Network Layer 4-1 Chapter 4: network layer chapter goals: understand principles behind network layer services: network layer service models forwarding versus routing how a router works routing (path selection) broadcast, multicast instantiation, implementation in the Internet
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Q: but what happens if ranges don’t divide up so nicely?
Datagram forwarding table
Network Layer 4-18
Longest prefix matching
Destination Address Range
11001000 00010111 00010*** *********
11001000 00010111 00011000 *********
11001000 00010111 00011*** *********
otherwise
DA: 11001000 00010111 00011000 10101010
examples:DA: 11001000 00010111 00010110 10100001 which interface?
which interface?
when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.
longest prefix matching
Link interface
0
1
2
3
Network Layer 4-19
Datagram or VC network: why?
Internet (datagram) data exchange among
computers “elastic” service, no strict
timing req. many link types
different characteristics uniform service difficult
“smart” end systems (computers) can adapt, perform
control, error recovery simple inside
network, complexity at “edge”
ATM (VC) evolved from
telephony human conversation:
strict timing, reliability requirements
need for guaranteed service
“dumb” end systems telephones complexity inside
network
Comparison of Virtual-Circuit and Datagram Networks
Network Layer 4-21
4.1 introduction4.2 virtual circuit and
datagram networks4.3 what’s inside a
router4.4 IP: Internet Protocol
datagram format IPv4 addressing ICMP IPv6
4.5 routing algorithms link state distance vector hierarchical routing
4.6 routing in the Internet RIP OSPF BGP
4.7 broadcast and multicast routing
Chapter 4: outline
Network Layer 4-22
Router architecture overviewtwo key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link
high-seed switching
fabric
routing processor
router input ports router output ports
forwarding data plane (hardware)
routing, managementcontrol plane (software)
forwarding tables computed,pushed to input ports
Network Layer 4-23
linetermination
link layer
protocol(receive)
lookup,forwarding
queueing
Input port functions
decentralized switching: given datagram dest., lookup output
port using forwarding table in input port memory (“match plus action”)
goal: complete input port processing at ‘line speed’
queuing: if datagrams arrive faster than forwarding rate into switch fabric
physical layer:bit-level reception
data link layer:e.g., Ethernetsee chapter 5
switchfabric
Network Layer 4-24
Switching fabrics transfer packet from input buffer to
appropriate output buffer switching rate: rate at which packets
can be transfer from inputs to outputs often measured as multiple of input/output line rate N inputs: switching rate N times line rate desirable
three types of switching fabrics
memory
memory
bus crossbar
Network Layer 4-25
Switching via memoryfirst generation routers: traditional computers with switching under direct control of CPU packet copied to system’s memory speed limited by memory bandwidth (2 bus crossings per datagram)
inputport
(e.g.,Ethernet)
memory
outputport
(e.g.,Ethernet)
system bus
Network Layer 4-26
Switching via a bus
datagram from input port memory
to output port memory via a shared bus
bus contention: switching speed limited by bus bandwidth
32 Gbps bus, Cisco 5600: sufficient speed for access and enterprise routers
bus
Network Layer 4-27
Switching via interconnection network overcome bus bandwidth
limitations banyan networks, crossbar,
other interconnection nets initially developed to connect processors in multiprocessor
advanced design: fragmenting datagram into fixed length cells, switch cells through the fabric.
Cisco 12000: switches 60 Gbps through the interconnection network
crossbar
Network Layer 4-28
Output ports
buffering required when datagrams arrive from fabric faster than the transmission rate
scheduling discipline chooses among queued datagrams for transmission
linetermination
link layer
protocol(send)
switchfabric
datagrambuffer
queueing
Network Layer 4-29
Output port queueing
buffering when arrival rate via switch exceeds output line speed
queueing (delay) and loss due to output port buffer overflow!
at t, packets morefrom input to output
one packet time later
switchfabric
switchfabric
Network Layer 4-30
How much buffering?
