Re: xAP EOM identifier in xAP v1.3

[Kevin Hawkins <yahoogroupskh@xxxxxxxxxxxxxx>]

Patrick Lidstone wrote:
>
>
> I don't think hubs currently send heartbeats? If they do, it's not
> explicit in the spec. (Mine doesn't). We also need a mechanism to
> differentiate between multiple hubs (since most people will generally
> run at least one hub per vm).
All the hubs I use (xFX) send heartbeats and so obviously have a unique
name/port/ip identifier.  I thought all xAP app's should send heartbeats.
>
> Is a TCP hub going to relay all traffic to all TCP endpoints? That
> won't scale very well... but it's also not clear how the filtering
> etc. will work. Administering filters manually works fine for the odd
> serial segment, but it's not going to be viable for a large number of
> endpoints.
There's an existing application called xServer (by mi4) that was created
for internet routing of xAP messages. It supports authentication of
clients and within the app you can setup device filters and the
allowable directions of traffic. It supports a couple of different
message formats depending on the client type connection.

I expanded this to become 'iServer' which supports some extra client
types (iPhone and C-Bus touch screens) and also added some inbuilt BSC
schema intelligence.  This was used to just transfer state change info
to dumber clients ie the client wasn't parsing xAP.   More significantly
I added a client protocol that allows the client to add/remove xAP
device or schema filters on the fly.  So a client that is specifically
interested in specific devices or schema can create those filters at
connect. The filters support wildcards too.    Now a hub version maybe
should, or shouldn't support such features but it is an option..

K

PS  For anyone interested a 'not quite the most recent' beta version of
iServer is downloadable here.  If you want the latest - which adds C-Bus
touchscreen support then drop me an email.  The interface needs an
overhaul too.

