CS计算机代考程序代写 database flex distributed system algorithm PowerPoint Presentation

PowerPoint Presentation

Computer Networking: A Top-Down Approach
8th edition
Jim Kurose, Keith Ross
Pearson, 2020
Chapter 5
Network Layer:
Control Plane
A note on the use of these PowerPoint slides:
We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you see the animations; and can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:

If you use these slides (e.g., in a class) that you mention their source (after all, we’d like people to use our book!)
If you post any slides on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

For a revision history, see the slide note for this page.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2020
J.F Kurose and K.W. Ross, All Rights Reserved

Version History

8.0 (May 2020)
All slides reformatted for 16:9 aspect ratio
All slides updated to 8th edition material
Use of Calibri font, rather that Gill Sans MT
Add LOTS more animation throughout
lighter header font
Re-do of network management slides; redo of Bellman-Ford slides
1

Network layer control plane: our goals
understand principles behind network control plane:
traditional routing algorithms
SDN controllers
network management, configuration

instantiation, implementation in the Internet:
OSPF, BGP
OpenFlow, ODL and ONOS controllers
Internet Control Message Protocol: ICMP
SNMP, YANG/NETCONF
Network Layer: 5-2

2

Network layer: “control plane” roadmap

network management, configuration
SNMP
NETCONF/YANG

introduction
routing protocols
link state
distance vector
intra-ISP routing: OSPF
routing among ISPs: BGP
SDN control plane
Internet Control Message Protocol

Network Layer: 5-3

3

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

Network-layer functions
Network Layer: 5-4
forwarding: move packets from router’s input to appropriate router output

data plane

control plane

routing: determine route taken by packets from source to destination

Per-router control plane
Individual routing algorithm components in each and every router interact in the control plane

Routing
Algorithm

data
plane
control
plane

1
2

0111

values in arriving
packet header
3
Network Layer: 5-5

5

Software-Defined Networking (SDN) control plane
Remote controller computes, installs forwarding tables in routers

data
plane
control
plane

Remote Controller

CA

CA

CA

CA

CA
1
2

0111

3
values in arriving
packet header
Network Layer: 5-6

6

Network layer: “control plane” roadmap

network management, configuration
SNMP
NETCONF/YANG

introduction
routing protocols
link state
distance vector
intra-ISP routing: OSPF
routing among ISPs: BGP
SDN control plane
Internet Control Message Protocol

Network Layer: 5-7

7

Routing protocol goal: determine “good” paths (equivalently, routes), from sending hosts to receiving host, through network of routers
path: sequence of routers packets traverse from given initial source host to final destination host
“good”: least “cost”, “fastest”, “least congested”
routing: a “top-10” networking challenge!

Routing protocols

mobile network
enterprise
network

national or global ISP

datacenter
network

application
transport
network
link
physical

application
transport
network
link
physical

network
link
physical

network
link
physical

network
link
physical

network
link
physical

network
link
physical

Network Layer: 5-8

8

Graph abstraction: link costs
Network Layer: 5-9

u

y

x

w

v

z
2
2
1
3
1
1
2
5
3
5
graph: G = (N,E)

ca,b: cost of direct link connecting a and b
e.g., cw,z = 5, cu,z = ∞

cost defined by network operator: could always be 1, or inversely related to bandwidth, or inversely related to congestion
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) }

9

Routing algorithm classification
Network Layer: 5-10
global or decentralized information?

global: all routers have complete topology, link cost info
“link state” algorithms

decentralized: iterative process of computation, exchange of info with neighbors
routers initially only know link costs to attached neighbors
“distance vector” algorithms
How fast do routes change?

dynamic: routes change more quickly
periodic updates or in response to link cost changes

static: routes change slowly over time

10

Network layer: “control plane” roadmap

network management, configuration
SNMP
NETCONF/YANG

introduction
routing protocols
link state
distance vector
intra-ISP routing: OSPF
routing among ISPs: BGP
SDN control plane
Internet Control Message Protocol

Network Layer: 5-11

11

Dijkstra’s link-state routing algorithm
Network Layer: 5-12
centralized: network 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
cx,y: direct link cost from node x to y; = ∞ if not direct neighbors
D(v): current estimate of cost of least-cost-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

notation

12

Dijkstra’s link-state routing algorithm
Network Layer: 5-13
1 Initialization:
2 N’ = {u} /* compute least cost path from u to all other nodes */
3 for all nodes v
4 if v adjacent to u /* u initially knows direct-path-cost only to direct neighbors */
5 then D(v) = cu,v /* but may not be minimum cost! */
6 else D(v) = ∞
7

8 Loop
9
10
11
12
13
14
15 until all nodes in N’

find w not in N’ such that D(w) is a minimum
add w to N’
update D(v) for all v adjacent to w and not in N’ :
D(v) = min ( D(v), D(w) + cw,v )
/* new least-path-cost to v is either old least-cost-path to v or known
least-cost-path to w plus direct-cost from w to v */

13

Dijkstra’s algorithm: an example
Network Layer: 5-14
Step
0
1
2
3
4
5
N’
D(v),p(v)
D(x),p(x)
D(y),p(y)
D(z),p(z)

u

y

x

w

v

z
2
2
1
3
1
1
2
5
3
5
4,y
D(w),p(w)
D(w),p(w)
5,u
4,x
3,y
3,y
4,y
3,y
5,u


1,u
2,u

2,x
4,x
2,u
4,y
3,y
2,u
uxyvwz
uxyvw
uxyv
uxy
ux
u
v
w
x
y
z

find a not in N’ such that D(a) is a minimum
add a to N’
update D(b) for all b adjacent to a and not in N’ :
D(b) = min ( D(b), D(a) + ca,b )
Initialization (step 0): For all a: if a adjacent to then D(a) = cu,a

