LDP
/
man-pages
mirror of https://github.com/mkerrisk/man-pages
0
1
Fork 0
Code Releases Activity

packet.7: Status in PACKET_RX_RING is actually a bit mask 

Stefan:
I'm playing with the PACKET_RX_RING option to packet sockets and I
noticed that the status field from the tpacket_hdr structures
inside the mmaped buffer is actually a bit mask. I'm testing with
4Gbps traffic rates and sometimes I get status=5. I'm using Ubuntu
12.04 with a 3.8.0-29 kernel, and at least from the code (I got it
using "sudo apt-get source linux-image-$(uname -r)") it seems the
5 means TP_STATUS_USER | TP_STATUS_LOSING. The 3.8.0 kernel code
has this line in tpacket_rcv():

    /*
     * LOSING will be reported till you read the stats,
     * because it's COR - Clear On Read.
     * Anyways, moving it for V1/V2 only as V3 doesn't need this
     * at packet level.
     */
            if (po->stats.tp_drops)
                    status |= TP_STATUS_LOSING;

Daniel:
Now to your question. It can easily be seen from the if_packet.h header
file http://lingrok.org/xref/linux-net-next/include/uapi/linux/if_packet.h#93
that TP_STATUS_* are individual bits that are set in tp_status field.

TP_STATUS_USER simply means a frame was received in the ring and is
ready for user space to be processed. If the field also indicates
TP_STATUS_LOSING then this means that there were packet drops in the
rx ring itself so a user knows it didn't get all traffic. This bit
is being reset on getsockopt() when querying PACKET_STATISTICS,
otherwise it stays. Drops occur when the ring buffer was not emptied
fast enough by user space (so no free slot with TP_STATUS_KERNEL), e.g.
due to high incoming traffic load. However, the current frame you're
reading that has TP_STATUS_USER|TP_STATUS_LOSING is fine by itself.

Acked-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Carsten Andrich <carsten.andrich@tu-ilmenau.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Stefan Puiu 2014-04-24 11:25:19 +02:00 committed by Michael Kerrisk
parent 84f573db60
commit a29edaeef6
1 changed files with 4 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

8
man7/packet.7
View File

@ -9,7 +9,7 @@
.\"
.\" $Id: packet.7,v 1.13 2000/08/14 08:03:45 ak Exp $
.\"
.TH PACKET 7 2014-04-23 "Linux" "Linux Programmer's Manual"
.TH PACKET 7 2014-04-24 "Linux" "Linux Programmer's Manual"
.SH NAME
packet \- packet interface on device level
.SH SYNOPSIS
@ -364,9 +364,9 @@ The packet socket owns all slots with status
.BR TP_STATUS_KERNEL .
After filling a slot, it changes the status of the slot to transfer
ownership to the application.
During normal operation, the new status is
.BR TP_STATUS_USER ,
to signal that a correctly received packet has been stored.
During normal operation, the new status has the
.BR TP_STATUS_USER
bit set to signal that a received packet has been stored.
When the application has finished processing a packet, it transfers
ownership of the slot back to the socket by setting the status to
.BR TP_STATUS_KERNEL .