X-Git-Url: https://www.tinc-vpn.org/git/browse?p=tinc;a=blobdiff_plain;f=doc%2FCONNECTIVITY;h=8ccc0de6f8b9424644ea2d20b77ab20a6aec25a8;hp=f504f25f14276af5dbfaa7be57ddd81555810986;hb=5333cada0dfc4dfc3be728e6c78d8d42dc2ace52;hpb=c1a98cd37ea20f6020487b2a5586e6de432398e7 diff --git a/doc/CONNECTIVITY b/doc/CONNECTIVITY index f504f25f..8ccc0de6 100644 --- a/doc/CONNECTIVITY +++ b/doc/CONNECTIVITY @@ -12,7 +12,7 @@ maintain a stable network. provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. - $Id: CONNECTIVITY,v 1.1.2.1 2001/07/22 14:04:38 guus Exp $ + $Id: CONNECTIVITY,v 1.1.2.5 2001/07/22 17:41:52 guus Exp $ 1. Problem ========== @@ -45,3 +45,189 @@ to C, both at the same time. The following loop will occur: The situation described here is totally symmetric, there is no preference to one connection over the other. The problem of resolving the loop, maintaining consistency and stability is therefore not a trivial one. + +What happens when A---D and C---F are connected to eachother? They exchange +lists of known hosts. A knows of B and C, and D knows of E and F. The protocol +defines ADD_HOST messages, from now on we will say that "node X sends and +ADD_HOST(Y) to Z". + +There are two possible scenarios: either both A---D and C---F finish +authentication at the same time, or A---D finishes first, so that ADD_HOST +messages will reach C and F before they finish authentication. + +1.1 A---D finishes first +------------------------ + +After A---D authentication finishes the following actions are taken: + + 1 A sends ADD_HOST(B) to D + A sends ADD_HOST(C) to D + D sends ADD_HOST(E) to A + D sends ADD_HOST(F) to A + + 2 A sends ADD_HOST(D) to B + A receives ADD_HOST(E) from D: + A sends ADD_HOST(E) to B + A receives ADD_HOST(F) from D: + A sends ADD_HOST(F) to B + D sends ADD_HOST(A) to E + D receives ADD_HOST(B) from A: + D sends ADD_HOST(B) to E + D receives ADD_HOST(C) from A: + D sends ADD_HOST(C) to E + + 3 B receives ADD_HOST(D) from A, + B sends ADD_HOST(D) to C + B receives ADD_HOST(E) from A: + B sends ADD_HOST(E) to C + B receives ADD_HOST(F) from A: + B sends ADD_HOST(F) to C + E receives ADD_HOST(A) from D: + E sends ADD_HOST(A) to F + E receives ADD_HOST(B) from D: + E sends ADD_HOST(B) to F + E receives ADD_HOST(C) from D: + E sends ADD_HOST(C) to F + + 4 C receives ADD_HOST(D) from B. + C receives ADD_HOST(E) from B. + C receives ADD_HOST(F) from B. + F receives ADD_HOST(A) from E. + F receives ADD_HOST(B) from E. + F receives ADD_HOST(C) from E. + +Then C---F authentication finishes, the following actions are taken: + + 1 C notes that F is already known: + Connection is closed. + F notes that C is already known: + Connection is closed. + +1.2 Both A---D and C---F finish at the same time. +------------------------------------------------- + + 1 A sends ADD_HOST(B) to D + A sends ADD_HOST(C) to D + D sends ADD_HOST(E) to A + D sends ADD_HOST(F) to A + + C sends ADD_HOST(A) to F + C sends ADD_HOST(B) to F + F sends ADD_HOST(D) to C + F sends ADD_HOST(E) to C + + 2 A sends ADD_HOST(D) to B + A receives ADD_HOST(E) from D: + A sends ADD_HOST(E) to B + A receives ADD_HOST(F) from D: + A sends ADD_HOST(F) to B + D sends ADD_HOST(A) to E + D receives ADD_HOST(B) from A: + D sends ADD_HOST(B) to E + D receives ADD_HOST(C) from A: + D sends ADD_HOST(C) to E + + C sends ADD_HOST(F) to B + C receives ADD_HOST(D) from F: + A sends