14

Dijkstra’s algorithm: an example
Network Layer: 5-15

u

y

x

w

v

z
2
2
1
3
1
1
2
5
3
5
D(w),p(w)
5,u
4,x
3,y
3,y

u

y

x

w

v

z
resulting least-cost-path tree from u:
resulting forwarding table in u:

v
x
y
w
x
(u,v)
(u,x)
(u,x)
(u,x)
(u,x)
destination
outgoing link
route from u to v directly
route from u to all other destinations via x

15

Dijkstra’s algorithm: another example
Network Layer: 5-16

w
3
4

v

x

u

5

3

7

4

y
8

z

2

7

9
Step

N’
D(v),
p(v)
0
1
2
3
4
5
D(w),
p(w)
D(x),
p(x)
D(y),
p(y)
D(z),
p(z)

u



7,u
3,u
5,u
uw

11,w
6,w

5,u
14,x
11,w
6,w

uwx

uwxv
14,x
10,v

uwxvy
12,y

notes:
construct least-cost-path tree by tracing predecessor nodes
ties can exist (can be broken arbitrarily)

uwxvyz
v
w
x
y
z

16

Dijkstra’s algorithm: discussion
Network Layer: 5-17
algorithm complexity: n nodes
each of n iteration: need to check all nodes, w, not in N
n(n+1)/2 comparisons: O(n2) complexity
more efficient implementations possible: O(nlogn)
message complexity:
each router must broadcast its link state information to other n routers
efficient (and interesting!) broadcast algorithms: O(n) link crossings to disseminate a broadcast message from one source
each router’s message crosses O(n) links: overall message complexity: O(n2)

17

Dijkstra’s algorithm: oscillations possible
Network Layer: 5-18
when link costs depend on traffic volume, route oscillations possible

