DHC Working Group Bernard Aboba INTERNET-DRAFT Microsoft Corporation Category: Proposed Standard 25 March 2005 Detection of Network Attachment (DNA) in IPv4 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. 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 October 22, 2005. Copyright Notice Copyright (C) The Internet Society (2005). All Rights Reserved. Abstract The time required to detect movement (or lack of movement) between subnets, and to obtain (or continue to use) a valid IPv4 configuration may be significant as a fraction of the total delay in moving between points of attachment. This document specifies a procedure for optimizing the detection of network attachment and IP configuration in order to decrease the delay in moving between points of attachment. Aboba Proposed Standard [Page 1] INTERNET-DRAFT DNAv4 25 March 2005 Table of Contents 1. Introduction.............................................. 3 1.1 Requirements .................................... 3 1.2 Terminology ..................................... 3 2. Overview ................................................. 4 2.1 Most Likely Point of Attachment ................. 5 2.2 Reachability Test ............................... 6 2.3 IPv4 Address Acquisition ........................ 9 2.4 IPv4 Link-Local Addresses ....................... 9 3. Constants ................................................ 11 4. IANA Considerations ...................................... 11 5. Security Considerations .................................. 11 6. References ............................................... 11 6.1 Normative references ............................ 11 6.2 Informative references .......................... 12 Acknowledgments .............................................. 13 Authors' Addresses ........................................... 13 Appendix A - Link Layer Hints ................................ 14 A.1 Introduction .................................... 14 A.2 Hints ........................................... 15 Intellectual Property Statement .............................. 16 Disclaimer of Validity ....................................... 16 Copyright Statement .......................................... 17 Aboba Proposed Standard [Page 2] INTERNET-DRAFT DNAv4 25 March 2005 1. Introduction The time required to detect movement (or lack of movement) between subnets, and to obtain (or continue to use) a valid IPv4 address may be significant as a fraction of the total delay in moving between points of attachment. As a result, optimizing detection of network attachment is important for mobile hosts. This document synthesizes experience in the deployment of hosts supporting ARP [RFC826], DHCP [RFC2131], and IPv4 Link-Local addresses [IPv4LL], specifying a procedure for optimizing detection of network attachment and IPv4 configuration, in order to minimize the latency in moving between points of attachment. Since this procedure is dependent on the ARP protocol, it is not suitable for use on media that do not support ARP [RFC826]. 1.1. Requirements In this document, several words are used to signify the requirements of the specification. 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]. 1.2. Terminology This document uses the following terms: ar$sha ARP packet field: Source Hardware Address [RFC826]. The hardware (MAC) address of the originator of an ARP packet. ar$spa ARP packet field: Source Protocol Address [RFC826]. For IP Address Resolution this is the IPv4 address of the sender of the ARP packet. If the sender address is unknown, this is set to 0.0.0.0. ar$tha ARP packet field: Target Hardware Address [RFC826]. The hardware (MAC) address of the target of an ARP packet, or the broadcast address if the target hardware address is unknown. ar$tpa ARP packet field: Target Protocol Address [RFC826]. For IPv4 Address Resolution, the IPv4 address for which one desires to know the hardware address. Aboba Proposed Standard [Page 3] INTERNET-DRAFT DNAv4 25 March 2005 DHCP client A DHCP client or "client" is an Internet host using the Dynamic Host Configuration protocol (DHCP) [RFC2131] to obtain configuration parameters such as a network address. DHCP server A DHCP server or "server" is an Internet host that returns configuration parameters to DHCP clients. Point of Attachment A location within the network where a host may be connected. This attachment point can be characterized by its address prefix and next hop routing information. Most Likely Point of Attachment (MLPA) The point of attachment heuristically determined by the host to be most likely, based on hints from the network. Routable address In this specification, the term "routable address" refers to any address other than an IPv4 Link-Local address. This includes private addresses as specified in [RFC1918]. Valid address In this specification, the term "valid address" refers to either a static IPv4 address, or an address assigned via DHCPv4 which has not been relinquished, and whose lease has not yet expired. 