+------------------------------------+
|             IP Header              |
+------------------------------------+
|      Transport Header (UDP)        |
+------------------------------------+
|             RTP Header             |
+------------------------------------+
|       Repair FEC Payload ID        |
+------------------------------------+
|          Repair Symbols            |
+------------------------------------+

Figure 10: Packet Format for FEC Repair Packets over RTP

5.5. FEC Framework Configuration Information

The FEC Framework Configuration Information is information that the FEC Framework needs in order to apply FEC protection to the ADU flows. A complete CDP specification that uses the framework specified here MUST include details of how this information is derived and communicated between sender and receiver.

The FEC Framework Configuration Information includes identification of the set of source flows. For example, in the case of UDP, each source flow is uniquely identified by a tuple {source IP address, source UDP port, destination IP address, destination UDP port}. In some applications, some of these fields can contain wildcards, so that the flow is identified by a subset of the fields. In particular, in many applications the limited tuple {destination IP address, destination UDP port} is sufficient.

A single instance of the FEC Framework provides FEC protection for packets of the specified set of source flows, by means of one or more packet flows consisting of repair packets. The FEC Framework Configuration Information includes, for each instance of the FEC Framework:

1. Identification of the repair flows.
2. For each source flow protected by the repair flow(s):
   1. Definition of the source flow.
   2. An integer identifier for this flow definition (i.e., tuple). This identifier MUST be unique among all source flows that are protected by the same FEC repair flow. Integer identifiers can be allocated starting from zero and increasing by one for each flow. However, any random (but still unique) allocation is also possible. A source flow identifier need not be carried in source packets, since source packets are directly associated with a flow by virtue of their packet headers.
3. The FEC Encoding ID, identifying the FEC scheme.
4. The length of the Explicit Source FEC Payload ID (in octets).
5. Zero or more FEC-Scheme-Specific Information (FSSI) elements, each consisting of a name and a value where the valid element names and value ranges are defined by the FEC scheme.

Multiple instances of the FEC Framework, with separate and independent FEC Framework Configuration Information, can be present at a sender or receiver. A single instance of the FEC Framework protects packets of the source flows identified in (2) above; i.e., all packets sent on those flows MUST be FEC source packets as defined in Section 5.3. A single source flow can be protected by multiple instances of the FEC Framework.

The integer flow identifier identified in (2B) above is a shorthand to identify source flows between the FEC Framework and the FEC scheme. The reason for defining this as an integer, and including it in the FEC Framework Configuration Information, is so that the FEC scheme at the sender and receiver can use it to identify the source flow with which a recovered packet is associated. The integer flow identifier can therefore take the place of the complete flow description (e.g., UDP 4-tuple).

Whether and how this flow identifier is used is defined by the FEC scheme. Since repair packets can provide protection for multiple source flows, repair packets either would not carry the identifier at all or can carry multiple identifiers. However, in any case, the flow identifier associated with a particular source packet can be recovered from the repair packets as part of a FEC decoding operation.

A single FEC repair flow provides repair packets for a single instance of the FEC Framework. Other packets MUST NOT be sent within this flow; i.e., all packets in the FEC repair flow MUST be FEC repair packets as defined in Section 5.4 and MUST relate to the same FEC Framework instance.

In the case that RTP is used for repair packets, the identification of the repair packet flow can also include the RTP payload type to be used for repair packets.

FSSI includes the information that is specific to the FEC scheme used by the CDP. FSSI is used to communicate the information that cannot be adequately represented otherwise and is essential for proper FEC encoding and decoding operations. The motivation behind separating the FSSI required only by the sender (which is carried in a Sender-Side FEC-Scheme-Specific Information (SS-FSSI) container) from the rest of the FSSI is to provide the receiver or the third-party entities a means of controlling the FEC operations at the sender. Any FSSI other than the one solely required by the sender MUST be communicated via the FSSI container.

The variable-length SS-FSSI and FSSI containers transmit the information in textual representation and contain zero or more distinct elements, whose descriptions are provided by the fully specified FEC schemes.

For the CDPs that choose the Session Description Protocol (SDP) [RFC4566] for their multimedia sessions, the ABNF [RFC5234] syntax for the SS-FSSI and FSSI containers is provided in Section 4.5 of [RFC6364].

5.6. FEC Scheme Requirements

In order to be used with this framework, a FEC scheme MUST be capable of processing data either arranged into blocks of ADUs (source blocks) in case of a Block FEC Code, or arranged as a continuous flow of ADUs in case of a Convolutional FEC Code.

A specification for a new FEC scheme MUST include the following:

1. The FEC Encoding ID value that uniquely identifies the FEC scheme. This value MUST be registered with IANA, as described in Section 11.
2. The type, semantics, and encoding format of the Repair FEC Payload ID.
3. The name, type, semantics, and text value encoding rules for zero or more FEC-Scheme-Specific Information elements.
4. A full specification of the FEC code.
   
