RFC 3208
PGM Reliable Transport Protocol Specification
3. Terms and Concepts
Before proceeding from the preceding overview to the detail in the subsequent Procedures, this section presents some concepts and definitions that make that detail more intelligible.3.1. Transport Session Identifiers
Every PGM packet is identified by a: TSI transport session identifier TSIs MUST be globally unique, and only one source at a time may act as the source for a transport session. (Note that repairers do not change the TSI in any RDATA they transmit). TSIs are composed of the concatenation of a globally unique source identifier (GSI) and a source-assigned data-source port. Since all PGM packets originated by receivers are in response to PGM packets originated by a source, receivers simply echo the TSI heard from the source in any corresponding packets they originate. Since all PGM packets originated by network elements are in response to PGM packets originated by a receiver, network elements simply echo the TSI heard from the receiver in any corresponding packets they originate.3.2. Sequence Numbers
PGM uses a circular sequence number space from 0 through ((2**32) - 1) to identify and order ODATA packets. Sources MUST number ODATA packets in unit increments in the order in which the corresponding application data is submitted for transmission. Within a transmit or
receive window (defined below), a sequence number x is "less" or "older" than sequence number y if it numbers an ODATA packet preceding ODATA packet y, and a sequence number y is "greater" or "more recent" than sequence number x if it numbers an ODATA packet subsequent to ODATA packet x.3.3. Transmit Window
The description of the operation of PGM rests fundamentally on the definition of the source-maintained transmit window. This definition in turn is derived directly from the amount of transmitted data (in seconds) a source retains for repair (TXW_SECS), and the maximum transmit rate (in bytes/second) maintained by a source to regulate its bandwidth utilization (TXW_MAX_RTE). In terms of sequence numbers, the transmit window is the range of sequence numbers consumed by the source for sequentially numbering and transmitting the most recent TXW_SECS of ODATA packets. The trailing (or left) edge of the transmit window (TXW_TRAIL) is defined as the sequence number of the oldest data packet available for repair from a source. The leading (or right) edge of the transmit window (TXW_LEAD) is defined as the sequence number of the most recent data packet a source has transmitted. The size of the transmit window in sequence numbers (TXW_SQNS) (i.e., the difference between the leading and trailing edges plus one) MUST be no greater than half the PGM sequence number space less one. When TXW_TRAIL is equal to TXW_LEAD, the transmit window size is one. When TXW_TRAIL is equal to TXW_LEAD plus one, the transmit window size is empty.3.4. Receive Window
The receive window at the receivers is determined entirely by PGM packets from the source. That is, a receiver simply obeys what the source tells it in terms of window state and advancement. For a given transport session identified by a TSI, a receiver maintains: RXW_TRAIL the sequence number defining the trailing edge of the receive window, the sequence number (known from data packets and SPMs) of the oldest data packet available for repair from the source
RXW_LEAD the sequence number defining the leading edge of the
receive window, the greatest sequence number of any
received data packet within the transmit window
The receive window is the range of sequence numbers a receiver is
expected to use to identify receivable ODATA.
A data packet is described as being "in" the receive window if its
sequence number is in the receive window.
The receive window is advanced by the receiver when it receives an
SPM or ODATA packet within the transmit window that increments
RXW_TRAIL. Receivers also advance their receive windows upon receipt
of any PGM data packet within the receive window that advances the
receive window.
3.5. Source Path State
To establish the repair state required to constrain RDATA, it's
essential that NAKs return from a receiver to a source on the reverse
of the distribution tree from the source. That is, they must return
through the same sequence of PGM network elements through which the
ODATA was forwarded, but in reverse. There are two reasons for this,
the less obvious one being by far the more important.
The first and obvious reason is that RDATA is forwarded on the same
path as ODATA and so repair state must be established on this path if
it is to constrain the propagation of RDATA.
The second and less obvious reason is that in the absence of repair
state, PGM network elements do NOT forward RDATA, so the default
behavior is to discard repairs. If repair state is not properly
established for interfaces on which ODATA went missing, then
receivers on those interfaces will continue to NAK for lost data and
ultimately experience unrecoverable data loss.
The principle function of SPMs is to provide the source path state
required for PGM network elements to forward NAKs from one PGM
network element to the next on the reverse of the distribution tree
for the TSI, establishing repair state each step of the way. This
source path state is simply the address of the upstream PGM network
element on the reverse of the distribution tree for the TSI. That
upstream PGM network element may be more than one subnet hop away.
