RFC 8175
Dynamic Link Exchange Protocol (DLEP)
Internet Engineering Task Force (IETF) S. Ratliff Request for Comments: 8175 VT iDirect Category: Standards Track S. Jury ISSN: 2070-1721 Cisco Systems D. Satterwhite Broadcom R. Taylor Airbus Defence & Space B. Berry June 2017 Dynamic Link Exchange Protocol (DLEP)Abstract
When routing devices rely on modems to effect communications over wireless links, they need timely and accurate knowledge of the characteristics of the link (speed, state, etc.) in order to make routing decisions. In mobile or other environments where these characteristics change frequently, manual configurations or the inference of state through routing or transport protocols does not allow the router to make the best decisions. This document introduces a new protocol called the Dynamic Link Exchange Protocol (DLEP), which provides a bidirectional, event-driven communication channel between the router and the modem to facilitate communication of changing link characteristics. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8175.
Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.Table of Contents
1. Introduction ....................................................4 2. Protocol Overview ...............................................7 2.1. Destinations ...............................................8 2.2. Conventions and Terminology ................................9 3. Requirements ....................................................9 4. Implementation Scenarios .......................................10 5. Assumptions ....................................................10 6. Metrics ........................................................11 7. DLEP Session Flow ..............................................12 7.1. Peer Discovery State ......................................12 7.2. Session Initialization State ..............................14 7.3. In-Session State ..........................................14 7.3.1. Heartbeats .........................................15 7.4. Session Termination State .................................15 7.5. Session Reset State .......................................16 7.5.1. Unexpected TCP Connection Termination ..............16 8. Transaction Model ..............................................16 9. Extensions .....................................................17 9.1. Experiments ...............................................18 10. Scalability ...................................................18 11. DLEP Signal and Message Structure .............................18 11.1. DLEP Signal Header .......................................19 11.2. DLEP Message Header ......................................20 11.3. DLEP Generic Data Item ...................................20 12. DLEP Signals and Messages .....................................21 12.1. General Processing Rules .................................21 12.2. Status Code Processing ...................................22 12.3. Peer Discovery Signal ....................................22 12.4. Peer Offer Signal ........................................23 12.5. Session Initialization Message ...........................23
12.6. Session Initialization Response Message ..................24
12.7. Session Update Message ...................................26
12.8. Session Update Response Message ..........................27
12.9. Session Termination Message ..............................28
12.10. Session Termination Response Message ....................28
12.11. Destination Up Message ..................................28
12.12. Destination Up Response Message .........................30
12.13. Destination Announce Message ............................30
12.14. Destination Announce Response Message ...................31
12.15. Destination Down Message ................................32
12.16. Destination Down Response Message .......................33
12.17. Destination Update Message ..............................33
12.18. Link Characteristics Request Message ....................35
12.19. Link Characteristics Response Message ...................35
12.20. Heartbeat Message .......................................36
13. DLEP Data Items ...............................................37
13.1. Status ...................................................38
13.2. IPv4 Connection Point ....................................41
13.3. IPv6 Connection Point ....................................42
13.4. Peer Type ................................................43
13.5. Heartbeat Interval .......................................45
13.6. Extensions Supported .....................................45
13.7. MAC Address ..............................................46
13.8. IPv4 Address .............................................47
13.8.1. IPv4 Address Processing ...........................48
13.9. IPv6 Address .............................................49
13.9.1. IPv6 Address Processing ...........................50
13.10. IPv4 Attached Subnet ....................................51
13.10.1. IPv4 Attached Subnet Processing ..................52
13.11. IPv6 Attached Subnet ....................................53
13.11.1. IPv6 Attached Subnet Processing ..................54
13.12. Maximum Data Rate (Receive) .............................55
13.13. Maximum Data Rate (Transmit) ............................56
13.14. Current Data Rate (Receive) .............................56
13.15. Current Data Rate (Transmit) ............................57
13.16. Latency .................................................58
13.17. Resources ...............................................59
13.18. Relative Link Quality (Receive) .........................60
13.19. Relative Link Quality (Transmit) ........................60
13.20. Maximum Transmission Unit (MTU) .........................61
14. Security Considerations .......................................62
15. IANA Considerations ...........................................63
