RFC 1326 (rfc1326) - Page 2 of 5

Mutual Encapsulation Considered Dangerous

Alternative Format: Original Text Document

```RFC 1326                Encapsulation Dangerous                 May 1992

protocol of Y.  Likewise, the right and left S(Y) stubs use protocol
Y, and the right and left S(X) stubs use protocol X.

:::  :::::          :::::   :::          :::
+------+ :Y   :X:Y  +------+ :X:Y    :Y  +------+ :Y   +------+
|      | :::  ::::: |      | :::::   ::: |      | :::  |      |
| S(Y) |-----Ra-----|      |-------Rb----|      |------| S(Y) |
|      |            |      |             |      |      |      |
+------+            |      |             |      |      +------+
| B(X) |             | B(Y) |
|      |             |      |
:::  |      | :::   ::::: |      | :::::  :::
+------+  X:  |      |  X:    X:Y: |      |  X:Y:   X: +------+
|      | :::  |      | :::   ::::: |      | :::::  ::: |      |
| S(X) |------|      |-----Rc------|      |------Rd----| S(X) |
|      |      |      |             |      |            |      |
+------+      |      |-----Re------|      |            +------+
+------+             +------+

LEGEND:

:::::
X:Y:  A packet with protocol X encapsulated in protocol
:::::  Y, moving left to right

Rx  Router x

S(Y)  A stub network whose native protocol is protocol Y

B(X)  A backbone network whose native protocol is protocol X

FIGURE 1:  MUTUAL ENCAPSULATION

Figure 1 shows how packets would travel from left S(X) to right S(X),
and from right S(Y) to left S(Y).  Consider a packet from left S(X)
to right S(X).  The packet from left S(X) has just a header of X up
to the point where it reaches router Rc.  Since B(Y) cannot forward
header X, Rc encapsulates the packet into a Y header with a
destination address of Rd.  When Rd receives the packet from B(Y), it
strips off the Y header and forwards the X header packet to right
S(X).  The reverse situation exists for packets from right S(Y) to
left S(Y).

In this example Rc and Rd treat B(Y) as a lower-level subnetwork in
exactly the same way that an IP router currently treats an Ethernet
as a lower-level subnetwork.  Note that Rc considers Rd to be the

Tsuchiya```

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