Date: Wed, 15 Sep 2010 12:12:52 -0400 From: John Baldwin <jhb@freebsd.org> To: Andre Oppermann <andre@freebsd.org> Cc: svn-src-head@freebsd.org, Lawrence Stewart <lstewart@freebsd.org>, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r212653 - head/sys/netinet Message-ID: <201009151212.52337.jhb@freebsd.org> In-Reply-To: <4C90D27D.4070306@freebsd.org> References: <201009151039.o8FAdU4H030416@svn.freebsd.org> <4C90B326.4000208@freebsd.org> <4C90D27D.4070306@freebsd.org>
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On Wednesday, September 15, 2010 10:04:45 am Andre Oppermann wrote: > Below is the wording proposed by Lawrence: > /* > * The default Maximum Segment Size (MSS) to use when we do not have specific > * knowledge (e.g. via path MTU discovery) that the destination host is prepared > * to accept larger datagrams. The smallest allowable IP datagram MTU and > * optionless IP/TCP header lengths are used for the calculation as per RFC879. > * For IPv4 (RFC791): 576 - 20 - 20 = 536. > * For IPv6 (RFC2460): 1280 - 40 - 20 = 1220. > */ > #define TCP_MSS 536 > #define TCP6_MSS 1220 I think the existing text is fine for this comment, with one nit: * For IPv4 the MSS is 576 - sizeof(struct tcpiphdr) I would find it clearer if it was 'sizeof(struct ip) - sizeof(struct tcphdr)' instead. > * Limit the lowest MSS we accept for path MTU discovery and the TCP SYN MSS > * option. Allowing low values of MSS can consume significant resources and be > * used to mount a resource exhaustion attack. Connections requesting lower MSS > * values will be rounded up to this value and the IP_DF flag will be cleared to > * allow fragmentation along the path. > * > * See tcp_subr.c tcp_minmss SYSCTL declaration for more comments. Setting this > * SYSCTL to "0" disables the minmss check. > * > * The default value is fine for TCP over IPv4 across the Internet's smallest > * known link MTU (256 bytes for AX.25 packet radio). However, a connection is > * very unlikely to come across such low MTU interfaces (anno domini 2003). > */ > #define TCP_MINMSS 216 I actually prefer the above text for this block. The 'amounts of resources' phrase is certainly redundant and just 'resources' is clearer. -- John Baldwin
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