[OpenAjaxSecurity] [OpenAjaxInterop] Cross-Frame Messaging (was SMash questions and issues)

Gideon Lee glee at openspot.com
Fri Sep 28 13:02:54 PDT 2007

My comment in blue...  your second question is worth everyone pitching in some thoughts.

----- Original Message ----- 
  From: Sumeer Bhola 
  To: OpenAjax Alliance Security Task Force 
  Cc: interop at openajax.org ; OpenAjax Alliance Security Task Force ; security-bounces at openajax.org 
  Sent: Friday, September 28, 2007 2:24 PM
  Subject: Re: [OpenAjaxSecurity] [OpenAjaxInterop] Cross-Frame Messaging (was SMash questions and issues)

  interesting discussion. A couple of questions/comments: 

  - I don't quite understand the performance implications of full-duplex versus two one-way channels. Since the browser modification proposals are for asynchronous messaging, and that is also what we can implement using fragments, it seems natural to require that the lower-level API support asynchronous messaging in both directions. 

  I agree with the last statement.  The lower-level API ought to be asynchronous messaging.  Both the outer and the inner frame should be able to send a string message.  Asychronously, the outer side receives a registered callback with the string value.  

  If everyone agrees above this point, the full-duplex vs.two channel issue is a less significant point.  It is just a question of how one would implement streaming over reliable asynchronous transport -- whether it is better to have two queues or a loop, etc.

  - One issue with the channel/pipe abstraction offered by the lower-level API is privacy. With SMash, the main mashup application can listen on any channel, so it is not possible to implement private point-to-point channels between two arbitrary frames in the mashup. If the HTML 5 proposal is implemented by browsers, this would be possible. 
  It seems that SMash has an A1-B-A2 frame hierarchy for each "widget".  When the host (A1) sends fragment to the widget, it waits for the widget frame (B) to sends an acknowledgment via sending a fragment to A2.   Correct me if I misunderstood.

  But this model might be insufficient when we get into the realm of composite widgets.  For example, in a multi-frame webtop, you can have compound document widgets like spreadsheets, which contain floating frames of chart/drawing widgets.  The webtop, the spreadsheet, and the charts might come from different domains.  The question, then, is do we allow the chart to communicate with the webtop directly, not letting the spreadsheet to see the message flying in between?  For security reason, it might be desirable to allow that.  More importantly, given that passing data across domains take so long, it is a good idea.

  We can let a deeply nested chart talk to the webtop by flipping the ABA hierarchy.  That is, we have the (chart) widget frame contains a frame of the toplevel (webtop) domain which contains a frame of the widget's domain. The first time the chart sends a message to the webtop, the webtop also tracks a pointer to the frame contained by the chart.  So subsequently, either side can send message to another.

  - Regarding standardizing the protocol itself, for example, the exact fragment formats that SMash and XDDE use, it might be useful. On the other hand, as these are hopefully temporary hacks till we get browser support, it may not be worth too much effort. At minimum, we should document the protocol names, so that different entities can communicate with each other if they use the same protocol name. For instance, the SMash protocol could be used by the hub, if the scheme in the hostname used in the OpenAjax.hub.connect call referred to SMash. 

  Well, given that the low-level is a temporary measure, we might as well standardize it.  It is not like people are going to compete for better implementations, anyway.


        "Gideon Lee" <glee at openspot.com> 
        Sent by: security-bounces at openajax.org 
        09/28/2007 10:48 AM Please respond to
              OpenAjax Alliance Security Task Force <security at openajax.org> 

       To "OpenAjax Alliance Security Task Force" <security at openajax.org>, <interop at openajax.org>  
              Subject Re: [OpenAjaxSecurity] [OpenAjaxInterop] Cross-Frame Messaging (was        SMash questions and issues) 



  My thoughts in blue. 
  ----- Original Message ----- 
  From: Howard Weingram 
  To: OpenAjax Alliance Security Task Force ; interop at openajax.org 
  Sent: Friday, September 28, 2007 4:20 AM 
  Subject: Re: [OpenAjaxInterop] Cross-Frame Messaging (was [OpenAjaxSecurity]SMash questions and issues) 

