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Network Layer: IP (contd.), ICMP, Overview of Network Control Plane and Routing Algorithms
UG3 Computer Communications & Networks(COMN)Mahesh Marina
Slides thanks to Myungjin Lee and copyright of Kurose and Ross
IP addresses: how to get one?
Q: how does network get subnet part of IP addr?A: gets allocated portion of its provider ISP�s address
space
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ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20
Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23
... ….. …. ….Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23
Hierarchical addressing: route aggregation
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�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:
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
...
...
Hierarchical addressing: more specific routes
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IP addressing: the last word...
Q: how does an ISP get block of addresses?A: ICANN: Internet Corporation for Assigned
Names and Numbers http://www.icann.org/– allocates addresses– manages DNS– assigns domain names, resolves disputes
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Network Address Translation (NAT)
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NAT: network address translation
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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
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)
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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
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
NAT translation tableWAN side addr LAN side addr
138.76.29.7, 5001 10.0.0.1, 3345…… ……
S: 128.119.40.186, 80 D: 10.0.0.1, 3345
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S: 138.76.29.7, 5001D: 128.119.40.186, 80
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2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table
S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3
3: reply arrivesdest. address:138.76.29.7, 5001
4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345
NAT: network address translation
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NAT Discussion
• 16-bit port-number field: – 60,000 simultaneous connections with a single LAN-side
address!• NAT is controversial:– routers should only process up to layer 3– address shortage should instead be solved by IPv6– violates end-to-end argument
• NAT possibility must be taken into account by app designers, e.g., P2P applications
– NAT traversal: what if client wants to connect to server behind NAT?
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Internet Control Message Protocol
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ICMP: internet control message protocol
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• 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 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
Traceroute and ICMP
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• when ICMP messages arrives, source records RTTs
• source sends series of UDP segments to dest– first set has TTL =1– second set has TTL=2, etc.– unlikely port number
• when nth set of datagrams arrives to nth router:– router discards datagrams– and sends source ICMP
messages (type 11, code 0)– ICMP messages includes
name of router & IP address
stopping criteria:v UDP segment eventually
arrives at destination hostv destination returns ICMP �port unreachable�message (type 3, code 3)
v source stops
3 probes
3 probes
3 probes
IPv6
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IPv6: motivation
• initial motivation: 32-bit address space soon to be completely allocated
• additional motivation:– header format helps speed processing/forwarding– header changes to facilitate QoS
IPv6 datagram format: – fixed-length 40 byte header– no fragmentation allowed
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IPv6 datagram format
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priority: identify priority among datagrams in flowflow Label: identify datagrams in same �flow.�
(concept of �flow� not well defined)next header: identify upper layer protocol for data
data
destination address(128 bits)
source address(128 bits)
payload len next hdr hop limitflow labelpriver
32 bits
Other changes from IPv4
• checksum: removed entirely to reduce processing time at each hop
• options: allowed, but outside of header, indicated by �Next Header� field
• ICMPv6: new version of ICMP– additional message types, e.g. �Packet Too Big�– multicast group management functions
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Transition from IPv4 to IPv6
• not all routers can be upgraded simultaneously– no �flag days�– how will network operate with mixed IPv4 and IPv6
routers? • tunneling: IPv6 datagram carried as payload in IPv4
datagram among IPv4 routers
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IPv4 source, dest addr IPv4 header fields
IPv4 datagramIPv6 datagram
IPv4 payload
UDP/TCP payloadIPv6 source dest addr
IPv6 header fields
Tunneling
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physical view:IPv4 IPv4
A B
IPv6 IPv6
E
IPv6 IPv6
FC D
logical view:IPv4 tunnel
connecting IPv6 routers E
IPv6 IPv6
FA B
IPv6 IPv6
flow: Xsrc: Adest: F
data
A-to-B:IPv6
Flow: XSrc: ADest: F
data
src:Bdest: E
B-to-C:IPv6 inside
IPv4
E-to-F:IPv6
flow: Xsrc: Adest: F
data
B-to-C:IPv6 inside
IPv4
Flow: XSrc: ADest: F
data
src:Bdest: E
physical view:A B
IPv6 IPv6
E
IPv6 IPv6
FC D
logical view:IPv4 tunnel
connecting IPv6 routers E
IPv6 IPv6
FA B
IPv6 IPv6
Tunneling
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IPv4 IPv4
IPv6: adoption
• Google: 8% of clients access services via IPv6• NIST: 1/3 of all US government domains are IPv6
capable
• Long (long!) time for deployment, use–20 years and counting!–think of application-level changes in last 20 years: WWW, Facebook, streaming media, Skype, …–Why?
