Difference between AVP and Grouped AVP ?

Question

I saw Capabilities-Exchange-Request and Capabilities-Exchange-Answer messages. In CER message there are two places where Vendor-Id AVP is present, one as an independent AVP and the other one as part of Vendor-Specific-Application-Id AVP. If both of them are present in request message then both will be having same value or can hold different values.

Answer 1

Hi Ganesh,
First we need to understand why Diameter uses Attribute-Value pair data structure?
If we see any Seagull scenario file,the Diameter AVPs are defined in the form of XML tags.

1.XML is extensible and flexible by design, so there isn’t really a problem with being constrained by a fixed set of fields.
2.New XML elements and attributes can be invented and added as needed,
specifically to deal with variable and evolving data formats. Many applications use XML exactly for this reason.

Lets take a relational database example, you want to maintain peer table information

Name | Host | Port | State |

---

| SER_GX | 10.122.65.39 | 3869 | Up |

Same above information can be written using Attribute-Value pair:
(Name,SER_GX)
(Host, 10.122.65.39)
(Port, 3869)
(State,Up)

Hope the above explanation gives you an idea of what AVP means.
Grouped AVP is nothing but an AVP within another AVP.
For example :
Single AVP:

<avp name="Auth-Application-Id" value="4"> </avp>

Grouped AVP:

<avp name="Subscription-Id">
    <avp name="Subscription-Id-Type" value="0"> </avp>
    <avp name="Subscription-Id-Data" value=""> </avp> 
  </avp>

Answer 2

A diameter AVP has pre-defined format. Every diameter AVP has following items
1. AVP code
2. AVP Flags (VMPRRRRR bits) - V stands for vendor-specific, M stands for mandatory and P for protected
3. Vendor -Id (optional - it is present only if V bit is set in AVP flag)
3. AVP Length - AVP header length + Data Length
4. Data field.

If Data field itself is an AVP then this AVP is known as Grouped AVP.