One document matched: draft-ietf-rmt-pi-alc-revised-05.xml
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<rfc category="std" docName="draft-ietf-rmt-pi-alc-revised-05" ipr="full3978"
obsoletes="3450">
<front>
<title abbrev="ALC Protocol Instantiation">Asynchronous Layered Coding
(ALC) Protocol Instantiation</title>
<author fullname="Michael Luby" surname="Luby">
<organization>Digital Fountain</organization>
<address>
<postal>
<street>39141 Civic Center Dr.</street>
<street>Suite 300</street>
<city>Fremont</city>
<region>CA</region>
<code>94538</code>
<country>US</country>
</postal>
<email>luby@digitalfountain.com</email>
</address>
</author>
<author fullname="Mark Watson" surname="Watson">
<organization>Digital Fountain</organization>
<address>
<postal>
<street>39141 Civic Center Dr.</street>
<street>Suite 300</street>
<city>Fremont</city>
<region>CA</region>
<code>94538</code>
<country>US</country>
</postal>
<email>mark@digitalfountain.com</email>
</address>
</author>
<author fullname="Lorenzo Vicisano" surname="Vicisano">
<organization>Digital Fountain</organization>
<address>
<postal>
<street>39141 Civic Center Dr.</street>
<street>Suite 300</street>
<city>Fremont</city>
<region>CA</region>
<code>94538</code>
<country>US</country>
</postal>
<email>lorenzo@digitalfountain.com</email>
</address>
</author>
<date day="16" month="November" year="2007" />
<area>Transport</area>
<workgroup>Reliable Multicast Transport (RMT) Working Group</workgroup>
<keyword>RFC</keyword>
<keyword>Request for Comments</keyword>
<keyword>I-D</keyword>
<keyword>Internet-Draft</keyword>
<keyword>XML</keyword>
<keyword>Extensible Markup Language</keyword>
<abstract>
<t>This document describes the Asynchronous Layered Coding (ALC)
protocol, a massively scalable reliable content delivery protocol.
Asynchronous Layered Coding combines the Layered Coding Transport (LCT)
building block, a multiple rate congestion control building block and
the Forward Error Correction (FEC) building block to provide congestion
controlled reliable asynchronous delivery of content to an unlimited
number of concurrent receivers from a single sender. This document
obsoletes RFC3450.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>This document describes a massively scalable reliable content
delivery protocol, Asynchronous Layered Coding (ALC), for multiple rate
congestion controlled reliable content delivery. The protocol is
specifically designed to provide massive scalability using IP multicast
as the underlying network service. Massive scalability in this context
means the number of concurrent receivers for an object is potentially in
the millions, the aggregate size of objects to be delivered in a session
ranges from hundreds of kilobytes to hundreds of gigabytes, each
receiver can initiate reception of an object asynchronously, the
reception rate of each receiver in the session is the maximum fair
bandwidth available between that receiver and the sender, and all of
this can be supported using a single sender.</t>
<t>Because ALC is focused on reliable content delivery, the goal is to
deliver objects as quickly as possible to each receiver while at the
same time remaining network friendly to competing traffic. Thus, the
congestion control used in conjunction with ALC should strive to
maximize use of available bandwidth between receivers and the sender
while at the same time backing off aggressively in the face of competing
traffic.</t>
<t>The sender side of ALC consists of generating packets based on
objects to be delivered within the session and sending the appropriately
formatted packets at the appropriate rates to the channels associated
with the session. The receiver side of ALC consists of joining
appropriate channels associated with the session, performing congestion
control by adjusting the set of joined channels associated with the
session in response to detected congestion, and using the packets to
reliably reconstruct objects. All information flow in an ALC session is
in the form of data packets sent by a single sender to channels that
receivers join to receive data.</t>
<t>ALC does specify the Session Description needed by receivers before
they join a session, but the mechanisms by which receivers obtain this
required information is outside the scope of ALC. An application that
uses ALC may require that receivers report statistics on their reception
experience back to the sender, but the mechanisms by which receivers
report back statistics is outside the scope of ALC. In general, ALC is
designed to be a minimal protocol instantiation that provides reliable
content delivery without unnecessary limitations to the scalability of
the basic protocol.</t>
<t>This document is a product of the IETF RMT WG and follows the general
guidelines provided in <xref target="RFC3269"></xref>.</t>
<t>RFC3450 <xref target="RFC3450"></xref>, which is obsoleted by this
document, contained a previous versions of the protocol. RFC3450 was
published in the "Experimental" category. It was the stated intent of
the RMT working group to re-submit these specifications as an IETF
Proposed Standard in due course.</t>
<t>This Proposed Standard specification is thus based on and backwards
compatible with the protocol defined in RFC3450 <xref
target="RFC3450"></xref> updated according to accumulated experience and
growing protocol maturity since its original publication. Said
experience applies both to this specification itself and to congestion
control strategies related to the use of this specification.</t>
<t>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 BCP 14, <xref
target="RFC2119"></xref>.</t>
<section title="Delivery service models">
<t>ALC can support several different reliable content delivery service
models as described in <xref
