MTU tweaking in Modem Mode

[moof]

I'm working through some tweaks with my VPN supplier...

but before I implement their advice I need to get my Draytek router set to the optimum MTU when connected to a Hub 3.0 in Modem Mode (on VIVID300)

tons of conflicting advice in the forums...   

the norm seems to be 1500, many suggest 1442, then I see lots of gamers down in the triple figures 

what should my MTU be set to, when the Hub 3.0 is in Modem Mode?

what pros and cons to adjusting this figure?

TIA

[ravenstar68]

You shouldn't need to touch the MTU.  The common MTU across the Internet is for the most part 1500.  There are some cases where this is different, but devices can detect if a Path MTU (PMTU) is lower and usually make adjustments for this on the fly.

To find your maximum MTU you can use ping to send an ICMP packet that with fixed length and the DNF (do not fragment) bit set.

For example.

C:\Users\timdu>ping -l 1472 -f google.com

Pinging google.com [216.58.206.78] with 1472 bytes of data:
Reply from 216.58.206.78: bytes=64 (sent 1472) time=31ms TTL=52
Reply from 216.58.206.78: bytes=64 (sent 1472) time=27ms TTL=52
Reply from 216.58.206.78: bytes=64 (sent 1472) time=26ms TTL=52
Reply from 216.58.206.78: bytes=64 (sent 1472) time=27ms TTL=54

Ping statistics for 216.58.206.78:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 26ms, Maximum = 31ms, Average = 27ms

The reason I use 1472 is because you need to take the headers into account which take up 28 bytes.

Therefore the MTU is the highest value that can be sent without fragmenting +28 - in this case 1500.

Tim

[moof]

cheers Tim

I followed a similar test and also got to the figure 1472, anything higher said Message to Long

i'm on a Mac but speak Windows as a second language 

Finding the largest MTU, by EXPERIMENT
ping -D -s 1472 www.dslreports.com 

--- www.dslreports.com ping statistics ---

1 packets transmitted, 0 packets received, 100.0% packet loss

mycomp:~ user$ ping -D -s 1474 www.dslreports.com

PING www.dslreports.com (64.91.255.98): 1474 data bytes

ping: sendto: Message too long

Request timeout for icmp_seq 0

^C

--- www.dslreports.com ping statistics ---

7 packets transmitted, 0 packets received, 100.0% packet loss

mycomp:~ user$ ping -D -s 1473 www.dslreports.com

PING www.dslreports.com (64.91.255.98): 1473 data bytes

ping: sendto: Message too long

Request timeout for icmp_seq 0

^C

--- www.dslreports.com ping statistics ---

4 packets transmitted, 0 packets received, 100.0% packet loss

mycomp:~ user$ ping -D -s 1472 www.dslreports.com

PING www.dslreports.com (64.91.255.98): 1472 data bytes

1480 bytes from 64.91.255.98: icmp_seq=0 ttl=49 time=112.737 ms

1480 bytes from 64.91.255.98: icmp_seq=1 ttl=49 time=176.851 ms

I seemed to be able to go lower and lower... without issue?

so setting my router to 1472 will be the optimum for Virgin, yes?

[ravenstar68]

No

because MTU (Maximum transmission unit) also needs to take the  ICMP headers into account.  1472 is the payload size.  The header size is 28 so the MTU is 1472 + 28 = 1500

Tim

[moof]

I C, so leave it on 1500? sorry my heads elsewhere today

[ravenstar68]

Yep - if you do end up visiting a site with a lower PMTU your operating system will handle it.

Tim

[moof]

gotcha 

so seeing as you seem to understand these three little letters better than i

whats your thoughts on the advice from my VPN provider:

"One last thing we can try, and it is usually the last thing on my list is to adjust the MTU settings so you are receiving the data in smaller chunks and therefore easier for your computer to process"

In a VPN-nut shell, I'm not seeing anywhere near 380Mbps when using a VPN, seeing more like 140Mbps... I was asking them about ways to improve the speed (if at all)

TIA

[ravenstar68]

I'm not sure changing MTU would have the desired effect.

MTU us the Maximum Transmission Unit for the medium you are using.  As a rule you want to leave this set as high as your connection will allow.  Remember your computer has to stitch packets back together and sometimes packets can be received out of order.  Smaller MTU means more packets for the same amount of data.  Including the additional headers.

@Sephiroth  I'd welcome your input.

Tim

[Sephiroth]

A pleasure, Tim. You've got it right as usual, both in your last post and the one previous to that.

Messing around with the MTU is inadvisable although I think I recall that some PS4 users reported better performance at a lower MTU. That, by the way, is down to how the game is written in terms of its packet size.

The MTU (Maximum Transmission Unit) is the largest size packet or frame, specified in octets (eight-bit bytes), that can be sent in a packet or frame-based network such as the Internet.

If you smaller packet sizes are needed for whatever reason, the IP layer (TCP/IP and UDP) breaks the data into smaller sizes.

[ravenstar68]

@moof  You also need to understand a little on how VPN's work.

VPN's essentially create a virtual network interface to "tunnel" the connection via your network card.  In this case it encrypts your connection and then adds the necessary headers in order to transmit the packet to the VPN server.

So lets consider two packets.  We'll use TCP/IP.

First without using a Tunnel

IP header - 20 bytes
TCP header - 20 bytes
Data segment 1460 bytes (In TCP terms the maximum size of the data segment is refered to as the MSS - Maximum Segment Size, and is actually negotiated by with the target server when the connection is set up.)

Now lets add a LT2P tunnel into the mix

IP header - 20 bytes
UDP Header - 8 bytes
LT2P Header - 12 bytes.

Encrypted packet containing

IP header - 20 bytes
TCP header 20 Bytes
Data Segment 1420 bytes

So the LT2P interface has an MTU of 1460 in order to take the additional headers into account, but your ethernet MTU remains unchanged.

Note as well that the TCP MSS will be negotiated between the VPN server and the destination server.  And will usually be transparent to you.

Tim