2. Overview DNAv4 consists of three phases: determination of the Most Likely Point of Attachment (MLPA), reachability testing, and IPv4 address acquisition. On connecting to a new point of attachment, the host responds to "Link Up" indications from the link layer by carrying out the DNAv4 procedure. As noted in Appendix A, hints about the point of attachment may be available from the link and Internet layers. Based on these hints, the host determines the "Most Likely Point of Attachment" (MLPA) and determines whether it has a valid IP configuration associated with it. If the host believes that it has attached to a network on which it has a valid IP configuration, then it performs a reachability test in order to confirm that configuration. In contrast to a DHCP exchange, which may be between a DHCP client and an off-link DHCP server, the reachability test is designed to verify bi-directional connectivity to the default gateway(s) on the MLPA. Aboba Proposed Standard [Page 4] INTERNET-DRAFT DNAv4 25 March 2005 If the reachability test is successful, the host MAY continue to use a valid routable IPv4 address without having to re-acquire it. This reduces roaming latency by allowing the host to bypass DHCP as well as subsequent Duplicate Address Detection (DAD). If the host believes that it has attached to a network on which it has no valid IPv4 configuration, or if the reachability test fails, then the host attempts to obtain an IPv4 configuration using DHCPv4. Since DNAv4 represents a performance optimization, it is important to avoid compromising robustness. In some circumstances, DNAv4 may result in a host successfully verifying an existing IPv4 configuration where attempting to obtain configuration via DHCPv4 would fail (such as when the DHCP server is down). To improve robustness, this document suggests that hosts behave conservatively with respect to assignment of IPv4 Link-Local addresses, configuring them only in situations in which they can do no harm. Experience has shown that IPv4 Link-Local addresses are often assigned inappropriately, and that inappropriate assignment can compromise both performance and connectivity. While the performance of DNAv4 is dependent on the reliability of the hints provided to the client, the host will ultimately determine the correct IPv4 configuration even in the presence of misleading hints. Where the host mistakenly concludes that it has attached to a network on which it has a valid configuration a timeout will occur, providing poorer performance than would be experienced by initially attempting to obtain IPv4 configuration using DHCPv4. However, after timing out, the host will obtain its configuration using DHCPv4, so that the correct configuration will eventually be obtained. DNAv4 does not increase the likelihood of an address conflict. The DNAv4 procedure is only carried out when the host has a valid IPv4 configuration on the MLPA, implying that duplicate address detection has previously been completed. Restrictions on sending ARP requests and replies are described in Section 2.2.1. 2.1. Most Likely Point of Attachment In order to determine the MLPA, it is assumed that the host saves to stable storage parameters relating to the networks it connects to: [1] Link and Internet layer hints associated with each network. For details, see Appendix A. [2] The IPv4 and MAC address of the default gateway(s) on each network. Aboba Proposed Standard [Page 5] INTERNET-DRAFT DNAv4 25 March 2005 [3] The link type, such as whether the link utilizes Ethernet, or 802.11 adhoc or infrastructure mode. Appendix A discusses hints useful for the determination of the MLPA. By matching received hints against network parameters previously stored, the host makes an an educated guess of which network it has attached to. In the absence of other information, the MLPA defaults to the network to which the host was most recently attached. Aside from utilizing link layer indications, a host may also be able to utilize Internet layer information in order to determine the MLPA. IPv4 ICMP Router Discovery messages [RFC1256] provide information relating to prefix(es) available on the link, as well as the routers that serve those prefix(es). A host may use ICMP Router Discovery to conclude that an advertised prefix is available; however it cannot conclude the converse -- that prefixes not advertised are unavailable. However, since [RFC1256] is not widely implemented, it is NOT RECOMMENDED that hosts utilize ICMP Router Discovery messages as an alternative to the reachability test outlined in Section 2.2. Instead, ICMP Router Advertisements can be used to obtain information on available prefixes and default gateway(s). This can provide additional resilience in the case where default gateway(s) become unavailable. Similarly hosts that support routing protocols such as RIP and OSPF can use these protocols to determine the prefix(es) available on a link and the default gateway(s) that serve those prefixes. Full support is not required to glean this information. A host that passively observes routing protocol traffic may deduce this information without supporting a fully conformant routing protocol implementation. 