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This blog post discusses the RTP in EIGRP. My question is how seq number is chosen and why in sending new packet the device should mention the seq number of last packet? for example

A-----seq:23 ack:0 ---->B
A<----seq:23 ack:23---- B

No problem so far, it says actually I have received the seq:23 and I acknowledge 23.

A-----seq:24 ack:23---->B

First problem is here, why should A put ack:23 in its packet? I mean it should be zero because A have not received ack for seq:24?

A<----seq:24  ack:24-----B

and now B acknowledges A seq:24 with ack:24? Please refer to the above link, I'm a bit confused.

  • sorry im new to this forum and its actually my second post,I should have add link description, – Mjaryan Apr 29 '14 at 13:04
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Нi! Look at 61 page of RFC1889

about your question: "is how seq number is chosen"

when device sending first RTP packet of the session, seq number must be random, to make known-plaintext attacks on encryption more difficult , in next packets seq is just increment

"B first problem is here", there is no problem, A tell to B, that A has recieved ack of the last B packet(which was seq 23)

"and now B acknowledge A seq:24 with ack:24???", yes ;-), B acknowledge to A, that seq 24 was recieved, by sending ack 24

added:

there is last version of Enhanced Interior Gateway Routing Protocol draft-savage-eigrp-02:

 The following sequence number rules must be met for the reliable
 EIGRP protocol to work correctly:

     o A sender of a packet includes its global sequence number
       in the sequence number field of the fixed header. The
       sender includes the receivers sequence number in the
       acknowledgment number field of the fixed header.
     o Any packets that do not require acknowledgment must be
       sent with a sequence number of 0.
     o Any packet that has an acknowledgment number of zero (0)
       indicates that sender is not expecting to explicitly
       acknowledging delivery. Otherwise, it is acknowledging
       a single packet.
     o Packets that are network layer multicast must contain
       acknowledgment number of 0.

But, in the rfc, there is no info about first seq number, so i`ve tested this on hardware. And as i see, seq number is starting from 1(not random, like in RTP). And it is intresting, looks like, that the seq number will be increment, until we will reboot device or restart eigrp process("no router eigrp"). "clear ip eigrp nei" wasnt drop seq num to zero, hm

  • the RFC 1889 discuss Real-time transport protocol, not Cisco Proprietary RTP in EIGRP "RTP as referenced by EIGRP refers to Cisco's proprietary Reliable Transport Protocol, not the much more commonly discussed Real-time Transport Protocol characteristic of VoIP communications" but thanks anyway – Mjaryan Apr 29 '14 at 13:38
  • as i understand, ciscos rtp is the same like "usual" rtp, with difference: cisco uses seq\ack only in packets which contain topology changes(like a tcp), in simple eigrp hello-packets there are no seq\ack fields(like a udp). Iam not sure about first sequence number, but i`ll check this tomorrow, on my test-stand(if nobody will answer) – pyatka Apr 29 '14 at 14:02
  • Dear friend VoIP's RTP is a wire-level protocol and Cisco RTP is a layer-4-like function!! that functions similar to TCP, but without any fancy windowing or congestion control,and it's proprietary.so there are 2 different protocol with 2 different function,anyway thanks again. – Mjaryan Apr 29 '14 at 14:56
  • Ive updated my answer ;-) – pyatka Apr 30 '14 at 6:08
  • Amazing my friend! the doc u have mentioned.answered almost all of my questions,thank you – Mjaryan May 1 '14 at 13:32

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