Reliable Multicast at Internet Scale (Part 4): The Gotchas

Freshlex LLC (should) architect the reliable multicast infrastructure for the putative John Carmack biopic, which will hit the Internet in December of 2018. The first blog post discusses two of the enabling technologies, FCAST and ALC. The second blog post discusses the remaining three technologies, LCT, WEBRC and the FEC building block. The third blog post discusses an Architecture that integrates the five technologies. This final architecture discusses possible integration issues and challenges.

WEBRC Integration Issues

Our FCAST/ALC architecture runs over a multicast IP network. An issue arises when the multicast IP network uses RFC112 Any Source Multicast (ASM). The WEBRC building block uses multicast round trip time (MRTT) and packet loss to compute a target reception rate. The WEBRC receiver uses this target reception rate for congestion control. ASM skews MRTT and packet loss, and thus gives receivers an erroneous target reception rate.

The WEBRC receiver computes MRTT as the time it takes the receiver to receive the first data packet after sending a join request to a channel. ASM, however, initiates multicast using rendezvous points (RP). All transmitters send their data packets to an RP (decided a priori by network engineers) that may be far away. The receivers send a join to this RP. Once data packets begin to flow to the receivers, the routers switch to a shortest path tree (SPT), finding the shortest path from the transmitter to the receiver, which does not need to include the RP. [RFC5775 6]

The following (see diagram) illustrates a scenario where switching from an RP to SPT skews the WEBRC receiver MRTT computation (and therefore target reception rate). We use ASM, so “any source” transmits to the multicast address. TX-A and TX-B both have data to multicast. They transmit to the rendezvous point. The RX has no idea who is sending, they just want to join the multicast, so they send a join to the rendezvous point. The RP is three hops away. Lets say for illustrative purposes each hop adds 10ms delay. The join takes 30ms to reach the RP and then the first data packets from the multicasts for TX-A and TX-B take 30ms each.

Thus, the RX computes the MRTT for TX-A as 60ms, and the MRTT for TX-B as 60ms. At this point the multicast enabled routers switch to shortest path tree. The multicast from TX-A to the RX now only takes one hop, so the MRTT would be 2 * 10ms or 20ms. The multicast from TXB to the RX is now four hops, so the actual MRTT should be 80ms. Thus, as a result of ASM, the RX sets the target reception rate for TX-A as 66% too low, and the target reception rate for TX-B as 33% too high.

The “saving grace” in the case for TX-B would be the dropped packets, since 1/3 would drop and the RX would change the target reception rate accordingly. WEBRC, however, adjusts rates at points in time that are separated by seconds. In addition, if we lost packets during the switch over from RP to SPT then the RX would have incorrect parameters for packet loss (based on receiving or not receiving monotonically increasing sequence numbers), which would skew the target reception rate. The solution to this issue is to use SSM multicast, which does not use RP. If we must use ASM, then have one RP (and thus multicast address) per sender and put the RP as close to the sender as possible (i.e. on the first hop router at the demarc). [RFC5775 6]

Another design issue with WEBRC deals with setting the appropriate wave channel rates. We need to set the base rate to the lowest common denominator, so that all users can subscribe to it. The main purpose of the base channel is to communicate timing information (CTSI) and wave channel rates to the receivers so they can sync their joins to wave channel periods and join enough channels to reach their target rates (RFC3738 8). We need to, however find the right balance for the wave channel data rates. We need to balance granularity against number of multicast channels. If we had a video stream at OC-192 rates, would it make sense to have 3.75e+4 channels? Would the joins flood the NW? It would make sense to tune the channel rates to the expected use case. If 99% of the users have the same capacity, then we can be coarse. If the bell curve of capacity is low and wide, then we need to be more granular. The only way to find the optimal channel rates is through off line analysis, either using mathematical analysis (Bertsekas, Kleinrock, Jackson etc.) or a discrete event simulation (DES) such as Riverbed SteelCentrall NetPlanner. Off line analysis, however, requires user profiles, use cases and real life network metrics.

The following diagram illustrates a poor design choice. We have three channels, the base channel is set to T1, wave 1 is an OC-12 and wave 2 is an OC-192. A receiver with an OC-12 does not have enough capacity to join the base and wave 1, so he is stuck with just the T1 rate, a very poor efficiency.

