Suggestion by Mathieu:


> in tcp.h:
> 
> class Tcp
> {
> public:
>   Tcp (void);
>   Ptr<Socket> CreateSocket (...);
> 
>   void SetDefaultSeqSize (uint32_t seqSize);
>   ...
> protected:
>   // make it available only to subclasses
>   uint32_t GetDefaultSeqSize (void);
>   ...
> private:
>   // hide the member.
>   uint32_t m_defaultSeqSize;
> };
> 
> in tcp.cc:
> 
> static NumericDefaultValue<uint32_t> g_defaultSegSize = ...;
> 
> Tcp::Tcp (void)
>   : m_defaultSeqSize (g_defaultSeqSize)
> {}
> 
> void 
> Tcp::SetDefaultSeqSize (uint32_t seqSize)
> {
>   m_defaultSeqSize = seqSize;
> }
> uint32_t 
> Tcp::GetDefaultSeqSize (void)
> {
>   return m_defaultSeqSize;
> }


I support this in principle, but the above suggestion needs a little massaging:
 TcpSocket is not a subclass of Tcp