Network Layer7-1 2010 session 1 TELE3118: Network Technologies Week 7: Network Layer Routing Protocols Some slides have been taken from: r Computer Networking:

Network Layer 7-1

2010 session 1TELE3118: Network

Technologies

Week 7: Network LayerRouting Protocols

Some slides have been taken from:Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition. Jim Kurose, Keith Ross. Addison-Wesley, July 2004. All material copyright 1997-2004. J.F Kurose and K.W. Ross, All Rights Reserved.

Network Layer 7-2

IP routing0.0.0.0 0 192.168.1.1

10.0.0.0 8 172.20.4.1

200.23.16.0 20 199.31.18.4

200.23.18.0 23 172.20.4.1

10.20.0.0 24 199.31.18.4

192.168.1.0 24 L 192.168.1.18

172.20.4.0 24 L 172.20.4.253

199.31.18.0 24 L 199.31.18.52

destination mask loca

l

next-hop

LAN

inte

rface

s172.20.4.253/24

192.168.1.18/24199.31.18.52/24

How is the routing table constructed? Static (manual) Dynamic (routing protocol)

Network Layer 7-3

The Internet Network layer

Note on terminology: “routing” vs. “forwarding” “routing table” vs. “forwarding table”

forwardingtable

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

Network Layer 7-4

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

“routing” and “forwarding” tables

Network Layer 7-5

Routing: abstract model

Graph abstraction for routing algorithms:

graph nodes are routers

graph edges are physical links link cost: delay, $

cost, or congestion level

Goal: determine “good” path

(sequence of routers) thru network from source to

dest.

Routing protocol

A

ED

CB

F

2

2

13

1

1

2

53

5

“good” path: typically means

minimum cost path other def’s possible

Network Layer 7-6

Routing algorithm classification

Distance-vector algorithm

Local information: router knows physically-

connected neighbors, link costs to neighbors

2 components: Neighbor routing-table

exchange Bellman-Ford (also

called Ford-Fulkerson) computation

E.g.: RIP

Link-state algorithm Global information:

router knows complete topology and link cost info of entire network

2 components: Reliable flooding Dijkstra shortest-path

tree (SPT) computation

E.g.: OSPF, IS-IS

Network Layer 7-7

Distance vector - RIP

Each node maintains a table of triples.

D

G

A

F

E

B

C

Destination Cost Next-hop

A 1 A

C 1 C

D 2 C

E 2 A

F 2 A

G 3 A

table at B:

Network Layer 7-8

RIP: overview

Iterative, asynchronous, distributed Directly connected neighbors exchange

updates periodically (on the order of several seconds) whenever table changes (called triggered update)

Each update is a vector of distances: (Destination, Cost)

Update local table if receive a “better” route smaller cost came from next-hop

Refresh existing routes; delete if they time out

Network Layer 7-9

RIP: example

Destination Cost Next-hop

B 1 B

C 1 C

D ∞ -

E 1 E

F 1 F

G ∞ -

D

G

A

F

E

B

C

Destination Cost Next-hop

B 1 B

C 1 C

D 2 C

E 1 E

F 1 F

G ∞ -

Destination Cost Next-hop

B 1 B

C 1 C

D 2 C

E 1 E

F 1 F

G 2 F

Initial table at A:

After receiving update from C:

After receiving update from F:

Network Layer 7-10

RIP: recovering from link failure

Dest Cost Nh

A 1 A

B 2 A

C 2 A

D ∞ -

E 2 A

G ∞ -

D

G

A

F

E

B

C

Dest Cost Nh

B 1 B

C 1 C

D 2 C

E 1 E

F 1 F

G ∞ -

Dest Cost Nh

B 1 B

C 1 C

D 2 C

E 1 E

F 1 F

G 3 C

At F:

At A:A receives update from C:

Dest Cost Nh

A 1 A

B 2 A

C 2 A

D 3 A

E 2 A

G 4 A

F receives update from A:

Network Layer 7-11

RIP: link cost decreases

X Z14

12

Y1

X 4 X

Z 1 Z

X 5 Y

Y 1 Y

X 1 X

Z 1 Z

X 5 Y

Y 1 Y

X 1 X

Z 1 Z

X 2 Y

Y 1 Y

At Y:

At Z:

Good news travels fast

Network Layer 7-12

RIP: link cost increases

X Z14

12

Y14

X 4 X

Z 1 Z

X 5 Y

Y 1 Y

X 6 Z

Z 1 Z

X 5 Y

Y 1 Y

X 6 Z

Z 1 Z

X 7 Y

Y 1 Y

At Y:

At Z:

X 8 Z

Z 1 Z

X 7 Y

Y 1 Yand so on

Bad news travels slow “count to infinity” problem loops!

