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Protocol Design - Spam, Work, & Prioritization

Part of work in progress Living Whitepaper

This page is part of the Living Whitepaper revisions currently underway to replace the original static whitepaper. These efforts include the Protocol Design and Node Implementation sections of the docs, which will cover and expand on details and topics covered in the original whitepaper.

See the bottom of the page for related whitepaper sections and other related details. Some of the sections and headers on this page may be in draft form or just suggestions/framework for later consideration. If you are interested in helping with revisions please connect with us on the #documentation channel on our Discord server.

Spam resistance

A spam transaction is loosely defined as a block broadcasted with the intent to saturate the network, reduce network availability, or increase the size of the ledger. In order to make spam more costly, each valid block in Nano requires a proof-of-work (PoW) solution to be attached to it - similar to the original Hashcash proposition1. Participants can compute the required work in a few seconds, and verification time for this work is negligible (to prevent invalid blocks, large work, and/or invalid work from becoming a denial-of-service attack vector). The cost of spamming the network then increases linearly with the number of spam transactions, reducing the impact of spam from theoretically infinite to a manageable amount.

In addition to proof-of-work, another key component of Nano's defense against spam is transaction prioritization using a round-robin balance-bucket system, combined with least-recently-used (LRU) prioritization within those buckets. This system ensures that spam does not prevent legitimate users from making transactions & achieving fast confirmation, which in turn removes some of the incentives to spam (e.g. network disruption). See the prioritization details & prioritization buckets sections below for more information. While prioritization can be considered a "Node Implementation" topic, it's included in this "Protocol Design" discussion due to its relevance to spam resistance.

Work algorithm details

Every block includes a work field that must be correctly populated. Valid work is obtained by randomly guessing a nonce such that:

H(\text{nonce} || \text{x}) \ge \text{threshold}

where H is an algorithm, usually in the form of a hash function, || is the concatenation operator, threshold is a parameter of the network that relates to the resources spent to obtain a valid work, and x is either:

  • The account's public key, in the case of the first block on the account, or
  • The previous block's hash

The following image illustrates the process by which valid work is obtained for Block 2.

generate-work

The work field is not used when signing a block. This design has two consequences:

  1. A block can be securely signed locally, while the work is requested from a remote server, with larger resources. This is especially important for devices with low resources.

  2. Since all inputs are known before generating a block, a user can precompute the work for the next block, eliminating any time between creating and broadcasting a block. After a block is created, the next block's work can be computed immediately, using the last block's hash as input.

Choosing an algorithm

While the specific algorithm used is an implementation decision, there is a minimal set of requirements that must be met for compatibility with the Nano protocol.

  1. Asymmetry. Verifying work should take the least amount of resources (including time) as possible.
  2. Small proof size. Work should take a minimal amount of a block's size compared to the resources required to generate it, in order to reduce overhead and maximize throughput.
  3. Amortization-free. The cost of obtaining work for multiple blocks should scale linearly with the number of blocks. This ensures fairness for all participants.
  4. Progress-free. Any attempt at obtaining work should follow a stochastic process, with no dependence on previous attempts.

Additional requirements of parameter flexibility, constrained parallelism, and being optimization-free, are desired but not required 2.

Prioritization details

As of V24, Nano representatives rotate (round-robin) through 62 balance-based buckets when voting on transactions, and the least-recently-used (LRU) accounts in each bucket have the most priority (within their bucket). For example, if an account with 1 XNO and another account with 5 XNO both make two transactions, Nano representatives will vote on one transaction from the 1 XNO bucket, and one transaction from the 5 XNO bucket, before voting on a second transaction from the same bucket. Furthermore, the least-recently-used account within a bucket has the most priority (in that bucket), so after an account makes a transaction it gets moved to the back of the line behind everyone else (in that bucket). This means that if an attacker tries to send thousands of transactions from an account that only has 0.00001 Nano (for example), other accounts in the 0.00001 bucket that don't make frequent transactions will have priority over the spam, and the 0.00001 bucket spam will have no impact on accounts in other balance-buckets (e.g. 1 XNO).

