[DCCP]: Increment sequence numbers on retransmitted Response packets Problem: -------- If DCCP B receives a retransmitted Request from DCCP A (with an increased sequence number), DCCP A sends one or more Responses without incrementing A's sent sequence number. This problem was first identified thanks to Vladimir M. in http://www.mail-archive.com/dccp@xxxxxxxxxxxxxxx/msg00570.html RFC 4340 says: * sec. 4.2: "Every DCCP packet increments the sequence number, whether or not it contains application data." * sec. 7 : "Even DCCP-Ack and DCCP-Sync packets, and other packets that don't carry user data, increment the Sequence Number. [...] retransmissions, such as retransmissions of DCCP-Request packets, also increment the Sequence Number." Cause: ------ The cause is that make_response keeps the ISS on each retransmission; only Responses to retransmitted Requests have ISS incremented in dccp_check_req(), keep-alive Responses triggered via dccp_response_timer() retain initial ISS. Solution: --------- This patch uses the unused `acked' field in the inet_request_sock() associated with the connection request: to remember whether an incoming Request has already been ack(nowledg)ed by way of sending a Response. This helps to distinguish initial from retransmitted Responses, and is used to increment the sequence numbers accordingly. In addition, to protect against Request floods, the number of retransmissions is also incremented when sending further Responses in reply to retransmitted (i.e., non-initial) Requests. Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx> --- net/dccp/minisocks.c | 16 +++++++++------- net/dccp/output.c | 4 ++++ 2 files changed, 13 insertions(+), 7 deletions(-) --- a/net/dccp/minisocks.c +++ b/net/dccp/minisocks.c @@ -196,15 +196,17 @@ struct sock *dccp_check_req(struct sock /* Check for retransmitted REQUEST */ if (dccp_hdr(skb)->dccph_type == DCCP_PKT_REQUEST) { - if (after48(DCCP_SKB_CB(skb)->dccpd_seq, - dccp_rsk(req)->dreq_isr)) { - struct dccp_request_sock *dreq = dccp_rsk(req); + struct dccp_request_sock *dreq = dccp_rsk(req); + if (after48(DCCP_SKB_CB(skb)->dccpd_seq, dreq->dreq_isr)) { dccp_pr_debug("Retransmitted REQUEST\n"); - /* Send another RESPONSE packet */ - dccp_set_seqno(&dreq->dreq_iss, dreq->dreq_iss + 1); - dccp_set_seqno(&dreq->dreq_isr, - DCCP_SKB_CB(skb)->dccpd_seq); + dreq->dreq_isr = DCCP_SKB_CB(skb)->dccpd_seq; + /* + * Send another RESPONSE packet + * To protect against Request floods, increment retrans + * counter (backoff, monitored by dccp_response_timer). + */ + req->retrans++; req->rsk_ops->rtx_syn_ack(sk, req, NULL); } /* Network Duplicate, discard packet */ --- a/net/dccp/output.c +++ b/net/dccp/output.c @@ -323,6 +323,8 @@ struct sk_buff *dccp_make_response(struc skb->dst = dst_clone(dst); dreq = dccp_rsk(req); + if (inet_rsk(req)->acked) /* increase ISS upon retransmission */ + dccp_inc_seqno(&dreq->dreq_iss); DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_RESPONSE; DCCP_SKB_CB(skb)->dccpd_seq = dreq->dreq_iss; @@ -346,6 +348,8 @@ struct sk_buff *dccp_make_response(struc dccp_csum_outgoing(skb); + /* We use `acked' to remember that a Response was already sent. */ + inet_rsk(req)->acked = 1; DCCP_INC_STATS(DCCP_MIB_OUTSEGS); return skb; } - To unsubscribe from this list: send the line "unsubscribe dccp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
