| DetNet | B. Varga, Ed. |
| Internet-Draft | J. Farkas |
| Intended status: Standards Track | Ericsson |
| Expires: May 4, 2017 | October 31, 2016 |
DetNet Service Model
draft-varga-detnet-service-model-01
This document describes service model for scenarios requiring deterministic networking.
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A Deterministic Networking (DetNet) service provides a capability to carry a unicast or a multicast data flow for an application with constrained requirements on network performance, e.g., low packet loss rate and/or latency. During the discussion of DetNet use cases, DetNet architecture, and various related networking scenarios, several confusions have been raised due to different service model interpretations. This document defines service reference points, service components and proposes naming for service scenarios to achieve common understanding of the DetNet service model.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
The lowercase forms with an initial capital "Must", "Must Not", "Shall", "Shall Not", "Should", "Should Not", "May", and "Optional" in this document are to be interpreted in the sense defined in [RFC2119], but are used where the normative behavior is defined in documents published by SDOs other than the IETF.
Additional terms to [I-D.ietf-detnet-architecture] used in this draft.
Deterministic connectivity service is required by time/loss sensitive application(s) running on an end system during communication with its peer(s). Such a data exchange has various requirements on delay and/or loss parameters.
A DetNet flow [I-D.ietf-detnet-architecture] can have different formats during while it is transported between the peer end systems. Therefore, the following possible formats of a DetNet flow are distinguished in this document:
App-flow and DetNet-s-flow are generated by end systems. DetNet-st-flow can be generated by a DetNet edge node or an end system that is an integral part of a DetNet domain. Further details are described below. This document uses the exact DetNet flow type where it is important to distinguish the flow type; otherwise, the generic term, i.e., DetNet flow is used.
The native data flow between the source/destination end systems is referred to as application-flow (App-flow) as shown in Figure 1. The traffic characteristics of an App-flow can be CBR (constant bit rate) or VBR (variable bit rate) and can have L1 or L2 or L3 encapsulation (e.g., TDM (time-division multiplexing), Ethernet, IP).
[Note: Interworking function for L1 application-flows is out-of-scope in this document, therefore, not depicted in figures.]
+-----------+ +-----------+
/ Application \<---native data flow--->/ Application \
|-------------| (App-flow) +-------------+
| End | | End |
| System | | System |
| -------- | | -------- |
| | NIC | | | | NIC | |
+-----+-------+ +-------+-----+
| ____ ____ |
| __/ \_____/ \____ |
| / \ |
+--------| DetNet |-----+
\ Networking /
\ ___ __ _/
\__/ \___/ \____/
Figure 1: End systems connected to DetNet
An end system may or may not be DetNet transport layer aware or DetNet service layer aware, see [I-D.ietf-detnet-architecture]. That is, an end system may or may not contain DetNet specific functionality. End systems with DetNet functionalities may have the same or different transport layer as the connected DetNet domain. Grouping of end systems are shown in Figure 2. (Note: A "TSN end system" of [I-D.ietf-detnet-dp-alt] is an example for a "DetNet unaware end system".)
+---------------------------------------+
No DetNet | DetNet unaware |
functions | end system |
+-------------------+-------------------+
With DetNet | DetNet aware | DetNet |
Functions | end system | end system |
+-------------------+-------------------+
Some DetNet DetNet Service
Service Layer and Transport Layer
Figure 2: Grouping of end systems
End system(s) may or may not be directly connected to the DetNet transport network. This document assumes direct connection in the remaining part. The end system types are: Figure 3. A DetNet-UNI ("U" on Figure 3) is assumed in this document to be a packet-based reference point and provides connectivity over the DetNet domain. A DetNet-UNI may add forwarding technology specific encapsulation to the App-flow / DetNet-s-flow and transport it as a DetNet-st-flow over the network.
