IPv6 Working Group Greg Daley INTERNET-DRAFT Nick "Sharkey" Moore Expires: December 2004 Monash University CTIE Erik Nordmark Sun Microsystems 9 June 2004 Tentative Source Link-Layer Address Options for IPv6 Neighbour Discovery Status of this Memo By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed and any of which I become aware will be disclosed, in accordance with RFC 3668 (BCP 79). By submitting this Internet-Draft, I accept the provisions of Section 3 of RFC 3667 (BCP 78). 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 cite them other than as "work in progress". The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/lid-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This document is an individual submission to the IETF. Comments should be directed to the authors. Definitions of requirements keywords are in accordance with the IETF Best Current Practice - RFC2119 [KEYW-RFC] Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 1] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 Abstract The proposed IPv6 Duplicate Address Detection (DAD) Optimization "Optimistic DAD" defines a set of recoverable procedures which allow a node to make use of an address before DAD completes. Essentially, Optimistic DAD forbids usage of certain Neighbour Discovery options which could pollute active neighbour cache entries, while an address is tentative. This document defines a new option and procedures to replace cache polluting options, in a way which is useful to tentative nodes. These procedures are designed to be to backward compatible with existing devices which support IPv6 Neighbour Discovery. 1.0 Introduction Source Link-Layer Address Options (SLLAOs) are sent in Neighbour discovery messages in order to notify neighbours of a mapping between a specific IPv6 Network layer address and a link-layer (or MAC) address. Upon reception of a neighbour discovery message containing such an option, nodes update their neighbour cache entries with the IP to link-layer address mapping in accordance with procedures defined in IPv6 Neighbour Discovery [RFC-2461]. Optimistic DAD [OPTIDAD] prevents usage of these options in Router and Neighbour Solicitation messages from a tentative address (while Duplicate Address Detection is occurring). This is because receiving a Neighbour Solicitation (NS) or Router Solicitation (RS) containing an SLLAO would otherwise overwrite an existing cache entry, even if the cache entry contained the legitimate address owner, and the solicitor was a duplicate address. Neighbour Advertisement (NA) messages don't have such an issue, since the Advertisement message contains a flag which explicitly disallows overriding of existing cache entries, by the target link-layer address option carried within. The effect of preventing SLLAOs for tentative addresses is that communications with these addresses are sub-optimal for the tentative period. Sending solicitations without these options causes an additional round-trip for neighbour discovery if the advertiser does not have an existing neighbour cache entry for the solicitor. Tentative Source Link-Layer Address Options are designed to replace the existing Source Link-Layer Address Options available in IPv6 Neighbour Discovery, when a device is performing Optimistic DAD, or a device is sending Router Solicitations from an unspecified source address. Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 2] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 1.1 Tentative Source Link-Layer Address Option Format 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 5 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Length | Link-Layer Address ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Fields: Type TBD Length The length of the option (including the type and length fields) in units of 8 octets [RFC-2461]. Link-Layer Address The variable length link-layer address. Description The Tentative Source Link-Layer Address option contains the link-layer address of the sender of the packet. It is used in the Neighbour Solicitation and Router Solicitation packets. 1.2 Tentative Source Link-Layer Address Option Semantics The Tentative Source Link-Layer Address option (TSLLAO) functions in the same role as the Source Link-Layer Address option defined for [RFC-2461], but it MUST NOT override an existing neighbour cache entry. The differing neighbour cache entry MUST NOT be affected by the reception of the Tentative Source Link-Layer Address option. This ensures that tentative addresses are unable to modify legitimate neighbour cache entries. In the case where an entry is unable to be added to the neighbour cache, a node MAY send responses direct to the link-layer address specified in the TSLLAO. For these messages, no Neighbour Cache entry may be created, although response messages may be directed to a particular unicast address. These procedures are discussed further in section 3.3. Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 3] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 2.0 Sending Solicitations containing TSLLAO Solicitations sent containing Tentative Source Link-Layer Address Options need to conform to IPv6 Neighbour Discovery procedures, since they are send without SLLAOs, and legacy nodes will ignore the new option. In a case where it is safe to send a Source Link-Layer Address Option, a host SHOULD NOT send a TSLLAO, since the message may be mis-interpreted by legacy nodes. Importantly, a node MUST NOT send a TSLLAO in the same message where a Source Link-Layer Address Option is sent. A node MUST NOT send TSLLAO options except where [RFC-2461] allows ommision of an SLLAO i.e. in the following cases: 2.1 Sending Neighbour Solicitations with TSLLAO Except for packets sent from an unspecified source address, Source Link-Layer Address options are mandatory in Neighbour Solicitation messages destined to multicast addresses. Neighbour Solicitations for the unspecified source address are typically used for Duplicate Address Detection. Any receiver of an unspecified source addressed Neighbour Solicitation with TSLLAO will believe the packet to be a DAD attempt if it is unable to interpret the TSLLAO option. Since many nodes will halt address configuration if they receive a DAD NS while an address is tentative, Tentative Source Link-Layer Address options MUST NOT be sent in Neighbour Solicitation messages from the unspecified source address. On the other hand, Neighbour Solicitation packets with unicast source and destination addresses are not explicitly required to include SLLAOs. Such packets may instead use a Tentative Source Link-Layer Address Option, which is safe when undergoing Duplicate Address Detection[OPTIDAD]. Since delivery of a packet to a unicast destination requires prior knowledge of the destination's hardware address, unicast Neighbour Solicitation packets may only be sent to destinations for which a neighbour cache entry already exists. For example, if checking bidirectional reachability to a router, it may be possible to send a Neighbour Solicitation with TSLLAO to the router's advertised address. Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 4] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 As discussed in [RFC-2461], the peer device may not have a cache entry even if the soliciting host does, in which case reception of TSLLAO may create a neighbour cache entry, without the need for neighbour discovering the original solicitor. If the device is a legacy [RFC-2461] device, which doesn't recognize TSLLAO options, it will perform a Neighbour Solicitation back to the tentative node. Hosts MUST NOT send Neighbour Solicitations with specified source addresses and TSLLAO to nodes for which there is no pre-existing neighbour cache entry, or state is INCOMPLETE, unless these nodes are known to support TSLLAOs. This is because such Neighbour Solicitations violate IPv6 Neighbour Discovery specifications since they contain no SLLAO, and may cause confusion or harm to nodes which receive them. 2.2 Sending Router Solicitations with TSLLAO Router Solicitations are always able to be sent without Source Link- Layer Address options. Some routers may choose to send a multicast response to devices which send Router Solicitations without SLLAOs when they do not have an existing neighbour cache entry. If a router does not understand Tentative Source Link-Layer Address Options, it MAY send a multicast solicitation in preference to sending Neighbour Solicitation packets to learn unicast address's link-layer address. Any router solicitation, including those from the unspecified address, MAY be sent with a TSLLAO. Responses from routers depend on existing neighbour cache state, and their ability to send packets to identified MAC addresses without using the neighbour cache. Such issues are discussed in sections 3.4 and 3.5. 3.0 Receiving Tentative Source Link-Layer Address Options In the case that a node receives a solicitation without a Link-Layer identifier it will determine if a responding Advertisement will be sent to a unicast or multicast address. If the advertisement is to be sent to the solicitor's unicast address, the node will consult its existing neighbour cache for the solicitor's information, and if not present, will undertake neighbour discovery. Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 5] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 Receiving a Tentative Source Link-Layer Address Option, avoids this neighbour discovery step, by allowing the host to create or update a matching entry, setting it to STALE state if it didn't previously exist. Additionally, TSLLAO messages may be used to direct advertisements to particular link-layer destinations without updating neighbour cache entries. This is described in section 3.4. 3.1 Handling messages with Tentative Source Link-Layer Address Options Use of Tentative Source Link-Layer Address Options is only defined for Neighbour and Router Solicitation messages. In any other received message, a Tentative Source Link-Layer Address Option MUST be treated as if it is an unknown option, and processed appropriately. It is REQUIRED that the same validation algorithms for Neighbour and Router Solicitations received with TSLLAO as in the IPv6 Neighbour Discovery specification [RFC-2461], are used. In the case that a solicitation containing a TSLLAO is received, this does not mean that the solicitor needs to be treated differently, except in the updating of the cache entry and processing of the option. Particularly, there is no reason to believe that the host will remain tentative after receiving a responding advertisement. As defined in Section 1.2, Tentative Source Link-Layer Address Options do not overwrite existing neighbour cache entries where the link-layer addresses differ. If a solicitation from a unicast source address is received where no conflict occurs between the TSLLAO and an existing neighbour cache entry, the option MUST be treated as if it were an SLLAO after message validation, and processed accordingly. In the case that a cache entry is unable to be created or updated, the receiving node MAY send a direct advertisement to the soliciting host by responding with an appropriate advertisement, where the link- layer address contained in the TSLLAO is copied into the destination address of the link-layer frame. This is described further in sections 3.4 and 3.5. 