Network Layer: Control Plane
Network-layer functions
• forwarding: move packets from router�s input to appropriate router output
data plane
control plane
Two approaches to structuring network control plane:§ per-router control (traditional) ç our initial focus§ logically centralized control (software defined networking)
Recall: two network-layer functions:
§ routing: determine route taken by packets from source to destination
Per-router control plane
RoutingAlgorithm
Individual routing algorithm components in each and every router interact in the control plane
dataplane
controlplane
4.1 • OVERVIEW OF NETWORK LAYER 309
tables. In this example, a routing algorithm runs in each and every router and both forwarding and routing functions are contained within a router. As we’ll see in Sec-tions 5.3 and 5.4, the routing algorithm function in one router communicates with the routing algorithm function in other routers to compute the values for its forward-ing table. How is this communication performed? By exchanging routing messages containing routing information according to a routing protocol! We’ll cover routing algorithms and protocols in Sections 5.2 through 5.4.
The distinct and different purposes of the forwarding and routing functions can be further illustrated by considering the hypothetical (and unrealistic, but technically feasible) case of a network in which all forwarding tables are configured directly by human network operators physically present at the routers. In this case, no routing protocols would be required! Of course, the human operators would need to interact with each other to ensure that the forwarding tables were configured in such a way that packets reached their intended destinations. It’s also likely that human configu-ration would be more error-prone and much slower to respond to changes in the net-work topology than a routing protocol. We’re thus fortunate that all networks have both a forwarding and a routing function!
Values in arrivingpacket’s header
1
23
Local forwardingtable
header
0100011001111001
1101
3221
output
Control plane
Data plane
Routing algorithm
Figure 4.2 ♦ Routing algorithms determine values in forward tables
M04_KURO4140_07_SE_C04.indd 309 11/02/16 3:14 PM
1
2
0111
values in arriving packet header
3
dataplane
controlplane
Logically centralized control planeA distinct (typically remote) controller interacts with local control agents (CAs)
Remote Controller
CA
CA CA CA CA
1
2
0111
3
values in arriving packet header
Routing protocols
Routing protocol goal: determine “good” paths (equivalently, routes), from sending hosts to receiving host, through network of routers• path: sequence of routers packets will traverse
in going from given initial source host to given final destination host
• “good”: least “cost”, “fastest”, “least congested”• routing: a “top-10” networking challenge!
1
23
IP destination address in arriving packet�s header
routing algorithm
local forwarding tabledest address output link
address-range 1address-range 2address-range 3address-range 4
3221
Interplay between routing, forwarding
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routing algorithm determinesend-end-path through network
forwarding table determineslocal forwarding at this router
u
yx
wv
z2
21
3
1
1
2
53
5
graph: G = (N,E)
N = set of routers = { u, v, w, x, y, z }
E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }
Graph abstraction
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Graph abstraction: costs
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u
yx
wv
z2
21
3
1
1
2
53
5 c(x,x�) = cost of link (x,x�)e.g., c(w,z) = 5
cost could always be 1, or inversely related to bandwidth,or related to congestion
cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)
key question: what is the least-cost path between u and z ?routing algorithm: algorithm that finds that least cost path
Routing algorithm classification
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Q: static or dynamic?static:• routes change slowly over
timedynamic: • routes change more
quickly– periodic update– in response to link
cost changes
Q: global or decentralized information?
global:• all routers have complete
topology, link cost info• �link state� algorithmsdecentralized: • router knows physically-
connected neighbors, link costs to neighbors
• iterative process of computation, exchange of info with neighbors
• �distance vector� algorithms
