Interworking between Locator/ID Separation Protocol (LISP) and Non&nbhy;LISP Sites

This document describes interoperation mechanisms between LISP sites that use EIDs that are not globally routed, and non-LISP sites. A key behavior of the separation of Locators and Endpoint IDs is that EID-Prefixes are normally not advertised into the Internet's Default-Free Zone (DFZ). (See for the exception case.) Specifically, only Routing Locators (RLOCs) are carried in the Internet's DFZ. Existing Internet sites (and their hosts) that do not run LISP must still be able to reach sites numbered from LISP EID space. This document describes three mechanisms that can be used to provide reachability between sites that are LISP-capable and those that are not. The first mechanism uses a new network element, the LISP Proxy Ingress Tunnel Router (Proxy-ITR), to act as an intermediate LISP Ingress Tunnel Router (ITR) for non-LISP-speaking hosts. The second mechanism adds a form of Network Address Translation (NAT) functionality to Tunnel Routers (xTRs, where "xTR" refers to either an ITR or ETR), to substitute routable IP addresses for non-routable EIDs. The final network element is the LISP Proxy Egress Tunnel Router (Proxy-ETR), which acts as an intermediate Egress Tunnel Router (ETR) for LISP sites that need to encapsulate LISP packets destined to non-LISP sites. More detailed descriptions of these mechanisms and the network elements involved may be found in the following sections: defines terms used throughout this document. describes the different cases where interworking mechanisms are needed. describes the relationship between the new EID-Prefix space and the IP address space used by the current Internet. introduces and describes the operation of Proxy-ITRs. introduces and describes the operation of Proxy-ETRs. defines how NAT is used by ETRs to translate non-routable EIDs into routable IP addresses. describes the relationship between asymmetric and symmetric interworking mechanisms (Proxy-ITRs and Proxy-ETRs vs. LISP-NAT). Note that any successful interworking model should be independent of any particular EID-to-RLOC mapping algorithm. This document does not comment on the value of any of the particular LISP mapping systems. Several areas concerning the interworking of LISP and non-LISP sites remain open for further study. These areas include an examination of the impact of LISP-NAT on Internet traffic and applications, understanding the deployment motivations for the deployment and operation of Proxy Tunnel Routers, the impact of EID routes originated into the Internet's Default-Free Zone, and the effects of Proxy Tunnel Routers or LISP-NAT on Internet traffic and applications. Until these issues are fully understood, it is possible that the interworking mechanisms described in this document will be hard to deploy or may have unintended consequences to applications.

The Default-Free Zone (DFZ) refers to the collection of all Internet autonomous systems that do not require a default route to route a packet to any destination. A LISP site whose addresses are used as both globally routable IP addresses and LISP EIDs. A LISP site whose addresses are EIDs only; these EIDs are not found in the legacy Internet routing table. Proxy-ITRs are used to provide connectivity between sites that use LISP EIDs and those that do not. They act as gateways between those parts of the Internet that are not using LISP (the legacy Internet). A given Proxy-ITR advertises one or more highly aggregated EID-Prefixes into the public Internet and acts as the ITR for traffic received from the public Internet. LISP Proxy-ITRs are described in . Network address translation between EID space assigned to a site and RLOC space also assigned to that site. LISP-NAT is described in . Proxy-ETRs provide a LISP (routable or non-routable EID) site's ITRs with the ability to send packets to non-LISP sites in cases where unencapsulated packets (the default mechanism) would fail to be delivered. Proxy-ETRs function by having an ITR encapsulate all non-LISP destined traffic to a pre-configured Proxy-ETR. LISP Proxy-ETRs are described in . A power-of-two block of aggregatable Locators set aside for LISP interworking. For definitions of other terms -- notably Map-Request, Map-Reply, Ingress Tunnel Router (ITR), and Egress Tunnel Router (ETR) -- please consult the LISP specification .