SPMs establish the identity of the upstream PGM network element on
the distribution tree for each TSI in each group in each PGM network
element, a sort of virtual PGM topology. So although NAKs are
unicast addressed, they are NOT unicast routed by PGM network
elements in the conventional sense. Instead PGM network elements use
the source path state established by SPMs to direct NAKs PGM-hop-by- PGM-hop toward the source. The idea is to constrain NAKs to the pure PGM topology spanning the more heterogeneous underlying topology of both PGM and non-PGM network elements. The result is repair state in every PGM network element between the receiver and the source so that the corresponding RDATA is never discarded by a PGM network element for lack of repair state. SPMs also maintain transmit window state in receivers by advertising the trailing and leading edges of the transmit window (SPM_TRAIL and SPM_LEAD). In the absence of data, SPMs MAY be used to close the transmit window in time by advancing the transmit window until SPM_TRAIL is equal to SPM_LEAD plus one.3.6. 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.3.6.1. Source Path Messages
3.6.1.1. SPMs
SPMs are transmitted by sources to establish source-path state in PGM network elements, and to provide transmit-window state in receivers. SPMs are multicast to the group and contain: SPM_TSI the source-assigned TSI for the session to which the SPM corresponds SPM_SQN a sequence number assigned sequentially by the source in unit increments and scoped by SPM_TSI Nota Bene: this is an entirely separate sequence than is used to number ODATA and RDATA. SPM_TRAIL the sequence number defining the trailing edge of the source's transmit window (TXW_TRAIL) SPM_LEAD the sequence number defining the leading edge of the source's transmit window (TXW_LEAD) SPM_PATH the network-layer address (NLA) of the interface on the PGM network element on which the SPM is forwarded
3.6.2. Data Packets
3.6.2.1. ODATA - Original Data
ODATA packets are transmitted by sources to send application data to receivers. ODATA packets are multicast to the group and contain: OD_TSI the globally unique source-assigned TSI OD_TRAIL the sequence number defining the trailing edge of the source's transmit window (TXW_TRAIL) OD_TRAIL makes the protocol more robust in the face of lost SPMs. By including the trailing edge of the transmit window on every data packet, receivers that have missed any SPMs that advanced the transmit window can still detect the case, recover the application, and potentially re-synchronize to the transport session. OD_SQN a sequence number assigned sequentially by the source in unit increments and scoped by OD_TSI3.6.2.2. RDATA - Repair Data
RDATA packets are repair packets transmitted by sources or DLRs in response to NAKs. RDATA packets are multicast to the group and contain: RD_TSI OD_TSI of the ODATA packet for which this is a repair RD_TRAIL the sequence number defining the trailing edge of the source's transmit window (TXW_TRAIL). This is updated to the most current value when the repair is sent, so it is not necessarily the same as OD_TRAIL of the ODATA packet for which this is a repair RD_SQN OD_SQN of the ODATA packet for which this is a repair3.6.3. Negative Acknowledgments
3.6.3.1. NAKs - Negative Acknowledgments
NAKs are transmitted by receivers to request repairs for missing data packets.
NAKs are unicast (PGM-hop-by-PGM-hop) to the source and contain:
NAK_TSI OD_TSI of the ODATA packet for which a repair is
requested
NAK_SQN OD_SQN of the ODATA packet for which a repair is
requested
NAK_SRC the unicast NLA of the original source of the missing
ODATA.
NAK_GRP the multicast group NLA
3.6.3.2. NNAKs - Null Negative Acknowledgments
NNAKs are transmitted by a DLR that receives NAKs redirected to it by
either receivers or network elements to provide flow-control feed-
back to a source.
NNAKs are unicast (PGM-hop-by-PGM-hop) to the source and contain:
NNAK_TSI NAK_TSI of the corresponding re-directed NAK.
NNAK_SQN NAK_SQN of the corresponding re-directed NAK.
NNAK_SRC NAK_SRC of the corresponding re-directed NAK.
NNAK_GRP NAK_GRP of the corresponding re-directed NAK.
3.6.4. Negative Acknowledgment Confirmations
3.6.4.1. NCFs - NAK confirmations
NCFs are transmitted by network elements and sources in response to
NAKs.
NCFs are multicast to the group and contain:
NCF_TSI NAK_TSI of the NAK being confirmed
NCF_SQN NAK_SQN of the NAK being confirmed
NCF_SRC NAK_SRC of the NAK being confirmed
NCF_GRP NAK_GRP of the NAK being confirmed
3.6.5. Option Encodings
OPT_LENGTH 0x00 - Option's Length OPT_FRAGMENT 0x01 - Fragmentation OPT_NAK_LIST 0x02 - List of NAK entries OPT_JOIN 0x03 - Late Joining OPT_REDIRECT 0x07 - Redirect OPT_SYN 0x0D - Synchronization OPT_FIN 0x0E - Session Fin receivers, conventional feedbackish OPT_RST 0x0F - Session Reset OPT_PARITY_PRM 0x08 - Forward Error Correction Parameters OPT_PARITY_GRP 0x09 - Forward Error Correction Group Number OPT_CURR_TGSIZE 0x0A - Forward Error Correction Group Size OPT_CR 0x10 - Congestion Report OPT_CRQST 0x11 - Congestion Report Request OPT_NAK_BO_IVL 0x04 - NAK Back-Off Interval OPT_NAK_BO_RNG 0x05 - NAK Back-Off Range OPT_NBR_UNREACH 0x0B - Neighbor Unreachable OPT_PATH_NLA 0x0C - Path NLA OPT_INVALID 0x7F - Option invalidated4. Procedures - General
Since SPMs, NCFs, and RDATA must be treated conditionally by PGM network elements, they must be distinguished from other packets in the chosen multicast network protocol if PGM network elements are to extract them from the usual switching path.