15.1. Registrations ............................................63
15.2. Signal Type Registrations ................................63
15.3. Message Type Registrations ...............................64
15.4. DLEP Data Item Registrations .............................65
15.5. DLEP Status Code Registrations ...........................66
15.6. DLEP Extension Registrations .............................67
15.7. DLEP IPv4 Connection Point Flags .........................68
15.8. DLEP IPv6 Connection Point Flags .........................68
15.9. DLEP Peer Type Flags .....................................68
15.10. DLEP IPv4 Address Flags .................................69
15.11. DLEP IPv6 Address Flags .................................69
15.12. DLEP IPv4 Attached Subnet Flags .........................69
15.13. DLEP IPv6 Attached Subnet Flags .........................70
15.14. DLEP Well-Known Port ....................................70
15.15. DLEP IPv4 Link-Local Multicast Address ..................70
15.16. DLEP IPv6 Link-Local Multicast Address ..................70
16. References ....................................................71
16.1. Normative References .....................................71
16.2. Informative References ...................................71
Appendix A. Discovery Signal Flows ................................73
Appendix B. Peer-Level Message Flows ..............................73
B.1. Session Initialization .....................................73
B.2. Session Initialization - Refused ...........................74
B.3. Router Changes IP Addresses ................................74
B.4. Modem Changes Session-Wide Metrics .........................75
B.5. Router Terminates Session ..................................75
B.6. Modem Terminates Session ...................................76
B.7. Session Heartbeats .........................................77
B.8. Router Detects a Heartbeat Timeout .........................78
B.9. Modem Detects a Heartbeat Timeout ..........................78
Appendix C. Destination-Specific Message Flows ....................79
C.1. Common Destination Notification ............................79
C.2. Multicast Destination Notification .........................80
C.3. Link Characteristics Request ...............................81
Acknowledgments ...................................................82
Authors' Addresses ................................................82
1. Introduction
There exist today a collection of modem devices that control links of
variable data rate and quality. Examples of these types of links
include line-of-sight (LOS) terrestrial radios, satellite terminals,
and broadband modems. Fluctuations in speed and quality of these
links can occur due to configuration, or on a moment-to-moment basis,
due to physical phenomena like multipath interference, obstructions,
rain fade, etc. It is also quite possible that link quality and
data rate vary with respect to individual destinations on a link and
with the type of traffic being sent. As an example, consider the
case of an IEEE 802.11 access point serving two associated laptop
computers. In this environment, the answer to the question "What is
the data rate on the 802.11 link?" is "It depends on which associated
laptop we're talking about and on what kind of traffic is being
sent." While the first laptop, being physically close to the access
point, may have a data rate of 54 Mbps for unicast traffic, the other laptop, being relatively far away or obstructed by some object, can simultaneously have a data rate of only 32 Mbps for unicast. However, for multicast traffic sent from the access point, all traffic is sent at the base transmission rate (which is configurable but, depending on the model of the access point, is usually 24 Mbps or less). In addition to utilizing links that have variable data rates, mobile networks are challenged by the notion that link connectivity will come and go over time, without an effect on a router's interface state (Up or Down). Effectively utilizing a relatively short-lived connection is problematic in IP routed networks, as IP routing protocols tend to rely on interface state and independent timers to maintain network convergence (e.g., HELLO messages and/or recognition of DEAD routing adjacencies). These dynamic connections can be better utilized with an event-driven paradigm, where acquisition of a new neighbor (or loss of an existing one) is signaled, as opposed to a paradigm driven by timers and/or interface state. DLEP not only implements such an event-driven paradigm but does so over a local (1-hop) TCP session, which guarantees delivery of the event messages. Another complicating factor for mobile networks are the different methods of physically connecting the modem devices to the router. Modems can be deployed as an interface card in a router's chassis, or as a standalone device connected to the router via Ethernet or serial link. In the case of Ethernet attachment, with existing protocols and techniques, routing software cannot be aware of convergence events occurring on the radio link (e.g., acquisition or loss of a potential routing neighbor), nor can the router be aware of the actual capacity of the link. This lack of awareness, along with the variability in data rate, leads to a situation where finding the (current) best route through the network to a given node is difficult to establish and properly maintain. This is especially true of demand-based access schemes such as Demand Assigned Multiple Access (DAMA) implementations used on some satellite systems. With a DAMA-based system, additional data rate may be available but will not be used unless the network devices emit traffic at a rate higher than the currently established rate. Increasing the traffic rate does not guarantee that additional data rate will be allocated; rather, it may result in data loss and additional retransmissions on the link.
