Re: [rtcweb] Network times . was SDP Security Descriptions (RFC 4568) and RTCWeb

[Tim Panton <[e-mail]>]

> 
> 2) Are packet drops during TLS-setup more likely over the UDP channel
> between browsers (as for DTLS) than over the signaling channel (for
> SRTP/DES). And are there more packets that safely need to arrive with DTLS
> setup?
> 
> 3) SRTP/DES setup rely on TCP retransmission if packets are lost (correct me
> if I am wrong), while DTLS must rely on some other higher level
> retransmission mechanisms. Are there longer timers involved then? I think
> this may be the real explanation for reports of slow DTLS setup, isn't it?
> 
> The only cure against dropped UPD packets is IP-level QoS, such as traffic
> shaping in firewalls/routers (but they need to be aware of traffic type) and
> diffserv or reservation over the transport network- but that is (currently)
> not enabled over the Internet, where all traffic is best effort class.


If we are seeing variability in DTLS setup times that don't seem to be justified 
by ping times it may be due to MTU issues.

DTLS has some rules (which I don't yet clearly understand) about how much
one can stuff into a packet based on an assumed/derived/calculated MTU. 
Guess too high and your packet will be dropped - forcing a retry with the data
broken over multiple packets. 

I suspect we may have a little learning to do in the implementations to select
an optimum MTU for the wild internet.

In these cases I'm sure packet captures set to the implementors would be most welcome.

Tim.