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Chapter goals: understand principles behind network layer services: routing (path selection) dealing with scale how a router works advanced topics: IPv6, multicast instantiation and implementation in the Internet. Chapter Overview: network layer services routing principle: path selection - PowerPoint PPT Presentation
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4: Network Layer 4a-1
Chapter 4: Network LayerChapter goals: understand principles
behind network layer services: routing (path
selection) dealing with scale how a router works advanced topics: IPv6,
multicast instantiation and
implementation in the Internet
Chapter Overview: network layer services routing principle: path
selection hierarchical routing IP Internet routing protocols
reliable transfer intra-domain inter-domain
what’s inside a router? IPv6 multicast routing
4: Network Layer 4a-2
Network layer functions
transport packet from sending to receiving hosts
network layer protocols in every host, router
three important functions: path determination: route
taken by packets from source to dest. Routing algorithms
switching: move packets from router’s input to appropriate router output
call setup: some network architectures require router call setup along path before data flows
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
4: Network Layer 4a-3
Network service model
Q: What service model for “channel” transporting packets from sender to receiver?
guaranteed bandwidth? preservation of inter-
packet timing (no jitter)? loss-free delivery? in-order delivery? congestion feedback to
sender?
? ??virtual circuit
or datagram?
The most important abstraction provided
by network layer:
serv
ice a
bst
ract
ion
4: Network Layer 4a-4
Virtual circuits
call setup, teardown for each call before data can flow each packet carries VC identifier (not destination host OD) every router on source-dest path s maintain “state” for each
passing connection transport-layer connection only involved two end systems
link, router resources (bandwidth, buffers) may be allocated to VC to get circuit-like perf.
“source-to-dest path behaves much like telephone circuit” performance-wise network actions along source-to-dest path
4: Network Layer 4a-5
Virtual circuits: signaling protocols
used to setup, maintain teardown VC used in ATM, frame-relay, X.25 not used in today’s Internet
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1. Initiate call 2. incoming call
3. Accept call4. Call connected5. Data flow begins 6. Receive data
4: Network Layer 4a-6
Datagram networks: the Internet model no call setup at network layer routers: no state about end-to-end connections
no network-level concept of “connection”
packets typically routed using destination host ID packets between same source-dest pair may take
different paths
application
transportnetworkdata linkphysical
application
transportnetworkdata linkphysical
1. Send data 2. Receive data
4: Network Layer 4a-7
Network layer service models:
NetworkArchitecture
Internet
ATM
ATM
ATM
ATM
ServiceModel
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constantrateguaranteedrateguaranteed minimumnone
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestionfeedback
no (inferredvia loss)nocongestionnocongestionyes
no
Guarantees ?
Internet model being extented: Intserv, Diffserv Chapter 6
4: Network Layer 4a-8
Datagram or VC network: why?
Internet data exchange among
computers “elastic” service, no
strict timing req. “smart” end systems
(computers) can adapt, perform
control, error recovery simple inside network,
complexity at “edge” many link types
different characteristics uniform service difficult
ATM evolved from telephony human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems telephones complexity inside
network
4: Network Layer 4a-9
Routing
Graph abstraction for routing algorithms:
graph nodes are routers
graph edges are physical links link cost: delay, $
cost, or congestion level
Goal: determine “good” path
(sequence of routers) thru network from source to
dest.
