Lecture 2: Basic routing, ARP, and basic IP

Transcription

1 Internetworking Lecture 2: Basic routing, ARP, and basic IP Literature: Forouzan, TCP/IP Protocol Suite: Ch 6-8

2 Basic Routing Delivery, Forwarding, and Routing of IP packets

3 Connection-oriented vs Connectionless Connection-Oriented Services The network layer establishes a connection between a source and a destination Packets are sent along the connection. The decision about the route is made once at connection establishment Routers/switches in connection-oriented networks are stateful Connectionless Services The network layer treats each packet independently Route lookup for each packet (routing table) IP is connectionless IP routers are stateless

4 Direct vs Indirect delivery Direct delivery The final destination is connected to the same physical network as the sender. IP destination address and local interface has same netmask Map IP address to physical address: ARP Indirect delivery From router to router, last delivery is direct Destination address and routing table: Routing A direct delivery B A indirect delivery R1 indirect delivery R2 indirect delivery R3 direct delivery direct delivery R B

5 Next-hop Routing How do you hold information about route from A to all other hosts? A R1 R2 R3 B Store table of host/network address and nexthop in every node N1, - N2, R1 N3, R1 N4, R1 A N1, - N2, R2 N3, R2 N4, R2 N1, R1 N2, R4 N3, R4 N4, R3 N1, R2 N2, R2 N3, R2 N4, - R1 R2 R3 N1, R3 N2, R3 N3, R3 N4, - B N1 N2 R4 N3 N4 C D E F

6 Routing Table Search - Classful Determine class from destination address Search within class Routing table often divided into buckets Class A bucket destination IP address Class B bucket Class C bucket

7 Routing Table Search - Classless Longest prefix first Conceptually: divide table in 32 buckets - one for each netmask length and match destination with longest prefixes first SW algorithms: tree, binary trees, tries (different data structures) HW support: TCAMs Content Addressable Memory Masklen 0 1 Netid Netid destination IP address

8 Routing Tables The basic idea with IP addressing (and CIDR) is to aggregate addresses more specific networks (with longer prefixes) less specific networks (with shorter prefixes) More aggregation leads to smaller routing tables The ideal situation is to have domains publishing (exporting) only a small set of prefixes Effective address assignment policy Some mechanisms lead to increased fragmentation # of available addresses decreasing distribution of long prefixes (/24) Multihoming - sites having several subnetworks from different providers Current routing tables (# of entries) is ~ (~60% are /24 prefixes)

9 Routing Table Common Fields Mask Network Address Next-hop Address Interface Flags Reference count Use Mask netmask applied for the entry [ ] Network address destination network [ ] Next-hop address next router [ ] Interface outgoing interface [eth0] Flags status/info [U(p), G(ateway), H(ost-specific)...] Reference count # of users using this route Use # of packets transmitted for this destination

10 Control Plane IP Router Model IP Routing RIB Routing Information Base Data Plane IP Forwarding FIB Forwarding Information Base Ethernet Interface Router FDDI Interface A Router can be partitioned into a dataplane and a controlplane The dataplane is fast and special purpose handles packet forwarding in real-time The control plane is general purpose handles routing in the background

11 IP Forwarding A router switches packets between network interfaces Extracts header information from the incoming datagram Destination IP address Makes a lookup in the forwarding information base by making a match against networks Next-Hop IP address, Outgoing interface,... Modifies datagram header Sends on outgoing interface But a router performs much more than IPv4 lookup Access lists, filtering Traffic management Other protocols: Bridging, MPLS, IPv6,...

12 ARP Mapping between logical IP addresses and physical addresses

13 Logical and Physical Addresses Name: bsdi sun svr4 MAC addr: IP addr: 0:0:c0:6f:2d:40 8:0:20:3:f6:42 0:0:c0:c2:9b: A host s network interface card (NIC) has: a hardcoded, physical MAC address e.g., 48-bit Ethernet address a configured, logical IP address a configured name

14 Communicating with a next-hop Name: bsdi sun svr4 MAC addr: IP addr: 0:0:c0:6f:2d:40 8:0:20:3:f6:42 0:0:c0:c2:9b: Problem: bsdi wants to send an IP packet to svr4 No routers between sender and receiver directly connected host Getting the IP address of svr4 Static configuration DNS: Name Address (Later lectures) Getting the MAC address of svr4 Static configuration Dynamic Address Resolution - ARP

15 ARP - Address Resolution Protocol Problem: we are to send a packet to an interface on a directly attached network - we know the IP-address of the destination but not the MAC address. Idea: Broadcast a request - On which MAC address can IP-address X be reached?. ARP request The host/router with the destination replies with its MAC address ARP reply This is the basic functionality of ARP

