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171 lines
5.2 KiB
Markdown
171 lines
5.2 KiB
Markdown
---
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title: "20-network-layer-control-plane"
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aliases:
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tags:
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- cosc203
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- lecture
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sr-due: 2022-11-23
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sr-interval: 22
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sr-ease: 250
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---
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[slides](https://blackboard.otago.ac.nz/bbcswebdav/pid-2982068-dt-content-rid-19209669_1/courses/COSC203_S2DNI_2022/COSC203_lecture22%281%29.pdf)
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# per-router control vs SDN
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## per router
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- each router runs a routing algorithm
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- router communicate with each other
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- and creates a forwarding table
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## SDN software defined networking
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a remote controller computes, and install fowarding tables in routers
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- much faster: less work for each router
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- separate control and data plane
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# routing algorithms
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- goal is to determine "good" paths from sending to recieving host through network of routers
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- path: sequence of routers
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- "good": least "cost", "fastest", "least congested"
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## graph abstraction
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- nodes: set of routers
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- edges: set of links
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$c_{a,b}$ cost the link directly connecting a and b. if there is no link the cost is ♾
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## algorithm classification
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global: all routers have complete toplogy, link cost into
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- link state algotihms
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decentralized: interative process of computation, exchange of into with neighbors
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- routers only know link costs to attacked neighbors
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- "distance vecotr algorithms
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static: routes change very slowly over time
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dynamic: routes change more quickly
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- periodic updates or in response to link cost changes
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## notations
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## link state routing
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centralized: network topology, link costs are known to all nodes
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- each node gathers informatin on each link to its neighbors
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- build link state packets and flood to all other nodes
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computes least costs paths from one node to all other nodes
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- gives a fowarding table for that node
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iterative: after k interations, know least cost path to k destinations
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### Dijkstra's algorithm
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foward search algorithm
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```
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///initialization
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N` = {u}
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for all nodes v
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if v adjacent to u
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then D(v) = Cuv
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else D(v) = infinity
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//loop
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loop until all nodes in N`
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find W not in N` such that D(w) is a min
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add w to N`
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update D(v) for all v adjacent to w and not in N`:
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D(v) = min(D(v), D(w)+Cwv)
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//new least-path-cost to v is either old least-cost-path to v or known least-path-cost to w plus direct-cost from w to v
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```
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algorithm complexity:
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- n nodes
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- for each of n interation: need to check all nodes, w, not in N
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- n(n+1)/2 comparisons: O(n²)
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- more efficient implementations possible: O(nlogn)
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message complexity
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- each router must broadcast its link state information to other n routers
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- efficient broadcast algorithms: O(n) link crossings to dissenimate a bradcast message from one source
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- each router's message crosses O(n) links: overall message compexity O(n²)
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## Distance vector routing
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### bellman-ford equation
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backward search algorithm
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$D_{x}(y) = min_{v} \{c_{x, v} + D_{v}(y)\}$
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### distance vector algorithm
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each node:
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- **wait** for change in local link cost or msg from neighbor
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- **recompute** DV estimate using DV recieved from neighbor
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- if DV to any destination had changed, **notify** neighbors
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- iterative, asynchronous
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- distributed, self-stopping
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- no notifications recieved, no action taken
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"good news (decrease on link cost) travels quickly"
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"bad news (increase on link cost) travels slowly" - count-to-infinity problem
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- if a link is broken other routers become aware slowly
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- routers unaware of broken link can "advertise" incorrect costs and create a routing loop
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- loop is broken if a hop count threshold is passed
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## link state (LS) vs Distance vector (DV)
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message complexity
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- LS: n routers O(n²) messages sent
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- DV: exhange between neighbors; convergence time varies
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speed of convergence:
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- LS: O(n²) algorithm, O(n²) messages
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- DV: varies
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- may have routing loops
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- count-to-infinity problem
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robustness
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- LS:
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- router can advertise incorrect link cost
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- each router computes only its own table
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- DV:
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- router can advertise incorrect path cost: black holing
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- each routers table is used by others: errors propagate through network
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# Routing in internet
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- autonomous system (AS): a group of networks and routers controled by a single administrative authority
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- Intra-AS routing:
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- routing information protocol (RIP): distance vector routing
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- open shortest path first (OSPF): link state routing
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- Inter-AS routing:
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- border gateway protocol (BGP)L path-vector routing
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# ICMP
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- used by hosts and routers to communicate netowrk level information
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- error reporting:
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- echo requeset/reply
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- network layer "above" IP
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- ICMP message carried in IP datagrams
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- ICMP message: type, code plus first 8 bytes of IP datagram causing error
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## traceroute
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- sends a set of UDP segments to destination
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- datagram in nth set arrives to nth router
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- router discards datagram and sends source ICMP message (type 11 code 0)
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- ICMP message possibly incldes name of router and IP
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- when ICMP message arrives at source: record RTTs
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stopping:
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- when a packet arrives that the destination, it returns ICMP "port unreachable message" (type 3 code 3)
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- source stops |