Addressing the challenges listed above, the Dynamic Link Exchange Protocol, or DLEP, has been developed. DLEP runs between a router and its attached modem devices, allowing the modem devices to communicate (1) link characteristics as they change and (2) convergence events (acquisition and loss of potential routing next hops). Figures 1 and 2 illustrate the scope of DLEP packets. |-------Local Node-------| |-------Remote Node------| | | | | +--------+ +-------+ +-------+ +--------+ | Router |=======| Modem |{~~~~~~~~}| Modem |=======| Router | | | | Device| | Device| | | +--------+ +-------+ +-------+ +--------+ | | | Link | | | |-DLEP--| | Protocol | |-DLEP--| | | | (e.g., | | | | | | 802.11) | | | Figure 1: DLEP Network In Figure 1, when the local modem detects the presence of a remote node, it (the local modem) sends a message to its router via DLEP. The message consists of an indication of what change has occurred on the link (e.g., the presence of a remote node detected), along with a collection of DLEP-defined Data Items that further describe the change. Upon receipt of the message, the local router may take whatever action it deems appropriate, such as initiating discovery protocols and/or issuing HELLO messages to converge the network. On a continuing, as-needed basis, the modem devices use DLEP to report any characteristics of the link (data rate, latency, etc.) that have changed. DLEP is independent of the link type and topology supported by the modem. Note that DLEP is specified to run only on the local link between router and modem. Some over-the-air signaling may be necessary between the local and remote modem in order to provide some parameters in DLEP Messages between the local modem and local router, but DLEP does not specify how such over-the-air signaling is carried out. Over-the-air signaling is purely a matter for the modem implementer. Figure 2 shows how DLEP can support a configuration where routers are connected with different link types. In this example, Modem Device Type A implements a point-to-point link, and Modem Device Type B is connected via a shared medium. In both cases, DLEP is used to report the characteristics of the link (data rate, latency, etc.) to routers. The modem is also able to use the DLEP session to notify the router when the remote node is lost, shortening the time required to reconverge the network.
+--------+ +--------+ +----+ Modem | | Modem +---+ | | Device | | Device | | | | Type A | <===== // ======> | Type A | | | +--------+ Point-to-Point Link +--------+ | +---+----+ +---+----+ | Router | | Router | | | | | +---+----+ +---+----+ | +--------+ +--------+ | +-----+ Modem | | Modem | | | Device | o o o o o o o o | Device +--+ | Type B | o Shared o | Type B | +--------+ o Medium o +--------+ o o o o o o o +--------+ | Modem | | Device | | Type B | +---+----+ | | +---+----+ | Router | | | +--------+ Figure 2: DLEP Network with Multiple Modem Devices2. Protocol Overview
DLEP defines a set of Messages used by modems and their attached routers to communicate events that occur on the physical link(s) managed by the modem: for example, a remote node entering or leaving the network, or that the link has changed. Associated with these Messages are a set of Data Items -- information that describes the remote node (e.g., address information) and/or the characteristics of the link to the remote node. Throughout this document, we refer to modems/routers participating in a DLEP session as "DLEP Participants", unless a specific distinction (e.g., modem or router) is required. DLEP uses a session-oriented paradigm between the modem device and its associated router. If multiple modem devices are attached to a router (as in Figure 2) or the modem supports multiple connections
(via multiple logical or physical interfaces), then separate DLEP sessions exist for each modem or connection. A router and modem form a session by completing the discovery and initialization process. This router-modem session persists unless or until it either (1) times out, based on the absence of DLEP traffic (including heartbeats) or (2) is explicitly torn down by one of the DLEP participants. While this document represents the best efforts of the working group to be functionally complete, it is recognized that extensions to DLEP will in all likelihood be necessary as more link types are used. Such extensions are defined as additional Messages, Data Items, and/or status codes, and associated rules of behavior, that are not defined in this document. DLEP contains a standard mechanism for router and modem implementations to negotiate the available extensions to use on a per-session basis.2.1. Destinations