The most obvious way for network elements to achieve this is to examine every packet in the network for the PGM transport protocol and packet types. However, the overhead of this approach is costly for high-performance, multi-protocol network elements. An alternative, and a requirement for PGM over IP multicast, is that SPMs, NCFs, and RDATA MUST be transmitted with the IP Router Alert Option [6]. This option gives network elements a network-layer indication that a packet should be extracted from IP switching for more detailed processing.5. Procedures - Sources
5.1. Data Transmission
Since PGM relies on a purely rate-limited transmission strategy in the source to bound the bandwidth consumed by PGM transport sessions, an assortment of techniques is assembled here to make that strategy as conservative and robust as possible. These techniques are the minimum REQUIRED of a PGM source.5.1.1. Maximum Cumulative Transmit Rate
A source MUST number ODATA packets in the order in which they are submitted for transmission by the application. A source MUST transmit ODATA packets in sequence and only within the transmit window beginning with TXW_TRAIL at no greater a rate than TXW_MAX_RTE. TXW_MAX_RTE is typically the maximum cumulative transmit rate of SPM, ODATA, and RDATA. Different transmission strategies MAY define TXW_MAX_RTE as appropriate for the implementation.5.1.2. Transmit Rate Regulation
To regulate its transmit rate, a source MUST use a token bucket scheme or any other traffic management scheme that yields equivalent behavior. A token bucket [7] is characterized by a continually sustainable data rate (the token rate) and the extent to which the data rate may exceed the token rate for short periods of time (the token bucket size). Over any arbitrarily chosen interval, the number of bytes the source may transmit MUST NOT exceed the token bucket size plus the product of the token rate and the chosen interval. In addition, a source MUST bound the maximum rate at which successive packets may be transmitted using a leaky bucket scheme drained at a maximum transmit rate, or equivalent mechanism.
5.1.3. Outgoing Packet Ordering
To preserve the logic of PGM's transmit window, a source MUST strictly prioritize sending of pending NCFs first, pending SPMs second, and only send ODATA or RDATA when no NCFs or SPMs are pending. The priority of RDATA versus ODATA is application dependent. The sender MAY implement weighted bandwidth sharing between RDATA and ODATA. Note that strict prioritization of RDATA over ODATA may stall progress of ODATA if there are receivers that keep generating NAKs so as to always have RDATA pending (e.g. a steady stream of late joiners with OPT_JOIN). Strictly prioritizing ODATA over RDATA may lead to a larger portion of receivers getting unrecoverable losses.5.1.4. Ambient SPMs
Interleaved with ODATA and RDATA, a source MUST transmit SPMs at a rate at least sufficient to maintain current source path state in PGM network elements. Note that source path state in network elements does not track underlying changes in the distribution tree from a source until an SPM traverses the altered distribution tree. The consequence is that NAKs may go unconfirmed both at receivers and amongst network elements while changes in the underlying distribution tree take place.5.1.5. Heartbeat SPMs
In the absence of data to transmit, a source SHOULD transmit SPMs at a decaying rate in order to assist early detection of lost data, to maintain current source path state in PGM network elements, and to maintain current receive window state in the receivers. In this scheme [8], a source maintains an inter-heartbeat timer IHB_TMR which times the interval between the most recent packet (ODATA, RDATA, or SPM) transmission and the next heartbeat transmission. IHB_TMR is initialized to a minimum interval IHB_MIN after the transmission of any data packet. If IHB_TMR expires, the source transmits a heartbeat SPM and initializes IHB_TMR to double its previous value. The transmission of consecutive heartbeat SPMs doubles IHB each time up to a maximum interval IHB_MAX. The transmission of any data packet initializes IHB_TMR to IHB_MIN once again. The effect is to provoke prompt detection of missing packets in the absence of data to transmit, and to do so with minimal bandwidth overhead.
5.1.6. Ambient and Heartbeat SPMs
Ambient and heartbeat SPMs are described as driven by separate timers in this specification to highlight their contrasting functions. Ambient SPMs are driven by a count-down timer that expires regularly while heartbeat SPMs are driven by a count-down timer that keeps being reset by data, and the interval of which changes once it begins to expire. The ambient SPM timer is just counting down in real-time while the heartbeat timer is measuring the inter-data-packet interval. In the presence of data, no heartbeat SPMs will be transmitted since the transmission of data keeps setting the IHB_TMR back to its initial value. At the same time however, ambient SPMs MUST be interleaved into the data as a matter of course, not necessarily as a heartbeat mechanism. This ambient transmission of SPMs is REQUIRED to keep the distribution tree information in the network current and to allow new receivers to synchronize with the session. An implementation SHOULD de-couple ambient and heartbeat SPM timers sufficiently to permit them to be configured independently of each other.5.2. Negative Acknowledgment Confirmation
A source MUST immediately multicast an NCF in response to any NAK it receives. The NCF is REQUIRED since the alternative of responding immediately with RDATA would not allow other PGM network elements on the same subnet to do NAK anticipation, nor would it allow DLRs on the same subnet to provide repairs. A source SHOULD be able to detect a NAK storm and adopt countermeasure to protect the network against a denial of service. A possible countermeasure is to send the first NCF immediately in response to a NAK and then delay the generation of further NCFs (for identical NAKs) by a small interval, so that identical NCFs are rate-limited, without affecting the ability to suppress NAKs.5.3. Repairs
After multicasting an NCF in response to a NAK, a source MUST then multicast RDATA (while respecting TXW_MAX_RTE) in response to any NAK it receives for data packets within the transmit window. In the interest of increasing the efficiency of a particular RDATA packet, a source MAY delay RDATA transmission to accommodate the arrival of NAKs from the whole loss neighborhood. This delay SHOULD not exceed twice the greatest propagation delay in the loss neighborhood.
