Media anchoring and media release on SBCs

See also

Sources

1. 1
   Asterisk res_pjsip endpoint option direct_media, added in 12.0.0, determines whether media may flow directly between endpoints and is listed with default yes; direct_media_method defaults to invite and also accepts update.

   res_pjsip - Asterisk Documentation · res_pjsip [endpoint] options table: direct_media, direct_media_method · Checked 2026-10-14
2. 2
   When anchoring media, the AudioCodes Mediant 500L SBC modifies all IP address fields in the SDP, including the origin line, session and media connection lines, media port and RTCP address and port.

   Media Anchoring (Mediant 500L SBC and Media Gateway User's Manual Version 7.6) · Media Anchoring, SDP field list · Checked 2026-10-14
3. 3
   The AudioCodes Mediant 500L SBC manual lists resolving NAT problems, enforcing media security policies, media transcoding and media monitoring as reasons media anchoring may be required.

   Media Anchoring (Mediant 500L SBC and Media Gateway User's Manual Version 7.6) · Media Anchoring, introductory paragraph · Checked 2026-10-14
4. 4
   AudioCodes Mediant Software SBC employs media anchoring by default, so the media session traverses the device unless direct media is enabled.

   Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, opening paragraph · Checked 2026-10-14
5. 5
   AudioCodes lists saved network bandwidth, lower device CPU usage and no interference with SDP negotiation or RTP header manipulation as direct-media benefits, typically for calls within the same LAN or domain that need no NAT traversal or transcoding.

   Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, benefits bullet list · Checked 2026-10-14
6. 6
   With direct media on AudioCodes Mediant Software SBC, SDP manipulation, forced transcoding, extension coders, DTMF extensions and SRTP/RTP extensions are not supported and media-related IP Profile parameters do not apply.

   Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, limitations notes · Checked 2026-10-14
7. 7
   AudioCodes calls the mode where RTP/SRTP flows directly between endpoints No Media Anchoring, also known as Anti-Tromboning or Direct Media, and SIP signaling still traverses the device in this mode.

   Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, opening paragraph · Checked 2026-10-14
8. 8
   AudioCodes Mediant Software SBC uses direct media between endpoints whose IP Profiles share the same non-empty Direct Media Tag value.

   Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, enabling conditions list (IP Profile) · Checked 2026-10-14
9. 9
   When direct media is enabled with the global SBCDirectMedia parameter on AudioCodes Mediant Software SBC, direct media is always enforced and calls are not recorded by SIPREC.

   Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, SIPREC note · Checked 2026-10-14
10. 10
    For an anchored call the AudioCodes Mediant 500L SBC allocates separate local ports per leg from the Media Realm assigned to the leg's IP Group, falling back to the Media Realm of the leg's SIP Interface if the IP Group has none.

    Media Anchoring (Mediant 500L SBC and Media Gateway User's Manual Version 7.6) · Media Anchoring, Media Realm selection paragraph · Checked 2026-10-14
11. 11
    For SIP Interface and IP Profile based direct media, AudioCodes Mediant Software SBC still allocates media ports because a call may switch to media-anchored mode mid-call for early media, forwarding, transfer or on-hold tones, so the Media Realm needs enough ports.

    Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, media port allocation note · Checked 2026-10-14
12. 12
    AudioCodes documents incomplete REFER call-transfer support with direct media: one of the SIP user agents in the call must support re-INVITE messages without SDP.

    Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, limitations notes (call transfer) · Checked 2026-10-14
13. 13
    On AudioCodes Mediant Software SBC the global SBCDirectMedia parameter applies direct media to all calls and overrides the other direct-media settings.

    Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, enabling conditions list (global parameter) · Checked 2026-10-14
14. 14
    The AudioCodes SIP Interface parameter SBC Direct Media can be set to Enable or to Enable When Single NAT for endpoints behind the same NAT, and both endpoints must be on the same SIP Interface.

    Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, enabling conditions list (SIP Interface) · Checked 2026-10-14
15. 15
    When a SIPREC recording rule applies, AudioCodes Mediant Software SBC automatically disables direct media for calls whose direct media was enabled by SIP Interface or IP Profile.

    Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, SIPREC note · Checked 2026-10-14
16. 16
    An initial INVITE carrying the header X-AC-Action: direct-media forces direct media on AudioCodes Mediant Software SBC regardless of other settings, and the device opens no media ports for such calls.

    Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, X-AC-Action note · Checked 2026-10-14
17. 17
    Cisco CUBE media anti-trombone, enabled with media anti-trombone under voice service voip, looks for looped or hairpin calls and triggers another media negotiation after which CUBE is no longer in the media path.

    Troubleshoot No-way Audio Issue with Hairpin Calls on CUBE · Solution section, Media Anti-Trombone · Checked 2026-10-14
18. 18
    Cisco states that both CUBE and the ITSP must support the anti-trombone feature for media anti-trombone to work.

