CSCI-1680 Network Layer: Wrapup

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CSCI-1680 Network Layer: Wrapup. Rodrigo Fonseca. Based partly on lecture notes by Jennifer Rexford, Rob Sherwood, David Mazières , Phil Levis, John Jannotti. Administrivia. Homework 2 is due today So we can post solutions before the midterm! Exam on Thursday
CSCI-1680Network Layer:WrapupRodrigo FonsecaBased partly on lecture notes by Jennifer Rexford, Rob Sherwood, David Mazières, Phil Levis, John JannottiAdministrivia
  • Homework 2 is due today
  • So we can post solutions before the midterm!
  • Exam on Thursday
  • All content up to today (no TCP!)
  • Questions similar to the homework
  • Book has some exercises, samples on the course web page
  • Today: IP Wrap-up
  • IP Service models
  • Unicast, Broadcast, Anycast, Multicast
  • IPv6
  • Tunnels
  • Different IP Service Models
  • Broadcast: send a packet to all nodes in some subnet. “One to all”
  • : all hosts within a subnet, never forwarded by a router
  • “All ones host part”: broadcast address
  • Host address | ( & ~subnet mask)
  • E.g.: mask
  • ~mask = => Bcast =
  • Example use: DHCP
  • Not present in IPv6
  • Use multicast to link local all nodes group
  • Anycast
  • Multiple hosts may share the same IP address
  • “One to one of many” routing
  • Example uses: load balancing, nearby servers
  • DNS Root Servers (e.g.
  • Google Public DNS (
  • IPv6 6-to-4 Gateway (
  • Anycast Implementation
  • Anycast addresses are /32s
  • At the BGP level
  • Multiple ASs can advertise the same prefixes
  • Normal BGP rules choose one route
  • At the Router level
  • Router can have multiple entries for the same prefix
  • Can choose among many
  • Each packet can go to a different server
  • Best for services that are fine with that (connectionless, stateless)
  • Multicast
  • Send messages to many nodes: “one to many”
  • Why do that?
  • Snowcast, Internet Radio, IPTV
  • Stock quote information
  • Multi-way chat / video conferencing
  • Multi-player games
  • What’s wrong with sending data to each recipient?
  • Link stress
  • Have to know address of all destinations
  • Multicast Service Model
  • Receivers join a multicast group G
  • Senders send packets to address G
  • Network routes and delivers packets to all members of G
  • Multicast addresses: class D (start 1110)
  • 224.x.x.x to 229.x.x.x
  • 28 bits left for group address
  • LAN Multicast
  • Easy on a shared medium
  • Ethernet multicast address range:
  • 01:00:5E:00:00:00 to 01:00:5E:7f:ff:ff
  • Set low 23 bits of Ethernet address to low bits of IP address
  • (Small problem: 28-bit group address -> 23 bits)
  • How about on the Internet?Use Distribution Trees
  • Source-specific trees:
  • Spanning tree over recipients, rooted at each source
  • Best for each source
  • Shared trees:
  • Single spanning tree among all sources and recipients
  • Hard to find one shared tree that’s best for many senders
  • State in routers much larger for source-specific
  • Source vs Shared TreesBuilding the Tree: Host to Router
  • Nodes tell their local routers about groups they want to join
  • IGMP, Internet Group Management Protocol (IPv4)
  • MLD, Multicast Listener Discovery (IPv6)
  • Router periodically polls LAN to determine memberships
  • Hosts are not required to leave, can stop responding
  • Building the Tree across networks
  • Routers maintain multicast routing tables
  • Multicast address -> set of interfaces, or
  • <Source, Multicast address> -> set of interfaces
  • Critical: only include interfaces where there are downstream recipients
  • Practical Considerations
  • Multicast protocols end up being quite complex
  • Introduce a lot of router state
  • Turned off on most routers
  • Mostly used within domains
  • In the department: Ganglia monitoring infrastructure
  • IPTV on campus
  • Alternative: do multicast in higher layers
  • IPv6
  • Main motivation: IPv4 address exhaustion
  • Initial idea: larger address space
  • Need new packet format:
  • REALLY expensive to upgrade all infrastructure!
  • While at it, why don’t we fix a bunch of things in IPv4?
