Slide 1

MTU Amit Dhamija amit.dhamija@colt.net Slide 2 MTU Calculation in IOS In cisco IOS physical MTU consists of L2 headers. Example :- MTU 1500 on Ethernet links includes 14 bytes of L2 header and pass traffic upto 1514 bytes or IP-MTU = Physical-MTU In Cisco IOS when we ping and give the total payload size then it accommodates the (IP+ICMP Header) Example :- If we ping 1500 bytes as payload in IOS it means payload is 1472 as it includes 1472+20+8. MRU in IOS is physical-MTU so if it receives packet higher then MTU it will drop it, However in egress it will fragment automatically. IP-MTU command works only in egress direction Slide 3 MTU Calculation in IOS-XR In cisco IOS-XR physical MTU is excluding the L2 headers. Example :- MTU 1500 on Ethernet links excludes 14 bytes of L2 header and pass traffic upto 1486 bytes or IP-MTU =Physical-MTU-14 Bytes ( Ethernet Header) RP/0/RP0/CPU0:crs1.BLB#sh interfaces Te0/1/1/0 | in MTU Fri May 23 15:45:36.197 IST MTU 4484 bytes, BW 10000000 Kbit (Max: 10000000 Kbit) RP/0/RP0/CPU0:crs1.BLB#sh ipv4 interface Te0/1/1/0 | in MTU Fri May 23 15:45:47.241 IST MTU is 4484 ( 4470 is available to IP ) In Cisco IOS-XR when we ping and give the total payload size then it accommodates the (IP+ICMP Header) Slide 4 MTU Calculation in JUNOS In JUNOS physical MTU is excluding the L2 headers. Example :- MTU 1500 on Ethernet links excludes 14 bytes of L2 header and pass traffic upto 1486 bytes or IP-MTU =Physical-MTU-14 Bytes ( Ethernet Header) lab@Mx-240-Test> show interfaces xe-0/0/0 | match MTU Link-level type: Ethernet, MTU: 1600, LAN-PHY mode, Speed: 10Gbps, Protocol inet, MTU: 1582 Including one Vlan TAG In JUNOS adds icmp and ip header to the payload size you specify. So 1500 becomes 1528 bytes of ip packet. MRU in JUNOS is physical-MTU so if it receives packet higher then MTU it will drop it, However in egress it will fragment automatically. Slide 5 MTU Calculation in SR-OS In SR-OS physical MTU is excluding the L2 headers. Example :- MTU 1500 on Ethernet links excludes 14 bytes of L2 header and pass traffic upto 1486 bytes or IP-MTU =Physical-MTU-14 Bytes ( Ethernet Header) *A:KJC_PE_00001# show port 5/1/2 | match MTU Physical Link : Yes MTU : 9212 *A:KJC_PE_00001# show router interface "To_FSS_PE_00004_IPE-4" detail | match MTU IP Oper MTU : 9198 ICMP Mask Reply : True In SR-OS it adds icmp and ip header to the payload size you specify. So 1500 becomes 1528 bytes of ip packet. Service MRU in SR-OS has no limit so if it receives packet higher then MTU it will pass it, However in egress it will fragment automatically and ip-mtu is similar to IOS/IOS-XR Slide 6 Fragmentation in Egress Router If IP packet is longer than the MTU, the router breaks packet into smaller packets Called IP fragments Fragments are still IP packets IP Packet 21 IP Packets Fragmentation MTU 3 Slide 7 Multiple Fragmentations Original packet may be fragmented multiple times along its route Destination Host Internet Process Source Host Internet Process Fragmentation Slide 8 Defragmentation Internet layer process on destination host defragments, restoring the original packet IP Defragmentation only occurs once Destination Host Internet Process Defragmentation Source Host Internet Process Slide 9 Fragmentation and IP Fields More Fragments field (1 bit) 1 if more fragments 0 if not Source host internet process sets to 0 If router fragments, sets More Fragments field in last fragment to 0 In all other fragments, sets to 1 0011 Original IP PacketFragments Slide 10 Identification Fields & Fragment Offset IP packet has a 16-bit Identification field Total Length in bytes (16) Time to Live (8) Options (if any) Version (4) Hdr Len (4) TOS (8) Indication (16 bits)Flags (3)Fragment Offset (13) Source IP Address Destination IP Address Header Checksum (16)Protocol (8) PAD Data Field Slide 11 Identification Fields Purpose Allows receiving hosts internet layer process know what fragments belong to each original packet Works even if an IP packet is fragmented several times 47 Original IP PacketFragments Slide 12 Fragment Offset Fragment offset field (13 bits) is used to reorder fragments with the same Identification field Contains the data fields starting point (in octets) from the start of the data field in the original IP packet Value of offset is measured in 8 bytes. This is done because length of offset is only 13 bits long and cannot be represent a sequence of bytes greater then 8191.This force host/router that fragment datagrams to choose the size of each fragment so that first byte number is divisible by 8 Slide 13 Fragment Offset Field Receiving hosts internet layer process assembles fragments in order of increasing fragment offset field value This works even if fragments arrive out of order! Works even if fragmentation occurs multiple times 0212730 Fragment Offset Field Slide 14 Fragmentation Rule First fragment will be equal to ip-mtu 20 bytes and then rest of fragments can be summed up. Example below a link with ip-mtu 1300 with a payload of 5008 fragments into 4 1280 ( 0 1279) frag offset 0 - 0 1280 ( 1280 2559) frag offset 1280 1280 ( 2560 3839) frag offset 2560 1168 ( 3840 1279) frag offset 3480 Slide 15 Slide 16 Slide 17 Slide 18 Thanks