The delivery of high priority traffic within predefined delay bounds on a slow speed last mile link is ensured by the correct QoS classification and prioritization. High priority traffic is interleaved with low priority fragments on a single link MLPPPoX bundle with LFI enabled. The classification of traffic into the correct (high or low priority) forwarding class is performed on the downstream ingress interface. However, traffic can be re-classified (re-mapped into another forwarding class) on the egress access interface of the carrier IOM, just before packets are transmitted to the BB-ISA for MLPPPoX processing. This can be achieved with a QoS sap-egress policy referenced in the LNS sla-profile.
The priority of the forwarding class in regular QoS (on IOM) is determined by the properties of the queue to which the forwarding class is mapped. Expedited, non-expedited queue type, CIR and PIR rates. In contracts, traffic prioritization in LFI domain (in BB-ISA) is determined by the outer dot1p bits that are set by the carrier IOM while transmitting packets toward the BB-ISA. The outer dot1p bits are marked based on the forwarding class information determined by classification/re-classification on ingress/carrier IOM. This marking of outer dot1p bits in the Ethernet header between the carrier IOM and the BB-ISA is fixed and defined in the default sap-egress LNS ESM policy 65537. The marking definition is as follows:
FC be -> dot1p 0
FC l2 -> dot1p 1
FC af -> dot1p 2
FC l1 -> dot1p 3
FC h2 -> dot1p 4
FC ef -> dot1p 5
FC h1 -> dot1p 6
FC nc -> dot1p 7
In LFI (on BB-ISA), dot1p bits [0,1,2 and 3] are considered low priority while dot1p bits (4,5,6 and 7) are considered high priority. Consequently, forwarding classes BE, L2, AF and L1 are considered low priority while forwarding classes H2, EF, H1 and NC are considered high priority. High priority traffic is interleaved with low priority traffic. Assuming that the packet size does not exceed maximum fragment size.
The following describes the reference points in traffic prioritization for the purpose of LFI in the 7750 SR:
Classification on downstream ingress interface (entrance point into the 7750 SR) - packets can be classified into one of the following eight forwarding classes: be, l2, af, l1, h2, ef, h1, and nc. Depending on the type of the ingress interface (access or network), traffic can be classified based on dot1p, exp, DSCP, ToS bits or ip-match criteria (dscp, dst-ip, dst-port, fragment, src-ip, src-port and protocol-id).
Re-classification on downstream access egress interface between the carrier IOM and the BB-ISA - in the carrier IOM, downstream traffic can be re-classified into another forwarding class, just before it is forwarded to the BB-ISA. Re-classification on access egress is based on the same fields as on ingress except for the dot1p and exp bits because Ethernet or MPLS headers from ingress are not carried from ingress to egress.
Marking on downstream access egress interface between the carrier IOM and the BB-ISA, after the forwarding class is available on the carrier IOM in the egress direction (toward BB-ISA), it is used to mark outer dot1p bits in the new Ethernet header that are used to transport the frame from the carrier IOM to the BB-ISA. The marking of the dot1p bits on the egress SAP between the carrier IOM and the BB-ISA cannot be changed for MLPPPoX even if the no qos-marking-from-sap command is configured under the sla-profile on egress.