There are 4 unicast connectivity cases that describe how sites can send packets to each other: non-LISP site to non-LISP site LISP site to LISP site LISP site to non-LISP site non-LISP site to LISP site Note that while Cases 3 and 4 seem similar, there are subtle differences due to the way packets are originated. The first case is the Internet as we know it today and as such will not be discussed further here. The second case is documented in , and there are no new interworking requirements because there are no new protocol requirements placed on intermediate non-LISP routers. In Case 3, LISP site to non-LISP site, a LISP site can (in most cases) send packets to a non-LISP site because the non-LISP site prefixes are routable. The non-LISP sites need not do anything new to receive packets. The only action the LISP site needs to take is to know when not to LISP-encapsulate packets. An ITR knows explicitly that the destination is non-LISP if the destination IP address of an IP packet matches a (negative) Map-Cache entry that has the action 'Natively-Forward'. There could be some situations where (unencapsulated) packets originated by a LISP site may not be forwarded to a non-LISP site. These cases are reviewed in (Proxy Egress Tunnel Routers). Case 4, typically the most challenging, occurs when a host at a non-LISP site wishes to send traffic to a host at a LISP site. If the source host uses a (non-globally routable) EID as the destination IP address, the packet is forwarded inside the source site until it reaches a router that cannot forward it (due to lack of a default route), at which point the traffic is dropped. For traffic not to be dropped, some mechanism to make this destination EID routable must be in place. Sections (Proxy-ITRs) and (LISP-NAT) describe two such mechanisms. Case 4 also applies to non-LISP packets (as in Case 3) that are returning to the LISP site.

An obvious way to achieve interworking between LISP and non-LISP hosts is for a LISP site to simply announce EID-Prefixes into the DFZ, much like the current routing system, effectively treating them as "Provider-Independent" (PI) prefixes. Having a site do this is undesirable, as it defeats one of the primary goals of LISP -- to reduce global routing system state.

If EID-Prefixes are announced into the DFZ, the impact is similar to the case in which LISP has not been deployed, because these EID-Prefixes will be no more aggregatable than existing PI addresses. Such a mechanism is not viewed as a viable long-term solution but may be a viable short-term way for a site to transition a portion of its address space to EID space without changing its existing routing policy.

Routable EIDs might require non-LISP sites today to use BGP to, among other things, originate their site's routes into the DFZ, in order to enable ingress Traffic Engineering. Relaxing this requirement (and thus potentially reducing global DFZ routing state) while still letting sites control their ingress Traffic Engineering policy is a design goal of LISP.

Two schemes are proposed to limit the impact of having EIDs announced in the current global Internet routing table: discusses the LISP Proxy Ingress Tunnel Router, an approach that provides ITR functionality to bridge LISP-capable and non-LISP-capable sites. discusses another approach, LISP-NAT, in which NAT is combined with ITR functionality to limit the impact of routable EIDs on the Internet routing infrastructure.

A primary design goal for LISP (and other Locator/ID separation proposals) is to facilitate topological aggregation of namespaces used for the path computation, and thus decrease global routing system overhead. Another goal is to achieve the benefits of improved aggregation as soon as possible. Individual sites advertising their own routes for LISP EID-Prefixes into the global routing system is therefore not recommended. That being said, single-homed sites (or multihomed sites that are not leaking more-specific exceptions) that are already using provider-aggregated prefixes can use these prefixes as LISP EIDs without adding state to the routing system. In other words, such sites do not cause additional prefixes to be advertised. For such sites, connectivity to a non-LISP site does not require interworking machinery because the "PA" (Provider-Assigned) EIDs are already routable (they are effectively LISP-R type sites). Their EIDs are found in the LISP mapping system, and their (aggregate) PA prefix(es) are found in the DFZ of the Internet. The continued announcements of an existing site's Provider-Independent (PI) prefix(es) is of course under the control of that site. Some period of transition, where a site is found both in the LISP mapping system, and as a discrete prefix in the Internet routing system, may be a viable transition strategy. Care should be taken not to advertise additional more-specific LISP EID-Prefixes into the DFZ.

Proxy Ingress Tunnel Routers (Proxy-ITRs) allow non-LISP sites to send packets to LISP-NR sites. A Proxy-ITR is a new network element that shares many characteristics with the LISP ITR. Proxy-ITRs allow non-LISP sites to send packets to LISP-NR sites without any changes to protocols or equipment at the non-LISP site. Proxy-ITRs have two primary functions: Proxy-ITRs advertise highly aggregated EID-Prefix space on behalf of LISP sites so that non-LISP sites can reach them. Proxy-ITRs also encapsulate non-LISP Internet traffic into LISP packets and route them towards their destination RLOCs.

