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Lehrstuhl für Informatik 4

Kommunikation und verteilte Systeme

Chapter 3.2: Routing in the Internet

Routing

• Very important functionality on layer 3 is routing

• Every router manages a table which indicates, which outgoing link has to be selected for a certain destination

• The routing tables can be determined statically; it is better however to adapt them continuously to the current network status (response times, throughput, avoidance of local overload situations, security requirements, …)

• The routing must be done for each packet separately. The routing decision can vary from packet to packet.



Page 2Chapter 3.2: Routing in the Internet

“Optimal” route decision is in principle not possible, since• no complete information about the network is present within the individual

nodes • a routing decision effects the network for a certain period

• conflicts between fairness and optimum can arise:

X X’

A B C

A’ B’ C’

• High traffic amount between A and A’, B and B’, C and C’

• In order to optimize the total data flow, no traffic may occur between X and X’

• X and X’ would see this totally different!

⇒ fairness criteria!

“Optimal” Routing




The Internet consists of a large number of autonomous systems. Each autonomous system is operated by its own operator and can use its own routing protocols.

By standardization of usable protocols, gateways can forward packets at the borders of the autonomous systems.

InternalProtocol 1

InternalProtocol 2

InternalProtocol 3

External Protocol

Routing in the Internet

Autonomous Systems




Interior Gateway Protocols (Routing in autonomous systems, for efficient transmission)

• Routing Information Protocol (RIP)

• Internet Gateway Routing Protocol (IGRP)

• Enhanced IGRP

• Open Shortest Path First (OSPF)

• Intermediate System to Intermediate System (IS-IS)

Exterior Gateway Protocols (Routing between domains, adherence of policies for the

domains)

• Border Gateway Protocol (BGP)

• Exterior Gateway Protocol (EGP)

Router Discovery Protocols

• ICMP Router Discovery Protocol (IRDP)

• Hot Standby Router Protocol (HSRP)

Routing Protocols




deterministic stochasticcentralized isolated

static adaptive

deterministic stochastic deterministic stochastic. local global

deterministic stochastic deterministic stochastic

Routing

Routing Algorithms

distributed




• Source Routing (“external routing”):Route is determined by the sender

• Internal Routing:- Routing decision by intermediate nodes

- Static tables are used within the routers, no reaction to changes in the network

- Stable

- Simple

- No reaction to changing network conditions

- Breakdowns in links or routers can have catastrophic results

Header 3,5,7,8 Data

1 2 3 4 5by nodevia line L 1L2 L2 L3 –

1

2 3

4

5

L

L

L

1

2

3

static

Static Routing




� Flooding of the network with copies of the data packet

� Router spreads packet over all links, except over the ingoing one

• High reliability (in case of failure of single routers)

• Meaningful for military applications (robustness)

• But: large number of copied packets

• Possible loops are problematic

→ Hop counter (TTL), list of all packets already sent

• Usable as reference for the quality of routing algorithms (delay)

Incoming packet

static

Flooding




Static variant

• Router administrates a table with distances to each destination and lines to be used

• Based on a constant metric (e.g. distance, line costs, transmission capacity etc.)

• Computation of the shortest path (regarding the metric) according to Dijkstra

Adaptive variant

• Dynamic routing algorithms

• Dynamic metrics (e.g. current delay, actual transmission capacity)

• (Regular) update of the routing tables

static

Routing: Shortest Path




Algorithm of Dijkstra (1959)

for static Routing

1. Mark source node (the “Work node”) as “permanent” (i.e. distance and line do not change any more)

2. Consider neighbored nodes of the node currently marked as “permanent”(i.e. the “Work node”) and compute the distance to them based on own knowledge and link costs

3. Choose the node with the smallest distance to the source from the nodes not marked yet and mark it as “permanent”, it becomes the new “Work node”. Goto 2.

static

Shortest Path: Dijkstra (1)




D

Example:

A

B

E

G

F

H

C

22

2

2 261

4

7

33

As metric, exemplarily thedistance in kilometers is chosen.

