Re: [RFC v2] RoCE v2.0 Entropy - IPv6 Flow Label and UDP Source Port

[Tom Talpey <tom@talpey.com>]

On 2/13/2020 6:03 AM, Alex Rosenbaum wrote:
> On Wed, Feb 12, 2020 at 5:47 PM Tom Talpey <tom@talpey.com> wrote:
>>
>> On 2/8/2020 4:58 AM, Alex Rosenbaum wrote:
>>> On Thu, Feb 6, 2020 at 5:19 PM Tom Talpey <tom@talpey.com> wrote:
>>>>
>>>> On 2/6/2020 9:39 AM, Alex Rosenbaum wrote:
>>>>> On Thu, Feb 6, 2020 at 4:18 PM Tom Talpey <tom@talpey.com> wrote:
>>>>>>
>>>>>> On 1/8/2020 9:26 AM, Alex Rosenbaum wrote:
>>>>>>> A combination of the flow_label field in the IPv6 header and UDP source port
>>>>>>> field in RoCE v2.0 are used to identify a group of packets that must be
>>>>>>> delivered in order by the network, end-to-end.
>>>>>>> These fields are used to create entropy for network routers (ECMP), load
>>>>>>> balancers and 802.3ad link aggregation switching that are not aware of RoCE IB
>>>>>>> headers.
>>>>>>>
>>>>>>> The flow_label field is defined by a 20 bit hash value. CM based connections
>>>>>>> will use a hash function definition based on the service type (QP Type) and
>>>>>>> Service ID (SID). Where CM services are not used, the 20 bit hash will be
>>>>>>> according to the source and destination QPN values.
>>>>>>> Drivers will derive the RoCE v2.0 UDP src_port from the flow_label result.
>>>>>>>
>>>>>>> UDP source port selection must adhere IANA port allocation ranges. Thus we will
>>>>>>> be using IANA recommendation for Ephemeral port range of: 49152-65535, or in
>>>>>>> hex: 0xC000-0xFFFF.
>>>>>>>
>>>>>>> The below calculations take into account the importance of producing a symmetric
>>>>>>> hash result so we can support symmetric hash calculation of network elements.
>>>>>>>
>>>>>>> Hash Calculation for RDMA IP CM Service
>>>>>>> =======================================
>>>>>>> For RDMA IP CM Services, based on QP1 iMAD usage and connected RC QPs using the
>>>>>>> RDMA IP CM Service ID, the flow label will be calculated according to IBTA CM
>>>>>>> REQ private data info and Service ID.
>>>>>>>
>>>>>>> Flow label hash function calculations definition will be defined as:
>>>>>>> Extract the following fields from the CM IP REQ:
>>>>>>>       CM_REQ.ServiceID.DstPort [2 Bytes]
>>>>>>>       CM_REQ.PrivateData.SrcPort [2 Bytes]
>>>>>>>       u32 hash = DstPort * SrcPort;
>>>>>>>       hash ^= (hash >> 16);
>>>>>>>       hash ^= (hash >> 8);
>>>>>>>       AH_ATTR.GRH.flow_label = hash AND IB_GRH_FLOWLABEL_MASK;
>>>>>>>
>>>>>>>       #define IB_GRH_FLOWLABEL_MASK  0x000FFFFF
>>>>>>
>>>>>> Sorry it took me a while to respond to this, and thanks for looking
>>>>>> into it since my comments on the previous proposal. I have a concern
>>>>>> with an aspect of this one.
>>>>>>
>>>>>> The RoCEv2 destination port is a fixed value, 4791. Therefore the
>>>>>> term
>>>>>>
>>>>>>            u32 hash = DstPort * SrcPort;
>>>>>>
>>>>>> adds no entropy beyond the value of SrcPort.
>>>>>>
>>>>>
>>>>> we're talking about the CM service ports, taken from the
>>>>> rdma_resolve_route(mca_id, <ip:SrcPort>, <ip:DstPort>, to_msec);
>>>>> these are the CM level port-space and not the RoCE UDP L4 ports.
>>>>> we want to use both as these will allow different client instance and
>>>>> server instance on same nodes will use differen CM ports and hopefully
>>>>> generate different hash results for multi-flows between these two
>>>>> servers.
>>>>
>>>> Aha, ok I guess I missed that, and ok.
>>>>
>>>>>> In turn, the subsequent
>>>>>>
>>>>>>            hash ^= (hash >> 16);
>>>>>>            hash ^= (hash >> 8);
>>>>>>
>>>>>> are re-mashing the bits with one another, again, adding no entropy.
>>>>
>>>> I still wonder about this one. It's attempting to reduce the 32-bit
>>>> product to 20 bits, but a second xor with the "middle" 16 bits seems
>>>> really strange. Mathematically, wouldn't it be better to just take
>>>> the modulus of 2^20? If not, are you expecting some behavior in the
>>>> hash values that makes the double-xor approach better (in which case
>>>> it should be called out)?
>>>>
>>>> Tom.
>>>
>>> The function takes into account creating a symmetric hash, so both
>>> active and passive can reconstruct the same flow label results. That's
>>> why we multiply the two CM Port values (16 bit * 16 bit). The results
>>> is a 32 bit value, and we don't want to lose any of of the MSB bit's
>>> by modulus or masking. So we need some folding function from 32 bit to
>>> the 20 bit flow label.
>>>
>>> The specific bit shift is something I took from the bond driver:
>>> https://elixir.bootlin.com/linux/latest/source/drivers/net/bonding/bond_main.c#L3407
>>> This proved very good in spreading the flow label in our internal
>>> testing. Other alternative can be suggested, as long as it considers
>>> all bits in the conversion 32->20 bits.
>>
>> I'm ok with it, but I still don't fully understand why the folding
>> is necessary. The multiplication is the important part, and it is
>> the operation that combines the two entropic inputs. The folding just
>> flips bits from what's basically the same entropy source.
>>
>> IOW, I think that
>>
>>          u32 hash = (DstPort * SrcPort) & IB_GRH_FLOWLABEL_MASK;
>>
>> would produce a completely equal benefit, mathematically.
>> Tom.
>>
> 
> If both src & dst ports are in the high value range you loss those
> hash bits in the masking.
> If src & dst port are both 0xE000, your masked hash equals 0. You'll
> get the same hash if both ports are equal 0xF000.

Sure, but this is because it's a 20-bit hash of a 32-bit object. There
will always be collisions, this is just one example. My concern is the
statistical spread of the results. I argue it's not changed by the
proposed bit-folding, possibly even damaged.

> The idea with the bit shift is to take the MSB hash bits (left from
> the 0XFFFFF mask) and fold them with the LSB in some way.

I get that, but it's only folding the "one" bits, and it's doing so in
a rather primitive way. For example, the ">> 8" term is folding the
high 4 of 20 bits twice - once in the >> 16 and again in the >> 8.

This value is only computed once, at QP creation, correct? Why not
compute a CRC-20, for example?

Tom.