    Troubleshoot No-way Audio Issue with Hairpin Calls on CUBE · Solution section, Media Anti-Trombone · Checked 2026-10-14
19. 19
    Cisco document 214622 configuration steps pair global media flow-around with a dial-peer voice class media flow-through and state that dial-peers without that voice class fall back to media flow-around, implying the dial-peer setting does take effect.disputed

    Troubleshoot No-way Audio Issue with Hairpin Calls on CUBE · Solution section, Media Flow-Around configuration steps · Checked 2026-10-14
20. 20
    Cisco document 214622 states that dial-peer media flow-through does not take effect when media flow-around is configured globally.disputed

    Troubleshoot No-way Audio Issue with Hairpin Calls on CUBE · Solution section, Media Flow-Around note · Checked 2026-10-14
21. 21
    Cisco's example enables media flow-around globally with media flow-around under voice service voip and applies flow-through to chosen dial-peers through a voice class media containing media flow-through.

    Troubleshoot No-way Audio Issue with Hairpin Calls on CUBE · Solution section, Media Flow-Around configuration steps · Checked 2026-10-14
22. 22
    With media flow-around on Cisco Unified Border Element, signaling still terminates and originates on CUBE while media packets bypass CUBE and flow directly between endpoints.

    Troubleshoot No-way Audio Issue with Hairpin Calls on CUBE · Solution section, Media Flow-Around · Checked 2026-10-14
23. 23
    Cisco warns that media flow-around can impact CUBE functionality because CUBE cannot terminate media for any call in that mode.

    Troubleshoot No-way Audio Issue with Hairpin Calls on CUBE · Solution section, Media Flow-Around · Checked 2026-10-14
24. 24
    FreeSWITCH bypass media, set with bypass_media or inbound-bypass-media (alias inbound-no-media), rewrites the SDP so endpoints exchange RTP directly, leaving transcoding, recording, RTP DTMF detection and hold music unavailable; resume-media-on-hold can pull the call back into the media path on hold.

    Chapter 17: Media Handling (FreeSWITCH documentation) · Chapter 17 Media Handling, Bypass Media Mode section · Checked 2026-10-14
25. 25
    By default FreeSWITCH terminates RTP on both call legs independently, and transcoding, recording, DTMF detection, jitter buffering and hold music all require this mode.

    Chapter 17: Media Handling (FreeSWITCH documentation) · Chapter 17 Media Handling, default media path section · Checked 2026-10-14
26. 26
    FreeSWITCH proxy media mode, set with the proxy_media channel variable or inbound-proxy-media profile parameter, passes RTP between endpoints without decoding the payload, so transcoding is not possible.

    Chapter 17: Media Handling (FreeSWITCH documentation) · Chapter 17 Media Handling, Proxy Media Mode section · Checked 2026-10-14
27. 27
    Oracle SIP Distributed Media Release lets RTP/RTCP flow directly between SIP endpoints even when signaling traverses multiple Oracle SBCs, by encoding the original media realm, IPv4 address and port into the SIP messages entering the backbone.

    SIP Distributed Media Release (Oracle Communications Session Border Controller Configuration Guide 9.3.0) · SIP Distributed Media Release, overview · Checked 2026-10-14
28. 28
    Oracle DMR can carry the encoded media information in an a=acme-media SDP attribute, an acme-media Contact header parameter or a P-Media-Release header, selected by the sip-config option media-release, with P-Media-Release used when no header is named.

    SIP Distributed Media Release (Oracle Communications Session Border Controller Configuration Guide 9.3.0) · SIP Distributed Media Release, encoding options and sip-config options field · Checked 2026-10-14
29. 29
    The Oracle SBC 9.3.0 SIP Distributed Media Release page states that if mm-in-network is enabled and the ingress and egress realms share a network interface, the SBC is allowed to release the media.disputed

    SIP Distributed Media Release (Oracle Communications Session Border Controller Configuration Guide 9.3.0) · SIP Distributed Media Release, realm-config mm in network field description · Checked 2026-10-14
30. 30
    On Oracle Enterprise Session Border Controller ECZ7.4.0 the realm-config parameter mm-in-system defaults to enabled, and setting it to disabled stops the ESBC steering media through the system.

    Media Release Configuration (Oracle Enterprise Session Border Controller ECZ7.4.0 Configuration Guide) · Media Release Configuration, mm-in-system parameter · Checked 2026-10-14
31. 31
    On Oracle Enterprise SBC 8.1.0, mm-same-ip is ignored when mm-in-realm is disabled; with mm-in-realm enabled and mm-same-ip disabled, media is managed in the realm but released when the two endpoints are behind the same IP address.