RFC 3439 rule of thumb: average buffering equal to “typical” RTT (say 250 msec) times link capacity C e.g., C = 10 Gpbs link: 2.5 Gbit buffer
recent recommendation: with N flows, buffering equal to
RTT C.N
Network Layer 4-31
Input port queuing fabric slower than input ports combined ->
queueing may occur at input queues queueing delay and loss due to input buffer
overflow! Head-of-the-Line (HOL) blocking: queued
datagram at front of queue prevents others in queue from moving forward
output port contention:only one red datagram can
be transferred.lower red packet is blocked
switchfabric
one packet time later: green
packet experiences HOL
blocking
switchfabric
Network Layer 4-32
4.1 introduction4.2 virtual circuit and
datagram networks4.3 what’s inside a
router4.4 IP: Internet Protocol
datagram format IPv4 addressing ICMP IPv6
4.5 routing algorithms link state distance vector hierarchical routing
4.6 routing in the Internet RIP OSPF BGP
4.7 broadcast and multicast routing
Chapter 4: outline
Network Layer 4-33
The Internet network layer
forwardingtable
host, router network layer functions:
routing protocols• path selection• RIP, OSPF, BGP
IP protocol• addressing conventions• datagram format• packet handling conventions
“type” of data forfragmentation/reassemblymax number
remaining hops(decremented at
each router)
e.g. timestamp,record routetaken, specifylist of routers to visit.
how much overhead? 20 bytes of TCP 20 bytes of IP = 40 bytes + app
layer overhead
Network Layer 4-35
IP fragmentation, 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
…
…
Network Layer 4-36
ID=x
offset=0
fragflag=0
length=4000
ID=x
offset=0
fragflag=1
length=1500
ID=x
offset=185
fragflag=1
length=1500
ID=x
offset=370
fragflag=0
length=1040
one large datagram becomesseveral smaller datagrams
example: 4000 byte
datagram MTU = 1500
bytes1480 bytes in data field
offset =1480/8
IP fragmentation, reassembly
Network Layer 4-37
Chapter 4: Network Layer
4. 1 Introduction4.2 Virtual circuit and
datagram networks4.3 What’s inside a
router4.4 IP: Internet Protocol
Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer 4-38
IPv6 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
Network Layer 4-39
IPv6 Header (Cont)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 dataData: extension headers + upper layer payload
data
destination address(128 bits)
source address(128 bits)
payload len next hdr hop limitflow labelpriver
32 bits
Network Layer 4-40
Extension Header
Network Layer 4-41
Other Changes from IPv4
Checksum: removed entirely to reduce processing time at each hop
Options: allowed, but outside of header (in the “extension headers” data portion), pointed to by “Next Header” field. Upper layer protocol info is put into “Next Header” field in the last extension header
ICMPv6: new version of ICMP additional message types, e.g. “Packet Too
Big” multicast group management functions
Network Layer 4-42
Transition From IPv4 To IPv6 Not all routers can be upgraded
simultaneous no “flag days” How will the network operate with mixed IPv4
and IPv6 routers? Tunneling: IPv6 carried as payload in IPv4
datagram among IPv4 routers
Network Layer 4-43
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
Network Layer 4-44
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
Network Layer 4-45
Chapter 4: Network Layer
4. 1 Introduction4.2 Virtual circuit and
datagram networks4.3 What’s inside a
router4.4 IP: Internet Protocol
Datagram format IPv4 addressing ICMP IPv6
4.5 Routing algorithms Link state Distance Vector Hierarchical routing
4.6 Routing in the Internet RIP OSPF BGP
4.7 Broadcast and multicast routing
Network Layer 4-46
ICMP: Internet Control Message Protocol used by hosts & routers to
communicate 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 the header and 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
Network Layer 4-47
Traceroute and ICMP Source sends series of
UDP segments to dest first has TTL =1 second has TTL=2, etc. unlikely port number
When nth datagram arrives to nth router: router discards
“Send me anythingwith addresses beginning 200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7Internet
Organization 1
ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”
200.23.20.0/23Organization 2
...
...
Hierarchical addressing allows efficient advertisement of routing information:
Network Layer 4-59
Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
“Send me anythingwith addresses beginning 200.23.16.0/20”
200.23.16.0/23
200.23.18.0/23
200.23.30.0/23
Fly-By-Night-ISP
Organization 0
Organization 7Internet
Organization 1
ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”
200.23.20.0/23Organization 2
...
...
IP Addresses-Examples
Splitting an IP prefix into separate networks with subnetting.
A set of IP address assignments
IP Addresses-Examples
Aggregation of IP prefixes
IP Addresses-Examples
Longest matching prefix routing at the New York router.
IP Addresses-Examples
Network Layer 4-64
Exercise—IP Addressing (1)
[Leon-Garcia 8.6]
A host in an organization has an IP address 150.32.64.34 and a subnet mask 255.255.240.0. What is the address of this subnet? What is the range of IP addresses that a host can have on this subnet?
Network Layer 4-65
Exercise—IP Addressing (2)
[Leon-Garcia 8.12, 8.13]
Perform CIDR aggregation on the following /24 IP addresses: 128.56.24.0/24; 128.56.25.0/24; 128.56.26.0/24; 128.56.27.0/24And the following /24 IP addresses: 200.96.86.0/24; 200.96.87.0/24; 200.96.88.0/24; 200.96.89.0/24
Network Layer 4-66
Exercise—IP Addressing (3)
[Tanenbaum 5.39]
A network on the Internet has a subnet mask of 255.255.240.0. What is the maximum number of hosts it can handle?
Network Layer 4-67
Exercise—IP Addressing (4)
[Tanenbaum 5.41]
A router has just received the following new IP addresses: 57.6.96.0/21, 57.6.104.0/21, 57.6.112.0/21, and 57.6.120.0/21. If all of them use the same outgoing line, can they be aggregated? If so, to what? If not, why not?
Network Layer 4-68
Exercise—IP Addressing (5)
[Tanenbaum 5.43]
A router has the following CIDR entries in its routing table:
DHCP: Dynamic Host Configuration Protocol: dynamically get address from a server “plug-and-play”
Network Layer 4-70
DHCP: Dynamic Host Configuration Protocol
Goal: allow host to dynamically obtain its IP address from network server when it joins networkCan renew its lease on address in useAllows reuse of addresses (only hold address while connected and “on”)Support for mobile users who want to join network (more shortly)
DHCP overview: host broadcasts “DHCP discover” msg [optional] DHCP server responds with “DHCP offer” msg [optional] host requests IP address: “DHCP request” msg DHCP server sends address: “DHCP ack” msg
DHCP: more than IP addressDHCP can return more than just allocated
IP address on subnet: address of first-hop router for client name and IP address of DNS sever network mask (indicating network versus
host portion of address)
Network Layer 4-74
DHCP: example
connecting laptop needs its IP address, addr of first-hop router, addr of DNS server: use DHCP
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCPDHCP
DHCP request encapsulated in UDP, encapsulated in IP, encapsulated in 802.3 Ethernet
Ethernet frame broadcast (dest: FFFFFFFFFFFF) on LAN, received at router running DHCP server
Ethernet demuxed to IP demuxed, UDP demuxed to DHCP
168.1.1.1
Network Layer 4-75
DHCP server formulates DHCP ACK containing client’s IP address, IP address of first-hop router for client, name & IP address of DNS server
router(runs DHCP)
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCPUDP
IPEthPhy
DHCP
DHCP
DHCP
DHCP
DHCP
encapsulation of DHCP server, frame forwarded (broadcast) to client, demuxing up to DHCP at client
client now knows its IP address, name and IP address of DNS server, IP address of its first-hop router
DHCP: example
Network Layer 4-76
DHCP: Wireshark output (home LAN)
Message type: Boot Reply (2)Hardware type: EthernetHardware address length: 6Hops: 0Transaction ID: 0x6b3a11b7Seconds elapsed: 0Bootp flags: 0x0000 (Unicast)Client IP address: 192.168.1.101 (192.168.1.101)Your (client) IP address: 0.0.0.0 (0.0.0.0)Next server IP address: 192.168.1.1 (192.168.1.1)Relay agent IP address: 0.0.0.0 (0.0.0.0)Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)Server host name not givenBoot file name not givenMagic cookie: (OK)Option: (t=53,l=1) DHCP Message Type = DHCP ACKOption: (t=54,l=4) Server Identifier = 192.168.1.1Option: (t=1,l=4) Subnet Mask = 255.255.255.0Option: (t=3,l=4) Router = 192.168.1.1Option: (6) Domain Name Server Length: 12; Value: 445747E2445749F244574092; IP Address: 68.87.71.226; IP Address: 68.87.73.242; IP Address: 68.87.64.146Option: (t=15,l=20) Domain Name = "hsd1.ma.comcast.net."