The router-modem session provides a carrier for information exchange concerning "destinations" that are available via the modem device. Destinations can be identified by either the router or the modem and represent a specific, addressable location that can be reached via the link(s) managed by the modem. The DLEP Messages concerning destinations thus become the way for routers and modems to maintain, and notify each other about, an information base representing the physical and logical destinations accessible via the modem device, as well as the link characteristics to those destinations. A destination can be either physical or logical. The example of a physical destination would be that of a remote, far-end router attached via the variable-quality network. It should be noted that for physical destinations the Media Access Control (MAC) address is the address of the far-end router, not the modem. The example of a logical destination is Multicast. Multicast traffic destined for the variable-quality network (the network accessed via the modem) is handled in IP networks by deriving a Layer 2 MAC address based on the Layer 3 address. Leveraging on this scheme, multicast traffic is supported in DLEP simply by treating the derived MAC address as any other destination in the network. To support these logical destinations, one of the DLEP participants (typically, the router) informs the other as to the existence of the logical destination. The modem, once it is aware of the existence of this logical destination, reports link characteristics just as it
would for any other destination in the network. The specific algorithms a modem would use to derive metrics on logical destinations are outside the scope of this specification; these algorithms are left to specific implementations to decide. In all cases, when this specification uses the term "destination", it refers to the addressable locations, either logical or physical, that are accessible by the radio link(s).2.2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.3. Requirements
DLEP MUST be implemented on a single Layer 2 domain. The protocol identifies next-hop destinations by using the MAC address for delivering data traffic. No manipulation or substitution is performed; the MAC address supplied in all DLEP Messages is used as the Destination MAC address for frames emitted by the participating router. MAC addresses MUST be unique within the context of the router-modem session. To enforce the single Layer 2 domain, implementations MUST support the Generalized TTL Security Mechanism [RFC5082], and implementations MUST adhere to this specification for all DLEP Messages. DLEP specifies UDP multicast for single-hop discovery signaling and TCP for transport of the Messages. Modems and routers participating in DLEP sessions MUST have topologically consistent IP addresses assigned. It is RECOMMENDED that DLEP implementations utilize IPv6 link-local addresses to reduce the administrative burden of address assignment. DLEP relies on the guaranteed delivery of its Messages between router and modem, once the 1-hop discovery process is complete -- hence, the specification of TCP to carry the Messages. Other reliable transports for the protocol are possible but are outside the scope of this document.
4. Implementation Scenarios
During development of this specification, two types of deployments were discussed. The first can be viewed as a "dedicated deployment". In this mode, DLEP routers and modems are either directly connected (e.g., using crossover cables to connect interfaces) or connected to a dedicated switch. An example of this type of deployment would be a router with a line-of-sight radio connected into one interface, with a satellite modem connected into another interface. In mobile environments, the router and the connected modem (or modems) are placed into a mobile platform (e.g., a vehicle, boat, or airplane). In this mode, when a switch is used, it is possible that a small number of ancillary devices (e.g., a laptop) are also plugged into the switch. But in either event, the resulting network segment is constrained to a small number of devices and is not generally accessible from anywhere else in the network. The other type of deployment envisioned can be viewed as a "networked deployment". In this type of scenario, the DLEP router and modem (or modems) are placed on a segment that is accessible from other points in the network. In this scenario, not only are the DLEP router and modem(s) accessible from other points in the network; the router and a given modem could be multiple physical hops away from each other. This scenario necessitates the use of Layer 2 tunneling technology to enforce the single-hop requirement of DLEP.5. Assumptions
DLEP assumes that a signaling protocol exists between modems participating in a network. This specification does not define the character or behavior of this over-the-air signaling but does expect some information to be carried (or derived) by the signaling, such as the arrival and departure of modems from this network, and the variation of the link characteristics between modems. This information is then assumed to be used by the modem to implement DLEP. This specification assumes that the link between router and modem is static with respect to data rate and latency and that this link is not likely to be the cause of a performance bottleneck. In deployments where the router and modem are physically separated by multiple network hops, served by Layer 2 tunneling technology, DLEP statistics on the RF links could be insufficient for routing protocols to make appropriate routing decisions. This would
especially become an issue in cases where the Layer 2 tunnel between router and modem is itself served in part (or in total) with a wireless backhaul link.6. Metrics
