packet-filtering-and-fec.md

SRT Packet Filtering & FEC

• Introduction
• Configuration
  • General syntax
  • Configuring the FEC filter
  • The motivation for staircase arrangement
• The Built-in FEC Filter
  • Sending
  • Receiving
  • FEC Packet Header
  • Cooperation with retransmission
  • FEC Group Dismissal and Deletion
• Packet Filter Framework
  • Basic types
  • Construction
  • Sending
  • Receiving

Introduction

SRT has a general-purpose mechanism for injecting extra processing instructions at the beginning and/or end of a transmission. This mechanism, based on packet filtering, was originally created as a means to implement Forward Error Correction (FEC) in SRT, but can be extended for other uses.

As of SRT version 1.4 there is one built-in filter ("fec") installed, but more can be added.

Configuration

General syntax

SRT packet filtering can be configured by an SRTO_PACKETFILTER socket option, which gets the configuration contents passed as a string. How this string is interpreted depends on the filter itself. However, there is an obligatory general syntax:

<filter-type>,<key>:<value>[,...]

The parts of this syntax are separated by commas. The first part is the name of the filter. This is followed by one or more key:value pairs, the interpretation of which depends on the filter type. Note that all keys and values are case-sensitive.

You can try this out using the SRTO_PACKETFILTER option, or the packetfilter parameter in an SRT URI in the applications.

The packet filter framework is open for extensions so that users may register their own filters. SRT provides also one built-in filter named "fec". This filter implements the FEC mechanism, as described in SMPTE 2022-1-2007.

On the input side, filtering occurs at the moment when a packet is extracted from the send buffer. A filter may then do two things:

• alter the packet before inserting it into the SRT channel (the built-in "fec" filter doesn't do it, though)

• insert another packet (such as an FEC control packet) into the channel ahead of the next waiting packet from the send buffer.

On the receiving side a packet is first reviewed by the filter, and then potentially passed to the receive buffer. In the case of FEC, the output of the filter may be:

• nothing (this happens when an FEC control packet is received but does not trigger rebuilding)

• one or more packets (when together with a newly received packet the FEC filter succeeded to rebuild a lost packet)

Configuring the FEC filter

To use the FEC filter, set <filter-type> in your configuration to fec. Then add the appropriate key:value pairs based on the following parameters:

• cols: The number of columns in your FEC matrix (which is the equivalent of the size of each row). This parameter is obligatory and must be a positive number >=2.

• rows: The number of rows in your FEC matrix (which is the equivalent of the size of each column). This parameter is optional and defaults to 1. If the value is >=2, this corresponds to the exact number of rows. Beside this, two other special cases are allowed:

  • 1: in this case you have a row-only configuration (no columns)

  • -N (where N >= 2): column-only configuration. In this case N designates the exact size of a column, but the FEC control packet for rows will not be generated (in other words, the cols parameter designates in this case only a number of columns in one series)

• layout: The format of the FEC matrix. The possible values are:

  • even: block aligned (default) - columns are arranged in a solid matrix; the first sequence numbers (SNbase) are all contained in one row:

  

  • staircase: non-block aligned - column starting points are staggered; the first sequence numbers (SNbase) have an offset equivalent to R+1:

  

• arq: Optional use of the Automatic Repeat Request (ARQ) protocol. The possible values are:

  • always: ARQ is done in parallel with FEC (a loss is always reported immediately once detected in SRT).

  • onreq: ARQ is allowed, but a loss is only reported when FEC fails to rebuild, at the moment when an incoming packet has a sequence number that exceeds the last in one of the column groups; such a packet, if still lacking at that moment, is considered no longer recoverable by FEC.

  • never: ARQ is not done at all. Packets not recovered by FEC undergo TLPKTDROP, just like those that fail ARQ recovery in a conventional SRT exchange.

For example, this is how it should be used in the URI:

srt://recv.com:5000?latency=500&packetfilter=fec,cols:10,rows:5

As there can only be one configuration for both parties, it is recommended that one party defines the full configuration while the other only defines the matching packet filter type (for example, the sender sets fec,cols:10,rows:-5,layout:staircase and the receiver sets just fec). Both parties can also set this option to the same value. The packet filter function will attempt to merge configuration definitions, but if the options specified are in conflict, the connection will be rejected.