Searched is the shortestpath from A to D

1. Step: Marking of node A as permanent

2. Step: Marking of theneighbor nodes of A

static





A

B (2, A)

E (∞, -)

G (6, A)

F (∞, -)

H (∞, -)

D (∞, -)

C (∞, -)

22

2

2 261

4

7

33

In order to be able to trace back the path later on, the predecessor node is stored

3. Step: B becomes permanent, because distance is 2 (< 6).

4. Step: Tentative labeling of the neighbor nodes of B

static





A

B (2, A)

E (4, B)

G (6, A)

F (∞, -)

H (∞, -)

D (∞, -)

C (9, B)

E (4, B): Distance of A to E sums up to 4, using the path BE

5. Step: E becomes permanent, since the distance to A is 4 (< 6 < 9).

6. Step: Tentative labeling of the neighbor nodes of E

22

2

2 261

7

33

4

static





A

B (2, A)

E (4, B)

G (5, E)

F (6, E)

H (∞, -)

D (∞, -)

C (9, B)

5. Step: Preliminary label of G is over-written

6. Step: G becomes permanent, since the distance of A is 5 (< 6 < 9).

7. Step: Preliminary labeling of the neigh-bor nodes of G

22

2

2 261

7

3 3

4

static





A

B (2, A)

E (4, B)

G (5, E)

F (6, E)

H (9, G)

D (∞, -)

C (9, B)

8. Step: Tentative label of G is overwritten

9. Step: F becomespermanent, since the of A is 6 (< 9).

10. Step: Tentativelabeling of theneighbor nodes of F

22

2

2 261

7

33

4

static





A

B (2, A)

E (4, B)

G (5, E)

F (6, E)

H (8, F)

D (10, H)

C (9, B)

22

2

2 261

7

33

11. Step: H becomes permanent, since the distance of A is 8 (< 9).

12. Step: Tentative labeling of the neighbor nodes of H

13. Step: C becomes permanent, since the distance of A is9 (< 10).

4

static





A

B (2, A)

E (4, B)

G (5, E)

F (6, E)

H (8, F)

C (9, B)

D (10, H)

The distance to D using C is larger than the tentative label of D. No more paths exist, no states are changed - the algorithm terminates.

22

2

2 261

7

3 3

4

static





A

B (2, A)

E (4, B)

G (5, E)

F (6, E)

H (8, F)

C (9, B)

D (10, H)

22

2

2 261

7

3 3

14. Step: D is reached on the shortest path and finally becomes permanent.

4

static





Problem: Static procedures are inflexible, they do not react to problems and must be computed again after each change of the topology etc.

Solution: Routers mutually exchange (regularly) information about the current state of outgoing lines

Adaptive variant of Shortest Path Routing:

Distance Vector Routing (Bellman et al. 1957)

Also: Distributed Bellmann-Ford Routing, Ford-Fulkerson Routing,

RIP (ARPANET, Internet); improved in Cisco routers

adaptive

Distance Vector Routing




Every router manages a table with (known/estimated) distances to each destination and the assigned lines to neighbor nodes.

Each router is assumed to know the distances to its neighbors.

Regularly, the distance information of the routing tables is communicated to the neighbors; due to the information from the neighbors and the known distances to the neighbors every router computes its routing table again(without use of the own old routing information).

Example of a Distance Vector: Vj = ((A=2), (B=3), (C=1),…)

(A is reachable with costs 2, B with costs 3, and C with costs 1)adaptive

Distance Vector Routing




Information Exchange

Essential here:

Global information is exchanged only between neighbors! adaptive




(D) L6 (E) L5

L1 (B) L2 (C)

L3 L4

From A to Link Costs

A locally 0

Table of router A after system initialization or “cold start”

Table of router B after system initialization or “cold start”

As “transmission costs” for each line, 1 is assumed.

Example

adaptive

(A)

From B to Link Costs

B locally 0





B locally 0A L1 1

Routing tables of routers B and D, after the vector of router A is processed

From D to Link Costs

D locally 0A L3 1

A transfers (A=0) to its

neighbors B and D

B and D know, from where the vector comes and so the costs to A can be computed

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4

Example

adaptive




Routing table of A after theactualization coming from D and B


A locally 0B L1 1D L3 1

B now transfers its vector (B=0, A=1) using link 1, 2 and 4 to its neighbors A, E and C

A receives this vector over link 1 and updates its table as follows:

