DHC Working Group Bernard Aboba INTERNET-DRAFT Microsoft Corporation Category: Proposed Standard
<draft-ietf-dhc-dna-ipv4-02.txt> 28 September<draft-ietf-dhc-dna-ipv4-03.txt> 9 October 2003 Detection of Network Attachment (DNA) in IPv4 This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC 2026. 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. Copyright Notice Copyright (C) The Internet Society (2003). 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 address may be significant as a fraction of the total delay in moving between points of attachment. This specification synthesizes experience garnered over the years in the deployment of hosts supporting ARP, DHCP and IPv4 Link-Local addresses, in order to optimize detection of network attachment by mobile hosts. Table of Contents 1. Introduction.............................................. 3 1.1 Requirements .................................... 4 1.2 Terminology ..................................... 4 2. Framework ................................................ 4 2.1 Reachability test ............................... 5 2.2 Packet format ................................... 5 2.3 IPv4 Address Acquisition ........................ 67 3. IANA Considerations ...................................... 8 4. Security Considerations .................................. 8 5. References ............................................... 8 5.1 Normative references ............................ 8 5.2 Informative references .......................... 9 Acknowledgments .............................................. 10 Authors' Addresses ........................................... 10 Appendix A - Hints ........................................... 11 Intellectual Property Statement .............................. 12 Full Copyright Statement ..................................... 12 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 Link-Local IPv4 addresses [IPv4LL], specifying a procedure to be performed for IPv4 detection of network attachment. The procedure consists of three phases: determination of the "most likely" point of attachment, reachability testing, and IPv4 address acquisition. On disconnection from a network, there is no need to take action until the host is reconnected, since it is typically not possible for a host to communicate until it has obtained connectivity. Therefore, contrary to [RFC2131] Section 3.7, no action need be taken on network disconnection. For Detection of Network attachment, the following basic principles are suggested: [a] Utilization of link layer hints. Link layers such as IEEE 802 [IEEE802] provide hints about whether a host remains on the same subnet despite changing its point of attachment, or even whether the host is connected to an adhoc or infrastructure network. Where available, these hints can be used to guide host behavior - with the understanding that they are not infallible and therefore that the host should be capable of making the correct determination even in the presence of misleading hints. Link layer hints are described in more detail in Appendix A. [b] Link-Local IPv4 addressing as a mechanism of last resort. Experience has shown that Link-Local IPv4 addresses are often assigned inappropriately. For example, an IPv4 host assigning an Link-Local IPv4 address may not be connected to any network, in which case assignment of a Link-Local IPv4 address does no good; or the host may be attached to a network with a DHCPv4 server but may not receive a response to a DHCPREQUEST or DHCPDISCOVER. As described in Appendix A of [IPv4LL] once a Link-Local IPv4 address is assigned, existing implementations do not query the DHCPv4 server again for 5 minutes. As noted in Section 2.3, this behavior is in violation of [RFC2131] Section 4.1. For hosts changing their point of attachment more frequently than this, inappropriate assignment of an Link-Local IPv4 address can result in an ongoing inability to connect. As a result, this document suggests that hosts behave conservatively with respect to assignment of Link-Local IPv4 addresses, using them only as a last resort. 1.1. Requirements In this document, several words are used to signify the requirements of the specification. These words are often capitalized. 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: DHCP client A DHCP client or "client" is an Internet host using DHCP 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. 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]. 2. Framework On connecting to a new point of attachment, the host attempts to determine the "most likely" configuration associated with the new point of attachment. In order to determine the "most likely" point of attachment it is assumed that the host is capable of obtaining and writing to stable storage parameters relating to networks that it connects to, including:  IP and link layer hints associated with each network. For details, see Appendix A.  The IP and MAC address of the primary default gateway on each network. By matching the received hints against information previously collected, the host may be able to make an educated guess of which network it has attached to. Where no additional information is available, by default the host assumes that the "most likely" point of attachment is the network to which it was most recently connected. If the host has a valid routable IPv4 address on the "most likely" point of attachment, the host performs a reachability test as described below. If the reachability test is not successful, or if the host does not have a valid routable IPv4 address on the "most likely" point of attachment, the host proceeds to the IPv4 address acquisition phase. 