RFC 3208
PGM Reliable Transport Protocol Specification
9. Options
PGM specifies several end-to-end options to address specific application requirements. PGM specifies options to support fragmentation, late joining, and redirection.
Options MAY be appended to PGM data packet headers only by their
original transmitters. While they MAY be interpreted by network
elements, options are neither added nor removed by network elements.
Options are all in the TLV style, or Type, Length, Value. The Type
field is contained in the first byte, where bit 0 is the OPT_END bit,
followed by 7 bits of type. The OPT_END bit MUST be set in the last
option in the option list, whichever that might be. The Length field
is the total length of the option in bytes, and directly follows the
Type field. Following the Length field are 5 reserved bits, the
OP_ENCODED flag, the 2 Option Extensibility bits OPX and the
OP_ENCODED_NULL flag. Last are 7 bits designated for option specific
information which may be defined on a per-option basis. If not
defined for a particular option, they MUST be set to 0.
The Option Extensibility bits dictate the desired treatment of an
option if it is unknown to the network element processing it.
Nota Bene: Only network elements pay any attention to these bits.
The OPX bits are defined as follows:
00 - Ignore the option
01 - Invalidate the option by changing the type to OPT_INVALID
= 0x7F
10 - Discard the packet
11 - Unsupported, and reserved for future use
Some options present in data packet (ODATA and RDATA) are strictly
associated with the packet content (PGM payload), OPT_FRAGMENT being
an example. These options must be preserved even when the data
packet that would normally contain them is not received, but its the
payload is recovered though the use of FEC. PGM specifies a
mechanism to accomplish this that uses the F (OP_ENCODED) and U
(OP_ENCODED_NULL) bits in the option common header. OP_ENCODED and
OP_ENCODED_NULL MUST be normally set to zero except when the option
is used in FEC packets to preserve original options. See Appendix A
for details.
There is a limit of 16 options per packet.
General Option Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| Option Type | Option Length |Reserved |F|OPX|U|Opt. Specific|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Value ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+...+-+-+
9.1. Option extension length - OPT_LENGTH
When option extensions are appended to the standard PGM header, the
extensions MUST be preceded by an option extension length field
specifying the total length of all option extensions.
In addition, the presence of the options MUST be encoded in the
Options field of the standard PGM header before the Checksum is
computed.
All network-significant options MUST be appended before any
exclusively receiver-significant options.
To provide an indication of the end of option extensions, OPT_END
(0x80) MUST be set in the Option Type field of the trailing option
extension.
9.1.1. OPT_LENGTH - Packet Extension Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Length | Total length of all options |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type = 0x00
Option Length = 4 octets
Total length of all options
The total length in octets of all option extensions including
OPT_LENGTH.
OPT_LENGTH is NOT network-significant.
9.2. Fragmentation Option - OPT_FRAGMENT
Fragmentation allows transport-layer entities at a source to break up application protocol data units (APDUs) into multiple PGM data packets (TPDUs) to conform with the MTU supported by the network layer. The fragmentation option MAY be applied to ODATA and RDATA packets only. Architecturally, the accumulation of TSDUs into APDUs is applied to TPDUs that have already been received, duplicate eliminated, and contiguously sequenced by the receiver. Thus APDUs MAY be reassembled across increments of the transmit window.9.2.1. OPT_FRAGMENT - Packet Extension Contents
OPT_FRAG_OFF the offset of the fragment from the beginning of the APDU OPT_FRAG_LEN the total length of the original APDU9.2.2. OPT_FRAGMENT - Procedures - Sources
A source fragments APDUs into a contiguous series of fragments no larger than the MTU supported by the network layer. A source sequentially and uniquely assigns OD_SQNs to these fragments in the order in which they occur in the APDU. A source then sets OPT_FRAG_OFF to the value of the offset of the fragment in the original APDU (where the first byte of the APDU is at offset 0, and OPT_FRAG_OFF numbers the first byte in the fragment), and set OPT_FRAG_LEN to the value of the total length of the original APDU.9.2.3. OPT_FRAGMENT - Procedures - Receivers
Receivers detect and accumulate fragmented packets until they have received an entire contiguous sequence of packets comprising an APDU. This sequence begins with the fragment bearing OPT_FRAG_OFF of 0, and terminates with the fragment whose length added to its OPT_FRAG_OFF is OPT_FRAG_LEN.
9.2.4. OPT_FRAGMENT - Packet Extension Format
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| Option Type | Option Length |Reserved |F|OPX|U| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | First Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type = 0x01 Option Length = 12 octets First Sequence Number Sequence Number of the PGM DATA/RDATA packet containing the first fragment of the APDU. Offset The byte offset of the fragment from the beginning of the APDU (OPT_FRAG_OFF). Length The total length of the original APDU (OPT_FRAG_LEN). OPT_FRAGMENT is NOT network-significant.9.3. NAK List Option - OPT_NAK_LIST
The NAK List option MAY be used in conjunction with NAKs to allow receivers to request transmission for more than one sequence number with a single NAK packet. The option is limited to 62 listed NAK entries. The NAK list MUST be unique and duplicate free. It MUST be ordered, and MUST consist of either a list of selective or a list of parity NAKs. In general, network elements, sources and receivers must process a NAK list as if they had received individual NAKs for each sequence number in the list. The procedures for each are outlined in detail earlier in this document. Clarifications and differences are detailed here.
9.3.1. OPT_NAK_LIST - Packet Extensions Contents
A list of sequence numbers for which retransmission is requested.9.3.2. OPT_NAK_LIST - Procedures - Receivers
Receivers MAY append the NAK List option to a NAK to indicate that they wish retransmission of a number of RDATA. Receivers SHOULD proceed to back off NAK transmission in a manner consistent with the procedures outlined for single sequence number NAKs. Note that the repair of each separate sequence number will be completed upon receipt of a separate RDATA packet. Reception of an NCF or multicast NAK containing the NAK List option suspends generation of NAKs for all sequence numbers within the NAK list, as well as the sequence number within the NAK header.9.3.3. OPT_NAK_LIST - Procedures - Network Elements
Network elements MUST immediately respond to a NAK with an identical NCF containing the same NAK list as the NAK itself. Network elements MUST forward a NAK containing a NAK List option if any one sequence number specified by the NAK (including that in the main NAK header) is not currently outstanding. That is, it MUST forward the NAK, if any one sequence number does not have an elimination timer running for it. The NAK must be forwarded intact. Network elements MUST eliminate a NAK containing the NAK list option only if all sequence numbers specified by the NAK (including that in the main NAK header) are outstanding. That is, they are all running an elimination timer. Upon receipt of an unsolicited NCF containing the NAK list option, a network element MUST anticipate data for every sequence number specified by the NAK as if it had received an NCF for every sequence number specified by the NAK.9.3.4. OPT_NAK_LIST - Procedures - Sources
A source MUST immediately respond to a NAK with an identical NCF containing the same NAK list as the NAK itself. It MUST then multicast RDATA (while respecting TXW_MAX_RTE) for every requested sequence number.
