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Chapter 4Internetworking

4.1 Simple Internetworking (IP) 4.2 Routing4.3 Global Internet4.4 Multicast4.5 Multiprotocol Label Switching (MPLS)

2

4.1 Simple Internetworking (IP)

Best Effort Service ModelGlobal Addressing SchemeARP (Address Resolution ProtocolICMP (Internet Message Control Protocol)

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IP Internet Concatenation of Networks

Protocol Stack

R2

R1

H4

H5

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

H6

Network 4(point-to-point)

H7 R3 H8

Network 3 (FDDI)

R1 R2 R3

H1 H8

ETH FDDI

IP

ETH

TCP

FDDI PPP PPP ETH

IP

ETH

TCP

IP IP IP

4

Service ModelConnectionless (datagram-based)Best-effort delivery (unreliable service)

packets are lostpackets are delivered out of orderduplicate copies of a packet are deliveredpackets can be delayed for a long time

Datagram formatVersion HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

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Fragmentation and ReassemblyEach network has some MTUDesign decisions

fragment when necessary (MTU < Datagram)try to avoid fragmentation at source hostre-fragmentation is possible fragments are self-contained datagramsuse CS-PDU (not cells) for ATMdelay reassembly until destination hostdo not recover from lost fragments

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Example

H1 R1 R2 R3 H8

ETH FDDI

PPP IP (376)

PPP IP (512)

PPP IP (512) (512)

ETH IP

ETH IP

(512)ETH IP

(376)

IP (1400) IP (1400)

R1 R2 R3

(a)

Ident = x

Start of header

Rest of header

1400 data bytes

Offset = 00

(b)

Ident = x

Start of header

Rest of header

512 data bytes

Offset = 01

Ident = x

Rest of header

512 data bytes

Offset = 641

Start of header

Ident = x

Start of header

Rest of header

376 data bytes

Offset = 1280

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Global Addresses

Properties globally unique hierarchical: network + host

Dot Notation 10.3.2.4 128.96.33.81 192.12.69.77

Network Host

7 24

0(a)

Network Host

14 16

1 0(b)

Network Host

21 8

1 1 0(c)

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Datagram Forwarding Strategy

every datagram contains destination’s address if connected to destination network, then forward to host if not directly connected, then forward to some router forwarding table maps network number into next hop each host has a default router each router maintains a forwarding table

Example (R2) Network Number Next Hop 1 R3 2 R1 3 interface 1 4 interface 0

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Address Translation Map IP addresses into physical addresses

destination host next hop router

Techniques encode physical address in host part of IP address table-based

ARP table of IP to physical address bindings broadcast request if IP address not in table target machine responds with its physical address table entries are discarded if not refreshed

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ARP Details

Request Format HardwareType: type of physical network (e.g., Ethernet) ProtocolType: type of higher layer protocol (e.g., IP) HLEN & PLEN: length of physical and protocol addresses Operation: request or response Source/Target-Physical/Protocol addresses

Notes table entries timeout in about 10 minutes update table with source when you are the target update table if already have an entry do not refresh table entries upon reference

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ARP Packet Format

TargetHardwareAddr (bytes 2 - 5)

TargetProtocolAddr (bytes 0 3)

SourceProtocolAddr (bytes 2 3)

Hardware type = 1 ProtocolType = 0x0800

SourceHardwareAddr (bytes 4-5)

TargetHardwareAddr (bytes 0 – 1)

SourceProtocolAddr (bytes 0 -1)

HLen = 48 PLen = 32 Operation

SourceHardwareAddr (bytes 0― 3)

0 8 16 31

-

-

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Internet Control Message Protocol (ICMP)Echo (ping)Redirect (from router to source host)Destination unreachable (protocol, port, or host)TTL exceeded (so datagrams don’t cycle forever)Checksum failed Reassembly failedCannot fragment

CS 461

Redirect

Network

G1

H2

Network

G2H1

Network

(1)

(2)

G2 finds that H1 is directly connected and will inform H1 to redirect the IP datagrams to G2.

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4.2 Routing Forwarding vs Routing

forwarding: to select an output port based on destination address and routing table

routing: process by which routing table is built Network as a Graph

Problem: Find lowest cost path between two nodes Factors

static: topology dynamic: load

4

3

6

21

9

1

1D

A

FE

B

C

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Distance VectorEach node maintains a set of triples

(Destination, Cost, NextHop)

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

Each update is a list of pairs: (Destination, Cost)

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

Refresh existing routes; delete if they time out

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Routing Table Example (Node B)

Destination Cost NextHop A 1 A C 1 C D 2 C E 2 A F 2 A G 3 A

D

G

A

F

E

B

C

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Routing LoopsExample 1

F detects that link to G has failed F sets distance to G to infinity and sends update to A A sets distance to G to infinity since it uses F to reach G A receives periodic update from C with 2-hop path to G A sets distance to G to 3 and sends update to F F decides it can reach G in 4 hops via A

D

G

A

F

E

B

C

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Routing LoopsExample 2

link from A to E fails A advertises distance of infinity to E B and C advertise a distance of 2 to E B decides it can reach E in 3 hops; advertises this to A A decides it can read E in 4 hops; advertises this to C C decides that it can reach E in 5 hops…

D

G

A

F

E

B

C

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Distance Vector: link cost changesLink cost changes: node detects local link cost change updates routing info, recalculates

distance vector if DV changes, notify neighbors

“ goodnews travelsfast”

x z14

50

y1

At time t0, y detects the link-cost change, updates its DV, and informs its neighbors.