target="I-D.ietf-rmt-bb-lct-revised"></xref>.</t>
</section>
<section title="Scalability">
<t>Massive scalability is a primary design goal for ALC. IP multicast
is inherently massively scalable, but the best effort service that it
provides does not provide session management functionality, congestion
control or reliability. ALC provides all of this on top of IP
multicast without sacrificing any of the inherent scalability of IP
multicast. ALC has the following properties:</t>
<t><list style="symbols">
<t>To each receiver, it appears as if though there is a dedicated
session from the sender to the receiver, where the reception rate
adjusts to congestion along the path from sender to receiver.</t>
<t>To the sender, there is no difference in load or outgoing rate
if one receiver is joined to the session or a million (or any
number of) receivers are joined to the session, independent of
when the receivers join and leave.</t>
<t>No feedback packets are required from receivers to the
sender.</t>
<t>Almost all packets in the session that pass through a
bottleneck link are utilized by downstream receivers, and the
session shares the link with competing flows fairly in proportion
to their utility.</t>
</list></t>
<t>Thus, ALC provides a massively scalable content delivery transport
that is network friendly.</t>
<t>ALC intentionally omits any application specific features that
could potentially limit its scalability. By doing so, ALC provides a
minimal protocol that is massively scalable. Applications may be built
on top of ALC to provide additional features that may limit the
scalability of the application. Such applications are outside the
scope of this document.</t>
</section>
<section title="Environmental Requirements and Considerations">
<t>All of the environmental requirements and considerations that apply
to the LCT building block <xref
target="I-D.ietf-rmt-bb-lct-revised"></xref>, the FEC building block
<xref target="RFC5052"></xref>, the multiple rate congestion control
building block and to any additional building blocks that ALC uses
also apply to ALC.</t>
<t>One issues that is specific to ALC with respect to the Any- Source
Multicast (ASM) model of IP multicast as defined in RFC 1112 <xref
target="RFC1112"></xref> is the way the multiple rate congestion
control building block interacts with ASM. The congestion control
building block may use the measured difference in time between when a
join to a channel is sent and when the first packet from the channel
arrives in determining the receiver reception rate. The congestion
control building block may also uses packet sequence numbers per
channel to measure losses, and this is also used to determine the
receiver reception rate. These features raise two concerns with
respect to ASM: The time difference between when the join to a channel
is sent and when the first packet arrives can be significant due to
the use of Rendezvous Points (RPs) and the MSDP protocol, and packets
can be lost in the switch over from the (*,G) join to the RP and the
(S,G) join directly to the sender. Both of these issues could
potentially substantially degrade the reception rate of receivers. To
ameliorate these concerns, it is RECOMMENDED that the RP be as close
to the sender as possible. SSM does not share these same concerns. For
a fuller consideration of these issues, consult the multiple rate
congestion control building block.</t>
</section>
</section>
<section title="Architecture Definition">
<t>ALC uses the LCT building block <xref
target="I-D.ietf-rmt-bb-lct-revised"></xref> to provide in-band session
management functionality. ALC uses a multiple rate congestion control
building block that is compliant with <xref target="RFC2357"></xref> to
provide congestion control that is feedback free. Receivers adjust their
reception rates individually by joining and leaving channels associated
with the session. ALC uses the FEC building block <xref
target="RFC5052"></xref> to provide reliability. The sender generates
encoding symbols based on the object to be delivered using FEC codes and
sends them in packets to channels associated with the session. Receivers
simply wait for enough packets to arrive in order to reliably
reconstruct the object. Thus, there is no request for retransmission of
individual packets from receivers that miss packets in order to assure
reliable reception of an object, and the packets and their rate of
transmission out of the sender can be independent of the number and the
individual reception experiences of the receivers.</t>
<t>The definition of a session for ALC is the same as it is for LCT. An
ALC session comprises multiple channels originating at a single sender
that are used for some period of time to carry packets pertaining to the
transmission of one or more objects that can be of interest to
receivers. Congestion control is performed over the aggregate of packets
sent to channels belonging to a session. The fact that an ALC session is
restricted to a single sender does not preclude the possibility of
receiving packets for the same objects from multiple senders. However,
each sender would be sending packets to a a different session to which
congestion control is individually applied. Although receiving
concurrently from multiple sessions is allowed, how this is done at the
application level is outside the scope of this document.</t>
<t>ALC is a protocol instantiation as defined in <xref
target="RFC3048"></xref>. This document describes version 1 of ALC which
MUST use version 1 of LCT described in <xref
target="I-D.ietf-rmt-bb-lct-revised"></xref>. Like LCT, ALC is designed
to be used with the IP multicast network service. This specification
defines ALC as payload of the UDP transport protocol <xref
target="RFC0768"></xref> that supports IP multicast delivery of packets.
