[Sis-csi] IP Header Compression

[Howard Weiss]

Keith,

When we first started working on SCPS (way, way, way back in what seems 
now like the stone ages) we were faced with this same dilemma - how to 
allow emergency mode messages in w/o having to deal with the on-board 
computer and the protocol stack.  Honestly, this was something we never 
looked at when we started researching the ways in which upper layer 
protocols would be used in a spacecraft.  I guess we were all network 
weenies who figured that the CD&H would be just /like /an IP router (or 
just an IP router if IP were used vs. NP) and all would be good.  So, 
the idea for hardware command decoders being able to handle 
emergency-mode commanding (e.g., reset, safe-mode) came up to bite us 
back in the mid-90s because of the kinds of heroic efforts that the UKs 
STRV team used to recover a tumbling micro-sat by constantly shooting 
CCSDS telecommands at it to cause it to go into safe-mode.  We realized 
at that time that if uppler layer protcols were to be used aboard 
spacecraft that the command decoders would have to be able to parse and 
handle these types of emergency mode commands outside of the protocol 
stack.  Security, however becomes paramount at the space link if such 
hardware command decoding is employed.

Howie Weiss

Keith Hogie wrote:

> Chris,
>
>   I agree we need to consider issues with small packets and low rates, 
> but how low do we need to go.  In all of the missions I have seen (non 
> deep space), the lowest data rates are 125 bps.  This is over an order 
> of magnitude difference from your 10 bps. 
>
>   For the Cislunar environment, we need to figure out what some of our 
> limits are.  Do we really want to burden the Cislunar design with 
> issues that only relate to Deep Space?
>
>   Also, for hardware decode commands like hardware reset, I'm not sure 
> if packet sizes or IP or CCSDS headers really matter.  Isn't a 
> hardware decode command just a string of bits (hopefully long enough 
> to be unique) that get grabbed by hardware without any special packet 
> knowledge.  That would mean that this bit string can be carried inside 
> any packet and the only length that matters is the length of the 
> hardware command bitstring.
>
> Keith Hogie
>
> Krupiarz, Christopher wrote:
>
>> I was curious about thoughts as to if and where we would address IP 
>> header compression in the Green Book.  On some our missions (and I 
>> think this is typical at least in deep space), if we have a reset of 
>> the system, the spacecraft may come up in an emergency mode of 
>> receiving 10 bps.  Hence, every bit is quite valuable at this point.  
>> With a CCSDS header, we're looking at 6 bytes (plus a couple of bytes 
>> if a secondary header is used).  If we move to IPv6, this becomes 
>> 40.  At 10 bps, that's an additional uplink time of 25-26 seconds 
>> before a command can be received which is long enough to envision 
>> some nightmare scenarios.  Clearly IP header compression would 
>> alleviate this concern but I'm not sure where it fits or if it is 
>> needed in this doc (I hope I didn't miss it somewhere).
>>
>> Chris
>>
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>----------------------------------------------------------------------
>  Keith Hogie                   e-mail: Keith.Hogie at gsfc.nasa.gov
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Howard Weiss
SPARTA, Inc.
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