Development

Q: How do I implement a new OpenFlow message?

A: Add your new message to enum ofpraw and enum ofptype in include/openvswitch/ofp-msgs.h, following the existing pattern. Then recompile and fix all of the new warnings, implementing new functionality for the new message as needed. (If you configure with --enable-Werror, as described in Open vSwitch on Linux, FreeBSD and NetBSD, then it is impossible to miss any warnings.)

To add an OpenFlow vendor extension message (aka experimenter message) for a vendor that doesn’t yet have any extension messages, you will also need to edit build-aux/extract-ofp-msgs and at least ofphdrs_decode() and ofpraw_put__() in lib/ofp-msgs.c. OpenFlow doesn’t standardize vendor extensions very well, so it’s hard to make the process simpler than that. (If you have a choice of how to design your vendor extension messages, it will be easier if you make them resemble the ONF and OVS extension messages.)

Q: How do I add support for a new field or header?

A: Add new members for your field to struct flow in include/openvswitch/flow.h, and add new enumerations for your new field to enum mf_field_id in include/openvswitch/meta-flow.h, following the existing pattern. If the field uses a new OXM class, add it to OXM_CLASSES in build-aux/extract-ofp-fields. Also, add support to miniflow_extract() in lib/flow.c for extracting your new field from a packet into struct miniflow, and to nx_put_raw() in lib/nx-match.c to output your new field in OXM matches. Then recompile and fix all of the new warnings, implementing new functionality for the new field or header as needed. (If you configure with --enable-Werror, as described in Open vSwitch on Linux, FreeBSD and NetBSD, then it is impossible to miss any warnings.)

If you want kernel datapath support for your new field, you also need to modify the kernel module for the operating systems you are interested in. This isn’t mandatory, since fields understood only by userspace work too (with a performance penalty), so it’s reasonable to start development without it. If you implement kernel module support for Linux, then the Linux kernel “netdev” mailing list is the place to submit that support first; please read up on the Linux kernel development process separately. The Windows datapath kernel module support, on the other hand, is maintained within the OVS tree, so patches for that can go directly to ovs-dev.

Q: How do I add support for a new OpenFlow action?

A: Add your new action to enum ofp_raw_action_type in lib/ofp-actions.c, following the existing pattern. Then recompile and fix all of the new warnings, implementing new functionality for the new action as needed. (If you configure with --enable-Werror, as described in the Open vSwitch on Linux, FreeBSD and NetBSD, then it is impossible to miss any warnings.)

If you need to add an OpenFlow vendor extension action for a vendor that doesn’t yet have any extension actions, then you will also need to add the vendor to vendor_map in build-aux/extract-ofp-actions. Also, you will need to add support for the vendor to ofpact_decode_raw() and ofpact_put_raw() in lib/ofp-actions.c. (If you have a choice of how to design your vendor extension actions, it will be easier if you make them resemble the ONF and OVS extension actions.)

Q: How do I add support for a new OpenFlow error message?

A: Add your new error to enum ofperr in include/openvswitch/ofp-errors.h. Read the large comment at the top of the file for details. If you need to add an OpenFlow vendor extension error for a vendor that doesn’t yet have any, first add the vendor ID to the <name>_VENDOR_ID list in include/openflow/openflow-common.h.

Q: What’s a Signed-off-by and how do I provide one?

A: Free and open source software projects usually require a contributor to provide some assurance that they’re entitled to contribute the code that they provide. Some projects, for example, do this with a Contributor License Agreement (CLA) or a copyright assignment that is signed on paper or electronically.

For this purpose, Open vSwitch has adopted something called the Developer’s Certificate of Origin (DCO), which is also used by the Linux kernel and originated there. Informally stated, agreeing to the DCO is the developer’s way of attesting that a particular commit that they are contributing is one that they are allowed to contribute. You should visit https://developercertificate.org/ to read the full statement of the DCO, which is less than 200 words long.

To certify compliance with the Developer’s Certificate of Origin for a particular commit, just add the following line to the end of your commit message, properly substituting your name and email address:

Signed-off-by: Firstname Lastname <email@example.org>

Git has special support for adding a Signed-off-by line to a commit message: when you run “git commit”, just add the -s option, as in “git commit -s”. If you use the “git citool” GUI for commits, you can add a Signed-off-by line to the commit message by pressing Control+S. Other Git user interfaces may provide similar support.

Q: How do I apply patches from email?

A: You can use git am on raw email contents, either from a file saved by or piped from an email client. In mutt, for example, when you are viewing a patch, you can apply it to the tree in ~/ovs by issuing the command |cd ~/ovs && git am. If you are an OVS committer, you might want to add -s to sign off on the patch as part of applying it. If you do this often, then you can make the keystrokes ,a shorthand for it by adding the following line to your .muttrc:

macro index,pager ,a “<pipe-message>cd ~/ovs && git am -s” “apply patch”

git am has a problem with some email messages from the ovs-dev list for which the mailing list manager edits the From: address, replacing it by the list’s own address. The mailing list manager must do this for messages whose sender’s email domain has DMARC configured, because receivers will otherwise discard these messages when they do not come directly from the sender’s email domain. This editing makes the patches look like they come from the mailing list instead of the author. To work around this problem, one can use the following wrapper script for git am:

#! /bin/sh
tmp=$(mktemp)
cat >$tmp
if grep '^From:.*via dev.*' "$tmp" >/dev/null 2>&1; then
   sed '/^From:.*via dev.*/d
        s/^[Rr]eply-[tT]o:/From:/' $tmp
else
   cat "$tmp"
fi | git am "[email protected]"
rm "$tmp"

Another way to apply emailed patches is to use the pwclient program, which can obtain patches from patchwork and apply them directly. Download pwclient at https://patchwork.ozlabs.org/project/openvswitch/. You probably want to set up a .pwclientrc that looks something like this:

[options]
default=openvswitch
signoff=true

[openvswitch]
url=https://patchwork.ozlabs.org/xmlrpc/

After you install pwclient, you can apply a patch from patchwork with pwclient git-am #, where # is the patch’s number. (This fails with certain patches that contain form-feeds, due to a limitation of the protocol underlying pwclient.)

Another way to apply patches directly from patchwork which supports applying patch series is to use the git-pw program. It can be obtained with pip install git-pw. Alternative installation instructions and general documentation can be found at https://patchwork.readthedocs.io/projects/git-pw/en/latest/. You need to use your openvswitch patchwork login or create one at https://patchwork.ozlabs.org/register/. The following can then be set on the command line with git config or through a .gitconfig like this:

[pw]
server=https://patchwork.ozlabs.org/api/1.0
project=openvswitch
username=<username>
password=<password>

Patch series can be listed with git-pw series list and applied with git-pw series apply #, where # is the series number. Individual patches can be applied with git-pw patch apply #, where # is the patch number.