Future versions of this specification, or companion documents may extend
ALC to use the IP network layer service directly. ALC could be used as
the basis for designing a protocol that uses a different underlying
network service such as unicast UDP, but the design of such a protocol
is outside the scope of this document.</t>
<t>An ALC packet header immediately follows the UDP header and consists
of the default LCT header that is described in <xref
target="I-D.ietf-rmt-bb-lct-revised"></xref> followed by the FEC Payload
ID that is described in <xref target="RFC5052"></xref>. The Congestion
Control Information field within the LCT header carries the required
Congestion Control Information that is described in the multiple rate
congestion control building block specified that is compliant with <xref
target="RFC2357"></xref>. The packet payload that follows the ALC packet
header consists of encoding symbols that are identified by the FEC
Payload ID as described in <xref target="RFC5052"></xref>.</t>
<t>Each receiver is required to obtain a Session Description before
joining an ALC session. As described later, the Session Description
includes out-of-band information required for the LCT, FEC and the
multiple rate congestion control building blocks. The FEC Object
Transmission Information specified in the FEC building block <xref
target="RFC5052"></xref> required for each object to be received by a
receiver can be communicated to a receiver either out-of-band or in-band
using a Header Extension. The means for communicating the Session
Description and the FEC Object Transmission Information to a receiver is
outside the scope of this document.</t>
<section title="LCT building block">
<t>LCT requires receivers to be able to uniquely identify and
demultiplex packets associated with an LCT session, and ALC inherits
and strengthens this requirement. A Transport Session Identifier (TSI)
MUST be associated with each session and MUST be carried in the LCT
header of each ALC packet. The TSI is scoped by the sender IP address,
and the (sender IP address, TSI) pair MUST uniquely identify the
session.</t>
<t>The LCT header contains a Congestion Control Information (CCI)
field that MUST be used to carry the Congestion Control Information
from the specified multiple rate congestion control protocol. There is
a field in the LCT header that specifies the length of the CCI field,
and the multiple rate congestion control building block MUST uniquely
identify a format of the CCI field that corresponds to this
length.</t>
<t>The LCT header contains a Codepoint field that MAY be used to
communicate to a receiver the settings for information that may vary
during a session. If used, the mapping between settings and Codepoint
values is to be communicated in the Session Description, and this
mapping is outside the scope of this document. For example, the FEC
Encoding ID that is part of the FEC Object Transmission Information as
specified in the FEC building block <xref target="RFC5052"></xref>
could vary for each object carried in the session, and the Codepoint
value could be used to communicate the FEC Encoding ID to be used for
each object. The mapping between FEC Encoding IDs and Codepoints could
be, for example, the identity mapping.</t>
<t>If more than one object is to be carried within a session then the
Transmission Object Identifier (TOI) MUST be used in the LCT header to
identify which packets are to be associated with which objects. In
this case the receiver MUST use the TOI to associate received packets
with objects. The TOI is scoped by the IP address of the sender and
the TSI, i.e., the TOI is scoped by the session. The TOI for each
object is REQUIRED to be unique within a session, but MAY NOT be
unique across sessions. Furthermore, the same object MAY have a
different TOI in different sessions. The mapping between TOIs and
objects carried in a session is outside the scope of this
document.</t>
<t>If only one object is carried within a session then the TOI MAY be
omitted from the LCT header.</t>
<t>The LCT header from version 1 of the LCT building block <xref
target="I-D.ietf-rmt-bb-lct-revised"></xref> MUST be used.</t>
<t>The LCT Header includes a two-bit Protocol Specific Indication
(PSI) field in bits 6 and 7 of the first word of the LCT header. These
two bits are used by ALC as follows: <figure anchor="psibitsfig"
title="PSI bits within LCT Headder">
<artwork><![CDATA[
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
+-+-+
...|A|B|...
