INTERNET-DRAFT Erik Nordmark Oct 27, 2003 Sun Microsystems Multihoming using 64-bit Crypto-based IDs Status of this Memo This document is an Internet-Draft and is subject to all provisions of Section 10 of RFC2026. 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 expires April 27, 2004. Abstract This document outlines a potential solution to IPv6 multihoming in order to stimulate discussion. This proposal is a middle ground between the NOID and CB128 proposals. This proposed solution relies on verification the crypto-based identifier properties (using public-key crypto during uncommon operations), while allowing locator rewriting by (border) routers, with no per-packet overhead. The solution does have something which could be viewed as a "stack name" type of identifier, but this isn't exposed to upper layer protocols. Instead it ensures that all upper layer protocols can operate unmodified in a multihomed setting while still seeing a stable IPv6 address, even though the address draft-nordmark-multi6-cb64-00.txt [Page 1] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 internally consists of 64-bits worth of subnet locator plus 64-bits of crypto-based identifier. This solution (and this draft) is remarkably similar to draft- nordmark-multi6-noid-00.txt; only issues related to prevention of redirection attacks differ. DISCLAIMER: This work has been discussed in a design team. The design team is still exploring multiple approaches and this is an attempt to capture one such approach on paper. Because of this and due to lack of time to review the document one can not say that this is a product of the DT; errors and confusions should be attributed to the scribe and not to the DT. draft-nordmark-multi6-cb64-00.txt [Page 2] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 Contents 1. INTRODUCTION............................................. 4 1.1. Non-Goals........................................... 4 1.2. Assumptions......................................... 5 2. TERMINOLOGY.............................................. 5 2.1. Notational Conventions.............................. 6 3. PROTOCOL OVERVIEW........................................ 7 3.1. Host-Pair Context................................... 10 3.2. Message Formats..................................... 12 4. PROTOCOL WALKTHROUGH..................................... 14 4.1. Initial Context Establishment....................... 14 4.2. Locator Change...................................... 17 4.3. Handling Locator Failures........................... 18 4.4. Locator Set Changes................................. 18 4.5. Preventing Premeditated Redirection Attacks......... 19 5. HANDLING STATE LOSS...................................... 20 6. ENCODING BITS IN THE IPv6 HEADER?........................ 22 7. COMPATIBILITY WITH STANDARD IPv6......................... 23 8. APPLICATION USAGE OF IDENTIFIERS......................... 24 9. CHECKSUM ISSUES.......................................... 24 10. IMPLICATIONS FOR PACKET FILTERING....................... 25 11. IPSEC INTERACTIONS...................................... 26 12. SECURITY CONSIDERATIONS................................. 26 13. DESIGN ALTERNATIVES..................................... 27 14. OPEN ISSUES............................................. 28 14.1. Initiator Confusion vs. "Virtual Hosting".......... 28 14.2. Using larger context tags.......................... 29 15. COMPARISON WITH NOID AND CB128.......................... 30 16. ACKNOWLEDGEMENTS........................................ 30 17. IPR CONSIDERATIONS...................................... 31 draft-nordmark-multi6-cb64-00.txt [Page 3] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 18. REFERENCES.............................................. 31 18.1. Normative References............................... 31 18.2. Informative References............................. 31 1. INTRODUCTION The goal of the IPv6 multihoming work is to allow a site to take advantage of multiple attachments to the global Internet without having a specific entry for the site visible in the global routing table. Specifically, a solution should allow users to use multiple attachments in parallel, or to switch between these attachment points dynamically in the case of failures, without an impact on the upper layer protocols. This proposed solution uses crypto-based identifiers [CBID] properties to perform enough validation to prevent redirection attacks. The goals for this proposed solution is to: o Have no impact on upper layer protocols in general and on transport protocols in particular. o Address the security threats in [M6SEC]. o Allow routers rewriting the (source) locators as a means of quickly detecting which locator is likely to work for return traffic. o No per-packet overhead. o No extra roundtrip for setup. o Take advantage of multiple locators/addresses for load spreading. 1.1. Non-Goals The assumption is that the problem we are trying to solve is site multihoming, with the ability to have the set of site locator prefixes change over time due to site renumbering. Further, we assume that such changes to the set of locator prefixes can be draft-nordmark-multi6-cb64-00.txt [Page 4] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 relatively slow and managed; slow enough to allow updates to the DNS to propagate. This proposal does not attempt to solve, perhaps related, problems such as host multihoming or host mobility. This proposal also does not try to provide an IP identifier to the upper layer protocols - even though it uses a 64-bit CBID internally to prevent redirection attacks. An IP identifier name space would be useful to ULPs and applications, especially if the management burden for such a name space was zero and there was an efficient yet secure mechanism to map from identifiers to locators, but exposing such a name space isn't necessary to solve the problem at hand. 1.2. Assumptions The main technical assumptions this proposal makes is that using public key signatures during the more uncommon operations would provide acceptable performance. The proposal doesn't require such operations during normal communication; only when a locator changes for a host will it need to be verified before return traffic will be sent to that locator, or when two hosts claim to use the same identifier. Another assumption is that where DNS is already used (normally at the initiating end of communication) it is acceptable to lookup a multihoming capability "flag" in the DNS in addition to the current AAAA lookup (of the addresses/locators). 2. TERMINOLOGY upper layer protocol (ULP) - a protocol layer immediately above IP. Examples are transport protocols such as TCP and UDP, control protocols such as ICMP, routing protocols such as OSPF, and internet or lower-layer protocols being "tunneled" over (i.e., encapsulated in) IP such as IPX, AppleTalk, or IP itself. interface - a node's attachment to a link. address - an IP layer name that contains both topological significance and acts as a unique identifier for an interface. 128 bits. locator - an IP layer topological name for an interface or a set of interfaces. 