Network Working Group B. Aboba, Ed. INTERNET-DRAFT Internet Architecture Board Category: Informational IAB <draft-iab-link-indications-01.txt> 10 January 2005 Architectural Implications of Link Indications 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 July 22, 2005. Copyright Notice Copyright (C) The Internet Society (2005). All Rights Reserved. Abstract This document provides an overview of the role of link indications within the Internet Architecture. While the judicious use of link indications can provide performance benefits, experience has also shown that that inapropriate use can degrade both robustness and performance. This document summarizes current proposals, describes the architectural issues and provides examples of appropriate and inappropriate uses of link layer indications. IAB Informational [Page 1]
INTERNET-DRAFT Link Indications 10 January 2005 Table of Contents 1. Introduction.............................................. 3 1.1 Requirements ....................................... 3 1.2 Terminology ........................................ 3 1.3 Link Indications ................................... 5 1.4 Proposals .......................................... 8 1.5 Layering ........................................... 10 2. Architectural considerations ............................. 14 2.1 Model Validation ................................... 14 2.2 Robustness ......................................... 17 2.3 Effectiveness ...................................... 20 2.4 Interoperability Issues ............................ 21 2.5 Race Conditions .................................... 22 2.6 Layer Compression .................................. 24 2.7 Transport of Link Indications ...................... 25 3. Future Work .............................................. 27 4. Security Considerations .................................. 28 5. References ............................................... 28 5.1 Informative References ............................. 28 Appendix A - IAB Members ..................................... 32 Intellectual Property Statement .............................. 33 Disclaimer of Validity ....................................... 33 Copyright Statement .......................................... 33 IAB Informational [Page 2]
INTERNET-DRAFT Link Indications 10 January 2005 1. Introduction As a performance optimization, proposals have been made for utilizing link indications (also known as "triggers" or "hints") to influence the behavior of the Internet, Transport or Application layers. This document provides an overview of the role of link indications within the Internet Architecture. While the judicious use of link indications can provide performance benefits, experience has also shown that that inapropriate use can degrade both robustness and performance. This document summarizes the current understanding of the role of link indications, and provides advice to document authors considering the role of link indications within their own work. In Section 1 of this document we present a brief overview of research on link behavior as well as proposals for utilization of link indications. Section 2 provides advice to document authors. Section 3 describes recommendations and future work. 1.1. Requirements 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 Access Point (AP) A station that provides access to the distribution services, via the wireless medium (WM) for associated stations. Association The service used to establish an access point/station (AP/STA) mapping and enable STA access to the Distribution System. Basic Service Set (BSS) A set of stations controlled by a single coordination function, where the coordination function may be centralized (e.g., in a single AP) or distributed (e.g., for an ad-hoc network). The BSS can be thought of as the coverage area of a single AP. Care of Address (CoA) A unicast routable address associated with a mobile node while visiting a foreign link; the subnet prefix of this IP address is a foreign subnet prefix. Among the multiple care-of addresses that a mobile node may have at any given time (e.g., with different subnet IAB Informational [Page 3]
INTERNET-DRAFT Link Indications 10 January 2005 prefixes), the one registered with the mobile node's home agent for a given home address is called its "primary" care-of address. Correspondent Node A peer node with which a mobile node is communicating. The correspondent node may be either mobile or stationary. Distribution System (DS) A system used to interconnect a set of basic service sets (BSSs) and integrated local area networks (LANs) to create an extended service set (ESS). Dynamic Host Configuration Protocol (DHCP) client A DHCP 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. Extended Service Set (ESS) A set of one or more interconnected basic service sets (BSSs) and integrated local area networks (LANs) that appears as a single BSS to the logical link control layer at any station associated with one of those BSSs. The ESS can be thought of as the coverage area provided by a collection of APs all interconnected by the Distribution System. It may consist of one or more IP subnets. Home Address (HoA) A unicast routable address assigned to a mobile node, used as the permanent address of the mobile node. This address is within the mobile node's home link. Standard IP routing mechanisms will deliver packets destined for a mobile node's home address to its home link. Mobile nodes can have multiple home addresses, for instance when there are multiple home prefixes on the home link. Inter-Access Point Protocol (IAPP) A protocol used between access points that assures that the station may only be connected to a single AP within the ESS at a time, and also provides for transfer of context to the new AP. Link A communication facility or medium over which nodes can communicate at the link layer, such as an Ethernet (simple or bridged). The link layer is the layer immediately below IP. Link indication Information provided by the link layer to higher layers relating to IAB Informational [Page 4]
INTERNET-DRAFT Link Indications 10 January 2005 the state of the link. Mobile Node A node that can change its point of attachment from one link to another, while still being reachable via its home address. 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 [RFC3927]. This includes private addresses as specified in [RFC1918]. Station (STA) Any device that contains an IEEE 802.11 conformant medium access control (MAC) and physical layer (PHY) interface to the wireless medium (WM). Valid address The term "valid address" refers to either a static address, or a dynamically assigned address which has not been relinquished, and has not expired. Weak End-System Model In the Weak End-System Model, packets sent out an interface need not necessarily have a source address configured on that interface. 1.3. Link Indications A link indication represents information provided by the link layer to higher layers relating to the state of the link. While link indications vary considerably between media, abstraction models have been proposed. For example, [GenTrig] defines "generic triggers", including "Link Up", "Link Down", "Link Going Down", "Link Going Up", "Link Quality Crosses Threshold", "Trigger Rollback", and "Better Signal Quality AP Available". Other link indications include the current link rate (which may vary with time and location), link identifiers (e.g. SSID, BSSID in 802.11), and statistics relating to link performance (such as the delay or loss rate). "Link Up" and "Link Down" indications were first developed for wired IAB Informational [Page 5]