The final issue for WEBRC deals with the length of periods for joins. We need to balance the join/leave times against available BW fluctuations. For example, if a receiver joins a channel and the BW drops significantly, the receiver can’t leave that channel until the next time slot (RFC3738 13). For the duration of the time slot, the traffic congestion may choke other congestion protocols (like TCP). The RFC recommends 10s/period (RFC3738 9). Since our data rate is constant, the receivers should not have any surprises, and this period duration should suffice. This however is still an issue and needs to be observed and addressed during live transmissions.

LCT Integration Issues

LCT provides a convenient mechanism for setting the mandatory transport session ID. As per the RFC, we have the option of using the 16-bit UDP port field to carry the TSI (RFC5651 9). I would recommend against this, since we cannot guarantee that downstream receivers would use some sort of port address translation or firewalling. Since the LCT header is mandatory and contains a field for the TSI, it’s best to just set the TSI there.

FCAST Integration Issues

To recap, FCAST uses sessions to send objects to receivers. FCAST sends objects by creating a carousel instance, filling the carousel instance with objects, and then using a carousel instance object to let receivers know which objects the carousel instance carries (RFC6968 8).

FCAST uses one session per sender, and in each session each object must have a unique Transport Object Identifier. Our integration engineers need to be aware of the potential for TOI wrapping, for long-lived sessions (RFC6968 10). FCAST gets the TOI from the LCT header.

The LCT RFC allows a finite number of bits in the LCT header for TOI (RFC5651 17). Thus, for long-lived sessions (days, weeks), TOI wrap and present ambiguity to receivers, similar to the issue of byte sequence number wrapping for TCP. A receiver may receive two separate objects with the same TOI in the course of a long-lived session. With “on demand” mode, carousels cycle through the same pieces of data a set number of times. Consider a large carousel instance, where FCAST sends an object with TOI “1”, followed by enough objects to wrap the TOI.

During the first cycle, the CI sends another, newer object with TOI “1.” The cycle finishes, and FCAST starts the cycle again, sending the original object with TOI “1.” The receiver has no idea what to do with the object of TOI “l,” since it alternates as a reference for two distinct objects. A way to prevent this issue is, once FCAST reaches the halfway point of sequence numbers, it resends any old data with a new TOI. Another way to prevent TOI wrap ambiguity is to have metadata associated with TOI, so the receiver can distinguish between two objects with the same TOI. [RFC6968 10-11]

Another integration issue relates to the Carousel Instance Object (CIO). As mentioned in the above paragraph, the CIO carries the list of the compound objects that comprise a Carouse1 Instance, and specifies the respective Transmission Object Identifiers (TOI) for the objects. The CIO contains a “complete” flag that informs the receiver that the CI will not change in the future (i.e. FCAST guarantees the sender will not add, remove or modify any objects in the current carousel instance). Consider a receiver that receives a CIO with a “complete” flag. We may be tempted to use the list of Compound Object TOI as a means to filter incoming data. The issue, however, is that FCAST treats the CIO (list of objects) as any other object, that is, there are no reserved TOI that designate an object as a CIO. Thus, the receiver will never know in advance the TOI of CIO, so the RFC recommends that the receivers do not filter based on TOI. If a receiver were to filter out all but the TOI received in CIO with a “complete” flag, that receiver would also filter out any new CIO for new carousel instances associated with the session, and the receiver may miss out on interesting objects. [RFC6968 9]

FCAST, finally, allows integrators to send an empty CIO during idle times. The empty CIO lets RX know all previous objects have been removed, and can be used as a heartbeat mechanism. [RFC6968 9-10]


Real time video content delivery systems are a challenge due to the constant, high data rates involved. RT Video CDP lend themselves to circuit switched networks that can reserve enough bandwidth from sender to receiver and let the data fly. The next best architecture would be packet switched networks that were designed for multimedia, such as the Integrated Services Data Network (ISDN) or ATM. Our customer, unfortunately, required us to deploy a CDP on the Internet. To make matters worse, they required our CDP to handle millions of simultaneous receivers. Normally, when delivering constant rate video on the Internet, engineers will use a signaling technology such as resource reservation protocol (RSVP) to guarantee bandwidth from sender to receiver. An end to end (E2E) reservation scheme, however, does not lend itself well to a system with one sender and millions of receivers.

