TCP/IP MTU size for EE

To ensure optimal performance, the TCP/IP maximum transmission unit (MTU) size should be greater than or equal to the RTP network layer packet (NLP) size. The MTU size (both IPv4 and IPv6) might change during the life of the EE connection. The MTU can change for the following reasons:

Initially, VTAM® queries the TCP/IP stack for its MTU size and sets the EE connection to use this value. This MTU size has already been reduced to account for various header lengths such as the IP, UDP and LLC headers necessary for EE traffic.
VTAM also takes into account the VTAM MTU operand value, if specified. The MTU operand may be specified on three types of VTAM major nodes:
- For EE connection networks, this parameter may be defined on the connection network GROUP definition statements in the EE XCA major node.
- For dial in Enterprise Extender connections which have their associated PUs dynamically created, this parameter may be defined on the model major node (DYNTYPE=EE) PU definition statement.
- For predefined Enterprise Extender connections, this parameter may be defined on the PU definition statement in the switched major node.
VTAM then takes the lesser of the TCP/IP stack's computed MTU size and the VTAM defined MTU operand value (if specified). If the TCP/IP stack presents a value less than 768 bytes, VTAM sets the MTU to 768 because this is the smallest packet size allowed by the HPR architecture.
The MTU size for an EE connection is fairly constant when the EE connection is established. However, in the event the TCP/IP stack's MTU size changes, RTP pipes with endpoints on the same node as the TCP/IP stack dynamically detect these changes when their outbound packets are being transmitted. Some reasons for MTU size changes include:
- New IP routes come available with different local MTU sizes
- Existing IP routes become unavailable.
- Path MTU discovery is enabled for IPv4 or IPv6 EE connections (see the PMTUD start option in z/OS Communications Server: SNA Resource Definition Reference for details), and path MTU changes are discovered in the IP network.

Table 1 shows connection conditions and related results. When establishing an RTP connection (for a CP-CP session, to transport ROUTE_SETUP GDS variables, or for an LU-LU session) over an EE connection, RTP pipes learn the MTU size when the pipes are being established (RSETUP flows). RTP then segments data to this size when transmitting outbound data.

Table 1. Connection conditions and results
If	Then
This node is the origin endpoint of the RTP connection	VTAM sets the maximum packet size equal to the lesser of the MTU size or the VTAM maximum data size
This node is an intermediate node or the destination node of an RTP connection	VTAM sets the maximum packet size equal to the lesser of the MTU size, VTAM maximum data size for the next hop, or the value received on the ROUTE_SETUP GDS variable
This node is one of the endpoints of the RTP connection and a change in the EE connection's MTU size occurs.	When VTAM detects this condition (the EE connection's MTU size changes during the transmission of an NLP) the MTU size is altered. This change is specified in message IST2029I when you issue the DISPLAY EE command. Also, if this change alters the permitted NLP size (NLP size cannot be increased beyond the originally negotiated value for the RTP connection) then this change is specified in message IST1511I which is displayed as the result of the DISPLAY ID=rtp-pu command.

If the MTU size is less than 768 bytes, VTAM sets the maximum packet size to 768 (this is the smallest maximum packet size allowed by VTAM for HPR packets). This limitation can cause TCP/IP to fragment but exists because the RTP layer cannot allow the HPR header to be segmented in the RTP layer.