+-+-+
]]></artwork>
</figure> <list>
<t>PSI bit A - Source Packet Indicator (SPI)</t>
<t>PSI bit B - Reserved</t>
</list></t>
<t>The Source Packet Indicator is used with systematic FEC Schemes
which define a different FEC Payload ID format for packets containing
only source data compared to the FEC Payload ID format for packets
containing repair data. For such FEC Schemes, then the SPI MUST be set
to 1 when the FEC Payload ID format for packets containing only source
data is used and the SPI MUST be set to zero, when the FEC Payload ID
for packerts containing repair data is used. In the case of FEC
Schemes which define only a single FEC Payload ID format, then the SPI
MUST be set to zero by the sender and MUST be ignored by the
receiver.</t>
<t>Support of two FEC Payload ID formats allows FEC Payload ID
information which is only of relevance when FEC decoding is to be
performed to be provided in the FEC Payload ID format for packets
containing repair data. This information need not be processed by
receivers which do not perform FEC decoding (either because no FEC
decoding is required or because the receiver does not support FEC
decoding).</t>
</section>
<section title="Multiple rate congestion control building block">
<t>Implementors of ALC MUST implement a multiple rate feedback-free
congestion control building block that is in accordance to <xref
target="RFC2357"></xref>. Congestion control MUST be applied to all
packets within a session independently of which information about
which object is carried in each packet. Multiple rate congestion
control is specified because of its suitability to scale massively and
because of its suitability for reliable content delivery. The multiple
rate congestion control building block MUST specify in-band Congestion
Control Information (CCI) that MUST be carried in the CCI field of the
LCT header. The multiple rate congestion control building block MAY
specify more than one format, but it MUST specify at most one format
for each of the possible lengths 32, 64, 96 or 128 bits. The value of
C in the LCT header that determines the length of the CCI field MUST
correspond to one of the lengths for the CCI defined in the multiple
rate congestion control building block, this length MUST be the same
for all packets sent to a session, and the CCI format that corresponds
to the length as specified in the multiple rate congestion control
building block MUST be the format used for the CCI field in the LCT
header.</t>
<t>When using a multiple rate congestion control building block a
sender sends packets in the session to several channels at potentially
different rates. Then, individual receivers adjust their reception
rate within a session by adjusting which set of channels they are
joined to at each point in time depending on the available bandwidth
between the receiver and the sender, but independent of other
receivers.</t>
</section>
<section anchor="SeFECBB" title="FEC building block">
<t>The FEC building block <xref target="RFC5052"></xref> provides
reliable object delivery within an ALC session. Each object sent in
the session is independently encoded using FEC codes as described in
<xref target="RFC3453"></xref>, which provide a more in-depth
description of the use of FEC codes in reliable content delivery
protocols. All packets in an ALC session MUST contain an FEC Payload
ID in a format that is compliant with the FEC Scheme in use. The FEC
Payload ID uniquely identifies the encoding symbols that constitute
the payload of each packet, and the receiver MUST use the FEC Payload
ID to determine how the encoding symbols carried in the payload of the
packet were generated from the object as described in the FEC building
block.</t>
<t>As described in <xref target="RFC5052"></xref>, a receiver is
REQUIRED to obtain the FEC Object Transmission Information for each
object for which data packets are received from the session. In the
context of ALC, the FEC Object Transmission Information includes:</t>
<t><list style="symbols">
<t>The FEC Encoding ID.</t>
<t>If an Under-Specified FEC Encoding ID is used then the FEC
Instance ID associated with the FEC Encoding ID.</t>
<t>For each object in the session, the transfer length of the
object in bytes.</t>
</list></t>
<t>Additional FEC Object Transmission Information may be required
depending on the FEC Scheme that is used (identified by the FEC
Encoding ID).</t>
<t>Some of the FEC Object Transmission Information MAY be implicit
based on the FEC Scheme and/or implementation. As an example, source
block lengths may be derived by a fixed algorithm from the object
length. As another example, it may be that all source blocks are the
same length and this is what is passed out-of-band to the receiver. As
another example, it could be that the full sized source block length
is provided and this is the length used for all but the last source
block, which is calculated based on the full source block length and
the object length. As another example, it could be that the same FEC
Encoding ID and FEC Instance ID are always used for a particular
application and thus the FEC Encoding ID and FEC Instance ID are
implicitly defined.</t>
<t>Sometimes the objects that will be sent in a session are completely
known before the receiver joins the session, in which case the FEC
Object Transmission Information for all objects in the session can be
communicated to receivers before they join the session. At other times
the objects may not know when the session begins, or receivers may
join a session in progress and may not be interested in some objects
for which transmission has finished, or receivers may leave a session
before some objects are even available within the session. In these
cases, the FEC Object Transmission Information for each object may be
dynamically communicated to receivers at or before the time packets
for the object are received from the session. This may be accomplished
using either an out-of-band mechanism, in-band using the Codepoint
field or a Header Extension, or any combination of these methods. How
the FEC Object Transmission Information is communicated to receivers
is outside the scope of this document.</t>
<t>If packets for more than one object are transmitted within a
session then a Transmission Object Identifier (TOI) that uniquely
identifies objects within a session MUST appear in each packet header.