128 bits. The locators are draft-nordmark-multi6-cb64-00.txt [Page 5] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 carried in the IP address fields as the packets traverse the network. identifier - an IP layer identifier for an IP layer endpoint (stack name in [NSRG]). In this proposal a 64-bit CBID i.e. a hash of a public key. Application identifier (AID) - an IP locator which has been selected for communication with a peer to be used by the upper layer protocol. 128 bits. This is used for pseudo-header checksum computation and connection identification in the ULP. Different sets of communication to a host (e.g., different connections) might use different AIDs in order to enable load spreading. The AIDs contain a 64-bit CBID in the low-order bits. address field - the source and destination address fields in the IPv6 header. As IPv6 is currently specified this fields carry "addresses". If identifiers and locators are separated these fields will contain locators. FQDN - Fully Qualified Domain Name 2.1. Notational Conventions A, B, and C are hosts. X is a potentially malicious host. FQDN(A) is the domain name for A. ID(A) is the 64 bit CBID for A. Ls(A) is the locator set for A, which consists of L1(A), L2(A), ... Ln(A). Each Lk(A) contains ID(A) in the low-order bits. The high- order 64 bits of a locator are sometimes referred to as the "subnet locator". (The protocol doesn't prevent a single host having multiple identities, but that can be viewed multiple entities A, B, etc existing on the same host.) AID(A) is an application ID for A. In this proposal, AID(A) is always one member of Ls(A). draft-nordmark-multi6-cb64-00.txt [Page 6] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 3. PROTOCOL OVERVIEW In order to prevent redirection attacks this protocol relies on the ability to verify (using public key crypto as in [CBID]) that the entity requesting redirection indeed holds the private key where the hash of the corresponding public key hashes to the ID itself. DNS is used to lookup an indication of the multihoming capability of a host, in addition to being used to lookup the AAAA records containing the locators. The details of this is TBD but a simple example would be to introduce a new M6 RR type in the DNS which has no RDATA; thus the mere existence of such a record at a FQDN would imply that the host supports the M6 protocol. In essence this proposal is the same as [NOID] except that the CBID property is used instead of DNS for verification. ----------------------- | Transport Protocols | ----------------------- ------ ------- -------------- ------------- | AH | | ESP | | Frag/reass | | Dest opts | ------ ------- -------------- ------------- ----------------- | M6 shim layer | ----------------- ------ | IP | ------ Figure 1: Protocol stack The proposal uses an M6 shim layer between IP and the ULPs as shown in figure 1, in order to provide ULP independence. Conceptually the M6 shim layer behaves as if it is an extension header, which would be ordered immediately after any hop-by-hop options in the packet. However, the amount of data that needs to be carried in an actual M6 extension header is close to zero. By using some encoding of the nexthdr value it is possible to carry the common protocols/extension headers without making the packets larger. The nexthdr encodings are discussed later in this document. We refer to packets that use this encoding to indicate to the receiver that M6 processing should be applied as "M6 packets" (analogous to "ESP packets" or "TCP packets"). draft-nordmark-multi6-cb64-00.txt [Page 7] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 Layering AH and ESP above the M6 shim means that IPsec can be made to be unaware of locator changes the same way that transport protocols can be unaware. Thus the IPsec security associations remain stable even though the locators are changing. Layering the fragmentation header above the M6 shim makes reassembly robust in the case that there is broken multi-path routing which results in using different paths, hence potentially different source locators, for different fragments. The proposal uses router rewriting of (source) locators as one way to determine which is the preferred (or only working) locator to use for return traffic. But not all packets can have their locators rewritten. In addition to existing IPv6 packets, the packets exchanged before M6 host-pair context state is established at the receiver can not have their locators rewritten. Thus a simple mechanism is needed to indicate to the routers on the path whether or not it is ok to rewrite the locators in the packet. Conceptually this is a single bit in the IPv6 header (we call it the "rewrite ok" bit) but there is no spare bit available. Later in the document we show how we solve this by allocating a range of next header values to denote this semantic bit. Applications and upper layer protocols use AIDs which the M6 layer will map to/from different locators. The M6 layer maintains state, called host-pair context, in order to perform this mapping. The mapping is performed consistently at the sender and the receiver, thus from the perspective of the upper layer protocols packets appear to be sent using AIDs from end to end, even though the packets travel through the network containing locators in the IP address fields, and even though those locators might be rewritten in flight. draft-nordmark-multi6-cb64-00.txt [Page 8] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 ---------------------- ---------------------- | Sender A | | Receiver B | | | | | | ULP | | ULP | | | src AID(A) | | ^ | | | dst AID(B) | | | src AID(A) | | v | | | dst AID(B) | | M6 | | M6 | | | src L1(A) | | ^ | | | dst L1(B) | | | src L2(A) | | v | | | dst L1(B) | | IP | | IP | ---------------------- ---------------------- | ^ -- cloud with some routers ------- src L2(A) [Rewritten] dst L1(B) Figure 2: Mapping with router rewriting of locators. The result of this consistent mapping is that there is no impact on the ULPs. In particular, there is no impact on pseudo-header checksums and connection identification. Conceptually one could view this approach as if both AIDs and locators being present in every packet, but with a header compression mechanism applied that removes the need for the AIDs once the state has been established. As we will see below the flowid will be used akin to a "compression tag" i.e., to indicate the correct context to use for decompression. The need for some "compression tag" is because the desire to allow load spreading and handle site renumbering. Without those desires it could have been possible to e.g. designate one fixed locator as the AID for a host and storing that in the DNS. But instead different connections between two hosts are allowed to use different AIDs and on reception of a M6 packet the correct AIDs must be inserted into the IP address fields before passing the packet to the ULP. The flowid serves as a convenient "compression tag" without increasing the packet size, and this usage doesn't conflict with other flowid usage. In addition to the zero overhead data messages, there are six different M6 message types introduced (which could be defined as new ICMPv6 messages). Three types are used to perform a 3-way handshake to