2.2. Reachability Test If the host has a valid routable IPv4 address on the MLPA, a host conforming to this specification SHOULD perform a reachability test, in order to to confirm that it is connected to network on which it has a valid routable IPv4 address. If the reachability test is not successful, the host proceeds to the IPv4 address acquisition phase, described in Section 2.3. The host skips the reachability test and proceeds to the IPv4 address acquisition phase if any of the following conditions are true: [a] The host does not have a valid routable IPv4 address on the MLPA. In this case, the reachability Aboba Proposed Standard [Page 6] INTERNET-DRAFT DNAv4 25 March 2005 test cannot confirm that the host has a valid routable IPv4 address, so that completing the reachability test would serve no purpose. [b] The host does not have information on the default gateway(s) on the MLPA. In this case, insufficient information is available to carry out the reachability test. [c] Reliable hints are unavailable. Since confirming failure of the reachability test requires a timeout, mistakes are costly. In the absence of reliable hints, a host SHOULD instead send a DHCPREQUEST from the INIT-REBOOT state, as described in [RFC2131], Section 3.2 and 4.3.2. Where reliable hints are unavailable, this will typically complete more quickly than the reachability test. [d] If secure detection of network attachment is required. The reachability test utilizes ARP which is insecure, whereas DHCP can be secured via DHCP authentication, described in [RFC3118]. See Section 5 for details. The host MAY probe only the primary default gateway, or it MAY probe primary and secondary default gateways in series or in parallel. In order to ensure configuration validity, the host SHOULD only configure default gateway(s) which pass the reachability test. 2.2.1. Packet Format The reachability test is performed by sending an ARP Request. The ARP Request SHOULD use the host's MAC address as the source, and the broadcast MAC address as the destination. The host sets the target protocol address (ar$tpa) to the IPv4 address of the primary default gateway, and uses its own MAC address in the sender hardware address field (ar$sha). The host sets the target hardware address field (ar$tha) to 0. If the host has a private address as defined in [RFC1918], then it SHOULD set the sender protocol address field (ar$spa) to the unspecified address (0.0.0.0). This is to avoid a potential address conflict when the host changes its point of attachment from one private network to another. Note: Some routers may refuse to answer an ARP Request sent with the sender protocol address field (ar$spa) set to the unspecified address (0.0.0.0). In this case the reachability test will fail. Aboba Proposed Standard [Page 7] INTERNET-DRAFT DNAv4 25 March 2005 If the host has a valid routable IPv4 address other than a private address on the MLPA, then it SHOULD set the sender protocol address field (ar$spa) to that address. It is assumed that the host had previously done duplicate address detection so that an address conflict is unlikely. If a valid ARP Response is received, the MAC address in the sender hardware address field (ar$sha) and the IPv4 address in the sender protocol address field (ar$spa) are matched against the list of networks and associated default gateway parameters. If a match is found, then if the host has a valid routable IPv4 address on the matched network, the host continues to use that IPv4 address, subject to the lease re-acquisition and expiration behavior described in [RFC2131], Section 4.4.5. Checking for a match on both the IPv4 address and MAC address of the default gateway enables the host to confirm reachability even where it has moved between two private networks. In this case the IPv4 address of the default gateway could remain the same, while the MAC address would change, so that both addresses need to be checked. The risk of an address conflict is greatest when the host moves between private networks, since in this case the completion of Duplicate Address Detection on the former network does not provide assurance against an address conflict on the new network. Until a host has conformed the validity of its IPv4 configuration, it SHOULD NOT respond to ARP requests. In addition, prior to confirming its IPv4 configuration, a host with a private address SHOULD NOT send ARP requests containing its address within the sender protocol address field (ar$spa); instead it SHOULD use the unspecified address, as described above. However, where the host has a valid routable non- private address on the MLPA, it MAY send ARP requests using its address within the sender protocol address field (ar$spa) prior to confirming its IPv4 configuration. Sending an ICMP Echo Request [RFC792] to the default gateway IPv4 address does not provide the same level of assurance since this may require an ARP Request/Response exchange. Where the host has moved between two private networks, this could result in ARP cache pollution. Where a host moves from one private network to another, an ICMP Echo Request can result in