The motivation for staircase arrangement

Normally, FEC is done using a solid (block aligned) matrix. Packet sequences are arranged in a two-dimensional array of R rows and C columns. With SRT, the problem is that the FEC control packets are only transmitted when the last row is retransmitted.

Let's imagine 10 columns and 5 rows starting from sequence 500. The rows begin with sequences numbers 500, 510, 520, 530 and 540 (where 540 is the start of the last row). Here is a representation of a series of packets transmitted starting from around the middle of the second-to-last row up until the end of the last row (H = horizontal position; V = vertical position):

...

<537>

<538>

<539>        -> end of the second-to-last row

<FEC/H:539>  -> row FEC packet for the second-to-last row

<540>        -> beginning of the last row; end of the first column

<FEC/V:540>  -> column FEC packet for the first column

<541>        -> end of the second column

<FEC/V:541>  -> column FEC packet for the second column

<542>

<FEC/V:542>  -> column FEC etc.

<543>

<FEC/V:543>

<544>

<FEC/V:544>

<545>

<FEC/V:545>

<546>

<FEC/V:546>

<547>

<FEC/V:547>

<548>

<FEC/V:548>

<549>        -> end of the last row; end of the last column

<FEC/H:549>  -> row FEC packet for the last row

<FEC/V:549>  -> column FEC packet for the last column

Given a constant bitrate at the input, there's a certain bandwidth normally used by the regularly transmitted data packets. But when the last row in a series is transmitted, the transmission will use twice the normal bandwidth because of all the column FEC packets.

There are three methods for mitigating this:

1. Limit the bandwidth in SRT using SRTO_MAXBW.

   This means that packets will not be transmitted as fast as the FEC mechanism requires, and will add extra delay to the transmission of regular packets. Normally, this shouldn't matter much, as there's already a very high minimum latency you must configure with FEC. This is based on the size of the matrix (50 in the above example), multiplied by the bitrate, divided by a bytes-per-packet factor. But if you use FEC and ARQ together, delaying a packet at the sender side may challenge the response time for retransmission.

2. Delay sending the FEC control packet itself. -> A concept, NOT IMPLEMENTED

   Sending an FEC control packet can be postponed for several data packets. The problem is that this would increase the time interval between the first packet in a group and the control packet by a factor based on twice the matrix size (100 in the above example), thus increasing the required latency penalty.

3. Use the staircase arrangement.

   While in a simple (block aligned) matrix, the packet sequence numbers at the start of each column increment by 1, in a staircase (non-block aligned) matrix arrangement, the packet sequence numbers at the start of each column increment by R+1, where R is the row size.

Let's again imagine a matrix of 10 columns and 5 rows starting from sequence 500. The rows begin with sequences numbers 500, 510, 520, 530 and 540. But the columns begin in staggered fashion, separated by an interval of R+1. The colours represent consecutive FEC groups (note the "staircase" pattern):

Here is a representation of a series of packets transmitted starting from packet 537 (H = horizontal position; V = vertical position):

...

<537>

<538>

<539>

<FEC/H:539>  -> row FEC packet

<540>

<FEC/V:540>  -> column FEC packet

<541>

<542>

<543>

<544>

<545>

<FEC/V:545>  -> column FEC packet

<546>

<547>

<548>

<549>

<FEC/H:549>  -> row FEC packet

<550>  -> start of next FEC group

<551>

<FEC/V:551>  -> column FEC packet

<552>

...

Unlike with the block-aligned arrangement, the transmission of the FEC packets is well spaced, thus avoiding spikes in the use of available bandwidth.

Note that with the staircase arrangement, the initial packets sent when a connection is established will not be covered by an FEC group. But it is generally not a problem when packets are lost in the first 2 seconds of a transmission. If needed, even this can be mitigated by adding a series of unused groups to the FEC matrix before the transmission starts (a concept potentially to be implemented).

Another advantage of the staircase arrangement is that it increases the probability of recovering a long sequence of a lost packets in a case when consecutive lost packets in a row will belong to different column groups. In the example below, it is likely that the entire missing sequence from 572 to 583 can be rebuilt, largely because packets 573 and 583 belong to different groups (once these are rebuilt, it becomes possible to rebuild packets 572 and 582 via row FEC):

Although in a case of even arrangement you still may have a good luck of having a long loss exactly at the border of two column series, the staircase arrangement slightly increases the range of prospective long losses that can be successfully recovered.

