CODEWITHSUNDEEP
linuxsocketsiptcpdumpmtu
user993833
user993833

Reputation: 81

how does the tcp packet “size” reported by tcpdump correlate to the actual IP packet sent?

When I run a tcpdump – what exactly does the number reported between the () after the lowest and highest sequence number mean?

  1. If it correlates to the IP packet sent, how can it be larger than the mtu of 1500?
  2. On the receive side of this flow tcpdump always sees packets of 1448 or smaller – where are these packets being defragmented and reassembled? Can I see that with tcpdump?
  3. Is there any relationship between this number and the size of the tcp socket buffer?

I.e: Typical tcpdump on the sender:

11:47:18.278352 IP 1.1.1.36.41034 > 1.1.1.37.60960: . 3263771:3272459(8688) ack 1 win 54 11:47:18.278371 IP 1.1.1.36.41034 > 1.1.1.37.60960: . 3272459:3282595(10136) ack 1 win 54 11:47:18.278620 IP 1.1.1.36.41034 > 1.1.1.37.60960: . 3282595:3298523(15928) ack 1 win 54 11:47:18.278727 IP 1.1.1.36.41034 > 1.1.1.37.60960: . 3298523:3301419(2896) ack 1 win 54 11:47:18.278731 IP 1.1.1.36.41034 > 1.1.1.37.60960: P 3301419:3301719(300) ack 1 win 54 11:47:18.777160 IP 1.1.1.36.41034 > 1.1.1.37.60960: P 3301719:3301723(4) ack 1 win 54 11:47:18.777175 IP 1.1.1.36.41034 > 1.1.1.37.60960: . 3301723:3303171(1448) ack 1 win 54

And the receiver:

11:47:18.277895 IP 1.1.1.36.41034 > 1.1.1.37.60960: P 990413:990417(4) ack 1 win 54 11:47:18.277948 IP 1.1.1.36.41034 > 1.1.1.37.60960: . 990417:991865(1448) ack 1 win 54 11:47:18.277953 IP 1.1.1.36.41034 > 1.1.1.37.60960: . 991865:993313(1448) ack 1 win 54 11:47:18.277958 IP 1.1.1.36.41034 > 1.1.1.37.60960: . 993313:994761(1448) ack 1 win 54 11:47:18.278028 IP 1.1.1.36.41034 > 1.1.1.37.60960: . 994761:996209(1448) ack 1 win 54

Upvotes: 0

Views: 4824

Answers (1)

ninjalj
ninjalj

Reputation: 43688

That is the length of the data, as you can see if you substract the two sequence numbers (after accounting +1 for SYN and FIN flags, if they are present in the packet).

As to why it's greater than the MTU, either your netword card can send jumbo-frames (quite normal on 1Gbps cards), or your card can accept large segments and divide them on hardware (called LSO/GSO or some similar acronym I cannot seem to recall).

Edit: See Large Segment Offload on Wikipedia.

Upvotes: 1

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