time stamps of skb packets in NF_HOOKS callbacks

time stamps of skb packets in NF_HOOKS callbacks

[John Que]

Hello,
I am using NF_HOOKS (a "PRE_ROUTING" hook).
When I try to print the time stamps (sec and usec) of skb packets
which I recieve
in the hook callback I get 0. Other skb fileds are correct.
According to my understanding, the time stamp should be updated
when the packet is received. Isn't it so ? Any idea why am I getting
"0" for tstamp fields (tstamp is a field in sk_buff and is  an intance
of skb_timeval).
(what I mean is that the values of skb->tstamp.off_sec and
skb>tstamp.off_usec are both 0 , for ongoing incoming packets)
Regards,
John

Re: time stamps of skb packets in NF_HOOKS callbacks

[Evgeniy Polyakov]

On Thu, Aug 17, 2006 at 05:37:15PM +0300, John Que (qwejohn@gmail.com) wrote:
> Hello,
> I am using NF_HOOKS (a "PRE_ROUTING" hook).
> When I try to print the time stamps (sec and usec) of skb packets
> which I recieve
> in the hook callback I get 0. Other skb fileds are correct.
> According to my understanding, the time stamp should be updated
> when the packet is received. Isn't it so ? Any idea why am I getting
> "0" for tstamp fields (tstamp is a field in sk_buff and is  an intance
> of skb_timeval).

Timestamps are disabled by default, if you have sockets which enable
SO_TIMESTAMP or netfilter queue target and other agents which can enable
it, you will see updated values in skb->tstamp.

-- 
	Evgeniy Polyakov

Re: time stamps of skb packets in NF_HOOKS callbacks

[John Que]

Evgeniy,
Thnks, I appreciate your (quick) answer;

Following your answer I tried adding a call to net_enable_timestamp() in
a module I wrote which use NF_HOOKS.
This is what is done , for example, in ipq_rcv_skb(struct sk_buff *skb)
in linux-2.6.14 in net/ipv4/netfilter/ip_queue.c.

After doing so I get timestamps which are not 0.

what I wonder now is :
1) does this have ANY performance implication (when dealing with a
high  network  load which can get to 50,000 packets per second?
2) It does work as I said, but I want to be cautios: Am I permitted to
add this method ? are  there any  implications for adding this method
which can create troubles ?
I must say that I am NOT





On 8/17/06, Evgeniy Polyakov <johnpol@2ka.mipt.ru> wrote:
> On Thu, Aug 17, 2006 at 05:37:15PM +0300, John Que (qwejohn@gmail.com) wrote:
> > Hello,
> > I am using NF_HOOKS (a "PRE_ROUTING" hook).
> > When I try to print the time stamps (sec and usec) of skb packets
> > which I recieve
> > in the hook callback I get 0. Other skb fileds are correct.
> > According to my understanding, the time stamp should be updated
> > when the packet is received. Isn't it so ? Any idea why am I getting
> > "0" for tstamp fields (tstamp is a field in sk_buff and is  an intance
> > of skb_timeval).
>
> Timestamps are disabled by default, if you have sockets which enable
> SO_TIMESTAMP or netfilter queue target and other agents which can enable
> it, you will see updated values in skb->tstamp.
>
> --
>         Evgeniy Polyakov
>

Re: time stamps of skb packets in NF_HOOKS callbacks

[John Que]

Evgeniy,
Sorry ! ; last sentence in my reply was chopped so here is my message
again with this last sentence (complete this time)

Thnks, I appreciate your (quick) answer;


Following your answer I tried adding a call to net_enable_timestamp() in
a module I wrote which use NF_HOOKS.
This is what is done , for example, in ipq_rcv_skb(struct sk_buff *skb)
in linux-2.6.14 in net/ipv4/netfilter/ip_queue.c.

After doing so I get timestamps which are not 0.

what I wonder now is :
1) does this have ANY performance implication (when dealing with a
high  network  load which can get to 50,000 packets per second?
2) It does work as I said, but I want to be cautios: Am I permitted to
add this method ? are  there any  implications for adding this method
which can create troubles ?
I must say that I am NOT changing anywhere the timestamp; it's only
for statistics.

Regards,
John




On 8/17/06, Evgeniy Polyakov <johnpol@2ka.mipt.ru> wrote:
> On Thu, Aug 17, 2006 at 05:37:15PM +0300, John Que (qwejohn@gmail.com) wrote:
> > Hello,
> > I am using NF_HOOKS (a "PRE_ROUTING" hook).
> > When I try to print the time stamps (sec and usec) of skb packets
> > which I recieve
> > in the hook callback I get 0. Other skb fileds are correct.
> > According to my understanding, the time stamp should be updated
> > when the packet is received. Isn't it so ? Any idea why am I getting
> > "0" for tstamp fields (tstamp is a field in sk_buff and is  an intance
> > of skb_timeval).
>
> Timestamps are disabled by default, if you have sockets which enable
> SO_TIMESTAMP or netfilter queue target and other agents which can enable
> it, you will see updated values in skb->tstamp.
>
> --
>         Evgeniy Polyakov
>

Re: time stamps of skb packets in NF_HOOKS callbacks

[Evgeniy Polyakov]

On Fri, Aug 18, 2006 at 08:43:08AM +0300, John Que (qwejohn@gmail.com) wrote:
> Following your answer I tried adding a call to net_enable_timestamp() in
> a module I wrote which use NF_HOOKS.
> This is what is done , for example, in ipq_rcv_skb(struct sk_buff *skb)
> in linux-2.6.14 in net/ipv4/netfilter/ip_queue.c.
> 
> After doing so I get timestamps which are not 0.
> 
> what I wonder now is :
> 1) does this have ANY performance implication (when dealing with a
> high  network  load which can get to 50,000 packets per second?

Yes, it has. do_gettimeofday() is quite slow on some processors and is
not SMP-friendly.

> 2) It does work as I said, but I want to be cautios: Am I permitted to
> add this method ? are  there any  implications for adding this method
> which can create troubles ?
> I must say that I am NOT changing anywhere the timestamp; it's only
> for statistics.

If you add ata least on socket with SO_TIMESTAMP option, it will enable
timestamps for all skbs, so you have not created any wrong thing, except
that slowed down your network processing.

> Regards,
> John

-- 
	Evgeniy Polyakov