CoAP Pub-Sub Profile for Authentication and Authorization for Constrained Environments (ACE)
draft-palombini-ace-coap-pubsub-profile-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|
|---|---|---|---|
| Author | Francesca Palombini | ||
| Last updated | 2018-03-02 (Latest revision 2017-09-28) | ||
| Replaced by | draft-ietf-ace-pubsub-profile, draft-ietf-ace-pubsub-profile | ||
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draft-palombini-ace-coap-pubsub-profile-02
ACE Working Group F. Palombini
Internet-Draft Ericsson
Intended status: Standards Track March 02, 2018
Expires: September 3, 2018
CoAP Pub-Sub Profile for Authentication and Authorization for
Constrained Environments (ACE)
draft-palombini-ace-coap-pubsub-profile-02
Abstract
This specification defines a profile for authentication and
authorization for publishers and subscribers in a pub-sub setting
scenario in a constrained environment, using the ACE framework. This
profile relies on transport layer or application layer security to
authorize the publisher to the broker. Moreover, it relies on
application layer security for publisher-broker and subscriber-broker
communication.
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
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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 on September 3, 2018.
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
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 2
2. Profile Overview . . . . . . . . . . . . . . . . . . . . . . 3
3. coap_pubsub Profile . . . . . . . . . . . . . . . . . . . . . 4
3.1. Retrieval of COSE Key for protection of content . . . . . 5
4. Publisher . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Subscriber . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Pub-Sub Protected Communication . . . . . . . . . . . . . . . 11
6.1. Using COSE Objects to protect the resource representation 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 13
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . 15
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
The publisher-subscriber setting allows for devices with limited
reachability to communicate via a broker that enables store-and-
forward messaging between the devices. The pub-sub scenario using
the Constrained Application Protocol (CoAP) is specified in
[I-D.ietf-core-coap-pubsub]. This document defines a way to
authorize nodes in a CoAP pub-sub type of setting, using the ACE
framework [I-D.ietf-ace-oauth-authz].
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Readers are expected to be familiar with the terms and concepts
described in [I-D.ietf-ace-oauth-authz],
[I-D.palombini-ace-key-groupcomm] and [I-D.ietf-core-coap-pubsub].
In particular, analogously to [I-D.ietf-ace-oauth-authz], terminology
for entities in the architecture such as Client (C), Resource Server
(RS), and Authorization Server (AS) is defined in OAuth 2.0 [RFC6749]
and [I-D.ietf-ace-actors], and terminology for entities such as the
Key Distribution Center (KDC) and Dispatcher in
[I-D.palombini-ace-key-groupcomm].
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2. Profile Overview
The objective of this document is to specify how to protect a CoAP
pub-sub communication, as described in [I-D.ietf-core-coap-pubsub],
using [I-D.palombini-ace-key-groupcomm], which itself expands the Ace
framework ([I-D.ietf-ace-oauth-authz]), and profiles
([I-D.ietf-ace-dtls-authorize], [I-D.ietf-ace-oscore-profile]).
The architecture of the scenario is shown in Figure 1.
+----------------+ +----------------+
| | | |
| Authorization | | Authorization |
| Server 1 | | Server 2 |
| | | |
+----------------+ +----------------+
^ ^ ^
| | |
+---------(A)----+ | +-----(D)------+
| +--------------------(B)--------+ |
v v v
+------------+ +------------+ +------------+
| CoAP | ----(C)---> | CoAP | | CoAP |
| Client - | ----(E)---> | Server - | | Client - |
| | | | <----(F)---- | |
| Publisher | | Broker | -----(G)---> | Subscriber |
+------------+ +------------+ +------------+
Figure 1: Architecture CoAP pubsub with Authorization Servers
The RS is the broker, which contains the topic. This node
corresponds to the Dispatcher, in [I-D.palombini-ace-key-groupcomm].
The AS1 hosts the policies about the Broker: what endpoints are
allowed to Publish on the Broker. The Clients access this node to
get write access to the Broker. The AS2 hosts the policies about the
topic: what endpoints are allowed to access what topic. This node
represents both the AS and Key Distribution Center roles from
[I-D.palombini-ace-key-groupcomm].
There are four phases, the first three can be done in parallel.
1. The Publisher requests publishing access to the Broker at the
AS1, and communicates with the Broker to set up security.
2. The Publisher requests access to a specific topic at the AS2
3. The Subscriber requests access to a specific topic at the AS2.
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4. The Publisher and the Subscriber securely post to and get
publications from the Broker.
