Hi, By default udp computes its mss starting from a 1500 mtu. You can avoid this by either changing the default, i.e., in startup.conf under the udp stanza add “mtu <value>”, or alternatively you can force mss on each connect, by explicitly setting it in the remote endpoint’s transport_endpoint_cfg_t structure.
If the dgram written through app_send_dgram is shorter than mss, the session-queue node sends it out as one “packet”, which could be a chained buffer if mss > ~2kB (default buffer size). IP layer could still decide to fragment the resulting buffer, subject to the output interface’s mtu. Otherwise, i.e. dgram larger than mss, session-queue forcefully fragments the dgram, this is probably something that you don’t want. Regards, Florin > On Jul 21, 2020, at 5:07 AM, [email protected] wrote: > > hi: > When sending 1500 bytes data with session app_send_dgram api, I find > session-queue node will split the data to two udp packets, then vpp will > sending to two separate IP packets out with no fragmentation. Even I set > interface MTU with 9000, It's still sending two separate IP packets. I think > the way session-queue process the app_send_dgram data have two logic logic > bug. > 1. If app_send_dgram's sending length more then snd_mss, session-queue > should do udp fragmentation, rather simplely separating to other udp packets. > 2. if app_send_dgram's sending length less then interface's MTU, > session-queue should sending one ip packet out. But the MTU seems have no > effect on session-queue. > > Below is the debug log: > DBGvpp# trace add session-queue 20 > DBGvpp# sh trace > ------------------- Start of thread 0 vpp_main ------------------- > No packets in trace buffer > ------------------- Start of thread 1 vpp_wk_0 ------------------- > Packet 1 > > 00:06:06:009843: session-queue > session index 0 thread index 1 > 00:06:06:009860: ip4-lookup > fib 0 dpo-idx 1 flow hash: 0x00000000 > UDP: 192.168.7.101 -> 192.168.7.100 > tos 0x00, ttl 255, length 1500, checksum 0x0000 (should be 0x25f7) dscp > CS0 ecn NON_ECN > fragment id 0x0000 > UDP: 12738 -> 500 > length 1480, checksum 0x0000 > 00:06:06:009868: ip4-rewrite > tx_sw_if_index 1 dpo-idx 1 : ipv4 via 192.168.7.100 eth0: mtu:9000 next:3 > 0000000000010000000000020800 flow hash: 0x00000000 > 00000000: 0000000000010000000000020800450005dc00000000ff110000c0a80765c0a8 > 00000020: 076431c201f405c80000010101010101010101010101010101010101 > 00:06:06:009873: eth0-output > eth0 > IP4: 00:00:00:00:00:02 -> 00:00:00:00:00:01 > UDP: 192.168.7.101 -> 192.168.7.100 > tos 0x00, ttl 255, length 1500, checksum 0x0000 (should be 0x25f7) dscp > CS0 ecn NON_ECN > fragment id 0x0000 > UDP: 12738 -> 500 > length 1480, checksum 0x0000 > 00:06:06:009878: eth0-tx > eth0 tx queue 1 > buffer 0x27685e: current data 98, length 1514, buffer-pool 0, ref-count 1, > trace handle 0x1000000 > local ip4 offload-ip-cksum offload-udp-cksum l3-hdr-offset > 112 l4-hdr-offset 132 > PKT MBUF: port 65535, nb_segs 1, pkt_len 1514 > buf_len 