INTERNET-DRAFT Link Indications 10 January 2005 networks, and assume an idealized link behavior model. This model assumes that links in the "up" state experience low frame loss in both directions and are ready to send and receive data frames. Similarly it is assumed that links in the "down" state are unsuitable for sending and receiving data frames in either direction. Link indications based on signal quality, such as "Link Doing Down", "Link Going Up", and "Link Quality Crosses Threshold" are primarily intended for use in handoff optimization. These indications assume an idealized model of radio propagation, where signal strength varies smoothly and frame loss is well predicted by signal strength and distance. A number of link performance studies shed light on the applicability of these assumptions. For the purposes of illustration, we will focus on literature relating to IEEE 802.11. In "Measurement and Analysis of the Error Characteristics of an In- Building Wireless Network" [Eckhardt], the authors characterize the performance of an AT&T Wavelan 2 Mbps in-building WLAN operating in Infrastructure mode on the Carnegie-Mellon Campus. In this study, very low frame loss was experienced. As a result, links could either be assumed to operate very well or not at all. In "Performance of Multihop Wireless Networks: Shortest Path is Not Enough" [Shortest] the authors studied the performance of both an indoor and outdoor mesh network. By measuring inter-node throughput, the best path between nodes was computed. The throughput of the best path was compared with the throughput of the shortest path computed based on a hop-count metric. In almost all cases, the shortest path route offered considerably lower throughput than the best path. In examining link behavior, the authors found that rather than exhibiting a bi-modal distribution between "up" (low loss rate) and "down" (high loss rates), many links exhibited intermediate loss rates. Asymmetry was also common, with 30 percent of links demonstrating substantial differences between in the loss rates in each direction. As a result, on wireless networks the measured throughput can differ substantially from the negotiated rate due to retransmissions, and successful delivery of routing packets is not necessarily an indication that the link is useful for delivery of data. "Link-level Measurements from an 802.11b Mesh Network" [Aguayo] analyzes the causes of frame loss in a 38-node urban multi-hop 802.11 ad-hoc network. In most cases, links that are very bad in one direction tend to be bad in both directions, and links that are very good in one direction tend to be good in both directions. However, IAB Informational [Page 6]
INTERNET-DRAFT Link Indications 10 January 2005 30 percent of links exhibited loss rates differing substantially in each direction. Signal to noise ratio and distance showed little value in predicting loss rates, and rather than exhibiting a step-function transition between "up" (low loss) or "down" (high loss) states, inter-node loss rates varied widely, demonstrating a nearly uniform distribution over the range at the lower rates. The authors attribute the observed effects to multi-path fading, rather than attenuation or interference. The findings of [Eckhardt] and [Aguayo] demonstrate the diversity of loss conditions observed in practice. There is a fundamental difference between indoor infrastructure networks in which site surveys and careful measurement can assist in promoting ideal behavior and ad-hoc/mesh networks in which node mobility and external factors such as weather may not be easily controlled. In "The mistaken axioms of wireless-network research" [Kotz], the authors conclude that mistaken assumptions relating to link performance may lead to the design of network protocols that may not work in practice. For example, [Kotz] notes that the three- dimensional nature of wireless propagation can result in large signal strength changes over short distances, generating short-lived "Link Down" and "Link Up" indications that are not be predicted by a two dimensional radio propagation model. The literature also describes variations in link indication behavior between implementations. "Techniques to reduce IEEE 802.11b MAC layer handover time" [Velayos] measured handover times for a stationary STA after the AP was turned off. This study divided handover times into detection (determination of disconnection from the existing point of attachment) search (discovery of alternative attachment points), and execution phases (connection to an alternative point of attachment). These measurements indicated that the duration of the detection phase (the largest component of handoff delay) is determined by the number of non-acknowledged frames triggering the search phase and delays due to precursors such as RTS/CTS and rate adaptation. Detection behavior varied widely between implementations. For example, NICs designed for desktops attempted more retransmissions prior to triggering search as compared with laptop designs, since they assumed that the AP was always in range, regardless of whether the Beacon was received. The study recommends that the duration of the detection phase be IAB Informational [Page 7]
INTERNET-DRAFT Link Indications 10 January 2005 reduced by initiating the search phase as soon as collisions can be excluded as the cause of non-acknowledged transmissions; the authors recommend three consecutive transmission failures as the cutoff. Where the STA is not sending or receiving frames, it is recommended that Beacon reception be tracked in order to detect disconnection, and that Beacon spacing be reduced to 60 ms in order to reduce detection times. In order to compensate for more frequent triggering of the search phase, the authors recommend algorithms for wait time reduction, as well as interleaving of search and data frame transmission. "An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process" [Mishra] investigates handoff latencies obtained with three mobile STAs implementations communicating with two APs. The study found that there is large variation in handoff latency among STA and AP implementations and that implementations utilize different message sequences. For example, one STA sends a Reassociation Request prior to authentication, which results in receipt of a Deauthenticate message. The study divided handoff latency into discovery, authentication and reassociation exchanges, concluding that the discovery phase was the dominant component of handoff delay. Latency in the detection phase was not investigated. "Roaming Interval Measurements" [Alimian] presents data on stationary STAs after the AP signal has been shut off. This study highlighted implementation differences in rate adaptation as well as detection, scanning and handoff. As in [Velayos], performance varied widely between implementations, from half an order of magnitude variation in rate adaptation to an order of magnitude difference in detection times, two orders of magnitude in scanning, and one and a half orders of magnitude in handoff times. "An experimental study of IEEE 802.11b handoff performance and its effect on voice traffic" [Vatn] describes handover behavior observed when the signal from AP is gradually attenuated, which is more representative of field experience than the shutoff techniques used in [Velayos]. Stations were configured to initiate handover when signal strength dipped below a threshold, rather than purely based on frame loss, so that they could begin handover while still connected to the current AP. It was noted that stations continue to receive data frames during the search phase. Station-initiated Disassociation and pre-authentication were not observed in this study. 1.4. Proposals Within the Internet layer, proposals have been made for utilizing link indications to optimize IP configuration, to improve the IAB Informational [Page 8]
INTERNET-DRAFT Link Indications 10 January 2005 usefulness of routing metrics, and to optimize aspects of Mobile IP handoff. In "Detection of Network Attachment (DNA) in IPv4" [DNAv4], link indications are utilized to optimize Internet layer configuration. This enables a host that has moved to a new point of attachment but remained within the same subnet to rapidly confirm a currently valid configuration, rather than utilizing the DHCP protocol [RFC2131]. "A High-Throughput Path Metric for Multi-Hop Wireless Routing" [ETX] describes how routing metrics can be improved by taking link layer frame loss rates into account, enabling the selection of routes maximizing available throughput. While the proposed routing metric utilizes the Expected Transmission Count (ETX), it does not take the negotiated rate into account, although this was noted as a subject for further study. In "L2 Triggers Optimized Mobile IPv6 Vertical Handover: The 802.11/GPRS Example" [Park] the authors propose that the mobile node send a router solicitation on receipt of a "Link Up" indication in order provide lower handoff latency than would be possible using generic movement detection [RFC3775]. The authors also suggest immediate invalidation of the Care-Of-Address (CoA) on receipt of a "Link Down" indication. Within the Transport layer, proposals have focused on countering the effects of handoff-induced packet loss. This includes proposals for improving transport parameter estimation, as well as triggering immediate retransmission on availability of an interface or intervening link. "Framework and Requirements for TRIGTRAN" [TRIGTRAN] discusses optimizations to recover earlier from a retransmission timeout incurred during a period in which an interface or intervening link was down. "End-to-end, Implicit 'Link-Up' Notification" [E2ELinkup] describes methods by which a TCP implementation that has backed off its retransmission timer due to frame loss on a remote link can learn that the link has once again become operational. This enables retransmission to be attempted prior to expiration of the backed off retransmission timer. "Link-layer Triggers Protocol" [Yegin] describes transport issues arising from lack of host awareness of link conditions on downstream Access Points and routers. Transport of link layer triggers is proposed to address the issue. In "TCP Extensions for Immediate Retransmissions" [Eggert], it is proposed that in addition to regularly scheduled retransmissions that IAB Informational [Page 9]
