Communication Networks

Transcription

1 Communication Networks Spring 2018 Q&A Session Rüdiger Birkner Tobias Bühler ETH Zürich August

2 Old exam from hours instead of 2.5 Topics which we did not discuss this year Security, SDN,

3 How do you guide IP packets from a source to the destination?

4 Essentially, there are three ways to compute valid routing state Intuition Example #1 Use tree-like topologies Spanning-tree #2 Rely on a global network view Link-State SDN #3 Rely on distributed computation Distance-Vector BGP

5 Essentially, there are three ways to compute valid routing state Intuition Example #1 Use tree-like topologies Spanning-tree #2 Rely on a global network view Link-State SDN #3 Rely on distributed computation Distance-Vector BGP

6 Distance-vector protocols are based on Bellman-Ford algorithm

7 Every router maintains a table of all received distance vectors X routing table dest. via Y Z Y 3 11 Z 13 1 U 30 2

8 Every router maintains a table of all received distance vectors Let dx(y) be the cost of the least-cost path known by x to reach y X routing table dest. via Y Z Y 3 11 Z 13 1 U 30 2

9 Every router maintains a table of all received distance vectors Let dx(y) be the cost of the least-cost path known by x to reach y until convergence Each node bundles these distances into one message (called a vector) that it repeatedly sends to all its neighbors X routing table dest. via Y Z X vector dest. cost Y 3 11 Z 13 1 U 30 2 Y 3 Z 1 U 2

10 Every router maintains a table of all received distance vectors Let dx(y) be the cost of the least-cost path known by x to reach y until convergence Each node bundles these distances into one message (called a vector) that it repeatedly sends to all its neighbors Each node updates its distances based on neighbors vectors: dx(y) = min{ c(x,v) + dv(y) } over all neighbors v

11 Whenever a router uses another one, it will announce it an infinite cost The technique is known as poisoned reverse

12 Internet routing comes into two flavors: intra- and inter-domain routing inter-domain routing intra-domain routing Find paths between networks Find paths within a network!12

13 inter-domain routing intra-domain routing Find paths between networks!13

14 Internet Border Gateway Protocol (BGP)

15 BGP is the routing protocol glueing the entire Internet together AS20 AS30 AS10 BGP sessions AS40 AS50

16 BGP announcements carry complete path information instead of distances AS20 AS30 AS /16 Path: 40 AS40 AS /16 Path: /16 ETH/UNIZH Camp Net

17 Each AS appends itself to the path when it propagates announcements AS20 AS30 AS10 AS40 AS /16 Path: /16 ETH/UNIZH Camp Net

18 There are 2 main business relationships today: customer/provider peer/peer many less important ones (siblings, backups, )

19 These policies are defined by constraining which BGP routes are selected and exported Selection Export which path to use? which path to advertise?

20 Selection Export which path to use? control outbound traffic which path to advertise?

21 Business relationships conditions route selection For a destination p, prefer routes coming from customers over peers over route type providers

22 Selection Export which path to use? which path to advertise? control inbound traffic

23 Routes coming from peers and providers are only propagated to customers send to customer peer provider customer from peer - - provider - -

24 On the wire, BGP is a rather simple protocol composed of four basic messages type used to OPEN NOTIFICATION UPDATE establish TCP-based BGP sessions report unusual conditions inform neighbor of a new best route a change in the best route KEEPALIVE inform neighbor that the connection is alive

25 UPDATE inform neighbor of a new best route a change in the best route

26 BGP UPDATEs carry an IP prefix together with a set of attributes IP prefix Attributes Describe route properties used in route selection/exportation decisions are either local or global (only seen on ibgp) (seen on ibgp and ebgp)

27 Attributes Usage NEXT-HOP egress point identification AS-PATH loop avoidance outbound traffic control inbound traffic control LOCAL-PREF outbound traffic control MED inbound traffic control

28 Prefer routes with higher LOCAL-PREF with shorter AS-PATH length with lower MED learned via ebgp instead of ibgp with lower IGP metric to the next-hop with smaller egress IP address (tie-break)

29 Each BGP router processes UPDATEs according to a precise pipeline

30 BGP sessions Adj-RIB-In Adj-RIB-Out BGP sessions Neighbor1 Input filters Attribute Manipulation All acceptable routes Output filters Attribute Manipulation Neighbor1 Input filters Output filters Neighbor2 Attribute Manipulation Attribute Manipulation Neighbor2... BGP Decision Process... Input filters Loc-Rib Output filters Neighborn Attribute Manipulation Best route to each destination Attribute Manipulation Neighborn forwarding entries IP packets IP forwarding table IP packets

31 routing-table BGP sessions Adj-RIB-In Adj-RIB-Out BGP sessions Neighbor1 Input filters Attribute Manipulation All acceptable routes Output filters Attribute Manipulation Neighbor1 Input filters Output filters Neighbor2 Attribute Manipulation Attribute Manipulation Neighbor2... BGP Decision Process... Input filters Loc-Rib Output filters Neighborn Attribute Manipulation Best route to each destination Attribute Manipulation Neighborn forwarding entries IP packets IP forwarding table IP packets

