Date: Thu, 17 Aug 2017 22:33:31 -0500 From: "Mike Karels" <mike@karels.net> To: "Gopakumar Pillai" <gpillai@vmware.com> Cc: "Bjoern A. Zeeb" <bzeeb-lists@lists.zabbadoz.net>, "freebsd-net@FreeBSD.org" <freebsd-net@FreeBSD.org> Subject: Re: Only last IP frag sent if ARP entry absent Message-ID: <9B1B1A12-CD9F-4A9F-B596-A2F6E5BAED1E@karels.net> In-Reply-To: <AFD0C317-D4E2-4A9E-B6F2-CCA2B0B7464F@vmware.com> References: <F9ABB88D-108D-4EF0-8962-091662F488FD@vmware.com> <43CC3432-DB42-4170-B3E7-E305561973F3@lists.zabbadoz.net> <AFD0C317-D4E2-4A9E-B6F2-CCA2B0B7464F@vmware.com>
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Another $.02 (inline): On 17 Aug 2017, at 18:39, Gopakumar Pillai wrote: > Thank You Bjoern. Could you please point me to the RFC? I don=E2=80=99t know if there is anything more recent than RFC1122 on thi= s. =20 IIRC, it requires queuing at least one packet. Queing one packet is=20 what BSD has done essentially since ARP was implemented. > If this is not a MUST behavior in RFC, would my fix be good? I agree=20 > that this would affect only ICMP/UDP traffic. People have been asking for queuing of multiple packets for years. That=20 is a more general change. Consider another dumb application that starts=20 out by sending multiple UDP packets back-to-back. However,=20 well-designed application protocols don=E2=80=99t experience problems lik= e=20 this. I=E2=80=99ll quickly note that ping isn=E2=80=99t an application, = but a=20 network measuring tool. If you ask the question =E2=80=9Cwhat happens if= I=20 start off a session with a single large packet and I don=E2=80=99t suppor= t=20 retransmission=E2=80=9D, ping answers that question correctly. If badly-designed protocols get bad performance, that doesn=E2=80=99t see= m=20 like a bug to me, but a feature. > On 8/17/17, 2:40 PM, "Bjoern A. Zeeb" <bzeeb-lists@lists.zabbadoz.net>=20 > wrote: > > On 17 Aug 2017, at 21:16, Gopakumar Pillai wrote: > > > Hi FreeBSD Networking Gurus, > > I came across an issue with an old version of FreeBSD and=20 > looking at > > the latest FreeBSD code, seems it exists even now. I am assuming=20 > that > > this issue is not reported. > > > > Observation: > > When a ping was performed with larger payload than MTU, the=20 > first ping > > failed when the ARP entry was absent for that IP. > > That is because ping/ICMP has no retransmit. > > > > Noticed on the wire that the last IP fragment was sent for the=20 > first > > request and then the subsequent requests were fine. > > > > Root Cause: > > * ip_output fragments the packets and loops through the=20 > fragments to > > send them to ether_output. > > * ether_output does an arpresolve and if there is no existing=20 > ARP > > entry it'll return EWOULDBLOCK after sending ARP Request. > > * ether_output ignores the error and propagates success to=20 > ip_output > > and it continues to send the remaining fragments. > > * llentry keeps only one mbuf and the last fragment is=20 > retained when > > the ARP Reply comes and the fragment is sent. > > Yes, according to the spec (RFC) we are supposed to throw the=20 > packet > away entirely and simply report that to the next upper layer. =20 > However > over the years people realised that this sucks for a TCP SYN=20 > packet with > a retransmit timer and hence we store one of them. > > A large UDP packet would btw see the same behaviour to your ping. > There=E2=80=99s no guarantee any of these packets will not be dropp= ed=20 > anywhere > on the network, so we can as well. > > Just my 2ct > > /bz Mike From owner-freebsd-net@freebsd.org Fri Aug 18 04:36:10 2017 Return-Path: <owner-freebsd-net@freebsd.org> Delivered-To: freebsd-net@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id C98B9DC94BA for <freebsd-net@mailman.ysv.freebsd.org>; Fri, 18 Aug 2017 04:36:10 +0000 (UTC) (envelope-from gpillai@vmware.com) Received: from NAM03-CO1-obe.outbound.protection.outlook.com (mail-co1nam03on0045.outbound.protection.outlook.com [104.47.40.45]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-SHA384 (256/256 bits)) (Client CN "mail.protection.outlook.com", Issuer "Microsoft IT SSL SHA2" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 724D172F99 for <freebsd-net@FreeBSD.org>; Fri, 18 Aug 2017 04:36:09 +0000 (UTC) (envelope-from gpillai@vmware.com) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=onevmw.onmicrosoft.com; s=selector1-vmware-com; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version; bh=r1I6uWVXJSgJhXhQqxf7LI8Vxr6shEwQsVu/8MS4UlY=; b=qt1eQ9+VzMiF7XqQftRtl0K2AAQkFLGb/Kl3R3FPFoYPq/MaRgsB2pOPBCTuUZ2AC/S9GeKJAwLPpJ6qLOnXFGo3aDHR1ZLocUcpo44I7JsPg6JSmELzb1D436WMveioyJ+El+LBKffdguLfLXGPXDt0MX2asw7M+dFpUAumYfM= Received: from DM5PR05MB3081.namprd05.prod.outlook.com (10.173.218.143) by DM5PR05MB3561.namprd05.prod.outlook.com (10.174.242.154) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384_P256) id 15.1.1385.4; Fri, 18 Aug 2017 04:36:07 +0000 Received: from DM5PR05MB3081.namprd05.prod.outlook.com ([10.173.218.143]) by DM5PR05MB3081.namprd05.prod.outlook.com ([10.173.218.143]) with mapi id 15.01.1385.005; Fri, 18 Aug 2017 04:36:07 +0000 From: Gopakumar Pillai <gpillai@vmware.com> To: Mike Karels <mike@karels.net> CC: "Bjoern A. 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