6. Procedures - Receivers
6.1. Data Reception
Initial data reception A receiver SHOULD initiate data reception beginning with the first data packet it receives within the advertised transmit window. This packet's sequence number (ODATA_SQN) temporarily defines the trailing edge of the transmit window from the receiver's perspective. That is, it is assigned to RXW_TRAIL_INIT within the receiver, and until the trailing edge sequence number advertised in subsequent packets (SPMs or ODATA or RDATA) increments past RXW_TRAIL_INIT, the receiver MUST only request repairs for sequence numbers subsequent to RXW_TRAIL_INIT. Thereafter, it MAY request repairs anywhere in the transmit window. This temporary restriction on repair requests prevents receivers from requesting a potentially large amount of history when they first begin to receive a given PGM transport session. Note that the JOIN option, discussed later, MAY be used to provide a different value for RXW_TRAIL_INIT. Receiving and discarding data packets Within a given transport session, a receiver MUST accept any ODATA or RDATA packets within the receive window. A receiver MUST discard any data packet that duplicates one already received in the transmit window. A receiver MUST discard any data packet outside of the receive window. Contiguous data Contiguous data is comprised of those data packets within the receive window that have been received and are in the range from RXW_TRAIL up to (but not including) the first missing sequence number in the receive window. The most recently received data packet of contiguous data defines the leading edge of contiguous data. As its default mode of operation, a receiver MUST deliver only contiguous data packets to the application, and it MUST do so in the order defined by those data packets' sequence numbers. This provides applications with a reliable ordered data flow.
Non contiguous data PGM receiver implementations MAY optionally provide a mode of operation in which data is delivered to an application in the order received. However, the implementation MUST only deliver complete application protocol data units (APDUs) to the application. That is, APDUs that have been fragmented into different TPDUs MUST be reassembled before delivery to the application.6.2. Source Path Messages
Receivers MUST receive and sequence SPMs for any TSI they are receiving. An SPM is in sequence if its sequence number is greater than that of the most recent in-sequence SPM and within half the PGM number space. Out-of-sequence SPMs MUST be discarded. For each TSI, receivers MUST use the most recent SPM to determine the NLA of the upstream PGM network element for use in NAK addressing. A receiver MUST NOT initiate repair requests until it has received at least one SPM for the corresponding TSI. Since SPMs require per-hop processing, it is likely that they will be forwarded at a slower rate than data, and that they will arrive out of sync with the data stream. In this case, the window information that the SPMs carry will be out of date. Receivers SHOULD expect this to be the case and SHOULD detect it by comparing the packet lead and trail values with the values the receivers have stored for lead and trail. If the SPM packet values are less, they SHOULD be ignored, but the rest of the packet SHOULD be processed as normal.6.3. Data Recovery by Negative Acknowledgment
Detecting missing data packets Receivers MUST detect gaps in the expected data sequence in the following manners: by comparing the sequence number on the most recently received ODATA or RDATA packet with the leading edge of contiguous data by comparing SPM_LEAD of the most recently received SPM with the leading edge of contiguous data In both cases, if the receiver has not received all intervening data packets, it MAY initiate selective NAK generation for each missing sequence number.
In addition, a receiver may detect a single missing data packet by
receiving an NCF or multicast NAK for a data packet within the
transmit window which it has not received. In this case it MAY
initiate selective NAK generation for the said sequence number.
In all cases, receivers SHOULD temper the initiation of NAK
generation to account for simple mis-ordering introduced by the
network. A possible mechanism to achieve this is to assume loss only
after the reception of N packets with sequence numbers higher than
those of the (assumed) lost packets. A possible value for N is 2.
This method SHOULD be complemented with a timeout based mechanism
that handles the loss of the last packet before a pause in the
transmission of the data stream. The leading edge field in SPMs
SHOULD also be taken into account in the loss detection algorithm.
Generating NAKs
NAK generation follows the detection of a missing data packet and is
the cycle of:
waiting for a random period of time (NAK_RB_IVL) while listening
for matching NCFs or NAKs
transmitting a NAK if a matching NCF or NAK is not heard
waiting a period (NAK_RPT_IVL) for a matching NCF and recommencing
NAK generation if the matching NCF is not received
waiting a period (NAK_RDATA_IVL) for data and recommencing NAK
generation if the matching data is not received
The entire generation process can be summarized by the following
state machine:
|
| detect missing tpdu
| - clear data retry count
| - clear NCF retry count
V
matching NCF |--------------------------|
<---------------| BACK-OFF_STATE | <----------------------
| | start timer(NAK_RB_IVL) | ^ ^
| | | | |
| |--------------------------| | |
| matching | | timer expires | |
| NAK | | - send NAK | |
| | | | |
| V V | |
| |--------------------------| | |
| | WAIT_NCF_STATE | | |
| matching NCF | start timer(NAK_RPT_IVL) | | |
|<--------------| |------------> |
| |--------------------------| timer expires |
| | | ^ - increment NCF |
| NAK_NCF_RETRIES | | | retry count |
| exceeded | | | |
| V ----------- |
| Cancelation matching NAK |
| - restart timer(NAK_RPT_IVL) |
| |
| |
V |--------------------------| |
--------------->| WAIT_DATA_STATE |----------------------->
|start timer(NAK_RDATA_IVL)| timer expires
| | - increment data
|--------------------------| retry count
| | ^
NAK_DATA_RETRIES | | |
exceeded | | |
| -----------
| matching NCF or NAK
V - restart timer(NAK_RDATA_IVL)
Cancellation
In any state, receipt of matching RDATA or ODATA completes data
recovery and successful exit from the state machine. State
transition stops any running timers.