http://www.ukusa.demon.co.uk/iServer.htm
>
>
> Patrick
>
> 2009/9/15 Edward Pearson <edward.mailgroup@xxxxxxx
> <mailto:edward.mailgroup@xxxxxxx>>
>
>
>
>     I don?t mean that packet fragmentation or re-ordering are myths ?
>     just that when dealing with xAP at the IP stack (socket) interface
>     you are dealing with datagrams not data packets so you don?t need
>     to worry about those packet aspects. There was a design decision
>     to limit xAP datagrams to 1500 bytes to improve the likely
>     coverage of correctly sent datagrams in a world of IP stacks of
>     varying quality. It?s no more important than that. Readers of the
>     spec seem to attach more importance to it than it needs (there?s
>     the myth aspect).  To program xAP you don?t need to worry about
>     fragmentation and reordering ? just keep your datagrams 1500 bytes
>     or less and let the stack do its thing.
>
>
>
>     Sometimes an app goes a bit wild (xAP news has occasionally been a
>     useful test source) and big xAP messages are generated ? and they
>     get delivered too! It?s normally the input buffer of the receiving
>     app (eg hub) that breaks first.
>
>
>
>     On the tcp framing, I?d suggest that implementing the CRC part
>     (irrelevant on a reliable stream) is a waste of most people?s
>     time; by far more use will be made of tcp than any kind of
>     serial/485/etc networks so they?d be sharing development of an
>     implementation with nobody.
>
>
>
>     Discovery can be done simply with the existing hub heartbeat
>     message. Just need to agree on an extra block that advertises the
>     port number of the tcp service ? which I assume, by default would
>     be 3639.
>
>
>
>     Having read the 802.11 spec, I now understand why broadcast udp
>     from an AP to a NIC is so un-reliable. And ad-hoc mode is a real
>     disaster!
>
>
>
>     *From:* xAP_developer@xxxxxxx
>     <mailto:xAP_developer@xxxxxxx>
>     [mailto:xAP_developer@xxxxxxx
>     <mailto:xAP_developer@xxxxxxx>]
*On Behalf Of *Patrick
>     Lidstone
>     *Sent:* 15 September 2009 14:15
>
>     *To:* xAP_developer@xxxxxxx
>     <mailto:xAP_developer@xxxxxxx>
>     *Subject:* Re: [xAP_developer] xAP EOM identifier in xAP v1.3
>
>
>
>
>
>     Hi Edward,
>     Please see in-line responses...
>
>     2009/9/15 Edward Pearson <edward.mailgroup@xxxxxxx
>     <mailto:edward.mailgroup@xxxxxxx>>
>
>
>
>     The double 0x0a terminator works for me, it?s simple to implement
>     ? already done for my stuff.
>
>
>
>     ...but it isn't robust unless the message is sent as a single
>     datagram, and if it is sent as a single datagram, the os should
>     deliver it as such -- and if it doesn't, adding an eom marker
>     isn't going to help.
>
>         For reliable streams, such as TCP, I generally frame the
>         messages with STX and ETX.
>
>
>
>         I thought all this business about 1500 bytes was somewhat
>         urban myth (at least as it applies to xAP). The data packet
>         size is not what?s important; it?s how the particular IP stack
>         implementation deals with >datagrams< that?s the key.
>
>
>     I don't understand what you are syaing here? The data packet size
>     is inextricably linked to the IP stack. What aspect is it that you
>     consider to be an urban myth?
>
>
>         I only have experience of the two most recent Windows stacks
>         (XP and Vista) which I agree are likely more capable than old
>         embedded stacks. My experiments with Wireshark show that those
>         stacks will happily deal with fragmentation and
>         defragmentation. 64KB in a single datagram? No problem if your
>         receive buffer is long enough (even if you force a small MTU).
>
>     Yes, agreed, OSes perform fragmentation for you. The individual
>     fragments have a maximum size as determined by the MTU. For
>     pragmatic, practical reasons, xAP needs to define a maximum
>     overall message size, and for convenience's sake that was set as
>     equal to the 'standard' MTU size. Devices which use a smaller MTU
>     /should /fragment and reassemble seamlessly provided that the
>     correct socket options have been set to define the maximum UDP
>     packet size. By electing to use a single MTU's worth of data, we
>     we avoid the overhead associated with fragmentation and reassembly
>     (principally memory buffering) which is a good thing. When
>     reassembled, the os should deliver a datagram as a complete entity
>     in one go (irrespective of the mtu size, assuming that the
>     datagram falls within the maximum datagram size defined for the
>     stack). If the sender doesn't send a message as a single datagram,
>     then the whole thing falls apart because effectively you are then
>     doing fragmentation and reassembly at the application layer, and
>     that won't work because the ordering across datagrams is not
>     guaranteed and individual datagrams may get lost.
>
>
>         If anything goes wrong (eg, missing fragment) the whole
>         datagram is dropped. I can?t see how packet re-ordering can
>         happen on a single LAN for broadcast UDP ? there are no
>         multiple paths and no retry mechanism. Certainly never
>         observed it ? I assume the stack would again just drop the
>         entire datagram.
>
>
>     Re-ordering of datagrams can occur for multiple reasons on a
>     single lan. A sender may send individual UDP packets in any order
>     it chooses. This commonly occurs with fragmented packets
>     originating from a multi-threaded sender, which can be interleaved
>     with smaller, non-fragmented datagrams as required (optimising
>     transient memory use, as soon as they are sent, the buffer can be