Balance-based Buckets

Prioritization buckets are split by account balances, not transaction amounts

Prior to V24, there were 129 balance buckets4, with the majority (89) of those buckets being for balances under 0.0003. Since most legitimate users tend to have balances larger than that, this meant that most of the balance buckets were minimally used. In V24 however, there are now 62 balance buckets35, & account balances under 0.0003 all share the same bucket. This means that there are a lot more buckets for typical real-world account balances now, which helps prioritize legitimate transactions over spam.

Prioritization buckets

Here are the 62 balance buckets in the reference node implementation, based on the V24 source code3.

Bucket Number Bucket Region (uint128) Bucket Range (Nano) Bucket Range (Raw)
Bucket 0 0 - 2^88 0.0 - 0.0003094850098213451 0 - 309485009821345068724781056
Bucket 1 2^88 - 2^92 0.0003094850098213451 - 0.002630622583481433 309485009821345068724781056 - 2630622583481433084160638976
Bucket 2 2^88 - 2^92 0.002630622583481433 - 0.0049517601571415215 2630622583481433084160638976 - 4951760157141521099596496896
Bucket 3 2^92 - 2^96 0.0049517601571415215 - 0.023520860746422224 4951760157141521099596496896 - 23520860746422225223083360256
Bucket 4 2^92 - 2^96 0.023520860746422224 - 0.042089961335702926 23520860746422225223083360256 - 42089961335702929346570223616
Bucket 5 2^92 - 2^96 0.042089961335702926 - 0.06065906192498363 42089961335702929346570223616 - 60659061924983633470057086976
Bucket 6 2^92 - 2^96 0.06065906192498363 - 0.07922816251426434 60659061924983633470057086976 - 79228162514264337593543950336
Bucket 7 2^96 - 2^100 0.07922816251426434 - 0.22778096722850996 79228162514264337593543950336 - 227780967228509970581438857216
Bucket 8 2^96 - 2^100 0.22778096722850996 - 0.3763337719427556 227780967228509970581438857216 - 376333771942755603569333764096
Bucket 9 2^96 - 2^100 0.3763337719427556 - 0.5248865766570012 376333771942755603569333764096 - 524886576657001236557228670976
Bucket 10 2^96 - 2^100 0.5248865766570012 - 0.6734393813712468 524886576657001236557228670976 - 673439381371246869545123577856
Bucket 11 2^96 - 2^100 0.6734393813712468 - 0.8219921860854925 673439381371246869545123577856 - 821992186085492502533018484736
Bucket 12 2^96 - 2^100 0.8219921860854925 - 0.9705449907997381 821992186085492502533018484736 - 970544990799738135520913391616
Bucket 13 2^96 - 2^100 0.9705449907997381 - 1.1190977955139838 970544990799738135520913391616 - 1119097795513983768508808298496
Bucket 14 2^96 - 2^100 1.1190977955139838 - 1.2676506002282295 1119097795513983768508808298496 - 1267650600228229401496703205376
Bucket 15 2^100 - 2^104 1.2676506002282295 - 2.4560730379421947 1267650600228229401496703205376 - 2456073037942194465399862460416
Bucket 16 2^100 - 2^104 2.4560730379421947 - 3.6444954756561594 2456073037942194465399862460416 - 3644495475656159529303021715456
Bucket 17 2^100 - 2^104 3.6444954756561594 - 4.8329179133701246 3644495475656159529303021715456 - 4832917913370124593206180970496
Bucket 18 2^100 - 2^104 4.8329179133701246 - 6.021340351084089 4832917913370124593206180970496 - 6021340351084089657109340225536
Bucket 19 2^100 - 2^104 6.021340351084089 - 7.209762788798055 6021340351084089657109340225536 - 7209762788798054721012499480576