In any state, if the trailing edge of the window moves beyond the
sequence number, data recovery for that sequence number terminates.
During NAK_RB_IVL a NAK is said to be pending. When awaiting data or an NCF, a NAK is said to be outstanding. Backing off NAK transmission Before transmitting a NAK, a receiver MUST wait some interval NAK_RB_IVL chosen randomly over some time period NAK_BO_IVL. During this period, receipt of a matching NAK or a matching NCF will suspend NAK generation. NAK_RB_IVL is counted down from the time a missing data packet is detected. A value for NAK_BO_IVL learned from OPT_NAK_BO_IVL (see 16.4.1 below) MUST NOT be used by a receiver (i.e., the receiver MUST NOT NAK) unless either NAK_BO_IVL_SQN is zero, or the receiver has seen POLL_RND == 0 for POLL_SQN =< NAK_BO_IVL_SQN within half the sequence number space. When a parity NAK (Appendix A, FEC) is being generated, the back-off interval SHOULD be inversely biased with respect to the number of parity packets requested. This way NAKs requesting larger numbers of parity packets are likely to be sent first and thus suppress other NAKs. A NAK for a given transmission group suppresses another NAK for the same transmission group only if it is requesting an equal or larger number of parity packets. When a receiver has to transmit a sequence of NAKs, it SHOULD transmit the NAKs in order from oldest to most recent. Suspending NAK generation Suspending NAK generation just means waiting for either NAK_RB_IVL, NAK_RPT_IVL or NAK_RDATA_IVL to pass. A receiver MUST suspend NAK generation if a duplicate of the NAK is already pending from this receiver or the NAK is already outstanding from this or another receiver. NAK suppression A receiver MUST suppress NAK generation and wait at least NAK_RDATA_IVL before recommencing NAK generation if it hears a matching NCF or NAK during NAK_RB_IVL. A matching NCF must match NCF_TSI with NAK_TSI, and NCF_SQN with NAK_SQN. Transmitting a NAK Upon expiry of NAK_RB_IVL, a receiver MUST unicast a NAK to the upstream PGM network element for the TSI specifying the transport session identifier and missing sequence number. In addition, it MAY
multicast a NAK with TTL of 1 to the group, if the PGM parent is not directly connected. It also records both the address of the source of the corresponding ODATA and the address of the group in the NAK header. It MUST repeat the NAK at a rate governed by NAK_RPT_IVL up to NAK_NCF_RETRIES times while waiting for a matching NCF. It MUST then wait NAK_RDATA_IVL before recommencing NAK generation. If it hears a matching NCF or NAK during NAK_RDATA_IVL, it MUST wait anew for NAK_RDATA_IVL before recommencing NAK generation (i.e. matching NCFs and NAKs restart NAK_RDATA_IVL). Completion of NAK generation NAK generation is complete only upon the receipt of the matching RDATA (or even ODATA) packet at any time during NAK generation. Cancellation of NAK generation NAK generation is cancelled upon the advancing of the receive window so as to exclude the matching sequence number of a pending or outstanding NAK, or NAK_DATA_RETRIES / NAK_NCF_RETRIES being exceeded. Cancellation of NAK generation indicates unrecoverable data loss. Receiving NCFs and multicast NAKs A receiver MUST discard any NCFs or NAKs it hears for data packets outside the transmit window or for data packets it has received. Otherwise they are treated as appropriate for the current repair state.7. Procedures - Network Elements
7.1. Source Path State
Upon receipt of an in-sequence SPM, a network element records the Source Path Address SPM_PATH with the multicast routing information for the TSI. If the receiving network element is on the same subnet as the forwarding network element, this address will be the same as the address of the immediately upstream network element on the distribution tree for the TSI. If, however, non-PGM network elements intervene between the forwarding and the receiving network elements, this address will be the address of the first PGM network element across the intervening network elements.
The network element then forwards the SPM on each outgoing interface for that TSI. As it does so, it encodes the network address of the outgoing interface in SPM_PATH in each copy of the SPM it forwards.7.2. NAK Confirmation
Network elements MUST immediately transmit an NCF in response to any unicast NAK they receive. The NCF MUST be multicast to the group on the interface on which the NAK was received. Nota Bene: In order to avoid creating multicast routing state for PGM network elements across non-PGM-capable clouds, the network- header source address of NCFs transmitted by network elements MUST be set to the ODATA source's NLA, not the network element's NLA as might be expected. Network elements should be able to detect a NAK storm and adopt counter-measure to protect the network against a denial of service. A possible countermeasure is to send the first NCF immediately in response to a NAK and then delay the generation of further NCFs (for identical NAKs) by a small interval, so that identical NCFs are rate-limited, without affecting the ability to suppress NAKs. Simultaneously, network elements MUST establish repair state for the NAK if such state does not already exist, and add the interface on which the NAK was received to the corresponding repair interface list if the interface is not already listed.7.3. Constrained NAK Forwarding
The NAK forwarding procedures for network elements are quite similar to those for receivers, but three important differences should be noted. First, network elements do NOT back off before forwarding a NAK (i.e., there is no NAK_BO_IVL) since the resulting delay of the NAK would compound with each hop. Note that NAK arrivals will be randomized by the receivers from which they originate, and this factor in conjunction with NAK anticipation and elimination will combine to forestall NAK storms on subnets with a dense network element population. Second, network elements do NOT retry confirmed NAKs if RDATA is not seen; they simply discard the repair state and rely on receivers to re-request the repair. This approach keeps the repair state in the network elements relatively ephemeral and responsive to underlying routing changes.