9.3.5. OPT_NAK_LIST - Packet Extension Format
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| Option Type | Option Length |Reserved |F|OPX|U| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Requested Sequence Number 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ..... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Requested Sequence Number N | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type = 0x02 Option Length = 4 + (4 * number of SQNs) octets Requested Sequence Number A list of up to 62 additional sequence numbers to which the NAK applies. OPT_NAK_LIST is network-significant.9.4. Late Joining Option - OPT_JOIN
Late joining allows a source to bound the amount of repair history receivers may request when they initially join a particular transport session. This option indicates that receivers that join a transport session in progress MAY request repair of all data as far back as the given minimum sequence number from the time they join the transport session. The default is for receivers to receive data only from the first packet they receive and onward.9.4.1. OPT_JOIN - Packet Extensions Contents
OPT_JOIN_MIN the minimum sequence number for repair9.4.2. OPT_JOIN - Procedures - Receivers
If a PGM packet (ODATA, RDATA, or SPM) bears OPT_JOIN, a receiver MAY initialize the trailing edge of the receive window (RXW_TRAIL_INIT) to the given Minimum Sequence Number and proceeds with normal data reception.
9.4.3. OPT_JOIN - Packet Extension Format
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| Option Type | Option Length |Reserved |F|OPX|U| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Minimum Sequence Number | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type = 0x03 Option Length = 8 octets Minimum Sequence Number The minimum sequence number defining the initial trailing edge of the receive window for a late joining receiver. OPT_JOIN is NOT network-significant.9.5. Redirect Option - OPT_REDIRECT
Redirection MAY be used by a designated local repairer (DLR) to advertise its own address as an alternative to the original source, for requesting repairs. These procedures allow a PGM Network Element to use a DLR that is one PGM hop from it either upstream or downstream in the multicast distribution tree. The former are referred to as upstream DLRs. The latter are referred to as off-tree DLRs. Off-Tree because even though they are downstream of the point of loss, they might not lie on the subtree affected by the loss. A DLR MUST receive any PGM sessions for which it wishes to provide retransmissions. A DLR SHOULD respond to NCFs or POLLs sourced by its PGM parent with a redirecting POLR response packet containing an OPT_REDIRECT which provides its own network layer address. Recipients of redirecting POLRs MAY then direct NAKs for subsequent ODATA sequence numbers to the DLR rather than to the original source. In addition, DLRs that receive redirected NAKs for which they have RDATA MUST send a NULL NAK to provide flow control to the original source without also provoking a repair from that source.
9.5.1. OPT_REDIRECT - Packet Extensions Contents
OPT_REDIR_NLA the DLR's own unicast network-layer address to which recipients of the redirecting POLR MAY direct subsequent NAKs for the corresponding TSI.9.5.2. OPT_REDIRECT - Procedures - DLRs
A DLR MUST receive any PGM sessions for which it wishes to provide a source of repairs. In addition to acting as an ordinary PGM receiver, a DLR MAY then respond to NCFs or relevant POLLs sourced by parent network elements (or even by the source itself) by sending a POLR containing an OPT_REDIRECT providing its own network-layer address. If a DLR can provide FEC repairs it MUST denote this by setting OPT_PARITY in the PGM header of its POLR response.9.5.2.1. Upstream DLRs
If the NCF completes NAK transmission initiated by the DLR itself, the DLR MUST NOT send a redirecting POLR. When a DLR receives an NCF from its upstream PGM parent, it SHOULD send a redirecting POLR, multicast to the group. The DLR SHOULD record that it is acting as an upstream DLR for the said session. Note that this POLR MUST have both the data source's source address and the router alert option in its network header. An upstream DLR MUST act as an ordinary PGM source in responding to any NAK it receives (i.e., directed to it). That is, it SHOULD respond first with a normal NCF and then RDATA as usual. In addition, an upstream DLR that receives redirected NAKs for which it has RDATA MUST send a NULL NAK to provide flow control to the original source. If it cannot provide the RDATA it forwards the NAK to the upstream PGM neighbor as usual. Nota Bene: In order to propagate on exactly the same distribution tree as ODATA, RDATA and POLR packets transmitted by DLRs MUST bear the ODATA source's NLA as the network-header source address, not the DLR's NLA as might be expected.
9.5.2.2. Off-Tree DLRs
A DLR that receives a POLL with sub-type PGM_POLL_DLR MUST respond with a unicast redirecting POLR if it provides the appropriate service. The DLR SHOULD respond using the rules outlined for polling in Appendix D of this text. If the DLR responds, it SHOULD record that it is acting as an off-tree DLR for the said session. An off-tree DLR acts in a special way in responding to any NAK it receives (i.e., directed to it). It MUST respond to a NAK directed to it from its parent by unicasting an NCF and RDATA to its parent. The parent will then forward the RDATA down the distribution tree. The DLR uses its own and the parent's NLA addresses in the network header for the source and destination respectively. The unicast NCF and RDATA packets SHOULD not have the router alert option. In all other ways the RDATA header should be "as if" the packet had come from the source. Again, an off-tree DLR that receives redirected NAKs for which it has RDATA MUST originate a NULL NAK to provide flow control to the original source. It MUST originate the NULL NAK before originating the RDATA. This must be done to reduce the state held in the network element. If it cannot provide the RDATA for a given NAK, an off-tree DLR SHOULD confirm the NAK with a unicast NCF as normal, then immediately send a NAK for the said data packet back to its parent.9.5.2.3. Simultaneous Upstream and Off-Tree DLR operation
Note that it is possible for a DLR to provide service to its parent and to downstream network elements simultaneously. A downstream loss coupled with a loss for the same data on some other part of the distribution tree served by its parent could cause this. In this case it may provide both upstream and off-tree functionality simultaneously. Note that a DLR differentiates between NAKs from an NE downstream or from its parent by comparing the network-header source address of the NAK with it's upstream PGM parent's NLA. The DLR knows the parent's NLA from the session's SPM messages.