a

d

c

b
1

1+e
e
0

e
1
1

0
0
initially

a

d

c

b

given these costs,
find new routing….
resulting in new costs

2+e
0
0
0
1+e
1

a

d

c

b

given these costs,
find new routing….
resulting in new costs

0
2+e
1+e
1
0
0

a

d

c

b

given these costs,
find new routing….
resulting in new costs

2+e
0
0
0
1+e
1
sample scenario:
routing to destination a, traffic entering at d, c, e with rates 1, e (<1), 1 link costs are directional, and volume-dependent e 1 1 e 1 1 e 1 1 18 Network layer: “control plane” roadmap network management, configuration SNMP NETCONF/YANG introduction routing protocols link state distance vector intra-ISP routing: OSPF routing among ISPs: BGP SDN control plane Internet Control Message Protocol Network Layer: 5-19 19 Based on Bellman-Ford (BF) equation (dynamic programming): Distance vector algorithm Network Layer: 5-20 Let Dx(y): cost of least-cost path from x to y. Then: Dx(y) = minv { cx,v + Dv(y) } Bellman-Ford equation min taken over all neighbors v of x v’s estimated least-cost-path cost to y direct cost of link from x to v Bellman-Ford Example Network Layer: 5-21 u y z 2 2 1 3 1 1 2 5 3 5 Suppose that u’s neighboring nodes, x,v,w, know that for destination z: Du(z) = min { cu,v + Dv(z), cu,x + Dx(z), cu,w + Dw(z) } Bellman-Ford equation says: Dv(z) = 5 v Dw(z) = 3 w Dx(z) = 3 x = min {2 + 5, 1 + 3, 5 + 3} = 4 node achieving minimum (x) is next hop on estimated least-cost path to destination (z) Distance vector algorithm Network Layer: 5-22 key idea: from time-to-time, each node sends its own distance vector estimate to neighbors under minor, natural conditions, the estimate Dx(y) converge to the actual least cost dx(y) Dx(y) ← minv{cx,v + Dv(y)} for each node y ∊ N when x receives new DV estimate from any neighbor, it updates its own DV using B-F equation: Distance vector algorithm: Network Layer: 5-23 iterative, asynchronous: each local iteration caused by: local link cost change DV update message from neighbor wait for (change in local link cost or msg from neighbor) each node: distributed, self-stopping: each node notifies neighbors only when its DV changes neighbors then notify their neighbors – only if necessary no notification received, no actions taken! recompute DV estimates using DV received from neighbor if DV to any destination has changed, notify neighbors DV in a: Da(a)=0 Da(b) = 8 Da(c) = ∞ Da(d) = 1 Da(e) = ∞ Da(f) = ∞ Da(g) = ∞ Da(h) = ∞ Da(i) = ∞ Distance vector: example Network Layer: 5-24 g h i 1 1 1 1 1 1 1 1 1 8 1 t=0 All nodes have distance estimates to nearest neighbors (only) A few asymmetries: missing link larger cost d e f a b c All nodes send their local distance vector to their neighbors 24 Distance vector example: iteration Network Layer: 5-25 All nodes: receive distance vectors from neighbors compute their new local distance vector send their new local distance vector to neighbors t=1 g h i 1 1 1 1 1 1 1 1 1 8 1 d e f a b c 25 Distance vector example: iteration Network Layer: 5-26 g h i 1 1 1 1 1 1 1 1 1 8 1 d e f a b c All nodes: receive distance vectors from neighbors compute their new local distance vector send their new local distance vector to neighbors t=1 compute compute compute compute compute compute compute compute compute 26 Distance vector example: iteration Network Layer: 5-27 g h i 1 1 1 1 1 1 1 1 1 8 1 d e f a b c All nodes: receive distance vectors from neighbors compute their new local distance vector send their new local distance vector to neighbors t=1 27 Distance vector example: iteration Network Layer: 5-28 g h i 1 1 1 1 1 1 1 1 1 8 1 d e f a b c All nodes: receive distance vectors from neighbors compute their new local distance vector send their new local distance vector to neighbors t=2 28 Distance vector example: iteration Network Layer: 5-29 g h i 1 1 1 1 1 1 1 8 1 2 1 d e f a b c All nodes: receive distance vectors from neighbors compute their new local distance vector send their new local distance vector to neighbors t=2 compute compute compute compute compute compute compute compute compute 29 Distance vector example: iteration Network Layer: 5-30 g h i 1 1 1 1 1 1 1 1 1 8 1 d e f a b c All nodes: receive distance vectors from neighbors compute their new local distance vector send their new local distance vector to neighbors t=2 30 Distance vector example: iteration Network Layer: 5-31 …. and so on Let’s next take a look at the iterative computations at nodes 31 DV in a: Da(a)=0 Da(b) = 8 Da(c) = ∞ Da(d) = 1 Da(e) = ∞ Da(f) = ∞ Da(g) = ∞ Da(h) = ∞ Da(i) = ∞ Distance vector example: computation Network Layer: 5-32 g h i 1 1 1 1 1 1 1 1 1 8 1 t=1 DV in b: Db(f) = ∞ Db(g) = ∞ Db(h) = ∞ Db(i) = ∞ Db(a) = 8 Db(c) = 1 Db(d) = ∞ Db(e) = 1 b receives DVs from a, c, e a b c d e f DV in c: Dc(a) = ∞ Dc(b) = 1 Dc(c) = 0 Dc(d) = ∞ Dc(e) = ∞ Dc(f) = ∞ Dc(g) = ∞ Dc(h) = ∞ Dc(i) = ∞ DV in e: De(a) = ∞ De(b) = 1 De(c) = ∞ De(d) = 1 De(e) = 0 De(f) = 1 De(g) = ∞ De(h) = 1 De(i) = ∞ 32 Distance vector example: computation DV in a: Da(a)=0 Da(b) = 8 Da(c) = ∞ Da(d) = 1 Da(e) = ∞ Da(f) = ∞ Da(g) = ∞ Da(h) = ∞ Da(i) = ∞ DV in b: Db(f) = ∞ Db(g) = ∞ Db(h) = ∞ Db(i) = ∞ Db(a) = 8 Db(c) = 1 Db(d) = ∞ Db(e) = 1 DV in c: Dc(a) = ∞ Dc(b) = 1 Dc(c) = 0 Dc(d) = ∞ Dc(e) = ∞ Dc(f) = ∞ Dc(g) = ∞ Dc(h) = ∞ Dc(i) = ∞ DV in e: De(a) = ∞ De(b) = 1 De(c) = ∞ De(d) = 1 De(e) = 0 De(f) = 1 De(g) = ∞ De(h) = 1 De(i) = ∞ Network Layer: 5-33 g h i 1 1 1 1 1 1 1 1 1 8 1 t=1 b receives DVs from a, c, e, computes: a b c d e f DV in b: Db(f) =2 Db(g) = ∞ Db(h) = 2 Db(i) = ∞ Db(a) = 8 Db(c) = 1 Db(d) = 2 Db(e) = 1 e compute b Db(a) = min{cb,a+Da(a), cb,c +Dc(a), cb,e+De(a)} = min{8,∞,∞} = 8 Db(c) = min{cb,a+Da(c), cb,c +Dc(c), c b,e +De(c)} = min{∞,1,∞} = 1 Db(d) = min{cb,a+Da(d), cb,c +Dc(d), c b,e +De(d)} = min{9,2,∞} = 2 Db(f) = min{cb,a+Da(f), cb,c +Dc(f), c b,e +De(f)} = min{∞,∞,2} = 2 Db(i) = min{cb,a+Da(i), cb,c +Dc(i), c b,e+De(i)} = min{∞, ∞, ∞} = ∞ Db(h) = min{cb,a+Da(h), cb,c +Dc(h), c b,e+De(h)} = min{∞, ∞, 2} = 2 Db(e) = min{cb,a+Da(e), cb,c +Dc(e), c b,e +De(e)} = min{∞,∞,1} = 1 Db(g) = min{cb,a+Da(g), cb,c +Dc(g), c b,e+De(g)} = min{∞, ∞, ∞} = ∞ 33 DV in a: Da(a)=0 Da(b) = 8 Da(c) = ∞ Da(d) = 1 Da(e) = ∞ Da(f) = ∞ Da(g) = ∞ Da(h) = ∞ Da(i) = ∞ Distance vector example: computation Network Layer: 5-34 g h i 1 1 1 1 1 1 1 1 1 8 1 t=1 DV in b: Db(f) = ∞ Db(g) = ∞ Db(h) = ∞ Db(i) = ∞ Db(a) = 8 Db(c) = 1 Db(d) = ∞ Db(e) = 1 c receives DVs from b a b c d e f DV in c: Dc(a) = ∞ Dc(b) = 1 Dc(c) = 0 Dc(d) = ∞ Dc(e) = ∞ Dc(f) = ∞ Dc(g) = ∞ Dc(h) = ∞ Dc(i) = ∞ DV in e: De(a) = ∞ De(b) = 1 De(c) = ∞ De(d) = 1 De(e) = 0 De(f) = 1 De(g) = ∞ De(h) = 1 De(i) = ∞ 34 Distance vector example: computation Network Layer: 5-35 g h i 1 1 8 1 t=1 DV in b: Db(f) = ∞ Db(g) = ∞ Db(h) = ∞ Db(i) = ∞ Db(a) = 8 Db(c) = 1 Db(d) = ∞ Db(e) = 1 c receives DVs from b computes: a b c d e f DV in c: Dc(a) = ∞ Dc(b) = 1 Dc(c) = 0 Dc(d) = ∞ Dc(e) = ∞ Dc(f) = ∞ Dc(g) = ∞ Dc(h) = ∞ Dc(i) = ∞ Dc(a) = min{cc,b+Db(a}} = 1 + 8 = 9 Dc(b) = min{cc,b+Db(b)} = 1 + 0 = 1 Dc(d) = min{cc,b+Db(d)} = 