Addressing and Routing

Transcription

1 Addressing and Routing Andrew Scott

2 Physical/ Hardware Addresses Aka MAC* or link(-layer) address Can only talk to things on same link Unique ID given to every network interface card (NIC) on manufacture Generally written in form: f0:1c:d:f5:a5:41 Flat addressing scheme IDs allocated sequentially as devices manufactured Cannot tell anything about ID by knowing of ID1 * From Medium Access Control function in networks

3 Internet Protocol Connectionless network protocol No attempt to build path prior to transmission Best-effort packets may be: Lost Delivered out of order Duplicated Delayed

4 Transmit : encapsulate Receive : de-capsulate Protocol Encapsulation IP packets sit within link-layer frames Data Application Transport layer Application header Data Network layer Transport header Application header Data Link-layer IP header Transport header Application header Data Data-link header IP header Transport header Application header Data

5 IP Addresses Hierarchical Not flat like Ethernet/ MAC address Topological Reflects network structure Not geographic Though topology might be constrained by geography

6 IPv4 Addresses 3 bit/ 4 byte identifier Identify network interfaces not hosts/ devices * A device will have multiple addresses At least one per virtual or physical network interface Notice all devices use same loopback address: * Physical interfaces can have multiple IP addresses but not generally useful

7 IPv4 Addresses Typically written as 4 bytes, e.g., Often treated as a 3 bit long, i.e., 0x Split into network and host parts Split can be made at different positions Network Identifier Host Identifier

8 IPv4 Addresses Interfaces on same link share network part Network Identifier Host

9 Private Addresses Available for (organisation) internal/ test networks Must not be made externally visible Free for use, but check your organisation isn t already using part of range, for example, University makes extensive use of 10.xxx addresses RFC1918

10 Private Addresses Allow autonomy from any numbering authority Note that private addresses are not globally unique Therefore meaningless outside of organisation Internet Service Provider (ISP) Private Internal Network Home Network A Home Network B

11 Special Addresses Multicast Packets delivered to a group of interested devices Hosts subscribe to group in order to receive packets Can be useful for media or data distribution services RFC5771 Broadcast For delivery to all devices on local network RFC919

12 IPv Address Representation Addresses 18 bits long Prefix/ length notation prefix /n Written in hexadecimal, in following format XXXX : XXXX : XXXX : XXXX : XXXX : XXXX : XXXX : XXXX/ n Leading zeros in group can be omitted, for example 1080:0:0:0:8:800:00C:417A/4 At most one set of contiguous zeros can be dropped 1080::8:800:00C:417A/4

13 Sending a Network (IP) Packet We must tell our link-layer to send packet to hardware (MAC) address of gateway router Packet is addressed to IP address of server and sent by link-layer to MAC address of gateway router IP: MAC: 08:00:7:00:34:ca Gateway Router IP: MAC: 08:00:0:11:4a:e9 Internet IP: MAC: unknown

14 Looking Hop-by-Hop Client Gateway Router Server Interface A Interface B Interface C Interface D :00:7:00:34:ca :00:0:11:4a:e :00:0:79:a: :00:7:33:7b:a9 Ethernet Ethernet IP src: A IP dst: D Eth src: A Eth dst: B IP src: A IP dst: D Eth src: C Eth dst: D

15 Mapping Addresses Names IP Addresses Domain Name Service (DNS) IP Addresses Hardware/ Link-Layer Addr Address Resolution Protocol (ARP) Link-Layer Address IP Address Reverse Address Resolution Protocol (RARP) Hardware addresses more commonly known as Medium Access Control (MAC) addresses See the IEEE LAN/RM in first lecture to see why

16 Cached mappings : Windows C:\> arp -a Interface: xc Internet Address Physical Address Type c-4-f8-b-e dynamic ff-ff-ff-ff-ff-ff static e static e fc static e-7f-ff-fa static ff-ff-ff-ff-ff-ff static Linux acs:~$ arp -a? ( ) at 00:07:b4:00:5:0 [ether] on eth0