ADD_HOST(D) to B + C receives ADD_HOST(E) from F: + A sends ADD_HOST(E) to B + F sends ADD_HOSTS(C) to E + F receives ADD_HOST(A) from C: + D sends ADD_HOST(A) to E + F receives ADD_HOST(B) from C: + D sends ADD_HOST(B) to E + + 3 B receives ADD_HOST(D) from A, + B sends ADD_HOST(D) to C + B receives ADD_HOST(E) from A: + B sends ADD_HOST(E) to C + B receives ADD_HOST(F) from A: + B sends ADD_HOST(F) to C + E receives ADD_HOST(A) from D: + E sends ADD_HOST(A) to F + E receives ADD_HOST(B) from D: + E sends ADD_HOST(B) to F + E receives ADD_HOST(C) from D: + E sends ADD_HOST(C) to F + + B receives ADD_HOST(F) from C, and notes that is is already known: + + B receives ADD_HOST(D) from C, and notes that is is already known: + + B receives ADD_HOST(E) from C, and notes that is is already known: + + E receives ADD_HOST(C) from F, and notes that is is already known: + + E receives ADD_HOST(A) from F, and notes that is is already known: + + E receives ADD_HOST(B) from F, and notes that is is already known: + + + 4 A receives ADD_HOST(D) from B, and notes that it is already known: + + A receives ADD_HOST(E) from B, and notes that it is already known: + + A receives ADD_HOST(F) from B, and notes that it is already known: + + F receives ADD_HOST(A) from E, and notes that it is already known: + + F receives ADD_HOST(B) from E, and notes that it is already known: + + F receives ADD_HOST(B) from E, and notes that it is already known: + + + ... + +1.2.1 Augmenting ADD_HOST +------------------------- + +A solution would be to augment ADD_HOST with an extra parameter, the nexthop of +the added host: + + 3 B receives ADD_HOST(D,A) from A, + B sends ADD_HOST(D,A) to C + B receives ADD_HOST(E,D) from A: + B sends ADD_HOST(E,D) to C + B receives ADD_HOST(F,E) from A: + B sends ADD_HOST(F,E) to C + E receives ADD_HOST(A,D) from D: + E sends ADD_HOST(A,D) to F + E receives ADD_HOST(B,A) from D: + E sends ADD_HOST(B,A) to F + E receives ADD_HOST(C,B) from D: + E sends ADD_HOST(C,B) to F + + B receives ADD_HOST(F,C) from C, and notes that F is already known: + + B receives ADD_HOST(D,E) from C, and notes that D is already known: + + B receives ADD_HOST(E,F) from C, and notes that E is already known: + + E receives ADD_HOST(C,F) from F, and notes that C is already known: + + E receives ADD_HOST(A,B) from F, and notes that A is already known: + + E receives ADD_HOST(B,C) from F, and notes that B is already known: + + +So, B and E have to make a choice. Which ADD_HOST is going to win? Fortunately, +since the ADD_HOST messages are augmented, they have an extra piece of +information they can use to decide in a deterministic way which one is going to +win. For example, B got ADD_HOST(F,E) and ADD_HOST(F,C). Since "E" > "C", it +could let ADD_HOST(F,E) win. + + B receives ADD_HOST(F,C) from C, and notes that F is already known: + since "C" < "E", B ignores ADD_HOST(F,E) + B sends ADD_HOST(F,C) to A + ... + E receives ADD_HOST(C,F) from F, and notes that C is already known: + since "F" > "B", E removes the ADD_HOST(C,B) in favour of the new one + E sends ADD_HOST(C,F) to D + + 4 A receives ADD_HOST(F,E) from B, and notes that F is already known: + since "E" < "D", A ignores ADD_HOST(F,D). + ... + D receives ADD_HOST(C,F) from E, and notes that C is already known: + since "F" > "B", D removes the ADD_HOST(C,B), + closes the connection with C, in favour of the new one.