[mosh-devel] mosh and SCTP.

[Mark Blackman]

On 11 Apr 2012, at 12:23, Hari Balakrishnan wrote:

> Hi Mark,
> 
> Good question.  Here are some relevant differences:
> 
> 1) As far as I know, SCTP doesn't support the ability to resume after suspension (and changing IP), particularly if data was outstanding over the connection when the suspension occurred.

Well, there's SCTP multihoming, but that probably requires knowing all the IPs in advance.

> 
> 2) SCTP delivers data in order and does not have the ability to skip data to allow the receiver to reach the current state promptly.  This capability is important for interactivity.

The wikipedia entry indicates "However, message ordering is optional in SCTP; a receiving application may choose to process messages in the order they are received instead of the order they were sent." and " As in UDP, in SCTP a sender sends a message in one operation, and that exact message is passed to the receiving application process in one operation." Perhaps there's a wrinkle I'm missing though.

> 
> 3) SCTP, like TCP is bulk-transfer-optimized.  So if a buffer were to fill up for whatever reason from the session, it would reduce interactivity.  Mosh's State Synchronization Protocol (SSP) has a different end-to-end flow control method than SCTP (or TCP), which optimizes for interactivity (rather than for bulk throughput). 

Ok, it's not clear to me what the flow-control method is in SCTP, so I can't really comment on that. :)

> 
> 4) I'm not sure how widely deployed SCTP is, across different operating systems.

Wikipedia suggests an option for most of the common ones, but I'm sure they're all a bit
on the immature side.

> 
> Please note that Mosh doesn't run atop UDP; there's a state synchronization protocol that sits in between.  That protocol uses the principles of "application-level framing", which is a design philosophy that says that one can get better behavior and performance by designing a protocol in concert with the application.  In our case, SSP uses out-of-order delivery (i.e., skipping data when appropriate), roaming across IPs, and a different flow control strategy, to better support an interactive terminal.

Thanks, I did appreciate that SSP was the key layer in this discussion. I suppose I was thinking that SSP might
be implemented in terms of SCTP instead of UDP. UDP is, of course, a very mature protocol, but I got the impression
the SSP layer could offload more work into the SCTP layer than it could to the UDP transport. Ultimately, it may
be just that SCTP would just represent an alternate UDP transport with no extra benefits and the disadvantage of
only partial or incomplete support across OSes.

Cheers,
Mark