    Media Release Behind the Same IP Address (Oracle Enterprise Session Border Controller ACLI Configuration Guide 8.1.0) · Media Release Behind the Same IP Address, mm-in-realm and mm-same-ip parameters · Checked 2026-10-14
32. 32
    The Oracle SBC 9.1.0 ACLI reference states that mm-in-network, default enabled, makes the SBC steer media between endpoints in different realms on the same network interface, and that when disabled the endpoints send media directly to each other.disputed

    realm-config (Oracle Communications Session Border Controller ACLI Reference Guide 9.1.0) · realm-config, mm-in-network parameter · Checked 2026-10-14
33. 33
    On Oracle Communications Session Border Controller the realm-config parameter mm-in-realm controls whether media is steered through the SBC when both endpoints are in the same realm, and defaults to disabled.

    realm-config (Oracle Communications Session Border Controller ACLI Reference Guide 9.1.0) · realm-config, mm-in-realm parameter · Checked 2026-10-14
34. 34
    On Oracle Communications Session Border Controller the realm-config parameter mm-in-system defaults to enabled (manage, latch and steer media); setting it to disabled releases media so no media flows through the SBC.

    Media Release Across SIP Network Interfaces (Oracle Communications Session Border Controller Configuration Guide 9.2.0) · Media Release Across SIP Network Interfaces, mm-in-system parameter · Checked 2026-10-14
35. 35
    Oracle SBC 9.2.0 adds the realm-config option release-media-at-same-nat, which releases media between endpoints sharing the same NAT IP address even when one endpoint is at, but not behind, that NAT; it is appended with options +release-media-at-same-nat.

    Media Release Behind the Same NAT IP Address (Oracle Communications Session Border Controller Configuration Guide 9.2.0) · Media Release Behind the Same NAT IP Address, overview and configuration steps · Checked 2026-10-14
36. 36
    Oracle SBC realm-config restricted-latching accepts none, sdp (latch only to the address in the SDP) or peer-ip (latch only to the Layer 3 peer address), with none as the default.

    realm-config (Oracle Communications Session Border Controller ACLI Reference Guide 9.1.0) · realm-config, restricted-latching parameter · Checked 2026-10-14
37. 37
    When an SBC releases media, RTP no longer traverses it, so SBC media statistics and packet captures cannot show audio faults such as one-way audio for that call; media evidence must come from the endpoints or the network path.inferred

    Media Release Across SIP Network Interfaces (Oracle Communications Session Border Controller Configuration Guide 9.2.0) · Media Release Across SIP Network Interfaces, mm-in-system disabled effect · Checked 2026-10-14
38. 38
    RFC 5853 states that SBC media traffic management based on the session description does not work if user agents encrypt or integrity-protect their message bodies end to end.

    RFC 5853: Requirements from Session Initiation Protocol (SIP) Session Border Control (SBC) Deployments · Section 3.2.2 (Problems with media traffic management) · Checked 2026-10-14
39. 39
    RFC 5853 is an Informational RFC and not an Internet Standards Track specification, so it describes SBC deployment practices rather than mandating that SBCs anchor media.

    RFC 5853: Requirements from Session Initiation Protocol (SIP) Session Border Control (SBC) Deployments · Status of This Memo · Checked 2026-10-14
40. 40
    RFC 5853 states that SBCs can force media through a media relay for NAT traversal and that relaying media in only one direction can be sufficient.

    RFC 5853: Requirements from Session Initiation Protocol (SIP) Session Border Control (SBC) Deployments · Section 3.4.1 (Background on NAT traversal) · Checked 2026-10-14
41. 41
    RFC 5853 does not appear to define any mechanism for an SBC to release media back to a direct path after setup; media release is therefore a vendor-specific feature rather than an IETF-specified procedure.inferred

    RFC 5853: Requirements from Session Initiation Protocol (SIP) Session Border Control (SBC) Deployments · Sections 3.2 and 4 (read for absence of a release mechanism) · Checked 2026-10-14
42. 42
    RFC 5853 notes that inserting a media relay can prevent non-media uses of the media path such as media-path key agreement, and that this is sometimes intentional but not always necessary.

    RFC 5853: Requirements from Session Initiation Protocol (SIP) Session Border Control (SBC) Deployments · Section 3.2.2 (Problems with media traffic management) · Checked 2026-10-14
43. 43
    RFC 5853 describes media traffic management in which an SBC that modifies the session description can force media to be sent through a media relay instead of directly between user agents.

    RFC 5853: Requirements from Session Initiation Protocol (SIP) Session Border Control (SBC) Deployments · Section 3.2.1 (Background on media traffic management) · Checked 2026-10-14
44. 44
    RFC 7092 defines a media-aware B2BUA as a media relay that also inspects and potentially modifies the payload carried in UDP or TCP.