1+ ∞ = ∞ Dc(e) = min{cc,b+Db(e)} = 1 + 1 = 2 Dc(f) = min{cc,b+Db(f)} = 1+ ∞ = ∞ Dc(g) = min{cc,b+Db(g)} = 1+ ∞ = ∞ Dc(i) = min{cc,b+Db(i)} = 1+ ∞ = ∞ Dc(h) = min{cbc,b+Db(h)} = 1+ ∞ = ∞ DV in c: Dc(a) = 9 Dc(b) = 1 Dc(c) = 0 Dc(d) = 2 Dc(e) = ∞ Dc(f) = ∞ Dc(g) = ∞ Dc(h) = ∞ Dc(i) = ∞ compute * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/ 35 Distance vector example: computation Network Layer: 5-36 1 1 1 1 1 1 1 1 1 8 1 t=1 DV in b: Db(f) = ∞ Db(g) = ∞ Db(h) = ∞ Db(i) = ∞ Db(a) = 8 Db(c) = 1 Db(d) = ∞ Db(e) = 1 e receives DVs from b, d, f, h a b c DV in f: Dc(a) = ∞ Dc(b) = ∞ Dc(c) = ∞ Dc(d) = ∞ Dc(e) = 1 Dc(f) = 0 Dc(g) = ∞ Dc(h) = ∞ Dc(i) = 1 DV in e: De(a) = ∞ De(b) = 1 De(c) = ∞ De(d) = 1 De(e) = 0 De(f) = 1 De(g) = ∞ De(h) = 1 De(i) = ∞ DV in h: Dc(a) = ∞ Dc(b) = ∞ Dc(c) = ∞ Dc(d) = ∞ Dc(e) = 1 Dc(f) = ∞ Dc(g) = 1 Dc(h) = 0 Dc(i) = 1 DV in d: Dc(a) = 1 Dc(b) = ∞ Dc(c) = ∞ Dc(d) = 0 Dc(e) = 1 Dc(f) = ∞ Dc(g) = 1 Dc(h) = ∞ Dc(i) = ∞ d e f g h i Q: what is new DV computed in e at t=1? compute 36 Distance vector: state information diffusion t=0 c’s state at t=0 is at c only g h i 1 1 1 1 1 1 1 1 1 8 1 d e f a b c c’s state at t=0 has propagated to b, and may influence distance vector computations up to 1 hop away, i.e., at b t=1 c’s state at t=0 may now influence distance vector computations up to 2 hops away, i.e., at b and now at a, e as well t=2 c’s state at t=0 may influence distance vector computations up to 3 hops away, i.e., at b,a,e and now at c,f,h as well t=3 c’s state at t=0 may influence distance vector computations up to 4 hops away, i.e., at b,a,e, c, f, h and now at g,i as well t=4 Iterative communication, computation steps diffuses information through network: t=1 t=2 t=3 t=4 37 Distance vector: link cost changes Network Layer: 5-38 “good news travels fast” 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. link cost changes: node detects local link cost change updates routing info, recalculates local DV if DV changes, notify neighbors x z 1 4 50 y 1 Distance vector: link cost changes Network Layer: 5-39 link cost changes: node detects local link cost change “bad news travels slow” – count-to-infinity problem: x z 1 4 50 y 60 y sees direct link to x has new cost 60, but z has said it has a path at cost of 5. So y computes “my new cost to x will be 6, via z); notifies z of new cost of 6 to x. z learns that path to x via y has new cost 6, so z computes “my new cost to x will be 7 via y), notifies y of new cost of 7 to x. y learns that path to x via z has new cost 7, so y computes “my new cost to x will be 8 via y), notifies z of new cost of 8 to x. z learns that path to x via y has new cost 8, so z computes “my new cost to x will be 9 via y), notifies y of new cost of 9 to x. … see text for solutions. Distributed algorithms are tricky! Comparison of LS and DV algorithms Network Layer: 5-40 message complexity LS: n routers, O(n2) messages sent DV: exchange between neighbors; convergence time varies speed of convergence LS: O(n2) algorithm, O(n2) messages may have oscillations DV: convergence time varies may have routing loops count-to-infinity problem robustness: what happens if router malfunctions, or is compromised? LS: router can advertise incorrect link cost each router computes only its own table DV: DV router can advertise incorrect path cost (“I have a really low cost path to everywhere”): black-holing each router’s table used by others: error propagate thru network Network layer: “control plane” roadmap network management, configuration SNMP NETCONF/YANG introduction routing protocols intra-ISP routing: OSPF routing among ISPs: BGP SDN control plane Internet Control Message Protocol Network Layer: 5-41 41 our routing study thus far - idealized all routers identical network “flat” … not true in practice Making routing scalable Network Layer: 5-42 scale: billions of destinations: can’t store all destinations in routing tables! routing table exchange would swamp links! administrative autonomy: Internet: a network of networks each network admin may want to control routing in its own network aggregate routers into regions known as “autonomous systems” (AS) (a.k.a. “domains”) Internet approach to scalable routing Network Layer: 5-43 intra-AS (aka “intra-domain”): routing among within same AS (“network”) all routers in AS must run same intra-domain protocol routers in different AS can run different intra-domain routing protocols gateway router: at “edge” of its own AS, has link(s) to router(s) in other AS’es inter-AS (aka “inter-domain”): routing among AS’es gateways perform inter-domain routing (as well as intra-domain routing) Interconnected ASes Network Layer: 5-44 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c intra-AS routing intra-AS routing intra-AS routing inter-AS routing forwarding table forwarding table configured by intra- and inter-AS routing algorithms Intra-AS Routing Inter-AS Routing intra-AS routing determine entries for destinations within AS inter-AS & intra-AS determine entries for external destinations Inter-AS routing: a role in intradomain forwarding Network Layer: 5-45 3b 1d 3a 1c 2a AS3 AS1 AS2 1a 2c 2b 1b 3c other networks other networks suppose router in AS1 receives datagram destined outside of AS1: AS1 inter-domain routing must: learn which destinations reachable through AS2, which through AS3 propagate this reachability info to all routers in AS1 router should forward packet to gateway router in AS1, but which one? Inter-AS routing: routing within an AS Network Layer: 5-46 most common intra-AS routing protocols: RIP: Routing Information Protocol [RFC 1723] classic DV: DVs exchanged every 30 secs no longer widely used EIGRP: Enhanced Interior Gateway Routing Protocol DV based formerly Cisco-proprietary for decades (became open in 2013 [RFC 7868]) OSPF: Open Shortest Path First [RFC 2328] link-state routing IS-IS protocol (ISO standard, not RFC standard) essentially same as OSPF OSPF (Open Shortest Path First) routing Network Layer: 5-47 “open”: publicly available classic link-state each router floods OSPF link-state advertisements (directly over IP rather than using TCP/UDP) to all other routers in entire AS multiple link costs metrics possible: bandwidth, delay each router has full topology, uses Dijkstra’s algorithm to compute forwarding table security: all OSPF messages authenticated (to prevent malicious intrusion) Hierarchical OSPF Network Layer: 5-48 two-level hierarchy: local area, backbone. link-state advertisements flooded only in area, or backbone each node has detailed area topology; only knows direction to reach other destinations area border routers: “summarize” distances to destinations in own area, advertise in backbone area 1 area 2 area 3 backbone internal routers backbone router: runs OSPF limited to backbone boundary router: connects to other ASes local routers: flood LS in area only compute routing within area forward packets to outside via area border router Network layer: “control plane” roadmap network