EIP-7594 - Update ENR record

[nalepae]

Proposal for #3744

The maximum encoded size of a node record is 300 bytes. (Source).

1. Field key

Reducing the custoding_subnet_count key to csc saves 19 bytes.

2. Field value type

Switching the value of custoding_subnet_count from SSZ uint64 to big endian integer has multiple advantages:

• ENR are already RLP encoded, so there is no need to both SSZ and RLP encode.
• RLP encoding stores positive integer efficiently. If a positive integer is less than 128 (which is likely to be the case for the subnets count), then the value needs exactly one byte of storage when encoded. (Source).

Switching the value of custoding_subnet_count from SSZ uint64 to big endian integer saves 7 bytes.

Total

Changing 1. and 2. saves a total of 19 + 7 = 26 bytes. 26 bytes represents about 8.5% of the maximum size of an encoded node record.

[ppopth]

Looks good to me

[hwwhww]

lgtm

[g11tech]

lgtm from lodestar

[cskiraly]

looks good

[nalepae]

Actually it seems we are heading to have exactly 128 custody subnets for a supernode.

• For a supernode we will save only 25 bytes. (Because 128 is 2-bytes RLP encoded.)
• For other nodes we will save 26 bytes. (Becase < 128 is 1-byte RLP encoded.)