reply
Message type: Boot Request (1)Hardware type: EthernetHardware address length: 6Hops: 0Transaction ID: 0x6b3a11b7Seconds elapsed: 0Bootp flags: 0x0000 (Unicast)Client IP address: 0.0.0.0 (0.0.0.0)Your (client) IP address: 0.0.0.0 (0.0.0.0)Next server IP address: 0.0.0.0 (0.0.0.0)Relay agent IP address: 0.0.0.0 (0.0.0.0)Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)Server host name not givenBoot file name not givenMagic cookie: (OK)Option: (t=53,l=1) DHCP Message Type = DHCP RequestOption: (61) Client identifier Length: 7; Value: 010016D323688A; Hardware type: Ethernet Client MAC address: Wistron_23:68:8a (00:16:d3:23:68:8a)Option: (t=50,l=4) Requested IP Address = 192.168.1.101Option: (t=12,l=5) Host Name = "nomad"Option: (55) Parameter Request List Length: 11; Value: 010F03062C2E2F1F21F92B 1 = Subnet Mask; 15 = Domain Name 3 = Router; 6 = Domain Name Server 44 = NetBIOS over TCP/IP Name Server ……
request
Network Layer 4-77
IP addressing: the last word...
Q: How does an ISP get block of addresses?
A: ICANN: Internet Corporation for Assigned Names and Numbers
allocates addresses manages DNS assigns domain names, resolves disputes
Network Layer 4-78
NAT: Network Address Translation
10.0.0.1
10.0.0.2
10.0.0.3
10.0.0.4
138.76.29.7
local network(e.g., home network)
10.0.0/24
rest ofInternet
Datagrams with source or destination in this networkhave 10.0.0/24 address for
source, destination (as usual)
All datagrams leaving localnetwork have same single source
NAT IP address: 138.76.29.7,different source port numbers
Network Layer 4-79
NAT: Network Address Translation
Motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP: just
one IP address for all devices can change addresses of devices in local network
without notifying outside world can change ISP without changing addresses of
devices in local network devices inside local net not explicitly
addressable, visible by outside world (a security plus).
Network Layer 4-80
NAT: Network Address Translation
16-bit port-number field: 60,000 simultaneous connections with a
single WAN-side address! NAT is controversial:
routers should only process up to layer 3 violates end-to-end argument
• NAT possibility must be taken into account by app designers, e.g., P2P applications
address shortage should instead be solved by IPv6
Network Layer 4-81
NAT: Network Address Translation
Implementation: NAT router must:
outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #). . . remote clients/servers will respond using (NAT IP
address, new port #) as destination addr.
remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair
incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table
Network Layer 4-82
NAT: Network Address Translation
10.0.0.1
10.0.0.2
10.0.0.3
S: 10.0.0.1, 3345D: 128.119.40.186, 80
1
10.0.0.4
138.76.29.7
1: host 10.0.0.1 sends datagram to 128.119.40.186, 80
4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345
Network Layer 4-83
Unregistered IP addresses
The sets of IP address used for private networks, i.e., networks not directly connected to Internet (e.g., home networks) Range 1: 10.0.0.0 to 10.255.255.255 Range 2: 172.16.0.0 to 172.31.255.255 Range 3: 192.168.0.0 to 192.168.255.255
(used in home LAN)
Network Layer 4-84
NAT traversal problem client wants to connect to
server with address 10.0.0.1 server address 10.0.0.1
local to LAN (client can’t use it as destination addr)
only one externally visible NATed address: 138.76.29.7
solution 1: statically configure NAT to forward incoming connection requests at given port to server e.g., (138.76.29.7, port
2500) always forwarded to 10.0.0.1 port 25000
10.0.0.1
10.0.0.4
NAT router
138.76.29.7
Client?
