RMT V. Roca
Internet-Draft INRIA
Intended status: Standards Track B. Adamson
Expires: January 28, 2012 Naval Research Laboratory
July 27, 2011

FCAST: Scalable Object Delivery for the ALC and NORM Protocols


This document introduces the FCAST object (e.g., file) delivery application on top of the ALC and NORM reliable multicast protocols. FCAST is a highly scalable application that provides a reliable object delivery service.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 28, 2012.

Copyright Notice

Copyright (c) 2011 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

This document introduces the FCAST reliable and scalable object (e.g., file) delivery application. Two variants of FCAST exist:

Hereafter, the term FCAST denotes either FCAST/ALC or FCAST/NORM. FCAST is not a new protocol specification per se. Instead it is a set of data format specifications and instructions on how to use ALC and NORM to implement a file-casting service.

Depending on the target use case, the delivery service provided by FCAST is more or less reliable. For instance, with FCAST/ALC used in ON-DEMAND mode over a time period that largely exceeds the typical download time, the service can be considered as fully reliable. Similarly, when FCAST is used along with a session control application that collects reception information and takes appropriate corrective measures (e.g., a direct point-to-point retransmission of missing packets, or a new multicast recovery session), then the service can be considered as fully reliable. On the opposite, if FCAST operates in PUSH mode, then the service is usually only partially reliable, and a receiver that is disconnected during a sufficient time will perhaps not have the possibility to download the object.

Depending on the target use case, the FCAST scalability is more or less important. For instance, if FCAST/ALC is used on top of purely unidirectional transport channels, with no feedback information at all, which is the default mode of operation, then the scalability is maximum since neither FCAST, nor ALC, UDP or IP generates any feedback message. On the opposite, the FCAST/NORM scalability is typically limited by NORM scalability itself. Similarly, if FCAST is used along with a session control application that collects reception information from the receivers, then this session control application may limit the scalability of the global object delivery system. This situation can of course be mitigated by using a hierarchy of feedback message aggregators or servers. The details of this are out of the scope of the present document.

A design goal behind FCAST is to define a streamlined solution, in order to enable lightweight implementations of the protocol stack, and limit the operational processing and storage requirements. A consequence of this choice is that FCAST cannot be considered as a versatile application, capable of addressing all the possible use-cases. On the opposite, FCAST has some intrinsic limitations. From this point of view it differs from FLUTE [RMT-FLUTE] which favors flexibility at the expense of some additional complexity.

A good example of the design choices meant to favor the simplicity is the way FCAST manages the object meta-data: by default, the meta-data and the object content are sent together, in a compound object. This solution has many advantages in terms of simplicity as will be described later on. However, as such, it also has an intrinsic limitation since it does not enable a receiver to decide in advance, before beginning the reception of the compound object, whether the object is of interest or not, based on the information that may be provided in the meta-data. Therefore this document defines additional techniques that may be used to mitigate this limitation. It is also possible that some use-cases require that each receiver download the whole set of objects sent in the session (e.g., with mirroring tools). When this is the case, the above limitation is no longer be a problem.

1.1. Applicability

FCAST is compatible with any congestion control protocol designed for ALC/LCT or NORM. However, depending on the use-case, the data flow generated by the FCAST application might not be constant, but instead be bursty in nature. Similarly, depending on the use-case, an FCAST session might be very short. Whether and how this will impact the congestion control protocol is out of the scope of the present document.

FCAST is compatible with any security mechanism designed for ALC/LCT or NORM. The use of a security scheme is strongly RECOMMENDED (see Section 6).

FCAST is compatible with any FEC scheme designed for ALC/LCT or NORM. Whether FEC is used or not, and the kind of FEC scheme used, is to some extent transparent to FCAST.

FCAST is compatible with both IPv4 and IPv6. Nothing in the FCAST specification has any implication on the source or destination IP address.

2. Requirements notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. Definitions, Notations and Abbreviations

3.1. Definitions

This document uses the following definitions:

denotes the FCAST application running on top of the ALC/LCT reliable transport protocol;
denotes the FCAST application running on top of the NORM reliable transport protocol;
denotes either FCAST/ALC or FCAST/NORM;
Compound Object:
denotes an ALC or NORM transport object composed of the Compound Object Header (Section 5.1), including any meta-data, and the content of the original application object (e.g., a file);
denotes the process of sending Compound Objects implemented by a FCAST sender;
Carousel Instance:
denotes a fixed set of registered Compound Objects that are sent by the carousel during a certain number of cycles. Whenever Compound Objects need to be added or removed, a new Carousel Instance is defined;
Carousel Instance Descriptor (CID):
denotes a special object that lists the Compound Objects that comprise a given Carousel Instance;
Carousel Cycle:
denotes a transmission round within which all the Compound Objects registered in the Carousel Instance are transmitted a certain number of times. By default, Compound Objects are transmitted once per cycle, but higher values are possible, that might differ on a per-object basis;
Transmission Object Identifier (TOI):
denotes the numeric identifier associated to a specific object by the underlying transport layer. In the case of ALC, this corresponds to the TOI described in that specification while for the NORM specification this corresponds to the NormTransportId described there.