2.1. Reachability Test The purpose of the reachability test is to determine whether the host is connected to a network on which it had previously obtained a still valid routable IPv4 address. The test is performed by attempting to verify reachability of a previously configured primary default gateway on a former point of attachment. If the 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 the DHCP exchange as well as subsequent Duplicate Address Detection (DAD). In contrast to a DHCP exchange, which may be between a DHCP client and an offlink DHCP server, the reachability test occurs between a host and its next hop router. The host skips the reachability test and proceeds to the address acquisition phase in the following circumstances: [a] If the host does not have information on the default gateway on the network. [b] If the host does not have a valid routable IPv4 address on the network. Since Link-Local IPv4 addresses are a last resort, these addresses do not count as a valid routable IPv4 address. 2.2. Packet Format To perform the reachability test, an ARP Request SHOULD be sent, using 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). SinceIf the host has not yet confirmed the subneta valid globally routable IP address on which it is attached,the most likely point of attachment, then it MUSTSHOULD set the sender protocol address field (ar$spa) to 0.0.0.0. This prevents poisoning ofthat address. It is assumed that the ARP cache withhost had previously done duplicate address detection so that an address conflict is unlikely. However if the host has a (potentially invalid) sender IPv4 address. Sinceprivate address as defined in [RFC1918], then it SHOULD set the ARP Responseprotocol address field (ar$spa) to 0.0.0.0. This is sentto avoid an address conflict in the sender hardware address,case where the contentshost has changed its point of attachment from one private network to another. Note: Some routers may refuse to answer an ARP Request sent with the senderprotocol address field do not affect delivery of(ar$spa) set to the ARP Response. Since existing implementations do not create an ARP cache entry for 0.0.0.0, usingunspecified address. In this ascase the sender protocol address is harmless.reachability test will fail. If a valid ARP Response is received, the MAC address in the target hardware address field (ar$tha) and the IPv4 address in the target protocol address field (ar$tpa) are matched against the list of networks and associated default gateway parameters. If a match is found, then if the host has a valid IPv4 address lease 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 and MAC addresses of the default gateway allows the host to confirm reachability even where the host moves 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. Sending an ICMP Echo Request to the default gateway IPv4 address does not provide the same level of assurance since this requires an ARP Request/Response to be sent first, and typically does not allow the MAC address to be checked as well. It therefore SHOULD NOT be used as a substitute. 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. If the initial ARP Request does not elicit a Response, the host waits 200ms and then sends another ARP Request. If no ARP Response is received in response to this second Request, the host 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 has moved subnets and moves on to the address acquisition phase. 2.3. IPv4 Address Acquisition If the host has a valid cached configuration on the "most likely" point of attachment, but is unable to confirm reachability to the primary default gateway, then the host seeks to verify the cached 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 cached configuration, or had not previously obtained a routable IPv4 address on the "most likely" point of attachment, then 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 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 IP broadcast address. However, sending a DHCPREQUEST to the unicast address when in INIT- REBOOT state is not appropriate since it is possible that the client has moved to another subnet, and therefore the DHCPREQUEST needs to be forwarded to and from the DHCP server by a DHCP Relay so that the response can be broadcast. This ensures that the host will receive a response regardless of whether the cached IP address is correct for the network to which it has connected. As described in [IPv4LL] Section 1.7, use of a routable address is preferred to a Link-Local IPv4 address whenever it is available. [RFC2131] Section 3.2 states that if the host possesses a valid routable IPv4 address on the "most likely" network and does not receive a response after employing the retransmission algorithm, the client MAY choose to use the previously allocated network address and configuration parameters for the remainder of the unexpired lease. This is preferable to assigning a Link-Local IPv4 address if the host has good reason to believe that it remains connected to a network on which it possesses a valid IPv4 address lease. This would be the case, for example, where a host has received hints that it believes to be "strong". See Appendix A for details. If the host does not have a valid IPv4 address lease on the "most likely" network and does not receive a response after employing the retransmission algorithm, it MAY assign a Link-Local IPv4 address. Since a Link-Local IPv4 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 Link-Local IPv4 address assignment as soon as a valid IPv4 address lease can be obtained. Where a Link-Local IPv4 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 Link-Local IPv4 address. Should the DHCP client succeed in obtaining a routable address, then as noted in [IPv4LL], the Link-Local IPv4 address is deprecated. In order to avoid inappropriate assignment of an IPv4 Link-Local address, it is RECOMMENDED that such an address not be assigned until the DHCP client has retransmitted at least 3 times. 