  A. (Two layers...) I agree that there should be two API layers. But when I think about OAH pub-sub in particular, I wonder if it might impose a performance penalty if the lower-level API is generalized to be a "TCP-like" full-duplex binary data tunneling model.  It seems to me that an "email-like" string-based, one-way, and acknolwedged delivery will already be sufficient.  We will need two such pipes to do PUB and SUB.  But going from outer-frame to inner-frame requires a different implementation from the vice-versa in any case. 
  Now, I am actually not advocating either string-based or one-way only model.  (See response to C below). I'm just saying that if every character counts, there is indeed a performance difference. 
  B. Different upper level protocol implementations could 
  in fact be layered over the same lower level, and 
  different lower level protocols could implement the 
  same lower level API. We should consider standardizing 
  the data stream protocol API as well as the higher level. 
  I agree. 
  C. In cases where data is highly redundant and large messages 
  are frequently sent, a data stream layer implemnentation might 
  include data compression. The cost of computation might be 
  much less than the cost associated with delivering fragmented data. 
  Note: I am NOT suggesting that we provide a reference implementation 
  that does this. It is simply a possibility that some implemneters might 
  choose for their own reasons. 
  Interesting.  Shouldn't compression be a feature of the OAH pub-sub implementations instead?  The pub-sub implementation has the whole JavaScript object/JSON structure to look at, so it could do many more clever things.  Indeed, in addition to intra-message compression, it can do inter-message compression, like building a string table dynamically. 
  At the lower level API, there really not much compression worth doing.  And we cannot forget that in the long term, the lower-level API is really meant as a lobbying effort for to the browser vendor anyway.  The browser itself can use native functionality to implement the lower level API. 
  But OAH-level compression has other significant uses in its own right -- server-side OAH in particular. 
  For instance, in a mashup, the host and the widgets should by default use URL-encoded JSON as you suggest to negotiate the best compression.  But beyond the first conversation, clever OAH implementations can use more application-optimized representations to talk to each other. 
  There is clearly an opportunity for commercial competition of high-level OAH implementations and performance/compression could be a differentiation factor.  In contrast, nobody can compete with the browser when it comes to the low leverl API. 
  D. Messages being sent between frames need to be URL-encoded 
  Well, I agree that URL-encoded JSON is probably the best choice for the initial exchange for representation negotiation.  But I do believe that we should introduce the concept for representation negotiation as part of the OAH 1.1 standard. 
  I am also wondering if there should be at least a place-holder for identity negotiation.  It may be too much for us to take on for 1.1 time frame.  But we should design the high-level protocol in such a way that identity negotiation can take place. 
  In particular, it seems that there may be some reason for us to talk to folks at MIT Kerberos Consortium.  Single sign-on and secure mash-up are really hand and glove.  One is really made for the other.   
  E. Gideon, what were your performance numbers for XDDE? Order of 
  magnitude, I mean? 
  For each fragment to be sent and acknowledged, XDDE needs two iframe location change.  (Like SMash A.com contains 2 B.com contains A.com).  On most browsers, it seems that we can get at most 10 location change per second. 
  So I say the best we can expect is 50K going one-way. 
  F. (your #3) That kind of clicking is a royal pain. 
  Yes, 20Hz... ugh.   
  G. Development of applications for a sandboxed mashup still requires 
  developers to be quite careful. Even if the mashup itself successfully 
  prevents iframe X from accessing the contents of iframe Y won't do 
  any good if iframe Y is vulnerable to CSRF attacks by X. 
  Best Regards, 
  Howard Weingram 

  From: security-bounces at openajax.org [mailto:security-bounces at openajax.org] On Behalf Of Gideon Lee
  Sent: Thursday, September 27, 2007 4:02 PM
  To: OpenAjax Alliance Security Task Force; interop at openajax.org
  Subject: [OpenAjaxSecurity] SMash questions and issues

  We have something overlapping SMash at the SameDesk project and we call it XDDE (Cross Domain Data Exchange).  I have a small demo at: 
  I haven't looked at the SMash code closely.  But as I listened to the SMash presentation, here are some questions/issues that we may need to sort through as we work towards Hub 1.1: 
  1. What is SMash using to have the outer frame signal an inner frame to check window.location changes? 
  XDDE has the parent frame resize the child frame, alternatiing between +1 and -1, for each subsequent "fragment".  The child frame has an onresize event handler for the document.body. 
  2. Does the SMash protocol allows arbitrarily long message and allow either side to initiatiate the conversation? In XDDE, that's what we do to allow that... 
  (a) We always break up a long message into fragments.  And in delivery, when one side sends a fragment to the other side, it always waits for an ACK(nowledgement) fragment from the other side before sending the next fragment. 
  (b) In a Parent-Child relationship, we want to allow either party to initiate a request-reply conversation.  So to make the +1/-1 trick work, we actually have 4-layer deep iframe enclosure A-B-A-B instead of the A-B-A pattern in SMash. 
  (c) A long message begins with a length and an encoding description of the data fragment, followed by zero or more data fragments, followed by an end of message checksum fragment.  The reason we have encoding description is that different data can benefit from different encoding strategy. 
  So in a way, the conversation pattern is sort of like HTTP over TCP rather than an bi-directional UDP.   
  3. Does SMash cause annoying click sounds in IE 7 when long message with many #fragmet are sent?  For a short message, a click-click may not be that annoying.  But if it is a long message with many fragments, XDDE is very annoying because it sounds like old typewriters gone mad. 
  BTW, since a fragment can contain at most around 4K characters, it doesn't really take a very long message before the click-clack-click-clack becomes annoying. 
  4. Are there any performance benchmark on SMash yet?  Even with the iframe resize trick (which helps the inner frame to respond almost immediately without installing a window timeout), we found that XDDE is not exactly fast enough for the type of transfer we were trying to do. 
  How we optimize the protocol depends a lot on the higher level model. When we did XDDE, it wasn't built around single-directional PUB-SUB, but more a request-reply pattern.   
  5. So given OpenAjax Hub 1.0 currently allows JavaScript objects to be passed, I'd assume that we support try to achieve that with SMash-based cross-domain-data-exchange too.  How do we plan to encode a JavaScript object so that it can go across frames?  Do we encode it in JSON over UTF8 over Base64? 
  These are my top five issues.  I'm sure I can come up with more when we go down to the detail.  Overall, I like the SMash idea because it is secure.  But I think performance and user experience should not be totally hand waved either.  Otherwise, people will just keep doing unsafe JSON. 


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