3.2. Abbreviations

This document uses the following abbreviations:

Carousel Instance Descriptor
Carousel Instance IDentifier
FEC Object Transmission Information
Transmission Object Identifier

4. FCAST Principles

4.1. FCAST Content Delivery Service

The basic goal of FCAST is to transmit objects to a group of receivers in a reliable way. The receiver set MAY be restricted to a single receiver or MAY include possibly a very large number of receivers. FCAST is specified to support two forms of operation:

  1. FCAST/ALC: where the FCAST application is meant to run on top of the ALC/LCT reliable multicast transport protocol, and
  2. FCAST/NORM: where the FCAST application is meant to run on top of the NORM reliable multicast transport protocol.

This specification is designed such that both forms of operation share as much commonality as possible.

While the choice of the underlying transport protocol (i.e., ALC or NORM) and its parameters may limit the practical receiver group size, nothing in FCAST itself limits it. The transmission might be fully reliable, or only partially reliable depending upon the way ALC or NORM is used (e.g., whether FEC encoding and/or NACK-based repair requests are used or not), the way the FCAST carousel is used (e.g., whether the objects are made available for a long time span or not), and the way in which FCAST itself is employed (e.g., whether there is a session control application that might automatically extend an existing FCAST session until all receivers have received the transmitted content).

FCAST is designed to be as self-sufficient as possible, in particular in the way object meta-data is attached to object data content. However, for some uses, meta-data MAY also be communicated by an out-of-band mechanism that is out of the scope of the present document.

4.2. Meta-Data Transmission

FCAST usually carries meta-data elements by prepending them to the object it refers to. As a result, a Compound Object is created that is composed of a header followed by the original object data. This header is itself composed of the meta-data as well as several fields, for instance to indicate the boundaries between the various parts of this Compound Object (Figure 1).

<------------------------ Compound Object ----------------------->
|  Compound Object Header |              Object Data             |
| (can include meta-data) |       (can be encoded by FCAST)      |

Attaching the meta-data to the object is an efficient solution, since it guaranties that meta-data be received along with the associated object, and it allows the transport of the meta-data to benefit from any transport-layer erasure protection of the Compound Object (e.g., using FEC encoding and/or NACK-based repair). However a limit of this scheme, as such, is that a client does not know the meta-data of an object before beginning its reception, and in case of erasures affecting the meta-data, not until the object decoding is completed. The details of course depend upon the transport protocol and the FEC code used.

In certain use-cases, FCAST can also be associated to another in-band (e.g., via NORM INFO messages, Section 4.8) or out-of-band signaling mechanism. In that case, this mechanism can be used in order to carry the whole meta-data (or a subset of it), possibly ahead of time.

4.3. Meta-Data Content

The meta-data associated to an object can be composed of, but are not limited to:

This list is not limited and new meta-data information can be added. For instance, when dealing with very large objects (e.g., that largely exceed the working memory of a receiver), it can be interesting to split this object into several sub-objects (or slices). When this happens, the meta-data associated to each sub-object MUST include the following entries:

When meta-data elements are communicated out-of-band, in advance of data transmission, the following pieces of information MAY also be useful:

4.4. Carousel Transmission

A set of FCAST Compound Objects scheduled for transmission are considered a logical "Carousel". A given "Carousel Instance" is comprised of a fixed set of Compound Objects. Whenever the FCAST application needs to add new Compound Objects to, or remove old Compound Objects from the transmission set, a new Carousel Instance is defined since the set of Compound Objects changes. Because of the native object multiplexing capability of both ALC and NORM, sender and receiver(s) are both capable to multiplex and demultiplex FCAST Compound Objects.

For a given Carousel Instance, one or more transmission cycles are possible. During each cycle, all of the Compound Objects comprising the Carousel are sent. By default, each object is transmitted once per cycle. However, in order to allow different levels of priority, some objects MAY be transmitted more often that others during a cycle, and/or benefit from higher FEC protection than others. This can be the case for instance for the CID objects (Section 4.5). For some FCAST usage (e.g., a unidirectional "push" mode), a Carousel Instance may be associated to a single transmission cycle. In other cases it may be associated to a large number of transmission cycles (e.g., in "on-demand" mode, where objects are made available for download during a long period of time).

4.5. Carousel Instance Descriptor Special Object

The FCAST sender CAN transmit an OPTIONAL Carousel Instance Descriptor (CID). The CID carries the list of the Compound Objects that are part of a given Carousel Instance, by specifying their respective Transmission Object Identifiers (TOI). However the CID does not describe the objects themselves (i.e., there is no meta-data). Additionally, the CID MAY include a "Complete" flag that is used to indicate that no further modification to the enclosed list will be done in the future. Finally, the CID MAY include a Carousel Instance ID that identifies the Carousel Instance it pertains to. These aspects are discussed in Section 5.2.

There is no reserved TOI value for the CID Compound Object itself, since this special object is regarded by ALC/LCT or NORM as a standard object. On the opposite, the nature of this object (CID) is indicated by means of a specific Compound Object header field (the "I" flag) so that it can be recognized and processed by the FCAST application as needed. A direct consequence is the following: since a receiver does not know in advance which TOI will be used for the following CID (in case of a dynamic session), he MUST NOT filter out packets that are not in the current CID's TOI list. Said differently, the goal of CID is not to setup ALC or NORM packet filters (this mechanism would not be secure in any case).