3.2 Receiving Neighbour Solicitations containing TSLLAO The TSLLAO option is only allowed in Neighbour Solicitations with specified source addresses for which SLLAO is not required. A Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 6] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 Neighbour Solicitation message received with TSLLAO and an unspecified source address MUST be silently discarded. Upon reception of a Tentative Source Link-Layer Address Option in a Neighbour Solicitation for which the receiver has the Target Address configured, a node checks to see if there is a neighbour cache entry with conflicting link-layer address. If no such entry exists, the neighbour cache of the receiver SHOULD be updated, as if the Tentative Source Link-Layer Address Option was a SLLAO. Sending of the solicited Neighbour Advertisement then proceeds normally, as defined in section 7.2.4 of [RFC-2461]. If there is a conflicting neighbour cache entry, a node processes the solicitation in a fashion defined in in sections 3.4 and 3.5. 3.3 Receiving a Router Solicitation containing TSLLAO In IPv6 Neighbour Discovery [RFC-2461], responses to router solicitations are either sent to the all-nodes multicast address, or may be sent to the solicitation's source address if it is a unicast address. Including a TSLLAO in the solicitation allows a router to choose to send a packet directly to the link-layer address even in situations where this would not normally be possible. For Router Solicitations with unicast source addresses, neighbour caches SHOULD be updated with the link-layer address from a TSLLAO if there is no conflicting neighbour cache entry. In this case, Router Advertisement continues as in section 6.2.6 of [RFC-2461]. For received solicitations with a conflicting neighbour cache entry or those containing a TSLLAO with an unspecified source address, responses can be generated in accordance with sections 3.4 and 3.5 below. 3.4 Sending Directed Advertisements without the Neighbour Cache. In the case where a received solicitation has a link-layer address specified in a TSLLAO which conflicts with an existing with a neighbour cache entry, modification of the neighbour cache MUST NOT occur. Router Solicitations with the unspecified source address MUST NOT create neighbour cache entires. Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 7] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 In both situations, it may be valuable to send a direct message to the soliciting nodes. In these cases a node MAY generate a responding advertisement according to the rules in [RFC-2461]. The responding packets MAY then have the unicast link-layer address from the TSLLAO inserted into the destination address of the link- layer frame used to transport this advertisement, without consulting the neighbour or destination caches for this entry. Such packets SHOULD scheduled as if they were unicast advertisements as specified in [RFC-2461]. 3.5 Alternatives to Sending Directed Advertisements. Some implementations will be unable to generate directed advertisements by copying the tentative source link-layer address into a packet. Also, some nodes will be unable to send unicast packets without consulting their neighbour caches. Alternative mechanisms for such nodes SHOULD perform at least as well as implementations where TSLLAO is not understood. For such implementations as these, it is not possible to send a response for Neighbour Solicitation messages without modifying the neighbour cache. Such nodes MAY send a responding NA message as if it did not understand the TSLLAO message. This will deliver the NA to the soliciting host if it has both the tentative link-layer address and the entry in the neighbour cache configured on the same link. Otherwise, the message will be sent to the originator of the neighbour cache entry, and the solicitor will receive no response. For Router Solicitations where no neighbour cache entry is able to be created, a multicast response SHOULD be sent in accordance with [RFC-2461]. This includes those solicitations sent from unicast sources, for which a conflicting neighbour cache entry was found. 4.0 IANA Considerations For standardization, it would be required that the IANA provide allocation of the Tentative Source Link-Layer Address Option (Section 1.1) from the IPv6 Neighbour Discovery options for IPv6. Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 8] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 Potential details of the allocation process for these options is detailed in the expired draft [IPv6-ALLOC]. 