an ICMP Echo Response even when the default gateway has changed, as long as the IPv4 address remains the same. This can occur, for example, where a host moves from one home network using prefix 192.168/16 to another one. In addition, if the ping is sent with TTL > 1, then an ICMP Echo Response can be received from an off-link gateway. As a result, if the MAC address of the Aboba Proposed Standard [Page 8] INTERNET-DRAFT DNAv4 25 March 2005 default gateway is not checked, the host can mistakenly confirm attachment to the MLPA, potentially resulting in an address conflict. As a result, sending of an ICMP Echo Request SHOULD NOT be used as a substitute for the DNAv4 procedure. If the initial ARP Request does not elicit a Response, the host waits for REACHABILITY_TIMEOUT and proceeds to the IPv4 address acquisition phase. If a valid ARP Response is received, but cannot be matched against known networks, the host assumes it does not have a valid IPv4 configuration and moves on to the IPv4 address acquisition phase. 2.3. IPv4 Address Acquisition If the host has a valid routable IPv4 address on the MLPA but the reachability test fails, the host SHOULD verify the configuration by entering the INIT-REBOOT state, and sending a DHCPREQUEST to the broadcast address as specified in [RFC2131] Section 4.4.2. If the host does not have a valid routable IPv4 address on the MLPA, the host enters the INIT state and sends a DHCPDISCOVER packet to the broadcast address, as described in [RFC2131] Section 4.4.1. If the host supports the Rapid Commit Option [RAPID], it is possible that the exchange can be shortened from a 4-message exchange to a 2-message exchange. If the host does not receive a response to a DHCPREQUEST or DHCPDISCOVER, then it retransmits as specified in [RFC2131] Section 4.1. As discussed in [RFC2131], Section 4.4.4, a host in INIT or REBOOTING state that knows the address of a DHCP server may use that address in the DHCPDISCOVER or DHCPREQUEST rather than the IPv4 broadcast address. In the INIT-REBOOT state a DHCPREQUEST is sent to the broadcast address so that the host will receive a response regardless of whether the previously configured IPv4 address is correct for the network to which it has connected. Sending a DHCPREQUEST to the unicast address in INIT-REBOOT state is not appropriate, since if the DHCP client has moved to another subnet, a DHCP server response cannot be routed back to the client since the DHCPREQUEST will bypass the DHCP relay and will contain an invalid source address. 2.4. IPv4 Link-Local Addresses To avoid inappropriate assignment of IPv4 Link-Local addresses, it is recommended that hosts behave conservatively with respect to Aboba Proposed Standard [Page 9] INTERNET-DRAFT DNAv4 25 March 2005 assignment of IPv4 Link-Local addresses. As described in [IPv4LL] Section 1.7, use of a routable address is preferred to a IPv4 Link- Local address whenever it is available. Where the host does not have a valid routable IPv4 address on the MLPA, the host MAY configure an IPv4 Link-Local address prior to entering the INIT state and sending a DHCPDISCOVER packet, as described in Section 2.3. However, should a routable IPv4 address be obtained, the IPv4 Link-Local address is deprecated, as noted in [IPv4LL]. Where a host has a valid routable IPv4 address on the MLPA, but the DHCP client does not receive a response after employing the retransmission algorithm, [RFC2131] Section 3.2 states that the client MAY choose to use the previously allocated network address and configuration parameters for the remainder of the unexpired lease. Where a host can confirm that it remains connected to a point of attachment on which it possesses a valid routable IPv4 address, that address SHOULD be used, rather than assigning a IPv4 Link-Local address. Since a IPv4 Link-Local address is often configured because a DHCP server failed to respond to an initial query or is inoperative for some time, it is desirable to abandon the IPv4 Link-Local address assignment as soon as a valid IPv4 address lease can be obtained. As described in [IPv4LL] Appendix A, once a Link-Local IPv4 address is assigned, existing implementations do not query the DHCPv4 server again for 5 minutes. This behavior violates [RFC2131] Section 4.1. Where a IPv4 Link-Local address is assigned, experience has shown that five minutes (see [IPv4LL] Appendix A.2) is too long an interval to wait until retrying to obtain a routable IPv4 address using DHCP. According to [RFC2131] Section 4.1: The retransmission delay SHOULD be doubled with subsequent retransmissions up to a maximum of 64 seconds. As a result, a DHCP client compliant with [RFC2131] will continue to retry every 64 seconds, even after allocating a IPv4 Link-Local address. Should the DHCP client succeed in obtaining a routable address, then as noted in [IPv4LL], the IPv4 Link-Local address is deprecated. Since it is inevitable that hosts will inappropriately configure IPv4 Link-Local addresses at some point, hosts with routable IPv4 addresses need to be able to respond to peers with IPv4 Link-Local addresses, as per [IPv4LL]. For example, a host configured with a Aboba Proposed Standard [Page 10] INTERNET-DRAFT DNAv4 25 March 2005 routable address may receive a datagram from a link-local source address. In order to respond, the host will use ARP to resolve the target hardware address and send the datagram directly, not to a router for forwarding. 