These end systems are shown in
DetNet unaware DetNet aware
end system end system
_ _
/ \ / \
/App\ /App\
/--O--\<-----App-flow-- /-(O)-\
| | |--V--|<--DetNet-s-flow--
| NIC | <-DetNet-st-flow- | NIC | <-DetNet-st-flow-
+--+--+ +----+ +--+--+ +----+
| | |T-PE| | | |T-PE|
+--------U +- +--------U +-
| | | | | |
| +----+ | +----+
Attachment Edge Attachment Edge
Circuit Node Circuit Node
DetNet
end system
_
/ \
/App\
/-(O)-\
|--W--|<--DetNet-st-flow--
| NIC |
+--+--+ +----+
| | |S-PE|
+--------+ +-
| | |
| +----+
Internal Relay
Link Node
Figure 3: Types of end systems
[Note: DetNet aware end systems can be also treated as a special mix of a DetNet unaware system and a DetNet end system. It is similar to a DetNet end system as its data flow contains DetNet attributes, however, those attributes cannot be used directly inside the DetNet domain, e.g., due to the different transport layer. Therefore, it is also similar to DetNet unaware end systems as it must be connected to a DetNet edge node to adapt, e.g., the encapsulation of the DetNet flow to the forwarding paradigm of the DetNet domain. A typical example showing a DetNet aware end system can be the following scenario: an end system encapsulates its App-flow in IP-RTP packets. It assumes a single connection to its peer, therefore, the Seq-num field is not used by the end-system. It is connected to an MPLS-based DetNet domain that has redundant paths and applies service protection via the duplication and elimination functionality. As per [I-D.ietf-detnet-architecture], the addition or removal of packet sequencing information is the job of a DetNet edge node. As forwarding is MPLS-based, the Seq-num required for service protection is created and added to the DetNet-s-flow by the DetNet edge node (in the PW control-word field).]
The DetNet service can be defined as a service that provides a capability to carry a unicast or a multicast data flow for an application with constrained requirements on network performance, e.g., low packet loss rate and/or latency.
Delay and loss parameters are somewhat correlated because the effect of late delivery can be equivalent to loss. However, not all applications require hard limits on both parameters (delay and loss). For example, some real-time applications allow graceful degradation if loss happens (e.g., sample-based processing, media distribution). Some others may require high-bandwidth connections that make the usage of techniques like flow duplication economically challenging or even impossible. Some applications may not tolerate loss, but are not delay sensitive (e.g., bufferless sensors).
Primary transport service attributes for DetNet transport are:
Time/loss sensitive applications may have somewhat special requirements especially for loss (e.g., no loss in two consecutive communication cycles; very low outage time, etc.).
Two connectivity types are distinguished: point-to-point (p2p) and point-to-multipoint (p2mp). Connectivity type p2mp is created by a transport layer function (e.g., p2mp LSP). (Note: mp2mp connectivity is a superposition of p2mp connections.)
The figures below show the DetNet service related reference points and components for various end system scenarios (Figure 4 and Figure 5).
DetNet DetNet
unaware aware
end system end system
_ _
/ \ / \
/App\ +----DetNet-UNI (U) DetNet-UNI (U) /App\
/--O--\ | [DetNet-NNI (N)] /-(O)-\
| | | | |--V--|
| NIC | v ________ | | NIC |
+--+--+ _____ / \ +------+--+---------+ +--+--+
| / \__/ \ | | | |
| / +----+ +----+ \_____ v | | |
| | / | | | | \_______ | | |
+--------U PE +----+ P +----+ \ v _ v |
| | | | | | | | ___/ \ |
| | +--+-+ +----+ | +----+ | / \_ | |
| \ | | | | | / \ | |
| \ | +----+ +--+-+ +--+PE U/N------N/U U------+
| \ | | | | | | | | | \_ _/ |
| \ +---+ P +----+ P +--+ +----+ | \____/ |
AC \___ | | | | / AC
\ +----+__ +----+ Domain-1 Domain-2
| \_____/ \___________/ |
| |
| | End-to-End-Service | | | |
<-------------------------------------------------------------------->
| | | | | |
| | DetNet-Service | | | |
| <--------------------------------> <---------> |
| <--------------------------------N---------N---------> |
| | | | | |
| DetLink| | | | |
<--------> <---------> <------>
| | DetNetwork | | | |
| <--------------------------------> <---------> |
| <----------------------------------------------------> |
| | | | | |
Figure 4: DetNet Service Reference Model (multi-domain)
DetNet
unaware
end system
_
/ \
/App\ +----DetNet-UNI (U)
/--O--\ | DetNet
| NIC | v ________ end system
+--+--+ _____ / \ _
| / \__/ \ / \
| / +----+ +----+ \_____ /App\
| | / | | | | \_______/-(O)-\
+--------U PE +----+ P +----+ |--W--|
| | | | | | | | NIC |
| | +--+-+ +----+ | +--+--+
| \ | | | |
| \ | +----+ +--+-+ +---------+ |
| \ | | | | | | |
| \ +---+ P +----+ P +----+ _/
AC \___ | | | | DetNet /
\ +----+__ +----+ Domain/
| \_____/ \_______________/ |
| |
| | End-to-End-Service |
<--------------------------------------------->
| | DetNet-Service | |
| <------------------------------------>
| | | |
| DetLink| | DetLink |
<--------> DetNetwork <-------------->
| <---------------------> |
| | | |
Figure 5: DetNet Service Reference Model (single domain)
From service model design perspective a fundamental question is the location of the service endpoints, i.e., where the service starts and ends. The following reference points can be distinguished for the DetNet use cases:
Data flow endpoints ("O", "V" and "W" in Figure 4 and Figure 5) are more challenging from control perspective as they are internal reference points of end systems. They are providing access to deterministic transport for the native data flow (App-flow).
