| Internet-Draft | DetNet Bounded Latency | November 2020 |
| Finn, et al. | Expires 6 May 2021 | [Page] |
This document presents a timing model for Deterministic Networking (DetNet), so that existing and future standards can achieve the DetNet quality of service features of bounded latency and zero congestion loss. It defines requirements for resource reservation protocols or servers. It calls out queuing mechanisms, defined in other documents, that can provide the DetNet quality of service.¶
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The ability for IETF Deterministic Networking (DetNet) or IEEE 802.1 Time-Sensitive Networking (TSN, [IEEE8021TSN]) to provide the DetNet services of bounded latency and zero congestion loss depends upon A) configuring and allocating network resources for the exclusive use of DetNet/TSN flows; B) identifying, in the data plane, the resources to be utilized by any given packet, and C) the detailed behavior of those resources, especially transmission queue selection, so that latency bounds can be reliably assured. Thus, DetNet is an example of an IntServ Guaranteed Quality of Service [RFC2212]¶
As explained in [RFC8655], DetNet flows are characterized by 1) a maximum bandwidth, guaranteed either by the transmitter or by strict input metering; and 2) a requirement for a guaranteed worst-case end-to-end latency. That latency guarantee, in turn, provides the opportunity for the network to supply enough buffer space to guarantee zero congestion loss.¶
To be of use to the applications identified in [RFC8578], it must be possible to calculate, before the transmission of a DetNet flow commences, both the worst-case end-to-end network latency, and the amount of buffer space required at each hop to ensure against congestion loss.¶
This document references specific queuing mechanisms, defined in other documents, that can be used to control packet transmission at each output port and achieve the DetNet qualities of service. This document presents a timing model for sources, destinations, and the DetNet transit nodes that relay packets that is applicable to all of those referenced queuing mechanisms.¶
Using the model presented in this document, it should be possible for an implementor, user, or standards development organization to select a particular set of queuing mechanisms for each device in a DetNet network, and to select a resource reservation algorithm for that network, so that those elements can work together to provide the DetNet service.¶
This document does not specify any resource reservation protocol or server. It does not describe all of the requirements for that protocol or server. It does describe requirements for such resource reservation methods, and for queuing mechanisms that, if met, will enable them to work together.¶
This document assumes that following paradigm is used for provisioning DetNet flows:¶
This paradigm can be implemented using peer-to-peer protocols or using a central server. In some situations, a lack of resources can require backtracking and recursing through this list.¶
Issues such as un-provisioning a DetNet flow in favor of another, when resources are scarce, are not considered, here. Also not addressed is the question of how to choose the path to be taken by a DetNet flow.¶
In this calculation, all of the DetNet flows are known before the calculation commences. This problem is of interest to relatively static networks, or static parts of larger networks. It provides bounds on delay and buffer size. The calculations can be extended to provide global optimizations, such as altering the path of one DetNet flow in order to make resources available to another DetNet flow with tighter constraints.¶
The static flow calculation is not limited only to static networks; the entire calculation for all flows can be repeated each time a new DetNet flow is created or deleted. If some already-established flow would be pushed beyond its latency requirements by the new flow, then the new flow can be refused, or some other suitable action taken.¶
This calculation may be more difficult to perform than that of the dynamic calculation (Section 3.1.2), because the flows passing through one port on a DetNet transit node affect each others' latency. The effects can even be circular, from Flow A to B to C and back to A. On the other hand, the static calculation can often accommodate queuing methods, such as transmission selection by strict priority, that are unsuitable for the dynamic calculation.¶
This calculation is dynamic, in the sense that flows can be added or deleted at any time, with a minimum of computation effort, and without affecting the guarantees already given to other flows.¶
The choice of queuing methods is critical to the applicability of the dynamic calculation. Some queuing methods (e.g. CQF, Section 6.6) make it easy to configure bounds on the network's capacity, and to make independent calculations for each flow. Some other queuing methods (e.g. strict priority with the credit-based shaper defined in [IEEE8021Q] section 8.6.8.2) can be used for dynamic flow creation, but yield poorer latency and buffer space guarantees than when that same queuing method is used for static flow creation (Section 3.1.1).¶
A model for the operation of a DetNet transit node is required, in order to define the latency and buffer calculations. In Figure 1 we see a breakdown of the per-hop latency experienced by a packet passing through a DetNet transit node, in terms that are suitable for computing both hop-by-hop latency and per-hop buffer requirements.¶
DetNet transit node A DetNet transit node B
+-------------------------+ +------------------------+
| Queuing | | Queuing |
| Regulator subsystem | | Regulator subsystem |
| +-+-+-+-+ +-+-+-+-+ | | +-+-+-+-+ +-+-+-+-+ |
-->+ | | | | | | | | | + +------>+ | | | | | | | | | + +--->
| +-+-+-+-+ +-+-+-+-+ | | +-+-+-+-+ +-+-+-+-+ |
| | | |
+-------------------------+ +------------------------+
|<->|<------>|<------->|<->|<---->|<->|<------>|<------>|<->|<--
2,3 4 5 6 1 2,3 4 5 6 1 2,3
1: Output delay 4: Processing delay
2: Link delay 5: Regulation delay
3: Preemption delay 6: Queuing delay.