The use of a CID remains optional. If it is not used, then the clients progressively learn what files are part of the carousel instance by receiving ALC or NORM packets with new TOIs. However using a CID has several benefits:

During idle periods, when the carousel instance does not contain any object, a CID with an empty TOI list MAY be transmitted. In that case, a new carousel instance ID MUST be used to differentiate this (empty) carousel instance from the other ones. This mechanism can be useful to inform the receivers that:

The decisions of whether a CID should be used or not, how often and when a CID should be sent, are left to the sender and depend on many parameters, including the target use case and the session dynamics. For instance it may be appropriate to send a CID at the beginning of each new carousel instance, and then periodically. These operational aspects are out of the scope of the present document.

4.6. Compound Object Identification

The FCAST Compound Objects are directly associated with the object-based transport service that the ALC and NORM protocols provide. In each of these protocols, the messages containing transport object content are labeled with a numeric transport object identifier (i.e., the ALC TOI and the NORM NormTransportId). For the purposes of this document, this identifier in either case (ALC or NORM) is referred to as the TOI.

There are several differences between ALC and NORM:

In both NORM and ALC, it is possible that the transport identification space may eventually wrap for long-lived sessions (especially with NORM where this phenomenon is expected to happen more frequently). This can possibly introduce some ambiguity in FCAST object identification if a sender retains some older objects in newer Carousel Instances with updated object sets. To avoid ambiguity the active TOIs (i.e., the TOIs corresponding to objects being transmitted) can only occupy half of the TOI sequence space. If an old object, whose TOI has fallen outside the current window, needs to be transmitted again, a new TOI must be used for it. In case of NORM, this constraint limits to 32768 the maximum number of objects that can be part of any carousel instance. In order to allow receivers to properly combine the transport packets with a newly-assigned TOI to those of associated to the previously-assigned TOI, a mechanism is required to equate the objects with the new and the old TOIs.

The preferred mechanism consists in signaling, within the CID, that the newly assigned TOI, for the current Carousel Instance, is equivalent to the TOI used within a previous Carousel Instance. By convention, the reference tuple for any object is the {TOI; CI ID} tuple used for its first transmission within a Carousel Instance. This tuple MUST be used whenever a TOI equivalence is provided.

An alternative solution, when meta-data can be processed rapidly (e.g., by using NORM-INFO messages), consists for the receiver in identifying that both objects are the same, after examining the meta-data. The receiver can then take appropriate measures.

4.7. FCAST/ALC Additional Specificities

There are no additional detail or option for FCAST/ALC operation.

4.8. FCAST/NORM Additional Specificities

The NORM Protocol provides a few additional capabilities that can be used to specifically support FCAST operation:

  1. The NORM_INFO message can convey "out-of-band" content with respect to a given transport object. With FCAST, it MAY be used to provide to the receivers a new, associated, Compound Object which contains the main Compound Object meta-data, or a subset of it. In that case the NORM_INFO Compound Object MUST NOT contain any Object Data field (i.e., it is only composed of the header), it MUST feature a non global checksum, and it MUST NOT include any padding field. The main Compound Object MUST in any case contain the whole meta-data (e.g., because a receiver MAY not support the NORM_INFO facility). Additionally, the meta-data entries contained in the NORM_INFO MUST be identical to the same entries in the main Compound Object. Finally, note that the availability of NORM_INFO for a given object is signaled through the use of a dedicated flag in the NORM_DATA message header. Along with NORM's NACK-based repair request signaling, it allows a receiver to quickly (and independently) request an object's NORM_INFO content. However, a limitation here is that the NORM_INFO Compound Object header MUST fit within the byte size limit defined by the NORM sender's configured "segment size" (typically a little less than the network MTU);
  2. The NORM_CMD(SQUELCH) messages are used by the NORM protocol sender to inform receivers of objects that have been canceled when receivers make repair requests for such invalid objects. Along with the CID mechanism, a receiver has two efficient and reliable ways to discover old objects that have been removed from the carousel instance;
  3. NORM also supports an optional positive acknowledgment mechanism that can be used for small-scale multicast receiver group sizes. Also, it may be possible in some cases for the sender to infer, after some period without receiving NACKs at the end of its transmission that the receiver set has fully received the transmitted content. In particular, if the sender completes its end-of-transmission series of NORM_CMD(FLUSH) messages without receiving repair requests from the group, it may have some assurance that the receiver set has received the content prior to that point. These mechanisms are likely to help FCAST in achieving fully reliable transmissions;

It should be noted that the NORM_INFO message header may carry the EXT_FTI extension. The reliable delivery of the NORM_INFO content allows the individual objects' FEC Transmission Information to be provided to the receivers without burdening every packet (i.e. NORM_DATA messages) with this additional, but important, content. Examples are provided in Appendix Appendix A.