create state at both endpoints without creating state on the first received packet (which would introduce a memory consumption DoS attack), 2 are used to do a challenge request/response exchange when a new locator is introduced, and finally a single message type to draft-nordmark-multi6-cb64-00.txt [Page 9] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 signal that state has been lost. The six message types are called: o Context request message; first message of the 3-way context establishment. Sent by the responder when a data packet arrives with no context state. An ULP packet can be piggybacked on this message. o Context response message; second message of the 3-way context establishment. Sent in response to a context request. An ULP packet can be piggybacked on this message. o Context confirm message; third message of the 3-way context establishment. Sent in response to a context response. An ULP packet can be piggybacked on this message. o Challenge request message; first message of the 2-way challenge. o Challenge response message; second message of the 2-way challenge. o Unknown context message; error which is sent when no state is found. Similar to MAST [MAST] the 3-way state creation exchange can be performed asynchronously with data packets flowing between the two hosts; until context state has been established at both ends the packets would flow without allowing router rewriting of locators and without the ability for the hosts to switch locators. Once the 3-way state creation exchange has completed there is host- pair context state at both hosts. At that point in time the hosts will use the challenge/response mechanism each time a new and unverified peer locator appears. 3.1. Host-Pair Context The host-pair context is established on the initiator of communication based on information learned from the DNS (either by starting with a FQDN or with an IP address -> FQDN lookup). The responder will establish some initial state using the context creation 3-way handshake. Both hosts later update the peer locators based on the source locator in received packets after having verified the new locator using a challenge exchange. The context state contains the following information: - the peer locator which the ULP uses as ID; AID(peer) draft-nordmark-multi6-cb64-00.txt [Page 10] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 - the local locator which the ULP uses as ID; AID(local) - the set of peer locators; Ls(peer). Each locator contains the same 64-bit ID. - for each peer locator, a bit whether it has been verified (by performing a challenge/response. TBD: do this even for the original locators that where learned from the DNS?) - the preferred peer locator - used as destination; Lp(peer) - the set of local locators; Ls(local). Each locator contains the same 64-bit ID. - the preferred local locator - used as source; Lp(local) - the flowid used to transmit packets; F(local) - the flowid to expect in receive packets; F(peer) - State about peer locators that are in the process of being verified. This state is accessed differently in the transmit and receive paths. In the transmit path when the ULP passes down a packet the key to the context state is the tuple ; this key must identify at most one state record. In the receive path it is the F(peer) plus the 64-bit peer and local IDs that are used to identify at most one state record. Thus the sender allocated flowid is part of the key for looking up the context state at the receiver. These uniqueness requirements imposed by those lookup keys uniquely identifying one state record means that one can not create multiple records (e.g. with different locator sets) that have the same AID pair, and the peer must pick a flowid so that host-pair contexts which have the same 64-bit ID pair but a different AID pair gets a different F(local). Note that the flowids could be selected to be finer grain than above; for instance having a different flowid for each connection. Doing so requires some efficient data structure organization at the receiver to map multiple F(peer) to the same context. draft-nordmark-multi6-cb64-00.txt [Page 11] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 3.2. Message Formats These message formats are largely the same as in [CB128] but the context request, response, and confirm are sent in the opposite direction. The base M6 header is an ICMPv6 header as follows: 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 | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ICMPv6 Fields: Type TBD [IANA] Code 8-bit field. The type of M6 message. The M6 header carries 6 different types of messages: o Context request message; first message of the 3-way context establishment. An ULP packet can be piggybacked on this message. o Context response message; second message of the 3-way context establishment. An ULP packet can be piggybacked on this message. o Context confirm message; third message of the 3-way context establishment. An ULP packet can be piggybacked on this message. o Challenge request message; first message of the 2-way challenge. o Challenge response message; second message of the 2-way challenge. o Unknown context message; error which is sent when no context state found. Checksum The ICMPv6 checksum. draft-nordmark-multi6-cb64-00.txt [Page 12] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 Future versions of this protocol may define message codes. Receivers MUST silently ignore? Reject? [TBD] any message code they do not recognize. This drafts doesn't contain actual message layout for code specific part. However, the content of these messages is specified below. The Context request message contains: - Sender Nonce - Sender AID - Receiver AID - Sender flowid (20 bits) The Context response message contains: - Receiver Nonce (copied from Sender Nonce in request) - Context state consisting of: the two AIDs, the two flowids, and the initial locators - A timestamp or nonce (for sender's benefit) - A hash over the context state and timestamp (to prevent modification) The Context confirm message contains: - The context state, timestamp/nonce, and hash copied from the context response. The Challenge request message contains: - Sender generated nonce/timestamp - The two AIDs - The 20-bit flowid from the received data message - The source locator from the received data message The Challenge response message contains: - The nonce/timestamp from the challenge request draft-nordmark-multi6-cb64-00.txt [Page 13] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 - The 20-bit flowid (from the challenge request) - The above locator - A hash value (H2) which proves that the sender knows the both flowids. This allows the receiver of the response to avoid verifying the PK signature generated by a host which can't be the original peer. - The public key of the sender. - The public key signature of all of the above fields. The Unknown context message contains: - The 20-bit flowid from the triggering packet. 