DetNet-UNI and DetNet-NNI ("U" and "N" in Figure 4) are assumed in this document to be packet-based reference points and provide connectivity over the packet network and between domains. A DetNet-UNI adds networking technology specific encapsulation to the App-flow / DetNet-s-flow in order to transport it as a DetNet-st-flow over the network. There are many similarities regarding the functions of a DetNet-st-flow endpoint ("W") and a DetNet-UNI ("U") but there may be some differences. For example, in-order delivery is expected in end system internal reference points, whereas it is considered optional over the DetNet-UNI.
Using the above defined reference points, two major service scenarios can be identified:
End-to-End-Service is defined between App-flow endpoints, whereas DetNet-Service is between DetNet-st-flow endpoints. This allows the peering of same layers/functions.
Three possible DetNet flow formats are distinguished for unambiguous references:
The follwoing building blocks are used as reference to service components/segments:
Any DetNet service scenario can be described using DetLink and DetNetwork components/segments. For example, the service between the App-flow endpoints in Figure 4 can be composed as a DetLink-1 (between the end system on the left and the edge node of Domain-1) + DetNetwork-1 (of Domain-1) + DetLink-2 (between Domain-1 and Domain-2) + DetNetwork-2 (of Domain-2) + DetLink-3 (between edge node of Domain-2 and the end system on the right).
The three DetNet functions (congestion protection, explicit routes, service protection) require two data flow related attributes to work properly:
These attributes are extracted from the ingress packets of the node [I-D.ietf-detnet-architecture]. Flow-ID is used by all the three DetNet functions, but sequence number is used only by the duplicate elimination functionality.
Flow-ID must be unique per network domain. Its encoding format is specific to the forwarding paradigm of the domain and to the capabilities of intermediate nodes to identify data flows. For example, in case of "PW over MPLS", one option is to construct the Flow-ID by the PW label and the LSP label (denoted as [PW-label;LSP-label]). In such a case, intermediate P nodes have to check all labels to identify a DetNet flow, what may not be a valid option in some deployment scenarios. Another possible option is to use a dedicated LSP per data flow, so the LSP label itself can be used as a Flow-ID (denoted as [LSP-label]). In such a case, the intermediate P nodes do not have to check the whole label stack to recognize a data flow (DetNet flow), however, it results in larger L-FIB tables on the MPLS nodes.
[Note: Seq-num requires a control-word in the label stack in MPLS domains, which should be recognized by intermediate S-PE (relay) nodes.]
The DetNet network reference model is shown in Figure 6 for a DetNet-Service scenario (i.e. between two DetNet-UNIs). In this figure, the end systems ("A" and "B") are connected directly to the edge nodes of the PSN ("PE1" and "PE2"). End-systems participating DetNet communication may require connectivity before setting up an App-flow that requires the DetNet service. Such a service instance and the one dedicated for DetNet service share the same attachment circuit. Packets belonging to a DetNet flow are selected by a filter configured on the attachment circuit ("F1" and "F2"). As a result, data flow specific attachment circuits ("AC-A + F1" and "AC-B + F2") are terminated in the flow specific service instance ("SI-1" and "SI-2"). A PSN tunnel is used to provide connectivity between the service instances. The encapsulation used over the PSN tunnel are described in [I-D.ietf-detnet-dp-alt].
The PSN tunnel is used to transport exclusivelly the packets of the DetNet flow between "SI-1" and "SI-2". The service instances are configured to implement a flow specific routing or bridging function depending on what connectivity the participating end systems require (L3 or L2). The service instance and the PSN tunnel may or may not be shared by multiple DetNet flows. Sharing the service istance by multiple DetNet flows requires properly populated forwarding tables of the service instance.
