Animated flows
140 drawings move a packet along the message. Reduced motion in Settings stops extra page motion. The SVG animation is in the file.
Sequences
OPTIONS across two transports
user=phone, then a redirect
Each arrow has a chapter
The map names the document
The m= line is not a license
Walk the first transaction
Redirect steps out, proxy stays
Busy is final
Compact request, long response
Retransmit, then a new ACK branch
Early, then confirmed, then BYE
Retransmit until 100, then wait
UDP in, TCP out
Hop-by-hop ACK of a 603
Two proxies stay on the dialog
183, then PRACK
Answer the INVITE, then UPDATE
422, then a longer interval
CANCEL, then 487
REFER with Replaces
Challenge, then a binding
SIP after the lookup
BYE with two loose routes
Two flows, one instance
Learn both vectors, then call
Register a GRUU, then invite it
Two early dialogs
SDP rides inside the INVITE
Two streams, one refused
96 and 111 are both opus
re-INVITE to sendonly
Second stream refused
183 with SDP, then 180 without it
Delayed offer, then the ACK answer
SIP finishes, then RTP starts
Static types need no guess
Pick a shared name
Reports follow the answer
Digits in RTP, named in SDP
ptime sits in the offer
Answer says sendonly
407 first, then a 401
INVITE over TLS
Two legs, two methods
PAI stops at the edge
No Identity header, no STIR result
A REGISTER that should have been challenged
Policy on one INVITE
Offer with a documentation address
Response follows the source port
Binding request, then a SIP offer
Allocate, then media via the relay
Gather, offer, check, nominate
UDP rewritten, TLS not rewritten
First packet, not c=
Requirements do not change the ladder
Two Call-IDs for one conversation
Hide the caller address
Anchored answer SDP
OPTIONS before the next INVITE
INVITE refused at the budget
The standby is not a SIP hop
CUBE between the phone and the carrier
session target becomes the Request-URI host
Diversion present, then removed on the copy you test
Delayed inbound, early outbound
CUCM trunk to CUBE
CUCM forwards to the Unity SIP peer
The messages the debug is showing
Two sides of one Oracle SBC
Server removed on the way out
SIP still looks like SIP on either family
Remove Server toward the carrier
One header, removed only by the grammar that is loaded
PAI arrives from the carrier
TLS INVITE with an FQDN Contact
INVITE to the primary proxy
Healthy OPTIONS, then a call
TLS and SRTP toward the Webex peer
This ladder is premises-based PSTN only
SIPREC toward the analytics connector
Request-URI matches the termination URI
BYOC Cloud signaling in the synthetic
Match this dialog, then the 2xx ACK
CANCEL, then 487
A capture of one answered call
Synthetic ccsip, then the response
422, then a taller session interval
OPTIONS to Teams after TLS
Each hop has a document
An intended path among lab addresses
Listed sources only
Synthetic emergency request
Fork to a disclosed SRS
SIP sets up the call. EF applies to the voice.
OPTIONS, then 503
Baseline OPTIONS, then one INVITE
Roles on a single call
The ladder stays SIP
Assistance is not a hop
The call the exams still expect you to read
Which page owns the hop
PSTN lands on CUBE, then CUCM
Premises PSTN through Local Gateway
The ladder the interview is about
A 488, named correctly
Two legs, two Call-IDs possible
Silence after a completed ladder
Checklist mapped onto one call
CUCM to CUBE to Teams
Which number is on which hop
The application picks SIP. The socket is only the transport.
Register on WSS, with the example's .invalid host
Gather, then signal the result in SDP
SIP offers the description. DTLS keys the packets.
Browser softphone, study sequence
SIP hits Kamailio. Media can be anchored in rtpengine.
A phone registers to Asterisk. The dialplan is a separate file.
SIP profile accepts the call. The dialplan routes it.
Edge proxy in front of a PBX
SIP on Kamailio, RTP toward rtpengine
A successful call is not a compliance result
Identity is a header on the INVITE, not a second call
An emergency study call still needs location, not just SIP
Two paths. Do not merge them in one ladder.
Four checks, one call trace
From a word to a document
The session completes. A neighbor hears an event.
Decide the plane, then the page
CUCM trunk toward an open SIP server
A control event is not the SIP call
User agent, proxy, registrar
Offer then answer
Signaling beside media
ICE candidate checklist
SBC as a back-to-back user agent
STIR identity on the INVITE
Trunk cutover
Browser media and a SIP trunk
Sources and go deeper
- RFC 3261 — last checked 2026-10