Networks & Operating Systems Essentials
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Source: https://xkcd.com/742/
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Based on slides © 2017
NETWORK LAYER (L3)
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The Network Layer
• First end-to-end layer in the OSI reference model
– Responsibleforend-to-enddeliveryofdata:
• Across multiple link-layer hops and technologies
• Across multiple autonomous systems
– BuildinganInternet:asetofinterconnectednetworks
• An internet comprises a set of interconnected networks
– Eachnetworkadministeredseparately
• An autonomous system (AS)
• Making independent policy and technology choices
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The Network Layer
• Components of an internet
– Acommonend-to-endnetworkprotocol
• Provide a single seamless service to transport layer – Deliveryofdatapackets/provisioningofcircuits
– Addressingofendsystems
– Asetofgatewaydevices(a.k.a.routers)
• Implement the common network protocol
• Hide differences in link layer technologies
– Framing,addressing,flowcontrol,errordetectionandcorrection – Desiretoperformtheleastamountoftranslationnecessary
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The Internet
• The globally interconnected networks running the Internet Protocol (IP)
– 1965: Concept of packet switching
• (RAND, USA), (NPL, UK)
– 1969: Wide-area packet networks • ARPANET (US), CYCLADES (France)
– 1973: First non-US ARPANET sites • UCL
– 1974: Initial version of the Internet Protocol
– 1981: Access to ARPANET broadened to non-DARPA-funded sites
• NSF funds access for universities; production internetworking starts
– 1983: Network switched to IPv4
– 1992: Development of IPv6 starts
• Initial IETF IPng effort led by and
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The Internet
ARPA Network Map, December 1972
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Based on slides © 2017
THE INTERNET PROTOCOL (IPV4 AND IPV6)
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The Internet Protocol
• IP provides an abstraction layer
– Transport protocols and applications above
– Assorted data link technologies and physical links below
– A simple, best effort, connectionless, packet delivery service
– Addressing, routing, fragmentation and reassembly
• Basic concepts:
– Global inter-networking protocol
– Hour glass protocol stack
• Many transport & application layer protocols
• Single standard network layer protocol (IP)
– Packet switched network, best effort service
– Uniform network and host addressing
– Uniform end-to-end connectivity
(subject to firewall policy)
• Range of link-layer technologies supported
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The Internet Protocol
• IP Service Model: Best effort, connectionless, packet delivery
– Just send – no need to setup a connection first
– Network makes its best effort to deliver packets, but
• Time taken to transit the network may vary
• Packets may be lost, delayed, reordered, duplicated or corrupted
• The network discards packets it can’t deliver
– Easy to run over any type of link layer
– Fundamental service: can easily simulate a circuit over packets, but simulating packets over a circuit difficult
• Two versions of IP in use
– IPv4 – the current production Internet
– IPv6 – the next generation Internet
• Compared to IPv4: simpler header format, larger addresses, removes support for fragmentation, adds flow label
provides no guarantees
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Addressing
• Every network interface on every host is intended to have a unique address
– Hosts may change address over time to give illusion of privacy
– Addressable ≠ reachable: firewalls exist in both IPv4 and IPv6
• IPv4 addresses are 32 bits
– Example: 130.209.241.197
– Significant problems due to lack of IPv4 addresses → details later
• IPv6 addresses are 128 bits
– Example: 2001:4860:4860::8844
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Fragmentation (placeholder)
• Link layer has a maximum packet size (MTU)
– What is this?
– Why is it needed?
– How is it used?
– Is it a good solution?
(you can post answers on SlidoJ)
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Fragmentation
• Link layer has a maximum packet size (MTU) – Ethernet: 1500 bytes by default
• IPv4 routers will fragment packets that are larger than the MTU
– MF bit is set if more fragments follow: reconstruct using fragment offset and fragment identifier
– DF bit is set → routers shouldn’t fragment, must discard large packets
• IPv6 doesn’t support fragmentation
– Hard to implement for high rate links
– End-to-end principle
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Loop Protection
• Packets include a forwarding limit:
– Set to a non-zero value when the packet
is sent (typically 64 or 128)
– Each router that forwards the packet reduces this value by 1
– If zero is reached, packet is discarded
• Why is it needed?