At time t1, z receives the update from y and updates its table. It computes a new least cost to x and sends its neighbors its DV.

At time t2, y receives z’s update and updates its distance table. y’s least costs do not change and hence y does not send any message to z.

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algorithmterminates

x z14

50

y1

Distance Vector: link cost changes

“ good news Travels fast”

Dy

Dz

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Distance Vector: link cost changesLink cost changes: bad news travels slow - “count to infinity” problem! 44 iterations before algorithm stabilizes z (y) does not know that the least distance from y (z)

to x that y (z) tells z (y) is the distance of the path y-z-y-x (z-y-x)

X Z14

50

Y60

algorithmcontinues

on!

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Distance Vector: poisoned reverseIf Z routes through Y to get to X : Z tells Y its (Z’s) distance to X is infinite (so Y

won’t route to X via Z) will this completely solve count to infinity

problem? Loops involving three or more nodes cannot be

solved using the technique

X Z14

50

Y60

algorithmterminates

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RIP ( Routing Information Protocol)

Distance vector algorithm Included in BSD-UNIX Distribution in 1982Distance metric: # of hops (max = 15 hops)

DC

BA

u v

w

x

yz

destination hops u 1 v 2 w 2 x 3 y 3 z 2

Source node: A

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RIP advertisements

Distance vectors: exchanged among neighbors every 30 sec via Response Message (also called advertisement)

Each advertisement: a list of up to 25 destination subnets within AS

Address of net 2

Distance to net 2

Command Must be zero

Family of net 2 Address of net 2

Family of net 1 Address of net 1

Address of net 1

Distance to net 1

Version

0 8 16 31

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RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B 7

x -- 1…. …. ....

w x y

z

A

C

D B

Routing table in D

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RIP: Example

Destination Network Next Router Num. of hops to dest. w A 2

y B 2 z B A 7 5

x -- 1…. …. ....Routing table in D

w x y

z

A

C

D B

Dest Next hops w - - x - - z C 4 …. … ...

Advertisementfrom A to D

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RIP: Link Failure and Recovery

If no advertisement heard after 180 sec --> neighbor or link declared deadroutes via neighbor invalidatednew advertisements sent to neighborsneighbors in turn send out new advertisements (if

tables changed) link failure info quickly propagates to entire netpoison reverse used to prevent ping-pong loops

(infinite distance = 16 hops)

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RIP Table processing

RIP routing tables managed by application-level process called route-d (daemon)

advertisements sent in UDP packets, periodically repeated

physical

link

network forwarding (IP) table

Transprt (UDP)

routed

physical

link

network (IP)

Transprt (UDP)

routed

forwardingtable

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Link State

Strategysend to all nodes (not just neighbors) information

about directly connected links (not entire routing table)

Link State Packet (LSP) id of the node that created the LSPcost of link to each directly connected neighborsequence number (SEQNO) time-to-live (TTL) for this packet

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Link State (cont)

Reliable floodingstore most recent LSP from each nodeforward LSP to all nodes but one that sent itgenerate new LSP periodically

increment SEQNOstart SEQNO at 0 when rebootdecrement TTL of each stored LSP

discard when TTL=0

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Reliable Flooding

(a)

X A

C B D

(b)

X A

C B D

(c)

X A

C B D

(d)

X A

C B D

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Route Calculation Dijkstra’s shortest path algorithm Let

N denotes set of nodes in the graph l (i, j) denotes non-negative cost (weight) for edge (i, j) s denotes this node M denotes the set of nodes incorporated so far C(n) denotes cost of the path from s to node n

M = {s}for each n in N - {s}

C(n) = l(s, n)while (N != M)

M = M union {w} such that C(w) is the minimum for all w in (N - M)

for each n in (N - M)C(n) = MIN(C(n), C (w) + l(w, n ))

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A Link-State Routing Algorithm

Dijkstra’s algorithm net topology, link costs known to

all nodes accomplished via “link state

broadcast” all nodes have same info

computes least cost paths from one node (‘source”) to all other nodes gives forwarding table for that

node iterative: after k iterations, know

least cost path to k destinations

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 destination v

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

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

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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'

u: source node

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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

