The Netwok 15 Layer IPv4 and IPv6 Part 3
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1 1 ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE The Netwok 15 Layer IPv4 and IPv6 Part 3 Jean Yves Le Boudec 2015
2 Contents 1. Fragmentation 2. Interworking h4 h6 with NATs 3. Proxy ARP Textbook Chapter 5: The Network Layer 2
3 1. Fragmentation Link layer networks have different maximum frame lengths MTU (maximum transmission unit) = maximum frame size usable for an IP packet MAC layer options and tunnels make this worse Link layer Network Ethernet, WiFi VPN Tunnel in IPv4 Token Ring 4 Mb/s 16 Mb/s FDDI X.25 Frame Relay ATM with AAL5 Hyperchannel PPP MTU to 1500 Z:\>netsh interface ipv6 show subinterfaces MTU MediaSenseState Bytes In Bytes Out Interface Loopback Pseudo Interface Wireless Network Connection VPN EPFL Local Area Connection Local Area Connection* 12 3
4 4 IPv4 Fragmentation IPv4 hosts or routers may have IP datagrams larger than MTU Fragmentation is performed when IP datagram too large re assembly is only at destination, never at intermediate points fragmentation is in principle avoided with TCP MTU = 1500 MTU = 620 MTU =1500 R1 R2 1 IP Header 1400 Bytes
5 5 IPv4 header fields 1 and 4 2a, 3a 2b,3b 2c, 3c Length Identification More Fragment flag Offset 8 * Offset IPv4 Header 1400 Bytes MTU = 1500 MTU = 620 MTU =1500 R1 R2 1 IPv4 Header 1400 Bytes 2a IPv4 Header 600 B 3a IPv4 Header 600 B 2b IPv4 Header 600 B 3b IPv4 Header 600 B 2c IPv4 Header 200 B 3c IPv4 Header 200 B
6 IPv4 Fragmentation (2) IP datagram is fragmented if MTU of interface < datagram total length all fragments are self contained IP packets fragmentation controlled by fields: Identification, Flag and Fragment Offset in IPv4 header; IP datagram = original ; IP packet = fragments or complete datagram 1 2a 2b 2c Length Identification More Fragment flag Offset 8 * Offset Fragment data size (here 600) is always a multiple of 8 Identification given by source 6
7 7 IPv6 Fragmentation Same as IPv4 except Routers never fragment drop packet if too large Fragmentation header is a «next header» Minimum MTU is 1280 Bytes (vs 68 for IPv4)
8 8 Fragmentation Considered Harmful Fragmentation requires re assembly at destination; this may cause deadlocks and identification wrapping problems unit of loss is smaller than unit of re transmission: can worsen congestion (avalanche effect) Solution = avoid fragmentation as much as possible by discovering Path MTU (= minimum MTU along one path)
9 9 Methods for setting Path MTU Path MTU Discovery (PMTUD) 1. Host sets Don t Fragment bit on all datagrams (IPv4 only with IPv6 routers never fragment) 2. sets PathMTU to local MTU 3. routers send ICMP message: destination unreachable/ fragmentation needed if MTU is too large 4. host reduces PathMTU estimate to next smallest value 5. after timeout, host increases PMTU estimate Packetization Layer Path MTU Discovery (PLPMTUD) does not require routers to send ICMP messages relies on TCP making Path MTU probes from time to time; estimate the largest possible Path MTU that works by observing packets that were acknowledged Path MTU is kept in destination cache by operating system
10 10 IPv6 example :\>netsh interface ipv6 show destinationcache Interface 12: Local Area Connection PMTU Destination Address Next Hop Address a00:1450:4001:c02::54 fe80::208:e3ff:feff:fc a00:1450:4002:801::100c fe80::208:e3ff:feff:fc a00:1450:4003:803::100f fe80::208:e3ff:feff:fc a00:1450:400a:804::1008 fe80::208:e3ff:feff:fc a00:1450:400a:805::100d fe80::208:e3ff:feff:fc a00:1450:400a:805::100f fe80::208:e3ff:feff:fc a00:1450:400a:807::1018 fe80::208:e3ff:feff:fc a00:1450:400a:807::101f fe80::208:e3ff:feff:fc a03:2880:f006:101:face:b00c:0:1 fe80::208:e3ff:feff:fc a02:26f0:12:198::eed fe80::208:e3ff:feff:fc50