Portions of the FEC Object Transmission Information could be the same
for all objects in the session, in which case these portions can be
communicated to the receiver with an indication that this applies to
all objects in the session. These portions may be implicitly
determined based on the application, e.g., an application may use the
same FEC Encoding ID for all objects in all sessions. If there is a
portion of the FEC Object Transmission Information that may vary from
object to object and if this FEC Object Transmission Information is
communicated to a receiver out-of-band then the TOI for the object
MUST also be communicated to the receiver together with the
corresponding FEC Object Transmission Information, and the receiver
MUST use the corresponding FEC Object Transmission Information for all
packets received with that TOI. How the TOI and corresponding FEC
Object Transmission Information is communicated out-of-band to
receivers is outside the scope of this document.</t>
<t>It is also possible that there is a portion of the FEC Object
Transmission Information that may vary from object to object that is
carried in-band, for example in the CodePoint field or in Header
Extensions. How this is done is outside the scope of this document. In
this case the FEC Object Transmission Information is associated with
the object identified by the TOI carried in the packet.</t>
</section>
<section anchor="SeSessDes" title="Session Description">
<t>The Session Description that a receiver is REQUIRED to obtain
before joining an ALC session MUST contain the following
information:</t>
<t><list style="symbols">
<t>The multiple rate congestion control building block to be used
for the session;</t>
<t>The sender IP address;</t>
<t>The number of channels in the session;</t>
<t>The address and port number used for each channel in the
session;</t>
<t>The Transport Session ID (TSI) to be used for the session;</t>
<t>An indication of whether or not the session carries packets for
more than one object;</t>
<t>If Header Extensions are to be used, the format of these Header
Extensions.</t>
<t>Enough information to determine the packet authentication
scheme being used, if it is being used.</t>
</list></t>
<t>How the Session Description is communicated to receivers is outside
the scope of this document.</t>
<t>The Codepoint field within the LCT portion of the header CAN be
used to communicate in-band some of the dynamically changing
information within a session. To do this, a mapping between Codepoint
values and the different dynamic settings MUST be included within the
Session Description, and then settings to be used are communicated via
the Codepoint value placed into each packet. For example, it is
possible that multiple objects are delivered within the same session
and that a different FEC encoding algorithm is used for different
types of objects. Then the Session Description could contain the
mapping between Codepoint values and FEC Encoding IDs. As another
example, it is possible that a different packet authentication scheme
is used for different packets sent to the session. In this case, the
mapping between the packet authentication scheme and Codepoint values
could be provided in the Session Description. Combinations of settings
can be mapped to Codepoint values as well. For example, a particular
combination of a FEC Encoding ID and a packet authentication scheme
could be associated with a Codepoint value.</t>
<t>The Session Description could also include, but is not limited
to:</t>
<t><list style="symbols">
<t>The mappings between combinations of settings and Codepoint
values;</t>
<t>The data rates used for each channel;</t>
<t>The length of the packet payload;</t>
<t>Any information that is relevant to each object being
transported, such as the Object Transmission Information for each
object, when the object will be available within the session and
for how long.</t>
</list></t>
<t>The Session Description could be in a form such as SDP as defined
in <xref target="RFC2327"></xref>, or XML metadata as defined in <xref
target="RFC3023"></xref>, or HTTP/Mime headers as defined in <xref
target="RFC2616"></xref>, etc. It might be carried in a session
announcement protocol such as SAP as defined in <xref
target="RFC2974"></xref>, obtained using a proprietary session control
protocol, located on a web page with scheduling information, or
conveyed via E-mail or other out-of-band methods. Discussion of
Session Description formats and methods for communication of Session
Descriptions to receivers is beyond the scope of this document.</t>
</section>
<section title="Packet authentication building block">
<t>It is RECOMMENDED that implementors of ALC use some packet
authentication scheme to protect the protocol from attacks. An example
of a possibly suitable scheme is described in <xref
target="PER2001"></xref>. Packet authentication in ALC, if used, is to
be integrated through the Header Extension support for packet
authentication provided in the LCT building block.</t>
</section>
</section>
<section title="Conformance Statement">
<t>This Protocol Instantiation document, in conjunction with the LCT
building block <xref target="I-D.ietf-rmt-bb-lct-revised"></xref>, the
FEC building block <xref target="RFC5052"></xref> and with a multiple
rate congestion control building block completely specifies a working
reliable multicast transport protocol that conforms to the requirements
described in <xref target="RFC2357"></xref>.</t>
</section>
<section title="Functionality Definition">
<t>This section describes the format and functionality of the data
packets carried in an ALC session as well as the sender and receiver
operations for a session.</t>
<section title="Packet format used by ALC">
<t>The packet format used by ALC is the UDP header followed by the LCT
header followed by the FEC Payload ID followed by the packet payload.