Network Layer 4-85
NAT traversal problem solution 2: Universal Plug
and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATed host to:learn public IP address
(138.76.29.7)add/remove port
mappings (with lease times)
i.e., automate static NAT port map configuration
10.0.0.1
10.0.0.4
NAT router
138.76.29.7
IGD
Network Layer 4-86
NAT traversal problem solution 3: relaying (used in Skype)
NATed client establishes connection to relay
External client connects to relay relay bridges packets between to
connections
138.76.29.7
Client
10.0.0.1
NAT router
1. connection torelay initiatedby NATed host
2. connection torelay initiatedby client
3. relaying established
Network Layer 4-87
Traffic Management Concerned with delivery of QoS to the end user and with efficient use of network resources Can be classified into packet level (queue scheduling and management) and flow level (congestion control)
Network Layer 4-88
Queue Scheduling and Mgmt FIFO Priority Queuing Round Robin Fair Queuing Weighted Fair Queuing (WFQ) Random Early Detection (RED)-mgmt
Network Layer 4-89
Congestion Control Algorithms Open-loop
Admission control-usually used in virtual circuit networks such as ATM Traffic shaping and policing
I-units of packet time for each incoming packet, L-depends on traffic bustiness. Here I=4 and L=6 packet times.
04:53 PM
Token Bucket Algorithm
Traffic shapersLeaky bucket traffic shaper
Token bucket traffic shaper
Locations of traffic policing and shaping
Choke Packets
Choke packets: packets carrying warning message for congestion
Node monitors outgoing link utilization U and updates its average utilization based on the instantaneous line utilization f: Unew = a Uold +(1-a) f where a is the forgetting factor, determining how fast the node forgets recent history
Choke Packets (cont)
Choke packet generation: If Unew > Uth, a warning state is on, the router generates a choke packet, and sends it to the source host
Source reduces the traffic rate by adjusting the policy parameters such as window size or leaky bucket output rate Variations: (1) use multiple thresholds; (2)
use queue length or buffer utilization
Backpressure
Choke packets is slow in resolving congestion
Hop-by-hop choke packets When congested (same method as in choke
packets), the choke packet will take effect at every hop it passes through, all the nodes on the path back to the source will all slow down
The net effect: quick relief at the point of congestion
Load Shedding
When all congestion controls fail, use load shedding: throw away packets whenever you could not handle
Real-time traffic such as voice is delay sensitive, each packet has a delay bound, each router may check whether a packet is on-time or not, scheduling may take this time constraint into the congestion control design
Control the delay variation to maintain the quality, e.g., for video
Network Layer 4-100
4.1 introduction4.2 virtual circuit and
datagram networks4.3 what’s inside a
router4.4 IP: Internet Protocol
datagram format IPv4 addressing ICMP IPv6
4.5 routing algorithms link state distance vector hierarchical routing
4.6 routing in the Internet RIP OSPF BGP
4.7 broadcast and multicast routing
Chapter 4: outline
Network Layer 4-101
1
23
IP destination address in arriving packet’s header
key question: what is the least-cost path between u and z ?routing algorithm: algorithm that finds that least cost path
Network Layer 4-104
Routing algorithm classificationQ: 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
Network Layer 4-105
4.1 introduction4.2 virtual circuit and
datagram networks4.3 what’s inside a
router4.4 IP: Internet Protocol
datagram format IPv4 addressing ICMP IPv6
4.5 routing algorithms link state distance vector hierarchical routing
4.6 routing in the Internet RIP OSPF BGP
4.7 broadcast and multicast routing
Chapter 4: outline
Network Layer 4-106
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 dest.’s
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
Network Layer 4-107
Dijsktra’s Algorithm
1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N'
Network Layer 4-108
w3
4
v
x
u
5
37 4
y
8
z2
7
9
Dijkstra’s algorithm: example