11 11 TCP, UDP and Fragmentation The UDP service interface accepts a datagram up to 64 KB (by default) or up to 4GB (with IPv6 jumbo payload extension header UDP datagram passed to the IP service interface as one SDU is fragmented at the source if resulting IP datagram is too large to fit on Path MTU The TCP service interface is stream oriented packetization is done by TCP fragmentation should in principle be avoided TCP connections negotiate an MSS (maximum segment size) Host operating system should verify that
12 12 When a host generates UDP traffic, the port number is always 59% present in all packets 1. True 2. False 3. True with IPv4, false with IPv6 4. True with IPv6, false with IPv4 5. I don t know 19% 15% 7% 0%
13 13 Solution: When a host generates UDP traffic, the port number is always present in all packets False If fragmentation occurs, only the first fragment contains the port number. Fragmentation may occur at the source with UDP and with IPv6 as well as IPv4
14 14 One TCP segment is contained in one IPv4 datagram that is fragmented by a router on its way from source to destination. One of the fragments is lost. What will TCP retransmit? 1. The bytes that were in the missing fragment 2. The bytes that were in all fragments of the datagram, missing or not 3. It depends whether the loss is detected by fast retransmit or 85% by timeout 4. I don t know 7% 4% 4%
15 Solution TCP segment TCP header If the TCP segment is in several fragments and one of the fragments is lost, the whole segment is lost and will be retransmitted fragmentation/re assembly is done at IP layer this is why fragmentation of IP packets that contain TCP should be avoided 15
16 2. IPv4 and IPv6 Interworking IPv4 and IPv6 are incompatible v4 only host cannot handle IPv6 packets v6 only host cannot handle IPv4 packets What needs to be solved: interworking: h6 to h4 like to like access 6 to 6 over 4 4 to 4 over 6 with NATs In this module we study interworking; In a later module we will study like to like 16
17 Reminder: Dual Stack Application Layer Gateways (ALG46s) can be used to solve h4 to h6 Interworking 17 Homer s PC Web proxy Web server Application ALG46 Application TCP TCP TCP TCP/ IP IPv6 IPv6 IPv4 IPv4 IPv6 IPv4 Application layer gateway (e.g. web proxy) relays HTTP questions / answers. ALG must be dual stack Simple but performance is not optimal (store and forward) Plus dependency on application
18 NAT64 (RFC6146) is an alternative solution to solve h6 to h4 interworking h6 (client6) NAT??? h4 s IPv 6 translated address h4 (server4) 2001:620:618:dede::baba IPv6 only host IPv6 Internet 1 2 IPv4 Internet IPv4 only host??? h6 s IPv 4translated address NAT64 translates an IPv4 packet into an IPv6 packet and vice versa; no encapsulation NAT uses an IPv4 address pool to translate IPv6 address (No need for IPv6 private pool) Port translation is used (as in any NAT) to save number of IPv4 addresses of the pool To h6, h4 appears under an IPv6 translated address To h4, h6 appears under an IPv4 translated address 18
19 Translation from IPv6 address to IPv4 translated address is stateful From: 2001:620:618:dede::baba Next hdr : tcp Srce port = 2345 h6 (client6) NAT64 From: Protocol type: tcp Srce port = 1763 h4 (server4) 2001:620:618:dede::baba IPv6 only host IPv6 Internet 1 2 IPv4 Internet IPv4 only host v6 v4 2001:620:618:dede::baba tcp port tcp port 1763 NAT table NAT owns pool /24 Packet from h6 is translated by NAT to next available IPv4 source address /port number combination this is the usual NAT s job Need to keep table of all existing mapping this is called stateful operation 19