Network Layer 7-13

Breaking the loop …

X Z

14

12

Y14

X 4 X

Z 1 Z

X 5 Y

Y 1 Y

X 14 X

Z 1 Z

X 5 Y

Y 1 Y

X 14 X

Z 1 Z

X 12 X

Y 1 Y

At Y:

At Z:

X 13 Z

Z 1 Z

X 12 X

Y 1 Y

Does this solve the “count to infinity” problem?

If next-hop to D is R: Split Horizon: do not include D

in update to R Split Horizon with Poison

Reverse: include D, but with metric = ∞

Network Layer 7-14

… is not always easy

Dest Cost Nh

B 1 B

C 1 C

D 2 C

E ∞ -

F 1 F

G 2 F

D

G

A

F

E

B

C

Dest Cost Nh

A 1 A

C 1 C

D 2 C

E 3 C

F 2 A

G 3 A

Dest Cost Nh

B 1 B

C 1 C

D 2 C

E 4 B

F 1 F

G 2 F

At A:

B receives update from C:A receives update from B:

Dest Cost Nh

A 1 A

B 1 C

D 1 D

E 5 A

F 2 A

G 2 D

C receives update from A:

Network Layer 7-15

RIPv2 (RFC 2453) details

Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (∞ = 16): why? Distance vectors only exchanged among

neighbors Up to 25 destinations per RIP update message Update-interval is 30 sec:

If too large, convergence is slow If too small, too much traffic

Triggered update whenever change in routing table

Split horizon mandatory, poison reverse optional

Network Layer 7-16

RIPv2 details (contd.)

Updates sent every 30 (+/- 5) seconds

Route not refreshed for 180 sec is timed-out Still included in update

messages Timed-out route is deleted

(garbage-collected) after 120 sec

Triggered update timer set for 1-5 sec Includes only changed routes Suppressed if regular update due

Address of net 2

Distance to net 2

Command Must be zero

Family of net 2 Must be zero

Family of net 1 Must be zero

Address of net 1

Distance to net 1

Version

0 8 16 31

subnet mask of net 1

subnet mask of net 2

next hop of net 1

next hop of net 2

Network Layer 7-17

RIP: where does it run? RIP runs as application-level process (route-d) Updates sent as UDP message (port 520) Multicast IP address 224.0.0.9 (with TTL=1)

physical

link

network forwarding (IP) table

Transprt (UDP)

routed

physical

link

network (IP)

Transprt (UDP)

routed

forwardingtable

Network Layer 7-18

Link State - OSPF

Strategy: each node learns complete topology send information about directly connected links (not

entire routing table) to entire network (not just neighbors)

Link State Advertisement (LSA) include Nodes (routers) and links (networks) Sequence number and age

Reliable flooding Store most recent LSA for each node Send LSA to all nodes except one that sent it Generate LSA periodically (with higher sequence

number) Age out each stored LSA

Network Layer 7-19

A Link-State Routing Algorithm

Notation: c(x,y): link cost from node x

to y; = ∞ if not direct neighbors

D(v): current value of cost of path from source to dest. v

p(v): predecessor node along path from source to v

N': set of nodes whose least cost path definitively known

Dijkstra’s algorithm Given: all nodes know full

topology and link costs Objective: compute least

cost paths from self to all other nodes routing table

iterative: after k iterations, know least cost path to k destinations

distributed: each node computes shortest-path tree from itself

Network Layer 7-20

Dijsktra’s Algorithm

1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N'

Network Layer 7-21

Dijkstra’s algorithm: example

Step012345

N'u

uxuxy

uxyvuxyvw

uxyvwz

D(v),p(v)2,u2,u2,u

D(w),p(w)5,u4,x3,y3,y

D(x),p(x)1,u

D(y),p(y)∞

2,x

D(z),p(z)∞ ∞

4,y4,y4,y

u

yx

wv

z2

2

13

1

1

2

53

5

Network Layer 7-22

Dijkstra’s algorithm, discussionAlgorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n2) more efficient implementations possible: O(nlogn)

Link Metric Static: link latency, link capacity, … Dynamic: based on load?

e.g.: link cost = amount of carried traffic oscillations!

