[InfiniBand Configuration] Why is the InfiniBand LRH called a local header?

The InfiniBand Local Route Header (LRH) is named "Local" because its function is to route packets exclusively within a single, local InfiniBand subnet. It uses a 16-bit Local Identifier (LID) for both the source and destination ports. These LIDs are unique only within that specific subnet and are used by the subnet's switches to forward packets to their destination. For communication that needs to cross subnet boundaries (inter-subnet routing), an additional Global Route Header (GRH) is required, which uses globally unique identifiers.

B. The Subnet Manager (SM) assigns LIDs, but the header's name describes its routing scope and function, not the origin of the identifiers it contains.

C. The LRH is used for routing across the entire subnet fabric, which can involve many switches and links, not just a single, local point-to-point link.

D. The LRH contains per-packet routing information (like LIDs and Virtual Lane), not static configuration parameters for the Host Channel Adapter (HCA).

1. InfiniBand™ Architecture Specification, Volume 1, Release 1.4, Section 9.5.2, "Local Route Header (LRH)": "The Local Route Header (LRH) is the first header of all IB packets. It is used by switches to route packets between a source port and a destination port that are in the same subnet."

2. NVIDIA Mellanox, "Introduction to InfiniBand for End Users" White Paper, Section "InfiniBand Addressing", Page 5: "The Local ID, or LID, is a 16-bit address assigned to each port by the Subnet Manager when the port comes up. It is used to deliver packets within the local subnet." This section clarifies that LIDs, the core component of the LRH, are for local subnet communication.

3. Pabsti, M., & Mueller, M. S. (2015). An Introduction to the InfiniBand Architecture. In High Performance Interconnects (pp. 1-18). Springer, Cham. Section 2.3, "Addressing and Routing": "The Local Route Header (LRH) is present in every packet and contains the source and destination LIDs. It is used for routing within a subnet." (DOI not typically available for book chapters, but this is a standard academic reference).