Lecture 4: Dynamic routing protocols Tuesday: 1.Overview of router architecture 2.RIP Thursday: 1.OSPF, BGP

Lecture 4: Dynamic routing protocols

Tuesday:1. Overview of router architecture2. RIPThursday:1. OSPF, BGP

2

Review of RIP

• A distance vector routing protocol

3

A router updates its distance vectors using bellman-ford equation

Bellman-Ford Equation

Define

dx(y) := cost of the least-cost path from x to y

Then

• dx(y) = minv{c(x,v) + dv(y) }, where min is taken over all neighbors of node x

4

Distance vector algorithm: initialization

• Let Dx(y) be the estimate of least cost from x to y

• Initialization:– Each node x knows the cost to each neighbor: c(x,v). For

each neighbor v of x, Dx(v) = c(x,v)

– Dx(y) to other nodes are initialized as infinity.

• Each node x maintains a distance vector (DV):

– Dx = [Dx(y): y 2 N ]

5

Distance vector algorithm: updates

• Each node x sends its distance vector to its neighbors, either periodically, or triggered by a change in its DV.

• When a node x receives a new DV estimate from a neighbor v, it updates its own DV using B-F equation:– If c(x,v) + Dv(y) < Dx(y) then

• Dx(y) = c(x,v) + Dv(y)• Sets the next hop to reach the destination y to the

neighbor v• Notify neighbors of the change

• The estimate Dx(y) will converge to the actual least cost dx(y)

6

The Count-to-Infinity Problem

AA BB CC1 1

A's Routing Table B's Routing Table

C

to costvia(next hop)

2B C

to costvia(next hop)

1C

now link B-C goes down

C 2 C

C 1-C 2B

C C 3

C 3AC -

C 4 C

C -C 4B

1

1

1

1

1

7

Count-to-Infinity

• The reason for the count-to-infinity problem is that each node only has a “next-hop-view”

• For example, in the first step, A did not realize that its route (with cost 2) to C went through node B

• How can the Count-to-Infinity problem be solved?

8

Count-to-Infinity



• How can the Count-to-Infinity problem be solved?• Solution 1: Always advertise the entire path in an update

message to avoid loops (Path vectors)– BGP uses this solution

9

Count-to-Infinity



• How can the Count-to-Infinity problem be solved?• Solution 2: Never advertise the cost to a neighbor if this

neighbor is the next hop on the current path (Split Horizon)– Example: A would not send the first routing update to B, since B

is the next hop on A’s current route to C– Split Horizon does not solve count-to-infinity in all cases!

» You can produce the count-to-infinity problem in Lab 4.

10

RIP - Routing Information Protocol

• A simple intradomain protocol• Straightforward implementation of Distance Vector Routing• Each router advertises its distance vector every 30 seconds

(or whenever its routing table changes) to all of its neighbors• RIP always uses 1 as link metric• Maximum hop count is 15, with “16” equal to “”• Routes are timeout (set to 16) after 3 minutes if they are not

updated

11

RIP - History

• Late 1960s : Distance Vector protocols were used in the ARPANET

• Mid-1970s: XNS (Xerox Network system) routing protocol is the ancestor of RIP in IP (and Novell’s IPX RIP and Apple’s routing protocol)

• 1982 Release of routed for BSD Unix• 1988 RIPv1 (RFC 1058)

- classful routing• 1993 RIPv2 (RFC 1388)

- adds subnet masks with each route entry - allows classless routing

• 1998 Current version of RIPv2 (RFC 2453)

12

RIPv1 Packet Format

IP header UDP header RIP Message

Command Version Set to 00...0

32-bit address

Unused (Set to 00...0)

address family Set to 00.00


metric (1-16)

one

rout

e en

try(2

0 by

tes)

Up to 24 more routes (each 20 bytes)

32 bits

One RIP message can have up to 25 route entries

1: request2: response

2: for IP

Address of destination

Cost (measured in hops)

1: RIPv1

13

RIPv2

• RIPv2 is an extends RIPv1:– Subnet masks are carried in the route information– Authentication of routing messages– Route information carries next-hop address– Uses IP multicasting

• Extensions of RIPv2 are carried in unused fields of RIPv1 messages

14

RIPv2 Packet Format

IP header UDP header RIP Message

Command Version Set to 00...0

32-bit address


address family Set to 00.00


metric (1-16)

one

rout

e en

try(2

0 by

tes)


32 bits

One RIP message can have up to 25 route entries

1: request2: response

2: for IP

Address of destination

Cost (measured in hops)

2: RIPv2

15

RIPv2 Packet Format

IP header UDP header RIPv2 Message

Command Version Set to 00.00

IP address

Subnet Mask

address family route tag

Next-Hop IP address

metric (1-16)

one

rout

e en

try(2

0 by

tes)


32 bits

Used to provide a method of separating "internal" RIP routes (routes for networks within the RIP routing domain) from "external" RIP routes

Identifies a better next-hop address on the same subnet than the advertising router, if one exists (otherwise 0….0)

2: RIPv2

Subnet mask for IP address

16

RIP Messages

• This is the operation of RIP in routed. Dedicated port for RIP is UDP port 520.