17 and Cisco IOS wallace# show ip arp Protocol Address Age(min) Hardware Addr Type Interface Internet b ARPA GigE0/1.70 Internet c.99f.9e80 ARPA GigE0/1.70 Internet b4 ARPA GigE0/1.70 Internet d1.541f ARPA GigE0/1.70 Internet c.9a8.39e ARPA GigE0/1.70 Internet cf ARPA GigE0/1.70 Internet c.9c.75b9 ARPA GigE0/1.70

18 Getting from A to B Given a packet destined for host B Is B on same physical link? send direct Is B on a remote network? send to router local network destination A router B

19 More Generally PC A may have many interfaces, thus options If we send to a gateway router what next? How should packet be forwarded toward B? local network destination network A Internet B

20 Getting from A to B Internet formed from a set of routers No global view (at IP level) Routers conspire to deliver packets to destination Each pushing packets closer to their destination A B

21 Internet Routing Two distinct parts to process Routing Application level process to determine routes Forwarding System level process directing packets according to learned routes

22 Routing and Forwarding User Space : applications System Space : OS Kernel Send Packet Queue Routing Process Forwarding Process Routing Table Periodic update Forwarding Table Network Interface Cards (NICs) eth0 eth1

23 IP Forwarding Network layer IP Data-link layer IP Ether IP Wi-Fi Ethernet link-layer Wi-Fi link-layer

24 IPv4 Address Format 3 bit address Typically represented in dotted-quad form Each part represents 8 bit (byte), thus 0 55 Network Part Host Part

25 Internet Address Allocation Internet Corporation for Assigned Names and Numbers (ICANN) Internet Assigned Numbers Authority (IANA) European Internet Service Providers (ISPs)

26 IP Addresses Network addresses allocated to organisations by their Internet Service Provider (ISP) Used to correctly forward traffic how we know destination Fixed and cannot be changed by organisation Host addresses belong to organisation Allocate/ change them at will Network Host

27 Internal IP Sub-netting Within organisation Borrow host bits to form internal (sub-)networks Network part is fixed what ISP allocated to org. 8 bits s bits 4 - s bits Network Subnet Host 1 bits (allocated by ISP) s bits 1 - s bits Network Subnet Host 4 bits (allocated by ISP) Network s bits SN 8 - s Host

28 Sub-netting Notice we can t determine network-host boundary Subnet address only makes sense within organisation We need some more information ISP will still treat this as 1bit network address The network address it allocated to organisation 1 bits s bits 1 - s bits Network Subnet Host Network part now

29 Subnet Masks Network Host Address Subnet mask Result Network Host Address Subnet mask Result

30 Variable Length Subnet Masking (VLSM) Subnet mask is used to split (sub-)net and host parts Mask must have set of ones followed by set of zeros Once a zero bit appears, all remaining bits must be zero OK illegal Any IP address AND d with its subnet mask gives network address on which it resides

31 Subnet Masks Mask: Network-host boundary

32 Masks and Subnets Address Mask Subnet mask Single available subnet

33 Masks and Subnets Address X Mask Host bits Subnet mask bit unset in address bit set in address

34 Masks and Subnets Address X X Mask Host bits Subnet mask (18 + 4)

35 Masks and Subnets Address X X X Mask Host bits

36 Prefix Notation Address Mask bytes, 4 bits +3 = 7 bits Address: Mask: Prefix: / = = /4 /5 / /7

37 Addresses are bound to Interfaces Addresses belong to interfaces not machines Note: Can have multiple virtual interfaces on single physical interface Share same link layer Loopback Interface lo0: /8 Virtual Interface gre0: /3 eth0: /4 eth1: /4 Router

38 One Hop at a Time IP depends on underlying data-link protocol Data-link protocol can only address devices on same physical link/ network segment IP header holds endpoint addresses Original source and final destination Data-link frames holding packet sent hop-by-hop Always sent to next-hop router, until last subnet Data sent on wire Ethernet (Layer ) header IP (Layer 3) header UDP (Layer 4) header Payload/ Application data