Step N'D(v)
p(v)
012345
D(w)p(w)
D(x)p(x)
D(y)p(y)
D(z)p(z)
u ∞ ∞ 7,u 3,u 5,uuw ∞ 11,w 6,w 5,u
14,x 11,w 6,wuwxuwxv 14,x 10,v
uwxvy 12,y
notes: construct shortest path
tree by tracing predecessor nodes
ties can exist (can be broken arbitrarily)
uwxvyz
Network Layer 4-109
Dijkstra’s algorithm: another example
Step012345
N'u
uxuxy
uxyvuxyvw
uxyvwz
D(v),p(v)2,u2,u2,u
D(w),p(w)5,u4,x3,y3,y
D(x),p(x)1,u
D(y),p(y)∞
2,x
D(z),p(z)∞ ∞
4,y4,y4,y
u
yx
wv
z2
2
13
1
1
2
53
5
Network Layer 4-110
Dijkstra’s algorithm: example (2)
u
yx
wv
z
resulting shortest-path tree from u:
vx
y
w
z
(u,v)
(u,x)
(u,x)
(u,x)
(u,x)
destination link
resulting forwarding table in u:
Network Layer 4-111
Dijkstra’s algorithm, discussionalgorithm complexity: n nodes each iteration: need to check all nodes, w, not in N n(n+1)/2 comparisons: O(n2) more efficient implementations possible: O(nlogn)
oscillations possible: e.g., support link cost equals amount of carried
traffic:
A
D
C
B1 1+e
e0
e
1 1
0 0
initially
A
D
C
B
given these costs,find new routing….
resulting in new costs
2+e 0
001+e 1
A
D
C
B
given these costs,find new routing….
resulting in new costs
0 2+e
1+e10 0
A
D
C
B
given these costs,find new routing….
resulting in new costs
2+e 0
001+e 1
Network Layer 4-112
4.1 introduction4.2 virtual circuit and
datagram networks4.3 what’s inside a
router4.4 IP: Internet Protocol
datagram format IPv4 addressing ICMP IPv6
4.5 routing algorithms link state distance vector hierarchical routing
Distance vector: link cost changeslink cost changes: node detects local link cost
change updates routing info,
recalculates distance vector
if DV changes, notify neighbors
“goodnews travelsfast”
x z14
50
y1
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.
Network Layer 4-121
Distance vector: link cost changeslink cost changes: node detects local link cost
change bad news travels slow -
“count to infinity” problem! 44 iterations before
algorithm stabilizes: see text
x z14
50
y60
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?
Network Layer 4-122
Comparison of LS and DV algorithmsmessage 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 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
Network Layer 4-123
4.1 introduction4.2 virtual circuit and
datagram networks4.3 what’s inside a
router4.4 IP: Internet Protocol
datagram format IPv4 addressing ICMP IPv6
4.5 routing algorithms link state distance vector hierarchical routing
4.6 routing in the Internet RIP OSPF BGP
4.7 broadcast and multicast routing
Chapter 4: outline
Network Layer 4-124
Hierarchical routing
scale: with 600 million destinations:
can’t store all dest’s in routing tables!
routing table exchange would swamp links!
administrative autonomy
internet = network of networks
each network admin may want to control routing in its own network
our routing study thus far - idealization
all routers identical network “flat”… not true in practice
Network Layer 4-125
aggregate routers into regions, “autonomous systems” (AS)
routers in same AS run same routing protocol “intra-AS” routing
protocol routers in different
AS can run different intra-AS routing protocol
gateway router: at “edge” of its own AS has link to router in
another AS
Hierarchical routing
Network Layer 4-126
3b
1d
3a
1c2aAS3
AS1
AS21a
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 sets
entries for internal dests
inter-AS & intra-AS sets entries for external dests
Network Layer 4-127
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
3c
3a
AS1
1c1a
1d1b
2a2c
2b
othernetworks
othernetworks
Network Layer 4-128
Example: setting forwarding table in router 1d
suppose AS1 learns (via inter-AS protocol) that subnet x reachable via AS3 (gateway 1c), but not via AS2 inter-AS protocol propagates reachability info to
all internal routers router 1d determines from intra-AS routing info that
its interface I is on the least cost path to 1c installs forwarding table entry (x,I)
AS3
AS2
3b
3c
3a
AS1
1c1a
1d1b
2a2c
2b
othernetworks
othernetworks
x…
Network Layer 4-129
Example: choosing among multiple ASes now suppose AS1 learns from inter-AS protocol
that subnet x is reachable from AS3 and from AS2.
