Date: Thu, 1 Jul 2010 07:23:19 GMT From: Andre Oppermann <andre@FreeBSD.org> To: Perforce Change Reviews <perforce@freebsd.org> Subject: PERFORCE change 180367 for review Message-ID: <201007010723.o617NJtu063386@repoman.freebsd.org>
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http://p4web.freebsd.org/@@180367?ac=10 Change 180367 by andre@andre_t61 on 2010/07/01 07:22:20 Add more comments. Affected files ... .. //depot/projects/tcp_new/netinet/tcp_sack.c#14 edit Differences ... ==== //depot/projects/tcp_new/netinet/tcp_sack.c#14 (text+ko) ==== @@ -67,14 +67,34 @@ #endif /* TCPDEBUG */ /* - * Implementation of Selective Acknowledgements (SACK) as described in - * RFC2018. + * Implementation of the data sender part of Selective Acknowledgements + * (SACK) as described in RFC2018 including detection of duplicate SACK + * (D-SACK) as described in RFC2883. * - * This file implements the data sender part of SACK. It stores all - * received SACK blocks in a scoreboard built on a ranged red-black tree. - * * The data receiver part (RFC2018: section 4) is part of the reassembly * queue. + * + * With SACK a receiver can signal the sender about the segments received + * beyond a lost one waiting in the reassembly queue. Based on this + * information the sender can make an informed decision about which parts + * of the send buffer to retransmit. Network resources are saved because + * only the missing parts are retransmitted. + * + * Implementation details and choices: + * + * The data sender part of SACK stores all received SACK blocks in a + * scoreboard. With large windows and large delay*bandwidth products and + * losses in the network the scoreboard can get quite large. To prevent + * long linked list chain traversals and complexity attacks a ranged + * red-black tree has been chosen. The overhead for small scoreboards + * is about the same as in a linked list but for larger ones it goes down + * to n log(n). All overlapping or contigous SACK blocks are automatically + * merged into one block at insert time. The red-black tree is layed out + * in a way to preserve the ordering of the SACK blocks upon lookup operations. + * + * To prevent resource exhaustion attacks a local and a global limit governs + * the SACK scoreboard. The local limits prevents single connections from + * (possibly maliciously) monopolizing the global limit. */ SYSCTL_NODE(_net_inet_tcp, OID_AUTO, sack, CTLFLAG_RW, 0, "TCP SACK");
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