   This specification MUST precisely define the valid FEC-Scheme-Specific Information values, the valid FEC Payload ID values, and the valid packet payload sizes (where packet payload refers to the space within a packet dedicated to carrying encoding symbols).
   
   Furthermore, given valid values of the FEC-Scheme-Specific Information, a valid Repair FEC Payload ID value, a valid packet payload size and in case of a Block FEC Code a source block as defined in Section 5.2, the specification MUST uniquely define the values of the encoding symbols to be included in the repair packet payload of a packet with the given Repair FEC Payload ID value.
   
   A common and simple way to specify the FEC code to the required level of detail is to provide a precise specification of an encoding algorithm that -- given valid values of the FEC-Scheme-Specific Information, a valid Repair FEC Payload ID value, a valid packet payload size, and in case of a Block FEC Code a source block as input -- produces the exact value of the encoding symbols as output.
5. A description of practical encoding and decoding algorithms.
   
   This description need not be to the same level of detail as for the encoding above; however, it has to be sufficient to demonstrate that encoding and decoding of the code are both possible and practical.

FEC scheme specifications MAY additionally define the following:

• Type, semantics, and encoding format of an Explicit Source FEC Payload ID.

Whenever a FEC scheme specification defines an 'encoding format' for an element, this has to be defined in terms of a sequence of bytes that can be embedded within a protocol. The length of the encoding format either MUST be fixed or it MUST be possible to derive the length from examining the encoded bytes themselves. For example, the initial bytes can include some kind of length indication.

FEC scheme specifications SHOULD use the terminology defined in this document and SHOULD follow the following format:

1. Introduction

<Describe the use cases addressed by this FEC scheme>

2. Formats and Codes

2.1. Source FEC Payload ID(s)

<Either define the type and format of the Explicit Source FEC Payload ID or define how Source FEC Payload ID information is derived from source packets>

2.2. Repair FEC Payload ID

<Define the type and format of the Repair FEC Payload ID>

2.3. FEC Framework Configuration Information

<Define the names, types, and text value encoding formats of the FEC-Scheme-Specific Information elements>

3. Procedures

<Describe any procedures that are specific to this FEC scheme, in particular derivation and interpretation of the fields in the FEC Payload IDs and FEC-Scheme-Specific Information>

4. FEC Code Specification

<Provide a complete specification of the FEC Code>

Specifications can include additional sections including examples.

Each FEC scheme MUST be specified independently of all other FEC schemes, for example, in a separate specification or a completely independent section of a larger specification (except, of course, a specification of one FEC scheme can include portions of another by reference). Where an RTP payload format is defined for repair data for a specific FEC scheme, the RTP payload format and the FEC scheme can be specified within the same document.

6. Feedback

Many applications require some kind of feedback on transport performance, e.g., how much data arrived at the receiver, at what rate, and when? When FEC is added to such applications, feedback mechanisms may also need to be enhanced to report on the performance of the FEC, e.g., how much lost data was recovered by the FEC?

When used to provide instrumentation for engineering purposes, it is important to remember that FEC is generally applied to relatively small sets of data (in the sense that each block or symbols of an encoding window is transmitted over a relatively small period of time). Thus, feedback information that is averaged over longer periods of time will likely not provide sufficient information for engineering purposes. More detailed feedback over shorter time scales might be preferred. For example, for applications using RTP transport, see [RFC5725].

Applications that use feedback for congestion control purposes MUST calculate such feedback on the basis of packets received before FEC recovery is applied. If this requirement conflicts with other uses of the feedback information, then the application MUST be enhanced to support information calculated both pre- and post-FEC recovery. This is to ensure that congestion control mechanisms operate correctly based on congestion indications received from the network, rather than on post-FEC recovery information that would give an inaccurate picture of congestion conditions.

New applications that require such feedback SHOULD use RTP/RTCP [RFC3550].

7. Transport Protocols

This framework is intended to be used to define CDPs that operate over transport protocols providing an unreliable datagram service, including in particular the User Datagram Protocol (UDP) and the Datagram Congestion Control Protocol (DCCP).

8. Congestion Control

This section starts with some informative background on the motivation of the normative requirements for congestion control, which are spelled out in Section 8.2.

