DHC Working Group Bernard Aboba INTERNET-DRAFT Microsoft Corporation Category: Best Current Practice
<draft-ietf-dhc-dna-ipv4-00.txt> 10 August<draft-ietf-dhc-dna-ipv4-01.txt> 11 September 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 This specification attempts to synthesize experience garnered over the years in the deployment of hosts supporting ARP, DHCP and IPv4 Link- Local addresses. Given this experience, this document suggests optimizations for detection of network attachment in IPv4 as well as heuristics for determining when assignment of an IPv4 Link-Local address is appropriate. Table of Contents 1. Introduction.............................................. 3 1.1 Requirements .................................... 3 1.2 Terminology ..................................... 3 2. Framework ................................................ 4 2.1 Most Likely Point of Attachment ................. 4 2.2 Reachability test ............................... 5 2.3 IPv4 Address Acquisition ........................ 6 3. Constants ................................................ 8 4. IANA Considerations ...................................... 8 4.5. Security Considerations .................................. 8 5.6. References ............................................... 8 5.16.1 Normative references ............................ 8 5.26.2 Informative references .......................... 9 Acknowledgments .............................................. 10 Authors' Addresses ........................................... 10 Appendix A - Hints ........................................... 1112 Intellectual Property Statement .............................. 1213 Full Copyright Statement ..................................... 1213 1. Introduction This draft attempts to synthesize experience garnered over the years in the deployment of hosts supporting ARP [RFC826], DHCP [RFC2131], and Link-Local IPv4 addresses [IPv4LL]. Experience has indicated the importance of several goals in detection of network attachment: [a] Avoiding inappropriate assignment of Link-Local IPv4 addresses. Experience has shown that in the vast majority of cases, the assignment of Link-Local IPv4 addresses is inappropriate. That is, the IPv4 host assigning an Link-Local IPv4 address either is not connected to a network at all, in which case assignment of an Link- LocalLink-Local IPv4 address does no good; or the host is in fact present on a network with a DHCPv4 server but for one reason or another does not receive a response to a DHCPREQUEST or DHCPDISCOVER. [b] Latency Optimization. The time required to detect movement (or lack of movement) between subnets, and to obtain (or continue to use) a valid IPv4 address represents a significant fraction of the overall latency resulting from movement between points of attachment on the network. As a result, optimization of detection of network attachment in IPv4 hosts is helpful, to the extent that it is achievable. In order to achieve these goals, this document specifies procedures conducted on connection to a network. 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. 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 This document specifies a procedure to be performed for IPv4 network attachment detection that depends on three phases: determination of the "most likely" point of attachment, a reachability test phase, and an IPv4 address acquisition phase. The following basic principles are suggested:  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.  Link-Local IPv4 addressing as a mechanism of last resort. In existing implementations of [IPv4LL], once a Link-Local IPv4 address is assigned, the DHCPv4 server may not be queried again for 5 minutes. As a result, the inappropriate assignment of a Link- LocalLink-Local IPv4 address results in an extended period of limited connectivity. For a host that may change its point of attachment more frequently than every 5 minutes, the inappropriate assignment of an Link-Local IPv4 address is more than just an annoyance - it 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. 2.1. Most Likely Point of Attachment 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 default gateway(s) 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. 2.2. Reachability Test The purpose of the reachability test is for the host to quickly determine whether it 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. 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. 2.2.1. 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). Since the host has not yet confirmed the subnet on which it is attached, it MUST set the sender protocol address field (ar$spa) to 0.0.0.0. This prevents poisoning of the ARP cache with a (potentially invalid) sender IPv4 address. If a valid ARP Response is received, the MAC address in the target hardware address field (ar$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 noteddescribed in [IPv4LL] Section 1.7, use of a routable address is preferred to use of a Link-Local IPv4 address. [RFC2131] Section 3.2,3.2 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. Alternatively, ifIf 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. This is the preferred behavior only in situations where an adhoc network is likely to be in operation. For example,Since a host might choose to assign an IPv4Link-Local address on an IEEE 1394 or adhoc IEEE 802.11 network, unless a routableIPv4 address had previously been assigned on that network. However even in this situation, itis possible that the failure to obtain a routable IPv4 address representsoften configured because a temporary aberration, rather than legitimate detection ofDHCP server failed to respond to an adhoc network. Itinitial query or is inoperative for some time, it is thereforedesirable to abandon the assignment of anLink-Local IPv4 address assignment as soon as a valid IPv4 address lease can be obtained. Where ana Link-Local IPv4 Link- Localaddress is assigned, it is therefore RECOMMENDEDexperience has shown that thefive minutes (see [IPv4LL] Appendix A.2) was too long an interval to wait and try to obtain a routable IPv4 address via DHCP. A host which has been configured with a Link-Local IPv4 address SHOULD periodically attempt to obtain ana routable IPv4 address at 30 second intervals.via DHCP. The recommended policy is to attempt to obtain an address via DHCP after waiting for RECONF_INTERVAL, plus a random number of seconds, uniformly distributed, between zero to RECONF_JITTER seconds. 3. Constants RECONF_INTERVAL 30 seconds RECONF_JITTER 10 seconds 4. IANA Considerations This specification does not request the creation of any new parameter registries, not does it require any other IANA assignments 4.5. 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]. 5.6. References 188.8.131.52. Normative References [RFC791] Postel, J., "Internet Protocol", RFC 791, USC/Information Sciences Institute, September 1981. [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-09.txt, AugustSeptember 2003. 184.108.40.206. Informative References [RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities", STD 13, RFC 1034, USC/Information Sciences Institute, November 1987. [RFC1035] Mockapetris, P., "Domain Names - Implementation and Specification", STD 13, RFC 1035, USC/Information Sciences Institute, November 1987. [RFC1058] Hedrick, C.L., "Routing Information Protocol", RFC 1058, Rutgers University, June 1, 1988. [RFC1332] McGregor, G., "PPP Internet Control Protocol", RFC 1332, Merit, May 1992. [RFC1661] Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, Daydreamer, July 1994. [RFC1877] Cobb, S., "PPP Internet Protocol Control Protocol Extensions for Name Server Addresses", RFC 1877, December 1995. [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. [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. [IEEE8023] ISO/IEC 8802-3 Information technology - Telecommunications and information exchange between systems - Local and metropolitan area networks - Common specifications - Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications, (also ANSI/IEEE Std 802.3- 1996), 1996. [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 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: email@example.com 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 wired networks, hints include link-layer discovery traffic as well as information exchanged as part of IEEE 802.1X authentication. Link-layer discovery traffic includes Link Layer Discovery Protocol [LLDP] traffic as well as network identification information passed in the EAP-Request/Identity or within an EAP method exchange. For example, LLDP advertisements can provide information on the IP address or VLANs supported by the device. These hints, if provided, are considered "strong"; all other hints are considered "weak". When used with IEEE 802.1X authentication, 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. 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 [Congdon], 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". Intellectual Property StatementThe 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. Full Copyright Statement Copyright (C) The Internet Society (2003). All Rights Reserved. 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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. 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-ipv4-00.txt>,<draft-ietf-dhc-dna-ipv4-01.txt>, and expires February 22, 2004.