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VIEWPOINT The Protocol Wedge Protocol, while giving us the benefits of security and reliability, kills performance and efficiency. By Ray Alderman, Executive Director, VITA When you look at all the interconnects in the market, things can get confusing and frustrating fast. The best way to untangle this morass, as a rule of thumb, is to look at the interconnects by the distance they are designed to run (http://en.wikipedia.org/wiki/ List_of_device_bit_rates .) The closer together the connected devices reside, the “lighter” the protocol stack, and less protocol latency will be experienced. The further away the devices are, the “heavier” the protocol stack, and more protocol latency will be induced. This observed phenomenon creates a reverse “wedge”, with the apex starting at chip-tochip connections, and the wide end of the wedge at the location-tolocation end, the Internet. Chip-to-chip connections don’t need a lot of protocol to control data flow. PCI and PCIExpress are some examples. You don’t have to worry about multiple packet recipients, you don’t need to worry too much about synchronization and error handling, and you don’t have big packet headers defining the origin of the data and who handled it before it was received. In a point-topoint connection like chip-to-chip, it can only come from the trusted device on that link. Board-to-board connections have heavier protocols. RapidIO and InfiniBand are examples. You could have multiple data senders, error handling gets messier, critical data may need time-stamping, and synchronization becomes necessary. Box-to-box connections get even heavier. Ethernet is a perfect example. Not only do you have the overheads of the board-to-board connections, but now you need to verify the sender as a valid data source, you need a history of who handled the data before it was received, error handling becomes more prolific, and packet headers become huge. Finally, we have location-to-location connections. Internet Protocol (IPv6) is an example here. The protocol stack is a monster; all the legacy protocol requirements are there plus many more. Packet headers are now massive, the history of the packet across all the previous internet systems involved in its transmission is needed, and byte counts plus packet numbers are needed, too (since most large packets are broken-up into smaller packets and must be reassembled, in order, by the receiver). “DATA CENTERS AND ‘CLOUD COMPUTING’ MACHINES ARE BEGINNING TO TELL US THAT WE NEED TO REDUCE THE PROTOCOL STACK OVERHEADS OF THE BOARD-TO-BOARD AND BOX-TO-BOX INTERCONNECTS.” Nicholas Negroponte warned us about this increasing protocol-stack burden in his 1996 book, “Being Digital”. He claims that “the bits about the bits” are becoming more important that the bits themselves (the real usable data). Protocol, while giving us the benefits of security and reliability, kills performance and efficiency. Data centers and “cloud computing” machines are beginning to tell us that we need to reduce the protocol stack overheads of the board-to-board and boxto-box (or rack-to-rack) interconnects. The way to do this is to make those interconnections look more like chip-to-chip connections, with their lighter protocol stacks. RDMA (Remote Direct Memory Access) used in InfiniBand and Ethernet today are doing just that. But, IPvx protocols continue to expand and become heavier. This modifies the protocol “wedge” somewhat, and gives us much better performance and efficiency once the internet connection protocols are strippedoff when they enter the data center. Ray Alderman is the Executive Director of VITA, an ANSI-certified standards developer for high-performance computer systems and architectures used in critical embedded applications. He is a recognized authority on embedded architectures and computing. 40 Engineers’ Guide to VME, VPX & VXS 2013
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Page 3 and 4: TAKE T TAKE YOUR YO YOUR R VIRTUAL
Page 5 and 6: The only compact, rugged chassis of
Page 8 and 9: SPECIAL FEATURE VME’s Long From D
Page 10 and 11: SPECIAL FEATURE We are also in the
Page 14 and 15: Advertorial 3U VPX Solutions from E
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Page 19 and 20: Table 1: Notional VITA 65 Channel G
Page 21 and 22: Also supported is VITA 48 REDI (Rug
Page 23 and 24: Tackling the Challenge of Building
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Page 31 and 32: intended message. To get past those
Page 33 and 34: Test and Analysis Teledyne LeCroy
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