INTERNET-DRAFT Link Indications 10 January 2005 retransmission be attempted by the Transport layer on receipt of an indication that connectivity to a peer node may have been restored. End-to-end connectivity restoration indications include "Link Up", confirmation of first-hop router reachability, confirmation of Internet layer configuration, and receipt of other traffic from the peer. In "The BU-trigger method for improving TCP performance over Mobile IPv6" [Kim], the authors note that handoff-related packet loss is interpreted as congestion by the Transport layer. In the case where the correspondent node is sending to the mobile node, it is proposed that receipt of a Binding Update by the correspondent node be used as a signal to the Transport layer to adjust cwnd and ssthresh values, which may have been reduced due to handoff-induced packet loss. The authors recommend that cwnd and ssthresh be recovered to pre-timeout values, regardless of whether the link parameters have changed. The paper does not discuss the behavior of a mobile node sending a Binding Update, in the case where the mobile node is sending to the correspondent node. At the Application layer, the usage of "Link Down" indications has been proposed to augment presence systems. In such systems, client devices periodically refresh their presence state using application layer protocols such as SIMPLE [RFC3428] or XMPP [RFC3921]. If the client should become disconnected, their unavailability will not be detected until the presence status times out, which can take many minutes. However, if a link goes down, and a disconnect indication can be sent to the presence server (presumably by the access point, which remains connected), the status of the user's communication application can be updated nearly instantaneously. 1.5. Layering A layered indication model is shown in Figure 1 which includes both internally generated link indications and indications arising from external interactions (such as receipt of Mobile IP Binding Updates, and detection of path changes via routing protocols and TTL changes). In this model, link indications include frame loss (before retransmissions), the current link rate, the link state (up/down), and link identifiers. The indications are inter-dependent, since rate adjustment and detection algorithms are typically influenced by frame loss, and in turn a "Link Down" indication may be influenced by the detection and search process. Link Identifiers are typically obtained in the process of bringing the link up. The Internet layer is the primary consumer of link indications, since one of its functions is to shield applications from the specifics of IAB Informational [Page 10]
INTERNET-DRAFT Link Indications 10 January 2005 link behavior. The Internet layer may utilize link indications to optimize aspects of IP configuration, routing and mobility. As noted in [DNAv4], "Link Up" indications and link identifiers may be useful in validating the IP configuration. Once the IP configuration is confirmed, it may be determined that an address change has occurred. As described in [ETX], the frame loss rate as well as the current link rate may be utilized in the calculation of routing metrics. Within "Weak End-System Model" implementations, changes in routing metrics may in turn result in a change in the outgoing interface for one or more transport connections. Routes may also be added or withdrawn, resulting in loss or gain of peer connectivity. The Internet layer may also become aware of path changes by other mechanisms, such as by running a routing protocol, receipt of a Router Advertisement or a change in the IP TTL of received packets. A change in the outgoing interface may in turn influence the mobility sub-layer, causing a change in the incoming interface. The mobility sub-layer may also become aware of a change in the incoming interface of a peer (via receipt of a Mobile IP binding update). However, "Link Up" indications need not result in a change to Internet layer configuration, and changes in link rate or frame loss need not result in a change of outgoing interface. By filtering "Link Up" indications, and selecting outgoing and incoming interfaces based on the link rate and frame loss, the Internet layer enables upper layers to avoid writing their own code to filter and validate link indications. The Transport layer processes Internet layer and link indications differently for the purposes of transport parameter estimation and connection management. For the purposes of parameter estimation, the Transport layer may be interested in a wide range of Internet and link layer indications. The Transport layer may wish to use path change indications from the Internet layer in order to rest parameter estimates. It may also be useful for the Transport layer to consume link layer indications such as link rate, frame loss rate and "Link Up"/"Link Down" in order to improve transport parameter estimates. However at this point, the algorithms for improving transport parameter estimates using link layer indications are not well understood. For example, in transport parameter estimation, layering considerations may not exist to the same extent as in connection management. For example, the Internet layer may receive a "Link Down" indication followed by a subsequent "Link Up" indication. This information may useful for transport parameter estimation even if IP configuration does not change, since it may indicate that packet loss is not caused by congestion. IAB Informational [Page 11]
INTERNET-DRAFT Link Indications 10 January 2005 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Application | | Layer | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ ^ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | + | | | | | ^ ^ | Transport | Transport Parameter + | Teardown | Layer | Estimation | | | | (MTU, RTT, RTO, cwnd, + Conxn.| Management| | ssthresh, Reset) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ^ ^ ^ ^ ^ | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Incoming | MIP | | | | | | Interface | BU | | | | | | Change |Receipt| | | | | ^ ^ ^ ^ | | | | | | | | | | | | | | | | | | Mobility | | | | Internet | | | | | | | Layer +-+- -+- - - - - -+- -+- -+- - - - -+- - - - - -+ | | | Outgoing | | | | IP | | | | Interface | + | | Address | | | ^ Change ^ | ^ ^ Config/ | | | Path + | Changes | | | Change | | | | | Routing + | IP Configuration | | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ^ ^ ^ | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | | | Link | V ^ ^ ^ | Layer + Frame -> Rate -> Link Link + | Loss Adjustment Up/Down Identifiers | | Rate | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 1. Layered Indication Model IAB Informational [Page 12]
INTERNET-DRAFT Link Indications 10 January 2005 For the purposes of connection management, the Transport layer typically only utilizes Internet layer indications such as changes in the incoming/outgoing interface and IP configuration changes. For example, the Transport layer may tear down transport connections due to invalidation of a connection endpoint IP address. However, even where an Internet layer configuration change has occurred, the configuration change may not be relevant for the purposes of connection management. For example, where the connection has been established based on the home address, a change in the care- of-address need not result in connection teardown, since the configuration change is masked by the mobility functionality within the Internet layer, and is therefore transparent to the Transport layer. Since a "Link Up" indication may or may not result in a change in Internet layer configuration, the Transport layer cannot draw conclusions about the implications of "Link Up" for connection management until the Internet layer has determined whether a configuration change has occurred. Similarly, the Transport layer does not tear down connections on receipt of a "Link Down" indication, regardless of the cause. Where the "Link Down" indication results from frame loss rather than an explicit exchange, the indication may be transient, to be soon followed by a "Link Up" indication. Even where the "Link Down" indication results from an explicit exchange such as receipt of a PPP LCP-Terminate or an 802.11 Disassociate or Deauthenticate frame, an alternative point of attachment may be available, allowing connectivity to be quickly restored. As a result, robustness is best achieved by allowing connections to remain up until an endpoint address changes, or the connection is torn down due to lack of response to repeated retransmission attempts. In addition to Internet layer indications propagated to the Application layer (such as IP address configuration and changes), the Transport layer provides its own indications to the Application layer, such as connection teardown. The Transport layer may also provide indications to the link layer. For example, to prevent excessive retransmissions within the link layer, the Transport layer may wish to control the maximum number of times that a link layer frame may be retransmitted, so that the link layer does not continue to retransmit after a Transport layer timeout. In 802.11, this can be achieved by adjusting the MIB variables dot11ShortRetryLimit (default: 7) and dot11LongRetryLimit (default: 4), which control the maximum number of retries for frames shorter and longer in length IAB Informational [Page 13]