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Dijkstra’s algorithm: example

u

yx

wv

z2

2

13

1

1

2

53

5

u

yx

wv

z2

2

13

1

1

2

53

5

u

yx

wv

z2

2

13

1

1

2

53

5

u

yx

wv

z2

2

13

1

1

2

53

5

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Dijkstra’s algorithm: example

u

yx

wv

z2

2

13

1

1

2

53

5

u

yx

wv

z2

2

13

1

1

2

53

5

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Dijkstra’s algorithm, discussion

Algorithm 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)

Oscillations possible: e.g., link cost = amount of carried traffic

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

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OSPF (Open Shortest Path First)

“open”: publicly available – defined in RFC 2328Uses Link State algorithm

Link-State packet dissemination Topology map at each node Route computation using Dijkstra’s algorithm

OSPF advertisement carries one entry per neighbor router

Advertisements disseminated to entire AS (via flooding) Carried in OSPF messages directly over IP (rather than TCP

or UDP)

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OSPF “advanced” features (not in RIP)

Security: all OSPF messages authenticated (to prevent malicious intrusion)

Load Balancing: Multiple same-cost paths allowed (only one path in RIP)

For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort; high for real time)

Integrated uni- and multicast support: Multicast OSPF (MOSPF) uses same topology data

base as OSPF Hierarchical OSPF in large domains.

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Hierarchical OSPF

An OSPF autonomous system (AS) can be configured into areas

Exactly one OSPF area in the AS is configured to be the backbone area

Each area runs its own OSPF link-state routing algorithm 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.

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Hierarchical OSPF

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Hierarchical OSPF

Four types of routersInternal routers: perform only intra AS routingArea border routers: belong to both an area

and the backboneBackbone routers: run OSPF routing limited to

backbone.Boundary routers: connect to other AS’s.

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OSPF Advertisement Format

Authentication

Version Type Message length

Checksum Authentication type

SourceAddr

AreaId

0 8 16 31LS Age Options Type=1

0 Flags 0 Number of links

Link type Num_TOS Metric

Link-state ID

Advertising router

LS sequence number

Link ID

Link data

Optional TOS information

More links

LS checksum Length

Header FormatLink-State Advertisement

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Comparison of LS and DV algorithms

Message complexity LS: with n nodes, E links, O(nE)

messages sent DV: exchange between

neighbors only convergence time varies

Speed of Convergence LS: O(n2) algorithm requires

O(nE) messages may have oscillations

DV: convergence time varies may be routing loops count-to-infinity problem

Robustness: what happens if router malfunctions?

LS: node can advertise incorrect

link cost each node computes only its

own table

DV: DV node can advertise

incorrect path cost each node’s table used by

others error propagate thru

network

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Metrics Original ARPANET metric

measures number of packets queued on each link took neither latency or bandwidth into consideration

New ARPANET metricstamp each incoming packet with its arrival time (AT)record departure time (DT)when link-level ACK arrives, compute

Delay = (DT - AT) + Transmit + Latency if timeout, reset DT to departure time for retransmission link cost = average delay over some time period

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Metrics Still has problems

Under light load, it works well since the two static factors of delay dominated the cost.

Under heavy load, a congested link would start to advertise a very high cost. This caused all the traffic to move off that link, leaving it idle, so then it advertise a low cost,…

The range of link values was much too large.Fine Tuning

compressed dynamic rangereplaced Delay with link utilization

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Revised ARPANET routing metric versus link utilization

225

140

907560

30

25% 50% 75% 100%

Utilization

9.6-Kbps satellite link

9.6-Kbps terrestrial link

56-Kbps satellite link

56-Kbps terrestrial link

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Revised ARPANET routing metric versus link utilizationA highly loaded link never shows a cost of

more than three times its cost when idleThe most expensive link is only seven times

the cost of least expensiveA high-speed satellite link is more attractive

than a low-speed terrestrial linkCost is a function of link utilization only at

moderate to high loads.

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Tree Structure of the Internet in 1990

NSFNET backboneStanford

BARRNETregional

BerkeleyPARC

NCAR

UA

UNM

Westnetregional

UNL KU

ISU

MidNetregional■ ■ ■

4.3 Global Internet Structure

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Global Internet One of the salient features of this topology is that it consists of

“end user” sites (e.g, Stanford university) that connect to “service provider” networks (e.g, BARRNET)

Each provider and end user is likely to be an administratively independent entity – Autonomous System (AS).

Scalability problems Scalability of routing Address utilization

Subnetting – deals with address space utilization Classless routing or supernetting – tackles both address

utilization and routing scalability

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Subnetting Inefficient use of Hierarchical Address Space

class C with 2 hosts (2/255 = 0.78% efficient) class B with 256 hosts (256/65535 = 0.39% efficient)

Still Too Many Networks routing tables do not scale route propagation protocols do not scale

Subnetting provides an elegantly simple way to reduce the total number of networks that are assigned

The idea is to take a single IP network number and allocate the IP addresses with that network number to several physical networks – subnets.

