From 8dd180f165eb8cd0a8475c0fd19bd6dbf17f95d8 Mon Sep 17 00:00:00 2001 From: Michael Brown Date: Wed, 4 Sep 2013 17:37:34 +0100 Subject: [PATCH] [tcp] Reduce path MTU to 1280 bytes The path MTU is currently hardcoded to 1460 bytes, which fails to allow space for TCP options. Sending a maximum-sized datagram (which is viable when using HTTP POST) will therefore fail since the Ethernet MTU will be exceeded. Reduce the hardcoded path MTU to produce a maximum datagram of 1280 bytes, which is the size required of data link layers by IPv6. It is a reasonable assumption that all intermediary data link layers will be able to convey this packet without fragmentation, even for IPv4. Note that this reduction has a minimal impact upon download throughput, since it affects only the transmit data path. Originally-fixed-by: Suresh Sundriyal Signed-off-by: Michael Brown --- src/include/ipxe/tcp.h | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/src/include/ipxe/tcp.h b/src/include/ipxe/tcp.h index 6b669117..eb4b7b22 100644 --- a/src/include/ipxe/tcp.h +++ b/src/include/ipxe/tcp.h @@ -316,10 +316,19 @@ struct tcp_options { /** * Path MTU * - * We really ought to implement Path MTU discovery. Until we do, - * anything with a path MTU greater than this may fail. + * IPv6 requires all data link layers to support a datagram size of + * 1280 bytes. We choose to use this as our maximum transmitted + * datagram size, on the assumption that any practical link layer we + * encounter will allow this size. This is a very conservative + * assumption in practice, but the impact of making such a + * conservative assumption is insignificant since the amount of data + * that we transmit (rather than receive) is negligible. + * + * We allow space within this 1280 bytes for an IPv6 header, a TCP + * header, and a (padded) TCP timestamp option. */ -#define TCP_PATH_MTU 1460 +#define TCP_PATH_MTU \ + ( 1280 - 40 /* IPv6 */ - 20 /* TCP */ - 12 /* TCP timestamp */ ) /** * Advertised TCP MSS