4. PROTOCOL WALKTHROUGH 4.1. Initial Context Establishment Here is the sequence of events when A starts talking to B: 1. A looks up FQDN(B) in the DNS which returns Ls(B) plus "B is M6 capable". The host verifies that all the locators contain the same 64-bit ID. One locator is selected to be returned to the application: AID(B) = L1(B). The others are installed in the M6 layer on the host with AID(B) being the key to find that state. To make sure that the lookup from AID(B) returns a single state record one has to catch the case when an attempt is to create different context state for the same AID; different locators sets. To make sure that the lookup from AID(B) returns a single state record in the M6 layer there has to be a check if there is already a record for that 64-bit ID with a different Ls. One could envision sending a PK challenge to the locators to resolve such a conflict, or doing a reverse lookup of AID(B) to get and verify the FQDN. See section 14.1 for more discussion. draft-nordmark-multi6-cb64-00.txt [Page 14] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 2. The ULP creates "connection" state between AID(A)=L1(A) and AID(B) and sends the first packet. L1(A) was picked using regular source address selection mechanisms. 3. The M6 layer matches on AID(B) and finds the proto-context state (setup in step #1) with Ls(B). The existence of that state will make the M6 layer send a M6 packet. The M6 layer selects a flowid F(A) consistent with the uniqueness requirements in section 3.1 (which ensure that the receiver will map to the correct AID pair). 4. The packet (TCP SYN or whatever) is sent to peer with locators L1(A) to L1(B) i.e., the same as the AIDs. Since B doesn't have any context state yet, A will not set the "rewrite ok" bit in the header. 5. Host B receives packet and sees it is a "M6 packet". Passes the packet to the M6 shim layer. The M6 layer tries to match a context state using the flowid and the bottom 64 bits of the address fields. Since this is the first packet in the communication between A and B no such state is found. The M6 layer can't create state on the first packet, but since the rewrite bit is not set in the packet it can pass the packet unmodified to the ULP. The ULP sees a packet identified by AID(A), AID(B). The M6 layer initiates a state creation 3-way exchange by forming a context request message. The same technique as in [MIPv6] can be used to securely do this exchange without any local state; use a local key which is never shared with anybody and pass the context state, a timestamp, and the keyed hash of the state+timestamp in the context request packet. When the state, timestamp, and keyed hash value is returned in the context response message, the hash is used to verify that the state hasn't been modified. The 3-way exchange is done asynchronously with ULP packets, but it is possible (assuming the MTU allows) to piggyback ULP packets on this exchange. Should ULP packets be passed down to the M6 layer on B before the context response message has been received there will be no context state and no state installed as a result of a DNS lookup (unlike on A). This will indicate that the ULP message should be passed as-is (not as an M6 message) to the peer. Thus during the 3-way exchange packets can flow in both directions using the original locators=AIDs. (However, this has some interactions with the suggestions in section draft-nordmark-multi6-cb64-00.txt [Page 15] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 5.) 6. Host A receives the context request message. It verifies that the message is related to something it sent by looking at the locators (should match the AIDs) and the flowid it sent (which is in the state in the context request message). If a ULP packet was piggybacked A will pass that to the ULP. Then A sends a content response which has the same information as the context request plus a nonce/timestamp that A selected. 7. Host B receives the context response message. It verifies that the hash of the state is correct using its per-host key. At this point in time it knows that A is at least not just blasting out packets as a DoS - A is also responding to context request messages. Thus B goes ahead and allocates state at this point in time using the state that is in the context response message. The M6 layer selects a flowid F(B) consistent with the uniqueness requirements in section 3.1 (which ensure that the receiver will map to the correct AID pair). At this point in time B has enough information to handle M6 packets from A. It has also the state (F(B) mainly) necessary send data packets to A with "rewrite ok" set. Thus B sends a context confirm message to A which contains A's nonce/timestamp from the context response and F(B). If a ULP packet was piggybacked on the context response B will pass that to the ULP. Note that B doesn't perform a public key challenge/response for the original AIDs since ULPs and applications don't seem assume that level of strength in the peer address/identity. If there will be such applications the applications could perhaps trigger such additional verification. 8. Host A receives the context confirm message, verifies the nonce/timestamp, and records F(peer) from the packet. If a ULP packet was piggybacked on the context confirm A will pass that to the ULP. At this point in time A knows that B has context state, thus it can start sending packets with "rewrite ok" set. draft-nordmark-multi6-cb64-00.txt [Page 16] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 4.2. Locator Change This is the sequence of events when B receives a packet with a previously unused source locator for A, for instance L2(A). 1. Host B receives M6 packet with source L2(A) and destination L1(B). Looks up context state using the flowid and the bottom 64-bits of the locators. If this lookup succeeds then the locator is acceptable for incoming packets (even though it might not have been verified for use as return traffic) and the packet is rewritten to contain the AIDs from the context state and passed to the ULP. If the source locator is in Ls(peer) and already verified then the preferred return locator (Lp(peer)) is updated to use it for return packets. If the source locator is previously unknown then it is added to the context state as a Ls(peer) awaiting verification and a Challenge Request packet is generated. The challenge request includes a nonce generated by B, F(A) (that was received in the packet from the unknown locator), the identifiers and the previously unknown peer locator. The challenge response needs to complete before using the locator as a destination in order to prevent redirection attacks and 3rd party DoS attacks. 