INTERNET-DRAFT Link Indications 10 January 2005 than dot11RTSThreshold, respectively. In most cases applications can obtain the information they need from Internet and Transport layer indications so that they do not need to directly consume link indications. For example, a "Link Up" indication typically only implies that the link was suitable for sending and receiving link layer control frames, not that it has been configured for and is capable of reliably sending and receiving IP data packets. As a result, applications will typically consume an Internet layer "IP Address Configured" event instead of a "Link Up" indication. Similarly, it is typically not useful for applications to consume "Link Down" indications, since these indications can be transient in nature. Instead, applications should consume Transport layer teardown indications. 2. Architectural considerations While the literature on the usage of Link indications provides persuasive evidence of their utility, experience shows that a number of difficulties can arise in making effective use of them. These issues include: a. Model validation b. Robustness c. Effectiveness d. Interoperability e. Race conditions f. Layer compression g. Transport of link indications The sections that follow discuss each of these issues in turn. 2.1. Model Validation Authors need to be careful to avoid use of simplified link models in circumstances where they do not apply. In order to avoid the pitfalls described in [Kotz], documents dependent on link indications should explicitly articulate the assumptions of the link model and describe the circumstances in which it applies. For example, generic "trigger" models often include implicit assumptions. The use of "Link Up" and "Link Down" indications implies that a link is either in a state experiencing low frame loss ("up") or in a state where few frames are successfully delivered ("down"). Symmetry may also be assumed, so that the link is either "up" in both directions or "down" in both directions. Link indications based on signal quality, such as "Link Doing Down", IAB Informational [Page 14]
INTERNET-DRAFT Link Indications 10 January 2005 "Link Going Up", and "Link Quality Crosses Threshold" typically assume the absence of multi-path interference, so that signal to noise ratio varies smoothly in space, and frame loss is well predicted by signal strength and distance. In wireless networks, particularly in outdoor or mesh deployments, the assumptions underlying generic trigger models may prove invalid. Where links may exist in intermediate states between "up" and "down" or asymmetry is encountered, generic "triggers" such as "Link Going Down", "Link Going Up", and "Link Quality Crosses Threshold" may be difficult to reliably define and may be unreliable predictors of future link performance. Once the network model is defined, considerable effort may be required to define the link indications model for a given link layer. For example, the definition of "Link Up" or "Link Down" varies considerably between link layers. Within PPP [RFC1661], either peer may send an LCP-Terminate frame in order to terminate the PPP link layer, and a link may only be assumed to be usable for sending network protocol packets once NCP negotiation has completed for that protocol. Unlike PPP, IEEE 802 does not include facilities for network layer configuration, and the definition of "Link Up" and "Link Down" varies between and even within Link types. For example, in IEEE 802.11, the definition of "Link Up" and "Link Down" depends on whether the station is mobile or stationary, whether infrastructure or ad-hoc mode is in use, and whether security and Inter-Access Point Protocol (IAPP) is implemented. Where a mobile 802.11 STA encounters a series of consecutive non- acknowledged frames, the most likely cause is that the station has moved out of range of the AP. As a result, [Velayos] recommends that the station begin the search phase after collisions can be ruled out, after three consecutive non-acknowledged frames. Only when no alternative point of attachment is found is a "Link Down" indication returned. In a stationary point-to-point installation, the most likely cause of an outage is that the link has become impaired, and alternative points of attachment may not be available. As a result, implementations configured to operate in this mode tend to be more persistent. For example, within 802.11 the short interframe space (SIFS) interval may be increased and MIB variables relating to timeouts (such as dot11AuthenticationResponseTimeout, dot11AssociationResponseTimeout, dot11ShortRetryLimit, and dot11LongRetryLimit) may be set to larger values. In addition a IAB Informational [Page 15]
INTERNET-DRAFT Link Indications 10 January 2005 "Link Down" indication may be returned later. In 802.11 ad-hoc mode with no security, reception of data frames is enabled in State 1 ("Unauthenticated" and "Unassociated"). As a result, reception of data frames is enabled at any time, and no explicit "Link Up" indication exists. In Infrastructure mode, IEEE 802.11-2003 enables reception of data frames only in State 3 ("Authenticated" and "Associated"). As a result, a transition to State 3 (e.g. completion of a successful Association or Reassociation exchange) enables sending and receiving of network protocol packets and a transition from State 3 to State 2 (reception of a "Disassociate" frame) or State 1 (reception of a "Deauthenticate" frame) disables sending and receiving of network protocol packets. As a result, IEEE 802.11 stations typically signal "Link Up" on receipt of a successful Association/Reassociation Response. As described within [IEEE80211F], after sending a Reassociation Response, an Access Point will send a frame with the station's source address to a multicast destination. This causes switches within the Distribution System (DS) to update their learning tables, readying the DS to forward frames to the station at its new point of attachment. Were the AP to not send this "spoofed" frame, the station's location would not be updated within the distribution system until it sends its first frame at the new location. Thus the purpose of spoofing is to equalize uplink and downlink handover times. This enables an attacker to deny service to authenticated and associated stations by spoofing a Reassociation Request using the victim's MAC address, from anywhere within the ESS. Without spoofing, such an attack would only be able to disassociate stations on the AP to which the Reassociation Request was sent. The signaling of "Link Down" is considerably more complex. Even though a transition to State 2 or State 1 results in the station being unable to send or receive IP packets, this does not necessarily imply that such a transition should be considered a "Link Down" indication. In an infrastructure network, a station may have a choice of multiple access points offering connection to the same network. In such an environment, a station that is unable to reach State 3 with one access point may instead choose to attach to another access point. Rather than registering a "Link Down" indication with each move, the station may instead register a series of "Link Up" indications. In [IEEE80211i] forwarding of frames from the station to the distribution system is only feasible after the completion of the 4-way handshake and group-key handshake, so that entering State 3 is IAB Informational [Page 16]