management, configuration SNMP NETCONF/YANG introduction routing protocols intra-ISP routing: OSPF routing among ISPs: BGP SDN control plane Internet Control Message Protocol Network Layer: 5-49 49 BGP (Border Gateway Protocol): the de facto inter-domain routing protocol “glue that holds the Internet together” allows subnet to advertise its existence, and the destinations it can reach, to rest of Internet: “I am here, here is who I can reach, and how” BGP provides each AS a means to: eBGP: obtain subnet reachability information from neighboring ASes iBGP: propagate reachability information to all AS-internal routers. determine “good” routes to other networks based on reachability information and policy Internet inter-AS routing: BGP Network Layer: 5-50 eBGP, iBGP connections Network Layer: 5-51 eBGP connectivity logical iBGP connectivity 1b 1d 1c 1a 2b 2d 2c 2a 3b 3d 3c 3a AS 2 AS 3 AS 1 1c ∂ ∂ gateway routers run both eBGP and iBGP protocols BGP basics Network Layer: 5-52 when AS3 gateway 3a advertises path AS3,X to AS2 gateway 2c: AS3 promises to AS2 it will forward datagrams towards X BGP session: two BGP routers (“peers”) exchange BGP messages over semi-permanent TCP connection: advertising paths to different destination network prefixes (BGP is a “path vector” protocol) 2b 2d 2c 2a AS 2 3b 3d 3c 3a AS 3 1b 1d 1c 1a AS 1 X BGP advertisement: AS3, X Path attributes and BGP routes Network Layer: 5-53 BGP advertised route: prefix + attributes prefix: destination being advertised two important attributes: AS-PATH: list of ASes through which prefix advertisement has passed NEXT-HOP: indicates specific internal-AS router to next-hop AS policy-based routing: gateway receiving route advertisement uses import policy to accept/decline path (e.g., never route through AS Y). AS policy also determines whether to advertise path to other other neighboring ASes 2b 2d 2c 2a AS 2 3b 3d 3c 3a AS 3 1b 1d 1c 1a AS 1 X BGP path advertisement Network Layer: 5-54 based on AS2 policy, AS2 router 2c accepts path AS3,X, propagates (via iBGP) to all AS2 routers AS2,AS3,X AS2 router 2c receives path advertisement AS3,X (via eBGP) from AS3 router 3a based on AS2 policy, AS2 router 2a advertises (via eBGP) path AS2, AS3, X to AS1 router 1c AS3, X BGP path advertisement (more) Network Layer: 5-55 AS2,AS3,X AS1 gateway router 1c learns path AS2,AS3,X from 2a gateway router may learn about multiple paths to destination: AS3,X AS1 gateway router 1c learns path AS3,X from 3a based on policy, AS1 gateway router 1c chooses path AS3,X and advertises path within AS1 via iBGP AS3, X 2b 2d 2c 2a AS 2 3b 3d 3c 3a AS 3 1b 1d 1c 1a AS 1 X AS3,X AS3,X AS3,X BGP messages Network Layer: 5-56 BGP messages exchanged between peers over TCP connection BGP messages: OPEN: opens TCP connection to remote BGP peer and authenticates sending BGP peer 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 2b 2d 2c 2a AS 2 3b 3d 3c 3a AS 3 1b 1d 1c 1a AS 1 X BGP path advertisement Network Layer: 5-57 AS2,AS3,X AS3,X AS3, X recall: 1a, 1b, 1d learn via iBGP from 1c: “path to X goes through 1c” at 1d: OSPF intra-domain routing: to get to 1c, use interface 1 1 2 1 2 dest interface … … … … local link interfaces at 1a, 1d at 1d: to get to X, use interface 1 1c 1 X 1 AS3,X AS3,X AS3,X 2b 2d 2c 2a AS 2 3b 3d 3c 3a AS 3 1b 1d 1c 1a AS 1 X BGP path advertisement Network Layer: 5-58 recall: 1a, 1b, 1d learn via iBGP from 1c: “path to X goes through 1c” at 1d: OSPF intra-domain routing: to get to 1c, use interface 1 1 2 at 1d: to get to X, use interface 1 dest interface … … … … 1c 2 X 2 at 1a: OSPF intra-domain routing: to get to 1c, use interface 2 at 1a: to get to X, use interface 2 Why different Intra-, Inter-AS routing ? Network Layer: 5-59 policy: inter-AS: admin wants control over how its traffic routed, who routes through its network intra-AS: single admin, so policy less of an issue scale: hierarchical routing saves table size, reduced update traffic performance: intra-AS: can focus on performance inter-AS: policy dominates over performance 2b 2d 2c 2a AS 2 3b 3d 3c 3a AS 3 1b 1d 1c 1a AS 1 X Hot potato routing Network Layer: 5-60 2d learns (via iBGP) it can route to X via 2a or 2c hot potato routing: choose local gateway that has least intra-domain cost (e.g., 2d chooses 2a, even though more AS hops to X): don’t worry about inter-domain cost! AS3,X AS1,AS3,X OSPF link weights 201 112 263 BGP: achieving policy via advertisements Network Layer: 5-61 B legend: customer network: provider network A advertises path Aw to B and to C B chooses not to advertise BAw to C! B gets no “revenue” for routing CBAw, since none of C, A, w are B’s customers C does not learn about CBAw path C will route CAw (not using B) to get to w ISP only wants to route traffic to/from its customer networks (does not want to carry transit traffic between other ISPs – a typical “real world” policy) w A y C x A,w A,w BGP: achieving policy via advertisements (more) Network Layer: 5-62 B ISP only wants to route traffic to/from its customer networks (does not want to carry transit traffic between other ISPs – a typical “real world” policy) w A y C x A,B,C are provider networks x,w,y are customer (of provider networks) x is dual-homed: attached to two networks policy to enforce: x does not want to route from B to C via x .. so x will not advertise to B a route to C legend: customer network: provider network router may learn about more than one route to destination AS, selects route based on: local preference value attribute: policy decision shortest AS-PATH closest NEXT-HOP router: hot potato routing additional criteria BGP route selection Network Layer: 5-63 Network layer: “control plane” roadmap network management, configuration SNMP NETCONF/YANG introduction routing protocols intra-ISP routing: OSPF routing among ISPs: BGP SDN control plane Internet Control Message Protocol Network Layer: 5-64 64 Internet network layer: historically implemented via distributed, per-router control approach: monolithic router contains switching hardware, runs proprietary implementation of Internet standard protocols (IP, RIP, IS-IS, OSPF, BGP) in proprietary router OS (e.g., Cisco IOS) different “middleboxes” for different network layer functions: firewalls, load balancers, NAT boxes, .. ~2005: renewed interest in rethinking network control plane Software defined networking (SDN) Network Layer: 5-65 Per-router control plane Individual routing algorithm components in each and every router interact in the control plane to computer forwarding tables Routing Algorithm data plane control plane 1 2 0111 values in arriving packet header 3 Network Layer: 4-66 66 Software-Defined Networking (SDN) control plane Remote controller computes, installs forwarding tables in routers data plane control plane Remote Controller CA CA CA CA CA 1 2 0111 3 values in arriving packet header Network Layer: 4-67 67 Why a logically centralized control plane? easier network management: avoid router misconfigurations, greater flexibility of traffic