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SubnettingAdd another level to address/routing hierarchy: subnetSubnet masks define variable partition of host partA single network number can be shared among multiple

networks involves configuring all the nodes on each subnet with a subnet mask.

Subnets visible only within siteNetwork number Host number

Class B address

Subnet mask (255.255.255.0)

Subnetted address

11111111111111111111111100000000

Network number Host IDSubnet ID

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Subnet Example

Forwarding table at router R1Subnet Number Subnet Mask Next Hop

128.96.34.0 255.255.255.128 interface 0

128.96.34.128 255.255.255.128 interface 1

128.96.33.0 255.255.255.0 R2

Subnet mask: 255.255.255.128Subnet number: 128.96.34.0

128.96.34.15128.96.34.1

H1 R1

128.96.34.130 Subnet mask: 255.255.255.128Subnet number: 128.96.34.128

128.96.34.129128.96.34.139

R2H2

128.96.33.1128.96.33.14

Subnet mask: 255.255.255.0Subnet number: 128.96.33.0

H3

H1 H2255.255.255.128128.96.34.139128.96.34.128

R1 255.255.255.128128.96.34.139128.96.34.128

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Forwarding AlgorithmD = destination IP addressfor each entry (SubnetNum, SubnetMask, NextHop) D1 = SubnetMask & D if D1 = SubnetNum if NextHop is an interface deliver datagram directly to D else deliver datagram to NextHop

Use a default router if nothing matches Not necessary for all 1s in subnet mask to be contiguous Can put multiple subnets on one physical network Subnets not visible from the rest of the Internet

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Classless Routing (CIDR) Supernetting CIDR: Classless Inter-Domain Routing A technique that addresses two scaling concerns:

the growth of backbone routing tables, and the potential for the 32-bit IP address space to be exhausted

well before the 4 billionth host is attached to the Internet.Even though subnetting can help to assign addresses

carefully, it does not get around the fact that any AS with more than 255 hosts wants a class B address – exhaustion of IP address space.

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Classless Routing (CIDR) Supernetting CIDR tries to balance the desire to minimize the

number of routes that a router needs to know against the need to hand out addresses efficiently

Assign block of contiguous network numbers to nearby networks

Represent blocks with a single pair (first_network_address, count)

Restrict block sizes to powers of 2Use a bit mask (CIDR mask) to identify block sizeAll routers must understand CIDR addressing

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Route aggregation with CIDR

AdvertiseISP

128.112.128/24

128.112.135/24

128.112.128/21

Customers

...

Since all of the customers are reachable through the same Provider network, it can advertise a single route to all of Them by just advertising the common 21-bit prefix they share

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IP Forwarding Revisited Find the network number in a packet and then lookup that number in a

forwarding table. Reexamine this assumption with CIDR Prefixes length 2-32 bits Prefixes may “overlap” Some addresses may match more than one prefix. Longest Prefix Matching (LPM) For example

171.69 (16-bit prefix) 171.69.10 (24-bit prefix) 171.69.10.5 matches both 171.69.20.5 only matches 171.69

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Interdomain Routing (BGP)

R1

Autonomous system 1R2

R3

Autonomous system 2R4

R5 R6

• AS = routing domain• Routing Policies• Two major Interdomain routing protocols -- Exterior gateway Protocol (EGP) -- Border gateway Protocol (BGP-4)

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BGP-4: Border Gateway ProtocolAS Types

stub AS: has a single connection to one other AScarries local traffic only

multihomed AS: has connections to more than one ASrefuses to carry transit traffic

transit AS: has connections to more than one AScarries both transit and local traffic

Each AS has:one or more border routersone BGP speaker that advertises:

local networksother reachable networks (transit AS only)gives path information

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Backbone service provider

Peeringpoint

Peeringpoint

Large corporation

Large corporation

Smallcorporation

“ Consumer” ISP

“ Consumer” ISP

“ Consumer” ISP

Today’s multibackbone Internet

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BGP Example Speaker for AS2 advertises reachability to P and Q

network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS2

Speaker for backbone advertises networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached

along the path (AS1, AS2). Speaker can cancel previously advertised paths

Regional provider A(AS 2)

Regional provider B(AS 3)

Customer P(AS 4)

Customer Q(AS 5)

Customer R(AS 6)

Customer S(AS 7)

128.96192.4.153

192.4.32192.4.3

192.12.69

192.4.54192.4.23

Backbone network(AS 1)

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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”

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

semi-permanent TCP connections: 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

External BGP (eBGP) session

Internal BGP (iBGP) session

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Aggregation of prefixes

138.16.64/24

138.16.65/24

138.16.66/24 ＝＞ 138.16.64/22

138.16.67/24

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Distributing reachability info With eBGP session between 3a and 1c, AS3 sends prefix reachability

information to AS1. 1c can then use iBGP to distribute this new prefix reachability

information to all routers in AS1 1b can then re-advertise the new reachability information to AS2 over

the 1b-to-2a eBGP session When router learns about a new prefix, it creates an entry for the

prefix in its forwarding table.

