233 lines
8 KiB
Text
233 lines
8 KiB
Text
This document describes how nodes in a VPN find and connect to eachother and
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maintain a stable network.
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Copyright 2001 Guus Sliepen <guus@sliepen.warande.net>
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Permission is granted to make and distribute verbatim copies of
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this documentation provided the copyright notice and this
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permission notice are preserved on all copies.
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Permission is granted to copy and distribute modified versions of
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this documentation under the conditions for verbatim copying,
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provided that the entire resulting derived work is distributed
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under the terms of a permission notice identical to this one.
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$Id: CONNECTIVITY,v 1.1.2.5 2001/07/22 17:41:52 guus Exp $
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1. Problem
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==========
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We have a set of nodes (A, B, C, ...) that are part of the same VPN. They need
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to connect to eachother and form a single graph that satisfies the tree
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property.
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There is the possibility that loops are formed, the offending connections must
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be eliminated.
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Suppose we start with two smaller graphs that want to form a single larger
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graph. Both graphs consist of three nodes:
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A-----B-----C
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D-----E-----F
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It is very well possible that A wants to connect to D, and F wants to connect
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to C, both at the same time. The following loop will occur:
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A-----B-----C
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| ^
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| |
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v |
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D-----E-----F
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The situation described here is totally symmetric, there is no preference to
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one connection over the other. The problem of resolving the loop, maintaining
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consistency and stability is therefore not a trivial one.
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What happens when A---D and C---F are connected to eachother? They exchange
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lists of known hosts. A knows of B and C, and D knows of E and F. The protocol
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defines ADD_HOST messages, from now on we will say that "node X sends and
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ADD_HOST(Y) to Z".
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There are two possible scenarios: either both A---D and C---F finish
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authentication at the same time, or A---D finishes first, so that ADD_HOST
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messages will reach C and F before they finish authentication.
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1.1 A---D finishes first
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------------------------
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After A---D authentication finishes the following actions are taken:
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1 A sends ADD_HOST(B) to D
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A sends ADD_HOST(C) to D
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D sends ADD_HOST(E) to A
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D sends ADD_HOST(F) to A
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2 A sends ADD_HOST(D) to B
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A receives ADD_HOST(E) from D:
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A sends ADD_HOST(E) to B
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A receives ADD_HOST(F) from D:
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A sends ADD_HOST(F) to B
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D sends ADD_HOST(A) to E
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D receives ADD_HOST(B) from A:
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D sends ADD_HOST(B) to E
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D receives ADD_HOST(C) from A:
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D sends ADD_HOST(C) to E
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3 B receives ADD_HOST(D) from A,
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B sends ADD_HOST(D) to C
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B receives ADD_HOST(E) from A:
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B sends ADD_HOST(E) to C
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B receives ADD_HOST(F) from A:
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B sends ADD_HOST(F) to C
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E receives ADD_HOST(A) from D:
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E sends ADD_HOST(A) to F
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E receives ADD_HOST(B) from D:
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E sends ADD_HOST(B) to F
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E receives ADD_HOST(C) from D:
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E sends ADD_HOST(C) to F
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4 C receives ADD_HOST(D) from B.
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C receives ADD_HOST(E) from B.
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C receives ADD_HOST(F) from B.
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F receives ADD_HOST(A) from E.
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F receives ADD_HOST(B) from E.
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F receives ADD_HOST(C) from E.
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Then C---F authentication finishes, the following actions are taken:
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1 C notes that F is already known:
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Connection is closed.
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F notes that C is already known:
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Connection is closed.
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1.2 Both A---D and C---F finish at the same time.
