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 DFCBDC47096 for ; Mon, 31 May 2021 10:41:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C139960FF3 for ; Mon, 31 May 2021 10:41:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231441AbhEaKnd convert rfc822-to-8bit (ORCPT ); Mon, 31 May 2021 06:43:33 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:35864 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231529AbhEaKnF (ORCPT ); Mon, 31 May 2021 06:43:05 -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-181-0wYY4wymNmmsRwAcyPod2Q-1; Mon, 31 May 2021 11:41:21 +0100 X-MC-Unique: 0wYY4wymNmmsRwAcyPod2Q-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) by AcuMS.aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Mon, 31 May 2021 11:41:18 +0100 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.015; Mon, 31 May 2021 11:41:18 +0100 From: David Laight To: 'Willy Tarreau' , "netdev@vger.kernel.org" , "linux-kernel@vger.kernel.org" CC: David Miller , Jakub Kicinski , Alexey Kuznetsov , Hideaki YOSHIFUJI , Amit Klein , Eric Dumazet Subject: RE: [PATCH net-next] ipv6: use prandom_u32() for ID generation Thread-Topic: [PATCH net-next] ipv6: use prandom_u32() for ID generation Thread-Index: AQHXVHsAigx754KkQkSlq/l9+mKws6r9Zzgg Date: Mon, 31 May 2021 10:41:18 +0000 Message-ID: References: <20210529110746.6796-1-w@1wt.eu> In-Reply-To: <20210529110746.6796-1-w@1wt.eu> 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: netdev@vger.kernel.org From: Willy Tarreau > Sent: 29 May 2021 12:08 > > This is a complement to commit aa6dd211e4b1 ("inet: use bigger hash > table for IP ID generation"), but focusing on some specific aspects > of IPv6. > > Contary to IPv4, IPv6 only uses packet IDs with fragments, and with a > minimum MTU of 1280, it's much less easy to force a remote peer to > produce many fragments to explore its ID sequence. In addition packet > IDs are 32-bit in IPv6, which further complicates their analysis. On > the other hand, it is often easier to choose among plenty of possible > source addresses and partially work around the bigger hash table the > commit above permits, which leaves IPv6 partially exposed to some > possibilities of remote analysis at the risk of weakening some > protocols like DNS if some IDs can be predicted with a good enough > probability. > > Given the wide range of permitted IDs, the risk of collision is extremely > low so there's no need to rely on the positive increment algorithm that > is shared with the IPv4 code via ip_idents_reserve(). We have a fast > PRNG, so let's simply call prandom_u32() and be done with it. > > Performance measurements at 10 Gbps couldn't show any difference with > the previous code, even when using a single core, because due to the > large fragments, we're limited to only ~930 kpps at 10 Gbps and the cost > of the random generation is completely offset by other operations and by > the network transfer time. In addition, this change removes the need to > update a shared entry in the idents table so it may even end up being > slightly faster on large scale systems where this matters. > > The risk of at least one collision here is about 1/80 million among > 10 IDs, 1/850k among 100 IDs, and still only 1/8.5k among 1000 IDs, > which remains very low compared to IPv4 where all IDs are reused > every 4 to 80ms on a 10 Gbps flow depending on packet sizes. The problem is that, on average, 1 in 2^32 packets will use the same id as the previous one. If a fragment of such a pair gets lost horrid things are likely to happen. Note that this is different from an ID being reused after a count of packets or after a time delay. So you still need something to ensure IDs aren't reused immediately. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)