9.5.3. OPT_REDIRECT - Procedures - Network Elements
9.5.3.1. Discovering DLRs
When a PGM router receives notification of a loss via a NAK, it SHOULD first try to use a known DLR to recover the loss. If such a DLR is not known it SHOULD initiate DLR discovery. DLR discovery may occur in two ways. If there are upstream DLRs, the NAK transmitted by this router to its PGM parent will trigger their discovery, via a redirecting POLR. Also, a network element SHOULD initiate a search for off-tree DLRs using the PGM polling mechanism, and the sub-type PGM_POLL_DLR. If a DLR can provide FEC repairs it will denote this by setting OPT_PARITY in the PGM header of its POLR response. A network element SHOULD only direct parity NAKs to a DLR that can provide FEC repairs.9.5.3.2. Redirected Repair
When it can, a network element SHOULD use upstream DLRs. Upon receiving a redirecting POLR, network elements SHOULD record the redirecting information for the TSI, and SHOULD redirect subsequent NAKs for the same TSI to the network address provided in the redirecting POLR rather than to the PGM neighbor known via the SPMs. Note, however, that a redirecting POLR is NOT regarded as matching the NAK that provoked it, so it does not complete the transmission of that NAK. Only a normal matching NCF can complete the transmission of a NAK. For subsequent NAKs, if the network element has recorded redirection information for the corresponding TSI, it MAY change the destination network address of those NAKs and attempt to transmit them to the DLR. No NAK for a specific SQN SHOULD be sent to an off-tree DLR if a NAK for the SQN has been seen on the interface associated with the DLR. Instead the NAK SHOULD be forwarded upstream. Subsequent NAKs for different SQNs MAY be forwarded to the said DLR (again assuming no NAK for them has been seen on the interface to the DLR). If a corresponding NCF is not received from the DLR within NAK_RPT_IVL, the network element MUST discard the redirecting information for the TSI and re-attempt to forward the NAK towards the PGM upstream neighbor.
If a NAK is received from the DLR for a requested SQN, the network element MUST discard the redirecting information for the SQN and re- attempt to forward the NAK towards the PGM upstream neighbor. The network element MAY still direct NAKs for different SQNs to the DLR. RDATA and NCFs from upstream DLRs will flow down the distribution tree. However, RDATA and NCFs from off-tree DLRs will be unicast to the network element. The network element will terminate the NCF, but MUST put the source's NLA and the group address into the network header and MUST add router alert before forwarding the RDATA packet to the distribution subtree. NULL NAKs from an off-tree DLR for an RDATA packet requested from that off-tree DLR MUST always be forwarded upstream. The network element can assume that these will arrive before the matching RDATA. Other NULL NAKs are forwarded only if matching repair state has not already been created. Network elements MUST NOT confirm or retry NULL NAKs and they MUST NOT add the receiving interface to the repair state. If a NULL NAK is used to initially create repair state, this fact must be recorded so that any subsequent non-NULL NAK will not be eliminated, but rather will be forwarded to provoke an actual repair. State created by a NULL NAK exists only for NAK_ELIM_IVL.9.5.4. OPT_REDIRECT - Procedures - Receivers
These procedures are intended to be applied in instances where a receiver's first hop router on the reverse path to the source is not a PGM Network Element. So, receivers MUST ignore a redirecting POLR from a DLR on the same IP subnet that the receiver resides on, since this is likely to suffer identical loss to the receiver and so be useless. Therefore, these procedures are entirely OPTIONAL. A receiver MAY choose to ignore all redirecting POLRs since in cases where its first hop router on the reverse path is PGM capable, it would ignore them anyway. Also, note that receivers will never learn of off-tree DLRs. Upon receiving a redirecting POLR, receivers SHOULD record the redirecting information for the TSI, and MAY redirect subsequent NAKs for the same TSI to the network address provided in the redirecting POLR rather than to the PGM neighbor for the corresponding ODATA for which the receiver is requesting repair. Note, however, that a redirecting POLR is NOT regarded as matching the NAK that provoked it, so it does not complete the transmission of that NAK. Only a normal matching NCF can complete the transmission of a NAK. For subsequent NAKs, if the receiver has recorded redirection information for the corresponding TSI, it MAY change the destination network address of those NAKs and attempt to transmit them to the
DLR. If a corresponding NCF is not received within NAK_RPT_IVL, the receiver MUST discard the redirecting information for the TSI and re-attempt to forward the NAK to the PGM neighbor for the original source of the missing ODATA.9.5.5. OPT_REDIRECT - Packet Extension Format
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| Option Type | Option Length |Reserved |F|OPX|U| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | NLA AFI | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DLR's NLA ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+ Option Type = 0x07 Option Length = 4 + NLA length DLR's NLA The DLR's own unicast network address to which recipients of the redirecting POLR may direct subsequent NAKs. OPT_REDIRECT is network-significant.9.6. OPT_SYN - Synchronization Option
The SYN option indicates the starting data packet for a session. It must only appear in ODATA or RDATA packets. The SYN option MAY be used to provide a useful abstraction to applications that can simplify application design by providing stream start notification. It MAY also be used to let a late joiner to a session know that it is indeed late (i.e. it would not see the SYN option).9.6.1. OPT_SYN - Procedures - Receivers
Procedures for receivers are implementation dependent. A receiver MAY use the SYN to provide its applications with abstractions of the data stream.
9.6.2. OPT_SYN - Procedures - Sources
Sources MAY include OPT_SYN in the first data for a session. That is, they MAY include the option in: the first ODATA sent on a session by a PGM source any RDATA sent as a result of loss of this ODATA packet all FEC packets for the first transmission group; in this case it is interpreted as the first packet having the SYN9.6.3. OPT_SYN - Procedures - DLRs
In an identical manner to sources, DLRs MUST provide OPT_SYN in any retransmitted data that is at the start of a session.9.6.4. OPT_SYN - Packet Extension Format
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| Option Type | Option Length |Reserved |F|OPX|U| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type = 0x0D Option Length = 4 OPT_SYN is NOT network-significant.9.7. OPT_FIN - Session Finish Option
This FIN option indicates the last data packet for a session and an orderly close down. The FIN option MAY be used to provide an abstraction to applications that can simplify application design by providing stream end notification. This option MAY be present in the last data packet or transmission group for a session. The FIN PGM option MUST appear in every SPM sent after the last ODATA for a session. The SPM_LEAD sequence number in an SPM with the FIN option indicates the last known data successfully transmitted for the session.