Our challenge, therefore, was to identify and deploy a one to many, massively scalable CDP that provides asynchronous, reliable and fair, multi-rate streaming data transport. We identified a solution based on IETF standards. This paper went through the solution intent, the technologies used, the integration choices made, the integration issues avoided and the validation steps performed to ensure the success.

Update: January 2019:

In the end, we successfully integrated the architecture and met all of Warner Brothers’ requirements. Averaged over the course of the movie, the average receiver ran at 90% of the available network capacity, and utilized 87% of processor resources. The bit error rate for the average receiver was 1e-13. Finally, both Anthony Michael Hall and Dwayne ‘The Rock” Johnson went on to win academy awards for best actor and best supporting actor.


[RFC3453] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, H. and J. Crowcroft, “The Use of Forward Error Correction (FEC) in Reliable Multicast”, RFC 3453 December 2002.

[RFC3738] Luby, M. and V. Goyal, “Wave and Equation Based Rate Control (WEBRC) Building Block”, RFC 3738, April 2004.

[RFC5651] Luby, M., Watson, M. and L. Vicisano, “Layered Coding Transport (LCT) Building Block”, RFC 5651, October 2009.

[RFC5775] Luby, P., Watson, P. and L. Vicisano, “Asynchronous Layered Coding (ALC) Protocol Instantiation”, RFC 5775, April 2010.

[RFC6968] Roca, V. and B. Adamson, “FCAST: Scalable Object Delivery for the ALC and NORM Protocols”, RFC 6968, July 2013.

Reliable Multicast at Internet Scale (Part 2): LCT, WEBRC and FEC

Freshlex LLC (should) architect the reliable multicast infrastructure for the putative John Carmack biopic, which will hit the internet in December of 2018. The first blog post discusses two of the enabling technologies, FCAST and ALC. This blog post discusses the three technologies that enable ALC: LCT, WEBRC and the FEC building block.

Reliable Multicast Building Block: LCT

LCT Description Since the 70’s engineers have for the most part associated the transport layer of the Open Systems Interconnect (OSI) protocol stack with either Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). More recently, we have real time protocol (RTP) as a session layer for real time media. This blog post introduces a new transport layer protocol, layered coding transport (LCT).

LCT acts as a building block for ALC. LCT provides a transport layer service that, in concert with FEC and WEBRC allows ALC to be a massively scalable and reliable content stream delivery protocol for IP multicast networks. RMT WG designs LCT for multicast protocols and designs LCT to be compatible with WEBRC and FEC. LCT does not require any backchannel and works well with any LAN, WAN, Intranet, Internet, asymmetric NW, wireless NW or Satellite NW (RFC5651 9). LCT works best for at least multi-GB objects that are transmitted for at least 10s of seconds. Streaming applications benefit greatly from LCT. [RFC5651 4]

LCT Architecture Definition

LCT uses a single sender that transmits objects (interesting to receivers) via packets to multiple channels for some period of time. These channels split the objects into packets and associate the packets with the object using headers. LCT works with WEBRC to provide multiple-rate congestion control. Receivers join and leave LCT layers (via ALC channels) during participation in a session to reach their target reception rate (see WEBRC).

As the name suggests, LCT uses layered coding to produce a coded stream of packets that LCT partitions into ordered sets of packets. The FEC building block codes the packets for reliability. For streaming media applications, layering allows variable transfer speeds and by extension image quality to RX with arbitrary NW capacity. The best example of LCT follows.

Imagine a web TV application split into three layers. A RX that joins the first channel would receive a black and white picture. An RX that had more capacity would join the first and second channel and receive a color picture. An RX with transparent capacity would be able to join all three layers, and receive a HD color picture. The key to this example is that the sender does not duplicate any data between layers. The RX joins successive layers to receive a higher quality picture at the cost of using more bandwidth. [RFC5651 6]




LCT Operations

The WEBRC building block sends packets associated with a single session to multiple LCT channels at rates computed to optimize multiple-rate congestion control (RFC3738 3). The receivers join one or more channels according to the NW congestion. The WEBRC building block provides LCT with information for the CCI field, which is opaque to LCT (RFC5651 16). The FEC building block codes the packets that LCT sends to channels for reliability.