3. Constants The suggested default value of REACHABILITY_TIMEOUT is 200 ms. This value was chosen so as to accommodate the maximum retransmission timer likely to be experienced on an Ethernet network. 4. IANA Considerations This specification does not request the creation of any new parameter registries, nor does it require any other IANA assignments. 5. Security Considerations Detection of Network Attachment (DNA) is based on ARP and DHCP. ARP [RFC826] traffic is inherently insecure, so that the reachability test described in Section 1.3 can be easily spoofed by an attacker, leading a host to falsely conclude that it is attached to a network that it is not connected to. Similarly, where DHCP traffic is not secured, an attacker could masquerade as a DHCP server, in order to convince the host that it was attached to a particular network. As a result, it is inadvisable for a host to adjust its security based on which network it believes it is attached to. For example, it would be inappropriate for a host to disable its personal firewall based on the belief that it had connected to a home network. Where secure detection of network attachment is required, a host SHOULD skip the reachability test since it cannot be secured, and instead utilize authenticated DHCP [RFC3118], possibly in combination with the Rapid Commit Option [RAPID]. 6. References 6.1. Normative References [RFC792] Postel, J., "Internet Control Message Protocol", RFC 792, USC/Information Sciences Institute, September 1981. [RFC826] D. Plummer, "An Ethernet Address Resolution Protocol -or- Converting Network Addresses to 48-bit Ethernet Address for Transmission on Ethernet Hardware", STD 37, RFC 826, November 1982. Aboba Proposed Standard [Page 11] INTERNET-DRAFT DNAv4 25 March 2005 [RFC1256] Deering, S., "ICMP Router Discovery Messages", RFC 1256, Xerox PARC, September 1991. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March, 1997. [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001. [RFC3927] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration of IPv4 Link-Local Addresses", RFC 3927, March 2005. 6.2. Informative References [RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, Daydreamer, July 1994. [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E. Lear, "Address Allocation for Private Internets", RFC 1918, February 1996. [RFC3580] Congdon, P., Aboba, B., Smith, A., Zorn, G., and J. Roese, "IEEE 802.1X Remote Authentication Dial In User Service (RADIUS) Usage Guidelines", RFC 3580, September 2003. [RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J. and H. Levkowetz, "Extensible Authentication Protocol (EAP)", RFC 3748, June 2004. [RAPID] Park, S., Kim, P., and B. Volz, "Rapid Commit Option for DHCPv4", Internet draft (work in progress), draft-ietf-dhc- rapid-commit-opt-05.txt, June 2004. [IEEE8021AB] IEEE Standards for Local and Metropolitan Area Networks: Station and Media Access Control - Connectivity Discovery, IEEE Std 802.1AB/D5, September 2003. [IEEE8021X] IEEE Standards for Local and Metropolitan Area Networks: Port based Network Access Control, IEEE Std 802.1X-2004, December 2004. [IEEE802] IEEE Standards for Local and Metropolitan Area Networks: Overview and Architecture, ANSI/IEEE Std 802, 1990. Aboba Proposed Standard [Page 12] INTERNET-DRAFT DNAv4 25 March 2005 [IEEE8021Q] IEEE Standards for Local and Metropolitan Area Networks: Draft Standard for Virtual Bridged Local Area Networks, P802.1Q, January 1998. [IEEE80211] Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE Std. 802.11-2003, 2003. Acknowledgments The authors would like to acknowledge Greg Daley of Monash University, Erik Guttman and Erik Nordmark of Sun Microsystems, Ted Lemon of Nominum Thomas Narten of IBM, and David Hankins of ISC for contributions to this document. Authors' Addresses Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 EMail: bernarda@microsoft.com Phone: +1 425 706 6605 Fax: +1 425 936 7329 Aboba Proposed Standard [Page 13] INTERNET-DRAFT DNAv4 25 March 2005 Appendix A - Link Layer Hints A.1 Introduction In order to assist in IPv4 network attachment detection, information associated with each network may be retained by the host. Based on link-layer information, the host may be able to make an educated guess as to whether it has moved between subnets, or has remained on the same subnet, as well as whether it has connected to an infrastructure or adhoc network. If the host is likely to have moved between subnets, it may be possible to make an educated guess as to which subnet it has moved to. Since an educated guess may be incorrect, prior to concluding that the host remains on the same subnet, further tests (such as a reachability test or a DHCPREQUEST sent from the INIT-REBOOT state) are REQUIRED. In practice, it is