In Figure 1, we see two DetNet transit nodes (typically, bridges or routers), with a wired link between them. In this model, the only queues, that we deal with explicitly, are attached to the output port; other queues are modeled as variations in the other delay times. (E.g., an input queue could be modeled as either a variation in the link delay [2] or the processing delay [4].) There are six delays that a packet can experience from hop to hop.¶
Not shown in Figure 1 are the other output queues that we presume are also attached to that same output port as the queue shown, and against which this shown queue competes for transmission opportunities.¶
The initial and final measurement point in this analysis (that is, the definition of a "hop") is the point at which a packet is selected for output. In general, any queue selection method that is suitable for use in a DetNet network includes a detailed specification as to exactly when packets are selected for transmission. Any variations in any of the delay times 1-4 result in a need for additional buffers in the queue. If all delays 1-4 are constant, then any variation in the time at which packets are inserted into a queue depends entirely on the timing of packet selection in the previous node. If the delays 1-4 are not constant, then additional buffers are required in the queue to absorb these variations. Thus:¶
End-to-end delay bounds can be computed using the delay model in Section 3.2. Here, it is important to be aware that for several queuing mechanisms, the end-to-end delay bound is less than the sum of the per-hop delay bounds. An end-to-end delay bound for one DetNet flow can be computed as¶
The two terms in the above formula are computed as follows.¶
First, at the h-th hop along the path of this DetNet flow, obtain an upperbound per-hop_non_queuing_delay_bound[h] on the sum of the bounds over the delays 1,2,3,4 of Figure 1. These upper bounds are expected to depend on the specific technology of the DetNet transit node at the h-th hop but not on the T-SPEC of this DetNet flow. Then set non_queuing_delay_bound = the sum of per-hop_non_queuing_delay_bound[h] over all hops h.¶
Second, compute queuing_delay_bound as an upper bound to the sum of the queuing delays along the path. The value of queuing_delay_bound depends on the T-SPEC of this flow and possibly of other flows in the network, as well as the specifics of the queuing mechanisms deployed along the path of this flow. The computation of queuing_delay_bound is described in Section 4.2 as a separate section.¶
For several queuing mechanisms, queuing_delay_bound is less than the sum of upper bounds on the queuing delays (5,6) at every hop. This occurs with (1) per-flow queuing, and (2) per-class queuing with regulators, as explained in Section 4.2.1, Section 4.2.2, and Section 6.¶
For other queuing mechanisms the only available value of queuing_delay_bound is the sum of the per-hop queuing delay bounds. In such cases, the computation of per-hop queuing delay bounds must account for the fact that the T-SPEC of a DetNet flow is no longer satisfied at the ingress of a hop, since burstiness increases as one flow traverses one DetNet transit node.¶
With such mechanisms, each flow uses a separate queue inside every node. The service for each queue is abstracted with a guaranteed rate and a latency. For every flow, a per-node delay bound as well as an end-to-end delay bound can be computed from the traffic specification of this flow at its source and from the values of rates and latencies at all nodes along its path. The per-flow queuing is used in IntServ. Details of calculation for IntServ are described in Section 6.5.¶
With such mechanisms, the flows that have the same class share the same queue. A practical example is the credit-based shaper defined in section 8.6.8.2 of [IEEE8021Q]. One key issue in this context is how to deal with the burstiness cascade: individual flows that share a resource dedicated to a class may see their burstiness increase, which may in turn cause increased burstiness to other flows downstream of this resource. Computing delay upper bounds for such cases is difficult, and in some conditions impossible [charny2000delay][bennett2002delay]. Also, when bounds are obtained, they depend on the complete configuration, and must be recomputed when one flow is added. (The dynamic calculation, Section 3.1.2.)¶