8.1. Motivation

• The enforcement of congestion control principles has gained a lot of momentum in the IETF over recent years. While the need for congestion control over the open Internet is unquestioned, and the goal of TCP friendliness is generally agreed upon for most (but not all) applications, the problem of congestion detection and measurement in heterogeneous networks can hardly be considered solved. Most congestion control algorithms detect and measure congestion by taking (primarily or exclusively) the packet loss rate into account. This appears to be inappropriate in environments where a large percentage of the packet losses are the result of link-layer errors and independent of the network load.
• The authors of this document are primarily interested in applications where the application reliability requirements and end-to-end reliability of the network differ, such that it warrants higher-layer protection of the packet stream, e.g., due to the presence of unreliable links in the end-to-end path and where real-time, scalability, or other constraints prohibit the use of higher-layer (transport or application) feedback. A typical example for such applications is multicast and broadcast streaming or multimedia transmission over heterogeneous networks. In other cases, application reliability requirements can be so high that the required end-to-end reliability will be difficult to achieve. Furthermore, the end-to-end network reliability is not necessarily known in advance.
• This FEC Framework is not defined as, nor is it intended to be, a quality-of-service (QoS) enhancement tool to combat losses resulting from highly congested networks. It should not be used for such purposes.
• In order to prevent such misuse, one approach is to leave standardization to bodies most concerned with the problem described above. However, the IETF defines base standards used by several bodies, including the Digital Video Broadcasting (DVB) Project, the Third Generation Partnership Project (3GPP), and 3GPP2, all of which appear to share the environment and the problem described.
• Another approach is to write a clear applicability statement. For example, one could restrict the use of this framework to networks with certain loss characteristics (e.g., wireless links). However, there can be applications where the use of FEC is justified to combat congestion-induced packet losses -- particularly in lightly loaded networks, where congestion is the result of relatively rare random peaks in instantaneous traffic load -- thereby intentionally violating congestion control principles. One possible example for such an application could be a no-matter-what, brute-force FEC protection of traffic generated as an emergency signal.
• A third approach is to require, at a minimum, that the use of this framework with any given application, in any given environment, does not cause congestion issues that the application alone would not itself cause; i.e., the use of this framework must not make things worse.
• Taking the above considerations into account, Section 8.2 specifies a small set of constraints for FEC; these constraints are mandatory for all senders compliant with this FEC Framework. Further restrictions can be imposed by certain CDPs.

8.2. Normative Requirements

• The bandwidth of FEC repair data MUST NOT exceed the bandwidth of the original source data being protected (without the possible addition of an Explicit Source FEC Payload ID). This disallows the (static or dynamic) use of excessively strong FEC to combat high packet loss rates, which can otherwise be chosen by naively implemented dynamic FEC-strength selection mechanisms. We acknowledge that there are a few exotic applications, e.g., IP traffic from space-based senders, or senders in certain hardened military devices, that could warrant a higher FEC strength. However, in this specification, we give preference to the overall stability and network friendliness of average applications.
• Whenever the source data rate is adapted due to the operation of congestion control mechanisms, the FEC repair data rate MUST be similarly adapted.

9. Security Considerations

First of all, it must be clear that the application of FEC protection to a stream does not provide any kind of security. On the contrary, the FEC Framework itself could be subject to attacks or could pose new security risks. The goals of this section are to state the problem, discuss the risks, and identify solutions when feasible. It also defines a mandatory-to-implement (but not mandatory-to-use) security scheme.

9.1. Problem Statement

A content delivery system is potentially subject to many attacks. Attacks can target the content, the CDP, or the network itself, with completely different consequences, particularly in terms of the number of impacted nodes.

Attacks can have several goals:

• They can try to give access to confidential content (e.g., in the case of non-free content).
• They can try to corrupt the source flows (e.g., to prevent a receiver from using them), which is a form of denial-of-service (DoS) attack.
• They can try to compromise the receiver's behavior (e.g., by making the decoding of an object computationally expensive), which is another form of DoS attack.
• They can try to compromise the network's behavior (e.g., by causing congestion within the network), which potentially impacts a large number of nodes.

These attacks can be launched either against the source and/or repair flows (e.g., by sending fake FEC source and/or repair packets) or against the FEC parameters that are sent either in-band (e.g., in the Repair FEC Payload ID or in the Explicit Source FEC Payload ID) or out-of-band (e.g., in the FEC Framework Configuration Information).

Several dimensions to the problem need to be considered. The first one is the way the FEC Framework is used. The FEC Framework can be used end-to-end, i.e., it can be included in the final end-device where the upper application runs, or the FEC Framework can be used in middleboxes, for instance, to globally protect several source flows exchanged between two or more distant sites.

A second dimension is the threat model. When the FEC Framework operates in the end-device, this device (e.g., a personal computer) might be subject to attacks. Here, the attacker is either the end-user (who might want to access confidential content) or somebody else. In all cases, the attacker has access to the end-device but does not necessarily fully control this end-device (a secure domain can exist). Similarly, when the FEC Framework operates in a middlebox, this middlebox can be subject to attacks or the attacker can gain access to it. The threats can also concern the end-to-end transport (e.g., through the Internet). Here, examples of threats include the transmission of fake FEC source or repair packets; the replay of valid packets; the drop, delay, or misordering of packets; and, of course, traffic eavesdropping.