A=2 is larger than A=0 ⇒ discardB=1 for link 1

Similar to it the processing of the vector of D takes place

Example

adaptive

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4




From C to Link Costs

C locally 0B L2 1A L2 2

From E to Link Costs

E locally 0B L4 1A L4 2

Router C receives (B=0, A=1)

Router E receives (B=0, A=1)

Example

adaptive

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4





E locally 0B L4 1A L4 2D L6 1

Router E receives the vector (D=0, A=1) which it uses to update the routing table with D=1 and A=2 using link 6

The entry for A over link 4 is already registered with costs of 2, therefore no new entry for A is necessary

Routing table of E after the update

A, C and E now have new routing tables

⇒ generate distance vectors

Example

adaptive

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4





B locally 0A L1 1D L1 2C L2 1E L4 1

From A: (A=0, B=1, D=1) over link 1 and 3

From C: (C=0, B=1, A=2) over link 2 and 5

From E: (E=0, B=1, A=2, D=1) over link 4, 5 and 6

Routing table of B:

Example

adaptive

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4





D locally 0A L3 1B L3 2E L6 1


E locally 0B L4 1A L4 2D L6 1C L5 1

From A: (A=0, B=1, D=1) over link 1 and 3

From C: (C=0, B=1, A=2) over link 2 and 5

From E: (E=0, B=1, A=2, D=1) over link 4, 5 and 6

Example

adaptive

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4





C locally 0B L2 1A L2 2E L5 1D L5 2


A locally 0B L1 1D L3 1C L1 2E L1 2

From B: (B=0, A=1, D=1, C=1, E=1) over linj1, 3 and 4

From D: (D=0, A=1, B=2, E=1) over link 3 and 6

From E: (E=0, B=1, A=2, D=1, C=1) over link 4, 5 and 6

Example

adaptive

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4





D locally 0A L3 1B L3 2E L6 1C L6 2

From B: (B=0, A=1, D=1, C=1, E=1) over link 1, 3 and 4

From D: (D=0, A=1, B=2, E=1) over link 3 and 6


Algorithm terminates since A, C and D create and send new vectors, but B, D and E do not have to apply updates any longer.

Example

adaptive

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4




Problem with this procedure: Information must be reliable

Otherwise: Christmas Deadlock

A router j announced Uj = (0,…, 0)

Consequence: Nearly the entire traffic was led over j

⇒ Collapse

Disadvantages:

- Unreliable information is dangerous- Cycling load conditions

- Additional overhead- And: information propagation lasts a certain time!

Successive Information Spreading




L1 (B) L2 (C)

Router A and B notice the interruption e.g. by control packets

⇒ Update of their own routing tables

⇒ inf = infinite


A locally 0B L1 infD L3 1C L1 infE L1 inf


B locally 0A L1 infD L1 infC L2 1E L4 1

Connection Loss

adaptive

(D) L6 (E) L5

L3 L4

(A)




(A) L1 (B) L2 (C)

