csc358 week 8ylzhang/csc358/files/lec07.pdfcsc358 week 8 adapted from slides by j.f. kurose and k....

CSC358 Week 8

Adapted from slides by J.F. Kurose and K. W. Ross.All material copyright 1996-2016 J.F Kurose and K.W. Ross, All Rights Reserved

4.1 Overview of Network layer• data plane• control plane

4.2 What�s inside a router4.3 IP: Internet Protocol• datagram format• fragmentation• IPv4 addressing• network address

translation• IPv6

4.4 Generalized Forward and SDN• match• action• OpenFlow examples

of match-plus-action in action

Outline

4-5Network Layer: Data Plane

IPv6: motivation§ initial motivation: 32-bit address space soon to be

completely allocated. § additional motivation:• header format helps speed processing/forwarding• header changes to facilitate QoS

IPv6 datagram format: • fixed-length 40 byte header• no fragmentation allowed


IPv6 datagram format

priority: identify priority among datagrams in flowflow Label: identify datagrams in same �flow.�

(concept of�flow� not well defined).next header: identify upper layer protocol for data

data

destination address(128 bits)

source address(128 bits)

payload len next hdr hop limitflow labelpriver

32 bits4-7Network Layer: Data Plane

How many IP addresses?

§ 2^128 is roughly 3 x 10^38

§ Assume 10 billion people on Earth

§ Each person gets 3 x 10^28

Network Layer 4-8

Other changes from IPv4

§ checksum: removed entirely to reduce processing time at each hop

§ options: allowed, but outside of header, indicated by �Next Header� field

§ ICMPv6: new version of ICMP• additional message types, e.g. �Packet Too Big�• No fragmentation in the intermediate routers. Sender

receives “Packet Too Big ” and resend smaller datagrams.


Transition from IPv4 to IPv6§ not all routers can be upgraded simultaneously

• no �flag days�• how will network operate with mixed IPv4 and

IPv6 routers? § tunneling: IPv6 datagram carried as payload in IPv4

datagram among IPv4 routers

IPv4 source, dest addr IPv4 header fields

IPv4 datagramIPv6 datagram

IPv4 payload

UDP/TCP payloadIPv6 source dest addr

IPv6 header fields


Tunneling

physical view:IPv4 IPv4

A B

IPv6 IPv6

E

IPv6 IPv6

FC D

logical view:

IPv4 tunnel connecting IPv6 routers E

IPv6 IPv6

FA B

IPv6 IPv6


flow: Xsrc: Adest: F

data

A-to-B:IPv6

Flow: XSrc: ADest: F

data

src:Bdest: E

B-to-C:IPv6 inside

IPv4

E-to-F:IPv6

flow: Xsrc: Adest: F

data

B-to-C:IPv6 inside

IPv4

Flow: XSrc: ADest: F

data

src:Bdest: E

physical view:A B

IPv6 IPv6

E

IPv6 IPv6

FC D

logical view:

IPv4 tunnel connecting IPv6 routers E

IPv6 IPv6

FA B

IPv6 IPv6

Tunneling

IPv4 IPv4


IPv6: adoption§ Google: 8% of clients access services via IPv6§ NIST: 1/3 of all US government domains are IPv6

capable

§ Long (long!) time for deployment, use•20 years and counting!•think of application-level changes in last 20 years: WWW, Facebook, streaming media, Skype, …


4.1 Overview of Network layer• data plane• control plane

4.2 What�s inside a router4.3 IP: Internet Protocol• datagram format• fragmentation• IPv4 addressing• network address

translation• IPv6

4.4 Generalized Forward and SDN• match• action• OpenFlow

examples of match-plus-action in action

Outline


Generalized Forwarding and SDN

230100 1101

values in arrivingpacket�s header

logically-centralized routing controller

1

control plane

data plane

Each router contains a flow table that is computed and distributed by a logically centralized routing controller

local flow tableheaders counters actions


OpenFlow data plane abstraction§ flow: defined by header fields§ generalized forwarding: simple packet-handling rules• Pattern: match values in packet header fields• Actions: for matched packet: drop, forward, modify,

matched packet or send matched packet to controller • Priority: disambiguate overlapping patterns• Counters: #bytes and #packets

Flow table in a router (computed and distributed by controller) define router’s match+action rules


OpenFlow data plane abstraction§ flow: defined by header fields§ generalized forwarding: simple packet-handling rules