9.7.1. OPT_FIN - Procedures - Receivers
A receiver SHOULD use receipt of a FIN to let it know that it can tear down its data structures for the said session once a suitable time period has expired (TXW_SECS). It MAY still try to solicit retransmissions within the existing transmit window. Other than this, procedures for receivers are implementation dependent. A receiver MAY use the FIN to provide its applications with abstractions of the data stream and to inform its applications that the session is ending. 9.7.2. OPT_FIN - Procedures - Sources Sources MUST include OPT_FIN in every SPM sent after it has been determined that the application has closed gracefully. If a source is aware at the time of transmission that it is ending a session the source MAY include OPT_FIN in, the last ODATA any associated RDATAs for the last data FEC packets for the last transmission group; in this case it is interpreted as the last packet having the FIN When a source detects that it needs to send an OPT_FIN it SHOULD immediately send it. This is done either by appending it to the last data packet or transmission group or by immediately sending an SPM and resetting the SPM heartbeat timer (i.e. it does not wait for a timer to expire before sending the SPM). After sending an OPT_FIN, the session SHOULD not close and stop sending SPMs until after a time period equal to TXW_SECS.9.7.3. OPT_FIN - Procedures - DLRs
In an identical manner to sources, DLRs MUST provide OPT_FIN in any retransmitted data that is at the end of a session.
9.7.4. OPT_FIN - Packet Extension Format
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| Option Type | Option Length |Reserved |F|OPX|U| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type = 0x0E Option Length = 4 OPT_FIN is NOT network-significant.9.8. OPT_RST - Session Reset Option
The RST option MAY appear in every SPM sent after an unrecoverable error is identified by the source. This acts to notify the receivers that the session is being aborted. This option MAY appear only in SPMs. The SPM_LEAD sequence number in an SPM with the RST option indicates the last known data successfully transmitted for the session.9.8.1. OPT_RST - Procedures - Receivers
Receivers SHOULD treat the reception of OPT_RST in an SPM as an abort of the session. A receiver that receives an SPM with an OPT_RST with the N bit set SHOULD not send any more NAKs for the said session towards the source. If the N bit (see 9.8.5) is not set, the receiver MAY continue to try to solicit retransmit data within the current transmit window.9.8.2. OPT_RST - Procedures - Sources
Sources SHOULD include OPT_RST in every SPM sent after it has been determined that an unrecoverable error condition has occurred. The N bit of the OPT_RST SHOULD only be sent if the source has determined that it cannot process NAKs for the session. The cause of the OPT_RST is set to an implementation specific value. If the error code is unknown, then the value of 0x00 is used. When a source detects that it needs to send an OPT_RST it SHOULD immediately send it. This is done by immediately sending an SPM and resetting the SPM heartbeat timer (i.e. it does not wait for a timer to expire before sending the SPM). After sending an OPT_RST, the session SHOULD not close and stop sending SPMs until after a time period equal to TXW_SECS.
9.8.3. OPT_RST - Procedures - DLRs
None.9.8.4. OPT_RST - Packet Extension Format
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| Option Type | Option Length |Reserved |F|OPX|U|N|Error Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type = 0x0F Option Length = 4 N bit The N bit is set to 1 to indicate that NAKs for previous ODATA will go unanswered from the source. The application will tell the source to turn this bit on or off. Error Code The 6 bit error code field is used to forward an error code down to the receivers from the source. The value of 0x00 indicates an unknown reset reason. Any other value indicates the application purposely aborted and gave a reason (the error code value) that may have meaning to the end receiver application. These values are entirely application dependent. OPT_RST is NOT network-significant.10. Security Considerations
In addition to the usual problems of end-to-end authentication, PGM is vulnerable to a number of security risks that are specific to the mechanisms it uses to establish source path state, to establish repair state, to forward NAKs, to identify DLRs, and to distribute repairs. These mechanisms expose PGM network elements themselves to security risks since network elements not only switch but also interpret SPMs, NAKs, NCFs, and RDATA, all of which may legitimately be transmitted by PGM sources, receivers, and DLRs. Short of full authentication of all neighboring sources, receivers, DLRs, and network elements, the protocol is not impervious to abuse.
So putting aside the problems of rogue PGM network elements for the
moment, there are enough potential security risks to network elements
associated with sources, receivers, and DLRs alone. These risks
include denial of service through the exhausting of both CPU
bandwidth and memory, as well as loss of (repair) data connectivity
through the muddling of repair state.
False SPMs may cause PGM network elements to mis-direct NAKs intended
for the legitimate source with the result that the requested RDATA
would not be forthcoming.
False NAKs may cause PGM network elements to establish spurious
repair state that will expire only upon time-out and could lead to
memory exhaustion in the meantime.
False NCFs may cause PGM network elements to suspend NAK forwarding
prematurely (or to mis-direct NAKs in the case of redirecting POLRs)
resulting eventually in loss of RDATA.
False RDATA may cause PGM network elements to tear down legitimate
repair state resulting eventually in loss of legitimate RDATA.
The development of precautions for network elements to protect
themselves against incidental or unsophisticated versions of these
attacks is work outside of this spec and includes:
Damping of jitter in the value of either the network-header source
address of SPMs or the path NLA in SPMs. While the network-header
source address is expected to change seldom, the path NLA is
expected to change occasionally as a consequence of changes in
underlying multicast routing information.
The extension of NAK shedding procedures to control the volume, not
just the rate, of confirmed NAKs. In either case, these procedures
assist network elements in surviving NAK attacks at the expense of
maintaining service. More efficiently, network elements may use the
knowledge of TSIs and their associated transmit windows gleaned from
SPMs to control the proliferation of repair state.
A three-way handshake between network elements and DLRs that would
permit a network element to ascertain with greater confidence that an
alleged DLR is identified by the alleged network-header source
address, and is PGM conversant.