• Two types of messages: – Request messages

• used to ask neighboring nodes for an update– Response messages

• contains an update

17

Routing with RIP

• Initialization: Send a request packet (command = 1, address family=0..0) on all interfaces:

• RIPv1 uses broadcast if possible, • RIPv2 uses multicast address 224.0.0.9, if possible

requesting routing tables from neighboring routers • Request received: Routers that receive above request send their entire

routing table• Response received: Update the routing table

• Regular routing updates: Every 30 seconds, send all or part of the routing tables to every neighbor in an response message

• Triggered Updates: Whenever the metric for a route change, send entire routing table.

18

RIP Security

• Issue: Sending bogus routing updates to a router• RIPv1: No protection• RIPv2: Simple authentication scheme

IP header UDP header RIPv2 Message

Command Version Set to 00.00

Password (Bytes 0 - 3)

Password (Bytes 4 - 7)

0xffff Authentication Type

Password (Bytes 8- 11)

Password (Bytes 12 - 15) Auth

etic

atio

nUp to 24 more routes (each 20 bytes)

32 bits

2: plaintext password

19

RIP Problems

• RIP takes a long time to stabilize– Even for a small network, it takes several minutes until the

routing tables have settled after a change• RIP has all the problems of distance vector algorithms, e.g.,

count-to-Infinity » RIP uses split horizon to avoid count-to-infinity

• The maximum path in RIP is 15 hops

Relates to Lab 4. This module covers link state routing and the Open Shortest Path First (OSPF) routing protocol.

Dynamic Routing Protocols IIOSPF

21

Distance Vector vs. Link State Routing

• With distance vector routing, each node has information only about the next hop:

• Node A: to reach F go to B• Node B: to reach F go to D• Node D: to reach F go to E• Node E: go directly to F

• Distance vector routing makespoor routing decisions if directions are not completelycorrect (e.g., because a node is down).

• If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged.

AA BB CC

DD EE FF

22

Distance Vector vs. Link State Routing

• In link state routing, each node has a complete map of the topology

• If a node fails, each node can calculate the new route

• Difficulty: All nodes need to have a consistent view of the network

AA BB CC

DD EE FF

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

23

Link State Routing: Properties

• Each node requires complete topology information• Link state information must be flooded to all nodes• Guaranteed to converge

24

Link State Routing: Basic principles

1. Each router establishes a relationship (“adjacency”) with its neighbors

2. Each router generates link state advertisements (LSAs) which are distributed to all routers

LSA = (link id, state of the link, cost, neighbors of the link)

Each router sends its LSA to all routers in the network (using a method called reliable flooding)

3. Each router maintains a database of all received LSAs (topological database or link state database), which describes the network has a graph with weighted edges

4. Each router uses its link state database to run a shortest path algorithm (Dijikstra’s algorithm) to produce the shortest path to each network

25

Link state routing: graphical illustration

a

b

c d

3 1

6

2

a

3

6

b

c

a

b

c

3 1

a

b

c d

1

6

c d

2

a’s view

b’s view

c’s view

d’s view

Collecting all pieces yielda complete view of the network!

26

Operation of a Link State Routing protocol

ReceivedLSAs

IP Routing Table

Dijkstra’s

Algorithm

Link StateDatabase

LSAs are flooded to other interfaces

27

Dijkstra’s Shortest Path Algorithm for a Graph

Input: Graph (N,E) with N the set of nodes and E the set of edges

cvw link cost (cvw = 1 if (v,w) E, cvv = 0)

s source node.