This exchange aims at setting up 2 different security associations:
on the one hand, the Publisher has a security association with the
Broker, to protect the communication and securely authorize the
Publisher to publish on a topic (Security Association 1). On the
other hand, the Publisher has a security association with the
Subscriber, to protect the publication content itself (Security
Association 2). The Security Association 1 is set up using AS1 and a
profile of [I-D.ietf-ace-oauth-authz], the Security Association 2 is
set up using AS2 and [I-D.palombini-ace-key-groupcomm].
Note that, analogously to the Publisher, the Subscriber can also set
up an additional security association with the Broker, using an AS,
in the same way the Publisher does with AS1. In this case, only
authorized Subscribers would be able to get notifications from the
Broker. The overhead would be that each Subscriber should access the
AS and get all the information to start a secure exchange with the
Broker.
+------------+ +------------+ +------------+
| CoAP | | CoAP | | CoAP |
| Client - | | Server - | | Client - |
| | | | | |
| Publisher | | Broker | | Subscriber |
+------------+ +------------+ +------------+
: : : :
: '------ Security -------' :
: Association 1 :
'------------------------------- Security --------------'
Association 2
Note that AS1 and AS2 might either be co-resident or be 2 separate
physical entities, in which case access control policies must be
exchanged between AS1 and AS2, so that they agree on rights for
joining nodes about specific topics. How the policies are exchanged
is out of scope for this profile.
3. coap_pubsub Profile
This profile uses [I-D.palombini-ace-key-groupcomm], which expands
the ACE framework. This document specifies which exact parameters
from [I-D.palombini-ace-key-groupcomm] have to be used, and the
values for each parameter.
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The Publisher and the Subscriber map to the Client in
[I-D.palombini-ace-key-groupcomm], the AS2 maps to the AS and to the
KDC, the Broker maps to the Dispatcher.
Note that both publishers and subscribers use the same profile,
called "coap_pubsub".
3.1. Retrieval of COSE Key for protection of content
This phase is common to both Publisher and Subscriber. To maintain
the generality, the Publisher or Subscriber is referred as Client in
this section.
Client Broker AS2
| [----- Resource Request ---->] | |
| | |
| [<-- AS1, AS2 Information ---] | |
| |
| -- Authorization + Key Distribution Request ---> |
| |
| <-- Authorization + Key Distribution Response -- |
| |
Figure 2: B: Access request - response
Complementary to what is defined in [I-D.ietf-ace-oauth-authz]
(Section 5.1.1), to determine the AS2 in charge of a topic hosted at
the Broker, the Broker MAY send the address of both the AS in charge
of the topic back to the Client in the 'AS' parameter in the AS
Information, as a response to an Unauthorized Resource Request
(Section 5.1.2). An example using CBOR diagnostic notation is given
below:
4.01 Unauthorized
Content-Format: application/ace+cbor
{"AS1": "coaps://as1.example.com/token",
"AS2": "coaps://as2.example.com/pubsubkey"}
Figure 3: AS1, AS2 Information example
Analogously to what is defined in [I-D.ietf-ace-oauth-authz], instead
of the initial Unauthorized Resource Request message, the Client MAY
look up the desired topic in a resource directory (see
[I-D.ietf-core-resource-directory]).
After retrieving the AS2 address, the Client sends an Authorization +
Key Distribution Request, which is an Authorization Request merged
with a Key Distribution Request, as described in
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[I-D.palombini-ace-key-groupcomm], Sections 3.1 and 4.1. The reason
for merging these two messages is that the AS2 is both the AS and the
KDC, in this setting, so the Authorization Response and the Post
Token message are not necessary.
More specifically, the Client sends a POST request to the /token
endpoint on AS2, that MUST contain in the payload (formatted as a
CBOR map):
o the following fields from the Authorization Request (Section 3.1
of [I-D.palombini-ace-key-groupcomm]):
* the grant type set to "client_credentials",
* OPTIONALLY, if needed, other additional parameters such as
"Client_id"
o the following fields from the Key Distribution Request
(Section 4.1 of [I-D.palombini-ace-key-groupcomm]):
* the client_cred parameter containing the Client's public key,
if the Client needs to directly send that to the AS2,
* the scope parameter set to a CBOR array containing the broker's
topic as first element and the string "publisher" for
publishers and "subscriber" for subscribers as second element
* the get_pub_keys parameter set to 0x01 if the Client needs to
retrieve the public keys of the other pubsub members
* OPTIONALLY, if needed, the pub_keys_repos parameters
Note that the alg parameter in the client_cred COSE_Key MUST be a
signing algorithm, as defined in section 8 of [RFC8152].