Routing protocol
A
ED
CB
F
22
13
1
1
2
53
5
“good” path: typically means
minimum cost path other def’s possible
4: Network Layer 4a-10
Routing Algorithm classification
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
Static or dynamic?Static: routes change slowly
over timeDynamic: routes change more
quickly periodic update in response to link
cost changes
4: Network Layer 4a-11
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 routing table for
that node iterative: after k iterations,
know least cost path to k dest.’s
Notation: c(i,j): link cost from node
i to j. cost infinite 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, that is next v
N: set of nodes whose least cost path definitively known
4: Network Layer 4a-12
Dijsktra’s Algorithm
1 Initialization: 2 N = {A} 3 for all nodes v 4 if v adjacent to A 5 then D(v) = c(A,v) 6 else D(v) = infty 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
4: Network Layer 4a-13
Dijkstra’s algorithm: example
Step012345
start NA
ADADE
ADEBADEBC
ADEBCF
D(B),p(B)2,A2,A2,A
D(C),p(C)5,A4,D3,E3,E
D(D),p(D)1,A
D(E),p(E)infinity
2,D
D(F),p(F)infinityinfinity
4,E4,E4,E
A
ED
CB
F
22
13
1
1
2
53
5
4: Network Layer 4a-14
Dijkstra’s algorithm, discussionAlgorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n*(n+1)/2 comparisons: O(n**2) 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
4: Network Layer 4a-15
Distance Vector Routing Algorithm
iterative: continues until no
nodes exchange info. self-terminating: no
“signal” to stop
asynchronous: nodes need not
exchange info/iterate in lock step!
distributed: each node
communicates only with directly-attached neighbors
Distance Table data structure each node has its own row for each possible destination column for each directly-
attached neighbor to node example: in node X, for dest. Y
via neighbor Z:
D (Y,Z)X
distance from X toY, via Z as next hop
c(X,Z) + min {D (Y,w)}Z
w
=
=
4: Network Layer 4a-16
Distance Table: example
A
E D
CB7
81
2
1
2
D ()
A
B
C
D
A
1
7
6
4
B
14
8
9
11
D
5
5
4
2
Ecost to destination via
dest
inat
ion
D (C,D)E
c(E,D) + min {D (C,w)}D
w== 2+2 = 4
D (A,D)E
c(E,D) + min {D (A,w)}D
w== 2+3 = 5
D (A,B)E
c(E,B) + min {D (A,w)}B
w== 8+6 = 14
loop!
loop!
4: Network Layer 4a-17
Distance table gives routing table
D ()
A
B
C
D
A
1
7
6
4
B
14
8
9
11
D
5
5
4
2
Ecost to destination via
dest
inat
ion
A
B
C
D
A,1
D,5
D,4
D,4
Outgoing link to use, cost
dest
inat
ion
Distance table Routing table
4: Network Layer 4a-18
Distance Vector Routing: overview
Iterative, asynchronous: each local iteration caused by:
local link cost change message from neighbor:
its least cost path change from neighbor
Distributed: each node notifies
neighbors only when its least cost path to any destination changes neighbors then notify
their neighbors if necessary
wait for (change in local link cost of msg from neighbor)
recompute distance table
if least cost path to any dest
has changed, notify neighbors
Each node:
4: Network Layer 4a-19
Distance Vector Algorithm:
1 Initialization: 2 for all adjacent nodes v: 3 D (*,v) = infty /* the * operator means "for all rows" */ 4 D (v,v) = c(X,v) 5 for all destinations, y 6 send min D (y,w) to each neighbor /* w over all X's neighbors */
XX
Xw
At all nodes, X:
4: Network Layer 4a-20
Distance Vector Algorithm (cont.):8 loop 9 wait (until I see a link cost change to neighbor V 10 or until I receive update from neighbor V) 11 12 if (c(X,V) changes by d) 13 /* change cost to all dest's via neighbor v by d */ 14 /* note: d could be positive or negative */ 15 for all destinations y: D (y,V) = D (y,V) + d 16 17 else if (update received from V wrt destination Y) 18 /* shortest path from V to some Y has changed */ 19 /* V has sent a new value for its min DV(Y,w) */ 20 /* call this received new value is "newval" */ 21 for the single destination y: D (Y,V) = c(X,V) + newval 22 23 if we have a new min D (Y,w)for any destination Y 24 send new value of min D (Y,w) to all neighbors 25 26 forever
w
XX
XX
X
w
w
4: Network Layer 4a-21
Distance Vector Algorithm: example
X Z12
7
Y
4: Network Layer 4a-22
Distance Vector Algorithm: example
X Z72
1
Y
D (Y,Z)X
c(X,Z) + min {D (Y,w)}w=
= 7+1 = 8
Z
D (Z,Y)X
c(X,Y) + min {D (Z,w)}w=
= 2+1 = 3
Y
4: Network Layer 4a-23
Distance Vector: link cost changes
Link cost changes: node detects local link cost
change updates distance table (line 15) if cost change in least cost path,
notify neighbors (lines 23,24)
X Z14
50
Y1
algorithmterminates“good
news travelsfast”
4: Network Layer 4a-24
Distance Vector: link cost changes
Link cost changes: good news travels fast bad news travels slow -
“count to infinity” problem! X Z14
50
Y60
algorithmcontinues
on!