20 Translation from IPv4 address to IPv6 translated address is stateless 64:ff9b::c000:201 h6 (client6) 2001:620:618:dede::baba IPv6 only host IPv6 Internet NAT h4 (server4) IPv4 Internet IPv4 only host IPv4 addresses are mapped to valid IPv6 addresses such as 64:ff9b::c00:201 The block 64:ff9b::/96 is a well know prefix reserved for that use; other prefixes (ISP specific) may be used This is called stateless operation 20
21 How is the v4 address translated to the v6 address 64:ff9b::c000:201? 64:ff9b::c000:201 h6 (client6) 2001:620:618:dede::baba IPv6 only host IPv6 Internet NAT h4 (server4) IPv4 Internet IPv4 only host 1. The NAT keeps a translation table 2. It is algorithmic 3. None of the above 4. I don t know 76% 20% 4% 0%
22 22 Solution 64:ff9b::c000:201 64:ff9b::/ c000: :ff9b:0000:0000:0000:0000:c000: :ff9b::c000:201 The mapping is algorithmic The IPv4 address is written in the last 32 bits
23 23 How does client6 knows that NAT64 should be used? infoscience.epfl.ch AAAA 64:ff9b::c000:201 DNS64 infoscience.epfl.ch A DNS A and AAAA infoscience.epfl.ch? infoscience.epfl.ch? h6 (client6) NAT64 h4 (server4) 2001:620:618:dede::baba IPv6 only host IPv6 Internet IPv4 Internet IPv4 only host DNS64 is used in combination with stateful NAT64 DNS64 responds with translated IPv6 address if no AAAA record is found This is transparent to client6
24 24 NAT64, putting things together infoscience.epfl.ch AAAA 64:ff9b::c000:201 DNS64 infoscience.epfl.ch A DNS h6 (client6) 2001:620:618:dede::baba IPv6 only host infoscience.epfl.ch? IPv6 Internet NAT64 A and AAAA infoscience.epfl.ch? h4 (server4) IPv4 Internet IPv4 only host To: 64:ff9b::c000:201 From: 2001:620:618:dede::baba Next Header : tcp Source port 2345 Dest Port : 80 GET / To: From: Protocol type: tcp Source port 1763 Dest Port : 80 GET /
25 25 How does NAT64 translate to 2001:620:618:dede::baba? h6 (client6) NAT64 h4 (server4) 2001:620:618:dede::baba IPv6 only host IPv6 Internet IPv4 Internet IPv4 only host To: 2001:620:618:dede::baba From: 64:ff9b::c000:201 Next Header : tcp Source port 80 Dest Port : 2345 <html><head>loula From: To: Protocol type: tcp Source port 80 Dest Port : 1763 <html><head>loula 1. Algorithmically using the 64:ff9b::/96 prefix 2. Algorithmically using a different prefix 3. Using its NAT table and with the help of the port numbers 8% 4. I don t know 13% 79% 0%
26 26 h6 (client6) Solution NAT64 h4 (server4) 2001:620:618:dede::baba IPv6 only host IPv6 Internet IPv4 Internet IPv4 only host To: 2001:620:618:dede::baba From: 64:ff9b::c000:201 Next Header : tcp Source port 80 Dest Port : 2345 <html><head>loula From: To: Protocol type: tcp Source port 80 Dest Port : 1763 <html><head>loula Answer 3 TheIPv4 address of client6 is kept in NAT table (stateful); several client6 s may use the same IPv4 address, as with regular NATs but, in the reverse direction, server4 s IPv4 address is algorithmically mapped to an IPv6 address (stateless) in some sense, NAT64 treats IPv4 as WAN and IPv6 as LAN
27 client4 to server6 with NAT64 NAT64 and DNS64 support client6 server4 The reverse client4 server6 works with NAT64 with static configuration of NAT64 mapping from server IPv6 address to server s IPv4 translated address is stateful and requires ad hoc configuration of NAT64; must be put in DNS as A record. mapping from client4 IPv4 address to client s IPv6 translated address is NAT64 stateless 2001:620:618:dede::baba server6 IPv6 Internet IPv4 Internet client4 To: 2001:620:618:dede::baba From: 64:ff9b::c217:1819 Next Header : tcp Source port: 1763 Dest Port : 80 GET / From: To: Protocol type: tcp Source port: 1763 Dest Port : 80 GET / IPv6 address and port 2001:620:618:dede::baba ; port x IPv4 address and port : port x, static NAT64 table 27
28 Pros and Cons of NAT64 28
