I think No. Difference between maximum segment size and maximum transmission unit? Labels: Labels: Cisco Cafe. I have this problem too.
All forum topics Previous Topic Next Topic. Accepted Solutions. Sunil Bhadauria. In response to Sunil Bhadauria. This is the link to the best document on this topic. Joseph W.
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Basically, MTU is determined by hardware, i. In response to Joseph W. Hi Joseph, Sorry I got buzy in other stuff and could not come back to earlier, Thanks for the clarification on the topic, I had a doubt on mss value but I think i was wrong becuase today we changed the DNS Message Length Maximum from default of to In response to Muhammad Rafi.
Posting Explanation? Recommended Action None required. And what the referenced RFC section says: 3. Name space definitions Domain names in messages are expressed in terms of a sequence of labels. Each label is represented as a one octet length field followed by that number of octets. Since every domain name ends with the null label of the root, a domain name is terminated by a length byte of zero.
The high order two bits of every length octet must be zero, and the remaining six bits of the length field limit the label to 63 octets or less.
To simplify implementations, the total length of a domain name i. Although labels can contain any 8 bit values in octets that make up a label, it is strongly recommended that labels follow the preferred syntax described elsewhere in this memo, which is compatible with existing host naming conventions.
Name servers and resolvers must compare labels in a case-insensitive manner i. Non-alphabetic codes must match exactly. Post Reply. Latest Contents. Cisco's sky clears for October's Spotlight Awards ! But, it has no way of knowing the MTU of a link further up the path.
This could leave us in a sticky situation. The workstation may send a byte packet. The solution is fragmentation. When a packet is larger than the MTU, a device often a router will break the packet into smaller fragments. Each of these fragments is still a packet, just smaller than the original. The packets move along the path to the destination just like normal.
The destination device reassembles the fragments into the original packet and processes it as normal. This might sound like a wonderful solution, but it comes with its drawbacks. For one, each fragment has duplicated IP headers, which results in more traffic begin sent. Also, this adds processing overhead on the router that fragments the packets, and on the destination that reassembles them. And what if one fragment goes missing or becomes corrupted? We need to resend the entire packet, not just the fragment.
Firewalls may also have problems with fragments. Some of them will drop fragments if they arrive out of order, seeing them as unsolicited traffic. But there are other causes as well. Tunnels, such as a GRE tunnel , will add additional headers to a packet.
This may push the packet size over the frame limit and cause fragmentation. Encryption, IPSec for example , may also add additional headers, with the same effect.
So what do we do about this? But that still leaves us with fragmentation. MSS is normally decided in the TCP three-way handshake, but some setup might yield path where the decided upon MSS is still too big, leading to dropped packets. When using PPPoE, all the overhead means you needs to reduce the MSS on the way, normally by specifying it on the router where the chokepoint is found, which will then replace the MSS of passing threeway handshake by the correct lower value if it's higher.
MTU is used for fragmentation i. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Learn more. Ask Question. Asked 7 years, 5 months ago. Active 6 months ago. Viewed k times. I'm confused as to the difference between maximum segment size and a maximum transmission unit.
Can someone please explain in relation to layers 2 and 3? Improve this question.
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