4.9. FCAST Sender Behavior

The following operations MAY take place at a sender:

  1. The user (or another application) selects a set of objects (e.g., files) to deliver and submits them, along with their meta-data, to the FCAST application;
  2. For each object, FCAST creates the Compound Object and registers this latter in the carousel instance;
  3. The user then informs FCAST that all the objects of the set have been submitted. If the user knows that no new object will be submitted in the future (i.e., if the session's content is now complete), the user informs FCAST. Finally, the user specifies how many transmission cycles are desired (this number may be infinite);
  4. At this point, the FCAST application knows the full list of Compound Objects that are part of the Carousel Instance and can create a CID if desired, possibly with the complete flag set;
  5. The FCAST application can now define a transmission schedule of these Compound Objects, including the optional CID. This schedule defines in which order the packets of the various Compound Objects should be sent. This document does not specify any scheme. This is left to the developer within the provisions of the underlying ALC or NORM protocol used and the knowledge of the target use-case.
  6. The FCAST application then starts the carousel transmission, for the number of cycles specified. Transmissions take place until:
  7. If the session is not finished, then continue at Step 1 above;

4.10. FCAST Receiver Behavior

The following operations MAY take place at a receiver:

  1. The receiver joins the session and collects incoming packets;
  2. If the header portion of a Compound Object is entirely received (which may happen before receiving the entire object with some ALC/NORM configurations), or if the meta-data is sent by means of another mechanism prior to the object, the receiver processes the meta-data and chooses to continue to receive the object content or not;
  3. When a Compound Object has been entirely received, the receiver processes the header, retrieves the object meta-data, perhaps decodes the meta-data, and processes the object accordingly;
  4. When a CID is received, which is indicated by the 'C' flag set in the Compound Object header, the receiver decodes the CID, and retrieves the list of objects that are part of the current carousel instance. This list CAN be used to remove objects sent in a previous carousel instance that might not have been totally decoded and that are no longer part of the current carousel instance;
  5. When a CID is received, the receiver also retrieves the list of TOI equivalences, if any, and takes appropriate measures, for instance by informing the transport layer;
  6. When a receiver has received a CID with the "Complete" flag set, and has successfully received all the objects of the current carousel instance, it can safely exit from the current FCAST session;
  7. Otherwise continue at Step 2 above.

5. FCAST Data Formats

This section details the various data formats used by FCAST.

5.1. Compound Object Header Format

In an FCAST session, Compound Objects are constructed by prepending the Compound Object Header (which may include meta-data) before the original object data content (Figure 2).

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ^
|Ver| Resvd |G|C| MDFmt | MDEnc |           Checksum            |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                 Compound Object Header Length                 |  h
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|  d
|         Object Meta-Data (optional, variable length)          |  r
|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               |      Padding (optional)       |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  v
|                                                               |
.          Object Data (optional, variable length)              .
.                                                               .
.                                                               .

The Compound Object Header fields are:

Field Description
Version 2-bit field that MUST be set to 0 in this specification and indicates the protocol version number.
Reserved 4-bit field that MUST be set to 0 in this specification and is reserved for future use. Receivers MUST ignore this field.
G 1-bit field that, when set to 1, indicates that the checksum encompasses the whole Compound Object (Global checksum). When set to 0, this field indicates that the checksum encompasses only the Compound Object header.
C 1-bit field that, when set to 1, indicates the object is a Carousel Instance Descriptor (CID). When set to 0, this field indicates that the transported object is a standard object.
Meta-Data Format (MDFmt) 4-bit field that defines the format of the object meta-data (see Section 7). An HTTP/1.1 metainformation format [RFC2616] MUST be supported and is associated to value 0. Other formats (e.g., XML) MAY be defined in the future.
Meta-Data Encoding (MDEnc) 4-bit field that defines the optional encoding of the Object Meta-Data field (see Section 7). By default, a plain text encoding is used and is associated to value 0. Gzip encoding MUST also be supported and is associated to value 1. Other encodings MAY be defined in the future.
Checksum 16-bit field that contains the checksum computed over either the whole Compound Object (when G is set to 1), or over the Compound Object header (when G is set to 0), using the Internet checksum algorithm specified in [RFC1071]. More precisely, the checksum field is the 16-bit one's complement of the one's complement sum of all 16-bit words to be considered. If a segment contains an odd number of octets to be checksummed, the last octet is padded on the right with zeros to form a 16-bit word for checksum purposes (this pad is not transmitted). While computing the checksum, the checksum field itself is set to zero.
Compound Object Header Length 32-bit field indicating total length (in bytes) of all fields of the Compound Object Header, except the optional padding. A header length field set to value 8 means that there is no meta-data included. When this size is not multiple to 32-bits words and when the Compound Object Header is followed by a non null Compound Object Data, padding MUST be added. It should be noted that the meta-data field maximum size is equal to (2^32 - 8) bytes.
Object Meta-Data Optional, variable length field that contains the meta-data associated to the object. The format and encoding of this field is defined by the MDFmt MDEnc fields. With the default HTTP/1.1 format and plain text encoding, the Meta-Data is NULL-terminated plain text that follows the "TYPE" ":" "VALUE" "<CR-LF>" format used in HTTP/1.1 for metainformation [RFC2616]. The various meta-data items can appear in any order. The associated string, when non empty, MUST be NULL-terminated. When no meta-data is communicated, this field MUST be empty and the Compound Object Header Length MUST be equal to 8.
Padding Optional, variable length field of zero-value bytes to align the start of the Object Data to 32-bit boundary. Padding is only used when the Compound Object Header Length value, in bytes, is not multiple of 4 and when the Compound Object Header is followed by non null Compound Object Data.

