1. 15-440 Inter-Domain Routing BGP (Border Gateway Protocol) DNS (Domain Name System) These slides proudly ripped from Srini Seshan and Dave Anderson and Seth Goldstein, 15-441 F’06 and F’08

2. Outline Internet Structure/Routing Hierarchy External BGP (E-BGP) Internal BGP (I-BGP)

3. A Logical View of the Internet? R R R RR After looking at RIP/OSPF descriptions End-hosts connected to routers Routers exchange messages to determine connectivity NOT TRUE!

4. Internet’s Area Hierarchy What is an Autonomous System (AS)? A set of routers under a single technical administration, using an interior gateway protocol (IGP) and common metrics to route packets within the AS and using an exterior gateway protocol (EGP) to route packets to other AS’s Each AS assigned unique ID AS’s peer at network exchanges

5. AS Numbers (ASNs) ASNs are 16 bit values64512 through 65535 are “private” Genuity: 1 MIT: 3 CMU: 9 UC San Diego: 7377 AT&T: 7018, 6341, 5074, … UUNET: 701, 702, 284, 12199, … Sprint: 1239, 1240, 6211, 6242, … … ASNs represent units of routing policy Currently over 15,000 in use

6. Example 12 3 1.1 1.2 2.1 2.2 3.1 3.2 2.2.1 4 4.1 4.2 5 5.1 5.2 EGP IGP EGP IGP EGP

7. A Logical View of the Internet? R R R RR RIP/OSPF not very scalable  area hierarchies NOT TRUE EITHER! ISP’s aren’t equal Size Connectivity ISP

8. A Logical View of the Internet Tier 1 Tier 2 Tier 3 Tier 1 ISP “Default-free” with global reachability info Tier 2 ISP Regional or country-wide Tier 3 ISP Local Customer Provider

9. Transit vs. Peering ISP X ISP Y ISP Z ISP P Transit ($$) Transit ($$$) Transit ($$ 1/2) Transit ($$) Peering Transit ($$$) Transit ($) Transit ($$) Transit ($$$)

10. Policy Impact “Valley-free” routing Number links as (+1, 0, -1) for provider, peer and customer In any path should only see sequence of +1, followed by at most one 0, followed by sequence of -1 WHY? Consider the economics of the situation

11. Outline Internet Structure/Routing Hierarchy External BGP (E-BGP) Internal BGP (I-BGP)

12. Choices Link state or distance vector? No universal metric – policy decisions Problems with distance-vector: Bellman-Ford algorithm may not converge Problems with link state: Metric used by routers not the same – loops LS database too large – entire Internet May expose policies to other AS’s

13. Solution: Distance Vector with Path Each routing update carries the entire path Loops are detected as follows: When AS gets route, check if AS already in path If yes, reject route If no, add self and (possibly) advertise route further Advantage: Metrics are local - AS chooses path, protocol ensures no loops

14. Interconnecting BGP Peers BGP uses TCP to connect peers Advantages: Simplifies BGP No need for periodic refresh - routes are valid until withdrawn, or the connection is lost Incremental updates Disadvantages Congestion control on a routing protocol? Poor interaction during high load

15. Hop-by-hop Model BGP advertises to neighbors only those routes that it uses Consistent with the hop-by-hop Internet paradigm e.g., AS1 cannot tell AS2 to route to other AS’s in a manner different than what AS2 has chosen (need source routing for that) BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS’s

16. Examples of BGP Policies A multi-homed AS refuses to act as transit Limit path advertisement A multi-homed AS can become transit for some AS’s Only advertise paths to some AS’s An AS can favor or disfavor certain AS’s for traffic transit from itself

17. BGP Messages Open Announces AS ID Determines hold timer – interval between keep_alive or update messages, zero interval implies no keep_alive Keep_alive Sent periodically (but before hold timer expires) to peers to ensure connectivity. Sent in place of an UPDATE message Notification Used for error notification TCP connection is closed immediately after notification

18. BGP UPDATE Message List of withdrawn routes Network layer reachability information List of reachable prefixes Path attributes Origin Path Metrics All prefixes advertised in message have same path attributes

