--- 1/draft-ietf-csi-proxy-send-01.txt 2010-03-03 17:10:55.000000000 +0100 +++ 2/draft-ietf-csi-proxy-send-02.txt 2010-03-03 17:10:55.000000000 +0100 @@ -1,403 +1,233 @@ -Network Working Group S. Krishnan -Internet-Draft Ericsson -Intended status: Standards Track J. Laganier -Expires: January 14, 2010 DoCoMo Euro-Labs - M. Bonola +CGA & SEND maintenance Working S. Krishnan +Group Ericsson +Internet-Draft J. Laganier +Intended status: Standards Track QUALCOMM Inc. +Expires: September 4, 2010 M. Bonola Rome Tor Vergata University - July 13, 2009 + A. Garcia-Martinez + UC3M + March 3, 2010 Secure Proxy ND Support for SEND - draft-ietf-csi-proxy-send-01 + draft-ietf-csi-proxy-send-02 + +Abstract + + Secure Neighbor Discovery (SEND) specifies a method for securing + Neighbor Discovery (ND) signaling against specific threats. As + defined today, SEND assumes that the node sending a ND message is the + owner of the address from which the message is send, so that it is in + possession of the private key used to generate the digital signature + on the message. This means that the Proxy ND signaling performed by + nodes that do not possess knowledge of the address owner's private + key cannot be secured using SEND. This document extends the current + SEND specification in order to support Proxy ND operation. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the - provisions of BCP 78 and BCP 79. This document may contain material - from IETF Documents or IETF Contributions published or made publicly - available before November 10, 2008. The person(s) controlling the - copyright in some of this material may not have granted the IETF - Trust the right to allow modifications of such material outside the - IETF Standards Process. Without obtaining an adequate license from - the person(s) controlling the copyright in such materials, this - document may not be modified outside the IETF Standards Process, and - derivative works of it may not be created outside the IETF Standards - Process, except to format it for publication as an RFC or to - translate it into languages other than English. + provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. - This Internet-Draft will expire on January 14, 2010. + This Internet-Draft will expire on September 4, 2010. Copyright Notice - Copyright (c) 2009 IETF Trust and the persons identified as the + Copyright (c) 2010 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 in effect on the date of - publication of this document (http://trustee.ietf.org/license-info). - Please review these documents carefully, as they describe your rights - and restrictions with respect to this document. - -Abstract - - Secure Neighbor Discovery (SEND) specifies a method for securing - Neighbor Discovery (ND) signaling against specific threats. As - specified today, SEND assumes that the node advertising an address is - the owner of the address and is in possession of the private key used - to generate the digital signature on the message. This means that - the Proxy ND signaling initiated by nodes that do not possess - knowledge of the address owner's private key cannot be secured using - SEND. This document extends the current SEND specification with - support for Proxy ND, the Secure Proxy ND Support for SEND. + Provisions Relating to IETF Documents + (http://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 BSD License. Table of Contents - 1. Requirements notation . . . . . . . . . . . . . . . . . . . . 4 - 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 - 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 4. Application Scenarios . . . . . . . . . . . . . . . . . . . . 7 - 4.1. Scenario 1: RFC 4389 Neighbor Discovery Proxy . . . . . . 7 - 4.2. Scenario 2: Mobile IPv6 . . . . . . . . . . . . . . . . . 8 - 4.3. Scenario 3: Proxy Mobile IPv6 . . . . . . . . . . . . . . 10 - 5. Secure Proxy ND Overview . . . . . . . . . . . . . . . . . . . 12 - 6. Secure Proxy ND Specification . . . . . . . . . . . . . . . . 14 - 6.1. Proxy Signature Option . . . . . . . . . . . . . . . . . . 14 - 6.2. Modified SEND processing rules . . . . . . . . . . . . . . 16 - 6.2.1. Processing rules for senders . . . . . . . . . . . . . 16 - 6.2.2. Processing rules for receivers . . . . . . . . . . . . 17 - 7. Backward Compatibility with legacy SEND nodes . . . . . . . . 18 - 8. Security Considerations . . . . . . . . . . . . . . . . . . . 19 - 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 - 10. Normative References . . . . . . . . . . . . . . . . . . . . . 21 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22 + 1. Requirements notation . . . . . . . . . . . . . . . . . . . . 3 + 2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 + 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 + 4. Secure Proxy ND Overview . . . . . . . . . . . . . . . . . . . 6 + 5. Secure Proxy ND Specification . . . . . . . . . . . . . . . . 8 + 5.1. Proxy Signature Option . . . . . . . . . . . . . . . . . . 8 + 5.2. Modified SEND processing rules . . . . . . . . . . . . . . 10 + 5.2.1. Processing rules for senders . . . . . . . . . . . . . 10 + 5.2.2. Processing rules for receivers . . . . . . . . . . . . 11 + 5.3. Proxying Link-Local Addresses . . . . . . . . . . . . . . 12 + 6. Application Scenarios . . . . . . . . . . . . . . . . . . . . 13 + 6.1. Scenario 1: Mobile IPv6 . . . . . . . . . . . . . . . . . 13 + 6.2. Scenario 2: Proxy Mobile IPv6 . . . . . . . . . . . . . . 15 + 6.3. Scenario 3: RFC 4389 Neighbor Discovery Proxy . . . . . . 17 + 7. Backward Compatibility with RFC3971 nodes and non-SEND + nodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 + 7.1. Backward Compatibility with RFC3971 nodes . . . . . . . . 20 + 7.2. Backward Compatibility with non-SEND nodes . . . . . . . . 20 + 8. Security Considerations . . . . . . . . . . . . . . . . . . . 23 + 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24 + 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25 + 10.1. Normative References . . . . . . . . . . . . . . . . . . . 25 + 10.2. Informative References . . . . . . . . . . . . . . . . . . 25 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26 1. Requirements notation 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 [RFC2119]. 