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From: David Laight <David.Laight@ACULAB.COM>
To: 'Xin Long' <lucien.xin@gmail.com>
Cc: network dev <netdev@vger.kernel.org>,
	"davem@davemloft.net" <davem@davemloft.net>,
	"kuba@kernel.org" <kuba@kernel.org>,
	"Marcelo Ricardo Leitner" <marcelo.leitner@gmail.com>,
	"linux-sctp@vger.kernel.org" <linux-sctp@vger.kernel.org>
Subject: RE: [PATCHv2 net-next 00/14] sctp: implement RFC8899: Packetization Layer Path MTU Discovery for SCTP transport
Date: Wed, 23 Jun 2021 09:50:25 +0000	[thread overview]
Message-ID: <8328f935ec4f488e8d95486a7564aec0@AcuMS.aculab.com> (raw)
In-Reply-To: <CADvbK_eeJVoWps8UrygEfNdXL76Q2XMoNOoELWHFqOTq2634cA@mail.gmail.com>

From: Xin Long
> Sent: 23 June 2021 04:49
...
> [103] pl_send: PLPMTUD: state: 1, size: 1200, high: 0 <--[a]
> [103] pl_recv: PLPMTUD: state: 1, size: 1200, high: 0
...
> [103] pl_send: PLPMTUD: state: 2, size: 1456, high: 0
> [103] pl_recv: PLPMTUD: state: 2, size: 1456, high: 0  <--[b]
> [103] pl_send: PLPMTUD: state: 2, size: 1488, high: 0
> [108] pl_send: PLPMTUD: state: 2, size: 1488, high: 0
> [113] pl_send: PLPMTUD: state: 2, size: 1488, high: 0
> [118] pl_send: PLPMTUD: state: 2, size: 1488, high: 0
> [118] pl_recv: PLPMTUD: state: 2, size: 1456, high: 1488 <---[c]
> [118] pl_send: PLPMTUD: state: 2, size: 1460, high: 1488
> [118] pl_recv: PLPMTUD: state: 2, size: 1460, high: 1488 <--- [d]
> [118] pl_send: PLPMTUD: state: 2, size: 1464, high: 1488
> [124] pl_send: PLPMTUD: state: 2, size: 1464, high: 1488
> [129] pl_send: PLPMTUD: state: 2, size: 1464, high: 1488
> [134] pl_send: PLPMTUD: state: 2, size: 1464, high: 1488
> [134] pl_recv: PLPMTUD: state: 2, size: 1460, high: 1464 <-- around
> 30s "search complete from 1200 bytes"
> [287] pl_send: PLPMTUD: state: 3, size: 1460, high: 0
> [287] pl_recv: PLPMTUD: state: 3, size: 1460, high: 0
> [287] pl_send: PLPMTUD: state: 2, size: 1464, high: 0 <-- [aa]
> [292] pl_send: PLPMTUD: state: 2, size: 1464, high: 0
> [298] pl_send: PLPMTUD: state: 2, size: 1464, high: 0
> [303] pl_send: PLPMTUD: state: 2, size: 1464, high: 0
> [303] pl_recv: PLPMTUD: state: 2, size: 1460, high: 1464  <--[bb]  <--
> around 15s "re-search complete from current pmtu"
> 
> So since no interval to send the next probe when the ACK is received
> for the last one,
> it won't take much time from [a] to [b], and [c] to [d],
> and there are at most 2 failures to find the right pmtu, each failure
> takes 5s * 3 = 15s.
> 
> when it goes back to search from search complete after a long timeout,
> it will take only 1 failure to get the right pmtu from [aa] to [bb].

What mtu is being used during the 'failures' ?
I hope it is the last working one.

Also, what actually happen if the network route changes from
one that supports 1460 bytes to one that only supports 1200
and where ICMP errors are not generated?

The first protocol retry is (probably) after 2 seconds.
But it will use the 1460 byte mtu and fail again.

Notwithstanding the standards, what pmtu actually exist
'in the wild' for normal networks?
Are there actually any others apart from 'full sized ethernet'
and 'PPPoE'?
So would it actually better to send two probes one for
each of those two sizes and see which ones respond?

(I'm not sure we ever manage to send full length packets.
Our data is M3UA (mostly SMS) and sent with Nagle disabled.
So even the customers sending 1000s of SMS/sec are unlikely
to fill packets.)

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

  reply	other threads:[~2021-06-23  9:50 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-22 18:04 Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 01/14] sctp: add pad chunk and its make function and event table Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 02/14] sctp: add probe_interval in sysctl and sock/asoc/transport Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 03/14] sctp: add SCTP_PLPMTUD_PROBE_INTERVAL sockopt for sock/asoc/transport Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 04/14] sctp: add the constants/variables and states and some APIs for transport Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 05/14] sctp: add the probe timer in transport for PLPMTUD Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 06/14] sctp: do the basic send and recv for PLPMTUD probe Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 07/14] sctp: do state transition when PROBE_COUNT == MAX_PROBES on HB send path Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 08/14] sctp: do state transition when a probe succeeds on HB ACK recv path Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 09/14] sctp: do state transition when receiving an icmp TOOBIG packet Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 10/14] sctp: enable PLPMTUD when the transport is ready Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 11/14] sctp: remove the unessessary hold for idev in sctp_v6_err Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 12/14] sctp: extract sctp_v6_err_handle function from sctp_v6_err Xin Long
2021-06-22 18:04 ` [PATCHv2 net-next 13/14] sctp: extract sctp_v4_err_handle function from sctp_v4_err Xin Long
2021-06-22 18:05 ` [PATCHv2 net-next 14/14] sctp: process sctp over udp icmp err on sctp side Xin Long
2021-06-22 18:40 ` [PATCHv2 net-next 00/14] sctp: implement RFC8899: Packetization Layer Path MTU Discovery for SCTP transport patchwork-bot+netdevbpf
2021-06-22 22:13 ` David Laight
2021-06-23  1:09   ` Xin Long
2021-06-23  3:48     ` Xin Long
2021-06-23  9:50       ` David Laight [this message]
2021-06-23 15:59         ` Xin Long

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