Change in osmo-gsm-manuals[master]: sigtran: Describe sctp/m3ua role, multi-homing, traffic-modes

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laforge gerrit-no-reply at lists.osmocom.org
Fri Nov 20 15:50:29 UTC 2020


laforge has uploaded this change for review. ( https://gerrit.osmocom.org/c/osmo-gsm-manuals/+/21267 )


Change subject: sigtran: Describe sctp/m3ua role, multi-homing, traffic-modes
......................................................................

sigtran: Describe sctp/m3ua role, multi-homing, traffic-modes

Change-Id: I4c54dbde5a0b0f6ae38efdd047b17b4dbaae12c3
---
M common/chapters/sigtran.adoc
1 file changed, 62 insertions(+), 0 deletions(-)



  git pull ssh://gerrit.osmocom.org:29418/osmo-gsm-manuals refs/changes/67/21267/1

diff --git a/common/chapters/sigtran.adoc b/common/chapters/sigtran.adoc
index 42135d5..cbf513e 100644
--- a/common/chapters/sigtran.adoc
+++ b/common/chapters/sigtran.adoc
@@ -333,3 +333,65 @@
 stackings) operates on top of both IPv4 and IPv6.  As the entire
 underlying IP transport is transparent to the SS7/SCCP applications,
 there is no restriction on whether to use SIGTRAN over IPv4 or IPv6.
+
+==== SCTP multi-homing in SIGTARN
+
+SCTP, unlike more traditional IP L4 protocols (TCP, UDP) doesn't work
+based on a _connection_ between source IP:port and Destination IP:port.
+
+Instead, SCTP creates _associations_ between two endpoints, both of which
+can have any number of IP addresses.  This means that in case of
+network outage, traffic can continue to flow through any of the IP
+addresses of that association.
+
+The Linux kernel by default advertises all IP addresses of the local
+system to the peer.  This can be seen when inspecting the SCTP INIT
+chunk e.g. in wireshark.  While this may be a reasonable default in some
+use cases, it is not always the best idea.  Imagine addresses of
+internal/private IP networks, for example local bridge devices between
+lxc or docker containers, or local VMs.  Such addresses have no
+significance beyond the local machine.
+
+Subsequently, libosmo-sigtran allows the user to explicitly select which
+local IP addresses shall be used in SCTP multi-homing for the SIGTRAN
+associations it manages.  The user can achieve this by specifying
+multiple `local-ip` VTY commands within one `asp` (SCTP client role) or
+within one `listen m3ua 2905` (SCTP server role).
+
+==== SCTP role
+
+The _SCTP role_ defines which of the two L4 protocol roles SCTP assumes:
+
+* The _SCTP server_ role binds to a local port and handles incoming
+  connections from clients
+* The _SCTP client_ role connects to a remote SCTP sever.
+
+==== M3UA/SUA role
+
+The _M3UA role_ (or _SUA role_) determines which role a given peer of a
+M3UA connection implements.  3GPP specifies the following role:
+
+* _SGP_ (Signaling Gateway): The entity connected to the larger SS7
+  network
+* _ASP_ (Application Server Process): A client application that connects
+  to the SGW to talk to the SS7 network
+* _IPSP_ (IP Server Process): M3UA in point-to-point mode
+
+Osmocom (libosmo-sigtran) implements both the SGP and ASP roles, but not
+the IPSP role.
+
+==== traffic modes in SIGTRAN
+
+Whenever an AS consists of multiple ASPs, the traffic mode expresses how
+messages are distributed between those ASPs.
+
+* _Override_: There is always one active ASP and multiple hot standby
+  ASPs.  If the active ASP fails, one of the remaining ASPs will become
+  the new active ASP.
+* _Loadshare_: The messages will be distributed between the different
+  ASPs in a way to distribute the load among them.  Details are
+  implementation specific.
+* _Broadcast_: A copy of every incoming signaling message is sent to
+  _all_ the ASPs in broadcast traffic mode.
+
+Osmocom (libosmo-sigtran) implements all above-mentioned traffic modes.

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Gerrit-Project: osmo-gsm-manuals
Gerrit-Branch: master
Gerrit-Change-Id: I4c54dbde5a0b0f6ae38efdd047b17b4dbaae12c3
Gerrit-Change-Number: 21267
Gerrit-PatchSet: 1
Gerrit-Owner: laforge <laforge at osmocom.org>
Gerrit-MessageType: newchange
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