4: Network Layer 4a-25
Distance Vector: poisoned reverse
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) will this completely solve count to infinity
problem? X Z
14
50
Y60
algorithmterminates
4: Network Layer 4a-26
Comparison of LS and DV algorithms
Message complexity LS: with n nodes, E links,
O(nE) msgs sent each DV: exchange between
neighbors only convergence time varies
Speed of Convergence LS: O(n**2) algorithm
requires O(nE) msgs 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
4: Network Layer 4a-27
Hierarchical Routing
scale: with 50 million destinations:
can’t store all dest’s in routing tables!
routing table exchange would swamp links!
administrative autonomy
internet = network of networks
each network admin may want to control routing in its own network
Our routing study thus far - idealization all routers identical network “flat”… not true in practice
4: Network Layer 4a-28
Hierarchical Routing
aggregate routers into regions, “autonomous systems” (AS)
routers in same AS run same routing protocol “inter-AS” routing
protocol routers in different AS
can run different inter-AS routing protocol
special routers in AS run inter-AS routing
protocol with all other routers in AS
also responsible for routing to destinations outside AS run intra-AS routing
protocol with other gateway routers
gateway routers
4: Network Layer 4a-29
Intra-AS and Inter-AS routing
Gateways:•perform inter-AS routing amongst themselves•perform intra-AS routers with other routers in their AS
inter-AS, intra-AS routing in
gateway A.c
network layer
link layer
physical layer
a
b
b
aaC
A
Bd
A.a
A.c
C.bB.a
cb
c
4: Network Layer 4a-30
Intra-AS and Inter-AS routing
Host h2
a
b
b
aaC
A
Bd c
A.a
A.c
C.bB.a
cb
Hosth1
Intra-AS routingwithin AS A
Inter-AS routingbetween A and B
Intra-AS routingwithin AS B
We’ll examine specific inter-AS and intra-AS Internet routing protocols shortly
4: Network Layer 4a-31
The Internet Network layer
routingtable
Host, router network layer functions:
Routing protocols•path selection•RIP, OSPF, BGP
IP protocol•addressing conventions•datagram format•packet handling conventions
ICMP protocol•error reporting•router “signaling”
Transport layer: TCP, UDP
Link layer
physical layer
Networklayer
4: Network Layer 4a-32
IP Addressing IP address: 32-bit
identifier for host, router interface
interface: connection between host, router and physical link router’s typically have
multiple interfaces host may have
multiple interfaces IP addresses
associated with interface, not host, router
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.1 = 11011111 00000001 00000001 00000001
223 1 11
4: Network Layer 4a-33
IP Addressing IP address:
network part (high order bits)
host part (low order bits)
What’s a network ? (from IP address perspective) device interfaces with
same network part of IP address
can physically reach each other without intervening router
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
network consisting of 3 IP networks(for IP addresses starting with 223, first 24 bits are network address)
LAN
4: Network Layer 4a-34
IP AddressingHow to find the
networks? Detach each
interface from router, host
create “islands of isolated networks
223.1.1.1
223.1.1.3
223.1.1.4
223.1.2.2223.1.2.1
223.1.2.6
223.1.3.2223.1.3.1
223.1.3.27
223.1.1.2
223.1.7.0
223.1.7.1223.1.8.0223.1.8.1
223.1.9.1
223.1.9.2
Interconnected system consisting
of six networks
4: Network Layer 4a-35
IP Addresses
0network host
10 network host
110 network host
1110 multicast address
A
B
C
D
class
1.0.0.0 to127.255.255.255
128.0.0.0 to191.255.255.255
192.0.0.0 to239.255.255.255
240.0.0.0 to247.255.255.255
32 bits
4: Network Layer 4a-36
Getting a datagram from source to dest.