Examples of the payload of a Authorization + Key Distribution Request
are specified in Figure 5 and Figure 8.
The AS2 verifies that the Client is authorized to access the topic
and, if the 'client_cred' parameter is present, stores the public key
of the Client.
The AS2 response is an Authorization + Key Distribution Response, see
Section 4.2 of [I-D.palombini-ace-key-groupcomm]. The payload
(formatted as a CBOR map) MUST contain:
o the following fields from the Authorization Response (Section 3.2
of [I-D.palombini-ace-key-groupcomm]):
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* profile set to "coap_pubsub"
* scope parameter (optionally), set to a CBOR array containing
the broker's topic as first element and the string "publisher"
for publishers and "subscriber" for subscribers as second
element
o the following fields from the Key Distribution Response
(Section 4.2 of [I-D.palombini-ace-key-groupcomm]):
* "key" parameter including:
+ kty with value 4 (symmetric)
+ alg with value defined by the AS2 (Content Encryption
Algorithm)
+ Base IV with value defined by the AS2
+ k with value the symmetric key value
+ OPTIONALLY, kid with an identifier for the key value
* "pub_keys", containing the public keys of all authorized
signing members, if the "get_pub_keys" parameter was present
and set to 0x01 in the Authorization + Key Distribution Request
Examples for the response payload are detailed in Figure 6 and
Figure 9.
4. Publisher
In this section, it is specified how the Publisher requests, obtains
and communicates to the Broker the access token, as well as the
retrieval of the keying material to protect the publication.
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+----------------+ +----------------+
| | | |
| Authorization | | Authorization |
| Server 1 | | Server 2 |
| | | |
+----------------+ +----------------+
^ ^
| |
+---------(A)----+ |
| +--------------------(B)--------+
v v
+------------+ +------------+
| CoAP | ----(C)---> | CoAP |
| Client - | | Server - |
| | | |
| Publisher | | Broker |
+------------+ +------------+
Figure 4: Phase 1: Publisher side
This is a combination of two independent phases:
o one is the establishment of a secure connection between Publisher
and Broker, using an ACE profile such as DTLS
[I-D.ietf-ace-dtls-authorize] or OSCOAP
[I-D.ietf-ace-oscore-profile]. (A)(C)
o the other is the Publisher's retrieval of keying material to
protect the publication. (B)
In detail:
(A) corresponds to the Access Token Request and Response between
Publisher and Authorization Server to retrieve the Access Token and
RS (Broker) Information. As specified, the Publisher has the role of
a CoAP client, the Broker has the role of the CoAP server.
(C) corresponds to the exchange between Publisher and Broker, where
the Publisher sends its access token to the Broker and establishes a
secure connection with the Broker. Depending on the Information
received in (A), this can be for example DTLS handshake, or other
protocols. Depending on the application, there may not be the need
for this set up phase: for example, if OSCOAP is used directly.
(A) and (C) details are specified in the profile used.
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(B) corresponds to the retrieval of the keying material to protect
the publication, and uses [I-D.palombini-ace-key-groupcomm]. The
details are defined in Section 3.1.
An example of the payload of an Authorization + Key Distribution
Request and corresponding Response for a Subscriber is specified in
Figure 5 and Figure 6.
{
"grant_type" : "client_credentials",
"scope" : ["Broker1/Temp", "publisher"],
"client_id" : "publisher1",
"client_cred" :
{ / COSE_Key /
/ type / 1 : 2, / EC2 /
/ kid / 2 : h'11',
/ alg / 3 : -7, / ECDSA with SHA-256 /
/ crv / -1 : 1 , / P-256 /
/ x / -2 : h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de1
08de439c08551d',
/ y /-3 : h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e
9eecd0084d19c'
}
}
Figure 5: Authorization + Key Distribution Request payload for a
Publisher
{
"profile" : "coap_pubsub",
"key" : {1: 4, 2: h'1234', 3: 12, 5: h'1f389d14d17dc7',
-1: h'02e2cc3a9b92855220f255fff1c615bc'}
}
Figure 6: Authorization + Key Distribution Response payload for a
Publisher
5. Subscriber
In this section, it is specified how the Subscriber retrieves the
keying material to protect the publication.