>     released). A switch is not obliged to retransmit packets in the
>     order in which they are received. And most fundamentally, the
>     specs do not require UDP packets to be ordered so, even if you
>     don't observe it, it /can/ happen, and sooner or later
>     interoperability issues will arise if the working assumption is
>     made that they always arrive in order.
>
>
>
>         A bigger issue for me, and the reason for me experimenting a
>         few times with TCP stream serving from hubs, is datagram loss
>         over WiFi networks. This is greatly accentuated for UDP when
>         you use broadcast (as we do) from wired to wireless (fine the
>         other way as it?s not really a broadcast packet till it gets
>         to the AP) as the access point and NIC can not longer apply
>         their ack/nak procedure at the transport level. I commonly see
>         xAP datagram loss rates from wired to wireless connections of
>         20%. So I?d like us to agree on a standard transport wrapper
>         for TCP streams which a lot of platforms would find useful.
>
>
>
>     I'd suggest using the same framing as the "transport
wrapper", as
>     this then allows for code to be shared across transports. If xAP
>     was extended to support TCP, then that should also include a
>     formal discovery mechanism by which the IP address/characteristics
>     of the hub can be discovered (over UDP xAP?)
>
>     Patrick
>
>
>         *From:* xAP_developer@xxxxxxx
>         <mailto:xAP_developer@xxxxxxx>
>         [mailto:xAP_developer@xxxxxxx
>         <mailto:xAP_developer@xxxxxxx>]
*On Behalf Of *Patrick
>         Lidstone
>         *Sent:* 14 September 2009 14:19
>         *To:* xAP_developer@xxxxxxx
>         <mailto:xAP_developer@xxxxxxx>
>         *Subject:* Re: [xAP_developer] xAP EOM identifier in xAP v1.3
>
>
>
>
>
>         So the xAP delimiters for serial are defined in the 1.2 xAP
>         spec here:
>         http://www.xapautomation.org/index.php?title=Protocol_definition#Transport_Wrapper
>
>         To the best of my knowledge, the MTU size for an ethernet
>         frame is 1518 bytes, which leads to a UDP packet MTU of 1500
>         bytes and this is the size that is adopted by the majority of
>         operating systems. Internet routers (i.e. ISPs) sometimes use
>         an MTU of 576 bytes, but this wouldn't be relevant to xAP
>         since the traffic doesn't pass over the wider net, or if it
>         does, it's generally gatewayed via a TCP/IP connection.
>
>         If a device is receiving fragmented UDP packets, I think the
>         same issues arise as those related to extending the xAP
>         message size beyond 1500 bytes - what happens if a fragment
>         gets discarded.
>
>         If you take the scenario:
>
>         Part 1(a), Part 1(b), Part 2(a), Part 2(b), Part 3(a), Part
3(b)
>
>         --- first there is no guarantee that the parts will be
>         delivered in order, and second, if part 1(b) was dropped, and
>         you were blindly assembling messages based on the proposed
>         double-0a terminator, you'd end up with a message comprising
>         part 1(a), 2(a) and 2(b) which is not only obviously corrupt,
>         but also possibly larger than the maximum xAP message size,
>         blowing away your buffers.
>
>         I think the solution is to probably parse incoming messages on
>         the fly, byte-by-byte. You can then at least reset your state
>         when you encounter the xap-header, and if you count open and
>         close curly braces, you can tell when you have an apparently
>         complete message. This won't solve the issue of UDP fragments
>         being potentially received out of order, but so long as we are
>         dealing with a single LAN, and fragmentation occurs at the
>         receiver, we will be ok I think.
>
>         It is absolutely possible for UDP packets to be discarded, and
>         the way we deal with this in xAP is to accept that this can
>         happen to xAP messages, and layer application level
>         acknowledgements where knowing that a message has been
>         received is critically important - whether explicitly or
>         implicitly through status messages. There are various schemes
>         that could be adopted to allow a receiver to detect lost
>         messages (e.g. sequence numbering), but they quickly become
>         quite onerous, and assume that the originator keeps a copy of
>         the original message or is able to reconstruct it - which is
>         non-trivial for the many xAP nodes.
>
>         Perhaps you can share more of the specific details of the
>         device(s) in question (manufacturer, docs, o/s etc), and their
>         specific behaviour, which seems a bit anomalous?
>
>         Patrick
>
>         2009/9/13 Kevin Hawkins <yahoogroupskh@xxxxxxx
>         <mailto:yahoogroupskh@xxxxxxx>>
>
>         One of the issues seems to be that there is conflicting views
>         as to the
>         length of a a UDP data packet payload.  Some people cite 500
>         or 512
>         characters and some 1500.  Regardless in some low
>         memory/performance
>         devices it is being reported, that even with UDP,  packets are
>         being
>         received from the buffer either truncated or appended back to
>         back.
>         The latter I assume is due to the speed of the device in
>         servicing the
>         buffer.
>
>         We have an opportunity to protect against this with v1.3 and
>         the double
>         '0A' seems the most compatible.   I would be loathe to support
>         anything
>         that wasn't backward compatible.
>
>           K
>
>
>         Patrick Lidstone wrote:
>         >
>         >
>         > I will dig it out - it included an optional checksum I
>         think, and IIRC
>         > was framed by stx and etx (a kind of pseudo industry
>         standard). I
>         > certainly used it with the PIC serial stuff and the
>         xAP-serial bridge.
>         > Re.: long message truncation and concatenation: If we
need
>         to support
>         > messages that are larger than one UDP packet, then there
are
>         > additional complexities and the proposed scheme won't
work
>         as intended
>         > as the ordering of UDP messages is not guaranteed. I'm
happy
>         to help
>         > refine a spec to support these capabilities, but it is
>         moving away
>         > from the basic ethos of xAP somewhat, as devices would
have
>         to be able
>         > to buffer received messages, and that raises the bar
>         considerably.
>         > Perhaps there is scope for co-existence of a long and
>         standard message
>         > protocol though?
>         >
>         > Patrick
>         >
>         > 2009/9/13 Kevin Hawkins <yahoogroupskh@xxxxxxx
>         <mailto:yahoogroupskh@xxxxxxx>
>
>         > <mailto:yahoogroupskh@xxxxxxx
>         <mailto:yahoogroupskh@xxxxxxx>>>
>
>         >
>         >     ... Oh ... where is that in the spec ?  it might be
all
>         we need.
>         >
>         >        This is also tied in with some aspects of long
>         message truncation
>         >     and concatenation of messages received in UDP receive
>         buffers
>         >     though...
>         >
>         >      K
>         >
>         >     Patrick Lidstone wrote:
>         >     >
>         >     >
>         >     > The original xap spec provides extensions for
framing
>         a message over
>         >     > async serial which also delimit the start of the
>         message - you don't
>         >     > need this 'hack' if you follow the original spec
for
>         non-UDP
>         >     transports.
>         >     >
>         >     > Patrick
>         >     >
>         >     > 2009/9/13 Kevin Hawkins
<yahoogroupskh@xxxxxxx
>         <mailto:yahoogroupskh@xxxxxxx>
>         >     <mailto:yahoogroupskh@xxxxxxx
>         <mailto:yahoogroupskh@xxxxxxx>>
>
>         >     > <mailto:yahoogroupskh@xxxxxxx
>         <mailto:yahoogroupskh@xxxxxxx>
>
>         >     <mailto:yahoogroupskh@xxxxxxx
>         <mailto:yahoogroupskh@xxxxxxx>>>>
>         >     >
>         >     >       We have been asked on several occasions
how to
>         detect the
>         >     end of a
>         >     >     xAP messager as there is no unique EOM
character.
>          Typically
>         >     in any
>         >     >     reasonable sized packet structured transport
eg
>         UDP then the
>         >     packet
>         >     >     provides such an indication but on systems
with
>         small packets or
>         >     >     non eg
>         >     >     asynchronous serial this is not useable.
>         >     >
>         >     >       In discussing this with the specification
team
>         we must
>         >     consider
>         >     >     backwards compatability and so we do not
wish to
>         alter the
>         >     >     specification
>         >     >     to include a unique EOM character.  What we
do propose
>         >     however is that
>         >     >     xAP v1.3 will specify that all messages
should end
>         with two
>         >     >     consecutive
>         >     >     chr(10)'s immediately after the closing '}'
>         >     >
>         >     >     ie  ..... 7D 0A 0A
>         >     >
>         >     >      Some apps, even v1.2 ones,  already do
this.  We
>         don't
>         >     believe this
>         >     >     will cause any backwards compatibility
issues and
>         it will
>         >     always be
>         >     >     unique within a xAP message.
>         >     >
>         >     >      So, in developing xAP v1.3 apps could you
>         therefore append
>         >      two 0A's
>         >     >     at the end of your messages , and of course
handle
>         incoming
>         >     messages
>         >     >     containing such.
>         >     >
>         >     >       K
>         >     >
>         >     >
>         >     >     ------------------------------------
>         >     >
>         >     >     Yahoo! Groups Links
>         >     >
>         >     >
>         >     >
>          mailto:xAP_developer-fullfeatured@xxxxxxx
>         <mailto:xAP_developer-fullfeatured@xxxxxxx>
>         >     <mailto:xAP_developer-fullfeatured@xxxxxxx
>         <mailto:xAP_developer-fullfeatured@xxxxxxx>>
>         >     >     <mailto:xAP_developer-fullfeatured@xxxxxxx
>         <mailto:xAP_developer-fullfeatured@xxxxxxx>
>         >     <mailto:xAP_developer-fullfeatured@xxxxxxx
>         <mailto:xAP_developer-fullfeatured@xxxxxxx>>>
>         >     >
>         >     >
>         >     >
>         >     >
>         >     >
>         >     >
>         >
>         >
>         >
>         >     ------------------------------------
>         >
>         >     Yahoo! Groups Links
>         >
>         >
>         >        mailto:xAP_developer-fullfeatured@xxxxxxx
>         <mailto:xAP_developer-fullfeatured@xxxxxxx>
>         >     <mailto:xAP_developer-fullfeatured@xxxxxxx
>         <mailto:xAP_developer-fullfeatured@xxxxxxx>>
>         >
>         >
>         >
>         >
>         >
>         >
>
>
>
>         ------------------------------------
>
>         Yahoo! Groups Links
>
>
>            mailto:xAP_developer-fullfeatured@xxxxxxx
>         <mailto:xAP_developer-fullfeatured@xxxxxxx>
>
>
>
>
>
>
>
>
>
>
>
>
>
>



------------------------------------