The third dimension consists in the desired security services. Among them, the content integrity and sender authentication services are probably the most important features. We can also mention DoS mitigation, anti-replay protection, or content confidentiality.

Finally, the fourth dimension consists in the security tools available. This is the case of the various Digital Rights Management (DRM) systems, defined outside of the context of the IETF, that can be proprietary solutions. Otherwise, the Secure Real-Time Transport Protocol (SRTP) [RFC3711] and IPsec/Encapsulating Security Payload (IPsec/ESP) [RFC4303] are two tools that can turn out to be useful in the context of the FEC Framework. Note that using SRTP requires that the application generate RTP source flows and, when applied below the FEC Framework, that both the FEC source and repair packets be regular RTP packets. Therefore, SRTP is not considered to be a universal solution applicable in all use cases.

In the following sections, we further discuss security aspects related to the use of the FEC Framework.

9.2. Attacks against the Data Flows

9.2.1. Access to Confidential Content

Access control to the source flow being transmitted is typically provided by means of encryption. This encryption can be done by the content provider itself, or within the application (for instance, by using SRTP [RFC3711]), or at the network layer on a per-packet basis when IPsec/ESP is used [RFC4303]. If confidentiality is a concern, it is RECOMMENDED that one of these solutions be used. Even if we mention these attacks here, they are neither related to nor facilitated by the use of FEC.

Note that when encryption is applied, this encryption MUST be applied either on the source data before the FEC protection or, if done after the FEC protection, on both the FEC source packets and repair packets (and an encryption at least as cryptographically secure as the encryption applied on the FEC source packets MUST be used for the FEC repair packets). Otherwise, if encryption were to be performed only on the FEC source packets after FEC encoding, a non-authorized receiver could be able to recover the source data after decoding the FEC repair packets, provided that a sufficient number of such packets were available.

The following considerations apply when choosing where to apply encryption (and more generally where to apply security services beyond encryption). Once decryption has taken place, the source data is in plaintext. The full path between the output of the deciphering module and the final destination (e.g., the TV display in the case of a video) MUST be secured, in order to prevent any unauthorized access to the source data.

When the FEC Framework endpoint is the end-system (i.e., where the upper application runs) and if the threat model includes the possibility that an attacker has access to this end-system, then the end-system architecture is very important. More precisely, in order to prevent an attacker from getting hold of the plaintext, all processing, once deciphering has taken place, MUST occur in a protected environment. If encryption is applied after FEC protection at the sending side (i.e., below the FEC Framework), it means that FEC decoding MUST take place in the protected environment. With certain use cases, this MAY be complicated or even impossible. In such cases, applying encryption before FEC protection is preferred.

When the FEC Framework endpoint is a middlebox, the recovered source flow, after FEC decoding, SHOULD NOT be sent in plaintext to the final destination(s) if the threat model includes the possibility that an attacker eavesdrops on the traffic. In that case, it is preferable to apply encryption before FEC protection.

In some cases, encryption could be applied both before and after the FEC protection. The considerations described above still apply in such cases.

9.2.2. Content Corruption

Protection against corruptions (e.g., against forged FEC source/repair packets) is achieved by means of a content integrity verification/source authentication scheme. This service is usually provided at the packet level. In this case, after removing all the forged packets, the source flow might sometimes be recovered. Several techniques can provide this content integrity/source authentication service:

• At the application layer, SRTP [RFC3711] provides several solutions to check the integrity and authenticate the source of RTP and RTCP messages, among other services. For instance, when associated with the Timed Efficient Stream Loss-Tolerant Authentication (TESLA) [RFC4383], SRTP is an attractive solution that is robust to losses, provides a true authentication/integrity service, and does not create any prohibitive processing load or transmission overhead. Yet, with TESLA, checking a packet requires a small delay (a second or more) after its reception. Whether or not this extra delay, both in terms of startup delay at the client and end-to-end delay, is appropriate depends on the target use case. In some situations, this might degrade the user experience. In other situations, this will not be an issue. Other building blocks can be used within SRTP to provide content integrity/authentication services.
• At the network layer, IPsec/ESP [RFC4303] offers (among other services) an integrity verification mechanism that can be used to provide authentication/content integrity services.

It is up to the developer and the person in charge of deployment, who know the security requirements and features of the target application area, to define which solution is the most appropriate. Nonetheless, it is RECOMMENDED that at least one of these techniques be used.

Note that when integrity protection is applied, it is RECOMMENDED that it take place on both FEC source and repair packets. The motivation is to keep corrupted packets from being considered during decoding, as such packets would often lead to a decoding failure or result in a corrupted decoded source flow.