Third, note that ODATA does NOT cancel NAK forwarding in network
elements since it is switched by network elements without transport-
layer intervention.
Nota Bene: Once confirmed by an NCF, network elements discard NAK
packets; they are NOT retained in network elements beyond this
forwarding operation.
NAK forwarding requires that a network element listen to NCFs for the
same transport session. NAK forwarding also requires that a network
element observe two time out intervals for any given NAK (i.e., per
NAK_TSI and NAK_SQN): NAK_RPT_IVL and NAK_RDATA_IVL.
The NAK repeat interval NAK_RPT_IVL, limits the length of time for
which a network element will repeat a NAK while waiting for a
corresponding NCF. NAK_RPT_IVL is counted down from the transmission
of a NAK. Expiry of NAK_RPT_IVL cancels NAK forwarding (due to
missing NCF).
The NAK RDATA interval NAK_RDATA_IVL, limits the length of time for
which a network element will wait for the corresponding RDATA.
NAK_RDATA_IVL is counted down from the time a matching NCF is
received. Expiry of NAK_RDATA_IVL causes the network element to
discard the corresponding repair state (due to missing RDATA).
During NAK_RPT_IVL, a NAK is said to be pending. During
NAK_RDATA_IVL, a NAK is said to be outstanding.
A Network element MUST forward NAKs only to the upstream PGM network
element for the TSI.
A network element MUST repeat a NAK at a rate of NAK_RPT_RTE for an
interval of NAK_RPT_IVL until it receives a matching NCF. A matching
NCF must match NCF_TSI with NAK_TSI, and NCF_SQN with NAK_SQN.
Upon reception of the corresponding NCF, network elements MUST wait
at least NAK_RDATA_IVL for the corresponding RDATA. Receipt of the
corresponding RDATA at any time during NAK forwarding cancels NAK
forwarding and tears down the corresponding repair state in the
network element.
7.4. NAK elimination
Two NAKs duplicate each other if they bear the same NAK_TSI and
NAK_SQN. Network elements MUST discard all duplicates of a NAK that
is pending.
Once a NAK is outstanding, network elements MUST discard all duplicates of that NAK for NAK_ELIM_IVL. Upon expiry of NAK_ELIM_IVL, network elements MUST suspend NAK elimination for that TSI/SQN until the first duplicate of that NAK is seen after the expiry of NAK_ELIM_IVL. This duplicate MUST be forwarded in the usual manner. Once this duplicate NAK is outstanding, network elements MUST once again discard all duplicates of that NAK for NAK_ELIM_IVL, and so on. NAK_RDATA_IVL MUST be reset each time a NAK for the corresponding TSI/SQN is confirmed (i.e., each time NAK_ELIM_IVL is reset). NAK_ELIM_IVL MUST be some small fraction of NAK_RDATA_IVL. NAK_ELIM_IVL acts to balance implosion prevention against repair state liveness. That is, it results in the elimination of all but at most one NAK per NAK_ELIM_IVL thereby allowing repeated NAKs to keep the repair state alive in the PGM network elements.7.5. NAK Anticipation
An unsolicited NCF is one that is received by a network element when the network element has no corresponding pending or outstanding NAK. Network elements MUST process unsolicited NCFs differently depending on the interface on which they are received. If the interface on which an NCF is received is the same interface the network element would use to reach the upstream PGM network element, the network element simply establishes repair state for NCF_TSI and NCF_SQN without adding the interface to the repair interface list, and discards the NCF. If the repair state already exists, the network element restarts the NAK_RDATA_IVL and NAK_ELIM_IVL timers and discards the NCF. If the interface on which an NCF is received is not the same interface the network element would use to reach the upstream PGM network element, the network element does not establish repair state and just discards the NCF. Anticipated NAKs permit the elimination of any subsequent matching NAKs from downstream. Upon establishing anticipated repair state, network elements MUST eliminate subsequent NAKs only for a period of NAK_ELIM_IVL. Upon expiry of NAK_ELIM_IVL, network elements MUST suspend NAK elimination for that TSI/SQN until the first duplicate of that NAK is seen after the expiry of NAK_ELIM_IVL. This duplicate MUST be forwarded in the usual manner. Once this duplicate NAK is outstanding, network elements MUST once again discard all duplicates of that NAK for NAK_ELIM_IVL, and so on. NAK_RDATA_IVL MUST be reset
each time a NAK for the corresponding TSI/SQN is confirmed (i.e., each time NAK_ELIM_IVL is reset). NAK_ELIM_IVL must be some small fraction of NAK_RDATA_IVL.7.6. NAK Shedding
Network elements MAY implement local procedures for withholding NAK confirmations for receivers detected to be reporting excessive loss. The result of these procedures would ultimately be unrecoverable data loss in the receiver.7.7. Addressing NAKs
A PGM network element uses the source and group addresses (NLAs) contained in the transport header to find the state for the corresponding TSI, looks up the corresponding upstream PGM network element's address, uses it to re-address the (unicast) NAK, and unicasts it on the upstream interface for the distribution tree for the TSI.7.8. Constrained RDATA Forwarding
Network elements MUST maintain repair state for each interface on which a given NAK is received at least once. Network elements MUST then use this list of interfaces to constrain the forwarding of the corresponding RDATA packet only to those interfaces in the list. An RDATA packet corresponds to a NAK if it matches NAK_TSI and NAK_SQN. Network elements MUST maintain this repair state only until either the corresponding RDATA is received and forwarded, or NAK_RDATA_IVL passes after forwarding the most recent instance of a given NAK. Thereafter, the corresponding repair state MUST be discarded. Network elements SHOULD discard and not forward RDATA packets for which they have no repair state. Note that the consequence of this procedure is that, while it constrains repairs to the interested subset of the network, loss of repair state precipitates further NAKs from neglected receivers.8. Packet Formats
All of the packet formats described in this section are transport- layer headers that MUST immediately follow the network-layer header in the packet. Only data packet headers (ODATA and RDATA) may be followed in the packet by application data. For each packet type, the network-header source and destination addresses are specified in
addition to the format and contents of the transport layer header.