A

D

C

B1 1+e

e0

e

1 1

0 0

A

D

C

B2+e 0

001+e1

A

D

C

B0 2+e

1+e10 0

A

D

C

B2+e 0

e01+e1

initially… recompute

routing… recompute … recompute

Network Layer 7-23

OSPF details RFC 2328 (244 pages long!) Neighbor up/down detected using “hello” packets LSA reliable flooding over entire AS

LSA includes sequence number and age LSA integrity using checksum (excludes age)

OSPF messages directly over IP (no UDP or TCP) Hierarchical OSPF: allow scaling to larger networks 5 types of LSAs:

1. Router LSA: set of nodes2. Network LSA: set of links3. Summary LSA: inter-area networks4. Summary LSA: area-border-routers5. External LSA: external to AS

Network Layer 7-24

Hierarchical OSPF

Network Layer 7-25

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.

Boundary routers: connect to other AS’s.

Network Layer 7-26

OSPF “advanced” features (not in RIP)

Authentication: prevents malicious intrusion Hierarchy: allows larger domains Load balancing: equal-cost multi-path (ECMP) Extensions to support:

Multicast: MOSPF Traffic-engineering: OSPF-TE

Network Layer 7-27

Comparison of LS and DV algorithms

Messaging DV: entire routing table,

but only exchanged between neighbors

LS: small messages, but flooded in whole network

Speed of Convergence DV: multiple iterations,

each requires recompute and transmit count-to-infinity

problem LS: flood and recalculate,

one shot, faster

Robustness: both LS and DV can be wrecked by one bad router.

In 1997 a bad router in a small ISP advertised a false cost, became flooded with traffic, disconnecting ISPs from most U.S. backbone providers for ~3 hours

Bottom line: No clear winner in terms of

complexity, robustness, etc LS often favored due to

faster convergence

Network Layer 7-28

Hierarchical Routing

scale: with 200 million destinations:

can’t store all dest’s in routing tables!

routing table exchange would swamp links!

administrative autonomy

internet = network of networks

each network admin may want to control routing in its own network

Our routing study thus far - idealization all routers identical network “flat”… not true in practice

Network Layer 7-29

Hierarchical Routing in the Internet Internet is organized as Autonomous Systems (AS)

Each AS is an independent administrative domain (e.g. ISP)

Intra-AS routing protocol All routers in an AS run same intra-AS routing protocol Routers in different AS can run different intra-AS routing

protocol

Inter-AS routing protocol Between routers in different AS

Gateway routers: run both intra-AS and inter-AS routing protocols

Network Layer 7-30

Intra-AS and Inter-AS routing

Gateways:•perform inter-AS routing amongst themselves•perform intra-AS routing with other routers in their AS

inter-AS, intra-AS routing in

gateway A.c

network layer

link layer

physical layer

a

b

b

aaC

A

Bd

A.a

A.c

C.bB.a

cb

c

Network Layer 7-31

IGP vs. EGP

Figure 4.5.2-new2: BGP use for inter-domain routing

AS2 (OSPF

intra-AS routing)

AS1 (RI P intra-AS

routing) BGP

AS3 (OSPF intra-AS

routing)

BGP

R1 R2

R3

R4

R5

Intra-area routing protocol also called Interior Gateway Protocol (IGP) Administrator can choose any: RIP, OSPF, ISIS, …

Inter-area routing protocol also called Exterior Gateway Protocol (EGP) Unique: Border Gateway Protocol (BGP)

Network Layer 7-32

Internet inter-AS routing: BGP

BGP (Border Gateway Protocol): the de facto standard

BGP provides each AS a means to:1. Obtain subnet reachability information from

neighboring ASs.2. Propagate the reachability information to all

routers internal to the AS.3. Determine “good” routes to subnets based

on reachability information and policy. Allows a subnet to advertise its

existence to rest of the Internet: “I am here”

