Date: Wed, 4 Feb 2015 00:34:53 +0000 From: "np (Navdeep Parhar)" <phabric-noreply@FreeBSD.org> To: freebsd-net@freebsd.org Subject: [Differential] [Changed Subscribers] D1761: Extend LRO support to accumulate more than 65535 bytes Message-ID: <beb189b34392dcdb4d3861c23b9a3021@localhost.localdomain> In-Reply-To: <differential-rev-PHID-DREV-wifan4aus5qtbwh2gnxc-req@FreeBSD.org> References: <differential-rev-PHID-DREV-wifan4aus5qtbwh2gnxc-req@FreeBSD.org>
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np added a subscriber: np. np added a reviewer: lstewart. np added a comment. LRO affects the kernel TCP code in subtle (and almost always undesirable) ways by "compressing" multiple TCP headers into one. Think TCP timestamps, bursty changes in sequence space as seen by the kernel, what happens to pure acks, etc. Our LRO implementation is even willing to combine multiple received TCP PSHes into one. All this is a decent tradeoff when a handful of segments are involved but the proposed LRO_PLEN_MAX (1MB) is 700+ MSS sized segments at 1500 MTU. I wonder how well the kernel TCP will deal with such big bubbles of data. Please do get the big picture reviewed by one of our TCP protocol experts. M_HASHTYPE_LRO_TCP isn't really a hash type and will likely confuse the RSS code. There is some value in providing the hash type to the stack but with your proposed change the hash type will be clobbered by tcp_lro_flush. Data for a single stream will show up in the stack with either the correct hash type or M_HASHTYPE_LRO_TCP. Not pretty. I wonder what one of these gigantic LRO'd packet looks like to bpf consumers? If they go by the ip header then they will likely get confused. A good test would be to see if wireshark is able to follow the TCP stream accurately or does it lose track when it encounters one of these VLRO (Very Large RO) packets? At the very least, allow drivers to opt out of this VLRO by a) making LRO_PLEN_MAX per lro_ctrl, to be set when the LRO structures are initialized by the driver. b) never clobbering the hash type in tcp_lro.c if the total length accumulated is <= 65535. REVISION DETAIL https://reviews.freebsd.org/D1761 To: hselasky, rmacklem, rrs, glebius, gnn, emaste, imp, adrian, bz, rwatson, lstewart Cc: np, freebsd-net
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