Recall from General Procedures that, for PGM over IP multicast, SPMs,
NCFs, and RDATA MUST also bear the IP Router Alert Option.
For PGM over IP, the IP protocol number is 113.
In all packets the descriptions of Data-Source Port, Data-Destination
Port, Type, Options, Checksum, Global Source ID (GSI), and Transport
Service Data Unit (TSDU) Length are:
Data-Source Port:
A random port number generated by the source. This port number
MUST be unique within the source. Source Port together with
Global Source ID forms the TSI.
Data-Destination Port:
A globally well-known port number assigned to the given PGM
application.
Type:
The high-order two bits of the Type field encode a version
number, 0x0 in this instance. The low-order nibble of the type
field encodes the specific packet type. The intervening two
bits (the low-order two bits of the high-order nibble) are
reserved and MUST be zero.
Within the low-order nibble of the Type field:
values in the range 0x0 through 0x3 represent SPM-like
packets (i.e., session-specific, sourced by a source,
periodic),
values in the range 0x4 through 0x7 represent DATA-like
packets (i.e., data and repairs),
values in the range 0x8 through 0xB represent NAK-like
packets (i.e., hop-by-hop reliable NAK forwarding
procedures),
and values in the range 0xC through 0xF represent SPMR-like
packets (i.e., session-specific, sourced by a receiver,
asynchronous).
Options:
This field encodes binary indications of the presence and
significance of any options. It also directly encodes some
options.
bit 0 set => One or more Option Extensions are present
bit 1 set => One or more Options are network-significant
Note that this bit is clear when OPT_FRAGMENT and/or
OPT_JOIN are the only options present.
bit 6 set => Packet is a parity packet for a transmission group
of variable sized packets (OPT_VAR_PKTLEN). Only present when
OPT_PARITY is also present.
bit 7 set => Packet is a parity packet (OPT_PARITY)
Bits are numbered here from left (0 = MSB) to right (7 = LSB).
All the other options (option extensions) are encoded in
extensions to the PGM header.
Checksum:
This field is the usual 1's complement of the 1's complement
sum of the entire PGM packet including header.
The checksum does not include a network-layer pseudo header for
compatibility with network address translation. If the
computed checksum is zero, it is transmitted as all ones. A
value of zero in this field means the transmitter generated no
checksum.
Note that if any entity between a source and a receiver
modifies the PGM header for any reason, it MUST either
recompute the checksum or clear it. The checksum is mandatory
on data packets (ODATA and RDATA).
Global Source ID:
A globally unique source identifier. This ID MUST NOT change
throughout the duration of the transport session. A
RECOMMENDED identifier is the low-order 48 bits of the MD5 [9]
signature of the DNS name of the source. Global Source ID
together with Data-Source Port forms the TSI.
TSDU Length:
The length in octets of the transport data unit exclusive of
the transport header.
Note that those who require the TPDU length must obtain it from
sum of the transport header length (TH) and the TSDU length.
TH length is the sum of the size of the particular PGM packet
header (type_specific_size) plus the length of any options that
might be present.
Address Family Indicators (AFIs) are as specified in [10].
8.1. Source Path Messages
SPMs are sent by a source to establish source path state in network
elements and to provide transmit window state to receivers.
The network-header source address of an SPM is the unicast NLA of the
entity that originates the SPM.
The network-header destination address of an SPM is a multicast group
NLA.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Options | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Global Source ID ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Global Source ID | TSDU Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPM's Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Trailing Edge Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Leading Edge Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NLA AFI | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Path NLA ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
| Option Extensions when present ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source Port:
SPM_SPORT
Data-Source Port, together with SPM_GSI forms SPM_TSI
Destination Port:
SPM_DPORT
Data-Destination Port
Type:
SPM_TYPE = 0x00
Global Source ID:
SPM_GSI
Together with SPM_SPORT forms SPM_TSI
SPM's Sequence Number
SPM_SQN
The sequence number assigned to the SPM by the source.