The Built-in FEC Filter

The built-in FEC filter implements the standard XOR-based FEC protection operation. It qualifies packets into groups (see Layout option). For every group, an FEC control packet is generated that can be used to rebuild a packet that was lost.

The built-in FEC filter allows for the following possible configurations:

• row only: a row group is a range of consecutive packets; no columns are used

• column only: the row FEC packet is never sent

• columns and rows

A row is a series of consecutive packets from a base sequence number up to a number (N) of packets, with N being equal to the size of the row (R).

A column is a series of packets that starts at a base sequence number, with subsequent packets in the column group separated over an interval N (which is also the size of the row).

It's important to note that the size of a row (R) is equal to the number of columns, and the size of a column (C) is equal to the number of rows.

Sending

Prior to sending, feedSource gets the packets to be sent so that their contents can be XORed and written into the FEC group's buffer. Data on which a protection operation is performed are:

• timestamp
• encryption flags
• content length
• contents - padded with zeros up to payloadSize()

For the timestamp recovery, the header field TIMESTAMP is reused. For all others there are extra fields in the payload space of the SRT packet:

• 8 bits: group index (for rows this is -1)
• 8 bits: flag recovery
• 16 bits: length recovery

When the number of packets, applied in a protection operation, reaches the desired size of the group, the FEC control packet is considered ready for extraction, and the current FEC buffer state (after processing all packets from the group) becomes the required contents of the FEC control packet. This will then be returned at the next call to packControlPacket. This function checks if the FEC control packet is ready for extraction, first for the row group, then for the column group, and provides it if it is ready.

An FEC control packet is distinguished from a regular data packet by having its message number equal to 0. This value isn't normally used in SRT (message numbers start from 1, increment to a maximum, and then roll back to 1).

Figure 1 - SRT data packet structure

   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|                     Packet Sequence Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |FF |O|KK |R|                 Message Number                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Time Stamp                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination Socket ID                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                             Data                              |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2 - FEC control packet structure

   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|                     Packet Sequence Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |FF |O|KK |R|               Message Number = 0                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Time Stamp Recovery                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination Socket ID                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Group Index  | Flags Recovery|        Length Recovery        | -> FEC header
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                     Payload recovery                          |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Receiving

The receiver must first determine whether the packet is a regular data packet or an FEC control packet.

If the getMsgSeq() function returns 0 (signifying the packet is an FEC control packet), then the first byte from the FEC header in that packet is extracted to determine if this corresponds to a column FEC (contains the column number) or row FEC (contains -1).

If the getMsgSeq() function identifies a regular packet, then the packet is inserted into the horizontal and vertical position of the FEC group to which it belongs by performing the XOR operation with the current contents of the recovery buffer and increasing the packets' count (the number of currently processed packets in the FEC group buffer). Note that at the receiver there is only one buffer for an FEC group. There's a counter that is incremented for each packet on which the XOR protection operation has been performed and added to that group's recovery buffer. This operation is done as well on the FEC control packet, although this is marked by a separate flag.

The FEC control packet is inserted into the group to which it is destined (its sequence number is the last sequence in the group). Once you have a state where the group buffer contains the result of the XOR operation of the FEC control packet and N-1 data packets from this group (with N being the size of the group), the group is rebuild-ready, and the contents of a missing packet can be rebuilt from the FEC group buffer contents.

Every incoming packet that is applied into the group also marks the reception bit flag for its sequence number. The sequence number of the missing packet is therefore determined by examining this flag for all these sequence numbers to find out which one is missing. The packet is then rebuilt by:

• setting default values in the header (socket ID for the connection, message number 1)

• setting the sequence number to the number found for the missing packet

• setting the flags directly from the corresponding contents of the FEC group buffer (Flag Recovery from the FEC header); the retransmission flag is always set

• setting the timestamp directly from the corresponding contents of Time Stamp Recovery

• reading the payload size from the corresponding contents of Length Recovery

• setting the payload contents from the corresponding contents of Data Recovery, up to the payload size as read above

If both columns and rows are used, the rebuilding happens recursively - that is, after a packet is rebuilt its contents are also XORed to the row or column in the FEC group to which it simultaneously belongs, and then that row or column is checked again.That may trigger the rebuilding of another packet, which in turn causes a new column or row to be checked, and so on.

