On my old Android phone, the APN has the following fields:

Name
APN
Proxy
Port
Username
Password Server
MMSC
MMS Proxy
MMS Port
MCC
MNC
Authentication type
APN type
APN protocol
APN roaming protocol
APN enable/disable
Bearer
MVNO type
MVNO value

My Ubuntu Touch phone has far fewer fields:

Used For
Name
APN
MMSC
Proxy
Proxy Port
Username
Password

How to these fields correspond to each other? Some are obvious (Name, APN, MMSC) but what about proxy and proxy port. Does MMS Proxy correspond to Proxy and MMS Port to Proxy Port?

The carrier is AT&T prepaid.