On Mon, Sep 02, 2013 at 03:58:25PM +0200, Alexander Sverdlin wrote: > From: Alexander Sverdlin <alexander.sverdlin@xxxxxxx> > > net: sctp: Fix data chunk fragmentation for MTU values which are not multiple of 4 > > Initially the problem was observed with ipsec, but later it became clear that > SCTP data chunk fragmentation algorithm has problems with MTU values which are > not multiple of 4. Test program was used which just transmits 2000 bytes long > packets to other host. tcpdump was used to observe re-fragmentation in IP layer > after SCTP already fragmented data chunks. > > With MTU 1500: > 12:54:34.082904 IP (tos 0x2,ECT(0), ttl 64, id 0, offset 0, flags [DF], proto SCTP (132), length 1500) > 10.151.38.153.39303 > 10.151.24.91.54321: sctp (1) [DATA] (B) [TSN: 2366088589] [SID: 0] [SSEQ 1] [PPID 0x0] > 12:54:34.082933 IP (tos 0x2,ECT(0), ttl 64, id 0, offset 0, flags [DF], proto SCTP (132), length 596) > 10.151.38.153.39303 > 10.151.24.91.54321: sctp (1) [DATA] (E) [TSN: 2366088590] [SID: 0] [SSEQ 1] [PPID 0x0] > 12:54:34.090576 IP (tos 0x2,ECT(0), ttl 63, id 0, offset 0, flags [DF], proto SCTP (132), length 48) > 10.151.24.91.54321 > 10.151.38.153.39303: sctp (1) [SACK] [cum ack 2366088590] [a_rwnd 79920] [#gap acks 0] [#dup tsns 0] > > With MTU 1499: > 13:02:49.955220 IP (tos 0x2,ECT(0), ttl 64, id 48215, offset 0, flags [+], proto SCTP (132), length 1492) > 10.151.38.153.39084 > 10.151.24.91.54321: sctp[|sctp] > 13:02:49.955249 IP (tos 0x2,ECT(0), ttl 64, id 48215, offset 1472, flags [none], proto SCTP (132), length 28) > 10.151.38.153 > 10.151.24.91: ip-proto-132 > 13:02:49.955262 IP (tos 0x2,ECT(0), ttl 64, id 0, offset 0, flags [DF], proto SCTP (132), length 600) > 10.151.38.153.39084 > 10.151.24.91.54321: sctp (1) [DATA] (E) [TSN: 404355346] [SID: 0] [SSEQ 1] [PPID 0x0] > 13:02:49.956770 IP (tos 0x2,ECT(0), ttl 63, id 0, offset 0, flags [DF], proto SCTP (132), length 48) > 10.151.24.91.54321 > 10.151.38.153.39084: sctp (1) [SACK] [cum ack 404355346] [a_rwnd 79920] [#gap acks 0] [#dup tsns 0] > > Here problem in data portion limit calculation leads to re-fragmentation in IP, > which is sub-optimal. The problem is max_data initial value, which doesn't take > into account the fact, that data chunk must be padded to 4-bytes boundary. > It's enough to correct max_data, because all later adjustments are correctly > aligned to 4-bytes boundary. > > After the fix is applied, everything is fragmented correctly for uneven MTUs: > 15:16:27.083881 IP (tos 0x2,ECT(0), ttl 64, id 0, offset 0, flags [DF], proto SCTP (132), length 1496) > 10.151.38.153.53417 > 10.151.24.91.54321: sctp (1) [DATA] (B) [TSN: 3077098183] [SID: 0] [SSEQ 1] [PPID 0x0] > 15:16:27.083907 IP (tos 0x2,ECT(0), ttl 64, id 0, offset 0, flags [DF], proto SCTP (132), length 600) > 10.151.38.153.53417 > 10.151.24.91.54321: sctp (1) [DATA] (E) [TSN: 3077098184] [SID: 0] [SSEQ 1] [PPID 0x0] > 15:16:27.085640 IP (tos 0x2,ECT(0), ttl 63, id 0, offset 0, flags [DF], proto SCTP (132), length 48) > 10.151.24.91.54321 > 10.151.38.153.53417: sctp (1) [SACK] [cum ack 3077098184] [a_rwnd 79920] [#gap acks 0] [#dup tsns 0] > > The bug was there for years already, but > - is a performance issue, the packets are still transmitted > - doesn't show up with default MTU 1500, but possibly with ipsec (MTU 1438) > > Signed-off-by: Alexander Sverdlin <alexander.sverdlin@xxxxxxx> > --- > > --- linux-3.10.orig/net/sctp/chunk.c > +++ linux-3.10/net/sctp/chunk.c > @@ -200,9 +200,9 @@ struct sctp_datamsg *sctp_datamsg_from_u > /* This is the biggest possible DATA chunk that can fit into > * the packet > */ > - max_data = asoc->pathmtu - > + max_data = (asoc->pathmtu - > sctp_sk(asoc->base.sk)->pf->af->net_header_len - > - sizeof(struct sctphdr) - sizeof(struct sctp_data_chunk); > + sizeof(struct sctphdr) - sizeof(struct sctp_data_chunk)) & ~3; > > max = asoc->frag_point; > /* If the the peer requested that we authenticate DATA chunks > -- > To unsubscribe from this list: send the line "unsubscribe linux-sctp" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > Acked-by: Neil Horman <nhorman@xxxxxxxxxxxxx> -- To unsubscribe from this list: send the line "unsubscribe linux-sctp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