flows table-based forwarding (recall OpenFlow API) allows “programming” routers centralized “programming” easier: compute tables centrally and distribute distributed “programming” more difficult: compute tables as result of distributed algorithm (protocol) implemented in each-and-every router open (non-proprietary) implementation of control plane foster innovation: let 1000 flowers bloom Software defined networking (SDN) Network Layer: 5-68 SDN analogy: mainframe to PC revolution Network Layer: 5-69 Vertically integrated Closed, proprietary Slow innovation Small industry Specialized Operating System Specialized Hardware App App App App App App App App App App App Specialized Applications Horizontal Open interfaces Rapid innovation Huge industry Microprocessor Open Interface * Slide courtesy: N. McKeown or or Open Interface Windows Linux MAC OS 2 2 1 3 1 1 2 5 3 5 v w u z y x Traffic engineering: difficult with traditional routing Network Layer: 5-70 Q: what if network operator wants u-to-z traffic to flow along uvwz, rather than uxyz? A: need to re-define link weights so traffic routing algorithm computes routes accordingly (or need a new routing algorithm)! link weights are only control “knobs”: not much control! 2 2 1 3 1 1 2 5 3 5 v w u z y x Traffic engineering: difficult with traditional routing Network Layer: 5-71 Q: what if network operator wants to split u-to-z traffic along uvwz and uxyz (load balancing)? A: can’t do it (or need a new routing algorithm) Traffic engineering: difficult with traditional routing Network Layer: 5-72 Q: what if w wants to route blue and red traffic differently from w to z? A: can’t do it (with destination-based forwarding, and LS, DV routing) 2 2 1 3 1 1 2 5 3 5 v w u z y x We learned in Chapter 4 that generalized forwarding and SDN can be used to achieve any routing desired Software defined networking (SDN) Network Layer: 5-73 data plane control plane Remote Controller CA CA CA CA CA 1: generalized “flow-based” forwarding (e.g., OpenFlow) 2. control, data plane separation 3. control plane functions external to data-plane switches … routing access control load balance 4. programmable control applications Software defined networking (SDN) Network Layer: 5-74 Data-plane switches: fast, simple, commodity switches implementing generalized data-plane forwarding (Section 4.4) in hardware flow (forwarding) table computed, installed under controller supervision API for table-based switch control (e.g., OpenFlow) defines what is controllable, what is not protocol for communicating with controller (e.g., OpenFlow) data plane control plane SDN Controller (network operating system) … routing access control load balance southbound API northbound API SDN-controlled switches network-control applications Software defined networking (SDN) Network Layer: 5-75 SDN controller (network OS): maintain network state information interacts with network control applications “above” via northbound API interacts with network switches “below” via southbound API implemented as distributed system for performance, scalability, fault-tolerance, robustness data plane control plane SDN Controller (network operating system) … routing access control load balance southbound API northbound API SDN-controlled switches network-control applications Software defined networking (SDN) Network Layer: 5-76 network-control apps: “brains” of control: implement control functions using lower-level services, API provided by SDN controller unbundled: can be provided by 3rd party: distinct from routing vendor, or SDN controller data plane control plane SDN Controller (network operating system) … routing access control load balance southbound API northbound API SDN-controlled switches network-control applications Components of SDN controller Network Layer: 5-77 Network-wide distributed, robust state management Communication to/from controlled devices Link-state info switch info host info statistics flow tables … … OpenFlow SNMP … network graph intent RESTful API … Interface, abstractions for network control apps SDN controller routing access control load balance communication: communicate between SDN controller and controlled switches network-wide state management : state of networks links, switches, services: a distributed database interface layer to network control apps: abstractions API OpenFlow protocol Network Layer: 5-78 operates between controller, switch TCP used to exchange messages optional encryption three classes of OpenFlow messages: controller-to-switch asynchronous (switch to controller) symmetric (misc.) distinct from OpenFlow API API used to specify generalized forwarding actions OpenFlow Controller 78 OpenFlow: controller-to-switch messages Network Layer: 5-79 Key controller-to-switch messages features: controller queries switch features, switch replies configure: controller queries/sets switch configuration parameters modify-state: add, delete, modify flow entries in the OpenFlow tables packet-out: controller can send this packet out of specific switch port OpenFlow Controller 79 OpenFlow: switch-to-controller messages Network Layer: 5-80 Key switch-to-controller messages packet-in: transfer packet (and its control) to controller. See packet-out message from controller flow-removed: flow table entry deleted at switch port status: inform controller of a change on a port. Fortunately, network operators don’t “program” switches by creating/sending OpenFlow messages directly. Instead use higher-level abstraction at controller OpenFlow Controller 80 SDN: control/data plane interaction example Network Layer: 5-81 Link-state info switch info host info statistics flow tables … … OpenFlow SNMP … network graph intent RESTful API … Dijkstra’s link-state routing s1 s2 s3 s4 S1, experiencing link failure uses OpenFlow port status message to notify controller 1 SDN controller receives OpenFlow message, updates link status info 2 Dijkstra’s routing algorithm application has previously registered to be called when ever link status changes. It is called. 