IP datagram:
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
BE
miscfields
sourceIP addr
destIP addr data
datagram remains unchanged, as it travels source to destination
addr fields of interest here
Dest. Net. next router Nhops
223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2
routing table in A
4: Network Layer 4a-37
Getting a datagram from source to dest.
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
BE
Starting at A, given IP datagram addressed to B:
look up net. address of B find B is on same net. as A link layer will send datagram
directly to B inside link-layer frame B and A are directly connected
Dest. Net. next router Nhops
223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2
miscfields 223.1.1.1223.1.1.3 data
4: Network Layer 4a-38
Getting a datagram from source to dest.
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
BE
Dest. Net. next router Nhops
223.1.1 1223.1.2 223.1.1.4 2223.1.3 223.1.1.4 2
Starting at A, dest. E: look up network address of E E on different network
A, E not directly attached routing table: next hop router
to E is 223.1.1.4 link layer sends datagram to
router 223.1.1.4 inside link-layer frame
datagram arrives at 223.1.1.4 continued…..
miscfields 223.1.1.1223.1.2.3 data
4: Network Layer 4a-39
Getting a datagram from source to dest.
223.1.1.1
223.1.1.2
223.1.1.3
223.1.1.4 223.1.2.9
223.1.2.2
223.1.2.1
223.1.3.2223.1.3.1
223.1.3.27
A
BE
Arriving at 223.1.4, destined for 223.1.2.2
look up network address of E E on same network as
router’s interface 223.1.2.9 router, E directly
attached link layer sends datagram to
223.1.2.2 inside link-layer frame via interface 223.1.2.9
datagram arrives at 223.1.2.2!!! (hooray!)
miscfields 223.1.1.1223.1.2.3 data network router Nhops interface
223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9
223.1.3 - 1 223.1.3.27
Dest. next
4: Network Layer 4a-40
IP datagram format
ver length
32 bits
data (variable length,typically a TCP
or UDP segment)
16-bit identifier
Internet checksum
time tolive
32 bit source IP address
IP protocol versionnumber
header length (bytes)
max numberremaining hops
(decremented at each router)
forfragmentation/reassembly
total datagramlength (bytes)
upper layer protocolto deliver payload to
head.len
type ofservice
“type” of data flgsfragment
offsetupper layer
32 bit destination IP address
Options (if any) E.g. timestamp,record routetaken, pecifylist of routers to visit.