Bucket 20 2^100 - 2^104 7.209762788798055 - 8.39818522651202 7209762788798054721012499480576 - 8398185226512019784915658735616
Bucket 21 2^100 - 2^104 8.39818522651202 - 9.586607664225985 8398185226512019784915658735616 - 9586607664225984848818817990656
Bucket 22 2^100 - 2^104 9.586607664225985 - 10.775030101939949 9586607664225984848818817990656 - 10775030101939949912721977245696
Bucket 23 2^100 - 2^104 10.775030101939949 - 11.963452539653915 10775030101939949912721977245696 - 11963452539653914976625136500736
Bucket 24 2^100 - 2^104 11.963452539653915 - 13.15187497736788 11963452539653914976625136500736 - 13151874977367880040528295755776
Bucket 25 2^100 - 2^104 13.15187497736788 - 14.340297415081846 13151874977367880040528295755776 - 14340297415081845104431455010816
Bucket 26 2^100 - 2^104 14.340297415081846 - 15.52871985279581 14340297415081845104431455010816 - 15528719852795810168334614265856
Bucket 27 2^100 - 2^104 15.52871985279581 - 16.717142290509774 15528719852795810168334614265856 - 16717142290509775232237773520896
Bucket 28 2^100 - 2^104 16.717142290509774 - 17.90556472822374 16717142290509775232237773520896 - 17905564728223740296140932775936
Bucket 29 2^100 - 2^104 17.90556472822374 - 19.093987165937705 17905564728223740296140932775936 - 19093987165937705360044092030976
Bucket 30 2^100 - 2^104 19.093987165937705 - 20.282409603651672 19093987165937705360044092030976 - 20282409603651670423947251286016
Bucket 31 2^104 - 2^108 20.282409603651672 - 39.297168607075115 20282409603651670423947251286016 - 39297168607075111446397799366656
Bucket 32 2^104 - 2^108 39.297168607075115 - 58.31192761049855 39297168607075111446397799366656 - 58311927610498552468848347447296
Bucket 33 2^104 - 2^108 58.31192761049855 - 77.32668661392199 58311927610498552468848347447296 - 77326686613921993491298895527936
Bucket 34 2^104 - 2^108 77.32668661392199 - 96.34144561734543 77326686613921993491298895527936 - 96341445617345434513749443608576
Bucket 35 2^104 - 2^108 96.34144561734543 - 115.35620462076888 96341445617345434513749443608576 - 115356204620768875536199991689216
Bucket 36 2^104 - 2^108 115.35620462076888 - 134.3709636241923 115356204620768875536199991689216 - 134370963624192316558650539769856
Bucket 37 2^104 - 2^108 134.3709636241923 - 153.38572262761576 134370963624192316558650539769856 - 153385722627615757581101087850496
Bucket 38 2^104 - 2^108 153.38572262761576 - 172.40048163103918 153385722627615757581101087850496 - 172400481631039198603551635931136
Bucket 39 2^104 - 2^108 172.40048163103918 - 191.41524063446263 172400481631039198603551635931136 - 191415240634462639626002184011776
Bucket 40 2^104 - 2^108 191.41524063446263 - 210.42999963788608 191415240634462639626002184011776 - 210429999637886080648452732092416
Bucket 41 2^104 - 2^108 210.42999963788608 - 229.44475864130953 210429999637886080648452732092416 - 229444758641309521670903280173056
Bucket 42 2^104 - 2^108 229.44475864130953 - 248.45951764473295 229444758641309521670903280173056 - 248459517644732962693353828253696
Bucket 43 2^104 - 2^108 248.45951764473295 - 267.4742766481564 248459517644732962693353828253696 - 267474276648156403715804376334336
Bucket 44 2^104 - 2^108 267.4742766481564 - 286.48903565157985 267474276648156403715804376334336 - 286489035651579844738254924414976