3 Dijkstra’s routing algorithm access network graph info, link state info in controller, computes new routes 4 1 2 3 4 81 SDN: control/data plane interaction example Network Layer: 5-82 Link-state info switch info host info statistics flow tables … … OpenFlow SNMP … network graph intent RESTful API … Dijkstra’s link-state routing s1 s2 s3 s4 link state routing app interacts with flow-table-computation component in SDN controller, which computes new flow tables needed 5 controller uses OpenFlow to install new tables in switches that need updating 6 5 6 1 2 3 4 82 OpenDaylight (ODL) controller Network Layer: 5-83 Network Orchestrations and Applications Southbound API Service Abstraction Layer (SAL) config. and operational data store REST/RESTCONF/NETCONF APIs messaging OpenFlow NETCONF SNMP OVSDB … Northbound API Traffic Engineering … Firewalling Load Balancing Basic Network Functions Enhanced Services … … Forwarding rules mgr. AAA Host Tracker Stats mgr. Switch mgr. Topology processing Service Abstraction Layer: interconnects internal, external applications and services 83 ONOS controller Network Layer: 5-84 Network Applications Southbound API Northbound API Traffic Engineering … Firewalling Load Balancing southbound abstractions, protocols OpenFlow Netconf OVSDB device link host flow packet northbound abstractions, protocols REST API Intent ONOS distributed core statistics devices hosts links paths flow rules topology control apps separate from controller intent framework: high-level specification of service: what rather than how considerable emphasis on distributed core: service reliability, replication performance scaling 84 hardening the control plane: dependable, reliable, performance-scalable, secure distributed system robustness to failures: leverage strong theory of reliable distributed system for control plane dependability, security: “baked in” from day one? networks, protocols meeting mission-specific requirements e.g., real-time, ultra-reliable, ultra-secure Internet-scaling: beyond a single AS SDN critical in 5G cellular networks SDN: selected challenges Network Layer: 5-85 SDN-computed versus router-computer forwarding tables: just one example of logically-centralized-computed versus protocol computed one could imagine SDN-computed congestion control: controller sets sender rates based on router-reported (to controller) congestion levels SDN and the future of traditional network protocols Network Layer: 5-86 How will implementation of network functionality (SDN versus protocols) evolve? Network layer: “control plane” roadmap network management, configuration SNMP NETCONF/YANG introduction routing protocols intra-ISP routing: OSPF routing among ISPs: BGP SDN control plane Internet Control Message Protocol Network Layer: 5-87 87 ICMP: internet control message protocol Network Layer: 4-88 used by hosts and routers to communicate network-level information error reporting: unreachable host, network, port, protocol echo request/reply (used by ping) network-layer “above” IP: ICMP messages carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header Traceroute and ICMP Network Layer: 4-89 when ICMP message arrives at source: record RTTs stopping criteria: UDP segment eventually arrives at destination host destination returns ICMP “port unreachable” message (type 3, code 3) source stops 3 probes 3 probes 3 probes source sends sets of UDP segments to destination 1st set has TTL =1, 2nd set has TTL=2, etc. datagram in nth set arrives to nth router: router discards datagram and sends source ICMP message (type 11, code 0) ICMP message possibly includes name of router & IP address Network layer: “control plane” roadmap network management, configuration SNMP NETCONF/YANG introduction routing protocols intra-ISP routing: OSPF routing among ISPs: BGP SDN control plane Internet Control Message Protocol Network Layer: 5-90 90 autonomous systems (aka “network”): 1000s of interacting hardware/software components other complex systems requiring monitoring, configuration, control: jet airplane, nuclear power plant, others? What is network management? Network Layer: 5-91 "Network management includes the deployment, integration and coordination of the hardware, software, and human elements to monitor, test, poll, configure, analyze, evaluate, and control the network and element resources to meet the real-time, operational performance, and Quality of Service requirements at a reasonable cost." Components of network management Network Layer: 5-92 managed device managed device managed device managed device managed device agent data agent data agent data agent data agent data managing server/controller data Managing server: application, typically with network managers (humans) in the loop Managed device: equipment with manageable, configurable hardware, software components Data: device “state” configuration data, operational data, device statistics Network management protocol: used by managing server to query, configure, manage device; used by devices to inform managing server of data, events. 92 Network operator approaches to management Network Layer: 5-93 managed device managed device managed device managed device managed device agent data agent data agent data agent data agent data managing server/controller data CLI (Command Line Interface) operator issues (types, scripts) direct to individual devices (e.g., vis ssh) SNMP/MIB operator queries/sets devices data (MIB) using Simple Network Management Protocol (SNMP) NETCONF/YANG more abstract, network-wide, holistic emphasis on multi-device configuration management. YANG: data modeling language NETCONF: communicate YANG-compatible actions/data to/from/among remote devices 93 SNMP protocol Network Layer: 5-94 managed device agent data managing server/controller data request response trap message Two ways to convey MIB info, commands: request/response mode managed device agent data managing server/controller data trap mode SNMP protocol: message types Network Layer: 5-95 GetRequest GetNextRequest GetBulkRequest manager-to-agent: “get me data” (data instance, next data in list, block of data). Message type Function SetRequest manager-to-agent: set MIB value Response Agent-to-manager: value, response to Request Trap Agent-to-manager: inform manager of exceptional event SNMP protocol: message formats Network Layer: 5-96 …. PDU type (0-3) Request ID Error Status (0-5) Error Index Name Value Name Value Get/set header Variables to get/set SNMP PDU message types 0-3 …. PDU type 4 Enterprise Agent Addr Trap Type (0-7) Specific code Time stamp Name Value Trap header Trap info message type 4 managed device’s operational (and some configuration) data gathered into device MIB module 400 MIB modules defined in RFC’s; many more vendor-specific MIBs SNMP: Management Information Base (MIB) Network Layer: 5-97 Object ID Name Type Comments 1.3.6.1.2.1.7.1 UDPInDatagrams 32-bit counter total # datagrams delivered 1.3.6.1.2.1.7.2 UDPNoPorts 32-bit counter # undeliverable datagrams (no application at port) 1.3.6.1.2.1.7.3 UDInErrors 32-bit counter # undeliverable datagrams (all other reasons) 1.3.6.1.2.1.7.4 UDPOutDatagrams 32-bit counter total # datagrams sent 1.3.6.1.2.1.7.5 udpTable SEQUENCE one entry for each port currently in use agent data Structure of Management Information (SMI): data definition language example MIB variables for UDP protocol: goal: actively manage/configure devices network-wide operates between managing server and managed network devices actions: retrieve, set, modify, activate configurations atomic-commit actions over multiple devices query operational data and statistics subscribe to notifications from devices remote procedure call (RPC) paradigm NETCONF protocol messages encoded in XML exchanged over secure, reliable transport (e.g., TLS) protocol NETCONF overview Network Layer: 5-98 NETCONF initialization, exchange, close Network Layer: 5-99 Session initiation, capabilities exchange:
Session close:















managing
server/controller

data

agent

data

Selected NETCONF Operations
Network Layer: 5-100
NETCONF Operation Description
Retrieve all or part of a given configuration. A device may have multiple configurations.
Retrieve all or part of both configuration state and operational state data.
Change specified (possibly running) configuration at managed device. Managed device contains or with rollback.
, Lock (unlock) configuration datastore at managed device (to lock out NETCONF, SNMP, or CLIs commands from other sources).
, Enable event notification subscription from managed device

Sample NETCONF RPC message
Network Layer: 5-101

note message id
change the running configuration
change MTU of Ethernet 0/0 interface to 1500
change a configuration

data modeling language used to specify structure, syntax, semantics of NETCONF network management data
built-in data types, like SMI
XML document describing device, capabilities can be generated from YANG description
can express constraints among data that must be satisfied by a valid NETCONF configuration
ensure NETCONF configurations satisfy correctness, consistency constraints

YANG
Network Layer: 5-102

agent

data

managing
server/controller

data

NETCONF RPC message


YANG-generated XML

YANG
generated

Network layer: Summary
Network Layer: 5-103
we’ve learned a lot!
approaches to network control plane
per-router control (traditional)
logically centralized control (software defined networking)
traditional routing algorithms
implementation in Internet: OSPF , BGP
SDN controllers
implementation in practice: ODL, ONOS
Internet Control Message Protocol
network management

next stop: link layer!

103

Network layer, control plane: Done!

network management, configuration
SNMP
NETCONF/YANG

introduction
routing protocols
link state
distance vector
intra-ISP routing: OSPF
routing among ISPs: BGP
SDN control plane
Internet Control Message Protocol

Network Layer: 5-104

104

Additional Chapter 5 slides
Network Layer: 5-105

105

Distance vector: another example
Network Layer: 5-106

x y z
x
y
z
0 2 7






from
cost to
from
from

x y z
x
y
z
0

x y z
x
y
z





cost to

x y z
x
y
z



7
1
0
cost to

2 0 1
∞ ∞ ∞
2 0 1
7 1 0

time

x

z
1
2
7

y
Dx()

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

Dx(z) = min{cx,y+ Dy(z), cx,z+ Dz(z)}
= min{2+1 , 7+0} = 3

3
2
Dy()
Dz()
cost to
from

106

Distance vector: another example
Network Layer: 5-107

x y z
x
y
z
0 2 7






cost to
from
from

x y z
x
y
z





cost to

x y z
x
y
z



7
1
0
cost to

2 0 1
∞ ∞ ∞

x

z
1
2
7

y
Dx()

Dy()
Dz()
from

x y z
x
y
z
0 2 3
from
cost to

x y z
x
y
z
0 2 7
from
cost to

x y z
x
y
z
0 2 3
from
cost to

x y z
x
y
z
0 2 3
from
cost to

x y z
x
y
z
0 2 7
from
cost to
2 0 1
7 1 0
2 0 1
3 1 0
2 0 1
3 1 0
2 0 1
3 1 0
2 0 1
3 1 0

from

x y z
x
y
z
0
2 0 1
7 1 0

3
2
cost to

time

107

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 arriving
packet’s header

1

2
3

Local forwarding
table

header

0100
0110
0111
1001

1101

3
2
2
1

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

/docProps/thumbnail.jpeg