Output: Dn cost of the least-cost path from node s to node n

M = {s};

for each n M Dn = csn;

while (M all nodes) do Find w M for which Dw = min{Dj ; j M};Add w to M;for each neighbor n of w and n M

Dn = min[ Dn, Dw + cwn ];Update route;

enddo

28

OSPF

• OSPF = Open Shortest Path First

• The OSPF routing protocol is the most important link state routing protocol on the Internet (another link state routing protocol is IS-IS (intermediate system to intermediate system)

• The complexity of OSPF is significant

– RIP (RFC 2453 ~ 40 pages)

– OSPF (RFC 2328 ~ 250 pages)

• History:– 1989: RFC 1131 OSPF Version 1 – 1991: RFC1247 OSPF Version 2– 1994: RFC 1583 OSPF Version 2 (revised)– 1997: RFC 2178 OSPF Version 2 (revised)– 1998: RFC 2328 OSPF Version 2 (current version)

29

Features of OSPF

• Provides authentication of routing messages• Enables load balancing by allowing traffic to be split evenly

across routes with equal cost• Type-of-Service routing allows to setup different routes

dependent on the TOS field• Supports subnetting• Supports multicasting• Allows hierarchical routing

30

Hierarchical OSPF

31

Hierarchical OSPF

• Two-level hierarchy: local area, backbone.

– Link-state advertisements only in area – each nodes has detailed area topology; only know

direction (shortest path) to nets in other areas.• Area border routers: “summarize” distances to nets in own area,

advertise to other Area Border routers.• Backbone routers: run OSPF routing limited to backbone.

32

Example Network

Router IDs can be selected independent of interface addresses, but usually chosen to be the smallest interface address

3

4 2

5

1

1

32

•Link costs are called Metric

• Metric is in the range [0 , 216]

• Metric can be asymmetric

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10.1.2.3 10.1.5.5

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Link State Advertisement (LSA)

• The LSA of router 10.1.1.1 is as follows:

• Link State ID: 10.1.1.1 = Router ID

• Advertising Router: 10.1.1.1 = Router ID• Number of links: 3 = 2 links plus router itself

• Description of Link 1: Link ID = 10.1.1.2, Metric = 4• Description of Link 2: Link ID = 10.1.2.3, Metric = 3• Description of Link 3: Link ID = 10.1.1.1, Metric = 0

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10.1.2.3 10.1.5.5

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34

Network and Link State Database

Each router has a database which contains the LSAs from all other routers

LS Type Link StateID Adv. Router Checksum LS SeqNo LS Age

Router-LSA 10.1.1.1 10.1.1.1 0x9b47 0x80000006 0

Router-LSA 10.1.1.2 10.1.1.2 0x219e 0x80000007 1618

Router-LSA 10.1.2.3 10.1.2.3 0x6b53 0x80000003 1712

Router-LSA 10.1.4.4 10.1.4.4 0xe39a 0x8000003a 20

Router-LSA 10.1.5.5 10.1.5.5 0xd2a6 0x80000038 18

Router-LSA 10.1.7.6 10.1.7.6 0x05c3 0x80000005 1680

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10.1.2.3 10.1.5.5

35

Link State Database

• The collection of all LSAs is called the link-state database• Each router has an identical link-state database

– Useful for debugging: Each router has a complete description of the network

• If neighboring routers discover each other for the first time, they will exchange their link-state databases

• The link-state databases are synchronized using reliable flooding

36

OSPF Packet Format

OSPF MessageIP header

Body of OSPF MessageOSPF MessageHeader

Message TypeSpecific Data

LSA LSALSA ...

LSAHeader

LSAData

...

Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)

TTL: set to 1 (in most cases)

OSPF packets are not carried as UDP payload!OSPF has its own IP protocol number: 89

37

OSPF Packet Format

source router IP address

authentication

authentication

32 bits

version type message length

Area ID

checksum authentication type

Body of OSPF MessageOSPF MessageHeader

2: current version is OSPF V2

Message types:1: Hello (tests reachability)2: Database description3: Link Status request4: Link state update5: Link state acknowledgement

ID of the Area from which the packet originated

Standard IP checksum taken over entire packet

0: no authentication1: Cleartext password2: MD5 checksum(added to end packet)

Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits)

KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits)

Prevents replay attacks

38

OSPF LSA Format

Link State ID

link sequence number

advertising router

Link Age Link Type

checksum length

Link ID

Link Data

Link Type Metric#TOS metrics

LSA

LSAHeader

LSAData

Link ID

Link Data

Link Type Metric#TOS metrics

LSA Header

Link 1

Link 2

39

Discovery of Neighbors

• Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces.

