Abstract
Traditional IP multicast (IPMC) maintains state per IPMC group in core devices to distribute one-to-many traffic along tree-like structures through the network. This limits its scalability because whenever subscribers of IPMC groups change, forwarding state in the core network needs to be updated. Bit Index Explicit Replication (BIER) has been proposed by the IETF for efficient transport of IPMC traffic without the need of IPMC-group-dependent state in core devices. However, legacy devices do not offer the required features to implement BIER. P4 is a programming language which follows the software-defined networking (SDN) paradigm. It provides a programmable data plane by programming the packet processing pipeline of P4 devices. The contribution of this article is threefold. First, we provide a hardware-based prototype of BIER and BIER fast reroute (BIER-FRR) which leverages packet recirculation. Our target is the P4-programmable high-performance switching ASIC Tofino; the source code is publicly available. Second, we perform an experimental evaluation, with regard to failover time and throughput, which shows that up to 100 Gb/s throughput can be obtained and that failures affect BIER forwarding for less than 1 ms. However, throughput can decrease if switch-internal packet loss occurs due to missing recirculation capacity. As a remedy, we add more recirculation capacity by turning physical ports into loopback mode. To quantify the problem, we derive a prediction model for reduced throughput whose results are in good accordance with measured values. Third, we provide a provisioning rule for recirculation ports, that is applicable to general P4 programs, to avoid switch-internal packet loss due to packet recirculation. In a case study we show that BIER requires only a few such ports under realistic mixes of unicast and multicast traffic.
Highlights
IP multicast (IPMC) has been proposed to efficiently distribute one-to-many traffic, e.g. for IPTV, multicast VPN, commercial stock exchange, video services, public surveillance data distribution, emergency services, telemetry, or content-delivery networks, by forwarding only one packet per link
IPMC traffic is organized in IPMC groups which are subscribed by hosts
This bit-forwarding egress routers (BFERs) is used for lookup in the Bit Index Forwarding Table (BIFT), which results in the F-BM 1010 and the NH Bit-forwarding routers (BFRs) 2
Summary
TO P4 we briefly review fundamentals of P4 [3]. First, we give an short overview of the P4 processing pipeline. The packet clone is processed by a second BIER control block in the egress pipeline which sets the BitString of the packet copy to the remaining bits. Since the egress port of the packet clone is a recirculation port, the packet is recirculated, i.e., it is processed by the ingress pipeline again. In the first and second pipeline iteration the original BIER packet is sent through a physical egress port towards a NH and the copied BIER packet is recirculated by sending the packet copy to a recirculation port. In the last iteration when the remaining bits contain no activated bits anymore, no further packet copy is required and only the original BIER packet is sent through the egress port. In general, a BIER packet with n NHs, has to be recirculated n − 1 times and the first NH can be served without packet recirculation
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