9.3. Attacks against the FEC Parameters

Attacks on these FEC parameters can prevent the decoding of the associated object. For instance, modifying the finite field size of a Reed-Solomon FEC scheme (when applicable) will lead a receiver to consider a different FEC code.

Therefore, it is RECOMMENDED that security measures be taken to guarantee the integrity of the FEC Framework Configuration Information. Since the FEC Framework does not define how the FEC Framework Configuration Information is communicated from sender to receiver, we cannot provide further recommendations on how to guarantee its integrity. However, any complete CDP specification MUST give recommendations on how to achieve it. When the FEC Framework Configuration Information is sent out-of-band, e.g., in a session description, it SHOULD be protected, for instance, by digitally signing it.

Attacks are also possible against some FEC parameters included in the Explicit Source FEC Payload ID and Repair FEC Payload ID. For instance, with a Block FEC Code, modifying the Source Block Number of a FEC source or repair packet will lead a receiver to assign this packet to a wrong block.

Therefore, it is RECOMMENDED that security measures be taken to guarantee the integrity of the Explicit Source FEC Payload ID and Repair FEC Payload ID. To that purpose, one of the packet-level source authentication/content integrity techniques described in Section 9.2.2 can be used.

9.4. When Several Source Flows Are to Be Protected Together

When several source flows, with different security requirements, need to be FEC protected jointly, within a single FEC Framework instance, then each flow MAY be processed appropriately, before the protection. For instance, source flows that require access control MAY be encrypted before they are FEC protected.

There are also situations where the only insecure domain is the one over which the FEC Framework operates. In that case, this situation MAY be addressed at the network layer, using IPsec/ESP (see Section 9.5), even if only a subset of the source flows has strict security requirements.

Since the use of the FEC Framework should not add any additional threat, it is RECOMMENDED that the FEC Framework aggregate flow be in line with the maximum security requirements of the individual source flows. For instance, if denial-of-service (DoS) protection is required, an integrity protection SHOULD be provided below the FEC Framework, using, for instance, IPsec/ESP.

Generally speaking, whenever feasible, it is RECOMMENDED that FEC protecting flows with totally different security requirements be avoided. Otherwise, significant processing overhead would be added to protect source flows that do not need it.

9.5. Baseline Secure FEC Framework Operation

The FEC Framework has been defined in such a way to be independent from the application that generates source flows. Some applications might use purely unidirectional flows, while other applications might also use unicast feedback from the receivers. For instance, this is the case when considering RTP/RTCP-based source flows.

This section describes a baseline mode of secure FEC Framework operation based on the application of the IPsec protocol, which is one possible solution to solve or mitigate the security threats introduced by the use of the FEC Framework.

Two related documents are of interest. First, Section 5.1 of [RFC5775] defines a baseline secure Asynchronous Layered Coding (ALC) operation for sender-to-group transmissions, assuming the presence of a single sender and a source-specific multicast (SSM) or SSM-like operation. The proposed solution, based on IPsec/ESP, can be used to provide a baseline FEC Framework secure operation, for the downstream source flow.

Second, Section 7.1 of [RFC5740] defines a baseline secure NACK-Oriented Reliable Multicast (NORM) operation, for sender-to-group transmissions as well as unicast feedback from receivers. Here, it is also assumed there is a single sender. The proposed solution is also based on IPsec/ESP. However, the difference with respect to [RFC5775] relies on the management of IPsec Security Associations (SAs) and corresponding Security Policy Database (SPD) entries, since NORM requires a second set of SAs and SPD entries to be defined to protect unicast feedback from receivers.

Note that the IPsec/ESP requirement profiles outlined in [RFC5775] and [RFC5740] are commonly available on many potential hosts. They can form the basis of a secure mode of operation. Configuration and operation of IPsec typically require privileged user authorization. Automated key management implementations are typically configured with the privileges necessary to allow the needed system IPsec configuration.

10. Operations and Management Considerations

The question of operating and managing the FEC Framework and the associated FEC scheme(s) is of high practical importance. The goals of this section are to discuss aspects and recommendations related to specific deployments and solutions.

In particular, this section discusses the questions of interoperability across vendors/use cases and whether defining mandatory-to-implement (but not mandatory-to-use) solutions is beneficial.

10.1. What Are the Key Aspects to Consider?

Several aspects need to be considered, since they will directly impact the way the FEC Framework and the associated FEC schemes can be operated and managed.

