Chapter 4 Hierarchy DHCP ICMP Professor Rick Han

Chapter 4 Hierarchy, DHCP, ICMP Professor Rick Han University of Colorado at Boulder rhan@cs. colorado. edu Prof. Rick Han, University of Colorado at Boulder

Announcements • Homework #3 on Web, due March 12 (two weeks), netstat portion online later today • Programming Assignment #2 coming… • Midterm March 14 • Last week’s lectures on Web • Next, more on hierarchy, DHCP, ICMP, … Prof. Rick Han, University of Colorado at Boulder

Recap of Previous Lecture • Link State vs. Distance Vector • • • Routing Update Size Routing Update Comm. Overhead Convergence Speed Complexity Space Robustness • • • Queue Length Delay Normalized Hop Count • BGP • Link State Cost Metric • Hierarchical Routing Prof. Rick Han, University of Colorado at Boulder

Scalability in Internet Routing (2) AS 1 Inter-Domain Routing Border/ RIP Gateway Router Border/ Gateway Router Intra-Domain Routing. Prof. Rick Han, University of Colorado at Boulder AS 2 OSPF

Border Gateway Protocol (BGP) • Interdomain Routing • “Path” Vector similar to Distance Vector – BGP router advertises only reachability info in its vector, not costs/hop counts • E. g. networks 128. 96, 192. 4. 153, and 192. 4. 3 can be reached from AS 2 – BGP router advertises its path to each destination in its vector • Avoids loops Prof. Rick Han, University of Colorado at Boulder

Interior Border Gateway Protocol • Each AS may have many border routers – Each border routers could inject 10000 prefixes from neighboring AS • LSP’s too large • Shortest path calculations too expensive • Border routers use interior BGP (IBGP) to limit routing info received by internal AS routers – IBGP routers determine best route to each destination – Only the best interior BGP router injects info into AS – Any router in AS learns one best border router to use when sending a packet externally Prof. Rick Han, University of Colorado at Boulder

Hierarchy In Addition To BGP • OSPF has its own hierarchy: group OSPF routers into areas – Hierarchy: AS –> OSPF area -> OSPF network • Subnets: – Fixed Classes A, B, C inefficient - Class B exhaustion – Subdivide a Class B IP address 128. 96. 34. 15 into <Network ID, Subnet ID, Host ID> • IP address is AND’ed with subnet mask to extract subnet address: – Subnet mask 255. 0 AND’ed with IP address 128. 96. 34. 15 gives subnet address 128. 96. 34 – Subnet mask 255. 128 AND’ed with IP address 128. 96. 34. 15 gives subnet address 128. 96. 34. 0 Prof. Rick Han, University of Colorado at Boulder

Additional Hierarchy (2) • Subnets: – When host 1 wants to send to host 2, AND the subnet mask with the destination IP address • If result is same subnet as sending host 1, then send over local LAN subnet • If result differs, then route to another subnet using subnet -to-subnet routing – Forwarding table changes from <destination IP, next hop> to <destination subnet, subnet mask, next hop> • For each entry, router AND’s subnet mask with dest. IP address and looks for match with destination subnet • Longest match breaks a tie Prof. Rick Han, University of Colorado at Boulder

Additional Hierarchy (3) • CIDR (Classless Interdomain Routing) Subnets: – When subnet mask is top N bits, then have a CIDR network prefix, • 192. 4. 16 with 20 bit prefix is written 192. 4. 16/20 • Approaches for fast prefix matching • How do nodes advertise their CIDR prefix/mask? – IP header only has 32 -bit address • Where is subnet mask? – BGP-4 path vectors and OSPF LSP’s carry the CIDR prefix along with the IP address, e. g. 192. 4. 16/20 Prof. Rick Han, University of Colorado at Boulder

Additional Hierarchy (4) • How do CIDR and non-CIDR routing stay compatible? – OSPF and BGP support CIDR, RIP does not – RIP builds a routing table by falling back to the old Class A, B, C network prefixes • makes RIP more inefficient • Packets are still routed correctly • CIDR Bottom line: – Improves address assignment efficiency – Helps aggregate routing to occur between networks rather than nodes Prof. Rick Han, University of Colorado at Boulder

Fast Matching of Variable Prefixes • Need to match CIDR network prefix with IP packet’s destination address – Brute force: for each destination router in list • apply mask to match prefix with destination address’s prefix • choose longest match Prof. Rick Han, University of Colorado at Boulder

Fast Matching of Variable Prefixes (2) • Speeding it up: Organize prefixes into a Patricia tree – If Nth bit is zero, go left, otherwise go right – Automatically finds longest match – Worst case = 32 bit tests 1 0 default 0/0 0 128. 2/16 Bit to test : 0 = left child, 1 = right child 0 1 1 0 163. 32/16 1 192. 3/20 Prof. Rick Han, University of Colorado at Boulder 252. 32. 150/24

Dynamic Host Configuration Protocol (DHCP) • RARP: A host knows a destination’s MAC address, but not destination’s IP address. • If destination=itself, then same goal as DHCP • BOOTP: similar goal to RARP, devised same time (1985) • DHCP: a host knows its own MAC address, but doesn’t have an IP address yet • Due to hierarchical addressing on network, can’t have manufacturer-preassigned IP addresses • Manual configuration is time-consuming, inflexible to changes, wastes addresses on disconnected nodes Prof. Rick Han, University of Colorado at Boulder