A solution to deal with this issue is to reshape the flows at every hop. This can be done with per-flow regulators (e.g. leaky bucket shapers), but this requires per-flow queuing and defeats the purpose of per-class queuing. An alternative is the interleaved regulator, which reshapes individual flows without per-flow queuing ([Specht2016UBS], [IEEE8021Qcr]). With an interleaved regulator, the packet at the head of the queue is regulated based on its (flow) regulation constraints; it is released at the earliest time at which this is possible without violating the constraint. One key feature of per-flow or interleaved regulator is that, it does not increase worst-case latency bounds [le_boudec_theory_2018]. Specifically, when an interleaved regulator is appended to a FIFO subsystem, it does not increase the worst-case delay of the latter.¶
Figure 2 shows an example of a network with 5 nodes, per-class queuing mechanism and interleaved regulators as in Figure 1. An end-to-end delay bound for flow f, traversing nodes 1 to 5, is calculated as follows:¶
In the above formula, Cij is a bound on the delay of the queuing subsystem in node i and interleaved regulator of node j, and S4 is a bound on the delay of the queuing subsystem in node 4 for flow f. In fact, using the delay definitions in Section 3.2, Cij is a bound on sum of the delays 1,2,3,6 of node i and 4,5 of node j. Similarly, S4 is a bound on sum of the delays 1,2,3,6 of node 4. A practical example of queuing model and delay calculation is presented Section 6.4.¶
f
----------------------------->
+---+ +---+ +---+ +---+ +---+
| 1 |---| 2 |---| 3 |---| 4 |---| 5 |
+---+ +---+ +---+ +---+ +---+
\__C12_/\__C23_/\__C34_/\_S4_/
REMARK: The end-to-end delay bound calculation provided here gives a much better upper bound in comparison with end-to-end delay bound computation by adding the delay bounds of each node in the path of a flow [TSNwithATS].¶
A sender can be a DetNet node which uses exactly the same queuing methods as its adjacent DetNet transit node, so that the delay and buffer bounds calculations at the first hop are indistinguishable from those at a later hop within the DetNet domain. On the other hand, the sender may be DetNet unaware, in which case some conditioning of the flow may be necessary at the ingress DetNet transit node.¶
This ingress conditioning typically consists of a FIFO with an output regulator that is compatible with the queuing employed by the DetNet transit node on its output port(s). For some queuing methods, simply requires added extra buffer space in the queuing subsystem. Ingress conditioning requirements for different queuing methods are mentioned in the sections, below, describing those queuing methods.¶
It is sometimes desirable to build a network that has both DetNet aware transit nodes and DetNet non-aware transit nodes, and for a DetNet flow to traverse an island of non-DetNet transit nodes, while still allowing the network to offer delay and congestion loss guarantees. This is possible under certain conditions.¶
In general, when passing through a non-DetNet island, the island causes delay variation in excess of what would be caused by DetNet nodes. That is, the DetNet flow is "lumpier" after traversing the non-DetNet island. DetNet guarantees for delay and buffer requirements can still be calculated and met if and only if the following are true:¶
The ingress conditioning is exactly the same problem as that of a sender at the edge of the DetNet domain. The requirement for bounds on the latency variation across the non-DetNet island is typically the most difficult to achieve. Without such a bound, it is obvious that DetNet cannot deliver its guarantees, so a non-DetNet island that cannot offer bounded latency variation cannot be used to carry a DetNet flow.¶
When the input rate to an output queue exceeds the output rate for a sufficient length of time, the queue must overflow. This is congestion loss, and this is what deterministic networking seeks to avoid.¶
To avoid congestion losses, an upper bound on the backlog present in the regulator and queuing subsystem of Figure 1 must be computed during resource reservation. This bound depends on the set of flows that use these queues, the details of the specific queuing mechanism and an upper bound on the processing delay (4). The queue must contain the packet in transmission plus all other packets that are waiting to be selected for output.¶
A conservative backlog bound, that applies to all systems, can be derived as follows.¶
The backlog bound is counted in data units (bytes, or words of multiple bytes) that are relevant for buffer allocation. For every class we need one buffer space for the packet in transmission, plus space for the packets that are waiting to be selected for output. Excluding transmission and preemption times, the packets are waiting in the queue since reception of the last bit, for a duration equal to the processing delay (4) plus the queuing delays (5,6).¶
Let¶
Then a bound on the backlog of traffic of all classes in the queue at this output port is¶