This section lists them as follows:

1. A Single Small Generic Component within a Larger (and Often Legacy) Solution: The FEC Framework is one component within a larger solution that includes one or several upper-layer applications (that generate one or several ADU flows) and an underlying protocol stack. A key design principle is that the FEC Framework should be able to work without making any assumption with respect to either the upper-layer application(s) or the underlying protocol stack, even if there are special cases where assumptions are made.
2. One-to-One with Feedback vs. One-to-Many with Feedback vs. One-to-Many without Feedback Scenarios: The FEC Framework can be used in use cases that completely differ from one another. Some use cases are one-way (e.g., in broadcast networks), with either a one-to-one, one-to-many, or many-to-many transmission model, and the receiver(s) cannot send any feedback to the sender(s). Other use cases follow a bidirectional one-to-one, one-to-many, or many-to-many scenario, and the receiver(s) can send feedback to the sender(s).
3. Non-FEC Framework Capable Receivers: With the one-to-many and many-to-many use cases, the receiver population might have different capabilities with respect to the FEC Framework itself and the supported FEC schemes. Some receivers might not be capable of decoding the repair packets belonging to a particular FEC scheme, while some other receivers might not support the FEC Framework at all.
4. Internet vs. Non-Internet Networks: The FEC Framework can be useful in many use cases that use a transport network that is not the public Internet (e.g., with IPTV or Mobile TV). In such networks, the operational and management considerations can be achieved through an open or proprietary solution, which is specified outside of the IETF.
5. Congestion Control Considerations: See Section 8 for a discussion on whether or not congestion control is needed, and its relationships with the FEC Framework.
6. Within End-Systems vs. within Middleboxes: The FEC Framework can be used within end-systems, very close to the upper-layer application, or within dedicated middleboxes (for instance, when it is desired to protect one or several flows while they cross a lossy channel between two or more remote sites).
7. Protecting a Single Flow vs. Several Flows Globally: The FEC Framework can be used to protect a single flow or several flows globally.

10.2. Operational and Management Recommendations

Overall, from the discussion in Section 10.1, it is clear that the CDPs and FEC schemes compatible with the FEC Framework differ widely in their capabilities, application, and deployment scenarios such that a common operation and management method or protocol that works well for all of them would be too complex to define. Thus, as a design choice, the FEC Framework does not dictate the use of any particular technology or protocol for transporting FEC data, managing the hosts, signaling the configuration information, or encoding the configuration information. This provides flexibility and is one of the main goals of the FEC Framework. However, this section gives some RECOMMENDED guidelines.

1. A Single Small Generic Component within a Larger (and Often Legacy) Solution: It is anticipated that the FEC Framework will often be used to protect one or several RTP streams. Therefore, implementations SHOULD make feedback information accessible via RTCP to enable users to take advantage of the tools using (or used by) RTCP to operate and manage the FEC Framework instance along with the associated FEC schemes.
2. One-to-One with Feedback vs. One-to-Many with Feedback vs. One-to-Many without Feedback Scenarios: With use cases that are one-way, the FEC Framework sender does not have any way to gather feedback from receivers. With use cases that are bidirectional, the FEC Framework sender can collect detailed feedback (e.g., in the case of a one-to-one scenario) or at least occasional feedback (e.g., in the case of a multicast, one-to-many scenario). All these applications have naturally different operational and management aspects. They also have different requirements or features, if any, for collecting feedback, processing it, and acting on it. The data structures for carrying the feedback also vary.
   
   Implementers SHOULD make feedback available using either an in-band or out-of-band asynchronous reporting mechanism. When an out-of-band solution is preferred, a standardized reporting mechanism, such as Syslog [RFC5424] or Simple Network Management Protocol (SNMP) notifications [RFC3411], is RECOMMENDED. When required, a mapping mechanism between the Syslog and SNMP reporting mechanisms could be used, as described in [RFC5675] and [RFC5676].
3. Non-FEC Framework Capable Receivers: Section 5.3 gives recommendations on how to provide backward compatibility in the presence of receivers that cannot support the FEC scheme being used or the FEC Framework itself: basically, the use of Explicit Source FEC Payload ID is banned. Additionally, a non-FEC Framework capable receiver MUST also have a means not to receive the repair packets that it will not be able to decode in the first place or a means to identify and discard them appropriately upon receiving them. This SHOULD be achieved by sending repair packets on a different transport-layer flow. In the case of RTP transport, and if both source and repair packets will be sent on the same transport-layer flow, this SHOULD be achieved by using an RTP framing for FEC repair packets with a different payload type. It is the responsibility of the sender to select the appropriate mechanism when needed.
4. Within End-Systems vs. within Middleboxes: When the FEC Framework is used within middleboxes, it is RECOMMENDED that the paths between the hosts where the sending applications run and the middlebox that performs FEC encoding be as reliable as possible, i.e., not be prone to packet loss, packet reordering, or varying delays in delivering packets.
   
   Similarly, when the FEC Framework is used within middleboxes, it is RECOMMENDED that the paths be as reliable as possible between the middleboxes that perform FEC decoding and the end-systems where the receiving applications operate.
   
