 0.00: 1} ML3 {2.05:0, 0.00:1} ML4 {NULL:0, k=AS:1, K=AZE:2} 9.+---------------+--+--+-----------+---------+-----------++------------+ Figure 16: SCHC-CoAP Rules (No OSCORE) This yields the results in Figure 17 for the Request, and Figure 18 for the Response. Compressed message: ================== 0x0114 0x01 = Rule ID Compression residue: 0b00010100 (1 byte) Compressed msg length: 2 Figure 17: CoAP GET Compressed without OSCORE Compressed message: ================== 0x010a32332043 0x01 = Rule ID Compression residue: 0b00001010 (1 byte) Payload 0x32332043 Compressed msg length: 6 Figure 18: CoAP CONTENT Compressed without OSCORE As can be seen, the difference between applying SCHC + OSCORE as compared to regular SCHC + COAP is about 10 bytes of cost. 8. Normative References[I-D.toutain-lpwan-ipv6-static-context-hc] Minaburo, A.[I-D.ietf-core-object-security] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", draft-ietf-core-object-security-13 (work in progress), June 2018. [I-D.ietf-lpwan-ipv6-static-context-hc] Minaburo, A., Toutain, L., Gomez, C., and D. Barthel, "LPWAN Static Context Header Compression (SCHC) and fragmentation for IPv6 and UDP",draft-toutain-lpwan- ipv6-static-context-hc-00draft-ietf-lpwan-ipv6- static-context-hc-16 (work in progress),September 2016.June 2018. [I-D.toutain-core-time-scale] Minaburo, A. and L. Toutain, "CoAP Time Scale Option", draft-toutain-core-time-scale-00 (work in progress), October 2017. [rfc7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014, <https://www.rfc-editor.org/info/rfc7252>. [rfc7641] Hartke, K., "Observing Resources in the Constrained Application Protocol (CoAP)", RFC 7641, DOI 10.17487/RFC7641, September 2015, <https://www.rfc-editor.org/info/rfc7641>. [rfc7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in the Constrained Application Protocol (CoAP)", RFC 7959, DOI 10.17487/RFC7959, August 2016, <https://www.rfc-editor.org/info/rfc7959>. [rfc7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T. Bose, "Constrained Application Protocol (CoAP) Option for No Server Response", RFC 7967, DOI 10.17487/RFC7967, August 2016, <https://www.rfc-editor.org/info/rfc7967>. Authors' Addresses Ana Minaburo Acklio2bis rue de la Chataigneraie1137A avenue des Champs Blancs 35510 Cesson-Sevigne Cedex France Email: ana@ackl.io Laurent Toutain Institut MINES TELECOM; IMT Atlantique 2 rue de la Chataigneraie CS 17607 35576 Cesson-Sevigne Cedex France Email: Laurent.Toutain@imt-atlantique.fr Ricardo Andreasen Universidad de Buenos Aires Av. Paseo Colon 850 C1063ACV Ciudad Autonoma de Buenos Aires Argentina Email: randreasen@fi.uba.ar