On 24/02/2016 10:33, Nick Cutting wrote:
Im an enterprise guy, so this is what I see for our clients.
I don't have exact info but if for example a TCP sessions stays up between
two end hosts (whether they are routers or windows boxes whatever..) and a path
changes from say a P2P to a routed VPN, quick enough that the tcp session
between the endpoints doesn't time out (I hope so) - the MSS is NOT
recalculated as, it is only set in the TCP handshake - which is what Adam is
referring to when you set the MSS on the interface.
The exact path mtu discovery technique doesn't seem to be well documented - the RFC is 26
years old - and unless your hosts are actively "checking" mtu using DF bit the
whole time a session is up (for changes) - I don't believe even half of RFC-1191 is
implemented in the real world.
If someone has knowledge of how cisco routers do it exactly - I would love to
hear.
They do appear to set DF bit on every IP packet (at least on a CSR1000v
this is true).
Packet dump shows DF being set on BGP keepalives:
dan@fluff:~$ sudo tcpdump -i virbr13 -v 'ip && tcp port 179'
tcpdump: listening on virbr13, link-type EN10MB (Ethernet), capture size
262144 bytes
11:06:36.922464 IP (tos 0xc0, ttl 63, id 1770, offset 0, flags [DF],
proto TCP (6), length 59)
172.16.0.6.59175 > 172.16.0.5.bgp: Flags [P.], cksum 0x4b81
(correct), seq 3523842787:3523842806, ack 1374955837, win 16384, length
19: BGP, length: 19
Keepalive Message (4), length: 19
11:06:37.651918 IP (tos 0xc0, ttl 63, id 1771, offset 0, flags [DF],
proto TCP (6), length 40)
172.16.0.6.59175 > 172.16.0.5.bgp: Flags [.], cksum 0x4f89
(correct), ack 20, win 16384, length 0
11:06:39.991295 IP (tos 0xc0, ttl 253, id 1831, offset 0, flags [DF],
proto TCP (6), length 59)
172.16.0.3.bgp > 172.16.0.5.53016: Flags [P.], cksum 0xe427
(correct), seq 579671816:579671835, ack 1679671767, win 16384, length
19: BGP, length: 19
Keepalive Message (4), length: 19
Whether they act on receiving an ICMP Frag Needed packet by decreasing
the MSS of the BGP TCP session is another question. When I get time I'll
try and test it.
Cheers,
Dan
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