necessary for hints to be highly reliable before they are worth considering, if the penalty paid for an incorrect hint is substantial. As an example, assume that a DHCPREQUEST requires DHCPREQUEST_TIME to determine if a host has remained on the same subnet, while a reachability test typically completes in REACH_TIME and times out in REACHABILITY_TIMEOUT, after which a DHCPREQUEST is sent. If a hint that the host has remained on the same subnet is wrong x fraction of the time, then it is only worth considering if: DHCPREQUEST_TIME = (1 - x) * REACH_TIME + x * (REACHABILITY_TIMEOUT + DHCPREQUEST_TIME) x = DHCPREQUEST_TIME - REACH_TIME ---------------------------------------------------- REACHABILITY_TIMEOUT + DHCPREQUEST_TIME - REACH_TIME If we assume that DHCPREQUEST_TIME = 100 ms, REACH_TIME = 10 ms, and REACHABILITY_TIMEOUT = 1000ms, then: x = (100 - 10)/(1000 + 100 - 10) = 8.2 percent Therefore the hint need only be wrong 8.2 percent of the time before it is not worth considering. Aboba Proposed Standard [Page 14] INTERNET-DRAFT DNAv4 25 March 2005 A.2 Hints For networks running IPv4 over PPP [RFC1661], IPv4 parameters negotiated in IPCP provide direct information on whether a previously obtained address remains valid on the link. On IEEE 802 [IEEE802] wired networks, hints include link-layer discovery traffic as well as information exchanged as part of IEEE 802.1X authentication [IEEE8021X]. Link-layer discovery traffic includes Link Layer Discovery Protocol (LLDP) [IEEE8021AB] traffic as well as network identification information passed in the EAP-Request/Identity or within an EAP method exchange, as defined in EAP [RFC3748]. For example, LLDP advertisements can provide information on VLANs supported by the device. When used with IEEE 802.1X authentication [IEEE8021X], the EAP-Request/Identity exchange may contain the name of the authenticator, also providing information on the potential network. Similarly, during the EAP method exchange the authenticator may supply information that may be helpful in identifying the network to which the device is attached. However, as noted in [RFC3580], it is possible for the VLANID defined in [IEEE8021Q] to be assigned dynamically, so that static advertisements may not prove definitive. In IEEE 802.11 [IEEE80211] stations provide information in Beacon and/or Probe Response messages, such as the SSID, BSSID, and capabilities, as well as information on whether the station is operating in Infrastructure or Ad hoc mode. As described in [RFC3580], it is possible to assign a Station to a VLAN dynamically, based on the results of IEEE 802.1X [IEEE8021X] authentication. This implies that a single SSID may offer access to multiple VLANs, and in practice most large WLAN deployments offer access to multiple subnets. Thus, associating to the same SSID is a necessary, but not necessarily a sufficient condition, for remaining within the same subnet: while a Station associating to the same SSID may not necessarily remain within the same subnet, a Station associating to a different SSID is likely to have changed subnets. In IEEE 802.11, the SSID is a non-unique identifier, and SSIDs such as "default", "linksys" and "tsunami" are often configured by manufacturers by default. As a result, matching an advertised SSID against those of previously encountered networks may be misleading. Where an SSID known to be configured by default is encountered, it is recommended that the BSSID be stored and subsequently compared against the advertised BSSID to determine whether a match exists. Aboba Proposed Standard [Page 15] INTERNET-DRAFT DNAv4 25 March 2005 In order to provide additional guidance on the subnets to which a given AP offers access, additional subnet-related Information Elements (IEs) have been proposed for addition to the IEEE 802.11 Beacon and Probe Response messages. As noted earlier, VLANs may be determined dynamically so that these information elements may not be reliable. In IEEE 802.11, the presence of an IBSS can be used as a hint that a point of attachment supports adhoc networking, and therefore that assignment of a IPv4 Link-Local address is likely. When running IPv4 over PPP, if an IP address is not statically configured or assigned via IPCP, this can also be taken as a hint that assignment of an IPv4 Link-Local address is likely. In addition, certain media such as USB or IEEE 1394 may be considered inherently more likely to support adhoc operation, so that connection to these networks may by itself be considered a hint. Intellectual Property Statement The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementors or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Disclaimer of Validity 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. Aboba Proposed Standard [Page 16] INTERNET-DRAFT DNAv4 25 March 2005 Copyright Statement Copyright (C) The Internet Society (2005). 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. Open issues Open issues relating to this specification are tracked on the following web site: http://www.drizzle.com/~aboba/DNA/dnaissues.html Aboba Proposed Standard [Page 17]