3. IANA Considerations Guidelines for IANA considerations are specified in [RFC2434]. This specification does not request the creation of any new parameter registries, nor does it require any other IANA assignments. 4. Security Considerations Detection of Network Attachment (DNA) is typically insecure, so that 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 believe that it had connected to a home network. 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 conclude that it remained attached to a former network. Similarly, where DHCP [RFC2131] 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. Where secure detection of network attachment is required, a host MAY wish to skip the ARP-based reachability test entirely since it cannot be secured, and go immediately to the IPv4 address acquisition phase, utilizing authenticated DHCP [RFC3118]. 5. References 5.1. 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. [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. [RFC2132] Alexander, S. and Droms, R., "DHCP Options and BOOTP Vendor Extensions", RFC 2132, Silicon Graphics, Inc., Bucknell University, March 1997. [RFC2434] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 2434, October 1998. [RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001. [IPv4LL] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration of IPv4 Link-Local Addresses", Internet draft (work in progress), draft-ietf-zeroconf-ipv4-linklocal-10.txt, October 2003. 5.2. Informative References [RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, Daydreamer, July 1994. [RFC1918] Rekhter, Y., et al., "Address Allocation for Private Internets", RFC 1918, February 1996. [RFC2284] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication Protocol (EAP)", RFC 2284, March 1998. [RFC3580] Congdon, P., et al., "IEEE 802.1X Remote Authentication Dial In User Service (RADIUS) Usage Guidelines", RFC 3580, September 2003. [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-2001, June 2001. [IEEE802] IEEE Standards for Local and Metropolitan Area Networks: Overview and Architecture, ANSI/IEEE Std 802, 1990. [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. 802.11-1999, 1999. Acknowledgments The authors would like to acknowledge Greg Daley of Monash University, Erik Guttman and Erik Nordmark of Sun Microsystems, Ted Lemon of Nominum and Thomas Narten of IBM for contributions to this document. Authors' Addresses Bernard Aboba Microsoft Corporation One Microsoft Way Redmond, WA 98052 EMail: firstname.lastname@example.org Phone: +1 425 706 6605 Fax: +1 425 936 7329 Appendix A - Hints In order to assist in IPv4 network attachment detection, information associated with each network may be retained by the host. Based on IP and link-layer information, the host may be able to make an educated guess as to whether it has moved between subnets, or remained on the same subnet. If it is likely to have moved between subnets, the host may have an educated guess as to which subnet it has moved to. The term "strong hint" refers to information which provides an unambiguous indication of the network to which a host has connected. "Weak hints" involve information which is inconclusive. IPv4 ICMP Router Discovery messages [RFC1256] provide information directly relevant to determining the network to which a host has connected. As such, information gleaned from Router Advertisements can be considered a "strong" hint. For networks running over PPP [RFC1661], "weak" hints include the link characteristics negotiated in LCP, and the associated phone number. The IP parameters negotiated in IPCP are considered a "strong" hint. 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 [IEEE8021AB] traffic as well as network identification information passed in the EAP- Request/Identity or within an EAP method exchange, as defined in EAP [RFC2284]. For example, LLDP advertisements can provide information on the IP address or VLANs supported by the device. These hints, if provided, are considered "strong". 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 this static information may not prove definitive. As a result, these hints are considered "weak". 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 Adhoc 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; on the other hand, a Station associating to a different SSID is likely to have changed subnets. 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. Such hints are considered "strong"; all other IEEE 802.11 hints are considered "weak". 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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. Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. 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. 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Open issues Open issues relating to this specification are tracked on the following web site: http://www.drizzle.com/~aboba/DNA/dnaissues.html Expiration Date This memo is filed as <draft-ietf-dhc-dna-ipv4-02.txt>,<draft-ietf-dhc-dna-ipv4-03.txt>, and expires FebruaryApril 22, 2004.