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MTU to CDN's

On Fri, Jan 19, 2018 at 8:58 AM, Jared Mauch <jared at puck.nether.net> wrote:
>> On Jan 18, 2018, at 8:44 PM, William Herrin <bill at herrin.us> wrote:
>>> Which packet?  Is there a specific CDN that does this?  Iâ??d be curious to see
>>> data vs speculation.
>> Path MTU discovery (which sets the DF bit on TCP packets) is enabled
>> by default on -every- operating system that's shipped for decades now.
> Iâ??m not seeing this in a PCAP capture to at least one CDN, either from my
> host or from the CDN endpoint.
> PCAP: https://puck.nether.net/~jared/akamai.pcap

Hi Jared,

tcpdump -v -n -nn -r akamai.pcap |more
reading from file akamai.pcap, link-type EN10MB (Ethernet)

08:54:48.611321 IP (tos 0x0, ttl 64, id 12596, offset 0, flags [DF],
proto TCP (6), length 60) > Flags [S], cksum 0x1504
(incorrect -> 0x5a14), seq 3315894416, win 29200, options [mss
1460,sackOK,TS val 3822930236 ecr 0,nop,wscale 7], length 0

08:54:48.633286 IP (tos 0x0, ttl 58, id 0, offset 0, flags [DF], proto
TCP (6), length 60) > Flags [S.], cksum 0x0972
(correct), seq 3383397658, ack 3315894417, win 28960, options [mss
1460,sackOK,TS val 2906475904 ecr 3822930236,nop,wscale 5], length 0

Note: "flags [DF]"

That means the don't fragment bit is set.

Bill Herrin

William Herrin ................ herrin at dirtside.com  bill at herrin.us
Dirtside Systems ......... Web: <http://www.dirtside.com/>