  • Work started in 1994, basic protocol published in 1998
  • The original expected plan From: The plan in 2011What is really happeningSource: potaroo.netCurrent Adoption (as seen by Google)Source: Key Features
  • 128-bit addresses
  • Autoconfiguration
  • Simplifies basic packet format through extension headers
  • 40-byte base header (fixed)
  • Make less common fields optional
  • Security and Authentication
  • IPv6 Address Representation
  • Groups of 16 bits in hex notation
  • 47cd:1244:3422:0000:0000:fef4:43ea:0001
  • Two rules:
  • Leading 0’s in each 16-bit group can be omitted
  • 47cd:1244:3422:0:0:fef4:43ea:1
  • One contiguous group of 0’s can be compacted
  • 47cd:1244:3422::fef4:43ea:1IPv6 Addresses
  • Break 128 bits into 64-bit network and 64-bit interface
  • Makes autoconfiguration easy: interface part can be derived from Ethernet address, for example
  • Types of addresses
  • All 0’s: unspecified
  • 000…1: loopback
  • ff/8: multicast
  • fe8/10: link local unicast
  • fec/10: site local unicast
  • All else: global unicast
  • IPv6 HeaderIPv6 Header Fields
  • Version: 4 bits, 6
  • Class: 8 bits, like TOSS in IPv4
  • Flow: 20 bits, identifies a flow
  • Length: 16 bits, datagram length
  • Next Header, 8 bits: …
  • Hop Limit: 8 bits, like TTL in IPv4
  • Addresses: 128 bits
  • What’s missing?
  • No options, no fragmentation flags, no checksum
  • Design Philosophy
  • Simplify handling
  • New option mechanism (fixed size header)
  • No more header length field
  • Do less work at the network (why?)
  • No fragmentation
  • No checksum
  • General flow label
  • No semantics specified
  • Allows for more flexibility
  • Still no accountability
  • With some content from Scott ShenkerInteroperability
  • RFC 4291
  • Every IPv4 address has an associated IPv6 address
  • Simply prefix 32-bit IPv4 address with 96 bits of 0
  • E.g., ::
  • Two IPv6 endpoints must have IPv6 stacks
  • Transit network:
  • v6 – v6 – v6 : ✔
  • v4 – v4 – v4 : ✔
  • v4 – v6 – v4 : ✔
  • v6 – v4 – v6 : ✗!!
  • IP Tunneling
  • Encapsulate an IP packet inside another IP packet
  • Makes an end-to-end path look like a single IP hop
  • Key issues: configuring the tunnelsDetermining addressesDetermining routesDeploying relays to encapsulate/forward/decapsulate6to4 is a standard to automate thisDeterministic address generationAnycast to find gateway into IPv6 networkDrawbacks: voluntary relays, requires public endpoint addressIPv6 in IPv4 TunnelingOther uses for tunneling
  • Virtual Private Networks
  • Use case: access CS network from the outside
  • Set up an encrypted TCP connection between your computer and Brown’s OpenVPN server
  • Configure routes to Brown’s internal addresses to go through this connection
  • Can connect two remote sites securely
  • Extension Headers
  • Two types: hop-by-hop and end-to-end
  • Both have a next header byte
  • Last next header also denotes transport protocol
  • Destination header: intended for IP endpoint
  • Fragment header
  • Routing header (loose source routing)
  • Hop-by-hop headers: processed at each hop
  • Jumbogram: packet is up to 232 bytes long!
  • Example Next Header Values
  • 0: Hop by hop header
  • 1: ICMPv4
  • 4: IPv4
  • 6:TCP
  • 17: UDP
  • 41: IPv6
  • 43: Routing Header
  • 44: Fragmentation Header
  • 58: ICMPv6
  • Fragmentation and MTU
  • Fragmentation is supported only on end hosts!
  • Hosts should do MTU discovery
  • Routers will not fragment: just send ICMP saying packet was too big
  • Minimum MTU is 1280-bytes
  • If some link layer has smaller MTU, must interpose fragmentation reassembly underneath
  • Current State
  • IPv6 Deployment has been slow
  • Most end hosts have dual stacks today (Windows, Mac OSX, Linux, *BSD, Solaris)
  • 2008 Google study:
  • Less than 1% of traffic globally
  • Requires all parties to work!
  • Servers, Clients, DNS, ISPs, all routers
  • IPv4 and IPv6 will coexist for a long time
  • Next time: Midterm
  • After that, transport layer and above!
  • UDP, TCP, Congestion Control
  • Application protocols
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