The rebuilt packets are stored in the provided array so that they can be picked up by SRT to insert them all into the receiver buffer.

FEC Packet Header

In SRT, FEC control packets have an extra header so that all the recovery information can be contained. This is the breakdown of the FEC control packet:

   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|                     Packet Sequence Number                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |FF |O|KK |R|                  Message Number                   |   Regular
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     SRT
   |                      Time Stamp Recovery                      |   header
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination Socket ID                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -------
   |  Group index  | Flags Recovery| Length Recovery               | Extra FEC header
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ -------
   |                    Payload Recovery...                        | Data

This is much like a regular data packet in SRT, except that there's an extra 32-bit special FEC header with additional information, and also many of the fields in the SRT header are used for a different purpose:

• Packet Sequence Number: Contains the sequence number of the last packet in the FEC group to which this FEC control packet is assigned.

• FF (Packet position flag): 11b (solo). FEC is currently used only in Live mode, where this is the only value that applies.

• O (Deliver in order flag): This flag is taken from the first data packet during sending and kept this way for all FEC packets, without checking other data packets. It's assumed that in Live mode all packets have this flag set the same way.

• KK (Encryption flag): 00b (unencrypted). The encryption specification doesn't apply to FEC control packets because the protection operation is being performed on the contents, whether they have been encrypted or not.

• R (Retransmission flag): 1 (set). An FEC control packet is marked as a retransmission to prevent it from being treated as a reordered packet.

• Message Number: 0 (a special value identifying an FEC control packet)

• Time Stamp: Contains the timestamp recovery (XOR)

• Destination Socket ID: Same as in regular SRT packets, as required to dispatch the packet to the correct socket.

• Group Index: Contains the column number, if it's for a column group, or -1, if it is for a row group. The only functional purpose of this field is to know whether this FEC control packet is for a row group or for a column group.

• Flag recovery: Contains the recovery bits to recover the flags preceding the Message Number field. Currently applies only to the KK flags, because encrypted packets may have KK set to 01 or 10 for the same transmission.

• Length recovery: 16-bit field containing the XOR of the packet length.

• Payload recovery: Contains the XOR-ed value of all payloads from packets in the group, each one padded with zeros before the operation up to payloadSize().

For reference, here is the FEC header for transmission over RTP according to RFC 2733:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      SN base                  |        length recovery        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |E| PT recovery |                 mask                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          TS recovery                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

• SNBase: Minimum sequence number of the packets associated to the FEC packet, and where 16 bit sequence numbers are sufficient, this parameter shall contain the entire sequence number. For transport protocols with longer sequence numbers this field shall contain the least significant 16 bits of the sequence number.

• Length Recovery: This field should be used to determine the length of any media packets associated with the FEC packet.

• PT recovery: This field should be used to determine the Payload Type of any media packets associated with the FEC packet.

• TS recovery: This field should be used to recover the timestamp of any media packets associated with the FEC packet.

These fields have equivalents in the FEC control packet in SRT:

• SNBase (both low bits and ext bits): This is equivalent to the Packet Sequence Number (which is already 32-bit) since the sequence numbers used for FEC in SRT are just the existing sequence numbers.

• Length recovery: There's a similar Length recovery field in the extra FEC header.

• PT recovery: This field is specific to RTP, but is similar to the Flag recovery field.

• TS recovery: In SRT FEC the timestamp recovery is stored in the SRT header's Timestamp field (being Time Stamp Recovery in this case)

• Index, Offset: Some similarities can be found in the Group index field. However its purpose is mainly to distinguish row and column groups. The index itself is only for prospective sanity checks.

Cooperation with retransmission

The ARQ level is a value that decides how the packet filtering should cooperate with retransmission. Possible values are:

• NEVER: Do not do retransmission at all. This means that packets are always ACK-ed up to the last sequence that is received, and all losses are ignored.

