path: root/p2p/discv5/table.go

// Copyright 2016 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

// Package discv5 implements the RLPx v5 Topic Discovery Protocol.
//
// The Topic Discovery protocol provides a way to find RLPx nodes that
// can be connected to. It uses a Kademlia-like protocol to maintain a
// distributed database of the IDs and endpoints of all listening
// nodes.
package discv5

import (
    "crypto/rand"
    "encoding/binary"
    "fmt"
    "net"
    "sort"

    "github.com/ethereum/go-ethereum/common"
)

const (
    alpha      = 3  // Kademlia concurrency factor
    bucketSize = 16 // Kademlia bucket size
    hashBits   = len(common.Hash{}) * 8
    nBuckets   = hashBits + 1 // Number of buckets

    maxFindnodeFailures = 5
)

type Table struct {
    count         int               // number of nodes
    buckets       [nBuckets]*bucket // index of known nodes by distance
    nodeAddedHook func(*Node)       // for testing
    self          *Node             // metadata of the local node
}

// bucket contains nodes, ordered by their last activity. the entry
// that was most recently active is the first element in entries.
type bucket struct {
    entries      []*Node
    replacements []*Node
}

func newTable(ourID NodeID, ourAddr *net.UDPAddr) *Table {
    self := NewNode(ourID, ourAddr.IP, uint16(ourAddr.Port), uint16(ourAddr.Port))
    tab := &Table{self: self}
    for i := range tab.buckets {
        tab.buckets[i] = new(bucket)
    }
    return tab
}

const printTable = false

// chooseBucketRefreshTarget selects random refresh targets to keep all Kademlia
// buckets filled with live connections and keep the network topology healthy.
// This requires selecting addresses closer to our own with a higher probability
// in order to refresh closer buckets too.
//
// This algorithm approximates the distance distribution of existing nodes in the
// table by selecting a random node from the table and selecting a target address
// with a distance less than twice of that of the selected node.
// This algorithm will be improved later to specifically target the least recently
// used buckets.
func (tab *Table) chooseBucketRefreshTarget() common.Hash {
    entries := 0
    if printTable {
        fmt.Println()
    }
    for i, b := range &tab.buckets {
        entries += len(b.entries)
        if printTable {
            for _, e := range b.entries {
                fmt.Println(i, e.state, e.addr().String(), e.ID.String(), e.sha.Hex())
            }
        }
    }

    prefix := binary.BigEndian.Uint64(tab.self.sha[0:8])
    dist := ^uint64(0)
    entry := int(randUint(uint32(entries + 1)))
    for _, b := range &tab.buckets {
        if entry < len(b.entries) {
            n := b.entries[entry]
            dist = binary.BigEndian.Uint64(n.sha[0:8]) ^ prefix
            break
        }
        entry -= len(b.entries)
    }

    ddist := ^uint64(0)
    if dist+dist > dist {
        ddist = dist
    }
    targetPrefix := prefix ^ randUint64n(ddist)

    var target common.Hash
    binary.BigEndian.PutUint64(target[0:8], targetPrefix)
    rand.Read(target[8:])
    return target
}

// readRandomNodes fills the given slice with random nodes from the
// table. It will not write the same node more than once. The nodes in
// the slice are copies and can be modified by the caller.
func (tab *Table) readRandomNodes(buf []*Node) (n int) {
    // TODO: tree-based buckets would help here
    // Find all non-empty buckets and get a fresh slice of their entries.
    var buckets [][]*Node
    for _, b := range &tab.buckets {
        if len(b.entries) > 0 {
            buckets = append(buckets, b.entries[:])
        }
    }
    if len(buckets) == 0 {
        return 0
    }
    // Shuffle the buckets.
    for i := uint32(len(buckets)) - 1; i > 0; i-- {
        j := randUint(i)
        buckets[i], buckets[j] = buckets[j], buckets[i]
    }
    // Move head of each bucket into buf, removing buckets that become empty.
    var i, j int
    for ; i < len(buf); i, j = i+1, (j+1)%len(buckets) {
        b := buckets[j]
        buf[i] = &(*b[0])
        buckets[j] = b[1:]
        if len(b) == 1 {
            buckets = append(buckets[:j], buckets[j+1:]...)
        }
        if len(buckets) == 0 {
            break
        }
    }
    return i + 1
}

func randUint(max uint32) uint32 {
    if max < 2 {
        return 0
    }
    var b [4]byte
    rand.Read(b[:])
    return binary.BigEndian.Uint32(b[:]) % max
}

func randUint64n(max uint64) uint64 {
    if max < 2 {
        return 0
    }
    var b [8]byte
    rand.Read(b[:])
    return binary.BigEndian.Uint64(b[:]) % max
}

