Re: [dpdk-dev] [PATCH 2/2] app/testpmd: compute checksum in icmpecho replies

[Adrien Mazarguil <adrien.mazarguil@6wind.com>]

On Fri, May 22, 2015 at 02:42:08PM +0200, Olivier MATZ wrote:
> Hi Adrien,

Hi Olivier,

> On 05/22/2015 01:07 PM, Adrien Mazarguil wrote:
> > ICMP echo replies with invalid checksums may be dropped by network nodes or
> > ignored by the ping utility.
> > 
> > Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
> > Acked-by: Ivan Boule <ivan.boule@6wind.com>
> > ---
> >  app/test-pmd/icmpecho.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/app/test-pmd/icmpecho.c b/app/test-pmd/icmpecho.c
> > index c5933f4..c278baf 100644
> > --- a/app/test-pmd/icmpecho.c
> > +++ b/app/test-pmd/icmpecho.c
> > @@ -445,7 +445,8 @@ reply_to_icmp_echo_rqsts(struct fwd_stream *fs)
> >  		 * - switch IPv4 source and destinations addresses,
> >  		 * - set IP_ICMP_ECHO_REPLY in ICMP header.
> >  		 * No need to re-compute the IP header checksum.
> > -		 * Reset ICMP checksum.
> > +		 * ICMP checksum is computed by assuming it is valid in the
> > +		 * echo request and not verified.
> >  		 */
> >  		ether_addr_copy(&eth_h->s_addr, &eth_addr);
> >  		ether_addr_copy(&eth_h->d_addr, &eth_h->s_addr);
> > @@ -454,7 +455,7 @@ reply_to_icmp_echo_rqsts(struct fwd_stream *fs)
> >  		ip_h->src_addr = ip_h->dst_addr;
> >  		ip_h->dst_addr = ip_addr;
> >  		icmp_h->icmp_type = IP_ICMP_ECHO_REPLY;
> > -		icmp_h->icmp_cksum = 0;
> > +		icmp_h->icmp_cksum += htons(IP_ICMP_ECHO_REQUEST << 8);
> 
> 
> I don't think this checksum calculation method is always correct.
> 
> Example of a working case:
> 
> >>> p=Ether(str(Ether()/IP()/ICMP(seq=0)))
> >>> p
> <Ether  dst=ff:ff:ff:ff:ff:ff src=00:00:00:00:00:00 type=0x800 |<IP
> version=4L ihl=5L tos=0x0 len=28 id=1 flags= frag=0L ttl=64 proto=icmp
> chksum=0x7cde src=127.0.0.1 dst=127.0.0.1 options=[] |<ICMP
> type=echo-request code=0 chksum=0xf7ff id=0x0 seq=0x0 |>>>
> >>> del(p[ICMP].chksum)
> >>> p[ICMP].type = 0
> >>> p=Ether(str(p))
> >>> p
> <Ether  dst=ff:ff:ff:ff:ff:ff src=00:00:00:00:00:00 type=0x800 |<IP
> version=4L ihl=5L tos=0x0 len=28 id=1 flags= frag=0L ttl=64 proto=icmp
> chksum=0x7cde src=127.0.0.1 dst=127.0.0.1 options=[] |<ICMP
> type=echo-reply code=0 chksum=0xffff id=0x0 seq=0x0 |>>>
> 
> We have 0xffff - 0xf7ff = 0x0800
> 
> 
> Example of a non-working case:
> 
> >>> p=Ether(str(Ether()/IP()/ICMP(seq=0xf800)))
> >>> p
> <Ether  dst=ff:ff:ff:ff:ff:ff src=00:00:00:00:00:00 type=0x800 |<IP
> version=4L ihl=5L tos=0x0 len=28 id=1 flags= frag=0L ttl=64 proto=icmp
> chksum=0x7cde src=127.0.0.1 dst=127.0.0.1 options=[] |<ICMP
> type=echo-request code=0 chksum=0xfffe id=0x0 seq=0xf800 |>>>
> >>> del(p[ICMP].chksum)
> >>> p[ICMP].type = 0
> >>> p=Ether(str(p))
> >>> p
> <Ether  dst=ff:ff:ff:ff:ff:ff src=00:00:00:00:00:00 type=0x800 |<IP
> version=4L ihl=5L tos=0x0 len=28 id=1 flags= frag=0L ttl=64 proto=icmp
> chksum=0x7cde src=127.0.0.1 dst=127.0.0.1 options=[] |<ICMP
> type=echo-reply code=0 chksum=0x7ff id=0x0 seq=0xf800 |>>>
> 
> Here, 0x7ff - 0xfffe != 0x08000
> 
> 
> A correct solution (not tested) would be something like:
> 
> 	uint32_t cksum;
> 	cksum = (~(icmp_h->icmp_cksum)) & 0xffff;
> 	cksum += (~htons(IP_ICMP_ECHO_REQUEST << 8)) & 0xffff;
> 	cksum += (htons(IP_ICMP_ECHO_REPLY << 8)) & 0xffff;
> 	cksum = (cksum & 0xffff) + (cksum >> 16);
> 	cksum = (cksum & 0xffff) + (cksum >> 16);
> 	icmp_h->icmp_cksum = (~cksum) & 0xffff;

You're right, my patch is taking too many shortcuts. I'll submit a new patch
with your suggestion.

Regards,

-- 
Adrien Mazarguil
6WIND