• Pattern: match values in packet header fields• Actions: for matched packet: drop, forward, modify,

matched packet or send matched packet to controller • Priority: disambiguate overlapping patterns• Counters: #bytes and #packets

1. src=1.2.*.*, dest=3.4.5.* à drop 2. src = *.*.*.*, dest=3.4.*.* à forward(2)3. src=10.1.2.3, dest=*.*.*.* à send to controller

* : wildcard

OpenFlow: Flow Table Entries

SwitchPort

MACsrc

MACdst

Ethtype

VLANID

IPSrc

IPDst

IPProt

TCPsport

TCPdport

Rule Action Stats

1. Forward packet to port(s)2. Encapsulate and forward to controller3. Drop packet4. Send to normal processing pipeline5. Modify Fields

Packet + byte counters

Link layer Network layer Transport layer

OpenFlow abstraction

§ Router• match: longest

destination IP prefix• action: forward out

a link§ Switch• match: destination

MAC address• action: forward or

flood

§ Firewall• match: IP addresses

and TCP/UDP port numbers• action: permit or

deny § NAT• match: IP address

and port• action: rewrite

address and port

§ match+action: unifies different kinds of devices


Summary: Network Layer Data Plane

Question: how do forwarding tables (destination-based forwarding) or flow tables (generalized forwarding) computed?Answer: by the control plane (next chapter)

4.1 Overview of Network layer: data plane and control plane

4.2 What�s inside a router4.3 IP: Internet Protocol• datagram format• fragmentation• IPv4 addressing• NAT• IPv6

4.4 Generalized Forward and SDN• match plus action• OpenFlow example


Network layer control plane

goals: understand principles behind network control plane

§ traditional routing algorithms§ SDN controllers§ Internet Control Message Protocol§ network management

and their instantiation, implementation in the Internet:§ OSPF, BGP, OpenFlow, ICMP, SNMP

5-21

Network Layer: Control Plane

5.1 introduction5.2 routing protocols§ link state§ distance vector5.3 intra-AS routing in the

Internet: OSPF5.4 routing among the ISPs:

BGP

5.5 The SDN control plane5.6 ICMP: The Internet

Control Message Protocol

5.7 Network management and SNMP

Outline

5-22


Network-layer functions

§ forwarding: move packets from router�s input to appropriate router output

data plane

control plane

Two approaches to structuring network control plane:§ per-router control (traditional)§ logically centralized control (software defined networking)

Recall: two network-layer functions:

5-23Network Layer: Control Plane

§ routing: determine route taken by packets from source to destination

Per-router control plane

RoutingAlgorithm

Individual routing algorithm components in each and every router interact with each other in control plane to compute forwarding tables

dataplane

controlplane

4.1 • OVERVIEW OF NETWORK LAYER 309

tables. In this example, a routing algorithm runs in each and every router and both forwarding and routing functions are contained within a router. As we’ll see in Sec-tions 5.3 and 5.4, the routing algorithm function in one router communicates with the routing algorithm function in other routers to compute the values for its forward-ing table. How is this communication performed? By exchanging routing messages containing routing information according to a routing protocol! We’ll cover routing algorithms and protocols in Sections 5.2 through 5.4.

The distinct and different purposes of the forwarding and routing functions can be further illustrated by considering the hypothetical (and unrealistic, but technically feasible) case of a network in which all forwarding tables are configured directly by human network operators physically present at the routers. In this case, no routing protocols would be required! Of course, the human operators would need to interact with each other to ensure that the forwarding tables were configured in such a way that packets reached their intended destinations. It’s also likely that human configu-ration would be more error-prone and much slower to respond to changes in the net-work topology than a routing protocol. We’re thus fortunate that all networks have both a forwarding and a routing function!

Values in arrivingpacket’s header

1

23

Local forwardingtable

header

0100011001111001

1101

3221

output

Control plane

Data plane

Routing algorithm

Figure 4.2 ♦ Routing algorithms determine values in forward tables

M04_KURO4140_07_SE_C04.indd 309 11/02/16 3:14 PM


dataplane

controlplane

Logically centralized control planeA distinct (typically remote) controller interacts with local control agents (CAs) in routers to compute forwarding tables

Remote Controller

CA

CA CA CA CA

5-25




BGP




Outline

5-26


Routing protocols

Routing protocol goal: determine “good” paths (equivalently, routes), from sending hosts to receiving host, through network of routers§ path: sequence of routers packets will traverse

in going from given initial source host to given final destination host

§ “good”: least “cost”, “fastest”, “least congested”