The Compound Object Header is then followed by the Object Data, i.e., the original object possibly encoded by FCAST. Note that the length of this content is the transported object length (e.g., as specified by the FEC OTI) minus the Compound Object Header Length and optional padding if any.

5.2. Carousel Instance Descriptor Format

The format of the CID, which is a special Compound Object, is given in Figure 3.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ^
|Ver| Resvd |G|C| MDFmt | MDEnc |           Checksum            |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                 Compound Object Header Length                 |  h
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|  d
|         Object Meta-Data (optional, variable length)          |  r
|                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               |      Padding (optional)       |  |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  v
.                                                               .  ^
.                Object List (variable length)                  .  |
.                                                               .  o
.                                               +-+-+-+-+-+-+-+-+  b
.                                               |                  j
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                  v 

Because the CID is transmitted as a special Compound Object, the following CID-specific meta-data entries are defined:

The motivation for making the Fcast-CID-Complete and Fcast-CID-ID entries optional is to simplify the simple case of a session consisting of a single, complete, carousel instance, with an Object List given in plain text, without any content encoding. In that case, the CID does not need to contain any meta-data entry.

        Content-Encoding: gzip  

Additionally, the following standard meta-data entries are often used (Section 4.3):

        Content-Length: 0  

An empty Object List is valid and indicates that the current carousel instance does not include any object (Section 4.5). This can be specified by using the following meta-data entry:

The non-encoded (i.e., plain text) Object List, when non empty, is a NULL-terminated ASCII string. It can contain two things:

First of all, this string can contain the list of TOIs included in the current carousel instance, specified either as the individual TOIs of each object, or as TOI intervals, or any combination. The format of the ASCII string is a comma-separated list of individual "TOI" values or "TOI_a-TOI_b" elements. This latter case means that all values between TOI_a and TOI_b, inclusive, are part of the list. In that case TOI_a MUST be strictly inferior to TOI_b. If a TOI wrapping to 0 occurs in an interval, then two TOI intervals MUST be specified, TOI_a-MAX_TOI and 0-TOI_b.

This string can also contain the TOI equivalences, if any. The format is a comma-separated list of "(" newTOI "=" 1stTOI "/" 1stCIID ")" elements. Each element says that the new TOI, for the current Carousel Instance, is equivalent to (i.e., refers to the same object as) the provided identifier, 1stTOI, for the Carousel Instance of ID 1stCIID.

The ABNF specification is the following:

cid-list   =  *(list-elem *( "," list-elem))
list-elem    =  toi-elem / toieq-elem
toi-elem     =  toi-value / toi-interval
toi-value    =  1*DIGIT
toi-interval =  toi-value "-" toi-value
                ; additionally, the first toi-value MUST be
                ; strictly inferior to the second toi-value
toieq-elem   =  "(" toi-value "=" toi-value "/" ciid-value ")"
ciid-value   = 1*DIGIT
DIGIT        =  %x30-39
                ; a digit between O and 9, inclusive

For readability purposes, it is RECOMMENDED that all the TOI values in the list be given in increasing order. However a receiver MUST be able to handle non-monotonically increasing values. It is also RECOMMENDED to group the TOI equivalence elements together, at the end of the list, in increasing newTOI order. However a receiver MUST be able to handle lists of mixed TOI and TOI equivalence elements. Specifying a TOI equivalence for a given newTOI relieves the sender from specifying newTOI explicitly in the TOI list. However a receiver MUST be able to handle situations where the same TOI appears both in the TOI value and TOI equivalence lists. Finally, a given TOI value or TOI equivalence item MUST NOT be included multiple times in either list.

For instance, the following object list specifies that the current Carousel Instance is composed of 8 objects, and that TOIs 100 to 104 are equivalent to the TOIs 10 to 14 of Carousel Instance ID 2 and refer to the same objects:


or equivalently:


6. Security Considerations

6.1. Problem Statement

A content delivery system is potentially subject to attacks. Attacks may target:

These attacks can be launched either:

In the following sections we provide more details on these possible attacks and sketch some possible counter-measures. We finally provide recommendations in Section 6.5.

6.2. Attacks Against the Data Flow

Let us consider attacks against the data flow first. At least, the following types of attacks exist:

6.2.1. Access to Confidential Objects

Access control to the object being transmitted is typically provided by means of encryption. This encryption can be done over the whole object (e.g., by the content provider, before submitting the object to FCAST), or be done on a packet per packet basis (e.g., when IPsec/ESP is used [RFC4303], see Section 6.5). If confidentiality is a concern, it is RECOMMENDED that one of these solutions be used.

6.2.2. Object Corruption

Protection against corruptions (e.g., if an attacker sends forged packets) is achieved by means of a content integrity verification/sender authentication scheme. This service can be provided at the object level, but in that case a receiver has no way to identify which symbol(s) is(are) corrupted if the object is detected as corrupted. This service can also be provided at the packet level. In this case, after removing all corrupted packets, the file may be in some cases recovered. Several techniques can provide this content integrity/sender authentication service:

Techniques relying on public key cryptography (digital signatures and TESLA during the bootstrap process, when used) require that public keys be securely associated to the entities. This can be achieved by a Public Key Infrastructure (PKI), or by a PGP Web of Trust, or by pre-distributing securely the public keys of each group member.