• ALWAYS: Do normal retransmission - that is, the retransmission request is sent immediately upon loss detection, possibly in parallel with FEC rebuilding. This might be useful for networks with high bandwidth capacity that happen to be very unstable; working with both FEC and ARQ on the same edge increases the probability that whatever can't be restored by one system will be restored by the other as fast as possible. However, both ARQ and FEC overheads will apply.

• ONREQ: Lost packets are recorded by SRT, but the loss report is not sent unless a sequence is reported in loss_seqs (see "Receiving" in the Packet Filter Framework section for details on how packets that are not recoverable by FEC are reported). Note that in this case the lost packets will be reported with a delay. At very low latency there is very little time to recover by ARQ in general, and with the added delay incurred by using this option there is even less, to the point where it may always be too late to retransmit.

Note that in ALWAYS and NEVER modes the filter should not return anything in loss_seqs.

It's very important that the latency is properly set. The FEC mechanism may rebuild a packet, but it's delivery will be delayed by the time it takes for enough packets to be accumulated in the corresponding group to trigger the rebuild. The minimum value for the delay due to rebuilding is based on the number of packets:

N = (R * (C-1)) + 2 

(where R = row size, and C = column size).

The SRT latency should be set such that, given the current bitrate, it is at least equal to the time it takes to send the N packets in an FEC group, and even this minimum should be increased by an extra safety margin. If this condition isn't met, the TLPKTDROP mechanism may drop packets even if they can be rebuilt.

The standard recommended minimum ARQ latency for SRT is estimated as 4 * RTT. If you choose to use FEC in cooperation with ARQ with the ALWAYS level, your latency penalty should be the maximum of the standard ARQ latency and the FEC latency (which usually means that only the FEC latency penalty should apply). If you use the ONREQ level, your latency penalty should be the sum of these two.

With the ONREQ setting, the built-in FEC filter collects the sequence numbers of all packets that are lost and no longer recoverable. This is recognized by the fact that a packet has come with a sequence number that is past the last sequence number for a particular group. All groups for which this packet is "in the future" are dismissed, that is, the irrecoverable packets are reported at this call and the dismissed flag is set so that it's not reported again. Note that this group isn't physically deleted at this time.

FEC Group Dismissal and Deletion

At some point, row and column groups are dismissed, which means that they are no longer of use. Missing packets are reported as irrecoverable, and the group is marked dismissed (so that this happens only once). This happens upon arrival of a packet with a sequence number that is higher than that of the last sequence number in the group.

Dismissed FEC groups (and objects representing them) are deleted when no longer needed. This happens when the whole series of groups is already in the past relative to the incoming packet.

Column and row groups are arranged in series, where the earliest active series is 0, the next is 1, and so on. When series 0 is deleted, series 1 becomes the new series 0. A column or row group is always deleted with the entire series to which it belongs (never by itself).

For block-aligned (even) FEC arrangements, a series is easily defined as all column and row groups that fit in a given size of matrix. All groups are deleted by deleting the entire matrix. In the figure below, with a matrix size of 5 rows by 10 columns, the green region of column and row groups (series 0) is deleted once a packet (#550 or later) from the red region (series 1) arrives.

For non block-aligned (staircase) FEC arrangements, a series has a more complex definition, and the trigger for deletion comes later (and may therefore use more memory for FEC groups than an even arrangement). Let's look at a specific example. In the figure below (also a matrix size of 5 rows by 10 columns), we can define packet #500 as base0 - the very first sequence number in a group that is still active. Red then represents series 0, blue series 1, and white series 2:

In a staircase arrangement, the minimum distance between base0 and the first incoming packet that might trigger deletion is two times the size of the matrix (an arbitrary value chosen for the sake of simplicity and to provide a small margin of safety; it doesn't impact the functionality because deletion is independent of dismissal, except for the fact that the latter must happen first). For example, deletion of the column and row groups in series 0 (the red region) will be triggered by reception of packet #600 (or greater, because packets can be lost).

Once the trigger packet arrives, all column groups belonging to series 0 (red) are deleted, as well as all rows that begin with packets from the base0 column group - that is, all rows whose first packet falls in the sequence from #500 to #540. In the figure below, deletion of series 0 corresponds to columns with a red background and rows with a red border:

After the red series 0 is deleted, packet #550 becomes the new base0, packets in the column from #550 to #559 define the new first row group, and the blue packets become series 0.