INTERNET-DRAFT Link Indications 10 January 2005 no longer sufficient. This has resulted in several observed problems. For example, where a "Link Up" indication is triggered on the station by receipt of an Association/Reassociation Response, DHCP [RFC2131] or RS/RA may be triggered prior to when the link is usable by the Internet layer, resulting in configuration delays or failures. Similarly, Transport layer connections will encounter packet loss, resulting in back-off of retransmission timers. 2.2. Robustness In some situations, improper use of Link indications can result in operational malfunctions. Given the potential problems, proposals for consideration of link indications must demonstrate robustness against misleading indications. Elements to consider include: a. Indication validation b. Recovery from invalid indications c. Damping and hysteresis 2.2.1. Indication Validation Radio propagation and implementation differences can impact the reliability of Link indications. As described in [Aguayo], wireless links often exhibit loss rates intermediate between "up" (low loss) and "down" (high loss) states, as well as substantial asymmetry. In these circumstances, a "Link Up" indication may not imply bi-directional reachability. Also, a reachability demonstration based on small packets may not mean that the link is suitable for carrying larger data packets. As a result, "Link Up" and "Link Down" indications may not reliably determine whether a link is suitable for carrying IP data packets. Where multi-path interference or hidden nodes are encountered, frame loss may vary widely over a short distance. While techniques such as use of multiple antennas may be used to reduce multi-path effects and RTS/CTS signaling can be used to address hidden node problems, these techniques may not be completely effective. As a result, a mobile host may find itself experiencing widely varying link conditions, causing the link to rapidly cycle between "up" and "down" states, with "Going down" or "Going up" indications providing little predictive value. Where the reliability of a link layer indication is suspect, it is best for upper layers to treat the indication as a "hint" (advisory in nature), rather than a "trigger" forcing a given action. In order to provide increased robustness, heuristics can be developed to assist upper layers in determining whether the "hint" is valid or IAB Informational [Page 17]
INTERNET-DRAFT Link Indications 10 January 2005 should be discarded. To provide robustness in the face of potentially misleading link indications, in [DNAv4] "Link Up" indications are assumed to be inherently unreliable, so that bi-directional reachability needs to be demonstrated in the process of validating an existing IP configuration. However, where a link exhibits an intermediate loss rate, the success of the [DNAv4] reachability test does not guarantee that the link is suitable for carrying IP data packets. Another example of link indication validation occurs occurs in IPv4 Link-Local address configuration [RFC3927]. Prior to configuration of an IPv4 Link-Local address, it is necessary to run a claim and defend protocol. Since a host needs to be present to defend its address against another claimant, and address conflicts are relatively likely, a host returning from sleep mode or receiving a "Link Up" indication could encounter an address conflict were it to utilize a formerly configured IPv4 Link-Local address without rerunning claim and defend. 2.2.2. Recovery From Invalid Indications Upper layers should utilize a timely recovery step so as to limit the potential damage from link indications determined to be invalid after they have been acted on. Recovery is supported within [DNAv4] in the case where link indications may lead a host to erroneously conclude that the link prefix remains unchanged when the host has in fact changed subnets. In this case, the bi-directional reachability test times out, and the host will eventually realize its mistake and obtain an IP address by normal means. Where a proposal involves recovery at the transport layer, the recovered transport parameters (such as the MTU, RTT, RTO, congestion window, etc.) must be demonstrated to remain valid. Congestion window validation is discussed in [RFC2861]. Where timely recovery is not supported, unexpected consequences may result. As described in [RFC3927], early IPv4 Link-Local implementations would wait five minutes before attempting to obtain a routable address after assigning an IPv4 Link-Local address. In one implementation, it was observed that where mobile hosts changed their point of attachment more frequently than every five minutes, they would never obtain a routable address. The problem was caused by an invalid link indication (signalling of "Link Up" prior to completion of link layer authentication), IAB Informational [Page 18]
INTERNET-DRAFT Link Indications 10 January 2005 resulting in an initial failure to obtain a routable address using DHCP. As a result, [RFC3927] recommends against modification of the maximum retransmission timeout (64 seconds) provided in [RFC2131]. 2.2.3. Damping and Hysteresis Damping and hysteresis can be utilized to ensure that stability is maintained in the face of jittery link indications. These limits typically place constraints on the number of times a given action can be performed within a time period or introduce damping mechanisms to prevent instability. While [Aguayo] found that frame loss was relatively stable for stationary stations, obstacles to radio propagation and multi-path interference can result in rapid changes in signal strength for a mobile station. As a result, it is possible for mobile stations to encounter rapid changes in link performance, including changes in the negotiated rate, frame loss and even "Link Up"/"Link Down" indications. Where link-aware routing metrics are implemented, this can result in rapid metric changes, potentially resulting in frequent changes in the outgoing interface for "Weak End-System" implementations. As a result, it may be necessary to introduce route flap dampening. However, the benefits of damping need to be weighed against the additional latency that can be introduced. For example, in order to filter out spurious "Link Down" indications, these indications may be delayed until it can be determined that a "Link Up" indication will not follow shortly thereafter. However, in situations where multiple Beacons are missed such a delay may not be needed, since there is no evidence of a suitable point of attachment in the vicinity. In some cases, it may be desirable to ignore link indications entirely. Since it is possible for a host to transition from an ad- hoc network to a network with centralized address management, a host receiving a "Link Up" indication cannot necessarily conclude that it is appropriate to configure a IPv4 Link-Local address prior to determining whether a DHCP server is available [RFC3927]. As noted in Section 1.5, the Transport layer does not utilize "Link Up" and "Link Down" indications for the purposes of connection management. In most cases applications can obtain the information they need from Internet and Transport layer indications so that they do not need to directly consume link indications. Where link indications are used to optimize transport performance, authors must demonstrate that effective congestion control is IAB Informational [Page 19]
INTERNET-DRAFT Link Indications 10 January 2005 maintained [RFC2914] in the face of rapidly changing link indications. Consider a proposal where a "Link Up" indication is used by a router to trigger retransmission of the last previously sent packet, in order to enable ACK reception prior to expiration of the host's retransmission timer. Where "Link Up" indications follow in rapid succession, this could result in a burst of retransmitted packets, violating the principle of "conservation of packets". At the Application Layer, Link indications have been utilized by applications such as Presence [RFC2778] in order to optimize registration and user interface update operations. For example, implementations may attempt presence registration on receipt of a "Link Up" indication, and presence de-registration by a surrogate receiving a "Link Down" indication. Presence implementations using "Link Up"/"Link Down" indications this way violate the principle of "conservation of packets" when link indications are generated on a time scale of RTO or less. The problem is magnified since for each presence update, notifications can be delivered to many watchers. In addition, use of a "Link Up" indication in this manner is unwise since the interface may not yet have a valid Internet layer configuration. The issue can be addressed by one or more of the following techniques: [a] Rate limiting. A limit of one packet per RTO can be imposed on packets generated from receipt of link indications. [b] Utilization of upper layer indications. Instead of consuming a "Link Up" indication, applications can consume alternative upper layer indications such as an IP address configuration/change notifications. [c] Keepalives. Instead of consuming a "Link Down" indication, an application can utilize an application keepalive or consume Transport layer indications such as connection teardown. 2.3. Effectiveness While link indications may show promise, it may be difficult to prove that processing of a given indication provides benefits in a wide variety of circumstances. Where link indications are utilized for the purpose of optimization, proposals need to carefully analyze the effectiveness of the optimizations in the face of unreliable link indications. Since optimizations typically bring with them increased IAB Informational [Page 20]