3b

1d

3a

1c2aAS3

AS1

AS21a

2c

2b

1b

3c

eBGP session

iBGP session

68

Path attributes & BGP routes When advertising a prefix, advertisement includes BGP

attributes. prefix + attributes = “route”

Two important attributes: AS-PATH: contains the ASs through which the advertisement for

the prefix passed: AS 67 AS 17 used to detect and prevent looping advertisement also use in choosing among multiple path to the same prefix

NEXT-HOP: Indicates the specific internal-AS router to next-hop AS. (There may be multiple links from current AS to next-hop-AS.)

When gateway router receives route advertisement, uses import policy to accept/decline.

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BGP route selection

Router may learn about more than 1 route to any one prefix. Router must select route.

Elimination rules invoked sequentially until one route remains:

1. Local preference value attribute: policy decision – AS’s network administrator

2. Shortest AS-PATH

3. Closest NEXT-HOP router: hot potato routing

4. Additional criteria

70

BGP messages

BGP messages exchanged using TCP. BGP 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 message; also

used to close connection

71

BGP routing policy

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

A,B,C are provider networks X,W,Y are customer (of provider networks) X is dual-homed: attached to two networks

X does not want to route from B via X to C .. so X will not advertise to B a route to C

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BGP routing policy (2)

Figure 4.5-BGPnew: a simple BGP scenario

A

B

C

W X

Y

legend:

customer network:

provider network

A advertises to B the path AW B advertises to X the path BAW Should B advertise to C the path BAW?

No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers

B wants to force C to route to w via A B wants to route only to/from its customers!

73

Why different Intra- and Inter-AS routing ?

Policy: Inter-AS: administrator 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 may dominate over performance

74

IP Version 6 Features

128-bit addresses (classless) multicast real-time service authentication and security autoconfiguration end-to-end fragmentation protocol extensions

Header 40-byte “base” header extension headers (fixed order, mostly fixed length)

fragmentationsource routingauthentication and securityother options

75

4.4 Broadcast/Multicast routing

Broadcast routing –- deliver a packet from a source node to all other nodes

Multicast routing – deliver a packet from a source node to a subset of other nodes

76

(a) source duplication, (b) in-network duplication

R1

R2

R3 R4

(a)

R1

R2

R3 R4

(b)

duplicatecreation/transmissionduplicate

duplicate

Source-duplication versus in-network duplication

77

Broadcast routing algorithms

Uncontrolled floodingControlled floodingSpanning-tree broadcast

78

Uncontrolled floodingThe source node sends a copy of the packet to all of its

neighborsWhen a node receives a broadcast packet, it duplicates

the packet and forwards it to all of its neighbors (except the neighbor from which it receives the packet)

Problems:If the graph has cycles, then one or more copies of each

broadcast packet will cycle indefinitelyBroadcast storm

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Controlled floodingSequence-number-controlled flooding

Source node puts its address and a broadcast sequence number into a broadcast packet

Each node maintains a list of the source address and sequence number of each packet it has received

When a node receives a broadcast packetIf the packet is in the list, the packet is droppedOtherwise, the packet is duplicated and forwarded

80

A

B

G

DE

c

F

Controlled flooding

Packet will be forwarded

Packet not forwarded beyond receiving router

Reverse path forwardingWhen a router receives a broadcast packet, it duplicates and

forwards the packet only if the packet arrives on the link that is on its own shortest unicast path back to the source

81

Controlled flooding

DrawbackSome of the nodes receive redundant packets

A

B

G

DE

c

F

Redundant packets

Ideally, every node should receive only one copy of the broadcast packet.

82

A

B

G

DE

c

F

A

B

G

DE

c

F

(a) Broadcast initiated at A (b) Broadcast initiated at D

Spanning-tree broadcastSpanning tree – a tree that contains all nodes in a graph

Minimum spanning tree – a spanning tree whose cost is the minimum among all the spanning trees of a graph

Broadcast along a spanning tree

83

(a) Stepwise construction of spanning tree

A

B

G

DE

c

F

(b) Constructed spanning tree

Construction of Spanning-treeMany algorithms have been developedCenter-based approach

Select a center node (rendezvous or core) Each node unicasts tree-join message to the center node

A

B

G

DE

c

F1

2

3

4

5Center node

84

Multicast Routing: Problem StatementGoal: find a tree (or trees) connecting routers

having local multicast group members tree: not all paths between routers used source-based: different tree from each sender to receivers shared-tree: same tree used by all group members

Shared treeSource-based trees

85

Approaches for building multicast trees

source-based tree: one tree per sourceshortest path treesreverse path forwarding

group-shared tree: group uses one treeminimal spanning (Steiner) center-based trees

…we first look at basic approaches, then specific protocols adopting these approaches

