Hi Venky,
Which nodes get called depends on the nature of the route. Take this trivial
example
DBGvpp# set int state loop0 up
DBGvpp# set int ip addr loop0 10.0.0.1/24
This route is non recursive
DBGvpp# ip route 1.1.1.1/32 via 10.0.0.2 loop0
This one is recursive
DBGvpp# ip route 2.2.2.2/32 via 10.0.0.2
You can see the DPO graph built for them is different.
DBGvpp# sh ip fib 1.1.1.1
ipv4-VRF:0, fib_index:0, flow hash:[src dst sport dport proto flowlabel ]
epoch:0 flags:none locks:[recursive-resolution:1, default-route:1, ]
1.1.1.1/32 fib:0 index:11 locks:2
CLI refs:1 src-flags:added,contributing,active,
path-list:[16] locks:2 flags:shared, uPRF-list:12 len:1 itfs:[1, ]
path:[18] pl-index:16 ip4 weight=1 pref=0 attached-nexthop:
oper-flags:resolved,
10.0.0.2 loop0
[@0]: arp-ipv4: via 10.0.0.2 loop0
forwarding: unicast-ip4-chain
[@0]: dpo-load-balance: [proto:ip4 index:13 buckets:1 uRPF:12 to:[0:0]]
[0] [@3]: arp-ipv4: via 10.0.0.2 loop0
This recursive prefix has an extra load-balanace object
DBGvpp# sh ip fib 2.2.2.2
ipv4-VRF:0, fib_index:0, flow hash:[src dst sport dport proto flowlabel ]
epoch:0 flags:none locks:[recursive-resolution:1, default-route:1, ]
2.2.2.2/32 fib:0 index:12 locks:2
CLI refs:1 src-flags:added,contributing,active,
path-list:[18] locks:2 flags:shared, uPRF-list:15 len:1 itfs:[1, ]
path:[20] pl-index:18 ip4 weight=1 pref=0 recursive: oper-flags:resolved,
via 10.0.0.2 in fib:0 via-fib:13 via-dpo:[dpo-load-balance:14]
forwarding: unicast-ip4-chain
[@0]: dpo-load-balance: [proto:ip4 index:15 buckets:1 uRPF:15 to:[0:0]]
[0] [@12]: dpo-load-balance: [proto:ip4 index:14 buckets:1 uRPF:14
to:[0:0]]. <<<< HERE
[0] [@3]: arp-ipv4: via 10.0.0.2 loop0
The first LB in the DPO graph will be the one used in ip4-lookup the second
one, when present, will be used in ip4-load-balance.
The golden rule of forwarding is to do all the heavy lifting in the
control-plane so you can do the absolute minimum in the DP, because down there
extra work means worse performance. The load-balance bucket choice is basically
choice = buckets[flow-hash & n_buckets], which is the minimum we can do. I
would advise that you try and keep it this way by:
1 – choosing how the flow hash is calculated (see the list of ‘algorithms’
available in ip_flow_hash_set)
2 – choosing in the CP how the buckets are populated.
If 2. Is an option we can talk more about how you might go about doing that 😊
Are you looking to change the bucket selection for all prefixes or most, some
or a few?
/neale
From: Venky Venkatesh <[email protected]>
Date: Tuesday, 7 September 2021 at 20:11
To: Neale Ranns <[email protected]>
Cc: [email protected] <[email protected]>
Subject: Re: [vpp-dev] Question regarding ip4/ip6 nexthop load balance
flexibility #vnet
Hi Neale,
Thanks for getting back. I thought as much that cloning code (even though I do
a graph node replacement) wouldn't have been the way it would be done by
someone knowledgeable in the area.
When I looked again at the code, the area I intended to change seems to be a
node in itself. Is this code for ECMP? (appears to be like that)
VLIB_REGISTER_NODE<https://docs.fd.io/vpp/21.06/d0/da8/vlib_2node_8h.html#a025e3596085258b3bbd8e1a99ab026f4>
(ip4_load_balance_node<https://docs.fd.io/vpp/21.06/dc/da9/ip4__forward_8c.html#a90b92667856f47252d614ba214ace26b>)
=
270 {
271 .name = "ip4-load-balance",
272 .vector_size = sizeof
(u32<https://docs.fd.io/vpp/21.06/de/deb/vppinfra_2types_8h.html#a162050b1a67fffab30498a67c9ab0f09>),
273 .sibling_of = "ip4-lookup",
274 .format_trace =
format_ip4_lookup_trace<https://docs.fd.io/vpp/21.06/dc/da9/ip4__forward_8c.html#af59e7cb8282da4ff432f4237f1ab1e4a>,
275 };
For some reason it doesn't show up in
https://docs.fd.io/vpp/21.06/d9/db0/nodes.html -- which is how I missed it the
1st time around.
The localized changes that I was intending to do are in
load_balance_get_bucket_i and load_balance_get_fwd_bucket (i.e. having computed
the hash, the way I select the next hop could differ). I could also entirely
outsource the hash computation and nexthop calculation to my code (in which
case, also gets included in my changes).
The load_balance_* code gets invoked from ip4_lookup_* and also from
ip4_load_balance_*. Am I right that if I want to change the next hop selection,
I would replace ip4_load_balance_* with my implementation? However I do not see
this chained in the graph.
Look forward to your response
Thanks
-Venky
On Sun, Sep 5, 2021 at 6:52 AM Neale Ranns
<[email protected]<mailto:[email protected]>> wrote:
Hi Venky,
There are several ways you might go about this but I would council that cloning
code is a last resort – it would be a mainanence headache if you replace the
current one or have i’s own set of challenges to use your replace node. if you
could give me some more detail on how you want to change the next-hop selection
I will gladly give you some specific advice.
/neale
From: [email protected]<mailto:[email protected]>
<[email protected]<mailto:[email protected]>> on behalf of Venky Venkatesh
via lists.fd.io<http://lists.fd.io>
<[email protected]<mailto:[email protected]>>
Date: Wednesday, 1 September 2021 at 16:48
To: [email protected]<mailto:[email protected]>
<[email protected]<mailto:[email protected]>>
Subject: [vpp-dev] Question regarding ip4/ip6 nexthop load balance flexibility
#vnet
Hi,
I am new to
fd.io<https://urldefense.proofpoint.com/v2/url?u=http-3A__fd.io_&d=DwMF-g&c=V9IgWpI5PvzTw83UyHGVSoW3Uc1MFWe5J8PTfkrzVSo&r=w2W5SR0mU5u5mz008DZNCsexDN1Lr9bpL7ZGKuD0Zd4&m=bdaAaiMxrwSAEn2IzzNT7qwU8Iy_G-G-eQlabvZRaDM&s=zLAgC1KZL_0sC1trbkMPHqZrVJHEWT5__qCEOOVPJ1U&e=>.
So pls pardon my ignorance.
I wanted to experiment with different next hop selection algorithms once the
destination IP is looked up. I looked at the ip4 code. My changes would be
localized to how the nexthop is determined after the hash is calculated
(currently it is going thru load_balance code). To my understanding it appears
that ip4_lookup graph node has both the lookup and the next hop selection all
bundled in one. If that is correct, then is the only way to try out alternate
next hop selection algorithms be to create a graph node which is practically a
clone of the ip4_lookup and then change the required portions in the cloned
node code? If so, is this workflow in line with the way flexibility was
intended?
Any ideas welcome.
Thanks
-Venky
#vnet
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