A key part of Proxy-ITR functionality is to advertise routes for highly aggregated EID-Prefixes into parts of the global routing system. Aggressive aggregation is performed to minimize the number of new announced routes. In addition, careful placement of Proxy-ITRs can greatly reduce the advertised scope of these new routes. To this end, Proxy-ITRs should be deployed close to non-LISP-speaking sites rather than close to LISP sites. Such deployment not only limits the scope of EID-Prefix route advertisements but also allows the traffic forwarding load to be spread among many Proxy-ITRs.

What follows is an example of the path a packet would take when using a Proxy-ITR. In this example, the LISP-NR site is given the EID-Prefix 192.0.2.0/24. For the purposes of this example, neither this prefix nor any covering aggregate are present in the global routing system. In other words, without the Proxy-ITR announcing 192.0.2.0/24, if a packet with this destination were to reach a router in the Default-Free Zone, it would be dropped. The following diagram describes a high-level view of the topology:

Internet DFZ .--------------------------------. / \ | Traffic Encap'd to Site's | | +-----+ RLOC(s) | LISP Site: | |P-ITR|=========> | | +-----+ +--+ +-----+ | | | |PE+------+CE 1 |-| | | Originated Route +--+ +-----+ | +----+ | V 192.0.2.0/24 | |-|Host| | +--| +-----+ | +----+ | |PE+------+CE 2 |-| 192.0.2.1 | +---+ +--+ +-----+ | \ |PE | / '---------------+-+-+------------' Site EID-Prefix: | 192.0.2.0/24 | ^ | | +--+--+ | Traffic Non LISP Site: | CE | | to +--+--+ | 192.168.2.1 | | ----------- | +----+ |Host| +----+ A full protocol exchange example follows: The source host makes a DNS lookup EID for the destination and gets 192.0.2.1 in return. The source host has a default route to the Customer Edge (CE) router and forwards the packet to the CE. The CE has a default route to its Provider Edge (PE) router and forwards the packet to the PE. The PE has a route to 192.0.2.0/24, and the next hop is the Proxy-ITR. The Proxy-ITR has or acquires a mapping for 192.0.2.1 and LISP-encapsulates the packet. The outer IP header now has a destination address of one of the destination EID's RLOCs. The outer source address of this encapsulated packet is the Proxy-ITR's RLOC. The Proxy-ITR looks up the RLOC and forwards the LISP packet to the next hop, after which it is forwarded by other routers to the ETR's RLOC. The ETR decapsulates the packet and delivers the packet to the 192.0.2.1 host in the destination LISP site. Packets from host 192.0.2.1 will flow back through the LISP site's ITR. Such packets are not encapsulated because the ITR knows that the destination (the original source) is a non-LISP site. The ITR knows this because it can check the LISP mapping database for the destination EID and on a failure determines that the destination site is not LISP enabled. Packets are then routed natively and directly to the destination (original source) site. Note that in this example the return path is asymmetric, so return traffic will not go back through the Proxy-ITR. This is because the LISP-NR site's ITR will discover that the originating site is not a LISP site and will not encapsulate the returning packet (see for details of ITR behavior). The asymmetric nature of traffic flows allows the Proxy-ITR to be relatively simple -- it will only have to encapsulate LISP packets.

Proxy-ITRs attract traffic by announcing the LISP EID namespace into parts of the non-LISP-speaking global routing system. There are several ways that a network could control how traffic reaches a particular Proxy-ITR to prevent it from receiving more traffic than it can handle: The Proxy-ITR's aggregate routes might be selectively announced, giving a coarse way to control the quantity of traffic attracted by that Proxy-ITR. For example, some of the routes being announced might be tagged with a BGP community and their scope of announcement limited by the routing policy of the provider. The same address might be announced by multiple Proxy-ITRs in order to share the traffic using IP Anycast. The asymmetric nature of traffic flows through the Proxy-ITR means that operationally, deploying a set of Proxy-ITRs would be very similar to existing anycasted services like DNS caches. Multiple Proxy-ITRs could advertise the same BGP Next Hop IP address as their RLOC, and traffic would be attracted to the nearest Next Hop according to the network's IGP.

