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Cut home/office device power consumption (Part 2)

Posted: 16 Jul 2013     Print Version  Bookmark and Share

Keywords:Packet classification  power consumption  eTSEC  autorespond proxy  Ethernet Controller 

Packet classification, as discussed in Part 1, ensures that the system only processes what it has to process, and therefore goes a long way to minimising power consumption in networked devices. However, further optimisation is not only possible, but desirable.

Real-world applications can contain extensive software footprints, and the time to boot is non-trivial. In systems where clock cycles are measured in nanoseconds, the time for software to boot may be measured in seconds.

Y. Agarwal [3] lists 10 seconds boot as the shortest achievable today for a PC, although Microsoft claims [8] that Windows 8 can resume an active state from its S3 sleep state in 2 seconds. Even lower boot times are achievable in embedded applications; for example, the Lineo Warp Website [9] lists an optimised boot of 1.07 seconds for X-Windows on the Armadillo 500-FX platform, and 1.9 seconds for Android on the same platform.

Even with packet classification, the arrival rate of packets that require further processing may approach the time it takes for the system to wake to process a packet. In such a scenario, the system will be continually waking and sleeping, without ever being able to spend any significant time in the lowest power dormant state.

This situation can be improved through packet accumulation, which allows multiple packets to be buffered until such time as the system is ready to wake to process them. This minimises the overhead of waking and sleeping, allowing the system to efficiently process a group of packets in bulk.

There are several caveats to be aware of when using packet accumulation:
 • The system must be able to help guarantee that its packet accumulation buffer does not overflow, regardless of whether that buffer is in dedicated on-chip SRAM in a SoC or in external DRAM. This implies that while performing packet accumulation, the system maintains a count of the number or size of received packets to help ensure that it does not exceed the available buffer, and that the system wakes before this occurs.
 • The system must be able to respond to packets within a defined maximum time, regardless of network traffic. This means that as the system starts to accumulate packets, a timer must be started. When the timer expires, the system needs to wake up, regardless of how many packets have been accumulated. This prevents network protocols from timing out if a packet is received and accumulated, but subsequently the network has relatively little relevant traffic to force the packet accumulation buffer to fill.
 • There may be certain types of packets for which it is desirable to wake immediately and process, rather than accumulate multiple packets to process. For example, for a networked printer it may be desirable to respond and accumulate multiple ARP requests before wake, but if a packet that looks like the beginning of a print job arrives, then the system should wake immediately in order to print as soon as possible. S Gobriel et. al. [2] uses heuristics to differentiate between packets that are "idle" (bufferable) and packets that are "active" (need fast response), but relatively simple deep packet inspection is also a workable solution.

Freescale's QorIQ P1022 Communications Processor [10] can classify packets with its eTSEC controller, as well as accumulate packets as needed, storing them in external DRAM, while maintaining counters in its eTSEC and timers in its interrupt controller to guarantee packet response within predefined maximum times.

Autorespond proxy
The shortcoming of packet classification and accumulation on larger networks is the amount of time spent servicing protocols such as ARP and SNMP that are required to maintain network connectivity. As an example, if it takes a system 500ms to go through a cycle of wake-up, message processing, and return-to-sleep, then even modest message frequency (<500ms) could force a system to stay permanently in a high power state.

For this reason, techniques recently introduced into network standards have added an intelligent proxy to the network interface to maintain network connectivity. The ECMA-393 standard [11] defines the concept of "Full Network Connectivity" as the ability of the computer to maintain network presence while in sleep and intelligently wake when further processing is required.

Microsoft [12] and B. Combs [13] provide a framework for protocol offload. In particular, they standardise the way that systems running Microsoft Windows 7 can allow IPv4 address resolution (ARP) and IPv6 network solicitation (NS) to be offloaded to an external Network Interface Controller (NIC), rather than the primary Windows host.

The ECMA-393 standard [14] is not tied to the Windows 7 operating system and is therefore more suitable to a wide range of embedded applications. Similar to [12] and [13], it also has the requirement for IPv4 ARP and IPv6 NS proxying. It goes further to provide options of further proxying of other protocols such as IGMP, DHCP, IPv4 SIP, IPv6 Teredo tunnelling, SNMP, mDNS, and LLMNR.

Fundamentally however, the concept of all such proxying is similar – to maintain the ECMA-393 standard 's[11] "Full Network Connectivity" by using some sort of hardware that is distinct from the primary processor in a system that would otherwise maintain network connectivity. The intent is for the proxying hardware to be much lower power than the primary processor, thereby allowing the primary processor to be in a low power state, or potentially even off, for extended periods of time.

S. Nedevschi et. al. [1] also implements several types of autorespond proxy in its "proxy_2" through "proxy_4" definitions, although there is no mention of packet accumulation. It analyses in detail the types of incoming packets and provides information as to which may be the best "low hanging fruit" protocols to proxy. This data shows that the best protocol to proxy is ARP, as it has the highest percentage of incoming broadcast traffic, and packets destined for the host cannot be ignored.

However, other packets, which they classify as both "Don't wake" because they may occur frequently, and "Mechanical response", for which a proxy autoresponse may be possible, includes the protocols SSDP, IGMP, ICMP, and NBDGM as well as ARP. All of these as well as others may be considered as candidates for an autorespond proxy, although a definitive list is highly network-dependent and an optimised proxy should be tuned to specific use cases.

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