Serving regular traffic and DetNet flows by the same service instance is out-of-scope in this draft, but some related thoughts are described in Annex 1. Such a combination can provide the required connectivity before setting up a DetNet service.
AC-A AC-B
<-----> <-------- PSN tunnel --------> <----->
+---------+ ___ _ +---------+
| +----+ | / \/ \_ | +----+ |
"A" ----F1+ | | / \ | | +F2---- "B"
| | +==========+ PSN +========+ | |
| |SI-1| | \__ _/ | |SI-2| |
| +----+ | \____/ | +----+ |
|PE1 | | PE2|
+---------+ +---------+
Figure 6: DetNet network reference model
[Note: There are differences in the usage of a "packet PW" for DetNet traffic compared to the network model described in [RFC6658]. In the DetNet scenario, the packet PW is used exclusivelly by the DetNet flow, whereas [RFC6658] states: "The packet PW appears as a single point-to-point link to the client layer. Network-layer adjacency formation and maintenance between the client equipments will follow the normal practice needed to support the required relationship in the client layer ... This packet pseudowire is used to transport all of the required layer 2 and layer 3 protocols between LSR1 and LSR2".]
Transport of DetNet flows over multiple technology domains may require that lower layers are aware of specific flows of higher layers. Such an "exporting of flow identification" (see section 4.7 in [I-D.ietf-detnet-architecture]) is needed each time when the forwarding paradigm is changed on the transport path (e.g., two LSRs are interconnected by a L2 bridged domain, etc.). The three main forwarding methods considered for deterministic networking are:
The simplest solution for generalized flow identification could be to define a unique Flow-ID triplet per DetNet flow (e.g., [IP: "IPv6-flow-label"+"IPv6-address"; MPLS: "PW-label"+"LSP-label"; Ethernet: "VLAN-ID"+"MAC-address"). This triplet can be used by the DetNet encoding function of technology border nodes (where forwarding paradigm changes) to adapt to capabilities of the next hop node. They push a further (forwarding paradigm specific) Flow-ID to packet header ensuring that flows can be easily recognized by domain internal nodes. This additional Flow-ID might be removed when the packet leaves a given technology domain.
[Note: Seq-num attribute may require a similar functionality at technology border nodes.]
The additional (domain specific) Flow-ID can be
so that it must be unique inside the given domain. Note, that the Flow-ID added to the App-flow is still present in the packet, but transport nodes may lack the function to recognize it; that's why the additional Flow-ID is added (pushed).
IP nodes and MPLS nodes are assumed to be configured to push such an additional (domain specific) Flow-ID when sending traffic to an Ethernet switch (as shown in the examples below).
Figure 7 shows a scenario where an IP end system ("IP-A") is connected via two Ethernet switches ("ETH-n") to an IP router ("IP-1").
IP domain
<-----------------------------------------------
+======+ +======+
|L3-ID | |L3-ID |
+======+ /\ +-----+ +======+
/ \ Forwards as | |
/IP-A\ per ETH-ID |IP-1 | Recognize
Push ------> +-+----+ | +---+-+ <----- ETH-ID
ETH-ID | +----+-----+ |
| v v |
| +-----+ +-----+ |
+------+ | | +---------+
+......+ |ETH-1+----+ETH-2| +======+
.L3-ID . +-----+ +-----+ |L3-ID |
+======+ +......+ +======+
|ETH-ID| .L3-ID . |ETH-ID|
+======+ +======+ +------+
|ETH-ID|
+======+
Ethernet domain
<---------------->
Figure 7: IP nodes interconnected by an Ethernet domain
End system "IP-A" uses the original App-flow specific ID ("L3-ID"), but as it is connected to an Ethernet domain it has to push an Ethernet-domain specific flow-ID ("VID + multicast MAC address", referred as "ETH-ID") before sending the packet to "ETH-1" node. Ethernet switch "ETH-1" can recognize the data flow based on the "ETH-ID" and it does forwarding towards "ETH-2". "ETH-2" switches the packet towards the IP router. "IP-1" must be configured to receive the Ethernet Flow-ID specific multicast stream, but (as it is an L3 node) it decodes the data flow ID based on the "L3-ID" fields of the received packet.