   
5. Management of Communication Issues before Reaching the Sending FECFRAME Instance: Let us consider situations where the FEC Framework is used within middleboxes. At the sending side, the general reliability recommendation for the path between the sending applications and the middlebox is important, but it may not guarantee that a loss, reordering, or long delivery delay cannot happen, for whatever reason. If such a rare event happens, this event SHOULD NOT compromise the operation of the FECFRAME instances, at either the sending side or the receiving side. This is particularly important with FEC schemes that do not modify the ADU for backward-compatibility purposes (i.e., do not use any Explicit Source FEC Payload ID) and rely on, for instance, the RTP sequence number field to identify FEC source packets within their source block (Block FEC Code) or source flow (Convolutional FEC Code). In this case, packet loss or packet reordering leads to a gap in the RTP sequence number space seen by the FECFRAME instance. Similarly, varying delay in delivering packets over this path can lead to significant timing issues. With FEC schemes for a Block FEC Code that indicate in the Repair FEC Payload ID, for each source block, the base RTP sequence number and number of consecutive RTP packets that belong to this source block, a missing ADU or an ADU delivered out of order could cause the FECFRAME sender to switch to a new source block. However, some FEC schemes and/or receivers may not necessarily handle such varying source block sizes. In this case, one could consider duplicating the last ADU received before the loss, or inserting zeroed ADU(s), depending on the nature of the ADU flow. Implementers SHOULD consider the consequences of such alternative approaches, based on their use cases.
6. Protecting a Single Flow vs. Several Flows Globally: In the general case, the various ADU flows that are globally protected can have different features, and in particular different real-time requirements (in the case of real-time flows). The process of globally protecting these flows SHOULD take into account the requirements of each individual flow. In particular, it would be counterproductive to add repair traffic to a real-time flow for which the FEC decoding delay at a receiver makes decoded ADUs for this flow useless because they do not satisfy the associated real-time constraints. From a practical point of view, this means that the source block creation process (Block FEC Code) or encoding window management process (Convolutional FEC Code) at the sending FEC Framework instance SHOULD consider the most stringent real-time requirements of the ADU flows being globally protected.
7. ADU Flow Bundle Definition and Flow Delivery: By design, a repair flow might enable a receiver to recover the ADU flow(s) that it protects even if none of the associated FEC source packets are received. Therefore, when defining the bundle of ADU flows that are globally protected and when defining which receiver receives which flow, the sender SHOULD make sure that the ADU flow(s) and repair flow(s) of that bundle will only be received by receivers that are authorized to receive all the ADU flows of that bundle. See Section 9.4 for additional recommendations for situations where strict access control for ADU flows is needed.
   
   Additionally, when multiple ADU flows are globally protected, a receiver that wants to benefit from FECFRAME loss protection SHOULD receive all the ADU flows of the bundle. Otherwise, the missing FEC source packets would be considered lost, which might significantly reduce the efficiency of the FEC scheme.

11. IANA Considerations

FEC schemes for use with this framework are identified in protocols using FEC Encoding IDs. Values of FEC Encoding IDs are subject to IANA registration. For this purpose, this document reuses the registry called the "FEC Framework (FECFRAME) FEC Encoding IDs".

The values that can be assigned within the "FEC Framework (FECFRAME) FEC Encoding IDs" registry are numeric indexes in the range (0, 255). Values of 0 and 255 are reserved. Assignment requests are granted on an IETF Review basis as defined in [RFC5226]. Section 5.6 defines explicit requirements that documents defining new FEC Encoding IDs should meet.

12. Acknowledgments

This document is based in part on [FEC-SF], and so thanks are due to the additional authors of that document: Mike Luby, Magnus Westerlund, and Stephan Wenger. That document was in turn based on the FEC Streaming Protocol defined by 3GPP in [MBMSTS], and thus, thanks are also due to the participants in 3GPP SA Working Group 4. Further thanks are due to the members of the FECFRAME Working Group for their comments and reviews.

13. References

13.1. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3411]	Harrington, D., Presuhn, R. and B. Wijnen, "An Architecture for Describing Simple Network Management Protocol (SNMP) Management Frameworks", STD 62, RFC 3411, DOI 10.17487/RFC3411, December 2002.
[RFC5052]	Watson, M., Luby, M. and L. Vicisano, "Forward Error Correction (FEC) Building Block", RFC 5052, DOI 10.17487/RFC5052, August 2007.
[RFC5226]	Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008.
[RFC5234]	Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5424]	Gerhards, R., "The Syslog Protocol", RFC 5424, DOI 10.17487/RFC5424, March 2009.
[RFC6363]	Watson, M., Begen, A. and V. Roca, "Forward Error Correction (FEC) Framework", RFC 6363, DOI 10.17487/RFC6363, October 2011.