2. Host A receives the challenge request packet. Verifies that it has state for those AIDs with the F(A) it received on the request. It computes the hash H2 to show to B that it knows the flowids for both directions by computing H2 = SHA1(nonce from request, F(A), F(B), AID(A), AID(B)) It includes its public key (the one whose hash is ID(A)) and signs the whole challenge response using its private key. 3. Host B receives the challenge response packet. It finds the context state using F(A). It verifies the nonce against what it used for sending the challenge request. It verifies H2. (Only devices on the path between A and B during the context establishment knows both F(A) and F(B), thus this check limits DoS attacks based on forcing PK signature verification to attackers on the path.) Then it verifies that the hash of the public key equals ID(A), and finally the public key signature using that public key. draft-nordmark-multi6-cb64-00.txt [Page 17] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 4.3. Handling Locator Failures Should not all locators be working when the communication is initiated some extra complexity arises, because the ULP has already been told which AIDs to use. If the locators that where selected to be AIDs are not working it isn't possible to send a zero-overhead initial packet from A to B. Instead both the AIDs and the working locators need to be conveyed. This could be done by either reusing IP-in-IP encapsulating or defining another M6 message type which carries both. Details TBD. After context setup the sender can use retransmit hints from the ULP to get the M6 layer to try a different verified locator. If one outbound path from the site fails and the border routers rewrite source locators then the peer in another site will see packets with the working source locators. Once that has locator been verified, the return path will switch to use the working locator. As long as both ends are transmitting packets this will relatively quickly switch to working locators except when both hosts experience a failing locator at the same time. Without locator rewriting one would need to add some notification e.g., by defining a new bit in the router advertisement prefixes (IMHO this is semantically different than the preferred vs. deprecated stuff), but we also need some mechanism to carry this info from the border routers to the routers on each subnet. 4.4. Locator Set Changes Should the set of locators change after the context has been established the ability to learn and verify new peer locators will handle this fine. The DNS only needs to be updated with new locators in order to allow new communication to be established. When a host sees (based on router advertisements [DISCOVERY]) that one of its locators has become deprecated and it has additional locators that are still preferred, it is recommended that the host start using the preferred locator(s) with the contexts that have already been established. This ensures that should the deprecated locator become invalid the peers have already verified other locator(s) for the host. draft-nordmark-multi6-cb64-00.txt [Page 18] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 4.5. Preventing Premeditated Redirection Attacks The threats document [M6SEC] talks of premeditated redirection attacks that is where an attacker claims to be a host before the real host appears. While an attacker can only use a given AID if it is on the path (so that return packets arrive at the attacker), an attacker can use a 64-bit ID combined with any 64-bit subnet locator. This is of concern since on the receive side the context state is identified by the 64-bit ID pair. Thus this proposal is potentially subject to this threat because for performance reasons there is no public-key challenge when the context state is initially established. The following sequence shows how such a redirection attack is detected when X pretends to be ID(A): 1. Host X with creates locator L1(X) using its "own" subnet locator and ID(A) as the bottom 64 bits. X selects F(X) to communicate with B and sends a M6 data message to B. 2. The context request, response and confirm messages are exchanged between B and X resulting on B selecting F(B) for communicating with X (which B believes has identifier ID(A)). 3. X and B happily communicate without B performing any higher- level, such as IKE/IPsec, identity check on its peer. 4. Host A tries to communicate with B. It sends an M6 packet with ID(A) and F(A) to B. 5. Host B receives this M6 packet and discovers it already has context state for ID(A). B doesn't do anything different than if there was no context state - the difference in processing happens when the context confirm is received - except that any piggybacked ULP packet is not passed to the ULP. Thus, as in section 4.1, a flowid is selected and the context request is sent, which makes A send back a context response. 6. Host B receives the context response and verifies it the same way as in section 4.1. Then it looks if there is already a context for ID(A) and finds the context which contains F(X) and L1(X). The existence of this "old" context could be due to multiple reasons: draft-nordmark-multi6-cb64-00.txt [Page 19] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 - The peer lost state while B retained the context state. In this case one would expect that the old context has not been used to receive packets for some time. (Having a protocol constant denoting the minimum time after sending a packet that state can be lost and later recreated would be helpful here.) In this case it would also be common that the source address of the packet would fall in the locator set for the old context. But it isn't impossible that a peer state loss and using a different locator happens at the same time. - The old host was performing a premediated redirection attack. - The new host is attempting a redirection attack. In all cases the approach consists of finishing the state setup by sending a context confirm message to A, and then sending a challenge to both the "new" A and the "old" A. But depending on the time since packets where last received from the "old" A the order can be different. The first peer locator which responds with a valid challenge response will "win" and the other context state will be deleted. TBD: The above has DoS concerns in terms of verifying the challenge response. Having both ends remove the context state at about the same time would be beneficial since it would reduce the frequency of this happening in the absence of attacks, thus it would be more realistic to apply resource limits for this type of challenges. 5. HANDLING STATE LOSS The protocol needs to handle two forms of state loss: - a peer loosing all state, - the M6 layer garbage collecting state too early due to not being aware of what all ULPs do. The first case is the already existing case of a host crashing and "rebooting" and as a result loosing transport and application state. In this case there are some added complications from the draft-nordmark-multi6-cb64-00.txt [Page 20] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 M6 layer since a peer will continue to send packets assuming the context still exists and due to the loss of state on the receiver it isn't even able to pass the correct packet up to the ULP (e.g., to be able to get TCP to generate a reset packet) since it doesn't know what AIDs to use when replacing the locators. The second case is a bit more subtle. Ideally an implementation shouldn't discard the context state when there is some ULP that still depends on this state. While this might be possible for some implementations with a fixed set of applications, it doesn't appear to be possible for implementations which provide the socket API; there can be things like user-level "connections" on top of UDP as well as application layer "session" above TCP which retain the identifiers from some previous communication and expect to use those identifiers