86

Shortest Path Treemulticast forwarding tree: tree of shortest path

routes from source to all receiversDijkstra’s algorithm

R1

R2

R3

R4

R5

R6 R7

21

6

3 4

5

i

router with attachedgroup member

router with no attachedgroup member

link used for forwarding,i indicates order linkadded by algorithm

LEGENDS: source

87

Reverse Path Forwarding

if (multicast datagram received on incoming link on shortest path back to sender)

then flood datagram onto all outgoing links

else ignore datagram

rely on router’s knowledge of unicast shortest path from it to sender

each router has simple forwarding behavior:

88

Reverse Path Forwarding: example

• result is a source-specific reverse SPT– may be a bad choice with asymmetric links

R1

R2

R3

R4

R5

R6 R7

router with attachedgroup member

router with no attachedgroup member

datagram will be forwarded

LEGENDS: source

datagram will not be forwarded

89

Reverse Path Forwarding: pruning forwarding tree contains subtrees with no multicast

group membersno need to forward datagrams down subtree“prune” messages sent upstream by router with

no downstream group members

R1

R2

R3

R4

R5

R6 R7

router with attachedgroup member

router with no attachedgroup member

prune message

LEGENDS: source

links with multicastforwarding

P

P

P

90

Shared-Tree: Steiner TreeSteiner Tree: minimum cost tree connecting all

routers with attached group membersproblem is NP-completeexcellent heuristics existsnot used in practice:

computational complexity information about entire network neededmonolithic: rerun whenever a router needs to join/leave

91

Center-based trees single delivery tree shared by allone router identified as “center” of tree to join:

edge router sends unicast join-message addressed to center router

join-message “processed” by intermediate routers and forwarded towards center

join-message either hits existing tree branch for this center, or arrives at center

path taken by join-message becomes new branch of tree for this router

92

Center-based trees: an example

Suppose R6 chosen as center:

R1

R2

R3

R4

R5

R6 R7

router with attachedgroup member

router with no attachedgroup member

path order in which join messages generated

LEGEND

21

3

1

93

Internet Multicasting Routing: DVMRP

DVMRP: distance vector multicast routing protocol, RFC1075

flood and prune: source-based tree, reverse path forwarding, RPF tree based on DVMRP’s own routing tables

constructed by communicating DVMRP routers no assumptions about underlying unicast initial datagram to multicast group flooded everywhere via

RPF routers not wanting group: send upstream prune messages

94

DVMRP: continued…

soft state: DVMRP router periodically (1 min.) “forgets” branches are pruned: multicast data again flows down unpruned branchdownstream router: reprune or else continue to receive

data

routers can quickly regraft to tree following IGMP join at leaf

odds and endscommonly implemented in commercial routersMbone routing done using DVMRP

95

Tunneling

Q: How to connect “islands” of multicast routers in a “sea” of unicast routers?

multicast datagram encapsulated inside “normal” (non-multicast-addressed) datagram

normal IP datagram sent thru “tunnel” via regular IP unicast to receiving multicast router

receiving multicast router decapsulates to get multicast datagram

physical topology logical topology

96

PIM: Protocol Independent MulticastNot dependent on any specific underlying unicast

routing algorithm (like RIP, OSPF, works with all)

Two different multicast distribution scenarios :

Dense: group members

densely packed, in “close” proximity.

Sparse: # of routers with group

members is small wrt total # of routers

group members “widely dispersed”

97

Consequences of Sparse-Dense Dichotomy: Dense group membership by

routers assumed until routers explicitly prune

data-driven construction of multicast tree (e.g., RPF)

bandwidth and non-group-router processing profligate

Sparse:no membership until

routers explicitly join receiver-driven

construction of multicast tree (e.g., center-based)

bandwidth and non-group-router processing conservative

98

PIM- Dense Mode

Flood-and-prune RPF, similar to DVMRP butunderlying unicast protocol provides RPF

information for incoming datagramless complicated (less efficient) downstream

flood than DVMRP reduces reliance on underlying routing

algorithmhas protocol mechanism for router to detect if

it is a leaf-node router

99

PIM - Sparse Mode

Center-based approachrouter sends join message

to rendezvous point (RP) intermediate routers update

state and forward join

after joining via RP, router can switch to source-specific tree

R1

R2

R3

R4

R5

R6R7

join

join

join

all data multicastfrom rendezvouspoint

rendezvouspoint

100

PIM - Sparse Mode

Sender(s):unicast data to RP, which

distributes down RP-rooted tree

RP can extend multicast tree upstream to source

RP can send stop message to the source if no attached receivers “no one is listening!”

R1

R2

R3

R4

R5

R6R7

join

join

join

all data multicastfrom rendezvouspoint

rendezvouspoint

CS 461

4.5 MultiProtocol Label Switching (MPLS)

Prior WorkMPLS OverviewMPLS Architecture

102

Prior Work

Tag Switching (Cisco)Aggregate Route-Based IP Switching (ARIS,

IBM)IP NavigatorIFMP-IP Switching (Ipsilon)Cell Switching Router (CSR, Toshiba)

103

Prior WorkTag switching is based on the control-driven approach. The set

up of LSPs (Label Switched Paths) closely follows control messages such as routing updates and RSVP messages.