4: Network Layer 4a-41
IP Fragmentation and Reassembly
network links have MTU (max.transfer size) - largest possible link-level frame. different link types,
different MTUs large IP datagram divided
(“fragmented”) within net one datagram
becomes several datagrams
“reassembled” only at final destination
IP header bits used to identify, order related fragments
fragmentation: in: one large datagramout: 3 smaller datagrams
reassembly
4: Network Layer 4a-42
IP Fragmentation and Reassembly
ID=x
offset=0
fragflag=0
length=4000
ID=x
offset=0
fragflag=1
length=1500
ID=x
offset=1480
fragflag=1
length=1500
ID=x
offset=2960
fragflag=0
length=1040
One large datagram becomesseveral smaller datagrams
4: Network Layer 4a-43
ICMP: Internet Control Message Protocol
used by hosts, routers, gateways to communication network-level information error reporting:
unreachable host, network, port, protocol
echo request/reply (used by ping)
network-layer “above” IP: ICMP msgs carried in
IP datagrams ICMP message: type,
code plus first 8 bytes of IP datagram causing error
Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header
4: Network Layer 4a-44
Routing in the Internet
The Global Internet consists of Autonomous Systems (AS) interconnected with eachother:
Stub AS: small corporation Multihomed AS: large corporation (no
transit) Transit AS: provider
Two level routing: Intra-AS: administrator is responsible
for choice Inter-AS: unique standard
4: Network Layer 4a-45
Internet AS Hierarchy
4: Network Layer 4a-46
Intra-AS Routing
Also known as Interior Gateway Protocol (IGP) Most common IGPs:
RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco
propr.)
4: Network Layer 4a-47
RIP ( Routing Info Protocol)
Distance vector type scheme Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops) Distance vector: exchanged every 30 sec via a
Response Message (also called Advertisement) Each Advertisement contains up to 25 destination nets
4: Network Layer 4a-48
RIP
4: Network Layer 4a-49
RIP
destination network next router number of hops to destination 1 A 2
20 B 2 30 B 7
10 -- 1…. …. ....
4: Network Layer 4a-50
RIP: Link Failure and Recovery
If no advertisement heard after 180 sec, neighbor/link dead
Routes via the neighbor are invalidated; new advertisements sent to neighbors
Neighbors in turn send out new advertisements if their tables changed
Link failure info quickly propagates to entire net Poison reverse used to prevent ping-pong loops (infinite
distance = 16 hops)
4: Network Layer 4a-51
RIP Table processing
RIP routing tables managed by an application process called route-d (demon)
advertisements encapsulated in UDP packets (no reliable delivery required; advertisements are periodically repeated)
4: Network Layer 4a-52
RIP Table processing
4: Network Layer 4a-53
RIP Table example
Destination Gateway Flags Ref Use Interface -------------------- -------------------- ----- ----- ------ --------- 127.0.0.1 127.0.0.1 UH 0 26492 lo0 192.168.2. 192.168.2.5 U 2 13 fa0 193.55.114. 193.55.114.6 U 3 58503 le0 192.168.3. 192.168.3.5 U 2 25 qaa0 224.0.0.0 193.55.114.6 U 3 0 le0 default 193.55.114.129 UG 0 143454
4: Network Layer 4a-54
RIP Table example (cont)
RIP Table example (at router giroflee):
Three attached class C networks (LANs) Router only knows routes to attached LANs Default router used to “go up” Route multicast address: 224.0.0.0 Loopback interface (for debugging)
4: Network Layer 4a-55
OSPF (Open Shortest Path First)
“open”: publicly available uses the Link State algorithm (ie, LS packet
dissemination; topology map at each node; route computation using Dijkstra’s alg)
OSPF advertisement carries one entry per neighbor router
advertisements disseminated to ENTIRE Autonomous System (via flooding)