2. Introduction - Secure Neighbor Discovery [RFC3971] specifies a method for securing - neighbor discovery signaling [RFC4861] against specific threats. As - specified today, SEND assumes that the node advertising an address is - the owner of the address and is in possession of the private key used - to generate the digital signature on the message. This means that - the Proxy ND signaling initiated by nodes that do not possess - knowledge of the address owner's private key cannot be secured using - SEND. + Secure Neighbor Discovery (SEND) [RFC3971] specifies a method for + securing Neighbor Discovery (ND) signaling [RFC4861] against specific + threats. As defined today, SEND assumes that the node sending a ND + message is the owner of the address from which the message is sent, + so that it is in possession of the private key used to generate the + digital signature on the message. This means that the Proxy ND + signaling performed by nodes that do not possess knowledge of the + address owner's private key cannot be secured using SEND. This document extends the current SEND specification with support for Proxy ND. From this point on we refer to such extension as "Secure Proxy ND Support for SEND". 3. Terminology Secure Proxy ND A node authorized to either modify or generate a SEND message without knowing the private key related to the source address of the ICMPv6 ND message. Proxied IPv6 address - An IPv6 address that doesn't belong to the Secure Proxy ND and for - which the Secure Proxy ND is advertising. - -4. Application Scenarios - - In this section we provide three different application scenarios for - which the ICMPv6 Neighbor Discovery signaling cannot be secured by - using the current SEND specification. - - Either of the entities described in the following three scenarios, - (i.e.: ND Proxy, MIPv6 Home Agent, PMIPv6 Mobile Access Gateway) can - be consider as a Secure Proxy ND. - -4.1. Scenario 1: RFC 4389 Neighbor Discovery Proxy - - Link 1 Link 2 - - Host A ND Proxy (P) Host B - | | | - | SRC = A | | - | DST = solicited_node(B) | | - | ICMPv6 NS | | - | TARGET = B | | - | SLLAO = B_LL | | - |------------------------->| | - | | SRC = A | - | | DST = solicited_node(B) | - | | ICMPv6 NS | - | | TARGET = B | - | | SLLAO = P_LL | - | |------------------------->| - | | | - | | SRC = B | - | | DST = A | - | | ICMPv6 NA | - | | TARGET = B | - | | TLLAO = B_LL | - | |<-------------------------| - | SRC = B | | - | DST = A | | - | ICMPv6 NA | | - | TARGET = B | | - | TLLAO = B_LL | | - |<-------------------------| | - | | | - - Figure 1: Proxy ND operations - - The Neighbor Discovery (ND) Proxy specification [RFC4389] provides a - method by which multiple link layer segments are bridged into a - single segment and specifies the IP-layer support that enables - bridging under these circumstances. - - A ND Proxy shall parse any IPv6 packet it receives on a proxy - interface to check whether it contains one of the following ICMPv6 - messages: Neighbor Solicitation (NS), Neighbor Advertisement (NA), - Router Advertisement, or Redirect. Since each of these messages - contains a link-layer address which might not be valid on another - segment, the ND Proxy proxies these packets as follows, and as - illustrated in Figure 1: - - 1. The source link layer address will be the address of the outgoing - interface. - - 2. The destination link layer address will be the address in the - neighbor entry corresponding to the destination IPv6 address. - - 3. A link layer address within the payload (that is, in a Source - Local Link Address option - SLLAO, or a Target Local Link Address - option - TLLAO) is substituted with the link-layer address of the - outgoing interface. - - Moreover, when any other IPv6 unicast packet is received on a proxy - interface, if it is not locally destined then it is forwarded - unchanged (other than using a new link-layer header) to the proxy - interface for which the next hop address appears in the neighbor - cache. If no neighbor cache entry is present, the ND proxy should - queue the packet and initiate a Neighbor Discovery signalling as if - the ICMPv6 NS message were locally generated. - - A ND proxy cannot protect proxied ND messages since protection of an - ND message as per the current SEND specification requires knowledge - of the private key of each node for which it is generating or - forwarding a ND message on the bridged link layer segments. - -4.2. Scenario 2: Mobile IPv6 - - The Mobile IPv6 protocol [RFC3775] allows a mobile node (MN) to move - from one link to another while maintaining reachability at a stable - address, the so-called MN's home address (HoA.) When a mobile node - attaches to a foreign network, all the packets sent to the MN's HoA - and forwarded on the home link by a correspondent node (CN) or a - router are intercepted by the home agent (HA) on that home link, - encapsulated and tunneled to the mobile node's registered care-of - address (CoA.) - The HA intercepts these packets by being a Neighbor Discovery proxy - for this MN. When a Neighbor Solicitation (NS) is intercepted on the - home link, the home agent checks if the Target address within the NS - matches with any of the MN's Home Address in the Binding Cache and if - so, it replies with a Neighbor Advertisement (NA) containing its own - link layer address (HA_LL) as the Target Link Layer Address Option - (TLLAO), as illustrated in Figure 2. - - Node (N) Home Agent (HA) Mobile Node (MN) - on Home Link on Home Link on Foreign Link - | | | - | SRC = N | | - | DST = solicited_node(MN) | | - | ICMPv6 NS | | - | TARGET = MN | | - | SLLAO = N_LL | | - |------------------------->| | - | | | - | SRC = MN | | - | DST = N | | - | ICMPv6 NA | | - | TARGET = MN | | - | TLLAO = HA_LL | | - |<-------------------------| | - | | | - | traffic | | - | dest= MN HoA | | - |------------------------->| | - | | | - | | tunnelled traffic | - | | dest= MN CoA | - | |------------------------->| - | | | - - Figure 2: Proxy ND role of the Home agent in MIPv6 + An IPv6 address that does not belong to the Secure Proxy ND and + for which the Secure Proxy ND is performing advertisements. - It is not possible to apply the current SEND specification to protect - the NA message issued by the HA. To generate an ICMPv6 NA with a - valid CGA option and the corresponding RSA Signature option, the HA - needs knowledge of the private key related to the MN's - Cryptographically Generated Address (CGA.) Any ICMPv6 NA without a - valid CGA and RSA signature option is to be treated as insecure by a - SEND receiver. + Non-SEND node -4.3. Scenario 3: Proxy Mobile IPv6 + An IPv6 node that does not implement the SEND [RFC3971] + specification but uses only the ND protocol defined in [RFC4861] + and [RFC4862], without security. - MN new MAG LMA - | | | - MN Attached | | - | | | - | MN Attached Event from MN/Network | - | | | - |--- ICMPv6 RS ------->| | - | | | - | |--- PBU ------------->| - | | | - | | Accept PBU - | | | - | |<------------- PBA ---| - | | | - | Accept PBA | - | | | - | |==== Bi-Dir Tunnel ===| - | | | - |<------ ICMPv6 RA ----| | - | | | - | | | - | | | + RFC3971 node - Figure 3: Mobile node's handover in PMIPv6 + An IPv6 node that does not implement the specification defined in + this document for Secure Proxy ND support, but uses only the SEND + specification as defined in [RFC3971]. - Proxy Mobile IPv6 [I-D.ietf-netlmm-proxymip6] is a network-based - mobility management protocol that provides an IP mobility management - support for MNs without requiring MNs being involved in the mobility - related signaling. The IP mobility management is totally hidden to - the MN in a Proxy Mobile IPv6 domain and is performed by two - functional entities: the Local Mobility Anchor (LMA) and the Mobile - Access Gateway (MAG.) + SPND node - When the MN connects to a new access link it will send a multicast - ICMPv6 Router Solicitation (RS.) The MAG on the new access link, - upon detecting the MN's attachment, will signal the LMA for updating - the binding state of the MN (Proxy Binding Update - PBU) and once the - signaling is complete (Proxy Binding Ack - PBA - received), it will - reply to the MN with a ICMPv6 Router Advertisement (RA) containing - its home network prefix(es) that were assigned to that mobility - session, making the MN believe it is still on the same link and not - triggering the IPv6 address reconfiguration (figure Figure 3.) - To avoid potential link-local address collisions between the MAG and - the MN after a handoff to a new link, the Proxy Mobile IPv6 - specification requires the MAG's link-local address configured on the - link to which the MN is attached to be generated once by the LMA when - the MN first attach to a PMIPv6 domain, and to be provided to the new - MN's serving MAG after each handoff. Thus, from the MN's point of - view, the MAG's link-local address remains constant for the duration - of that MN's session. + An IPv6 node that implements the specification defined in this + document for Secure Proxy ND support. - The approach described above and the current SEND specification are - incompatible since: +4. Secure Proxy ND Overview - Sharing the same link-local address on different MAGs would - require all MAGs of a PMIPv6 domain to construct the CGA and the - RSA Signature option with the same public-private key pair, which - is not acceptable from a security point of view. + The original SEND specification [RFC3971] has implicitly assumed that + only the node sending a ND message is the owner of the address from + which the message is sent. This assumption does not allow proxying + of ND messages since the advertiser is required to generate a valid + RSA Signature option, which in turns requires possession of the + public-private key pair that was used to generate a CGA, or that was + associated to a router certificate. - Using different public-private key pairs on different MAGs would - mean different MAGs use different CGAs as link-local address. - Thus the serving MAG's link-local address changes after each - handoff of the MN which is contradiction with the way MAG link- - local address assignment occurs in a PMIPv6 domain. + To be able to separate the roles of ownership and advertiser the + following extensions to the SEND protocol are defined: -5. Secure Proxy ND Overview + o A Secure Proxy ND certificate, which is a certificate authorizing + an entity to act as an ND proxy. It is a X509v3 certificate in + which the purpose for which the certificate is issued has been + specified explicitly as described in a companion document + [I-D.ietf-csi-send-cert]. Briefly, a KeyPurposeID value is + defined for authorizing proxies. The inclusion of the proxy + authorization value allows the certificate owner to perform + proxying of SEND messages for a range of addresses indicated in + the same certificate. This certificate can be exchanged as a + result of the Authorization Delegation Discovery process defined + in [RFC3971]. - The original SEND specification [RFC3971] has implicitly assumed that - the owner of the address was the one who was advertising the prefix. - This assumption does not allow proxying of a CGA based address as the - receiver requires the advertiser to generate a valid CGA and RSA - Signature option, which in turns requires possession of the public- - private key pair that was used to generate the CGA. + o A new Neighbor Discovery option called Proxy Signature option + (PSO). This option contains the hash value of the public key of + the proxy, and the digital signature of the SEND message computed + with the private key of the proxy. The hash of the public key of + the proxy is computed over the public key contained in the Secure + Proxy ND's certificate. When a ND message contains a PSO, it MUST + NOT contain CGA and RSA Signature options. This option can be + appended to any ND message: NA, NS, RS, RA and Redirect. - This specification explicitly separates the roles of ownership and - advertiser by extending the SEND protocol as follows: + o A modification of the SEND processing rules for all ND messages: + NA, NS, RS, RA, and Redirect. When any of these messages is + received with a valid Proxy Signature option, it is considered as + secure. - o A certificate authorizing an entity to act as an ND proxy is - introduced. This is achieved via specifying explicitly in the - X509v3 certificate the purpose for which the certificate is - issued, as described in a companion document - [I-D.krishnan-cgaext-send-cert-eku]. Briefly, two KeyPurposeID - values are defined: one for authorizing routers, and one for - authorizing proxies. The inclusion of the proxy authorization - value allows the certificate owner to perform proxying of SEND - messages for a set of prefixes indicated in the same certificate. + These extensions are applied in the following way: - o A new option called Proxy Signature option (PSO) is defined. This - option contains the key hash value of the Secure Proxy ND's public - key and the digital signature computed over the SEND message. The - key has value is computed over the public key within the Secure - Proxy ND's certificate. + o A Secure Proxy ND which proxies ND messages on behalf of a node + can use the PSO option to protect the proxied messages. This + Secure Proxy ND becomes part of the trusted infrastructure just + like a SEND router. - o The SEND processing rules are modified for all Neighbor Discovery - messages: NA, NS, RS, RA, and Redirect. When any of these - messages is received with a valid