Lower the default MTU

diff --git a/NEWS b/NEWS
index dc19521..388dfbb 100644 (file)
--- a/NEWS
+++ b/NEWS
@@ -12,6 +12,10 @@ Changes in 2.06:
          actually process comments in the header and odd
          alignments of the various parameters, as well as
          "plain" (not raw) files and PBM and PGM files.
+       * PXELINUX: Lower the default MTU to 1472 in order to deal
+         with systems with slightly nonstandard MTUs, and PXE
+         stacks which don't defragment correctly.  Unfortunately this
+         is very hard to test dynamically.
 
 Changes in 2.05:
        * PXELINUX: Add a default query based on the hardware address

diff --git a/pxelinux.asm b/pxelinux.asm
index 337dcc6..3becef2 100644 (file)
--- a/pxelinux.asm
+++ b/pxelinux.asm
@@ -42,7 +42,11 @@ TFTP_PORT    equ htons(69)           ; Default TFTP port
 PKT_RETRY      equ 6                   ; Packet transmit retry count
 PKT_TIMEOUT    equ 12                  ; Initial timeout, timer ticks @ 55 ms
 ; Desired TFTP block size
-TFTP_LARGEBLK  equ (1500-20-8-4)       ; MTU 1500 - IP hdr - UDP hdr - TFTP hdr
+; For Ethernet MTU is normally 1500.  Unfortunately there seems to
+; be a fair number of networks with "substandard" MTUs which break.
+; The code assumes TFTP_LARGEBLK <= 2K.
+TFTP_MTU       equ 1472
+TFTP_LARGEBLK  equ (TFTP_MTU-20-8-4)   ; MTU - IP hdr - UDP hdr - TFTP hdr
 ; Standard TFTP block size
 TFTP_BLOCKSIZE_LG2 equ 9               ; log2(bytes/block)
 TFTP_BLOCKSIZE equ (1 << TFTP_BLOCKSIZE_LG2)