DHCP (2) • Goal: Automatic configuration of a host’s IP address – A host queries a DHCP server to obtain an IP address • How does a host find the address of a DHCP server? – Host sends a DHCPDISCOVER “limited IP broadcast packet”, with destination address 255 – Routers never forward such a packet, so it stays within LAN IP Router 255 LAN 1 Requesting Host LAN 2 DHCP Server Prof. Rick Han, University of Colorado at Boulder

DHCP (3) • DHCP relays enable one DHCP server per administrative domain, rather than one server per network – Requires a DHCP relay on each network – DHCP relay sends a unicast IP packet to DHCP server when it hears a local IP broadcast packet with DHCPDISCOVER 255 IP Router LAN 1 Requesting Host DHCP Relay Prof. Rick Han, University of Colorado at Boulder LAN 2 DHCP Server

DHCP (4) • DHCP server selects a dynamic IP addr. from pool – maps host’s MAC address to the dynamic IP address • Another advantage of relays: enable DHCP responses to get back to requesting host – Server can’t send directly back using host’s MAC address – DHCP server sends unicast to known IP address of DHCP relay, which sends to host’s local MAC address IP Router LAN 1 Requesting Host DHCP Relay Prof. Rick Han, University of Colorado at Boulder LAN 2 DHCP Server

DHCP (5) • Hosts cannot keep dynamic IP addresses indefinitely – Timeout/lease by DHCP • 3 days for Windows NT, 8 days for Windows 2000, 1 day… • Configurable when starting DHCP server – Host must periodically renew lease, otherwise IP address goes back into pool of available addresses • DHCP is implemented as an application-level protocol on top of UDP and IP Prof. Rick Han, University of Colorado at Boulder

Internet Control Message Protocol (ICMP) • Used for reporting errors in the Internet – Most ICMP packets contain diagnostic info sent back to source • Destination unreachable • TTL expired • Implemented at the same level as transport protocols, just above IP – Nevertheless, all IP routers are expected to speak ICMP IP Header ICMP message Protocol=ICMP Prof. Rick Han, University of Colorado at Boulder

ICMP (2) • Already seen it in use: – Ping – Traceroute – Discovery of local routers on a LAN • Format of an ICMP message: – Some Types: • • Type Code Cksum ICMP body Echo & Echo Reply *Destination Unreachable : dest not in routing table, or down *Source Quench : sent by router during congestion Redirect Router Advertisement Router Solication *Time Exceeded : TTL Expired Prof. Rick Han, University of Colorado at Boulder * = most frequently used

ICMP (3) • ICMP body often contains a copy of IP header (+ first 8 bytes of payload) of packet that generated the ICMP message • Ping: – A host sends an ICMP “echo” message – As IP packet, “echo” message gets routed to destination – At destination, respond by sending an ICMP “echo reply” message • Swap source and destination IP addresses and recompute checksum Prof. Rick Han, University of Colorado at Boulder

“Smurf” Denial of Service Attack via ICMP echo • Ping an IP broadcast address using spoofed source IP addr, e. g. ping 255 – All nodes on LAN respond to ICMP echo with ICMP echo request, directed at source – LAN and especially source are flooded • Solutions: – Patch OS to disallow ICMP echo request to ICMP echo using IP broadcast address – Don’t allow router to forward external IP broadcast addresses into your LAN Prof. Rick Han, University of Colorado at Boulder

Traceroute and ICMP • Trace the route of an IP packet – A host sends a regular IP packet to destination IP address with TTL of one – First router in path decrements TTL to zero, and sends back to source a “Time exceeded” ICMP message • Source address in ICMP message is first router on path ! – Increment TTL by one (TTL=2), next ICMP error message sent by second router in path – Keep incrementing TTL to find routers in path Prof. Rick Han, University of Colorado at Boulder

Traceroute and ICMP (2) • Trace the route of an IP packet Source Router 1 Timeline: Router 2 TTL=1 Router 1 known Router 2 known Destination known TTL=2 TTL=3 Prof. Rick Han, University of Colorado at Boulder Destination

Traceroute and ICMP (3) • Trace the route of an IP packet – Upon reaching destination, • No “Time exceeded” message generated • How do you know when final destination is reached? – Traceroute sends to unused UDP port (>30000), generating an ICMP “destination unreachable” message • With code “port unreachable” Prof. Rick Han, University of Colorado at Boulder

Router Discovery via ICMP • Routers periodically broadcast their ICMP router advertisement to local LAN – About every 7 minutes – Lifetime of 30 minutes • New hosts can broadcast ICMP router solicitation message, to avoid waiting 7 minutes • ICMP Redirect – Sent when there are two or more routers on the same LAN – Each router will know whether its neighbors on a LAN are closer to a destination • When source sends to higher cost router, that router sends an ICMP Redirect message to the source Prof. Rick Han, University of Colorado at Boulder

IP Tunnelling and VPN’s • IP router 1 builds an IP tunnel to IP router 2 – Router 1 encapsulates packets destined for network/LAN 2 with router 2’s IP address – Router 2 de-encapsulates • Advantages – Secure tunnels = Virtual Private Networks (VPNs) for corporations – Layered functionality = multicast/MBone – Encapsulate non-IP protocols LAN 1 R 1 Internet Prof. Rick Han, University of Colorado at Boulder R 2 LAN 2