11. Appendix A - Forward Error Correction
11.1. Introduction
The following procedures incorporate packet-level Reed Solomon Erasure correcting techniques as described in [11] and [12] into PGM. This approach to Forward Error Correction (FEC) is based upon the computation of h parity packets from k data packets for a total of n packets such that a receiver can reconstruct the k data packets out of any k of the n packets. The original k data packets are referred to as the Transmission Group, and the total n packets as the FEC Block. These procedures permit any combination of pro-active FEC or on- demand FEC with conventional ARQ (selective retransmission) within a given TSI to provide any flavor of layered or integrated FEC. The two approaches can be used by the same or different receivers in a single transport session without conflict. Once provided by a source, the actual use of FEC or selective retransmission for loss recovery in the session is entirely at the discretion of the receivers. Note however that receivers SHOULD NOT ask for selective retransmissions when FEC is available, nevertheless sources MUST provide selective retransmissions in response to selective NAKs from the leading partial transmission group (i.e. the most recent transmission group, which is not yet full). For any group that is full, the source SHOULD provide FEC on demand in response to a selective NAK. Pro-active FEC refers to the technique of computing parity packets at transmission time and transmitting them as a matter of course following the data packets. Pro-active FEC is RECOMMENDED for providing loss recovery over simplex or asymmetric multicast channels over which returning repair requests is either impossible or costly. It provides increased reliability at the expense of bandwidth. On-demand FEC refers to the technique of computing parity packets at repair time and transmitting them only upon demand (i.e., receiver- based loss detection and repair request). On-demand FEC is RECOMMENDED for providing loss recovery of uncorrelated loss in very large receiver populations in which the probability of any single packet being lost is substantial. It provides equivalent reliability to selective NAKs (ARQ) at no more and typically less expense of bandwidth. Selective NAKs are NAKs that request the retransmission of specific packets by sequence number corresponding to the sequence number of any data packets detected to be missing from the expected sequence (conventional ARQ). Selective NAKs can be used for recovering losses
occurring in leading partial transmission groups, i.e. in the most
recent transmission group, which is not yet full. The RECOMMENDED
way of handling partial transmission groups, however, is for the data
source to use variable-size transmission groups (see below).
Parity NAKs are NAKs that request the transmission of a specific
number of parity packets by count corresponding to the count of the
number of data packets detected to be missing from a group of k data
packets (on-demand FEC).
The objective of these procedures is to incorporate these FEC
techniques into PGM so that:
sources MAY provide parity packets either pro-actively or on-
demand, interchangeably within the same TSI,
receivers MAY use either selective or parity NAKs interchangeably
within the same TSI (however, in a session where on-demand parity
is available receivers SHOULD only use parity NAKs).
network elements maintain repair state based on either selective
or parity NAKs in the same data structure, altering only search,
RDATA constraint, and deletion algorithms in either case,
and only OPTION additions to the basic packet formats are
REQUIRED.
11.2. Overview
Advertising FEC parameters in the transport session
Sources add OPT_PARITY_PRM to SPMs to provide session-specific
parameters such as the number of packets (TGSIZE == k) in a
transmission group. This option lets receivers know how many packets
there are in a transmission group, and it lets network elements sort
repair state by transmission group number. This option includes an
indication of whether pro-active and/or on-demand parity is available
from the source.
Distinguishing parity packets from data packets
Sources send pro-active parity packets as ODATA (NEs do not forward
RDATA unless a repair state is present) and on-demand parity packets
as RDATA. A source MUST add OPT_PARITY to the ODATA/RDATA packet
header of parity packets to permit network elements and receivers to
distinguish them from data packets.
Data and parity packet numbering Parity packets MUST be calculated over a fixed number k of data packets known as the Transmission Group. Grouping of packets into transmission groups effectively partitions a packet sequence number into a high-order portion (TG_SQN) specifying the transmission group (TG), and a low-order portion (PKT_SQN) specifying the packet number (PKT-NUM in the range 0 through k-1) within that group. From an implementation point of view, it's handy if k, the TG size, is a power of 2. If so, then TG_SQN and PKT_SQN can be mapped side-by- side into the 32 bit SQN. log2(TGSIZE) is then the size in bits of PKT_SQN. This mapping does not reduce the effective sequence number space since parity packets marked with OPT_PARITY allow the sequence space (PKT_SQN) to be completely reused in order to number the h parity packets, as long as h is not greater than k. In the case where h is greater than k, a source MUST add OPT_PARITY_GRP to any parity packet numbered j greater than k-1, specifying the number m of the group of k parity packets to which the packet belongs, where m is just the quotient from the integer division of j by k. Correspondingly, PKT-NUM for such parity packets is just j modulo k. In other words, when a source needs to generate more parity packets than there were original data packets (perhaps because of a particularly lossy line such that a receiver lost not only the original data but some of the parity RDATA as well), use the OPT_PARITY_GRP option in order to number and identify the transmission group of the extra packets that would exceed the normal sequential number space. Note that parity NAKs (and consequently their corresponding parity NCFs) MUST also contain the OPT_PARITY flag in the options field of the fixed header, and that in these packets, PKT_SQN MUST contain PKT_CNT, the number of missing packets, rather than PKT_NUM, the SQN of a specific missing packet. More on all this later. Variable Transmission Group Size The transmission group size advertised in the OPT_PARITY_PRM option on SPMs MUST be a power of 2 and constant for the duration of the session. However, the actual transmission group size used MAY not be constant for the duration of the session, and MAY not be a power of 2. When a TG size different from the one advertised in OPT_PARITY_PRM is used, the TG size advertised in OPT_PARITY_PRM MUST be interpreted as specifying the maximum effective size of the TG.
When the actual TG size is not a power of 2 or is smaller than the
max TG size, there will be sparse utilization of the sequence number
space since some of the sequence numbers that would have been
consumed in numbering a maximum sized TG will not be assigned to
packets in the smaller TG. The start of the next transmission group
will always begin on the boundary of the maximum TG size as though
each of the sequence numbers had been utilized.
When the source decides to use a smaller group size than that
advertised in OPT_PARITY_PRM, it appends OPT_CURR_TGSIZE to the last
data packet (ODATA) in the truncated transmission group. This lets
the receiver know that it should not expect any more packets in this
transmission group, and that it may start requesting repairs for any
missing packets. If the last data packet itself went missing, the
receiver will detect the end of the group when it receives a parity
packet for the group, an SPM with SPM_LEAD equal to OD_SQN of the
last data packet, or the first packet of the next group, whichever
comes first. In addition, any parity packet from this TG will also
carry the OPT_CURR_TGSIZE option as will any SPM sent with SPM_LEAD
equal to OD_SQN of the last data packet.
Variable TSDU length
If a non constant TSDU length is used within a given transmission
group, the size of parity packets in the corresponding FEC block MUST
be equal to the size of the largest original data packet in the
block. Parity packets MUST be computed by padding the original
packets with zeros up to the size of the largest data packet. Note
that original data packets are transmitted without padding.