39 Forwarding Table Each device maintains a forwarding table This is one of the device s network interfaces - can be interface name or address Net. Address/ Prefix Subnet Mask Next Hop Router Interface Metric ( cost ) eth is router that can get our packets to network This router MUST be on same (virtual) link/ subnet as eth0

40 Forwarding Table Many nodes have a default route prefix: /0 Route of last resort, when no other route available Default route typically toward our ISP s router Net. Address/ Prefix Subnet Mask Next Hop Router Interface Metric ( cost ) eth eth0 0

41 Checking Host Table route Kernel IP routing table U route up G gateway route H host route Destination Gateway Genmask Flags Metric Ref Use Iface default UG eth0 link-local * U eth * U eth0 root@acs:~# C:\> route print Network Destination Netmask Gateway Interface Metric On-link On-link On-link On-link On-link On-link On-link On-link

42 Updating Routing Table: Linux route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface default UG eth0 link-local * U eth * U eth0 root@as:~# root@as:~# route add -net netmask gw root@as:~# root@as:~# route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface default UG eth U eth0 link-local * U eth * U eth0 root@as:~# root@as:~# route del -net netmask

43 Multiple Next Hop Matches Likely to be more than one matching entry If only due to default route Which always matches Always select longest matching prefix Most specific entry Should be best path to destination

44 Longest Prefix Match Apply mask to address and compare with prefix Destination Address = Prefix Mask Masked Address =? Options are: shortest of two longest of two we select this = Prefix?

45 Why Multiple Matches? Two equivalent scenarios : 1. Fully enumerated table / / /4 All addresses bar one / / /4 A B Network Address N.H /4 A /4 A /4 A /4 B /4 B /4 B

46 Why Multiple Matches? Two equivalent scenarios : /1 with exception for / / / / / / /4 A B Simpler routing table (fewer to check = faster) Network Address N.H /1 A /4 B /4 B /4 B If we always select Longest Prefix Match we ll always get to correct destination

47 Building Tables Forwarding Table Subnet Nxt Hop / A / A / B / B A B

48 Route Summarisation Looking at that last example: We can combine these routes Two routes differ only in last bit Each pair has the same next hop * aka. Route Aggregation though this is really a specific wide area mechanism Initial Forwarding Table Subnet Nxt Hop A A B B New Forwarding Table Subnet Nxt Hop A B

49 Route Summarisation Notice we change mask as we combine routes R A /5 B / R /4 A W / X / B Y / Z / W X Y Z

50 Completing the tables A /5 B /5 A W / X / B /5 B Y / Z / A /5 W X Y Z

51 Question A router supporting variable length subnet masks and classless inter-domain routing (CIDR) has the following forwarding entries: Showing full details of your working identify: Which next hop entries would match each of the following destination IP addresses, and In each case, which next hop would the router select? i ii iii iv Address/ Prefix Length Next Hop /0 A /17 B /18 C /1 D / E

52 First WHICH ADDRESSES MATCH?

53 / matches to 17 bits doesn t match to 17 bits

54 / matches to 18 bits doesn t match to 18 bits

55 / matches to 1 bits doesn t match to 1 bits

56 / doesn t match to bits does match to bits

57 So WHAT CHOICES DO WE HAVE?

58 Choice is at 0 bits at 17 bits at 18 bits Select longest prefix (18 bits): C Address Prefix Length Next Hop Matches A Yes, always! B Yes C Yes D Obviously not E Obviously not

59 Choice is at 0 bits Select longest prefix (0 bits): A We use the default route Address Prefix Length Next Hop Matches A Yes, always! B No C No D Obviously not E Obviously not

60 Choice is at 0 bits at 1 bits Longest prefix is (1 bits): D Address Prefix Length Next Hop Matches A Yes, always! B Obviously not C Obviously not D Yes E No

61 Choice is at 0 bits at bits Longest prefix is ( bits): E Address Prefix Length Next Hop Matches A Yes, always! B Obviously not C Obviously not D No E Yes

62 Summary Destination Valid next hops Next hop A, B, C C A A A, D D A, E E