Re: Failure to send fragmented IP packet in case of missing ARP entry

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2012-09-10 at 12:59 +0400, Andrei Dolnikov wrote:
> Hello all,
> 
> The following issue is observed on most Linux distributions:
> Transmission of fragmented IP packets in case of missing ARP entry for 
> destination IP fails.
> Actually ARP request is sent, and, once ARP response is received, only 
> few queued fragments are transmitted. Remaining fragments are lost.
> It can be easily reproduced as follows:
>      # arp -d <dst IP>
>      # ping -s 65000 -c 1 <dst IP>
> Ping result is: "1 packets transmitted, 0 received, 100% packet loss, 
> time 0ms".
> 
> The latest kernel version I tried was 3.5.0-1 x86_64, but I also was 
> able to reproduce it with 3.2.x, 3.0.x and 2.6.32.
> It doesn't depend on hardware: was able to reproduce with VMWare Player, 
> Intel based laptop, Intel Atom and ARM based custom boards.
> As I'm not a networking standards expert I'm not sure if it's a real bug 
> or acceptable behaviour, but decided to raise the issue here as I can't 
> reproduce this anomaly with the Windows 7 PC.
> 
> Thanks,
> Andrei.
> --

Its a bit better with linux-3.3, with commit
8b5c171bb3dc0686b2647a84e990199c5faa9ef8
(neigh: new unresolved queue limits)

+neigh/default/unres_qlen_bytes - INTEGER
+       The maximum number of bytes which may be used by packets
+       queued for each unresolved address by other network layers.
+       (added in linux 3.3)
+
+neigh/default/unres_qlen - INTEGER
+       The maximum number of packets which may be queued for each
+       unresolved address by other network layers.
+       (deprecated in linux 3.3) : use unres_qlen_bytes instead.


Problem is : unres_qlen_bytes default value is 65536, so its a bit too
small once you take into account truesize overhead

I guess following patch would be needed :

diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
index 4780045..3395bb6 100644
--- a/net/ipv4/arp.c
+++ b/net/ipv4/arp.c
@@ -171,7 +171,7 @@ struct neigh_table arp_tbl = {
 		.gc_staletime		= 60 * HZ,
 		.reachable_time		= 30 * HZ,
 		.delay_probe_time	= 5 * HZ,
-		.queue_len_bytes	= 64*1024,
+		.queue_len_bytes	= 64 * SKB_TRUESIZE(1024),
 		.ucast_probes		= 3,
 		.mcast_probes		= 3,
 		.anycast_delay		= 1 * HZ,