(D) L6 (E) L5


D locally 0A L3 1B L3 infE L6 1C L6 2

From A: (A=0, B=inf, D=1, C=inf, E=inf) over link 3

From B: (B=0, A=inf, D=inf, C=1, E=1) over link 2 and 4

D receives the vector from Aand updates with A=1, B=inf, D=2, C=inf, E=inf

Connection Loss

adaptive

L3 L4




From E to link costs

E locally 0B L4 1A L4 infD L6 1C L5 1

From C to link costs

C locally 0B L2 1A L2 infE L5 1D L5 2

From A: (A=0, B=inf, D=1, C=inf, E=inf) over link 3

From B: (B=0, A=inf, D=inf, C=1, E=1) over link 2 und 4

Connection Loss

adaptive

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4




From D: (D=0, A=1, B=inf, E=1, C=2) over link 3 and 6

From C: (C=0, B=1, A=inf, E=1, D=1) over link 2 and 5

From E: (E=0, B=1, A=inf, D=1, C=1) over link 4, 5 and 6

Connection Loss

adaptive


A locally 0B L1 infD L3 1C L3 3E L3 2


B locally 0A L1 infD L4 2C L2 1E L4 1

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4




From D: (D=0, A=1, B=inf, E=1, C=2) over link 3 and 6

From C: (C=0, B=1, A=inf, E=1, D=1) over link 2 and 5

From E: (E=0, B=1, A=inf, D=1, C=1) over link 4, 5 and 6

Connection Loss

adaptive


D locally 0A L3 1B L6 2E L6 1C L6 2


E locally 0B L4 1A L6 2D L6 1C L5 1

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4




From A: (A=0, B=inf, D=1, C=3, E=2) over link 3

From B: (B=0, A=inf, D=2, C=1, D=1) over link 2 and 4

From D: (D=0, A=1, B=2, E=1, C=2) over link 3 and 6


Connection Loss

adaptive


B locally 0A L4 3D L4 2C L2 1E L4 1


A locally 0B L3 3D L3 1C L3 3E L3 2

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4




From A: (A=0, B=inf, D=1, C=3, E=2) over link 3

From B: (B=0, A=inf, D=2, C=1, D=1) over link 2 and 4

From D: (D=0, A=1, B=2, E=1, C=2) over link 3 and 6


Algorithm terminates, because A, B and C create and send new vectors, but don´t cause further updates in the routing tables.

Connection Loss

adaptive


C locally 0B L2 1A L5 3E L5 1D L5 2

(A) L1 (B) L2 (C)

(D) L6 (E) L5

L3 L4




(D) L6 (E) L5

So far: Costs of 1 for each link

Reality: Different costs per link

Example: Link 5 has costs of 10

In the following, only the paths to router C are examined.

In the case of stable conditions the tables of the other routers have these entries for C:

The Bouncing Effect

adaptive

L1 (B) L2 (C)

L3 L4

(A)1

1

1

1

1

10

From Link Costs

A to C L1 2B to C L2 1C to C locally 0D to C L3 3E to C L4 2




Assumption 1: Connection 2 breaks down.

B detects the failure and sets its costs to inf.

Temporarily, the following table results:

Assumption 2: A has sent its vector before B.

The Bouncing Effect

adaptive

From Link Costs

A to C L1 2B to C L2 infC to C locally 0D to C L3 3E to C L4 2

(in case of regular transmission)

(D) L6 (E) L5

L1 (B) L2 (C)

L3 L4

(A)1

1

1

1

1

10




⇒ A reports C over link 1 with costs of 2

⇒ Router B adds costs of 1 to link 1 for C

⇒ Value is lower than inf

⇒ Table entry for C is replaced by link 1 and costs of 3

⇒ Passing on the table to A and E

⇒ Message comes over link 1 resp. 4, which A and E are using for C

⇒ Update of A and E

The Bouncing Effect

adaptive

From Link Costs


(D) L6 (E) L5

L1 (B) L2 (C)

L3 L4

(A)1

1

1

1

1

10




⇒ Routing tables contain a loop!

⇒ Packets from C are bouncing between A and B

⇒ E adds costs of 10 to its own costs of 0

⇒ E ignores the message, because the costs are higher as before

C sends its vector over link 5

A and E send vectors

⇒ Update of B and D

The Bouncing Effect

adaptive

From Link Costs


(D) L6 (E) L5

L1 (B) L2 (C)

L3 L4

(A)1

1

1

1

1

10




After several iterations, the following tables result:

Entries depend however on random processes (e.g. on the order of the update messages, the arrival times of the vectors, losses of vectors etc.)

The Bouncing Effect

adaptive

From Link Costs





Count to Infinity problem:

The Distance Vector Routing achieves a correct solution, but possibly many (up to infinite) update steps are necessary.

Example:5 routers A, B, C, D, E, are connected linearly, the distance between neighbor routers in each case is 1

A B C D E1 1 11

Count to Infinity

adaptive




Example 1: A at first is switched off, then it is switched on

Count to Infinity

adaptive

43214. Update

∞3213. Update

∞∞212. Update

∞∞∞1A switched on

∞∞∞∞to A

A B C D E




87876. Update

65654. Update

45453. Update

43432. Update

43231. Update

4321A switched off

Example 2: A is switched off

Count to Infinity

adaptive

A B C D E

::∞

......

...... :

:∞

::∞

::∞




A B

C

D

First solution approach: Split-Horizon Algorithm

Do not send the distance to X over that link used to transfer packets for X (resp. path is reported as inf).