When column groups are in the staircase arrangement, the penalty for loss detection (encountered with the ONREQ level) shall be always counted as the size of the matrix (a product of both group sizes). A certain number of packets must be received after the packet that caused a loss detection in order for the FEC facility to report it as lost and not recoverable. In other words, the FEC penalty is the number of packets between the lost packet and the first packet that triggered sending a loss report for that lost packet. The FEC mechanism always waits for the moment when the lost packet is declared irrecoverable.

Packet Filter Framework

The built-in FEC facility is connected with SRT through a mechanism called "packet filtering". This mechanism relies on the following checkpoints which allow for packet filter injection:

• Sending:

  • a filter is first asked if it is ready to deliver a control packet; if so it is expected to deliver it, and this packet is sent instead of a data packet waiting in the sender buffer

  • when a new packet in the sender buffer is about to be sent over the network, it's first passed to the filter

• Receiving:

  • Every packet received from the network is passed to a filter, which can:

    • pass it through

    • provide extra packets

The built-in FEC filter doesn't use all capabilities of Packet Filtering, in particular:

• a packet may be altered prior to sending; FEC only reads the data.

• a receiver does allow packet passthrough; it must process each packet and put all results into a provisional buffer(*). Note that the FEC filter does allow passthrough for regular data packets, but traps all FEC control packets and provides rebuilt packets.

  (*) This provisional buffer is the socket's receiver buffer that is passed to the packet filter framework to accommodate packets coming out of the packet filter (see the provided constructor parameter in the Construction section below).

The sending and receiving mechanism of packet filtering is defined in one class. SRT is generally bidirectional, so both directions must be covered in a single mechanism.

A user-defined packet filter should be a class derived from SrtPacketFilterBase class, and it should override several virtual methods, as specified below.

Basic types

The basic packet structures are:

• CPacket is an internal SRT class, that is, it gives you access to the exact packet to be used in the operation.

• SrtPacket is a special intermediate class, which has a static definition and is used as an intermediate space to be copied to the CPacket structure when needed, automatically. This is required for cases when SRT is doing some specific memory management. The packet filter framework cannot reuse the same memory management for keeping the packets in the receiver buffer, or have access to it. Therefore a filter should provide packets using appropriate methods, and SRT internals will take care of copying it to the receiver buffer. This is why the CPacket structure cannot be used for that purpose.

Both classes - while characterized by completely independent contents and methods - implement a common C++ concept consisting of the following methods:

• char* data();

  • Returns the buffer in the packet.

• size_t size();

  • Returns the size of the contents in the buffer. Note that this is not the size of the buffer itself.

• uint32_t header(SrtPktHeaderFields field);

  • This accesses the header. The field type specifies the field index in the header. Note that if a function gives you writable access to CPacket, you can modify the payload, but you can't modify the header. With SrtPacket you are free to modify any contents, but to access the header you should use the SrtPacket::hdr field, which is the array to be indexed by values of SrtPktHeaderFields type.

Construction

The constructor of the packet filter class should have the following signature:

MyFilter(const SrtFilterInitializer &init, std::vector<SrtPacket>& provided, const string &confstr);

Here MyFilter is the example filter class derived from SrtPacketFilterBase. The following parameters are required:

• init

  • This should be passed to the constructor of SrtPacketFilterBase. It will provide you with the basic data you will need when creating a packet at the receiver.

• provided

  • This is the provisional buffer where you have to store packets that your filter will generate as extra packets (in the case of FEC, this is where the rebuilt packets will be supplied).

• confstr

  • This is a configuration string. You should parse it using ParseFilterConfig so that you can use it for your purposes. Note that this configuration string is still parsed by this function internally in SRT, so the official syntax must be preserved.

The base class will provide you with important data from the socket through the following methods:

• socketID()

  • The socket ID that you should write into the ID header field on the receiver

• sndISN() and rcvISN()

  • It returns the very first sequence number in a particular direction (FEC uses it to initialize the base sequence numbers for FEC groups).