There are several approaches that a network could take in placing Proxy-ITRs. Placing the Proxy-ITR near the source of traffic allows the communication between the non-LISP site and the LISP site to have the least "stretch" (i.e., the least number of forwarding hops when compared to an optimal path between the sites). Some proposals, for example the Core Router-Integrated Overlay , have suggested grouping Proxy-ITRs near an arbitrary subset of ETRs and announcing a 'local' subset of EID space. This model cannot guarantee minimum stretch if the EID-Prefix route advertisement points are changed (such a change might occur if a site adds, removes, or replaces one or more of its ISP connections).

When packets destined for LISP-NR sites arrive and are encapsulated at a Proxy-ITR, a new LISP packet header is prepended. This causes the packet's destination to be set to the destination ETR's RLOC. Because packets are thus routed towards RLOCs, it can potentially better follow the Proxy-ITR network's Traffic Engineering policies (such as closest exit routing). This also means that providers that are not default-free and do not deploy Proxy-ITRs end up sending more traffic to expensive transit links (assuming their upstreams have deployed Proxy-ITRs) rather than to the ETR's RLOC addresses, to which they may well have cheaper and closer connectivity (via, for example, settlement-free peering). A corollary to this would be that large transit providers deploying Proxy-ITRs may attract more traffic, and therefore more revenue, from their customers.

Proxy Egress Tunnel Routers (Proxy-ETRs) allow LISP sites to send packets to non-LISP sites in the case where the access network does not allow the LISP site to send packets with the source address of the site's EID(s). A Proxy-ETR is a new network element that, conceptually, acts as an ETR for traffic destined to non-LISP sites. This also has the effect of allowing an ITR to avoid having to decide whether to encapsulate packets or not -- it can always encapsulate packets. An ITR would encapsulate packets destined for LISP sites (no change here), and these would be routed directly to the corespondent site's ETR. All other packets (those destined to non-LISP sites) will be sent to the originating site's Proxy-ETR. There are two primary reasons why sites would want to utilize a Proxy-ETR: Some providers' access networks require the source of the packets emitted to be within the addressing scope of the access networks (see ). A LISP site may want to transmit packets to a non-LISP site where some of the intermediate network does not support the particular IP protocol desired (v4 or v6). Proxy-ETRs can allow this LISP site's data to 'hop over' this by utilizing LISP's support for mixed-protocol encapsulation.

Packets from a LISP site can reach a non-LISP site with the aid of a Proxy-ETR. An ITR is simply configured to send all non-LISP traffic, which it normally would have forwarded natively (non-encapsulated), to a Proxy-ETR. In the case where the ITR uses one or more Map-Resolvers, the ITR will encapsulate packets that match the received Negative Map-Cache to the configured Proxy-ETR(s). In the case where the ITR is connected to the mapping system directly, it would encapsulate all packets to the configured Proxy-ETR that are cache misses. Note that this outer encapsulation to the Proxy-ETR may be in an IP protocol other than the (inner) encapsulated data. Routers then use the LISP (outer) header's destination address to route the packets toward the configured Proxy-ETR. A Proxy-ETR should verify the (inner) source EID of the packet at the time of decapsulation in order to verify that this is from a configured LISP site. This is to prevent spoofed inner sources from being encapsulated through the Proxy-ETR. What follows is an example of the path a packet would take when using a Proxy-ETR. In this example, the LISP-NR (or LISP-R) site is given the EID-Prefix 192.0.2.0/24, and it is trying to reach a host at a non-LISP site with the IP prefix 198.51.100.0/24. For the purposes of this example, the destination (198.51.100.0/24) is found in the Internet's routing system. A full protocol exchange example follows: The source host makes a DNS lookup for the destination and gets 198.51.100.100 (an IP address of a host in the non-LISP site) in return. The source host has a default route to the Customer Edge (CE) router and forwards the packet towards the CE. The CE is a LISP ITR and is configured to encapsulate traffic destined for non-LISP sites to a Proxy-ETR. The Proxy-ETR decapsulates the LISP packet and forwards the original packet to its next hop. The packet is then routed natively and directly to the destination (non-LISP) site 198.51.100.0/24. Note that in this example the return path is asymmetric, so return traffic will not go back through the Proxy-ETR. This means that in order to reach LISP-NR sites, non-LISP sites must still use Proxy-ITRs.

