PCP is a protocol that operates between subscribers and the NAT directly. This makes the protocol similar to DHCP or PPP in that the subscriber has a limited but direct control over the NAT behavior.
PCP is designed to allows the configuration of static port-forwards, obtain information about existing port forwards and to obtain the outside IP address from software running in the home network or on the CPE.
PCP runs on each MS-ISA as its own process and make use of the same source-IP hash algorithm as the NAT mappings themselves. The protocol itself is UDP based and is request/response in nature, in some ways, similar to UPnP.
PCP operates on a specified loopback interface in a similar way to the local DHCP server. It operates on UDP and a specified (in CLI) port. As Epoch is used to help recover mappings, a unique PCP service must be configured for each NAT group.
When epoch is lowered, there is no mechanism to inform the clients to refresh their mappings en masse. External synchronization of mappings is possible between two chassis (epoch does not need to be synchronized). If epoch is unsynchronized then the result is clients re-creating their mapping on next communication with the PCP server.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version = 1 |R| OpCode | Reserved (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) opcode-specific information :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) PCP Options :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The R-bit (0) indicates request and (1) indicates response. This is a request so (0).
OpCode defined as:
Requested Lifetime: Lifetime 0 means delete.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version = 1 |R| OpCode | Reserved | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) OpCode-specific response data :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (optional) Options :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
As this is a response, R = (1).
The Epoch field increments by 1 every second and can be used by the client to determine if state needs to be restored. On any failure of the PCP server or the NAT to which it is associated Epoch must restart from zero (0).
Result Codes:
0 SUCCESS, success.
1 UNSUPP_VERSION, unsupported version.
2 MALFORMED_REQUEST, a general catch-all error.
3 UNSUPP_OPCODE, unsupported OpCode.
4 UNSUPP_OPTION, unsupported option. Only if the Option was mandatory.
5 MALFORMED_OPTION, malformed option.
6 UNSPECIFIED_ERROR, server encountered an error
7 MISORDERED_OPTIONS, options not in correct order
Creating a Mapping
Client Sends
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | Reserved (24 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internal port | Suggested external port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Suggested External IP Address (32 or 128, depending on OpCode):
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MAP4 opcode is (1). Protocols: 0 – all; 1 – ICMP; 6 – TCP; 17 – UDP.
MAP4 (1), PEER4 (3) and PREFER_FAILURE are supported. FILTER and THIRD_PARTY are not supported.