In this section, for simplicity of delay computation, we assume that the T-SPEC or arrival curve [NetCalBook] for each flow at source is leaky bucket. Also, at each relay node, the service for each queue is abstracted with a guaranteed rate and a latency.¶
Sophisticated queuing mechanisms are available in Layer 3 (L3, see, e.g., [RFC7806] for an overview). In general, we assume that "Layer 3" queues, shapers, meters, etc., are precisely the "regulators" shown in Figure 1. The "queuing subsystems" in this figure are not the province solely of bridges; they are an essential part of any DetNet transit node. As illustrated by numerous implementation examples, some of the "Layer 3" mechanisms described in documents such as [RFC7806] are often integrated, in an implementation, with the "Layer 2" mechanisms also implemented in the same node. An integrated model is needed in order to successfully predict the interactions among the different queuing mechanisms needed in a network carrying both DetNet flows and non-DetNet flows.¶
Figure 3 shows the general model for the flow of packets through the queues of a DetNet transit node. Packets are assigned to a class of service. The classes of service are mapped to some number of regulator queues. Only DetNet/TSN packets pass through regulators. Queues compete for the selection of packets to be passed to queues in the queuing subsystem. Packets again are selected for output from the queuing subsystem.¶
|
+--------------------------------V----------------------------------+
| Class of Service Assignment |
+--+------+----------+---------+-----------+-----+-------+-------+--+
| | | | | | | |
+--V-+ +--V-+ +--V--+ +--V--+ +--V--+ | | |
|Flow| |Flow| |Flow | |Flow | |Flow | | | |
| 0 | | 1 | ... | i | | i+1 | ... | n | | | |
| reg| | reg| | reg | | reg | | reg | | | |
+--+-+ +--+-+ +--+--+ +--+--+ +--+--+ | | |
| | | | | | | |
+--V------V----------V--+ +--V-----------V--+ | | |
| Trans. selection | | Trans. select. | | | |
+----------+------------+ +-----+-----------+ | | |
| | | | |
+--V--+ +--V--+ +--V--+ +--V--+ +--V--+
| out | | out | | out | | out | | out |
|queue| |queue| |queue| |queue| |queue|
| 1 | | 2 | | 3 | | 4 | | 5 |
+--+--+ +--+--+ +--+--+ +--+--+ +--+--+
| | | | |
+----------V----------------------V--------------V-------V-------V--+
| Transmission selection |
+----------+----------------------+--------------+-------+-------+--+
| | | | |
V V V V V
DetNet/TSN queue DetNet/TSN queue non-DetNet/TSN queues
Some relevant mechanisms are hidden in this figure, and are performed in the queue boxes:¶
Ideally, neither of these actions are performed on DetNet packets. Full queues for DetNet packets should occur only when a flow is misbehaving, and the DetNet QoS does not include "yellow" service for packets in excess of committed rate.¶
The Class of Service Assignment function can be quite complex, even in a bridge [IEEE8021Q], since the introduction of per-stream filtering and policing ([IEEE8021Q] clause 8.6.5.1). In addition to the Layer 2 priority expressed in the 802.1Q VLAN tag, a DetNet transit node can utilize any of the following information to assign a packet to a particular class of service (queue):¶
The "Transmission selection" function decides which queue is to transfer its oldest packet to the output port when a transmission opportunity arises.¶
In [IEEE8021Q] and [IEEE8023], the transmission of a frame can be interrupted by one or more "express" frames, and then the interrupted frame can continue transmission. This frame preemption is modeled as consisting of two MAC/PHY stacks, one for packets that can be interrupted, and one for packets that can interrupt the interruptible packets. The Class of Service (queue) determines which packets are which. Only one layer of preemption is supported -- a transmitter cannot have more than one interrupted frame in progress. DetNet flows typically pass through the interrupting MAC. For those DetNet flows with T-SPEC, latency bound can be calculated by the methods provided in the following sections that accounts for the affect of preemption, according to the specific queuing mechanism that is used in DetNet nodes. Best-effort queues pass through the interruptible MAC, and can thus be preempted.¶
In [IEEE8021Q], the notion of time-scheduling queue gates is described in section 8.6.8.4. On each node, the transmission selection for packets is controlled by time-synchronized gates; each output queue is associated with a gate. The gates can be either open or close. The states of the gates are determined by the gate control list (GCL). The GCL specifies the opening and closing times of the gates. Since the design of GCL should satisfy the requirement of latency upper bounds of all time-sensitive flows, those flows travers a network should have bounded latency, if the traffic and nodes are conformant.¶
