Virtual circuits¶
- In a network that uses virtual circuits, the forwarding paths depend on the label tables installed inside each network node.
![\tikzstyle{arrow} = [thick,->,>=stealth]
\tikzset{router/.style = {rectangle, draw, text centered, minimum height=2em}, }
\tikzset{host/.style = {circle, draw, text centered, minimum height=2em}, }
\tikzset{ftable/.style={rectangle, dashed, draw} }
\node[host] (A) {A};
\node[router, right=of A] (R1) { R1 };
\node[ftable, above=of R1] (FR1) { \begin{tabular}{l|l|l}
InLabel & OutPort & OutLabel \\
\hline
1 & E & 2 \\
2 & W & 1 \\
3 & S & 3 \\
\end{tabular}};
\node[router,right=of R1] (R2) {R2};
\node[ftable, right=of R2] (FR2) { \begin{tabular}{l|l|l}
InLabel & OutPort & OutLabel \\
\hline
1 & S-W & 4 \\
2 & W & 2 \\
\end{tabular}\\};
\node[router,below=of R1] (R3) {R3};
\node[ftable, below=of R3] (FR3) { \begin{tabular}{l|l|l}
InLabel & OutPort & OutLabel \\
\hline
1 & N-E & 1 \\
3 & N-E & 2 \\
4 & E & 1 \\
\end{tabular}\\};
\node[host, right=of R3] (B) {B};
\path[draw,thick]
(A) edge (R1)
(R1) edge (R2)
(R2) edge (R3)
(R1) edge (R3)
(R3) edge (B);
\draw[arrow, dashed] (FR1) -- (R1);
\draw[arrow, dashed] (FR2) -- (R2);
\draw[arrow, dashed] (FR3) -- (R3);](_images/tikz-5e9a787415e18dbdcfed2b82f91e4afb34bf14b8.png)
In this network, only one of the affirmations about the forwarding paths is correct. Which one ?
To reach B, A must send packets with label=3 and the path is R1->R3
To reach A, B must send packets with label=1 and the path is R3->R2->R1
- The forwarding paths used in a virtual circuits network depend on the label forwarding tables installed in the network nodes. Sometimes, these tables must be configured manually.
![\tikzstyle{arrow} = [thick,->,>=stealth]
\tikzset{router/.style = {rectangle, draw, text centered, minimum height=2em}, }
\tikzset{host/.style = {circle, draw, text centered, minimum height=2em}, }
\tikzset{ftable/.style={rectangle, dashed, draw} }
\node[host] (A) {A};
\node[router, right=of A] (R1) { R1 };
\node[ftable, above=of R1] (FR1) { \begin{tabular}{l|l|l}
InLabel & OutPort & OutLabel \\
\hline
1 & W & 1 \\
2 & S & 1 \\
3 & E & 2 \\
\end{tabular}};
\node[router,right=of R1] (R2) {R2};
\node[router,below=of R1] (R3) {R3};
\node[router,below=of R2] (R4) {R4};
\node[ftable, below right=of R4] (FR4) { \begin{tabular}{l|l|l}
InLabel & OutPort & OutLabel \\
\hline
1 & E & 1 \\
2 & N & 1 \\
3 & W & 3 \\
\end{tabular}\\};
\node[host, right=of R4] (B) {B};
\path[draw,thick]
(A) edge (R1)
(R1) edge (R2)
(R2) edge (R3)
(R1) edge (R3)
(R4) edge (R3)
(R2) edge (R4)
(R4) edge (B);
\draw[arrow, dashed] (FR1) -- (R1);
\draw[arrow, dashed] (FR4) -- (R4);](_images/tikz-7eb976bf4317a372532cdfa6ad5278945dd38002.png)
In this network, which of the label forwarding tables below ensures that A and B can exchange packets in both directions.
To reach B, A sends packet with label=3. To reach A, B sends packets with label=2. New label forwarding table for R2:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 2 | S | 1 |
| 1 | W | 1 |
The label forwarding table for R3 remains empty.