Network Layer 7-33

BGP basics Pairs of routers (BGP peers) exchange routing info over

semi-permanent TCP conctns: BGP sessions Note that BGP sessions do not correspond to physical links. When AS2 advertises a prefix to AS1, AS2 is promising it

will forward any datagrams destined to that prefix towards the prefix. AS2 can aggregate prefixes in its advertisement

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

Network Layer 7-34

Path attributes & BGP routes

When advertising a prefix, advert includes BGP attributes. prefix + attributes = “route”

Path Vector protocol: similar to Distance Vector protocol each Border Gateway broadcast to

neighbors (peers) entire path (i.e., sequence of AS’s) to destination

• E.g., Gateway X may send its path to dest. Z:

Path (X,Z) = X,Y1,Y2,Y3,…,Z when gateway router receives route advert,

uses import policy to accept/decline.

Network Layer 7-35

BGP operation Point-to-point peering BGP peers explicitly configured

Lack of trust no auto-discovery! BGP session runs over TCP

Reliable Can detect neighbor/link down

4 types of messages: OPEN: opens TCP connection to peer and

authenticates sender UPDATE: advertises new path (or withdraws old) KEEPALIVE keeps connection alive in absence of

UPDATES; also ACKs OPEN request NOTIFICATION: reports errors in previous msg; also

used to close connection

Network Layer 7-36

BGP operation (contd.)

BGP peers exchange route prefixes AS-path Route attributes No cost included!

Route prefixes received from peer are filtered and selected (based on AS-path and route attributes) for installation in RIB

Route prefixes from RIB are sent to peer after filtering and selection

All the complexity is in the use of policies for filtering and selection

Network Layer 7-37

BGP attribute: AS-path Prevents looping!

Prefix 138.39.0.0/16, AS1 AS2: AS-path = AS1 AS2 AS3: AS-path = AS2-AS1 AS3 AS1: AS-path = AS3-AS2-AS1 AS1 detects loop, and can reject the route

AS 1

AS 2

AS 3

138.39.0.0/16

(a)

AS 2

AS 3

138.39.0.0/16

(b)AS 1

Partition healing:rare case where AS1 mayaccept “loop” route:

Network Layer 7-38

BGP attribute: Multi-Exit-Discriminator Used when two AS connect to each other in more than

one place Used by AS to advertise degree of preference of each

link to reach a particular prefix Example:

AS1 and AS2 have 2 BGP sessions: one on each link AS2 advertises prefixes of AS3 to AS1 on both links

• MED advertised on link A better than MED advertised on link B

AS 1 AS 2

AS 3

AS 4

Link A

Link B

Network Layer 7-39

MED (contd.) ISP-1 and ISP-2 connect in New York and San Francisco ISP-1 has customer-1 in San Francisco ISP-2 has customer-2 in New York What happens if:

Case A: Both ISPs set and accept MED? Case B: Both ISP-1 and ISP-2 ignore MED? Case C: ISP-1 accepts MED but ISP-2 ignores MED?

ISP 1

ISP 2

Cust 2

Cust 1

Case A:

Network Layer 7-40

BGP attribute: Local-Pref Most commonly used attribute Determines local (i.e. within AS) preference of use of

received route E.g.: say AS3 provides better service than AS2 to AS4

AS4 can configure local-pref of routes from AS3 to be higher (better) than those heard from AS2

AS1 advertises prefix 138.39.0.0.16 to AS2 and AS3 AS4 receives the prefix from both, but chooses the AS3-

AS1 path since it has better local-pref

AS 1

AS 2

AS 3

138.39.0.0/16

AS 4

Network Layer 7-41

BGP policies Can be complex, yet are key to flexibility

and control of inter-AS routing Examples:

Avoid using competitor’s network• avoid routes with AS-n in AS-Path

Avoid transit service, i.e. do not carry any traffic that does not have source or destination within AS

• Do not advertise any non-local routes to peers Let another ISP carry most cross-country load

• Use of MED was shown earlier More examples in subscriber-ISP connection next

Network Layer 7-42

Subscriber connection: singly-homed Easy case! Possible options:

Static configuration: easiest• Customer has default route via R2• ISP configures static route to customer’s prefix

Include customer in ISP’s IGP (too risky!) Run a small IGP (say RIP) on R1-R2 link, leak that into BGP Run a single BGP session

• customer will still likely use a default route or a small set of filtered routes and not absorb the entire Internet routing table

customer ISPR1 R2

AS1

AS2

138.39.2.0/23

BGP session

Network Layer 7-43

Multi-homed subscriber Multiple customer links to one or more ISPs Why?