Techniques relying on symmetric key cryptography (Group MAC) require that a secret key be shared by all group members. This can be achieved by means of a group key management protocol, or simply by pre-distributing securely the secret key (but this manual solution has many limitations).

It is up to the developer and deployer, who know the security requirements and features of the target application area, to define which solution is the most appropriate. In any case, whenever there is any concern of the threat of file corruption, it is RECOMMENDED that at least one of these techniques be used.

6.3. Attacks Against the Session Control Parameters and Associated Building Blocks

Let us now consider attacks against the session control parameters and the associated building blocks. The attacker has at least the following opportunities to launch an attack:

The consequences of these attacks are potentially serious, since they can compromise the behavior of content delivery system or even compromise the network itself.

6.3.1. Attacks Against the Session Description

An FCAST receiver may potentially obtain an incorrect Session Description for the session. The consequence of this is that legitimate receivers with the wrong Session Description are unable to correctly receive the session content, or that receivers inadvertently try to receive at a much higher rate than they are capable of, thereby possibly disrupting other traffic in the network.

To avoid these problems, it is RECOMMENDED that measures be taken to prevent receivers from accepting incorrect Session Descriptions. One such measure is the sender authentication to ensure that receivers only accept legitimate Session Descriptions from authorized senders. How these measures are achieved is outside the scope of this document since this session description is usually carried out-of-band.

6.3.2. Attacks Against the FCAST CID

Corrupting the FCAST CID is one way to create a Denial of Service attack. For example, the attacker can set the "Complete" flag to make the receivers believe that no further modification will be done.

It is therefore RECOMMENDED that measures be taken to guarantee the integrity and to check the sender's identity of the CID. To that purpose, one of the counter-measures mentioned above (Section 6.2.2) SHOULD be used. These measures will either be applied on a packet level, or globally over the whole CID object. When there is no packet level integrity verification scheme, it is RECOMMENDED to digitally sign the CID.

6.3.3. Attacks Against the Object Meta-Data

Corrupting the object meta-data is another way to create a Denial of Service attack. For example, the attacker changes the MD5 sum associated to a file. This possibly leads a receiver to reject the files received, no matter whether the files have been correctly received or not. When the meta-data are appended to the object, corrupting the meta-data means that the Compound Object will be corrupted.

It is therefore RECOMMENDED that measures be taken to guarantee the integrity and to check the sender's identity of the Compound Object. To that purpose, one of the counter-measures mentioned above (Section 6.2.2) SHOULD be used. These measures will either be applied on a packet level, or globally over the whole Compound Object. When there is no packet level integrity verification scheme, it is RECOMMENDED to digitally sign the Compound Object.

6.3.4. Attacks Against the ALC/LCT and NORM Parameters

By corrupting the ALC/LCT header (or header extensions) one can execute attacks on the underlying ALC/LCT implementation. For example, sending forged ALC packets with the Close Session flag (A) set to one can lead the receiver to prematurely close the session. Similarly, sending forged ALC packets with the Close Object flag (B) set to one can lead the receiver to prematurely give up the reception of an object. The same comments can be made for NORM.

It is therefore RECOMMENDED that measures be taken to guarantee the integrity and to check the sender's identity of each ALC or NORM packet received. To that purpose, one of the counter-measures mentioned above (Section 6.2.2) SHOULD be used.

6.3.5. Attacks Against the Associated Building Blocks

Let us first focus on the congestion control building block that may be used in an ALC or NORM session. A receiver with an incorrect or corrupted implementation of the multiple rate congestion control building block may affect the health of the network in the path between the sender and the receiver. That may also affect the reception rates of other receivers who joined the session.

When congestion control is applied with FCAST, it is therefore RECOMMENDED that receivers be required to identify themselves as legitimate before they receive the Session Description needed to join the session. If authenticating a receiver does not prevent this latter to launch an attack, it will enable the network operator to identify him and to take counter-measures. This authentication can be made either toward the network operator or the session sender (or a representative of the sender) in case of NORM. The details of how it is done are outside the scope of this document.

When congestion control is applied with FCAST, it is also RECOMMENDED that a packet level authentication scheme be used, as explained in Section 6.2.2. Some of them, like TESLA, only provide a delayed authentication service, whereas congestion control requires a rapid reaction. It is therefore RECOMMENDED [RFC5775] that a receiver using TESLA quickly reduces its subscription level when the receiver believes that a congestion did occur, even if the packet has not yet been authenticated. Therefore TESLA will not prevent DoS attacks where an attacker makes the receiver believe that a congestion occurred. This is an issue for the receiver, but this will not compromise the network. Other authentication methods that do not feature this delayed authentication could be preferred, or a group MAC scheme could be used in parallel to TESLA to prevent attacks launched from outside of the group.

6.4. Other Security Considerations

Lastly, we note that the security considerations that apply to, and are described in, ALC [RFC5775], LCT [RFC5651], NORM [RFC5740] and FEC [RFC5052] also apply to FCAST as FCAST builds on those specifications. In addition, any security considerations that apply to any congestion control building block used in conjunction with FCAST also applies to FCAST. Finally, the security discussion of [RMT-SEC] also applies here.

