UDP packet chaining

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When UDP Datagrams to be transmitted are larger than the adapters MTU size, the IP protocol layer will fragment the datagram into MTU size fragments. Ethernet interfaces include a UPD packet chaining feature. This feature is enabled by default in AIX®.

UDP packet chaining causes IP to build the entire chain of fragments and pass that chain down to the Ethernet device driver in one call. This improves performance by reducing the calls down through the ARP and interface layers and to the driver. This also reduces lockand unlock calls in SMP environment. It also helps the cache affinity of the code loops. These changes reduce the CPU utilization of the sender.

You can view the UDP packet chaining option with the ifconfig command. The following example shows the ifconfig command output for the en0 interface, where the CHAIN flag indicates that packet chaining in enabled:

# ifconfig en0
en0: flags=5e080863,80<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST,GROUPRT,64BIT,CHECKSUM_OFFLOAD,PSEG,CHAIN>
         inet 192.1.6.1 netmask 0xffffff00 broadcast 192.1.6.255
         tcp_sendspace 65536 tcp_recvspace 65536 tcp_nodelay 1

Packet chaining can be disabled by the following command:

# ifconfig en0 -pktchain

# ifconfig en0
en0: flags=5e080863,80<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST,GROUPRT,64BIT,CHECKSUM_OFFLOAD,PSEG>
         inet 192.1.6.1 netmask 0xffffff00 broadcast 192.1.6.255
         tcp_sendspace 65536 tcp_recvspace 65536 tcp_nodelay 1

Packet chaining can be re-enabled with the following command:

# ifconfig en0 pktchain

# ifconfig en0
en0: flags=5e080863,80<UP,BROADCAST,NOTRAILERS,RUNNING,SIMPLEX,MULTICAST,GROUPRT,64BIT,CHECKSUM_OFFLOAD,PSEG,CHAIN>
         inet 192.1.6.1 netmask 0xffffff00 broadcast 192.1.6.255
         tcp_sendspace 65536 tcp_recvspace 65536 tcp_nodelay 1