2176, data_len 1514, ol_flags 0xf0000000000000, data_off 226, > phys_addr 0x47a1800 > packet_type 0x0 l2_len 14 l3_len 20 outer_l2_len 0 outer_l3_len 0 > rss 0x0 fdir.hi 0x0 fdir.lo 0x0 > Packet Offload Flags > PKT_TX_IP_CKSUM (0x0000) IP cksum of TX pkt. computed by NIC > PKT_TX_TCP_CKSUM (0x0000) TCP cksum of TX pkt. computed by NIC > PKT_TX_SCTP_CKSUM (0x0000) SCTP cksum of TX pkt. computed by NIC > IP4: 00:00:00:00:00:02 -> 00:00:00:00:00:01 > UDP: 192.168.7.101 -> 192.168.7.100 > tos 0x00, ttl 255, length 1500, checksum 0x0000 (should be 0x25f7) dscp > CS0 ecn NON_ECN > fragment id 0x0000 > UDP: 12738 -> 500 > length 1480, checksum 0x95f3 > > Packet 2 > > 00:06:06:009940: session-queue > session index 0 thread index 1 > 00:06:06:009949: ip4-lookup > fib 0 dpo-idx 1 flow hash: 0x00000000 > UDP: 192.168.7.101 -> 192.168.7.100 > tos 0x00, ttl 255, length 66, checksum 0x0000 (should be 0x2b91) dscp CS0 > ecn NON_ECN > fragment id 0x0000 > UDP: 12738 -> 500 > length 46, checksum 0x0000 > 00:06:06:009952: ip4-rewrite > tx_sw_if_index 1 dpo-idx 1 : ipv4 via 192.168.7.100 eth0: mtu:9000 next:3 > 0000000000010000000000020800 flow hash: 0x00000000 > 00000000: 00000000000100000000000208004500004200000000ff110000c0a80765c0a8 > 00000020: 076431c201f4002e0000010101010101010101010101010101010101 > 00:06:06:009954: eth0-output > eth0 > IP4: 00:00:00:00:00:02 -> 00:00:00:00:00:01 > UDP: 192.168.7.101 -> 192.168.7.100 > tos 0x00, ttl 255, length 66, checksum 0x0000 (should be 0x2b91) dscp CS0 > ecn NON_ECN > fragment id 0x0000 > UDP: 12738 -> 500 > length 46, checksum 0x0000 > 00:06:06:009956: eth0-tx > eth0 tx queue 1 > buffer 0x276837: current data 98, length 80, buffer-pool 0, ref-count 1, > trace handle 0x1000001 > local ip4 offload-ip-cksum offload-udp-cksum l3-hdr-offset > 112 l4-hdr-offset 132 > PKT MBUF: port 65535, nb_segs 1, pkt_len 80 > buf_len 2176, data_len 80, ol_flags 0xf0000000000000, data_off 226, > phys_addr 0x47a0e40 > packet_type 0x0 l2_len 14 l3_len 20 outer_l2_len 0 outer_l3_len 0 > rss 0x0 fdir.hi 0x0 fdir.lo 0x0 > Packet Offload Flags > PKT_TX_IP_CKSUM (0x0000) IP cksum of TX pkt. computed by NIC > PKT_TX_TCP_CKSUM (0x0000) TCP cksum of TX pkt. computed by NIC > PKT_TX_SCTP_CKSUM (0x0000) SCTP cksum of TX pkt. computed by NIC > IP4: 00:00:00:00:00:02 -> 00:00:00:00:00:01 > UDP: 192.168.7.101 -> 192.168.7.100 > tos 0x00, ttl 255, length 66, checksum 0x0000 (should be 0x2b91) dscp CS0 > ecn NON_ECN > fragment id 0x0000 > UDP: 12738 -> 500 > length 46, checksum 0x9059 > > DBGvpp# sh int > Name Idx State MTU (L3/IP4/IP6/MPLS) > Counter Count > eth0 1 up 9000/0/0/0 rx > packets 164 > rx bytes > 16136 > tx > packets 9 > tx bytes > 6428 > drops > 164 > ip4 > 102 > ip6 > 62 > local0 0 down 0/0/0/0 drops > 2 > DBGvpp# sh session verbose > Thread 0: no sessions > > Connection State Rx-f > Tx-f > [1:0][U] 192.168.7.101:12738->192.168.7.100:500 OPENED 0 0 > > Thread 1: active sessions 1 > DBGvpp# >
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