From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4E697C2B9F4 for ; Tue, 22 Jun 2021 22:13:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 2FDB26113E for ; Tue, 22 Jun 2021 22:13:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229955AbhFVWPe convert rfc822-to-8bit (ORCPT ); Tue, 22 Jun 2021 18:15:34 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:58736 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229675AbhFVWPe (ORCPT ); Tue, 22 Jun 2021 18:15:34 -0400 Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-231-IXZpIp62M8ys0FTGHGmJgA-1; Tue, 22 Jun 2021 23:13:14 +0100 X-MC-Unique: IXZpIp62M8ys0FTGHGmJgA-1 Received: from AcuMS.Aculab.com (10.202.163.4) by AcuMS.aculab.com (10.202.163.4) with Microsoft SMTP Server (TLS) id 15.0.1497.18; Tue, 22 Jun 2021 23:13:14 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.018; Tue, 22 Jun 2021 23:13:14 +0100 From: David Laight To: 'Xin Long' , network dev , "davem@davemloft.net" , "kuba@kernel.org" , Marcelo Ricardo Leitner , "linux-sctp@vger.kernel.org" Subject: RE: [PATCHv2 net-next 00/14] sctp: implement RFC8899: Packetization Layer Path MTU Discovery for SCTP transport Thread-Topic: [PATCHv2 net-next 00/14] sctp: implement RFC8899: Packetization Layer Path MTU Discovery for SCTP transport Thread-Index: AQHXZ5Gk1m/0BhrSYEC4nBP7A0Tb5KsglL6Q Date: Tue, 22 Jun 2021 22:13:14 +0000 Message-ID: References: In-Reply-To: Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-sctp@vger.kernel.org From: Xin Long > Sent: 22 June 2021 19:05 > > Overview(From RFC8899): > > In contrast to PMTUD, Packetization Layer Path MTU Discovery > (PLPMTUD) [RFC4821] introduces a method that does not rely upon > reception and validation of PTB messages. It is therefore more > robust than Classical PMTUD. This has become the recommended > approach for implementing discovery of the PMTU [BCP145]. > > It uses a general strategy in which the PL sends probe packets to > search for the largest size of unfragmented datagram that can be sent > over a network path. Probe packets are sent to explore using a > larger packet size. If a probe packet is successfully delivered (as > determined by the PL), then the PLPMTU is raised to the size of the > successful probe. If a black hole is detected (e.g., where packets > of size PLPMTU are consistently not received), the method reduces the > PLPMTU. This seems to take a long time (probably well over a minute) to determine the mtu. What is used for the actual mtu while this is in progress? Does packet loss and packet retransmission cause the mtu to be reduced as well? I can imagine that there is an expectation (from the application) that the mtu is that of an ethernet link - perhaps less a PPPoE header. Starting with an mtu of 1200 will break this assumption and may have odd side effects. For TCP/UDP the ICMP segmentation required error is immediate and gets used for the retransmissions. This code seems to be looking at separate timeouts - so a lot of packets could get discarded and application timers expire before if determines the correct mtu. Maybe I missed something about this only being done on inactive paths? David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)