Figure 8 shows a scenario where MPLS domain nodes ("PE-n" and "P-m") are connected via two Ethernet switches ("ETH-n").
MPLS domain
<----------------------------------------------->
+=======+ +=======+
|MPLS-ID| |MPLS-ID|
+=======+ +-----+ +-----+ +=======+ +-----+
| | Forwards as | | | |
|PE-1 | per ETH-ID | P-2 +-----------+ PE-2|
Push -----> +-+---+ | +---+-+ +-----+
ETH-ID | +-----+----+ | \ Recognize
| v v | +-- ETH-ID
| +-----+ +-----+ |
+---+ | | +----+
+.......+ |ETH-1+----+ETH-2| +=======+
.MPLS-ID. +-----+ +-----+ |MPLS-ID|
+=======+ +=======+
|ETH-ID | +.......+ |ETH-ID |
+=======+ .MPLS-ID. +-------+
+=======+
|ETH-ID |
+=======+
Ethernet domain
<---------------->
Figure 8: MPLS nodes interconnected by an Ethernet domain
"PE-1" uses the MPLS specific ID ("MPLS-ID"), but as it is connected to an Ethernet domain it has to push an Ethernet-domain specific flow-ID ("VID + multicast MAC address", referred as "ETH-ID") before sending the packet to "ETH-1". Ethernet switch "ETH-1" can recognize the data flow based on the "ETH-ID" and it does forwarding towards "ETH-2". "ETH-2" switches the packet towards the MPLS node ("P-2"). "P-2" must be configured to receive the Ethernet Flow-ID specific multicast stream, but (as it is an MPLS node) it decodes the data flow ID based on the "MPLS-ID" fields of the received packet.
This document describes DetNet service model.
N/A.
N/A.
The authors wish to thank Lou Berger, Norman Finn, Jouni Korhonen and the members of the data plane design team for their various contributions, comments and suggestions regarding this work.
This Annex contains some thoughts about scenarios where the service instance is shared by DetNet and regular traffic.
In case of a L2 VPN transport, the service instance implements bridging. In MPLS-based PSN, there is a full mesh of PWs between service instances of PE nodes. Adding DetNet flows to the network results in a somewhat modified PW structure, as a DetNet flow requires its unique Flow-ID to be encoded in the labeled packet.
+---------+
| PE2|
| +----+ |
PW-12 | |SI-2| |
+----------------+ | |
+-----|---+ | +-+--+ |
| +--+-+ | +---|-----+
"A" ------+ | | |
| |SI-1| | |
| +-+-++ | | PW-23
|PE1 . | | |
+----.-|- + |
. | + --|-----+
. | PW-13 | +-+--+ |
. +---------------+ | |
. | | +------ "B"
+.................+SI-3| |
PW-AB | +----+ |
| PE3|
+ --------+
Figure 9: DetNet L2 VPN Service
Figure 9 shows a scenario where there is a DetNet flow between the end systems ("A" and "B"). "SI-n" denotes the L2 VPN service instance of "PEn". Regular traffic of the L2 VPN instance use "PW-12", "PW-13" and "PW-23". However, for transport of DetNet traffic between "A" and "B" a separate PW ("PW-AB") has to be used. "PW-AB" is a somewhat special PW (called here "virtual PW") and it is treated differently than PWs used by regular traffic (i.e., PW-13, PW-12, and PW-23). Namely, "PW-AB" is used exclusivelly by the DetNet flow between "A" and "B". "PW-AB" does not participate in flooding and no MAC addresses are associated with it (not considered for the MAC learning process). "PW-AB" may use the same LSP as "PW-13" or a dedicated one.
Regular traffic between "A" and "B" has an encapsulation [PW-13_label ; LSP_label], whereas DetNet flow has [PW-AB_label ; LSP_label].
In case of a L3 DetNet service, the service instance implements routing. In MPLS-based PSN, such a "routing service" can be provided by IP VPNs ([RFC4364]). However, the IP VPN service adds only a single label (VPN label) during forwarding, therefore, the label stack does not contain a "control word" (i.e., there is no field to encode a sequence number). Therefore, transport of DetNet flows requires the combination of IP VPN and PW technologies.
Adding DetNet flows to the network results in a somewhat modified label stack structure, as a DetNet flow requires its packet PW encapsulation ([RFC6658]).
