Musings on zero-copy TCP
In light of ongoing work to improve the TCP Tx zero-copy efficiency  one begins to wonder what can be done on the Rx side. Tx zero-copy is generally easier to implement because it requires no extra HW support. It's primarily a SW exercise in keeping references to user data rather than owning a copy of it.
There had been efforts to support direct Rx to user space buffers by performing header-data splitting in the HW and sending headers (for kernel consumption) to a different buffer than the data. There are two known implementations which depend on header-data splitting (HDS).
First one, which is upstream today  depends on data payloads being received in page-sized chunks and mapping (mmap'ing?) the data into the process's virtual address space. Even though modifying the virtual address map is not cheap this scheme works well for large transfers. Importantly applications which use zero-copy are often more DRAM bandwidth constrained than CPU bound. Meaning that even for smaller transfers they may prefer to burn extra cycles modifying page tables than copying data and using up the precious DRAM transfers.
The second approach is to pre-register user memory and let the NIC DMA the data directly there . The memory in this case can be main DRAM or accelerator memory, that's not really important for us here. The model is similar to that of AF_XDP UMEMs. It still depends on header-data split as we don't want the application to be able to modify the headers, as they flow thru the networking stack and crash the kernel. Additionally the device must provide sufficient flow steering to be able to direct traffic for an application to its buffers / queues – we don't want the data to end up in memory of the wrong application. Once the TCP stack is done processing the headers, it simply tells the application where the data is.
Apart from the ability to direct the data to memory other than DRAM (compute accelerators, disks) the second approach has the advantage of not requiring page table changes and neatly page-sized payloads. It is harder to implement in SW because the stack does not necessarily have access to the payload memory. Although netgpu/zctap patches have stalled the idea may have sufficient merit to eventually come to fruition.
Both of the approaches described above deal with packet-sized chunks of data. Admittedly some HW supports data coalescing (GRO-HW/LRO) but it's limited in size, best effort, latency inducing and generally unproven at scale.
Given that we already modify the HW to support zero-copy Rx (HDS for the former case, HDS+steering for the latter) what modifications would help us get to the next level? The data coalescing can certainly be improved.
LRO depends on intelligence in the NIC and maintaining state about connections which always poses scaling challenges. At the same time modern applications most often know the parameters of the transfer upfront, so they can put processing hints in the packet.
The receiver needs to know (1) which memory pool / region to place the data into and (2) at what offset. We can assume the sender gets these parameters thru the RPC layer. Sender can insert the placement information into the packet it sends. Either wrapped in a UDP header in front of the TCP header, or as a TCP option.
One useful modification may be to express the offset in terms of the TCP sequence number. Instead of providing the absolute offset where the packet data has to land (i.e. “data of this packet has to land in memory X at offset Y”) provide a base TCP sequence number and offset. The destination address would then be computed as
address = mem_pool[packet.dma.mem_id].base + packet.dma.offset + packet.tcp.seq_no - packet.dma.base_tcp_seq
This simplifies the sender's job as it no longer has to change the offset as it breaks up a TSO super-frame into MTU-sized segments.
Last thing – we must protect applications from rogue writes. This can be done by programming an allow list of flow to memory pool pairs into the NIC. Unfortunately the full list scales linearly with the number of flows. A better approach would be to depend on a security protocol like Google's PSP . PSP hands out association IDs to the senders, and only after authentication. The PSP IDs are much smaller (4B) than a IPv6 flow key (36B) if we have to keep a full list. We can try to be clever about how we hand them out – for instance allocating the “direct write” IDs from high numbers and less trusted connections from low numbers. We can also modify PSP to maintain a key per memory pool rather than per device, or incorporate the memory region ID in the key derivation algorithm. I'm not sufficiently crypto-savvy to know if the latter would weaken the protection too much.
To sum up in the scheme I'm dreaming up we add the following fields after the PSP header:
- 16b – memory id – ID of the region;
- 64b – base offset – absolute offset within the memory region;
- 32b – TCP sequence number base for the transfer.
Since we have 16 extra bits to round up to full 128b header we can consider adding a generation number for the memory region. This could be useful if memory region configuration / page table update is asynchronous so that we can post the read request before NIC confirmed the configuration is complete. In most cases configuration should be done by the time the data arrives, if it's not we can fall back to non-zero-copy TCP.
I don't work for Netronome/Corigine any more but it's certainly something their HW can easily support with a FW update. Much like PSP itself. What an amazing piece of HW that is...