Reliability (redundancy) Performance (load-sharing)

Challenging Static routing often doesn’t suffice (why?) Want to minimize routing prefixes injected into customer

network BGP configuration requires thought and planning, taking into

account both traffic directions (to and from the customer)

customer

ISP-2ISP-1

Network Layer 7-44

Multi-homing to a single provider

Example 1: same router in ISP, different routers in customer ISP to customer traffic:

customer sets MED Customer to ISP traffic: 2

default routes!

Example 2: different routers in ISP, same router in customer ISP to customer traffic: as

before Customer to ISP traffic:

customer may have to get BGP prefixes from ISP

138.39/16

R1ISP

R3

customerR2

204.70/16

138.39/16

R1ISP

R3

customer

R2

204.70/16

Network Layer 7-45

Multi-homing to multiple providers

Options for customer address space: Exclusively from ISP1 (or from ISP2)

• E.g.: customer uses 138.39.1/24 and advertises this prefix to ISP2• ISP3 gets prefixes 138.39/16 from ISP1 and 138.39.1.24 from ISP2• ISP3 traffic to customer will go via ISP2 (longest prefix match)• Aggregation is pushing traffic away?!

From both ISP1 and ISP2• E.g.: customer uses 138.39.1/24 and 204.70.1/24• Good load-sharing if traffic to these prefixes is about the same

Independently from address registry• Can manipulate load-sharing better, but bad for aggregation!

Bottom line: it all depends on the traffic patterns!

ISP1

customer

ISP3

ISP2138.39/16 204.70/16

Network Layer 7-46

Interaction among routing protocols Every routing protocol is computing its own

routes: how does it all fit? Question: do they interact with each other? Yes! Question: which route is inserted in the forwarding

tables? If conflict, priority mechanism is used

Question: how does IGP fill its routing table? Direct routes: directly-connected interfaces Static routes: user configured

Question: How does BGP fill it routing table? Learns AS local networks from IGP

Network Layer 7-47

E-BGP vs. I-BGP Question: How do BGP routes get propagated within AS?

E.g.: how does B.b learn about routes from AS-A and AS-B? Inject BGP routes into IGP? bad idea – IGPs don’t scale Preferred way of distributing externally learnt prefixes

within an AS:• Internal-BGP (I-BGP): full-mesh within AS

Our earlier discussion on BGP peering between different AS• Technically correct to call it External-BGP (E-BGP)

a

b

b

aaC

A

Bd

A.a

A.c

C.bB.a

cb

c

Network Layer 7-48

Configuring routing In your organization you have to install a new PC in a

server-farm. The PC is multi-homed on two LANs. What static routes do you need to configure on the PC for shortest-path routing to all destinations? Assume: The PC is not routing between LANs The PC is not running any routing protocols Pick any IP addresses for the router interfaces consistent with

the LAN subnets

LAN193.1.1.32/28

LAN202.1.1/24

LAN193.1.1.0/28

LAN193.1.1.16/28

ISPR1 R2

serverfarm

new PC

Network Layer 7-49

Configuring routing (contd.) Now suppose your organization gets a second link to the

ISP via a new router R3. Your PC now has 3 LAN interfaces, and your organization has two links to the Internet. Can you suggest ways of load-balancing traffic to/from your organization?

LAN193.1.1.32/28

LAN202.1.1/24

LAN202.1.2/24

LAN193.1.1.0/28

LAN193.1.1.16/28

ISPR1 R2

R3

serverfarm

new PC

Network Layer 7-50

Summary Hierarchical routing: intra-AS versus inter-AS Policy:

Inter-AS: admin wants control over how its traffic routed, who routes through its net.

Intra-AS: single admin, so no policy decisions needed

Scale: hierarchical routing saves table size, reduced update

traffic

Performance: Intra-AS: can focus on performance Inter-AS: policy dominates over performance

Network Layer 7-51

Summary (contd.) Principles of BGP operation

Path-vector Configuration driven Route attributes (AS-Path, MED, Local-Pref,

…) Policies dictate everything! How does a customer connect to ISP? Examples of single and multi-homing

Interaction between routing protocols How does it all fit?

Design examples Finished with IP routing - whew!