13.2. Informative References

[FEC-SF]	Watson, M., Luby, M., Westerlund, M. and S. Wenger, "Forward Error Correction (FEC) Streaming Framework", Work in Progress, July 2005.
[FECFRAMEv2-Motivations]	IRTF NetWork Coding Research Group (NWCRG), "FECFRAMEv2: Adding Sliding Encoding Window Capabilities to the FEC Framework: Problem Position", May 2016.
[MBMSTS]	3GPP, "Multimedia Broadcast/Multicast Service (MBMS); Protocols and codecs", 3GPP TS 26.346, March 2009.
[RFC3095]	Bormann, C., Burmeister, C., Degermark, M., Fukushima, H., Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le, K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke, T., Yoshimura, T. and H. Zheng, "RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed", RFC 3095, DOI 10.17487/RFC3095, July 2001.
[RFC3550]	Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003.
[RFC3711]	Baugher, M., McGrew, D., Naslund, M., Carrara, E. and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, DOI 10.17487/RFC3711, March 2004.
[RFC4303]	Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005.
[RFC4383]	Baugher, M. and E. Carrara, "The Use of Timed Efficient Stream Loss-Tolerant Authentication (TESLA) in the Secure Real-time Transport Protocol (SRTP)", RFC 4383, DOI 10.17487/RFC4383, February 2006.
[RFC4566]	Handley, M., Jacobson, V. and C. Perkins, "SDP: Session Description Protocol", RFC 4566, DOI 10.17487/RFC4566, July 2006.
[RFC4588]	Rey, J., Leon, D., Miyazaki, A., Varsa, V. and R. Hakenberg, "RTP Retransmission Payload Format", RFC 4588, DOI 10.17487/RFC4588, July 2006.
[RFC5675]	Marinov, V. and J. Schoenwaelder, "Mapping Simple Network Management Protocol (SNMP) Notifications to SYSLOG Messages", RFC 5675, DOI 10.17487/RFC5675, October 2009.
[RFC5676]	Schoenwaelder, J., Clemm, A. and A. Karmakar, "Definitions of Managed Objects for Mapping SYSLOG Messages to Simple Network Management Protocol (SNMP) Notifications", RFC 5676, DOI 10.17487/RFC5676, October 2009.
[RFC5725]	Begen, A., Hsu, D. and M. Lague, "Post-Repair Loss RLE Report Block Type for RTP Control Protocol (RTCP) Extended Reports (XRs)", RFC 5725, DOI 10.17487/RFC5725, February 2010.
[RFC5740]	Adamson, B., Bormann, C., Handley, M. and J. Macker, "NACK-Oriented Reliable Multicast (NORM) Transport Protocol", RFC 5740, DOI 10.17487/RFC5740, November 2009.
[RFC5775]	Luby, M., Watson, M. and L. Vicisano, "Asynchronous Layered Coding (ALC) Protocol Instantiation", RFC 5775, DOI 10.17487/RFC5775, April 2010.
[RFC6364]	Begen, A., "Session Description Protocol Elements for FEC Framework", RFC 6364, October 2011.

Appendix A. Possible management within a FEC Scheme of the Encoding Window with Convolutional FEC Codes (non Normative)

The FEC Framework does not specify the management of the encoding window, which is left to the FEC scheme associated to a Convolutional FEC Code. This section is therefore non normative. On the opposite, the FEC scheme associated to a Convolution FEC Code:

• MUST defined an encoding window that slides over the set of ADUs and its management.
• MUST define the relationships between ADUs and the associated source symbols (as with Block FEC Codes).

Source symbols are added to the sliding encoding window each time a new ADU arrives, where the following information is provided for this ADU by the FEC Framework:

• A description of the source flow with which the ADU is associated.
• The ADU itself.
• The length of the ADU.

Source symbols and the corresponding ADUs are removed from the sliding encoding window, for instance:

• after a certain delay, for situations where the sliding encoding window is managed on a time basis. The motivation is that an old ADU of a real-time flow becomes useless after a certain delay. The ADU retention delay in the sliding encoding window is therefore initialized according to the real-time features of incoming flow(s).
• once the sliding encoding window has reached its maximum size, since there is usually an upper limit to the sliding encoding window size.
• when the sliding encoding window is of fixed size or has reached its maximum size, the oldest symbol is removed each time a new symbol is added.

Limitations MAY exist that impact the encoding window management. For instance:

• at the FEC Framework level: the source flows can have real-time constraints that limit the number of ADUs in the encoding window.
• at the FEC scheme level: there may be theoretical or practical limitations (e.g., computational complexity) that limit the number of ADUs in the encoding window.

The most stringent limitation defines the maximum encoding window size, either in terms of number of source symbols or number of ADUs, whichever applies.

Authors' Addresses