Protocols and Standards

Space link protocols are published as CCSDS Blue Books, which are Recommended Standards with an issue number and a date. A mission cites the specific issue it implements, because the frame formats change between issues.

The layers a Mars rover uses

Layer	Standard	Issue	Role
Application data unit	Space Packet Protocol, CCSDS 133.0-B-2	June 2020	The packet a science or engineering data product is broken into
Space data link, orbiter to Earth	AOS Space Data Link Protocol, CCSDS 732.0-B-4	October 2021	Transfer frames on the deep space link
Proximity link, surface to orbiter	Proximity-1 Space Link Protocol, Data Link Layer, CCSDS 211.0-B-6	July 2020	The UHF relay hop
File delivery	CCSDS File Delivery Protocol, CCSDS 727.0-B-5	July 2020	Reliable file transfer across one or more hops
Networking overlay	CCSDS Bundle Protocol, CCSDS 734.2-B-1	September 2015	Store-and-forward networking across scheduled contacts

Sources: the Blue Books themselves, [1] through [4].

Space Packet Protocol

A Space Packet is a 6 octet primary header plus a data field of 1 to 65,536 octets, so a packet is at least 7 and at most 65,542 octets, with individual projects free to set a lower maximum [1]. The primary header is four contiguous fields: packet version number, 3 bits; packet identification, 13 bits, of which 1 bit is packet type, 1 bit the secondary header flag and 11 bits the application process identifier; packet sequence control, 16 bits; and packet data length, 16 bits.

The application process identifier is the field that makes multiplexing work operationally. MER relay data arriving at Odyssey was divided into fixed length packets with a distinct identifier per rover, and the MER ground system queried the Odyssey ground system after each pass for the packets carrying its identifiers, stripped the headers and fed the result to frame synchronization, at which point the data looked as though it had arrived directly [7].

Proximity-1

Proximity-1 is the protocol on the UHF hop, and it is a session protocol rather than a framing convention. The Version-3 transfer frame carries a mandatory 5 octet header of ten fields: transfer frame version number, 2 bits; quality of service indicator, 1 bit; PDU type identifier, 1 bit; data field construction identifier, 2 bits; spacecraft identifier, 10 bits; port identifier, 3 bits; frame length, 11 bits; and frame sequence number, 8 bits, among others [2]. On the physical channel each frame is wrapped in an attached synchronization marker and an attached CRC-32.

Two services are defined: sequence controlled, which is reliable and retransmits from a sent frame queue against status reports, and expedited, which is not [2]. On Curiosity the sequence controlled link is the nominal protocol, with an unreliable bit stream mode that bypasses Proximity-1 available for off-nominal cases and for entry, descent and landing [6]. Of all MSL relay sessions flown to that point, 99.9 percent used the reliable mode, bitstream being reserved for a handful of demonstrations [10].

The session model is what enables adaptive data rates. Every link is opened by the orbiter hailing the surface vehicle, on MER always at 8 kbit/s, carrying the communications mode, data rates and coding to use [7]. Because rate is a negotiable session parameter, Proximity-1 can command a different return rate during a pass, which is how the Curiosity to MRO link tracks the changing geometry within a single overflight; only the return rate is adapted, and the forward rate stays at 32 kbit/s [6].

CCSDS File Delivery Protocol

CFDP moves files rather than packets, and it carries its own PDU header so it is independent of the underlying protocol [3]. It offers an unacknowledged mode and several acknowledged modes. In acknowledged mode the receiver informs the sender of undelivered file segments by negative acknowledgment, with sub-options ranging from immediate NAK release to deferred release in which NAKs are held until the assumed end of transmission. A transaction closure option lets unacknowledged mode still confirm delivery. Annex B specifies store and forward overlay operations, which is the multi-hop case a Mars relay chain represents.

The core Flight System ships CFDP as one of its standard applications; on the Global Precipitation Measurement observatory it was 8559 logical lines of code and 85,812 bytes of EEPROM, the largest single application in that build [9].

Delay tolerant networking, and where it has actually flown

The Bundle Protocol adds a store-and-forward overlay above the link layer, so a node can take custody of data and forward it when the next contact opens rather than requiring an end-to-end path to exist at any instant [4]. The CCSDS Blue Book is a profile of the Internet experimental RFC 5050.

DTN’s deep space flight validation is DINET, run on the Deep Impact spacecraft under the EPOXI project in October and November 2008 [5]. About 300 images were sent from JPL ground nodes to the spacecraft and automatically forwarded back over 27 days across eight transmission opportunities, exercising bundle origination, transmission, acquisition, dynamic route computation, congestion control, prioritization, custody transfer and automatic retransmission. All transmitted bundles were received without corruption.

Four validation metrics were defined in advance [5]:

Metric	Criterion	Result
Path utilization rate, configuration a	greater than 90 percent	76.2 percent raw, 97.4 percent excluding underutilized passes and non-DTN overhead
Path utilization rate, configuration b, higher loss	greater than 90 percent	72.4 percent raw, 92.5 percent on the same basis
Delivery acceleration ratio, configuration a	greater than 1.05	1.10
Delivery acceleration ratio, configuration b	greater than 1.1	1.12
Node storage utilization	zero custody refusals for depleted storage, and stabilizing utilization	satisfied, except node 10 changed by 156,816 bytes between passes 6 and 7
Multipath advantage	greater than 20 percent	35 percent

About 20 percent of the available uplink capacity was consumed by link service overhead, mainly telecommand coding, which is why the raw and corrected utilization figures differ so much [5]. Errors in the dynamic routing implementation meant the measured multipath advantage did not translate into improved delivery acceleration.

ION’s design constraints are stated explicitly and are the reason the protocol looks as it does: interplanetary downlink on the order of 256 kbit/s to 6 Mbit/s, uplink on the order of 1 to 2 kbit/s, and flight processors that are invariably slower than ground equivalents because radiation hardening lags commercial process nodes, so the cost per processing cycle is high and processors are heavily subscribed [5]. The Interplanetary Overlay Network implementation runs on VxWorks 5.4 among other targets, avoids unrestrained dynamic allocation, and uses a shared memory model rather than protected memory to keep the cost of passing data between tasks down.

The operational DTN deployment with published throughput is the International Space Station, where bundle storage covers 4.4 hours at 60 Mbit/s and measured throughput was about 15 Mbit/s down and 4 Mbit/s up [8]. No Mars surface vehicle has flown DTN operationally.