Re: MTU to CDN's

[William Herrin <bill () herrin us>]

On Fri, Jan 19, 2018 at 8:58 AM, Jared Mauch <jared () puck nether net> wrote:
> > On Jan 18, 2018, at 8:44 PM, William Herrin <bill () herrin us> wrote:
> > > Which packet?  Is there a specific CDN that does this?  I’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.
>
> I’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)
    204.42.254.5.60262 > 23.0.51.165.80: 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)
    23.0.51.165.80 > 204.42.254.5.60262: 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.

Regards,
Bill Herrin


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