§ routing: a “top-10” networking challenge!


u

yx

wv

z2

21

3

1

1

2

53

5

graph: G = (N,E)

N = set of routers = { u, v, w, x, y, z }

E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

Graph abstraction of the network


Graph abstraction: costs

u

yx

wv

z2

21

3

1

1

2

53

5 c(x,x�) = cost of link (x,x�)e.g., c(w,z) = 5

cost could be inversely related to bandwidth, or inversely related to congestion

cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)

key question: what is the least-cost path between u and z ?routing algorithm: algorithm that finds that least cost path


Routing algorithm classification

Q: global or decentralized information?

global:§ all routers have complete

topology, link cost info§ �link state� algorithmsdecentralized: § router knows physically-

connected neighbors, link costs to neighbors

§ iterative process of computation, exchange of info with neighbors

§ �distance vector� algorithms

Q: static or dynamic?

static:§ routes change slowly over

timedynamic: § routes change more

quickly• periodic update• in response to link

cost changes




BGP




Outline


A link-state routing algorithmDijkstra�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 dests

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 5-32Network Layer: Control Plane

Dijsktra�s algorithm1 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 Loop9 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'


Dijkstra�s algorithm: example

Step012345

N'uuxuxyuxyvuxyvwuxyvwz

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

21

3

1

1

2

53

5


u

yx

wv

z

Dijkstra�s algorithm: example (2)

u

yx

wv

z

resulting shortest-path tree from u:

vxywz

(u,v)(u,x)

(u,x)(u,x)(u,x)

destination link

resulting forwarding table in u:


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)

Each node knows the entire graph’s topology.




BGP




Outline


Distance vector algorithm

Bellman-Ford equation (dynamic programming)

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

thendx(y) = min {c(x,v) + dv(y) }

v

cost to neighbor v

min taken over all neighbors v of x

cost from neighbor v to destination y


Bellman-Ford example

u

yx

wv

z2

21

3

1

1

2

53

5clearly, dv(z) = 5, dx(z) = 3, dw(z) = 3

du(z) = min { c(u,v) + dv(z),c(u,x) + dx(z),c(u,w) + dw(z) }

= min {2 + 5,1 + 3,5 + 3} = 4

node achieving minimum is nexthop in shortest path, used in forwarding table

B-F equation says:



§ Dx(y) = estimate of least cost from x to y• x maintains distance vector Dx = [Dx(y): y є N ]

§ node x:• knows cost to each neighbor v: c(x,v)• maintains its neighbors� distance vectors. For

each neighbor v, x maintains Dv = [Dv(y): y є N ]


key idea:§ from time-to-time, each node sends its own

distance vector estimate to neighbors§ when x receives new DV estimate from neighbor,

it updates its own DV using B-F equation:

Dx(y) ← minv{c(x,v) + Dv(y)} for each node y ∊ N

v under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y)



iterative, asynchronous:each local iteration caused by:

§ local link cost change § DV update message from

neighbordistributed:§ each node notifies

neighbors only when its DV changes• neighbors then notify their

neighbors if necessary

wait for (change in local link cost or msg from neighbor)

recompute estimates

if DV to any dest has changed, notify neighbors

each node:



Each node only needs to know their neighbours.

x y zxyz

0 2 7∞ ∞ ∞∞ ∞ ∞

from

cost to

from

from

x y zxyz

0

x y zxyz

∞ ∞

∞ ∞ ∞

cost to

x y zxyz

∞ ∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

time

x z12

7

y

node xtable

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}= min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) +Dy(z), c(x,z) + Dz(z)}

= min{2+1 , 7+0} = 3

32

node ytable

node ztable

cost to

from


x y zxyz

0 2 3

from

cost to

x y zxyz

0 2 7

from

cost tox y z

xyz

0 2 3

from

cost to

x y zxyz

0 2 3fro

mcost to

x y zxyz

0 2 7

from

cost to

2 0 17 1 0

2 0 13 1 0

2 0 13 1 0

2 0 1

3 1 02 0 1

3 1 0

time

x y zxyz

0 2 7∞ ∞ ∞∞ ∞ ∞

from

cost to

from

from

x y zxyz

0

x y zxyz

∞ ∞

∞ ∞ ∞

cost to

x y zxyz

∞ ∞ ∞7 1 0

cost to

∞2 0 1

∞ ∞ ∞

2 0 17 1 0

time

x z12

7

y

node xtable

Dx(y) = min{c(x,y) + Dy(y), c(x,z) + Dz(y)}= min{2+0 , 7+1} = 2

Dx(z) = min{c(x,y) +Dy(z), c(x,z) + Dz(z)}

= min{2+1 , 7+0} = 3

32

node ytable

node ztable

cost to

from


Distance vector: link cost changes

link cost changes:v node detects local link cost change v updates routing info, recalculates

distance vectorv if DV changes, notify neighbors

�goodnews travelsfast�

x z14

50

y1

t0 : y detects link-cost change, updates its DV, informs its neighbors.