It should be noted that scheduled traffic service relies on a synchronized network and coordinated GCL configuration. Synthesis of GCL on multiple nodes in network is a scheduling problem considering all TSN/DetNet flows traversing the network, which is a non-deterministic polynomial-time hard (NP-hard) problem. Also, at this writing, scheduled traffic service supports no more than eight traffic classes, typically using up to seven priority classes and at least one best effort class.¶
In the cosidered queuing model, there are four types of flows, namely, control-data traffic (CDT), class A, class B, and best effort (BE) in decreasing order of priority. Flows of classes A and B are together referred to AVB flows. This model is a subset of Time-Sensitive Networking as described next.¶
Based on the timing model described in Figure 1, the contention occurs only at the output port of a relay node; therefore, the focus of the rest of this subsection is on the regulator and queuing subsystem in the output port of a relay node. The output port performs per-class scheduling with eight classes (queuing subsystems): one for CDT, one for class A traffic, one for class B traffic, and five for BE traffic denoted as BE0-BE4. The queuing policy for each queuing subsystem is FIFO. In addition, each node output port also performs per-flow regulation for AVB flows using an interleaved regulator (IR), called Asynchronous Traffic Shaper [IEEE8021Qcr]. Thus, at each output port of a node, there is one interleaved regulator per-input port and per-class; the interleaved regulator is mapped to the regulator depicted in Figure 1. The detailed picture of scheduling and regulation architecture at a node output port is given by Figure 4. The packets received at a node input port for a given class are enqueued in the respective interleaved regulator at the output port. Then, the packets from all the flows, including CDT and BE flows, are enqueued in queuing subsytem; there is no regulator for such classes.¶
+--+ +--+ +--+ +--+
| | | | | | | |
|IR| |IR| |IR| |IR|
| | | | | | | |
+-++XXX++-+ +-++XXX++-+
| | | |
| | | |
+---+ +-v-XXX-v-+ +-v-XXX-v-+ +-----+ +-----+ +-----+ +-----+ +-----+
| | | | | | |Class| |Class| |Class| |Class| |Class|
|CDT| | Class A | | Class B | | BE4 | | BE3 | | BE2 | | BE1 | | BE0 |
| | | | | | | | | | | | | | | |
+-+-+ +----+----+ +----+----+ +--+--+ +--+--+ +--+--+ +--+--+ +--+--+
| | | | | | | |
| +-v-+ +-v-+ | | | | |
| |CBS| |CBS| | | | | |
| +-+-+ +-+-+ | | | | |
| | | | | | | |
+-v--------v-----------v---------v-------V-------v-------v-------v--+
| Strict Priority selection |
+--------------------------------+----------------------------------+
|
V
Each of the queuing subsystems for class A and B, contains Credit-Based Shaper (CBS). The CBS serves a packet from a class according to the available credit for that class. The credit for each class A or B increases based on the idle slope, and decreases based on the send slope, both of which are parameters of the CBS (Section 8.6.8.2 of [IEEE8021Q]). The CDT and BE0-BE4 flows are served by separate queuing subsystems. Then, packets from all flows are served by a transmission selection subsystem that serves packets from each class based on its priority. All subsystems are non-preemptive. Guarantees for AVB traffic can be provided only if CDT traffic is bounded; it is assumed that the CDT traffic has leaky bucket arrival curve with two parameters r_h as rate and b_h as bucket size, i.e., the amount of bits entering a node within a time interval t is bounded by r_h t + b_h.¶
Additionally, it is assumed that the AVB flows are also regulated at their source according to leaky bucket arrival curve. At the source, the traffic satisfies its regulation constraint, i.e. the delay due to interleaved regulator at source is ignored.¶
At each DetNet transit node implementing an interleaved regulator, packets of multiple flows are processed in one FIFO queue; the packet at the head of the queue is regulated based on its leaky bucket parameters; it is released at the earliest time at which this is possible without violating the constraint. The regulation parameters for a flow (leaky bucket rate and bucket size) are the same at its source and at all DetNet transit nodes along its path.¶
A delay bound of the queuing subsystem ([4] in Figure 1) for an AVB flow of class A or B can be computed if the following condition holds:¶
If the condition holds, the delay bounds for a flow of class X (A or B) is d_X and calculated as:¶
where L_min_X is the minimum packet lengths of class X (A or B); c is the output link transmission rate; b_t_X is the sum of the b term (bucket size) for all the flows of the class X. Parameters R_X and T_X are calculated as follows for class A and class B, separately:¶
If the flow is of class A:¶