// closest returns the n nodes in the table that are closest to the
// given id. The caller must hold tab.mutex.
func (tab *Table) closest(target common.Hash, nresults int) *nodesByDistance {
    // This is a very wasteful way to find the closest nodes but
    // obviously correct. I believe that tree-based buckets would make
    // this easier to implement efficiently.
    close := &nodesByDistance{target: target}
    for _, b := range &tab.buckets {
        for _, n := range b.entries {
            close.push(n, nresults)
        }
    }
    return close
}

// add attempts to add the given node its corresponding bucket. If the
// bucket has space available, adding the node succeeds immediately.
// Otherwise, the node is added to the replacement cache for the bucket.
func (tab *Table) add(n *Node) (contested *Node) {
    //fmt.Println("add", n.addr().String(), n.ID.String(), n.sha.Hex())
    if n.ID == tab.self.ID {
        return
    }
    b := tab.buckets[logdist(tab.self.sha, n.sha)]
    switch {
    case b.bump(n):
        // n exists in b.
        return nil
    case len(b.entries) < bucketSize:
        // b has space available.
        b.addFront(n)
        tab.count++
        if tab.nodeAddedHook != nil {
            tab.nodeAddedHook(n)
        }
        return nil
    default:
        // b has no space left, add to replacement cache
        // and revalidate the last entry.
        // TODO: drop previous node
        b.replacements = append(b.replacements, n)
        if len(b.replacements) > bucketSize {
            copy(b.replacements, b.replacements[1:])
            b.replacements = b.replacements[:len(b.replacements)-1]
        }
        return b.entries[len(b.entries)-1]
    }
}

// stuff adds nodes the table to the end of their corresponding bucket
// if the bucket is not full.
func (tab *Table) stuff(nodes []*Node) {
outer:
    for _, n := range nodes {
        if n.ID == tab.self.ID {
            continue // don't add self
        }
        bucket := tab.buckets[logdist(tab.self.sha, n.sha)]
        for i := range bucket.entries {
            if bucket.entries[i].ID == n.ID {
                continue outer // already in bucket
            }
        }
        if len(bucket.entries) < bucketSize {
            bucket.entries = append(bucket.entries, n)
            tab.count++
            if tab.nodeAddedHook != nil {
                tab.nodeAddedHook(n)
            }
        }
    }
}

// delete removes an entry from the node table (used to evacuate
// failed/non-bonded discovery peers).
func (tab *Table) delete(node *Node) {
    //fmt.Println("delete", node.addr().String(), node.ID.String(), node.sha.Hex())
    bucket := tab.buckets[logdist(tab.self.sha, node.sha)]
    for i := range bucket.entries {
        if bucket.entries[i].ID == node.ID {
            bucket.entries = append(bucket.entries[:i], bucket.entries[i+1:]...)
            tab.count--
            return
        }
    }
}

func (tab *Table) deleteReplace(node *Node) {
    b := tab.buckets[logdist(tab.self.sha, node.sha)]
    i := 0
    for i < len(b.entries) {
        if b.entries[i].ID == node.ID {
            b.entries = append(b.entries[:i], b.entries[i+1:]...)
            tab.count--
        } else {
            i++
        }
    }
    // refill from replacement cache
    // TODO: maybe use random index
    if len(b.entries) < bucketSize && len(b.replacements) > 0 {
        ri := len(b.replacements) - 1
        b.addFront(b.replacements[ri])
        tab.count++
        b.replacements[ri] = nil
        b.replacements = b.replacements[:ri]
    }
}

func (b *bucket) addFront(n *Node) {
    b.entries = append(b.entries, nil)
    copy(b.entries[1:], b.entries)
    b.entries[0] = n
}

func (b *bucket) bump(n *Node) bool {
    for i := range b.entries {
        if b.entries[i].ID == n.ID {
            // move it to the front
            copy(b.entries[1:], b.entries[:i])
            b.entries[0] = n
            return true
        }
    }
    return false
}

// nodesByDistance is a list of nodes, ordered by
// distance to target.
type nodesByDistance struct {
    entries []*Node
    target  common.Hash
}

// push adds the given node to the list, keeping the total size below maxElems.
func (h *nodesByDistance) push(n *Node, maxElems int) {
    ix := sort.Search(len(h.entries), func(i int) bool {
        return distcmp(h.target, h.entries[i].sha, n.sha) > 0
    })
    if len(h.entries) < maxElems {
        h.entries = append(h.entries, n)
    }
    if ix == len(h.entries) {
        // farther away than all nodes we already have.
        // if there was room for it, the node is now the last element.
    } else {
        // slide existing entries down to make room
        // this will overwrite the entry we just appended.
        copy(h.entries[ix+1:], h.entries[ix:])
        h.entries[ix] = n
    }
}