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+----------------+
| |
| Authorization |
| Server 2 |
| |
+----------------+
^
|
+-----(D)------+
|
v
+------------+
| CoAP |
| Client - |
| |
| Subscriber |
| |
+------------+
Figure 7: Phase 2: Subscriber side
Step (D) between Subscriber and AS2 corresponds to the retrieval of
the keying material to verify the publication. The details are
defined in Section 3.1
This step is the same as (B) between Publisher and AS2 (Section 3.1),
with the following differences:
o The Authorization + Key Distribution Request MUST NOT contain the
client_cred parameter, the role element in the 'scope' parameter
MUST be set to "subscriber". The Subscriber MUST have access to
the public keys of all the Publishers; this MAY be achieved in the
Authorization + Key Distribution Request by using the parameter
get_pub_keys set to 0x01.
o The Authorization + Key Distribution Response MUST contain the
pub_keys parameter.
An example of the payload of an Authorization + Key Distribution
Request and corresponding Response for a Subscriber is specified in
Figure 8 and Figure 9.
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{
"grant_type" : "client_credentials",
"scope" : ["Broker1/Temp", "subscriber"],
"get_pub_keys" : 0x01
}
Figure 8: Authorization + Key Distribution Request payload for a
Subscriber
{
"profile" : "coap_pubsub",
"scope" : ["Broker1/Temp", "subscriber"],
"key" : {1: 4, 2: h'1234', 3: 12, 5: h'1f389d14d17dc7',
-1: h'02e2cc3a9b92855220f255fff1c615bc'},
"pub_keys" : [
{
1 : 2, / type EC2 /
2 : h'11', / kid /
3 : -7, / alg ECDSA with SHA-256 /
-1 : 1 , / crv P-256 /
-2 : h'65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de43
9c08551d', / x /
-3 : h'1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd
0084d19c' / y /
}
]
}
Figure 9: Authorization + Key Distribution Response payload for a
Subscriber
6. Pub-Sub Protected Communication
This section specifies the communication Publisher-Broker and
Subscriber-Broker, after the previous phases have taken place.
+------------+ +------------+ +------------+
| CoAP | | CoAP | | CoAP |
| Client - | | Server - | | Client - |
| | ----(E)---> | | | |
| Publisher | | Broker | <----(F)---- | Subscriber |
| | | | -----(G)---> | |
+------------+ +------------+ +------------+
Figure 10: Phase 3: Secure communication between Publisher and
Subscriber
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The (E) message corresponds to the publication of a topic on the
Broker. The publication (the resource representation) is protected
with COSE ([RFC8152]). The (F) message is the subscription of the
Subscriber, which is unprotected. The (G) message is the response
from the Broker, where the publication is protected with COSE.
The flow graph is presented below.
Publisher Broker Subscriber
| --- PUT /topic ----> | |
| protected with COSE | |
| | <--- GET /topic ----- |
| | |
| | ---- response ------> |
| | protected with COSE |
Figure 11: (E), (F), (G): Example of protected communication
6.1. Using COSE Objects to protect the resource representation
The Publisher uses the symmetric COSE Key received from AS2 in
exchange B (Section 3.1) to protect the payload of the PUBLISH
operation (Section 4.3 of [I-D.ietf-core-coap-pubsub]).
Specifically, the COSE Key is used to create a COSE_Encrypt0 with
algorithm specified by AS2. The Publisher uses the private key
corresponding to the public key sent to the AS2 in exchange B
(Section 3.1) to countersign the COSE Object as specified in
Section 4.5 of [RFC8152]. The CoAP payload is replaced by the COSE
object before the publication is sent to the Broker.
The Subscriber uses the kid in the countersignature field in the COSE
object to retrieve the right public key to verify the
countersignature. It then uses the symmetric key received from AS2
to verify and decrypt the publication received in the payload of the
CoAP Notification from the Broker.
The COSE object is constructed in the following way:
o The protected Headers (as described in Section 3 of [RFC8152]) MAY
contain the kid parameter, with value the kid of the symmetric
COSE Key received in Section 3.1 and MUST contain the content
encryption algorithm
o The unprotected Headers MUST contain the Partial IV and the
counter signature that includes:
* the algorithm (same value as in the asymmetric COSE Key
received in (B)) in the protected header
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* the kid (same value as the kid of the asymmetric COSE Key
received in (B)) in the unprotected header
* the signature computed as specified in Section 4.5 of [RFC8152]
o The ciphertext, computed over the plaintext that MUST contain the
CoAP payload.