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-------------------------------------------------
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1 A sends ADD_HOST(B) to D
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A sends ADD_HOST(C) to D
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D sends ADD_HOST(E) to A
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D sends ADD_HOST(F) to A
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C sends ADD_HOST(A) to F
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C sends ADD_HOST(B) to F
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F sends ADD_HOST(D) to C
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F sends ADD_HOST(E) to C
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2 A sends ADD_HOST(D) to B
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A receives ADD_HOST(E) from D:
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A sends ADD_HOST(E) to B
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A receives ADD_HOST(F) from D:
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A sends ADD_HOST(F) to B
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D sends ADD_HOST(A) to E
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D receives ADD_HOST(B) from A:
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D sends ADD_HOST(B) to E
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D receives ADD_HOST(C) from A:
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D sends ADD_HOST(C) to E
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C sends ADD_HOST(F) to B
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C receives ADD_HOST(D) from F:
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A sends ADD_HOST(D) to B
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C receives ADD_HOST(E) from F:
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A sends ADD_HOST(E) to B
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F sends ADD_HOSTS(C) to E
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F receives ADD_HOST(A) from C:
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D sends ADD_HOST(A) to E
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F receives ADD_HOST(B) from C:
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D sends ADD_HOST(B) to E
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3 B receives ADD_HOST(D) from A,
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B sends ADD_HOST(D) to C
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B receives ADD_HOST(E) from A:
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B sends ADD_HOST(E) to C
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B receives ADD_HOST(F) from A:
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B sends ADD_HOST(F) to C
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E receives ADD_HOST(A) from D:
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E sends ADD_HOST(A) to F
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E receives ADD_HOST(B) from D:
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E sends ADD_HOST(B) to F
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E receives ADD_HOST(C) from D:
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E sends ADD_HOST(C) to F
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B receives ADD_HOST(F) from C, and notes that is is already known:
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<insert solution here>
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B receives ADD_HOST(D) from C, and notes that is is already known:
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<insert solution here>
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B receives ADD_HOST(E) from C, and notes that is is already known:
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<insert solution here>
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E receives ADD_HOST(C) from F, and notes that is is already known:
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<insert solution here>
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E receives ADD_HOST(A) from F, and notes that is is already known:
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<insert solution here>
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E receives ADD_HOST(B) from F, and notes that is is already known:
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<insert solution here>
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4 A receives ADD_HOST(D) from B, and notes that it is already known:
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<insert solution here>
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A receives ADD_HOST(E) from B, and notes that it is already known:
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<insert solution here>
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A receives ADD_HOST(F) from B, and notes that it is already known:
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<insert solution here>
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F receives ADD_HOST(A) from E, and notes that it is already known:
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<insert solution here>
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F receives ADD_HOST(B) from E, and notes that it is already known:
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<insert solution here>
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F receives ADD_HOST(B) from E, and notes that it is already known:
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<insert solution here>
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...
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1.2.1 Augmenting ADD_HOST
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-------------------------
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A solution would be to augment ADD_HOST with an extra parameter, the nexthop of
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the added host:
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3 B receives ADD_HOST(D,A) from A,
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B sends ADD_HOST(D,A) to C
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B receives ADD_HOST(E,D) from A:
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B sends ADD_HOST(E,D) to C
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B receives ADD_HOST(F,E) from A:
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B sends ADD_HOST(F,E) to C
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E receives ADD_HOST(A,D) from D:
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E sends ADD_HOST(A,D) to F
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E receives ADD_HOST(B,A) from D:
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E sends ADD_HOST(B,A) to F
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E receives ADD_HOST(C,B) from D:
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E sends ADD_HOST(C,B) to F
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B receives ADD_HOST(F,C) from C, and notes that F is already known:
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<insert solution here>
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B receives ADD_HOST(D,E) from C, and notes that D is already known:
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<insert solution here>
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B receives ADD_HOST(E,F) from C, and notes that E is already known:
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<insert solution here>
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E receives ADD_HOST(C,F) from F, and notes that C is already known:
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<insert solution here>
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E receives ADD_HOST(A,B) from F, and notes that A is already known:
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<insert solution here>
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E receives ADD_HOST(B,C) from F, and notes that B is already known:
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<insert solution here>
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So, B and E have to make a choice. Which ADD_HOST is going to win? Fortunately,
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since the ADD_HOST messages are augmented, they have an extra piece of
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information they can use to decide in a deterministic way which one is going to
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win. For example, B got ADD_HOST(F,E) and ADD_HOST(F,C). Since "E" > "C", it
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could let ADD_HOST(F,E) win.
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B receives ADD_HOST(F,C) from C, and notes that F is already known:
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since "C" < "E", B ignores ADD_HOST(F,E)
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B sends ADD_HOST(F,C) to A
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...
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E receives ADD_HOST(C,F) from F, and notes that C is already known:
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since "F" > "B", E removes the ADD_HOST(C,B) in favour of the new one
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E sends ADD_HOST(C,F) to D
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4 A receives ADD_HOST(F,E) from B, and notes that F is already known:
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since "E" < "D", A ignores ADD_HOST(F,D).
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...
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D receives ADD_HOST(C,F) from E, and notes that C is already known:
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since "F" > "B", D removes the ADD_HOST(C,B),
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closes the connection with C, in favour of the new one.
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