• If two routers share a link, they can become neighbors, and establish an adjacency

• After becoming a neighbor, routers exchange their link state databases

OSPF Hello

OSPF Hello: I heard 10.1.10.2

10.1.10.1 10.1.10.2

Scenario:Router 10.1.10.2 restarts

40

Neighbor discovery and database synchronization

OSPF Hello

OSPF Hello: I heard 10.1.10.2

Database Description: Sequence = X

10.1.10.1 10.1.10.2

Database Description: Sequence = X, 5 LSA headers = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

Database Description: Sequence = X+1, 1 LSA header= Router-LSA, 10.1.10.2, 0x80000005

Database Description: Sequence = X+1

Sends empty database description

Scenario:Router 10.1.10.2 restarts

Discovery of adjacency

Sends database description. (description only contains LSA headers)

Database description of 10.1.10.2Acknowledges

receipt of description

After neighbors are discovered the nodes exchange their databases

41

Regular LSA exchanges

10.1.10.2 explicitly requests each LSA from 10.1.10.1

10.1.10.1 sends requested LSAs

10.1.10.1 10.1.10.2

Link State Request packets, LSAs = Router-LSA, 10.1.10.1, Router-LSA, 10.1.10.2, Router-LSA, 10.1.10.3, Router-LSA, 10.1.10.4, Router-LSA, 10.1.10.5, Router-LSA, 10.1.10.6,

Link State Update Packet, LSAs = Router-LSA, 10.1.10.1,0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

42

Routing Data Distribution

• LSA-Updates are distributed to all other routers via Reliable Flooding

• Example: Flooding of LSA from 10.10.10.1

LSA

LSA

Updatedatabase

Updatedatabase

ACK

AC

K

LSA

LS

A

LSA

LS

A AC

K

AC

K

ACK

ACK

LSA

LS

A

LSA

LS

A

Updatedatabase

Updatedatabase

ACK

AC

K

ACK

AC

K

Updatedatabase

10.1.1.1 10.1.2.2 10.1.3.4 10.1.7.6

10.1.1.2 10.1.4.5

43

Dissemination of LSA-Update

• A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet)

• Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes.

• Acknowledgements of LSA-updates:• explicit ACK, or• implicit via reception of an LSA-Update

• Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play?

Border Gateway protocol (BGP)

45

BGP

• BGP = Border Gateway Protocol . Currently in version 4, specified in RFC 1771. (~ 60 pages)

• Interdomain routing protocol for routing between autonomous systems

• Uses TCP to establish a BGP session and to send routing messages over the BGP session

• BGP is a path vector protocol. Routing messages in BGP contain complete routes.

• Network administrators can specify routing policies

46

BGP policy routing

• BGP’s goal is to find any path (not an optimal one). Since the internals of the AS are never revealed, finding an optimal path is not feasible.

• Network administrator sets BGP’s policies to determine the best path to reach a destination network.

47

BGP basics

• A route is defined as a unit of information that pairs a destination with the attributes of a path to that destination.

• EBGP session is a BGP session between two routers in different ASes.

• IBGP session is a BGP session between internal routers of an AS.

48

EBGP and IBGP

• IBGP is organized into a full mesh topology, or IBGP sessions are relayed using a route reflector.

128.195.0.0/16 0128.195.0.0/16 0

128.195.0.0/16 1 0AS 0

AS 1

AS 2

AS 3

128.195.0.0/16 2 1 0

R1R2 R3

R4

R5R6

R7

R8

49

Commonly BGP attributes

• Origin: indicates how BGP learned about a particular route – IGP

– EGP

– Incomplete

• AS path : – When a route advertisement passes through an autonomous system, the AS

number is added to an ordered list of AS numbers that the route advertisement

has traversed • Next hop• Multi_Exit_Disc (MED, multiple exit discriminator):

-used as a suggestion to an external AS regarding the preferred route into the AS • Local_pref: is used to prefer an exit point from the local autonomous

system • Community: apply routing decisions to a group of destinations

50

BGP route selection process

• Input/output engine may filter routes or manipulate their attributes

InputPolicyEngine

Decisionprocess

Bestroutes

OutPolicyEnigne

Routes recved from peers Routes sentto peers

51

Best path selection algorithm

1. If next hop is inaccessible, ignore routes2. Prefer the route with the largest local preference value.3. If local prefs are the same, prefer route with the shortest AS

path4. If AS_path is the same, prefer route with lowest origin (IGP

< EGP < incomplete)5. If origin is the same, prefer the route with lowest MED6. IF MEDs are the same, prefer EBGP paths to IBGP paths7. If all the above are the same, prefer the route that can be

reached via the closest IGP neighbor.8. If the IGP costs are the same, prefer the router with lowest

router id.

52

Summary

• Router architectures• Dynamic routing protocols: RIP, OSPF, BGP• RIP uses distance vector algorithm, and converges slow (the

count-to-infinity problem)• OSPF uses link state algorithm, and converges fast. But it is

more complicated than RIP.• Both RIP and OSPF finds lowest-cost path.• BGP uses path vector algorithm, and its path selection

algorithm is complicated, and is influenced by policies.

Documents

Lecture 4: Dynamic routing protocols Tuesday: 1.Overview of router architecture 2.RIP Thursday: 1.OSPF, BGP