Receivers using a combination of original packets and FEC packets to
rebuild missing packets MUST pad the original packets in the same way
as the source does. The receiver MUST then feed the padded original
packets plus the parity packets to the FEC decoder. The decoder
produces the original packets padded with zeros up to the size of the
largest original packet in the group. In order for the receiver to
eliminate the padding on the reconstructed data packets, the original
size of the packet MUST be known, and this is accomplished as
follows:
The source, along with the packet payloads, encodes the TSDU
length and appends the 2-byte encoded length to the padded FEC
packets.
Receivers pad the original packets that they received to the
largest original packet size and then append the TSDU length to
the padded packets. They then pass them and the FEC packets to
the FEC decoder.
The decoder produces padded original packets with their original
TSDU length appended. Receivers MUST now use this length to get
rid of the padding.
A source that transmits variable size packets MUST take into account
the fact that FEC packets will have a size equal to the maximum size
of the original packets plus the size of the length field (2 bytes).
If a fixed packet size is used within a transmission group, the
encoded length is not appended to the parity packets. The presence
of the fixed header option flag OPT_VAR_PKTLEN in parity packets
allows receivers to distinguish between transmission groups with
variable sized packets and fixed-size ones, and behave accordingly.
Payload-specific options
Some options present in data packet (ODATA and RDATA) are strictly
associated with the packet content (PGM payload), OPT_FRAGMENT being
an example. These options must be preserved even when the data
packet that would normally contain them is not received, but its the
payload is recovered though the use of FEC.
To achieve this, PGM encodes the content of these options in special
options that are inserted in parity packets. Two flags present in
the the option common-header are used for this process: bit F
(OP_ENCODED) and bit U (OP_ENCODED_NULL).
Whenever at least one of the original packets of a TG contains a
payload-specific option of a given type, the source MUST include an
encoded version of that option type in all the parity packets it
transmits. The encoded option is computed by applying FEC encoding
to the whole option with the exception of the first three bytes of
the option common-header (E, Option Type, Option Length, OP_ENCODED
and OPX fields). The type, length and OPX of the encoded option are
the same as the type, length and OPX in the original options.
OP_ENCODED is set to 1 (all original option have OP_ENCODED = 0).
The encoding is performed using the same process that is used to
compute the payload of the parity packet. i.e. the FEC encoder is fed
with one copy of that option type for each original packet in the TG.
If one (or more) original packet of the TG does not contain that
option type, an all zeroes option is used for the encoding process.
To be able to distinguish this "dummy" option from valid options with
all-zeroes payload, OP_ENCODED_NULL is used. OP_ENCODED_NULL is set
to 0 in all the original options, but the value of 1 is used in the
encoding process if the option did not exist in the original packet.
On the receiver side, all option with OP_ENCODED_NULL equal to 1 are
discarded after decoding.
When a receiver recovers a missing packet using FEC repair packets, it MUST also recover payload-specific options, if any. The presence of these can be unequivocally detected through the presence of encoded options in parity packets (encoded options have OP_ENCODED set to 1). Receivers apply FEC-recovery to encoded options and possibly original options, as they do to recover packet payloads. The FEC decoding is applied to the whole option with the exception of the first three bytes of the option common-header (E, Option Type, Option Length, OP_ENCODED and OPX fields). Each decoded option is associated with the relative payload, unless OP_ENCODED_NULL turns out to be 1, in which case the decoded option is discarded. The decoding MUST be performed using the 1st occurrence of a given option type in original/parity packets. If one or more original packets do not contain that option type, an option of the same type with zero value must be used. This option MUST have OP_ENCODED_NULL equal to 1.11.3. Packet Contents
This section just provides enough short-hand to make the Procedures intelligible. For the full details of packet contents, please refer to Packet Formats below. OPT_PARITY indicated in pro-active (ODATA) and on-demand (RDATA) parity packets to distinguish them from data packets. This option is directly encoded in the "Option" field of the fixed PGM header OPT_VAR_PKTLEN MAY be present in pro-active (ODATA) and on-demand (RDATA) parity packets to indicate that the corresponding transmission group is composed of variable size data packets. This option is directly encoded in the "Option" field of the fixed PGM header OPT_PARITY_PRM appended by sources to SPMs to specify session- specific parameters such as the transmission group size and the availability of pro-active and/or on- demand parity from the source OPT_PARITY_GRP the number of the group (greater than 0) of h parity packets to which the parity packet belongs when more than k parity packets are provided by the source
OPT_CURR_TGSIZE appended by sources to the last data packet and any
parity packets in a variable sized transmission
group to indicate to the receiver the actual size
of a transmission group. May also be appended to
certain SPMs
11.3.1. Parity NAKs
NAK_TG_SQN the high-order portion of NAK_SQN specifying the
transmission group for which parity packets are
requested
NAK_PKT_CNT the low-order portion of NAK_SQN specifying the
number of missing data packets for which parity
packets are requested
Nota Bene: NAK_PKT_CNT (and NCF_PKT_CNT) are 0-based counters,
meaning that NAK_PKT_CNT = 0 means that 1 FEC RDATA is being
requested, and in general NAK_PKT_CNT = k - 1 means that k FEC
RDATA are being requested.
11.3.2. Parity NCFs
NCF_TG_SQN the high-order portion of NCF_SQN specifying the
transmission group for which parity packets were
requested
NCF_PKT_CNT the low-order portion of NCF_SQN specifying the
number of missing data packets for which parity
packets were requested
Nota Bene: NCF_PKT_CNT (and NAK_PKT_CNT) are 0-based counters,
meaning that NAK_PKT_CNT = 0 means that 1 FEC RDATA is being
requested, and in general NAK_PKT_CNT = k - 1 means that k FEC
RDATA are being requested.