5.0 Security Considerations The use of the TSLLAO in Neighbour and Router Solicitation messages acts in a similar manner to SLLAO, updating neighbour cache entries, in a way which causes packet transmission. Particular care should be taken that transmission of messages complies with existing IPv6 Neighbour Discovery Procedures, so that unmodified hosts do not receive invalid messages. An attacker may cause messages may be sent to another node by an advertising node (a reflector), without creating any ongoing state on the reflector. This is attack requires one solicitation for each advertisement and the advertisement has to go to a unicast MAC destination. That said, the size of the advertisement may be significantly larger than the solicitation, or the attacker and reflector may be on a medium with greater available bandwidth than the victim. For link-layers where it isn't possible to spoof the link-layer source address this allows a slightly increased risk of reflection attacks from nodes which are on-link. Additionally, since a SEND host must always advertise using SEND options and signatures, a non-SEND attacker may cause excess computation on both a victim node and a router by causing SEND advertisement messages to be transmitted to a particular MAC address and the all-nodes multicast. [SEND] specifies guidelines to hosts receiving unsolicited advertisements in order to mitigate such attacks. While this is the same effect as experienced when accepting SLLAO from non-SEND nodes, the lack of created neighbour cache entries on the advertiser may make such attacks more difficult to trace. Modification of Neighbour Discovery messages on the network is possible, unless SEND is used. [SEND] provides a protocol specification in which soliciting nodes sign ND messages with a private key and use addresses generated from this key. Even if SEND is used, the lifetime of a neighbour cache entry may be extended by continually replaying a solicitation message to a particular router or hosts. Since this may be achieved for any Neighbour or Router Solicitation message, corresponding Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 9] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 advertisements to the original transmitters of these solicitation messages may occur. SEND defines use of Timestamp values to protect a device from attack through replay of previously sent messages. Although this applies to Neighbour and Router Solicitation messages, granularity of the timestamp allows the messages to be used for up to two hours in extreme cases [SEND]. All Router and Neighbour Solicitations using SEND contain a Nonce option, containing a random identifier octet string. Since SEND messages are digitally signed, and may not be easily modified, replay attacks will contain the same Nonce option, as was used in the original solicitation. While the Nonce Option included in a transmission to another node may not vary within one short solicitation period (the host may itself replay solicitations in the case of packet loss), the presence of the timestamp option ensures that for later solicitations, a different Timestamp and Nonce will be used. Therefore, a receiver seeing a solicitation with the same Timestamp and Nonce (and signature) for more than either of MAX_RTR_SOLICITATIONS (for router solicitations), MAX_UNICAST_SOLICIT or MAX_MULTICAST_SOLICIT (for Neighbour Solicitations), SHOULD ignore further solicitations with this (Nonce,Timestamp,Source) triple, ensuring that no modification is made to neighbour cache entries. This applies to any solicitation packet capable of carrying a SEND payload, whether they use TSLLAO or SLLAO. Stations noticing such an attack SHOULD notify their administrator of the attempt at Denial-of-service. Normative References [KEYW-RFC] S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. Request for Comments (Best Current Practice) 2119 (BCP 14), Internet Engineering Task Force, March 1997 [OPTIDAD] N. Moore. Optimistic Duplicate Address Detection. Internet Draft (work in progress), March 2004. [RFC-2461] T. Narten, E.Nordmark, W. Simpson. Neighbour Discovery for IP Version 6 (IPv6). Request for Comments (Draft Standard) 2461, Internet Engineering Task Force, December 1998. [SEND] J. Arkko (Editor) et al. SEcure Neighbour Discovery (SEND). Internet Draft (work in progress), April 2004. Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 10] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 Non-Normative References [IPv6-ALLOC] T. Narten. "IANA Allocation Guidelines for Values in IPv6 and Related Headers", Internet Draft (work in progress), October 2002. www.watersprings.org/pub/id/draft-narten- ipv6-iana-considerations-00.txt Acknowledgments Erik Nordmark coined a proposal for TSLLAO during a conversation with JinHyeock Choi and Greg Daley. Authors' Addresses Greg Daley greg.daley@eng.monash.edu.au Nick "Sharkey" Moore nick.moore@eng.monash.edu.au Centre for Telecommunications and Information Engineering Department of Electrical and Computer Systems Engineering Monash University Clayton 3800 Victoria Australia Erik Nordmark erik.nordmark@sun.com Sun Microsystems, Inc. 17 Network Circle Mountain View, CA USA phone: +1 650 786 2921 fax: +1 650 786 5896 Intellectual Property Rights By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 11] INTERNET-DRAFT Tentative Source Link-Layer Address Options June 2004 Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. This document expires in December 2004. Daley et al. draft-daley-ipv6-tsllao-00.txt [Page 12]