MTU Amit Dhamija amit.dhamija@colt.net

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) 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

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)

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.

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

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 2 1 IP Packets Fragmentation MTU 3

Original packet may be fragmented multiple times along its route Multiple Fragmentations Original packet may be fragmented multiple times along its route Source Host Internet Process Destination Host Internet Process Fragmentation

IP Defragmentation only occurs once Internet layer process on destination host defragments, restoring the original packet IP Defragmentation only occurs once Destination Host Internet Process Defragmentation Source

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 1 Original IP Packet Fragments

Identification Fields & Fragment Offset IP packet has a 16-bit Identification field Version (4) Hdr Len (4) TOS (8) Total Length in bytes (16) Indication (16 bits) Flags (3) Fragment Offset (13) Time to Live (8) Protocol (8) Header Checksum (16) Source IP Address Destination IP Address Options (if any) PAD Data Field

Identification Fields Purpose Allows receiving host’s internet layer process know what fragments belong to each original packet Works even if an IP packet is fragmented several times 47 Original IP Packet Fragments

Fragment Offset Fragment offset field (13 bits) is used to reorder fragments with the same Identification field Contains the data field’s 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

This works even if fragments arrive out of order! Fragment Offset Field Receiving host’s 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 212 730 Fragment Offset Field

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

Thanks