asterisk users - Jul 2016 - how to read sip debug

Generally, what am I looking for when turning SIP debug on?  More
specifically, the provider says that I'm returning a 404 when they try to
call me.  Now, I had inbound working, literally, the other day.  Outbound
works fine.  I "may" have broken it either through Asterisk config or
the
providers portal with settings.  Ok, I broke it -- not sure how.

comments interspersed:

mordor*CLI>
Reliably Transmitting (NAT) to 192.76.120.10:5060:

I think/infer/assume that this is the IP address for telnyx SIP servers

OPTIONS sip:sip.telnyx.com SIP/2.0

What does OPTIONS mean?

Via: SIP/2.0/UDP <externip>:5060;branch=z9hG4bK28142189;rport

rport relates to NAT?  The message is via SIP UPD from my externip ....
what is branch?

Max-Forwards: 70

70 hops max?

From: "asterisk" <sip:asterisk@<externip>>;tag=as1a7aca46

from my externip, with a hash to keep the calls straight?

To: <sip:sip.telnyx.com>

easy, to telnyx

Contact: <sip:asterisk@<externip>:5060>

from me

Call-ID: 6fce72627f253b7f2e15dac713b52392@<externip>:5060

another hashcode, Call-ID ?

CSeq: 102 OPTIONS

?

User-Agent: Asterisk PBX 13.1.0~dfsg-1.1ubuntu3

easy enough, my system

Date: Wed, 06 Jul 2016 02:17:12 GMT

easy, date

Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY, INFO,
PUBLISH, MESSAGE

enumerating accepted replies?

Supported: replaces

?

Content-Length: 0

no data, just "hi"


---
mordor*CLI>


If I see https://wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions in
a SIP trace, that's relatively clear.  But what am I looking for with
regards to receiving calls?


thanks,


Thufir
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This is interesting:

"Note that the To and From header fields are not reversed in the response
message as one might expect them to be. This is because the To and From
header fields in SIP are defined to indicate the direction of the request,
not the direction of the message. "  -Cisco

so, when I'm receiving an inbound call, the direction would be telnyx
first, then me.  Regardless of whether the ?message? is from me or the
provider.


-Thufir
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