+---------+
| PE2|
| +----+ |
VPN-12 | |SI-2| |
+----------------+ | |
+-----|---+ | +-+--+ |
| +--+-+ | +---|-----+
"A" ------+ | | |
| |SI-1| | |
| +-+-++ | | VPN-23
|PE1 . | | |
+----.-|- + |
. | + --|-----+
. | VPN-13 | +-+--+ |
. +---------------+ | |
. | | +------ "B"
+.................+SI-3| |
PW-AB | +----+ |
| PE3|
+ --------+
Figure 10: DetNet L3 VPN Service
Figure 10 shows a scenario where there is a DetNet flow between the end systems ("A" and "B"). "SI-n" denotes the L3 VPN service instance of "PEn". Regular traffic of the L3 VPN instance use as service label "VPN-12", "VPN-13" and "VPN-23". However, for transport of DetNet traffic between "A" and "B" a PW ("PW-AB") has to be used, what ensures that DetNet flow can be recognized by intermediate P nodes and a control world can be also present. "PW-AB" is used exclusivelly by the DetNet flow between "A" and "B". "PW-AB" may use the same LSP as regular traffic (labeled by "VPN-13") or a dedicated one.
Regular traffic between "A" and "B" has an encapsulation [VPN-13_label ; LSP_label], whereas DetNet flow has [PW-AB_label ; LSP_label].
Sophisticated QoS mechanisms are available in Layer 3 (L3), see, e.g., [RFC7806] for an overview. Although, Layer 2 (L2) QoS and queuing used to be simpler; it has been evolving, it is now equipped with Time-Sensitive Networking (TSN) features [IEEE8021TSN]. The TSN features may be beneficial or even essential for DetNet flows if Layer 2 links or sub-networks are included in their path. Therefore, it is worth investigating the problems arising when both Layer 3 and Layer 2 QoS features are supported by a node; even without diving deep into solution/implementation details.
In IEEE Std 802.1Q-2005, eight traffic classes are supported, allowing separate queues for each priority as illustrated in Figure 11. Any traffic class-based transmission selection algorithm can be implemented in addition to the strict priority algorithm mandated by IEEE Std 802.1Q-2005. The priority information is encoded in the 3-bit field carried in a tag in the frame header. Note that the IEEE 802.1Q architecture specifies queuing at the output port; however, implementations may differ. Consequently, the following figures only show the queuing at the output port that is selected by the forwarding decision for the transmission of a frame.
| | | | |
+---v---+ +---v---+ +---v---+ +---v---+ +---v---+
| Queue | | Queue | | Queue | | Queue | | Queue |
| for | | for | | for | | for | ... | for |
|Traffic| |Traffic| |Traffic| |Traffic| |Traffic|
|Class 7| |Class 6| |Class 5| |Class 4| |Class 0|
+---+---+ +---+---+ +---+---+ +---+---+ +---+---+
| | | | |
+---v----------v----------v----------v-------------v---+
| Transmission Selection |
+--------------------------+---------------------------+
|
v
Figure 11: Queuing in IEEE 802.1Q-2005
The Layer 2 QoS architecture has been evolving, see, e.g., IEEE Std 802.1Q-2014 [IEEE8021Q], which specifies the Credit-Based Shaper (originally specified by IEEE Std 802.1Qav). There are recent IEEE 802.3 and 802.1 standards and ongoing projects to enhance the QoS supported by Ethernet and Layer 2 networks. For instance, frame preemption is specified by IEEE Std 802.3br ([IEEE8023br], to be amended to [IEEE8023]) and IEEE Std 802.1Qbu ([IEEE8021Qbu], to be amended to [IEEE8021Q]) where time-critical (express) frames can suspend the transmission of non-time-critical (preemptable) frames while one or more time-critical frames are transmitted. Another recently published specification is IEEE Std 802.1Qbv [IEEE8021Qbv], which specifies time-aware queue-draining controlled by transmission gates in order to schedule the transmission of frames relative to a known timescale, which can be provided by time synchronization. The architecture extended with time-aware queuing and frame preemption is illustrated in Figure 12. These time-sensitive networking extensions provide deterministic behavior in Layer 2 networks. The ongoing IEEE 802.1 projects provide further extensions to the QoS architecture, e.g., ingress filtering and policing (P802.1Qci), cyclic queuing and forwarding (P802.1Qch), and asynchronous traffic shaping (P802.1Qcr), see [IEEE8021TSN].
...express traffic...|..........preemtable traffic......
| | | | | .
