Link-Layer Event Notifications for Detecting Network Attachments

Link-layer event notifications are considered to be one of the inputs to the DNA process. A DNA process is likely to take other inputs (e.g., presence of advertised prefixes, reachability of default gateways) before determining whether IP-layer configuration must be updated. It is expected that the DNA process can take advantage of link-layer notifications when they are made available to IP. While by itself a link-layer notification may not constitute all the input DNA needs, it can at least be useful for prompting the DNA process to collect further information (i.e., other inputs to the process). For example, the node may send a router solicitation as soon as it learns that a new link-layer connection is established. The link-layer event that is considered most useful to DNA process is the link up event. The associated notifications can be provided to the IP-layer after the event concludes successfully. The link up events and notifications are associated with a network interface on the node. The IP module may receive simultaneous independent notifications from each one of the network interfaces on the node. The actual event is managed by the link layer of the node through execution of link-layer protocols and mechanisms. Once the event successfully completes within the link layer, its notification is delivered to the IP-layer. By the time the notification is delivered, the link layer of the node must be ready to accept IP packets from the IP and the physical layers. Each time an interface changes its point of attachment, a link up event should be generated. There is a non-deterministic usage of the link up notification to accommodate implementations that desire to indicate the link is up, but the data transmission may be blocked in the network (see IEEE 802.3 discussion). A link up notification may be generated with an appropriate attribute, conveying its non-deterministic nature, to convey the event. Alternatively, the link-layer implementation may choose to delay the link up notification until the risk conditions cease to exist. If a non-deterministic link up was generated, another link up must follow as soon as the link layer is capable of generating a deterministic notification. The event attributes may indicate whether the packets transmitted since the previous notification were presumed to be blocked or allowed by the network, if the link layer could determine the exact conditions. The deterministic link up event following a non-deterministic link up event can be treated differently by consumers of the link up event. For example, the second link up event need not trigger a confirmation process, if the first one already did. A node may have to change its IP-layer configuration even when the link-layer connection stays the same. An example scenario is the IPv6 subnet renumbering . Therefore, there exist cases where IP-layer configuration may have to change even without the IP layer receiving a link up notification. Therefore, a link-layer notification is not a mandatory indication of a subnet change. A link up notification may optionally deliver information relating to the attachment point. Such auxiliary information may include the identity of the attachment point (e.g., base station identifier), or the IP-layer configuration parameters associated with the attached subnet (e.g., subnet prefix, default gateway address, etc.). While merely knowing that a new link-layer connection is established may prompt the DNA process to immediately seek other clues for detecting a network configuration change, auxiliary information may constitute further clues (and even the final answers sometimes). In cases where there is a one-to-one mapping between the attachment point identifiers and the IP-layer configurations, learning the former can reveal the latter. Furthermore, IP-layer configuration parameters obtained during the link-layer connection may be exactly what the DNA process is trying to discover. The link-layer process leading to a link up event depend on the link technology. While a link-layer notification must always indicate that the link up event occurred, the availability and types of auxiliary information on the attachment point depends on the link-layer technology as well. The following subsections examine four link-layer technologies and describe when a link-layer notification is generated and what information is included in it.

GSM Packet Radio System (GPRS) provides packet-switched data transmission over a cellular network . The GPRS architecture consists of a Radio Access Network and a packet domain Core Network. The GPRS Radio Access Network is composed of Mobile Terminals (MTs), a Base Station Subsystem and Serving GPRS Support Nodes (SGSNs). An IP Core Network that acts as the transport backbone of user datagrams between SGSNs and Gateway GPRS Support Nodes (GGSNs). The GGSN ensures the GPRS IP core network connectivity with external networks, such as the Internet or Local Area Networks. The GGSN acts as the default IP gateway for the MT. A GPRS MT that wants to establish IP connectivity establishes first a connection to the GPRS network and one or more PDP Context associations between the MT and the GGSN. It is only after the PDP Context has been established and after address autoconfiguration and tunneling mechanism have taken place that the MT's IP packets can be forwarded to and from its remote IP peers. The aim of PDP Context establishment is also to provide IP-level configuration on top of the GPRS link-layer attachment. Successful establishment of a PDP Context on a GPRS link signifies the availability of IP service to the MT. Therefore, this link-layer event generates a link up event notification sent to the IP layer. An MT may establish a secondary PDP Context while reusing the IP configuration acquired from a previously established and active PDP Context. Such a secondary PDP Context does not provide additional information to the IP layer and only allows another quality-of-service (QoS) profile to be used. The activation of such a secondary PDP context does not usually generate a link up event since it does not require new IP parameters. However, other additional PDP Context activations are to be treated as indicated earlier. With IPv4, the auxiliary information carried along with this notification is the IPv4 address of the MT that is obtained as part of the PDP Context. With IPv6, the PDP Context activation response does not come along with a usable IPv6 address. Effectively, the IPv6 address received from the GGSN in the PDP address field of the message does not contain a valid prefix. The MN actually only uses the interface identifier extracted from that field to form a link-local address that it uses afterwards to obtain a valid prefix (e.g., by stateless or stateful address configuration). Therefore, no IPv6-related auxiliary information is provided to the IP layer.