To reach B, A sends packet with label=3. To reach A, B sends packets with label=3. New label forwarding table for R2:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 2 | S-W | 4 |
New label forwarding table for R3:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 4 | E | 1 |
| 3 | N | 1 |
To reach B, A sends packet with label=2. To reach A, B sends packets with label=1. New label forwarding table for R2:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 2 | S | 1 |
| 1 | W | 1 |
New label forwarding table for R3:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 2 | E | 1 |
| 1 | N | 1 |
To reach B, A sends packet with label=3. To reach A, B sends packets with label=2. New label forwarding table for R2:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 2 | S-W | 4 |
| 1 | S-W | 2 |
New label forwarding table for R3:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 4 | E | 1 |
| 2 | E | 2 |
- The forwarding paths used in a virtual circuits network depend on the label forwarding tables installed in the network nodes. Sometimes, these tables must be configured manually.
![\tikzstyle{arrow} = [thick,->,>=stealth]
\tikzset{router/.style = {rectangle, draw, text centered, minimum height=2em}, }
\tikzset{host/.style = {circle, draw, text centered, minimum height=2em}, }
\tikzset{ftable/.style={rectangle, dashed, draw} }
\node[host] (A) {A};
\node[router, right=of A] (R1) { R1 };
\node[ftable, above=of R1] (FR1) { \begin{tabular}{l|l|l}
InLabel & OutPort & OutLabel \\
\hline
1 & W & 1 \\
2 & S & 2 \\
3 & E & 3 \\
\end{tabular}};
\node[router,right=of R1] (R2) {R2};
\node[router,below=of R1] (R3) {R3};
\node[router,below=of R2] (R4) {R4};
\node[ftable, below right=of R4] (FR4) { \begin{tabular}{l|l|l}
InLabel & OutPort & OutLabel \\
\hline
1 & E & 4 \\
2 & N & 5 \\
3 & W & 6 \\
\end{tabular}\\};
\node[host, right=of R4] (B) {B};
\path[draw,thick]
(A) edge (R1)
(R1) edge (R2)
(R2) edge (R3)
(R1) edge (R3)
(R4) edge (R3)
(R2) edge (R4)
(R4) edge (B);
\draw[arrow, dashed] (FR1) -- (R1);
\draw[arrow, dashed] (FR4) -- (R4);](_images/tikz-4d7f3c8a3021d25cb465f5a563ec629b8880af2b.png)
In this network, which of the label forwarding tables below ensures that : both A and B can exchange packets in both directions and that the path from A to B is the reverse of the path from B to A
To reach B, A sends packets with label=2. To reach A, B sends packets with label=3. New label forwarding table for R3:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 2 | E | 1 |
| 6 | N | 1 |
New label forwarding table for R2:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 3 | S-W | 2 |
| 5 | W | 2 |
To reach B, A sends packets with label=3. To reach A, B sends packets with label=2. New label forwarding table for R2:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 5 | W | 1 |
| 3 | S | 1 |
New label forwarding table for R3:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 3 | N-W | 2 |
| 5 | N | 2 |
To reach B, A sends packets with label=2. To reach A, B sends packets with label=2. New label forwarding table for R2:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 5 | W | 1 |
| 6 | S | 1 |
New label forwarding table for R3:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 2 | E | 1 |
| 5 | N | 2 |
To reach B, A sends packets with label=3. To reach A, B sends packets with label=3. New label forwarding table for R2:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 5 | W | 1 |
| 3 | S | 1 |
New label forwarding table for R3:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 6 | N-W | 5 |
| 5 | N | 2 |
To reach B, A sends packets with label=2. To reach A, B sends packets with label=3. New label forwarding table for R2:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 2 | S | 1 |
| 1 | W | 1 |
New label forwarding table for R3:
| InLabel | OutPort | OutLabel |
|---|---|---|
| 6 | N | 1 |
| 1 | E | 1 |