In MPLS, packets are encapsulated with labels that add domain-specific forwarding information. Special purpose labels were introduced to trigger special behavior in MPLS nodes but their number is limited. Therefore, the IETF proposed the MPLS Network Actions (MNA) framework. It extends MPLS with new features, some of which have already been defined to support relevant use cases. This paper provides a comprehensive technological overview of MNA concepts and use cases. It compares MNA to IPv6 extension headers (EHs) that serve a similar purpose, and argues that MNA can be better deployed than EHs. It then presents P4-MNA, a first hardware implementation running at 400 Gb/s per port. Scalability and performance of P4-MNA are evaluated, showing negligible impact on processing delay caused by network actions. Moreover, the applicability of MNA is demonstrated by implementing the use cases of link-specific packet loss measurement using the alternate-marking-method (AMM) and bandwidth reservation using network slicing. We identify header stacking constraints resulting from hardware resources and from the number of network actions that must be supported according to the MNA encoding. They make an implementation for hardware that can only parse a few MPLS headers infeasible. We propose to make the number of supported network actions a node parameter and signal this in the network. Then, an upgrade to MNA is also feasible for hardware with fewer available resources. We explain that for MNA with in-stack data (ISD), some header bits must remain unchanged during forwarding, and give an outlook on post-stack data (PSD).

翻译：在MPLS中，数据包携带添加域特定转发信息的标签进行封装。引入特殊用途标签以触发MPLS节点中的特殊行为，但其数量有限。因此，IETF提出了MPLS网络动作（MNA）框架，该框架通过新功能扩展了MPLS，其中部分功能已定义以支持相关用例。本文对MNA概念及用例进行了全面的技术综述，将MNA与具有类似用途的IPv6扩展头（EH）进行比较，并论证MNA比EH更易于部署。随后，本文提出P4-MNA——首个以每端口400 Gb/s运行且基于硬件的实现。我们对P4-MNA的可扩展性与性能进行了评估，结果表明网络动作对处理延迟的影响可忽略不计。此外，通过使用备用标记法（AMM）测量链路特定丢包率以及使用网络切片预留带宽这两个用例，验证了MNA的适用性。我们识别出由硬件资源及MNA编码要求支持的网络动作数量导致的头部堆叠限制，这使得仅能解析少量MPLS头部的硬件实现不可行。我们建议将支持的网络动作数量作为节点参数，并在网络中进行信令通告，从而使资源较少的硬件也能升级支持MNA。对于包含堆栈内数据（ISD）的MNA，我们解释了转发过程中必须保持某些头部比特不变，并给出了堆栈后数据（PSD）的未来展望。