cdma2000-based 3GPP2 packet data services provide mobile users wide area high-speed access to packet switched networks . Some of the major components of the 3GPP2 packet network architecture consist of: Mobile Station (MS), which allows mobile access to packet-switched networks over a wireless connection. Radio Access Network, which consists of the Base Station Transceivers, Base Station Controllers, and the Packet Control Function. Network Access Server known as the Packet Data Switching Node (PDSN). The PDSN also serves as default IP gateway for the IP MS. 3GPP2 networks use the Point-to-Point Protocol (PPP ) as the link-layer protocol between the MS and the PDSN. Before any IP packets may be sent or received, PPP must reach the Network-Layer Protocol phase, and the IP Control Protocol (IPCP , IPV6CP ) must reach the Opened state. When these states are reached in PPP, a link up event notification is delivered to the IP layer. When the PPP is used for 3GPP2 Simple (i.e., non-Mobile) IPv4 Service, IPCP enables configuration of an IPv4 address on the MS. This IPv4 address is provided as the auxiliary information along with the link up notification. IPV6CP used for Simple IPv6 service does not provide an IPv6 address, but the interface identifiers for local and remote endpoints of the PPP link. Since there is no standards-mandated correlation between the interface identifier and other IP-layer configuration parameters, this information is deemed not useful for DNA (nevertheless, it may be provided as auxiliary information for other uses).

IEEE 802.11-based WiFi networks are the wireless extension of the Local Area Networks. Currently available standards are IEEE 802.11b , IEEE 802.11g , and IEEE 802.11a . The specifications define both the MAC layer and the physical layer. The MAC layer is the same for all these technologies. Two operating modes are available in the IEEE 802.11 series, either infrastructure mode or ad-hoc mode. In infrastructure mode, all link-layer frames are transmitted to an access point (AP) that then forwards them to the final receiver. A station (STA) establishes an IEEE 802.11 association with an AP in order to send and receive IP packets. In a WiFi network that uses Robust Secure Network (RSN ), successful completion of the 4-way handshake between the STA and AP commences the availability of IP service. The link up event notification is generated upon this event. In non-RSN-based networks, successful association or re-association events on the link layer causes a link up notification sent to the IP layer. As part of the link establishment, the STA learns the BSSID and SSID associated with the AP. The BSSID is a unique identifier of the AP, usually set to the MAC address of the wireless interface of the AP. The SSID carries the identifier of the Extended Service Set (ESS) -- the set composed of APs and associated STAs that share a common distribution system. The BSSID and SSID may be provided as auxiliary information along with the link up notification. Unfortunately, this information does not provide a deterministic indication of whether the IP-layer configuration must be changed upon movement. There is no standards-mandated one-to-one relation between the BSSID/SSID pairs and IP subnets. An AP with a given BSSID can connect a STA to any one of multiple IP subnets. Similarly, an ESS with the given SSID may span multiple IP subnets. And finally, the SSIDs are not globally unique. The same SSID may be used by multiple independent ESSs. Nevertheless, BSSID/SSID information may be used in a probabilistic way by the DNA process; hence, it is provided with the link up event notification. In ad-hoc mode, mobile stations (STA) in range may directly communicate with each other, i.e., without any infrastructure or intermediate hop. The set of communicating STAs is called IBSS for Independent Basic Service Set. In an IBSS, only STA services are available, i.e., authentication, deauthentication, privacy, and MAC Service Data Unit (MSDU) delivery. STAs do not associate with each other, and therefore may exchange data frames in state 2 (authenticated and not associated) or even in state 1 (unauthenticated and unassociated) if the Distribution System is not used (i.e., "To DS" and "From DS" bits are clear). If authentication is performed, a link up indication can be generated upon authentication. Concerning the link layer identification, both the BSSID (which is a random MAC address chosen by a STA of the IBSS) and SSID may be used to identify a link, but not to make any assumptions on the IP network configuration.

IEEE 802.3 CSMA/CD (commonly referred to as Ethernet) is the most commonly deployed Local Area Network technology in use today. As deployed today, it is specified by a physical layer/medium access control (MAC) layer specification . In order to provide connection of different LANs together into a larger network, 802.3 LANs are often bridged together . In this section, the terms 802.3 and Ethernet are used interchangeably. This section describes some issues in providing link-layer indications on Ethernet networks, and shows how bridging affects these indications. In Ethernet networks, hosts are connected by wires or by optic fibre to a switch (bridge), a bus (e.g., coaxial cable), a repeater (hub), or directly to another Ethernet device. Interfaces are symmetric, in that while many different physical layers may be present, medium access control is uniform for all devices. In order to determine whether the physical medium is ready for frame transfer, IEEE 802.3 Ethernet specifies its own link monitoring mechanism, which is defined for some, but not all, classes of media. Where available, this Link Integrity Test operation is used to identify when packets are able to be received on an Ethernet segment. It is applicable to both wired and optical physical layers, although details vary between technologies (link pulses in twisted pair copper, light levels in fibre).