11.3.3. On-demand Parity
RDATA_TG_SQN the high-order portion of RDATA_SQN specifying the
transmission group to which the parity packet
belongs
RDATA_PKT_SQN the low-order portion of RDATA_SQN specifying the
parity packet sequence number within the
transmission group
11.3.4. Pro-active Parity
ODATA_TG_SQN the high-order portion of ODATA_SQN specifying the transmission group to which the parity packet belongs ODATA_PKT_SQN the low-order portion of ODATA_SQN specifying the parity packet sequence number within the transmission group11.4. Procedures - Sources
If a source elects to provide parity for a given transport session, it MUST first provide the transmission group size PARITY_PRM_TGS in the OPT_PARITY_PRM option of its SPMs. This becomes the maximum effective transmission group size in the event that the source elects to send smaller size transmission groups. If a source elects to provide proactive parity for a given transport session, it MUST set PARITY_PRM_PRO in the OPT_PARITY_PRM option of its SPMs. If a source elects to provide on-demand parity for a given transport session, it MUST set PARITY_PRM_OND in the OPT_PARITY_PRM option of its SPMs. A source MUST send any pro-active parity packets for a given transmission group only after it has first sent all of the corresponding k data packets in that group. Pro-active parity packets MUST be sent as ODATA with OPT_PARITY in the fixed header. If a source elects to provide on-demand parity, it MUST respond to a parity NAK for a transmission group with a parity NCF. The source MUST complete the transmission of the k original data packets and the proactive parity packets, possibly scheduled, before starting the transmission of on-demand parity packets. Subsequently, the source MUST send the number of parity packets requested by that parity NAK. On-demand parity packets MUST be sent as RDATA with OPT_PARITY in the fixed header. Previously transmitted pro-active parity packets cannot be reused as on-demand parity packets, these MUST be computed with new, previously unused, indexes. In either case, the source MUST provide selective retransmissions only in response to selective NAKs from the leading partial transmission group. For any group that is full, the source SHOULD provide FEC on demand in response to a selective retransmission request. In the absence of data to transmit, a source SHOULD prematurely terminate the current transmission group by including OPT_CURR_TGSIZE to the last data packet or to any proactive parity packets provided.
If the last data packet has already been transmitted and there is no provision for sending proactive parity packets, an SPM with OPT_CURR_TGSIZE SHOULD be sent. A source consolidates requests for on-demand parity in the same transmission group according to the following procedures. If the number of pending (i.e., unsent) parity packets from a previous request for on-demand parity packets is equal to or greater than NAK_PKT_CNT in a subsequent NAK, that subsequent NAK MUST be confirmed but MAY otherwise be ignored. If the number of pending (i.e., unsent) parity packets from a previous request for on-demand parity packets is less than NAK_PKT_CNT in a subsequent NAK, that subsequent NAK MUST be confirmed but the source need only increase the number of pending parity packets to NAK_PKT_CNT. When a source provides parity packets relative to a transmission group with variable sized packets, it MUST compute parity packets by padding the smaller original packets with zeroes out to the size of the largest of the original packets. The source MUST also append the encoded TSDU lengths at the end of any padding or directly to the end of the largest packet, and add the OPT_VAR_PKTLEN option as specified in the overview description. When a source provides variable sized transmission groups, it SHOULD append the OPT_CURR_TGSIZE option to the last data packet in the shortened group, and it MUST append the OPT_CURR_TGSIZE option to any parity packets it sends within that group. In case the the last data packet is sent before a determination has been made to shorten the group and there is no provision for sending proactive parity packets, an SPM with OPT_CURR_TGSIZE SHOULD be sent. The source MUST also add OPT_CURR_TGSIZE to any SPM that it sends with SPM_LEAD equal to OD_SQN of the last data packet. A receiver MUST NAK for the entire number of packets missing based on the maximum TG size, even if it already knows that the actual TG size is smaller. The source MUST take this into account and compute the number of packets effectively needed as the difference between NAK_PKT_CNT and an offset computed as the difference between the max TG size and the effective TG size.11.5. Procedures - Receivers
If a receiver elects to make use of parity packets for loss recovery, it MUST first learn the transmission group size PARITY_PRM_TGS from OPT_PARITY_PRM in the SPMs for the TSI. The transmission group size is used by a receiver to determine the sequence number boundaries between transmission groups.
Thereafter, if PARITY_PRM_PRO is also set in the SPMs for the TSI, a receiver SHOULD use any pro-active parity packets it receives for loss recovery, and if PARITY_PRM_OND is also set in the SPMs for the TSI, it MAY solicit on-demand parity packets upon loss detection. If PARITY_PRM_OND is set, a receiver MUST NOT send selective NAKs, except in partial transmission groups if the source does not use the variable transmission-group size option. Parity packets are ODATA (pro-active) or RDATA (on-demand) packets distinguished by OPT_PARITY which lets receivers know that ODATA/RDATA_TG_SQN identifies the group of PARITY_PRM_TGS packets to which the parity may be applied for loss recovery in the corresponding transmission group, and that ODATA/RDATA_PKT_SQN is being reused to number the parity packets within that group. Receivers order parity packets and eliminate duplicates within a transmission group based on ODATA/RDATA_PKT_SQN and on OPT_PARITY_GRP if present. To solicit on-demand parity packets, a receiver MUST send parity NAKs upon loss detection. For the purposes of soliciting on-demand parity, loss detection occurs at transmission group boundaries, i.e. upon receipt of the last data packet in a transmission group, upon receipt of any data packet in any subsequent transmission group, or upon receipt of any parity packet in the current or a subsequent transmission group. A parity NAK is simply a NAK with OPT_PARITY and NAK_PKT_CNT set to the count of the number of packets detected to be missing from the transmission group specified by NAK_TG_SQN. Note that this constrains the receiver to request no more parity packets than there are data packets in the transmission group. A receiver SHOULD bias the value of NAK_BO_IVL for parity NAKs inversely proportional to NAK_PKT_CNT so that NAKs for larger losses are likely to be scheduled ahead of NAKs for smaller losses in the same receiver population. A confirming NCF for a parity NAK is a parity NCF with NCF_PKT_CNT equal to or greater than that specified by the parity NAK. A receiver's NAK_RDATA_IVL timer is not cancelled until all requested parity packets have been received. In the absence of data (detected from SPMs bearing SPM_LEAD equal to RXW_LEAD) on non-transmission-group boundaries, receivers MAY resort to selective NAKs for any missing packets in that partial transmission group.
