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2.7. Tunnels and Hierarchy |
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A router Ru could take explicit action to cause a particular packet
to be delivered to a router Rd, even though Ru and Rd
are not consecutive routers on the hop-by-hop path for that packet,
and Rd is not the packet's ultimate destination. This could be done
by encapsulating the packet inside a network layer header whose destination
is Rd address. This is called a tunnel from Ru to Rd.
The packet is a tunneled-packet. |
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There are two types of tunnels: when a tunneled-packet follows a
hop-by-hop path from Ru to Rd it is called a "Hop-by-Hop
Routed Tunnel"; when the packet travels over a path other than the
hop-by-hop path, we say that it is an "Explicit Routed Tunnel". |
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LSP Tunnels |
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| Using label switching rather that network switching we can
implement an LSP tunnel. The tunnel would be the LSP <R1,
R2, ..., Rn>, where R1 is the transmit endpoint and Rn
the receive endpoint. |
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| A set of packets sent through the LSP tunnel constitutes a FEC,
and each LSR in the tunnel must assign a label to that FEC (a
label to the tunnel). The criteria for assigning a packet to an LSP
tunnel is a local matter at the tunnel's transmit endpoint. To
put a packet into the tunnel, the transmit endpoint pushes a label
for the tunnel onto the label stack, and sends the labeled packet to the
next hop in the tunnel. The receive endpoint doesn't need to know
which packets it receives through the tunnel, since the label stack may be
popped at the penultimate LSR in the tunnel. |
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| A "Hop-by-Hop Routed LSP Tunnel" is a tunnel that is implemented as
an hop-by-hop routed LSP between endpoints. An "Explicit
Routed LSP Tunnel" is a tunnel that is an Explicit Routed LSP. |
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Hierarchy: LSP tunnels within LSPs |
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| Consider a LSP <R1,R2,R3,R4>. R1 receives unlabeled packet
P and pushes on its label stack the label to cause it to follow this
path. Let's suppose that R2 and R3 are not directly
connected, but are neighbors by virtue of being endpoints of an LSP
tunnel. So the actual sequence of LSRs traversed by P is
<R1,R2,R21,R22,R23,R3,R4>. |
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| When P travels from R1 to R2, it will have a label
stack of depth 1. R2, switching on the label, determines that
P must enter the tunnel. Then, it replaces the incoming label with a
label that is meaningful to R3 and pushes on a new label. This new
level 2 label, has a value which is meaningful to R21. Switching
is done on the level 2 label by R21, R22, and R23.
R23, which is the penultimate hop in the R2-R3 tunnel,
pops the label stack before forwarding the packet to R3. When R3
sees packet P, P has only a level 1 label, having now
exited the tunnel. Since R3 is the penultimate hop in P's level 1
LSP, it pops the label stack, and R4 receives P unlabeled. |
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| The label stack mechanism allows LSP tunneling to nest any
depth. |
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Label Distribution Peering and Hierarchy |
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Suppose that packet P travels along a Level 1 LSP <R1,R2,R3,R4>,
and when going from R2 to R3, travels along a Level 2 LSP
<R2,R21,R22,R3>. Seing from the Level 2 LSP perspective, R2's
label distribution peer is R21. From the Level 1 perspective,
R2's label distribution peers are R1 and R3. One can
have label distribution peers at each layer of hierarchy. In
the former example, R2 and R21 must be IGP neighbors,
but R2 and R3 need not be. |
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When two label distribution peer LSRs are IGP neighbors, we
will refer to them as "local distribution peers". When they are not
IGP neighbors, we will refer to them as "remote label distribution
peers". Above, R2 and R21 are local label distribution
peers, but R2 and R3 are remote label distribution
peers. |
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Label distribution between local distribution peers is done by
sending label distribution protocol messages which are addressed to
the peer. Label distribution between remote distribution peers
is done in one of two ways: |
- Explicit peering: Same as local distribution peers, this
means, protocol messages are addressed to the remote peer.
This technique is used when the number of remote peers is small, or when
the number of higher level label binding is large, or when the remote peer
is in a distinct routing area or domain. Of course, it should be known
which label to distribute to which peer.
- Implicit peering: Protocol messages are not addressed to
one's peer. Instead, one encodes a higher level label as an
attribute of a lower level label, and then distribute the lower
level label, allong with this attribute, to one's local label
distribution peer. The local peer propagates the information to
its local label distribution peers. This process continues till the
information reaches the remote peer.
This technique is most useful when the number of remote label
distribution peers is large. It does not require an n-square
peering mesh to distribute labels to the remote label distribution
peers because the information is piggybacked through the local
label distribution peering. However, implicit peering requires
the intermediate nodes to store information that they might not be
directly interested in.
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Important keywords to understand and remember |
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Tunnel
Tunneled packet
Hop-by-Hop routed tunnel
Explicit routed tunnel
Hop-by-Hop routed LSP tunnel
Explicit routed LSP tunnel
Tunnel's transmit endpoint
Tunnel's receive endpoint
Tunnel's hierarchy
Layer of hierarchy
Local distribution peers
Remote distribution peers
Explicit peering
Implicit peering |
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