Link Integrity Tests in 802.3 networks typically occur at initial physical connection time (for example, at the auto-negotiation stage) and periodically afterwards. They make use of physical-layer specific operations to determine if a medium is able to support link-layer frames . The status of the link as determined by the Link Integrity Test is stored in the variable 'link_status'. Changes to the value of link_status (for example due to Link Integrity Test failure) will generate link indications if the technology-dependent interface is implemented on an Ethernet device . The link_status has possible values of FAIL, READY, and OK. In FAIL state, Link Integrity Tests have failed. In READY state, the link segment has passed integrity tests, but auto-negotiation has not completed. In OK state, the medium is able to send and receive packets. Upon transition to a particular state, the Physical Medium Attachment subsystems generates a PMA_LINK.indicate(link_status). Indications of OK state may be used to generate a link up event notification. These indications do not definitively ensure that packets will be able to be received through the bridge domain, though (see the next section). Such operations are governed by bridging.

Ethernet networks commonly consist of LANs joined together by transparent bridges (usually implemented as switches). Transparent bridges require the active topology to be loop free. This is achieved through the Spanning Tree Protocol (STP) or the Rapid Spanning Tree Protocol (RSTP). These protocols exchange Bridge Protocol Data Units (BPDUs), as defined in ; this leads to the blocking of ports (i.e., not forwarding), where required. By default, the spanning tree protocol does not know whether a particular newly connected piece of Ethernet will cause a loop. Therefore, it will block all traffic from and to newly connected ports with the exception of some unbridged management frames. The STP will determine if the port can be connected to the network in a loop-free manner. For these technologies, even though the link layer appears available, no data packet forwarding will occur until it is determined that the port can be connected to the network in a loop-free environment. For hosts that are providing indications to upper-layer protocols, even if the host itself does not implement bridging or STP, packet delivery across the network can be affected by the presence of bridges. A host connected to a bridge port does not receive any explicit indication that the bridge has started forwarding packets. Therefore, a host may not know when STP operations have completed, or when it is safe to inform upper layers to transmit packets. Where it is not known that forwarding operations are available, a host should assume that RSTP or STP is being performed. Hosts may listen to STP/RSTP and 802.1AB messages to gain further information about the timing of full connectivity on the link, for example, to override an existing indication. Notably, though, it is not easy for a host to distinguish between disabled bridge ports and non-bridge ports with no active transmitters on them, as Disabled ports will have no traffic on them, and incur 100% sender loss. If no bridge configuration messages are received within the Bridge_Max_Age interval (default 20s) then it is likely that there is no visible bridge whose port is enabled for bridging (S8.4.5 of ), since at least two BPDU hello messages would have been lost. Upon this timeout, a link up notification is generated, if one has not been already. If a BPDU is received, and the adjacent bridge is running the original Spanning Tree Protocol, then a host cannot successfully send packets until at least twice the ForwardDelay value in the received BPDU has elapsed. After this time, a link up notification is generated. If the previous link up notification was non-deterministic, then this notification includes an attribute signifying that the packets sent within the prior interval were lost. If the bridge is identified as performing Rapid Spanning Tree Protocol (RSTP), it instead waits Bridge_Max_Age after packet reception (advertised in the BPDU's Max Age field), before forwarding. For ports which are known to be point-to-point through auto-negotiation, this delay is abbreviated to 3 seconds after auto-negotiation completes .

The recently defined 802.1AB Link-Layer Discovery Protocol (LLDP) provides information to devices that are directly adjacent to them on the local LAN . LLDP sends information periodically and at link status change time to indicate the configuration parameters of the device. Devices may send or receive these messages, or do both. The LLDP message may contain a System Capabilities TLV, which describes the MAC- and IP-layer functions that a device is currently using. Where a host receives the System Capabilities TLV indicating that no Bridging is occurring on the LLDP transmitter, no delays for STP calculation will be applied to packets sent through this transmitter. This would allow the generation of a link up notification. Additionally, if a host receives a System Capabilities TLV indicating that the LLDP transmitter is a bridge, the host's advertisement that it is an (end-host) Station-Only may tell the bridge not to run STP and may immediately allow forwarding. Proprietary extensions may also indicate that data forwarding is already available on such a port. Discussion of such optimizations is out of scope for this document. Because the protocol is new and not widely deployed, it is unclear how this protocol will eventually affect DNA in IPv4 or IPv6 networks.

In 802.3 networks, Network Interface Cards (NICs) are often capable of returning a speed and duplex indication to the host. Changes in these characteristics may indicate a connection to a new layer 2 network.

Link-layer indications in Ethernet-like networks are complicated by additional unadvertised delays due to spanning tree calculations. This may cause re-indication or retraction of indications previously sent to upper layer protocols.