+---v---+ +---v---+ +---v---+ +---v---+ +---v---+ .
| Queue | | Queue | | Queue | | Queue | | Queue | .
| for | | for | | for | | for | ... | for | .
|Traffic| |Traffic| |Traffic| |Traffic| |Traffic| .
|Class 7| |Class 6| |Class 5| |Class 4| |Class 0| .
+---+---+ +---+---+ +---+---+ +---+---+ +---+---+ .
| | | | | .
+---v---+ +---v---+ +---v---+ +---v---+ +---v---+ .
|Transm.| |Transm.| |Transm.| |Transm.| |Transm.| .
| Sel. | | Sel. | | Sel. | | Sel. | ... | Sel. | .
| Alg. | | Alg. | | Alg. | | Alg. | | Alg. | .
+---+---+ +---+---+ +---+---+ +---+---+ +---+---+ .
| | | | | 802.1Q
+---v---+ +---v---+ +---v---+ +---v---+ +---v---+ .
|Transm.| |Transm.| |Transm.| |Transm.| |Transm.| .
| Gate | | Gate | | Gate | | Gate | ... | Gate | .
+---+---+ +---+---+ +---+---+ +---+---+ +---+---+ .
| | | | | .
+---v----------v---+ +---v----------v-------------v---+ .
| Transmission | | Transmission | .
| Selection | | Selection | .
+--------+---------+ +----------------+---------------+ .
| | --x--
+--------v---------+ +----------------v---------------+ .
| Express MAC | | Preemptable MAC | .
+--------+---------+ +----------------+---------------+ .
| | 802.3
+--------v-----------------------------v---------------+ .
| MAC merge sublayer | .
+--------------------------+---------------------------+ .
| .
+--------------------------v---------------------------+ .
| PHY (unaware of preemption) | .
+------------------------------------------------------+ .
Figure 12: L2 queuing and frame preemption
A QoS architecture integrating both Layer 3 and Layer 2 features is necessary to exploit the benefits provided by the different layers if a DetNet network includes link(s) or sub-network(s) equipped with TSN features. For instance, it can be crucial for a time-critical DetNet flow to leverage TSN features in a Layer 2 sub-network in order to meet the DetNet flow's requirements, which may be spoiled otherwise.
Figure 13 provides a theoretical illustration for the integration of the Layer 3 and Layer 2 QoS architecture. The figure only shows the queuing after the routing decision. The figure also illustrates potential implementation dependent borders (Brdr). The borders shown in the figure are critical in the sense that the high priority DetNet flows have to be transferred via a different Service Access Points (SAPs) through these borders than the low priority (background) flows. Having a single SAP for these very different traffic types may result in possible QoS degradation for the DetNet flows because packets of other flows could delay the transmission of DetNet packets. For instance, different SAPs are needed for the DetNet flows and other flows when they get to Layer 3 queuing after the routing decision via Brdr-d. Furthermore, a different SAP is needed for DetNet packets than other packets when they get to Layer 2 queuing from Layer 3 queuing via Brdr-c. Similarly, different SAPs are needed for the express and for the preemptable frames when they get to the MAC layer from Layer 2 queuing via Brdr-b, which is provided by the IEEE 802.1Q architecture as shown in Figure 12. It depends on the implementation whether or not Brdr-a exists.
..priority (DetNet)..|............low priority......... .
| traffic | | traffic | .
xxx|xxxxxxxxxxx|xxxxxxxxxxx|xxxxxxxxxxxxxxxxxxxxxxx|xxxxx Brdr-d
| | | | .
+--v--+ +--v--+ +--v--+ +--v--+ .
|Queue| ... |Queue| |Queue| ... |Queue| .
| i | | j | | m | | n | .
+--+--+ +--+--+ +--+--+ +--+--+ .
| | | | L3
+--v-----------v--+ +--v-----------------------v--+ .
| Selection | | Selection | .
+--+-----------+--+ +--+---------+----------------+ .
| | | | | .
xxx|xxxxxxxxxxx|xxxxxxxxxxx|xxxxxxxxx|xxxxxxxxxxxxx|xxxxx Brdr-c
| | | | | .
+--v----+ +---v---+ +----v--+ +---v---+ +---v---+ .
| Queue | | Queue | | Queue | | Queue | ... | Queue | .
| (7) | | (6) | | (5) | | (4) | | (0) | .
+---+---+ +---+---+ +---+---+ +---+---+ +---+---+ .
| | | | | .