    RFC 7092: A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents · Section 3.2.2 (Media-Aware Relay) · Checked 2026-10-14
45. 45
    RFC 7092 defines a media-relay B2BUA as one that terminates media at the IP and transport layers but neither modifies nor restricts the payload carried in the packets.

    RFC 7092: A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents · Section 3.2.1 (Media Relay) · Checked 2026-10-14
46. 46
    RFC 7092 defines a media-termination B2BUA as one operating at the media payload layer such as the RTP/RTCP codec layer, which is the role needed for transcoding.

    RFC 7092: A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents · Section 3.2.3 (Media Termination) · Checked 2026-10-14
47. 47
    RFC 7092 states that by default most SBCs act as media-relay or media-aware B2BUAs, and that depending on local policy they can perform every B2BUA role in the taxonomy.

    RFC 7092: A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents · Section 4.3 (Session Border Controller) · Checked 2026-10-14
48. 48
    RFC 7362 states that hosted NAT traversal causes media to go through a relay instead of the direct IP-routed path between the user agents, and that operators view this as a benefit because it lets them monitor and measure media.

    Latching: Hosted NAT Traversal (HNT) for Media in Real-Time Communication · Section 4 (Latching) · Checked 2026-10-14
49. 49
    RFC 7362 records that the IETF advises against using latching over the Internet and recommends ICE instead.

    Latching: Hosted NAT Traversal (HNT) for Media in Real-Time Communication · Abstract · Checked 2026-10-14
50. 50
    RFC 7362 defines latching as a media relay locking onto the source address and port of the first received media packets and, in symmetric latching, sending media back to that address and port.

    Latching: Hosted NAT Traversal (HNT) for Media in Real-Time Communication · Section 4 (Latching) · Checked 2026-10-14
51. 51
    RFC 7362 describes restricted latching as the main mitigation for media hijacking: the SBC does not latch to packets from any source public IP address other than the one the SIP UA uses for signaling.

    Latching: Hosted NAT Traversal (HNT) for Media in Real-Time Communication · Section 5 (Security Considerations) · Checked 2026-10-14
52. 52
    Even on a media-bypass trunk, Teams inserts Media Processors when a call escalates from 1:1 to a group call, goes to a federated Teams user, or is forwarded or transferred to a Skype for Business user, and converts calls from unsupported endpoints such as web clients to non-bypass.

    Plan for media bypass with Direct Routing · Use of Teams Media Processors if trunk is configured for media bypass; Client endpoints supported with media bypass · Checked 2026-10-14
53. 53
    Teams Direct Routing media bypass uses ICE on the Teams client and ICE Lite on the SBC to find the most direct media path.

    Plan for media bypass with Direct Routing · About media bypass with Direct Routing, paragraph on ICE · Checked 2026-10-14
54. 54
    Teams Direct Routing media bypass is not SBC media release: it removes Microsoft Media Processors from the path while the SBC remains a media endpoint, so SBC-side anchoring features such as recording or transcoding are governed by the SBC's own media mode, not by the MediaBypass setting.inferred

    Plan for media bypass with Direct Routing · About media bypass with Direct Routing; Media Processors vs Transport Relays table · Checked 2026-10-14
55. 55
    With Teams Direct Routing media bypass, media is kept between the SBC and the Teams client instead of passing through Microsoft media components, while SIP signaling always flows via the Microsoft cloud.

    Plan for media bypass with Direct Routing · About media bypass with Direct Routing; Call flow and firewall planning · Checked 2026-10-14
56. 56
    Microsoft Teams Media Processors are always in the media path for non-bypassed end-user calls and for voice applications such as Auto Attendant, Call Queues and Call Park, but never in the path for bypassed end-user calls.

    Plan for media bypass with Direct Routing · Use of Media Processors and Transport Relays, bullet list and comparison table · Checked 2026-10-14
57. 57
    Teams Transport Relays are not B2BUAs and never change the codec between client and SBC, whereas Media Processors are B2BUAs that can transcode, for example SILK toward the client and G.711 toward the SBC.

    Plan for media bypass with Direct Routing · Use of Media Processors and Transport Relays, transcoding explanation · Checked 2026-10-14
58. 58
    Across the documented implementations, SBC-side recording and transcoding are only available while the SBC or B2BUA anchors and terminates media, so releasing media trades those capabilities for bandwidth and CPU savings.inferred

    Direct Media Calls (Mediant Software SBC User's Manual Version 7.6) · Direct Media Calls, limitations and SIPREC notes · Checked 2026-10-14

Documents

Cite this page

WarmTransfer. (2026, October 14). Media anchoring and media release on SBCs. WarmTransfer. https://warmtransfer.net/knowledge/sbc-media-anchoring

@misc{warmtransfer-sbc-media-anchoring,
  title  = {Media anchoring and media release on SBCs},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/sbc-media-anchoring},
  note   = {Verified 2026-10-14}
}