Aggregate route-based IP switching (ARIS) is based on the control-driven approach. Very similar to tag switching. ARIS introduces the concept of an “egress identifier” (FECs) to express the granularity of LSPs.

IP Navigator is again a control-driven protocol. Use OSPF as the internal routing protocol used within a routing domain. Explicit routing is used to setting up the VCs.

104

Prior Work

Ipsilon Flow Management Protocol (IFMP) is a traffic driven protocol. When the number of packets from a flow exceeds a predetermined threshold, the controller uses IFMP to set up an LSP for the particular flow.

Cell switch router (CSR) proposal is similar to IP switching. CSR is primarily designed as a device for interconnecting ATM clouds. Within an LIS (logical IP subnet), ATM forum standards are used to connection hosts and switched together.

Multiple LISs are then interconnected with CSRs that are capable of running both IP forwarding and cell forwarding. The setup of LSPs is data-driven for best effort traffic and RSVP-driven for flows that require resource reservation.

CS 461

MPLS Overview

RFC 3812 The IETF MPLS working group is to standardize a

base technology that integrates the label swapping forwarding paradigm with network layer routing.

Cisco is the major contributor to the MPLS working group.substitute “Label” for “Tag” in Tag Switching MPLS

CS 461

Core mechanisms of MPLS

Semantics assigned to a stream labelLabels are associated with specific streams of data.

Forwarding MethodsForwarding is simplified by the use of the short

fixed length labels to identify streams.Forwarding may require simple functions such as

looking up a label in a table, swapping labels, and possibly decrementing and checking a TTL.

Label Distribution MethodsAllow nodes to determine which labels to use for

specific streams.

107

Native IP ForwardingIP routing: both the packet forwarding and

route determination process in an IP network.Native IP forwarding (NIF): hop-by-hop,

destination-based packet forwarding.Each packet’s next hop and output port are

determined by a longest-prefix-match forwarding table lookup.

Additional packet classification may also be performed to derive output port queuing and scheduling rules.

108

A Simplified NIF forwarding engine

IP Header IP payload

ForwardingTable

ForwardingTable

PacketClassification

PacketClassification

InputPorts

OutputPorts

Next hop + port

Queuing andScheduling rules

Longest Prefix Match lookup

Packet Classification keys: IP source and destination addresses, IP protocol type, DiffServ (DS) or TOS byte, and TCP/UDP port numbers.

109

Per-Hop classification, queuing, and scheduling

Queue

S

ClassifyPort 1

Port N

Port M

110

A Simplified LSR forwarding engine

MPLS label MPLS payload

SwitchingTable

SwitchingTable

InputPorts

OutputPorts

Next hop + portQueuing and

Scheduling rules

111

Traffic Engineering

Conventional IP routing attempts to find the shortest path between a packet’s current location and its intended destination.

“Hot spots” and packet loss rates, latency, and jitter increase as the average load on a router rises.

Solutions: (1) Faster routers, (2) Alternate routes.Routing policy may also require traffic engineering. For

example, the external link between R6 and A3 may have been funded solely by A2 and A3. Therefore, A1’s traffic must not be allowed to traverse it.

112

Traffic Engineering

R1

Access 3

Access 1

Access 2

IP Backbone

R2 R3

R4

R5

R6

Route from A2 to D

Desired route from A1 to D

Actual route from A1 to D

Destination D

-- Override the shortest path route

113

Signaling and ProvisioningSignaling: when network (re)configuration can be requested by

users at any time and achieved within milliseconds or seconds.Provisioning: When the reaction time for (re)configuration

becomes measured in minutes or hours.In either case, the (re)configuring action involves establishing

(or modifying) information used by routers or switches to control their forwarding actions, including forwarding (routing) information, classification rules, and/or queuing and scheduling parameters.

CS 461

Core MPLS ComponentsThe basic routing approach

Routing is accomplished through the use of standard L3 routing protocols (e.g. OSPF and BGP).

The information maintained by the L3 routing protocols is then used to distribute labels to neighboring nodes that are used in the forwarding of packets.

LabelsLabel semantics, Label granularity, Label assignment,

Label stack and forwarding operations.

CS 461

Label Semantics The label is nothing more than a shorthand for an

aggregate stream of user data. The meaning of the label is a strictly local issue between

two neighboring nodes. MPLS could be employed between any two neighboring

nodes, even if no other nodes in the network participate in MPLS.

When MPLS is used between more than two nodes, then the operation between any two neighboring nodes could be interpreted as independent of the operation between any other pair of nodes.

CS 461

Label Granularity The device uses the label to forward packets will

forward all packets with the same label in the same way.