to configure forwarding table, router 1d must determine which gateway it should forward packets towards for dest x this is also job of inter-AS routing protocol!
AS3
AS2
3b
3c
3a
AS1
1c1a
1d1b
2a2c
2b
othernetworks
othernetworks
x ……
…
?
Network Layer 4-130
learn from inter-AS protocol that subnet x is reachable via multiple gateways
use routing infofrom intra-AS
protocol to determinecosts of least-cost
paths to eachof the gateways
hot potato routing:choose the gateway
that has the smallest least cost
determine fromforwarding table the interface I that leads
to least-cost gateway. Enter (x,I) in
forwarding table
Example: choosing among multiple ASes now suppose AS1 learns from inter-AS protocol
that subnet x is reachable from AS3 and from AS2.
to configure forwarding table, router 1d must determine towards which gateway it should forward packets for dest x this is also job of inter-AS routing protocol!
hot potato routing: send packet towards closest of two routers.
Network Layer 4-131
4.1 introduction4.2 virtual circuit and
datagram networks4.3 what’s inside a
router4.4 IP: Internet Protocol
datagram format IPv4 addressing ICMP IPv6
4.5 routing algorithms link state distance vector hierarchical routing
4.6 routing in the Internet RIP OSPF BGP
4.7 broadcast and multicast routing
Chapter 4: outline
Network Layer 4-132
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 IGRP: Interior Gateway Routing
Protocol (Cisco proprietary)
Network Layer 4-133
RIP ( Routing Information Protocol)
included in BSD-UNIX distribution in 1982 distance vector algorithm
distance metric: # hops (max = 15 hops), each link has cost 1 DVs exchanged with neighbors every 30 sec in response message (aka
advertisement) each advertisement: list of up to 25 destination subnets (in IP addressing
sense)
DC
BA
u v
w
x
yz
subnet hops u 1 v 2 w 2 x 3 y 3 z 2
from router A to destination subnets:
Network Layer 4-134
RIP: example
destination subnet next router # hops to dest
w A 2y B 2
z B 7x -- 1…. …. ....
routing table in router D
w x yz
A
C
D B
Network Layer 4-135
w x yz
A
C
D B
destination subnet next router # hops to dest
w A 2y B 2
z B 7x -- 1…. …. ....
routing table in router D
A 5
dest next hops w - 1 x - 1 z C 4 …. … ...
A-to-D advertisement
RIP: example
Network Layer 4-136
RIP: link failure, recovery if no advertisement heard after 180 sec -->
neighbor/link declared dead routes via neighbor invalidated new advertisements sent to neighbors neighbors in turn send out new advertisements
(if tables changed) link failure info eventually propagates to entire
net poison reverse used to prevent ping-pong
loops (infinite distance = 16 hops)
Network Layer 4-137
RIP table processing
RIP routing tables managed by application-level process called route-d (daemon)
advertisements sent in UDP packets, periodically repeated
physical
link
network forwarding (IP) table
transport (UDP)
routed
physical
link
network (IP)
transprt (UDP)
routed
forwardingtable
Network Layer 4-138
OSPF (Open Shortest Path First) “open”: publicly available uses link state algorithm
LS packet dissemination topology map at each node route computation using Dijkstra’s algorithm
OSPF advertisement carries one entry per neighbor
advertisements flooded to entire AS carried in OSPF messages directly over IP
(rather than TCP or UDP IS-IS routing protocol: nearly identical to
OSPF
Network Layer 4-139
OSPF “advanced” features (not in RIP) security: all OSPF messages authenticated