where L_nA is the maximum packet length of class B and BE packets; L_n is the maximum packet length of classes A,B, and BE.¶
If the flow is of class B:¶
where L_A is the maximum packet length of class A; L_BE is the maximum packet length of class BE.¶
Then, an end-to-end delay bound of class X (A or B)is calculated by the formula Section 4.2.2, where for Cij:¶
More information of delay analysis in such a DetNet transit node is described in [TSNwithATS].¶
The delay bound calculation requires some information about each node. For each node, it is required to know the idle slope of CBS for each class A and B (I_A and I_B), as well as the transmission rate of the output link (c). Besides, it is necessary to have the information on each class, i.e. maximum packet length of classes A, B, and BE. Moreover, the leaky bucket parameters of CDT (r_h,b_h) should be known. To admit a flow/flows, their delay requirements should be guaranteed not to be violated. As described in Section 3.1, the two problems, static and dynamic, are addressed separately. In either of the problems, the rate and delay should be guaranteed. Thus,¶
The choice of the static values of R and b_t at all nodes and classes must be done in a prior configuration phase; R controls the bandwidth allocated to this class at this node, b_t affects the delay bound and the buffer requirement. R must satisfy the constraints given in Annex L.1 of [IEEE8021Q].¶
Integrated service (IntServ) is an architecture that specifies the elements to guarantee quality of service (QoS) on networks.¶
The flow, at the source, has a leaky bucket arrival curve with two parameters r as rate and b as bucket size, i.e., the amount of bits entering a node within a time interval t is bounded by r t + b.¶
If a resource reservation on a path is applied, a node provides a guaranteed rate R and maximum service latency of T. This can be interpreted in a way that the bits might have to wait up to T before being served with a rate greater or equal to R. The delay bound of the flow traversing the node is T + b / R.¶
Consider an IntServ path including a sequence of nodes, where the i-th node provides a guaranteed rate R_i and maximum service latency of T_i. Then, the end-to-end delay bound for a flow on this can be calculated as sum(T_i) + b / min(R_i).¶
If more information about the flow is known, e.g. the peak rate, the delay bound is more complicated; the detail is available in Section 1.4.1 of [NetCalBook].¶
Annex T of [IEEE8021Q] describes Cyclic Queuing and Forwarding (CQF), which provides bounded latency and zero congestion loss using the time-scheduled gates of [IEEE8021Q] section 8.6.8.4. For a given DetNet class of service, a set of two or more buffers is provided at the output queue layer of Figure 3. A cycle time T_c is configured for each class c, and all of the buffer sets in a class swap buffers simultaneously throughout the DetNet domain at that cycle rate, all in phase. In such a mechanism, the regulator, mentioned in Figure 1, is not required.¶
In the case of two-buffer CQF, each class c has two buffers, namely buffer1 and buffer2. In a cycle (i) when buffer1 accumulates received packets from the node's reception ports, buffer2 transmits the already stored packets from the previous cycle (i-1). In the next cycle (i+1), buffer2 stores the received packets and buffer1 transmits the packets received in cycle (i). The duration of each cycle is T_c.¶
The per-hop latency is trivially determined by the cycle time T_c: the packet transmitted from a node at a cycle (i), is transmitted from the next node at cycle (i+1). Hence, the maximum delay experienced by a given packet is from the beginning of cycle (i) to the end of cycle (i+1), or 2T_c; also, the minimum delay is from the end of cycle (i) to the beginning of cycle (i+1), i.e., zero. Then, if the packet traverses h hops, the maximum delay is:¶
and the minimum delay is:¶
which gives a latency variation of 2T_c.¶
The cycle length T_c should be carefully chosen; it needs to be large enough to accomodate all the DetNet traffic, plus at least one maximum interfering packet, that can be received within one cycle. Also, the value of T_c includes a time interval, called dead time (DT), which is the sum of the delays 1,2,3,4 defined in Figure 1. The value of DT guarantees that the last packet of one cycle in a node is fully delivered to a buffer of the next node is the same cycle. A two-buffer CQF is recommended if DT is small compared to T_c. For a large DT, CQF with more buffers can be used.¶
Ingress conditioning (Section 4.3) may be required if the source of a DetNet flow does not, itself, employ CQF. Since there are no per-flow parameters in the CQF technique, per-hop configuration is not required in the CQF forwarding nodes.¶