19. Path Selection Criteria Attributes + external (policy) information Examples: Hop count Policy considerations Preference for AS Presence or absence of certain AS Path origin Link dynamics

20. LOCAL PREF Local (within an AS) mechanism to provide relative priority among BGP routers (e.g. R3 over R4) R1R2 R3R4 I-BGP AS 256 AS 300 Local Pref = 500Local Pref = 800 AS 100 R5 AS 200

21. LOCAL PREF – Common Uses Peering vs. transit Prefer to use peering connection, why? In general, customer > peer > provider Use LOCAL PREF to ensure this

22. AS_PATH List of traversed AS’s AS 500 AS 300 AS 200AS 100 180.10.0.0/16 300 200 100 170.10.0.0/16 300 200 170.10.0.0/16180.10.0.0/16

23. Multi-Exit Discriminator (MED) Hint to external neighbors about the preferred path into an AS Non-transitive attribute Different AS choose different scales Used when two AS’s connect to each other in more than one place

24. MED Hint to R1 to use R3 over R4 link Cannot compare AS40’s values to AS30’s R1R2 R3R4 AS 30 AS 40 180.10.0.0 MED = 120 180.10.0.0 MED = 200 AS 10 180.10.0.0 MED = 50

25. MED MED is typically used in provider/subscriber scenarios It can lead to unfairness if used between ISP because it may force one ISP to carry more traffic: SF NY ISP1 ignores MED from ISP2 ISP2 obeys MED from ISP1 ISP2 ends up carrying traffic most of the way ISP1 ISP2

26. Decision Process Processing order of attributes: Select route with highest LOCAL-PREF Select route with shortest AS-PATH Apply MED (if routes learned from same neighbor)

27. CIDR and BGP AS X 197.8.2.0/24 AS Y 197.8.3.0/24 AS T (provider) 197.8.0.0/23 AS Z What should T announce to Z?

28. Options Advertise all paths: Path 1: through T can reach 197.8.0.0/23 Path 2: through T can reach 197.8.2.0/24 Path 3: through T can reach 197.8.3.0/24 But this does not reduce routing tables! We would like to advertise: Path 1: through T can reach 197.8.0.0/22

29. Sets and Sequences Problem: what do we list in the route? List T: omitting information not acceptable, may lead to loops List T, X, Y: misleading, appears as 3-hop path Solution: restructure AS Path attribute as: Path: (Sequence (T), Set (X, Y)) If Z wants to advertise path: Path: (Sequence (Z, T), Set (X, Y)) In practice used only if paths in set have same attributes

30. Important Concepts Wide area Internet structure and routing driven by economic considerations Customer, providers and peers BGP designed to: Provide hierarchy that allows scalability Allow enforcement of policies related to structure Mechanisms Path vector – scalable, hides structure from neighbors, detects loops quickly

31. History Mid-80s: EGP Reachability protocol (no shortest path) Did not accommodate cycles (tree topology) Evolved when all networks connected to NSF backbone Result: BGP introduced as routing protocol Latest version = BGP 4 BGP-4 supports CIDR Primary objective: connectivity not performance

32. Outline Internet Structure/Routing hierarchy External BGP (E-BGP) Internal BGP (I-BGP)

33. Internal vs. External BGP R3R4 R1 R2 E-BGP BGP can be used by R3 and R4 to learn routes How do R1 and R2 learn routes? AS1 AS2

34. Internal BGP (I-BGP) Same messages as E-BGP Different rules about re-advertising prefixes: Prefix learned from E-BGP can be advertised to I-BGP neighbor and vice-versa, but Prefix learned from one I-BGP neighbor cannot be advertised to another I-BGP neighbor Reason: no AS PATH within the same AS and thus danger of looping.