Trailing Edge Sequence Number:
SPM_TRAIL
The sequence number defining the current trailing edge of the
source's transmit window (TXW_TRAIL).
Leading Edge Sequence Number:
SPM_LEAD
The sequence number defining the current leading edge of the
source's transmit window (TXW_LEAD).
If SPM_TRAIL == 0 and SPM_LEAD == 0x80000000, this indicates that
no window information is present in the packet.
Path NLA:
SPM_PATH
The NLA of the interface on the network element on which this SPM
was forwarded. Initialized by a source to the source's NLA,
rewritten by each PGM network element upon forwarding.
8.2. Data Packets
Data packets carry application data from a source or a repairer to
receivers.
ODATA:
Original data packets transmitted by a source.
RDATA:
Repairs transmitted by a source or by a designated local
repairer (DLR) in response to a NAK.
The network-header source address of a data packet is the unicast NLA
of the entity that originates the data packet.
The network-header destination address of a data packet is a
multicast group NLA.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Options | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Global Source ID ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Global Source ID | TSDU Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data Packet Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Trailing Edge Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Extensions when present ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ... -+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Data ...
+-+-+- ...
Source Port:
OD_SPORT, RD_SPORT
Data-Source Port, together with Global Source ID forms:
OD_TSI, RD_TSI
Destination Port:
OD_DPORT, RD_DPORT
Data-Destination Port
Type:
OD_TYPE = 0x04 RD_TYPE = 0x05
Global Source ID:
OD_GSI, RD_GSI
Together with Source Port forms:
OD_TSI, RD_TSI
Data Packet Sequence Number:
OD_SQN, RD_SQN
The sequence number originally assigned to the ODATA packet by the
source.
Trailing Edge Sequence Number:
OD_TRAIL, RD_TRAIL
The sequence number defining the current trailing edge of the
source's transmit window (TXW_TRAIL). In RDATA, this MAY not be
the same as OD_TRAIL of the ODATA packet for which it is a repair.
Data:
Application data.
8.3. Negative Acknowledgments and Confirmations
NAK: Negative Acknowledgments are sent by receivers to request the repair of an ODATA packet detected to be missing from the expected sequence. N-NAK: Null Negative Acknowledgments are sent by DLRs to provide flow control feedback to the source of ODATA for which the DLR has provided the corresponding RDATA. The network-header source address of a NAK is the unicast NLA of the entity that originates the NAK. The network-header source address of NAK is rewritten by each PGM network element with its own. The network-header destination address of a NAK is initialized by the originator of the NAK (a receiver) to the unicast NLA of the upstream PGM network element known from SPMs. The network-header destination address of a NAK is rewritten by each PGM network element with the unicast NLA of the upstream PGM network element to which this NAK is forwarded. On the final hop, the network-header destination address of a NAK is rewritten by the PGM network element with the unicast NLA of the original source or the unicast NLA of a DLR. NCF: NAK Confirmations are sent by network elements and sources to confirm the receipt of a NAK. The network-header source address of an NCF is the ODATA source's NLA, not the network element's NLA as might be expected. The network-header destination address of an NCF is a multicast group NLA. Note that in NAKs and N-NAKs, unlike the other packets, the field SPORT contains the Data-Destination port and the field DPORT contains the Data-Source port. As a general rule, the content of SPORT/DPORT is determined by the direction of the flow: in packets which travel down-stream SPORT is the port number chosen in the data source (Data-Source Port) and DPORT is the data destination port number (Data-Destination Port). The opposite holds for packets which travel upstream. This makes DPORT the protocol endpoint in the recipient host, regardless of the direction of the 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port | Destination Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Options | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Global Source ID ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Global Source ID | TSDU Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NLA AFI | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source NLA ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
| NLA AFI | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group NLA ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+
| Option Extensions when present ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- ...
Source Port:
NAK_SPORT, NNAK_SPORT
Data-Destination Port
NCF_SPORT
Data-Source Port, together with Global Source ID forms NCF_TSI
Destination Port:
NAK_DPORT, NNAK_DPORT
Data-Source Port, together with Global Source ID forms:
NAK_TSI, NNAK_TSI
NCF_DPORT
Data-Destination Port
Type:
NAK_TYPE = 0x08 NNAK_TYPE = 0x09
NCF_TYPE = 0x0A
Global Source ID:
NAK_GSI, NNAK_GSI, NCF_GSI
Together with Data-Source Port forms
NAK_TSI, NNAK_TSI, NCF_TSI
Requested Sequence Number:
NAK_SQN, NNAK_SQN
NAK_SQN is the sequence number of the ODATA packet for which a
repair is requested.
NNAK_SQN is the sequence number of the RDATA packet for which a
repair has been provided by a DLR.
NCF_SQN
NCF_SQN is NAK_SQN from the NAK being confirmed.
Source NLA:
NAK_SRC, NNAK_SRC, NCF_SRC
The unicast NLA of the original source of the missing ODATA.
Multicast Group NLA:
NAK_GRP, NNAK_GRP, NCF_GRP
The multicast group NLA. NCFs MAY bear OPT_REDIRECT and/or
OPT_NAK_LIST
(page 40 continued on part 3)