t1 : z receives update from y, updates its table, computes new least cost to x , sends its neighbors its DV.

t2 : y receives z�s update, updates its distance table. y�s least costs do not change, so y does not send a message to z.


Dy(x) Dz(x)

4 5

1 5

1 2

1 2

Distance vector: link cost changes

link cost changes:v node detects local link cost change v bad news travels slow - �count to

infinity� problem!v 44 iterations before algorithm

stabilizes: see text x z14

50

y60

solution: poisoned reverse:v If 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)

v will this completely solve count to infinity problem?


Dy(x) Dz(x)

4 5

6 5

6 7

… …

48 49

50 49

50 50

51 50

51 50

Dy(x) Dz(x)

4 5 (inf)

60 5 (inf)

60 50

51 50

This table is just meant to give a rough idea of the change of the values. To be more precise, you should put a distance table next to each node in the graph and update/trace them.

Comparison of LS and DV algorithms

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

msgs sent § DV: exchange between neighbors

only• convergence time varies

speed of convergence§ LS: O(n2) algorithm requires

O(nE) msgs

§ 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 tableDV:

• DV node can advertise incorrect path cost

• each node�s table used by others

• error propagate thru network


No clear obvious winner. Both are used in the Internet.

5.1 introduction5.2 routing protocols§ link state§ distance vector5.3 intra-AS routing in

the Internet: OSPF5.4 routing among the ISPs:

BGP




Outline


Making routing scalable

scale: with billions of destinations:

§ can�t store all destinations 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 - idealized § all routers identical§ network �flat�… not true in practice


aggregate routers into regions known as �autonomous systems� (AS) (a.k.a. “domains”)

inter-AS routing§ routing among AS’es§ gateways perform inter-

domain routing (as well as intra-domain routing)

Internet approach to scalable routing

intra-AS routing§ routing among hosts, routers

in same AS (“network”)§ all routers in AS must run same intra-domain protocol

§ routers in differentAS can run different intra-domain routing protocol

§ gateway router: at “edge” of its own AS, has link(s) to router(s) in other AS’es


3b

1d

3a

1c2aAS3

AS1AS2

1a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASes

§ forwarding table configured by both intra-and inter-AS routing algorithm• intra-AS routing

determine entries for destinations within AS

• inter-AS & intra-AS determine entries for external destinations


Inter-AS tasks§ suppose router in AS1

receives datagram destined outside of AS1:• router should forward

packet to gateway router, but which one?

AS1 must:1. learn which dests are

reachable through AS2, which through AS3

2. propagate this reachability info to all routers in AS1

job of inter-AS routing!

AS3

AS2

3b

3c3a

AS1

1c1a

1d1b

2a2c

2bothernetworks

othernetworks


Intra-AS Routing

§ also known as interior gateway protocols (IGP)§ most common intra-AS routing protocols:• RIP: Routing Information Protocol• OSPF: Open Shortest Path First (IS-IS protocol

essentially same as OSPF)• IGRP: Interior Gateway Routing Protocol

(Cisco proprietary for decades, until 2016)


OSPF (Open Shortest Path First)

§ �open�: publicly available§ uses link-state algorithm • link state packet dissemination• topology map at each node• route computation using Dijkstra�s algorithm

§ router floods OSPF link-state advertisements to all other routers in entire AS• carried in OSPF messages directly over IP (rather than

TCP or UDP• link state: for each attached link


OSPF �advanced� features

§ security: all OSPF messages authenticated (to prevent malicious intrusion)

§ 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 ToS; high for real-time ToS)

§ integrated uni- and multi-cast support: • Multicast OSPF (MOSPF) uses same topology data

base as OSPF§ hierarchical OSPF in large domains.


csc358 week 8ylzhang/csc358/files/lec07.pdfcsc358 week 8 adapted from slides by j.f. kurose and k....

Documents