Written down a possible solution.

[tinc] / doc / CONNECTIVITY

diff --git a/doc/CONNECTIVITY b/doc/CONNECTIVITY
index f504f25..8ccc0de 100644 (file)
--- 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.
 
    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
 ==========
 
 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.
 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:
+      <insert solution here>
+    B receives ADD_HOST(D) from C, and notes that is is already known:
+      <insert solution here>
+    B receives ADD_HOST(E) from C, and notes that is is already known:
+      <insert solution here>
+    E receives ADD_HOST(C) from F, and notes that is is already known:
+      <insert solution here>
+    E receives ADD_HOST(A) from F, and notes that is is already known:
+      <insert solution here>
+    E receives ADD_HOST(B) from F, and notes that is is already known:
+      <insert solution here>
+
+  4 A receives ADD_HOST(D) from B, and notes that it is already known:
+      <insert solution here>
+    A receives ADD_HOST(E) from B, and notes that it is already known:
+      <insert solution here>
+    A receives ADD_HOST(F) from B, and notes that it is already known:
+      <insert solution here>
+    F receives ADD_HOST(A) from E, and notes that it is already known:
+      <insert solution here>
+    F receives ADD_HOST(B) from E, and notes that it is already known:
+      <insert solution here>
+    F receives ADD_HOST(B) from E, and notes that it is already known:
+      <insert solution here>
+
+    ...
+
+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:
+      <insert solution here>
+    B receives ADD_HOST(D,E) from C, and notes that D is already known:
+      <insert solution here>
+    B receives ADD_HOST(E,F) from C, and notes that E is already known:
+      <insert solution here>
+    E receives ADD_HOST(C,F) from F, and notes that C is already known:
+      <insert solution here>
+    E receives ADD_HOST(A,B) from F, and notes that A is already known:
+      <insert solution here>
+    E receives ADD_HOST(B,C) from F, and notes that B is already known:
+      <insert solution here>
+
+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.