Date: Wed, 27 Sep 2017 05:52:37 +0000 (UTC) From: Sepherosa Ziehau <sephe@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r324050 - head/sys/netinet Message-ID: <201709270552.v8R5qbLU071173@repo.freebsd.org>
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Author: sephe Date: Wed Sep 27 05:52:37 2017 New Revision: 324050 URL: https://svnweb.freebsd.org/changeset/base/324050 Log: tcp: Don't "negotiate" MSS. _NO_ OSes actually "negotiate" MSS. RFC 879: "... This Maximum Segment Size (MSS) announcement (often mistakenly called a negotiation) ..." This negotiation behaviour was introduced 11 years ago by r159955 without any explaination about why FreeBSD had to "negotiate" MSS: In syncache_respond() do not reply with a MSS that is larger than what the peer announced to us but make it at least tcp_minmss in size. Sponsored by: TCP/IP Optimization Fundraise 2005 The tcp_minmss behaviour is still kept. Syncookie fix was prodded by tuexen, who also helped to test this patch w/ packetdrill. Reviewed by: tuexen, karels, bz (previous version) MFC after: 2 week Sponsored by: Microsoft Differential Revision: https://reviews.freebsd.org/D12430 Modified: head/sys/netinet/tcp_syncache.c Modified: head/sys/netinet/tcp_syncache.c ============================================================================== --- head/sys/netinet/tcp_syncache.c Wed Sep 27 05:44:50 2017 (r324049) +++ head/sys/netinet/tcp_syncache.c Wed Sep 27 05:52:37 2017 (r324050) @@ -1634,9 +1634,7 @@ syncache_respond(struct syncache *sc, struct syncache_ tlen = hlen + sizeof(struct tcphdr); /* Determine MSS we advertize to other end of connection. */ - mssopt = tcp_mssopt(&sc->sc_inc); - if (sc->sc_peer_mss) - mssopt = max( min(sc->sc_peer_mss, mssopt), V_tcp_minmss); + mssopt = max(tcp_mssopt(&sc->sc_inc), V_tcp_minmss); /* XXX: Assume that the entire packet will fit in a header mbuf. */ KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN, @@ -1985,7 +1983,7 @@ syncookie_mac(struct in_conninfo *inc, tcp_seq irs, ui static tcp_seq syncookie_generate(struct syncache_head *sch, struct syncache *sc) { - u_int i, mss, secbit, wscale; + u_int i, secbit, wscale; uint32_t iss, hash; uint8_t *secbits; union syncookie cookie; @@ -1995,8 +1993,8 @@ syncookie_generate(struct syncache_head *sch, struct s cookie.cookie = 0; /* Map our computed MSS into the 3-bit index. */ - mss = min(tcp_mssopt(&sc->sc_inc), max(sc->sc_peer_mss, V_tcp_minmss)); - for (i = nitems(tcp_sc_msstab) - 1; tcp_sc_msstab[i] > mss && i > 0; + for (i = nitems(tcp_sc_msstab) - 1; + tcp_sc_msstab[i] > sc->sc_peer_mss && i > 0; i--) ; cookie.flags.mss_idx = i;
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