Added use case Shared service functions

Change-Id: Id06570995c112bda391689f80265d596c4ac43fa
Signed-off-by: Georg Kunz <georg.kunz@ericsson.com>

1 files changed, 42 insertions, 6 deletions

diff --git a/docs/requirements/use_cases/shared-service-functions.rst b/docs/requirements/use_cases/shared-service-functions.rst
index 12b2f5c..3d370c0 100644
--- a/docs/requirements/use_cases/shared-service-functions.rst
+++ b/docs/requirements/use_cases/shared-service-functions.rst
@@ -6,21 +6,56 @@
 Shared Service Functions
 ------------------------
 
-
 Description
 ^^^^^^^^^^^
 
-TBD
+This use case aims at binding multiple networks or network services to a single
+vNIC (port) of a given VM. There are several specific application scenarios for
+this use case:
+
+* Shared Service Functions: A service function connects to multiple (tenant)
+  networks by means of a single vNIC.
+
+  Typically, a vNIC is bound to a single (tenant) network. Hence, in order to
+  directly connect a service function to multiple (tenant) networks at the same
+  time, multiple vNICs are needed - each vNIC binding the service function to a
+  separate network. For service functions requiring connectivity to a large
+  number of networks, this approach does not scale as the number of vNICs per VM
+  is limited and additional vNICs occupy additional resources on the hypervisor.
+
+  A more scalable approach is to bind multiple networks to a single vNIC
+  and let the service function, which is now shared among multiple networks,
+  handle the separation of traffic itself.
+
+
+* Multiple network services: A service function connects to multiple different
+  network types such as a L2 network, a L3(-VPN) network, a SFC domain or
+  services such as DHCP, IPv6 NDP, firewall/security, etc.
+
+
+In order to achieve a flexible binding of multiple services to vNICs, a logical
+separation between a vNIC (instance port) - that is, the entity that is used by
+the compute service as hand-off point between the network and the VM - and a
+service interface - that is, the interface a service binds to - is needed.
+
+Furthermore, binding network services to service interfaces instead of to the
+vNIC directly enables a more dynamic management of the network connectivity of
+network functions as there is no need to add or remove vNICs.
 
 
 Requirements
 ^^^^^^^^^^^^
 
-
 Northbound API
 """"""""""""""
 
-TBD
+The API has to cover the following functionality:
+
+* management of instance ports
+
+* management of service ports
+
+* binding of services to service ports
 
 
 Data models
@@ -32,13 +67,14 @@ TBD
 Orchestration
 """""""""""""
 
-TBD
+None.
 
 
 Dependencies on other resources
 """""""""""""""""""""""""""""""
 
-TBD
+The compute service needs to be enabled to consume instance ports instead of
+classic Neutron ports.
 
 
 Implementation Proposal