INTERNET-DRAFT Link Indications 10 January 2005 complexity, an optimization that does not bring about a performance improvement is not useful. As with any optimization, the usefulness of link indications lies in demonstrated effectiveness of the optimization under consideration. This in turn may depend heavily on the penalty to be paid for false positives and false negatives. As noted in [DNAv4], it is simultaneously possible for a link indication to be highly reliable and provide no net benefit, depending on the probability of a false indication and the penalty paid for the false indication. In the case of [DNAv4], the benefits of successful optimization are modest, but the penalty for falsely concluding that the subnet remains unchanged is a lengthy timeout. The result is that link indications may not be worth considering if they are incorrect even just a small fraction of the time. For example, it can be argued that a change in the Service Set Identifier (SSID) in [IEEE80211] is not a sufficiently reliable indication of a prefix change. Within IEEE 802.11, the Service Set Identifier (SSID) functions as a non-unique identifier of the administrative domain of a Wireless LAN. Since the SSID is non- unique, many different operators may share the same SSID, and Access Points typically ship with a default value for the SSID (e.g. "default"). Since the SSID relates to the administrative domain and not the network topology, multiple SSIDs may provide access to the same prefix, and a single SSID may provide access to multiple prefixes at one or multiple locations. Given this, it is unreliable to use the SSID alone for the purpose of movement detection. A host moving from one point of attachment to another, both with the same SSID, may have remained within the same subnet, or may have changed subnets. Similarly, a host discovering that the SSID has changed may have changed subnets, or it may not have. Moreover, where private address space is in use, it is possible for the SSID, the prefix (e.g. 192.168/16) and even the default gateway IP address to remain unchanged, yet for the host to have moved to a different point of attachment. Were the host to make decisions relating to configuration of the IP layer (such as address assignment) based solely on the SSID, address conflicts are likely. 2.4. Interoperability In general, link indications should only be incorporated by upper layers for performance optimization, but should not be required, in order to main link independence. IAB Informational [Page 21]
INTERNET-DRAFT Link Indications 10 January 2005 To avoid compromising interoperability in the pursuit of performance optimization, proposals must demonstrate that interoperability remains possible (though potentially with degraded performance) even if one or more participants do not implement the proposal. For example, if link layer prefix hints are provided as a substitute for Internet layer configuration, hosts not understanding those hints would be unable to obtain an IP address. Where link indications are proposed to optimize Internet layer configuration, proposals must demonstrate that they do not compromise robustness by interfering with address assignment or routing protocol behavior, making address collisions more likely, or compromising Duplicate Address Detection (DAD). 2.5. Race Conditions It is possible for link indications to be utilized directly by multiple layers of the stack in situations in which strict layering may not be observed. In these situations, it is possible for race conditions to occur. For example, as discussed earlier, link indications have been shown to be useful in optimizing aspects of Internet Protocol layer addressing and configuration as well as routing. Although [Kim] describes situations in which link indications are first processed by the Internet Protocol layer (e.g. MIPv6) before being consumed by the Transport layer, for the purposes of parameter estimation, it may be desirable for the Transport layer to consume link indications directly. For example, in situations where the "Weak End-System Model" is implemented, a change of outgoing interface may occur at the same time the Transport layer is modifying transport parameters based on other link indications. As a result, transport behavior may differ depending on the order in which the link indications are processed. Where a multi-homed host experiences increasing frame loss on one of its interfaces, a routing metric taking frame loss into account will rise, potentially causing a change in the outgoing interface for one or more transport connections. This may trigger Mobile IP signaling so as to cause a change in the incoming path as well. As a result, the transport parameters for the original interface (MTU, congestion state) may no longer be valid for the new outgoing and incoming paths. To avoid race conditions, the following measures are recommended: IAB Informational [Page 22]
INTERNET-DRAFT Link Indications 10 January 2005 a. Path change processing b. Layering c. Metric consistency 2.5.1. Path Change Processing When the Internet layer detects a path change, such as a change in the outgoing or incoming interface of the host or the incoming interface of a peer, or perhaps a substantial change in the TTL of received IP packets, it may be worth considering whether to reset transport parameters to their initial values and allow them to be re- estimated. This ensures that estimates based on the former path do not persist after they have become invalid. 2.5.2. Layering Another technique to avoid race conditions is to rely on layering to damp transient link indications and provide greater link layer independence. The Internet layer is responsible for routing as well as IP configuration, and mobility, providing higher layers with an abstraction that is independent of link layer technologies. Since one of the major objectives of the Internet layer is maintaining link layer independence, upper layers relying on Internet layer indications rather than consuming link indications directly can avoid link layer dependencies. As described in Section 1.5, it is advisable for applications to utilize indications from the Internet or Transport layers rather than consuming link indications directly. 2.5.3. Metric Consistency Once a link is in the "up" state, its effectiveness in transmission of data packets can be determined. For example, frame loss may be used to assist in rate adjustment and to determine when to select an alternative point of attachment. Also, the effective throughput depends on the negotiated rate and frame loss, and can be used in calculation of the routing metric, as described in [ETX]. However, prior to sending data packets over the link, other metrics are required to determine suitability. As noted in [Shortest], a link that can successfully transmit the short frames utilized for control, management or routing may not necessarily be able to reliably transport data packets. Since the negotiated rate and frame loss typically cannot be IAB Informational [Page 23]