The LCT header is defined in the LCT building block <xref
target="I-D.ietf-rmt-bb-lct-revised"></xref> and the FEC Payload ID is
described in the FEC building block <xref target="RFC5052"></xref>.
The Congestion Control Information field in the LCT header contains
the REQUIRED Congestion Control Information that is described in the
multiple rate congestion control building block used. The packet
payload contains encoding symbols generated from an object. If more
than one object is carried in the session then the Transmission Object
ID (TOI) within the LCT header MUST be used to identify which object
the encoding symbols are generated from. Within the scope of an
object, encoding symbols carried in the payload of the packet are
identified by the FEC Payload ID as described in the FEC building
block.</t>
<t>The version number of ALC specified in this document is 1. The
version number field of the LCT header MUST be interpreted as the ALC
version number field. This version of ALC implicitly makes use of
version 1 of the LCT building block defined in <xref
target="I-D.ietf-rmt-bb-lct-revised"></xref>.</t>
<t>The overall ALC packet format is depicted in <xref
target="ALCpfmt"></xref>. The packet is an IP packet, either IPv4 or
IPv6, and the IP header precedes the UDP header. The ALC packet format
has no dependencies on the IP version number.</t>
<figure anchor="ALCpfmt" title="Overall ALC packet format">
<artwork><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| UDP header |
| |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| LCT header |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FEC Payload ID |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encoding Symbol(s) |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>In some special cases an ALC sender may need to produce ALC packets
that do not contain any payload. This may be required, for example, to
signal the end of a session or to convey congestion control
information. These data-less packets do not contain the FEC Payload ID
either, but only the LCT header fields. The total datagram length,
conveyed by outer protocol headers (e.g., the IP or UDP header),
enables receivers to detect the absence of the ALC payload and FEC
Payload ID.</t>
<t>For ALC the length of the TSI field within the LCT header is
REQUIRED to be non-zero. This implies that the sender MUST NOT set
both the LCT flags S and H to zero.</t>
</section>
<section anchor="SeHeHex" title="LCT Header-Extension Fields">
<t>All senders and receivers implementing ALC MUST support the EXT_NOP
Header Extension and MUST recognize EXT_AUTH, but MAY NOT be able to
parse its content. The EXT_NOP and EXT_AUTH LCT Header Extensions are
defined in <xref target="I-D.ietf-rmt-bb-lct-revised"></xref></t>
<t>This specification defines a new LCT Header Extension, "EXT_FTI",
for the purpose of communicating the FEC Object Transmission
Information in association with data packets of an object. The LCT
Header Extension Type for EXT_FTI is 64.</t>
<t>The Header Extension Content (HEC) field of the EXT_FTI LCT Header
Extension contains the encoded FEC Object Transmission Information as
defined in <xref target="RFC5052"></xref>. The format of the encoded
FEC Object Transmission Information is dependent on the FEC Scheme in
use and so is outside the scope of this document.</t>
</section>
<section title="Sender Operation">
<t>The sender operation when using ALC includes all the points made
about the sender operation when using the LCT building block <xref
target="I-D.ietf-rmt-bb-lct-revised"></xref>, the FEC building block
<xref target="RFC5052"></xref> and the multiple rate congestion
control building block.</t>
<t>A sender using ALC MUST make available the required Session
Description as described in <xref target="SeSessDes"></xref>. A sender
also MUST make available the required FEC Object Transmission
Information as described in <xref target="SeFECBB"></xref>.</t>
<t>Within a session a sender transmits a sequence of packets to the
channels associated with the session. The ALC sender MUST obey the
rules for filling in the CCI field in the packet headers and MUST send
packets at the appropriate rates to the channels associated with the
session as dictated by the multiple rate congestion control building
block.</t>
<t>The ALC sender MUST use the same TSI for all packets in the
session. Several objects MAY be delivered within the same ALC session.