On the RX side, the RX must first join an LCT session. The RX must obtain enough of the session description parameters to start the session. Once the RX has all the session description parameters the RX begins to process packets. The RX must identify & de-multiplex the packets associated the LCT session. Each LCT session must have a unique Transport Session Identifier (TSI). The LCT session scopes the TSI by the (Sender IP Address, TSI) pair. LCT stamps each packet’s LCT header with the appropriate TSI. [RFC5651 25-26]

The RMT WG designed LCT for best effort (BE) service. BE service does not guarantee packet reception or packet reception order. BE service does not provide support for reliability or flow/ congestion control. LCT does not provide any of these services on its own. ALC, however, uses LCT along with FEC and WEBRC to provide reliable, multi-rate congestion controlled layered transport. [RFC5651 27]

Reliable Multicast Building block: WEBRC

WEBRC Description

As per RFC 2357, the use of any reliable multicast protocol in the Internet requires an adequate congestion control scheme. Furthermore, ALC must support RFC3738, the Wave and Equation Based Rate Congestion Control (WEBRC) Building Block (RFC5775 10). WEBRC provides multiple rate congestion control for data delivery. Similar to FCAST, ALC, LCT and multicast FEC, the RMT WG designs WEBRC to support protocols for IP Multicast. In the spirit of massive scalability, WEBRC requires no feedback and uses a completely receiver driven congestion control protocol. WEBRC enables a single sender to deliver data to individual receivers at the fastest possible rate, even in a highly heterogeneous network architecture. In other words, WEBRC dynamically varies the reception rate of each RX independent of other receivers (RFC3738 1). WEBRC competes fairly with TCP and similar congestion control sessions (RFC3738 4).

WEBRC Architecture Definition

A single sender transmits packets to multiple channels. The sender designates one channel as the base channel, the remaining are called wave channels. Each channel starts off at a high packet rate, after each equal-spaced period of time, the packet rate of that channel reduces until the channel is quiescent. A channel’s cycle from full rate to quiescence takes a configurable number of periods, by default their aggregate summing to a long duration of time (several minutes). At the end of each period, the RX joins or leaves channels depending on if the aggregate of the current TX rates allows the RX to reach its target RX rate. At the end of each period the RX orders each wave channel into layers, based on their TX rates (the higher the rate, the higher the layer). The designation of wave channel to a layer, therefore, varies cyclically over time. Once joined, an RX stays with a channel until that channel becomes quiescent. [RFC3738 8]

A key metric for each receiver, therefore, is the target reception rate. The target reception rate drives the number of layers (and by extension, channels) that a receiver must join. The RX measures and performs calculations on congestion control parameters (e.g. the average loss probability and the average RTT) and makes decisions on how to increase or decrease its reception rate based on these parameters. The RX based approach of WEBRC suits itself to protocols where the sender handles multiple concurrent connections and therefore WEBRC is suitable as a building block for multicast congestion control. An RX with a slow connection does not slow down RX with faster connections. [RFC3738 13-23]

WEBRC Operations

When WEBRC receives packets from ALC, WEBRC first checks to see that the packets belong to the appropriate session before applying WEBRC. ALC uses LCT, so WEBRC looks to the LCT header to find the (sender IP address, TSI) tuple that denotes what session a received packet belongs to (RFC5651 12). The multicast network identifies a channel to receivers via a (sender IP address, multicast group address) pair, and the receiver sends messages to join and leave the channel to the multicast group address. When the RX initiates a session, it must join the base channel. The packets on the base channel help the RX orient itself in terms of what the current time slot index is, which in turn allows the RX to know the relative rates on the wave channels. The RX orders these wave channels into layers, from lowest to highest rates. The RX remains joined to the base channel for the duration of its participation in the session. [RFC3738 8]

As mentioned earlier, the lowest layer has lowest rate and highest layer has highest rate. Each time a wave channel becomes active, it becomes the highest layer. At the end of each time slot the lowest-layer wave channel deactivates and all channels move down a layer. A RX always leaves the lowest layer when it deactivates.

After joining a session, the RX adjusts its rate upwards by joining wave channels in sequence, starting with the lowest layer and moving towards the highest. The rates on the active wave channels are decreasing with time so the receiver adjusts its rate downward simply by refraining from joining additional wave channels. The layer ordering among the channels changes dynamically with time so the RX must monitor the Current Time Slot Indicator (CTSI).

Once the receiver joins a wave channel, the receiver remains joined to the wave channel until it deactivates (RFC3738 8). The following diagram illustrates the relationship between wave channels, layers and target reception rate.



