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Topic # 159933 17-Dec-2014 17:35
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I'm on Spark DSL. Anybody else noticing MTU issues on Spark? Have had issues with connectivity for past 3 days or so. Looking at it now I can only pass up to 1272 bytes unfragmented implying an MTU of 1300.
 
Haven’t made any router changes for over a month. Anyone else (clients or at home) having this issue?
 
ping -l 1273 -f google.com
 
Pinging google.com [216.58.220.96] with 1273 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
 
Ping statistics for 216.58.220.96:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

ping -l 1272 -f google.com
 
Pinging google.com [216.58.220.96] with 1272 bytes of data:
Reply from 216.58.220.96: bytes=64 (sent 1272) time=55ms TTL=53
Reply from 216.58.220.96: bytes=64 (sent 1272) time=54ms TTL=53
Reply from 216.58.220.96: bytes=64 (sent 1272) time=56ms TTL=53
Reply from 216.58.220.96: bytes=64 (sent 1272) time=55ms TTL=53
 
Ping statistics for 216.58.220.96:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 54ms, Maximum = 56ms, Average = 55ms

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  Reply # 1199377 17-Dec-2014 17:45
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On Spark UFB, my MTU appears to be 1492 (which is expected based on my router config).

Pinging google.com [74.125.237.192] with 1464 bytes of data:
Reply from 74.125.237.192: bytes=1464 time=28ms TTL=53
Reply from 74.125.237.192: bytes=1464 time=30ms TTL=53
Reply from 74.125.237.192: bytes=1464 time=28ms TTL=53
Reply from 74.125.237.192: bytes=1464 time=29ms TTL=53

Ping statistics for 74.125.237.192:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 28ms, Maximum = 30ms, Average = 28ms




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  Reply # 1199419 17-Dec-2014 18:48
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I can get my byte size up to 1472 with spark VDSL, which would be 1500?

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  Reply # 1199428 17-Dec-2014 19:01
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Depending on the router you should be able to get a MTU of 1500, but worst case 1492 with the 8 bytes for PPPoE header.

When using the recent spark modems you shouldn't have an issue with 1500 MTUs on ADSL, VDSL and UFB.

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  Reply # 1199496 17-Dec-2014 20:13
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base64nz:
ping -l 1273 -f google.com
 
Pinging google.com [216.58.220.96] with 1273 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
 
Ping statistics for 216.58.220.96:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),


I'm almost certain that output indicates the MTU on your PC has been set to 1300.  If it were an upstream device in Spark, I'd expect the IP address of the reporting device to appear in the output.

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  Reply # 1200935 20-Dec-2014 08:29
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hashbrown:
base64nz:
ping -l 1273 -f google.com
 
Pinging google.com [216.58.220.96] with 1273 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
 
Ping statistics for 216.58.220.96:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),


I'm almost certain that output indicates the MTU on your PC has been set to 1300.  If it were an upstream device in Spark, I'd expect the IP address of the reporting device to appear in the output.


Correction.  Windows caches the path MTU once discovered for a given destination, so the output above could be the result of a cached response from an upstream device.  If you try pinging a host you haven't accessed though, it should give you something like below.  In this case it's my routers IP sending the ICMP unreachable for the first ping.

ping -l 1470 -f google.com

Pinging google.com [74.125.237.166] with 1470 bytes of data:
Reply from 192.168.1.1: Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 74.125.237.166:
    Packets: Sent = 4, Received = 1, Lost = 3 (75% loss),

Edited for formatting

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