• payloadSize()

  • The maximum size of a single packet. This determines how large a content payload should be used for the FEC control packet. The FEC control packet's payload recovery field must be of the maximum size of any packet, and any shorter packets being protected must be padded with zeros up to this size.

The following virtual methods (to be overridden) are not direction related:

• size_t extraSize() [REQUIRED]

  • Should return the size of the extra header that your filter will use, in the count of 32-bit data. The current FEC implementation returns 4 here, which is the size of the extra header in FEC control packets. Your implementation may use some extra header in regular packets as well, so this should be taken into account here. This is required for checking if the SRTO_PAYLOADSIZE socket option is properly set (it puts an extra limitation on the value for this option beside the overall maximum of 1456 bytes).

• SRT_ARQLevel arqLevel() [OPTIONAL; default implementation returns the value extracted earlier from the configuration string under the arq key]

  • Should return the ARQ level in the case where you want it to be specified differently than the default (for example, if you don't want it to be configurable in your implementation).

Sending

Sending in SRT is driven by a congestion control mechanism that determines when a socket is ready to send data at the currently defined speed. There is a function called at the precise moment when a socket should provide a packet to be sent over the UDP link.

There are currently three packet providers. When one provides a packet, the provision is done and the others will have to wait until the next opportunity. The providers are checked in a specific order:

• Case #1: A packet required for retransmission. This is based on the sender loss list, updated from UMSG_LOSSREPORT messages.

• Case #2: If a Packet Filter is installed, the filter's control packet, if one is ready.

• Case #3: The next waiting data packet.

For Case #3, this function is called on the filter (if it is installed):

void feedSource(CPacket& pkt);

This function is called at the moment when a packet (already submitted by the srt_sendmsg call) is picked up from the sender buffer and is going to be sent. Note that this packet is bound to a buffer representing the input, and its size is set to the maximum possible (see NOTE below). Since this packet is allowed to be altered, data can also be extended up to this size. Note that if such a packet is altered, it will stay in this form in the sender buffer and be retransmitted, if needed.

The current FEC filter implementation uses this function only to collect the contents of the packet to be XORed into an FEC group buffer.

For Case #2, a filter control packet is provided by this function:

bool packControlPacket(SrtPacket& packet, int32_t seq);

If the function returns true, it means that it has supplied the packet. The function should return false when there is no such packet ready, in which event Case #3 (new data packet) will be attempted.

NOTE: If true, the contents of the filter control packet are to be written in a special packet buffer, from which the contents will be copied to the target packet. This buffer is already the maximum possible size of a single packet in SRT.

Special control packets are distinguished from regular data packets by the Message Number (MSGNO) field. For control packets the Message Number is always 0, while regular data packets have Message Numbers from 1 up to a maximum (after which they roll back to 1). For that reason, you are free to store special values in TIMESTAMP and SEQNO fields, but the MSGNO and ID fields must be left alone (they will be overwritten anyway).

Receiving

There is just one function for receiving:

bool receive(const CPacket& pkt, loss_seqs_t& loss_seqs);

This function is called for any data packet received on a socket. It is understood that this is a data packet, the rest is up to the receive() function.

A packet received here must not be altered. However, you can decide whether a given packet should be passed through, or dismissed, based on the return value. Returning true means that the received packet should be passed on to the receiver buffer; otherwise it is discarded. If you have some special contents in a packet that you want removed, simply recreate the packet, then discard the original.

If you want to inject a packet from a filter (such as an FEC-rebuilt packet), you store it in a provisional buffer (the reference to which you received in the constructor). This method can also be used when you want to replace a packet. The order in which packets are stored in this array doesn't matter, because they will be sorted by sequence number order before they are returned to SRT.

Here are some additional considerations related to the receive() function:

1. It's up to you to distinguish regular data packets and filter control packets by checking the message number. It is important to use the CPacket::getMsgSeq function for that purpose because this extracts the right part of the MSGNO field from the header - pkt.header (SRT_PH_MSGNO) will return just the field, which contains extra flags.

2. If you use an extra header for every packet, be sure that you can recognize it correctly as you have created it.

3. When creating (reconstructing) a packet, you have to correctly set the sequence number, the ID field (get the value from socketID()), timestamp, and the encryption flags (other flags are not important, they are set as default).