35. Internal BGP (I-BGP) R3R4 R1 R2 E-BGP I-BGP R3 can tell R1 and R2 prefixes from R4 R3 can tell R4 prefixes from R1 and R2 R3 cannot tell R2 prefixes from R1 R2 can only find these prefixes through a direct connection to R1 Result: I-BGP routers must be fully connected (via TCP)! contrast with E-BGP sessions that map to physical links AS1 AS2

36. 15-441 © 200836 What is DNS? DNS (Domain Name Service) is primarily used to translate human readable names into machine usable addresses, e.g., IP addresses. DNS goal: Efficiently locate resources. E.g., Map name  IP address Scale to many users over a large area Scale to many updates Lecture 13

37. How resolve name  IP addr? Lecture 1315-441 © 200837

38. 15-441 © 200838 Obvious Solutions (1) Why not centralize DNS? Single point of failure Traffic volume Distant centralized database Single point of update Doesn’t scale! Lecture 13

39. 15-441 © 200839 Obvious Solutions (2) Why not use /etc/hosts? Original Name to Address Mapping Flat namespace /etc/hosts SRI kept main copy Downloaded regularly Mid 80’s this became untenable. Why? Count of hosts was increasing: machine per domain  machine per user Many more downloads Many more updates /etc/hosts still exists. Lecture 13

40. 15-441 © 200840 Domain Name System Goals Basically a wide-area distributed database (The biggest in the world!) Scalability Decentralized maintenance Robustness Global scope Names mean the same thing everywhere Don’t need all of ACID Atomicity Strong consistency Do need: distributed update/query & Performance Lecture 13

41. 15-441 © 200841 Programmer’s View of DNS Conceptually, programmers can view the DNS database as a collection of millions of host entry structures: in_addr is a struct consisting of 4-byte IP addr Functions for retrieving host entries from DNS: gethostbyname: query key is a DNS host name. gethostbyaddr: query key is an IP address. /* DNS host entry structure */ struct hostent { char *h_name; /* official domain name of host */ char **h_aliases; /* null-terminated array of domain names */ int h_addrtype; /* host address type (AF_INET) */ int h_length; /* length of an address, in bytes */ char **h_addr_list; /* null-termed array of in_addr structs */ }; Lecture 13

42. 15-441 © 200842 DNS Message Format Identification No. of Questions No. of Authority RRs Questions (variable number of answers) Answers (variable number of resource records) Authority (variable number of resource records) Additional Info (variable number of resource records) Flags No. of Answer RRs No. of Additional RRs Name, type fields for a query RRs in response to query Records for authoritative servers Additional “helpful info that may be used 12 bytes Lecture 13

43. 15-441 © 200843 DNS Header Fields Identification Used to match up request/response Flags 1-bit to mark query or response 1-bit to mark authoritative or not 1-bit to request recursive resolution 1-bit to indicate support for recursive resolution Lecture 13

44. 15-441 © 200844 DNS Records RR format: (class, name, value, type, ttl) DB contains tuples called resource records (RRs) Classes = Internet (IN), Chaosnet (CH), etc. Each class defines value associated with type For “IN” class: Type=A name is hostname value is IP address Type=NS name is domain (e.g. foo.com) value is name of authoritative name server for this domain Type=CNAME name is an alias name for some “canonical” name value is canonical name Type=MX value is hostname of mailserver associated with name Lecture 13

45. 15-441 © 200845 Properties of DNS Host Entries Different kinds of mappings are possible: 1-1 mapping between domain name and IP addr: provolone.crcl.cs.cmu.edu maps to 128.2.218.81 Multiple domain names maps to the same IP addr: www.scs.cmu.edu and www.cs.cmu.edu both map to 128.2.203.164 Single domain name maps to multiple IP addresses: aol.com and www.aol.com map to multiple IP addrs. Some valid domain names don’t map to any IP addr: crcl.cs.cmu.edu doesn’t have a host Lecture 13

46. 15-441 © 200846 DNS Design: Hierarchy Definitions root edunetorgukcom gwuucbcmubu mit cs ece crcl Each node in hierarchy stores a list of names that end with same suffix Suffix = path up tree E.g., given this tree, where would following be stored: Fred.com Fred.edu Fred.cmu.edu Fred.crcl.cs.cmu.edu Fred.cs.mit.edu Lecture 13