32 BGP sessions Adj-RIB-In Adj-RIB-Out BGP sessions route-map IN Neighbor1 Input filters Attribute Manipulation All acceptable routes Output filters Attribute Manipulation Neighbor1 Input filters Output filters Neighbor2 Attribute Manipulation Attribute Manipulation Neighbor2... BGP Decision Process... Input filters Loc-Rib Output filters Neighborn Attribute Manipulation Best route to each destination Attribute Manipulation Neighborn forwarding entries IP packets IP forwarding table IP packets

33 BGP sessions Adj-RIB-In Adj-RIB-Out BGP sessions Neighbor1 Input filters Attribute Manipulation All acceptable routes Output filters Attribute Manipulation route-map OUT Neighbor1 Input filters Output filters Neighbor2 Attribute Manipulation Attribute Manipulation Neighbor2... BGP Decision Process... Input filters Loc-Rib Output filters Neighborn Attribute Manipulation Best route to each destination Attribute Manipulation Neighborn forwarding entries IP packets IP forwarding table IP packets

34 Life of a BGP router is made of three consecutive steps while true: receives routes from my neighbors select one best route for each prefix export the best route to my neighbors

35 An AS is more than just one router AS30 AS20 AS50 AS10 AS40

36 BGP sessions come in two flavors AS30 AS20 AS50 AS10 AS40

37 external BGP (ebgp) sessions connect border routers in different ASes ebgp session

38 internal BGP (ibgp) connect the routers in the same AS ibgp sessions

39 ibgp sessions are used to disseminate externally-learned routes internally /16 Path: /16 Path: /16 Path: /16 Path: /16 Path: 20

40 Putting it all together Route A /16 Path: Route B /16 Path:

41 Putting it all together Route A /16 Path: Route B /16 Path:

42 Putting it all together Route A /16 Path: Route B /16 Path:

43 Putting it all together Route A /16 Path: Route B /16 Path: /16 Path: /16 Path:

44 Assignment 1 - Internet Communication packet link layer protocol packet frame frame sending node adapter adapter receiving node

45 Assignment 1 - Internet Communication Link Communication medium Wifi Ethernet Fiber and Network adapter

46 Accessing a website: DNS & HTTP step 1 Open browser and enter the URL step 2 Browser invokes DNS to resolve the URL into an IP step 3 Browser creates a HTTP request to retrieve the website GET / HTTP/1.1 Host:

47 Accessing a website: DNS & HTTP What if we do the DNS resolution ourselves? step 1 Perform a DNS lookup for the given URL dig step 2 Open browser and enter the IP address step 3 Browser creates a HTTP request to retrieve the website GET / HTTP/1.1 Host:

48 In practice, multiple URLs can be mapped to the same IP name DNS IP address How does a web server receiving an HTTP request know, which website you want to access?

49 The host field tells the server which website it should serve HTTP request: GET / HTTP/1.1 Host:

50 Anycast, Unicast, Multicast one-to-one-of-many Important, discussed in lecture Used for scalability, load-balancing (e.g. DNS root server) Routing finds shortest-paths Seamless replication But, potential problems for stateful applications

51 Anycast, Unicast, Multicast / /24

52 Anycast, Unicast, Multicast / /24

53 Anycast, Unicast, Multicast one-to-one Destination address uniquely identifies a single receiver No replication

54 Anycast, Unicast, Multicast /24

55 Anycast, Unicast, Multicast one-to-many-of-many ( many-to-many-of-many ) Not important for exam E.g. useful to stream the same video to multiple receivers

56 Anycast, Unicast, Multicast / / /24

57 Assignment 5 - Detective Work Poor choice of IP subnets from our side Indeed, /16 is a private subnet space normally not routed in the Internet

58 Assignment 5 - Detective Work

59 Assignment 5 - Detective Work Router interface MAC address

60 Assignment 5 - Detective Work Router interface MAC address Dst does not go over router => internal

61 Assignment 5 - Detective Work Router interface MAC address Dst does not go over router => internal Dst reaches router and has to be in the same subnet as => is the IP of the router

62 DHCP (assignment 10 and book chapter 4) Assignment uses 2 packets, book 4 Assignment simplifies the process slightly assumes only one DHCP server The sender recognizes its response based on the transaction ID therefore not a problem that also the response is broadcasted

63 DHCP (assignment 10 and book chapter 4)

64 VLAN Access link: part of only one VLAN normally connects hosts with switches (to get access ) Trunk link: can carry traffic for multiple VLANs normally connects switches to other switches or routers The per-vlan spanning tree still spans the entire network even if some of the switches do not have hosts in all VLANs => better optimized paths for hosts in one VLAN => ready for new hosts in the future

65 VLAN (spanning tree from the slides)

66 TCP Congestion Window Additive Increase pseudo code from the slides: CWND = CWND + 1/CWND

67 TCP Congestion Window Additive Increase pseudo code from the slides: CWND = CWND + 1/CWND More precise computation: { CWND(t+1) = CWND(t) + a CWND(t) * b if no congestion detected if congestion detected With a = MSS and b = 1/2 (t = current RTT)

68 TCP Congestion Window (assignment 8) Question c): how much time elapsed between E and F?

69 TCP Congestion Window (assignment 8) Question c): how much time elapsed between E and F? CWND S R 5 RTTs 6 RTTs => depends on when F is exactly happening

70 TCP Congestion Window - diagrams The presented diagrams do not capture all the details, e.g. D timeout E We will make sure that future question precisely define what the marked points represent.

71 Individual Questions