4: Network Layer 4a-56
OSPF “advanced” features (not in RIP)
Security: all OSPF messages are authenticated (to prevent malicious intrusion); TCP connections used
Multiple same-cost paths allowed (only one path in RIP)
For each link, multiple cost metrics for different TOS (eg, 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
4: Network Layer 4a-57
Hierarchical OSPF
4: Network Layer 4a-58
Hierarchical OSPF
Two level hierarchy: local area and backbone Link state advertisements do not leave
respective areas Nodes in each area have detailed area
topology; they only know direction (shortest path) to networks in other areas
Area Border routers “summarize” distances to networks in the area and advertise them to other Area Border routers
Backbone routers run an OSPF routing alg limited to the backbone
Boundary routers connect to other ASs
4: Network Layer 4a-59
IGRP (Interior Gateway Routing Protocol)
CISCO proprietary; successor of RIP (mid 80’s) Distance Vector, like RIP several cost metrics (delay, bandwidth,
reliability, load etc) uses TCP to exchange routing updates routing tables exchanged only when costs
change Loop free routing achieved by using a
Distributed Updating Alg. (DUAL) based on diffused computation
In DUAL, after a distance increase, the routing table is frozen until all affected nodes have learned of the change
4: Network Layer 4a-60
Inter-AS routing
4: Network Layer 4a-61
Inter-AS routing (cont)
BGP (Border Gateway Protocol): the de facto standard
Path Vector protocol: and extension of Distance Vector
Each Border Gateway broadcast to neighbors (peers) the entire path (ie, sequence of AS’s) to destination
For example, Gwy X may store the following path to destination Z:
Path (X,Z) = X,Y1,Y2,Y3,…,Z
4: Network Layer 4a-62
Inter-AS routing (cont)
Now, suppose Gwy X send its path to peer Gwy W Gwy W may or may not select the path offered by Gwy
X, because of cost, policy or loop prevention reasons If Gwy W selects the path advertised by Gwy X, then: Path (W,Z) = w, Path (X,Z)Note: path selection based not so much on cost (eg,# ofAS hops), but mostly on administrative and policy issues(eg, do not route packets through competitor’s AS)
4: Network Layer 4a-63
Inter-AS routing (cont)
Peers exchange BGP messages using TCP OPEN msg opens TCP connection to peer and
authenticates sender UPDATE msg advertises new path (or
withdraws old) KEEPALIVE msg keeps connection alive in
absence of UPDATES; it also serves as ACK to an OPEN request
NOTIFICATION msg reports errors in previous msg; also used to close a connection
4: Network Layer 4a-64
Address Management
As Internet grows, we run out of addresses Solution (a): subnetting. Eg, Class B Host
field (16bits) is subdivided into <subnet;host> fields
Solution (b): CIDR (Classless Inter Domain Routing): assign block of contiguous Class C addresses to the same organization; these addresses all share a common prefix
repeated “aggregation” within same provider leads to shorter and shorter prefixes
CIDR helps also routing table size and processing: Border Gwys keep only prefixes and find “longest prefix” match
4: Network Layer 4a-65
Why different Intra- and Inter-AS routing ?
Policy: Inter is concerned with policies (which provider we must select/avoid, etc). Intra is contained in a single organization, so, no policy decisions necessary
Scale: Inter provides an extra level of routing table size and routing update traffic reduction above the Intra layer
Performance: Intra is focused on performance metrics; needs to keep costs low. In Inter it is difficult to propagate performance metrics efficiently (latency, privacy etc). Besides, policy related information is more meaningful.
We need BOTH!