47. 15-441 © 200847 DNS Design: Zone Definitions Single node Subtree Complete Tree Zone = contiguous section of name space E.g., Complete tree, single node or subtree A zone has an associated set of name servers Must store list of names and tree links root edunetorgukcom gwuucbcmubu mit cs ece crcl Lecture 13

48. 15-441 © 200848 DNS Design: Cont. Zones are created by convincing owner node to create/delegate a subzone Records within zone stored in multiple redundant name servers Primary/master name server updated manually Secondary/redundant servers updated by zone transfer of name space Zone transfer is a bulk transfer of the “configuration” of a DNS server – uses TCP to ensure reliability Example: CS.CMU.EDU created by CMU.EDU admins Who creates CMU.EDU or.EDU? Lecture 13

49. 15-441 © 200849 DNS: Root Name Servers Responsible for “root” zone 13 root name servers Currently {a-m}.root-servers.net Local name servers contact root servers when they cannot resolve a name Why 13? Lecture 13

50. Not really 13! Lecture 1315-441 © 200850 10/08, from www.root-servers.orgwww.root-servers.org Check out anycast)

51. So Far Database structure Hierarchy of labels x.y.z Organized into zones Zones have nameservers (notice plural!) Database layout Records which map names  names, names  ip, etc. Programmer API: gethostbyname, … Lecture 1315-441 © 200851

52. 15-441 © 200852 Servers/Resolvers Each host has a resolver Typically a library that applications can link to Local name servers hand-configured (or DHCP) (e.g. /etc/resolv.conf) Name servers Either responsible for some zone or… Local servers Do lookup of distant host names for local hosts Typically answer queries about local zone Lecture 13

53. 15-441 © 200853 Typical Resolution Client Local DNS server root & edu DNS server ns1.cmu.edu DNS server www.cs.cmu.edu NS ns1.cmu.edu www.cs.cmu.edu NS ns1.cs.cmu.edu A www=IPaddr ns1.cs.cmu.edu DNS server Hmm: Notice root server returned NS ns1.cmu.edu Lecture 13

54. 15-441 © 200854 Typical Resolution Steps for resolving www.cmu.edu Application calls gethostbyname() (RESOLVER) Resolver contacts local name server (S 1 ) S 1 queries root server (S 2 ) for (www.cmu.edu)www.cmu.edu S 2 returns NS record for cmu.edu (S 3 ) What about A record for S 3 ? This is what the additional info section is for (PREFETCHING) S 1 queries S 3 for www.cmu.eduwww.cmu.edu S 3 returns A record for www.cmu.eduwww.cmu.edu Can return multiple A records  What does this mean? Lecture 13

55. 15-441 © 200855 Lookup Methods Recursive query: Server goes out and searches for more info Only returns final answer or “not found” Iterative query: Server responds with as much as it knows. “I don’t know this name, but ask this server” Workload impact on choice? Root/distant server does iterative Local server typically does recursive requesting host surf.eurecom.fr gaia.cs.umass.edu root name server local name server dns.eurecom.fr 1 2 3 4 5 6 authoritative name server dns.cs.umass.edu intermediate name server dns.umass.edu 7 8 iterated query Lecture 13

56. How to manage workload? Does root nameserver do recursive lookups? What about other zones? What about imbalance in popularity?.com versus.dj google.com versus bleu.crcl.cs.cmu.edu? How do we scale query workload? Lecture 1315-441 © 200856

57. 15-441 © 200857 Workload and Caching DNS responses are cached Quick response for repeated translations Other queries may reuse some parts of lookup E.g., NS records for domains DNS negative queries are cached Don’t have to repeat past mistakes E.g., misspellings, search strings in resolv.conf How do you handle updates? Lecture 13

58. 15-441 © 200858 Workload and Caching DNS responses are cached Quick response for repeated translations Other queries may reuse some parts of lookup E.g., NS records for domains DNS negative queries are cached Don’t have to repeat past mistakes E.g., misspellings, search strings in resolv.conf Cached data periodically times out Lifetime (TTL) of data controlled by owner of data TTL passed with every record Lecture 13

