Transport and TCP. EE122 Fall 2011 Scott Shenker

Transcription

1 Transport and TCP EE122 Fall 2011 Scott Shenker Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxson and other colleagues at Princeton and UC Berkeley 1

2 Announcements All account issues should be resolved by now Please check your accounts one last time Project 2 released on Wednesday HW2 has been released HW1 key has been released We now have a grader (probably two!) Will grade HW1 asap No office hours this week Will be available M, W, and Th of next week But not at normal office hours.so contact me for time 2

3 Agenda Review of addressing Forwarding Transport Next lecture: TCP and starting DNS 3

4 Review of Addressing Notation: dotted quad (e.g., ) Set of four 8-bit numbers Structure: (prefix, suffix) Network component (prefix) Host component (suffix) Slash notation: /x means that prefix is x bits long Addressing schemes: Original: prefix of length 8 (all addresses in /8s) Classful: opening bits determined length of prefix E.g., 0 meant /8, 10 meant /16, 110 meant /24, 1110 meant mcast Classless (CIDR): explicit mask defines prefix 4

5 CIDR Addressing Use two 32-bit numbers to represent a network location Address + Mask IP Address : IP Mask: Address Mask Network Prefix for hosts Written as /15 or 12.4/15 5

6 Special-Purpose Address Blocks Private addresses By agreement, not routed in the public Internet For networks not meant for general Internet connectivity Blocks: /8, /12, /16 Link-local By agreement, not forwarded by any router Used for single-link communication only Intent: autoconfiguration (especially when DHCP fails) Block: /16 Loopback Address blocks that refer to the local machine Block: /8 Usually only /32 is used Limited broadcast Sent to every host attached to the local network Block: /32 6

7 Allocation Done Hierarchically ICANN gives large blocks to... Regional Internet Registries, which give blocks to... Large institutions (ISPs), which give addresses to... Individuals and smaller institutions Examples: ICANN ARIN AT&T Customer ICANN ARIN UCB Department 7

8 FAKE Example in More Detail ICANN gives ARIN several /8s, including 12.0/8 Network Prefix: ARIN gives ACME Internet a /16, /16 Network Prefix: ACME give XYZ Hosting a /24, /24 Network Prefix: XYZ gives customer specific address Address:

9 Addressing Structure All about address aggregation Only way to make Internet scalable (2 billion users!) Want to represent routing tables more compactly Using aggregation, which involves special structure Otherwise, MAC or other random addresses would be ok Will now discuss this twice General observations on route aggregation Structure of forwarding tables 9

10 Aggregation in Phone Network? My work number is Country code 1 Area/City code 510 Exchange 643 Number

11 Scalability via Address Aggregation Provider is given /21 ( x x) Provider Each customer given smaller prefix / / / /23 Routers in the rest of the Internet just need to know how to reach /21. The provider can direct the IP packets to the appropriate customer. 11

12 Global Picture /21 Port /21 Port 2 202/8 Port 4.. Router in Internet Core Only /21 listed in core /22 Port /24 Port /24 Port /23 Port 4 Router in ISP /22, /23, /24 only listed in ISP s router 12

13 Prefix Expansion Original Prefix: /21= *** ******* Subprefixes: (disjoint coverage of original prefix) /22= ** ******* /24= ******* /24= ******* /23= * ******* 13

14 Aggregation Not Always Possible /21 Provider 1 Provider / / / /23 Multi-homed customer with /23 has two providers. Other parts of the Internet need to know how to reach these destinations through both providers. /23 route must be globally visible 14

15 Multihoming Global Picture /21 Port /23 Port /21 Port 3.. Router in Internet Core /23 Port /21 Port /21 Port /21 Port 4 Router in ISP /22 Port /24 Port /24 Port /23 Port 4 Router in ISP1 Which ISP does core send /23 to? It depends.. 15

16 Addresses Advertised in Two Places? Provider 1 and Provider 2 both advertise prefix That is, they both claim they can reach prefix What problems does this cause? None, in terms of basic connectivity! DV: routers often offered two paths to destination Pick the shorter path Here, situation is complicated by: Length of prefix Policy We will return to this example. Focus now on multihoming as impediment to aggregation 16

17 Two Countervailing Forces Aggregation reduces number of advertised routes Multi-homing increases number of routes 17

18 Growth in Routed Prefixes ( ) Dot-com implosion; Internet bubble bursts Advent of CIDR allows aggregation: linear growth Initial growth super-linear; no aggregation Back in business Internet boom: multihoming drives superlinear growth 18

19 Same Table, Extended to Present Linear growth Superlinear growth What Stock Happened Market Here? Crash of

20 Summary of Addressing Hierarchical addressing Critical for scalable system Don t require everyone to know everyone else Reduces amount of updating when something changes Non-uniform hierarchy Useful for heterogeneous networks of different sizes Class-based addressing was far too coarse Classless InterDomain Routing (CIDR) more flexible Any questions? 20

21 Conceptual Problems with IP Addressing 21

22 What s Wrong with IP Addressing? Multihoming not naturally supported Causes aggregation problems No binding to identity (spoofing, etc.) Scarce (IPv6 solves this) Forwarding hard (discuss later).. 22

23 Design Exercise: Design better addressing scheme Take five minutes Work in groups Will take three proposals We will then vote on the winner. 23

24 A Better Form of Addressing? Two (or more) layers of cryptographic names Network name (domain, subdomain, etc.) Host name Examples: N:H N1:N2:H Both tied to keys (e.g., hash of public key) 24

