General Switch Management Protocol (GSMP) Packet Encapsulations for Asynchronous Transfer Mode (ATM), Ethernet and Transmission Control Protocol (TCP)
Extracted elements (18)
TCP was chosen for IP-based GSMP transport because it provides reliable transport, network flow control, and end-system flow control suitable for networks that may have high loss and variable or unpredictable delay.
The Sender Instance and Receiver Instance fields are included in the Ethernet encapsulation (beyond what is in the adjacency protocol message) specifically to offer additional protection against the introduction of corrupt state, a consideration not needed in TCP where the connection provides ordering guarantees.
The type code used in the TCP TLV header (0x880C) is identical to GSMP's assigned Ethertype, providing a consistent protocol identifier across all encapsulations.
After adjacency protocol synchronisation, the receiver MUST discard incoming GSMP messages if the Sender Instance and Source Address do not match values stored by the adjacency protocol's 'Update Peer Verifier' operation, if the message arrives on the wrong port, or if the Receiver Instance does not match the current Sender Instance value.
GSMP messages other than adjacency protocol messages MUST NOT be sent until after the adjacency protocol has achieved synchronisation.
The MTU of the GSMP Message field is 1492 bytes for both ATM and Ethernet encapsulations.
When GSMP messages are transported over an IP network, they MUST be transported using TCP encapsulation. TCP provides reliable transport, network flow control, and end-system flow control.
When GSMPv3 is implemented for use in IP networks, security between the controller and client MUST be available and MUST be provided by IPsec. The IPsec Encapsulation Security Payload (ESP) MUST be used to provide both integrity and confidentiality.
Ethernet padding after adjacency is established must ensure the data field meets the minimum 46-byte Ethernet frame size. Padding bytes MUST be zero and are not considered part of the GSMP message.
For TCP encapsulations, the controller runs client code and the switch runs server code, listening on TCP port 6068. The switch MUST be a multi-connection server to allow multiple control sessions from N GSMP controller instances.
For the SYN message of the adjacency protocol in Ethernet encapsulation, the Destination Address is the broadcast address 0xFFFFFFFFFFFF. Alternatively, a configured unicast 48-bit IEEE MAC address may be used. For all other messages, the unicast MAC address of the destination is used.
The default GSMP control channel for LLC/SNAP encapsulated GSMP messages on ATM uses VPI=0 and VCI=15. This default MAY be changed using the GSMP MIB.
The LLC/SNAP header for ATM encapsulation contains the bytes 0xAA 0xAA 0x03 0x00 0x00 0x00 0x88 0x0C, where 0x880C is the assigned Ethertype for GSMP.
The Sender Instance and Receiver Instance fields are appended to all non-adjacency GSMP messages in the Ethernet encapsulation to offer additional protection against the introduction of corrupt state. These fields are already present in adjacency protocol messages.
For all uses of GSMP over an IP network, GSMP MUST be run over TCP/IP with IPsec ESP providing integrity and confidentiality. For ATM and Ethernet encapsulations, security MAY be provided at the link layer, but for secure operation over any media, the IP encapsulation with IPsec SHOULD be used.
ATM encapsulation wraps GSMP packets in AAL-5 CPCS-PDU with an LLC/SNAP header. The LLC field is 0xAA-AA-03 (3 bytes) and the SNAP field is 0x00-00-00-88-0C (5 bytes), followed by the GSMP message, 0–47 bytes of padding, and an 8-byte AAL-5 CPCS-PDU trailer.
Ethernet encapsulation includes a 6-byte Destination Address, 6-byte Source Address, 2-byte Ethertype (0x880C), variable-length GSMP Message, 4-byte Sender Instance, 4-byte Receiver Instance, variable Pad, and 4-byte Frame Check Sequence.
TCP encapsulation prepends a 4-byte TLV header to each GSMP message: a 2-byte Type field (value 0x880C, same as the GSMP Ethertype) and a 2-byte unsigned integer Length field indicating the total length of the GSMP message only, excluding the TLV header.