– Stops packets circling forever if a network problem causes a loop
• Assumption: network diameter is smaller than initial value of forwarding limit
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Header Checksum
• IPv4 header contains a checksum to detect transmission errors
– Conceptually similar to link-layer checksum, although uses a different algorithm
– Protects the IP header only, not the payload data protected
• Payload data must be protected by upper layer protocol, if needed
– Isn’tthechecksumpartoftheIP header? (you can tell me what you think during the Q&A sessionJ)
• IPv6 does not contain checksum
– Assumes the data is protected by a link layer checksum
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Transport Layer Protocol Identifier
• Network layer packets include the transport layer data as payload
• Must identify what transport layer protocol is used, to pass the data to the correct upper-layer protocol
– DCCP = 33
• Protocols managed by the IANA
– http://www.iana.org/assignments/proto col-numbers/
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Aside: Misc
• Differentiated services (DiffServ)
– End systems can request special service from the network
• Telephony or gaming might prefer low latency over high bandwidth
• Emergency traffic could be prioritised
• Background software updates might ask for low
– Signalled by differentiated service code point (DSCP) field in header
– Provides a hint to the network, not a guarantee
• Often stripped at network boundaries
• Difficult economic and network neutrality issues
– Who is allowed to set the DSCP and what are they charged for doing so?
– IPv6 provides a flow label to group related traffic flows together
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Aside: Misc
• Explicit Congestion Notification
– Routers typically respond to network congestion by dropping packets
• A “best effort” packet delivery service
• Transport protocols detect the loss, and
can request a retransmission if necessary
– Explicit congestion notification gives routers a way to signal congestion is approaching
– If a sending host enables ECN, routers monitor link usage and can indicate congestion is imminent
– A host seeing that congestion is imminent needs to reduce it’s sending rate – or the congested router will start dropping packets
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IPv4 or IPv6?
• IPv4 has reached end-of-life: insufficient addresses
• IPv6 intended as long term replacement for IPv4
– Primary goal: increase the size of the address space, to allow more hosts on the network
– Also simplifies the protocol, makes high-speed implementations easier
• Not yet clear if IPv6 will be widely deployed
– But, straight-forward to build applications that work with both IPv4 and IPv6
– DNS query will return IPv6 address if it exists, else IPv4 address; all other communication calls use the returned value
– Write new code to support both IPv6 and IPv4
• To get a better idea on the IPv6 deployment status check :
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IP Addresses
• How to name hosts in a network?
– Is the address an identity or a location?
• Does it name the host, or the location at which it attaches to the network – How should addresses be allocated?
• Hierarchical or flat?
– What is the address format?
• Human or machine readable?
• Textual or binary? Structured or unstructured?
• Fixed or variable length? How large?
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Identity and Location
• Addresses can denote host identity
– Give hosts a consistent address, irrespective of where or when they attach to
the network
– Simple upper-layer protocols
• Transport layer and applications unaware of multi-homing or mobility – Puts complexity in network layer
• Network must determine location of host before it can route data
• Often requires in-network database to map host identity to routable address
• E.g., mobile phone numbers
• Alternatively, an address can indicate the location at which a host attaches to the network
– Address structure matches the network structure
• Network can directly route data given an address
• E.g., geographic phone numbers: +44 141 330 4256
– Simplifies network layer, by pushing complexity to the higher layers
• Multi-homing and mobility must be handled by transport layer or applications –
transport layer connections break when host moves
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Address Allocation & Format
• Are addresses allocated hierarchically? – Allows routing on aggregate addresses
• E.g., phone call to +1 703 243 9422
– Forces address structure to match network topology
– Requires rigid control of allocations
• Or is there a flat namespace?
– Flexible allocations, no aggregation → not scalable
• How about format? Textual or binary? Fixed or variable length?
– Fixed length binary easier (faster) for machines to process
– Variable length textual easier for humans to read
– Which are you optimising for?