Bucket 45 2^104 - 2^108 286.48903565157985 - 305.5037946550033 286489035651579844738254924414976 - 305503794655003285760705472495616
Bucket 46 2^104 - 2^108 305.5037946550033 - 324.51855365842675 305503794655003285760705472495616 - 324518553658426726783156020576256
Bucket 47 2^108 - 2^112 324.51855365842675 - 932.9908417679768 324518553658426726783156020576256 - 932990841767976839501573559156736
Bucket 48 2^108 - 2^112 932.9908417679768 - 1,541.4631298775269 932990841767976839501573559156736 - 1541463129877526952219991097737216
Bucket 49 2^108 - 2^112 1,541.4631298775269 - 2,149.935417987077 1541463129877526952219991097737216 - 2149935417987077064938408636317696
Bucket 50 2^108 - 2^112 2,149.935417987077 - 2,758.407706096627 2149935417987077064938408636317696 - 2758407706096627177656826174898176
Bucket 51 2^108 - 2^112 2,758.407706096627 - 3,366.8799942061773 2758407706096627177656826174898176 - 3366879994206177290375243713478656
Bucket 52 2^108 - 2^112 3,366.8799942061773 - 3,975.3522823157273 3366879994206177290375243713478656 - 3975352282315727403093661252059136
Bucket 53 2^108 - 2^112 3,975.3522823157273 - 4,583.824570425278 3975352282315727403093661252059136 - 4583824570425277515812078790639616
Bucket 54 2^108 - 2^112 4,583.824570425278 - 5,192.296858534828 4583824570425277515812078790639616 - 5192296858534827628530496329220096
Bucket 55 2^112 - 2^116 5,192.296858534828 - 24,663.41007804043 5192296858534827628530496329220096 - 24663410078040431235519857563795456
Bucket 56 2^112 - 2^116 24,663.41007804043 - 44,134.52329754603 24663410078040431235519857563795456 - 44134523297546034842509218798370816
Bucket 57 2^112 - 2^116 44,134.52329754603 - 63,605.63651705164 44134523297546034842509218798370816 - 63605636517051638449498580032946176
Bucket 58 2^112 - 2^116 63,605.63651705164 - 83,076.74973655725 63605636517051638449498580032946176 - 83076749736557242056487941267521536
Bucket 59 2^116 - 2^120 83,076.74973655725 - 706,152.3727607365 83076749736557242056487941267521536 - 706152372760736557480147500773933056
Bucket 60 2^116 - 2^120 706,152.3727607365 - 1,329,227.995784916 706152372760736557480147500773933056 - 1329227995784915872903807060280344576
Bucket 61 2^120 - 2^128 1,329,227.995784916 - 340,282,366.9209385 1329227995784915872903807060280344576 - 340282366920938463463374607431768211456

Existing whitepaper sections related to this page:

Existing content related to this page:

  1. A. Back, “Hashcash - a denial of service counter-measure,” 2002. [Online]. Available: http://www.hashcash.org/papers/hashcash.pdf ↩

  2. For more details on these requirements, refer to A. Biryukov, "Equihash: Asymmetric Proof-of-Work Based on the Generalized Birthday Problem" 2017. [Online]. Available: https://doi.org/10.5195/ledger.2017.48 ↩

  3. C. LeMahieu et al, "Nanocurrency/Nano-Node - Prioritization.cpp". [Online]. Available: https://github.com/clemahieu/nano-node/blob/releases/v24/nano/node/prioritization.cpp#L46-L73 ↩↩

  4. A. Titan, "All 129 prioritization buckets in Nano", 2021. [Online]. Available: https://www.reddit.com/r/nanocurrency/comments/myf9c2/all_129_prioritization_buckets_in_nano/ ↩

  5. P. Luberus, "Visualizing the updated V24 balance buckets (62 prioritization buckets)", 2022. [Online]. Available: https://www.reddit.com/r/nanocurrency/comments/zydm1w/visualizing_the_updated_v24_balance_buckets_62/ ↩