Proxy Signature option, it is - considered as secure even if it doesn't contain a CGA option. + o In order to allow nodes to successfully validate secured proxied + messages, the nodes must know the Secure Proxy ND certificate (in + the format described in [I-D.ietf-csi-send-cert]) and must apply + the modified processing rules specified in this document. We call + this nodes 'SPND nodes'. Note that the rules for generating ND + messages in SPND nodes do not change, so these nodes behave as + defined in [RFC3971] for sending ND messages. - The Secure Proxy ND becomes part of the trusted infrastructure just - like a SEND router. The Secure Proxy ND is granted a certificate - that specifies the range of addresses for which it is allowed to - perform proxying of SEND messages. Hosts can use the same process to - discover the certification path between a proxy and one of the host's - trust anchors as the one defined for routers in Section 6 of SEND - specification [RFC3971]. + o To allow SPND nodes to know the certificate path required to + validate the public key of the proxy, devices responding to CPS + (Certification Path Solicitation) messages with CPA (Certification + Path Advertisements) as defined in Section 6 of SEND specification + [RFC3971] must handle the certificate format specified in + [I-D.ietf-csi-send-cert], and must be configured with the + appropriate certification path. The proposed approach resolves the incompatibilities between the current SEND specification and the application scenarios described in - Section 4. Since SEND messages containing a Proxy Signature option - are not required to carry a CGA option, the IPv6 source address is no - longer cryptographically bound to the signature, and the sender of a - Neighbor Discovery message is not required to be the owner of the - claimed address. Thus, the Secure Proxy ND is able to either forward - and generate SEND messages for a proxied address within the set of - prefixes for which it is authorized. + Section 6. -6. Secure Proxy ND Specification +5. Secure Proxy ND Specification - A Secure ND Proxy performs all the operation described in the SEND + A Secure Proxy ND performs all the operation described in the SEND specification [RFC3971] with the addition of new processing rules to ensure that the receiving node can differentiate between an authorized proxy generating or forwarding a SEND message for a proxied address, and a malicious node doing the same. This is accomplished by signing the message with the public key of - the authorized Secure Proxy ND. The signature of the neighbor - discovery proxy is included in a new option called Proxy Signature - option (PSO.) The signature is performed over all the NDP options - present in the message and the PSO is appended as the last option in - the message. + the authorized Secure Proxy ND. The signature of the ND Proxy is + included in a new option called Proxy Signature option (PSO). The + signature is performed over all the NDP options present in the + message and the PSO is appended as the last option in the message. -6.1. Proxy Signature Option +5.1. Proxy Signature Option The Proxy Signature option allows public key-based signatures to be attached to NDP messages. The format of the PSO is described in the following diagram: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ @@ -412,249 +242,686 @@ . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Padding . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ - Figure 4: PSO layout + Figure 1: PSO layout Type TBA Length The length of the option (including the Type, Length, Reserved, Key Hash, Digital Signature, and Padding fields) in units of 8 octets. Reserved - A 16-bit field reserved for future use. The value MUST be + A 11-bit field reserved for future use. The value MUST be initialized to zero by the sender, and MUST be ignored by the receiver. Key Hash A 128-bit field containing the most significant (leftmost) 128 bits of a SHA-1 [SHA1] hash of the public key used for constructing the signature. Its purpose is to associate the signature to a particular key known by the receiver. Such a key - MUST be the same one within the Secure Proxy ND's certificate. + MUST be the same one within the corresponding Secure Proxy ND's + certificate. Digital Signature A variable-length field containing a PKCS#1 v1.5 signature, constructed by using the sender's private key over the following sequence of octets: 1. The 128-bit CGA Message Type tag [RFC3972] value for Secure Proxy ND, 0x09F5 2BE5 3B62 4C76 CB96 4E7F CDC9 2804 (The tag value has been generated randomly by the editor of this - specification.) + specification). 2. The 128-bit Source Address field from the IP header. 3. The 128-bit Destination Address field from the IP header. 4. The 8-bit Type, 8-bit Code, and 16-bit Checksum fields from the ICMP header. 5. The NDP message header, starting from the octet after the ICMP Checksum field and continuing up to but not including NDP options. 6. All NDP options preceding the Proxy Signature option. The signature value is computed with the RSASSA-PKCS1-v1_5 algorithm and SHA-1 hash, as defined in [RSA]. This field starts after the Key Hash field. The length of the - Digital Signature field is determined by the length of the RSA - Signature option minus the length of the other fields (including - the variable length Pad field.) + Digital Signature field is determined by the ASN.1 BER coding of + the PKCS#1 v1.5 signature. Padding - This variable-length field contains padding, as many bytes long as - remain after the end of the signature. + This variable-length field contains padding. The length of the + padding field is determined by the length of the Proxy Signature + Option minus the length of the other fields. -6.2. Modified SEND processing rules +5.2. Modified SEND processing rules The modifications described in the following section applies when a SEND message contains the Proxy Signature option (PSO), i.e. the message was sent by a Secure Proxy ND. This specification modifies the sender and receiver processing rules - for the following options defined in the SEND specification - [RFC3971]: CGA option, RSA option. + for the CGA and RSA options defined in the SEND specification + [RFC3971]. -6.2.1. Processing rules for senders +5.2.1. Processing rules for senders A ICMPv6 message sent by a Secure Proxy ND for a proxied address MUST contain a Proxy Signature option (PSO) and MUST NOT contain CGA and RSA Signature options. A Secure Proxy ND sending a SEND message with the PSO Signature option MUST construct the message as follows: - 1. The SEND message is constructed without the PSO as follow: + 1. The SEND message is constructed without the PSO as follows: A. If the Secure Proxy ND is locally generating the SEND message for a proxied address, the message is constructed as described in Neighbor Discovery for IP version 6 specification [RFC4861]. B. If the Secure Proxy ND is forwarding a SEND message, first the authenticity of the intercepted message is verified as - specified in SEND specification [RFC3971] Section 5. If the + specified in SEND specification [RFC3971], Section 5. If the SEND message is valid, any CGA or RSA option MUST be removed from the message. The intercepted message is finally modified as described in Section 4 of the ND Proxy specification [RFC4389]. - 2. The Proxy Signature option is added as the last option in the + C. If the Secure Proxy ND is forwarding a SEND message already + containing a PSO, first the authenticity of the intercepted + message is verified as specified in Section 6.2.2 of this + draft. If the SEND message is valid, the original PSO MUST + be removed from the message. The intercepted message is + finally modified as described in Section 4 of the ND Proxy + specification[RFC4389]. + + 2. Timestamp and Nonce options are included according to the rules + specified in SEND [RFC3971]. The value in the Timestamp option + MUST be generated by the Proxy. If the proxy is forwarding a + message, the Nonce value in the proxied message MUST be the same + as in the forwarded message. + + 3. The Proxy Signature option is added as the last option in the message. - 3. The data is signed as explained in Section 6.1. + 4. The data is signed as explained in Section 5.1. -6.2.2. Processing rules for receivers +5.2.2. Processing rules for receivers Any SEND message without a Proxy Signature option MUST be treated as specified in the SEND specification [RFC3971]. A SEND message including a Proxy Signature option MUST be processed as specified below: 1. The receiver MUST ignore any RSA and CGA options, as well as any options that might come after the first PSO. The options are ignored for both signature verification and NDP processing purposes. 2. The Key Hash field MUST indicate the use of a known public key. A valid certification path (see [RFC3971] Section 6.3) between the receiver's trust anchor and the sender's public key MUST be known. The Secure Proxy ND's X509v3 certificate MUST contain a extended key usage extension including the KeyPurposeId value for the proxy authorization. - 3. The Digital Signature field MUST have correct encoding and MUST - NOT exceed the length of the Proxy Signature option minus the - Padding. + 3. The Digital Signature field MUST have correct encoding. 4. The Digital Signature verification MUST show that the signature - has been calculated as specified in Section 6.1. + has been calculated as specified in Section 5.1. + + 5. Timestamp and Nonce options MUST be processed as specified in + [RFC3971] Section 5.3.4, except for replacing 'RSA Signature + option' by 'PSO option'. + + 6. Messages with the Override bit [RFC4861] set should override an + existing cache entry regardless if it was created as a result of + a RSA Signature option or a PSO option validation. When it is + not set the advertisement will not update a cached link-layer + address created securily by means of RSA Signature option or PSO + option validation. Messages that do not pass all the above tests MUST be silently discarded if the host has been configured to accept only secured ND messages. -7. Backward Compatibility with legacy SEND nodes +5.3. Proxying Link-Local Addresses - The PSO added by a Secure Proxy ND will be ignored by nodes - implementing the original SEND specification and hence will not cause - any interoperability problems. Since the Secure Proxy ND also - removes the original RSA option, these messages will be treated as - "unsecured" message as described in Section 8 "Transitions Issues" of - the SEND specification [RFC3971]. Thus, this specification does not - introduce any new transition issue compared to the original SEND - specification. + Secure Neighbor Discovery [RFC3971] relies on certificates to prove + that routers are authorized to announce a certain prefix. However, + Neighbor Discovery [RFC4861] states that router does not announce the + Link-Local prefix (fe80::/64). Hence, it is not required for a SEND + certificate to hold a X.509 IP address extensions that authorizes the + fe80::/64 prefix. Some scenarios ([RFC4389], [RFC5213]) impose that + the Secure ND proxy provides proxying function for the Link-Local + address of a node. When Secure ND proxy functionality on a Link- + Local address is required, either a list of link-local addresses, or + the fe80::/64 prefix MUST be explicitly authorized to be proxied in + the corresponding certificate. + +6. Application Scenarios + + In this section we describe three different application scenarios for + which Secure Proxy ND Support for SEND can be applied. Note that the + particular way in which Secure Proxy ND support is applied (which ND + messages are proxied, in which directions, how the interaction with + non-SEND hosts and RFC3971 hosts is handled, etc.) largely depends on + the particular scenario considered. In the first two scenarios + presented below, ND messages are synthesized on behalf of off-link + nodes. In the third one, ND messages are generated in reaction to ND + messages being received by other interfaces of the proxied node. + +6.1. Scenario 1: Mobile IPv6 + + The description of the problems for deploying SEND in this scenario + can be found in [I-D.ietf-csi-sndp-prob]. + + The Mobile IPv6 protocol (MIPv6) [RFC3775] allows a Mobile Node (MN) + to move from one link to another while maintaining reachability at a + stable address, the so-called MN's Home Address (HoA). When a MN + attaches to a foreign network, all the packets sent to the MN's HoA + by a Correspondent Node (CN) on the home link, or a router, are + intercepted by the Home Agent (HA) on that home link, encapsulated + and tunneled to the MN's registered Care-of Address (CoA). + + The HA intercepts these packets acting as a ND proxy for this MN. + Lets assume that the nodes in the home link use SEND. When a secured + NS is intercepted on the home link, the HA checks the validity of the + received message according to the rules stated in [RFC3971]. If the + message is valid, it checks if the Target address within the NS + matches with any of the MN's Home Address in the Binding Cache and if + so, it replies with a Neighbor Advertisement (NA) constructed as + described in [RFC4861], so it contains its own link layer address + (HA_LL) as the Target Link Layer Address Option (TLLAO), with a PSO + option signing the message, added as the last option of the message. + + Node (N) Home Agent (HA) Mobile Node (MN) + on Home Link on Home Link on Foreign Link + | | | + | SRC = N | | + | DST = solicited_node(MN) | | + | ICMPv6 NS | | + | TARGET = MN | | + | SLLAO = N_LL | | + | [CGA] | | + | RSA signature | | + |------------------------->| | + | | | + | SRC = HA | | + | DST = N | | + | ICMPv6 NA | | + | TARGET = MN | | + | TLLAO = HA_LL | | + | PSO signature | | + |<-------------------------| | + | | | + | traffic | | + | dest= MN HoA | | + |------------------------->| | + | | | + | | tunnelled traffic | + | | dest= MN CoA | + | |------------------------->| + | | | + + Figure 2: Proxy ND role of the Home agent in MIPv6 + + A node receiving the NA containing the PSO (e.g.: the CN in the home + link, or a router) must have a certificate of the public key of the + HA acting as a Secure Proxy ND. To do so, a certificate for the HA + could be made available through the Authorization Delegation + Discovery process [RFC3971] performed at the home link, i.e. the + devices responding to CPS messages should be configured to include in + CPA messages information about the HA certificate. + + The Override bit of the NA message is used to control which messages + should prevail each case: the message generated by the proxy once the + MN moves from the home network, or the MN if it come back to the home + link, as defined in the MIPv6 specification [RFC3775] + +6.2. Scenario 2: Proxy Mobile IPv6 + + Proxy Mobile IPv6 [RFC5213] is a network-based mobility management + protocol that provides an IP mobility management support for MNs + without requiring MNs being involved in the mobility related + signaling. The IP mobility management is totally hidden to the MN in + a Proxy Mobile IPv6 domain and is performed by two functional + entities: the Local Mobility Anchor (LMA) and the Mobile Access + Gateway (MAG). + + When the MN connects to a new access link it will send a multicast + ICMPv6 Router Solicitation (RS). The MAG on the new access link, + upon detecting the MN's attachment, will signal the LMA for updating + the binding state of the MN (Proxy Binding Update - PBU) and once the + signaling is complete (it receives a Proxy Binding Ack - PBA), it + will reply to the MN with a ICMPv6 Router Advertisement (RA) + containing the home network prefix(es) that were assigned to that + mobility session, making the MN believe it is still on the same link + and not triggering the IPv6 address reconfiguration (figure + Figure 3). + + MN new MAG LMA + | | | + MN Attached | | + | | | + | MN Attached Event from MN/Network | + | | | + | SRC = MN | | + | DST = all-routers | | + | ICMPv6 RS | | + | [CGA] | | + | RSA signature | | + |--------------------->| | + | | | + | |--- PBU ------------->| + | | | + | | Accept PBU + | | | + | |<------------- PBA ---| + | | | + | Accept PBA | + | | | + | |==== Bi-Dir Tunnel ===| + | | | + | SRC = MAG4MN | | + | DST = MN | | + | ICMPv6 RA | | + | SLL = MAG_LL | | + | PSO | | + |<---------------------| | + | | | + | | | + | | | + + Figure 3: Mobile node's handover in PMIPv6 + + To avoid potential link-local address collisions between the MAG and + the MN after a handoff to a new link, the Proxy Mobile IPv6 + specification requires the MAG's link-local address configured on the + link to which the MN is attached to, to be generated once by the LMA + when the MN first attach to a PMIPv6 domain, and to be provided to + the new MN's serving MAG after each handoff. Thus, from the MN's + point of view, the MAG's link-local address remains constant for the + duration of that MN's session. + + Each MAG can be granted a certificate per each link-local address + expected by any MN that could attach to the link. However, the use + of Secure Proxy ND can greatly reduce the number of certificates + needed. In this case, each MAG is configured to act as a proxy by + means of a certification path from a trust anchor associated to the + PMIPv6 domain, authorizing each MAG to proxy securely ND messages. + + When a secured RS message is issued by the MN, the MAG checks its + validity according to the rules stated in [RFC3971]. If the message + is valid, it replies with a RA with source address equal to the MAG + link-local address associated to the MN in this PMIPv6 domain and its + own link layer address as Source link-layer address, with the PSO + option signing the message, added as the last option of the message. + + When the MN receives this message, it may issue a CPS message in + order to obtain the certification path associated to the public key + of the PSO (or may have this certification path already available). + The MN node must be configured with a trust anchor related with the + MAG's certificate. The MAG (or other device) could be configured to + provide its certification path in a CPS message as a response to a + CPA message issued by the MN. With this information, the MN can + validate the RS information, and use the same link-local address to + access to the MAG. + + The MAG will intercept secured NS messages and reply with NA messages + containing its own link layer address as the Target Link Layer + Address Option (TLLAO), with a PSO option signing the message, added + as the last option of the message. The same applies for Redirect + messages. + +6.3. Scenario 3: RFC 4389 Neighbor Discovery Proxy + + The description of the problems for deploying SEND in this scenario + can be found in [I-D.ietf-csi-sndp-prob]. + + Link 1 Link 2 + + Host A ND Proxy (P) Host B + | | | + | SRC = A | | + | DST = solicited_node(B) | | + | ICMPv6 NS | | + | TARGET = B | | + | SLLAO = A_LL | | + |------------------------->| | + | | SRC = A | + | | DST = solicited_node(B) | + | | ICMPv6 NS | + | | TARGET = B | + | | SLLAO = P_LL | + | |------------------------->| + | | | + | | SRC = B | + | | DST = A | + | | ICMPv6 NA | + | | TARGET = B | + | | TLLAO = B_LL | + | |<-------------------------| + | SRC = B | | + | DST = A | | + | ICMPv6 NA | | + | TARGET = B | | + | TLLAO = P_LL | | + |<-------------------------| | + | | | + + Figure 4: Proxy ND operations + + The Neighbor Discovery (ND) Proxy specification [RFC4389] provides a + method by which multiple link layer segments are bridged into a + single segment and specifies the IP-layer support that enables + bridging under these circumstances. + + A Secure ND Proxy shall parse any IPv6 packet it receives on a