at a later date. But the M6 layer has no ability to be aware of this. Thus an implementation shouldn't discard context state when it knows it has ULP connection state (which can be checked in e.g., Unix for TCP), or when there is active communication (UDP packets being sent to AID(A) recently), but when there is an infrequently communicating user-level "connection" over UDP or "session" over TCP the context state might be garbage collected even though it shouldn't. For instance, if B crashes and rebooted and A retransmits a packet with flowid, L3(B), L2(A) then what is needed is a packet to L1(B) from L1(A) passed to the ULP so that the ULP can send an error (such as a TCP reset). But B has no matching state thus it needs to send an Unknown context error back. (Should the packet not have "rewrite ok" set host B can pass it to the ULP since it knows that such packets contain locators that are AIDs. But once the context has been established the peer is likely to send all packets with "rewrite ok" set.) If host B instead only lost (garbage collected too early) the M6 context state things are a bit more complicated for packets passed down from the ULP. Without without any context state the M6 layer on B can not determine whether packets to AID(A) coming from the ULP are destinated to a standard IPv6 host or a host which supports multihoming. B can determine this by performing some packet exchange with A to ask it whether it supports multihoming. If B is communicating with both standard IPv6 hosts and hosts which support multihoming then it has to avoid doing these peer queries for every packet sent to a standard IPv6 host. Implementation tricks (such as "has this socket ever used M6" flag at the socket layer, and "negative caching" of peers that do not draft-nordmark-multi6-cb64-00.txt [Page 21] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 support M6) can be useful to avoid performance overhead. Potentially this state recovery can result in A having two contexts for B; one with the old flowid and one with a new flowid. This must be avoided so that packets to B only use the flowid that is known to B. Specifying this is for further study. 6. ENCODING BITS IN THE IPv6 HEADER? The idea is to pick extra IP protocol values for common combinations, and have a designated protocol value to capture the uncommon IP protocols which might use M6. The uncommon IP protocol values would require an additional extension header when used over M6. We pick two unused ranges of IP protocol values with 8 numbers each (assuming we will not need more than 7 common transport protocols). The ranges start at P1 and P2, respectively: P1 TCP over M6 - rewrite ok P1+1 UDP over M6 - rewrite ok P1+2 SCTP over M6 - rewrite ok P1+3 RDDP over M6 - rewrite ok P1+4 ESP over M6 - rewrite ok (...) P1+7 escape - any protocol over M6 - rewrite ok In this case we spend another 8 bytes (minimum IPv6 extension header size due to alignment rule) to carry the actual IP protocol. This causes some mtu concerns for those protocols, but they aren't very likely to be used with M6? P2 TCP over M6 - no rewrite P2+1 UDP over M6 - no rewrite P2+2 SCTP over M6 - no rewrite P2+3 RDDP over M6 - no rewrite P2+4 ESP over M6 - no rewrite (...) P2+7 escape - any protocol over M6 - no rewrite In this case we spend another 8 bytes (minimum IPv6 extension header size due to alignment rule) to carry the actual IP protocol. This causes some mtu concerns for those protocols, but they aren't very likely to be used with M6? Thus a router would check if the protocol is in the P1 range and draft-nordmark-multi6-cb64-00.txt [Page 22] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 if so, it can rewrite the locator(s). A host would check a received packet against both P1 and P2 ranges and if so pass it to the M6 shim layer. Some possible alternatives to the above encoding is to: - use some combination of the universal/local and group bit in the interface id of the source address to indicate "rewrite ok". - steal the ECN bits from the traffic class before ECN becomes a proposed standard? Don't think this will be popular! - always have a shim header - adds 8 bytes overhead per packet. 7. COMPATIBILITY WITH STANDARD IPv6 A host can easily implement M6 in a way that interoperates with current IPv6 as follows. When the DNS lookup routines do not find an M6 record for the peer they will return the AAAA resource record set to the application; those would be the IPv6 addresses. When the ULP passes down these addresses the M6 layer will not have any state generated by the DNS lookup code, thus no M6 processing will take place on the sender. (Note that this relates to the M6 layer state recovery in section 5.) The receive side handles both standard IPv6 and M6 since it demultiplexing on whether a packet is an M6 packet. draft-nordmark-multi6-cb64-00.txt [Page 23] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 8. APPLICATION USAGE OF IDENTIFIERS The upper level protocols will operate on AIDs which are mere locators. Thus as long as a site hasn't renumbered the AID can be used to either send packets to the host, or (e.g. if that locator isn't working), it is possible for an application to do a reverse lookup plus forward lookup of the AID to get the set of locators for the peer. Once a site has been renumbered the AIDs which contain the old prefix will no longer be useful. Hence applications must try to honor the DNS TTL somehow. Applications which use to map the peer's IP address to a domain name today perform a reverse lookup in the DNS (e.g., using the getnameinfo() API). This proposal doesn't add or subtract to the benefits of performing such reverse lookups. 9. CHECKSUM ISSUES The IPv6 header does not have a checksum field; the IPv6 address fields are assumed to be protected by the ULP pseudo-header checksum. The general approach of an M6 shim which replaces locators with identifiers (where only the identifiers are covered by the ULP checksum) raises the potential issue of robustly handling bit errors in the address fields. With the definition of the M6 shim there can be undetectable bit errors in the flowid field or the nexthdr field which might adversely affect the operation of the protocol. And since the AIDs are what's covered by the ULP's pseudo-header checksum the locators in the address fields are without checksum protection. An undetected bit error in the source locator would look like an unverified source locator to the receiver. Thus the packet would (after replacing locators with identifiers based on the context) be passed to the ULP and a challenge response exchange be triggered. In the case of a bit error in the locator this challenge isn't likely to receive a response; and if there is a response by someone it wouldn't be from the actual peer thus the verification would fail. Thus such an undetected bit error is harmless. Except for the obscure case when Ls(A) contains multiple verified draft-nordmark-multi6-cb64-00.txt [Page 24] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 locators, one or more of those are not working, and the bit error causes L1(A) to be replaced by L2(A). That would make the return traffic go to L2(A), but that might be a non-functioning locator. In this case the mistake will be corrected when a subsequent packet is received from A. An undetected bit error in the destination address field is also harmless; it might cause misdelivery of the packet to a host which has no context but the reception of the resulting Unknown context error message will show that it arrives from the incorrect locator thus it will be ignored. An undetected bit error in the IPv6 next header field can potentially make a M6 packet appear as a non-M6 packet and vice versa. This isn't any different than undetected bit errors in IPv6 next header field without multihoming support. An undetected bit error in the flowid in a data message could have two possible effects: not finding any context state, or finding the incorrect context state. In the first case the Unknown context error message would be dropped by the peer since the flowid included in the error message doesn't match the flowid that was originally sent. In the second case this will result in a packet with incorrect identifiers being delivered to the ULP which most like will drop it due to ULP checksums not matching. 10. IMPLICATIONS FOR PACKET FILTERING Ingress filtering should be replaced by locator rewrite when the "rewrite ok" bit is set. Locator rewriting (when the bit is set) can be applied at places where ingress filtering isn't currently performed (e.g., due to multihoming issues). Firewall filtering potentially require modifications to be aware of M6. All the packets contain locator thus a firewall would need to be aware of the context state to let the correct packets through. Such firewalls could optionally perform their own verification by issuing challenge request messages (the protocol doesn't explicitly allow for this; they would have to pretend being the actual endpoint sending the challenge). draft-nordmark-multi6-cb64-00.txt [Page 25] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 11. IPSEC INTERACTIONS As specified all of ESP, AH, and key management is layered above the M6 layer. Thus they benefit from the stable identifiers provided above the M6 layer. This means the IPsec security associations are unaffected by switching locators. The alternative would be to layer M6 above IPsec, but that doesn't seem to provide any benefits. Since we want to allow routers performing locator rewriting it wouldn't be possible to take advantage of for instance AH to protect the integrity of the IP headers. 12. SECURITY CONSIDERATIONS This analysis is far from complete. Early analysis indicates this addresses the issues in [M6SEC]. Just as in today's Internet hosts on the path can inject bogus packets; in this proposal they need to extract the flowids from the packets in order to do this which wouldn't be hard. Packet injection from off-path places becomes harder since it requires guessing the 20 bit flowid together with the 64-bit ID pair. Hosts on the path can also launch PK signature verification DoS attacks against either end since they can observe the flowids from the establishment and therefor compute the H2 hash in the challenge response packet. This would force the endpoint to run the signature verification algorithm which is expensive. If we don't expect the locator sets to be very dynamic one could restrict the rate at which such verification takes place, at least after the first few locators have been verified for a peer. The initial setup of a host-pair context does not perform any verification using public key crypto, but this does not seem to make the result less secure than today's Internet. Applications which do not perform access control based on it's notion of the peer wouldn't care about the strength of the peer's identifier. And applications which perform strict access control hopefully do this using strong crypto (IPsec, TLS, etc) today and would continue to do so. That leaves applications which perform the questionable practise of merely verifying the forward plus reverse lookups in the DNS and then using the IP address (or resulting draft-nordmark-multi6-cb64-00.txt [Page 26] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 FQDN) for access control discussions. As discussed in section 6 the application's lookup of locator->FQDN->ID and verifying that the identifier matches provides about the same strength. [TBD are we really sure?] The CBIDs are only statistically unique. But 64 bit identifiers seems large enough to make collisions unlikely enough to keep the protocol simple. (If not one could envision complications like making the protocol capable of detecting collisions by storing the public key in the context state and seeing if a host claims to use the same ID but has a different public key.) While at about 4 Billion hosts in the Internet there is approximately a 50% probability that there exists 2 hosts with the same 64-bit identifier this has no effect on the protocol per see. It is not until a single host has that order of magnitude of context state records that it could get confused due to collisions. 13. DESIGN ALTERNATIVES Use an actual extension header for M6 and use a context tag in that header instead of using the flowid. This would make the packets 8 bytes larger since the minimum extension header size is 8 bytes due to the alignment rules for extension headers in IPv6. Make the flowid allocation be done by the receiver of the flowid (as is done for the context tags in [CB128]) instead of by the sender as in this proposal. It is possible to use these mechanisms together with the structured 64/80 bit identifiers suggested in [CRYPTOID]. draft-nordmark-multi6-cb64-00.txt [Page 27] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 14. OPEN ISSUES Some protocol complexity is added by not performing a mutual public-key challenge immediately when a context is created. At the expensive of a performance hit one could simplify the protocol to always to these challenges. Is it possible to facilitate transition to M6 using some "M6 proxy" at site boundaries until all important hosts in a site have been upgraded to support M6? Would would be the properties of such a proxy? Would it place any additional requirements on the protocol itself? One of the issues with FQDNs mapping to AAAA records is that in some cases multiple AAAA records mean a multihomed host and in other cases it means multiple hosts providing the same service. If we need to introduce a new RR type for M6, would it be useful to try to make this host/service distinction more clear at the same time? An example solution would be that the M6 record would by its existence indicate the M6 capability, and its RDATA would contain a list of host names which would be used to resolve the AAAA records for each host implementing the service. Another possibility related to this proposal would be to use the fact that a given host would have the same 64-bit ID in its locators, thus after retrieving the locators for a FQDN it is possible to group those into "host sets" by comparing the bottom 64-bits (assuming the node is M6 capable). This grouping could be performed in routines like getaddrinfo() transparent to the applications. Would destination locator rewriting be a useful way for the routing system to pass some information to the host? Or is source locator rewriting sufficient? The mechanisms allow adding locators to a locator set but there is currently no mechanism for removing a locator (e.g., when a host renumbers). Does it make sense to add such a mechanism? The responder only discovers the peer's locators once they are used as sources in packets. Would it make sense to allow the initiator to pass a list of its locators to the responder? (They would still need to be verified before use to prevent 3rd party DoS attacks [M6SEC]). 