However, it does not work always:• Link CD is switched off• C announces A and B that D is unreachable• B announces however to A: D has distance 2• A updates and has D with distance 3• B updates for D with distance 4 ......• Count to Infinity

Split Horizon Algorithm

adaptive

Router




• Early internal gateway routing protocol used in the Internet

• Bases on the Distance Vector Protocol• RIP messages are sent every 30 seconds as UDP datagrams• As metric for the evaluation of the paths the number of hops is used (The

maximal possible number of hops is limited to 15!)• In a message (only) up to 25 entries of the routing table can be sent• Fits good for small systems

• Problems: slow convergence (duration of minutes), Count to Infinity, no addressing of subnets

• RIPv2: subnets, authentication, multicast,… however: the maximal possible number of hops is still limited to 15.

• As reaction to the restrictions of RIP, Cisco introduced the Internet Gateway Routing Protocol (IGRP): Extension of the metric, load sharing, more efficient packet format. The protocols did not become generally accepted, because they were Cisco-specific…

• Replacement by a Link State Protocol (OSPF)

Routing Information Protocol (RIP)




1.) Information exchange with neighborsDistance Vector Routing

2.) Global information exchangeLink State Routing

Routers exchange Link State Advertisements (LSA)

Extension

adaptive




Every router

• determines its neighbors and their addresses (with so-called HELLO packets)

• measures the distances to the neighbors (with so-called ECHO packets)

• sends these information in a packet to all other routers (LSA – Link State Advertisement)

• computes on the basis of all the received LSAs from other routers the shortest paths to the other routers (e.g. with the Dijkstra algorithm)

This is repeated regularly.

Link State Routing

adaptive




A

B

C

D E

F

G

H

I

LAN

First step:

Determination of all neighbored routers

• Sending of a HELLO message on all links

• Routers at the other end answer with their identification

• If several routers are connected in a (broadcast) network, a new “artificial”node is introduced for simplification

A

B

C

D E

F

G H

IN

Link State Routing

adaptive




A

B C

D

E

F

G

H

IJ

Second step:

Discovery of link costs

• Transmission of ECHO messages

• Routers at the other end answer immediately (measurement of the delay)

• Inclusion of load leads to the choice of the lowest loaded link

Side-effect in having two possible links: the less loaded link is loaded immediately heavily and the other link becomes free, with the next measurement the same happens for the other link, … (cycling load)

Link State Routing

adaptive




A

B C

D

E F5

1

24

67

8

ASeq.No.

Age

B 4E 5

BSeq.No.

Age

A 4C 2

F 6

DSeq.No.

Age

C 3F 7

CSeq.No.

Age

B 2D 3

E 1

ESeq.No.

Age

A 5C 1

F 8

FSeq.No.

Age

B 6D 7

E 8

Third step:

Create link state messages

• Contain list of neighbors with appropriate “link costs” (Delay, queue length, jitter etc.)

• Messages additionally contain sender identification, sequence number, age

Link State Routing

adaptive

3




Fourth step:

Sending of link state messages

• Flooding (problem: loops, duplicates, packet losses etc.)

• Sequence numbers are counted up, packets with outdated numbers (duplicates) are discarded

• Every router reduces the “age” “by one, with zero a packet is discarded

• Each router confirms the arrival of a link state packet to the sending router A

F

E

C

D

21

21

21

20

21

60

60

59

60

59

0

1

0

1

1

1

1

1

0

0

1

0

0

1

0

1

0

1

0

0

0

0

0

1

1

0

1

1

0

1

Source Seq.No. Age A C AF FC

Transmission flags Confirmation flags

Data

Data structure of router B

Packet Buffer for Router B

adaptive




• Link state message from A arrives directly, therefore Age=60, Seq.No.=21

• Message is sent to C and F and confirmed for A

• Message of F must be transferred to A and Cand be confirmed for F

• Message of E cametwice (over EAB and EFB) therefore onlyforwarding to C, confirmation forA and F

A

F

E

C

D

21

21

21

20

21

60

60

59

60

59

0

1

0

1

1

1

1

1

0

0

1

0

0

1

0

1

0

1

0

0

0

0

0

1

1

0

1

1

0

1

Source Seq.No. Age A C AF FC

Transmission flags Confirmation flags

Data

Packet Buffer for Router B

adaptive

A

B C

D

E F5

1

24

67

8

3




Fifth step:

Enquiry of best routes

• Router collects link state information from all other routers

• A path graph for the entire sub-network is determined

• Local execution of e.g. the Dijkstra algorithm for the determination of the optimal route

• Results are written into the routing table

Problems:

• With n routers and m neighbors, nm table entries are necessary

• In the case of router failures, the graphs of all routers are outdated

• Extremely susceptibly to attacks

Route Enquiry

adaptive




Problem: extensive routing tables

⇒ hierarchical routing

Large tables are needing too much memory, CPU time, transmission capacity for link state messages etc. ⇒ virtual division of the network

(Regions)

Routing Tables

adaptive




1A 1C

1B 2A

2C

2B

2D

5C

5D5E

4B

5B

5A4C

4A3A

3B

Region 1

Region 5Region 4

Region 2

Region 3

Hierarchical Routing

adaptive




Destination Link to Hops

- -1B 1

123343234445565

1C1B1B1B1B1C1C1C1C1C1C1C1B1C1C

1A1B1C2A2B2C2D3A3B4A4B4C5A5B5C5D5E

Full routing table for 1A Hierarchical routing table for 1A

- -1B 1

12234

1C1B1C1C1C

1A1B1C2345

1A1C

1B 2A

2C

2B

2D

5C

5D5E

4B

5B

5A4C

4A3A

3B

Region 1

Region 5Region 4

Region 2

Region 3

Disadvantage: Possibly increasing path length

Hierarchical Routing

adaptive

Destination Link to Hops




Routing Tables

137.226.0.0

142.117.0.0

12.0.0.0

194.52.124.0

194.52.124.1

142.117.1.10.0.0.0/0 (default)

194.52.124.1194.52.124.0/24

142.117.1.1142.117.0.0/16

142.117.1.1137.226.0.0/16

194.52.124.112.0.0.0/8

Next HopDestination Address

194.52.124.10

142.117.1.7

142.117.1.1




Open Shortest Path First

• Links State protocol

• 1990 standardized by IETF (RFC 1247)

• Open protocol (not manufacturer specific)

• Supports a multiplicity of metrics (distance, delay etc.)

• Dynamic algorithm for fast adjustment to changing conditions in the network

• Load sharing between redundant links

• Supports hierarchical systems

• Contains security mechanisms to protect routers from wrong routing information or attacks

• Three types of connections are supported:

- Point-to-point links between routers- Broadcast networks (mostly LANs)- Multi-access networks without broadcasting (e.g. packet switching

WANs)

Internal Gateway Routing Protocol - OSPF




• The Internet is divided into autonomous systems (AS)

• Very large autonomous systems are divided in areas

• Each autonomous system has a backbone, which connects all parts of the AS

• Every router, which belongs to two or more areas, is part of the backbone

• Within these areas every router has the same link state database and implements the same algorithm for determination of the shortest path

• A router, which connect two areas, needs the link state databases from both areas

• OSPF distinguishes four router classes (for reducing the extent of routing tables):

- Internal routers, which only belong to one area- Area routers at the border of areas, which connect two or more areas- Backbone routers, which are placed at the backbone- AS border routers, which mediate between several autonomous systems

OSPF




Area router

External protocols connectthe autonomous systems

Backbone

Backbonerouter

Area

Internal router

Border router of theautonomous system

AS 1 AS 2

AS 3 AS 4

Relationship between autonomous systems, backbones and areas in OSPF

OSPF




External Gateway Protocol - BGP

Internal gateway protocols are designed for efficiency: find the best way to the destination host.

External gateway protocols must have to consider policies (political, economical,…)

BGP - Border Gateway Protocol

An external routing protocol

• Variant of the Distance Vector Protocol: not the costs of a transmission path are being monitored and exchanged, but the complete description of paths

• Considers security and other rules (Routing Policies)

• Communicates the neighbor routers the whole path which is to be used (deterministically)

• Uses TCP for data exchange




H

A

CB

G

F

E

D

JI

Information sent to F

F receives the following information about D from its neighbors :

from B: “I use BCD”from G: “I use GCD”from I: “I use IFGCD”from E: “I use EFGCD”

Assumption: F uses FGCD to reach D

F searches for the optimal route

• Paths of I and E are directly discarded, because they cross F

• B and G are possible options

• Application of Policies

• Routes, which violate policies, are being set to inf

External Gateway Protocol - BGP

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