proxy + interface to check whether it contains one of the following secured + ICMPv6 messages: NS, NA, RA, or Redirect. The Secure ND Proxy MUST + verify the authenticity of the received ND message, according to + [RFC3971]. If the SEND message is valid, then it proxied the + original message with the following changes: + + 1. The message is processed according to [RFC4389]. This includes + changing the source link layer address will be the address of the + outgoing interface, maintaining the destination link layer + address as the address in the neighbor entry corresponding to the + destination IPv6 address, etc. In particular any link layer + address within the payload (that is, in a Source Local Link + Address option - SLLAO, or a Target Local Link Address option - + TLLAO) is substituted with the link-layer address of the outgoing + interface. + + 2. Any CGA or RSA option is removed. + + 3. If a Nonce option existed in the original message, its value is + preserved in the proxied message. The Timestamp is generated by + the proxy. + + 4. The PSO option is added as the last option in the message, + signing all the information contained so far in the message. + + Moreover, when any other IPv6 unicast packet is received on a proxy + interface, if it is not locally destined then it is forwarded + unchanged (other than using a new link-layer header) to the proxy + interface for which the next hop address appears in the neighbor + cache. If no neighbor cache entry is present, the ND proxy should + queue the packet and initiate a Neighbor Discovery signalling as if + the ICMPv6 NS message were locally generated. + + In order to deploy this scenario, nodes in proxied segments MUST know + the certificate authorizing proxy operation. To do so it could be + required to configure at least one device per each proxied segment + (may be the proxy itself) to propagate the required certification + path to authorize proxy operation by means of a CPS/CPA exchange. + + While more robust mechanisms could be developed for securing the + scenario described in [RFC4389], if hosts have been upgraded to apply + the rules stated in Section 5.2.2, for example, to benefit from + secure support for other scenarios, the application of this mechanism + is straighforward. + +7. Backward Compatibility with RFC3971 nodes and non-SEND nodes + + In this section we discuss the interaction of Secure Proxy ND nodes + and SPND nodes with RFC3971 nodes and non-SEND nodes. + +7.1. Backward Compatibility with RFC3971 nodes + + RFC3971 nodes, i.e. SEND nodes not compliant with the modifications + required in Section 5 cannot interpret correctly a PSO option + received in a proxied ND message. These SEND nodes silently discard + the PSO option, as specified in [RFC4861] for any unknown option. As + a result, these messages will be treated as unsecured as described in + Section 8 "Transitions Issues" of the SEND specification [RFC3971]. + + When RFC3971 nodes and SPND nodes exchange ND messages (without proxy + intervention), in either direction, messages are generated according + to the SEND specification [RFC3971], so these nodes interoperate + seamlessly. + + In the scenarios in which the proxy translates ND messages, the + messages to translate can either be originated in a RFC3971 node or + in an SPND node, without interoperability issues. + +7.2. Backward Compatibility with non-SEND nodes + + Plain ND nodes receiving NDP packets silently discard PSO options, as + specified in [RFC4861] for any unknown option. Therefore, these node + interpret messages proxied by a Secure Proxy ND as any other ND + message. + + When non-SEND nodes and SPND nodes exchange ND messages (without + proxy intervention), in either direction, the rules specified in + section 8 of [RFC3971] apply. + + A secure Proxy ND SHOULD support the use of secured and unsecured NDP + messages at the same time, although it MAY have a configuration that + causes not to perform proxing for unsecured NDP messages. A secure + Proxy ND MAY also have a configuration option whereby it disables + secure ND proxying completely. This configuration SHOULD be switched + off by default, that is SEND is used. In the next paragraphs we + discuss the recommended behavior of the Secure Proxy ND regarding to + which protection level provide to proxied messages in a mixed + scenario involving SPND/RFC3971 nodes and non-SEND nodes. In + particular, two different situations occur depending on if the + proxied nodes are RFC3971 or SPND, or if they are non-SEND nodes. + + As a rule of thumb, the Secure Proxy ND should only generate PSO + options for nodes which have SEND capabilities (i.e. that they could + use SEND to defend their messages if being in the same link than the + proxy, either RFC3971 nodes or SPND nodes). This is relevant to + allow nodes preferring secured information over unsecured one, and + for executing the DAD procedure, as specified in [RFC3971]. + Therefore, the Secure Proxy ND SHOULD generate messages containing + the PSO option for SPND and RFC3971 hosts, and SHOULD NOT generate + messages containing the PSO option for non-SEND nodes. Note that ND + advertisements in response to solicitations generated by a Secure + Proxy ND must be secured or not according to the previous + considerations (i.e. to the nature of the proxied node), and not + according to the secure or unsecure nature of the solicitation + message. + + To apply this rule, we have to consider that depending on the + application scenario the proxy may translate ND messages generated by + a node or synthetise ND messages on behalf of a node. + + o For ND translated messages, the rule can be easily applied: only + messages validated in the Secure Proxy ND according to the SEND + specification [RFC3971] MUST be proxied securely by the inclusion + of a PSO option. Unsecured ND messages could be proxied if + unsecured operation is enabled in the proxy, but the message + generated by the Secure Proxy ND for the received message MUST NOT + include a PSO option. + + o For ND messages synthesised on behalf of remote nodes, different + considerations should be made according to the particular + application scenario. + + * For MIPv6, if the MN can return to the home link, it is + required for the proxy to know if the node could use SEND to + defend its address or not. A mismatch between the proxy and + proxied node behavior regarding to SEND operation would result + in unaproppriate operation. A HA including the PSO option for + proxying a non-SEND MN would make ND messages sent by the proxy + to be more preferred than ND message of the non-SEND MN if the + MN returns to the home link (even if the proxied messages have + the Override bit set to 1). Not using the PSO option for a + RFC3971 or SPND MN would make more vulnerable the address in + the home link when the MN is away than when it is in the home + link (and would defeat the purpose of the Secure Proxy ND + mechanism). Therefore, in this case the HA must know the SEND + capabilities of the MN. + + * We can state the same for the Proxy Mobile IPv6 scenario as for + the MIPv6 scenario. Note that a node moving from a link in + which PSO has been used to protect a link-layer address to a + link in which ND messages are not SEND-enabled would prevent + the node from adquiring the new information until the + corresponding cache expires. However, in this case it is + reasonable to consider that all MAGs provide the same security + for protecting ND messages, and that either all MAGs will + behave as Secure Proxy ND, or none, so configuration could be + easier. 8. Security Considerations - The mechanism described in this document introduce a new Proxy + The mechanism described in this document introduces a new Proxy Signature Option (PSO) allowing a Secure Proxy ND to generate or - modify a SEND message for a proxied address. A node will only accept - such a message if it includes a valid PSO generated by an authorized - Secure Proxy ND. + modify a SEND message for a proxied address. An SPND node will only + accept such a message if it includes a valid PSO generated by an + authorized Secure Proxy ND. Such a message has equivalent protection + to the threats presented in section 9 of [RFC3971] as a message + signed with a RSA Signature option. - If, on the other hand, a message does not include a PSO, then the - Secure Proxy ND support doens't introduce any further security issues - since this specification does not modify the SEND processing rules if - an ICMPv6 ND message does not contain a PSO. Thus, the same security - considerations than that of SEND applies (cf. Section 9 of the SEND - specification [RFC3971].) + The security of proxied ND messages not including a PSO option is the + same of an unsecured ND message. The security of a proxied ND + message received by a non-SEND host or RFC3971 host is the same of an + unsecured ND message. + + Thanks to the authorization certificate it is provisioned with, a + proxy ND is authorized to issue ND signaling on behalf of nodes on + the subnet. Thus, a compromised proxy is able, like a compromised + router, to siphon off traffic from the host, or mount a man-in-the- + middle attack. However, when two on-link hosts communicate using + their respective link-local addresses, the threats involved with a + compromised router and a compromised proxy ND differs because the + router is not able to siphon off traffic exchanged between the hosts + or mount a man-in-the-middle attack, while the proxy ND can, even if + the hosts use SEND. + + The messages for which a Secure Proxy ND perform its function, and + the link for which this function is performed MUST be configured + appropriately for each proxy and scenario. This configuration is + specially relevant if Secure Proxy ND is used for translating ND + messages from one link to another. + + Proper configuration of when the PSO option must or must not be + included, depending on the proxied node being SEND or non-SEND may + result in security considerations, as discussed in Section 7. + + Attacks to the timestamp of the secured ND message can be performed + as describe in section 7.3 of [I-D.ietf-csi-sndp-prob]. 9. IANA Considerations IANA is requested to allocate: - A new IPv6 Neighbor Discovery Option types for the PSO, as TBA. + A new IPv6 Neighbor Discovery Option type for the PSO, as TBA. The value need to be allocated from the namespace specified in the IANA registry IPv6 NEIGHBOR DISCOVERY OPTION FORMATS located at http://www.iana.org/assignments/icmpv6-parameters. A new 128-bit value under the CGA Message Type [RFC3972] namespace, 0x09F5 2BE5 3B62 4C76 CB96 4E7F CDC9 2804. -10. Normative References +10. References - [I-D.ietf-netlmm-proxymip6] - Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K., - and B. Patil, "Proxy Mobile IPv6", - draft-ietf-netlmm-proxymip6-18 (work in progress), - May 2008. +10.1. Normative References - [I-D.krishnan-cgaext-send-cert-eku] - Krishnan, S., Kukec, A., and K. Ahmed, "Certificate + [I-D.ietf-csi-send-cert] + Gagliano, R., Krishnan, S., and A. Kukec, "Certificate profile and certificate management for SEND", - draft-krishnan-cgaext-send-cert-eku-03 (work in progress), - March 2009. + draft-ietf-csi-send-cert-02 (work in progress), + February 2010. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support in IPv6", RFC 3775, June 2004. [RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)", RFC 3971, March 2005. [RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)", RFC 3972, March 2005. [RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery Proxies (ND Proxy)", RFC 4389, April 2006. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, September 2007. + [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless + Address Autoconfiguration", RFC 4862, September 2007. + + [RFC5213] Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K., + and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008. + [RSA] RSA Laboratories, "RSA Encryption Standard, Version 2.1", PKCS 1 , November 2002. [SHA1] National Institute of Standards and Technology, "Secure Hash Standard", FIPS PUB 180-1 , April 1995. +10.2. Informative References + + [I-D.ietf-csi-sndp-prob] + Combes, J., Krishnan, S., and G. Daley, "Securing Neighbor + Discovery Proxy: Problem Statement", + draft-ietf-csi-sndp-prob-04 (work in progress), + January 2010. + Authors' Addresses Suresh Krishnan Ericsson 8400 Decarie Blvd. Town of Mount Royal, QC Canada Phone: +1 514 345 7900 x42871 Email: suresh.krishnan@ericsson.com Julien Laganier - DoCoMo Communications Laboratories Europe GmbH - Landsberger Strasse 312 - Munich D-80687 - Germany + QUALCOMM Incorporated + 5775 Morehouse Dr + San Diego, CA 92121 + USA - Phone: +49 89 56824 231 - Email: julien.ietf@laposte.net - URI: http://www.docomolab-euro.com/ + Phone: +1 858 658 3538 + Email: julienl@qualcomm.com Marco Bonola Rome Tor Vergata University Via del Politecnico, 1 Rome I-00133 Italy Phone: Email: marco.bonola@gmail.com + + Alberto Garcia-Martinez + U. Carlos III de Madrid + Av. Universidad 30 + Leganes, Madrid 28911 + Spain + + Phone: +34 91 6248782 + Email: alberto@it.uc3m.es + URI: http://www.it.uc3m.es/