LISP Network Address Translation (LISP-NAT) is a limited form of NAT . LISP-NAT is designed to enable the interworking of non-LISP sites and LISP-NR sites by ensuring that the LISP-NR's site addresses are always routable. LISP-NAT accomplishes this by translating a host's source address from an 'inner' (LISP-NR EID) value to an 'outer' (LISP-R) value and keeping this translation in a table that it can reference for subsequent packets. In addition, existing RFC 1918 sites can use LISP-NAT to talk to both LISP and non-LISP sites. The basic concept of LISP-NAT is that when transmitting a packet, the ITR replaces a non-routable EID source address with a routable source address, which enables packets to return to the site. Note that this section is intended as a rough overview of what could be done and is not an exhaustive guide to IPv4 NAT. There are two main cases that involve LISP-NAT: Hosts at LISP sites that use non-routable global EIDs speaking to non-LISP sites using global addresses. Hosts at LISP sites that use RFC 1918 private EIDs speaking to other sites, who may be either LISP or non-LISP sites. Note that LISP-NAT is not needed in the case of LISP-R (routable global EIDs) sources. This case occurs when a site is announcing its prefix into both the LISP mapping system and the Internet DFZ. This is because the LISP-R source's address is routable, and return packets will be able to natively reach the site.

LISP-NAT allows a host with a LISP-NR EID to send packets to non-LISP hosts by translating the LISP-NR EID to a globally unique address (a LISP-R EID). This globally unique address may be either a PI or PA address. An example of this translation follows. For this example, a site has been assigned a LISP-NR EID of 203.0.113.0/24. In order to utilize LISP-NAT, the site has also been provided the PA EID 192.0.2.0/24 and uses the first address (192.0.2.1) as the site's RLOC. The rest of this PA space (192.0.2.2 to 192.0.2.254) is used as a translation pool for this site's hosts who need to send packets to non-LISP hosts. The translation table might look like the following:

Site NR-EID Site R-EID Site's RLOC Translation Pool ============================================================== 203.0.113.0/24 192.0.2.0/24 192.0.2.1 192.0.2.2-254 The host 203.0.113.2 sends a packet (which, for the purposes of this example, is destined for a non-LISP site) to its default route (the ITR). The ITR receives the packet and determines that the destination is not a LISP site. How the ITR makes this determination is up to the ITR's implementation of the EID-to-RLOC mapping system used (see, for example, ). The ITR then rewrites the source address of the packet from 203.0.113.2 to 192.0.2.2, which is the first available address in the LISP-R EID space available to it. The ITR keeps this translation in a table in order to reverse this process when receiving packets destined to 192.0.2.2. Finally, when the ITR forwards this packet without encapsulating it, it uses the entry in its LISP-NAT table to translate the returning packets' destination IPs to the proper host.

In the case where hosts using RFC 1918 addresses desire to send packets to non-LISP hosts, the LISP-NAT implementation acts much like an existing IPv4 NAT device that is doing address translation only (not port translation). The ITR providing the NAT service must use LISP-R EIDs for its global address pool and also provide all the standard NAT functions required today. Note that the RFC 1918 addresses above are private addresses and not EIDs, and that these RFC 1918 addresses are not found in the LISP mapping system. The source of the packet must be translated to a LISP-R EID in a manner similar to that discussed in , and this packet must be forwarded to the ITR's next hop for the destination, without LISP encapsulation.

LISP-NAT allows a host with an RFC 1918 address to send packets to LISP hosts by translating the RFC 1918 address to a LISP EID. After translation, the communication between the source and destination ITR and ETRs continues as described in . While the communication of LISP EIDs to LISP EIDs is, strictly speaking, outside the scope of interworking, it is included here in order to complete the conceptual framework of LISP-NAT. An example of this translation and encapsulation follows. For this example, a host has been assigned an RFC 1918 address of 192.168.1.2. In order to utilize LISP-NAT, the site also has been provided the LISP-R EID-Prefix 192.0.2.0/24 and uses the first address (192.0.2.1) as the site's RLOC. The rest of this PA space (192.0.2.2 to 192.0.2.254) is used as a translation pool for this site's hosts who need to send packets to both non-LISP and LISP hosts. The host 192.168.1.2 sends a packet destined for a non-LISP site to its default route (the ITR). The ITR receives the packet and determines that the destination is a LISP site. How the ITR makes this determination is up to the ITR's implementation of the EID-to-RLOC mapping system. The ITR then rewrites the source address of the packet from 192.168.1.2 to 192.0.2.2, which is the first available address in the LISP EID space available to it. The ITR keeps this translation in a table in order to reverse this process when receiving packets destined to 192.0.2.2. The ITR then LISP-encapsulates this packet (see for details). The ITR uses the site's RLOC as the LISP outer header's source and the translation address as the LISP inner header's source. Once it decapsulates returning traffic, it uses the entry in its LISP-NAT table to translate the returning packet's destination IP address and then forwards it to the proper host.