16 ARP Example bsdi intends to send an IP datagram to svr4 ( ) 1. Send an ARP request on broadcast to all stations: who has ? 2. svr4 identifies it as its own address and sends an ARP reply on unicast back to bsdi I have and its mac address is 0:0:c0:c2:9b:26 3. bsdi sends the datagram to svr4 using the resolved mac address 4. Note that sun and svr4 can update their ARP caches with bsdi! bsdi sun svr

17 Two length fields ARP Packet Hardware (Ethernet address length: 6) Protocol (IP address length: 4) Sender Ethernet and IP address Target Ethernet and IP address ARP is encapsulated directly into a data link frame (e.g., Ethernet) hardware size hw type type prot 2 2 hw len prot len op sender Ethernet addr sender IP addr target Ethernet addr protocol size target IP addr

18 ARP Optimizations ARP cache Resolved addresses are saved in a cache. Works because of correlations in use of addresses Limits ARP traffic Entries in the ARP cache times out Network is snooped Since the sender s Internet-to-Physical address binding is in every ARP broadcast; (all) receivers update their caches before processing an ARP packet

19 ARP Timeouts If there is no reply to an ARP request The machine is down or not responding Request was lost, therefore retry (but not too often) Eventually give up (When?) ARP cache timeouts completed entry in 20 minutes (BSD Unix) incomplete entry in 3 minutes (BSD Unix)

20 Indirect/Direct Delivery and ARP A sends an IP packet to B through router R Ethernet links to connect A and B to R IP A IP R IP B MAC a MAC r1 MAC r2 MAC b IP Header Src: A, Dst: B Src: A, Dst: B Ethernet Header Src: a, Dst: r1 Src: r2, Dst: b Indirect delivery Direct delivery

21 Proxy ARP (RFC 826) Proxy ARP - someone responds to ARP requests on someone else s behalf Allows sub-networks to be hidden Example: sun is hidden behind netb: Netb responds on behalf of sun. gemini arp reply arp request for netb sun slip

22 Gratuitous ARP Host sends an ARP request of its own address Generally done at boot time to inform other machines of its address (possibly a new address) - they get a chance to update their cache entries immediately Lets hosts check to see if there is another machine claiming the same address duplicate IP address sent from Ethernet address a:b:c:d:e:f As noted before, hosts have paid the price by servicing the broadcast, so they can cache this information - this is one of the ways the proxy ARP server could know the mapping Note that faking that you are another machine can be used to provide failover for servers

23 RARP: Reverse Address Resolution Protocol (RFC 903) How to get your own IP address, when all you know is your link address Necessary if you don t have a disk or other stable storage RARP request - broadcast to every host on the network (i.e., EtherDST=0xFFFFFF), TYPE=0x8035 RARP server: I know that address! and sends an RARP reply Source host - receives the RARP reply, and now knows its own IP addr RARP packet has exactly the same format as ARP packet BOOTP/DHCP is a more powerful alternative to RARP

24 RARP Server Someone has to know the mappings - quite often this is in the file /etc/ethers Since this information is generally in a file, RARP servers are generally implemented as user processes Unlike ARP responses which are generally part of the TCP/IP implementation (often part of the kernel) How does the process get the packets - since they aren t IP and won t come across a socket? PCAP Packet Capture (used by Tcpdump/Ethereal) BPF Berkeley Packet Filter (older) RARP requests are sent as hardware level broadcasts - therefore are not forwarded across routers

25 IP Basic functionality and the IP packet header

26 Issues in IP Following the end2end argument, only the absolutely necessary functionality is in IP Best Effort Service: Unreliable and Connectionless Application or Transport layer handles reliability How to deliver datagrams over multiple links (hops) in an internetwork? Addressing Best-effort delivery service Forwarding of packets from one link to another Error handling

27 IPv4 Header RFC 791 Version HLEN Header Length Type of Service Total Length Header + Payload Fragmentation ID, Flags, Offset TTL Time To Live Limits lifetime Protocol Higher level protocol Header checksum IP Addresses Source, Destination Options The McGraw-Hill Companies, Inc., 2000

28 Version 3 (IEN 21) The Version Field Stems from when TCP was being split into one component handling hop-by-hop communication (IP) and one component handling endto-end communication (TCP). IEN 21 1 February Version 4 (RFC 791) IPv4 Version 5 (RFC 1190) ST-II - Multimedia streaming protocol Version 6 (RFC 2460) IPv6

29 Header Length (4 bits) The Length Fields Size of IPv4 header including options. Expressed in number of 32-bit words (4-byte words) Min is 5 words (=20 bytes) Max is 15 words (=60 bytes) limited size limited use Total Length (16 bits) Total length of datagram including header. If datagram is fragmented: length of fragment. Expressed in bytes. Max: bytes. (This is IPs length limit) Many systems only accept 8K bytes.

