354 lines
13 KiB
Text
354 lines
13 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 2001-2002 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 1.1.2.8 2002/02/10 21:57:51 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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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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Ok, time to forget this crap.
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1.2.2
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-----
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The problem with the current ADD/DEL_HOST technique is that each host only
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knows the general direction in which to send packets for the other hosts. It
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really doesn't know much about the true topology of the network, only about
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it's direct neighbours. With so little information each host cannot make a
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certain decision which it knows for sure all the others will decide too.
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Let's do something totally different. Instead of notifying every host of the
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addition of a new host, which is represented by a vertex in a graph, lets send
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out notifications of new connections, which are the edges in a graph. This is
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rather cheap, since our graphs are (almost) spanning trees, there is
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approximately one edge for each vertex in the graph, so we don't need to send
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more messages. Furthermore, an edge is characterized by two vertices, so we
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only send a fixed amount of extra information. The size/complexity of the
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problem therefore does not increase much.
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What is the advantage of notifying each vertex of new edges instead of new
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vertices? Well, all the vertices now know exactly which connections are made
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between each host. This was not known with the former schemes.
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Ok back to our problem:
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A-----B-----C
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D-----E-----F
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Edges are undirected, and are characterised by the vertices it connects, sorted
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alphabetically, so the edges in the two graphs are:
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(A,B), (B,C), (D,E) and (E,F).
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So again we have that A wants to connect to D, and F wants to connect to C,
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both at the same time. The following loop will occur:
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A-----B-----C
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D-----E-----F
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Instead of sending ADD_HOSTs, lets assume the hosts send ADD_EDGEs. So, after
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making the connections:
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1 A sends ADD_EDGE(A,D) to B
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A sends ADD_EDGE(A,B) to D
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A sends ADD_EDGE(B,C) to D
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D sends ADD_EDGE(A,D) to E
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D sends ADD_EDGE(D,E) to A
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D sends ADD_EDGE(E,F) to A
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C sends ADD_EDGE(C,F) to B
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C sends ADD_EDGE(A,B) to F
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C sends ADD_EDGE(B,C) to F
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F sends ADD_EDGE(C,F) to E
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F sends ADD_EDGE(D,E) to C
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F sends ADD_EDGE(E,F) to C
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2 B receives ADD_EDGE(A,D) from A:
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B sends ADD_EDGE(A,D) to C
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B receives ADD_EDGE(D,E) from A:
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B sends ADD_EDGE(D,E) to C
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B receives ADD_EDGE(E,F) from A:
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B sends ADD_EDGE(E,F) to C
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...
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B receives ADD_EDGE(C,F) from C, notes that both C and F are already known,
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but that the edge (C,F) was not known, so a loop has been created:
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<resolve loop here>
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Ok, how to resolve the loop? Remeber, we want to do that in such a way that it
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is consistent with the way all the other hosts resolve the loop. Here is the
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things B does when it notices that a loop is going to be formed:
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B performs a Breadth First Search from the first element of the list of all
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known hosts sorted alfabetically, in this case A, and thereby finds a
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spanning tree. (This might later be changed into a minimum spanning tree
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alhorithm, but the key point here is that all hosts do this with exactly the
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same starting parameters.) All known edges that are not in the spanning tree
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are marked inactive.
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An edge marked inactive does not mean anything, unless this edge is connected
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to B itself. In that case, B will stop sending messages over that edge. B might
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consider closing this edge, but this is not really needed. Keeping it means no
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DEL_EDGE has to be sent for it, and if another edge is removed (which will
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quite certainly split the graph if it's a spanning tree), this edge might be
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reactivated, without the need of sending a new ADD_EDGE for it. On the other
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hand, we mustn't keep to many inactive edges, because we want to keep the
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number of known edges linear to the number of hosts (otherwise the size of the
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problem will grow quadratically).
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So, since B didn't deactivate one of it's own edges, it forwards the
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ADD_EDGE(C,F) to A, which also does a BFS, and so on, until it reaches F. F of
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course also does a BFS, notes that is is one of it's own edges. It deactivates
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the edge (C,F), and consequently will not forward the ADD_EDGE(C,F) to C
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anymore. In the mean time, C got messages from B which will make C do the same.
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Ok, suppose a DEL_EDGE was sent, and it means an inactive edge has to be
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reactivated. The vertices connected by that edge must exchange their entire
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knowledge of edges again, because in the mean time other messages could have
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been sent, which were not properly forwarded. Take this example:
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X C-----D
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A-----B- - -E
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The edge (B,E) is inactive. X is trying to make a new connection with A. A
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sends an ADD_EDGE(A,X) to B, which forwards it to C. At that time, the
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connection between C and D goes down, so C sends a DEL_EDGE(C,D) to B, and D
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sends a DEL_EDGE(C,D) to E. If we just allow (B,E) to be reactivated again
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without anything else, then E and D will never have received the ADD_EDGE(A,X).
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So, B and E have to exchange edges again, and propagate them to the hosts they
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already know.
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