4: Network Layer 4a-66
Router Architecture Overview
Router main functions: routing algorithms and protocols processing, switching datagrams from an incoming link to an outgoing link
Router Components
4: Network Layer 4a-67
Input Ports
Decentralized switching: perform routing table lookup using a copy of the node routing table stored in the port memory
Goal is to complete input port processing at ‘line speed’, ie processing time =< frame reception time (eg, with 2.5 Gbps line, 256 bytes long frame, router must perform about 1 million routing table lookups in a second)
Queuing occurs if datagrams arrive at rate higher than can be forwarded on switching fabric
4: Network Layer 4a-68
Speeding Up Routing Table Lookup
Table is stored in a tree structure to facilitate binary search
Content Addressable Memory (associative memory), eg Cisco 8500 series routers
Caching of recently looked-up addresses Compression of routing tables
4: Network Layer 4a-69
Switching Fabric
4: Network Layer 4a-70
Switching Via Memory
First generation routers: packet is copied under system’s (single) CPU control; speed limited by Memory bandwidth. For Memory speed of B packet/sec or pps, throughput is B/2 pps
InputPort
OutputPort
Memory
System Bus
• Modern routers: input ports with CPUs that implement output port lookup, and store packets in appropriate locations (= switch) in a shared Memory; eg Cisco Catalyst 8500 switches
4: Network Layer 4a-71
Switching Via Bus
Input port processors transfer a datagram from input port memory to output port memory via a shared bus
Main resource contention is over the bus; switching is limited by bus speed
Sufficient speed for access and enterprise routers (not regional or backbone routers) is provided by a Gbps bus; eg Cisco 1900 which has a 1 Gbps bus
4: Network Layer 4a-72
Switching Via An Interconnection Network
Used to overcome bus bandwidth limitations Banyan networks and other interconnection networks were
initially developed to connect processors in a multiprocessor computer system; used in Cisco 12000 switches provide up to 60 Gbps through the interconnection network
Advanced design incorporates fragmenting a datagram into fixed length cells and switch the cells through the fabric; + better sharing of the switching fabric resulting in higher switching speed
4: Network Layer 4a-73
Output Ports
Buffering is required to hold datagrams whenever they arrive from the switching fabric at a rate faster than the transmission rate
4: Network Layer 4a-74
Queuing At Input and Output Ports Queues build up whenever there is a rate mismatch or
blocking. Consider the following scenarios: Fabric speed is faster than all input ports combined; more
datagrams are destined to an output port than other output ports; queuing occurs at output port
Fabric bandwidth is not as fast as all input ports combined; queuing may occur at input queues;
HOL blocking: fabric can deliver datagrams from input ports in parallel, except if datagrams are destined to same output port; in this case datagrams are queued at input queues; there may be queued datagrams that are held behind HOL conflict, even when their output port is available
4: Network Layer 4a-75
IPv6 Initial motivation is 32 bit address space is estimated to get
used up either by 2008 or 2018; opportunity for changes to achieve faster processing and provision of differentiated services
Packet Format: fixed header of 40 bytes + option; Fixed header fields:
Version: indicates IPv6 Priority: 4 bits, to give priority to certain packets within a
flow; values 0 to 7 for congestion-controlled traffic, while values 8 to 15 is for other traffic (eg constant bit rate)
Flow Label: intended to help with differentiating services based on flows, a flow is not strictly defined in IPv6 proposal, it can be traffic from a user who paid more, traffic that is real-time, etc.
Payload Length: 16 bit value identifying the number of bytes following the 40 bytes of the fixed IPv6 header
Next Header: same as Protocol field in IPv4, identifies higher layer protocol to process the contents (TCP or UDP, or?)
4: Network Layer 4a-76
IPv6 Header (Cont)
Hop Limit: same as TTL, still one byte! Source and Destination Addresses: 128
bits, with a new hierarchical structure (address can imply geographical location, not in IPv4); includes new type of address: anycast, delivery is to one of a number of destinations
4: Network Layer 4a-77
Other Changes from IPv4
Fragmentation: none provided, router which has a packet longer than the maximum allowed on a the next hop drops the packet, and sends an ICMP message “Packet Too Big” to the packet source; reduces processing time of packets
Checksum: removed entirely to reduce processing time at each hop
Options: Options are allowed and indicated by the header field “Next Header”, the content of this field indicates the higher level protocol or the existence of an option after the 40 bytes IPv6 header
ICMPv6: new version of ICMP, with additional message types, eg “Packet Too Big”; and group management function for multicast groups (Under IPv4 done by the protocol Internet Group Management Protocol IGMP to be discussed shortly)
4: Network Layer 4a-78
Transition From IPv4 To IPv6
During the transition, not all routers will be upgraded to IPv6; How will the network operate?
Two proposed approaches: Dual Stack and Tunneling
Dual Stack: Some routers with dual stack (v6, v4); others are only
v4 routers Dual stack routers translate the packet to v4 packet if
the next router is v4 only DNS can be used to determine whether a router is dual
stack or not Some info and v6 features will be lost if a packet has
to go through any v4 only router; eg Flow Identification
4: Network Layer 4a-79
Dual Stack Approach
4: Network Layer 4a-80
Tunneling Routers are as before v4/v6 or v4 only A v4/v6 router “encapsulates” the IPv6 packet inside
an IPv4 envelop before communication to a v4 only router
A v4/v6 router receiving an encapsulated packet from a “tunnel”, remove the envelop and forwards the IPv6 to next router if the next router is v4/v6 capable
4: Network Layer 4a-81
Multicast Routing
Multicast: delivery of same packet to a group of receivers
Multicasting is becoming increasingly popular in the Internet (video on demand; whiteboard; interactive games)
Multiple unicast vs multicast
4: Network Layer 4a-82
Multicast Group Address
M-cast group address “delivered” to all receivers in the group
Internet uses Class D for m-cast M-cast address distribution etc.
managed by IGMP Protocol
4: Network Layer 4a-83
IGMP Protocol
IGMP (Internet Group Management Protocol) operates between Router and local Hosts, typically attached via a LAN (e.g., Ethernet)
Router queries the local Hosts for m-cast group membership info
Router “connects” active Hosts to m-cast tree via m-cast protocol
Hosts respond with membership reports: actually, the first Host which responds (at random) speaks for all
Host issues “leave-group” mssg to leave; this is optional since router periodically polls anyway (soft state concept)
4: Network Layer 4a-84
IGMP message types
GMP Message type Sent by Purpose
membership query: general router query for current active multicast groups
membership query: specific router query for specific m-cast group
membership report host host wants to join goup
leave group host host leaves the group
4: Network Layer 4a-85
The Multicast Tree problem
Problem: find the best (e.g., min cost) tree which interconnects all the members
4: Network Layer 4a-86
Multicast Tree options
GROUP SHARED TREE: single tree; the root is the “CORE” or the “Rendez Vous” point; all messages go through the CORE
SOURCE BASED TREE: each source is the root of its own tree connecting to all the members; thus N separate trees
4: Network Layer 4a-87
Group Shared Tree
Predefined CORE for given m-cast group (eg, posted on web page)
New members “join” and “leave” the tree with explicit join and leave control messages
Tree grows as new branches are “grafted” onto the tree
CBT (Core Based Tree) and PIM Sparse-Mode are Internet m-cast protocols based on GSTree
All packets go through the CORE
4: Network Layer 4a-88
Source Based Tree
Each source is the root of its own tree: the tree of shortest paths
Packets delivered on the tree using “reverse path forwarding” (RPF); i.e., a router accepts a packet originated by source S only if such packet is forwarded by the neighbor on the shortest path to S
In other words, m-cast packets are “forwarded” on paths which are the “reverse” of “shortest paths” to S
4: Network Layer 4a-89
Source-Based tree: DVMRP
DVMRP was the first m-cast protocol deployed on the Internet; used in Mbone (Multicast Backbone)
Initially, the source broadcasts the packet to ALL routers (using RPF)
Routers with no active Hosts (in this m-cast group) “prune” the tree; i.e., they disconnect themselves from the tree
Recursively, interior routers with no active descendents self-prune After timeout (2 hours in Internet) pruned branches “grow back”
Problems: only few routers are mcast-able; solution: tunnels
4: Network Layer 4a-90
PIM (Protocol Independent Multicast) PIM (Protocol Independent Multicast) is
becoming the de facto intra AS m-cast protocol standard
“Protocol Independent” because it can operate on different routing infrastructures (as a difference of DVMRP)
PIM can operate in two modes: PIM Sparse and PIM dense Mode.
Initially, members join the “Shared Tree” centered around a Randez Vous Point
Later, once the “connection” to the shared treee has been established, opportunities to connet DIRECTLY to the source are explored (thus establishing a partial Source Based tree
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