6.5. Minimum Security Recommendations

We now introduce a mandatory to implement but not necessarily to use security configuration, in the sense of [RFC3365]. Since FCAST/ALC relies on ALC/LCT, it inherits the "baseline secure ALC operation" of [RFC5775]. Similarly, since FCAST/NORM relies on NORM, it inherits the "baseline secure NORM operation" of [RFC5740]. More precisely, in both cases security is achieved by means of IPsec/ESP in transport mode. [RFC4303] explains that ESP can be used to potentially provide confidentiality, data origin authentication, content integrity, anti-replay and (limited) traffic flow confidentiality. [RFC5775] specifies that the data origin authentication, content integrity and anti-replay services SHALL be used, and that the confidentiality service is RECOMMENDED. If a short lived session MAY rely on manual keying, it is also RECOMMENDED that an automated key management scheme be used, especially in case of long lived sessions.

Therefore, the RECOMMENDED solution for FCAST provides per-packet security, with data origin authentication, integrity verification and anti-replay. This is sufficient to prevent most of the in-band attacks listed above. If confidentiality is required, a per-packet encryption SHOULD also be used.

7. IANA Considerations

7.1. Namespace declaration for Object Meta-Data Format

This document requires a IANA registration for the following name-space: "Object Meta-Data Format" (MDFmt). Values in this namespace are 4-bit positive integers between 0 and 15 inclusive and they define the format of the object meta-data ((see Section 5.1).

Initial values for the LCT Header Extension Type registry are defined in Section 7.1.1. Future assignments are to be made through Expert Review [RFC5226].

7.1.1. Object Meta-Data Format registration

This document registers one value in the "Object Meta-Data Format" namespace as follows:

format name Value
as per HTTP/1.1 metainformation format 0 (default)

All implementations MUST support format 0 (default).

7.2. Namespace declaration for Object Meta-Data Encoding

This document requires a IANA registration for the following name-space: "Object Meta-Data Encoding" (MDEnc). Values in this namespace are 4-bit positive integers between 0 and 15 inclusive and they define the optional encoding of the Object Meta-Data field (see Section 5.1).

Initial values for the LCT Header Extension Type registry are defined in Section 7.2.1. Future assignments are to be made through Expert Review [RFC5226].

7.2.1. Object Meta-Data Encoding registration

This document registers two values in the "Object Meta-Data Encoding" namespace as follows:

Name Value
plain text 0 (default)
gzip 1

All implementations MUST support both value 0 (plain-text, default) and value 1 (gzip).

8. Acknowledgments

The authors are grateful to the authors of [ALC-00] for specifying the first version of FCAST/ALC. The authors are also grateful to Gorry Fairhurst and Lorenzo Vicisano for their valuable comments.

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[RFC1071] Braden, R., Borman, D., Partridge, C. and W. Plummer, "Computing the Internet checksum", RFC 1071, September 1988.
[RFC5651] Luby, M., Watson, M. and L. Vicisano, "Layered Coding Transport (LCT) Building Block", RFC 5651, October 2009.
[RFC5740] Adamson, B., Bormann, C., Handley, M. and J. Macker, "NACK-Oriented Reliable Multicast (NORM) Transport Protocol", RFC 5740, November 2009.
[RFC5775] Luby, M., Watson, M. and L. Vicisano, "Asynchronous Layered Coding (ALC) Protocol Instantiation", RFC 5775, April 2010.
[RFC1952] Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L.P. and G. Randers-Pehrson, "GZIP file format specification version 4.3", RFC 1952, May 1996.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

9.2. Informative References

[ALC-00] Luby, M., Gemmell, G., Vicisano, L., Crowcroft, J. and B. Lueckenhoff, "Asynchronous Layered Coding: a Scalable Reliable Multicast Protocol", March 2000.
[RMT-FLUTE] Paila, T., Walsh, R., Luby, M., Roca, V. and R. Lehtonen, "FLUTE - File Delivery over Unidirectional Transport", Work in Progress, February 2011.
[RFC3365] Schiller, J., "Strong Security Requirements for Internet Engineering Task Force Standard Protocols", BCP 61, RFC 3365, August 2002.
[RMT-SEC] Roca, V., Adamson, B. and H. Asaeda, "Security and Reliable Multicast Transport Protocols: Discussions and Guidelines", Work in progress, draft-ietf-rmt-sec-discussion-06.txt, March 2011.
[RFC2104] Krawczyk, H., Bellare, M. and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997.
[RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J.D. and B. Briscoe, "Timed Efficient Stream Loss-Tolerant Authentication (TESLA): Multicast Source Authentication Transform Introduction", RFC 4082, June 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, December 2005.
[RFC5052] Watson, M., Luby, M. and L. Vicisano, "Forward Error Correction (FEC) Building Block", RFC 5052, August 2007.
[RFC5510] Lacan, J., Roca, V., Peltotalo, J. and S. Peltotalo, "Reed-Solomon Forward Error Correction (FEC) Schemes", RFC 5510, April 2009.
[RFC5776] Roca, V., Francillon, A. and S. Faurite, "Use of Timed Efficient Stream Loss-Tolerant Authentication (TESLA) in the Asynchronous Layered Coding (ALC) and NACK-Oriented Reliable Multicast (NORM) Protocols", RFC 5776, April 2010.
[RMT-SIMPLE-AUTH] Roca, V., "Simple Authentication Schemes for the ALC and NORM Protocols", Work in progress draft-ietf-rmt-simple-auth-for-alc-norm-04.txt, July 2011.