25 Advantages Addresses are verifiable (challenge-response) Prove to me that this is your address! Hosts can prove this by signing a nonce Multihoming natural: host is both N1:H and N2:H Routing is exact match (much easier) If you have stack of network addresses Keep pointer to where in the stack of addresses you are Scaling not a problem Not that many network addresses And can aggregate them as necessary Explicit hierarchies more flexible than implicit ones 25

26 Forwarding 26

27 Forwarding Table Plays Crucial Role Table maps IP addresses into output interfaces Forwards packets based on destination address

28 Hop-by-Hop Packet Forwarding Forwarding table derived from: Routing algorithms (or static configuration) Upon receiving a packet Inspect the destination IP address in the header Index into the forwarding table Forward packet out appropriate interface If no match, take default route Default route Configured to cover cases where no matches Allows small tables at edge (w/o routing algorithms) o if it isn t on my subnet, send it to my ISP 28

29 Using the Forwarding Table With classful addressing, this is easy: Early bits in address specify mask o Class A [0]: /8 Class B [10]: /16 Class C [110]: /24 Can find exact match in forwarding table o Use prefix as index into hash table Why won t this work for CIDR? What s the network prefix in this address?

30 Finding Matches If address fields contained masks we could do an exact match on network portion! But address in packet doesn t specify mask! Would just take five bits! All delicacy of forwarding lookups due to CIDR Lack of mask prevents easy exact match over prefix 30

31 Example #1: Provider w/ 4 Customers Port 1 Provider Port 2 Port 3 Port / / / /23 Prefix Port /22 Port /24 Port /24 Port /23 Port 4 31

32 Finding the Match (at ISP s Router) No address matches more than one prefix But can t easily find match / / / / Consider First 21 bits match 4 partial prefixes First 22 bits match 3 partial prefixes First 23 bits match 2 partial prefixes First 24 bits match exactly one full prefix 32

33 Finding Match Efficiently Testing each entry to find a match scales poorly On average: (number of entries) ½ (number of bits) Leverage tree structure of binary strings Set up tree-like data structure Return to example: Prefix Port ********** ******** ******** ********* 4 33

34 Consider four three-bit prefixes Just focusing on the bits where all the action is. 0** Port Port Port 3 11* Port 4 34

35 Tree Structure *** 0 1 0** 0 1 1** * * * *

36 Walk Tree: Stop at Prefix Entries *** 0 1 0** 0 1 1** * * * *

37 Walk Tree: Stop at Prefix Entries *** * ** 0 1 P1 01* * ** * 0 1 P P2 P3 37

38 Slightly Different Example Several of the unique prefixes go to same port 0** Port Port Port 1 11* Port 1 38

39 Prefix Tree *** * ** 0 1 P1 01* * ** * 0 1 P P2 P1 39

40 More Compact Representation P1 *** Record port associated with first match, and only over-ride when it matches another prefix during walk down tree If you ever leave path, you are done, last matched prefix is answer 1 0 1** This is longest prefix match (LPM) * P2 40

41 Longest Prefix Match Representation *** Port Port 2 If address matches both, then take longest match 41

42 We Use LPM Every Day.. Everyone go outside to play. except for John, who has to stay inside We routinely insert an except whenever we make a general statement and then a contradictory specific statement Point: we would never explicitly list the members of the class, but instead use the term for the aggregate and then specify the exceptions 42

43 Example #2: Aggregating Customers Prefix Port /21 Provider /21 Provider / /21 Provider 1 Provider / / / / / / / /23 43

44 Global Picture /21 Port /21 Port /21 Port 3.. Router in Internet Core /22 Port /24 Port /24 Port /23 Port 4 Router in ISP /22 Port /24 Port /24 Port /23 Port 4 Router in ISP1 44

45 Example #3: Complications Forwarding table more complicated when addressing is non-topological / /21 Provider 1 Provider / / / / / / / /23 45

46 Global Picture /22 Port /24 Port /24 Port /23 Port /22 Port /24 Port /24 Port /23 Port 2.. Router in Internet Core /22 Port /24 Port /24 Port /23 Port 4 Router in ISP /22 Port /24 Port /24 Port /23 Port 4 Router in ISP1 46

47 Matching disjoint prefixes If match any of these prefixes, go to Provider If match any of these prefixes, go to Provider

48 Focusing Only on Crucial Bits Prefix destined for Provider 1 Prefix destined for Provider No packet will match more than one prefix All paths reach a unique prefix 48

49 More Compact Representation Prefix destined for Provider 1 Prefix destined for Provider

50 Arriving New Arriving packet: packet: Longest Prefix Match Provider / /24 Provider / /24 50

51 Return to multihoming example /21 Provider 1 Provider / / / /23 51

52 Global Picture with Multihoming /21 Port /23 Port /21 Port 3.. Router in Internet Core /23 Port /21 Port /21 Port /21 Port 4 Router in ISP2 Which ISP does core send /23 to? LPM says ISP /22 Port /24 Port /24 Port /23 Port 4 Router in ISP1 Need explicit decisions about prefix granularity ISP1 might also advertise specific prefix 52

53 Forwarding Summary Nontrivial to find matches in CIDR Because can t tell where network address ends Must walk down bit-by-bit LPM decreases size of routing table Reducing memory consumption Multihoming and LPM might have unintended consequences. 53

54 5 Minute Break 54

55 Anagram Contest What does this numerical anagram have to do with this alphabetical one? Don t ignore capitals. Alphabetical: Stern Alpaca Numerical: