From: Marcelo Ricardo Leitner <marcelo.leit...@gmail.com> Date: Sat, 19 Mar 2016 12:17:20 -0300
> SCTP is a protocol that is aligned to a word (4 bytes). Thus using bare > MTU can sometimes return values that are not aligned, like for loopback, > which is 65536 but ipv4_mtu() limits that to 65535. This mis-alignment > will cause the last non-aligned bytes to never be used and can cause > issues with congestion control. > > So it's better to just consider a lower MTU and keep congestion control > calcs saner as they are based on PMTU. > > Same applies to icmp frag needed messages, which is also fixed by this > patch. > > One other effect of this is the inability to send MTU-sized packet > without queueing or fragmentation and without hitting Nagle. As the > check performed at sctp_packet_can_append_data(): > > if (chunk->skb->len + q->out_qlen >= transport->pathmtu - packet->overhead) > /* Enough data queued to fill a packet */ > return SCTP_XMIT_OK; > > with the above example of MTU, if there are no other messages queued, > one cannot send a packet that just fits one packet (65532 bytes) and > without causing DATA chunk fragmentation or a delay. > > v2: > - Added WORD_TRUNC macro > > Signed-off-by: Marcelo Ricardo Leitner <marcelo.leit...@gmail.com> Applied.