14.1. Initiator Confusion vs. "Virtual Hosting" When A wants to communicate with host B and host X at the same draft-nordmark-multi6-cb64-00.txt [Page 28] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 time there can be some confusion since the DNS could return the same identifier for B and X while returning different locator sets. For example, The lookup of FQDN(B) returns Ls(B) where each locator has a 64- bit ID(B) The lookup of FQDN(X) returns Ls(X) where each locator has a 64- bit ID(B) The result is that connections that could be intended to go to B and to X would both end up with different AIDs ID at the ULP, but the multihoming shim layer would have two separate locator sets associated with ID(B). Thus at a minimum when the second of the two communications starts there has to be some way to resolve this conflict. If multiple FQDNs map to the same host, which is common in virtual hosting using IPv4 today, and the locator set is being modified for that host then this could be quite normal; looking up www.foo.com would provide the ID of the peer and a perhaps staler set of locators for the ID than looking up www.bar.com. Is it reasonable to assume when there is some overlap between Ls(B) and Ls(X) above that this is a normal condition? Should one form the intersection of Ls(B) and Ls(X) and use that for the existing context state? Or at least purge unverified peer locators, those from which the host hasn't seen a challenge response, that are not in the intersection from the locator set Section 4.1 suggests using a challenge request/response exchange when the second lookup takes place. Should the challenge be performed with the newer or older locator sets? What are the DoS issues in performing such a challenge? 14.2. Using larger context tags The 128-bit identifier proposal [CB128] uses a larger context tag as part of the context setup than is used in the data packets to identify the context. This larger context tag is useful to prevent off-path attackers from launching "verify signature" DoS attacks; a more inexpensive check can be performed to verify that the peer knows the full context tag. That approach could easily be adopted to this proposal as well with the flowid carrying the low-order 20 bits of e.g. a 64 bit draft-nordmark-multi6-cb64-00.txt [Page 29] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 context tag. Would this be beneficial? 15. COMPARISON WITH NOID AND CB128 This approach represents at some level a middle ground between [NOID] and [CB128] in that: - Not using DNS for verification as in [NOID] but using the CBID property plus public key crypto as in [CB128]. Once DNSsec is used there are less public key operations to verify using CBID than for all the delegations in the ip6.arpa tree for a previously unknown peer locator. - Like [NOID] there are no extra bytes added to the packets. - Like [NOID] the AIDs are useful in referrals. - Uses similar messages as in [CB128] but the context request/response/confirm is sent in the reverse direction with the context request being sent by the responder (in [CB128] the context request is sent by the initiator). 16. ACKNOWLEDGEMENTS This document is the result of discussions in a MULTI6 design team but is not the "product" of that design team. The scribe wishes to acknowledge the contributions of (in alphabetical order): Iljitsch van Beijnum, Brian Carpenter, Tony Li, Mike O'Dell, and Pekka Savola. The idea to allow locator rewriting by routers was first presented by Mike O'Dell [ODELL96]. The techniques for avoiding state DoS attacks on the first packet are patterned after [MIPv6]. draft-nordmark-multi6-cb64-00.txt [Page 30] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 17. IPR CONSIDERATIONS When IP addresses that have a hash of a public key in the low order 64 bits were discussed in the Mobile IP WG and in the SEND WG two vendors asserted IPR. It is not known to the author whether such IPR claims would apply to this specification as well. 18. REFERENCES 18.1. Normative References [M6SEC] Nordmark, E., and T. Li, "Threats relating to IPv6 multihoming solutions", draft-nordmark-multi6-threats- 00.txt, October 2003. [CBID] G. Montenegro and C. Castelluccia, "Statistically Unique and Cryptographically Verifiable Identifiers and Addresses", ISOC NDSS02, San Diego, February 2002. [ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6 Addressing Architecture", RFC 3513, April 2003. [IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2461. 18.2. Informative References [NSRG] Lear, E., and R. Droms, "What's In A Name: Thoughts from the NSRG", draft-irtf-nsrg-report-09.txt (work in progress), March 2003. [MIPv6] Johnson, D., C. Perkins, and J. Arkko, "Mobility Support in IPv6", draft-ietf-mobileip-ipv6-24.txt (work in progress), June 2003. [AURA02] Aura, T. and J. Arkko, "MIPv6 BU Attacks and Defenses", draft-aura-mipv6-bu-attacks-01 (work in progress), March 2002. [NIKANDER03] Nikander, P., T. Aura, J. Arkko, G. Montenegro, and E. Nordmark, "Mobile IP version 6 Route Optimization Security Design Background", draft-nikander-mobileip- draft-nordmark-multi6-cb64-00.txt [Page 31] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 v6-ro-sec-01 (work in progress), June 2003. [ODELL96] O'Dell M., "8+8 - An Alternate Addressing Architecture for IPv6", draft-odell-8+8-00.txt, October 1996, [MAST] D. Crocker, "MULTIPLE ADDRESS SERVICE FOR TRANSPORT (MAST): AN EXTENDED PROPOSAL", draft-crocker-mast- protocol-01.txt, October, 2003. [CRYPTOID] I. van Beijnum, "CRYPTO-BASED HOST IDENTIFIERS", draft-van-beijnum-multi6-cryptoid-00.txt, (unpublished) [CB128] E. Nordmark, "Strong Identity Multihoming using 128 bit Identifiers (SIM/CBID128)", draft-nordmark-multi6-sim- 00.txt, October 2003. [NOID] E. Nordmark, "Multihoming without IP Identifiers", draft-nordmark-multi6-noid-00.txt, October 2003. [DISCOVERY] T. Narten, E. Nordmark, and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [IPv6-SA] R. Atkinson. "Security Architecture for the Internet Protocol". RFC 2401, November 1998. [IPv6-AUTH] R. Atkinson. "IP Authentication Header", RFC 2402, November 1998. [IPv6-ESP] R. Atkinson. "IP Encapsulating Security Payload (ESP)", RFC 2406, November 1998. AUTHORS' ADDRESSES Erik Nordmark Sun Microsystems, Inc. 17 Network Circle Mountain View, CA USA phone: +1 650 786 2921 fax: +1 650 786 5896 email: erik.nordmark@sun.com Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. draft-nordmark-multi6-cb64-00.txt [Page 32] INTERNET-DRAFT Multihoming using 64-bit Crypto-based IDs Oct 27, 2003 This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assignees. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. draft-nordmark-multi6-cb64-00.txt [Page 33]