In the above figure, assume the receiver wants a target rate of 7λ/4 packets per second (pps). This means the receiver must join the base (λ/4pps), layer 0 (λ/4pps), layer 1 (λ/2pps) and layer 2 (3λ/4pps). The receiver joins layers by joining underlying channels, sending joins and leaves to their respective multicast addresses. We see in the figure that for time t, layer 2 contains wave channel 4, layer 1 contains wave channel 3 and layer 0 contains wave channel 2. The receiver leaves channel 1 (which is now quiescent). The receiver stays joined to the base and wave channels 3 and 2. The receiver sends a join to wave channel 4. At time t+1, the layers change again. The receiver stays joined to the base, 4 and 3. The receiver leaves channel 2 and joins channel 0. For time t+2, the receiver stays joined to the base, 0 and 4. The receiver leaves channel 3 and joins channel 1.

Reliable Multicast Building Block: FEC

FEC Building Block Description

Content Delivery Protocols (CDP) have many options available to them to increase reliability. We’ll first read about two non-forward error correction (FEC) based options: automatic request for retransmission (ARQ) and data carousels. First, consider ARQ. If an ARQ receiver does not receive a packet or receives a corrupted packet, the receiver asks the sender to re-transmit the packet. ARQ therefore, requires a back channel and does not scale well for one to many CDP. Using ARQ on one to many CDP sets the architecture up for feedback implosions and “NACK of death” (imagine 1e+7 receivers simultaneously detecting dropped data and asking for a re-transmission). In addition, in a network where different receivers have different loss patterns, ARQ wastes resources. RX would need to wait for the re-transmissions of packets that other receivers lost, even if the RX already have those data. [RFC5052 2]

A data carousel solution partitions objects into equal length pieces of data (source symbols), puts them into packets and cycles through and sends these packets. Each RX receives the packets until they have a copy of every packet. While the data carousel solution requires no back channel, if an RX misses a packet, the RX has to wait on Carousel until it’s sent again. [RFC6968 8]



RFC 3454 describes, therefore, how to use FEC codes to augment/ provide reliability for one-to-many reliable data transport using IP multicast. RFC 3454 uses the same packets containing FEC data to simultaneously repair different packet loss patterns at multiple RX. [RFC3453 4]

FEC has multiple benefits for our FCAST/ALC architecture. FEC augments reliability and overcomes erasures (losses) and bit level corruption. The primary application of FEC to IP multicast, however, is an erasure code since the IP multicast NW layers detect (bit level) corrupted packets and discard them (or the transport layers will use packet authentication to discard corrupted packets) (RFC3453 3).

FEC Operation

The data source inputs into FEC some number k of equal length source symbols. The FEC encoder then generates some number of encoding symbols that are of the same length as the source symbols. The packets are placed into packets and then sent. On the receiving side, the RX feeds the encoded symbols into a decoder to recreate an exact copy of the k source symbols. ALC can use block or expandable FEC codes for the underlying FEC building block. [RFC5775 11]

With a block encoder, we input k source symbols and a constant number n. The encoder generates a total of n encoding symbols. The encoder is systematic if it generates n-k redundant symbols yielding an encoding block of n encoding symbols in total composed of the k source symbols and the n-k redundant symbols. With a block encoder, any k of the n encoding symbols in the encoding block is sufficient to reconstruct the original k source symbols. [RFC3453 5-6]

An expandable FEC encoder takes input of k source symbols and generates as many unique encoding symbols as requested on demand. At the receiver side, any k of the unique encoding symbols is enough to reconstruct the original k source symbols. [RFC3453 7]


This post discusses three technologies that enable ALC for reliable multicast: LCT, WEBRC and the FEC building block. The next blog post discusses an Architecture that integrates all of the enabling technologies.


[RFC3453] Luby, M., Vicisano, L., Gemmell, J., Rizzo, L., Handley, H. and J. Crowcroft, “The Use of Forward Error Correction (FEC) in Reliable Multicast”, RFC 3453 December 2002.

[RFC3738] Luby, M. and V. Goyal, “Wave and Equation Based Rate Control (WEBRC) Building Block”, RFC 3738, April 2004.

[RFC5651] Luby, M., Watson, M. and L. Vicisano, “Layered Coding Transport (LCT) Building Block”, RFC 5651, October 2009.