29 29 Pros and Cons of NAT64 Transparent to end users (no software change) No change in server software easy deployment Packet switching, no store and forward Stateful operation from IPv6 address to IPv4 translated address
30 30 Reminder: IP principle says one subnet = one LAN Assume you want to cheat with this principle e.g is in the wrong place, but you want to keep it that way You can support this configuration with /32 entries (with IPv4) in forwarding tables at R2 and R3 ( Host Routes ) dest next-hop interface 3. Proxy ARP / /24 R1 R2 R /24 On-link eth /24 On-link eth / eth1 dest next-hop interface / eth /24 On-link eth /32 On-link eth
31 31 sic500cs should be a. Modem + PPP sic500cs stisun greenmac ed0-ext EPFL-Backbone ed2-in All subnets have netmask Subnet 84 is on both sides of sic500cs 1. Router 2. Bridge 3. Application Layer gateway 4. I don t know 25% 25% 25% 25%
32 32 Solution Since greenmac is on same subnet as here sics500 should be a bridge
33 33 Assume you want to have sic500cs operating Modem + PPP greenmac as a router sic500cs ed0-ext stisun ed2-in EPFL-Backbone This deviates from the IP principle: one subnet = one LAN Therefore we need a second deviation (at least) to compensate We can use PROXY ARP sic500cs answers for ARP REQUESTS / NS on behalf of greenmac
34 ed0 ext has a packet to and sends an ARP REQUEST. Modem + PPP sic500cs stisun greenmac ed0-ext EPFL-Backbone ed2-in 1. All subnets have netmask 2. Subnet 84 is on both sides of sic500cs 3. sic500cs does PROXY- ARP on behalf of greenmac 53% 1. sic500cs replies with his own MAC address 2. sic500cs replies with the MAC address of greenmac 3. Greenmac replies with his MAC address 4. Greenmac replies with the MAC address of sic500cs 5. I don t know 20% 20% 7% 0%
35 35 Modem + PPP greenmac sic500cs ed0-ext stisun ed2-in Solution EPFL-Backbone dest next-hop interface /24 On-link eth /25 On-link eth / eth0 Answer 2 sic500cs responds with own MAC adddress ed0 ext now believes that greenmac has the same MAC address as sic500cs All traffic to greenmac is sent to sic500cs sic500cs needs appropriate forwarding table configuration (e.g. host routes)
36 The forwarding table at sic500cs is correct Modem + PPP sic500cs stisun greenmac ed0-ext EPFL-Backbone ed2-in 1. Yes, all packets to greenmac will reach greenmac 2. No, the second line should be / 23 for example 3. I don t know 0% 0% 0%
37 All links are Ethernet v2 with MTU = 1500 Bytes. Assume all hosts perform Path MTU and discover the best possible Path MTU value. What is the value of Path MTU at server4 for the interaction with client6? Bytes Bytes Bytes 4. I don t know 0% 0% 0% 0%
38 38 Solution IPv4 packet of server4 is translated into IPv6 packet. The translation adds 20 bytes. The Path MTU at server4 is 1480 Bytes for its interaction with client6 Note: If TCP is used, it is likely that client6 will advertise MSS = 1220 Bytes. server4 will accept this MSS and there will be no fragmentation problem.
39 Conclusion Fragmentation is due to different MAC layers having different packet sizes; MTU is by definition the max length of an IP packet at a given interface. When a packet is too large, it can be either discarded or fragmented. Fragmentation occurs only at IPv6 hosts, IPv4 hosts or IPv4 routers. Re assembly is never done by routers. Fragmentation may cause problems and should be avoided if possible. Translation between IPv4 and IPv6 is a possible solution to the h4 h6 interworking problem. It uses NAT64. Translation is stateless between an IPv4 address and a translated IPv6 address; it is stateful between an IPv6 address and a translated IPv4 address. NAT64 is automatic for client6< > server4 and static for server6 < > client4. Both require some manipulation of DNS. 39
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