[openib-general] Re: [mgtwg] Payload Length in first RMPP sent segment

[Greg Pfister]

Hal,
My take is that there's no ambiguity. Then again, I wrote it, so I would 
think that, right? :-)
The idea is that we're trying to allow *either* of the usual two options 
for specifying a string of stuff: (a) Start out by giving the length; or 
(b) go until you reach a special mark meaning "the end."
The thing is it gets complicated when there is only one packet. So take 
two cases: >1 packet, and ==1 packet.
length >1 packet: 
-- PayloadLength <> 0 on 1st packet means case (a). Just read until you 
get that many bytes, which may use only part of the last packet. If the 
last packet isn't also marked last, scream about inconsistency. 
-- PayloadLength=0 on first packet - case (b). Read until you get a marked 
last packet. PayloadLength in that last packet tells you how many are 
valid in that packet (zero in that case -- I'm not sure; whole packet, I 
think).
length ==1 packet meaning RMPPFlags.Last=1 and RMPPFlags.First=1 in the 
same packet. 
-- Interpretation is the same as the "last packet" case above, i.e., 
RMPPFlags.Last=1 dominates the interpretation.
As far as I know, that's it. Any comments from others?
(This may not forward to openib-general, since I'm not on that list; if it 
doesn't please forward.)

Greg Pfister
IBM Distinguished Engineer, Member IBM Academy of Technology
IBM Systems & Technology Group, Server Technology & Architecture
(512) 838-8338 | IBM tieline 678-8338 | FAX (512) 838-3418
Sic Crustulum Frangitur



Hal Rosenstock <halr at voltaire.com> 
08/29/2005 08:14 AM

To
mgtwg at infinibandta.org
cc
openib-general at openib.org
Subject
[mgtwg] Payload Length in first RMPP sent segment





Hi,

On the RMPP send side, while the Payload Length field in the last
segment is clear that it indicates the number of valid bytes in
Transferred Data, there seems to be some ambiguity in the optional
Payload Length field in the first segment. I think it can work either
way but I also think the intent was to reflect the valid bytes. Maybe it
is this way to allow flexibility (choice in the implementation). What is
the correct interpretation ? Should I enter a comment on this ? Thanks.

-- Hal

IBA 1.2 p.775 line 37

In the first packet of an RMPP transfer (RMPPFlags.First=1),
PayloadLength may indicate the sum of the lengths, in bytes, of the
TransferredData fields in all packets of the entire multipacket
response; this is done by using a nonzero value for PayloadLength in the
first packet. 

IBA 1.2 p. 776 line 8

In the last packet of an RMPP transfer (RMPPFlags.Last=1), PayloadLength
indicates the number of valid bytes in the TransferredData field,
allowing data transfers that are not an integral multiple of the length
of the TransferredData field. A transfer terminates when either: (a) a
packet containing RMPPFlags.Last=1 is received; or (b) a nonzero
PayloadLength was given in the first packet of a transfer, and a packet
is received containing sufficient TransferredData bytes to equal or
exceed the PayloadLength originally provided. If case (b) occurs and
RMPPFlags.Last is not 1 for that packet, the Receiver sends an ABORT
packet with RMPPStatus of "Inconsistent Last and PayloadLength" and
terminates the transfer.




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