Hi Siva, Your content on the each topic and explanation is very good. Incase if you get a chance , Please upload , OSPF , BGP , Multicast and QoS related Interview questions & Answers videos as well.
Hi Siva, thanks for the nice explanation. I have one question related to the MF bit set, consider initially the first packet sent from R1 is 2000 and it gets fragmented into two packets at the next-hop R2 exit interface as 1500 and 500 (Two packets). Suppose there happens another fragmentation at R3 for the 1500 bytes packet, then it fragment it to 1000 and 500, as per the R3 device it is fragmenting the single 1500 to 1000 and 500. In that case, how does the fragmented packet 500 knows that there is another fragment packet of 500 is on the link? i.e. how does it set the MF bit to 1.
if we initiate icmp from source to destination, based on the outgoing interface mtu fragmentation will happen. so icmp reply from destinatiin to source also get fragmented based on the mtu size configured in the outgoing inteface ???
Here are the fields that are changed after fragmentation: Flags Field: The 3-bit Flags field in the IP header is altered during fragmentation. The More Fragments (MF) flag is set to 1 in all fragments except the last one, indicating that more fragments are following. In the last fragment, this flag is set to 0. The Don't Fragment (DF) flag is relevant before fragmentation. If set, it indicates that the packet should not be fragmented. Fragment Offset Field: This 13-bit field is crucial in fragmentation. It indicates where in the original data stream this fragment belongs. The Fragment Offset is measured in units of 8 bytes (64 bits). The first fragment has an offset of 0. Subsequent fragments' offset values indicate their relative position in the original packet. Total Length Field: This field represents the total length of each fragment, including the header and the data. It changes for each fragment because fragments typically do not carry the same amount of data. Header Checksum: Since the fragmentation alters the header, the Header Checksum field is recalculated for each fragment to ensure integrity. Fields Not Changed: Identification Field: This remains the same in all fragments to identify that they are parts of the same original packet. Source and Destination IP Address: These fields are not altered during fragmentation. Other Fields: Fields like the TTL, Protocol, and others not directly involved in the fragmentation process remain unchanged.
I really liked the way you are explaning these small things we miss lot of time. But i have question regarding the PMTU do we have any effects when we are enabling the PMTU ? 2nd question in cisco SDWAN world how does the PMTU works does it use the BFD packets ? it would great if you can explain MSS and what relation is brings between the MTU. Thank you
Thanks Robin, PMTU does not affect the application, it really happens very quickly and in SDWAN also it works in a similar way once enabled, questions related to MSS and MTU is in the pipeline will upload once ready in this same playlist.....
When an IP packet is fragmented, each fragment will have its own IP header (20 bytes) but will share the same Ethernet header (14 bytes) when transmitted over the same Ethernet segment.
@@Mr.Acharyya the point to point connected interface MTU should match , otherwise it will create congestion, some vendor the interface itself won’t come up if mtu mismatch is there
By far one of the best video on this topic
Its really a good video. Looking forward to see the concepts like this.
Thanks for the video.
@@zahirhussain5482 thanks makesure subscribe to my channel I will upload more videos in coming days
Best explanation of fragmentation seen so far
Thanks Vinesh, do share with your friends and colleagues !!!!
Hi Siva, Your content on the each topic and explanation is very good. Incase if you get a chance , Please upload , OSPF , BGP , Multicast and QoS related Interview questions & Answers videos as well.
Superb Video seen so far...!
Hi Siva, thanks for the nice explanation. I have one question related to the MF bit set, consider initially the first packet sent from R1 is 2000 and it gets fragmented into two packets at the next-hop R2 exit interface as 1500 and 500 (Two packets). Suppose there happens another fragmentation at R3 for the 1500 bytes packet, then it fragment it to 1000 and 500, as per the R3 device it is fragmenting the single 1500 to 1000 and 500. In that case, how does the fragmented packet 500 knows that there is another fragment packet of 500 is on the link? i.e. how does it set the MF bit to 1.
if we initiate icmp from source to destination, based on the outgoing interface mtu fragmentation will happen.
so icmp reply from destinatiin to source also get fragmented based on the mtu size configured in the outgoing inteface ???
Very Nice explanation !!!
Annae mass...really great explanation.
Please explain MSS vs MTU
In PMTU, only catch is DF bit, Each Route sets its own MTU in reply to source.
In 1528 case ..1500 is data ,20 is ip header ,8 is ICMP header..i think simulator is not adding 8 bytes
Yes it’s cisco packet tracer tool, hence difference from the real-time values
Nice exlanation
I thought the 8 bit on top of the IP header of 20 is for the DF bit which is 8 bits.
Siva.. what are the fields changes in IP header after fragmentation ?
Here are the fields that are changed after fragmentation:
Flags Field:
The 3-bit Flags field in the IP header is altered during fragmentation.
The More Fragments (MF) flag is set to 1 in all fragments except the last one, indicating that more fragments are following. In the last fragment, this flag is set to 0.
The Don't Fragment (DF) flag is relevant before fragmentation. If set, it indicates that the packet should not be fragmented.
Fragment Offset Field:
This 13-bit field is crucial in fragmentation. It indicates where in the original data stream this fragment belongs.
The Fragment Offset is measured in units of 8 bytes (64 bits).
The first fragment has an offset of 0. Subsequent fragments' offset values indicate their relative position in the original packet.
Total Length Field:
This field represents the total length of each fragment, including the header and the data. It changes for each fragment because fragments typically do not carry the same amount of data.
Header Checksum:
Since the fragmentation alters the header, the Header Checksum field is recalculated for each fragment to ensure integrity.
Fields Not Changed:
Identification Field: This remains the same in all fragments to identify that they are parts of the same original packet.
Source and Destination IP Address: These fields are not altered during fragmentation.
Other Fields: Fields like the TTL, Protocol, and others not directly involved in the fragmentation process remain unchanged.
I really liked the way you are explaning these small things we miss lot of time.
But i have question regarding the PMTU do we have any effects when we are enabling the PMTU ?
2nd question in cisco SDWAN world how does the PMTU works does it use the BFD packets ?
it would great if you can explain MSS and what relation is brings between the MTU.
Thank you
Thanks Robin, PMTU does not affect the application, it really happens very quickly and in SDWAN also it works in a similar way once enabled, questions related to MSS and MTU is in the pipeline will upload once ready in this same playlist.....
@@SivakumarNetLabs Did you get a chance to create the content on questions related to MSS and MTU ? Please upload if it is done
Sir thank for explaining.. one question when ip header is added which is 20 bytes then what about ethernet header ??
When an IP packet is fragmented, each fragment will have its own IP header (20 bytes) but will share the same Ethernet header (14 bytes) when transmitted over the same Ethernet segment.
what if my outgoing interface MTU is 1500 and my next hop connected interface MTU is 1000?
@@Mr.Acharyya the point to point connected interface MTU should match , otherwise it will create congestion, some vendor the interface itself won’t come up if mtu mismatch is there
@SivakumarNetLabs absolutely right. But I saw in my practical that it is working unfortunately I can't capture the packet that project
@@Mr.Acharyya it might come up and work but recommend setting is to match the mtu where the interfaces connected point to point
@@SivakumarNetLabs Thanks
super
router doesn't reassemble the packet as far i know. Please check
The end destination device will do the reassemble it could be any layer 3 devices
@@SivakumarNetLabs Thanks
Spr sir
JAI SHREE RAM🙏