30 The Type of Service Field Type of Service (ToS): 8 bits Intended as a field for specifying Quality of Service on a per-packet basis. Few applications set the TOS field. Unless an added cost/policy check/ associated with usage of a precedence level - it is very likely going to be abused. Long history of experimental use RFC 791 original RFC 1122, 1349, 1455 modified the meaning of the ToS field Current proposal: RFC 2474 Differentiated Services Early Congestion Notification (ECN): RFC 2481, 3168

31 The ToS Byte Original proposal Bit 0 Bit 7 Precedence TOS Original Proposal RFC 791 Bits 0-2: Precedence Defines priority e.g., when packets must be dropped Bits 3-5: TOS Bit 3: 0 = Normal Delay, 1 = Low Delay Bit 4: 0 = Normal Throughput, 1 = High Throughput Bit 5: 0 = Normal Reliability, 1 = High Reliability.

32 DSField Current Proposal Bit 0 Bit 7 DSCP ECN Differentiated Services (DiffServ) proposes to use 6 of these bits to provide 64 priority levels - calling it the Differentiated Service (DS) field RFC 2474 Bits 0-6: Differentiated Services CodePoint (DSCP) The DSCP is set when entering an area and determines the QoS handling of the IP datagram in the routers within that area Scheduling Shaping Queue Dropping Explicit Congestion Avoidance (ECN) ECN Capable Transport (ECT) Congestion Experienced (CE)

33 Fragmentation MTU The McGraw-Hill Companies, Inc., 2000 If the IP datagram is larger than the MTU of the link layer, it must be divided into several pieces to fit the MTU this is called fragmentation

34 Fragmentation cont d Physical networks maximum frame size MTU Maximum Transfer Unit. A host or router transmitting datagram larger than MTU of link must divide it into smaller pieces - fragments. Both hosts and router may fragment But only destination host reassemble! Each fragment routed separately as independent datagram In effect, only datagram service (e.g. UDP) TCP uses 576 byte MTU or path MTU discovery 3 fields of the IP header concerns fragmentation

35 Identification: 16 bits The Fragmentation Fields ID + src IP addr uniquely identifies each datagram sent by a host The ID is copied to all fragments of a datagram upon fragmentation Flags: 3 bits RF (Reserved Fragment) for future use (set to 0) DF (Dont Fragment). Set to 1 if datagram should not be fragmented. If set and fragmentation needed, datagram will be discarded and an error message will be returned to the sender MF (More Fragments) Set to 1 for all fragments, except the last. Fragmentation Offset: 13 bits 8-byte units: (ip ip_frag << 3) Shows relative position of a fragment with respect to the whole datagram

36 Fragmentation Example Offset The McGraw-Hill Companies, Inc., 2000

37 Fragmentation Example Detailed MTU = 1500 bytes IPv4 hdr id=0, DF=0 UDP hdr Data 20 bytes 8 bytes 1473 bytes IPv4 hdr id=n, DF=0 MF=1, off=0 UDP hdr Data IPv4 hdr id=n, DF=0 MF=0, off=185 Data 20 bytes 8 bytes 1472 bytes 20 bytes 1 byte Offset = x8 = 1480 bytes

38 TTL - Time To Live: 8 bits The TTL field Limit the lifetime of a datagram - avoid infinite loops A router receiving a TTL>1 decrements the TTL and forwards it A TTL <= 1 shall not be forwarded ICMP time exceeded is returned to the sender (later slide) Recommended value is 64 Should really be called Hop Limit (as in IPv6) Historically: Every router holding a datagram for more than 1 second should decrement the TTL by the number of seconds.

39 The Protocol Field Demultiplexing to higher layers Assigned by IANA Internet Assigned Numbers Authority A subset (out of 134) assigned decimal keyword ICMP IP TCP UDP IPv6 RSVP protocol Internet Control Message IP in IP (encapsulation) Transmission Control User Datagram IPv6 in IPv4 Reservation Protocol

40 Header Checksum Ensures integrity of header fields Hop-by-hop (not end-to-end) The header fields must be correct for proper and safe processing. The payload is not covered. Other checksums Link-level CRC. IP assumes a strong L2 checksum/crc. Hop-by-hop. L4 checksums, eg TCP/ICMP/UDP checksums cover payload. End-to-end. Internet Checksum Algorithm, RFC 1071 Treat header as sequence of 16-bit integers. Add them together Take the one s complement of the result.

41 Basic Routing Summary Connectionless, next-hop routing Routing tables: RIBs and FIBs Longest prefix match Address resolution ARP RARP IP Internet Protocol Basic functionality Header fields