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Tuning Techniques

2.3 Solving Performance Problems

Restart TCP/IP Services after you enable jumbo frames dynamically.

To verify that TCP/IP Services now sees a jumbo frame size IPMTU, use a

command similar to:

$ ifconfig we0

WE0: flags=c43<UP,BROADCAST,RUNNING,MULTICAST,SIMPLEX>

*inet 10.0.0.4 netmask ffff0000 broadcast 10.0.255.255 ipmtu 9000

You can verify jumbo frame conﬁguration using the

tcpdump

command. For

example, consider two systems connected with Gigabit Ethernet, GIGA1 and

GIGA2:

On GIGA1, enter the following command:

$ tcpdump -c 2 -t port telnet

On GIGA2, enter the following command:

$ TELNET giga1

The

tcpdump

command displays the two packets from the TCP connection

establishment and reveals the negotiated maximum segment size (

mss

). For

example, on a correctly conﬁgured system the

tcpdump

trace from the TELNET

command shows:

$ tcpdump -c 2 -t port telnet

tcpdump: listening on

Filtering in user process

GIGA2.49188 > GIGA1.23: S 165176320:165176320(0) win 3000

GIGA1.23 > GIGA2.49188: S 1890602256:1890602256(0) ack 165176321 win 3000

2 packets (out of 5 examined)

Note that the requested

mss

of 8960 in the SYN packet is also accepted by

the server in the SYN ACK packket. On earlier versions of OpenVMS, the

jumbo-frame

mss

may be less than 8960 bytes.

If one of the systems is not conﬁgured with jumbo frames, the

tcpdump

utility

shows the following:

$ tcpdump -c 2 -t port telnet

tcpdump: listening on

Filtering in user process

GIGA2.49187 > GIGA1.23: S 142920192:142920192(0) win 3000

GIGA1.23 > GIGA2.49187: S 2953104448:2953104448(0) ack 142920193 win 4380

2 packets (out of 5 examined)

The TELNET client requests an

mss

of 8960 bytes in the SYN packet, but the

server responds with an

mss

of 1460 bytes in the SYN ACK packet. Therefore,

GIGA1 is not correctly conﬁgured for jumbo frames.

Tuning Techniques 2–19