1. Review of the SpaceFibre standard draft
St. Petersburg State University of Aerospace Instrumentation Institute for High-Performance Computer and Network Technologies
Elena Suvorova, Yuriy Sheynin

2. Broadcast mechanism (1)
Page 219(J1):
Broadcast mechanism
a. When a broadcast message arrives at a routing switch, the port on which it arrived, port of arrival, shall be compared to the port on which the previous broadcast message arrived for the same broadcast channel, previous port of arrival.
b. The broadcast message shall be accepted to be broadcasted on any of the following conditions:
1. It is the first broadcast message on that broadcast channel following reset of the routing switch.
2. The port of arrival is the same as the previous port of arrival of the last accepted broadcast.
3. It has been longer than the broadcast time-out interval since the last broadcast message on the same broadcast channel was accepted, and the LATE flag of the broadcast message is clear, indicating that the broadcast message is not late.
27th SpaceWire Working Group Meeting

3. Broadcast mechanism problems(3)
Page 219(J1):
2. The port of arrival is the same as the previous port of arrival of the last accepted broadcast.
NOTE This method guarantees that the same broadcast message cannot be received more than once by the same node. It also does not limit the rate at which broadcast messages with the same channel can be generated, except in the case of a permanent link error or network topology change, in which case, the node has to wait for the duration of the broadcast time-out interval before sending a new broadcast message to ensure that the broadcast is not discarded.
While in SpW Time-codes and Interrupts will reach destinations if at least one of paths operate, in the last SpFi specifications they may be lost even though there are some operating paths.
The source may not immediately know about a permanent link error or network topology change and a number of Broadcasts will be lost:
Delivery of Broadcasts is not guaranteed in such case
27th SpaceWire Working Group Meeting

4. Broadcast mechanism problems(4)
Page 219(J1):
3. It has been longer than the broadcast time-out interval since the last broadcast message on the same broadcast channel was accepted, and the LATE flag of the broadcast message is clear, indicating that the broadcast message is not late.
Page 79(J1):
NOTE 1 The LATE flag set to one indicates that the broadcast frame has been delayed. If the broadcast message contains time synchronisation the end user application can decide to ignore the broadcast message because it was late. If the broadcast message contains event signalling information it can still be useful to the application even though it arrives late.
We propose not to control the LATE flag in routers
27th SpaceWire Working Group Meeting

5. Broadcast mechanism problems(2)
Page 219(J1):
2. The port of arrival is the same as the previous port of arrival of the last accepted broadcast.
NOTE This method guarantees that the same broadcast message cannot be received more than once by the same node. It also does not limit the rate at which broadcast messages with the same channel can be generated, except in the case of a permanent link error or network topology change, in which case, the node has to wait for the duration of the broadcast time-out interval before sending a new broadcast message to ensure that the broadcast is not discarded.
But this method does not provide protection against an Babbling idiot.
(Only Broadcast Bandwidth Credit Counter protects the network from a Broadcast Babbling idiot, but in worst case a Broadcast will be sent after every several words (dependently on Normalised Expected Broadcast Bandwidth ).
If Broadcasts from a Babbling idiot has highest priority, Broadcasts from other sources would be blocked)
27th SpaceWire Working Group Meeting

6. Scheduled QoS problems (1)
a. Time shall be split into 64 time-slots of equal duration specified at the system level.
For many Networks fixed quantity of 64 timeslots unacceptable
On-board systems engineers used to calculate number and duration of time slots according to their applications.
They will not accept fixing the number of time slots to 64 (or any other fixed number).
It is inconvenient also:
Example 1:
The Network includes 11 sources, and one timeslot is required for every source.
64/11 = 5, modulo 9:
We can not rationally distribute timeslots between nodes.
Example 2:
The Network includes 80 sources, and one timeslot is required for every source.
In such case we can not distribute 64 timeslots between 80 sources, therefore more that one source will be send data in one slot, that may lead to problems with guaranteed delivery time.
27th SpaceWire Working Group Meeting

7. 27th SpaceWire Working Group Meeting
Multilane (1)
Hot redundant lanes
k. When an active sending lane fails and is disconnected, the Multi-Lane layer shall automatically promote a hot redundant lane to an active sending lane.
Question:
We don’t understand how the receiver recognize the moment when hot redundant lane become an active sending lane: when the first word that should be transmitted to the data link layer is received?
27th SpaceWire Working Group Meeting

8. 27th SpaceWire Working Group Meeting
Multilane (2)
Hot redundant lanes
Hot redundant lane can be broken like an active sending lane. It can be broken in both directions or in only one direction.
Example:
Question:
If the hot redundant lane is broken in only one direction (from the Side A to the Side B), how the Side A will know about this problem?
27th SpaceWire Working Group Meeting

9. 27th SpaceWire Working Group Meeting
Multilane (3)
Page 146 (J4)
i. The maximum number of words in a data frame shall be Nx64, where N is an integer less than or equal to the number of sending lanes in a link, including both active lanes and inactive lanes.
Question:
For example, the link includes 16 lanes, but only 2 lanes are active.
In this case the length of frame part transmitted via one lane is 8*64 – it is in 8 times longer, than normal frame length:
The size of data object protected by one CRC grows essentially
The retry retransmission time grows essentially in case of single errors
27th SpaceWire Working Group Meeting

10. 27th SpaceWire Working Group Meeting
Summary
We propose :
Some changes to Broadcast mechanism
We have some comments to :
Broadcast mechanism
Scheduling quality of service
Multilane mechanism
Слайд презентации
Сейчас вопрос в том, какой сценарий поведения (или различные сценарии) предпочтительнее для пользователя. Если нужны сценарии, которые сейчас не заложены в стандарте, то нужно будет думать как их реализовать с разумными аппаратными затратами
27th SpaceWire Working Group Meeting

11. 27th SpaceWire Working Group Meeting
Thank you !
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Справа внизу указывается номер слайда / общее количество слайдов
27th SpaceWire Working Group Meeting