59. 15-441 © 200859 Typical Resolution Client Local DNS server root & edu DNS server ns1.cmu.edu DNS server www.cs.cmu.edu NS ns1.cmu.edu www.cs.cmu.edu NS ns1.cs.cmu.edu A www=IPaddr ns1.cs.cmu.edu DNS server Lecture 13

60. 15-441 © 200860 Subsequent Lookup Example Client Local DNS server root & edu DNS server cmu.edu DNS server cs.cmu.edu DNS server ftp.cs.cmu.edu A ftp=IPaddr ftp.cs.cmu.edu Lecture 13

61. 15-441 © 200861 Reliability DNS servers are replicated Name service available if ≥ one replica is up Queries can be load balanced between replicas UDP used for queries Need reliability  must implement this on top of UDP! Why not just use TCP? Try alternate servers on timeout Exponential backoff when retrying same server Same identifier for all queries Don’t care which server responds Lecture 13

62. So far Hierarchial name space Lecture 1315-441 © 200862

63. 15-441 © 200863 Reverse DNS Task Given IP address, find its name Method Maintain separate hierarchy based on IP names Write 128.2.204.27 as 27.204.2.128.in-addr.arpa Why is the address reversed? Managing Authority manages IP addresses assigned to it E.g., CMU manages name space 2.128.in-addr.arpa edu cmu cs bleu 128.2.204.27 crcl unnamed root arpa in-addr 128 2 204 27 Arpa: backronym  Address and Routing Parameter Area Lecture 13

64. 15-441 © 200864.arpa Name Server Hierarchy At each level of hierarchy, have group of servers that are authorized to handle that region of hierarchy 128 2 204 bleu 128.2.204.27 in-addr.arpa a.root-servers.net m.root-servers.net chia.arin.net (dill, henna, indigo, epazote, figwort, ginseng) cucumber.srv.cs.cmu.edu, t-ns1.net.cmu.edu t-ns2.net.cmu.edu mango.srv.cs.cmu.edu (peach, banana, blueberry) Lecture 13

65. 15-441 © 200865 Prefetching Name servers can add additional data to response Why would they? Lecture 13

66. 15-441 © 200866 Prefetching Name servers can add additional data to response Why would they? Typically used for prefetching CNAME/MX/NS typically point to another host name Responses include address of host referred to in “additional section” Lecture 13

67. 15-441 © 200867 Mail Addresses MX records point to mail exchanger for a name E.g. cmu.edu. 2590 IN MX 10 CMU-MX4.ANDREW.cmu.edu. cmu.edu. 2590 IN MX 10 CMU-MX5.ANDREW.cmu.edu. Addition of MX record type proved to be a challenge How to get mail programs to lookup MX record for mail delivery? Needed critical mass of such mailers Could we add a new one now? Lecture 13

68. 15-441 © 200868 Outline DNS Design DNS Today Lecture 13

69. 15-441 © 200869 Root Zone Generic Top Level Domains (gTLD) =.com,.net,.org, etc… Country Code Top Level Domain (ccTLD) =.us,.ca,.fi,.uk, etc… Root server ({a-m}.root-servers.net) also used to cover gTLD domains Load on root servers was growing quickly! Moving.com,.net,.org off root servers was clearly necessary to reduce load  done Aug 2000 How significant an effect would this have? On load? On performance? Lecture 13

70. 15-441 © 200870 gTLDs Unsponsored.com,.edu,.gov,.mil,.net,.org.biz  businesses.info  general info.name  individuals Sponsored (controlled by a particular association).aero  air-transport industry.cat  catalan related.coop  business cooperatives.jobs  job announcements.museum  museums.pro  accountants, lawyers, and physicians.travel  travel industry Starting up.mobi  mobile phone targeted domains.post  postal.tel  telephone related Proposed.asia,.cym,.geo,.kid,.mail,.sco,.web,.xxx Whatever you want! Is there anything special about.com? What about adding.goldstein as a gTLD? Lecture 13