If more than one object is to be delivered within a session then the
sender MUST use the TOI field and each object MUST be identified by a
unique TOI within the session, and the sender MUST use corresponding
TOI for all packets pertaining to the same object. The FEC Payload ID
MUST correspond to the encoding symbol(s) for the object carried in
the payload of the packet.</t>
<t>It is RECOMMENDED that packet authentication be used. If packet
authentication is used then the Header Extensions described in <xref
target="SeHeHex"></xref> MUST be used to carry the authentication.</t>
</section>
<section title="Receiver Operation">
<t>The receiver operation when using ALC includes all the points made
about the receiver operation when using the LCT building block <xref
target="I-D.ietf-rmt-bb-lct-revised"></xref>, the FEC building block
<xref target="RFC5052"></xref> and the multiple rate congestion
control building block.</t>
<t>To be able to participate in a session, a receiver MUST obtain the
REQUIRED Session Description as listed in <xref
target="SeSessDes"></xref>. How receivers obtain a Session Description
is outside the scope of this document.</t>
<t>To be able to be a receiver in a session, the receiver MUST be able
to process the ALC header. The receiver MUST be able to discard,
forward, store or process the other headers and the packet payload. If
a receiver is not able to process the ALC header, it MUST drop from
the session.</t>
<t>As described in <xref target="SeFECBB"></xref>, a receiver MUST
obtain the required FEC Object Transmission Information for each
object for which the receiver receives and processes packets.</t>
<t>Upon receipt of each packet the receiver proceeds with the
following steps in the order listed.</t>
<t><list style="numbers">
<t>The receiver MUST parse the packet header and verify that it is
a valid header. If it is not valid then the packet MUST be
discarded without further processing. If multiple packets are
received that cannot be parsed then the receiver SHOULD leave the
session.</t>
<t>The receiver MUST verify that the sender IP address together
with the TSI carried in the header matches one of the (sender IP
address, TSI) pairs that was received in a Session Description and
that the receiver is currently joined to. If there is not a match
then the packet MUST be discarded without further processing. If
multiple packets are received with non-matching (sender IP
address, TSI) values then the receiver SHOULD leave the session.
If the receiver is joined to multiple ALC sessions then the
remainder of the steps are performed within the scope of the
(sender IP address, TSI) session of the received packet.</t>
<t>The receiver MUST process and act on the CCI field in
accordance with the multiple rate congestion control building
block.</t>
<t>If more than one object is carried in the session, the receiver
MUST verify that the TOI carried in the LCT header is valid. If
the TOI is not valid, the packet MUST be discarded without further
processing.</t>
<t>The receiver SHOULD process the remainder of the packet,
including interpreting the other header fields appropriately, and
using the FEC Payload ID and the encoding symbol(s) in the payload
to reconstruct the corresponding object.</t>
</list></t>
<t>It is RECOMMENDED that packet authentication be used. If packet
authentication is used then it is RECOMMENDED that the receiver
immediately check the authenticity of a packet before proceeding with
step (3) above. If immediate checking is possible and if the packet
fails the check then the receiver MUST discard the packet and reduce
its reception rate to a minimum before continuing to regulate its
reception rate using the multiple rate congestion control.</t>
<t>Some packet authentication schemes such as TESLA <xref
target="PER2001"></xref> do not allow an immediate authenticity check.
In this case the receiver SHOULD check the authenticity of a packet as
soon as possible, and if the packet fails the check then it MUST be
discarded before step (5) above and reduce its reception rate to a
minimum before continuing to regulate its reception rate using the
multiple rate congestion control.</t>
</section>
</section>
<section title="Security Considerations">
<t>The same security consideration that apply to the LCT, FEC and the
multiple rate congestion control building blocks also apply to ALC.</t>
<t>Because of the use of FEC, ALC is especially vulnerable to denial-
of-service attacks by attackers that try to send forged packets to the
session which would prevent successful reconstruction or cause
inaccurate reconstruction of large portions of the object by receivers.
ALC is also particularly affected by such an attack because many
receivers may receive the same forged packet. There are two ways to
protect against such attacks, one at the application level and one at
the packet level. It is RECOMMENDED that prevention be provided at both
levels.</t>
<t>At the application level, it is RECOMMENDED that an integrity check
on the entire received object be done once the object is reconstructed
to ensure it is the same as the sent object. Moreover, in order to
obtain strong cryptographic integrity protection a digital signature
verifiable by the receiver SHOULD be used to provide this application
level integrity check. However, if even one corrupted or forged packet
is used to reconstruct the object, it is likely that the received object
will be reconstructed incorrectly. This will appropriately cause the
integrity check to fail and in this case the inaccurately reconstructed
object SHOULD be discarded. Thus, the acceptance of a single forged
packet can be an effective denial of service attack for distributing
objects, but an object integrity check at least prevents inadvertent use
of inaccurately reconstructed objects. The specification of an
application level integrity check of the received object is outside the
scope of this document.</t>
<t>At the packet level, it is RECOMMENDED that a packet level
authentication be used to ensure that each received packet is an
authentic and uncorrupted packet containing FEC data for the object
arriving from the specified sender. Packet level authentication has the
advantage that corrupt or forged packets can be discarded individually
and the received authenticated packets can be used to accurately
reconstruct the object. Thus, the effect of a denial of service attack
that injects forged packets is proportional only to the number of forged
packets, and not to the object size. Although there is currently no IETF
standard that specifies how to do multicast packet level authentication,
TESLA <xref target="PER2001"></xref> is a known multicast packet
authentication scheme that would work.</t>
<t>In addition to providing protection against reconstruction of
inaccurate objects, packet level authentication can also provide some
protection against denial of service attacks on the multiple rate
congestion control. Attackers can try to inject forged packets with
incorrect congestion control information into the multicast stream,
thereby potentially adversely affecting network elements and receivers
downstream of the attack, and much less significantly the rest of the
network and other receivers. Thus, it is also RECOMMENDED that packet
level authentication be used to protect against such attacks. TESLA
<xref target="PER2001"></xref> can also be used to some extent to limit
the damage caused by such attacks. However, with TESLA a receiver can
only determine if a packet is authentic several seconds after it is
received, and thus an attack against the congestion control protocol can
be effective for several seconds before the receiver can react to slow
down the session reception rate.</t>
<t>Reverse Path Forwarding checks SHOULD be enabled in all network
routers and switches along the path from the sender to receivers to
limit the possibility of a bad agent injecting forged packets into the
multicast tree data path.</t>
</section>
<section title="IANA Considerations">
<t>This specification registers the following LCT Header Extensions
Types in namespace ietf:rmt:lct:headerExtensionTypes:variableLength:</t>
<texttable>
<ttcol>Value</ttcol>
<ttcol>Name</ttcol>
<ttcol>Reference</ttcol>
<c>64</c>
<c>EXT_FTI</c>
<c>This specification</c>
</texttable>
</section>
<section title="Acknowledgments">
<t>This specification is substantially based on RFC3450 <xref
target="RFC3450"></xref> and thus credit for the authorship of this
document is primarily due to the authors of RFC3450: Mike Luby, Jim
Gemmel, Lorenzo Vicisano, Luigi Rizzo and Jon Crowcroft. Vincent Roca,
Justin Chapweske and Roger Kermode also contributed to RFC3450.
Additional thanks are due to Vincent Roca and Rod Walsh for
contributions to this update to Proposed Standard.</t>
</section>
<section anchor="changes" title="Changes from RFC3450">
<t>This section summarises the changes that were made from the
Experimental version of this specification published as RFC3450 <xref
target="RFC3450"></xref>: <list style="symbols">
<t>Update all references to the obsoleted RFC 2068 to RFC 2616</t>
<t>Removed the 'Statement of Intent' from the introduction (The
statement of intent was meant to clarify the "Experimental" status
of RFC3450.)</t>
<t>Removed the 'Intellectual Property Issues' Section and replaced
with a standard IPR Statement</t>
<t>Remove material duplicated in LCT</t>
<t>Update references for LCT and FEC Building Block to new versions
and align with changes in the FEC Building Block.</t>
<t>Split normative and informative references</t>
<t>Material applicable in a general LCT context, not just for ALC
has been moved to LCT</t>
<t>The first bit of the “Protocol Specific Indication”
in the LCT Headert is defined as a “Source Packet
Indication”. This is used in the case that an FEC Scheme
defines two FEC Payload ID formats, one of which is for packets
containing only source symbols which can be processed by receivers
that do not support FEC Decoding.</t>
<t>Definition and IANA registration of the EXT_FTI LCT Header
Extension</t>
</list></t>
</section>
</middle>
<back>
<references title="Normative references">
&rfc5052;
&lctbb;
&rfc2119;
&rfc1112;
&rfc2616;
&rfc2327;
&rfc2974;
&rfc2357;
&rfc3023;
&rfc0768;
</references>
<references title="Informative references">
&rfc3269;
&rfc3450;
&rfc3453;
&rfc3048;
<reference anchor="PER2001">
<front>
<title>Efficient and Secure Source Authentication for
Multicast</title>
<author fullname="A. Perrig" initials="A." surname="Perrig">
<organization></organization>
</author>
<author fullname="R. Canetti" initials="R." surname="Canetti">
<organization></organization>
</author>
<author fullname="D. Song" initials="D." surname="Song">
<organization></organization>
</author>
<author fullname="J. D. Tygar" initials="J. D." surname="Tygar">
<organization></organization>
</author>
<date month="February" year="2001" />
</front>
<seriesInfo name="Network and Distributed System Security Symposium, NDSS 2001, pp. 35-46"
value="" />
</reference>
</references>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-23 05:04:19 |