DLEP includes the ability for the router and modem to communicate metrics that reflect the characteristics (e.g., data rate, latency) of the variable-quality link in use. DLEP does not specify how a given metric value is to be calculated; rather, the protocol assumes that metrics have been calculated by a "best effort", incorporating all pertinent data that is available to the modem device. Metrics based on large-enough sample sizes will preclude short traffic bursts from adversely skewing reported values. DLEP allows for metrics to be sent within two contexts -- metrics for a specific destination within the network (e.g., a specific router), and "per session" (those that apply to all destinations accessed via the modem). Most metrics can be further subdivided into transmit and receive metrics. In cases where metrics are provided at the session level, the router propagates the metrics to all entries in its information base for destinations that are accessed via the modem. DLEP modems announce all metric Data Items that will be reported during the session, and provide default values for those metrics, in the Session Initialization Response Message (Section 12.6). In order to use a metric type that was not included in the Session Initialization Response Message, modem implementations terminate the session with the router (via the Session Termination Message (Section 12.9)) and establish a new session. A DLEP modem can send metrics in both (1) a session context, via the Session Update Message (Section 12.7) and (2) a specific destination context, via the Destination Update Message (Section 12.17), at any time. The most recently received metric value takes precedence over any earlier value, regardless of context -- that is: 1. If the router receives metrics in a specific destination context (via the Destination Update Message), then the specific destination is updated with the new metric. 2. If the router receives metrics in a session-wide context (via the Session Update Message), then the metrics for all destinations accessed via the modem are updated with the new metric.
It is left to implementations to choose sensible default values based on their specific characteristics. Modems having static (non-changing) link metric characteristics can report metrics only once for a given destination (or once on a session-wide basis, if all connections via the modem are of this static nature). In addition to communicating existing metrics about the link, DLEP provides a Message allowing a router to request a different data rate or latency from the modem. This Message is the Link Characteristics Request Message (Section 12.18); it gives the router the ability to deal with requisite increases (or decreases) of allocated data rate/latency in demand-based schemes in a more deterministic manner.7. DLEP Session Flow
All DLEP participants of a session transition through a number of distinct states during the lifetime of a DLEP session: o Peer Discovery o Session Initialization o In-Session o Session Termination o Session Reset Modems, and routers supporting DLEP discovery, transition through all five of the above states. Routers that rely on preconfigured TCP address/port information start in the Session Initialization state. Modems MUST support the Peer Discovery state.7.1. Peer Discovery State
Modems MUST support DLEP Peer Discovery; routers MAY support the discovery signals or rely on a priori configuration to locate modems. If a router chooses to support DLEP discovery, all signals MUST be supported. In the Peer Discovery state, routers that support DLEP discovery MUST send Peer Discovery Signals (Section 12.3) to initiate modem discovery.
The router implementation then waits for a Peer Offer Signal (Section 12.4) response from a potential DLEP modem. While in the Peer Discovery state, Peer Discovery Signals MUST be sent repeatedly by a DLEP router, at regular intervals. It is RECOMMENDED that this interval be set to 60 seconds. The interval MUST be a minimum of 1 second; it SHOULD be a configurable parameter. Note that this operation (sending Peer Discovery and waiting for Peer Offer) is outside the DLEP transaction model (Section 8), as the transaction model only describes Messages on a TCP session. Routers receiving a Peer Offer Signal MUST use one of the modem address/port combinations from the Peer Offer Signal to establish a TCP connection to the modem, even if a priori configuration exists. If multiple Connection Point Data Items exist in the received Peer Offer Signal, routers SHOULD prioritize IPv6 connection points over IPv4 connection points. If multiple connection points exist with the same transport (e.g., IPv6 or IPv4), implementations MAY use their own heuristics to determine the order in which they are tried. If a TCP connection cannot be achieved using any of the address/port combinations and the Discovery mechanism is in use, then the router SHOULD resume issuing Peer Discovery Signals. If no Connection Point Data Items are included in the Peer Offer Signal, the router MUST use the source address of the UDP packet containing the Peer Offer Signal as the IP address, and the DLEP well-known port number. In the Peer Discovery state, the modem implementation MUST listen for incoming Peer Discovery Signals on the DLEP well-known IPv6 and/or IPv4 link-local multicast address and port. On receipt of a valid Peer Discovery Signal, it MUST reply with a Peer Offer Signal. Modems MUST be prepared to accept a TCP connection from a router that is not using the Discovery mechanism, i.e., a connection attempt that occurs without a preceding Peer Discovery Signal. Implementations of DLEP SHOULD implement, and use, Transport Layer Security (TLS) [RFC5246] to protect the TCP session. The "dedicated deployments" discussed in "Implementation Scenarios" (Section 4) MAY consider the use of DLEP without TLS. For all "networked deployments" (again, discussed in "Implementation Scenarios"), the implementation and use of TLS are STRONGLY RECOMMENDED. If TLS is to be used, then the TLS session MUST be established before any Messages are passed between peers. Routers supporting TLS MUST prioritize connection points using TLS over those that do not. Upon establishment of a TCP connection, and the establishment of a TLS session if TLS is in use, both modem and router enter the Session Initialization state. It is up to the router implementation if Peer Discovery Signals continue to be sent after the device has
transitioned to the Session Initialization state. Modem implementations MUST silently ignore Peer Discovery Signals from a router with which a given implementation already has a TCP connection.7.2. Session Initialization State
On entering the Session Initialization state, the router MUST send a Session Initialization Message (Section 12.5) to the modem. The router MUST then wait for receipt of a Session Initialization Response Message (Section 12.6) from the modem. Receipt of the Session Initialization Response Message containing a Status Data Item (Section 13.1) with status code set to 0 'Success' (see Table 2 in Section 13.1) indicates that the modem has received and processed the Session Initialization Message, and the router MUST transition to the In-Session state. On entering the Session Initialization state, the modem MUST wait for receipt of a Session Initialization Message from the router. Upon receipt of a Session Initialization Message, the modem MUST send a Session Initialization Response Message, and the session MUST transition to the In-Session state. If the modem receives any Message other than Session Initialization or it fails to parse the received Message, it MUST NOT send any Message, and it MUST terminate the TCP connection and transition to the Session Reset state. DLEP provides an extension negotiation capability to be used in the Session Initialization state; see Section 9. Extensions supported by an implementation MUST be declared to potential DLEP participants using the Extensions Supported Data Item (Section 13.6). Once both DLEP participants have exchanged initialization Messages, an implementation MUST NOT emit any Message, Signal, Data Item, or status code associated with an extension that was not specified in the received initialization Message from its peer.7.3. In-Session State
In the In-Session state, Messages can flow in both directions between DLEP participants, indicating changes to the session state, the arrival or departure of reachable destinations, or changes of the state of the links to the destinations.
The In-Session state is maintained until one of the following
conditions occurs:
o The implementation terminates the session by sending a Session
Termination Message (Section 12.9), or
o Its peer terminates the session, indicated by receiving a Session
Termination Message.
The implementation MUST then transition to the Session Termination
state.
7.3.1. Heartbeats
In order to maintain the In-Session state, periodic Heartbeat
Messages (Section 12.20) MUST be exchanged between router and modem.
These Messages are intended to keep the session alive and to verify
bidirectional connectivity between the two DLEP participants. It is
RECOMMENDED that the interval timer between Heartbeat Messages be set
to 60 seconds. The interval MUST be a minimum of 1 second; it SHOULD
be a configurable parameter.
Each DLEP participant is responsible for the creation of Heartbeat
Messages.
Receipt of any valid DLEP Message MUST reset the heartbeat interval
timer (i.e., valid DLEP Messages take the place of, and obviate the
need for, additional Heartbeat Messages).
An implementation MUST allow a minimum of 2 heartbeat intervals to
expire with no Messages from its peer before terminating the session.
When terminating the session, a Session Termination Message
containing a Status Data Item (Section 13.1) with status code set to
132 'Timed Out' (see Table 2) MUST be sent, and then the
implementation MUST transition to the Session Termination state.
7.4. Session Termination State
When an implementation enters the Session Termination state after
sending a Session Termination Message (Section 12.9) as the result of
an invalid Message or error, it MUST wait for a Session Termination
Response Message (Section 12.10) from its peer. A sender SHOULD
allow 4 heartbeat intervals to expire before assuming that its peer
is unresponsive and before continuing with session termination. Any
other Message received while waiting MUST be silently ignored.
When the sender of the Session Termination Message receives a Session Termination Response Message from its peer or times out, it MUST transition to the Session Reset state. When an implementation receives a Session Termination Message from its peer, it enters the Session Termination state, and then it MUST immediately send a Session Termination Response and transition to the Session Reset state.7.5. Session Reset State
In the Session Reset state, the implementation MUST perform the following actions: o Release all resources allocated for the session. o Eliminate all destinations in the information base represented by the session. Destination Down Messages (Section 12.15) MUST NOT be sent. o Terminate the TCP connection. Having completed these actions, the implementation SHOULD return to the relevant initial state: o For modems: Peer Discovery. o For routers: either Peer Discovery or Session Initialization, depending on configuration.7.5.1. Unexpected TCP Connection Termination
If the TCP connection between DLEP participants is terminated when an implementation is not in the Session Reset state, the implementation MUST immediately transition to the Session Reset state.8. Transaction Model
DLEP defines a simple Message transaction model: only one request per destination may be in progress at a time per session. A Message transaction is considered complete when a response matching a previously issued request is received. If a DLEP participant receives a request for a destination for which there is already an outstanding request, the implementation MUST terminate the session by issuing a Session Termination Message (Section 12.9) containing a Status Data Item (Section 13.1) with status code set to 129 'Unexpected Message' (see Table 2) and transition to the Session
Termination state. There is no restriction on the total number of Message transactions in progress at a time, as long as each transaction refers to a different destination. It should be noted that some requests may take a considerable amount of time for some DLEP participants to complete; for example, a modem handling a multicast Destination Up request may have to perform a complex network reconfiguration. A sending implementation MUST be able to handle such long-running transactions gracefully. Additionally, only one session request, e.g., a Session Initialization Message (Section 12.5), may be in progress at a time per session. As noted above for Message transactions, a session transaction is considered complete when a response matching a previously issued request is received. If a DLEP participant receives a session request while there is already a session request in progress, it MUST terminate the session by issuing a Session Termination Message containing a Status Data Item with status code set to 129 'Unexpected Message' and transition to the Session Termination state. Only the Session Termination Message may be issued when a session transaction is in progress. Heartbeat Messages (Section 12.20) MUST NOT be considered part of a session transaction. DLEP transactions do not time out and are not cancellable, except for transactions in flight when the DLEP session is reset. If the session is terminated, canceling transactions in progress MUST be performed as part of resetting the state machine. An implementation can detect if its peer has failed in some way by use of the session heartbeat mechanism during the In-Session state; see Section 7.3.9. Extensions
Extensions MUST be negotiated on a per-session basis during session initialization via the Extensions Supported mechanism. Implementations are not required to support any extensions in order to be considered DLEP compliant. If interoperable protocol extensions are required, they will need to be standardized as either (1) an update to this document or (2) an additional standalone specification. The IANA registries defined in Section 15 of this document contain sufficient unassigned space for DLEP Signals, Messages, Data Items, and status codes to accommodate future extensions to the protocol. As multiple protocol extensions MAY be announced during session initialization, authors of protocol extensions need to consider the interaction of their extensions with other published extensions and specify any incompatibilities.
9.1. Experiments
This document registers Private Use [RFC5226] numbering space in the DLEP Signal, Message, Data Item, and status code registries for experimental extensions. The intent is to allow for experimentation with new Signals, Messages, Data Items, and/or status codes while still retaining the documented DLEP behavior. During session initialization, the use of the Private Use Signals, Messages, Data Items, status codes, or behaviors MUST be announced as DLEP extensions, using extension identifiers from the Private Use space in the "Extension Type Values" registry (Table 3), with a value agreed upon (a priori) between the participants. DLEP extensions using the Private Use numbering space are commonly referred to as "experiments". Multiple experiments MAY be announced in the Session Initialization Messages. However, the use of multiple experiments in a single session could lead to interoperability issues or unexpected results (e.g., clashes of experimental Signals, Messages, Data Items, and/or status code types) and is therefore discouraged. It is left to implementations to determine the correct processing path (e.g., a decision on whether to terminate the session or establish a precedence of the conflicting definitions) if such conflicts arise.10. Scalability
The protocol is intended to support thousands of destinations on a given modem/router pair. On a large scale, an implementation should consider employing techniques to prevent flooding its peer with a large number of Messages in a short time. For example, a dampening algorithm could be employed to prevent a flapping device from generating a large number of Destination Up / Destination Down Messages. Also, the use of techniques such as a hysteresis can lessen the impact of rapid, minor fluctuations in link quality. The specific algorithms for handling flapping destinations and minor changes in link quality are outside the scope of this specification.
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