71. 15-441 © 200871 New Registrars Network Solutions (NSI) used to handle all registrations, root servers, etc… Clearly not the democratic (Internet) way Large number of registrars that can create new domains  However NSI still handles A root server Lecture 13

72. 15-441 © 200872 Measurements of DNS No centralized caching per site Each machine runs own caching local server Why is this a problem? How many hosts do we need to share cache?  recent studies suggest 10-20 hosts “Hit rate for DNS:1 - (#DNS/#connections)  80% Is this good or bad? Most Internet traffic was Web with HTTP 1.0 What does a typical page look like?  average of 4-5 imbedded objects  needs 4-5 transfers This alone accounts for 80% hit rate! Lower TTLs for A records does not affect performance DNS performance really relies more on NS-record caching Lecture 13

73. 15-441 © 200873 Measurements of DNS No centralized caching per site Each machine runs own caching local server Why is this a problem? How many hosts do we need to share cache?  recent studies suggest 10-20 hosts “Hit rate for DNS:1 - (#DNS/#connections)  80% Is this good or bad? Most Internet traffic was Web with HTTP 1.0 What does a typical page look like?  average of 4-5 imbedded objects  needs 4-5 transfers This alone accounts for 80% hit rate! Lower TTLs for A records does not affect performance DNS performance really relies more on NS-record caching Lecture 13

74. 15-441 © 200874 Tracing Hierarchy (1) Dig Program Allows querying of DNS system Use flags to find name server (NS) Disable recursion so that operates one step at a time All.edu names handled by set of servers unix> dig +norecurse @a.root-servers.net NS kittyhawk.cmcl.cs.cmu.edu ;; AUTHORITY SECTION: edu. 172800 IN NS L3.NSTLD.COM. edu. 172800 IN NS D3.NSTLD.COM. edu. 172800 IN NS A3.NSTLD.COM. edu. 172800 IN NS E3.NSTLD.COM. edu. 172800 IN NS C3.NSTLD.COM. edu. 172800 IN NS F3.NSTLD.COM. edu. 172800 IN NS G3.NSTLD.COM. edu. 172800 IN NS B3.NSTLD.COM. edu. 172800 IN NS M3.NSTLD.COM. Zone TTL Class Type Value Lecture 13

75. 15-441 © 200875 Tracing Hierarchy (2) 3 servers handle CMU names unix> dig +norecurse @e3.nstld.com NS kittyhawk.cmcl.cs.cmu.edu ;; AUTHORITY SECTION: cmu.edu. 172800 IN NS CUCUMBER.SRV.cs.cmu.edu. cmu.edu. 172800 IN NS T-NS1.NET.cmu.edu. cmu.edu. 172800 IN NS T-NS2.NET.cmu.edu. Lecture 13

76. 15-441 © 200876 Tracing Hierarchy (3 & 4) 4 servers handle CMU CS names Quasar is master NS for this zone unix> dig +norecurse @t-ns1.net.cmu.edu NS kittyhawk.cmcl.cs.cmu.edu ;; AUTHORITY SECTION: cs.cmu.edu. 86400 IN NS MANGO.SRV.cs.cmu.edu. cs.cmu.edu. 86400 IN NS PEACH.SRV.cs.cmu.edu. cs.cmu.edu. 86400 IN NS BANANA.SRV.cs.cmu.edu. cs.cmu.edu. 86400 IN NS BLUEBERRY.SRV.cs.cmu.edu. unix>dig +norecurse @blueberry.srv.cs.cmu.edu NS kittyhawk.cmcl.cs.cmu.edu ;; AUTHORITY SECTION: cs.cmu.edu. 300 IN SOA QUASAR.FAC.cs.cmu.edu. Lecture 13

77. 15-441 © 200877 DNS (Summary) Motivations  large distributed database Scalability Independent update Robustness Hierarchical database structure Zones How lookups are done Caching/prefetching and TTLs Reverse name lookup What are the steps to creating your own domain? Lecture 13