[RFC5775] Luby, P., Watson, P. and L. Vicisano, “Asynchronous Layered Coding (ALC) Protocol Instantiation”, RFC 5775, April 2010.

[RFC6968] Roca, V. and B. Adamson, “FCAST: Scalable Object Delivery for the ALC and NORM Protocols”, RFC 6968, July 2013.

[RFC5052] Watson, M., Luby, M. and L. Vicisano, “Forward Error Correction (FEC) Building Block”, RFC 5052, August 2007.

Reliable Multicast at Internet Scale (Part 1): FCAST and ALC

Reliable multicast?!? How on earth can you guarantee transport using a unidirectional, asynchronous delivery method? Could you scale your solution to support ten million downstream users, each with a different capacity, varying from dial up to 100GbE metro Ethernet? Surprisingly, you can!

In this series of blog posts I discuss a few interesting technologies that provide massively salable reliable multicast. In summary, you can guarantee reliable multicast using a “data carousel,” and can handle the non-uniform capacity issues with the idea of layered coding. Users receive the layers that their capacity supports, the more layers they subscribe to, the higher quality video stream they receive. Read the next few blog posts to dive into the fascinating details.


Warner Brothers pictures begins filming the John Carmack biopic (starring Anthony Michael Hall as John Carmack and Dwayne “The Rock” Johnson as John Romero) next month. The film depicts Carmack’s life, from shareware coder to superstar “Doom” engine developer to the founder of Armadillo aerospace. To feed the geek buzz surrounding the picture, Warner Brothers will debut the film on the Internet, providing a one time, free Multicast on December 10th, 2018… the 25th anniversary of the initial release date of “Doom”.

Warner Brothers predicts millions of subscribers to this one time Multicast, and (should) contract Freshlex, LLC to develop the enabling architecture. In other words, Warner Brothers wants to transmit a single video feed to millions of Internet receivers, each with arbitrary network capacity. Warner Brothers needs a reliable, massively scalable solution.

These blog posts demonstrates the use of IETF standard protocols to provide a reliable, massively scalable solution with session management and multiple rate congestion control that stresses network fairness. Our solution looks at FCAST/ Asynchronous Layered Coding (ALC), designed by the IETF Reliable Multicast Transport (RMT) Working Group (WG) to provide just that.

This blog post describes FCAST and ALC, which enable reliable multicast. The next blog post describes the building blocks of ALC: LCT, WEBRC and FEC.

Reliable Multicast Building Block: FCAST


FCAST provides object delivery over asynchronous layered coding (ALC). FCAST uses a lightweight implementation of the user datagram protocl (UDP)/ Internet protocol (IP) protocol stack to provide a highly scalable, reliable object delivery service that limits operational processing and storage requirements. Engineers should not consider FCAST as highly versatile, but for appropriate uses cases (such as the streaming video use case this paper discusses), FCAST is massively scalable and robust. [RFC6968 3]


FCAST uses purely unidirectional transport channels for massive scalability. An engineer could hack FCAST to collect reception metrics but this limits scalability. FCAST favors simplicity, sending metadata and object content together in a compound object. The in-band approach, however does not allow a receiver (RX) to decide in advance if an object is of interest until the RX joins the session and processes the metadata portion of the compound object (RFC6968 9). An out-of-band metadata approach would obviate this setback, but remember, the driving requirement of the effort is massive scalability (RFC6968 4). The Reliable Multicast Transport Working Group (RMT WG) designs FCAST to be compatible with ALC and the ALC building blocks: FEC, WEBRC and LCT (RFC5775 1).

Architecture Definition

FCAST provides a content delivery service and transmits objects to a (very large) group of receivers in a reliable way. An engineer could use FCAST over negative acknowledgement (NACK) Oriented Reliable Multicast (NORM) but since the RMT WG designed NORM to use NACK, NORM does not fit the spirit of the Architecture. The Architecture, therefore integrates FCAST to use ALC. Nothing about FCAST limits the maximum number of receivers. Using ALC provides the FEC building block and thus a measure of reliability. In addition, FCAST uses the concept of data carousels (described below) and the longer a carousel runs, the more reliable the content delivery service becomes. [RFC6968 6]

Components: The bullets below describe the FCAST components [RFC6968 5]:

  • Compound Object: Header (Includes metadata) + Object
  • Carousel: Compound object transmission system
  • Carousel Instance
    • Transmission system containing a collection of compound objects
    • Fixed set of registered compound objects that are sent by the carousel during a
      certain number of cycles
    • Note: whenever objects need to be added or removed, a new carousel object is
  • Carousel Instance Object (CIO): List of objects in the carousel instance
    • Note: The CIO is itself an object
    • Note: The CIO does not describe the objects themselves (e.g. no metadata)
  • Carousel Cycle: A period of time when all the objects are sent once
    • Transmission round within which all the registered objects in a Carousel Instance
      are transmitted a certain amount of times
    • By default, objects are sent once per cycle
  • Transmission Object Identifier (TOI)
    • The ID number associated to an object at the lower LCT (Transport) layer
  • FEC Object Transmission Information (FEC TOI)
    • Information required for coding


On the sender side of FCAST, a user first selects a set of objects to deliver to the receivers and submits the objects and the object metadata to the FCAST application. For each object, FCAST creates the compound object (header, metadata and the original object) and registers the compound object in the carousel instance. The user informs FCAST when he completes submission of all the objects in the set. If the user knows that no other object will be submitted then it informs FCAST accordingly. The user then specifies the desired number of cycles. For the most part the user can correlate the number of cycles with reliability. FCAST, nonetheless, now knows the full list of compound objects that are part of the carousel instance and creates a CIO (if desired) with a complete flag (if appropriate). The FCAST application then defines a TX schedule of these compound objects, including the CIO. The schedule defines which order the packets of the various compound objects are sent. FCAST now starts the carousel transmission for the number of cycles specified and continues until (1) FCAST completes the desired number of TX cycles (2) the user wants to kill FCAST or (3) the user wants to add or remove objects, in which case FCAST must create a new CI. [RFC6968 12]

On the receiver side of FCAST, the RX joins the session and collects encoded symbols. Once the RX receives the header the RX processes the metadata and chooses to continue or not. Once the RX receives the entire object the RX process the headers retrieves the metadata, decodes the metadata and processes the object. When the RX receives a CIO (a compound object with the “I” bit set) the receiver decodes the CIO and retrieves the list of compound objects that are part of the current carousel instance (and can also determine which compound objects have been removed). If the RX receives a CIO with the complete flag set, and the RX has successfully received all the objects of the current carousel instance, the RX can safely exit the current FCAST session. [RFC6968 13]

Reliable Multicast Building Block: ALC


Asynchronous Layered Coding (ALC) provides massively scalable, asynchronous, multirate, reliable, network friendly content delivery transport to an unlimited number of concurrent receivers from a single sender. Three building blocks comprise ALC: (1) IETF RFC5651 Layered Coding Transport (LCT) for Transport Layer control, (B) IETF RFC3738 Wave and Equation Based Rate Control (WEBRC) for multi-rate congestion control and (3) IETF RFC3454 IP multicast forward error correction (FEC) for reliability [RFC5775 1]. The Reliable Multicast Transport (RMT) working group (WG) designs ALC for IP multicast, although an engineer can use it for unicast. ALC has no dependencies on IP version.

The diagram below shows the FCAST/ALC architecture and packet format.




ALC has advantageous features. The RMT WG designates scalability as the primary design goal of ALC. IP multicast by design is massively scalable, however, IP multicast only provides a best effort (BE) service devoid of session management, congestion control or reliability. ALC augments IP multicast with session management, congestion control and reliability without sacrificing massive scalability. As a result, the number of concurrent receivers for an object is theoretically infinite, and in practice potentially in the millions. [RFC5775 4]

ALC provides reliable asynchronous transport for a wide range of objects. The aggregate size of delivered objects can vary from hundreds of kilobytes (KB) to terabytes (TB). Each receiver (RX) initiates reception asynchronously and the reception rate for each RX is the maximum fair bandwidth available between the receiver and sender. In other words, each RX believes it has a dedicated session from TX to RX, with rate adjustments that match the available bandwidth at any given time. The building blocks of ALC allow it to perform congestion control, reliable transport and session layer control without the need for any feedback packets. The lack of any channel from receiver to sender enables ALC to be massively scalable. [RFC5775 5]