A Forwarding Equivalence Class (FEC) is a set of L3 packets which are all forwarded in the same manner by a particular Label Switching Router (LSR).

For unicast IP traffic, the granularity of a label allows various levels of aggregation in a Label Information Base (LIB).

For IP multicast, the natural binding of a label would be to a multicast tree.

117

Label assignment

Label assignment involves allocating a label, and then binding a label to a route.

Label assignment can be driven by control traffic or data traffic. (discussed later.)

Label withdrawal is primarily a matter of garbage collection, that is collecting up unused labels so that they may be reassigned.

118

Routing Aggregation

R1

Access 1

Access 2 R2 R3

R4

R5

R6

Destination D

Access 3

1

2

35

4

CS 461

Forwarding ComponentLabel Stack and Forwarding Operations

label swap : looking up the incoming label to determine the outgoing label, encapsulation, port, and any additional information which may pertain to the stream such as a particular queue or other QoS related treatment.

label push : When a packet first enters an MPLS domain, the packet is associated with a label.

label pop : When a packet leaves an MPLS domain, the label is removed.

The label stack is useful within hierarchical routing domain.

CS 461

EncapsulationLabel-based forwarding makes use of various pieces of

information, including a label or stack of labels, and possibly additional information such as a TTL field.

“MPLS encapsulation” : encapsulate the label information and information used for label based forwarding.

An encapsulation scheme may make use of the following fields:label, TTL, class of service, stack indicator, next

header type indicator, and checksum

121

MPLS label stack encoding

Label(20 bits)

Exp(3 bits)

S(1 bit)

TTL(8 bits)

Label(20 bits)

Exp(3 bits)

S(1 bit)

TTL(8 bits)

Label(20 bits)

Exp(3 bits)

S(1 bit)

TTL(8 bits)

Original Packet

...

Stack top Stack bottom

MPLS frame delivered to link layer

COS

122

Label AssignmentTopology driven (Tag)

In response to normal processing of routing protocol control traffic Labels are pre-assigned; no label setup latency at forwarding time.

Request driven (RSVP)In response to normal processing of request based control trafficMay require a large number of labels to be assigned.

Traffic driven (Ipsilon)The arrival of data at an LSR triggers label assignment and

distribution.Label setup latency; potential for packet reordering.

123

Label Distribution

Explicit Label DistributionDownstream label allocation

label allocation is done by the downstream LSRmost natural mechanism for unicast traffic

Upstream label allocationlabel allocation is done by the upstream LSRmay be used for optimality for some multicast traffic

A unique label for an egress LSR within the MPLS domainAny stream to a particular MPLS egress node could use the label of

that node.

124

Label DistributionExplicit Label Distribution Protocol (LDP)

Reliability : by transport protocol or as part of LDP.Separate routing computation and label distribution.

Piggybacking on Other Control MessagesUse existing routing/control protocol for distributing routing/control and

label information.OSPF, BGP, RSVP, PIM

Combine routing and label distribution.Label purge mechanisms

By time outExchange of MPLS control packets

125

Label Distribution ProtocolLDP Peer:

Two LSRs that exchange label/stream mapping information via LDP

LDP messages Discovery messages (via UDP)

announce and maintain the presence of LSR Session messages

maintain session between LDP peers Advertisement message

label operation (Label distribution) Notification message

advisory information and signal error information Error notification: signal fatal errors Advisory notification: status of the LDP session or some previous message received from the peer.

126

Label Swapping

Labeled Packet Map the incoming label to a next

hop label, determines where to forward the packet.

Encodes the new label stack into the packet, and then forwards it.

Unlabeled Packet LSR analyzes the L3 header, to

determine the packet’s stream. Map the stream to a next hop,

determines where to forward the packet.

Encodes the new label stack into the packet, and then forwards it.

L a b e l S w itc h in g R o u te r(L S R )

IP R o u te rM o d u le

1 2

L a b e l

L 3 H e a d e r

P o rt1

L a b e l4

P o rt2

L a b e l6

O u tp u tIn p u t

In c o m in g L a b e l M a p (IL M )

E xam p le : Fo rw ard in g a Lab eled P ack et

D at H 3 6 H 2D at H 3 4 H 2

L 2 H e a d e r

127

Use of MPLS in a Hierarchy

R 3R 2

R 6R 5R 4

R 1

D om ain 1 D om ain 2

O S P F

B G PL 1

P u shL 2

L 1

L 1S w a p

L 4

L 1P o p

L 3

L 1

S w a pL 3

L 1

L 2

L 1

O U T

L 2

IN

L 3

O U T

L 1

IN

L 1

O U T

L 4

IN

L 2

O U T

L 3

128

ConclusionMPLS improves the scalability of hop-by-hop

routing and forwarding, and provides traffic engineering capabilities for better network provisioning.

It decouples forwarding from routing and allows multi-protocol support without requiring changes to the basic forwarding paradigm.

Generalized MPLS (GMPLS)λMPLS (Optical wavelength-based)