+---v---+ +---v---+ +---v---+ +---v---+ +---v---+ .
|Transm.| |Transm.| |Transm.| |Transm.| |Transm.| .
|Sel. A.| |Sel. A.| |Sel. A.| |Sel. A.| ... |Sel. A.| .
+---+---+ +---+---+ +---+---+ +---+---+ +---+---+ .
| | | | | L2
+---v---+ +---v---+ +---v---+ +---v---+ +---v---+ .
| Gate | | Gate | | Gate | | Gate | ... | Gate | .
+---+---+ +---+---+ +---+---+ +---+---+ +---+---+ .
| | | | | .
+---v----------v---+ +---v----------v-------------v---+ .
|Transmission Sel. | | Transmission Selection | .
+--------+---------+ +----------------+---------------+ .
xxxxxxxxx|xxxxxxxxxxxxxxxxxxxxxxxxxxxxx|xxxxxxxxxxxxxxxxxx Brdr-b
+--------v---------+ +----------------v---------------+
| Express MAC | | Preemptable MAC |
+--------+---------+ +----------------+---------------+
| |
+--------v-----------------------------v---------------+
| MAC merge sublayer |
+--------------------------+---------------------------+
xxxxxxxxxxxxxxxxxxxxxxxxxxx|xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Brdr-a
+--------------------------v---------------------------+
| PHY |
+------------------------------------------------------+
Figure 13: Integrated L3/L2 queuing architecture and implementation options
Not all the functions depicted in Figure 13 are necessarily present in an implementation. A function may be combined with another one or may be completely missing. For instance, it may be the case that there is no Layer 3 queuing for DetNet packets, but they get directly to the Layer 2 queues. Alternatively, an implementation may combine the Layer 3 queues and the Layer 2 queues such that there is a single level of queues. There are further alternatives in addition to the ones mentioned here.
Different implementation approaches, i.e., different node designs are illustrated in Figure 14 and Figure 15. Figure 14 illustrates a monolithic node design where there is a single feature rich chip and relatively simple interfaces. The single chip implements all routing (and/or bridging) features as well as almost all QoS features. (Some aspects of frame preemption may be implemented on the interface.) Figure 15 illustrates a linecard-based design where each linecard has its own chip, which implements routing and QoS features.
+---------+ +----------------+ +---------+
|Interface| | | |Interface|
+---------+ | | +---------+
| |
+---------+ | | +---------+
|Interface| | | |Interface|
+---------+ | | +---------+
| Chip |
. | | .
. | | .
. | | .
| |
+---------+ | | +---------+
|Interface| | | |Interface|
+---------+ +----------------+ +---------+
Figure 14: Monolithic node design
+-----------------+ +---------+ +-----------------+
+---------+ +----+| | | |+----+ +---------+
|Interface| | || | | || | |Interface|
+---------+ | || | | || | +---------+
| . | || | | || | . |
| . |Chip|| | | ||Chip| . |
| . | || | | || | . |
+---------+ | || | | || | +---------+
|Interface| | || | | || | |Interface|
+---------+ +----+| | | |+----+ +---------+
+-----------------+ | | +-----------------+
. | | .
. |Backplane| .
. | | .
+-----------------+ | | +-----------------+
+---------+ +----+| | | |+----+ +---------+
|Interface| | || | | || | |Interface|
+---------+ | || | | || | +---------+
| . | || | | || | . |
| . |Chip|| | | ||Chip| . |
| . | || | | || | . |
+---------+ | || | | || | +---------+
|Interface| | || | | || | |Interface|
+---------+ +----+| | | |+----+ +---------+
+-----------------+ +---------+ +-----------------+
Figure 15: Linecard-based node design
Different implementations have different physical borders, which imply that different borders out of the ones illustrated in Figure 13 exist in a given implementation. For instance, there is no physical border corresponding to Brdr-d (Figure 13) in the monolithic implementation approach (Figure 14). However, Brdr-d is inevitably there in the linecard-based implementation approach (Figure 15) due to the backplane.
Altogether, it is essential to leverage the benefits of both Layer 3 and Layer 2 QoS features if Layer 2 is also involved in the support of a DetNet flow. Exploiting both layers requires attention to the aspects explained related to Figure 12. Nevertheless, the actually important aspects largely depend on the implementation approach chosen, see, e.g., Figure 14 vs. Figure 15.