ALC transports one or more objects to multiple receivers using a single sender and a single session. An application (such as FCAST) provides data objects to ALC. ALC generates packets from these objects, formats them and then hands them off to the lower layer building blocks. The FEC building block encodes them for reliability. The LCT building block provides in-band session management and places the objects onto multiple transmission channels. The WEBRC building block places the data onto the channels at rates optimized for multiple-rate, feedback free congestion control. The RX joins appropriate channels associated with the session, joins or leaves channels for congestion control and uses the ALC, LCT and FEC information to reliably reconstruct the packets into objects. Thanks to the FEC building block, the RX simply waits for enough packets to arrive to reliably reconstruct the object. The ALC architecture does not provide any ability for a RX to request a re-transmission. Thanks to the focus on massive scalability the rate of transmission out of the TX is independent of the number and individual reception experience of the RX. [RFC5775 7]

ALC Session

The concept of an ALC session matches that of LCT. A session contains multiple channels from a single sender used for some period of time to carry packets pertaining to the TX of objects interesting to receivers (RFCS5651 4-5). ALC performs congestion control over the aggregate of the packets sent to channels belonging to a session (RFC5775 7).

ALC Session Description

An ALC session requires a session description. Any receiver that wants to join an ALC session must first obtain the session description. A discussion of how to get this session description to the receivers follows in the “integration choices” section of this paper. The session description contains the following information (RFC5775 12):

  • Sender IP Address
  • Number of channels in the session
  • Multicast address and port # for each channel in the session
  • Data Rates used for each channel
  • Length of each packet payload
  • Transport Session Identifier (TSID) for the session
  • An indication if the session carries packets for more than one object
  • Whether the session describes required FEC information (RFC5052) out of band or in-band (using header extensions)
  • Whether the session uses Header Extensions, and if so the format
  • Whether the session uses packet authentication, and if so the scheme
  • The MRCC building block used (The ALC RFC recommends WEBRC, so we use that in this paper)
  • Mappings between settings and Codepoint Value (for example, if different objects use different FEC or authentication schemes, the Codepoint values distinguish them)
  • Object metadata such as when the objects will be available and for how long


The integration of three building blocks defines ALC, so first and foremost, the sender follows all operations associated with the LCT, FEC and WEBRC building blocks. ALC nonetheless, first makes available the required session description and FEC Object transmission information. As mentioned earlier, the session description contains the sequence of channels associated with the sender. ALC fills in the congestion control indication (CCI) field with information provided by the WEBRC building block. ALC then sends packets at appropriate rates to the channels as dictated by the WEBRC building block. ALC stamps every packet with the Transport Session ID (TSI), in case the receivers join sessions from other senders. If this particular session contains more than one object, then ALC stamps each packet with the appropriate transport object ID (TOI). ALC stamps the packet payload ID based on information from the FEC building block. As discussed in the “Security Validation” section of this paper, the IETF recommends packet authentication as a precaution. If an ALC instance does use packet authentication, it uses a header extension to carry the authentication information. [RFC5775 16]

The ALC RX also conforms to all operations required by LCT, FEC and WEBRC. The RX first obtains a session description and joins the session. The RX then obtains the in-band FEC Object Transmission Information for each object the RX wants. Upon receiving a packet the RX parses the packet header, and verifies that it is a valid header (discards packet if invalid). The RX verifies that the (Sender IP Address, TSI) tuple matches one of the pairs received in Session Description for the session the RX is currently joined to (if not, discard). The RX then proceeds with packet authentication and discards the packet if invalid. After valid packet authentication, the RX processes and acts on the CCI field in accordance with the WEBRC building block. If ALC carries more than one object in session, the RX verifies the TOI (and discards if not valid). The RX finally processes the remainder of the packet, interpreting other header fields and uses the FEC Payload ID and the encoding symbols in the payload to reconstruct the object. [RFC5775 17]


This blog post describes FCAST and ALC, which enable reliable multicast. The next blog post describes the building blocks of ALC: LCT, WEBRC and FEC.


[RFC5651] Luby, M., Watson, M. and L. Vicisano, “Layered Coding Transport (LCT) Building Block”, RFC 5651, October 2009.
[RFC5775] Luby, P., Watson, P. and L. Vicisano, “Asynchronous Layered Coding (ALC) Protocol Instantiation”, RFC 5775, April 2010.
[RFC6968] Roca, V. and B. Adamson, “FCAST: Scalable Object Delivery for the ALC and NORM Protocols”, RFC 6968, July 2013.