With LISP-NAT, there are two EIDs possible for a given host: the LISP-R EID and the LISP-NR EID. When a site has two addresses that a host might use for global reachability, name-to-address directories may need to be modified. This problem -- global vs. local addressability -- exists for NAT in general, but the specific issue described above is unique to location/identity separation schemes. Some of these have suggested running a separate DNS instance for new types of EIDs. This solves the problem but introduces complexity for the site. Alternatively, using Proxy-ITRs can mitigate this problem, because the LISP-NR EID can be reached in all cases.

In summary, there are three suggested mechanisms for interworking LISP with non-LISP sites (for both IPv4 and IPv6). In the LISP-NAT option, the LISP site can manage and control the interworking on its own. In the Proxy-ITR case, the site is not required to manage the advertisement of its EID-Prefix into the DFZ, with the cost of potentially adding stretch to the connections of non-LISP sites sending packets to the LISP site. The third option is Proxy-ETRs, which are optionally used by sites relying on Proxy-ITRs to mitigate two caveats for LISP sites sending packets to non-LISP sites. This means Proxy-ETRs are not usually expected to be deployed by themselves; rather, they will be used to assist LISP-NR sites that are already using Proxy-ITRs.

There is a subtle difference between symmetrical (LISP-NAT) and asymmetrical (Proxy-ITR and Proxy-ETR) interworking techniques. Operationally, Proxy-ITRs and Proxy-ETRs can (and likely should) be decoupled, since Proxy-ITRs are best deployed closest to non-LISP sites and Proxy-ETRs are best located close to the LISP sites they are decapsulating for. This asymmetric placement of the two network elements minimizes the stretch imposed on each direction of the packet flow while still allowing for coarsely aggregated announcements of EIDs into the Internet's routing table.

Like any router or LISP ITR, Proxy-ITRs will have the opportunity to inspect traffic at the time that they encapsulate. The location of these devices in the network can have implications for discarding malicious traffic on behalf of ETRs that request this behavior (by setting the ACT (action) bit in Map-Reply packets to "Drop" for an EID or EID-Prefix). This is an area that would benefit from further experimentation and analysis. LISP interworking via Proxy-ITRs should have no impact on the existing network beyond what LISP ITRs and ETRs introduce when multihoming. That is, if a site multihomes today (with LISP or BGP), there is a possibility of asymmetric flows. Proxy-ITRs and Proxy-ETRs will likely be operated by organizations other than those of the end site receiving or sending traffic. Care should be taken, then, in selecting a Proxy-ITR/Proxy-ETR provider to insure that the quality of service meets the site's expectations. Proxy-ITRs and Proxy-ETRs share many of the same security issues as those discussed for ITRs and ETRs. For further information, see the security considerations section of . As with traditional NAT, LISP-NAT will obscure the actual host LISP-NR EID behind the LISP-R addresses used as the NAT pool. When LISP sites send packets to non-LISP sites (these non-LISP sites rely on Proxy-ITRs to enable interworking), packets will have the site's EID as the source IP address. These EIDs may not be recognized by their ISP's Unicast Reverse Path Forwarding (uRPF) rules enabled on the Provider Edge router. Several options are available to the service provider. For example, they could enable a less strict version of uRPF, where they only look for the existence of the EID-Prefix in the routing table. Another option, which is more secure, is to add a static route for the customer on the PE router but not redistribute this route into the provider's routing table. Finally, Proxy-ETRs can enable LISP sites to bypass this uRPF check by encapsulating all of their egress traffic destined to non-LISP sites to the Proxy-ETR (thus ensuring that the outer IP source address is the site's RLOC).

Thanks go to Christian Vogt, Lixia Zhang, Robin Whittle, Michael Menth, Xuewei Wang, and Noel Chiappa, who have made insightful comments with respect to LISP interworking and transition mechanisms. A special thanks goes to Scott Brim for his initial brainstorming of these ideas and also for his careful review.