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IP Addresses
• IP addresses have the following characteristics:
– They specify location of a network interface
– They are allocated hierarchically
– They are fixed length binary values
• IPv4: 32 bits
• IPv6: 128 bits
• Domain names are a separate application level namespace
• Both IPv4 and IPv6 addresses encode location
– Addresses are split into a network part and a host part
• A netmask describes the number of bits in the network part
• The network itself has the address with the host part equal to zero
• The broadcast address for a network has all bits of host part equal to one
– Allows messages to be sent to all hosts on a network
– A host with several network interfaces will have one IP addresses per interface
• E.g., laptop with an Ethernet interface and a Wi-Fi interface will have two IP addresses
• Example:
– IP address: 130.209.241.197 => 10000010 11010001 11110001 11000101
– Netmask: 255.255.240.0 => 11111111 11111111 11110000 00000000
– Network = 130.209.240.0/20 => 10000010 11010001 11110000 00000000
– Broadcast: 130.209.255.255 => 10000010 11010001 11111111 11111111
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Aside: Classes of IP Addresses
• IP addresses used to be allocated so the netmask was a multiple of 8 bits
• Class A → a /8 network (~16 million addresses)
• Class B → a /16 network (65536 addresses)
• Class C → a /24 network (256 addresses)
– Old terminology, still used sometimes
– Inflexible, and wasted addresses
• Arbitrary length netmask allowed since 1993:
– The Glasgow SoCS network is a /20
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Aside: IPv6 Addresses
• IPv6 uses 128 bit binary addresses, written as 8 “:” separated 16 bit hexadecimal fields • 2a00:1098:0000:0086:1000:0000:0000:0010
• Usually written in a shortened form [RFC 5952]
– Leading zeros in each 16 bit field are suppressed
– A run of more than one consecutive 16 bit field that is all 0 is omitted and replaced with a “::”
• If there is more than one such run, the longest is replaced
• If there are several runs of equal length, the first is replaced
– The “::” must not be used to replace a single 16 bit field • 2a00:1098:0:86:1000::10
• Local identifier part of IPv6 address is 64 bits
• 2001:0db8:85a3:08d3:1319:8a2e:0370:7334
– Can be derived from Ethernet/Wi-Fi MAC address
• 48-bit IEEE MAC: 0014:5104:25ea
• Expand to 64 bits: 0014:51ff:fe04:25ea
• Invert bit 6: 0214:51ff:fe04:25ea
– Or randomly chosen, with bit 6 set to zero, to give illusion of privacy
• Routers advertise network part, hosts auto-configure address
• 2001:0db8:85a3:08d3:1319:8a2e:0370:7334
• Network part is split into global routing prefix (up to 48 bits) and a subnet identifier
• 130.209.247.112 = 10000010 11010001 11110111 01110000
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IP Address Management
• IPv4 has 232 = 4,294,967,296 addresses
– IANA administers the pool of unallocated addresses
• Historically would assign addresses directly to ISPs, large enterprises, etc.
• Now, addresses assigned to regional Internet registries (RIRs) as needed:
– Allocations made one /8 (224 = 16,777,216 addresses) at a time
– RIRs allocate addresses to ISPs and large enterprises within their region; ISPs allocate to their customers
• IANA has allocated all available addresses to RIRs – Last allocation on 3 February 2011
• In practical terms, we have run out of IPv4 address space
• IPv6 provides 128 bit addresses
– If deployed it will solve address shortage for a long time
• 2128 = 340,282,366,920,938,463,463,374, 607,431,768,211,456 addresses
• Approx. 665,570,793,348,866,943,898,599 addresses per m2 of the Earth’s surface
Source: http://ipv4.potaroo.net/
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IPv6 Deployment Issues
• IPv6 requires changes to every single host, router, firewall, and application…
– Significant deployment challenge!
– Host changes done: MacOS X, Windows, Linux, FreeBSD, Symbian, iOS,
Android, etc.
– Backbone routers generally support IPv6, home routers and firewalls are starting to be updated
– Many applications have been updated
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Reading Material
• Peterson & Davie “Computer Networks: A systems approach”: Chapter 3, sections 3.2, Chapter 4, section 4.1
• Kurose & Ross “Computer Networking: A top-down approach” : Chapter 4, sections 4.1, 4.4, 4.6
• Tanenbaum & Wetherall “Computer Networks” 5th edition: Chapter 5, sections 5.1, 5.5, 5.6
• Bonaventure “Computer Networking” : https://www.computer- networking.info/1st/html/network/network.html
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Coming up next…
Source: https://xkcd.com/399/
Source: https://xkcd.com/85/
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