The external_aad, when using AEAD, is an empty string.
An example is given in Figure 12
16(
[
/ protected / h'a2010c04421234' / {
\ alg \ 1:12, \ AES-CCM-64-64-128 \
\ kid \ 4: h'1234'
} / ,
/ unprotected / {
/ iv / 5:h'89f52f65a1c580',
/ countersign / 7:[
/ protected / h'a10126' / {
\ alg \ 1:-7
} / ,
/ unprotected / {
/ kid / 4:h'11'
},
/ signature / SIG / 64 bytes signature /
]
},
/ ciphertext / h'8df0a3b62fccff37aa313c8020e971f8aC8d'
]
)
Figure 12: Example of COSE Object sent in the payload of a PUBLISH
operation
The encryption and decryption operations are described in sections
5.3 and 5.4 of [RFC8152].
7. Security Considerations
In the profile described above, the Publisher and Subscriber use
asymmetric crypto, which would make the message exchange quite heavy
for small constrained devices. Moreover, all Subscribers must be
able to access the public keys of all the Publishers to a specific
topic to be able to verify the publications. Such a database could
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be set up and managed by the same entity having control of the topic,
i.e. AS2.
An application where it is not critical that only authorized
Publishers can publish on a topic may decide not to make use of the
asymmetric crypto and only use symmetric encryption/MAC to
confidentiality and integrity protect the publication, but this is
not recommended since, as a result, any authorized Subscribers with
access to the Broker may forge unauthorized publications without
being detected. In this symmetric case the Subscribers would only
need one symmetric key per topic, and would not need to know any
information about the Publishers, that can be anonymous to it and the
Broker.
Subscribers can be excluded from future publications through re-
keying for a certain topic. This could be set up to happen on a
regular basis, for certain applications. How this could be done is
out of scope for this work.
The Broker is only trusted with verifying that the Publisher is
authorized to publish, but is not trusted with the publications
itself, which it cannot read nor modify. In this setting, caching of
publications on the Broker is still allowed.
TODO: expand on security and Privacy considerations
8. IANA Considerations
The following registrations are done for the ACE OAuth Profile
Registry following the procedure specified in
[I-D.ietf-ace-oauth-authz].
Note to RFC Editor: Please replace all occurrences of "[[This
document]]" with the RFC number of this specification and delete this
paragraph.
Name: coap_pubsub
Description: Profile for delegating client authentication and
authorization for publishers and subscribers in a pub-sub setting
scenario in a constrained environment.
CBOR Key: TBD
Reference: [[This document]]
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9. References
9.1. Normative References
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE)", draft-ietf-ace-oauth-
authz-10 (work in progress), February 2018.
[I-D.ietf-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-ietf-core-coap-pubsub-03 (work in
progress), February 2018.
[I-D.palombini-ace-key-groupcomm]
Palombini, F. and M. Tiloca, "Key Provisioning for Group
Communication using ACE", draft-palombini-ace-key-
groupcomm-00 (work in progress), March 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<https://www.rfc-editor.org/info/rfc6749>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
9.2. Informative References
[I-D.ietf-ace-actors]
Gerdes, S., Seitz, L., Selander, G., and C. Bormann, "An
architecture for authorization in constrained
environments", draft-ietf-ace-actors-06 (work in
progress), November 2017.
[I-D.ietf-ace-dtls-authorize]
Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
L. Seitz, "Datagram Transport Layer Security (DTLS)
Profiles for Authentication and Authorization for
Constrained Environments (ACE)", draft-ietf-ace-dtls-
authorize-02 (work in progress), October 2017.
Palombini Expires September 3, 2018 [Page 15]
Internet-Draft coap-pubsub-profile March 2018
[I-D.ietf-ace-oscore-profile]
Seitz, L., Palombini, F., and M. Gunnarsson, "OSCORE
profile of the Authentication and Authorization for
Constrained Environments Framework", draft-ietf-ace-
oscore-profile-00 (work in progress), December 2017.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
resource-directory-13 (work in progress), March 2018.
Acknowledgments
The author wishes to thank Ari Keraenen, John Mattsson, Ludwig Seitz,
Goeran Selander, Jim Schaad and Marco Tiloca for the useful
discussion and reviews that helped shape this document.
Author's Address
Francesca Palombini
Ericsson
Email: francesca.palombini@ericsson.com
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