INTERNET-DRAFT Link Indications 10 January 2005 predicted prior to utilizing the link for data traffic, existing implementations often utilize metrics such as signal strength and access point load in handoff decisions. The "Link Going Down", "Link Going Up", "Link Quality Crosses Threshold" indications were developed primarily to assist with handoff between interfaces, and are oriented toward inferred rather than measured suitability. Research indicates that this approach may have some promise. In order to enable stations to roam prior to encountering packet loss, studies such as [Vatn] have suggested using signal strength as a detection mechanism, rather than frame loss, as suggested in [Velayos]. [Vertical] proposes use of signal strength and link utilization in order to optimize vertical handoff and demonstrates improved TCP throughput. However, without careful design, potential differences between link indications used in routing and those used in roaming and/or link enablement can result in instability, particularly in multi-homed hosts. For example, receipt of "Link Going Down" or "Link Quality Crosses Threshold" indications could be used as a signal to enable another interface. However, unless the new interface is the preferred route for one or more destination prefixes, a "Weak End- System" implementation will not use the new interface for outgoing traffic. Where "idle timeout" functionality is implemented, the unused interface will be brought down, only to be brought up again by the link enablement algorithm. As noted in [Aguayo], signal strength and distance are not good predictors of frame loss or negotiated rate, due to the potential effects of multi-path interference. As a result a link brought up due to good signal strength may subsequently exhibit significant frame loss, and a low negotiated rate. Similarly, an AP demonstrating low utilization may not necessarily be the best choice, since utilization may be low due to hardware or software problems. As noted in [Villamizar], link utilization-based routing metrics have a history of instability, so that they are rarely deployed. 2.6. Layer compression In many situations, the exchanges required for a host to complete a handoff and reestablish connectivity are considerable. This includes link layer scanning, authentication and connectivity establishment; Internet layer configuration, routing and mobility exchanges; Transport layer retransmission and recovery; security association re- establishment; application protocol re-authentication and re- registration exchanges, etc. Given this, it is natural to consider combining exchanges occurring within multiple layers into a single exchange. IAB Informational [Page 24]
INTERNET-DRAFT Link Indications 10 January 2005 Often this combined exchange occurs within the link layer. For example, in [EAPIKEv2], a link layer EAP exchange may be used for the purpose of IP address assignment, potentially bypassing Internet layer configuration. Within [PEAP], it is proposed that a link layer EAP exchange be used for the purpose of carrying Mobile IPv6 Binding Updates. [MIPEAP] proposes that EAP exchanges be used for configuration of Mobile IPv6. While the goals of layer compression are laudable, care needs to be taken to avoid compromising interoperability and introducing link layer dependencies into the Internet and Transport layers. For example, where link layer and Internet or Transport layer mechanisms are combined, it is necessary for hosts to maintain the ability to interoperate without layer compression schemes, in order to permit operation on networks where they are not available. Layer compression schemes may also negatively impact robustness. For example, in order to optimize IP address assignment, it has been proposed that prefixes be advertised at the link layer, such as within the 802.11 Beacon and Probe Response frames. However, [IEEE8021X] enables the VLANID to be assigned dynamically, so that prefix(es) advertised within the Beacon and/or Probe Response may not correspond to the prefix(es) configured by the Internet layer after the host completes link layer authentication. Were the host to handle IP configuration at the link layer rather than within the Internet layer, the host might be unable to communicate due to assignment of the wrong IP address. 2.7. Transport of Link Indications Proposals including the transport of link indications beyond the local host need to carefully consider the layering, security and transport implications. In general, implicit signals are preferred to explicit transport of link indications since they add no new packets in times of network distress, operate more reliably in the presence of middle boxes such as NA(P)Ts, and are more likely to be backward compatible. While facilities such as ICMP "source quench" were originally provided at the Internet layer, these facilities have fallen into disuse due to their questionable value for the Transport layer. In general, the Transport layer is able to determine an appropriate (and conservative) response to congestion based on packet loss or explicit congestion notification, so that ICMP "source quench" indications are not needed, and in fact the sending of additional "source quench" packets during periods of congestion may be detrimental. IAB Informational [Page 25]
INTERNET-DRAFT Link Indications 10 January 2005 Where explicit signalling is required, existing facilities should be used rather than creating new ones. For example, "TCP Extensions for Immediate Retransmissions" [Eggert] describes how a Transport layer implementation may utilize existing "end-to-end connectivity restored" indications. For example, routing metrics incorporating link layer indications such as [ETX] enable hosts participating in the routing mesh to gain knowledge of path changes and remote link conditions. This can be accomplished securely if routing protocol security is implemented. When a link experiences frame loss, routing metrics incorporating frame loss increase, possibly resulting in selection of an alternate route. If the troubled link represents the only path to a prefix and the link experiences high frame loss ("down"), the route will be withdrawn or the metric will become infinite. Similarly, when the link becomes operational, the route will appear again. Proposals involving transport of link indications need to demonstrate the following: [a] Absence of alternatives. By default, alternatives not requiring explicit signalling are preferred. Where these solutions are shown to be inadequate, proposals must prove that existing explicit signalling mechanisms (such as path change processing and link-aware routing metrics) are inadequate. [b] Conservative behavior. Due to experience with ICMP "source quench", proposals must demonstrate that they do not violate conservation of packets. [c] Security. Proposals need to describe how security issues can be addressed. Where link indications are transported over the Internet, an attack can be launched without requiring access to the link. [d] Identifiers. When link indications are transported, it is generally for the purposes of saying something about Internet, Transport or Application layer operations at a remote element. These layers use different identifiers, and so it is necessary to match the link indication with relevant higher layer state. Therefore proposals need to demonstrate how the link indication can be mapped to the right higher layer state. For example, if a presence server is receiving remote indications of "Link Up"/"Link Down" status for a particular MAC address, the presence server will need to associate that MAC address with the identity of the user (pres:user@example.com) to whom that link status change is relevant. IAB Informational [Page 26]
INTERNET-DRAFT Link Indications 10 January 2005 3. Future Work While Figure 1 presents an overview of how link indications are consumed by the Internet, Transport and Application layers, further work is needed to investigate this in more detail. Since recent proposals such as [IEEE80211e] incorporate burst ACKs, the relationship between 802.11 link throughput and frame loss is growing more complex, which may necessitate the development of revised routing metrics, taking the more complex transmission behavior as well as the negotiated rate into account. At the Link and Internet layers, more work is needed to reconcile pre and post-connection metrics, such as reconciling metrics utilized in handoff (e.g. signal strength and link utilization) with link-aware routing metrics (e.g. frame loss and negotiated rate). At the Transport layer, more work is needed to understand how to react to Internet layer indications such as path changes. For example, in an early draft of [DCCP], a "Reset Congestion State" option was proposed in Section 4. This option was removed in part because the use conditions were not fully understood: An Half-Connection Receiver sends the Reset Congestion State option to its sender to force the sender to reset its congestion state -- that is, to "slow start", as if the connection were beginning again. ... The Reset Congestion State option is reserved for the very few cases when an endpoint knows that the congestion properties of a path have changed. Currently, this reduces to mobility: a DCCP endpoint on a mobile host MUST send Reset Congestion State to its peer after the mobile host changes address or path. It may also make sense for the Transport layer to adjust transport parameter estimates in response to "Link Up"/"Link Down" indications and frame loss. For example, it is unclear that the Transport layer should adjust transport parameters as though congestion were detected when loss is occurring in the link layer or a "Link Down" indication has been received. Finally, more work is needed to determine how link layers may utilize information from the Transport layer. For example, it is undesirable for a link layer to retransmit so aggressively that the link layer round-trip time approaches that of the end-to-end transport connection. IAB Informational [Page 27]