When a receiver handles parity packets belonging to a transmission group with variable sized packets, (detected from the presence of the OPT_VAR_PKTLEN option in the parity packets), it MUST decode them as specified in the overview description and use the decoded TSDU length to get rid of the padding in the decoded packet. If the source was using a variable sized transmission group via the OPT_CURR_TGSIZE, the receiver might learn this before having requested (and received) any retransmission. The above happens if it sees OPT_CURR_TGSIZE in the last data packet of the TG, in any proactive parity packet or in a SPM. If the receivers learns this and determines that it has missed one or more packets in the shortened transmission group, it MAY then NAK for them without waiting for the start of the next transmission group. Otherwise it will start NAKing at the start of the next transmission group. In both cases, the receiver MUST NAK for the number of packets missing assuming that the size of the transmission group is the maximum effective transmission group. In other words, the receivers cannot exploit the fact that it might already know that the transmission group was smaller but MUST always NAK for the number of packets it believes are missing, plus the number of packets required to bring the total packets up to the maximum effective transmission group size. After the first parity packet has been delivered to the receiver, the actual TG size is known to him, either because already known or because discovered via OPT_CURR_TGSIZE contained in the parity packet. Hence the receiver can decode the whole group as soon as the minimum number of parity packets needed is received.11.6. Procedures - Network Elements
Pro-active parity packets (ODATA with OPT_PARITY) are switched by network elements without transport-layer intervention. On-demand parity packets (RDATA with OPT_PARITY) necessitate modified request, confirmation and repair constraint procedures for network elements. In the context of these procedures, repair state is maintained per NAK_TSI and NAK_TG_SQN, and in addition to recording the interfaces on which corresponding NAKs have been received, records the largest value of NAK_PKT_CNT seen in corresponding NAKs on each interface. This value is referred to as the known packet count. The largest of the known packet counts recorded for any interface in the repair state for the transmit group or carried by an NCF is referred to as the largest known packet count.
Upon receipt of a parity NAK, a network element responds with the corresponding parity NCF. The corresponding parity NCF is just an NCF formed in the usual way (i.e., a multicast copy of the NAK with the packet type changed), but with the addition of OPT_PARITY and with NCF_PKT_CNT set to the larger of NAK_PKT_CNT and the known packet count for the receiving interface. The network element then creates repair state in the usual way with the following modifications. If repair state for the receiving interface does not exist, the network element MUST create it and additionally record NAK_PKT_CNT from the parity NAK as the known packet count for the receiving interface. If repair state for the receiving interface already exists, the network element MUST eliminate the NAK only if NAK_ELIM_IVL has not expired and NAK_PKT_CNT is equal to or less than the largest known packet count. If NAK_PKT_CNT is greater than the known packet count for the receiving interface, the network element MUST update the latter with the larger NAK_PKT_CNT. Upon either adding a new interface or updating the known packet count for an existing interface, the network element MUST determine if NAK_PKT_CNT is greater than the largest known packet count. If so or if NAK_ELIM_IVL has expired, the network element MUST forward the parity NAK in the usual way with a value of NAK_PKT_CNT equal to the largest known packet count. Upon receipt of an on-demand parity packet, a network element MUST locate existing repair state for the corresponding RDATA_TSI and RDATA_TG_SQN. If no such repair state exists, the network element MUST discard the RDATA as usual. If corresponding repair state exists, the largest known packet count MUST be decremented by one, then the network element MUST forward the RDATA on all interfaces in the existing repair state, and decrement the known packet count by one for each. Any interfaces whose known packet count is thereby reduced to zero MUST be deleted from the repair state. If the number of interfaces is thereby reduced to zero, the repair state itself MUST be deleted. Upon reception of a parity NCF, network elements MUST cancel pending NAK retransmission only if NCF_PKT_CNT is greater or equal to the largest known packet count. Network elements MUST use parity NCFs to anticipate NAKs in the usual way with the addition of recording NCF_PKT_CNT from the parity NCF as the largest known packet count with the anticipated state so that any subsequent NAKs received with NAK_PKT_CNT equal to or less than NCF_PKT_CNT will be eliminated, and
any with NAK_PKT_CNT greater than NCF_PKT_CNT will be forwarded.
Network elements which receive a parity NCF with NCF_PKT_CNT larger
than the largest known packet count MUST also use it to anticipate
NAKs, increasing the largest known packet count to reflect
NCF_PKT_CNT (partial anticipation).
Parity NNAKs follow the usual elimination procedures with the
exception that NNAKs are eliminated only if existing NAK state has a
NAK_PKT_CNT greater than NNAK_PKT_CNT.
Network elements must take extra precaution when the source is using
a variable sized transmission group. Network elements learn that the
source is using a TG size smaller than the maximum from
OPT_CURR_TGSIZE in parity RDATAs or in SPMs. When this happens, they
compute a TG size offset as the difference between the maximum TG
size and the actual TG size advertised by OPT_CURR_TGSIZE. Upon
reception of parity RDATA, the TG size offset is used to update the
repair state as follows:
Any interface whose known packet count is reduced to the TG size
offset is deleted from the repair state.
This replaces the normal rule for deleting interfaces that applies
when the TG size is equal to the maximum TG size.
11.7. Procedures - DLRs
A DLR with the ability to provide FEC repairs MUST indicate this by
setting the OPT_PARITY bit in the redirecting POLR. It MUST then
process any redirected FEC NAKs in the usual way.
11.8. Packet Formats
11.8.1. OPT_PARITY_PRM - Packet Extension Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| Option Type | Option Length |Reserved |F|OPX|U| |P O|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Transmission Group Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type = 0x08
Option Length = 8 octets
P-bit (PARITY_PRM_PRO)
Indicates when set that the source is providing pro-active parity
packets.
O-bit (PARITY_PRM_OND)
Indicates when set that the source is providing on-demand parity
packets.
At least one of PARITY_PRM_PRO and PARITY_PRM_OND MUST be set.
Transmission Group Size (PARITY_PRM_TGS)
The number of data packets in the transmission group over which
the parity packets are calculated. If a variable transmission
group size is being used, then this becomes the maximum effective
transmission group size across the session.
OPT_PARITY_PRM MAY be appended only to SPMs.
OPT_PARITY_PRM is network-significant.
11.8.2. OPT_PARITY_GRP - Packet Extension Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| Option Type | Option Length |Reserved |F|OPX|U| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parity Group Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type = 0x09
Option Length = 8 octets
Parity Group Number (PRM_GROUP)
The number of the group of k parity packets amongst the h parity
packets within the transmission group to which the parity packet
belongs, where the first k parity packets are in group zero.
PRM_GROUP MUST NOT be zero.
OPT_PARITY_GRP MAY be appended only to parity packets.
OPT_PARITY_GRP is NOT network-significant.
11.8.3. OPT_CURR_TGSIZE - Packet Extension Format
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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |E| Option Type | Option Length |Reserved |F|OPX|U| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Actual Transmission Group Size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type = 0x0A Option Length = 8 octets Actual Transmission Group Size (PRM_ATGSIZE) The actual number of data packets in this transmission group. This MUST be less than or equal to the maximum transmission group size PARITY_PRM_TGS in OPT_PARITY_PRM. OPT_CURR_TGSIZE MAY be appended to data and parity packets (ODATA or RDATA) and to SPMs. OPT_CURR_TGSIZE is network-significant except when appended to ODATA.
(page 72 continued on part 4)
