Benefits of “Don't Fragment” on TCP Packets?

Question

One of our customers is having trouble submitting data from our application (on their PC) to a server (different geographical location). When sending packets under 1100 bytes everything works fine, but above this we see TCP retransmitting the packet every few seconds and getting no response. The packets we are using for testing are about 1400 bytes (but less than 1472). I can send an ICMP ping to www.google.com that is 1472 bytes and get a response (so it's not their router/first few hops).

I found that our application sets the DF flag for these packets, and I believe a router along the way to the server has an MTU less than/equal to 1100 and dropping the packet.

This affects 1 client in 5000, but since everybody's routes will be different this is expected.

The data is a SOAP envelope and we expect a SOAP response back. I can't justify WHY we do it, the code to do this was written by a previous developer.

So... Are there any benefits OR justification to setting the DF flag on TCP packets for application data?

I can think of reasons it is needed for network diagnostics applications but not in our situation (we want the data to get to the endpoint, fragmented or not). One of our sysadmins said that it might have something to do with us using SSL, but as far as I know SSL is like a stream and regardless of fragmentation, as long as the stream is rebuilt at the end, there's no problem.

If there's no good justification I will be changing the behaviour of our application.

Thanks in advance.

Answer 1

The operation of Path-MTU discovery is described in RFC 1191, https://tools.ietf.org/html/rfc1191. It is better for TCP to discover the Path-MTU than to have every packet over a certain size fragmented into two pieces (typically one large and one small).

Answer 2

The DF flag is typically set on IP packets carrying TCP segments.

This is because a TCP connection can dynamically change its segment size to match the path MTU, and better overall performance is achieved when the TCP segments are each carried in one IP packet.

So TCP packets have the DF flag set, which should cause an ICMP Fragmentation Needed packet to be returned if an intermediate router has to discard a packet because it's too large. The sending TCP will then reduce its estimate of the connection's Path MTU (Maximum Transmission Unit) and re-send in smaller segments. If DF wasn't set, the sending TCP would never know that it was sending segments that are too large. This process is called PMTU-D ("Path MTU Discovery").

If the ICMP Fragmentation Needed packets aren't getting through, then you're dealing with a broken network. Ideally the first step would be to identify the misconfigured device and have it corrected; however, if that doesn't work out then you add a configuration knob to your application that tells it to set the TCP_MAXSEG socket option with setsockopt(). (A typical example of a misconfigured device is a router or firewall that's been configured by an inexperienced network administrator to drop all ICMP, not realising that Fragmentation Needed packets are required by TCP PMTU-D).

Answer 3

Apparently, some protocols like NFS benefit from avoiding fragmentation (link text). However, you're right in that you typically shouldn't be requesting DF unless you really require it.