INTERNET-DRAFT Link Indications 10 January 2005 4. Security Considerations Since link indications are typically insecure, proposals incorporating them need to consider the potential security implications of spoofed or modified link indications, as well as potential denial of service attacks. This is particularly important in situations where insecure link indications are used as a substitute for secure mechanisms operating at a higher layer. For example, within [IEEE80211F], "Link Up" is considered to occur when an Access Point sends a Reassociation Response. At that point, the AP sends a frame with the station's source address to a multicast address, thereby causing switches within the Distribution System to learn the station's MAC address, enabling forwarding of frames to the station at the new point of attachment. Unfortunately, this does not take security into account, since the station is not capable of sending and receiving IP packets on the link until completion of the key exchange protocol defined in [IEEE80211i]. As a result, link indications as implemented in [IEEE80211F] enable an attacker to disassociate a station located anywhere within the ESS, by sending a Reassociation Request frame. Another example of the potential security implications of link indications occurs within DNAv4, where link indications are used for optimization of IP configuration, rather than using a secured configuration mechanism such as authenticated DHCP [RFC3118], thereby increasing vulnerability to spoofing. 5. References 5.1. Informative References [RFC1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, D. and E. Lear, "Address Allocation for Private Internets", RFC 1918, February 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997. [RFC2778] Day, M., Rosenberg, J. and H. Sugano, "A Model for Presence and Instant Messaging", RFC 2778, February 2000. IAB Informational [Page 28]
INTERNET-DRAFT Link Indications 10 January 2005 [RFC2861] Handley, M., Padhye, J. and S. Floyd, "TCP Congestion Window Validation", RFC 2861, June 2000. [RFC2914] Floyd, S., "Congestion Control Principles", RFC 2914, BCP 41, September 2000. [RFC3118] Droms, R. and B. Arbaugh, "Authentication for DHCP Messages", RFC 3118, June 2001. [RFC3428] Campbell, B., Rosenberg, J., Schulzrinne, H., Huitema, C. and D. Gurle, "Session Initiation Protocol (SIP) Extension for Instant Messaging", RFC 3428, December 2002. [RFC3775] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in IPv6", RFC 3775, June 2004. [RFC3921] Saint-Andre, P., "Extensible Messaging and Presence protocol (XMPP): Instant Messaging and Presence", RFC 3921, October 2004. [RFC3927] Cheshire, S., Aboba, B. and E. Guttman, "Dynamic Configuration of Link-Local IPv4 Addresses", RFC 3927, October 2004. [Alimian] Alimian, A., "Roaming Interval Measurements", 11-04-0378-00-roaming-intervals-measurements.ppt, IEEE 802.11 submission (work in progress), March 2004. [Aguayo] Aguayo, D., Bicket, J., Biswas, S., Judd, G. and R. Morris, "Link-level Measurements from an 802.11b Mesh Network", SIGCOMM '04, September 2004, Portland, Oregon. [DCCP] Kohler, E., Handley, M. and S. Floyd, "Datagram Congestion Control Protocol (DCCP)", Internet drafts (work in progress), draft-ietf-dccp-spec-08.txt, October 2004. [DNAv4] Aboba, B., "Detection of Network Attachment in IPv4", draft- ietf-dhc-dna-ipv4-09.txt, Internet draft (work in progress), October 2004. [E2ELinkup] Dawkins, S. and C. Williams, "End-to-end, Implicit 'Link-Up' Notification", draft-dawkins-trigtran-linkup-01.txt, Internet draft (work in progress), October 2003. [EAPIKEv2] Tschofenig, H., D. Kroeselberg and Y. Ohba, "EAP IKEv2 Method", draft-tschofenig-eap-ikev2-05.txt, Internet draft (work in progress), October 2004. IAB Informational [Page 29]
INTERNET-DRAFT Link Indications 10 January 2005 [Eckhardt] Eckhardt, D. and P. Steenkiste, "Measurement and Analysis of the Error Characteristics of an In-Building Wireless Network", SIGCOMM '96, August 1996, Stanford, CA. [Eggert] Eggert, L., Schuetz, S. and S. Schmid, "TCP Extensions for Immediate Retransmissions", draft-eggert-tcpm-tcp-retransmit- now-01.txt, Internet draft (work in progress), September 2004. [ETX] Douglas S. J. De Couto, Daniel Aguayo, John Bicket, and Robert Morris, "A High-Throughput Path Metric for Multi-Hop Wireless Routing", Proceedings of the 9th ACM International Conference on Mobile Computing and Networking (MobiCom '03), San Diego, California, September 2003. [GenTrig] Gupta, V. and D. Johnston, "A Generalized Model for Link Layer Triggers", submission to IEEE 802.21 (work in progress), March 2004, available at: http://www.ieee802.org/handoff/march04_meeting_docs/ Generalized_triggers-02.pdf [IEEE8021X] Institute of Electrical and Electronics Engineers, "Local and Metropolitan Area Networks: Port-Based Network Access Control", IEEE Standard 802.1X, December 2004. [IEEE80211] Institute of Electrical and Electronics Engineers, "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications", IEEE Standard 802.11, 2003. [IEEE80211e] Institute of Electrical and Electronics Engineers, "Draft Amendment 7: Medium Access Control (MAC) Quality of Service (QoS) Enhancements", IEEE 802.11e Draft 10.0, October 2004. [IEEE80211F] Institute of Electrical and Electronics Engineers, "IEEE Trial-Use Recommended Practice for Multi-Vendor Access Point Interoperability via an Inter-Access Point Protocol Across Distribution Systems Supporting IEEE 802.11 Operation", IEEE 802.11F, June 2003. [IEEE80211i] Institute of Electrical and Electronics Engineers, "Supplement to Standard for Telecommunications and Information Exchange Between Systems - LAN/MAN Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer IAB Informational [Page 30]
INTERNET-DRAFT Link Indications 10 January 2005 (PHY) Specifications: Specification for Enhanced Security", IEEE 802.11i, November 2004. [Kim] Kim, K., Park, Y., Suh, K., and Y. Park, "The BU-trigger method for improving TCP performance over Mobile IPv6", draft- kim-tsvwg-butrigger-00.txt, Internet draft (work in progress), August 2004. [Kotz] Kotz, D., Newport, C. and C. Elliot, "The mistaken axioms of wireless-network research", Dartmouth College Computer Science Technical Report TR2003-467, July 2003. [MIPEAP] Giaretta, C., Guardini, I., Demaria, E., Bournelle, J. and M. Laurent-Maknavicius, "MIPv6 Authorization and Configuration based on EAP", draft-giaretta-mip6-authorization-eap-02.txt, Internet draft (work in progress), October 2004. [Mishra] Mitra, A., Shin, M., and W. Arbaugh, "An Empirical Analysis of the IEEE 802.11 MAC Layer Handoff Process", CS-TR-4395, University of Maryland Department of Computer Science, September 2002. [PEAP] Palekar, A., Simon, D., Salowey, J., Zhou, H., Zorn, G. and S. Josefsson, "Protected EAP Protocol (PEAP) Version 2", draft- josefsson-pppext-eap-tls-eap-10.txt, Internet draft (work in progress), October 2004. [Park] Park, S., Njedjou, E. and N. Montavont, "L2 Triggers Optimized Mobile IPv6 Vertical Handover: The 802.11/GPRS Example", draft-daniel-mip6-optimized-vertical-handover-00.txt, July 2004. [Shortest] Douglas S. J. De Couto, Daniel Aguayo, Benjamin A. Chambers and Robert Morris, "Performance of Multihop Wireless Networks: Shortest Path is Not Enough", Proceedings of the First Workshop on Hot Topics in Networking (HotNets-I), Princeton, New Jersey, October 2002. [TRIGTRAN] Dawkins, S., Williams, C. and A. Yegin, "Framework and Requirements for TRIGTRAN", draft-dawkins-trigtran- framework-00.txt, Internet draft (work in progress), August 2003. [Vatn] Vatn, J., "An experimental study of IEEE 802.11b handover performance and its effect on voice traffic", TRITA-IMIT- TSLAB R 03:01, KTH Royal Institute of Technology, Stockholm, IAB Informational [Page 31]
INTERNET-DRAFT Link Indications 10 January 2005 Sweden, July 2003. [Yegin] Yegin, A., "Link-layer Triggers Protocol", draft-yegin- l2-triggers-00.txt, Internet Draft (work in progress), June 2002. [Velayos] Velayos, H. and G. Karlsson, "Techniques to Reduce IEEE 802.11b MAC Layer Handover Time", TRITA-IMIT-LCN R 03:02, KTH Royal Institute of Technology, Stockholm, Sweden, April 2003. [Vertical] Zhang, Q., Guo, C., Guo, Z. and W. Zhu, "Efficient Mobility Management for Vertical Handoff between WWAN and WLAN", IEEE Communications Magazine, November 2003. [Villamizar] Villamizar, C., "OSPF Optimized Multipath (OSPF-OMP)", draft- ietf-ospf-omp-02.txt, Internet draft (work in progress), February 1999. Appendix A. IAB Members at the time of this writing Bernard Aboba Rob Austein Leslie Daigle Patrik Falstrom Sally Floyd Mark Handley Bob Hinden Geoff Huston Jun-Ichiro Itojun Hagino Eric Rescorla Pete Resnick Jonathan Rosenberg IAB Informational [Page 32]
INTERNET-DRAFT Link Indications 10 January 2005 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. 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. IAB Informational [Page 33]