Appendix A. FCAST Examples

Appendix A.1. Basic Examples

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| 0 |   0   |1|0|   0   |   0   |           Checksum            |
|                               44                              | 
.                                                               .
.       meta-data ASCII null terminated string (33 bytes)       .
.                                                               .
+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               |                    Padding                    |
.                                                               .
.                         Object data                           .
.                                                               .

Figure 9 shows a regular Compound Object where the meta-data ASCII string, in HTTP/1.1 meta-information format (MDFmt=0) contains:

Content-Location: example.txt <CR-LF>

This string is 33 bytes long, including the NULL-termination character. There is no gzip encoding of the meta-data (MDEnc=0) and there is no Content-Length information either since this length can easily be calculated by the receiver as the FEC OTI transfer length minus the header length. Finally, the checksum encompasses the whole Compound Object (G=1).

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| 0 |   0   |1|0|   0   |   0   |           Checksum            |
|                               8                               |
.                                                               .
.                         Object data                           .
.                                                               .

Figure 11 shows a Compound Object without any meta-data. The fact there is no meta-data is indicated by the value 8 of the Compound Object Header Length field. No padding is required.

Figure 12 shows an example CID object, in the case of a static FCAST session, i.e., a session where the set of objects is set once and for all. There is no meta-data in this example since Fcast-CID-Complete and Fcast-CID-ID are both implicit.

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
| 0 |   0   |1|1|   0   |   0   |           Checksum            |
|                               8                               |
.                                                               .
.                Object List string                             .
.                                                               .
.                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.                               |

The object list contains the following 26 byte long string, including the NULL-termination character:


There are therefore a total of 3+5+4+1 = 13 objects in the carousel instance, and therefore in the FCAST session. There is no meta-data associated to this CID. The session being static and composed of a single Carousel Instance, the sender did not feel the necessity to carry a Carousel Instance ID meta-data.

Appendix A.2. FCAST/NORM with NORM_INFO Examples

In case of FCAST/NORM, the FCAST Compound Object meta-data (or a subset of it) can be carried as part of a NORM_INFO message, as a new Compound Object that does not contain any Compound Object Data. In the following example we assume that the whole meta-data is carried in such a message for a certain Compound Object. Figure 14 shows an example NORM_INFO message that contains the FCAST Compound Object Header and meta-data as its payload. In this example, the first 16 bytes are the NORM_INFO base header, the next 12 bytes are a NORM EXT_FTI header extension containing the FEC Object Transport Information for the associated object, and the remaining bytes are the FCAST Compound Object Header and meta-data. Note that "padding" MUST NOT be used and that the FCAST checksum only encompasses the Compound Object Header (G=0).

0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --
|version| type=1|  hdr_len = 7  |          sequence             |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                           source_id                           |  n
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  o
|          instance_id          |     grtt      |backoff| gsize |  r
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  m
|     flags     |  fec_id = 5   |     object_transport_id       |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --
|   HET = 64    |    HEL = 3    |                               |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +  f
|                     Transfer Length = 41                      |  t
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  i
|   Encoding Symbol Length (E)  | MaxBlkLen (B) |     max_n     |  v
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --
| 0 |   0   |0|0|   0   |   0   |           Checksum            |  ^
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|                               41                              |  f
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|  c
.                                                               .  a
.       meta-data ASCII null terminated string (33 bytes)       .  s
.                                                               .  t
+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  |
|               |                                                  v
+-+-+-+-+-+-+-+-+                                                 --

The NORM_INFO message shown in Figure 14 contains the EXT_FTI header extension to carry the FEC OTI. In this example, the FEC OTI format is that of the Reed-Solomon FEC coding scheme for fec_id = 5 as described in [RFC5510]. Other alternatives for providing the FEC OTI would have been to either include it directly in the meta-data of the FCAST Compound Header, or to include an EXT_FTI header extension to all NORM_DATA packets (or a subset of them). Note that the NORM "Transfer_Length" is the total length of the associated FCAST Compound Object, i.e., 41 bytes.

The FCAST Compound Object in this example does contain the same meta-data and is formatted as in the example of Figure 9. With the combination of the FEC_OTI and the FCAST meta-data, the NORM protocol and FCAST application have all of the information needed to reliably receive and process the associated object. Indeed, the NORM protocol provides rapid (NORM_INFO has precedence over the associated object content), reliable delivery of the NORM_INFO message and its payload, the FCAST Compound Object Header.

Authors' Addresses

Vincent Roca INRIA 655, av. de l'Europe Inovallee; Montbonnot ST ISMIER cedex, 38334 France EMail: vincent.roca@inria.fr URI: http://planete.inrialpes.fr/people/roca/
Brian Adamson Naval Research Laboratory Washington, DC, 20375 USA EMail: adamson@itd.nrl.navy.mil URI: http://cs.itd.nrl.navy.mil