The presentation by Martin Pieuchot at EuroBSDCon discusses the refactoring of the OpenBSD network stack, focusing on addressing and routing improvements to facilitate parallel execution and modern needs. It highlights challenges faced due to legacy code and the necessity for changes while ensuring developers understand the intricacies involved. The conclusion emphasizes the complexity of updating 30-year-old code but notes progress made towards eliminating inefficiencies and improving data structures.

1. Taming OpenBSD Network Stack Dragons
Martin Pieuchot
mpi@openbsd.org
EuroBSDcon, Sofia
September 2014

2. Taming OpenBSD Network Stack Dragons
sys/net/radix mpath.c
/*
* Stolen from radix.c rn addroute().
* This is nasty code with a certain amount of magic and dragons.
[...]
*/
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3. Agenda
Motivation
Representing Addresses & Routes
Stack Metamorphosis
Where are we now?
Conclusion
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4. Agenda
Motivation
Representing Addresses & Routes
Stack Metamorphosis
Where are we now?
Conclusion
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5. Motivation
1. Give a talk at EuroBSDCon
2. Enjoy code from the 80’s
3. Make it easier to run it in parallel
Execute (some parts of) the forwarding path on > 1 CPUs
Cleaning from the “top”: ioctl and ipforward paths
4. Adapt it to a Plug & Play world
5. Development process: commit early, revert, fix, commit
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6. Agenda
Motivation
Representing Addresses & Routes
Stack Metamorphosis
Where are we now?
Conclusion
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7. What do we use addresses for?
Identify peers
Who is the receiver?
Who is the sender?
Direct packets
Where is the destination? IPv4 header
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8. Journey of a packet
Input For me?
Forward?
no
Deliver
yes
Select interface
yes
OutputSend
Identify
Direct
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9. Representation of an address
Interface address (ifa)
struct ifaddr {
struct sockaddr *ifa_addr; /* address of interface */
struct sockaddr *ifa_dstaddr; /* other end of p-to-p link */
#define ifa_broadaddr ifa_dstaddr /* broadcast address interface */
struct sockaddr *ifa_netmask; /* used to determine subnet */
struct ifnet *ifa_ifp; /* back-pointer to interface */
TAILQ_ENTRY(ifaddr) ifa_list; /* list of addresses for interface */
[...]
};
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10. Global data structures
Interface without address
Int. List
ifp
lladdr
if_sadl ifa_ifp
RB-tree
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11. Global data structures
Interface with an address
Int. List
ifp
lladdr
if_sadl ifa_ifp
ifa
ifa_ifp
rtentry
Routing Table
RB-tree
Addr. List
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12. When are they accessed?
Input For me?
Forward?
no
Deliver
yes
Select interface
yes
OutputSend
ip input
RB-tree
Addr. List
Int. List
ip forward
Routing table*
ip output
Routing table*
Addr. List
Int. List
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13. Agenda
Motivation
Representing Addresses & Routes
Stack Metamorphosis
Where are we now?
Conclusion
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14. Global lists
1. Get rid of link-layer address lookups
2. Use local (per ifp) lists instead of global ones
3. Or simply rewrite the code without the lookup
4. Otherwise (in the process context) use the Interface List
Some modified functions
carp set addr(), ether output(), ifa ifwithnet(), ifa ifwithroute(), IFP TO IA(),
in localaddr(), in pcbbind(), in selectsrc(), ipv4 input(), ip output(), m cldrop(),
rip usrreq(), rt getifa()...
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15. Interface list
ifa ifwithaddr(), ifa ifwithdstaddr() and ifa ifwithnet()
1981: One address per interface (struct ifnet)
1985: Per interface list of addresses (struct ifaddr)
2010: Global RB-Tree of addresses
1985: Global list of addresses per protocol (i.e. struct in ifaddr)
1999: KAME uses the routing table to forward or deliver
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16. Routing table
Use the routing table for address lookups
Consolidate KAME’s “loobpack” hack
RTF LOCAL For each configured address
RTF BROADCAST For every IPv4 subnet
Only one global structure
Easier than maintaining coherency between various structures
Needs some love to be accessed in parallel
Not slower/faster than the actual RB-tree
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17. Protocol multicast addresses
ifp
ifa0
ifa1 mcast
ifa2
all hosts
mcast
Addr. List
OpenBSD 5.4
ifp
ifa0 all hosts
ifa1
ifa2
mcast
mcast
Addr. List
OpenBSD 5.5
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18. Related changes
The link-layer address has been remove from all the lists
No need to move this information to the routing table
Many many dragons in this code
SO DONTROUTE is no longer supported
No option to bypass the routing table
Interface indexes are now unique
Avoid dangling pointers
inet ntop() replaces inet ntoa() in the kernel
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19. Agenda
Motivation
Representing Addresses & Routes
Stack Metamorphosis
Where are we now?
Conclusion
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20. Global data structures
Interface with an address
ifa
ifp
ifa_ifp
Int. List
rtentry
Routing Table
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21. When are they accessed?
Input For me?
Forward?
no
Deliver
yes
Select interface
yes
OutputSend
ip input
Routing table*
ip forward
ip output
Routing table*
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22. Well, we’re almost there
Diff to kill the RB-tree is on tech@
RTF LOCAL routes still points to lo0
Still doing 2 lookups in the forwarding case
Hopefully integrated for OpenBSD 5.7
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23. Agenda
Motivation
Representing Addresses & Routes
Stack Metamorphosis
Where are we now?
Conclusion
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24. Conclusion
Refactoring 30 years old code is hard
But we have a pretty good history
Very few people care because
It’s not a “feature”
There’s no visible speed gain
Changes always find some dragons
Understanding what you’re changing is important
Future developers won’t hate you (or not that much)
Still plenty of dragons
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25. Questions?
Slides on http://www.openbsd.org/papers/
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